Solar A/C and Trigeneration
can be used in the dryer parts of the country by means of evaporating systems driven by solar thermal energy or waste heat from generator sets. Each installation has to be planned to the specific needs.
A good overview is given in the article from solarmagazin below.
Cooling with Solar Heat: Growing Interest in Solar Air Conditioning
Sunny summer days are beautiful, yet in the office a hot day can be altogether stressful.
Because productivity can suffer under such conditions, more and more buildings are being
fitted with air-conditioning systems. This is where solar air conditioning comes in: The
summer sun, which heats up offices, also delivers the energy to cool them. The thermal use
of solar energy offers itself: Days that have the greatest need for cooling are also the very
same days that offer the maximum possible solar energy gain.
The demand for air conditioning in offices, hotels, laboratories or public buildings such as
museums is considerable. This is true not only in southern Europe, but also in Germany and
middle Europe. Under adequate conditions, solar and solar-assisted air conditioning systems
can be reasonable alternatives to conventional air conditioning systems. Such systems have
advantages over those that use problematic coolants (CFCs), not to mention the incidental
CO2 emissions that are taking on increasingly critical values.
What is Solar Air Conditioning?
Should buildings be cooled with the help of solar energy, then water-assisted air
conditioning systems or ventilation systems can be powered with heat that is made available
by solar collectors. No long-term intermediate storage is necessary in months of high solar
energy gain or in southern lands. The sun can, at least seasonally at our latitudes, provide a
substantial part of the energy needed for air conditioning.
How does Solar Air Conditioning Work?
The basic principle behind (solar-) thermal driven cooling is the thermo-chemical process of
sorption: a liquid or gaseous substance is either attached to a solid, porous material
(adsorption) or is taken in by a liquid or solid material (absorption).
The sorbent (i.e. silica gel, a substance with a large inner surface area) is provided with heat
(i.e. from a solar heater) and is dehumidified. After this "drying", or desorption, the process
can be repeated in the opposite direction. When providing water vapor or steam, it is stored
in the porous storage medium (adsorption) and simultaneously heat is released.
Processes are differentiated between closed refrigerant circulation systems (for producing
cold water) and open systems according to the way in which the process is carried out: that
is, whether or not the refrigerant comes into contact with the atmosphere. The latter is used
for dehumidification and evaporative cooling. Both processes can further be classified
according to either liquid or solid sorbents. In addition to the available refrigerating capacity,
the relationship between drive heat and realized cold energy (coefficient of performance;
COP) is also an essential performance figure of such systems (see Table 1 at end of article).
Absorption Refrigeration Machines
Closed absorption refrigeration machines with liquid sorbent (water-lithium
bromide) are most often operated in combination with heat and power generation
(cogeneration) (i.e. with block unit heating power plants, district heating), but can also be
assisted by vacuum tube solar collectors (operating temperature above 80 °C). With
single-step process the COP is 0.6-0.75, or up to 1.2 for a two-step process. A market
overview is available from the Consortium for Economical and Environmentally Friendly
Energy Use (Arbeitsgemeinschaft für sparsamen und umweltfreundlichen Energieverbrauch
(ASUE)).
Adsorption Refrigeration Machines
Closed processes with solid sorbents work with so-called adsorption refrigeration machines
(operating temperatures 60° - 95°; COP = 0.3 - 0.7). Solar energy can easily be used in the
form of vacuum tube or flat plate collectors. A pilot system used for a laboratory's climate
control at the University Clinic of Freiburg is fitted with tube collectors; the Fraunhofer ISE
also took part in its scientific conception. The refrigerating machine is composed of two
adsorbers, one an evaporator and the other a condenser. An adsorber chamber takes up
the water vapor, which is transformed into the gas phase under low pressure and low
temperatures (about 9°C) within the evaporator. Granulated silicate gel, well known as an
environmentally friendly drying agent, then accumulates it (adsorbs the water vapor). In the
other sorption chamber the water vapor is set free again (the chamber is regenerated or
"charged") by the hot water from the solar collector (about 85°C). The pressure increases
and at the temperature of the surroundings (30°C) the water vapor can be transformed once
again into a fluid within a cooling tower (condensed). Through a butterfly valve the water is
led back into the evaporator and the cycle begins from the beginning. Both the condensed
water (low temperature) and the sorption heat (high temperature) are discharged.
Main components of the system at the University Clinic of Freiburg: Adsorption refrigeration machine (left) and solar thermal system (right).