Efficiency of Electrical Conversion

  • #1
I completed a solar energy lab, where we looked at power flows, which were represented through different parts of a circuit measured with 3 separate ammeters. We used four different modes and varied the irradiance and load.

only charging = energy from solar panel to circuit without battery switched on
charging + consumption = " " + battery switched on
discharging + consumption = some lights from solar panel switched off + battery switched on
only discharging = battery + solar panel switched off

I've had to calculate the overall efficiency of each setup too and have done so by:
[ (dis)charging + consumption ] / [ PV Power + (dis) charging ] = overall efficiency

I've attached the results to this thread, but I don't understand much about them and haven't been given any information about the theory behind it. If anyone could either point me in the direction of some literature, or help me answer the questions I have below I would very much appreciate it. Then I could interpret the data a bit better. :)

1) Why are only 2/4 (dis)charging results negative? The negative was a result of the current being negative, but I don't understand why 'only charging' and 'charging+consumption' had negative current, and 'discharging+consumption' and 'only discharging' had a positive current.

2) Is the overall efficiency different for each setup because of the battery using power generated by the solar panel? And because we changed the irradiance of the cell by switching some cells off?


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Answers and Replies

  • #2
Could you post a diagram of your setup, including the power measurement components? Also, what is the difference between "discharging" and "consumption"? Is discharging just self-discharging of the battery?
  • #3
Your terminology is counter-intuitive and confuses me. Is it possible that is the source of your uncertainty as well ?
only charging = energy from solar panel to circuit without battery switched on
I'd have thought "Charging" would refer to battery, so this would be panel to battery with circuit off.

charging + consumption = " " + battery switched on
Solar panel providing current for BOTH circuit AND charging battery??

discharging + consumption = some lights from solar panel switched off + battery switched on
Panel not keeping up with circuit so drawing some current from battery

only discharging = battery + solar panel switched off
If battery and solar panel are both switched off, from where does ANY current come ?

As Berkeman suggests, a picture is worth a thousand words...
  • #4
I've attached a photo of the circuit to this message. Sorry, writing is in czech! :)


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  • #5
spotrebic = appliance
stridac = inverters
  • #6
Ok, from what I understand now from reading a bit more about off-grid systems.

Charging = PV array is not powering anything
Charging + consumption = PV array is powering appliance
Discharging + consumption = PV array isn't totally irradiated, is still powering appliance
Only discharging = PV array isn't powering anything.
  • #7
Ahh i see .

I'm sorry if my post seemed impolite , it never dawned on me English might be a second language for you.

The picture and your additional posts helped a lot, thank you !

I often anthropomorphize things and talk them through as if i were a circuit element - "what do i see from its perspective ?"
Your regulator seems central so i'll pick his perspective.

Usually one figures 'efficiency' as Output/Input

and since there's no irradiance metering shown of the light striking the solar cell
i'd say solar panel power is always an input to your regulator

and since inverter doesn't have a meter on its output side,
i'd say power to the inverter is always an output from your regulator

and since power sometimes flows from regulator to battery and sometimes the other way
i'd say battery power is sometimes an output from your regulator and sometimes an input to it, and sometimes zero(neither input nor output).

And i'd calculate efficiency of the regulator for each case as ∑Output(s) / ∑Inputs(s)

If you do have an irradiance meter or know the input to your lamps you can put a number on the solar panel too.
  • #8
Thank you :)
output power = load, battery
input power = PV, battery
Now I just have to understand the distribution of power from the controller.
When discharging+consumption the battery and inverter (row 4, column 2 and 3) share an almost equal amount of power, and their signs are positive, while the PV has almost no power.
I assume this means the battery is outputting power to the controller, which in turn is distributing it to the inverter to power it, with small losses?

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