Sunday, January 8, 2012

Well perhaps no tank....

8 Jan 2012: I worked out that a 117 litre tank would hold 22 kWh of heat if it was raised to 60ºC (quite possible in summer with evacuated tubes) and then lowered at night to 15ºC by a few hours of circulating through the ground loop. With a specific heat of 4.186 kJ/kg K it's about half a kilowatt hour per degree change. However, I have to return to the key idea of the 'Surya' technology that I am using on the existing Sunbox is that it does not need a tank, and that the ground gives an excellent delta-T.
There is a 3m x 1.3m space on the east roof
    If the pipe diameter is large, it connects by the 'trickle and whoosh' serial loop design and if it is smaller pipe diameter then it can work on the 'trickle all the time' parallel loop design. Simples!
    If a water tank was getting into high temperature, the amount of heat it would store would decrease progressively as the delta-T decreased - as the temperature of the tank approached that of the panel. If the solar panel is receiving liquid at only 12-15ºC even in summer, it would perform far more efficiently than it would from a tank that was warming up.
  The next problem is that the space available on my roof is 3m wide by 1.3m high. I don't want to add more to the south wall because it could become 'target practice' for kids on the field behind, and glass tubes are tempting - and being vertical, they would be less efficient anyway. Putting them there looks neat - see the diagram - but most tubes are manufactured in a 1.6m length. The tubes could go on the west roof, but it is West facing, with a 10º tilt to the north - but there is a clear 1.6m high space on that roof. The inefficiency of its facing west is balanced by the high efficiency of an excellent delta-T to the ground.

   It's possible to buy an array of tubes reasonably cheaply - this diagram on the Navitron website shows a 30 tube array, with 1500mm tubes, 2.5m wide is about £700 including VAT.


  1. David,

    Looks as if through a matter of consideration of a number of processes you are going all the way to accepting that you could just charge the borehole with evacuated tubes!


  2. Quite right Chris, it was a sequence of thought, but in the interests of future discussion (e.g. with people who ask "Why aren't you using a Tank?") I thought it best to leave the debate visible.
    It has caused me some doubt about the idea of using PCM though for the skyscrapers. The cost and difficulty of long term storage look prohibitive. PCM can be used for cooling, and some same day heating, e.g. for HW. One could use facade solar thermal to store heat during the day to large water tanks, and pump it at night down a very well insulated duct to underground boreholes.

  3. Chris, my wondering now is the simpler one of the circuitry, and the controls.... the IF... THEN... conditions for running the two systems, where to fit the connection to the tubes, and how to energy meter them to get separate readings. I am close to having an entire two-year sequence of the present Sunboxes, so feel that I have that data recorded. Therefore I could now have another system to add. The important thing would be to do it after March 7 (2-yr date of Sunbox) but before the Summer.... to get the benefit of a summer's heating. It would be great to add another 1000 kWh to the borehole, annually (or more)


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