PV Geyser Element System Sizing and Savings

Energy cannot be created or destroyed but only converted from one form to another. So there is nothing special or magical about a PV geyser element. If we assume it is 100% efficient then it has a “COP” of 1. In other words, if it is a 1kW element it will consume 1kW of electricity to produce 1kW of thermal output. This power must come from a PV panel.

The PV panels are typically around 17% efficient (this is of course under lab conditions with the cell at 25°C which will seldom happen in real life). If we have 3 x 300W perfectly facing PV panels on the roof we should in theory be able to get a maximum of about 5 hours of 900W output (annual average – winter you will have less because of less sunlight hours and in summer you will have more. Again this is mismatched to when the most hot water is needed). We therefore have a theoretical maximum of 4.5kWh but with panel temperature derating (0.4%/°C with 20°C NOCT) you will typically get only 4.1kWh. Next you need a MPPT to ensure optimal power transfer from the panels to the element. A good MPPT is typically be around 95% efficient and therefore the power available to the element will be around 3.9kWh.

The thermal standing loss on a 200L Kwikot B-rated geyser is about 1.5kWh (in real life with piping connected it could easily be double). You therefore have a maximum of 3.9-1.5=2.4kWh left to heat the water with which for 200L will give you a temperature increase of 11°C (so if the incoming water in the tank was 15°C you now will have only 26°C). Of course if you only use 100L/day from the system you only need to heat 100L plus standing losses then your temperature increase will be about double and you end up with 37°C. The element controller will use Eskom to do the rest of the work and get the tank to 60°C (SANS 151 requires water to be stored at 60°C for Legionella disease prevention).

This 900W system has a physical footprint of 6 m2 (3 x 300W panels).

Flat Plate Solar Collector System Sizing and Saving

From the TUV Rheinland test reports of the ITS 2.5 m2 flat plate solar collector it can be seen that it will give a thermal output of around 9kWh for the same 5 hours of annual average usable sunlight per day as used with the PV example above.

Now the flat plate collector’s output also de-rate based on the temperature difference between the water in the collector and the ambient temperature. An average temperature delta of 17.5°C (heating water from 15°C to 60°C @20°C ambient) will give about a 10% reduction in output and therefore 8.1kWh. Piping thermal losses on the system for pipe runs of around 10m total both ways will further reduce the energy arriving at the tank to around 7.6kWh. Using the same 200L Kwikot B-rated geyser as in the above example you are left with 7.6-1.5 = 6.1kWh to heat the water with which for 200L will give you a temperature increase of around 27°C (so if the incoming water in the tank was 15°C you now will have 42°C).

Of course if you only use 100L for that day from the system you only need to heat 100L plus standing losses then your temperature increase will be about double and you will end up with 68°C. The solar controller will use Eskom to do the rest of the work when required.

Conclusion

It can therefore be seen that to get the same heating from a PV element system than an ITS 2.5 m2 flat plate solar collector you will need about 5.8 x 300W panels on the roof (1740W). This requires about 11 m2 of north facing unshaded roof space. Good quality flat plates have efficiencies of more than 70% compared to 17% on the PV panels. The roof space required by the flat plate is therefore only 2.5m2. Any shade or dirt on the PV has the potential to kill the whole output. With the flat plate a 10% shading, for example, will result in only a 10% loss in output power.

Flat Plate Solar Collector Advantages:

  1. Lower cost for the same thermal output.
  2. Much smaller footprint on the roof.
  3. Relatively insensitive to dirt and shading on the panels.

Flat Plate Solar Collector Disadvantages:

  1. On installations where the geyser is very far from the panel the piping thermal losses can seriously affect the system performance and the solar panel size must be increased to compensate.

PV Element Advantages:

  1. Installation is only electrical and do not involve plumbing.
  2. When the geyser is very far from the panels only cable losses need to be compensated for. This can be done by simply using lower resistance (thicker) cables.

PV Element Disadvantages:

  1. Higher cost of equivalent output system.
  2. Large physical footprint on the roof.
  3. Very sensitive to shading and dirt on the panels.
  4. High voltage DC from the panels is lethal and proper DC installation techniques and safety mechanism must be implemented.