Sunday, March 6, 2011

Effect of Mirrors, delta-T and time delay

6 Mar '11: Chris Wood posted replies to my earlier article about the efficacy of the mirrors,
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And as I haven't seen the sun for about 2 weeks in this grey February, I am beginning to wonder myself! Logically, they will only work in sunshine. The weather forecast keeps promising a bit of Sun, but it doesn't come - yet! We are approaching the anniversary of the sunboxes very soon, so we can compare their performance with identical weather conditions of a year ago.

As for the other mode of operation, based on Delta-T, these cold days do not result in much action - we are living off our reserves of heat in the ground. I have remembered that in the late autumn I changed the delta-T to trigger the sunboxes, to 6.0 degs C, to reduce wasteful pumping. This may have been overhigh. When it does work, we do get a good delivery, perhaps explaining the higher kilowatt delivery. However, the number of hours are too small. I changed this delta-T to 5.0 degs today.
    Also, when it starts off, based on delta-T, there are about 4 metres of pipe that are indoors, and the glycol in that pipe has warmed up. At startup, the rush of 4 m of room temperature glycol is so warm that it reduces the delta-T so quickly that the sunboxes turn off again. So I have added a one minute delay to the thermostat's operation, so that that first flush of warm glycol gets buried and the glycol from the boxes and the pipes in the loft have time to get down.

8 Mar '11 Postscript: 7 and 8 Mar of 2010 were also extremely sunny, with PV captures of over 14 kWh, higher than in 2011. But the Sunbox capture in 2011 (with mirrors) was significantly higher. Let's hope this continues!

9 Mar '11 Postscript: I swivelled the foot mirrors of the Sunboxes to the Equinox setting - about 20-25º tilt - this is to recognise the higher solar angle and increase reflection into the box. Later, in summer, the mirrors need to be nearly flat, to reflect the nearly 60º angle of midday sun. I have to remember, it is not only about midday sun, it should be at the optimum angle for a combination of solar altitudes from 10am to 2pm.

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