Microcontrollers > Data Logging
Solar hot water heater – Data Logging
David HK:
Sparweb,
Perhaps its me doing the misleading. There is no pump nor circulating system. This is still planned as phase 2 if I ever get round to it. The water in the box is what is heated and that's it.
This is drained off on demand and blended with cool water as required. In the summer the water is damn hot and I have seen 116 Celsius on the LCD displays - remember the system is pressurised to 64 psi so true boiling point is around 147 Celsius. It frightens me when I see temperatures this high.
At such high levels of temperature only a small amount of water is bled off to blend with cool water for showers and dish washing so its rare that we do not have enough warm to hot water on demand.
Trust this clears things up.
Dave
SparWeb:
Aha, now I see.
I thought you were storing the hot water in a tank, and circulating it through the panel. Now I can figure this out:
The total volume is about 35 Liters, or 35 kilograms. The specific heat of water is 4.187 kJ/kgK.
Raising the temperature of the water from 15C to 65C (from your graph posted to the Pub) is 50K.
(4.187) * (35) * (50) = 7327 kiloJoules
The panel was exposed to the sun from 10AM to 2PM (4 hours = 240 minutes = 14,400 seconds)
1 kJ = 1kW * sec so 1 kW = 1kJ / sec
7327 kiloJoules / 14,400 seconds = 0.5 kW
This result mashes together a lot of factors and can't even be considered an average input, because the rate of heat being radiated away increases as the panel gets hotter. Basically the panel becomes inefficient when it gets hot, even if it starts out efficiently in the morning. You already mentioned that the panel is shaded, but I wonder if it's being tracked to face the sun or not. You've posted photos of your tracked solar PV in the past, and I don't believe the water panel was on the rack.
These numbers from a no-flow situation shouldn't be discouraging. If this solar heater were to be hooked up to a water system continually circulating through, the wide temperature differential would significantly improve the efficiency, and the lower surface temperature of the panel, despite the insulation, would reduce losses too. I expect that once your project enters Phase 2 you will see even more benefit from the panel! Collecting data from this stage of the project will provide a valuable lesson on the effect these changes to your system will have.
Good luck!
David HK:
Lets see if Sparweb's excellent suggestion has worked. A selection of different days showing what the weather does to solar water heating.
David in HK
David HK:
I never seem to get things 100% right with picture posting, but for clarity on the above:-
First chart - a cold front arriving from North China - everything falling.
Second Chart - with self explanatory comments.
Third Chart - the second again except that a very bright whizz kid in the Excel Forum has gone to town on it and added a slide bar (top). this is mouse cursor controlled, and as it moves a blue drop line indicates the data line being read, it also automatically changes data in the box. This really does show the power of Excel for those who know how to use its very advanced features.
The jagged dark blue line shows coll water going into the pipes to replace hot being taken out.
I can promise some spectacular charts once Hong Kong reaches July and August. From 0830 am the temperatures in all pipes almost rise vertically.
That's about it folks. If you have any questions please post them.
David in HK
David HK:
Sparweb,
The solar heater is only partly shaded in the winter time. In mid summer its never shaded.
There will be a substantial difference between your calculations now and in mid summer.
I would never even think of this as a tracking system - too complicated and dangerous. Imagine a burst pipe at 116 Celsius!
Dave
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