Reply To: Using PV systems to heat hot water

October 7, 2013 at 4:40 pm #1443

Participant

In a nutshell yes, all the subsequent posts have been brilliant.

This how I would do it if starting from scratch:

· Use a large (depending on household size) 270 or 300 litre dual bottom element/ mid element hot water cylinder (with thermostat control on both elements) & two bottom entry wetback ports (explained below).

· Connect the PV supply to the bottom element, with timer control so that this element can only operate during the day when the PV system is producing, initially say from 10.30am to 3.30pm (the duration of this period can be fined tuned to fit higher or lower demands). You could also install a relay so that if the PV system stopped producing (due to cloud cover), it also cuts out this element to prevent it “sucking” power from the grid supply. So essentially this element would only work when the sun is shining and the water temperature in the bottom of the HWC is below the thermostat set point.

The bottom thermostat will cut off the power supply once the water temperature reaches the set point, so if it only needs two hours of input, that is all it will get, to avoid the HWC overheating. This will then free up further day time power generation from the PV system for other uses in the home.

· Connect the mid element & mid thermostat to the standard ripple control (or night rate) electricity supply. This will boost the top half of the cylinder only, up to 60oC, at night time using the “cheaper” ripple controlled electricity, if there have been some inclement days and the water in the top half of the cylinder drops to below 60oC. This also satisfies the Legionella protection requirements.

Normally this element would never work, only in the event of bad weather or high water draw off. This is similar to how you should configure a solar water heating system, but using PV instead.

· Ideally I would also have a wood-burner with a wetback also connected to the HWC (for South Island and cooler N.I. locations), which would help boost the hot water temperature in winter when the PV system might struggle to provide enough power.

On that note it is very important to use wetback connections fitted to the bottom of the HWC, not side connections (as are common on solar ready cylinders). This is to prevent reverse circulation occurring when the fire is not going. With side entry connections, the temperature differential that occurs between the side entry ports causes reverse circulation to occur. Side entry connected solar systems get around this problem by including non-return valves. That is fine for a pumped solar system, but with natural thermo-siphon wetback systems the pressure differentials are too small to overcome the resistance in most non-return valves. Yes you could fit a pump to the wetback circuit, but I would be trying to keep the system as simple and robust as possible by avoiding pumps and valves, which all have a finite life and will eventually fail.

The other point with wet-backs is that the base of the hot water cylinder must be a minimum of 300mm above the level of the top of the wetback, also to prevent reverse circulation occurring. So where you position the hot water cylinder is important in the planning stages.

If anyone wants further information on the intricacies of thermo-siphon systems and problems that can occur, I can bore you to tears on that subject.

So for a small extra investment in the right sort of hot water cylinder with the correct connections and ports/ pockets would allow a simple robust system to be installed. The only other bits (apart from the PV system and the wood burner) are a simple timer, a relay, and setting it up correctly.

Obviously there are other ways of doing it and I welcome others thoughts and opinions on alternatives, after all it is an ever changing field as we know.

I am a great believer in keeping things as simple as possible, and to avoid unnecessary mechanical devices which add complication and generally have a relatively short life before they need servicing or replacement.