Monday, October 6, 2014

Detail: a solar hot water system

Solar hot water is such a good idea that it amazes me that there are houses in Australia without it!
A solar hot water system (HWS) is so cheap and easy that it can be built DIY as my friends Roman and Jana Spur have done in Brisbane (I don't think they've documented theirs, so I've included a link to another).
Solar hot water encapsulates some of the good things I've discussed in recent posts, such as:
  • It is extremely simple, and is thus cheap, robust, repairable, low-energy
  • It uses the sun's energy directly, without any conversions, and so is using low-grade energy that would otherwise be wasted. This energy that is falling on your roof anyway, and using it liberates high-grade energy for high-grade applications (eg. computers). It also shades the roof, and so helps keep the house cooler in summer.
Despite this, paying to have a solar hot water system installed in Australia costs a lot of money -- typically it is nearly $5000. In days gone by, most solar HWSs had a tank on the roof, coupled directly to the solar collector, through which the water flows by thermosiphon -- there is no pump required. These days, installers are less keen to install tanks on roofs because of the weight, so the tank typically sits on the ground and the water circulates with a pump. I view the pump as just another thing to break (and use power), and was not keen to get such a system. It's also possible to just install a larger solar PV system, and use that extra electricity to heat the water, but I didn't want to do that because it would be using high-grade energy (electricity) to produce heat, which is wasteful.

This article goes part-way to explaining how I have reduced my power consumption to $1/day.

After a bit of searching, I became aware of some Chinese designed and built systems that are available in Australia for around $1000 (depending on size). For some, it's possible to add collectors to a standard electric storage unit, but I decided to buy an integrated system, which is shown in the photo.

The solar hot water system, installed on my roof and plumbed in. This system cost me ~$1300, not including installation costs.

This system is differs from typical Australian systems in a few ways:


The biggest difference, is that the tank is at atmospheric pressure (not mains pressure). The water is just sitting there, and the evacuated tubes are filled with water. Thus, the sun's heat directly heats the water, which sits in the tank. The small header tank you can see on top contains a float valve which de-pressurises the supply and ensures that the tank is always full. This has some advantages -- as water heats, it expands (some systems are known to dump all their water when it boils on sunny days). Because this system is open (note the upside-down U copper pipe at the top -- that's a vent), it can just vent steam if it boils. A downside of this I can see is that if I ever need to replace an evacuated tube, I'll have to drain the tank first.

Heat exchanger

So, if the system is low pressure, how do I use the water?
Separate to the supply to keep the tank topped up via the header tank, is a pressurised supply that passes through a heat exchanger within the tank. The cold enters, goes through a 30m copper coil within the tank, and by the time it comes out it has been heated by the hot water in the tank. The water in the tank acts as a heat reservoir -- that water never comes out the tap. The water that comes out the tap is instantaneously heated by the heat exchanger. A big benefit of this is that the hot water has no taste, since it hasn't been sitting in a hot tank, so I often use it to fill the kettle so it boils more quickly and uses less power.

The installation

This wasn't very easy, largely because I wanted the system to be on the steepest angle practicable. This is because in Adelaide, the optimal angle for a solar HWS is determined by the angle of the sun on the shortest day of the year. In Adelaide, this means a collector at 60° from horizontal (very steep!), so I had to "jack-up" the angle of the tank. I didn't get it as steep as 60° -- it's more like 45°, but trigonometry (performance = cos(15)) tells me that this should be about 96% performance at the winter solstice (vs a 60° tank). I made a big deal of getting this detail right, because I want to avoid using electric boosting as much as possible. The collectors face due North, which is also very important, and in winter it won't be shaded between 10am and 5pm.
I installed a heavy aluminium post horizontally on the roof to bear the weight. There is a wall directly beneath it, so the weight passes straight through the roof, through the rafters, and is borne by the wall. I've also put some tensioned wires to increase the strength of the frame when it is windy.


The system does not, to my knowledge, qualify for any rebates of this kind.

Total cost

I think the total cost, including getting it plumbed in, was about $2500 (about 1/2 the cost of a typical installation). This doesn't include the time I spent messing around. I estimate it's saving us about 6 kWh/day (that's what our old electric storage unit was using), which equates to about $2.10/day power saved [3]. This has a naive payback of 3.2 years at current power prices.


It works well. It was installed on September 21 (equinox), and has been boiling regularly since then. I will put shadecloth over the collectors in summer to reduce water wastage..

Future plans

I have a second smaller tank mounted on the roof nearer to the kitchen (to reduce the amount of hot water wasted in the pipes) that will be plumbed in soon. I will talk about this more in a future post.

[3] Our electric storage was not connected to off-peak power, which would have been much cheaper to run and would have made the economics less favourable

This article was written by Angus Wallace and first appeared at

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