How to choose your Solar Heating Panel - There is no simple rule... it is based on where you live, how much hot water you require and your water quality

Coast vs inland, south versus north, east versus west, evacuated tubes versus flat plates

 Firstly it is important to realise that ALL TYPES OF SOLAR WATER HEATING WORK.

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.

 

Radiation map

 

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 58-18	150	2000	1630	160	60
AKT 58-24	200	2000	2100	160	83
AKH 58-18	150	2000	1630	160	60
AKH 58-24	200	2000	2100	160	83
PG1.5	100	1500	1000	80	35
PG2.0	150	2000	1000	80	42

Below are pictures for a collectors with 12 and 18 tubes and a 2.0m2 flat plate

Advantages of evacuated tubes over flat plat panels

 

Reason for Evacuated tubes being successful and taking over from flat plate collectors:

 

1. Evacuated tubes have better insulation than flat plate collectors
2. They often use cheaper materials that flat plate collectors 
3. They have a better shape for solar absorption than flat plates
4. The water does not run through the glass tubes, so in the case of failure, no water is lost
5. They are more easily produced than flat plates
6. They are more easily replaced than flat plates-generally only one tube will need replacing
7. More easily customized by size and location than flat plates

1.      Better insulation that flat plates

 

The vacuum ensures that all round heat loss is minimal. The glass is coated with a low emissivity material that loses very little heat.

 

2.      Cheaper materials that flat plates

 

Evacuated tubes heat pipes use a small amount of copper, coated glass and aluminium heat gathering fins. Flat plates use mainly copper which is more expensive. The amount of material used in evacuated tubes is less per m² than for flat plates.

 

3.      Better shape for solar absorption than 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.

1. More easily replaced than flat plates

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

1. Some types are made in South Africa
2. They have worked well here for many years and have a proven track record
3. In summer they absorb heat from the ambient air to add to the radiation from the sun