Tuesday, October 26, 2010

Sunny cold days in October - Sunbox performance

25 Oct'10: This has been an unusually cold October. Last year, evening temperatures did not go into single figures until November, whereas we have had weather in the last two weeks that has been definitely wintry. October day temperatures should be similar to May, but haven't been! The length of days is now less than 10 hours, compared with the 18 hours of midsummer. October gets 1/3 the insolation of June, but when it is cold, the PV does well. We have had several days in the last week with over 10-11 kWh of PV, but temperatures at freezing for much of the 24 hours with only a brief daily rise, not even touching 10ยบ C.
  The Sunboxes have performed incredibly well, with days of 17, 19, 20 and 21 kWh, it has been a week of all-time high scores, hugely outperforming the best thermal harvests of mid June. On a day without sun, the harvest is never above 6kWh at this time of year - with a bit of sun, the harvest is never below 10kWh. This change can only be due to the mirrors. As there can no longer be doubt about it, this harvest encourages me to speed up installing of side mirrors. I was expecting marginal improvements, and the people on Navitron forum said that it wasn't worth the cost of the metal - how wrong that was.
  There is a tree to the southwest that is never a problem in summer, but cuts off the sun in the afternoon at this winter season. What with Sharphill to the south east making the sunrise later, the tree shortens the day even more than normal seasonal variation. I might have to make some long-reach clippers. These two day-shorteners makes our enhanced thermal harvest even more surprising.
  Once the side mirrors are up, I shall have run out of any more ways to boost these sunboxes. If only I could think of a way of fitting convector fins on the back of the existing black collectors.

What difference will this make to the performance of the Heat pump? This will really not show fully until deeper into winter, and the cold October is temporarily making our annual performance look to be worsening. But if I compare the recent daily consumption with days last year of similar evening air temperatures, then it is performing more economically. If I compare deep ground temperatures now with the same time last year (when the weather was warmer), the ground is better now.

Dr Chris Wood has posted some interesting queries, see the Comment below and my answer.


  1. David,

    You say the harvest is higher since the installing the mirrors - i would guess that these could not make this much difference. I would think that the actual increase in the energy harvest is due to an other reason. If I understand correctly you caclulate the kWh by measuring the flow and return (and flow rate) to and from the sunboxes? Some of the time, the circulation is directly between the ground and the sunboxes and at other times through the heat pump also i.e. colder glycol goes through the panels when the heat pump is running. As we are moving into much colder days the period when the heat pump is running is consequently increased hence the sun boxes have a greater proportion of the day 'feeling' a colder glycol flow, hence the potential greater delta T and therefore potential for greater heat yield. Even without the boxes the 'air panels' would experience exactly the same kind of increase in energy yield - especially on cold clear days, where the fabric heat loss from the building is high but there is good solar irradiance upon the panels. Unfortunately, there are a number of variables that make it difficult for you to conclude that the mirrors are the sole driver attributable to the increase. If you have time or the inclination, a small experiment using two mocked up sun boxes (only have to be a small proprtion of the size of yours, with some sort of water conctainer behind - both with thermocouples. Only one has 'mirrors'. Place in the same location and then monitor for a few weeks. Alternatively, you could put a flow meter on each of your sun boxes and measure flow and return temp for each and cover up the mirrors on one. (you would need to have a flow meter on each panel to ensure that you have an accurately balanced flow.

  2. Hi Chris, I am considering extending two datalogger wires to the return pipes from each box, and take the mirror off one of them, to compare the difference on sunny days. The top mirrors are riveted (because at that height, I dont want to fiddle around with bolts), but the rivets can be drilled out, or i can put a black sleeve over the top mirror.
    I cant get info out of this datalogger without the help of Blaise. I have to check up there to see if i already have enough datalogger wires, else i might have to ask you for a wire.
    I am going to increase the delta T by a degree, so that when the SBs are working, they are more of a help. If the increase was solely down to the GSHP being more busy, I would expect a more even gradient of daily harvests, eg on cloudy bright days, but I am getting a bunching of readings - either they are 6 or below on non sunny days, or are in double figures and above if any sun appears. So far, with day and evening temperatures sometimes as cold as last Nov or December, the daily GSHP consumption is acceptably lower than last year.

  3. Chris, you are too easily dismissing the experience of millions of Indians and Africans, that the simple technology of a Solar Cooker really works. One of my (and your) colleagues here, a Ghanaian remembers them being used in his village in his boyhood.
    On the 27th June, the longest sunny day of the whole year, when the Sunbox pump was actually running for 20 hours, it only got 21 kWh. Last week, with Sharphill and Wilford Hill shortening an already short day length, and a small tree to the west, the boxes picked up 21 kWh in only 9 hours. Some of the increase is definitely due to the busier HP putting out colder glycol. But on sunny days, I can see much higher air temperatures up there than used to be the case. I am convinced enough to be considering side mirrors too. As i cannot enlarge these, I can only enhance them.
    If we ever had a fear that this system would only work well in winter, we can dismiss that worry. Of course it is bad on dark cloudy days, but on sunny days, the difference is amazing. There have been no 'middling' results since they were fitted.


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