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Solar Water Heating
By John Herbert | DIRECTOR
Capturing solar radiation from the sun is on of the
most cost effective methods to reduce your energy bills - solar
energy from the sun is free!
Sun powered your water heating needs, providing trouble
free operation for many years, whilst dramatically reduce
your energy bills.
Solar water heaters are not suitable for every location,
before considering a solar thermal system consult the
experts. Remember too that lower utility bills provides
indirect benefits - reducing pollution and your environmental
footprint.
Operation
The radiant energy from the sun harnessed can provide free hot water for your
home or business, significantly reducing your energy bills and helping the
environment too.
During the sunshine hours, cold water is circulated and heated within a solar
collector panel, typically installed on the roof.
Solar energy strikes the earth at a constant rate, approximately 1Kw per
square metre, this energy can be absorbed, captured, and transferred to heat
your hot water.
The graph (above) indicates the monthly average solar energy and accumulated
hours of sunshine recorded by the Hong Kong Observatory at Kings Park during
2003.
The graph indicates that the monthly average incident solar radiation exceeds
10 MJ / square metre, providing a year around energy source. As expected the
summer months show the annual peak.
Solar Collection
If the value solar energy is to be captured, effect collectors are required. There are two basic types of solar collectors
Most collectors have a standard warrantee from the manufacturer, for up to 15
years - providing free hot water for years and years.
Flat Plat Solar Collectors
The flat plate collectors comprise water channels or piping, bonded to a
large flat absorption plate, thermal insulation beneath and covered
with glass or polycarbonate.
The absorption plate and water channels exposed to the sun are finished with
a selective coating or paint to enhance solar radiation absorption properties.
Flat plate collectors are less expensive than evacuated tube and the design
being very robust. However, the absorbed energy is easily released,
re-transmitted due to high emissivity coefficient - significantly lower
efficiency than evacuated tube collectors.
Evacuated Tube
To overcome the deficiency of flat plate collectors, namely the energy losses,
alternate solutions were sought to prevent re-transmission of the captured
energy.
The evacuated tube collector evolved, using the same principal as a flask,
incorporating a vacuum to minimise energy losses.
Therefore, radiant energy from the sun still can be readily absorbed with a
vacuum preventing convective and conductive losses.
The tube is constructed from two glass concentric glass tubes, with a
vacuum space. Typically, borosilicate glass is used for toughness and
mitigating potential weather damage. Most collector designs allow
individual tube removal on site for maintenance inspection or replacement
of damaged tube.
With conduction and convection losses minimised, a selective coating is
applied internally to restrict radiation losses. Over the years, evacuated tubes
have been continuously developed increasing efficiencies and have been
manufactured for the mass market. Different manufacturer's offer variants citing
different selective surfaces, connection details. etc.
There are two basic types of evacuated tube - Wet and Dry.
Wet Type Evacuated Tube
The wet type evacuate tube type, as one might expect, water is circulated
inside the core. Where properly mounted, at an angle, thermo-syphon effect
circulates water. This tube type is most often used in conjunction with a
high level storage tank.
Preventative measures and maintenance are required to deal with, and prevent
accumulation of debris at the base of the tubes.
Modern wet type evacuated tubes are very efficient, with absorption
coefficient between 0.94-0.96 and low emissivity typically between 0.04-0.06.
Dry Type Evacuated Tube
A variant of the classic evacuated tube, uses basically same principle,
except an internal heat pipe is fitted to transfer the heat energy from the tube
area to a common water filled header. Some manufacturers offer models with heat
pipes fitted with extended heat transfer surfaces, others offer U shaped heat
pipe tubing.
Without water circulating inside the tube, the risk of water pressure damage
and debris accumulation at the base is averted.
Further information
Call Kelcroft today - (852) 2335 9830 and start saving money tomorrow.
Author
Mr John Herbert is the Managing Director at Kelcroft. He was educated and trained in the United Kingdom, he has managed and engineered solutions for more than twenty years, the last twelve years here in Asia. An international designer, designed solutions across three continents, whilst retaining a litigation free record.