Off-grid solar hot water

The running of an off-grid solar hot water service, in a cool climate, with only occasional electric heating is described.

Introduction

I have supplemented my large 4.6kw PV solar system with a solar/mains powered hot water service to reduce my use of mains power for water heating.

“The backroom Boffins at the company that supplied my equipment assured me that the solar hot water service could give a better ‘bang for my buck’ than my PV system with regard to electricity savings.”

This comment was encouraging. However, the truth or otherwise of this would depend very much on factors such as:

• the size of each system,
• feed-in tariff for excess electricity,
• the quantity of electricity & hot water that I use,
• and importantly, how I run the hot water service.

Nevertheless, my old second-hand hot water service was inefficient and overdue for replacement and I received a very significant grant to replace it (proving that it was rendered unusable by drilling a hole through it).

My solar hot water system

[Photo of hot water service & pump]

Living in a cool Gippsland climate I chose to get an AAE solar hot water system. It has a smallish stainless steel lined 250L hot water tank with an electric booster element in the top half.

The tank was mounted at ground level and the rather oversized heat collector comprised 30 AAE evacuated tubes (double the number of tubes that were recommended for this tank size).

The tubes were mounted on my roof at ~60 degrees from horizontal. The circulation pump was the SolaStat-AAE system. The hot water in the collector manifold/heat exchanger (above the tubes) is pumped down to the storage tank when it gets to 12 C degree above the storage tank temperature. I figured that this combination should give me a good chance of getting adequate water heating, from the sun on most days, for daily showers.

“Again the back-room Boffins applauded my choice and indicated that I was on the right track oversizing the collectors .”

System performance

“Little did I know that my choice meant that I could, without using mains heating, maintain adequate water temperature for showers, rather indulgent long ones, in my own collected rainwater, where I perform my daily stretch routines.”

Some times, when I have a week of bad weather, the booster must be used.

“Luckily for me, such bad weather comes with snow which usually calls me away to my skiing adventures. Then a shortage of hot water at home is not an issue.”

With my electric heating elements mostly turned off I have had a unique opportunity to monitor the solar (only) water temperature. It has revealed that with minimal solar input (in continuous bad weather) that water temperature drops by about 7 C degree per 24h (without water usage). I estimate that most of this heat loss is happening during winter evenings and early mornings when air temperatures are lowest.

Such heat loss would normally be invisible when off-peak electricity is used to top up the temperature daily. I was amazed that such wastage could occur in such ‘swish looking’ hot water storage system.

“Luckily the loss in my case cost nothing as the sun does not charge for its generous energy from fusion where hydrogen joins to form helium. Did you know that the hot rocks in Australia that have been touted as a wonderful source of clean energy. They actually derive their heat from nuclear fission? What would the anti-nuclear ‘nay-sayers’ think about that if they knew?”

Back into hot water. For others, who leave the booster on, this daily loss would be significantly higher and of considerable cost.

I have searched for the source of heat leakage and it is not obvious to me. The insulation around the tank looks and feels excellent. The fittings and pipes are well insulated and I have added extra wool insulation. I have ruled out reverse thermo-syphon effects.

Unlikely as it may seem, there is the possibility that heat from the tank is conducted along the copper pipes and travels along the copper to someplace where a thermosyphon in the water can take place.

I have also observed that as little as 1.5- 2h of sunshine per day will recover the water temperature to provide hot showers the next morning. If the booster is left on, water heating is already done with electricity (before the sun comes up) then there is not much scope to do solar heating. “This does not seem to be a smart design to me.”

Overheating

I expected the solar heater to perform so well during summer that it would boil the water in the pipe that leads to the storage tank. There is an automatic ‘blow-off’ valve to provide degassing and steam relief at the head of the collector manifold/heat exchanger. However, this has not to my knowledge activated itself.

“A ‘blow-off’ happened during commissioning when there was a failure to install a degassing valve. The circulation pump cavitated and the manifold got up to 160 C degrees. It issued forth a shocking ‘hissss‘ sound that could be easily heard from 200 m away during the correction of the system.”

Without any observation of overheating, I did not implement my plan to deploy a ‘pull-down’ shade cloth screen to cover the collectors in summer when I was away on extended adventures. Possibly the thermal heat loss described above is saving my system from overheating?

Extended mains power outages are common in my rural area. I was initially apprehensive about the system overheating because the circulation pump can not work without power. However, this has not been a problem over many years and many outages on hot sunny days.

With no power, I think the system simply functions in this unusual way. The pressurised water becomes superheated in the heat exchanger manifold. Heat flows down through the wall of the insulated copper tubing by conduction. At the same time steam slowly makes its way down to the storage tank as the copper heats up to the boiling point. Eventually, the steam meets tank water and condenses and delivers it latent heat of vaporization to the water. Then more liquid water moves up to the manifold to replenish the water that was lost as steam.

“It may be similar to the heat transfer mechanism that operates within the heat transfer tubes that are mounted down the length of each evacuated tube.” I think this means that when deprived of pump circulation (or a thermosyphon) heat can still move down to the tank (probably not very efficiently) and thereby prevent steam ‘blow-off’.”

On a hot day with no power, if I put my ear against the tank wall I can even hear the steam bubbles collapsing noisily in the tank (like a coffee steam frother). I think this is evidence in support of my ‘blow-off’ protection theory.

“The good news is that the loss of power is no big deal and there is still plenty of hot water to have a nice shower after all the sweat of heavy investigation and thinking.”

Other observations

During very hot days, just after turning on a hot water tap, I can sense a short ‘hiss sound’ and strong momentary flow of water.

“By the sound of it, I expect to see steam, but I never do.”

I don’t think this ‘hissss‘ could be caused by momentary boiling of water that would result from the rapid decompression of my hot water pipes. This is because, with the circulation pump functioning normally, none of the water should exceed 100 C degree. For the water in the manifold to reach 100 C the water in the bottom of the tank would need to be 88 C (100-12 where 12 is the differential temperature to start the pump).

A more likely explanation may be that some small pocket of air/gas is trapped somewhere in the system. It simply gets compressed by the heat expansion of the water, against the strong overpressure safety valve in the HWS. I speculate that when the tap is turned on, high-pressure water is momentarily expelled from the tap until the decompression is finished.

I have a bucket that collects the water of expansion that is expelled daily through the HWS pressure safety valve. This expulsion is a normal part of the cycle of water heating, use and then replenishment with cold water. The fresh cold water must expand when heated and a volume of water equivalent to this expansion comes out through the pressure safety valve.

Initially, the bucket was there to save the water making a mess in the back veranda. However, it quickly became a simple way of estimating if significant water heating had occurred.

“When I come back from a trip the quantity of water added to the bucket, in a given period, usually provides me with a good indication of whether or not there is a nice hot shower waiting for me.”

Tim

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PERFORMANCE OF A LARGE DOMESTIC SOLAR POWER SYSTEM