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Solar Energy Storage Of all the energy sources
alternative to fossil fuel, the most promising are the various forms of solar
energy. Solar energy in most of its forms is inexpensive, renewable, and non-polluting.
The widespread adoption of solar energy has been inhibited
in part by economic considerations. The sunshine is free, of course, but its
capture involves some initial installation costs and some maintenance, and
there are vested interests in preserving the status
quo. Besides, the real cost of using fossil fuels is higher than appears in
the retail price, since the costs of resulting pollution are ignored. Beyond the economic barriers, there is a fundamental
drawback in an obvious characteristic of solar energy, the fact that it is
intermittent; the sun does not shine at night nor on
cloudy days. Our need, on the other hand, is continuous, and, if anything,
increases at night. So a critical aspect of solar
energy is the need for some form of storage. Active solar thermal energy consists of “greenhouse”
panels on the roof which heat circulating fluid which then provides heat for
domestic hot water and possibly (in warmer climates) interior heating.
Storage is provided by relatively large tanks of
water, similar to hot water boilers, which heat up during the day and may be
drawn on at night. For most applications, particularly in the temperate or
colder zones, solar energy is not used as the
exclusive source but is backed up by conventional gas or oil. In such a case,
the use of fossil fuel allows the storage tanks to be
scaled down to a more realistic size. Passive thermal solar energy depends on heat-retaining
aspects of the architectural design of the house without using any
circulating fluid. It may include large southerly-facing windows, thick
walls, and extensive insulation. Storage is provided
by the thick walls, which heat up in the day and release the heat at night.
There may also be additional storage in the form of rocks in the basement or
columns of water exposed to sunlight. Judicious use of insulating curtains to
keep the heat in at night is also common. One of the most promising forms of solar energy is the
photovoltaic cell, in which incident light produces electricity. The
resulting direct current is easily converted to
alternating current through an alternator. The electricity so produced can be
used not only for interior heat and also domestic
hot water, but also for the countless other applications requiring
electricity. Storage is often in the form of banks of storage batteries, both
bulky and relatively expensive. An alternative is to use a reversible
electric meter, whereby excess electricity is sent
back to the utility company and at night is drawn back from the grid. In
effect, the utility company is used as the
equivalent of storage batteries. Utility companies, of course, want to sell
electricity, not buy it, so they are not always agreeable to this
arrangement. In several states, however, legislation has been adopted which
requires the utilities to enter into such agreements. A related application is the hybrid automobile. Hybrids
run on gasoline when cruising, when gas mileage is most favorable, at the
same time charging a bank of storage batteries. During stop-and-go traffic,
and particularly when idling, the internal combustion engine shuts down and
the car runs on the stored electricity. An indirect form of solar is wind energy, since wind
results from the heating of the surface of the earth by absorbed solar
radiation and the resulting convection currents. Although there are other
configurations possible, particularly some which might be
useful for decentralized generation of electricity, the predominant
application today is the large propeller-like generator situated high
on a windy hill. Wind generators are most efficient in a steady wind, and the
velocity of wind tends to rise with altitude, so the
usual installation has generators linked together, each mounted on a tall
tower. As used by utility companies, they are connected
to the grid, so that storage is not an issue. For an installation on a single
dwelling, however, it would be necessary to have some form of storage.
Typically, there would be storage batteries or, if available, a reversible
meter connected to the power grid Another form of indirect solar is hydroelectric power.
It is considered solar because the hydrologic cycle
whereby evaporation from bodies of water returns as rain is powered by
sunlight. The rate of flow of water in a stream is fairly
steady from day to day, but it does vary seasonally, particularly in
the spring when the upstream snow melts. In some cases
a utility will use electricity during a high-flow period to pump water
upstream to a lake or pond and then release the water to flow downstream when
the natural flow decreases. Perhaps the most varied and promising form of indirect
solar energy is biomass, the form in which plant material formed through
photosynthesis is used for fuel in one form or another. Here the storage is
intrinsic to the biomass itself, the captured solar radiation having been used to build plant tissue. The most prevalent use of biomass today is the ethanol
used at about 10% in gasoline. Ethanol may be used
in internal combustion engines at concentrations ranging up to 100%, as is
common in some other countries. Another application is biomethane,
natural gas produced by anaerobic fermentation of waste material such as
sludge. The prospect of fermenting weeds, burning wood pellets, processing
food waste, and numerous other possibilities hold promise of simple, low-tech
ways to use indirect solar. The fossil fuels are running out. The various forms of
solar energy can have a large role to play in our transition, and the
problems of storage need not be an obstacle. Dom
Roberti |