Absorption Refrigeration

Absorption Refrigeration

Absorption refrigeration is the least intuitive of the solar refrigeration alternatives. Unlike the PV and solar mechanical refrigeration options, the absorption refrigeration system is considered a “heatdriven” system that requires minimal mechanical power for the compression process. It replaces the energy-intensive compression in a vapor compression system with a heatactivated “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

6. Absorption cooling systems that use lithium bromide-water absorption-refrigerant working fluids can not be used at temperatures below 0°C (32°F).

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 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 is required to carry away the heat of absorption. The principle governing this phase of the operation is that a vapor is 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 minimum mechanical power needed to operate the pump 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. It is, in fact, possible to design an absorption system that does not require any mechanical power input relying instead on gravity. However, grid-connected systems usually rely on the use of a small pump.

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 fl at-plate collectors as their effi ciency is less sensitive to operating temperature.

The overall effi ciency of a solar refrigeration system is the product of the solar collection effi ciency and the coeffi cient of performance of the absorption system. The effi ciency of an evacuated tubular collector for different levels of solar radiation and energy delivery temperatures is given in Figure 5. The COP for a single-stage ammonia-water system depends on the evaporator and condenser temperatures. The COP for providing refrigeration at –10°C (14°F) with a 35°C (95°F) condensing temperature is approximately 0.50. Advanced absorption cycle confi gurations have been developed that could achieve higher COP values. The absorption cycle will operate with lower temperatures of thermal energy supplied from the solar collectors with little penalty to the COP, although the capacity will be significantly reduced.

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, 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 longterm 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. Of the three solar refrigeration concepts presented here, the photovoltaic system is most appropriate for small capacity portable systems located in areas not near conventional energy sources (electricity or gas). Absorption and solar mechanical systems are necessarily larger and bulkier and require extensive plumbing as well as electrical connections. In situations where the cost of thermal energy is high, absorption systems may be viable for larger stationary refrigeration systems.

The solar mechanical refrigeration systems would require tracking solar collectors to produce high temperatures at which the heat power cycle effi ciency becomes competitive. If the capital cost and effi ciency of tracking solar collectors can be signifi cantly reduced, this refrigeration system option could be effective in larger scale refrigeration applications.