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The Greening of IT
278 How Companies Can Make a Difference for the Environment

expanding air flows toward the central tower, where a turbine converts
the air flow into electricity. A 50 kW prototype was constructed in
Ciudad Real, Spain, and operated for eight years before decommission-
ing in 1989.
Space solar power systems would use a large solar array in geosynchro-
nous orbit to collect sunlight and beam this energy in the form of
microwave radiation to receivers (rectennas) on Earth for distribution.
This concept was first proposed by Dr. Peter Glaser in 1968, and since
then a wide variety of systems have been studied, with both photovoltaic
and concentrating solar thermal technologies being proposed. Although
still in the concept stage, these systems offer the possibility of delivering
power approximately 96 percent of the time.

Energy Storage Methods
Solar Two’s thermal storage system generated electricity during
cloudy weather and at night. Storage is an important issue in the devel-
opment of solar energy because modern energy systems usually assume
continuous availability of energy. Solar energy is not available at night,
ptg
and the performance of solar power systems is affected by unpredictable
weather patterns; therefore, storage media or backup power systems
must be used.
Thermal mass systems can store solar energy in the form of heat at
domestically useful temperatures for daily or seasonal durations.
Thermal storage systems generally use readily available materials with
high specific heat capacities such as water, earth, and stone. Well-
designed systems can lower peak demand, shift time-of-use to off-peak
hours, and reduce overall heating and cooling requirements.
Phase change materials such as paraffin wax and Glauber’s salt are
another thermal storage media. These materials are inexpensive, readily
available, and can deliver domestically useful temperatures (approxi-
mately 64°C). The Dover House (in Dover, Massachusetts) was the first
to use a Glauber’s salt heating system, in 1948.
Solar energy can be stored at high temperatures using molten salts.
Salts are an effective storage medium because they are low-cost, have a
high specific heat capacity, and can deliver heat at temperatures compat-
ible with conventional power systems. The Solar Two used this method
3
of energy storage, allowing it to store 1.44 TJ in its 68 m storage tank
with an annual storage efficiency of about 99 percent.