Friday, May 13, 2011

Solar Oven Tests, Saturday 30th April, 2011 by Denis

In order to gain an idea of the ability of the " Solar Oven" to convert the passive solar gain from the sun into useful heat energy inside the oven, a series of tests were devised.

Test 1.
A jug containing 1 litre ( 1 kg ) of water was placed inside the sealed oven. The oven was placed in an optimum position to gain the maximum amount of direct sunlight and moved at regular intervals to maintain this position in relation to the sun.  The test duration was 6.5 hours.
- Temperature of water at test start. T1 = 23 Deg C
- Temperature of water at end of test. T2 = 38 Deg C
- Mass of water. m = 1 kg ( 1 Liter )
- Specific heat capacity of water. Cp = 4.18 kJ/(kg.k) = 4,180 J/(kg.k)
- Quantity of heat. = Q ( W.s ) Watt.seconds
- The general heat energy equation:-
- Q = mCp(T2-T1)
- Q = 1 x 4,180 x ( 38 - 23
- Q = 1 x 4,180 x 15
- Q = 62,700 Watts
- Q = 62.7 kWatts.

Total heat energy transfered into the jug of water over the 6 1/2 hour test was 62.7 kiloWatts.  This is equivalent to a transfer rate of 62,700 Watts divided by 23,400 seconds = 2.68 Watts.
The theoretical maximum temperature inside the solar oven when it contains only air with this 62.7 kWatts of heat energy input is 261 Deg C.  This value ignores heat loss thru the insulated polystyrene solar oven box to atmosphere. Also, loss from the matt black aluminium passive solar gain plate through the clear polycarbonate double glazing window to atmosphere
At some stage, as the internal temperature of the box rises the heat loss through both of these systems would balance the heat gain from the sun through the solar gain window. At this stage the box would reach state of thermal equilibrium at a temperature below this theoretical 261 Deg C.

Test 2
Repeat Test 1 with the solar oven containing air only to ascertain maximum temperature reached.

Test 3
Cook some food!
To be continued.

Unfortunately due to recent extended periods of overcast weather we have been unable to conduct these test yet !
Even in Sunny Otaki !


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