Even more solar power

Even more solar power

So far we had two rather small setups that would be enough for auxilary building but not really enough for a (tiny) house. So lets make an even bigger setup and add more solar power.

One of the things that I keep wondering myself about is that most people seem to stick with a 12V system. This is a bad idea since the amount of amps needed becomes higher and higher. So why not use a 24Volt or a 48Volt system. Very large scale could even go higher with the DC Voltage. Products for 24- and 48Volt systems are easy to acquire nowadays and the price is very reasonable.

If 12V is used, it takes 100A to provide 1200W, now if the voltage goes up to 48Volt we can use upto 4800Watts and still keep those 100A on the DC side.

New Shopping list:

  • More and bigger solar panels
  • An 48Volt charge controller (preferable MPPT)
  • More batteries
  • An inverter from 48Vdc to 110Vac (lets say 5KW)

In this bigger setup using an MPPT controller brings a lot of extra efficiency. A MPPT controller is a variable DC-DC converter that finds the optimal conversion for the current state of the batteries.

To reach 48V in batteries it nessecary to make a (serial) chain of batteries. That can be:

  • Four 12Volt Batteries
  • Eight 6Volt Batteries as found in golf carts.
  • Twentyfour 2Volt Batteries, Single cell from 200Ah upto 2000+ for large capacities.

For this example lets stick to the 12Volt / 200Ah batteries, now just use four pieces. The battery bank can store 48V x 200Ah =  9.6kWh so almost 2kWh of daily use. This is sufficient for a (tiny) household. If needed extra strings of batteries can be added in parallel or use eight 6 Volt 390Ah golf cart batteries.

One string of batteries requires the same amount of current as in the 12Volt setup, so between 16.7 and 25 Amps. But we need it now to be 48Volt.
Bigger PV-panels also have a voltage higher output simply because they are build with more cells.  An average 250Wp panel has ratings of 30Volt and 8.2Amps so we need at least four.

A good MPPT controller can handle upto 150Volt at it’s PV-input so we can put the whole series of 4 panels in 1 string. The output of the string wil be 120V at 8.2A but the MPPT controller converts this to the battery voltage. Ohms law dictates that the power will be the same so the charge current has to go up since the voltage drops. Voltage drops 2.4 times, so current wil increase by this number getting 8.2 x 2.4 = 19.7A (Due to losses and higher charge voltage the reality wil be a fraction lower, but this is in theory).

Charging 19.7A is well within the batteries charging margin so perfect.

Download the PDF

 

Notes:

Comment the inverter directly on the batteries with thick (supplied with the inverter) wires and keep those as short as possible. If no wires are supplied check the correct guage The charge controller only has 20Amps output which is next to nothing compared to the 100+ Amps that lead to the converter.

Remember to ground the required parts.

 

Now what about 12Volt ?

Even in larger set-ups with operating voltage of 48 Volt (or 24V) a demand for some 12V might be there. LED lighting for example. Far better than using just one battery is the solution to use a DC-DC step-down converter from 48v to 12V. Good available models go up to 20Amps, so upto 240Watts of LED lighting can be connected. The unloaded loss in these DC-DC converters is very low (1-2 Watts)

DC DC Step-down
DC-DC Step-down Converter 48V to 12V / 20A

 

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