Tuesday, September 28, 2010

Watts per Square Metre?

27 Sept '10: Someone on Navi tron forum said I was claiming more heat capture than the sun could provide. Errr?
 What I have been averaging from the sunboxes is about 1.1 kW when the pump is working - day or night, sun or cloud. Best days are averaging 1.7 kW during the day, and about 0.8 kW on a poor day. If conditions are worse, the thermostat doesn't let the pump run. There are four square metres of black collector facing south, vertically mounted. Averaging at 1.1 kW, that is 275 W/sqm. As my panels are vertical, there is a natural bias for them to collect less in summer (I don't want so much of that anyway...) and to collect more in winter (when it is more desirable).
  According to my solar radiation meter on the roof, the peak insolation at the best of the middle of the day in summer is 900 watts/sqm, on the very best day of the year, 17 June. The average for the days seems to be about 275-300 w/sqm, and we have to consider that the sunbox pump sometimes works for 5-12 hours a day. As I have 4 sqm, I am getting an average of 275 w/sqm, and of course this ought to be impossible, suggesting that on average, they collect everything that the sun is sending.
  However!  the panels are also getting much of their heat from the air temperature, indeed, on cloudy days, all of their heat is from the air (and it is helped by sky brightness). Their efficiency is enhanced by the fact that there is a powerful water pump driving chilled glycol through them and the ground loop when the heat pump is working, exploiting the delta-T whenever there is one. (when the heat pump is not running, the circulation is reduced to a humble 30W water pump).
   If I had convector fins on the back like you have on a steel radiator, the capture might be even better! The side panels of the sunboxes are transparent, so they pick up sunlight when it is at a slanting angle earlier than one that has opaque sides.
   When the sun does shine, their output boosts to well over 1.0 kW, and the recent addition of the mirrors has boosted their sunny performance even more, to nearly 2.0 kW.


  1. You said on the naviton forum this

    "as these panels, with 4 squ metres are lining up to bring in THREE times the energy of 4 sqm of industry standard panels "

    typical solar collectors are 60-80% efficient and you claim then three times more performance

    You should not wonder , why someone questions this

  2. Hi, these are not normal panels, as they have 1-4 tons of glycol pushed through them in a day, there is a good delta-T much of the time, and there is never stasis.

  3. Dear anon,
    I wont have egg on my face, as they have already taken over 2,000 kWh of solar energy in their first six months... so something has been achieved. something will be gained during the winter, as the sun does shine then, and they did well after being first installed in March, which was sunny but cold.

  4. If you read the excellent Navitron forum on the subject of heat transfer in solar systems, shovelling vast quantities of heat transfer fluid round the system is actually counter-productive - think trickle..........
    And just to clarify - you pay for electricity to run a heat pump, which captures heat which you then just put under the dwelling, and it sits there patiently waiting there for you to reuse it some months later?

  5. Anon Writes: "And just to clarify - you pay for electricity to run a heat pump, which captures heat which you then just put under the dwelling, and it sits there patiently waiting there for you to reuse it some months later?"

    Answer: No! Mostly, we run a 30W mini water pump that is in the loft, powered much of its lifetime by PV. When I mention the 'heat pump' driving the glycol through the panels, that is not the heating part of the pump, its the ground loop pump, that is simply an independent energy efficient water pump that normally pumps round the ground loop when the GSHP is running.

    Yes! that is burying solar heat 15 storeys down under the house, for use during the winter.


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