S ERI/TP-631-1065

UC CATEGORY : 59c

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AN OVERVIEW OF OPEN-CYCLE

D ESICCANT COOLING SYSTEMS

AND MAT ERIALS

R. COLLIER F. ARNOLD R. BARLOW

SEPT EMBER 1981

SUBMITTED TO THE JOURNAL OF SOLAR ENERGY ENGINEERING

PREPARED UNDER TASK Nb. 1131.00 WAPA No. 256 81

Solar Energy Research Institute A Division of Midwest Research Institute

1617 Cole Boulevard Golden, Colorado 80401

Prepared for the U.S. Department of Energy Contract No. EG-77-C-01-4042

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INTRODUCTION

Desiccant cooling systems process water vapor- in the earth's atmosphere . to produce cooling. Since mass transfer occurs between the system and its environment, they are commonly referred to as " open-cycle" systems. These systems all use a liquid or solid material called a desiccant to remove water vapor from the air. The process by which water is removed is most often adsorption on the solid desiccants and absorption in the liquid desiccants.

Desiccant systems are presently used in industrial air-drying applications. There are solid systems marketed by Bry-Air and Cargocaire and liquid systems marketed by Niagara and Kathabar. The first two use a desiccant-laden wheel in which air may flow in the axial direction only. The solid desiccant (lithium chloride salt or silica gel) is impregnated into the wheel material or encapsulated as a packed bed. Air to be dried flows through one side of the wheel, while the desiccant on the other side of the wheel is being dried by an externally heated air stream. These two air streams must be kept physically separate in order to maintain the distinctly separate functions of air drying and desiccant regeneration.

The liquid systems use two separate spray chambers for the processes of drying and regeneration. The "strong, " concentrated solution '(triethylene glycol or a lithium chloride solution in water) is sprayed over cooling coils whose temperature is maintained by water from a cooling tower or some other lowtemperature source. Air is dried as it passes through the spray. When the desiccant solution absorbs sufficient moisture from the air, it. is pumped to another chamber where it is sprayed into an externally heated air stream or over heated coils. This regenerates the solution, which is used again in the drying process.

These commercial systems are intended as air driers only and do not produce a s;ignificant net cooling. There are important differences in design philosophy between solar-regenerated desiccant _cooling systems and commercial desiccant air driers. The most important difference involves thermal and .. electrical coefficients of performance (COPs). Commercial desiccant air drier manufacturers have chosen markets in which vapor-compression equipment cannot compete: applications where extremely dry air ( 2-10% relative humidity) is required. The energy requirements to achieve these conditions have not been a major concern, and, as a consequence, the desiccant dehumidifiers often have

very low thermal and electrical COPs. Since solar-regenerated desiccant cooling machines must compete with vapor compression as well as all other spacecooling technologies, both electrical and thermal COPs are of primary concern.

Desiccant cooling systems are attractive areas of research because of their inherent ability to use air as the working fluid. In addition, they do not have to be hermetically sealed and are adaptable to applications with high ventilation loads.

One of the earliest solar-regenerated desiccant systems was built and tested by Lof [1]. This was a liquid, triethylene-glycol system using air heated directly in a solar collector for generating the desiccant. A problem with this system was the migration of glycol into the building space. More

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recently, Johannsen [2] has reported on a glycol system in which the weak solution is regenerated in the solar collector itself. The solution flows as a fluid film in contact with a solar absorbing surface and the ambient air.

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The heated solution releases its absorbed moisture to the atmosphere. A similar system using calcium chloride and water as the desiccant solution has been reported by Mullick & Gupta [3].

Various solid-desiccant systems employing rotary desiccant wheels or drums have been proposed and studied [ 4, 5, 6, 7, 8] In addition, researchers [ 9, 10] have also considered stationary beds that are cycled between drying and regeneration. In all these systems, it is possible to use solar collectors to vide heat for the regeneration of the desiccant. Solar air heaters are ticularly suited to these desiccant systems because air is the medium. An additional heat exchanger would be needed if a liquid the working fluid in the solar collectors.

par-regeneration were used as

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'lllERMODYNAMIC ANALYSIS

Block diagrams of the most common rotating solid-desiccant systems are shown in Figs. 1(a), 2(a) and 3(a) [11]. The symbols EC, DR and HE indicate evaporative coolers, dehumidifiers, and heat exchangers, and Q represents heat input. Psychrometric charts with state points corresponding to locations on the block diagrams are shown in Figs. 1(b), 2(b) and 3(b). These were calculated for ambient conditions of 35C and 0.021 kg/kg absolute humidity (95F, 60% RH), room conditions of 24C and 0.009 kg/kg absolute humidity (75F, 50% RH) and for a regeneration temperature of 95C (203F). The effectiveness of all components is 90% for the three cycles.

The cycle shown in Fig. 1 is called the ventilation mode. In this case, room air is used to regenerate the dehumidifier bed and outdoor air is cooled. The air leaving the room (6) is evaporatively cooled (7) and used as the cold sink for the dried room-return air. The room-exit air is heated during the heatexchange. process (8). It is then further heated by an external source of energy (Q) for desiccant drying (9). Drying the desiccant cools and humidifies the air (10). To supply room make-up air, ambient air (1) is dried by the desiccant (2) and cooled by heat exchange (3) with room-exit air. This dried and cooled ambient air is then further chilled by evaporative cooling (4). Just before point 5, remix air is introduc-ed. posed by Dunkle [6], mixes evaporatively cooled room dried room make-up air in order to control

The recirculation mode shown in Fig. 2 uses the same lation mode except that room air is constantly reconditioned and outdoor air is used for regeneration. Thermodynamically, the advantage of processing air with greater availability for cooling. the disadvantage of having a higher cold-sink temperature than the ventilation mode. The desirability of either cycle isa trade-off which depends upon room and ambient conditions. This trade-off will be discussed in greater detail later in the paper. Another important difference between this mode and the ventilation mode is that there is no direct fresh-air . supply, whereas the ventilation mode uses all fresh air. For the recirculation mode, as well as for most vapor-compression cycles, fresh air to the building space is supplied by normal infiltration. In an era of tighter buildings, systems which do not allow and control ventilation will be at a disadvantage.

The Dunkle cycle shown in Fig. 3 is an attempt to combine the thermodynamic advantages of both the ventilation and recirculation modes. The cycle uses the advantage of processing the higher cooling availability room air as in the recirculation mode, while retaining the lower cold-sink temperature of the ventilation mode. This advantage in performance comes at the cost of increased complexity and an additional sensible heat exchanger. As with the recirculation mode, the lack of controlled f.resh air to the building space may be a disadvantage.

The cooling capacity and thermal COP of these cycles can be calculated with the aid of the psychrometric diagrams. The amount of cooling delivered to the building for the ventilation mode would be defined as (see Fig. 1(b))

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cooling capacity

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(1)

where m is the mass flow rate of dry air.

If the mass flow rate out of the building equals the mass flow rate int