Thoughts regarding running a low voltage DC immersion heating element directly from a solar panel?

[michael123123]

Just as a little project, I was thinking about getting one or two solar panels (2-400 Watts) and hooking them up to a DC heating element in my hot water cylinder. I could potentially mount the panel(s) on the shed, which happens to be right next to the external wall behind which the hot water cylinder is located.

I've done a bit of googling, and whilst I can see that this is at least doable, I haven't found a huge amount of information.

In terms of my current situation, I don't use the electric immersion heater, and I pretty much never run the oil-fired boiler for hot water in my open-vented system. We have an electric shower and don't use the bath. We use a dishwasher mainly for washing up.

It would be nice quality of life improvement to have some warm/hot water from time to time for washing hands/washing up.

I'm just after a bit of a DIY project, so I'm not looking at a full PV or solar hot water system.

Anyone done anything similar?

 

[ericmark]

My immersion is DC powered, using an iboost+ which is fed to 230 volt AC and connects to a current transformer so it knows when there is spare power and although the element is 3 kW it can just use 100 watt if that is all the spare power available.

I can see your thoughts, but the problem is solar panels need a varying voltage to get the maximum out of them, building a controller will not be easy. Maybe a simple direct connection will work, I find my system does not work as intended as the immersion heater and thermostat are two short, so only heats two 9" of the tank.

You can look as the willis system with the immersion exterior to the tank, think these are now marketed as for solar, although many Irish homes have them, seems too complicated for the plumbers on the mainland to work out how the work, and then we say the Irish at tick?

 

[bernardgreen]

I came up with the idea of a solar powered Willis heater using NiChrome heater wire wound around a length of copper pipe. But with no way to install solar panels ( thatched roof ) it was of no use to me. But a friend did take it up and in summer he has ample hot/warm water for a farmer's field shelter.

 

[ericmark]

I have I think 14 panels on the roof and this is how there are doing now. The question is how many panels would you need, the ones I

have are split into two banks of 7 so to get around 230 volt it would need a fair few panels. As the day goes on the output increases, worse day this month was 7.5 kWh I found to have hot water before solar was installed in the summer for hand washing only I needed to run the boiler 3 times a week for 30 minutes a time, in the main only ran for 20 minutes but water more like warm to hot.

So 20 kW boiler for an hour to maintain the water temperature over a week. So would consider you need about 3 kWh a day. so 1/3 of the power I got on the worst day, best day this month 10 kWh so 1/4 so would need 3 to 4 panels in winter. I am in mid wales. 240 volt / 7 x 3 = 100 volt, a 3 kw immersion at 100 volt is around 500 watt, so yes it does seem about right, but seems rather an expensive project, not sure how the thermostat will work with DC and I am looking at November, I got 481.6 kWh last month so averaged 15.5 kWh per day so three panels around 3.3 kW per day, not had solar before that to be able to give figures.

 

[JohnW2]

Just as a little project, I was thinking about getting one or two solar panels (2-400 Watts) and hooking them up to a DC heating element in my hot water cylinder. I could potentially mount the panel(s) on the shed, which happens to be right next to the external wall behind which the hot water cylinder is located. ... I've done a bit of googling, and whilst I can see that this is at least doable, I haven't found a huge amount of information.

I can think of no reason why it would not be theoretically 'doable'. As eric has implied, if you could find an immersion element designed for roughly the right voltage ('intended for' AC or DC doesn't really matter) then a simple 'direct connection' may well be adequate, otherwise you would need dome sort of inverter.

In terms of my current situation, I don't use the electric immersion heater, and I pretty much never run the oil-fired boiler for hot water in my open-vented system. We have an electric shower and don't use the bath. We use a dishwasher mainly for washing up. It would be nice quality of life improvement to have some warm/hot water from time to time for washing hands/washing up.

Are you saying that your current 'quality of life' rarely has warm/hot water available other than from the electric shower?

Kind Regards, John

 

[JohnW2]

.... I find my system does not work as intended as the immersion heater and thermostat are two short, so only heats two 9" of the tank.

So 20 kW boiler for an hour to maintain the water temperature over a week. So would consider you need about 3 kWh a day ...

If I'm understand correctly, the OP's current situation is that he rarely has any hot/warm water other than that from an electric shower. If that's the case, then heating just 9" of his cylinder (or less) might be a major step forward in his 'quality of life'!

Kind Regards, John

 

[michael123123]

Currently looking up about these Willis systems - there's been some interesting previous discussion on them.

Are you saying that your current 'quality of life' rarely has warm/hot water available other than from the electric shower?

Correct, we turn on the hot water once a year or so when guests with a baby visit. At present, all hot water taps are always cold. Any hot water is electric shower (also used to fill up the recently acquired baby bath), and the dishwasher handles washing up anything greasy. The odd bit of cutlery or glass just gets washed up with cold water.

 

[JohnW2]

Correct, we turn on the hot water once a year or so when guests with a baby visit. At present, all hot water taps are always cold. Any hot water is electric shower (also used to fill up the recently acquired baby bath), and the dishwasher handles washing up anything greasy. The odd bit of cutlery or glass just gets washed up with cold water.

In that case, as I've just written to eric, I think that what you are proposing would be a definite 'step forward for you' - and I suspect that, in that situation, a Willis system might not offer much in the way of advantages.

Kind Regards, John

 

[ericmark]

How easy/hard to mount a panel and try it? An immersion heater is normally 3 kW and as long as there is a thermostat in it and voltage does not exceed 253 volt I see no problem

It it runs at 25 volt no harm, it does not care if AC or DC, but I have no idea what keeps the voltage of my panels down, I assume the inverter.

So if you can add panels latter then start with one, and add once you have had chance to see how it goes.

The danger is if the voltage exceeds 253 which is the top end of voltage range allowed. Over 253 volts the immersion heater could burn out.

 

[JohnW2]

How easy/hard to mount a panel and try it? An immersion heater is normally 3 kW ...

At 230V. I suspect that, unless the OP uses an inverter, his (probably DC) voltage will be much less than that.

The danger is if the voltage exceeds 253 which is the top end of voltage range allowed. Over 253 volts the immersion heater could burn out.

Do you really believe that one or two solar panels would be able to supply enough current to 'burn out' a 3kW element? For that to happen would obviously require the one or two panels to be supplying appreciably more than 3kW.

Kind Regards, John

 

[ericmark]

At 230V. I suspect that, unless the OP uses an inverter, his (probably DC) voltage will be much less than that.

Do you really believe that one or two solar panels would be able to supply enough current to 'burn out' a 3kW element? For that to happen would obviously require the one or two panels to be supplying appreciably more than 3kW.

Kind Regards, John

He has talked about 3 panels, one panel no problem, but three not so sure.

 

[fourtytwo]

Just as a little project, I was thinking about getting one or two solar panels (2-400 Watts) and hooking them up to a DC heating element in my hot water cylinder. I could potentially mount the panel(s) on the shed, which happens to be right next to the external wall behind which the hot water cylinder is located.

I've done a bit of googling, and whilst I can see that this is at least doable, I haven't found a huge amount of information.

In terms of my current situation, I don't use the electric immersion heater, and I pretty much never run the oil-fired boiler for hot water in my open-vented system. We have an electric shower and don't use the bath. We use a dishwasher mainly for washing up.

It would be nice quality of life improvement to have some warm/hot water from time to time for washing hands/washing up.

I'm just after a bit of a DIY project, so I'm not looking at a full PV or solar hot water system.

Anyone done anything similar?

I have had my system running since 2015 doing exactly this but there are a few pitfalls :-
An immersion heater thermostat is not designed to break DC current and will arc and catch fire, an iboost DOES NOT use DC current for the immersion heater.
To get the best out of a solar panel it requires a variable load impedance according to the amount of sun falling on it, a 3Kw immersion heater has a fixed load of around 19Ohms, any other voltage/wattage heater will also have a fixed impedance hence some form of converter is required between the PV array and the heater.
It is not possible to burn out or otherwise damage a heater unless the source of power exceeds it's rating so if your immersion is rated at 3Kw you would need to install in excess of that power in solar panels to be in danger of damaging it.
I have not seen commercially available converters for PV to heater conversion although there may be some however if you are capable of DIY electronics there are plenty of sites describing converters.
The immersion if fitted in the top of the tank as is usual will heat the tank from the top down so you will get your hot tap water first, however it is possible for the top of the tank to become dangerously hot (like burnt skin) so although the built in thermostat cannot be used directly it is essential that some thermal control is used.
Good luck

 

[JohnW2]

He has talked about 3 panels, one panel no problem, but three not so sure.

He originally said 'one or two' but, even if it were three, is 1 kW per panel a reasonable expectation these days - I have no idea, since I've never really had much interest in these things, but I thought that they were still well under half of that.

If 3 panels could generate 1 kW, then just 3 panels would probably be achieving something like 10 kWh per day on sunny days in mid-Summer, which sounds extremely optimistic to me!

Kind Regards, John

 

[JohnW2]

An immersion heater thermostat is not designed to break DC current and will arc and catch fire ...

That sounds reasonable, other than I'm not sure what it would be that would 'catch fire' (but the thermostat contacts may well weld together. However ...

,To get the best out of a solar panel it requires a variable load impedance according to the amount of sun falling on it, a 3Kw immersion heater has a fixed load of around 19Ohms, any other voltage/wattage heater will also have a fixed impedance hence some form of converter is required between the PV array and the heater.

If one is going to manufacture a 'converter' (which, as you say, ought not to be too difficult for someone with 'electronic skills'), would it not make sense for it to be a DC->AC 'converter' (a.k.a an 'inverter'), in which case the immersion's thermostat would be happy?

Kind Regards, John

 

[StephenStephen]

Just a thought... In terms of efficiency, I believe that solar thermal is more effective at water heating, even in winter, than solar pv...