This last week I have lined the large trench running the length of my Solarbubblebuild in order to create a larger 'warm' thermal mass capacity than my existing set-up. This tank is below grade and insulated. I have also installed a bladder tube which runs the length of this tank forming a thermally coupled soap tank.

Filling this tank to its 1300ltr capacity, provided me with an opportunity to test my liquid cooling system with water supplied at a constant cool temperature. My idea was to supply my roof cavity 'cooling film' nozzle array with rising mains and drain this water into my new tank. Thus filling my tank, via the roof cavity. This gave me the opportunity to record some temperatures over time with a constant temperature of cooling film. My thermal mass has been insufficient in volume to do this before until now.



During the 80minutes it took to fill my tank I recorded the data represented above in the graphs. This experiment was conducted on the 15-10-05 on a beautifully clear and sunny October afternoon. The outside temperature was an un-typical 21C (70F).

Mains water here in the south of England usually comes out of the tap at around 13C (55F). The mains supply I had on this day at my build was a constant 17C (63F), due to the long journey it takes above ground in a hose pipe in direct sun light, and in contact with warmed soil.

What was most notable from this test was the immediate and sustained drop in Relative Humidity that occurred during the cooling process. Whilst this produced the effect of feeling cooler, the actual temperature dropped by only a 1C (2F). However the temperature of the water leaving the cavity was on average, 4.4C (8F) warmer than the water going into the cavity. This is a clear example of the latent heat of evaporation at work within a solaroof building.

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Once full, my tank had a temperature of 22.2C (72F).

My tank capacity: 1270 litres = 1270kgs of water

90mins to fill thus flow rate of film = 14.1 litres/min = 3.7gallon/min

Without looking very hard I found this 24volt 'Solar Slowpump' pump that would produce a similar flow at a similar pressure. 3.65GPM at 140 feet of head pressure. This is at 195watts.

200watts x 1.5hours = 0.3 kwh (kilowatt hour)

Specific Heat Capacity
The specific heat capacity of a substance with specific mass is the heat energy required to change its temperature by a given temperature.
The specific heat capacity of water is 4180 J / kg K. (Joule/Kilogram Kelvin)

Q = m*c*(Rise in temperature / Fall in temperature).
Q is the heat required.
m is the mass of the substance in kg.
c is the specific heat capacity.

Using Q = m*c*(Rise in temperature / Fall in temperature)
To bring 1270kg of water from 17.8ºC to 22.2ºC... a raise of 4.4C
Energy needed = (1270)(4180)(4.4) see footer note
Energy needed = 23357840 Joules = 6.5 kwh (kilowatt hour)

Thus, in this specific test on this day, by pumping the equivalent water to the roof would have taken approx 0.3kwh and during the test we raised 1270 litres of water by 4.4C which is equivalent to 6.5kwh of energy.

However, circulating a liquid film from my soaptank which is thermally coupled to the thermal mass would not have produced the same results. This is because, as the thermal mass temperature would have risen this would in turn increase the temperature of the cooling film decreasing its ability to collect heat from the interior space.

If I was only cooling and not collecting heat from the interior space I could use a the external cold water pond I am building outside to cool the in-going film water and maintaining its cooling efficiency. Using this configuration, I would expect to see greater cooling capacity than the above test, being as this external cold water 3500ltr tank is below-grade and uninsulated. It is also in constant shade from the sun. I would expect the temperature of this tank to stay around 13C (55F) year round.

I don't want to draw too many conclusions from just this one short test as data collected over an entire year, would provide a much better picture of the solaroof system at work. It is one of my goals to get a system capable of recording a reliable data from the build by the end of the year.

Footer Note:-
17.8C = 290.95 Kelvin
22.2C = 295.35 Kelvin
4.4C = 4.4k