Heating Panel Sizes - No simple rule...
Coast vs inland, south versus north
South Africa has a wide variation of solar radiation, ranging from the lowest ( Durban area ) to the highest (Northern Cape and Limpopo).
This variation can be as much as 30%. As a result, there is no one standard across the country.
You can also use the Eskom calculator
Winter vs summer
In winter you can get as little as 33% of the radiation in summer. You should therefore have a backup system in place for winter conditions.
The table below is for high insolation areas with well insulated storage tanks. These are the minimum sizes recommended for the storage tank sizes. For lower insolation areas or higher hot water usage, we suggest you take the next size up.
Vacuum tubes vs flat plates
BOTH work - we prefer vacuum tubes, but willingly supply flat plates too
Advantages of evacuated tubes over flat plat panels
Advantages of Flat plates over vacuum tubes
These are the MINIMUM sizes for heating panels - both vacuum tube and flat plates - if in doubt, size up
| Length mm | Width mm | Height mm (off roof) | Weight kg | ||
| Panel name | Storage tank size (litres) | From top of manifold to bottom of frame | From end of copper pipe | From top of manifold to bottom of frame | |
| AKT 47-12 | 100 | 1705 | 1075 | 155 | 33 |
| AKT 47-18 | 150 | 1705 | 1460 | 155 | 43 |
| AKT 47-24 | 200 | 1705 | 1900 | 155 | 65 |
| AKT 58-18 | 200 | 2000 | 1630 | 160 | 60 |
| AKT 58-24 | 250 | 2000 | 2100 | 160 | 83 |
| AKH58-18 | 220 | 2000 | 1630 | 160 | 60 |
| AKH 58-24 | 300 | 2000 | 2100 | 160 | 83 |
| PG1.5 | 100 | 1500 | 1000 | 80 | 35 |
| PG2.0 | 150 | 2000 | 1000 | 80 | 42 |
Below are pictures for a AKT 47-12, AKT 47-18 and PG 2.0
Advantages of evacuated tubes over flat plat panels
Reason for Evacuated tubes being successful and taking
over from flat plate collectors:
The vacuum ensures that all round heat loss is minimal.
The glass is coated with a low emissivity material that loses very little heat.
Evacuated tubes heat pipes use a small amount of copper, coated glass and aluminium heat gathering fins. Flat plates use mainly copper which has risen in price over 60% recently. The amount of material used in evacuated tubes is less per m2 than for flat plates.
Evacuated tubes have a surface area that is directly facing the sun at all angles as the sun tracks across the sky. Flat plate collectors have a long period when the sun is tranmitting heat to a small area.
4.
No liquid loss in the case of failure
Evacuated tube heat pipes transfer the heat to a manifold. In the event of the failure of an individual tube, the system works less effectively, but there is no loss of liquid and the Solar water heater continues to work.
In the case of a failure of a flat plate collector, the
complete system may lose its liquid and the water heater will not work until
repairs have been made.
5.
More easily produced than flat plates
The process to produce
evacuated tubes and heat pipes is highly mechanized and automated which allows
for continuous process manufacture and reduces the cost of manufacture.
A single evacuated tube heat pipe be can be replaced on
site by an unskilled person within minutes. A flat plat collector needs to be
removed from its location, stripped, repaired and replaced.
7.
More easily customized by size and location than flat plates
Evacuated tube heat pipes can be arranged in an array of as many tubes as are calculated to be necessary to heat the water. This can be incremented by pairs and each heat pipe has an absorbing area of 0.08 m2 . In contrast flat plate collectors are sized in a minimum area of 1 m2 and cannot easily be made smaller.
How Evacuated
tube heat pipes work
1.Vacuum
tube
Evacuated
tubes are the absorber of the solar water heater. They absorb solar energy
converting it into heat for use in water heating. Each evacuated tube consists
of two glass tubes made from extremely strong borosilicate glass. The outer tube
is transparent allowing light rays to pass through with minimal reflection. The
inner tube is coated with a special selective coating (Al-N/Al) which features
excellent solar radiation absorption and minimal reflection properties. The top
of the two tubes are fused together and the air contained in the space between
the two layers of glass is pumped out while exposing the tube to high
temperatures. This "evacuation" of the gasses forms a vacuum, which is
an important factor in the performance of the evacuated tubes. The vacuum in the
evacuated glass tube is less than 5×10-3pa This can only be reached and
maintained over a long period of time through a specialised evacuation process
in production resulting in an almost total elimination of convection and
conduction losses from the tube.
2.Heat pipes
The
heat pipes used in solar collectors have a boiling point of only 25°C. So when
the heat pipe is heated above 25°C the water vaporizes. This vapour rapidly
rises to the top of the heat pipe transferring heat. As the heat is lost at the
condenser (top), the vapour condenses to form a liquid (water) and returns to
the bottom of the heat pipe to once again repeat the process. Each heat pipe is
tested for heat transfer performance and exposed to 250°C temperatures prior to
being approved for use. For this reason the copper heat pipes are relatively
soft. Heat pipes that are very stiff have not been exposed to such stringent
quality testing. Given this strict quality control and high copper purity, the
life expectancy of the heat pipe is even longer than that of the solar tube.
Advantages of Flat plates over vacuum tubes