Sunday, September 25, 2011

Solar charging in Bulwell

The house will appear in this gap
25 Sept 2011: Earlier in September (7th), I spent a couple of hours with a heat pump expert Mo Kelly of Sasie, and his colleague Simon. They are building a Code 6 house in Bulwell, Nottingham (built to Passivhaus standards are far as possible), and it will use Solar Earth Charging. It is designed by Mike Siebert of Ecologic.
   They wanted me to know what is proposed so it can add to my research, but also, were keen to test out the ideas on me. They are building their own heat pump, so that it will be air source in summer and ground source in winter. They haven't yet settled on the type of collector they will use for the solar heat, but with ground charging this is less important - it does not have to be high temperature (like tubes). Perhaps they will use an adaptation of my sunbox!

Rendering in ArchiCAD by Mike Siebert
    The site is between two terraced houses, so it is too small for a horizontal ground loop and they wish to avoid using a conventional borehole. With Solar charging and exceptionally well insulated buildings, there are other opportunities to try besides a borehole. In the centre of the plan, underneath the ground floor, they are proposing an uninsulated pit 2.4m x 2.4m x 2.0m with a large amount of horizontal slinky ground loop pipe buried in a helical stack - back filling carefully with the spoil that comes out.  The building will be like a thermal tea-cosy, nursing the heat that is buried. Surplus heat will move outwards into the volume around the pit, under the house. This size of pit isn't big enough for an entire winter, let alone a month, or even a week, having a capacity of about 8.5 kWh/degK (CWood). However, it will provide diurnial assistance to the GSHP as a thermal buffer. If the heat loss of the building is very low due to Passivhaus design, it may be sufficient.
  At the moment, the building is not clear of the ground, all that can be seen is foundations and ground insulation. So there is time to make decisions. Simon is an expert on data collection and circuit design, and will be planting thermal sensors in many places in the ground under the house.
  I hope to be able to report on this in future months.
 (See comments below from Chris Wood, and I have amended the text above to include his observation)

27 Sept Postscript, email from Mo Kelly: "just to be clear our main heat delivery for the house will be what I describe as my Mono Energy System, the main components of this will be PVT panels tied directly into the HP the 2 main benefits of this is an all year round PV energy collection and between 10 and 15% better performance and night time usable primary source for the evaporation process of HP. I will update you on my plans for the overall storage of excess energy."


  1. Just a quick calc....
    assuming soil/clay is backfilled then the thermal capacitance of the store will be around 8.5kWh/K. So this will provide a useful diurnal storage i.e. solar store daily - useful for optimising heat pump in shoulder months. Though in terms of deepest winter, the amount of influenced ground could be an issue when peak loading for extended periods of time. I suppose having the heat pump being both air source and ground will enable the source to be switched if the ground becomes too cold in anycase. and the low heat load of the code 6 will also help of course.

    cheers Chris

  2. Hi Chris, agree with all you say. I think the pit is uninsulated allowing surplus heat to move out, but if there is a tough cold snap like last December, there would still be a heavily localised chilling in the dense coil of pipes just in the volume of the pit, chilling too quickly to allow the heat in the wider volume to return (they should consider swelling of the fill in the pit). I hope that their house is well enough insulated for this not to be a major problem.


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