Tuesday, December 7, 2010

What is the effect of the Mirrors?

Rendering of mirrors before they went up,
including the vertical corner ones
7 Dec '10: For the last few weeks, I notice that when there is sunshine and the GSHP is demanding heat, the SBs always work at over 2 kW ! Even at this cold time of year. (I am so glad that the black collectors were contained in glassy boxes, not left naked on the wall.)

  As an assessment of the contribution of my 'diddly-squat' mirrors', let's look at total kWh divided by total number of hours, from the time that I installed the hour-timer in mid-April. This rules out freak results from single exceptional high or low days, by having time periods that are measured in months, not hours.

The average capture of the Sunboxes:
  • with No mirrors from late April -mid Sept, nearly 5 mos =1.11 kW
  • with Top Mirror only from mid Sept -mid Oct, 1 mo =1.43 kW
  • with both Top and Bottom Mirror from mid Oct -early Dec, nearly 2 mos =1.93 kW
Now I have fitted side mirrors, but these are going to be of marginal effect, intending to kick start the boxes on summer mornings by directing early morning heat into the boxes.
As Chris points out below, the increase in kW is also connected to pump speeds and hours of activity of the GSHP, which we discuss in the comments, and I add a bit more in a posting of 8th Dec.

Even if the bigger idea of interseasonal charging does not produce an improvement in the COP of the heat pump, I think and hope I have the confidence of my immediate technical group (Chris, David A and Blaise) in the general idea, and time will prove us right or wrong.... by next May for getting through the winter, and by next October for assessing the summer build up. So far, the results of my deep ground temperatures are proving me right, but I need longer to know if the electrical consumption of the GHSP will be reduced annually.

  But let's rejoice at small victories...
....with the months of testing before and after, I would declare the Mirrors to be a success.


  1. Sorry to sound so negative David, but as I have said previously, I believe that your increase energy yield from the boxes is due to the fact that as you approach deeper winter the heat pump runs for longer. The conseqence being that the mean circulating glycol temperature is reducing and the potential for a greater heat input from the boxes is increased. (ok we could also argue that the air temperature is also reducing and this has the opposite effect, but I would think on average you would have a greater temperature difference between the glycol temperature and the air in the boxes as the heat pump is used more... if i was you i would be checking this trend from your data i.e. plot sunbox temp minus glycol temp vs time... i reckon the gradient would be positive...
    sorry David but there is more than one variable to consider. and think about it logically, you show that the energy yield has increased by nearly double... but your mirrors do not provide double the irradiance to the boxes.. just look at the surface area of mirrors vs panel size. So the effect can not be entirely due to the mirrors... (ok there are angles of reflection to think about etc, but at this time of year the declination angle is small, so the vertical panels would be gaining the optimum irradiance capture and not the angled mirrors.


  2. It's not at all negative, it is all something I have been thinking about, trying to work out why the capture is so much higher.

    The vertical angle is undoubtedly helpful, and that was one of the ideas - get heat when its needed, reduce reflection off the front face.... and avoid overheating in summer by having an angle that isnt good in summer. The plastic panels are not idea as they do not have convector fins like a metal radiator has. They do not admit heat from the air well.

    Secondly, I recognise that the highly active GSHP is forcing them to provide more - although i countered this by requiring a delta-T of 6.0 degrees for them to work. This would reduce parasitic pump costs, but would also mean that when they were working, they would be more effective, hour by hour.

    The test of our discussion will be proven next summer when they have to work on their own with the GSHP asleep most of the time. I can tilt the lower mirrors to optimise to each month's solar angle.

  3. I am simply reporting the figures... so it requires some thought to work out why the average has doubled.

    There are some other simple facts....

    1. the deep ground temperature is still in double figures - so defrosting seems to be working.
    2. the GSHP is still working in the Plus temperature range even after 2 weeks of this below freezing temperatures.
    3. The daily consumption of the GSHP has only exceeded 30 kWh/day twice in this half winter, even though i extended the working hours of the GHSP by 3 hours/day and gave my wife the an extra degree on the thermostat.
    4. The GSHP hasnt needed additional heat, and i am determined that it should get through the winter without.

  4. Thankyou! You will be welcome back as often as you like, and I hope to report some good results as the Spring days lengthen and we get more sunshine


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