07-12-2012, 05:54 PM
Solar refrigeration options – a state-of-the-art review
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abstract
A state-of-the-art review is presented of the different technologies that are available to deliver
refrigeration from solar energy. The review covers solar electric, solar thermal and
some new emerging technologies. The solar thermal systems include thermo-mechanical,
absorption, adsorption and desiccant solutions. A comparison is made between the different
solutions both from the point of view of energy efficiency and economic feasibility. Solar
electric and thermo-mechanical systems appear to be more expensive than thermal
sorption systems. Absorption and adsorption are comparable in terms of performance
but adsorption chillers are more expensive and bulkier than absorption chillers. The total
cost of a single-effect LiBr–water absorption system is estimated to be the lowest.
Introduction – solar refrigeration in
a warming globe
Since the beginning of the last century, average global temperature
has risen by about 0.6 K according to UN Intergovernmental
Panel on Climate Change (IPCC). It is also warned
that the temperature may further increase by 1.4–4.5 K until
2100 (Climate Change, 2001). Having realized the seriousness
of the situation, the world community decided to take initiatives
to stop the process. One of such efforts is the Kyoto Protocol,
a legally binding agreement under which industrialized
countries will reduce their collective emissions of greenhouse
gases by 5.2% compared to the year 1990. Especially regarding
the reduction of carbon dioxide, being an inevitable byproduct
Solar thermal refrigeration
Solar thermal systems use solar heat rather than solar electricity
to produce refrigeration effect.
Flat-plate solar collectors are the most common type,
which consists of a metallic absorber and an insulated casing
topped with glass plate(s). Evacuated collectors have less heat
loss and perform better at high temperatures. Evacuated collectors
are typically made in a glass tube design, i.e. ametallic
absorber inserted in an evacuated glass tube, to withstand the
pressure difference between the vacuum and the atmosphere.
Fig. 3 shows schematic diagrams of these two collectors.
Thermo-mechanical refrigeration
In a solar thermo-mechanical refrigeration system, a heat engine
converts solar heat to mechanical work, which in turn
drives a mechanical compressor of a vapour compression refrigeration
machine. A schematic diagram of such a cooling
system is shown in Fig. 4. In the figure, a solar collector receives
solar radiation Qs [the surface area As (m2) multiplied
by the solar radiation perpendicular to the surface Ip (kW/
m2), see Eq. (4)] from the sun and supplies Qg to a heat engine
at the temperature TH. The ratio of supply heat Qg to the radiation
Qs is defined as the thermal efficiency of a solar thermal
collector,