Tuesday, May 3, 2011

Why use Sunboxes?


3 May '11: Lyndsey (writing a dissertation about the Peveril Solar house) brings me back to one of the fundamental questions : why have a Sunbox at all? 
Dr Chris Wood believes that we could use black chillers nakedly on the wall or roof - he is building a test rig in Burton on Trent using the same black panels as on the Peveril Solar House, but is omitting the glazed boxes. His intention is also to charge the ground.

So why enclose? Well, we could use unglazed black chillers and with enough of them, they could partially or completely take the place of the ground, making the GHSP into an Air source. But for this to happen they would be coated with Ice for much of the winter because they could benefit from the latent heat of freezing and thawing. But I believe that they would be prey to all the faults that ASHPs have in winter. The pipes running through the house would be dripping with condensation every metre along the way, and the GSHP would have to work hard to get the loop temperature low enough for form that ice.
Using Sunboxes, we are still reliant on the ground as our primary source and battery, but we are now making the ground less reliant on itself - it gets actively recharged all the year round instead of just in the summer in a very slow natural way. During the winter of 2010-2011, the deep temperature never went below 10 degs.
The Sunbox system is a warm, not icy, form of augmentation. The thermostatically controlled pump ensures that it is never wasting energy pumping for futile or marginal benefits - when there is no warmth to collect, it turns off!

2 comments:

  1. David,

    The idea is not to coat the panels in ice - for this to happen would mean that i would have glycol at below zero going through the piles for a long period also.. This can not happen with piles. A well designed system would ensure that the mean glycol tempertaure stays above zero. the panels are purely a means to recharge the ground, both seasonally and also diurnally in the shoulder months of winter. If there is any benefit to running the glycol through the panels even on cold winter nights - well thats another argument and indeed if the mean glcyol temp is 4degc and the outside air is 7degc then yes it may be worth it - in this instance heat would flow into the panel and also condensation may form on the panel - heat gain by the latent heat of condensation. But by no means would we ice the panels up. The design would have to ensure that we have enough ground loop and recharge capability to ensure that the mean glycol tempertaure does not fall towards zero or below. For sure we can allow short periods of zero degrees at the end of long heating cycles in janaury, but if we can aviod it - we will! The heat pump does not work hard to drive the glycol temperature down - this is not a choice of the heat pump. This is a consequence of the overall design of the ground loop (and recharging capability). A system which spends most of the winter with a mean glycol temp below zero degrees is not very well designed! The ourcome of low glycol tempertaure is reducing heat ouput and COP, not increasing compressor power.

    Chris

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  2. Hi Chris, when I first discussed these with David Atkins, the black panels were seen as an extension of the ground loop and he was clear that we can benefit from the latent heat of freezing and thawing. I was a bit horrified at this. We did see the sunboxes running at below zero on a few occasions during last winter, but not for long. This is real-time augmentation to the GSHP, quite different temperature ranges to the summertime behaviour which we are seeing at the moment.

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