Author : Aroune Ghadbane, Rome, Italy
Providing cooling from solar power is not as complicated as it sounds. The easiest and most
straightforward way to provide cooling power from solar energy is referred to as “solar
electric cooling” where the solar radiation is captured and converted to electricity through a
PV system feeding the compressor in a vapor compressor system. The vapor compressor
system is composed of a condenser, expansion valve, evaporator, and compressor. Work from
the PV system is used by the compressor to compress the refrigerant (Ammonia for instance)
and send it to the condenser where heat is released to the atmosphere and the refrigerant
changes its phase to a liquid state. At the expansion valve, the pressure is decreased and when
the refrigerant reaches the evaporator it absorbs latent heat and changes its phase again to
vapor, this system can also be used as a heat pump.
Thermoelectric cooling occurs as well through the Peltier effect, where when a DC voltage is
supplied across two different conductors of semiconductors, heat transfer occurs from one
junction to another. This method is only used in small portable applications.
Thermal energy produced by the sun (or any other low-temperature heat source) can be
exploited and transformed into useful cooling using thermally activated energy conversion
systems. This can be done through either a closed cycle, an open cycle, or a hybrid system
combining the two. Closed cycle systems use either liquid sorption (absorption cycle) or solid
sorption (Adsorption cycle). The Absorption cycle is the most common and it is quite similar
to the vapor compression cycle explained above, except that it doesn’t have a compressor.
Instead of the compressor, a generator-absorber system is used, and knowing that the
refrigerant is highly soluble in cold water, for instance, Ammonia and that when heated it
evaporates resulting in high-pressure ammonia gas, we can use the same vapor compression
cycle except that the vapor coming to the evaporator goes to the absorber where it is mixed
with water and is pumped to the generator where heat is supplied, the ammonia is then
evaporated at high pressure moving towards the condenser where it gives up its latent heat
and goes to the expansion valve as a liquid then to the evaporator where it absorbs heat and
so on. This cycle occurs within a thermally driven absorption chiller that can exploit
industrial waste heat, solar thermal power, or any other heat source. The adsorption cycle is
similar, except that the refrigerant adsorbs onto the surface of a solid instead of dissolving in
a liquid. About 88% of chillers commercialized are absorption chillers, due to their high
efficiency, low cost, and compact size.
Example of Application : North High School In Arizona, USA
The harsh climate of Arizona state, which often exceeds 40 C implies the need for cooling
systems to maintain thermal comfort. Desert Mountain High school in Arizona uses a 1,75
MWth solar cooling system established over an area of 4865 m2 (Roofs & Parkings). A
lithium bromide chiller is used. The system provides solar cooling to the school covering
100% of its needs during summertime and 30% of its annual demand.
Picture from : https://www.energy-innovation-austria.at/article/desert-mountain-high-schoolarizona-usa-2/?lang=en
Open cycle systems also called desiccant cooling systems refers to the use of solid or liquid
desiccant materials for the dehumidification of air. The desiccant system absorbs moisture
from an air stream and releases it into another using sorption and desorption, this
regeneration requires thermal energy. The Sorption process is the transfer of moisture from
the air to the desiccant material, In the regeneration process the moisture in the temperature
of the desiccant system is increased and it releases moisture into the air stream.