Tuesday, October 27, 2009

Thermal battery update

27 Oct 2009: I think the balls I have contain Phase Change Salts, not Wax. That's not a problem as their thermal retention can be even better than paraffin wax. See the Wikipedia on Phase Change (PCM). Having discussed with BASF experts, it seems that good old WATER is still the best medium for conveying heat from the copper element to the PCM balls. There's plenty of it, we have rainwater butts full of it.
   It may need some antifreeze for the winter, but BASF have a product called Glythermin P44 which can be mixed in solution with water, and is a non toxic antifreeze, and supports heat transfer to the balls. I am more worried about a heat build up in the summer causing the water to evaporate. I shall have to check the water level regularly, and could fit a Ballcock to the tank to trickle water in, that would keep it topped up. (see interesting discussion, US patents). It will be covered and insulated, and the balls will cool the water, and some of the evaporation will condense on the lid and fall back.
    I have purchased some ball valves, so that the piping can be dual use - allowing the solar panels to charge the earth direct (as previously intended) and by changing the valves, to allow the solar panels to charge the battery, and the heatpump to draw heat from the same battery, with a different circuit.
My earlier advice to myself was to keep things simple and not introduce so many variables that proper data analysis becomes too difficult - so I ought to take that advice. Therefore, I could work with the same 4 panels as originally intended, and have the ability to switch the battery in or out just by moving the lever-ball valves.
Illustration from Colin of BASF. He writes:
"Below is a table of typical values for specific heat capacity, thermal
conductivity and freezing point for various fluids at 20 deg C. Some are more accurate than others. Water, ethylene glycol, propylene glycol are defined liquids so the values are pretty accurate. Mineral oil and vegetable oil are generic terms, and for example the specific heat capacity of linseed oil is 1.84, olive oil is 1.97. You could expect bigger variations between the many grades of mineral oil, paraffin has a specific heat capacity of 2.13, the mineral oil in the table at 1.67 is probably a typical lubricating oil fraction. The Glythermin figures are from BASF data, the glycerol data are extrapolated from published values. What can clearly be seen is the higher the water content the higher the specific heat capacity and thermal conductivity."

3 comments:

  1. What temperature does the PCM phase over at? If you are wanting to store the energy from the low temperature solar collectors, then I am unsure how effectively this will be done unless you can reach the melting point. If melting point is never reached then you may as well use water, considering the specific heat of water will be higher?? In order to retain that heat as a store then it would be much better to have these balls inside a modern unvented cylinder - though getting them inside would be tricky - perhaps they would fit through the hole for the immersion heater - though such a tank would add some cost!

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  2. Chris, would it be possible to meet up? (If you are still in Nottingham) I have spent money on the cold water tank, and will work on the piping element. All this is more in faith than in 100% technical certainty. I will design a dual circuit so that the thermal battery can be actively connected, or disconnected, using lever-ball-valves, and with compression joints so it can be taken off altogether.

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  3. maybe employ a linear fresnel lens on a solar tracker to help with heat collection (keep it out of the focal point so you don't burn something though).

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