I have recently built a ‘PV Power Diverter’, which diverts my excess PV power to a 250 liter electric storage heater with 3.6kW element, that provides my entire hot water needs.
For those not familiar with PV diverters, a current xformer is used to monitor the net power flowing into the grid, and a triac is used to regulate the amount of power fed to the hot water (HW) heater such that only excess PV power is fed to the heater. In my case, using excess PV power in this way cost me nothing, because my feed in tariff pays me for every kWh of power generated, regardless of how it is used. More commonly, the feed in tariff is almost negligibly small, so it makes good economic sense to usefully use excess PV power for hot water, that would otherwise need to be paid for at a higher rate.
I live in Canberra, Australia, where the winters are moderately harsh, typically zero to -6 DegC overnight, though we do often get a run of clear days with daytime max of around 12 DegC. My PV installation has moderately severe shading issues in winter, but a lot of panels, so performance is probably equivalent to a 6kW system with little shading. The hot water serves 2 adults.
So how well does this PV hot water system perform? How often do I need to boost, and how does it perform compare to an evacuated tube system in the same city?
I have had it running now for around 3 weeks, in the middle of winter, and it’s very good indeed. On clear sunny winter days we run a 2.4kW (input), 7.2kW (heating) reverse cycle aircon all day to heat the 45 sqm living area, and there is still enough excess PV power to do the hot water without boosting, though only just. On cloudy/raining days we don’t use the R/C, and on most such days the cloud-shine and occasional appearance of sun also provides enough excess PV for the hot water. Without any PV input at all, the 250L tank, with setpoint at 80 DegC, will last for about 3 days. So far, we have not needed to boost, and I estimate boosting may be necessary around 4 days per year on average. I can ‘boost’ by choosing to draw power from the grid, or by using gas, but would choose gas because it’s cheaper and environmentally better than using grid power, though for a few days per year either is fine. It’s free hot water, for an initial capital cost of ~AUD$1000 for the 250L, stainless steel storage tank, plus my time and ~$100 of parts to build the diverter.
So how does this compare with an evacuated tube (ET) system? Turns out I have a good mate in Canberra with an ET system, and we know exactly how well his system performs, and how often it needs boosting, because his Resol solar HW controller logs all the key temperatures, and even plots them over a 30 day period on a web page. He started out with a 30 tube system, and it was pathetic, requiring very frequent gas boosting over winter. As he uses gas only for HW boosting, and the fixed costs of providing gas are $300 PA, he really wanted to eliminate the need for gas boosting and terminate his gas supply altogether, so he applied a ‘big hammer’ to the problem, and installed another 30 tubes, optimally pointed and angled for maximum winter performance. He has separate pumps for each bank of tubes, and his Resol controller is able to control both banks independently. That’s a 60 tube system, and optimally set up, at that! In summer he has to put black plastic over one bank, or the system will boil its brains out.
Even to my own surprise, the comparison is chalk and cheese. It’s been an unusually wet and cloudy winter here in Canberra, and when we get 2 or more days with heavy cloud and rain, the ET system just isn’t up to the task, even with 60 tubes, and gas boosting becomes necessary. Physics cannot be cheated. Even with 60 tubes, the collection area is still relatively small, and on days with little sunlight, the ET system can capture little heat energy, at best. But it is worse than that, because to prevent freezing at night, the tank water needs to be pumped through the collectors, destroying much of what little energy was collected. Also, the collection efficiency is very poor on cold, cloudy days, because on such days, the collector heat loss during the day is very significant compared to the amount of heat being collected. In contrast, PV panels don’t need defrost heating, and efficiency does not drop off under conditions of low temperature and low solar insolation, and nor does the efficiency drop off when producing water at high temperatures.
The practical result is that my PV hot water system leaves this 60-tube ET system for dead under all conditions. In summer, both systems have more than enough capacity, except of course that the excess PV power can be sold, or used for air conditioning or whatever. In the depths of a Canberra winter though, my PV system pumps out wonderfully hot water under nearly all conditions, when the 60-tube ET system has collapsed and needs significant boosting. I have had a lot of fun watching the day-by-day data plots of the water temperatures in this 60-tube ET system, and comparing with my PV system, and am quite surprised by just how much better the PV system performs. If you have the roof area, and are thinking about installing a PV system, then don’t waste your time with a traditional solar-collector HW system, with all of it’s additional complexity, freezing, boiling, and inferior performance that demands significant backup. Just add a few more kW of panels to your PV system, build or buy a PV power diverter, and take the PV hot water route. I’m laughing all the way to the bank. Hot showers never felt so good.
In warmer climates, or where roof area is scarce, or where you don’t have or want a PV system in the first place, the optimum choice may be different. Also, while for practical purposes I never need boost backup, that is with only 2 people. With a family of 4 or more, then my experience indicates that you will need some sort of boost backup with pretty much any solar HWS setup, at least in Canberra.
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