The Solar Power Expert Blog

Oversizing array and reducing battery cost

by GuestPoster - January 28th, 2013.
Filed under: Guest Posts.

Hi everyone,

I’m working on a oversized solar project to reduce battery size and winter geektime privation 😉

Let me explain.

First its not mppt, neither pwm. only shunt at the moment wich is not cool
Second I’d like to handle as many PV as I want by adding with the needed relay boards. wich is cool

I live in a off-grid place, so I listen to solar pro and bought 800w
of solar pannel, a 30A 24v mppt controler and a 275Ah 24v battery.

Most of the time its perfect. but sometime in winter, it can be cloudy for weeks.

On mppt controler, its specified X watts max and you can configure the charge rate so if you dont have a big battery no problem.
But they are expensive, no modular and most of them didnt even monitor and log.
worst you must absolutly have exactly the same panel spec and the same orientation in order to calcul the mpp. so you can forget second hand,or a new brand lower in price to complete your system later.

For exemple with my system, now in winter, with a bright day I only charge 15A. I could add twice the number of panel but in a month, they will produce more than 30A and the controler will likely cut all the
input or even die.

So I was thinking to throw my controler away and use a diversion load when batteries are full, just like a windmill.
But, you cant charge a battery at more than 10% of its capacity. So this idea doesnt work as I cant add more panel either.

Well I thought we can use relais to plug/unplug dynamicaly the number
of panel needed to stay the closest to 30A at possible (panel are cheap now, under 1$ /watt so I can by a lot more, than battery and expensive controler, they live 30 years, controleur 10 and battery even less)

SO in winter, let say I have 3kw pluged and in cloudy days I charge.
In half season some pannel are pluged the morning but unpluged the afternoon. I charge.
In summer nearly all panel are unpluged but I still charge. full battery @ 8am :)

As the relay board have 3 holes (common no nf), we can use it to charge the battery or to dump to a load (heating water or being connected to a inverter to
get the full 3kw power if we need it)

The relay board would be controled either by a arduino or a raspberry (I prefer raspi as I can do mesurement, relay control, distant acces, remote webcam etc all in one)

I was thinking to use a tinkerforge voltagecurrent bricklet and a temp sensor as well. this is cheap modulable opensource hardware/software.

Relay board from denkovi (4 8 ou 16 relay, more relay more precise would be the control as we could put les amps on each) so there will be not much electric arc and we avoid the high cost mosfet relay.

Everybody tells me, dont bother,buy a bigger controler, but the point is not here.
The point is to do a as more opensource as possible hardware/software, modular cheap controler either for solar windturbine or hydrogenerator.

What would be cool is also to find a way to use the same inverter, from the battery and from the diversion load, so at day time, we dont use the battery at all. but the problem is that the open voltage will
make the inverter suffer for a millisec each time we start using it, and condensator can really help with the battery but not with a open circuit solar array.

Like that a person who want to be off grid can just have a small battery who hold only 1 day of power. Its reducing the initial and maintenance cost tu up to 80% over 30 years of lifetime.
Its greener as battery are not green thing
With a device like this we can give acces to solar to everybody, not only well payd people, and we put less polluant in rivers 😉

What do you think about that? I really like you to show me the right way on this project.

thanks all


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