Monday, August 16, 2010

Performance comparison of Solar Thermal

15 Aug 2010: I was looking for information about calculation of output from panels based on the SAP assessment system. This site,  is a useful one which calculates output from solar thermal panels for water heating for the UK generally.

 I am seriously delighted and amazed!
1. The Valentin/Sap calculator indicates that Flat Plate panels on the roof for water heating will collect 1,000-1,100 kWh in a year - (about the same for evacuated tubes or flat plate). I think this low figure arises from the assumption that in summer, the tank reaches temperature quickly, and unless someone has a shower or a bath, no more heat can be stored in the tank.

2. My Surya Sunbox panels are 4 sqm and put down an average of about 9.5 kWh a day, and will continue do so so through the winter because they work on Delta-T - they will work even in cold weather so long as the heat pump provides a temperature difference. They continue to put heat into a cold thermal mass, the deep ground, so they can never reach stasis. That is likely to total about 3,000 kWh in a year - I very much hope so.

Surya Sunbox panels are more efficient!
What this appears to mean is that:
•  over the year my Surya panels appear to collect THREE times as much energy as a professional Flat Plate panel.
As they can do this, and do not need an intermediate copper hot water tank, that makes them even more cost effective. Another benefit is that the Surya panels are better on the wall, as they prefer an airspace next to a wall with high thermal capacity - and they work better in winter when the heat is needed. It frees up all the roof for PV! It has always seemed obvious to me that a water tank is so small relative to the deep ground that any useful heat it had would be rapidly downloaded by the GSHP and no more would be coming fast enough to be useful - whereas the Sunboxes will work at low temperature. It is easier to find somewhere for one cubic metre externally than for a 300 litre tank of water within the house. Only one pump and thermostat are needed, whereas a tank would need to have two coils, with a pump and thermostat in each loop.

Could we manage with more panels?
I now wish I had space for 6 or 8 sqm of panel on my south wall. It would be great to have 12 panels, and put down 9,000 - equivalent to the entire heating requirement for the house. I do not think there is a linear relationship - delivery of heat to the ground depends on the delta-T, and if you warm up the ground enough it would no longer accept further heat unless it is a notably higher temperature. As the liquid moved through the panels, it would rapidly reach the optimum temperature - and for the remaining panels would not pick up much more. With my 4 panels working as two pairs, I guess that there isn't time or space for dead spots or flat spots to appear.
   No matter how many you put down, you still need the heat pump to get them back up, so adding more would not reduce the electricity to zero. However, the hope is that heating cycles will be faster and more efficient, and will reduce the workload enough to reduce the electric consumption more than in 2009-2010.
  Having 12 sqm would increase the risk of the heat escaping to the greater mass of surrounding soil, as one could build up the temperature enough for conductive heatloss to take that heat away altogether. My 4 sqm of panels put down 3,000, and the heat pump needs to get 9,000 up during the year. The Push-Pull ratio of 1 to 3 means that none of the 3,000 has a chance to escape, making it 100% efficient (apart from pump costs which are mostly PV powered).

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