Solar thermal energy
Practical Action
Solar thermal energy applications
Solar energy reaches the earth’s surface as short wave radiation, absorbed by the earth and
objects on the earth that heat up and re-radiated as long-wave radiation. Obtaining useful power
from solar energy is based on the principle of capturing the short wave radiation and preventing
it from radiating away into the atmosphere. For storage of this trapped heat, a liquid or solid
with a high thermal mass is used. In a water heating system this will be the fluid that runs
through the collector, whereas in a building the walls will act as the thermal mass. Pools or
lakes are sometimes used for seasonal storage of heat.
Glass will allow short wave radiation to pass through it but prevents long wave radiation heat
escaping.
If this energy is being used to heat water with a collector panel, then the tilt and orientation of
the panel is critical to the level of energy captured and hence the temperature of the water. The
collector surface should be orientated towards the sun as much as is possible. Most solar
water-heating collectors are fixed permanently to roofs of buildings and cannot be adjusted.
More sophisticated systems for power generation use tracking devices to follow the sun through
the sky during the day.
There are many methods available for aiding system design and for predicting the performance
of a system. The variability of the solar resource is such that any accurate prediction requires
complex analytical techniques. Simpler techniques are available for an approximate analysis.
Water heating
The most common use for solar
thermal technology is for domestic
water heating. Hundreds of
thousands of domestic hot water
systems are in use throughout the
world, especially in areas such as the
Mediterranean and Australia where
there is high solar insolation (the
total energy per unit area received
from the sun). Presently, domestic
water heaters are usually only found
amongst wealthier sections of the
community in developing countries.
Low temperature (below 100ºC)
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water heating is required in most countries of the world for both domestic and commercial use.
There are a wide variety of solar water heaters available. The simplest is a piece of black plastic
pipe, filled with water, and laid in the sun for the water to heat up. Simple solar water heaters
usually comprise a series of pipes that are painted black, sitting inside an insulated box fronted
with a glass panel, this is known as a solar collector. The fluid to be heated passes through the
collector and into a tank for storage. The fluid can be cycled through the tank several times to
raise the heat of the fluid to the required temperature. There are two common simple
configurations for such a system and they are outlined below.
The thermosyphon system makes use of the natural tendency of hot water to rise above
cold water. The tank in such a system is always placed above the top of the collector
and as water is heated in the collector it rises and is replaced by cold water from the
bottom of the tank. This cycle will continue until the temperature of the water in the
tank is equal to that of the panel. Where there is a main water supply fresh cold water
is fed into the system from the mains as hot water is drawn off for use. A one-way valve
is usually fitted in the system to prevent the reverse occurring at night when the
temperature drops.
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