Cheap Solar Cooling Solution?

An anonymous reader asks: "I would like to use solar panels to generate enough power to run an air-conditioner that is only needed when the sun is out. The problem lies in the cost of the battery bank, where one can quickly run to thousands if not tens of thousands of dollars. Given the state of current solar panel technology, what kind of unique solutions have people come up with to handle the voltages of this setup WITHOUT the use of a battery bank?"

"I realize that the photovoltaic array will have to be of sufficient size to offer more power than will be consumed by peak load, causing excess power, that will need to be dealt with. Also, there will need to be some monitor so that if the available energy doesn't meet the minimum threshold, then the appliance is shut off (or the juice to the circuit is cut), and vice versa. As temperatures approach 120F and more this summer, I'm putting more aside for this project and at this point am not concerned with any but simple methods of using up 'excess' energy. Though thoughts have rattled around about a Linux controlled shading system to adjust the raw juice coming off the panels through selective shading."

If the concern is cooling, then one way of burning off any "excess" power might be creative use of fans (either single fans or a bank of them; small or large, depending on the amount of excess power available). What other ways might such "excess" power be used?

Comments

Flywheels!

[stick_figure_of_doom]

Flywheels are a really cool idea. I've never heard of one in practice, but they have been touted as a solution to peak hour power shortages. Just spin a heavy disk up with extra electricity in a low friction environment, and then siphon it off during peak hours. As a resident or a power company you could save money. It's not an option available right now, however.

There is more than one kind of energy storage

[El]

Use excess power to pump water 300 ft uphill, then use the water to drive a turbine to power the A/C. From what I know of air conditioners, it would take a LOT of photovoltaics to drive one... I'd guess at least $20,000 worth.

Re:There is more than one kind of energy storage

[Tuxinatorium]

No, actually modern power-efficient air conditioners can run on about 1 kilowatt. Solar panels run about $3 per watt these days if you get big ones.

Re:There is more than one kind of energy storage

[Walt Dismal]

One solution is to not use electricity. Build a solar-powered steam engine and use it to run a Carnot cycle in reverse (heat pump) during the day. All mechanical, no electricity. Cheaper than solar cells plus batteries plus charger/inverter.

Store it in the electric company

[orthogonal]

As temperatures approach 120F and more this summer, I'm putting more aside for this project and at this point am not concerned with any but simple methods of using up 'excess' energy.

Don't most state regulatory bodies require that electrical utilities purchase any power that a customer generates?

Just feed your excess into the power grid, and let it offset the power you buy from the utility. Pay the difference on your lower utility bill.

Re:Store it in the electric company

[jordandeamattson]

This is the right solution to this problem. I installed a 2.5 KW solar array this last year (December) and moved to time of day metering.

Under time of day metering, we are charged/paid 35 cents a kilowatt hour during "prime time" (Noon to 6 PM) and are charged/paid 7 cents a kilowatt hour during "non-prime time".

Since the array went online in December, I have seen the electric component of our utility bill drop from approximately $100.00 a month to between $20.00 (in December, when we were home during prime time due to the Christmas break) to $5.00 a month (which is what they charge us to send a bill with $0.00) in February.

I am currently generating and selling 6-8 net prime kilowatts a day, which offsets 42-56 non-prime kilowatts a day.

My investment in the system was approximately $11K after buy-downs (based on the number of watts I produce) and tax credits starting from a base costof $22K.

On this $11K investment I am seeing a projected after-tax return of $1080 a year or 9.8%. If you take a look at the pre-tax return I would have to get to match this, I would have to be getting an ROI of between 15-18%!

This is the best money I have spent ever! The grid tie setup is the way to go, since it lets me use the grid as my battery!

couple ideas..

[molo]

IANAEE (I am not an Electrical Engineer), so feel free to correct me where I stray from reality.. but here's some ideas:

1. Sell it back to the grid. Use the electrical power grid as a battery, drawing from it when you need and selling your power back when you don't. You can actually make some money this way, offsetting the cost of the solar panels.

2. Break the circuit when you don't need the cooling. (Any EEs want to comment on if this can damage the solar cells?) I believe this will just create a DC potential difference across the cells, and since the circuit is broken, there's no current flowing around to worry about storing.

Good luck.

Think Outside the Suburb

[kentborg]

Traditional air conditioning assumes cheap electricity, and plenty of peak capacity. (Ever try to start a compressor motor?)

You want to reduce you air conditioning need. Think awnings that reduce solar gain. Think reducing heat generation. Think insulation. Think how to be a bit more clever. There is a lot of work available on this aspect.

Once you do do some air conditioning, consider finding a cooler hot side for your heat pump. Something my wife wants us to look at for both heating and cooling is "geothermal", that is using the earth for the both hot side (in winter) and cold side (summer) of the heat pump. Also, if you are in a dry climate, consider if a little evaporative cooling might boost efficiency.

-kb

Re:Think Outside the Suburb

[Anonymous Coward]

If you've ever been to a _really_ hot country, the first thing you do is put a canopy over your house. Something like the tented airport in Saudi Arabia. Get everything to be light colored, to reflect the sun. Painting the streets white sounds like a stupid idea, but those streets soak up a lot of solar energy, and re-radiate it into the surroundings. Cities are much hotter than the surrounding area, and part of the problem is all that pavement and dark-colored roofs heating things up.

If you want solar, then build the solar array a couple of feet, at least, above your house. That way, your house will be in the shade created. You're smart enough to not stand out in the sun when it's really hot, but why do you make your house stand out in the sun all day, then expect it to be cool? If there's enough water around, plant tall trees that will shade the house. Keep windows and drapes closed on the sunny side of the house. Make sure your attic space is ventilated, or even power vented. Have canopies, or shades, on the outside of the house, that keep sun out of the windows.

Next, _build_ your own house. Use stack effect cooling. Insulate heavily. Insulation slows heat transfer, which means it will slow down the heat getting in. (Most people think you should only insulate in cold climates.)

Somehow, people survived for thousands of years without air conditioning. Could it be that "modern" housing design, where one style suits the entire country, isn't the best idea?

Re:Think Outside the Suburb

[stefanlasiewski]

Somehow, people survived for thousands of years without air conditioning. Could it be that "modern" housing design, where one style suits the entire country, isn't the best idea?

There's a place in Arizona called Arcosanti. It was designed around the philosophy that "modern" housing design is wrong and inefficient. The architect made alot of intelligent design decisions to keep the complex cool, all without Air Conditioning.

I was there a few years ago. It was 105 in the sun. Within the complex, in certain buildings or under the giant arches, it was only 90.

All without air conditioning.

Re:Darn batteries

[MBCook]

I agree about hydrogent fuel cells being a good battery replacement. When they start to be mass-manufactured (like for fuel cell cars) they'll be better.

As for alternatives, I agree. Solar isn't very efficent. As for wind, in the right area and done the right way it can be fanstastic. I live in eastern Kansas and let me tell you it can get VERY windy on some of the highways that I drive that are surounded (basically) by farms. Now, you would lose some crop if you put the windmills up in the middle of your fields, but there are also a large number of cow farms around (mostly meat cows, not dairy I think). Now you have all this land that's just used for grazing. You add some windmills and all of a sudden your land is now producing energy for free for you. You'd only lose a tiny ammount of grass (wouldn't effect the herd). You get money (that will cover the maintence) and it works year round. As for the cost of putting up the mills, you could almost certainly get grants from the Government and private groups to help pay for 'em (or band with other farmers to buy in "bulk"). The biggest problem is "environmentalists" complaining about the what it will do to the view (I think they're neat, personally) or how it makes the landscape unnatural (and ripping everything down for cattle grazing doesn't, let alone building "real" power plants). With all I've heard about the plight of family farms, this could be a fantastic way to suppliment income. Wind won't work everywhere, but in some areas you could get a LOT of power.

In fact, just 10 minutes or so down the road from me is a company's headquarters that has a few small windmills outside their building. I think they design/manufacture them.

architecture

[hak1du]

People have known for thousands of years how to build buildings that stay pleasantly cool on hot days. So, if you want to do something environmentally friendly, design and build your house accordingly. A good architect and landscaper should be able to advise you.

There are also some things you can do after the building has been built, like planting trees, improving insulation, and putting reflective coatings on your windows. If you have lots of space in your yard, you can also use a bed of rocks together with some kind of ventilation system as heat storage, cooling your house during the day and warming it at night.

If that isn't enough and you just can't cope with the remaining heat, then you might as well do what everybody else is doing: install air conditioning and pay the power company. It seems doubtful that any kind of solar-energy drive air conditioner would, on balance, be more environmentally friendly than that anyway.

You're crazy to suggest it...

[Tau Zero]

because you need economies of scale to make anything like that work, and unless you can get an old MRI machine as scrap and one of your in-laws is a liquid helium vendor, you're just not going to be able to get the hardware and keep it anywhere near running condition by your lonesome.

If you're trying to store cold, do what they did 200 years ago: put ice inside something insulated. Back then they used barns with walls full of sawdust, but we can do a lot better today.

You don't need electrical storage!

[taharvey]

There are many ways to design a solar system, depending on application.

Yours is an ideal application: the load is directly proportional to the solar gain. So there is no need for storage because your cooling needs track the solar gain. Less sun, less cooling needed. There can be some lag time, depending insulation and thermal mass in the house but these things are best solved with insulation and thermal mass as your "storage".

For an air conditioner, use an evaporative cooler (many times more efficient than a conventional air-con, very little solar to run). If you live somewhere humid use an desiccant adsorption cycle evaporative cooler (search google -lots of work done on these for solar powered cooling).

And finally, the best "solar cooling" is no active cooling. Build or modify your house with appropriate insulation, thermal mass, shading, and architectural passive solar heating and cooling elements - and your house will stay cozy all year around with little active cooling required.

-TAH

Re:Darn batteries

[Hardwyred]

I work in that building. They are actually there just to power the parking lot lights, but they do make a noticable impact on the power bill (My dad works for the company that tends the building). The whole building was designed to be energy efficient. That big lake out front actually holds a bunch of coils for the cooling/heating system. I believe they call it a hydrothermal heatpump and it is the same concept as a geothermal heatpump. Inside, everything is designed to be as energy efficient as possible. Unfortunatly, not many people have moved in as it was real estate that really wasn't needed (look at all the other office buildings right around it!) but I think its pretty slick.