MPPT maximum power point tracking; can it be repurposed ?

Thread Starter

Externet

Joined Nov 29, 2005
1,817
Hi.
An array of solar panels on a roof can vary its power delivery by the amount of insolation, angle of incidence, cleanliness, temperature...

Can MPPT equipment be applied to manage a fixed load from the above variables ?
Example : The purpose is only to electrically solar heating water in a domestic 3KW 230V water heater as load. When shady or fully insolated; the amount of power differs.
If at a moment of poor sun light the panel array produces 200 Volts and 4 Amperes, the internal resistance of the heating element will be limited to perform. At full sun, if the production is 230Volts and 13 Amperes, the set performs very well.

Or what kind of equipment would be convenient to match the resistances of panels and load for a given generation maximum power transfer ? When generation is say 1KW; to present a 1KW load.
 

Ian0

Joined Aug 7, 2020
4,838
Yes, in theory, but MPPT controllers are usually incorporated within battery chargers, so are optimised for fixed voltage not fixed resistance loads.
If you take an old fashioned approach that originated in the days of valves, and plot the 230V/13A=18ohms load-line on a I/V graph of the solar panel you will see how it quickly departs from the optimal load as the insolation varies.
The current varies linearly with the amount of insolation, but the voltage varies with its logarithm, so can almost be regarded as constant. In fact the voltage varies more with temperature than it does with insolation.
Ideally you could have ten 230V 100W immersion heaters and switch them in and out to keep the panel voltage constant, but that is a bit impractical.
A simple MPPT system uses a buck regulator with feedback to keep the INPUT voltage constant, and that will do a pretty good job of extracting the maximum power from the panel, but it really could do with temperature compensation.


PS. Long live the deciBel.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,817
Thanks, gentlemen.
That link hits the nail almost perfectly. The use is storage of hot water to feed exchanger circulation in house heating duct winter day and night from $0.3/Watt 400W panels array that runs airconditioning during daytime in summer. (no heat pump)
Perhaps a series of smaller heaters switched-on/off depending on sensing generated current, as that article suggests.

Edited... 5 of these 600W screwed in a tank and 5 controlling relays could work .
----> https://www.ebay.com/itm/1116882308...MIxNXd_LnW7gIV6-HACh3gpwyoEAQYAiABEgJXQvD_BwE

When current in first heater surpasses certain Amperes, the next heater gets its relay energized and so on... = variable load for variable generation.
 
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Ian0

Joined Aug 7, 2020
4,838
It's simpler than that - when the voltage exceeds Vmp (in @nsaspook 's article) then switch another heater on. If the voltage goes below Vmp switch a heater off. You can see that Vmp varies very little with light level. It does vary with temperature, so for an improved system, use a thermistor to reduce your reference voltage as the temperature increases. You'll need a bit of hysteresis so that the relays are not forever turning on and off.

If you've plenty of outputs a couple of dozen of these
https://www.rapidonline.com/arcol-hs100-470r-j-100w-aluminium-clad-resistor-62-8204
stuck to the outside of the tank under the insulation might work!
 

opera

Joined Jul 4, 2021
4
I've been doing it for years. The real question is why have panels just for water heating? I run heaters off the same raw panel volts as my charger. When the voltage exceeds the power point voltage, divert to heating water. Just a PWM off a capacitor bank. Need an inductor, not really. When delta V is kept really close losses are low. At high currents it is mostly pass thru. How do you know power point? It is an educated guess based on temperature. Sense a metal plate seeing the same sun. I use it at my seasonal camp and only use a fixed voltage for the season. Close enough to heat a lot of water. Garage washing machine has its own 40 gallon tank and all cycles use hot water. PV systems wast a lot of power just waiting for that rainy day. Arc interrupt can be added by including deadtime to PWM.
Current is a great way to switch in multiple heaters, voltage is not. Once an additional element is switched in, voltage drops dramatically. Current does not change. PWN high power is not electron appropriate. Sense current and then switch in additional heater.
 
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