The absorption refrigeration system is considered a heat driven system that requires minimal to no mechanical power for the compression process. It replaces the energy-intensive compression in a vapor compression system with a heat — activated thermal compression system. A schematic of a single-stage absorption system using ammonia as the refrigerant and ammonia-water as the absorbent is shown in Figure 1. The condenser, throttle and evaporator operate in the exactly the same manner as for the vapor compression system. In place of the compressor, however, the absorption system uses a series of three heat exchangers (absorber, regenerating intermediate heat exchanger and a generator) and may require a small solution pump. Ammonia vapor exiting the evaporator (State 6) is absorbed in a liquid solution of water-ammonia in the absorber. The absorption of ammonia vapor into the water-ammonia solution is analogous to a condensation process. The process is exothermic and so cooling water, or a liquid-to-air exchanger, is required to carry away the heat of absorption. The principle governing this phase of the operation is that a vapor is

2

more readily absorbed into a liquid solution as the temperature of the liquid solution is reduced. The ammonia-rich liquid solution leaving the absorber (State 7) is pumped to a higher pressure, passed through a heat exchanger and delivered to the generator (State 1). The mechanical power needed to operate the pump, if used, is given by Equation 1, the same equation that applies to the minimum power needed by a compressor. However, the power requirement for the pump is much smaller than that for the compressor since v, the specific volume of the liquid solution, is much smaller than the specific volume of a refrigerant vapor. Absorptions systems that do not require the use of a mechanical pump, such as the system used in the present study, rely instead on gravity. In the generator, the liquid solution is heated, which promotes desorption of the refrigerant (ammonia) from the solution. Unfortunately, some water also is desorbed with the ammonia, and it must be separated from the ammonia using the rectifier. Without the use of a rectifier, water exits at State 2 with the ammonia and travels to the evaporator, where it increases the temperature at which refrigeration can be provided. This solution temperature needed to drive the desorption process with ammonia-water is in the range between 120°C to 130°C (248°F to 266°F). Temperatures in this range can be obtained using low cost non-tracking solar collectors. At these temperatures, evacuated tubular collectors may be more suitable than flat-plate collectors as their efficiency is less sensitive to operating temperature. p2

W comp, min = m I vdP Equation 1

p;

A number of barriers have prevented more widespread use of solar refrigeration systems. First, solar refrigeration systems necessarily are more complicated, costly, and bulky than conventional vapor compression systems because of the necessity to locally generate the power needed to operate the refrigeration cycle. Second, solar refrigeration systems operate at a much lower COP, 0.5 versus 3 for more conventional compressor based refrigeration systems.5 Although this not a critical flaw as the fuel for solar systems is free solar energy. Third, the ability of a solar refrigeration system to function is driven by the availability of solar radiation. Because this energy resource is variable, some form of redundancy or energy storage (electrical or thermal) is required for most applications, which further adds to the system size and cost. The advantage of solar refrigeration systems is that they displace some or all of the conventional fuel use. The operating costs of a solar refrigeration system should be lower than that of conventional systems, but at current and projected fuel costs, this operating cost savings would not likely compensate for their additional capital costs, even in a long term life-cycle analysis. The major advantage of solar refrigeration is that it can be designed to operate independent of a utility grid. Applications exist in which this capability is essential, such as storing medicines in remote areas.