Environmental Engineering Reference
In-Depth Information
water and space heating) that are simple and cheap. Examples include
storage in building materials, water, crushed rock and phase-change materi-
als. The latter are materials that melt and freeze at a particular temperature
(e.g. 31°C).
Another method of solar energy storage is via chemical reactions driven
by concentrated solar heat or light (solar thermochemistry). One example
that is being worked on at the Australian National University is ammonia
(Lovegrove, Luzzi & Kretz 1999). Ammonia can be disassociated into hydro-
gen and nitrogen at the focus of a dish solar concentrator system. These
gases can be pumped long distances in natural gas pipelines and recombined
when convenient to form ammonia and to yield high temperature steam
that is suitable for industrial use or electricity generation.
Storage of renewable energy electricity is not a serious issue until the
fraction of solar electricity reaches 10-20 per cent of the total. This will not
happen (except in a few places) for many years, giving time for improved
storage technologies to be developed.
The use of a variety of solar conversion technologies minimises storage
requirements. For example, the probability that neither wind energy nor
photovoltaic energy will be available at a particular time is lower than the
probability that either is not available. Wide geographical dispersal of solar
energy collectors, the use of a variety of different solar technologies, the use
of energy storage and the judicious use of relatively small quantities of
natural gas and bio fuels minimises the probability of a shortfall in electric-
ity supply.
Energy 'payback' time
It is important that the amount of energy produced by a solar energy collector
over its lifetime be much larger than the amount of energy used in its manu-
facture. Some people have erroneously claimed that more energy is used to
manufacture solar collectors than is generated by the collector in its lifetime.
Fortunately, the time required to recover the energy investment in solar energy
equipment is typically one-tenth of the lifetime of the equipment. One excep-
tion is photovoltaics, which currently has an energy payback time of about five
years compared with an expected system life of 20 to 30 years. However, the
energy payback time and cost of photovoltaic systems are closely linked, and
both are falling. By the end of the decade the energy payback time of photo-
voltaic systems is expected to be less than two years.
