Absorption

Absorption

Absorption refrigeration machines work with minimal use of electric energy. Instead of the electrically driven mechanical compressor, which works typically in compression refrigeration machines, a thermal compressor is used in the absorption machines (see figure below). Absorption refrigeration machines are driven by thermal energy, e.g. solar radiation or waste heats, at various temperatures typically in the range of 90-140°C. .

Most absorption refrigeration machines work with two popular working pairs: ammonia/water and water/lithium bromide. While ammonia/water systems are used for both cold production and air conditioning, water/lithium bromide systems are used mostly for air conditioning with their evaporator temperatures above 0°C due to freezing of the refrigerant, water.

In the absorber, the refrigerant is absorbed by the absorbent fluid while rejecting heat. In the generator, separation of the refrigerant and the absorbent fluid is effected by heat input as mentioned above. In the condenser, the refrigerant is liquified while heat is rejected and then supplied to evaporator through the throttle.

Nowadays multi-stage or multi-effect absorption refrigeration machines become of increasing interest, as they demonstrate high COPs ranging 1.1 to 1.5. A two-stage water/lithium bromide machine, for example, has two generators running on different pressure levels. In the 2nd generator, high-temperature steam from the 1st generator condenses to boil off extra refrigerant vapour from the absorbent fluid yielding a COP of 1.2. Single-stage machines show, however, COPs up to 0.8 at best. This means that a two-stage machine can provide 50% more cooling than a single-stage machine with the same heat input. The cost of a solar cooling system with two-stage absorption machine is, however, higher than of single-stage system due to its high driving temperature requirement and complexity.

Adsorption

Adsorption refrigeration machine works with refrigerant/adsorbent working pairs such as water/silica gel in a quasi-continuous process. Unlike its absorption counterpart, refrigerant is absorbed by a solid material.

In a water/silica gel machine, water is evaporated from an evaporator and then absorbed by silica gel. In the process, the cold water is cooled down to approximately 5-15°C.

Due to its low driving temperature requirement ranging 50-90°C, adsorption systems are ideal for use with solar collectors. Adsorption systems work with COP of approximately 0.65.

Open sorption

The driving temperature level of 60 to 90°C of an open-sorption air conditioning system is an ideal basis for the exploitation of the heat from solar collectors. The term “desiccative and evaporative cooling”, DEC in short, is common for these systems.

In DEC systems, dehumidification and cooling take place in separate steps. There are two forms of desiccants, solid and liquid. Here the former is described in detail.

Humidity of air is reduced through an adiabatic external dehumidification process by a desiccant such as silica gel or lithium chloride in a slowly rotating sorption wheel. At a regeneration temperature of 70°C, a dehumidification output of up to 6g of water per kilogram of dry air is typical. In order to ensure continuous operation, desorption, i.e. regeneration with warm air is carried out in counter flow. For this process, energy from solar systems or waste heat may be used.

After dehumidification wheel, intake air has a higher temperature than the return air from the space to be air-conditioned due to the latent heat of dehumidification. This hot intake air is cooled down by the return air in a rotating heat recovery wheel for “cold recovery”. Then the pre-cooled air is further cooled down to the desired temperature of 16 to 18°C by evaporative cooling and blown into the space to be air-conditioned. For the operation during winter months, it is possible to integrate an afterheater.

The figure shows a schematic diagram of a simple DEC system with solar air collectors. In the system with closed exhaust air circuit shown in the figure, the exhaust air is warmed up in solar collectors placed at downstream of the heat recovery wheel and then blown out into the environment through the dehumidification wheel.