How does a
solar collector work?
A solar collector is basically a flat box and are composed
of three main parts, a transparent cover, tubes which carry a coolant and an
insulated back plate. The solar collector works on the green house effect
principle; solar radiation incident upon the transparent surface of the solar
collector is transmitted through though this surface. The inside of the solar
collector is usually evacuated, the energy contained within the solar collect
is basically trapped and thus heats the coolant contained within the tubes. The
tubes are usually made from copper, and the backplate is painted black to help
absorb solar radiation. The solar collector is usually insulated to avoid heat
losses.
The main components on an active solar water heating system
are
The two main types of solar water heating systems are the
closed loop system and the open loop system. The open loop system used water as
the coolant, the water circulates between the solar collector and the storage
tank.
There are two main types of open loop system these are the
draindown system and the recirculating system, the main principle behind both
systems is the activation of circulation from the collector to the storage tank
when the temperature within the solar collector reaches a certain value.
In the drain down system a valve is used to allow the solar
collector to fill with water when the collector reaches a certain temperature.
In the recirculating system water is pumped through the
collector when the temperature in the storage tank reaches a certain critical
value.
In applications where there is likely to be a temperature
drop below zero degrees then it is necessary to use a closed loop system. The
main difference between the open loop system is the water is replaced with a
coolant which will not freeze in the tempeture range which the solar collector
may be subject to. The coolant will usually be refrigerant, oil or distilled
water. Closed loop systems are generally more costly than their open loop
counter parts and great care must be taken to avoid contamination of the water
with refrigerant. The energy captured by the coolent is then transferred to the
hotwater via a heat exchanger. In a
drainback system the coolant may be distilled water. The system works on
the principle that there is only water in the collector when the pump is
operating. This has the benefit that the coolant used in the sytem will not
have the chance to cool down during the night when temperature may drop to a
level which may cause the coolant to increase in density and thus perhaps cause
is not be as free flowing as it should. The only necessary feature on the
drainback system is that the solar collector are elevated from the heat
exchanger or drainbak tank in order for the coolant to flow out of the
collector. This sytem again workson the principle that the water is circulted
between the collector and the drainback tank when the designated temperature is
reached between the solar collector and the hot water.
The system components in a space heating application are the same for water heating with the addition of radiators for space heating or under floor heating coils or even forced air systems.
A radiator system will generally work in a very simmialr
mannor to the hot water application, the main deffernece is the inclusion of a
boiler, heated water from the collector is passed through the heat exchanger or
drainback tank and is then passed to a boier with is used to supplement the
water hearing requirements before passing into the radiators to be used for
spaceheating.
Air distribution systems.
Again the air distribution system works in a manor very
similar to the hot water system, the main difference is the inclusion of a
blower and an air duct. The system uses an additional controlled which will
allow air flow over the coil when the temperature in the storage tank is high
enough that passing air over the coils in the return duct of the apparatus will
allow the system to make a positive contribution to the heating space heating
demand.
In large commercial or industrial applications system design
is slightly different from residential applications. It is worth noting that
the temperature rise across a collector is fairly constant to use an example if
the temperature of supply to the collector is around 60°F and the
temperature of return is around 73oC or the return is 173°F and the
supply is 160°C, this
basically means that high and low temperature applications should not be put in
series inside a loop. The low temperature application would basically drag down
the higher temperature application. Vacuum collectors are excellent performers
in high temperature applications the collector loop should be dedicated to the
higher temperature application until the load is satisfied. In applications
such as for hospitals, hotels or commercial office blocks is may be necessary
for the installation of two or more tanks connected in series.
1. storage tank 2. preheat tank 3. cold feed 4.mixing valve
5. supply and return to collector 6. hot water out
system operation: Hot water from the collector passes
through the coil in tank one (1),
Then, depending on it's temperature, it is diverted by a three way valve (4)
to either: the coil in tank (2) if it is above the set
temperature, (meaning tank (1) is hot) or the collector, if it
is below the set temperature of the mixing valve.
Commercial and industrial design considerations: The system
can be expanded to include more than one preheat tanks, the heat exchange coils
are linked by three way valves and the water which is to be heated runs in
series through the tanks in the opposite direction. The three way valve can
either be thermally controlled or electrically operated. No more than 100 tubes
should be plumbed in series. Care must be taken when designing the pipe work in
each section to ensure that each section receives equal flow.