SOLID SORPTION COOLER WITH COMPOSITE SORBENT BED AND HEAT PIPE THERMAL CONTROL LEONARD VASILIEV, ALIAKSANDR TSITOVICH, ALEXANDER ZHURAVLYOV, AND ALEXANDER ANTOUGH Luikov Heat & Mass Transfer Institute, P. Brovka, 15, , Minsk, Belarus. Tel/ Fax (375) , HTRSE – 2012,Międzyzdroje, , Poland

2 Refrigeration technologies have been critical in the evolution of production and distribution systems a long period of time. Reduction in use of synthetic refrigerants and production of CO 2 provide a new opportunity for solar cooling and refrigeration. Adsorbents like zeolite, silica gel, activated carbon and alumina oxide are considered as physical adsorbents having highly porous structures with surface-volume ratios in the order of several hundreds that can selectively catch and hold refrigerants. When saturated, they can be regenerated simply by being heated. The process is intermittent because the adsorbent must be regenerated when it is saturated. For this reason, multiple adsorbent beds are required for continuous operation. The combined action of physical adsorption and chemical reactions for the cold production in the same space and at the same time is attractive initiative to enhance the COP of a system. Introduction

3 The schematic of the three adsorbers cooler The original heat pipes (thermosyphon) are used as heat exchangers for external heat recovery and adsorbers thermal control. Activated carbon fibre (ACF) “Busofit” is used for ammonia adsorption/desorption. The micro/nano crystals of MnCl 2 and BaCl 2 are used as the chemical sorption material to increase the sorption capacity of the sorbent bed. The experimental set-up includes two medium temperature adsorbers (ACF + MnCl 2 ), one low temperature adsorber (ACF + BaCl 2 ). The thermal management system consists of four unites: vapordynamic thermosyphon, two loop thermosyphons and a loop heat exchanger joint to the low temperature adsorber.

4 Schematic of three adsorbers cycle with complex utilization of energy of low temperature adsorber The main idea is application of sorption/desorption and condensation/evaporation phenomena in the same volume. It increases the COP and specific cooling power of the system.

5 А А 34 Q Q Q Q А–А 22 4 7 LкLк H0H0 1 – evaporator, 2 – liquid pipe, 3 – vapor pipe, 4 – compensation chamber, 5 – heat sink, 6 – condenser, 7 – annular channel, H 0 – hydrostatic pressure drop Vasiliev, L.L., Morgun V.A., Rabetsky M.I (1985), US Patent No , Vapor-dynamic thermosyphon schematic

Thermal resistance R of VDT as a function of heat load (W): 1- water; 2 - propane; 3 - water with air; 4 – propane with an air in the gas trap VDT with liquid mini heat exchanger 7 (tube-in-tube) inside. a - liquid; b –vapor 1- cartridge heater; 2 - evaporator; 3 - condenser; 4 - liquid pipe; 5 – vapor pipe ; 6 – NCG trap 7- liquid heat exchanger Vapor –dynamic thermosyphon (VDT) with liquid mini heat exchanger inside it propane water The liquid heat exchanger (efficiency ε = 0.8) is made as a 2 mm SS tube placed inside the annular channel of the VDT condenser.

7 Vapour-dynamic thermosyphon 1. condensers, 2. valves, 3. liquid line, 4. vapour line, 5. evaporator, 6. liquid pool of the evaporator Vapor-dynamic thermosyphon is made from SS steel and used for thermal coupling of solar collector and two adsorbers. The working fluid is water. The evaporators of the vapour-dynamic thermosyphon are disposed inside the vacuum glass solar collectors. The thermosyphon condensers are placed inside two medium temperature adsorbers.

8 Solar heater with two adsorbers, solar collectors (with flame as the back-up), vapor-dynamic thermosyphon (VDT), two loop thermosyphons and two valves VDT thermosyphon consists of four evaporators (gas flame heated evaporator as a back-up), the vapor and liquid minichannels and two-condensers switched on and off alternatively

VDT with mini gas burner as a source of energy to heat the solid sorption cooler VDT (stainless-steel/water) with mini gas burner. Sorbent material is disposed between fins inside the adsorbers. Active carbon fibre – sorbent material VDT SS/water Mini gas burner Adsorber envelope

Solar cooler with two adsorbers. VDT has two condensers. The temperature evolution of VDT evaporator 1, adsorber 2 and adsorber 3, two VDT condensers and two liquid mini heat exchangers inside. During the tests the temperature of the evaporator envelope (curve 1) was constantly near C, while the temperature of adsorbers envelope (curves 2-3) was periodically changing from ambient temperature 20 0 C (adsorption) up to 90 0 C (desorption).

11 Dynamic ammonia sorption capacity (adsorption/desorption) for ACF “Busofit” at different temperatures versus pressure In order to study the sorption capacity of the sorbent material it is necessary to know the quantity of gas adsorbed on each point of the cycle. There is a general need to have a good fit of experimental isotherms and temperature and to extrapolate some isotherms. For ACF “Busofit” the approach of Dubinin is well adapted and allows linking quite simply the physical properties of “Busofit” to the capacity of adsorption of this carbon fiber.

12 Dubinin-Radushkevich equation is a special case of Dubinin-Astakhov equation, (n=2): a – sorption capacity, g/g, mmole/g, B – structural constant, which characterizes the size and distribution of micro pores, K -2 ; E 0 – characteristic energy of standard gas (usually – benzene) kJ/mole; P – pressure, Pa, kPa, MPa; Т – temperature,  C, K; R – universal gas constant, kJ/(mole K); V – volume, m 3 ; W o – micro porous volume limit, cm 3 /g; z – compressibility factor;  – affined coefficient, – adsorptive substance molar volume, cm 3 /mmole. – characteristic sorption energy, kJ/mole Dubinin-Astakhov equation:

13 Enlarged view of activated carbon fibre “Busofit” The activated carbon filaments and micro/nano crystals enhanced the COP of the system to compare with conventional chemical heat pumps. To minimize a void space and increase the adsorbent capacity of the active carbon fibre we need to compress “Busofit” together with a binder (monolithic material).

14 Enlarged view of activated carbon fibre “Busofit” with micro crystals of BaCl 2 on the filament surface Even for maximal concentration of salt BaCl 2 (low temperature adsorber) on the filament surface its structure around the filament rest porous. It is convenient for heat and mass transfer enhancement.

15 Temperature field evolution on solar cooler adsorbers and loop heat exchanger as the function of cycle The low temperature (T 1, T 2 ) adsorber (ACF +BaCl 2 ), T 3 – mean temperature of liquid inside the loop heat exchanger, q – heat flow to/from the liquid heat exchanger to the sorbent bed inside the low temperature adsorber

16 Temperature profiles in the low temperature adsorber, loop heat exchanger during the rate of cold (q) production (desorption) Temperature profiles of the low temperature adsorber (T 1 - beginning of adsorber, T 2 – end of adsorber), liquid in the loop heat exchanger (T 3 ), and mean heat flow q from the liquid heat exchanger to the sorbent bed inside the low temperature adsorber

17 The surface (T 1, T 2 ) of the low temperature adsorber, as the time of, T 3 – mean temperature inside the liquid heat exchanger, q – heat flow from the liquid heat exchanger to the sorbent bed inside the low temperature adsorber Temperature profiles in the low temperature adsorber, loop heat exchanger during the rate of cold (q) production (desorption+evaporation).

18  1. A novel three bed cooler based on two (ACF + MnCl 2 ) bed and a low temperature bed (ACF + BaCl 2 ) was suggested and experimentally investigated.  2. A new stream in the solid sorption chillers application is related with complex compounds sorbent materials development (for example, as active carbon fibre and microcristals of the salts on its surface) and heat pipe thermal management of adsorbers. CONCLUSIONS

19  3. The above mentioned sorbent bed has the advantages of chemical coolers (high sorption capacity) and high speed of adsorption, typical for adsorption coolers. Simultaneously there is a strong interaction (intensive heat and mass transfer) between adsorptive materials and chemical materials (active carbon/microcrystals of salts) during the cycle of heating/cooling.  4. Three bed cycle has higher performance of cooling compared with double stage cycle, using four adsorbers. The mass recovery process is the key to improve cooling capability of adsorption cycle with low temperature heat sources. CONCLUSIONS

20  5. The specific cooling power (SCP) of about 100 W/kg was achieved for solar cooler with complex sorbet bed (ACF+CaCL2), which is two times more comparing with conventional coolers.  6. The main idea is application of sorption/desorption and condensation/evaporation phenomena in the same volume. It increases the COP and specific cooling power of the system.  7. The heat pipe technique ensures an efficient thermal control of adsorbers and eliminates hot spots in the sorbent bed. CONCLUSIONS

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