Experimental Study of A Solar-Powered Adsorption Cooling Tube

To evaluate the performance of this solar adsorption cooling tube, an experiment was conducted on a typical day (total solar energy to the solar adsor...
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Energy & Fuels 2006, 20, 1738-1743

Experimental Study of A Solar-Powered Adsorption Cooling Tube Xiaodong Ma,* Zhenyan Liu, and Huizhong Zhao School of Mechanical Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200030, People’s Republic of China ReceiVed December 27, 2005. ReVised Manuscript ReceiVed March 24, 2006

This work presents the description and experimental investigation of a solar-powered adsorption cooling tube using the working pair of zeolite13X-water. The operating principle of this solar cooling tube, which consists of four major components, a solar collector, an adsorbent bed, a condenser, and an evaporator all in one glass tube, is also presented. To evaluate the performance of this solar adsorption cooling tube, an experiment was conducted on a typical day (total solar energy to the solar adsorption cooling tube was about 23.5 MJ/m2 kJ and the variation of ambient temperature was from 25 to 34 °C). The experimental results show that a solar adsorption cooling tube is capable of heating 4 kg of water to about 50 °C in daytime and 4 kg of water to about 39 °C at night, as well as producing a refrigeration capacity of about 276 kJ. Its coefficient of performance (COPsys) can reach 0.22.

1. Introduction The ecological problems and energy crisis in the world have induced scientists to develop sustainable energy utilization systems, in which solar energy holds much attraction for these scientists. Solar refrigeration is an important use of solar energy because the supply of solar energy and the demand for refrigeration are the greatest during the same season. Even though the coefficient of performance (COP) of the solar refrigerator is low compared to that of the conventional vapor compression refrigerator, the solar-powered adsorption refrigerator uses free energy for its operation. It is also noiseless, lowcost, simple to manufacture, and environmentally friendly. The adsorption refrigeration cycle can be achieved by a desorption process (when heated) and an adsorption process (when cooled) with working pairs, such as zeolite-water,1,2 activated carbonmethanol,3-5 silica gel-water,6,7 and activated carbon-ammonia.8,9 * Author to whom correspondence should be addressed. Tel.: 0086021-6407-6226. E-mail: [email protected], [email protected]. (1) Tchernev, D. I. Natural Zeolites: Occurrence Properties and Use; Pergamon Press: London, 1978. (2) Lu, Y. Z.; Wang, R. Z.; Zhou, S. J.; Xu, Y. X.; Wu, J. Y. Practical experiments on an adsorption air conditioner powered by exhausted heat from a diesel locomotive. Appl. Therm. Eng. 2004, 24, 1051-1059. (3) Wang, R. Z.; Li, M.; Xu, Y. X.; Wu, J. Y. An energy efficient hybrid system of solar powered water heater and adsorption ice maker. Sol. Energy 2000, 68 (2), 189-195. (4) Anyanwu, E. E.; Ezekwe, C. I. Design, construction and test run of a solid adsorption solar refrigerator using activated carbon/methanol as adsorbent/adsorbate pair. Energy ConVers. Manage. 2003, 44 (18), 287992. (5) Lemmini, F.; Errougani, A. Building and experimentation of a solar powered adsorption refrigerator. Renewable Energy 2005, 30, 1989-2003. (6) Bucher, F.; Hildbrand, C.; Dind, P.; Pons, M. Experimental data on an advanced solar-powered adsorption refrigerator. International Conference on Heat Powered Cycles, Paris, Sept. 5-7, 2001; pp 61-8. (7) Saha, B. B.; Akisawa, A.; Kashiwagi, T. Solar/waste heat driven twostage adsorption chiller: the prototype. Renewable Energy 2001, 23, 93101. (8) Critoph, R. E. Laboratory testing of an ammonia carbon solar refrigerator. ISES, Solar World Congress, Budapest, Hungary, August 2326, 1993. (9) Vasiliev, L. L.; Mishkinis, D. A.; Antukh, A. A.; Vasiliev, L. L., Jr. Solar-gas solid sorption refrigerator. Adsorption 2001, 7, 149-61.

Most solar-powered adsorption refrigerators contain three major components: a solar collector/adsorbent bed container, a condenser, and an evaporator, which are connected by steel tubes and valves. Because of air leakage through the joints and valves, the vacuum in such a system is difficult to maintain over a long period. In the solid adsorption refrigeration system, the vacuum is one of the most important factors affecting the desorption/adsorption rate.10 Recently, some tubular adsorption refrigerators, which have the adsorption bed, condenser, and evaporator all in one tube, have been used in the adsorption system.11,12 The adsorption generator is at one end of the tube, and a combined evaporator and condenser are at the other end. In this work, a new solar-powered adsorption cooling tube (both for cooling and heating water) named a “solar cooling tube” is presented.13 This solar-powered adsorption cooling tube has the adsorbent bed, solar collector, condenser, and evaporator all in one glass tube, and no joints and valves are used. The experimental study is carried out to test the performance of this solar-powered adsorption cooling tube. 2. Working Principle of a Solar Adsorption Refrigerator The main components of a solar-powered adsorption refrigerator are a solar collector, an adsorbent bed, a condenser, and an evaporator. It makes use of the working pair adsorbentrefrigerant in the process of desorption and adsorption. Figure 1 shows the ideal cycle of the adsorption refrigeration in the Clapeyron diagram (LnP vs -1/T). In an ideal process, the cycle of adsorption refrigeration consists of two periods: the heatingdesorption-condensation period and the cooling-adsorptionevaporation period. The process goes as follows. (10) Anyanwu, E. E. Review of solid adsorption solar refrigerator I: An overview of the refrigeration cycle. Energy ConVers. Manage. 2003, 44, 301-312. (11) Critoph, R. E. Simulation of a continuous multiple-bed regenerative adsorption cycle. Int. J. Refrig. 2001, 24, 428-437. (12) Khattab, N. M. A novel solar-powered adsorption refrigeration module. Appl. Therm. Eng. 2004, 24, 2747-2760. (13) Liu, Z. Y.; Lu, Y. Z.; Zhao, J. X. Zeolite-active carbon compound adsorbent and its use in adsorption solar cooling tube. Sol. Energy Mater. Sol. Cells 1998, 52 (1-2), 45-53.

10.1021/ef050437y CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006

A Solar-Powered Adsorption Cooling Tube

Energy & Fuels, Vol. 20, No. 4, 2006 1739

[ (

X ) x0 exp -k

T -1 TS

)] n

(4)

In this equation, k and n are the characteristic parameters of the adsorptive working pair. X0 is the adsorption capacity when T ) TS and P ) PS (TS is the saturation temperature at pressure PS). T is the adsorption temperature. The cooling production produced in the evaporator during the adsorption process is

Qre ) Mad(Xa - Xd)Lre

(5)

where Lre is the latent heat of the evaporation of the refrigerant. Some of the cooling production will be consumed to cool the refrigerant liquid from condensing temperature Tc to evaporation temperature Te Figure 1. Clapeyron diagram (LnP versus -1/T) of ideal adsorption cycle.

During the daytime (heating-desorption-condensation period), sunlight falls on the solar collector, which contains the adsorbent bed and refrigerant (state A). The solar energy heats the adsorbent bed until its temperature rises to the temperature Tb (state B), which raises the vapor pressure of the desorbed refrigerant to the condensing pressure (Pc); desorption at a constant pressure is initiated; the desorbed refrigerant vapor is condensed in the condenser and flows into the evaporator. As the temperature of the adsorbent bed continues rising because of the solar heating, the maximum temperature Td could be achieved at the end of this period (state D). During the night (cooling-adsorption-evaporation period), the adsorbent bed is cooled, and its temperature drops after sunset (states D-F). The decrease in temperature from Td to Tf induces the decrease in pressure from Pc to Pe. Then, adsorption and evaporation occur while the adsorbent bed is cooled from Tf to Ta (state F to state A). During this cooling period, the refrigerant begins to evaporate and is readsorbed by the adsorbent bed. The adsorption refrigeration will continue for the whole night until the next morning. From the Clapeyron diagram, the total solar energy that the solar adsorption refrigerator gained during the heating period QAD will be the sum of the energy QAB and the energy QBD. The energy QAB is used to raise the temperature of the adsorbent bed and refrigerant from point A to point B, and the energy QBD is used for progressive heating of the adsorbent bed to point D and desorption of the refrigerant.

QAD ) QAB + QBD

(1)

QAB ) Mad(CP‚ad + XaCP‚re)(Tb - Ta)

(2)

Here, Mad is the mass of the adsorbent bed, CP‚ad is the specific heat of the adsorbent bed, Xa is the adsorption capacity before desorption, and CP‚re is the specific heat of the refrigerant in the adsorbed state.

QBD ) Mad[CP‚ad + (Xa + Xd)CP‚re/2](Td - Tb) + HdMad(Xa - Xd) (3) Here, Xd is the adsorption capacity after desorption. Hd is the desorption heat. The adsorption capacity X can be expressed using the Dubinin-Astakhov equation:

Qce ) Mad(Xa - Xd)CP‚re(Tc - Te)

(6)

The COP of the adsorption refrigeration cycle can be expressed as

COPcyc )

Qre - Qce QAD

(7)

The efficiency of the solar collector is

ηcol )

QAD Qsol

(8)

where Qsol is the total solar energy input to the solar collector. It can be expressed as

Qsol )

∫σsolAc dt

(9)

where σsol is the solar flux density to the solar collector and Ac is the area of the solar collector. The COP of solar-powered adsorption refrigeration system is

COPsys )

Qre - Qce Qsol

(10)

3. Description of the Solar Cooling Tube The main components of the solar cooling tube are the solar collector, adsorbent bed, condenser, and evaporator, which are all in one glass tube. Figure 2 shows a sketch of the solar cooling tube and its cross section. A photo of the solar cooling tube is shown in Figure 3. Because zeolite13X-water is chosen as the working pair in the solar cooling tube, the adsorption refrigeration cycle can be made without noise and pollution. The solar cooling tube has outer and inner glass tubes. Both of the lower ends of the two glass tubes are closed. The upper end of the outer glass tube is jointed with the same end of the inner glass tube. The solar collector and adsorbent bed are in the outer glass tube, and the inner glass tube is inserted into the absorbent bed and used to cool the adsorbent bed. When the adsorbent bed is heated by solar energy in the daytime, the desorbed refrigerant vapor flows upward in the vapor passages and is condensed in the upper end of the outer glass tube. Then, condensed refrigerant flows into the lower end of the outer glass tube by gravity. When the adsorbent bed is cooled at night, the condensed refrigerant in the lower end of the outer glass tube begins to evaporate and to be readsorbed by the adsorbent bed. So, the upper end and lower end of the

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Figure 2. Sketch of the solar cooling tube and its cross section.

outer glass tube are used as the condenser and evaporator, respectively. To enhance the efficiency of the solar collector and reduce the heat loss, the all-glass evacuated solar collector tube has been applied to heat the adsorbent bed. It has a total surface area of 0.07 m2, of which 0.055 m2 is the actual absorber surface. The adsorbent bed is a cylinder, with an inside diameter of 10 mm, an outside diameter of 36 mm, and an axial length of 1400 mm. Moreover, for better mass transfer in the adsorbent bed, three vapor passages are arranged symmetrically in the adsorbent bed. Each of them is 4 mm in diameter and 1400 mm in length. All of the tubes are made of high-quality borosilicate glass with a high transmittance and low absorbance to solar radiation. The solar-selective coating material of the all-glass evacuated solar collector tube is aluminum with an aluminum nitride coating with a high absorbance to solar radiation and a low thermal emittance. Some technical parameters of the solar cooling tube are given in Table 1. The features of the solar cooling tube are (1) solar refrigeration and water heating occur; (2) an all-glass evacuated solar collector tube is used, so high-efficiency heating can be achieved; (3) the adsorbent bed/solar collector, condenser, and evaporator are all in one glass tube, and the vacuum is maintained easily in the solar cooling tube.

Ma et al.

packing rings (one for the cooling water tank, one for the condensing water tank, and one for the chilling water tank) were applied to guarantee the tightness when the solar cooling tube was inserted into the three water tanks. The experimental setup was mounted facing south at an angle of 30° from the horizontal on a steel shelf. The latitude of the location is about 32°. Copper-constantan thermocouples of type T were used to measure the temperatures with an accuracy of (0.5 °C. A Keithley multichannel recorder programmed from a PC was used to record the temperatures. A TRM-123 solar pyranometer was used to measure the incident solar radiation on the solar cooling tube with an uncertainty of (2%. Figure 5 shows the daily evolution of solar radiation and ambient temperature. The solar radiation varied from 270 to 1030 W/m2, and the ambient temperature varied within the range 24.2-34.4 °C. The solar radiation reached its peak value at 12: 30 a.m., while the ambient temperature came to its peak value at 2:00 p.m. The measured solar energy to the solar cooling tube was 23.5 MJ/m2 in the daytime. The daily desorption process proceeded from 7:30 a.m. to 5:00 p.m. Before the experiment started, 4 kg of city water at 25 °C was filled into the condensing water tank, and the initial temperature of the adsorbent bed was about 30 °C. The solar radiation that the adsorbent bed gained was used to heat the adsorbent bed and make the refrigerant desorbed. Figure 6 shows the daily evolution of the temperature of the adsorbent bed. Though the solar radiation reached its peak at about 12:00 a.m., the temperature of the adsorbent bed, however, was not in-phase with the solar radiation, because of the thermal inertia of the adsorbent bed. At about 3:30 p.m., the temperature of the adsorbent bed reached its peak value: 230 °C. When the adsorbent bed was heated by solar energy, the desorbed refrigerant vapor flowed upward in the vapor passages and conglomerated in the condenser. The 4 kg of water in the condensing water tank was heated by the latent heat that was discharged from the refrigerant vapor in the condenser, and its temperature could come to about 50 °C at the end of the daily desorption process. Figure 7 shows the daily evolution of the water temperature and heating power. The heating power at first rose rapidly as more desorbed refrigerant was condensed and more latent heat was discharged, but then as the desorbed refrigerant and discharged latent heat were decreasing, the heating power was falling. During the desorption process, the maximum heating power was 41 W. The condensed refrigerant flowed into the evaporator by gravity. In the daily desorption process, the heating power is expressed as

4. Experiments The performance of this solar cooling tube has been tested on a clear day in Huaian, Jiangsu province. The experimental setup is shown in Figure 4a. The condensing water tank and cooling water tank are jointed together and given in Figure 4b, and the chilling water tank is shown in Figure 4c. The condenser of the solar cooling tube was inserted into the condensing water tank and cooling water tank. The condensing water tank was used to collect the sensible heat and condensing heat discharged from the desorbed refrigerant vapor in the daytime, and the cooling water tank was used to collect the sensible heat and adsorption heat at night. Inserted into the chilling water tank, the evaporator of the solar cooling tube was used to collect the refrigeration capacity produced by the evaporation of the refrigerant at night. Each of the three water tanks was made of 1-mm-thick sheet iron framing with polyurethane foam of 50 mm thickness for the insulation. Three

dx dt

Phd ) Mad[Lre + CP‚re(Tre - Tc)]

(11)

Here, Tre denotes the temperature of the desorbed refrigerant vapor and Tc is the condensing temperature of the desorbed refrigerant vapor. In this experiment, the condensing water tank was insulated with 50 mm of polyurethane foam. If the heat loss was neglected, the heat gained by the water was the output heat of the solar cooling tube during the daily desorption process. It can be expressed as

Phd ) Mwd

dTwd dt

(12)

Here, Mwd is the mass of water in the condensing water tank. Twd is the water temperature.

A Solar-Powered Adsorption Cooling Tube

Energy & Fuels, Vol. 20, No. 4, 2006 1741

Figure 3. Photo of the solar cooling tube. Table 1. Main Parameters of Solar Cooling Tube parameters

value

parameters

outside diameter of outer glass tube outside diameter of evacuated solar collector tube inside diameter of evacuated solar collector tube outside diameter of adsorbent bed inside diameter of adsorbent bed outside diameter of inner glass tube

58 mm

wall thickness of glass tube

1.5 mm

value

47 mm

length of condenser

200 mm

38 mm

length of evaporator

200 mm

35 mm

1400 mm

8 mm

length of solar collector/ adsorbent bed length of inner glass tube

1550 mm

8 mm

mass of adsorbent bed

1.15 kg

The night adsorption process was from 5:00 p.m. to 7:30 a.m. of the next morning. The heated water in the condensing water tank was taken out at 5:00 p.m., and then 4 kg of city water at 26 °C was filled into the cooling water tank to cool the adsorbent bed through the inner glass tube. At the same time, 4 kg of city water was filled into the chilling water tank in order to take out the cooling production. Figure 8 shows the night evolution of the temperature of the adsorbent bed. At the beginning, the temperature of the adsorbent bed decreased rapidly because of the cooling effect of the cooling water in the cooling water tank; then, the decrease in temperature of the adsorbent bed slowed because of the adsorption heat discharged from the readsorbed refrigerant. During the adsorption process, the temperature of the adsorbent bed decreased from a temperature of 200 °C to a temperature of 50 °C. Figure 9 shows the night evolution of the temperature of the cooling water and heating power. During the adsorption process, the sensible heat of the adsorbent bed and the adsorption heat caused the water in the cooling water tank to raise its temperature from 25 °C to about 39 °C. The heating power at first increased quickly as much sensible heat and adsorption heat was released, but then, it fell as the sensible heat and adsorption heat of the adsorbent bed decreased. During the adsorption process, the heating power is expressed as

dTad dx + HaMad dt dt

Pha ) Mad(CP‚ad + xCp‚re)

(13)

Here, Tad denotes the temperature of the adsorbent bed and Ha is the adsorption heat. The cooling water tank was also insulated with 50 mm of polyurethane foam. If the heat loss was neglected, the heat

Figure 4. (a) Sketch of the experimental setup of the solar cooling tube. (b) Sketch of the cooling water tank and condensing water tank. (c) Sketch of the chilling water tank.

1742 Energy & Fuels, Vol. 20, No. 4, 2006

Ma et al.

Figure 5. Daily evolution of solar radiation and ambient temperature. Figure 8. Night evolution of temperature of adsorbent bed.

Figure 6. Daily evolution of temperature of adsorbent bed. Figure 9. Night evolution of temperature of cooling water and heating power.

Figure 7. Daily evolution of water temperature and heating power.

gained by the water was the output heat of the solar cooling tube during the adsorption process. It can be expressed as

dTwa Pha ) Mwa dt

(14)

Here, Mwa is the mass of the water in the cooling water tank. Twa is the water temperature. As the temperature of the adsorbent bed decreased, the vapor pressure of the refrigerant began to reduce. So the refrigerant

Figure 10. Night evolution of temperature of chilling water, ambient, and refrigeration power.

boiled in the evaporator and began to absorb the heat from the water in the chilling water tank. As the evaporation of refrigerant went on, the temperature of the chilling water decreased continuously. The refrigeration power at first increased rapidly, but later, it decreased because of the decreasing of the evaporation rate (see Figure 10). During the adsorption process, the maximum refrigeration power was about 32 W, and the temperature of the chilling water came to about 9.5 °C in the

A Solar-Powered Adsorption Cooling Tube

end. This adsorption refrigeration could last until the second morning. The ambient temperature changed within the range 24-30 °C. 5. Conclusion A solar-powered adsorption cooling tube is presented in this work. This solar cooling tube consists of four major components: a solar collector, an adsorbent bed, a condenser, and an evaporator all in one glass tube. The performance of the solar cooling tube both for cooling and for heating water was tested. Under the condition that the total solar energy to the solar cooling tube was about 23.5 MJ/m2 and the ambient temperature varied from 25 to 34 °C, a solar cooling tube was capable of heating 4 kg of water to about 50 °C in the daytime and 4 kg of water to about 39 °C at night, as well as producing a refrigeration capacity of about 276 kJ. Its coefficient of performance (COPsys) could reach 0.22. Each solar cooling tube is a solar refrigerator. A solar refrigeration system consisting of such refrigerators has the advantages of simple structure, low cost, and high efficiency. Acknowledgment. The authors thank Huaiyin Huihuang Solar Energy Co., Ltd for its support of the research work.

Nomenclature A-F ) points on Clapeyron diagram Ac ) area of the collector (m2)

Energy & Fuels, Vol. 20, No. 4, 2006 1743 CP‚ad ) specific heat of adsorbent bed (kJ/kg K) CP‚re ) specific heat of refrigerant (kJ/kg K) COPcyc ) COP of adsorption refrigeration cycle COPsys ) COP of adsorption refrigeration system Ha ) adsorption heat (kJ/kg) Hd ) desorption heat (kJ/kg) k,n ) characteristic parameters of adsorptive working pair Lre ) latent heat of refrigerant (kJ/kg) Mad ) mass of adsorbent bed (kg) Mwd ) mass of water in the condensing water tank (kg) Mwa ) mass of water in the cooling water tank (kg) Phd ) heating power during desorption process (kW) Pha ) heating power during adsorption process (kW) Q ) heat (kJ) Qre ) cooling production (kJ) Qsol ) total solar energy input to the solar cooling tube (kJ) Xa ) adsorption capacity before desorption (kg/kg) Xd ) adsorption capacity after desorption (kg/kg) X0 ) adsorption capacity at T ) TS and P ) PS (kg/kg) T ) temperature (°C) Tc ) condensing temperature (°C) Te ) evaporation temperature (°C) TS ) saturation temperature at pressure PS (°C) Twd ) temperature of water in the condensing water tank (°C) Twa ) temperature of water in the cooling water tank (°C) t ) time (s) σsol ) solar flux density radiation (W/m2) EF050437Y