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diy solar

Heater element matched to solar panel output = diode string

MattiFin

Solar Enthusiast
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Dec 31, 2020
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Was thinking of solar panel matching to resistive heater when it occurred to me that diode string as a heater would be almost perfect match to solar panel output characteristics (at least compared to resistive heater)

Comes with some obvious limitations like limited temperature range and need to connect gazillion diodes in series but might actually prove usable in some small scale project? 🫣
 
I think it's easier to match a set of panels to the resistive heating element. Then, adding a capacitor and PWM the output, and you've got a pretty effective system.
 
Resistive load in partial sun would get less current, operate at lower voltage further from maximum power point.

Diodes only decrease in voltage a little bit when current decreases a lot, closer match to PV curve. Doesn't adjust for shaded panel in a string, however.

The fancier alternative is the PWM circuit you mention. For a resistive load, it can operate as MPPT without inductors needed.
 
Diodes would work, but there are two disadvantages.
Max temperature of the silicon junction, about 160 Celsius, above which your diode fails short circuit.
Cost of a very large number of high power diodes.

A much better solution would be to use a simple resistive heating element, and a switching voltage regulator, to adjust the loading on the solar panels to keep the solar panel voltage very close to the maximum power voltage.
 
some more info might be nice...


It's a way to hook up solar panels to water heating elements. I'm building two of these to help offset electricity use at my in-laws, with an 8kW array hooked up to the 3-phase heating elements in the 2500L tank (used for domestic hot water and the floor heating). @Steve T - IoW UK is the person behind the LoadMaster.
 

It's a way to hook up solar panels to water heating elements. I'm building two of these to help offset electricity use at my in-laws, with an 8kW array hooked up to the 3-phase heating elements in the 2500L tank (used for domestic hot water and the floor heating). @Steve T - IoW UK is the person behind the LoadMaster.
Thanks.. whats your real name again? sorry I am a retard but I like to say thanks to people in real time...
 
All this seems excessively complicated given the application...

Why not just buy more cheap panels from SanTan than you know you will actually need, hook 'em up to more resistive elements than the minimum necessary to meet your requirements, be aware that there will be a mismatch, and get on with it?

I don't get it...in my experience simple is better. Less points of failure, less cost.
 
Because the amount of space for panels is limited, because matching resistance of a heating element with panels means a certain amount of panels (and panel combination) is needed, because even if you match the panels with the resistance it's only optimal in one case and will produce far less than the panels are capable of when it's not exactly noon, because the amount of heating elements in a tank is limited and adding another one is expensive and difficult or impossible, etc.
 
All this seems excessively complicated given the application...

Why not just buy more cheap panels from SanTan than you know you will actually need, hook 'em up to more resistive elements than the minimum necessary to meet your requirements, be aware that there will be a mismatch, and get on with it?

I don't get it...in my experience simple is better. Less points of failure, less cost.
Mismatch gets pretty awful when conditions get worse. If light levels are 10% of the rated STC conditions the direct resistive connection needs 10 times more panels than MPPT matched panel. 1kW vs 10kW of panels might sting your wallet even with the current prices.

I guess you could also match the resistive load to bad weather conditions and take the hit on good weather performance... could make sense in some cases if you have no use for the excess solar energy during nice weather.
 
Okay, points taken. All that makes sense for northerly locations with limited roof space.

I am not constrained by either, so was coming from that point of view. I am also 20 min. drive from SanTan, so the ability to throw cheap power around tends to shape how I do things.

I need to get out more ;-) There's a whole world out there...
 
I guess you could also match the resistive load to bad weather conditions and take the hit on good weather performance... could make sense in some cases if you have no use for the excess solar energy during nice weather.

With two elements in a water heater, you've got at least three different I/V operating points available by switching.

Extra PV panels in parallel, and of different orientations, would be a good way to maintain one operating point (plus about 10% ~ 15% voltage increase Voc vs. Vmp).
 
With two elements in a water heater, you've got at least three different I/V operating points available by switching.

Extra PV panels in parallel, and of different orientations, would be a good way to maintain one operating point (plus about 10% ~ 15% voltage increase Voc vs. Vmp).
Good thinking...But how do you switch DC without killing the switchgear due to arcing?
 

It's a way to hook up solar panels to water heating elements. I'm building two of these to help offset electricity use at my in-laws, with an 8kW array hooked up to the 3-phase heating elements in the 2500L tank (used for domestic hot water and the floor heating). @Steve T - IoW UK is the person behind the LoadMaster.

https://www.bel-shop.eu/controller-for-boiler-suplying-from-solar-panels-mr4316ac-ng-to-switchboard/

This is a commercially available alternative to Loadmaster allowing use of standard immersions inc mechanical thermostats. Not seemingly as elegant but available off the shelf or as a kit. It allows connection of AC resistive elements, to which it feeds 122Hz PWM, plus a DC connection, I assume to existing MPPT. It periodically looks for the presence/absence of each of the loads & will prioritise whichever one you choose.

I would much rather use Loadmaster, not least because all the documentation is in English, but I just can't get motivated to get everything lined up to build, test deploy. I would enjoy it but I just can't squeeze as many productive hours into a day as ai used to!
 
It's a way to hook up solar panels to water heating elements. I'm building two of these to help offset electricity use at my in-laws, with an 8kW array hooked up to the 3-phase heating elements in the 2500L tank (used for domestic hot water and the floor heating). @Steve T - IoW UK is the person behind the LoadMaster.
If you have 8kW array, rather just a panel or two, would it not be more useful to connect them to an inverter to power the house and then use an AC based immersion diverter? Surely an 8kW array would generate more hot water than anyone could usefully use - unless you are talking of swimming pools or hot tubs?
 
If you have 8kW array, rather just a panel or two, would it not be more useful to connect them to an inverter to power the house and then use an AC based immersion diverter? Surely an 8kW array would generate more hot water than anyone could usefully use - unless you are talking of swimming pools or hot tubs?
Increasing the inverter size to cope with extra heating (which is probably the biggest load) has several disadvantages
1. You might not be allowed to connect the larger inverter to the grid
2. Having a larger inverter used with a smaller load means you're operating in a very inefficient region of operation
3. You can be looking at over 100W of parasitic load on the larger inverters, just to operate maybe a 3W router and a 20W fridge
4. it costs more
 
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