Solar Energy Perspectives - IEA

Chapter 4: Buildings
Most widespread air-conditioning systems today are chillers, which work exactly as heat
pumps for space heating but function in reverse, moving heat from the interior to the outer
environment. A simple unique investment for both heating and cooling is a reversible heat
pump. Run by electricity and rejecting the heat in the outer air, most have not much to do
with solar energy, unless the electricity itself comes from the sun. Efficiency of cooling is
always lower than heating with heat pumps, as the mechanical energy of the heat pump is
not part of the desired result. Rejecting outdoor heat when it is quite warm further increases
the energy consumption (here again, ground-source reversible heat pumps could be
preferable). Rejecting the heat in a colder environment increases the SPF of the heap pump
during cooling. This heat, of solar origin, is not lost in this case, as when it is rejected in the
air; instead, a significant proportion can be recaptured during the winter. Finally, at times
where only mild cooling is needed, the heat pump itself can be bypassed; direct heat
exchanges between the fluid from the borehole and the fluid that cools the buildings can
further reduce electricity consumption.
Thermally driven heat pumps can also be run on solar heat. As often pointed out, the demand
for cooling matches the solar resource better than the demand for heating. Most current solar
cooling installations are based on absorption machines with closed cycles, with only a few
based on adsorption closed cycles. 1 Most are in Europe, especially small-scale systems in
Spain. A handful of large-scale open-cycle systems (based on desiccant materials) directly
produce cooled air, while solar energy regenerates the sorbent. One effective use of these
systems is to dry the air for environmental comfort in hot, damp climates. Running airconditioners
or chillers from the sun reduces the need for electricity to run a compressor, but
some electricity is still needed for pumps and fans. The electrical CoP usually reaches 8,
i.e. one kWh of electricity is used to produce 8 kWh of cold. The thermal CoP (kWh of cold
produced from 1 kWh of heat from the solar collector) is less than 1, except for double-effect
chillers run by concentrating solar collectors.
The economics of solar collectors is improved if they provide domestic hot water, space
cooling in summer, space heating in winter and, where possible, refrigeration services. There
are many large-scale examples in various countries; the world’s largest system is being built
in Singapore for a new campus of 2 500 students. However, solar thermally driven air
conditioning and cooling systems are still under development, in particular for individual
houses. The investment costs are five to ten times higher than standard air-conditioning
systems, and, despite electricity savings, the cost-effectiveness is low. Standard cooling
systems run from PV panels, perhaps with some cold storage, may be less costly.
Significant improvements seem needed in either compact thermal storage and/or solar thermally
driven cooling systems, to make large-scale solar thermal collectors cost-effective, by comparison
to renewable electricity-driven, reversible heat pumps (preferably GSHP). Combined with
smaller solar collectors they can cover a significant proportion of water heating loads, helping
to boost the performance of the heat pumps and stabilise long-term ground temperatures.
One emerging technology that could enhance the value of large-scale solar thermal systems
is the co-generation of electricity with lower temperature heat for space or water heating from
no- or low-concentrating collectors at temperatures of about 160°C.
1. Adsorption is the bonding of a gas or other material on the surface of a solid; in the absorption process a new compound is formed
from the absorbent and working fluids.
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© OECD/IEA, 2011