Friday, July 20, 2007

Why even small amounts of corrosion, rusting, fading or bleaching may seriously affect the performance of solar water heating panels

This video explains why even slight corrosion, bleaching, discolouration or fading of the collector surfaces of a solar water heating system may cause significant and unacceptable losses in performance.


John said...

Please explain further the "base temperature" of 50°. I would expect that after everyone has had their shower in the morning, the tank water temperature would have an average of more like 10-20°C. Why would the solar water heating therefor not contribute anything until the solar heater has reached 50°? Is this 50°C threshold set by the electronic controller? If your family only uses hot water in the evening, then your explanation makes good sense, since the water will already be at 50°C in the morning, having been electrically heated overnight.

Gary Moller said...

John, Fair question. You are right that the temperatures will of course drop below this base temperature whatever it might be and depending upon hot water use by the family. Unless one is constantly switching their electric element off and on then the main contributor would surely be the electrical element (unless it is a really small one!) which kicks in the instant the tank temperature drops below the thermostat setting. Solar water heating will only be a significant contributor above that once the element shuts off and only when the panel water is several degrees higher. Unless the electric heater is controlled by some sort of device that automatically shuts it off during the day, then my explanation stands.

John said...

Thanks for the explanation. I was assuming that the electric element was on ripple control (night rate) or was very slow to heat the cylinder. The scenario you describe is probably far more common.

Anonymous said...

Gary, I cannot see how spots on the panel will reduce "peak" temperature or the panel. I can however see how it MAY affect the RATE at which the panel gathers energy (same as if you partially cover the panel). Panels will usually get to temperatures in excess of 100 degrees in the summer. The only reason they shoule peak at less than that is if the panels reaches a point where it is losing energy as fast as it is gaining it.
Also, I assume the peak temperatures in your example is measured when the pump has been disconnected (ie. not taking the heat away).

Gary Moller said...

Good points and it shows that the actual truth is more complex than it may first seem. However; I think my explanation is essentially right.

First of all, we are hardly talking about "spots". We are talking about damage to the panel surfaces that renders some as good as dirty white all over within 5-8 years. Some clearly have ceased to work at all and definitely not during most of the winter and progressively so.

I wonder if you are describing temperatures by flat panel or solar tubes? I understand there is a difference.

And I should explain that I was talking about my observations from our system. I have an office at home and was able to monitor the pump and temperatures each hour on many days. What I noticed was the pump progressively worked less and less. When the temperature of the panels got several degrees above the base temperature in the tank, the pump would kick in and run for as long as the panels were able to return water several degrees above the base temp. As the panels lost their heating capacity, the duration of the pump being on became steadily shorter. In winter, it barely worked at all as the corrosion progressed. I have seen other examples of the exact same kind of panels as ours which have as good as ceased to work even on sunny days.

Poor lagging as seen on as many as 80% of panels in NZ will contribute to this problem as water cools slightly as it is sent from the tank to the panels and then back to the tank. While this might be a small heat loss, it all adds up.

I hope that this has clarified things.

Incidentally, we are about to reinvest in solar water heating and we will be placing a big emphasis on the quality of the lagging and other insulation to minimise heat loss. We are sold on solar water heating; but so long as it is of the kind of quality that really does give a return to the environment and to the consumer.