Solar Electricity

[bokbadok]

Does anyone have experience with solar collectors as a backup electricity source? It seems to me to be the ideal system (with generator backup) because there is no fuel to store.

[magnet]

It is quite simple to build a solar power project to store electricity. I will probably try my first attempt at it within the year. However the cost of buying the components seem a bit pricy. I understand the prices are a bit lower and the technology has shot up, so I can't wait to get started. My basic goal is to have a battery bank complete with converter and the works for an emergency power supply to run small appliances, radios, etc. My only big item would be a water pump for my well. Converting to a DC pump will do the trick much better by drawing less energy from the bank. With limited running water and my air tight wood stove I could easily survive our cold canadian winter. Of course along with my food storage supply, and all the rest.

[Julie]

I use solar up to my ranch and it is ideal. As soon as my son gets home from his mission (2 weeks - but whose counting:l0 (20):) I am going to have the rest of my cabin set up. I have a freezer that will run off a 75 watt pannel and it is a good sized freezer. I can't wait to hook that up. If you shop carefully, you can find what you need for good prices.
If there is no money to pay your power bill, it would be nice to have free electricity to run your house.

[bokbadok]

I'd like more information about the "if you shop carefully" issue, if you don't mind, Julie. That's where my plan ends - I have no idea how to go about shopping for components to a system. I've done some reading and have a general idea of what's needed, but I worry about getting taken advantage of because I still know so little. Where do I start?

[BackBlast]

I'd like more information about the "if you shop carefully" issue, if you don't mind, Julie. That's where my plan ends - I have no idea how to go about shopping for components to a system. I've done some reading and have a general idea of what's needed, but I worry about getting taken advantage of because I still know so little. Where do I start?

First you need to size the system you want. What devices do you want to run, how often, etc. Next you need to figure out the power requirements for those devices. You can purchase a Kill-a-Watt device that will measure power consumption at the wall.

Essentially you want to break this down to kilowatt hours/day. Then you can decide the size of battery bank, solar array, and inverter you need to make it work.

Brandon

[BackBlast]

Subsequent steps are to pick the major components of the system.

Lets say you want to occasionally run a wheat grinder, charge batteries for flashlights and radios, provide some limited fixed lighting, and perhaps charge some battery powered power tools. This would be a small system to run essentials in an emergency.

Your biggest energy consumer is the wheat grinder, so a cheap 25$ 400 watt inverter will suit you. You'll probably only use a grinder for a few minutes a day, lets say 12 minutes or 0.2 hours. 250 watts (my grinder) at 0.2 hours is 50 watt hours. You charge 6 AA cells per day, and one power tool. I'm guessing 15 watt hours for the AA cells, and 40 for the power tool. Lets say 4 hours of fixed lighting at 25 watts. This gives us ~200 watt hours of power required per day.

In my location, I believe I see about 5 full sun-hours per day in the winter (rough guess, if I were to finalize this sytem I would verify this more closely). So I need a 40 watt solar panel minimum to take care of my daily requirements. I should oversize this slightly to make up for poor days and days of extra use, and losses introduced by the lines, panel degredation, and main bank battery charging. So call it 50-60 watts. This will provide me on average 250-300 watt hours of energy per day during the poor part of the year.

Now, because I want to use my fixed lighting at night, and my power requirements are above the output capacity of the panel, I need a battery bank.

Traditional solar battery systems are lead-acid. Lead acid cells like to remain fully charged, this is because the process of discharge destroys the plate and recharge reforms the plate. This process causes wear. High depth of discharge is undesireable. To maximize the life of the bank I should size the system so I do not go below 50% depth of charge, preferably 75% depth of charge. Which means I need at least 400 watt hours of capacity and preferably 800. I'm using a 12v system, as is very common for small to medium system, so 400/12 = 33 ah minimum and 800/12 = 66 ah preferred. However I should also consider the speed of discharge, the grinder at 250 watts will be pulling at least 20 Amps from the cells. Lead acid batteries are usually rated at C/8 or 1/8th capacity drain. Or 33 ah/8 = 4.1 Amps to get rated capacity, 66ah/8 = 8.2 Amps to get rated capacity. This really makes the 33 ah cell too small, it will have a compromised lifespan. The 66 ah cell would probably serve, but we might do well to oversize this a little bit to 75-80AH to make the load a little nicer to our cells. Using the grinder during full sunlight will also help the cells since the panel itself will help with the load and it won't be pure battery power.

The estimated price of our system?

60 watt solar panel: $350
>80ah AGM battery: $100
2 400 watt inverters: $50
Misc cables, connectors, and a float charger: $50

Total: $550

Gotchas

When installing, you want to maximize solar energy on the panel (obviously). That means a good angle, no trees in the way, etc.

The battery system likes room temperature to a little bit cool. Too cold and it will have dramatically reduced capacity and you do not want to charge the cells below freezing. So, be mindful when you pick your battery bank location. The battery bank will last about 4-7 years before it needs to be replace. If using a multi cell bank you should not mix old and new cells - they must be replaced as a set. You do not want to leave a battery sitting for long periods - it needs to be trickle charged constantly to maximize lifespan. It's total lifespan clock is started the second it's done being manufactured - you cannot store a spare bank in the next room.

A cigarette lighter 12v plug will not supply the 20A required to run the grinder - you need a more direct connection for the inverter to run the grinder right.

Alternatives:

You could omit the battery bank and run direct drive off the panel. If you have a generator, you can use that to power the grinder and run the grinder for an hour or so to get a weeks worth of flour. This brings your power requirements down to about 60 watts at a time, but you also get no fixed lighting options and will need to rely 100% on flashlights and similar. You can also revert to this operating method if your battery bank fails.

You could opt for a NiCD battery bank system. The pocket plates are not destroyed during discharge, cells have expected service lives of 25 years commonly. 50 year old cells have been found to work. It's more robust so you do not need to oversize the battery bank so extensively, and retains capacity well into freezing temperatures. Cells of different ages can be swapped in and out to keep a bank running. The con is it's expensive. Used cells are 2x the cost of a new lead acid system -- if you can find them (Though if you don't oversize the battery - the battery cost may be nearly the same). New cells are prohibitively expensive. You may need to replace the electrolyte in used cells to restore capacity. Nasty chemicals are involved in this process. Better understanding of cell maintenance is required - how and when to water cells.

You may notice that if you include some large appliances, like a freezer, your power requirements go way up and thus the cost of your system. You will quickly realize that it's worth investing in extreemly power efficient appliances to minimize your power requirements.

You can go with a system that is not oversized if you plan on skimping on power when you're not getting good solar days. This requires you to be aware of your battery charge level and to skip out of the things you can do without. Probably using the fixed lighting less at night or skipping a power tool charge. Unfortunately less solar means less light, and will usually mean more desire to run the fixed lighting. So the nature of the weather is working against you.

Summary:

You may note that the largest initial cost of the system is the panel. This is true - but if you run the numbers over the life of the panel, the battery bank ultimately drives cost. You can expect the panel to give good service for 20-30 years. A lead acid battery bank, only 7 when you do your part.

This is not "free" power, it cost money. Lets say you replace the bank 3 times and use it for 30 years.

4*100 = $400 battery costs for $850 total system cost. 30*365*400 = 4,380 kwh or $0.195/kwh. Grid power runs around $0.08-0.10. So we're about double the current cost of grid power with this sytem. You may ask where the 400 comes form, this is a guess at the daily average production number in watts hours of the 60 watt panel. You'll make more in the summer than the winter where our panel was sized. You may also note that if you omit the battery cost, you come very close to grid prices. In good areas with good sun you can beat grid prices and save money.

I hope that provides a better picture of the process. My numbers are rough for sake of time, so I would not garantee them. I believe them to be close though.