OVERVIEW OF DEVELOPING PROGRAMS IN SOLAR DESICCANT COOLING FOR RESIDENTIAL BUILDINGS TO BE PRESENTED AT THE ANNUAL MEETING OF THE AMERICAN SOCIETY OF HEATING, REFR~CERATING, AND AIR-CONDITIONING ENGINEERS DETROIT, MICHIGAN Solar Energy Research Institute 1536 Cole Boulevard Golden, Colorado 80401 A Division of Midwest Research Institute Prepared for the U.S. Department of Energy Contract No. EG.77C.01.4042 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. 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OVERVIEW OF DEVELOPING PROGRAMS IN SOLAR DESICCANT COOLING FOR RESIDENTIAL BUILDINGS TO 8E PRESENTED AT THE ANNUAL MEETING OF THE AMERICAN SOCIETY OF HEATING, REFRIGERATING, AND AIR-CONDITIONING ENGINEERS Solar Energy Research.lnstitute 1536 Cole ~oulevard Golden, Colorado 80401 . A Division of Midwest Research Institute prgPared for the U!S:Department of Energy Contract No. EG.77.C.01.4042 p?m?nn.p(" -'.rer * 7.. , ,.n..,vF. ?,r,pT.,,187. \7ri, v(N -.k&k,~r LA ..)L.+ 4- 2% + ~rnL<.:&&A~>~~v.~ \ / This paper grovides an overview of the ongoing work in desiccant cooling under the national solar heating and cooling research program. Open cycle adsorption and absorption systems are examined. The different dehumidifier bed configurations are the distinguishing features of these systems. The basic operating principles of each dehumidifier concept are explained along with some discussion of their ccmparative features. Performance predictions developed by SERI for a solar desiccant solar system employing an axial-flow desiccant wheel de- humidifier are presented. In terms of life-cycle cost and displaced fossil-fuel energy, the results indicate that it should be beneficial to use solar desiccant coolers in residential applications. Although no prototype testing of any of these concepts is currently underway, test results are expected and xi11 be reported within one year. INTRCDUCTION Severai air-conditioning configurations involving different physical, chemical, and electrical 9rocesses can be used to produce cooling effects adequate for residential buildings. These systems are summarized in Table 1. TABLE 1 POTENTIAL AIR-CONDITIONING PROCESSES Process Type Open Cycle Closed Cycle Mecnanical compression Rankine cycle Brayton Stir1ing Absorption Desiccant Absorption Adsorption Cesiccant Adsorption Electronic transport Thermionic emission Pel tier effect The processes can be implemented in open or closed cycles. The closed cycle involves two separate process loops coupled by heat exchangers, with one loop for the refriaeration process and :he other for the transfer of heat from the load. The open cycle eliminates the inter- facing heat exchanger by combining the process and heat transfer loops. Benjamin C. She1 puk, senior Engineer, and Douglas !d. Hooker, Associate Engineer, Solar Enerqy qesearch Institute, Golden, Colorado. Contractors for the U.S. Department of Energy (DOE) are performing research and development work on solar cooling systems involving desiccation processes. In the open cycle adsorption processss, three contractors are developing solar cooling systems with different dehumidifier bed concepts. These projects are in various stages of completion, ranging from basic research and analysis to cooling system development and commercialization studies. Several other organizations have studied the open cycle absorption process, with proposed differences in the systems mainly in the method of regenerating the desiccant material. DOE has given the Solar Energy Research !nstitute (SERI) program management responsibilities for national solar cooling programs using desiccant processes, and SERI is continuing its own research program in desiccant cooling. OPEN CYCLE ADSORPTION PROCESSES The open cycle adsorption desiccant processes in Table 1, when coupled to a solar energy sujply system, represent a potentially attractive alternative to conventional air-conditioning systems. Most existing refrigeration and air-conditioning equipment operates on a closed mechanical compression cycle, specifically the Rankine vapor compression cycle. The major advantaaes of a solar desiccant cooler based on a recyclable air desiccation process include: e It uses inexpensive materials and has potentially low manufacturing cost. It usually uses air and inert inorganic materials and thus may not present corrosion or environmental problems. r It can tolerate air leakage and be serviced easily. e It can tolerate a wide range in solar input and still generate a useable output. i4ost of the research and development work on the adsorption process for solar cooling aoplications has focused on the desiccant cycle. The early worx was done in Australia , (Commonwealth Scientific and Industrial Research Organization) and the United States (Institute of Gas Technology) on systems using two open process air streams thermally coupled through rotary regenerators. The building air stream was dried in a desiccant bed, cooled in a reaenerative heat exchanger, and refrigerated Sy evaporative cooling. The air dryer was reactivated by an outside air stream that was solar heated to supply the desorption energy. In both cases, the desiccant bed operated in an adiabatic process. Although many desiccant materials can adsorb water vapor from air, attention has focused on molecular sieve and silica gel. The equilibrium adsorption curves for molecular sieve and silica gel are given in Fig. 1 [I]. Molecular sieve has better adsorption characteristics at low relative humidities or .at high temperatures. Thus, dehumidifiers using molecular sieves can operate successfully in the adiabatic mode where both the dehumidified air and the desiccant bed are at high temgeratures. Molecular sieves, however, require relatively high regeneration temperatures (150 C) for desorbing which for solar energy applications means using high performance collectors or a solar system cam~ined with an auxiliary heat source. Silica gel, on the other hand, can be desorbed at relatively low temperatures. Fig. 1 snows thzt che eauilibrium water capacity of silica gel is very mall at relatively low irmperatures (80%). which a1 lows silica gel dehumfdif iers to utilize 5imple flat-plate collectors for regeneration. However, silica gel a1 so has good adsorption capacity only at iow temperatures, and its adsorption capacity decreases rapidly as the bed temperature increases. The performance of a silica gel bed is improved if the adsorbent is naintained at a low temperature during dehumidification by removal of the heat of adsorption. Ces I,c$.ant .&stem A1 ternatives The major, unique component in all desiccant cooling systems is the dryer bed, which removes moisture from the air being processed in the system. If this step can be accomplished without a substantial fraction sf the neat of condensation remaining in the process stream, all of the cooling in the cycle is completed. The subsequent isoenthalpi'c evaporative process simply converts lazent cs?acity into sensible coolinp. The various on~oingprojects in desiccant cooling can be described in terms of the approacn they take to the dryer bed design. Four concepts are depicted in Figs. 2 to 5 and are compared in Table 2. TABLE 2 DEHUMIDIFIER BE3 CONCEPTS Approach Fig. Material Advantages Concerns 2otary axial 2 molecular compact design ; requires gas boost; flow sieve prototype model large auxi 1iary power; exists large air duct system Rotary radial a3 slllca reduced auxiliary. silica gel withcut outflow gel power ; cross cooling; large 1ow temperature air duct system reactivation Cross-cooled silica low temperature added auxil iary fixed bed gel reactivation