Combining DC power input from multiple erratic sources (solar / wind turbine)

Question

My understanding level: I'm reasonably familiar with simple electrics and electronics - volts, amps, watts, simple circuits etc.

I'm thinking my question is either really simple, or complicated - but hoping it's simple.

So I have a 2400W inverter for converting battery to AC. The inverter will use grid power when battery is low. It will not feed excess energy back to the grid.

Batteries are currently 2 x 12V 100 Ah deep-cycle lead-acid connected in series. (The inverter takes 24V battery input). (I have another 2, but have not plugged them in yet because of concerns with plugging batteries in parallel.)

The inverter has a 600W solar charger which supplements the battery power - and the inverter will run off the solar / battery when the solar is generating power. The panels can nominally product 660W, but generally (so far) have been making 400W at best. (I'm just coming out of winter.) Excess solar is sent to the battery.

I have an Arduino, with voltmeter and ammeter electronics that I made to measure the solar output.

To this setup I wish to add a small (24V, 400W max) wind turbine. The turbine has a MPPT charge controller. My goal at this point is to feed this into the system, and at the same time measure the size, and time-of-day of the output, comparing that to solar, to determine whether I should invest in more solar or more wind.

Obviously I'd prefer not to just "dump" the wind power right now - I'd like to feed it into the system. My instinct is to simply plug the output from the turbine to the batteries, but I'm not sure this is electronically correct, and I'm hoping for circuit advice here.

1. Of course the batteries are not exactly 24V, but a bit more - ditto with the turbine. So how will that affect the inverter - and specifically would the inverter potentially "not notice" the battery was getting flat? (I don't want the battery to drop below about 60%).

2. I presume the MPPT would prevent the battery from driving the wind turbine as a motor - or should I add a diode to prevent that?

Am I even a little bit on the right track?

Electrically, my question I guess is as follows - given multiple power sources (battery, solar, wind) all with slightly different voltages, and different power outputs, can I combine them to create one "source" for the inverter? What would this circuit look like?

As an analogy if I was doing this with water I could combine rain-water with ground water with mains water into a single tank, which then aggregated those supplies into a single source for say my garden.

Answer 1

The drawback in connecting the two MPPT chargers is that you have two contradictory purposes: (a) you want to use the wind power, not dump it and (b) you want to monitor the energy production of the two systems.

These contradict because MPPT controllers are only truly MPPT when there is adequate demand. Otherwise they throttle back, to protect batteries against overcharging.

In which situation : how do you tell from the measured results. whether the wind turbine isn't generating because there isn't wind, or because the batteries are charged? (Same applies to solar after a stormy night).

I think you have two options : either set up a separate system using the alternate batteries, and load both systems to give each a fair trial.

Or set up an intelligent load : perhaps a water heater? that cuts in about the time the batteries drop into float (or controlled by knowledge of the MPPT charger outputs) to maintain enough load to keep both systems fully employed. In which case you should be fine to connect both MPPT chargers.

Answer 2

The big unknown here is that you have two MPPT chargers, one for the solar and one for the wind. These are fairly complicated devices with a microprocessor inside making decisions, both with regard to finding the maximum power point of the input and assessing the state of charge of the battery on the output.

The good news is that they should themselves act as diodes and not allow backfeed to the generators.

As you say, the most natural approach is to connect both sets of chargers to the battery and let the voltage find its own level. If power out > power in then this voltage will be determined by the battery state of charge. On the other hand, if power in > power out the battery will be charging but you can't necessarily say how charged it currently is.

All I can say is to check the manuals for the two chargers to see if they have any guidance on connecting to a battery with another source of charging. If they say not to do this, then don't. If they say how to do this, you're fine. If they say nothing (the most likely outcome), you have to decide whether you feel lucky - it will probably work fine but nothing is guaranteed any more.