PV Gap

Darren Said:

Do you have an off-grid home in a cloudy, rainy climate? Would solar panels be worth it?

We Answered:

There are a lot of off-grid houses in Oregon, and many sell their power back during the sunny days to balance the rest of the time. Just do a search on green houses in Oregon to find some of the projects listed. There are also cob houses in Coquille that are quite interesting. But at present. it costs more to get these off grid houses built than they recover. Most folks just starting out can't afford to do it.

I'm looking for ways and colaborators to take an existing house and pull it off the grid - affordably. Have lots of ideas, but little free time to play with doing them. Would love to share ideas.

Gloria Said:

Items needed for an off grid solar panel?

We Answered:

Firstly you need to set a budget, both for $$$$$ and power requirements. This will tell you how much money you need to spend and whether you can afford it.

This exercise will lead you to what scale of system you need: how big an inverter, battery capacity, solar panel output etc.

If that doesn't put you off the project, the only extras you might need are a switch to prevent battery overcharge and overdrain (cuts off if voltage is not within preset limits) These are called regulators and could be sourced from auto spares.

Below are a couple of links you might be interested in.


http://www.rpc.com.au/company.html

http://www.solaronline.com.au/

Hazel Said:

How can I live off grid without solar panels or wind generators? And no septic systems for sewage?

We Answered:

How about a hydro power system. A stream, diversion, penstock, and a turbine. Thats how the old miners did it.
http://www.aurorapower.net/alternative-e…

You could also utilize geothermal heating.
http://www.aurorapower.net/alternative-e…

The answer your teach might be fishing for is a bio-digester. Those teachers are always encouraging creativity arnt they. Basically waste gets turned into methane which gets burned for heat or generates electricity in a turbine. Looks like these guys know how its done:
http://www.valleyairsolutions.com/methan…

Best of luck to you!

Hilda Said:

how to select a suitable battery for a off grid solar PV system and which is suitable tubular or flat plate?

We Answered:

The battery capacity is determined by the load and the number of days it will work without the sun.

Battery capacity is usually determined in ampere hours. First determine what load you have in Watt hours or kilowatt hours over a selected 24 hour period. If you have 800W and it is on for 4 hours a day, that is 3200Wh. Allow a figure for the losses between the battery and the load. This is cables and inverter. Maybe 20%. So now the battery has to supply 4000Wh.

Now what is the nominal battery voltage, under load, and not under charge. Lets say it is a 24V battery which is more suitable for this load than a 12V battery. The capacity is 4000Wh/24V = 167Ah. That is a battery large enough to carry the load for a single day. Now allow for a number of days without charging (no sun). This might end up being a compromise. Perhaps 5 days. The total capacity is then 5 x 167 = 835Ah. Perhaps the next nearest size is 1000Ah capacity at 24 Volts.

Checking the maximum load current...
The peak load is at least 800W (we started with that) but maybe there are short term peaks that are larger while the fridge starts etc. Let's say 1600W. This represents a current of 67A with a 24V battery. A 1000Ah battery should easily handle that, as it is less than 1/10 of its capacity. Cables and circuit breakers and inverter must be sized for the 800W load, but also capable of handling this surge. Usually the voltage drop determines what size cable, rather than the current carrying capability of the cable, so it will be bigger than the 33A cable expected to carry 800W. Use the 67A figure to calculate voltage drop.

At this size I doubt there are any tubular types of batteries. I think that is more a personal choice than anything else. I am not a fan of sealed lead acid batteries. I would tend to use a flooded electrolyte type intended for deep cycle usage, especially in a hot climate. Note that batteries need to be well ventilated (hydrogen).

The solar panels themselves are a separate issue, used to recharge the batteries. There is a loss due to the battery chemistry. Consider using a Maximum Power Point (MPP) charger for efficiency. The panels have to charge the 24V battery with 1.4 x 167Ah = 234Ah per day minimum. This charge has to occur during the equivalent full sun hours in your location. It is a good idea to allow more, for aging, losses, rainy days etc. This tends to be all about the cost in the end. For this case you need at least 234Ah per day, so if there is 5 equivalent hours that is 46.8A from the panels for 5h per day. This is 4 sets of 12A panels in parallel.

Deanna Said:

How do I figure out how many solar panels are needed for off grid solar system?

We Answered:

look at your electric bill and see how many kilowatt hours ( kWh) you use a month

divide by 30 for daily use. Most people are in the 20kW a day range

So you need to put that much into a battery bank each day

the Dept of Energy has a calculator for how much usable sun a day you have where you live ( like say 8 hours a day year round in Key West, 3 hours a day average in Seattle)

4 hours a day? you need a 5 kW system...more is better for there are various losses and inefficiencies to deal with.......

panels: this would mean about 30 2x3 foot 175 watt panels; add wire, battery bank, inverters to take the 14 volt of the battery system back up to 1120 volt home use......add 10% for "stuff".......and then the labor for the carpenters roofers and electricians to put it all in, plans permits and gov regulations.....

and time and again the number always comes out to about $6.00 a Watt installed..

so for a 5,000 Watt system plan on $30,000.00