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Solar panels for water heating

Solar panels for water heating

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Joseph Noci 106/04/2021 10:11:52
1028 forum posts
1272 photos
Posted by Peter Bell on 05/04/2021 13:10:59:

Thanks for all the constructive replies. To give a bit more of the picture I initially bought a "piped" panel system but it proved difficult to plumb in to my system as it was then. We now use an air source heat pump to a thermal store and have underfloor heating from that, all home grown and installed. The hot water cylinder is heated by the ASHP with the supply controlled by motorised valves on demand. It's stored at 35c and the final water to taps is boosted to 50c by on demand by elec heaters.

The on demand elec heaters can take feed water at up to 80c but obviously would be switched off so I decided if I could boost the cyl temp feeding them anything would be a bonus. I have never achieved more than 60c even in the summer hence my question about better matching of my panels to an immersion heater. I'm trying to avoid batteries and grid tie and had wondered if one of the solar type inverters would accept a dc supply direct from the panels and output 240v for the immersion?



And Also:

John, my panels produce 8.69a at 31v each according to the spec. I did some testing when I first got them and I'm sure they produced something like that but I need to do more. This video goes into it and the chap also has an interesting spread sheet.

What is important to realise with solar panels is that they are primarily a constant current device, but with a very non-linear delivery curve.

Connecting a solar panel directly to a heating element is the most inefficient way of doing this - Under bright sun conditions this is the mode that the panel will deliver its maximum current, but at very reduced panel voltage. Since Watts = Volts X Amps...

Cheap solar chargers are a PWM based direct connect, ie, connect the panel to the battery till the voltage = cutoff then disconnect, so the delivered watts can only be BatVoltage X charge current.

Cheap PWM convertor chargers are a 'proper' DC-DC converter, taking in panel volts and converting to battery volts ( or the voltage required by your heating element) and trying to maintain that, regardless of panel input voltage - also very inefficient - still Watts = BatVoltage X charge current

DC-DC MPPT converters however make the most of the panel available Power - MPPT = Mean Power Point Tracking. The converters monitor the output voltage to make sure it maintains that voltage best it can, and does all the monitoring of output current, etc to ensure the attached battery, or element, is not over stressed. The latter functions are also 'sort off' performed by the cheap chargers mentioned above.

However, the big difference is that an MPPT converter will manipulate the output voltage and current to maintain a INPUT current and INPUT voltage, such that the product of these is a maximum. If the panel in good sun can deliver 10amps, and we place a 1 ohm resistor across it, we get 10amps at 10volts from the panel. That's 100watts.

That same panel will however be able to deliver 10amps at its nominal 14.5volt output as well ( assuming a 10amp '12volts' type panel - around 22-23 volts open circuit) That's 145 watts = 50% more. It cannot generate a 14.5v potential on its output with a 10ohm resistor, since the output limit is 10amps...So an MPPT charger will increase or reduce the output voltage to ensure the input watts are max, and therefore the delivered output watts are max as well. The MPPT device is also a DC-DC converter so will step up or down the panel voltage to track the optimum power point

So, you should really get an MPPT element controller for a water heating element - An inverter will 'work' but suffers the same problem as the cheap PWM converter - it simply loads the panel as much as possible to get 220VAC out...

If you have agents for VICTRON, contact them - they have some good solutions.


EDIT - Just to add - 

If you use mains to supplement the water heating, ie, for sunless days, there are also simple timers available that you can easily connect to the mains supply so that, for instance, the mains is only enabled for water heating, perhaps 1 or 2 hours before shower/bath time, etc. ( and add a temp sense to the same timer so that if the water is hot, the mains does nothing..) Easier than trying to know what the day's WX is going to be..


Edited By Joseph Noci 1 on 06/04/2021 10:30:04

Peter Bell06/04/2021 13:57:33
372 forum posts
164 photos

Thanks for the reply Joseph.

I looked at various contollers but thought they did not apply to this instance as I am not charging a battery, also as I have 4 panels which are rated at 270w each it looks like I would need 4 controllers?

I suppose I need to experiment to find what the optimum resistive load is for the panel but I cannot find the restance wire I have safely stored also my clamp on meter has decided to pack in!


Joseph Noci 106/04/2021 14:25:15
1028 forum posts
1272 photos


Obviously your specific needs would dictate the controller choice, but there is nothing strange about your setup - there are MPPT Heater element controller types accommodating input voltages from 48 volts ( even 36 volts, I believe) all the way up to 340volts. Not sure what your panels are - 24v/36v/48v ? Only one, suitably sized, controller would be needed, so if your panels are say 24v each, it would be more efficient and make wiring easier to series them all into a controller able to take 72V to 140V inputs.

Your idea of finding an 'optimum' load resistance value won't really help much - the load resistor value will change very little between 20-60degC, but the Solar Panel ability to deliver the energy is so dependant on the sun - slight variations, esp on the murky days, has a huge effect on volts out from the panel. So a fixed resistance value is only applicable to the level of sun when you did your test. Thats what the MPPT controller does - it is continually changing the output voltage/current, and computing the input volt/amp product, and riding that curve to get the best watts from the panel..

Your statement;

I suppose I need to experiment to find what the optimum resistive load is for the panel

is not the way to go about it...All the panel can do is under the design point irradiance is to deliver X amps - if you look at the specs sticker on the panel underside you will see the max amps that that panel can deliver - INTO A SHORT...That is how they measure max current. But Max amps into 0 ohms is 0 watts - not very useful...And the sweet point on the watts curve for most panels lies between 14v and 18v output ( again, for a nominal 12v panel) so the controller must find a point where the panel voltage stays in that range, with the max amps it can extract - which in max sun, will be close to or equal to the short circuit current.

No short cuts here I fear, if you wish to extract the best from the panels - which I need to do since on the coast here we have more than 200 days/year of mist...


Nigel Graham 206/04/2021 20:53:19
1897 forum posts
26 photos


Your " The panel mentioned by NG 2 is huge, as panels go. "

I will admit I was guessing somewhat, and at the array size. I don't know the individual panels. I saw it last in 2019, and as it was facing away from the garden had only a rough idea from the assembly, about 8 feet above ground on an outhouse roof.

I think my brother also built a reasonably effective solar water-heater.


Neil -

That installation in Hamilton sounds interesting though seems needlessly complicated. I am pretty sure my brother's unit has a simple motor and gearbox so the array moves continually - perhaps I'll suggest he fits its base with numbers and pointer so it doubles as a garden clock!

This means it is always perpendicular to the sun, though a small angle off won't make much difference, and once started there are no stop-start intervals each needing a little extra woof to overcome the inertia.

Works for Jodrell Bank! (Albeit they'd drive it to the starting-bearing and elevation for whatever it to be observed.)

Peter Bell06/04/2021 20:55:54
372 forum posts
164 photos

Thanks for the guidance Joe. Spent some time researching what you have said and I came across this unit. Not fully sure how it operates but looks like a cross between an inverter and a buck converter?

Does this fit with your description of a MPPT Heater element controller, but seems to have charger in the title as well.


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