grid tie reccomendations are above 12 volts-why

Re: grid tie reccomendations are above 12 volts-why

Power = Voltage * Current

So, power can be 120 volts * 1 amp or 12 volts * 10 amps -- They both equal 120 Watts.

However, for wiring, transformers and such--they mostly are affected by current. When you cut the current by 1/2, you cut the area required by the wiring by 1/2 (~3 AWG smaller wire size) and you cut the size of the transformer iron, etc.

On the other side, as you increase voltage, you need better insulation and the very small dimensions in transistors/electronics make for very high voltage/inch numbers. In fact, for high power/voltage transistors, the older semiconductor processes from 20+ years ago are actually better than the new processes that use smaller dimensions/insulation thickness, etc. (makes it easier for excessive/high voltages to "punch through" semiconductor insulation).

36 volts was more used in marine industry (boats). Also, I think I have seen golf carts and possibly other electric vehicles use 36 volts even today.

32 volts was actually (as far as I can tell) very popular for farms/pre-Rural Electrification programs (and old rail road passenger cars too). There was a whole bunch of 8 volt batteries, 32 volt tools and appliances (even toasters) that were powered by generators and wind turbines.

But today... Notice that we use 6/12/24/48 volts as our basic building blocks. Each is 2x higher than the lower voltage before it. Frankly, 36 volts is only a 1/2 step change (but still somewhat popular in golf carts--I believe). Note that building higher voltage transistor switches is more expensive and less popular/volume than the lower voltage products.

This thread has some historical information and pictures in it:

Why are standard battery bank voltages so low?

-Bill

Re: grid tie reccomendations are above 12 volts-why

The general 140-150 limitation on most MPPT charge controller is the voltage rating of input electronics.

Why does the C40 PWM controller have such a high voltage rating when the maximum "nomimal"/"Useful" input voltage is around 80-90VDC... Probably because they had a choice of 80 volt or 140 volt components and just chose the next higher voltage rating (may have even been cheaper for higher volume/higher voltage components).

In any case, a C40 type PWM charger should follow the normal PWM controller rules for best efficiency. roughly 17.5/12; 35/24; 70/48 Vmp/Vatt ratios.

Yes, you could run a Vmp of ~100 volts on the C40 with a 12 to 48 volt battery bank, but you will still lose lots of power due to the mismatch of Vmp to Vbatt-charging.

Nothing is "for free".

-Bill

Re: grid tie reccomendations are above 12 volts-why

Actually, that is not a bad analogy... Have a hose with the water valve cracked open--flows 2 gpm onto the ground with almost no force. (all flow, no pressure)

Put your thumb over the hose, it still flows at 2 gallons per minute but will shoot 10' through the air--more "energy" drawn from the faucet/flow. (optimum flow * optimum pressure = maximum energy available from the faucet/hose system).

Keep clamping down more on the end of the hose, the water flow decreases and the total amount of energy drops as the flow drops to zero (no flow, no energy).

-Bill

Re: grid tie reccomendations are above 12 volts-why

The bucket fill almost the same speed if your thumb is there or not (say you have 100 psi water, and your thumb can only block 20-40 PSI worth of pressure, without using a real valve/nozzle).

Anyway--Look at the equation for power again:

Power = Voltage (aka pressure) * Current (aka water flow)

Depending on the energy/water source, V and C may change with back pressure--The MPPT controller just finds the "optimum" point where restricting water flow brings the pressure up. Past some point, you restrict so much water flow, the power curve falls again.

If Volts/pressure goes to zero (open hose), there is no power developed.

If the current/water flow goes to zero (blocked hose), there is no power developed either.

-Bill