Let's look at a wind and/or solar power experiment

Factual Questions
1

First of all, this is only a hobby project, not a serious one to replace my connection with the grid.

I have always been interested in solar and wind power for residential use, and I thought I’d just do some experiments. The result could be useful, but I don’t intend for it to be an all-comprehensive solution to lengthy power outages.

Let’s say I want to erect a small wind turbine. I could put it on top of a 25-ft pole, braced against my house, and have it charge a deep-cycle, 12v lead-acid battery. Cost of 650 watt turbine, battery, charging circuitry and mounting, $1000.

Then, if the power went out, I could take the battery, connect it to an inverter, and use it to power essential AC appliances like a computer, refrigerator or furnace blower, disconnected from the grid, of course. Cost of 1KW inverter, $200.

Meanwhile, a second battery could be charging from the wind generator as long as the wind was blowing.

So see if I have my math right. A 20AH 12v battery could provide 240 Watt-hours of power, neglecting losses in the system and decline of battery juice during discharge, of course. So I build in a 100% allowance for that. A 20AH batt could provide 120WH of practical power.

In addition, I could set up a similar arrangement with a 1KW solar panel, charging a 12v battery. Give me sun or wind, I’m covered!

Does this make any sense? Does my math work?

2

120 WH isn’t all that much energy. I don’t know what your power requirements are, but you mentioned a 1 kW inverter: 120 WH means you’ll only be able to sustain that 1 kW for less than 10 minutes. Even taking the 1 kW as a peak power and an average somewhat lower than that, it won’t last long.

3

  1. You don’t want to deep-cycle (100% to 0% to 100% to 0%) lead-acid batteries like that or it’ll significantly affect their capacity. A charge controller and low voltage disconnect can help with this. Helpful page: Photovoltaics on Appropedia (A DIY wiki for renewable energy). Ideally you want a bank of batteries properly sized to your generation capacity and estimated load; keep in mind that’ll you have to replace them every few years or so.

  2. Make sure solar and/or wind even make sense in your area. For wind, you can look at regional wind maps. For solar, you can look for online solar calculators and/or use a solar pathfinder to estimate annual/seasonal shading for any particular installation site.

  3. Unless off-grid usage is important to you, you might consider a grid-intertied system. In states/areas that allow this setup, it’ll roll back your electric meter and potentially even get you refund checks from the local utility.

  4. 120Wh of power is not a lot at all. It’s lot of work and money for a few hours of low-usage electricity. It’ll power a laptop for maybe 3 hours, a refrigerator for maybe half that (when it’s actively cooling) unless you have SunFrost or similar.

  5. For that amount of money, you would probably get more bang for your buck to just buy a bank of batteries (or maybe Uninterruptible Power Supplies, which are basically batteries enclosed in a charge controller shell) and keep them charged from the grid.

4

Indeed - it will light a 60-watt bulb for 2 hours. Seems a small return of the investment.

5

Yeah, the battery in my computer UPS (800 VA) is rated at 164 watt-hours; you’d want far more battery capacity, probably enough for at least several hours, maybe even an entire day, which would require 30 kWh, assuming 80% efficiency, double that if you want 100% redundancy; you’d also want to double the power output of the generator/solar panel so it can supply your needs and recharge the batteries, plus an additional factor for actual output, which can be very low; even large wind farms average only 15-30% of rated output, solar cells have similar figures, a 2.2 kW installation may produce 5 kWh per day, so for 1.25 kW continuous (30 kWh per day*, including charger and inverter losses), you’d need about 8.33 kW of wind turbines (averaging 15% peak) or 13.2 kW of solar panels.

*The average American home uses about 32 kWh per day, which would be about 40 kWh or 33% higher with losses included, but I suppose you could get by with less by using more efficient appliances and/or more efficient inverter/charging system (the battery itself has losses however, but it can approach 100% with good battery and charger designs).

6

You’re “covered” in the sense that you could power a couple light bulbs for an hour, or maybe make two pieces of toast before switching batteries.

To put it in an economic perspective, you’d spend about $1,500-2,000 for your system, which would then pay you back in little 3-cent chunks of electricity. Granted during a power outage a few hours of light can seem like a luxury, but a $500 generator from Lowe’s would give you way more useful power, and the 300 gallons or so of fuel you could buy with the savings over your solar/wind system would last you through quite a few power outages.

Outages only happen maybe a couple days a year, meaning you have a good 360 days to store up power within your system. Why not gradually charge a much larger bank of batteries (or gradually produce a bunch of hydrogen, or pump water up a hill, or however else you can think to store energy) to give you a larger store of power to tap into?

ETA: If you kept track of what you produced and just put it back into the grid, you could calculate how much money you saved on your electric bill that month. Put that money in an account, and when the power goes out use that money to buy gas for the generator.

7

One reason is that the cost of implementing any of these energy-storage schemes tends to be high.

This is quite a bit more practical.

8

One must look at what electrical equipment is important to you. i would say the ability to switch on a light is worth a lot to me, while the ability to run a microwave oven isn’t. Maybe a lap top is important but energy guzzling heat, which can be got elsewhere, isn’t. I can even see a case for installing 12v lighting and not much else.

In the UK the government recently had a “scheme” where by one borrowed cash to buy solar panels against what you fed back into the system while getting credits to future electricity. I usually ask how its going and often he says “the meter is going backwards and i like that!”…so the national grid is used as a battery without much expense but obviously it will go down with all the rest of the grid if that happens. In theory this is the sunniest place in the UK,but even on a very good year it is very very far from most places considered sunny!

9

I think many are missing the point. It’s not to provide a cost-effective, complete, or even efficient solution. It’s a hobby idea, an experiment, something to play with and gain a little experience on alternative energy hookups. I’ve already discarded the idea of a whole-house solution other than a backup gas generator.

If I’m without power in February for 2 days, as long as I have fireplace wood, it’s not a life-threatening situation, just a serious inconvenience.

Feeding the electricity back to the grid isn’t a bad idea, but it would take additional hardware, and probably not pay back with such a small source plant.

The idea of charging multiple batteries makes sense. I see 14AH units are available for $50. I could charge several at once or sequentially, since the time for charging would be months, and the time for using, only hours.

ETA: spam above reported.

10

25 ft. for a wind machine is not tall enough for a good system. attaching it to your house is not good structurally.

photovoltaic panels are easily ground mounted if your roof is not suitable. mast mounted tracking collectors have a small footprint.

making renewable energy is a very good thing. having backup energy is a good thing. they are sometimes the same and sometimes different.

solar electricity (photovoltaic) is a good thing. it has become more popular with costs dropping part of which is not using batteries but tying to the grid and selling your surplus electricity to the power utility.

energy conservation is a good thing, often referred to as a source of energy because if we conserved we (as some collective group) might save about a quarter to a third of our electricity. conserving energy during a power outage would be using battery LED lanterns for light rather than your wired in incandescent room lighting. running your laptop computer off a deep cycle battery with small inverter and doing some email and weather site checking and not catching last season of some tv show.

your power backup plan could become a couple deep cycle batteries and/or a small generator. using a gas cooktop, either your kitchen one or a camp stove. dressing warmer inside if needed. opening the refrigerator or freezer only as needed. using a 7" LCD tv (battery powered) for essential tv watching.

being green and emergency preparations can be a whole range of things. there is some detail for some aspects requiring developing some knowledge. for large aspects for most people hiring people skilled in that area is needed.

11

I’d say a vertical axis windmill mounted on the roof will be more cost effective. It will cost less, doesn’t have overspeed issues, and takes advantage of turbulence of wind passing over the roof.

The big problems are that windmills produce wild AC which must be rectified to DC for storage in batteries. There’s plenty of loss in that process. Then to use the stored electricity it must be converted back to AC which also has a lot of loss.

I think the best use of windmill power would be to directly use the wild AC to generate heat. An electric hot water heater could easily run off the wild AC. Possibly ventilation fans and water pumps could be powered my motors that accept the wild AC as well. These are things that could run intermittently.

12

The losses aren’t as bad as you suggest; an AC/DC rectifier can be over 99% efficient, depending on the voltage (AC voltage and voltage drop of diodes; you can also use semiconductor switches to reduce losses even further); converting this variable rectified voltage to the battery voltage isn’t that lossy either with a good DC/DC converter, which can easily be in the 95+% range. Similarly, efficient DC/AC conversion is possible, especially with all of the push towards energy efficiency.

Some references (not necessarily for the intended application, but the same basic designs are used):

98% Efficient Single-Stage AC/DC Converter Topologies (this design also eliminates the bridge rectifier; an example for solar cell use is also shown, with a 15-105 volt input range)

A High-Efficiency Soft-Switched DC/AC Inverter for SOFC Power Conditioning Systems (High efficiency (99% without filter, 98% with filter))

The total efficiency with those numbers would be around 96%, leaving the battery as the most significant cause of loss, even with a more typical (less efficient, cheaper) setup, but as previously posted, good battery designs can also approach this level of efficiency, especially with controlled slow charging, another reason to have lots of capacity. A good design will also work at the maximum power point of the wind turbine or solar cell (maximum power point tracking).

13

The Sunforce company seems to have a lot of what I am looking for. Here’s a 600 watt wind turbine, about 4 ft blade diameter. It’s an AC output, but comes with a controller to charge 12 or 24 v batteries. It’s designed to be mounted on a 1.5 inch dia. pole. They claim max output is reached at 14mph, with a starting speed at 4mph.

I’m curious about some calculations. Assuming no loss in the system, and a steady 600w 12v output, would it be reasonable that a 20AH battery would fully charge in 4 hours, at least in a nominal sense if not a practical one?

14

You’re kinda into “assuming a spherical cow” territory there, but yes, with those assumptions, that would be reasonable.

Along more practical lines though, you should probably regulate the charging so that it’s not pushing so much charging current through the battery. The battery will last a lot longer if you don’t charge it quite that fast (you don’t mention the specific battery type, but typically you want to aim for more like a 6 hour charge time or greater for longest battery life). You are also assuming that your turbine will run at max power continuously. In most places, the wind isn’t that consistent.

15

It’d be better to use a higher voltage, but not really high, say 48 volts, since at 12 volts and 600 watts you are dealing with 50 amps; if you used 48 volts, you’d only have 12.5 amps, which would reduce resistive losses by a factor of 16 (P = R x I^2); even if you used thinner wires and higher voltage semiconductors (with higher internal resistance) by a factor of 4, you’d still reduce losses by 4-fold.

Also, a 12 volt 20 amp-hour battery (240 watt-hours), with no losses, would charge in just 24 minutes on 600 watts (240 / 600), or 48 minutes for a 24 volt battery of the same amp-hour capacity (480 / 600).

As stated previously, you’d be better off with a much larger battery, say 100 amp-hours at 48 volts, which would be 4.8 kWh and take at least 8 hours to charge, which improves efficiency and life (you’d want an even larger battery, and larger turbine/solar panel if you wanted to be completely off the grid).

16

It will likely put out less than that. For car batteries, amp hours are usually given over a 20 hour period, i.e. a 20ah battery can put out one amp for 20 hours. If you pull more than one amp an hour, it will die faster.

I’ve always used cold cranking amps to estimate how much I can get for a few minutes. CCA are how many amps it can put out over 30 seconds. If it puts out 600cca, you can assume it can do around 300 amps for a minute, 150 amps for 2 minutes, 75 amps for 4 minutes, etc. If you take that up to an hour, it would be about 50 watts, but I’d say it’s pretty inaccurate over that much time and you might get more out of it. It’s not going to be 240 or even 120 though.

A group of us tested batteries with winches once. An alleged electrical engineer just refused to believe amphours were given over 20 hours, even when given a link to the manufacturer’s site that said as much. He said a 50ah battery would put out 50 amps per hour, thus 200 amps in 15 minutes, so by gum it would run a winch pulling 200 amps for 15 minutes. During testing, our batteries would run the winches pulling half that load for about 4 minutes before they died.

17

Quite likely you will have some very annoying noise if you brace it against the house.

18

Technology like that will make a big difference in making small scale wind and solar power economically practical.

19

I built a home-brew system at my cottage much like the one you are describing.
You’d be better off running a 24V system and finding DC appliances. Large electric motors such as your furnace fan or refridgerator compressor will drain the batteries quite quickly.

I also learned from experience that as cool as watching the game off the grid on a massive screen in the woods, a DLP will drain all your batteries long before halftime.:frowning:

20

Yeah, but aren’t DLPs great! I forget what type of bulb that is, but they burn bright and hot.