Solar Water Heater
heating is one of the most common applications of solar energy
for domestic and industrial applications. Like solar dryers the
water-heating systems are also available in natural convection
and forced convection designs. Natural convection water heating
system is also known as thermo- siphon water heating system that
consists of a flat plate solar collector, insulated water storage
tank and necessary insulated pipe fittings. Solar water heaters
collect sun energy. They usually have rooftop solar collectors
with arrays of piping and net of metal sheets.
Solar water heaters
collect the sun’s energy
usually with rooftop solar collectors with arrays of piping and net
metal sheets, painted
black to absorb as much radiation as possible, and a hot water storage
tank to enable hot water use at night. Usually the collectors are encased
in glass or plastic and angled towards south to catch maximum sunshine.
They are painted black to absorb as much radiation as possible. Usually
the collectors are encased in glass or plastic and angled towards south
to absorb maximum sunshine. The collectors act as miniature greenhouses
trapping heat under their glass plates. In areas where air freezes the
solar collectors must be filled with anti-freeze or emptied when frost
risk occurs. In warmer areas, the water for use can be circulated directly
through the collector. If the storage tank is above the collector, the
hot water can circulate from the collector to the storage having natural
convection’ no pump is required.
The size of solar
collectors needed vary greatly with location and the acceptance of
less hot water some days.
In a location
with a minimum
insolation of 4 kWh/day in monthly average, such as most of South Asia,
choose 3 m2 pr. 100 ltr of daily hot water use (tjek). That will give
sufficient hot water except for a few days a year. The hot water tank
should be large enough to store hot water from day till night, often
is used the same size as the daily hot water use; but if most hoit water
is used during the day, it can be smaller. The tank must be insulated
well enough to keep the temperature during the night, with for instance
maximal 5’C temperature loss.
Efficiency of solar water heaters to convert sun energy into heat energy
is 25-50%. The efficiency actually depends on the system and how much
the water temperature is higher than the surrounding temperature. Daily
solar energy inflow varies between 4 and 8 kWh/m2 as monthly average
in typical South Asian locations. It gives heat production of 1-4 kWh/m2
per day depending on type, usage, etc.
The system need to be installed by people with plumbing skills. A short
training (a few days) on special requirements for solar water heaters
is an advantage, in particular to avoid design flaws where the hot water
is cooled by unwanted circulation or un-insulated pipes or tank. The
pipes and system must be good enough to withstand corrosion of the water
used, and the pipe-combination that can cause corrosion must be avoided
(such as iron and cupper in the self-circulating system). The collectors
must be placed in free sunshine, faced south or horizontally. The inlets
and outlets of the tank should be placed so cold water is not mixed with
hot water outlet. Cold water should be let into the tank in the bottom,
preferably in a horizontal inlet.
A regular inspection by skilled people is recommended, for instance every
2. year. In systems with back-up heating, such as electric water heating,
it is recommended to turn off the back-up heating from time to time to
test the output of the solar heating. Lifetime solar hot waters are often
20 years if well maintained and if corrosion problems are avoided.
Solar hot water is used for washing, showers and cleaning, as well as
for industrial appliances. An important application is in health facilities.
In off-grid installations a ‘high hot water tank’ can be
used as storage tank, filled with hot water once a day in the morning.
High-temperature collectors with mirrors to concentrate the sunlight
(parabolic dish or trough shaped) can produce steam that can be used
in industries or in special solar power plants.
Life of solar water heaters is often 15-20 years if well maintained
and if corrosion problems are avoided. Estimated cost is approximately
Rs. 18000 for a 100 litres capacity SWH. For higher capacity it ranges
from Rs. 110 to Rs. 150 per installed litre.
Residential Buildings – Domestic Systems: The domestic solar hot
water systems substitute or supplement electrical geysers in bathrooms
and kitchens used for bathing and washing purposes.
Institutional/Commercial SHW Systems: Commercial uses of solar hot water
systems are in hotels, hostels, hospitals, holiday resorts and pilgrim
centers. Most of the institutional systems are large-scale hot water
plants ranging from 1,000 to 10,000 liters storage capacity. The systems
are ‘forced circulation’ ones fitted with electrical pumps
to aid the circulation.
Industrial Systems: Boilers supply low-pressure steam ranging from 110°C
to 150°C. Source of energy in the conventional boilers is furnace
oil or coal. As energy conservation measure, solar hot water systems
can be used for pre-heating of boiler feed water to meet the energy requirement
partly. Flat plate collectors can provide hot water at about 80°C
on sunny days at locations having favorable climatic conditions.