SOLAR WATER HEATING HOUSEHOLD /INDUSTRIAL USE.
Solar water heating, an example of solar thermal
technology, has become increasingly popular with Kenyan homeowners.
Using the sun to heat water is efficient, saves money, and
reduces pollution. There are over 65,000 solar water heaters
in use in Kenya today. In fact, Nairobi ranks number one in
Eastern Africa nation when it comes to using energy from the
sun to heat water.
HOW SOLAR WATER HEATING WORKS
Sunlight is absorbed by one or more solar collectors,
which transfer the absorbed solar heat to water circulated
through the collector(s). The heated water is then stored
for use throughout the day and night in a hot water storage
tank that is highly insulated for it not to loose heat..
THERE ARE TWO BASIC KINDS OF SOLAR WATER HEATERS:
Active (forced circulation) systems that use pumps to circulate
the water, and
passive (thermosiphon) systems that rely on natural convection
for water circulation.
Both types of systems include solar thermal collectors and
storage tanks. The collectors can range in size from 3 ft.
x 7 ft. up to 4 ft. x 12 ft. and are often used in groups.
The storage tanks are usually larger than conventional electric
water heaters and generally hold 80 or 120 gallons. The tanks
may be plumbed together to increase the storage capacity.
The heating efficiencies of either system are approximately
the same. In areas that experience freezing temperatures,
a heat exchanger system is often used but in tropical areas
like Kenya, they are not needed.
An active system (forced circulation) uses an electrical pump
to circulate water between the solar collectors and the storage
tank. The pump may be either a 120V AC pump or a DC pump.
An AC pump is plugged into regular house current and relies
on temperature sensors and a differential controller to turn
the pump on and off. The sensors indicate whether the water
in the collector is sufficiently hotter than the water at
the bottom of the storage tank. If it is, the differential
controller turns the pump on. Water will then circulate through
the system with the hot water in the collectors flowing to
the storage tank and being replaced by colder water; then
the pump will automatically turn off while the water in the
collector heats again. The pump will cycle on and off repeatedly
during the day, depending on the amount of heat generated
by the sun.
A DC pump gets its power from a small photovoltaic
(PV) panel usually mounted on the roof next to the solar collectors.
At the same time electricity is being generated to power the
pump, sunlight is shining on the solar collectors heating
water. When the sunlight is bright enough to generate electricity
and power the pump, it is usually also hot enough to heat
water. This synchronous use of the sunlight ensures that when
the water in the collector is hot, electricity is generated
to pump the hot water to the storage tank. The more sunlight
received, the faster the water heats and the faster the pump
works. A passive system.
A passive (thermosiphon) system relies on natural convection
to circulate the hot water. Hot water naturally rises so no
mechanical pump (AC or DC) or access to electricity for a
pump is needed. It is essential, however, that in a passive
system the storage tank be higher than the collectors. As
long as the sun shines, water in the collector will heat and
move slowly upward into the tank with the colder water descending
to replace it. As the storage tank is usually mounted horizontally
above the collectors on the roof, the added weight is a consideration.
SOLAR WATER HEATING POOLS
Not only is Kenya an ideal location for solar water heating
for household use, it is also an ideal location for solar
pool heating. Solar pool heaters generally do not have to
heat the water to the temperature used in most households
and the type of solar collector which is used for pool heating
is usually different than the glass covered collectors used
for domestic water heating.
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