Extraction of Energy from Small Thermal Differences near Room

Aug 22, 2014 - Department of Electrical and Computer Engineering, HiST, Sør-Trøndelag University College, 7004 Trondheim, Norway. ∥. INM - Leibniz ...
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Extraction of Energy from Small Thermal Differences near Room Temperature Using Capacitive Membrane Technology Bruno B. Sales,†,‡ Odne S. Burheim,§ Slawomir Porada,∥ Volker Presser,∥,⊥ Cees J. N. Buisman,†,# and Hubertus V. M. Hamelers*,† †

Wetsus, Centre of Excellence for Sustainable Water Technology, Oostergoweg 7, 8911 MA Leeuwarden, The Netherlands Flemish Institute for Technological Research (VITO), Boeretang 200, 2400 Mol, Belgium § Department of Electrical and Computer Engineering, HiST, Sør-Trøndelag University College, 7004 Trondheim, Norway ∥ INM - Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany ⊥ Saarland University, Campus D2 2, 66123 Saarbrücken, Germany # Sub-Department of Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands ‡

S Supporting Information *

ABSTRACT: Extracting electric energy from small temperature differences is an emerging field in response to the transition toward sustainable energy generation. We introduce a novel concept for producing electricity from small temperature differences by the use of an assembly combining ion exchange membranes and porous carbon electrodes immersed in aqueous electrolytes. Via the temperature differences, we generate a thermal membrane potential that acts as a driving force for ion adsorption/desorption cycles within an electrostatic double layer, thus converting heat into electric work. We report for a temperature difference of 30 °C a maximal energy harvest of ∼2 mJ/m2, normalized to the surface area of all the membranes.

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INTRODUCTION In response to the tremendous level of global energy consumption and its continuing growth, advanced energyharvesting technologies are in great demand.1 Among the large group of energy harvesters, extracting energy from thermal reservoirs is particularly attractive when we consider the vast abundance and availability of industrial waste heat or lowtemperature geothermal sites.2,3 The phenomenon of thermoelectricity, that is, the effect of an induced electric potential because of a temperature gradient, was first observed by Seebeck4 and correctly explained by Ørsted5 in the 19th Century, yet the thermoelectric effect is not limited to p- or n-type semiconductors6−8 and can be found in materials such as carbons,9 polymer membranes,10 or liquids such as ionic liquids.11 Utilizing this effect makes it possible to convert heat directly into electric work (“Seebeck machine”).12,13 In this letter, we introduce a new concept to directly convert small temperature heat differences (e.g., Δ30 °C) near room temperature into electric work with a capacitive membrane system immersed in saline water. The existence of a temperature gradient across ion exchange membranes leads to a voltage difference known as the thermal membrane potential that can be utilized, in combination with capacitive electrodes, to generate electricity from any source of low-temperature waste heat.14−16 Thermal membrane potentials are well-known © 2014 American Chemical Society

to arise when a membrane is subject to a thermal differential in an electrolyte with a constant concentration.15,17 The magnitude of such thermal membrane potentials is strongly influenced by the type of membrane and electrolyte but commonly is in the range of millivolts per degree Celsius.16 An intermittent operation of this process is achieved through alternately dipping a pair of titanium rods coated with a layer of activated carbon and sealed by ion exchange membranes in two separated electrolyte reservoirs at different temperatures. Thus, in our approach, we successfully and synergistically combine the migration of ions across the membranes with fully reversible ion electrosorption in the electrical double layer inside the porous electrodes.18 The term electrical double layer refers to the electrons immobilized within carbon electrodes and electrosorbed ions in the electrolyte. Ion adsorption occurs at the fluid−solid interface in the solution with a lower temperature, while desorption takes place in the solution with a higher temperature, allowing dissipation of the harvested power in an external load. Received: Revised: Accepted: Published: 356

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Figure 1. Illustration of the experimental setup, temperature profiles, and mass transport.

Figure 2. Change of system voltage and temperature (example shown, CEM-coated electrode) of the core of the wire electrode response during ion adsorption/desorption cycles measured in a 0.5 M NaCl solution and evolution of the temperature in the core of the wire. The gray areas underlining the data on the left side denote the times between the electrodes were swapped.

AEMcold with AEMhot and CEMcold with CEMhot. The electrical potential between the electrodes was measured via a potentiostat (Autolab PGSTAT 30, Metrohm Autolab B. V.), and the temperature inside the wire was accessed via a type K thermocouple that was placed between the carbon layer and the ion exchange membrane for each of the electrodes.

To demonstrate the validity of our approach, we have adapted a setup recently developed for power extraction from salinity gradients19,20 and desalination.21 Employing wire electrodes22 unlocks the possibility of implementing this technology to industrial processes, where water with different temperatures would flow by the electrodes rather than exchanging the electrodes between two different thermal reservoirs.

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RESULTS AND DISCUSSION Previous work has shown that a temperature gradient across a membrane leads to the formation of the so-called thermal membrane potential, and one can observe an ionic current passing through the membrane.23 However, we have to find a way to maintain the rule of charge neutrality to capitalize on this phenomenon for actual energy harvesting. When either cations or anions pass across a membrane, the net charge on either side changes. In the concept presented here, the local charge imbalance can be compensated by the flow of electrons and the buildup of an electrical charge at the surface of an electrode. Thus, charge balance is maintained as long as the charge of electrodes fully screens (i.e., counterbalances) the ionic charge imbalance in a solution. In our approach, this has been achieved by a combination of membrane technology with capacitive electrodes, as seen in Figure 1. To capitalize on the thermal membrane potential, the key aspect of our approach is to switch pairs of membrane-coated carbon wire electrodes (CEM and AEM) between electrolyte reservoirs with the same chemical composition of 0.5 M NaCl

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MATERIALS AND METHODS Platinum-coated titanium wires (2 mm thick, Magneto Special Anodes BV) were used as the current collector core of the carbon electrodes.19 The procedure for electrode manufacturing is outlined in more detail in the Supporting Information. In short, the electrodes had an average thickness of 200 μm along a length of ∼10 cm and consisted of 90 wt % activated carbon (YP50-F, Kuraray Chemical Co.) and 10 wt % polyvinylidene fluoride (PVDF) (Arkema Inc.). The surface of the electrodes was coated by a layer of ion exchange membranes that was hot pressed around the capacitive carbon coating.22 Two pairs of conductive rods were prepared: one coated with an anion exchange membrane (AEM) and the other with a cation exchange membrane (CEM). Each AEM/CEM pair was immersed in separate beakers with aqueous 0.5 M NaCl at different temperatures (e.g., 25 and 40 °C; the lower and higher temperatures are termed “cold” and “hot”, respectively). This was done while the electrodes were connected as follows: 357

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kept at different temperatures, for example, 25 and 40 °C. While larger temperature differences can be utilized, we limited this proof of concept to a maximal ΔT of 30 °C. The wire electrodes kept in the hot reservoir are then immersed in the cold electrolyte, while at the same time, the opposite is done for the second pair of electrodes. This way, one side of the membrane will be exposed to the hot or cold electrolyte, while the electrode on the other side of the membrane will still have the correspondingly opposite temperature. This effect is transient in nature, and once a thermal equilibrium has been established, the thermal membrane potential will be zero.17 However, until then, we observe the flow of an electrical current that mirrors ions being adsorbed by the cold electrode and desorbed by the hot electrode. Such a case is presented in Figure 2 for a temperature difference ΔT of 15 °C where a maximal system voltage of ∼0.2 mV can be seen until thermal equilibrium has been achieved within approximately 0.5 min. Because we operate the system at different temperatures, we also have to investigate the thermal behavior of the electrical double layer itself once the latter (i.e., thermal equilibrium) has been formed. Recent work by Janssen et al.24 has shown that charging and discharging capacitive electrodes at different temperatures allows a Carnot-like cycle in which each step exchanges either heat or work. In detail, porous carbon electrodes are charged, for example, in a cold saline solution and subsequently placed in a hot reservoir of the same electrolyte where an increase in electric potential (U + ΔU) is observed due to double layer expansion. Because the charge, Q, is preserved during this process, discharging from the now higher potential yields a higher energy, E, according eq 1: E=

1 Q (U + ΔU )2 2

uncharged capacitive electrodes at open cell voltage (see the Supporting Information). We then investigated in more detail the temperature dependence of the measured electrical potential for our system (Figure 2). We observed a direct and immediate response of the system in the form of a potential change between approximately ±0.2 mV during intermittent operation of swapping the electrodes back and forth between the two reservoirs. This change in potential is related to the transient nature of the thermal membrane potential. When the pair of electrodes is switched from hot to cold and cold to hot, the core of the electrodes will still be at the original temperature of the reservoir from which it has been removed. Over time, a thermal equilibrium is established and the electrode will show the same temperature as the surrounding beaker. The electrode temperature was tracked with a thermocouple inserted between the membrane coating and the top of the carbon electrode. The energy and power performance of our system were investigated next. For that, we normalized the data by the surface area of membranes. Extending the range to a ΔT of 30 °C, we correlated the amount of extracted energy with the temperature difference between the hot and cold reservoir when the electric current passes through an external resistor of 15 Ω (Figure 3). The energy is calculated per cycle by

(1)

It is important to note that we have to consider the interplay between the charge and double layer potential in more detail, as demonstrated in eq 2: UEDL =

⎛ Q /A 2F × arcsinh⎜⎜ RT ⎝ 8εrε0RTc inf

⎞ ⎟⎟ ⎠

(2)

where UEDL is the double layer potential, T the temperature, R the universal gas constant, F Faraday’s constant, Q the charge, A the surface, εr the dielectric permittivity of the electrolyte, ε0 the dielectric permittivity of vacuum, and cinf the bulk ion concentration. As a first-order approximation, one can see that the double layer potential is proportional to the temperature as long as the concentration is kept constant (note also εr is actually temperature-dependent). However, we also see that the entire potential term rapidly approaches zero when the charge becomes small. Thus, to exploit the effect of double layer expansion,25 we have to externally charge and discharge our electrodes: without external charge, the double-layer effect is minor, and henceforth, the electrode potential at the fluid− solid interface becomes insignificant. Our approach is different: here, the potential difference is generated by the membrane exposed to a temperature differential, and the double layer forms to maintain charge neutrality. As such, the accumulated charge is rather small and the effect of double layer expansion can be neglected at least for small thermal gradients, rather low values of thermal membrane potential and still high thermal conductivity of used membranes. This was confirmed by a control experiment in which our system was operated with fully

Figure 3. Extracted energy per immersion as a function of the temperature difference between the two electrolyte reservoirs (i.e., across the membrane once the electrodes are swapped and immersed in the hot or cold reservoir). Experiments were performed in a 0.5 M sodium chloride solution with an external resistance of 15 Ω and an initial temperature of 25 °C. The data were normalized per membrane area (accounting for all four membrane-coated electrodes). The dashed lines represents the fit of a quadratic equation (R2 = 0.9813).

integrating the power values obtained by multiplying the current and the voltage logged every second. As expected from Ohms law, the energy follows a second-order polynomial trend with a correlation coefficient R2 of 0.9813 with an increasing ΔT. This behavior was confirmed when conducting several cycles within an error margin of approximately 10%. The performance can be lowered by partial immersion of the electrodes in the electrolyte (see the Supporting Information), yet we see that the amount of generated energy is approximately equal to 2 mJ/m2. In a Carnot cycle, work with an efficiency of 9.1% would be extracted from a thermal difference of 30 °C and an initial temperature of 25 °C. If we would push the system close to the 358

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edges financial support from the Alexander von Humboldt Foundation.

boiling point of water at 100 °C, the efficiency would increase to 25% (although such a system would be difficult to operate given the high volatility). In contrast, the efficiency of the system presented here strongly depends on available thermal energy. For a small temperature difference of 30 °C and an optimized setup with a limited amount of electrolyte (water), we estimate our performance to reach 0.002‰ of Carnot efficiency. The low performance of thermoelectric technologies is a well-known and notorious problem8,13 (see the Supporting Information). In summary, we propose a novel concept based on capacitive membrane technology that allows us to extract electric work from low-grade heat by capitalizing on a combination of thermal membrane potential and adsorption and desorption of ions from an electrical double layer. As such, it is related to technologies such as “Capmix” (i.e., extracting electric work from a salinity gradient using a double-layer capacitor)25,26 and thermal double layer expansion24 but significantly distinct from both. The exact potential of this technology remains to be determined, but there are several important directions that our proof-of-concept study has now revealed. For example, adding a thermal gradient to the capmix process would enhance the generated electric energy not only by the contribution of double layer expansion but also by the thermal membrane potential of the AEM and CEM. Thus, salinity and temperature gradients should be considered, and the intrinsic Seebeck coefficient of the membrane and materials selection for advanced ion exchange membranes may be guided in that direction. Indeed, low-grade heat sources are vastly abundant, and virtually any industrial facility, especially when located near natural thermal sinks, such as rivers or lakes, can be a suitable user of the novel technology of thermal membrane potentialenabled energy harvesting.

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ASSOCIATED CONTENT

S Supporting Information *

Description of the material. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +31-58-2843000. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed in the TTIW-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (http://www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Friesland, the City of Leeuwarden, and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. We thank Natalia Grzywaczewska, Sebastien Ars, Dhan Prasam Gautam, Fei Liu, and Olivier Schaetzle (Wetsus) for assistance with the experiments and Michel Saakes for valuable discussions. We also thank the members of the research theme “Blue Energy” in Wetsus for their participation and contributions. The INM (http://www.inm-gmbh.de) is part of the Leibniz Research Alliance Energy Transition (LVE). S.P. and V.P. thank Prof. Eduard Arzt (INM) for his continuing support. S.P. acknowl359

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(23) Tasaka, M. Thermal membrane potential and thermoosmosis across charged membranes. Pure Appl. Chem. 1986, 58 (12), 1637− 1646. (24) Janssen, M.; Härtel, A.; van Roij, R. Boosting capacitive blueenergy and desalination devices with waste heat. arXiv 2014, 1405.5830. (25) Brogioli, D. Extracting Renewable Energy from a Salinity Difference Using a Capacitor. Phys. Rev. Lett. 2009, 103 (5), 058501. (26) Liu, F.; Schaetzle, O.; Sales, B. B.; Saakes, M.; Buisman, C. J. N.; Hamelers, H. V. M. Effect of additional charging and current density on the performance of capacitive energy extraction based on Donnan potential. Energy Environ. Sci. 2012, 5 (9), 8642−8650.

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