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Solar hot water: Reducing our reliance on coal

A couple of years ago our government offered rebates to replace electric hot water systems with solar and we used this offer to replace our aging electric hot water system. We wanted a solar hot water system for a while to reduce the amount of fossil fuels that are needlessly used to heat our hot water. Our new system uses just over a quarter of the electricity that our previous electric system used and it saves us some money too. In the right climate this is an easy way to significantly reduce the amount of fossil fuels use.

Here is what we have learned from the experience.

Our family

There are two adults and two children in our house. We have a dishwasher that is connected to the hot water tap to take advantage of the new solar system, but it does heat water using its own internal element if the hot water supply runs low. We have a 5kg front loading washing machine that we have connected to the cold water system and wash with cold water (although we sometimes pour in a bucket of hot water if we think the wash may need it).

The systems

Our old hot system was a gravity fed system with the tank sitting on a platform in the roof space of our house. It worked reasonably well, but it was 30 years old and I have heard these have a usual life span of 25 years, so we decided to look for a replacement rather than risk a failure. Urgently arranging a replacement was not something I wanted to do even. To take advantage of the government rebates we had to replace all the current system with a new one – we couldn’t use the existing tank, which was ok because it was old anyway.

There are a number of different systems, but the type that we decided to go for was a roof mounted system. To be honest I think those with the tank up on the roof are a bit ugly, but there were only two other alternatives that I discovered. The first was to replace the existing systems tank with a new one that used thermosiphoning to circulate water with a couple of panels on the roof. This would keep the system simple, but I could not find an installer that would replace the tank in the roof space.

The other alternative is to have the tank on the ground or indoors and use a pump to circulate water through the panels. I decided against this as it is more complicated, given that there is already piping and power in the roof space. Also it would more expensive and would have ended up with more water pipe between the tank and taps which would mean more wasted water waiting for the hot to come through.

Panels or tubes

Not that long ago the choice was simpler – you just got panels. Now there is a new system on the market based around evacuated tubes. These are a bit like the inside of old thermos flasks, a glass tube inside another with a vacuum in between. Thermal energy can enter the tube, but cannot easily pass back out due to the vacuum. A heat pipe located in the centre of each tube has a partial vacuum and a small amount of evaporative liquid. The top of the heat pipe fits into a fitting at the top. Cold water in the tank is circulated through the fitting and absorbs this heat. The heated water is then returned to the tank. There are a lot of good things about them. The installers I talked to like them because they are lighter and are easier to fit.

I wanted to get a system based on the evacuated tubes, but I couldn’t find a system with a roof mounted hot water tank at the time, but I believe these systems are now available.

Frost

It gets cold here, sometimes below -10°C (14°F), so the system would need some sort of frost protection. The evacuated tube systems that I looked at with the tank on ground use the pump to circulate a small amount of water to the panels when the temperature gets near or below freezing.

In the end we chose a Solarhart 302J system that does not circulate water through the panels. Instead it circulates a mixture of water and propylene glycol which has a low freezing point. The hot water cylinder sits inside a slightly larger cylinder and the glycol circulates through the panel and around in this outer tank with the heated water stored in the inner tank.

Our solar hot water system

Backup heating

The water in the tank can get surprisingly hot from the sun – up to 95°C (203°F). A temperature limiter is used to mix cold water with hot so that it is not dangerously hot at the taps. Of course it is not always sunny and that affects its operation.

If there is sunshine for at least half the day we usually have enough hot water for the day. There is usually plenty of hot water during a cloudy day if there has been a full day of sunshine the previous day, however by the second day of cloudy weather the electric backup element kicks in.

Some people turn off the backup heater and only turn it on when needed. That is probably not a bad plan if the electric element is connected to the normal (rather than off-peak) supply. However, our system is connected to the off-peak supply which usually only supplies power during the night. If we run out of hot water we don’t want to wait until the next day before getting more so we leave it on all the time. This works well with our system as the backup heater only heats the tank to 60°C (140°F), but the sun can heat it up to 95°C. So, it does not heat every night, rather only if the hot water drops below 60°C. It does mean that on extended cloudy weather we have less and cooler hot water. This doesn’t sound great but in practice it works well for us. During these periods we don’t put the dishwasher on until after showers are over because the dishwater will heat water for itself if needed.

How energy and money is saved

If you installed one just to save money you may be disappointed, particularly if you cannot get any government assistance to install it. As mentioned earlier we also wanted to reduce the amount of fossil fuels we use. Before the solar system was installed hot water accounted for around half of our electricity consumption.  However, the hot water was heated by off-peak electricity which is much cheaper than the usual domestic supply ($0.1229 kWh compared to $0.3129 kWh), so hot water was only ever a smaller portion of our electricity bill.

During 2008 before we got the new system we used 3545 kWh for hot water and in 2010 with the solar this dropped to 961 kWh. At current prices that is $435.68 in 2008 and $118.10 in 2010. A saving of $317.58 for a year.

Things to keep in mind if you are considering solar hot water

  • You need a good spot that gets a reasonable number of hours of sunshine on the panels per day
  • Connecting the backup heater to off-peak instead of the regular supply will help keep running costs down to a minimum but you may run low on hot water after days of cloudy weather
  • Check how the system will run with your hot water usage patterns. If you have showers at night you may find that the whole of the tank is heated during the night using the regular supply and is hot before morning giving little advantage for having the solar panels.
  • Get a reputable installer. We didn’t and it was not fastened to the roof properly.

Would we do it again?

Definitely. There are more systems to choose from now and I would consider an evacuated tube one, however the Solarhart does the job and we are happy with it.

After getting the solar hot water system we also got a grid connect solar electric system which will be the topic for a later post