Monday, 9 August 2010

Solar cold store with control system only

On further consideration it may be possible to maintain a constant temperature inside the cold store using only solar power by either:

1) Combined sweat cooling and semi-continuous solar ice maker powered control system:
For this method first we would encapsulate the entire chamber it in a moisture containing layer, supplied with water by a drip line fed by a raised reservoir.
This would reduce to the load on the active cooling cycle as it would reduce the surrounding temperature to the wet bulb temperature of the ambient surrounding (which can be as much as 10-15oC cooler).

Now to reduce the temperature further (as much as is required), the modified version of the solar absorption ice maker system which allows for semi-continuous operation (continuous during day - at variable rates depending on climate) using the same water reservoir feeding the sweat cooling system

2) Einstein’s fridge powered control system :
Alternatively we could simply use Einstein's refrigeration cycle to drive the system. This not require any water reservoir but the cycle itself more technically complex (but you can get it ready made from any gas fired camper-van fridge)

In either case, the evaporators would not cool the cold storage directly but would cool in intermediate low temperature thermal storage fluid (eg water/ice). The design of the evaporator tanks would be optimised to facilitate efficient heat transfer.

The cooling delivered by this system as it is would be variable as the power delivered from the sun will always be variable which would not ideal as for proper cold storage a constant temperature should be maintained.
This could be possible by controlling the rate of heat transfer (using an electrconic control system run on a car battery) from the thermal storage fluid to the air in the cold storage chamber by pumping (pump also run on car battery) a fluid a controlled rate through a heat exchanger inside the thermal fluid and in turn through a heat exchanger with the cold storage chamber air (which will have fins to increase surface area so a fan would not be required (thank you Sampal for the fins idea :) )

So when there is surplus heat from the sun it would deliver extra cooling to the thermal fluid so the pump would run slower and when there is less cooling delivered by the evaporators, the pump would run faster to compensate. Therefore we could maintain any constant specified temperature inside the cold store with only solar power...

Now lets build this thing :)

6 comments:

  1. Tancio suggested that the control system could alternatively be run using a high temperature thermal reservoir (i.e solar concentration used to heat up thermal storage fluid which is supplied to generator at controlled rate to maintain constant operation) instead of low temp store as I originally suggested above.

    This would give the same result (i.e maintain constant temperature inside cold store) and would probably be easier to design as this is much more common process used in solar electrical power generation plants, so is likely to be more information available (working fluids, heat exchangers designs, control systems etc).

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  2. On further consideration, the problem with using a high temp thermal store is that there will inevitably be heat losses to the outside surroundings which will reduce the efficiency of the system. Even if the hot thermal fluid is well insulated, there will still be significant heat lost to the surrounding as there will be a large temperature differential (typically hot thermal store = 120oC , while outside surroundings = 30oC).

    On the other hand if we were to use a low temperature thermal storage fluid if well insulated inside the cold store the 'losses' (actualy would be heat gain in this case) would be much less (typically cold thermal store = 0oC , while cold store surroundings = 7oC). Furthermore any heat gain in this case would not equate to reduction in efficiency of the system as the this heat transfer process (from the cold store air to the cold thermal fluid) is actually desired (but should be controlled).

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