The liquid desiccant system is designed to serve as an open-cycle absorption system that can operate with low-grade solar heat. A schematic description of the final design version of the system is given in Figure 1. The system consists of six major components: an air dehumidifier or absorber, a solution regenerator or desorber, two water-to-solution heat exchangers, a solution-to-solution heat exchanger, and an air-to-air heat exchanger. Arabic numerals indicate working fluids state points at specific locations. Air flow is represented by thick solid lines, solution flow by thin solid lines and water flow by dashed lines.

The dehumidifier (absorber) consists of a packed tower and operates in an adiabatic mode. Ambient air at state 13 entering the bottom of the absorber packed section is brought into contact with a concentrated absorbent solution entering the unit at state 8. Water vapor is removed from the air stream by being absorbed into the solution stream. The dehumidified warm air leaving the absorber passes through the blower and leaves the system toward the air-conditioned space at state 14. The blower controls the flow of air, while raising its temperature slightly. Solution is pumped from the absorber pool at the bottom of the tower into the plate heat exchanger (state 7), where it is cooled by water from a cooling tower. The solution leaving the heat exchanger (state 8) then proceeds to the distributor at the top of the packing, from where it trickles down in counter-flow to the air stream and collects in the pool. A controlled solution stream is transferred from the absorber pool to the regenerator, as shown (state 11). The return (pumped) stream from the regenerator (10) goes directly into the pool.

As evident, the regenerator (desorber) device is very similar to the dehumidifier, and so are the flow system and associated components. The solution is heated in the liquid-to — liquid heat exchanger by solar-heated water (states 1-2). Ambient air is pre-heated in the air-to-air heat exchanger by recovering heat from the exhaust air leaving the desorber. After pre-heating, the air stream (state 15) enters the desorber where it serves to re­concentrate the solution (state 3). The exhaust air leaves the desorber, passing through the blower, then pre-heats the entering air stream and is rejected to the environment. The solution-to-solution heat exchanger facilitates pre-heating of the weak solution leaving the dehumidifier (states 11 to 12) and recovers heat from the hot strong solution leaving the regenerator (states 9 to 10).

Figure 1: Schematic description of the liquid desiccant system

The above brief description of the system already reveals a number of advantages of this system over conventional absorption heat pump cycles: (1) The number of main components is reduced by one by transferring condensation of the refrigerant from a condenser to the environment. (2) Capital-intensive pressure-sealed units are avoided as the whole system operates at atmospheric pressure. (3) The amount of refrigerant (water) evaporated in the regenerator is independent of an evaporator, providing greater flexibility. (4) Efficient utilization of very low heat source temperatures is possible.

In the overall setup, the liquid desiccant system is connected in a flow arrangement allowing storage of concentrated solution and a capability to work in three different modes. The first is a manual mode used for testing individual components of the system. The other two modes are automatic, as may be selected by the user. One automatic mode is for full operation of the system (FOP) and the second is for regeneration only (REG). In the automatic FOP mode, all system components operate, including the solution storage circuit, if required. In this case, the absorber solution pump may supply the dehumidifier with solution from both the absorber pool and from the solution storage tank, in parallel. Thus, dehumidification can continue independent of regeneration. If the solar collectors cannot supply water at sufficiently high temperature, or if the concentration of the solution in the storage tank and/or the regenerator pool rises above a set limit, the regeneration side of the system will shut down for a certain time. In the REG mode, only the regenerator (desorber) side of the system operates. The system shuts down automatically when the concentration of the solution in the storage tank reaches a certain high value or when the temperature of the hot water drops below a certain limit. At the end of days of
high insolation, when a large amount of solar heat has been collected, the user can set the system to operate in the automatic REG mode before leaving the site.