Sustainable Cooling Strategies Using New ... - ACS Publications

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Sustainable Cooling Strategies Using New Chemical System Solutions Matthias Seiler,*,† Annett Kühn,‡ Felix Ziegler,‡ and Xinming Wang§ †

Evonik Industries AG, BU Advanced Intermediates, Rodenbacher Chaussee 4, 63457 Hanau, Germany Technische Universität Berlin, Institut für Energietechnik, KT2, Marchstrasse 18, 10587 Berlin, Germany § Evonik Degussa Japan Co., Ltd., 2-3-1, Nishi-Shinjuku, Shinjuku-ku, Tokyo, 163-0938 Japan ‡

S Supporting Information *

ABSTRACT: Energy-efficient cooling concepts play an important role for numerous applications in the area of comfort and industrial cooling. In this regard, it represents a huge challenge to ensure that the cooling demand of industrialized countries is covered in a sustainable manner with a drastically reduced carbon footprint. In recent years, several innovations in the field of absorption have been introduced. While some of them are based on process improvements, other innovations are based on newly developed chemical system solutions. One example is the use of new working pairs for absorption chillers. In the past, working pair related drawbacks such as crystallization, corrosion, and instability led to a number of innovation barriers. One very promising way to overcome these drawbacks and thereby to allow for new or improved absorption chiller processes is the replacement of the state-of-the-art absorbent LiBr by a suitable ionic liquid (IL). This review aims at discussing new, energyefficient cooling concepts with a special focus on using ionic liquid based working pairs in absorption chillers/absorption heat pumps. The most relevant research and development activities are analyzed, a patent overview is provided, and both new technological opportunities and remaining scientific challenges are identified. Although a large number of ionic liquid papers have been published in this field during the last 10 years, many contributions do not adequately address the interdisciplinary set of application requirements and do not fully recognize that most imidazole-based ionic liquids are not suitable for being used in multieffect absorption cycles since their (in)stability at high temperatures in the presence of water and the resulting cooling capacity do not meet the industrial requirements.

1. INTRODUCTION Cooling, today, is mainly provided by chillers and airconditioners with electrically driven compressors. In Germany for example, the air-conditioning and refrigeration sector requires around 15% of the final electrical energy.1 In countries with higher ambient temperatures, this share might be significantly higher. As shown in Figure 1.1, there is a steady increase in the world market for air-conditioning (insignificantly interrupted by the downturn in 2009). In 2011, the world market for air conditioning was valued at US$ 88.2 billion; 60% higher compared to US$ 55 billion in 2006.2 Asia Pacific (mainly China and Japan) is still the largest world region in terms of air conditioning related sales with close to 55% of the world market in 2011.3 If the global community is to achieve the ambitious objective of reducing the greenhouse gas emissions a far greater weight has to be given to sustainable energy-saving cooling concepts. The energy-saving potential of the air-conditioning sector is large. In new or reconstructed buildings, solar and internal heat gains can be minimized and passive cooling techniques can be applied. But if active cooling is necessary, thermally instead of electrically driven cooling systems are an interesting alternative. Solar cooling or trigeneration (combined heat, power and cooling process) applications benefit from the simultaneity of cooling demand peak load and availability of redundant solar thermal energy or rejected heat in summer months. Thermally driven cooling devices therefore are able to reduce the peak electricity loads in summer and to alleviate the growing © 2013 American Chemical Society

problem of power supply shortages during summer months in warm climates, which are caused by the worldwide market increase of the number of operating air-conditioning appliances. An additional advantage is the rise in competitiveness of solar thermal collector systems and cogeneration units due to the allseason use. Another driving heat source for thermally driven cooling devices with a considerable energy-saving potential is industrial waste heat. It is often available throughout the whole year but in many cases, it remains unused while cooling is needed. Different technologies of thermally driven cooling are known. In this paper heat transformation processes with closed cycles and liquid sorbents (absorbents) are considered. Cooling devices working according to this principle are called absorption chillers and they are in widespread use. Additionally to the two well-known temperature levels of chilled water (low) and reject heat (medium), driving heat is supplied at a third (high) temperature level to energize the cycle. There are different absorption chiller cycle configurations possible, some driven with low temperature heat, others with high temperature heat. Type of heat rejection technology and location has an impact on the heat rejection temperature. Different chilled water temperatures result from different fields Received: Revised: Accepted: Published: 16519

April 24, 2013 October 18, 2013 October 18, 2013 November 13, 2013 dx.doi.org/10.1021/ie401297u | Ind. Eng. Chem. Res. 2013, 52, 16519−16546

Industrial & Engineering Chemistry Research

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Figure 1.1. Development of the world air conditioning market. Diagram created with data from ref 2.

of application, e.g. air-conditioning and refrigeration. For all these various cases different combinations of refrigerant and absorbent would be appropriate. Many working pairs for absorption chillers and refrigerators have been investigated and tested. Market available machines, however, predominantly work with water/lithium bromide and ammonia/water. Nevertheless, both working pairs have drawbacks which exclude the use of absorption chillers in many economically rewarding fields and impede the development of new cycles or applications. Therefore, it is about time to intensify the search for new working pairs. A promising family for new working pairs is based on ionic liquid as absorbent and another component like water or an alcohol as refrigerant. Ionic liquids are considered to have a high potential to overcome the deficiencies of the prevalent working pairs. Ionic liquids are most commonly defined as substances composed entirely of ions with melting points below 100 °C. More specifically, ionic liquids are organic salts that are fluid at (or close to) ambient temperature. Being a laboratory curiosity few decades ago, ionic liquids are now available on large scale and some industrial applications are in the stage of commercialization. Additional applications are currently being discussed and developed and much work remains to fully uncover the large potential of this innovative class of liquids. Their scientific and technical appeal is predominantly based on their variability and versatility. By selecting a suitable combination of cation and anion, the properties of ionic liquids can be adjusted over a relatively wide range. Among the most interesting and characteristic properties of ionic liquids are their extremely low volatility, their high thermal and electrochemical stability, their wide liquid ranges, their absorption capacity, as well as their comparatively small corrosion rates. On the basis of these qualities, the consideration of ionic liquids in the search for improved chiller absorbents, e.g. for the replacement of LiBr, appears rather self-evident. This review intends to discuss new promising cooling concepts with a special focus on using ionic liquid based working pairs in absorption chillers. The most relevant research and development activities in this field are evaluated and a patent overview is provided. However, since a number of ionic liquid/refrigerant systems have been recently suggested as new

industrial working pair without evaluation of the most important requirements, this review will also address a number of existing misconceptions in various papers in order to motivate additional research and development (RD) studies with a stronger linkage and alignment to the industrial application. Furthermore, new technological opportunities and remaining scientific challenges are identified.

2. STATE-OF-THE-ART OVERVIEW This chapter gives an overview about different refrigeration methods and explains the fundamentals of absorption cooling cycles more in-depth. After a general overview of requirements for working fluids the current state-of-the-art and advantages and drawbacks of working fluids for absorption chillers are discussed. Finally, a short market overview of absorption cooling systems is given. 2.1. Overview of Refrigeration Methods. The refrigeration methods can be divided into open processes and thermodynamic cycles. Open cooling processes are for example evaporative cooling or cooling by dissolution of salt mixtures. There are also combinations of open and cyclic processes such as desiccant evaporative cooling (DEC). The refrigeration cycles can further be classified according to the type of driving energy as mechanically and thermally driven cycles (see Figure 2.1). Of course, combinations of both mechanical and thermal driving source are also possible (e.g., Rankine cycle + vaporcompression cycle or absorption−compression hybrid cycles258). The main methods of cooling are the compression refrigeration process and the absorption refrigeration process. The most widely used process is the vapor-compression cycle. It can be divided further into subcritical and transcritical, single and multistage cycles. Figure 2.2 shows the scheme of a basic subcritical vapor compression cycle in which the refrigerant vaporizes in the evaporator E (by absorbing heat Q0 from e.g. the room to be chilled) is compressed (by input of mechanical energy W) and, then, condensed in the condenser C at a higher pressure and, hence, temperature level. Heat Q1 is released e.g. to the environment. Afterward, the refrigerant expands in a valve (EV) to the evaporator pressure. A detailed overview about compression cycles is provided for instance by Wang.4 Further information can be found in the ASHRAE Handbook− 16520

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of the refrigerant at a higher pressure level is identical, too. As essential difference, in the absorption cooling process the refrigerant vapor is not compressed by a mechanically driven compressor to overcome the pressure difference but pumped in a liquid state. Due to the considerably lower specific volume of liquid compared to vapor refrigerant the electrical energy input required is very small. To liquefy the refrigerant drawn off the evaporator a suitable liquid, the absorbent, is used. During the absorption process heat is generated. As absorption can only occur if the absorbent is sufficiently subcooled, the heat has to be released to an external sink. Usually, the same heat sink as in the case of the condenser is usedthe environment. Hence, to liquefy the refrigerant at the same temperature (ambient temperature) but at a lower pressure (evaporation pressure), the absorbent has to provide an adequate vapor pressure depression. The large negative deviation of aqueous LiBr solution from Raoult’s law (solid line) is displayed in Figure 2.4. Another representation is given in Figure 2.5 where the pressure is plotted over the absorbent mass fraction (ratio of absorbent mass to total mass of solution). While pure water (absorbent mass fraction x = 0) evaporates at 56 mbar, the solution with a mass fraction of x = 0.5 evaporates at 15 mbar. With increasing dilution of the absorbent due to the absorption process, the absorbent’s capacity of vapor pressure depression decreases. In order to maintain the process, the absorbent has to be regenerated. Therefore, the liquid solution is pumped to a higher pressure (condensation pressure), where heat is supplied to boil off the refrigerant. The concentrated absorbent flows back to the absorber, and the vapor refrigerant flows to the condenser and, after throttling, back to the evaporator. Both the refrigerant and the solution circuit are closed. As shown in Figure 2.3, the solution circuit which substitutes the electrical compressor contains two additional main heat exchangers, the absorber (A) and the desorber (D). Another heat exchanger, the so-called solution heat exchanger (SHX), is added to increase the efficiency of the process by internal heat exchange. The absorption cooling cycle is usually displayed in a vapor pressure diagram according to Figure 2.6. The vapor pressure curves described by the Clausius−Clapeyron relation

Figure 2.1. Types of refrigeration cycles.

Figure 2.2. Compression cooling cycle.

Refrigeration5 and in the proceedings of the International Congress of Refrigeration6 run every four years. Most vaporcompression refrigeration systems still use fluorinated gases as refrigerants. In order to reduce global warming, natural refrigerants like ammonia, CO2, hydrocarbons, or water are gaining popularity (again).7 2.2. Absorption Cooling Cycle. Just as in the conventional compression cooling process, in the absorption cooling process useful cold is produced by evaporation of the refrigerant at low pressure (see Figure 2.3). The condensation

⎧ ⎪ d ln p ⎨ ⎪d − 1 ⎩ T

(

)

⎫ r⎪ = ⎬ R⎪ ⎭

are almost straight lines when using a logarithmic pressure and a negative inverse temperature scale. In this diagram the two pressure and the three temperature levels are easy to recognize. xw and xs are the absorbent mass fractions of the weak (high concentration of refrigerant) and the strong (low concentration of refrigerant) solution. If the volatility of the absorbent is small as compared to that of the refrigerant, the absorbent mass fraction of the refrigerant xR is 0, i.e. there is no absorbent in the refrigerant cycle between condenser (C) and evaporator (E). The basic cycle described so far is the single-effect cycle. The efficiency of the cycle is described by the coefficient of performance (COP), the ratio of gain (cooling energy) to effort (driving heat)

Figure 2.3. Absorption cooling cycle. 16521



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Figure 2.4. Pressure plotted over absorbent mole fraction of water/LiBr solution at 35 °C.

Figure 2.5. Pressure plotted over absorbent mass fraction of water/LiBr solution at 35 °C.

negative heat of solution would allow COPs > 1, also for singleeffects cycles. From Figure 2.5, it can be derived that r = r+l

Q0 Q2

If we assume that at the evaporator basically heat of evaporation r is transferred and at the desorber heat of evaporation r and heat of solution l, the reversible COP results to COPrev =

− −

1 T2 1 T1

another frequent definition of COPrev, which can be derived from an energy and entropy balance, also. Advanced cycles are presented in section 4.2. More information about absorption refrigeration technology can be found in the work of Herold et al.8 Current research and development information is given in the proceedings of the International Sorption Heat Pump Conference9 run every three years. 2.3. Working Fluids. Refrigerants and absorbents should meet certain property requirements as they influence power density (capital expenditure) and efficiency (operational expenditure) of the cooling machines. Also, the operational conditions resulting from the specific application of the cooling system, i.e. the chilled water, the cooling water, and the driving temperature level require certain desirable properties of the working fluid(s). In addition to typical requirements for working fluids like a low GWP (global warming potential), zero ODP (ozone

Figure 2.6. Absorption cooling cycle in the ln p/(−1/T) diagram.

COP =

1 T1 1 T0

r r+l

As long as l ≥ 0, the COPrev for single-effect cycles is limited to 1. For known absorption working pairs, l is in the order of 10% of r and COPrev is therefore about 0.9. Working pairs with 16522



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Figure 2.7. Absorption cooling cycle plotted over the vapor pressure lines of water/LiBr. Diagram created with data from ref 14.

Figure 2.8. Absorption cooling cycle plotted over the vapor pressure lines of NH3/water. Diagram created with data from ref 15.

absorbent with a negligibly low vapor pressure is favorable for the process to avoid the need of rectification. Process limiting factors like crystal formation are disadvantageous. As heat and mass transfer is essential for the chiller performance for both, the refrigerant and the absorbent, a low viscosity, a high density, a high thermal conductivity, and a high diffusivity are favorable. In addition, the surface tension should be low to ensure a good wetting of the heat exchangers. A low specific heat capacity and a low specific heat of solution in relation to the latent heat of evaporation increase the process efficiency. However, a high vapor pressure depression which comes from a high deviation of Raoult’s law implies a high specific heat of solution. A high specific heat of solution decreases the necessary driving temperature (for more details see ref 236). Finally, the working fluid sources should not be limited and transport and storage possibilities should be costeffective and simple. Although many working pairs for absorption cooling systems have been suggested over the last half century,10 only two prevail: water/LiBr and ammonia/water.

depletion potential), compatibility with other materials, and nontoxicity, suitable refrigerants and absorbents should not be flammable, have to be chemically stable, should have a low water pollution class, and must not decompose in the relevant pressure and temperature range. To keep the volumetric refrigerant flowrate low, the refrigerant should have a high specific heat of evaporation and, at the same time, a small specific volume of the of the vapor. The latter, of course, priorizes high pressure refrigerants. On the other hand, the vapor pressure should be moderate to allow an operation around atmospheric pressure. The difference between evaporation and condensation pressure should be low with regard to electricity consumption of the solution pump. The freezing temperature of the refrigerant determines the application temperature. It should be as low as possible to not be limiting. Most importantly, the absorbent should absorb the refrigerant with an adequate capacity. In other words, a significant decrease in the partial pressure of the refrigerant in the relevant range for cooling applications is essential. An 16523



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Figure 2.9. Absorption cooling system.

refrigeration purposes or industrial processes to 6 to 18 °C for air-conditioning. For compression chillers and refrigeration systems, the lower the chilled water temperature level, the less efficient is the cooling process. In absorption cooling systems this is also valid but to a much lower extent. The efficiency stays almost constant, however, the higher the chilled water temperature, the lower driving temperatures are required. Condensation and absorption heat usually is rejected to the ambient air (dry cooler, wet, or hybrid cooling tower) or to the ground (ground probes, ground collector, groundwater) via a cooling water cycle. Sewage water is an interesting alternative. Ideally, heat at medium temperature is not rejected but also used, e.g., for heating purposes. However, in most cases, cooling and heating demand do not occur at the same time. Cooling water temperature range is mainly between 23 and 40 °C but may also be higher. Driving heat preferably is provided by solar or waste heat. For reasons of energy efficiency, the use of fossil fuels to drive an absorption chiller should be avoided.16 The driving heat level required depends on the temperature lift. A high temperature lift requires a high driving temperature. Temperature from typical driving heat sources ranges from around 55 °C (solar cooling) up to 95 °C (district heat). Higher driving temperatures, e.g. from industrial processes or concentrating solar collectors above 120 °C allow for the use of advanced absorption cycles, e.g. double-effect cycles with higher efficiency. 2.4.2. Evolution of the Market. The principles of absorption cooling were discovered in the second half of the 18th century. In 1777, Gerald Nairne used sulfuric acid to absorb water. The first absorption ice maker was built in 1810 by Sir John Leslie. In 1878, Franz von Windhausen built the first continuously working absorption refrigerator. Shortly later, the first large absorption machines were being manufactured for ice making and process cooling in the chemical and petroleum industries. The availability of cheap electricity then promoted the introduction of electrically driven compression chillers and refrigeration systems which stopped the dissemination of absorption cooling machines. They only regained popularity to a significant extent in periods of high electricity rates. The first large water/LiBr chiller for air conditioning has been marketed by the US company Carrier in 1945. Today, the absorption cooling technology is considered to be mature. The market for large absorption chillers (up to 30 MW) is mainly concentrated in China, Japan, and South Korea. The absorption chiller are mainly manufactured in those countries (e.g. by Hitachi, Kawasaki, Yazaki, LG, Ebara, Panasonic, Shuangliang, Broad, and Century). A few absorption chillers are manufactured in the United States (e.g., by Carrier, Trane, and York/ Johnson Controls) and in India (e.g., by Thermax). The major

Water has a high latent heat, is chemically stable, nontoxic, environmentally neutral, and economically feasible. A drawback, however, is the low vapor pressure which requires a vacuum tight construction of the vessels. The freezing temperature of 0 °C limits the application. The production of chilled water around or lower than 0 °C is possible only by adding freezing point decreasing substances like salts.11−13 An aqueous LiBr solution has a negligible vapor pressure, a low viscosity, and is nontoxic. Both the formation of crystals at higher absorbent concentration and increasing corrosion rates at higher driving temperatures are unfavorable. Therefore, the possible temperature lift, the difference between low and medium temperature level, is limited (see Figure 2.7). If low chilled water temperatures are required (e.g., for air dehumidification), the cooling water temperature must be low as well, which often requires the use of wet cooling towers. This restricts the application of water/LiBr chillers in regions with water scarcity (e.g., Arabian Peninsula). Nevertheless, the working pair water/LiBr permits the highest energetic and economic efficiency using simple, well-engineered, and relatively compact systems. Ammonia, in contrast, is toxic, flammable and explosive. Depending on the charge special safety precautions are necessary. The vapor pressure is high (see Figure 2.8). Therefore, pressure vessels are needed and the solution pump requires more energy. Water has a significant vapor pressure as compared to ammonia, and consequently, a rectification unit is essential. It is an advantage that the ammonia/water solution does not crystallize. Furthermore, ammonia/water permits the generation of very low refrigeration temperatures down to −40 °C and the use of high cooling water temperatures if driving temperature is high enough. Ammonia/water refrigeration systems are slightly more complex, not as efficient as water/ LiBr chillers and need more auxiliary power. Water/LiBr and ammonia/water meet the hitherto existing demands on working pairs for absorption chillers and refrigeration machines quite wellor engineers have become used to their restrictions. In the past decade, only few research groups investigated alternative working pairs. Nevertheless, in light of the current energy and climate discussion some interesting applications are discussed (e.g., heat transformers) which revive the research on new working pairs. In particular, ionic liquids are given increased attention. 2.4. Absorption Cooling Systems. 2.4.1. System Aspects. To be operated, the absorption chiller has to be connected to a high temperature heat source, a low temperature heat source, and a medium temperature heat sink. Potential system configurations are presented in Figure 2.9. The low temperature heat source is the cold distribution system. The chilled water temperature level provided by absorption cooling machines ranges from −40 °C for 16524



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research groups started focusing on evaluating water/ionic liquid-systems for their potential use in absorption chillers.20−27 Since then, many more academics and companies joined this field and have made valuable contributions. A wide variety of ionic liquids has been investigated as potential absorbent for chiller absorbents. Cationwise, the following prominent candidates were often involved: [bmim] 1-butyl-3-methylimidazolium n-butyl-4-methylpyridinium [bmpy] [bmpyrr] 1-butyl-1-methyl-pyrrolidinium [Cholin] 2-hydroxy-N,N,N-trimethylethanaminium [DMEA] N,N-dimethylethanolammonium [emim] 1-ethyl-3-methylimidazolium [eeim] 1-ethyl-3-ethylimidazolium [empy] 1-ethyl-3-methylpyridinium [epy] 1-ethylpyridinium [EtOHmim] 1-methyl-3-2-hydroxyethyl-imidazolium [DMEA] n,n-dimethylethanolammonium [H2NC3H6mim] 1-(3aminopropyl)-3-methylimidazolium [HDEA] diethanolammonium [HEMA] monoethanolammonium [hmim] 1-hexyl-3-methylimidazolium [HTEA] triethanolammonium [mmim] 1,3-dimethylimidazolium [mmpy] 1,3-dimethylpyridinium [OHemim] 1-(2-hydroxyethyl)-3-methylimidazolium [omim] 1-octyl-3-methylimidazolium [P2444] ethyl(tributyl)phosphonium Anionwise, a lot of attention was directed toward the following prominent candidates: [Ac] acetate [B(CN)4] tetracyanoborate [BF4] tetrafluoroborat [Br] bromide [Cl] chloride [DBP] dibutylphosphate [DEP] diethylphosphate [DMP] dimethylphosphate [EtSO4] ethylsulfate [HSO4] hydrogensulfate [I] iodide [MeSO4] methylsulfate [MeSO3] methanesulfonate [NO3] nitrate [OTf] trifluoromethanesulfonate [PF6] hexafluorophosphate [SCN] thiocyanate [SO4] sulfate [TFA] trifluoroacetate [Tf2N] bis(trifluoromethylsulfonyl)imide [TOS] tosylate (p-toluenesulfonate) For water/ionic liquid systems the experimental and theoretical results obtained so far can be subdivided into the following groups: (1) Thermodynamic evaluations focusing on measuring and modeling vapor−liquid equilibria (VLE) or selected P,T,x-data20−157 (and references therein, see also the Supporting Information), (2) Thermophysical properties32,34,60,94,102,158−166 (see also the Supporting Information), (3) Interactions and orientational dynamics,51,105,167−172

growth area is still supposed to be the Asia-Pacific region, led by China, India, and South Korea.17 Up to now, gas-fired absorption chillers have been used for economic reasons in areas with high electrical loads in summer. Currently, numbers of waste heat driven and combined cold/ heat/power (trigeneration) installations are increasing, worldwide. Energy strategies of governments also play an increasing role. Large ammonia/water machines are produced in smaller quantities and are often tailor-made. They are mainly used in the industry for low temperature applications. Ammonia/water absorption refrigerators were common in the domestic market up to the 1940s. Featuring a thermally driven solution pump, they are still produced in larger quantities for the recreational vehicle market and the camping sector. Another market is hotel room refrigerators due to their silent operation. In the past decade, for both working pairs several small to medium scale absorption chillers (10−50 kW cooling capacity) for solar cooling or the use in trigeneration systems have been developed, mainly in Europe where academic RD has always been relatively strong in this field. Some models were already launched onto the market (e.g., by EAW Energieanlagenbau, Pink, and AGO). In 2009, in a joint project of Technische Universität Berlin, Bavarian Center for Applied Energy Research (ZAE Bayern), and Vattenfall Europe (a utility company which operates large district heating systems) a 50 kW water/LiBr absorption chiller to be driven by district heat has been developed.18 The focus of development was set on a large temperature glide in the district heat between supply and return, a high efficiency over a large driving and chilled water temperature range, a low specific volume and footprint, and low manufacturing costs. The aim was also to develop an efficient overall system including heat rejection, hydraulic components, and control to ensure a low parasitic energy demand. The low minimum driving temperature of only 55 °C is a novelty, also. The chiller, therefore, is also very suitable for solar cooling. To date, the chiller is installed in several demonstration sites including the German Federal Environment Agency. A photograph of the prototype and the specifications are presented in Figure 2.10.

3. ABSORPTION COOLING USING IONIC LIQUID BASED WORKING PAIRS 3.1. Water/Ionic Liquid Systems. During the last 60 years, a number of working fluids have been suggested for absorption cycles.10,19 Around the turn of the century, the first

Figure 2.10. Photo and specifications of a 50 kW absorption chiller. 16525



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(4) Process simulation and heat and mass transfer aspects of absorption chillers/absorption heat pumps,33,41,44,48,97,99,100,109−114,153,154,173−188 (5) Experimental absorption chiller studies focusing on the evaluation of water/ionic liquid working pairs,12,48,86,103,106,109,112,173−176,189−199 (6) General discussions about working pair requirements, chiller characteristics, IL-based patent applications, and valuable adjacent RD aspects19,28,29,89,104,190,192,198−270 (see also the Supporting Information). In this subchapter, only selected studies on water/ionic liquid systems are briefly summarized and discussed. The aim is to highlight new results in this area and to draw the attention to selected aspects that have not been adequately addressed so far. Additional studies can be found elsewhere.10,28,29,257−269 Several thermophysical properties of ionic liquids are strongly influenced if only traces of water or impurities are present in the sample. Therefore, the quality of the data often depends on the awareness and capability of the respective research group to identify and quantify impurities and/or the amount of water being present in the respective ionic liquid sample. Regarding the use of ionic liquids in absorption chillers or absorption heat pumps this is crucial in many respects: • The accuracy in determining the water concentration of ionic liquid/water systems is important for accuracy of the vapor−liquid equilibria (Dühring plot). • Determining COP, cooling capacity, and the heat and mass transfer in chillers and heat pumps is closely connected to the discussion about the film thickness, the solution viscosity, and the wetting behavior in the heat exchangers. All these aspects are greatly influenced by the water concentration. • Stability issues (thermal and chemical stability) of ionic liquid based working pairs are often discussed by focusing on analyzing pure ionic liquids, e.g. by thermogravimetric analysis (TGA), differential thermal analysis, and thermogravimetry (DTA-TG), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), triple quadrupole liquid chromatography−mass spectrometry, or headspace gas chromatography (HSGC). Investigations on pure ILs do not reflect the industrial reality since the refrigerant is always present in absorption chillers or heat pumps. The long-term stability needs to be evaluated under generator conditions which are typical for e.g. direct-fired doubleeffect absorption chillers. Both the water concentration and also low-concentrated impurities of ionic liquids can trigger ionic liquid degradation and the formation of noncondensable gases. Therefore, they can have an enormous impact on the long-term stability of an ionic liquid based working pair as well as on the process performance. The thermal decomposition temperature for selected water miscible ionic liquids is listed in Table 3.1. As described by Ficke, it is well-known that the decomposition temperatures are highest for ionic liquids containing the bis(trifluoromethylsulfonyl)imide anion.30 There is a substantial influence of the anion on the decomposition temperature as ionic liquids with an [emim]+ cation span in temperature from 178 to 388 °C (see Table 3.1). Increasing alkyl chains has little effect. For [emim][EtSO4], [emim][MeSO4], and [emim][HSO4] the decomposition temperature is 355, 362, and 359

Table 3.1. Decomposition Temperatures of Water Miscible Ionic Liquids39 IL

decomposition temperature, °C

[emim][TFA] [OHemim][TFA] [emJm][EtS04] [emim][HS04] [emim][MeS04] [emim][OTf] [emim][MeS03] [emim][SCN] [emim][DEP] [P2444][DEP]

178 187 355 359 362 388 335 281 273 314

°C, respectively.31 [emim][TFA] and [OHemim][TFA] show even lower decomposition temperatures. Keil et al.32 investigated the long-term stability and the regeneration and recycling of imidazolium-based ionic liquids and indicated that there is a poor understanding so far about which decomposition products are formed in dependence of the process conditions. They suggest analytical approaches for an adequate evaluation of the long-term stability of ionic liquid systems. Huangfu et al.33 investigated the thermal stability, of pure [bmim][BF4] and [bmim]2[SO4] . The temperature range of the respective liquid state was found to reach from −36 to 475 °C and from −93 to 263 °C, respectively. Moreover, [bmim][BF4] showed a much higher decomposition temperature of around 433 °C. Other investigations about thermal stability and related reviews can be found elswhere.31,32,34,35 However, when evaluating water/ionic liquid working pairs in direct fired double-effect absorption chillers even those ionic liquids of Table 3.1 with a comparatively high decomposition temperature show an insufficient (chemical) stability.36−38 This is why it needs to be stressed that the suitability of an ionic liquid in terms of stability cannot be judged on the basis of analytical TGA measurements of the pure ionic liquid. Ideally, chiller experiments are necessary to confirm the long-term stability (thermal and chemical stability) of an ionic liquid based working pair. If these chiller trials cannot be carried out (e.g., due to complexity and/or cost reasons) a method needs to be set up where (i) the ionic liquid is mixed with a representative water concentration (e.g., 20 wt %), (ii) this mixture is kept for a longer period of time (e.g., several months) at representative generator conditions (e.g., 180 °C), (iii) samples are eventually taken from this mixture and analyzed for thermal and chemical stability using the well established analytical methods. Ficke and Brennecke also carried out an evaluation of a variety of H2O/IL-systems focusing on thermo-physical properties and thermodynamics. Density, thermal decomposition, melting point, glass transition temperatures, excess enthalpy, heat capacity, and phase equilibria were measured. The experimental VLE data were also described using the nonrandom two-liquid (NRTL) model.30,31,39 Ficke and Brennecke also investigated the interactions between ionic liquids and water for aqueous binary systems containing [OHemim] [TFA], [emim] [HSO4], [emim] [MeSO4], and [emim] [MeSO3]. Adding a hydroxyl group to the ethyl chain of the [emim] cation results in increased hydrogen bond 16526



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Table 3.2. Partial Pressure of Water for Different Absorbent Concentrations at 308.5 K29 partial pressure of refrigerant/Pa absorbent 1 2 3 4 5

refrigerant

LiBr [C1mim][Me2PO4] [C2mim][CH3COO] choline acetate customized, proprietary IL-absorbent formulation by Evonik

H2O H2O H2O H2O H2O

temperature/K

at 60 wt % of absorbent

at 90 wt % of absorbent

308.5 308.5 308.5 308.5 308.5

5.2 × 10 46.9 × 102 30.8 × 102 27.0 × 102 4.9 × 102

LiBr crystallization 20.8 × 102 4.4 × 102 3.5 × 102 1.0 × 102

2

Bhargava et al. reviewed recent developments in the studies of H2O/IL mixtures using computational methodologies.40 Danten et al. investigated the interaction of water diluted in 1butyl-3-methyl imidazolium ionic liquids by vibrational spectroscopy modeling.171 Sturlaugson et al.51 presented results of optically heterodyne detected optical Kerr effect experiments on a series of pure 1-alkyl-3-methylimidzolium tetrafluoroborate room temperature ionic liquids (RTILs) and their mixtures with water. Freire et al.52−56 investigated the solubilities of nonaromatic piperidinium- and pyrrolidinium-based ionic liquids in water and predicted the mutual IL/water solubilities by using a quantitative structure−property relationship. Li et al.57 investigated the thermodynamic performance of 1,3-dimethylimidazolium chloride [Dmim][Cl] or of 1,3dimethylimidazolium tetrafluoroborate [Dmim][BF4] as an absorbent enhancing additive and carried out vapor pressure measurements of the ternary systems H2O + LiBr + [Dmim] [Cl], H 2O + LiBr + [Dmim][BF4], H 2 O + LiCl + [Dmim][Cl], and H2O + LiCl + [Dmim][BF4]. The results indicate that the effect on decreasing the partial pressure of water follows the order LiCl + [Dmim][Cl] > LiCl + [Dmim][BF4] > LiBr + [Dmim][Cl] > LiBr + [Dmim][BF4]. In summary, it needs to be pointed out that most stability investigations that have been carried out so far are not adequate to discuss the suitability of H2O/IL working pairs since the test conditions chosen are not representative for the generator in terms of concentration and temperature. Further investigations are necessary that take the aforementioned recommendations into account. Additional studies focusing on refrigerants other than water or on process simulation and experimental chiller studies are discussed in the following subsections. 3.2. Nonaqueous Refrigerant/Ionic Liquid Systems. The thermodynamic properties of pure ionic liquids58−60 and their mixtures with molecular solvents61−63 have been investigated intensely in the past (see also references listed in the Supporting Information). Most of them describe the vapor−liquid equilibria of systems containing ILs and light alcohols. Different systems consisting of ILs and methanol were investigated by various research groups28,41,42,64−67,202−213,267 including the ILs [bmim][Ac], [bmim][BF4], [bmim][DBP], [bmim][HSO4], [bmim][Tf2N], [emim][DMP], [emim][Tf2N], [mmim][DMP]. The best results, i.e. the lowest partial pressure of the alcohol, were reported for [bmim][Ac] by Revelli et al.,67−69 [mmim][DMP] by Kato and Gmehling64,70 and He et al.,42 and [emim][DMP] by Ren et al.41 Especially the system ethanol/IL has been extensively studied28,41,42,64−67,71,72,202−213,267 involving the ionic liquids [bmim][Ac], [bmim][BF4], [bmim][DBP], [bmim][HSO4], [bmim][Tf2N], [bmpy][TOS], [emim][DMP], [emim][Tf2N], [emim][TOS], [mmim][DMP]. Similar to the results

formation. Increasing the alkyl chain on the sulfate anion from [HSO4] over [MeSO4] to [EtSO4] decreases the excess enthalpy due to decreasing negative charges on the oxygen atoms of the anions and increasing hydrophobicity.39 Ren et al.41 measured vapor pressures, excess enthalpies, and specific heat capacities of the binary working pairs containing the ionic liquid ethyl-3-methylimidazolium dimethylphosphate. He et al.42 suggest 1,3-dimethylimidazolium dimethylphosphate [MMIM][DMP] as absorbent for the refrigerants water, ethanol, or methanol and discussed new results including vapor pressure, density, viscosity, and heat capacity as well as excess enthalpy. Zuo et al.43 presented experimental vapor pressure, heat capacity, and density data for the water/1-ethyl3-methylimidazolium ethylsulfate (EMISE) system and concluded that EMISE is a promising absorbent for absorption chillers. Yokozeki and Shiflett investigated selected H2O/IL-systems for absorption cooling. They found high solubilities of water in selected imidazole-based and choline-based ionic liquids and correlated the VLE data successfully by using an equation of state model.44 Freire et al.45 evaluated the predictive capability of COSMORS for liquid−liquid equilibria (LLE) and vapor−liquid equilibria (VLE) of water and several imidazolium-based ionic liquid mixtures. Other groups demonstrated that the anion is essential when aiming at lowering the partial pressure of water p(H2O). While, for instance, an ethylsulfate anion of the ionic liquid [C2mim][C2H5SO4] corresponds to a moderate decrease in p(H2O), the dimethylphosphate anion of the ionic liquid [C1mim][dimethylphosphate] leads to a stronger attraction of the water molecules corresponding to a smaller activity coefficient of H2O. However, with increasing compactness of the anion, the partial pressure can be decreased further.46 Also for systems illustrated in Table 3.2, the partial pressure of water is comparatively low when using ionic liquids with small and compact anions such as acetate or chloride based ionic liquids. However, neither acetate-based nor chloride-based ionic liquids are suitable for multieffect absorptions cycles due to an insufficient stability and/or too high corrosion rates. Schaber et al. experimentally determined the wetting behavior and the VLE of the systems H2O/ethylmethylmidazolium acetate and H2O/diethylmethylammonium methansulfonate by Fourier transform infrared (FT-IR) spectroscopy in a dynamic cell and via Raman-spectroscopy in a static cell.47,48 For a given absorbent mass fraction, the LiBr-solution showed a much lower water activity in comparison to the ethylmethylimidazolium acetate solution or the diethylmethylammonium methansulfonate system. The vapor−liquid equilibria of the systems water/[emim][TFA], water/[OHemim][TFA],30,31 water/[EMIM][DMP],41 water/[MMIM][DMP],49 water/[EMIM][EtSO4], and water/ [BEIM][EtSO4]50 have also been investigated recently. 16527



Review

concentration. Besides all, at the temperature of 298 K (0.00378 at the x-axis), the methanol vapor pressure of 17 kPa can be reduced to 13 kPa by the presence of 40 wt % of [mmim][DMP]. Beside of all the systems containing imidazolium based ILs, Shen et al.73 reported about the effect of the ionic liquid triethylmethylammonium dimethylphosphate on the partial pressure of water, methanol, ethanol, and their binary mixtures. As the systems containing [mmim][DMP] and water, ethanol or methanol showed a promising thermodynamic performance, additional properties including density, viscosity, heat capacity, and excess enthalpy of these systems were measured and correlated with different equations at various temperatures and concentrations.42 When discussing the development of new chiller working pairs, alcohol/IL mixtures seem to be particularily promising since they allow for cooling applications also below 0 °C at reasonable costs. However, as described in the following sections, more experimental chiller studies are needed to properly benchmark suitable alcohol/IL systems with the established state-of-the-art working pairs. 3.2.1. Modeling Vapor−Liquid Equilibria (VLE). Due to the large number of possible combinations of binary mixtures containing an alcohol as refrigerant and an ionic liquid as absorbent, the experimental evaluation of all of these systems is a tedious task. Therefore, several research groups started to present modeling results characterizing the VLE of several alcohol/IL systems. Some groups could demonstrate a good agreement between experimental and gE-model results when using the NRTL model for different systems. However, some studies do not account for the ionic nature of these systems since they do not use gE-models or equation of states that have been particularly designed for electrolyte systems. Zhao66 correlated the vapor pressure data of the binary systems methanol/IL and ethanol/ IL for the ILs [bmim][DBP], [emim][DEP], and [mmim][DMP], respectively. Furthermore, the binary NRTL parameters were used to describe the vapor pressure of the ternary system ethanol/water/[MMIM][DMP] with an ARD of 2.8% and the maximum relative deviation of −5.4%. Verevkin et al.65 discussed activity coefficients γi of methanol, ethanol, propanol-1 and benzene for mixtures with the ionic liquid [bmim][Tf2N] and the correlation of vapor−liquid equilibria by using the NRTL model. Revelli67 used the same approach for the describing the VLE data of binary mixtures containing methanol or ethanol and the imidazolium based ionic liquids [bmim][Ac], [bmim][BF4], [bmim][DBP], or [bmim][HSO4]. He et al.42 used binary mixtures of [mmim][DMP] and methanol or ethanol. Ren41 correlated the VLE data of the systems consisting of [emim][DMP] and methanol or ethanol. A comprehensive paper using the UNIQUAC model was provided by Kato et al.64 The respective binary refrigerant/ absorbent VLEs have been measured for the refrigerants methanol, ethanol, 2-propanol, acetone, tetrahydrofuran (THF), and water mixed with one of the ionic liquid absorbents [bmim][Tf2N], [emim][Tf2N], or [mmim][DMP] at 80 °C with the help of a static apparatus. Additionally, a comparison of different models for the prediction of the thermodynamic behavior of IL-mixtures using original UNIFAC, modified UNIFAC (Dortmund), and COSMO-RS (Ol) can be found.74

of methanol/IL systems the strongest decrease in vapor pressure was reported for [bmim][Ac] by Revelli et al.,67 [mmim][DMP] by Kato and Gmehling64 and He et al.,42 and [emim][DMP] by Ren et al.41 (see also the Supporting Information). As an example, Figure 3.1 shows the vapor−liquid equilibria of the systems methanol/[bmim][Ac] and ethanol/[bmim]-

Figure 3.1. Experimental vapor−liquid equilibria for alcohol/ [BMIM][Ac] systems at different temperatures. Unfilled symbols refer to the system methanol/[BMIM][Ac]. Filled symbols refer to the system ethanol/[BMIM][Ac]. Temperatures: rhombus = 283.15 K, square = 288.15 K, triangle = 293.15 K, circle = 298.15 K. Adapted with permission from ref 67. Copyright 2010 Elsevier.

[Ac] at various temperatures.67 The black curves were obtained by fitting the experimental data using the NRTL model. The negative deviation from the Raoult’s law was found for the methanol solutions, while a negative temperature dependence of excess Gibbs free energy was found for the ethanol systems. Zhao et al.66 could show that a decrease in vapor pressure follows the order [bmim][DBP] > [emim][DEP] > [mmim][DMP] for methanol and ethanol. Figure 3.2 shows the variation of vapor pressure with temperature at different ILconcentration for the binary mixture methanol/[mimm][DMP]. The vapor pressure decreases with an increasing IL

Figure 3.2. Experimental and correlative vapor pressure of the binary system methanol/[MMIM][DMP] at different mass percent of [MMIM][DMP]. Reprinted with permission from ref 66. Copyright 2006 Elsevier. 16528



Review

Alevizou et al.75 also used the UNIFAC model to describe the phase equilibria of solvent/ionic liquid systems. While the ionic liquids were based on an imidazolium cation and a hexafluorophosphate anion, alkanes, cycloalkanes, alcohols, or water were regarded as refrigerants. Two new main groups, the imidazolium and the hexafluorophosphate groups, were introduced in UNIFAC. Nebig et al.76 presented measurements of vapor−liquid equilibria and excess enthalpies (HE) for binary systems containing [bmim][Tf2N], [emim][Tf2N], or [hmim][Tf2N] in combination with a variety of alkanes, alkenes, aromatics, alcohols, ketones, or water. Moreover, the modified UNIFAC (Dortmund) model was used to describe the underlying VLE data. Correlations of osmotic coefficients using the extended Pitzer model modified by Archer, and the modified NRTL (MNRTL) model were successfully applied by Calvar77−79 for binary refrigerant/IL mixtures containing [bmim][MeSO4], [emim][EtSO4], [empy][EtSO4], [epy][EtSO4], or [hmim][Tf2N], and several primary and secondary alcohols as refrigerants. Regarding the partial pressure of the respective alcohols, they concluded that secondary alcohols undergo stronger interactions with [hmim][Tf2N] than primary alcohols. Furthermore, the higher the alkyl chain of the alcohol, the weaker the interactions with the respective ionic liquid. Calvar et al.77 showed that the vapor pressure decreases with increasing length of the cation side chain. Additionally, they pointed out that there is only a very small influence on a cation exchange between [empy][EtSO4] and [emim][EtSO4]. Gomez et al.80 used the same approach for binary mixtures of several primary and secondary alcohols in the ionic liquid [mmpy][MeSO4]. They could show that the vapor pressure depression for ILs consisting of a pyridinium cation is higher compared to those systems containing imidazolium based ILs. Wilson, NRTL, and UNIQUAC models have been used by Domanska et al. to model experimental VLE and other phase equilibria with the same parameters of the binary alcohol/ [bmpy][TOS] system. As alcohols ethanol, 1-propanol, and 1butanol were used. For these systems, it was noticed that with increasing chain length of alcohol the vapor pressure of the mixture and the solubility of the IL decreases.71 Freire et al.56 evaluated the predictive capability of COSMORS for the description of the liquid−liquid equilibria and the vapor−liquid equilibria of binary mixtures of alcohols and several imidazolium and pyridinium-based ILs. The influence of the ions and alcohol conformers on the quality of the modeling results was assessed and the quantum chemical COSMO calculation using the Turbomole BP density functional theory with the TZVP basis set derived from the lower energy conformations was adopted. In general, a reasonable qualitative agreement between the model results and experimental data for a large combination of structural variations of both alcohols and ILs was obtained. From the modeling studies above, it becomes obvious that a number of suitable gE-models or equations of states have been identified that allow for a good description of the underlying phase behavior. When it comes to circumvent the experimental screening effort by using a priori methods to identify the most thermodynamically suitable ionic liquid for a certain refrigerant, it becomes obvious that models like COSMO-RS can make a valuable contribution in terms of a rough prescreening. However, they cannot replace the final VLE measurements of the top 20 IL candidates since the required accuracy for

deriving the Dühring plot can often not be reached by these models. 3.2.2. Hydrofluorocarbons (HFCs) in Compression/Absorption Hybrid Cycles. The compression/absorption hybrid cycle, which combines a vapor compression cycle with an absorption cycle, has been investigated81−89 to improve energy efficiency by utilizing waste heat or extending the operating temperature range. The performance of the hybrid cycle is influenced significantly by the absorption performance of the absorbent for the refrigerant. Working pairs consisting of an IL and hydrofluorocarbons as refrigerants can be interesting for absorption refrigeration. However, they can hardly compete with the pairs discussed so far. For the use in compression/absorption hybrid cycles, they are more competitive and were described by different groups.81−85 Of course, this discussion may become obsolete if the group of simple HFC’s is banned totally. Combinations with HFO’s such as R1234yf have not been reported yet. Shifflet and Yokozeki85 measured the solubility of six different HFCs, trifluoromethane (HFC-23), difluoromethane (HFC-32), pentafluoroethane (HFC-125), 1,1,1,2-tetrafluoroethane (HFC-134a), 1,1,1-trifluoroethane (HFC-143a), and 1,1-difluoroethane (HFC-152a), in the ionic liquids [bmim][PF6] and [bmim][BF4], respectively. In this study HFC-32 and HFC-152a showed the best solubility. Investigations of the phase behavior and equilibrium data for various HFC/IL systems could show that HFC solubility is higher in ILs with anions consisting of fluorinated groups, such as [BF4], [PF6], and [Tf2N].61−63,90,92 However, ILs containing [PF6] or [BF4] anions show a high viscosity; they are relatively unstable and degrade rapidly.93,94 Ren and Scurto83,95 compared the phase equilibria (VLE, VLLE) of 1,1,1,2-tetrafluoroethane (R-134a) with the imidazolium based ionic liquids [hmim][BF4], [bmim][PF6], [hmim][PF6], [emim][Tf2N], and [hmim][Tf2N]. Additionally, the volume expansion and molar volumes were measured. From the investigated systems [hmim][Tf2N] showed the highest R-134a solubility. To model the vapor−liquid equilibrium (VLE) and the vapor−liquid−liquid-equilibrium (VLLE), the Peng−Robinson equation of state with the van der Waals 2-parameter mixing rule was used. Dong et al.82 reported about the vapor−liquid equilibrium of difluoromethane (HFC-32) or difluoroethane (HFC-152a) with the ionic liquids [emim][OTf] or [bmim][OTf]. They correlated the experimental solubility data with the NRTL activity coefficient model. The results indicate that the solubility of HFC-32 is higher than that of HFC-152a for the same IL. Also, the solubility in [bmim][OTf] is larger than in [emim][OTf] for the same HFC. The results show that the combinations of ILs based on fluorinated sulfonate anions and selected refrigerants such as HFC-32 and HFC-152a are interesting alternative working fluids for compression/absorption hybrid cycles and should be investigated further. A method for selecting novel IL/HFC working pairs for compression/absorption hybrid cycles using the UNIFAC model has been proposed by Dong et al.84 Figure 3.3 shows the plots of the normalized fugacity ( f/f 0) of various HFCs in [bmim][pf6]. The terms f and f 0 here denote the vapor phase and saturation fugacities, respectively. The solubility of those HFCs at a given fugacity can be compared using the plots. The results indicate that there is a positive deviation from Raoult’s law for most HFC/[bmim] [pf6] mixture with limited usability. So far, only the two systems R32/[bmim] [pf6] and R134/ 16529



Review

Figure 3.3. Normalized fugacity versus mole fraction of various HFCs in [Bmim][PF6] at 298.2 K: (symbols) experimental data ■ R23,85 □ R32,85 ● R41,62 ○ R125,85 ▲ R134,62 Δ R134a,85 ▼ R143a,62 ▽ R152a,85 ⧫ R161;62 (solid lines) UNIFAC model, (dotted line) Raoult’s law. Reprinted from ref 84. Copyright 2012 American Chemical Society.

Figure 3.4. PTx (pressure−temperature−liquid composition) phase diagram for the CO2 + [eeim][Ac] system: (solid and dotted lines) calculated by the present EOS model; (solid circles) the present experimental VLE data. Solid squares and broken lines: the present experimental VLLE data and the LLE tie lines. Reprinted with permission from ref 96. Copyright 2012 Wiley.

[bmim] [pf6] seem promising since they show the desired decrease in partial pressure of the refrigerant. However, further experimental cycle investigations are necessary to evaluate whether or not these two systems allow for an improved performance of compression/absorption hybrid cycles (see also the Supporting Information). 3.2.3. NH3/IL Systems. Shiflett and Yokozeki96,97 determined solubilities of ammonia in different ionic liquids such as [bmim][BF4], [bmim][PF6], [emim][Tf2N], [hmim][Cl], [emim][ACO], [emim][SCN], [emim][EtSO4], and [DMEA][ACO] at isothermal conditions between 10 and 100 °C and for concentrations ranging from 10 to 85 mol % of ammonia. Very high solubilities of ammonia in various ionic liquids have been observed. The PTx data has been successfully correlated with an equation of state. All the excess properties (enthalpy, entropy, and Gibbs energy) showed negative values, reflecting strong intermolecular complex formation. Shi and Maginn98 computed isotherms for ammonia absorption in [emim][Tf2N] using osmotic ensemble Monte Carlo simulations and could show a good agreement with experimental measurements. The simulations also indicated that strategies aiming at changing the solubility of ammonia should focus on altering the hydrogen bond donating ability of the cation. Altering the anion will have a more modest impact. Kotenko et al. studied the performance of a single-effect absorption cycle for different NH3/IL systems.99 In this context, [bmim][pf6] proved to be the most promising ionic liquid allowing for improved COP values. Further details are provided in subsection 3.3.1. Despite the aforementioned research work, more NH3/IL studies need to be carried out to obtain an accurate and comprehensive performance benchmark between the NH3/ H2O and NH3/IL systems and to derive a final conclusion about the potential of IL-based absorbents for the field of refrigeration. 3.2.4. CO2/IL Systems. Shiflett et al.96 measured the solubility of CO2 in [eeim][Ac] using a gravimetric microbalance, and analyzed the VLE data using the EOS model to predict VLLE in CO2-rich solutions. Figure 3.4 shows the isothermal VLE at four temperatures, and the calculated results from the EOS model are compared with experimental data. A highly asymmetric phase behavior is observed with respect to concentration. At CO2 concentration lower than 20 mol %, the binary system shows very low vapor pressure, which reflects a reaction of CO2 with [eeim][Ac]. At CO2 concentration higher than 70 mol %, however, the binary system shows the

formation of a vapor−liquid−liquid equilibrium. Other investigations on possible CO2/IL working pairs and related issues can be found by Cai et al.100 and Aki et al.93 In summary, it is worth mentioning that some ionic liquids such as [eeim][Ac], [hmim][Tf2N], [bmim][PF6], or [NH2pbim][BF4] offer a promising absorption capacity for CO2. Furthermore, CO2 is an A1 refrigerant, indicating minimal toxicity and nonflammability. It also offers a low global warming potential of 1 and is available at comparatively low cost. But again, without the availability of experimental chiller studies discussing the related heat and mass transfer phenomena at higher system pressures and their impact on COP and cooling capacity no benchmark and no final conclusion about the competitiveness and the market potential for the aforementioned systems can be derived. 3.2.5. Trifluoroethanol (TFE)/IL Systems. The possible working pair consisting of an ionic liquid based absorbent and 2,2,2-trifluoroethanol (TFE) as refrigerant has been investigated by various groups.24,26,101−107 Kim et al.24 investigated TFE/[bmim][Br] and TFE/[bmim][BF4] as potential working pairs for an absorption heat pump. Refractive indices and heat capacities were investigated in the temperature range of 25−50 °C. Vapor pressures were measured using the boiling point method in the concentration range of 40−90 mass % of ionic liquid and were successfully correlated with an Antoine-type correlation. Due to the vapor pressure results the TFE/[bmim][Br] system was found to be more favorable than the TFE/[bmim][BF4] system. However, it is noted that more thermophysical properties are required for a detailed discussion of the suitability of these working pairs. Additionally to the aforementioned paper, Curras et al.101 measured the densities of binary mixtures consisting of TFE and [bmim][BF4] or [emim][BF4] over the whole composition range from 10 to 60 °C at atmospheric pressure. The excess molar volumes were calculated from the density data. Then enthalpies of mixing for the same systems at 20 and 50 °C were experimentally determined. Curras et al. repeat the well-known statement that with respect to absorption refrigeration, an absorbent solution with higher density and with negative or small positive mixing enthalpy is preferred. 16530



Review

ature, pressure, concentration, and flow rate are measured at designated locations, the approach can provide intuitive output that lends itself more easily to qualification and explanation. Preparing the test facility, however, is costly and timeconsuming, and measurements usually need to be carefully taken to obtain results that are of acceptable accuracy and reproducibility. The following section summarizes recent theoretical and experimental work dealing with ionic liquid based working pairs for absorption cooling cycles (see also the Supporting Information). 3.3.1. Theoretical Analysis and Process Simulation. The studies described below focus on simulating the efficiency of IL based working pairs in absorption chillers, heat pumps or heat transformers on basis of mass and energy balance equations for each component and compare it to the common working pairs H2O/LiBr and/or NH3/H2O. They apply mostly the same assumptions such as phase equilibrium at the outlet of evaporator, absorber, condenser and desorber, neglection of heat losses, and incompressible and frictionless flows. Mainly, the source of the fluid properties differs from one work to the other. Yokozeki and Shiflett investigated a series of mixtures27,44,61−63,81,85,90−93,96,97,115,177,184,221−233 of ionic liquids and water using an equation-of-state model to correlate their vapor liquid equilibria.44,153 The following conditions have been applied to simulate an absorption cooling cycle: desorption temperature TD = 100 °C, condensation temperature TC = 40 °C, absorption temperature TA = 30 °C, evaporation temperature TE = 10 °C, mass flow rate of the refrigerant ṁ R = 1 kg/s, and solution heat exchanger efficiency ηSHX = 1. The simulation results of thermal cycle efficiency (COP), specific circulation ratio xs x ṁ f= w = = s ṁ R xs − x w Δx

Therefore, the potential absorbent [emim][BF4] seems to be more promising than [bmim][BF4]. A detailed analysis of the system TFE/[emim][BF4] is given by Curras et al.107 Density data are presented for several temperatures (10 < T < 60 °C) and pressures up to 40 MPa. Experimental data have been used to study the dependence of the isothermal compressibility and the isobaric thermal expansion coefficient on temperature, pressure, and composition. In addition, vapor pressures and density data of the refrigerant TFE were used to determine perturbed-chain statistical associating fluid theory (PC-SAFT) parameters. For the absorbent 1-butyl-3-methylimidazolium tetrafluoroborate the equation of state (EOS) parameters were optimized using density data at atmospheric pressure. Using these PC-SAFT parameters, the pvT behavior was reasonably well described. Wang et al.108 measured the vapor pressure for the system consisting of TFE and [emim][BF4] . An Antoine-type equation and the NRTL model were selected to correlate the experimental data. They point out that [emim][BF4] has a large absorption capacity for TFE and therefore should be considered as promising absorbent. On the basis of the TFE-based studies mentioned above and in the Supporting Information, it can be concluded that some ionic liquids have been identified with an attractive thermodynamic performance. However, experimental chiller studies are missing to fully assess the potential of TFE/IL working pairs. Furthermore, from an industrial point of view, it seems worth questioning whether the advantages of a potential TFE/IL working pair can offset the following two disadvantages from an economic point of view: (1) TFE is more expensive than the refrigerant water. (2) A TFE/IL working pair cannot be used as a drop-in solution for existing H20/LiBr absorption chillers. The construction of a new, customized chiller would be necessary accouting for the higher TFE volatility, the system-specific heat and mass transport and also the TFE-related safety issues. 3.3. Process Simulation and Experimental Chiller Trials Involving Ionic Liquids. As shown before researchers have collected a lot of data. Therefore, the idea of introducing IL based working pairs into industrial applications is becoming a realistic proposition. From an industrial point of view, a qualification step is apparently necessary, where the thermophysical properties must be linked to the real performances of absorption cooling cycles at the conditions of chiller or heat pump operation. Both theoretical and experimental approaches can be effectively employed in this critical step. From a theoretical perspective, the typical procedure is to establish a set of equations relevant to the process by considering the conservation of variables, such as mass and energy, together with the phase equilibrium of the working pair, and to solve the equation with numerical techniques. This approach makes it possible to arrive at a relatively quick estimate of the cooling performance and provides clues to understanding the inherent relationship between the characteristics of working pairs and the cooling performance. On the other hand, since simulations always necessarily generate results that are completely dependent on the physical models themselves, it is desirable to compare them with experimental results under certain operating conditions for validation before using them as universal simulators. As for experimental work, where temper-

and solution concentration difference Δx are presented in Table 3.3. In practice, it is rather unusual to choose a three Table 3.3. Results of Absorption Chiller Process Simulation at TD = 100 °C, TC = 40 °C, TA = 30 °C, TE = 10 °C, ṁ R = 1 kg/s, ηSHX = 144,153 water/[mmim] [(CH3)2PO4] xstrong [mass %] Δx [%] f [−] COP [−]

water/[emim] [(CH3)2PO4]

water/LiBr

94

98

66

18 5 0.66

11 9 0.69

16 4 0.7844/0.83153

times higher temperature thrust (TD − TC) compared to the temperature lift (TA − TE). With a lower driving temperature, Δx would be lower, f higher, and the COP lower. A concentration change Δx of 16% for H2O/LiBr is untypically high, resulting in a very low specific circulation ratio f of 4 that impedes a sufficient wetting of the heat exchange tubes. Among the IL based working pairs studied, [mmim][(CH3)2PO4] and [emim][(CH3)2PO4] performed best (COP = 0.66 and 0.69, respectively). The COP given for the H2O/LiBr system under these conditions in their earlier work with 0.83 and in their later work with 0.78 is inconsistent, although f and Δx are equal in both papers. This inconsistency might suggest checking again 16531



Review

the very high earlier value. Depending on the reference COP of H2O/LiBr used, the highest COP of the IL based working pairs is 11% or 17% lower. While the mean solution concentration of the water/LiBr system was 58% it was between 83% and 97% for the water/IL systems, and thus, considerably higher. The high solution concentration associated with a lower solution concentration difference always results in a high specific circulation ratio which here was 5 to 24. Normally, the higher the specific circulation ratio is the lower is the COP. Therefore, candidates with a smaller salt concentration at operation conditions have to be developed to achieve higher COPs. Zhang and Hu173 also conducted cycle simulations with the working pair H2O/[emim][(CH3)2PO4]. They used the NRTL model, the details of which were discussed in their previous study,41 to describe the thermodynamic behavior of the system. The cycle conditions TC = 40 °C, TA = 35 °C, TE = 5−12 °C, mass flow rate of the refrigerant = 1 kg/s, solution concentration difference Δx = 5%, and closest approach temperature at the solution heat exchanger CAT = 10 K are well chosen. The heat loads of the four main heat exchangers, the driving temperature required, the COP, and the specific circulation ratio f were calculated. The specific circulation ratio in general is higher compared to the study of Yokozeki and Shiflett44,153 due to the fixed (low) solution concentration difference. For H2O/[emim][(CH3)2PO4] it is more than 50% higher compared to H2O/LiBr. On the other hand, the generation temperature for H2O/[emim][(CH3)2PO4] was significantly lower than for H2O/LiBr (see Table 3.4), which

Table 3.5. Results of Absorption Heat Transformer Process Simulation at TC = 35 °C, TD = TE = 90 °C, TA = 140 °C, ΔTmin,SHX = 5 K174 xstrong [mass %] Δx [%] f [−] COP [−]

water/[emim][(CH3)2PO4]

water/LiBr

73 85 17 0.74

79 57 11 0.80

water/LiBr

TFE/E181

92 7 15 0.46

64 7 11 0.50

90 10 9 0.42

Römich et al.48 calculated the performance of an absorption chiller running on a H2O/[dema][CH3SO3] mixture and compared the COP with that of LiBr. They used the cooling capacity, the driving temperature, the cooling water temperature, and the chilled water outlet temperature as input parameters and assumed constant minimal temperature differences between external and internal flows in the apparatuses of 2 K for evaporator and condenser and 3 K for absorber and desorber. At operating conditions of 75 °C driving temperature tD,in, 24 °C cooling water temperature tA,in, and chilled water outlet temperatures tE,out from 7 to 10.5 °C, the COP of the H2O/[dema][CH3SO3] working pair resulted in slightly lower values (around 6%) than that of H2O/LiBr (see Table 3.6). In both cases, the COP is quite high. Table 3.6. Results of Absorption Chiller Process Simulation at tD,in = 75 °C, tA,in = 24 °C, tE,out = 10 °C, Q̇ E = 12 kW48 COP [−]

Table 3.4. Results of Absorption Chiller Process Simulation at TC = 40 °C, TA = 35 °C, TE = 10 °C, ṁ R = 1 kg/s, Δx = 5%, ΔTmin,SHX = 10 K173 TD [°C] xstrong [mass %] f [−] COP [−]

water/[emim][(CH3)2PO4]

water/[dema][CH3SO3]

water/LiBr

0.80

0.85

Kotenko et al.99 investigated the performance of ammonia/ ionic liquid working pairs for absorption heat pump processes by means of thermodynamic simulations using ASPEN Plus. The thermodynamic properties found in literature were regressed in ASPEN Plus with the NRTL model. The absorption cooling process has been calculated at absorber/ evaporator outlet temperatures of 25/5, 35/5, and 45/5 °C and desorber outlet temperatures between 60 and 160 °C. Among the ILs analyzed [bmim][PF6] performed best. As can be seen from Table 3.7, the IL concentration, again, is very high. The

implies that this ionic liquid could be used in absorption chillers driven by low-grade waste heat or hot water generated by solar collectors. The COP of H2O/[emim][(CH3)2PO4] was 0.74 for an evaporation temperature of 10 °C. Compared to that of H2O/LiBr for this condition (0.80), it was lower by about 7%. Following the same approach and using the same working pair (H2O/[emim][(CH3)2PO4]), Zhang and Hu174 also investigated the performance of an absorption heat transformer and compared the results to those using H2O/LiBr, as well as to using organic working pair trifluoroethanol/tetraethylenglycol dimethylether (TFE/E181). The following cycle conditions have been chosen: TD = TE = 90 °C, TC = 30/35/40 °C, TA = 110−160 °C, and closest approach temperature at the solution heat exchanger CAT = 5 K. Again, the heat flows are normalized to the mass flow rate of the refrigerant. As to the specific circulation ratio, H2O/[emim][(CH3)2PO4] resulted in a higher value than LiBr, which showed a tendency similar to the case of the aforementioned chiller studies. The reason, again, is the large salt concentration (92 mass % of the strong solution compared to 64 mass % for water/LiBr). The COP of the H2O/[emim][(CH3)2PO4] mixture was found to be about 8% lower than that of H2O/LiBr but higher than that of TFE/ E181 (see Table 3.5).

Table 3.7. Results of Absorption Chiller Process Simulation at TD,out = 160 °C, TA,out = 35 °C, TE,out = 5 °C99 xstrong [mass %] Δx [%] f [-] COP [−]

NH3/[bmim][PF6]

NH3/water

98 4 23 0.70

88 36 2.3 0.65

concentration difference of the IL based working pair is significantly smaller than the concentration difference for NH3/ H2O. Therefore, the solution circulation ratio is ten times higher. The COP is about 8% higher for NH3/[bmim][PF6] (0.70) compared to NH3/H2O (0.65), in spite of the high solution circulation ratio and although the authors included the solution pump power demand in the definition of the COP (high power demand for solution circulation). The reason for this is most probably the very large rectification need of the NH3/H2O system with the simulation conditions chosen. The advantage of using ILs as absorbents for ammonia instead of water is that no rectification is necessary. Nevertheless, in 16532



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However, the differences in the COP raise serious questions regarding the accuracy of the coefficients used. Using a group-contribution equation-of-state model to correlate the phase equilibrium properties, Martin and Bermejo154 studied a series of ionic liquids with supercritical CO2 as refrigerant under conditions of TD = 120 °C/pD = 10 MPa, TC = 40 °C, TA = 37 °C/pA = 4 MPa, and TE = 5 °C. Again, the strong solution concentration was found to be between 84% and 98%. Similar to the CO2/IL study of Cai et al.,100 the specific circulation ratio was high (in most cases between 20 and 60). Among the ILs studied, [bmpyrr][Tf2N] was cited as the most favorable absorbent for CO2 as refrigerant, because it seemed possible to operate the generator at lower temperatures. When the COP was compared to that of the NH3/H2O system, however, CO2/[bmpyrr][Tf2N] showed a much lower value of about 0.25 (unlike in the paper, the pump energy has not been considered in order to obtain the thermal COP), which was explained by its high specific circulation ratio. However, it may be also due to a nonoptimized choice of the upper pressure. As well as Cai et al.,100 the authors studied the CO2/[bmim][PF6] working pair. The results presented in Table 3.10 are different but this may be due to the different cycle conditions applied.

practice, driving temperatures for NH3/H2O systems are much lower for low temperature lifts which considerably decreases the demand for rectification. The simulations show that for TD,out = 90 °C and absorber/evaporator outlet temperatures of 35/5 °C, the COP of NH3/H2O is 0.68 compared to 0.4 of NH3/[bmim][PF6]. Liang et al.175 compared CH3OH/[mmim][(CH3)2PO4] to the NH3/H2O and H2O/LiBr simulation results of Yokozeki and Shiflett.97 They used the same untypical operating conditions which result in a very high Δx and a very low f: TD = 100 °C, TC = 40 °C, TA = 30 °C, TE = 10 °C. In contrast to the other simulation papers, the methanol/ionic liquid working pair showed about the same specific circulation rate f and a COP higher than LiBr (see Table 3.8). Compared to the Table 3.8. Results of Absorption Chiller Process Simulation at TD = 100 °C, TC = 40 °C, TA = 30 °C, TE = 10 °C175

xstrong [mass %] Δx [%] f [−] COP [−]

methanol/[mmim] [(C3)2PO4]

NH3/ water

water/ LiBr

89 24 4 0.87

61 24 2.5 0.65

65 16 4 0.83

Table 3.10. Results of Absorption Chiller Process Simulation at TD = 120 °C/pD = 10 MPa, TC = 40 °C, TA = 37 °C/pA = 4 MPa, and TE = 5 °C154

simulation result of Yokozeki and Shiflett44 for H2O/[mmim][(CH3)2PO4] the COP for CH3OH/[mmim][(CH3)2PO4] was 30% higher. The reason for the high COP is not discussed. It seems to be worth double-checking this result since the heat of vaporization of methanol is much smaller than that of water. Therefore, a lower COP might be expected. The concentration difference is very large, which may be a reason for a large COP, but which can only be achieved with a high driving temperature, which is not given. Another reason may be a low heat of solution; however, data are not given. While Shelide176 proposed the CO2/[bmim][PF6] working pair for absorption refrigeration, Cai et al.100 compared its performance with NH3/H2O. They used the same conditions as Yokozeki and Shiflett:97 TD = 100 °C, TC = 40 °C, TA = 30 °C, TE = 10 °C, and ṁ R = 1 kg/s. The authors state that for the NH3/H2O working pair simulation results were in good accordance with that of Yokozeki and Shiflett. Similar to the simulation results of the water/IL working pairs presented in this section the salt concentration of the IL based working pair was high (xstrong = 89 mass %, see Table 3.9). However, specific circulation ratio is significantly higher ( f = 26). Depending on the ideal-gas heat capacity coefficient applied (from two different sources), they got a much lower COP of 0.08 and −0.01, respectively. In the latter case the process operates so badly that no cooling effect is achieved. The low COP value is most probably a result of a nonoptimized upper pressure.



89 3 26 0.08/−0.01 (depending on ideal-gas heat capacity coefficients applied)

CO2/[bmim][PF6]

89 3 24 0.25

93 2 46 not specified

All simulations of water/IL working pairs resulted in slightly lower COPs (in the order of 10%) for the best of the investigated pairs compared to water/LiBr. This was attributed to the necessity of operating with a higher specific circulation ratio. However, compared to organic working pairs the COP is higher. Deviations about the simulation results can be ascribed to differences in IL properties as well as to differences regarding the operational conditions. 3.3.6. Experimental Chiller Studies. A large collection of thermo-physical properties for ionic liquids have been published in the past few decades. Considering the importance for industrial purposes, Aparicio et al.166 pointed out that some of them should be handled with caution, because sufficient data accuracy is frequently not achieved. This leads to design errors or it requires overdimensioning the equipment or relaxing design conditions. It is obvious that experimental verification using pilot or even small-scale commercial chillers is a necessity in order to pursue the commercialization of ionic liquid based working pairs. Experimental studies for such purpose, however, are found only scarcely in literature. TU Berlin and Evonik Industries AG109,110,112,189 jointly investigated the operational behavior as well as the heat and mass transfer characteristics of different ionic liquids with water as refrigerant. They used a small-scale single-effect absorption chiller that had a glass shell for visual inspection. Even though further optimization work was expected, the preliminary experimental screening provided promising results. Ionic liquids have been identified which experimentally showed about the same efficiency (COP) as LiBr solution. Without having added

Table 3.9. Results of Absorption Chiller Process Simulation at TD = 100 °C, TC = 40 °C, TA = 30 °C, TE = 10 °C, ṁ R = 1 kg/s100 CO2/[bmim][PF6]

CO2/[bmpyrr][Tf2N]

NH3/ water 59 24 2.5 0.64

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based working pair was about the same as for H2O/LiBr. Nevertheless, the cooling capacity obtained was considerably lower (one-third at 95 °C driving temperature, 27 °C cooling water temperature, and 17 °C chilled water inlet temperature). All experiments show that water/IL working pairs can reach about the same absorption chiller efficiency as the established water/LiBr work pair. However, under comparable conditions and in the absence of suitable performance additives for ionic liquids, the heat and/or mass transfer for most IL-systems was inferior to the established H2O/LiBr working pair resulting also in a lower cooling capacity. As a conclusion we can state that COP simulations using plain energy balances and phase equilibria to evaluate the suitability of IL based absorbents for absorption chillers, heat pumps, or heat transformers are not essential. A first screening of the COP can rapidly be effected with few experimental data required using COPrev = r/(r + l) as described in Alefeld and Radermacher.192 Using the same refrigerant, the COPrev only depends on the specific heat of solution l. If it is in the same order, the same COPrev can be expected. This consideration can continually be improved by adding effects like specific heat required for heating or cooling of the mass flows involved, throttling losses, etc.234 A more profound evaluation is only possible if heat and mass transfer is investigated as well. Therefore, one major field of future research should be the investigation of the underlying heat and mass transfer phenomena. Additional studies in this area are mentioned in the Supporting Information. 3.4. Industrial Activities and Patent Overview. Ionic liquids have been attracting not only scientists and researchers for their unique physical and chemical features, but also industrial companies because of their wide range of potential applications. In this section selected industrial activities in the area of ionic liquid based working fluids for absorption cooling are summarized. Many chemical and material companies have been involved in the development and commercialization of ionic liquids since the early 2000s. Focusing on the application of absorption refrigeration, the industrial information becomes rather limited, and only a few companies indicated their interest in applying ILs to this industrial field. Evonik Industries AG seems to be the only company so far that officially stated the motivation for commercializing IL based working pairs. In a technical paper published in the corporate quarterly science newsletter, a clear vision to launch working pairs in the near future was described.114 Performances of ionic liquids in absorption cycles with consideration of industrial application have been discussed. The promising results encouraged Evonik and a number of industrial partners to jointly carry out field trials to further qualify the prototypes. ILs are also discussed in the context of sorption cooling media200 or thermal media201 by other companies such as Iolitec and Koei as described on the corresponding company’s Web sites. Industrial activities regarding the application of ionic liquids as a new absorbent for absorption chillers and heat pumps date back about 10 years as discussed in the patent literature.23,26,27,202−233 Around the turn of the century, scientists from universities (e.g., Korea Advanced Institute of Science and Technology, Technical University of Berlin, Technical University of Karlsruhe, University of Erlangen-Nuremberg) and from industrial companies (e.g., Evonik Degussa, BASF,

suitable performance additives, the maximum cooling capacity achieved for the ionic liquid based working pairs was approximately two-third of the established H2O/LiBr/performance additive solution. To verify their simulation results, Römich et al.48 performed measurements on an absorption chiller with a nominal cooling capacity of 10 kW190 using the working pairs H2O/LiBr and H2O/[dema][CH3SO3]. Figure 3.5 shows a comparison of

Figure 3.5. COP of an absorption chiller: Comparison of experimental results and the corresponding simulation. Reprinted with permission from ref 48. Copyright 2011 International Institute of Refrigeration.

simulation and measurements of the COP for H2O/[dema][CH3SO3] at driving temperatures of 74−84 °C, 24 °C cooling water temperature, and 11 °C chilled water outlet temperature. The COP simulations of the authors for water/LiBr did not agree with their measurements due to simplified assumptions in the simulation model, e.g. neglected heat losses. Therefore, the authors use this deviation as an uncertainty rangealso for the IL working pair. Measured data as cooling capacity and temperature difference for the heat transport in evaporator, absorber, and desorber have been used as simulation input parameter. This may be the reason for the dip of the simulated COP at 75.5 °C driving temperature in Figure 3.5 which otherwise would not be reasonable. Apart from that, the simulated COP is quite constant in the whole driving temperature range at 0.8 unlike the experimental data which scatter between 0.6 and 0.8. This may be due to the small heat flows and therefore high uncertainty. Kühn et al.12 reported a measured COP value of around 0.74 for this chiller with the working pair H2O/LiBr and the aforementioned conditions. That means that the measured COP of the H2O/[dema][CH3SO3] working pair is nearly in the same range. The measured cooling capacity using H2O/[dema][CH3SO3], however, was significantly lower (1.3 kW compared to 10 kW), which was explained by insufficient heat and mass transfer due to higher viscosity and low wettability of the ionic liquid tested. It might also be due to the fact that the chiller designed for H2O/LiBr was not in every aspect suitable to be operated with this H2O/IL working pair. Radspieler and Schweigler191 investigated the suitability of [emim][(CH3)2SO4] as sorbent in an experimental absorption chiller with adiabatic absorber and desorber (spray chambers with external plate heat exchangers). The general feasibility of this concept was shown and again, the COP of the ionic liquid 16534



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Other companies filed similar patent applications. For example, [bmim][PF6] and [emim][NTf2]225 were also suggested to act as absorbent for CO2 as refrigerant. Adding [dmim][Cl] to LiBr was proposed to reduce crystallization and corrosion and to improve the COP.242 In summary, it can be stated that the patent activities described above reflect a strong industrial interest in the field of absorption cooling using ionic liquid based working pairs. Many companies seem to be confident that new or modified energyefficient cooling concepts can be leveraged and will play stronger role for numerous applications in the area of comfort and industrial cooling in the future. In this regard, some system solutions described in the following chapter could allow for making a valuable controbution to ensure that the cooling demand of industrialized countries is covered in a more sustainable manner with a reduced carbon footprint.

IoLiTec, and DuPont) began to realize the potential of using ionic liquids in absorption cycles. Subsequently, several patent applications were filed by Evonik,25,26,29,36−38,46,109−114,136,143,185,189,199,202−215 BASF,23,216−220 DuPont,27,44,61−63,81,85,90−93,96,97,115,177,184,221−233 and others240,241 (see also references therein). In the initial patent applications, companies illustrated the principles of using IL based working pairs, listed numerous IL-based absorbents, disclosed a large variety of combinations of anions and cations while also discussing absorbents for water,36−38,202−233 for fluorocarbons,222,224,225 for SO2,217 for CO2,226,240 for ammonia,227 and for polyols.228 To date Evonik Industries, DuPont, and BASF are considered the only companies worldwide which hold approved patents in this field. Evonik26,36−38,202−215 indicated that the working pairs claimed have sufficient thermal stability even at temperatures above 150 °Ca crucial prerequisite for indirect fired and direct fired double-effect absorption chillers. The specifications related to toxicity, corrosiveness, and saturated vapor pressure were also disclosed in a number of Evonik patent applications. For different types of refrigerants such as different alcohols, R134a, ammonia, and water, specified groups of ionic liquids were identified based on vapor pressure investigation. In a patent application filed by DuPont,224 which focuses on mixtures of IL and hydrofluorocarbon (HFC) based refrigerants, methods of correlating thermal property measurement and simulation models for absorption cycle were described. By comparing the calculated COPs of HFC/ILs systems to that of H2O/LiBr and NH3/H2O systems, DuPont found that COPs for all kinds of HFC/ILs mixtures were below 0.39, while H2O/ LiBr and NH3/H2O lead to COP values around 0.83 and 0.65, respectively. For fluoroolefins based refrigerants, [emim][Tf2N] seems to be a particular promising absorbent.225 The solubility was examined by measuring the vapor pressure at temperature from 20 to 80 °C, and the minimum vapor pressure of 0.5 bar was obtained with 28 mol % of cis-1,1,1,4,4,4-hexafluoro-2-butene (cis-HFO-1336mzz) at 20 °C. For the refrigerant methanol, [emim][Ac] was suggested220 as absorbent because the working pair may have advantages in terms of crystallization limit, vapor pressure, operating temperature, and heat and mass transfer. It was stated that the mixture is thermally stable up to 140 °C. For the refrigerant trifluoroethanol, a number of imidazolium salts showed a drastic reduction of the partial pressure of the refrigerant.211 Compared to inorganic absorbents, IL based absorbents usually have higher surface tension and low wettability, which in many cases results in an inferior heat and mass transfer performance. Additives like polyether−polysiloxane copolymer,36 polyethylene glycol, and zeolites212 were proposed to improve the thermodynamic performance. As far as machines and systems using IL based working pairs are concerned, several configurations, such as conventional type,206,207,209,230 dual absorption circuit type,231 and compression−absorption hybrid type,232 were described respectively. Using IL-based working pairs in combination with membrane absorption modules could be another interesting option.212 ILs were also proposed as working pair additives to improve the performance of the conventional LiBr solution in terms of preventing crystallization233 and reducing fluid friction.213

4. PROCESS DEVELOPMENT AND EMERGING COOLING TECHNOLOGIES There are economically rewarding fields of absorption cooling which cannot be covered satisfactory by the two common working pairs water/LiBr and ammonia/water due to their drawbacks described in section 2.3. One example is the use of absorption machines for solar thermal cooling in hot regions. These markets can only be entered by using new working pairs. Moreover, cycles such as highly efficient multieffect absorption chillers (e.g., triple-effect chillers) may become feasible whose development has failed up to now due to working pair related problems like corrosion or crystallization. Ionic liquids are supposed to offer advantages in these respects. 4.1. Solar Cooling and Novel Process Options. The main reasons for the interest in solar cooling systems are the correlation between insolation and cooling requirement, especially in the air-conditioning sector, and the worldwide growing energy and environmental concerns. Solar cooling is not a new business. We quote from the work of Albers et al.:244 Thévenot states in his ‘History of refrigeration’243 that as early as in 1872 Pifre produced in Paris some ice using a solar boiler to regenerate the absorbing solution via steam. Probably a batch system operating on water/sulfuric acid was used. In the 1970s in the US some 500 solar airconditioners had already been installed. They ran to about 75 to 80% on solar and the rest of the time on electricity or fuel oil. That means that a significant share of these installations was no big energy saver. A single-effect chiller with a COP of 0.7 operated on electricity requires about 5 times the primary energy than a compression chiller with a COP of 3.5. Consequently, all the energy saving by solar operation will be compensated by a 20% share of electrical operation. Probably most of them are not in operation anymore. In the last decades, interest in solar cooling has been steady. The International Energy AgencySolar Heating and Cooling Programme (IEA-SHC) Task 25: Solar Assisted Air Conditioning of Buildings (1999−2002)245 has been followed by Task 38: Solar Air-Conditioning and Refrigeration (2006− 2010) where experts of eleven countries worked together to improve conditions for the market introduction of solar airconditioning and refrigeration systems for residential and small commercial buildings, mainly by improving components, system concepts, and design tools.246 16535



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Figure 4.1. Typical solar air-conditioning system layout with absorption chiller.

Figure 4.2. Four-pressure triple-effect absorption cooling cycle plotted over the vapor pressure lines of water/LiBr. Diagram created with data from ref 14.

temperature.250−253 In addition, although today many of the solar air-conditioning systems are installed in Europe, the main market is in sunny, hot climate. As described in section 2.3, the use of the established absorption cooling working pairs is limited in such an application: at high reject temperature level (ambient temperature) water/LiBr chillers risk crystallization and ammonia/water chillers reach a very high upper pressure level. Since decades several approaches have been developed and tested to inhibit crystallization of LiBr solution including chemical (additives that shift the crystallization curve) and mechanical (improved heat exchangers) cycle modifications and hybrid absorption−compression cycles. While such modifications usually reduce the efficiency and improved or simply bigger heat exchangers increase the cost, many crystallization inhibitors involve problems like corrosion, reduced heat and mass transfer, or incompatibility with the conventional heat and mass transfer additive 2-ethyl-1hexanol.242 Carrier developed and tested thoroughly the water/LiBr working pair in combination with ethylene glycol as crystallization inhibitor and 1-nonalymine (which has later been replaced by phenylmethylcarbinol) as additive to enhance heat and mass transfer. They called this solution “Carrol”.254 Yazaki developed and patented an aqueous solution containing lithium bromide, lithium chloride, and lithium nitrate with a

Today, the technology is considered to be mature. In the past decade, around 600 systems have been installed worldwide247 with a market growth between 40 and 70% per year in the last six years.248 However, a cost-effective and energy saving solar cooling installation is not easy to devise. The recently started IEA-SHC Task 48 (2011−2015), therefore, has the focus on quality assurance and support measures for solar cooling.249 Moreover, several companies began to offer standardized solar cooling packages including solar collector, storages, heat rejection device, pumps and system control (e.g., Solarnext and Schüco from Germany, Solution, Gasokol, and Startec from Austria, Kloben from Italy, Kingspan from the UK, and Thermax from India248). Nevertheless, innovative research and development still is required. Figure 4.1 shows a general layout of a solar air-conditioning system with absorption chiller. The system is driven by heat from a solar collector, the chilled water is used in fan-coil units, chilled ceilings, or the like, and the reject heat is conveyed to the ambient usually via a heat rejection circuit (cooling tower etc.). For small systems for, e.g., detached houses, however, the use of a separate cooling tower seems to be too complex. Therefore, there are increased research activities focusing on air-cooled absorption chillers with relatively high absorber 16536



Review

Figure 4.3. Three-pressure triple-effect absorption cooling cycle plotted over the vapor pressure lines of water/LiBr. Diagram created with data from ref 14.

mixing ratio of 1:0.375−0.425:0.225−0.275.255 Ring et al.256 tested various organic crystallization inhibitors. None of these solutions are reported to be used in significant amount probably due to the aforementioned problems. 4.2. High-Temperature Working Pairs for Multistaging and Heat Transformers. The COP of single-effect absorption chillers is limited to maximum 0.6 to 0.8 depending on the temperature lift, the temperature levels, and the working pair. Higher efficiencies can be achieved using multieffect cycles, where the term “effect” originally describes the number of regeneration steps which is achieved with the driving heat input.257 In the 1980s, directly gas, steam, or hot water fired doubleeffect absorption chillers were produced in large numbers for the Asian market mainly in Japan. Later they were produced in North America and other Asian countries, too. Several of the leading absorption chiller manufacturers were also active in the development of triple-effect chillers.8 While today double-effect water/LiBr absorption chillers with a COP of 1.0 to 1.4 are mature, the development of chillers with higher effects is mainly restricted due to problems of corrosion at high temperatures (above 150 °C with common construction materials). High temperature resistant and low corrosive ionic liquids would be very suitable to substitute the hitherto discussed and tested absorbents (LiBr, sulfuric or phosphoric acid) in triple or even quadruple-effect cycles with high efficiencies. There are several cycle configurations of triple-effect chillers. When using LiBr solution, due to the small solution field the four-pressure triple-effect cycle (as an extension of the doubleeffect cycle) is feasible (see Figure 4.2). Driving heat is only supplied to the highest pressure desorber (D3). Therefore, driving temperatures higher than 160 °C are required. Desorbers D1 and D2 are heated by internal heat transfer from the condensers C2 and C3. Another cycle configuration consisting of a single-effect cycle and a topping stage with only three different pressure levels is presented in Figure 4.3. High temperature heat, again, is

supplied to the high pressure desorber (D2). The low pressure desorber (D1) is heated by internal heat transfer from condenser (C2) and absorber (A2). The advantage of this cycle configuration is that the single-effect chiller can also be operated independently, e.g. by solar thermal power, and that the maximum pressure is lower. However, as it can be seen in Figure 4.3 the use of the working pair water/LiBr is not possible due to crystallization. Once more, ionic liquids could be helpful to realize this cycle. Moreover, the same topping stage can be used to drive a double-effect chiller achieving a quadruple-effect with COPs around 2.0. Nevertheless, even higher driving temperatures are required. Heat transformers (also referred to as heat pump type II) operate according to the same principle as absorption chillers and heat pumps, just in the reverse thermodynamic sense. They also work between three different temperature levels (see Figure 4.4) and consist of the same main components as absorption heat pumps: evaporator, condenser, absorber, and desorber.

Figure 4.4. Operating principle of absorption chillers and heat pumps (left) and heat transformers (right).

Heat transformers are used to upgrade waste heat of a medium temperature level to a usable higher temperature level with virtually no external energy input except auxiliaries. Medium temperature heat is supplied to the evaporator and the generator, the useful heat is given off from the absorber, and low temperature waste heat is released to the environment from 16537



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Figure 4.5. Heat transformer cycle plotted over the vapor pressure lines of water/LiBr. Diagram created with data from ref 14.

the condenser (see Figure 4.5). The cycle efficiency (the COP) is defined by the ratio of high temperature useful heat output and low temperature driving heat input. Single-effect heat transformers can achieve COPs in the range of 0.45. While there has been considerable research and development activities on heat transformers in the 1980s, there are currently just a few cases of operation in Asian countries. Nevertheless, in the course of the environmental debate, the recovery and efficient use of low temperature (waste) heat is now being discussed again. The main application field is the upgrade of industrial waste heat. Another possible application is the temperature lift of district heat. The few heat transformers which have been realized almost exclusively used water/LiBr as working pair. As for absorption chillers the temperature lift is limited due to the danger of crystallization. More crucial, however, is the problem of corrosion at high temperatures. This restricts the range of application to a heat supply temperature of around 150 °C. High temperature resistant and low corrosive ionic liquids would be very suitable to substitute the LiBr solution in this process. This allows a new discussion of the use of heat transformers and opens the process to a much broader range of applications.

(iii) the lack of an intense dialogue between industry and academia also in order to allocate RD resources more efficiently. Just by screening the participant list of international conferences during the last 10 years in this field, it becomes obvious that the IL community and the (absorption) refrigeration community did not mix at all, although the challenges are highly interdisciplinary. In other words, there is an enormous potential that future interdisciplinary collaborations could leverage: There are about 1018 different ionic liquids. There are also numerous suitable refrigerants for absorption cooling machines. The number of possible combinations is overwhelming. To evaluate the most adequate working pairs, first and foremost, reliable property data are required. As it is not feasible to measure such a huge number of properties, models are needed (and sometimes already available) which are able to predict thermodynamic data of multicomponent systems (such as the activity coefficient of refrigerants in IL/refrigerant/performance additive systems) and transport properties such as diffusion coefficient, viscosity, and thermal conductivity. The models, of course, have to be verified by qualified measurements. The optimization of the equipment for heat and mass transfer depends on said properties but usually not all desired fluid properties can be realized simultaneously (e.g., low viscosity and large vapor pressure depression often do not coincide in the relevant range of cooling applications). Therefore, fundamental knowledge about strict physical interactions between the properties is required. Subsequently, these findings have to be implemented into the design of absorption chillers and heat pumps, which, in essence, is the design of the heat and mass exchange within the different components. To this end, additional knowledge about the surface properties of ionic liquids is necessary, namely, to evaluate surface wetting or droplet formation. This requirement leads to the research on additives, for example to induce Marangoni convection, to enhance surface wetting properties,

5. SCIENTIFIC CHALLENGES Most of the IL-based working pairs that have been suggested in the literature so far do not meet the industrial requirements. The main reason for this are (i) the complexity of working pair requirements that in most cases is not fully addressed or understood, (ii) the lack of collaboration of a number of specialized research groups (from the areas of thermodynamics, heat and mass transfer, process engineering, chemistry, modeling and simulation) that work independently from each other instead of leveraging their interdisciplinary competencies by closer collaboration, 16538



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Willmes, O. Zehnacker, M.-C. Schneider, R. Schneider, and S. Muenzner (all Evonik Industries AG) and to O. Buchin, J. L. Corrales Ciganda, F. Cudoc, H. Kemmer, and T. Meyer (all TU Berlin).

or to safeguard materials against corrosion. Research on these additives suggests an additional work stream for the chemical industry, as the additives usually are an indispensable part of the product and have to undergo the same testing procedures as the working fluids before the latter can enter the market. Depending on the application, additional properties such as stability at high temperature in aqueous surroundings are of further interest. In this context, especially the driving temperatures for direct-fired multieffect cycles (T > 160 °C) need to be addressed. There are attractive energy saving processes such as high efficient triple or quadruple effect absorption chillers or heat transformers, which up to now did not make the way from laboratory to the market because the available working pairs do not stand the required temperatures. Any progress in this respect will foster the development of new heat transformation processes and, consequently, open new market segments.

■

Symbols

COP = coefficient of performance [−] γ = activity coefficient [−] η = efficiency [−] η = viscosity [Pas] f = specific circulation ratio [−] f = fugacity [−] l = specific heat of solution [kJ/kg] ṁ = mass flow rate [g/s] p = pressure [Pa] Q = heat [kWh] r = specific heat of evaporation [kJ/kg] R = gas constant [J/(kg K)] T = temperature [°C] v = specific volume [m3/kg] W = work [kWh] x = absorbent mass fraction [gabsorbent/gsolution]

6. SUMMARY AND OUTLOOK About 10 years after the research on IL-based working pairs for absorptions chillers started many scientific and industrial challenges in this field are not sufficiently addressed and the literature about the performance of chillers is to a large extent confusing and contradictory. However, the working pair requirements are complex and highly interdisciplinary. This review summarized the state-of-the-art results of IL-based working pairs for absorption chillers, discussed several unmet needs, and discussed new promising and energy-efficient system solutions for high-temperature working pairs. Further RD collaborations are required to identify and develop suitable IL-based working pairs that show (i) a suitable decrease of the partial pressure of the refrigerant at relevant absorbent concentration (e.g., PH2O < 10 mbar at 70 wt % of ionic liquid and T = 40 °C) and relevant solution viscosities (η < 7 mPas under aforementioned conditions), (ii) an adequate IL-stability at 200 °C in aqueous solutions, (iii) wetting behavior and heat and mass transfer characteristics comparable to the state-of-the-art working pair H2O/LiBr/additives. Research needs to be adequately aligned to the full set of application requirements and strongly focused on leveraging the required interdisciplinary competencies in this field.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

NOMENCLATURE

Indices

■

0 = low temperature level 1 = medium temperature level 2 = high temperature level A = absorber C = condenser D = desorber E = evaporator R = refrigerant rev = reversible s = strong (low concentration of refrigerant) SHX = solution heat exchanger w = weak (high concentration of refrigerant)

REFERENCES

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S Supporting Information *

List of additional research work in this field as this review covers only a selection of studies regarding the use of ionic liquid based working pairs for absorption chillers. This material is available free of charge via the Internet at http://pubs.acs.org Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank the Federal Ministry of Economics and Technology of Germany (BMWi) for funding selected project activities under grant 0327472 A and B. Several colleagues supported our work in this field. Special thanks to S. 16539



Review

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