Tuesday, November 27, 2012

Solar Fraction - how are we doing?

27 Nov 2012: I am occasionally asked how we make the claim to be 'solar heated all year round' and perhaps that can be explained with the concept of 'solar fraction' - comparing what power we can capture from the sun with what we need to provide heating and hot water. Typically we are averaging 120% over the first 32 months of the Sunbox installation.
    There are some interpretations of Solar Fraction, but if you Google the expression, this seems to be the most frequent answer. See for example the 'Encyclopaedia of Alternative Energy and Sustainable Living'.
    As we live in an electrically heated house, it is possible to read the meters, deduct from the previous year's reading and then calculate the solar fraction precisely to a decimal point, because it is kilowatt hours against kilowatt hours.
   We do not have gas or biomass boilers or wood burning stoves to confuse the issue. Of course, we are getting thermal energy from the ground (which comes from long term solar originally). The whole point of using a Heat pump is to have a coefficient of performance that is better than 1:1 heating. It's classed as a renewable technology because it is drawing heat energy from a renewable source, and we aim for between 3:1 and hope to achieve better than 4:1 using the solar charging technology.

    In doing this calculation, I am simply looking at metered quantities: Electrical Energy we can generate annually divided by Electrical Energy we must use to heat the house and hot water.

Before the Sunboxes were installed, the Heat pump averaged 4,800-5,600 kWh/yr.
(This can vary by 800-1,000 simply depending on averaged weather over the year.)
The PV has averaged 3,300 kWh/yr in its first two years.
This would give a Solar Fraction range of 59-68%. (depending on weather)

Since the Sunbox was fitted, contributing its direct solar heat to reduce the GSHP consumption, the figures have improved significantly. 

The best ever solar fraction was in February of this year, when the sunny months and mild temperatures of 2011 had their good effect, by increasing PV capture and reducing heat loss in the house, thus reducing heating demand. At this moment in 2012, we are at a bad moment in time after a year of early winter, continuous rain through spring and summer with only the odd days here and there of sunshine. The only prolonged sunny periods were a bit in March, a bit in May and a short bit in August.
  • Since the Sunbox was fitted, the 'Best ever' solar fraction was:
    PV capture 3,450 kWh / entire Heating+hot water 2,625 kWh = a solar fraction of 130%
  • At the moment, we are struggling after the year of rain and the solar fraction is:
    PV capture 3,050 kWh / entire Heating+hot water 3,350 kWh, giving a solar fraction of 91%
  • Averaged over the 32 months from 11 April 2010 to 25 November 2012, the figures are PV 9,050 kWh, GSHP 7,567 kWh, giving a solar fraction of 119.6%.
We cannot enlarge the PV as this size is set to 4kW, as defined in the Feed in Tariff table. We can increase the solar capture of the Sunbox by adding in the Surya-4. The extra 2 square metres will contribute a fraction of better performance, but are not expected to make a huge difference overall - the purpose of installing them is to rehearse the installation difficulties and methodology of the roof mounted unit. 

1 comment:

  1. The solar savings fraction of a particular system is dependent on many factors such as the load, the collection and storage sizes, the operation, and the climate.


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