what are some methods for capturing low voltage electricity from solar panels?
Most solar panels (the flat ones that go onto roofs of houses) reject electricity produced by the panels if they are too low in voltage. For example, on a cloudy day, although the panels may still get some sunlight to produce some electricity, it is rejected because its too low in voltage. So, does anyone have any ideas?
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I have a 2.25KW system which uses micro inverters. Instead of one inverter powered by all panels, each (Canadian Solar CSp6-225W 36V no load 225W) PV panel has its own grid tie micro inverter. Each PV cell acts as its own independent system and avoids issues a traditional single inverter system would have. Single inverter systems must rely upon all of the PV cells in the array operating under identical conditions. If not then the entire array gets pulled down to the level of the weakest link. (A single PV cell partially shaded takes the whole system down for example)
http://www.enphaseenergy.com/products/moreinfo/howitworks.cfm
Each inverter is operated with a large voltage overhead which allows for low power operation and allows the panel to supply as little as 2W. My 10 panel system is being logged as coming on line with less than 8W yield and peaks over 2KW in bright sun.
Check out the logs:
http://enlighten.enphaseenergy.com/public/systems/xRm89776
http://www.enphaseenergy.com/products/index.cfm
Put one more panel in series with other one to increase the voltage output during cloudy day.
The suggestion above is very commonly applied in installations. Also, consider using power electronics to boost the voltage to a usable level, either via charge pumps or inductive boost.
On a day that’s really dark, I don’t think there is anything that can reasonably be done. The voltage is low because the power coming out of the panels is low. A circuit could step that voltage up, but could not increase the power.
According to our user’s manual, it takes 30 to 40 watts to run our inverter. So if the panels are putting out more than that, it’s worth running the inverter. If not, then it’s better to not even run it, so the inverter cuts out. A charge controller for a battery would have similar behavior if well-designed.
Our array is large enough that during even relatively dark days, there is enough power so that the inverter won’t cut out.
What about a battery bank as a sink? By that, I mean the batteries absorb the low/high voltage, and if the solar panels do cut out, then the batteries pick up the load and keep the inverter going. Just a thought.