Hot water heated by the sun is used in many ways. In order to heat water using solar energy, a collector, often fastened to a roof or a wall facing the sun, heats working fluid that is either pumped (active system) or driven by natural convection
(passive system) through it. The collector could be made of a simple
glass topped insulated box with a flat solar absorber made of sheet
metal attached to copper pipes and painted black, or a set of metal
tubes surrounded by an evacuated (near vacuum) glass cylinder. In
industrial cases a parabolic mirror can concentrate sunlight on the
tube. Heat is stored in a hot water storage tank. The volume of this
tank needs to be larger with solar heating systems in order to allow for
bad weather, and because the optimum final temperature for the solar
collector is lower than a typical immersion or combustion heater. The
heat transfer fluid (HTF) for the absorber may be the hot water from the
tank, but more commonly (at least in active systems) is a separate loop
of fluid containing anti-freeze and a corrosion inhibitor which delivers heat to the tank through a heat exchanger
(commonly a coil of copper tubing within the tank). Another
lower-maintenance concept is the 'drain-back': no anti-freeze is
required; instead all the piping is sloped to cause water to drain back
to the tank. The tank is not pressurized and is open to atmospheric
pressure. As soon as the pump shuts off, flow reverses and the pipes are
empty before freezing could occur.
Residential solar thermal installations fall into two groups: passive
(sometimes called "compact") and active (sometimes called "pumped")
systems. Both typically include an auxiliary energy source (electric
heating element or connection to a gas or fuel oil central heating
system) that is activated when the water in the tank falls below a
minimum temperature setting such as 55°C. Hence, hot water is always
available. The combination of solar water heating and using the back-up
heat from a wood stove chimney to heat water[2]
can enable a hot water system to work all year round in cooler
climates, without the supplemental heat requirement of a solar water
heating system being met with fossil fuels or electricity.
When a solar water heating and hot-water central heating system are
used in conjunction, solar heat will either be concentrated in a
pre-heating tank that feeds into the tank heated by the central heating,
or the solar heat exchanger will replace the lower heating element and
the upper element will remain in place to provide for any heating that
solar cannot provide. However, the primary need for central heating is
at night and in winter when solar gain is lower. Therefore, solar water
heating for washing and bathing is often a better application than
central heating because supply and demand are better matched. In many
climates, a solar hot water system can provide up to 85% of domestic hot
water energy. This can include domestic non-electric concentrating solar thermal systems. In many northern European countries, combined hot water and space heating systems (solar combisystems) are used to provide 15 to 25% of home heating energy.
