Category Archives: EuroSun2008-7

Solar cooling systems

1.2. Technologies

The following cooling technologies are feasible to be combined with solar thermal collectors in order to save energy:

• Evaporative cooling: by pre-treating the inlet air stream to improve its thermodynamic conditions

• Absorption: by providing thermal energy to the generator to separate the refrigerant from the absorbent

• Adsorption: similar to the absorption process, external heat helps to separate the refrigerant from the adsorbent

Evaporative cooling systems have high values of COP, which are optimal if the entering air has a temperature higher than 35°C with a relative humidity lower than 20%. Changes in these conditions entail a quick decrease of COP. Unfortunately, these optimum conditions are not registered in a zone of Spain wide enough to generalize them, being the reason of not having considered this technology as an alternative in the analysis.

Absorption process is based on the quality of some liquid substances of being able to absorb particles of other fluids into its molecular structure under certain conditions of pressure and temperature. These substances are classified in pairs corresponding to their affinity to absorb/being absorbed, being most common pairs water — lithium bromide and ammonia — water, absorbed and absorbent respectively.

The thermodynamic cycle is similar to a traditional vapour compression cycle. It has four main elements: absorber, generator, condenser and evaporator; and has four stages: the refrigerant (absorbed) enters into the absorber and it is circulated by an electric pump into a transport solution (absorbent) to the generator at a high pressure. Inside the generator, the solution receives heat from a thermal energy source, causing a desorption of the refrigerant from the transport solution. The high pressured refrigerant is conducted to the condenser as saturated vapour, while the transport solution goes back to the absorber through a pressure reducer valve. The refrigerant changes its state from vapour to liquid in the condenser, passing through an expansion valve to the evaporator as low — pressured liquid. Within the evaporator, the refrigerant is converted into vapour again by extracting heat from a secondary flow stream, obtaining the final cooling effect.

The highest energy demand takes place in the generator, where heat is needed to run the desorption process. The key of this technology is that the thermal energy can be freely supplied by a thermal solar field, resulting in an energy cost saving of almost 100%.

Adsorption process is similar to the absorption one, with the exception that the adsorption process uses a solid instead of a fluid to adsorb the refrigerant. As occurs in the absorption technology, the main energy consumption comes from a thermal energy source (i. e. solar collector field), using electric energy only for the recirculation pumps.

T. Soutsos et al. [1], in a comparative analysis between adsorption and absorption systems in Greece (similar latitude than southern Spain), showed that the economic results of absorption are clearly more favourable than those corresponding to adsorption.

For the purposes of this report, only absorption technology has been considered because of the fact that (i) the range of small powered absorption machines is wider than the adsorption one, (ii) the absorption market is more developed and (iii) the results of Soutsos on the payback period and the NPV for both technologies.