Solar Hot Water
Solar hot water (or solar water heating) is a method of using solar energy to heat
water. Solar water heating systems are typically composed of solar panels, called
solar thermal collectors, which directly collect heat from the Sun and then use this
heat to warm water kept in a storage tank. This is a thermodynamic process in which
no electricity is directly generated, though electricity may be used to pump water
(or another liquid such as an antifreeze solution) from the collectors to the storage
cylinder. Within the solar thermal collectors is an absorber plate. It is this
coated metal plate (usually black) that absorbs the Sun’s radiation causing its temperature
to rise. Heat is then transmitted from the plate, through radiation and convection,
to a “heat-transfer” fluid. This fluid then in turn feeds the hot water system.
The solar panels used will typically be one of two types: either evacuated tubes,
or, more commonly, conventional flat-plate collectors. Flat-plate collectors consist
of a thin, flat, rectangular box with a transparent cover facing the Sun. This cover
allows light in but prevents most of the heat escaping (the greenhouse effect) whilst
also protecting the absorber from external influences such as the weather. Within
the box runs a small tube system to carry the heat-transfer fluid. These tubes are
attached to the absorber plate allowing the heat absorbed to be passed to the fluid.
In evacuated tube collectors, the absorber strip is located in an evacuated and
pressure proof glass tube. The heat is transmitted to the heat-transfer fluid which
flows through an inner tube system. Several tube units are interconnected making
up the entire solar panel. Evacuated tube collectors are more costly than flat-plate
collectors but do have advantages: they can achieve higher temperatures and higher
efficiencies; they can perform well in direct or diffuse sunlight, making them ideal
for a climate such as that of the UK; and, the circular shape of the tube means that
for most of the day the sunlight is perpendicular to the absorber (with a flat-plate
collector the sun is only perpendicular at noon).
The installation of evacuated tube solar panels. Very often solar panels are attached
to the roof of a building, but not always - they could also be set up on the ground
or (in the northern hemisphere) attached to a south facing wall allowing them to
receive direct sunlight.
