Experiment No.
4: To study solar collector and solar water heater
Solar collector (Liquid flat plate collector):
The collector is placed at a location in a position such that its length aligns with the
line of longitude and is suitably tilted towards south to have maximum collection.
Construction:
1. Transparent cover (one or two sheets) of glass or plastic.
2. Blackened absorber plate usually of copper, aluminium or steel
3. Tubes, channels or passages in thermal contact with the absorber plate- in some
designs the tubes form an integral part of absorber plate.
4. Weather tight, insulated container to enclose the above components.
Working:
Liquid, most commonly water is used as heat transport medium from the collector to
the next stage of the system.
Mixture of water and ethylene glycol (anti-freeze mixture) used if the ambient
temperature are likely to drop below 0°C during nights.
As solar radiation strike on a specially treated metallic absorber plate, it is absorbed
and raises the plates temperature.
Absorber plate is made up of metal sheet ranging in thickness 0.2 to 1mm.
Heat is transferred to the heat transfer liquid circulating in the tube (or channels)
beneath the absorber plate and in intimate contact with it.
Metallic tubes are 1 to 1.5cm in diameters and are soldered, brazed, welded or
pressure bonded to absorber plate with a pitch of 5 to 12cm.
The glass cover permits the entry of solar radiation as it is transparent for incoming
short wavelength but is largely opaque to the longer infrared radiation reflected from
the absorber.
As a result, the heat remains trapped in the airspace between the absorber plate and
glass cover in a similar manner to a green house.
Glass cover also prevents heat loss due to convection by keeping the air stagnant. It
may reflect some 15% of incoming solar radiation which can be reduced by applying
anti-reflective coating on the outer surface of the glass.
Solar water heater:
Construction & working:
A tilted flat-plate solar collector with water as a heat- transfer fluid is used.
A thermally insulated hot-water storage tank is mounted above the collector.
The heated water of the collector rises up to the hot water tank and replaces an
equal quantity of cold water, which enters the collector.
The cycle repeats, resulting in all the water of the hot water tank getting heated
up.
When hot water is taken out from a hot water outlet, the same is replaced by cold
water from a cold-water make up tank fixed above the hot water tank.
The scheme is known as active heating scheme, as water is circulated in the loop
naturally due to thermos-siphon action.
When the collector is fixed above the level of the hot-water tank, a pump is
required to induce circulation of water in the loop and the scheme will be known
as active (or forced) solar thermal.
An auxiliary electrical emersion heater may be used as a back-up for use during
cloudy periods.
In average Indian climatic conditions, a solar water heater can be used for about
300 days in a year.
A typical 100 litres per day (LPD) rooftop solar water heater costs approximately
Rs 18,000- 21,900 (year 2008) and delivers water at 60-80 °C.
It has a life span of 10-12 years and a payback period of 2-6 years.
Sometimes, anti-freeze solution may be used as the heat transport medium t avoid
freezing during cold nights.
