PV --> Converter --> Inverter for short periods of direct sunlight

Caneman
Caneman Solar Expert Posts: 71 ✭✭✭✭
Not sure why this would not work to connect a PV to a Converter then to an Inverter for short periods of time:

PV output = 30 to 38 v, 8 amps (1 panel)
Converter = input voltage range is 30-60 vdc, output voltage is 12vdc +/- 2%
Inverter = 12vdc to 120vac

If you can have a steady sunlight source for a few hours, why wouldn't this work to generate a few hours of 150w of power during the day? Seems like the converter can handle the swings in voltage and feed the inverter with a steady supply of 12dc?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: PV --> Converter --> Inverter for short periods of direct sunlight

    Without battery? No battery = no power.

    PV's output is not a stable Voltage source; it is a current source that allows Voltage to drift from zero to Voc. Quite simply the panel will not put out "30 to 38 Volts @ 8 Amps".

    So there is nothing to mitigate the difference in varying supplied power and varying load demands.

    That's the second time today I've written that too, so you're not alone. ;)
  • Caneman
    Caneman Solar Expert Posts: 71 ✭✭✭✭
    Re: PV --> Converter --> Inverter for short periods of direct sunlight

    I guess I don't really understand how the panel works... if the sun is hitting it directly during the "10-2" peak hours won't the voltage/current be fairly stable?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: PV --> Converter --> Inverter for short periods of direct sunlight
    Caneman wrote: »
    I guess I don't really understand how the panel works... if the sun is hitting it directly during the "10-2" peak hours won't the voltage/current be fairly stable?

    No, because part of the formula is how much load is on it.
    The power output of a PV is a bell curve over time, peaking at midday (and also the Voltage being flattened by the higher temp).
    From Voc @ 0 Amps in the first morning light to a point where it hits Vmp @ Imp (ideally; it rarely happens in real life and doesn't last long when it does) to maintaining Imp as Voltage drops from increasing heat to a decline in current as the sun goes down (with possible lowering of temperatures resulting in Voltage increase) the power output is all over the place if left on its own. Change the resistance across the leads (load) at any time and the current will vary; if there is no load there is no current and this can occur at any time during the day.

    So if it doesn't have the battery to pump power in to its output becomes very unstable. It's like having a power supply whose output is all over the place. When you do have the battery then whatever power the PV can produce at whatever time can be "stored up" and released as needed by varying loads.

    There are direct from solar applications such as pumps and fans. They rely on linear current boosters and the fact the motors do not have to run at a consistent speed. For powering "unknown" (variable) loads direct-from-PV doesn't work.
  • marsofold
    marsofold Solar Expert Posts: 45 ✭✭
    Re: PV --> Converter --> Inverter for short periods of direct sunlight

    Could the poster's intent be approximately satisfied by utilizing an absolute minimum battery just to keep the solar charge controller running, but use 90%+ of the charge controller's output to run loads?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: PV --> Converter --> Inverter for short periods of direct sunlight
    marsofold wrote: »
    Could the poster's intent be approximately satisfied by utilizing an absolute minimum battery just to keep the solar charge controller running, but use 90%+ of the charge controller's output to run loads?

    This can be done but ... there is the potential problem of the controller providing too much current to the battery unless it is limited/controlled somehow. And that means possibly limiting the current available to run loads. You don't want some massive C/# charge rate hitting the battery. Probably the best solution there is to use a dump controller like you would with wind/hydro. That's more money and complexity, and you'd be better off buying a bigger battery to begin with.
  • mejustme
    mejustme Registered Users Posts: 17
    Re: PV --> Converter --> Inverter for short periods of direct sunlight

    Hi

    While using an inverter without a battery is impractical for most things.

    It MIGHT work to power something that only requires power for a short period. I would not want to try anything that was voltage sensitive though.

    What DOES work is using 12 volt appliances directly from a panel but again as long as it is a purely resistive load and you really have to use a higher rated panel (match to the amps x 12 instead of the 18 or whatever it is)....of course do this at your own risk but it is fine for things like 12 volt cooking.
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: PV --> Converter --> Inverter for short periods of direct sunlight
    marsofold wrote: »
    Could the poster's intent be approximately satisfied by utilizing an absolute minimum battery just to keep the solar charge controller running, but use 90%+ of the charge controller's output to run loads?

    You could but you'd need a way to dump the extra power - you can't just put 100 amps into a 10 amp hour battery and expect it to survive. What you really want is a charge controller that has two current sensors, one of which is used to limit current to the battery. I don't know of any such charge controller though.
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    Re: PV --> Converter --> Inverter for short periods of direct sunlight
    You could but you'd need a way to dump the extra power - you can't just put 100 amps into a 10 amp hour battery and expect it to survive.

    That might be fun to watch, from a distance!