In my last entry I talked about the need for ways to store energy to better manage supply and demand. This is necessary for home renewable energy systems as well as for the nations power grid. In this post I wanted to talk a little about the alternatives to batteries for storing energy. Specifically I’m looking at how utility companies do it. Then we will see if there is a similar method available at residential level.
Batteries
As I mentioned in an earlier post batteries are not used very often on a national power grid scale primarily due to costs and maintenance issues. A new possibility has presented itself though thanks to electric vehicles. Every electric car with a full charge represents a battery with enough power to run the average home for 2 to 5 days. (Still think you can power your house and a car with solar panels on your roof?) This means that if electric cars ever take off, the power company could use them as batteries for the grid. I’m not sure how this would be handled as I don’t want to wake up in the morning and find my car out of fuel because my local utility needed the juice. Still promising nonetheless.
I don’t think I need to talk much about batteries for residential use as its really common technology. Unfortunately it is very expensive. In the future using your car as at least part of your battery back up could probably make sense to reduce costs.
Pumped Water Storage
This is a very popular method of energy storage for utilities. Whenever there is excess power being generated it is used to pump water uphill to a large reservoir. When additional power is needed gates are opened in the reservoir and water runs through turbines dumping energy back into the grid. Obviously this takes a lot of water (billions of gallons)and a very large reservoir (hundreds of acres) but as a storage medium it’s 80 percent efficient which is fantastic compared to other storage methods. A number of pumped water storage systems use Wind turbines to generate the power that pumps the water up hill effectively spreading out the sporadic electricity generation of the wind. This also represents an extremely clean method of generating continuous electricity.
Could you use a method like this at home. As a thought experiment I tried picturing a home with two swimming pools. One pool being built substantially higher then the other and water being pumped from the lower pool to the higher pool when there is excess energy and then dumped back through a hydro generator when extra power is needed. After a little napkin math here is what I came up with. If the water is pumped to the higher pool at a level of 15 feet above the generator and flows at 15 gallons per second the possible energy generation is 2kw. To run this full tilt for an hour would require 54,000 gallons of water. This would require two pools that are about 6′ deep and 25′ by 30′ in size. Please check my math but while its technically possible I don’t think it is economically feasible. On the other hand who doesn’t want two pools. If anyone is aware of someone actually doing this please let me know.
Compressed Air
As far as I can tell this is not used to much by power companies. Where it is used the process is usually to pump air into an abandoned mine to a set pressure and then released on demand to create electricity. I believe this isn’t used often due to the difficulties in accounting fro pressure and temperature fluctuations which can greatly effect the energy stored in the compressed air.
Can this be used at home. I didn’t find any examples of this in residential use but there are companies building cars today that are powered by compressed air. You can read up on this here.
Thermal Storage
Thermal energy storage is most often used in relation to power generation from the sun. Some of the most effective solar power systems do not generate electricity directly but use the suns rays to heat a liquid to steam which runs a turbine. In this case liquid can also be super heated and stored for later use. One proposal is to use molten salt as storage medium for the heat. You can read about this in one of my earlier posts.
This is the easiest technology for the average person to take advantage of. excess energy can be used to heat or cool dense materials (thermal masses) allowing for space heating or cooling when energy is not available. An example of this would be solar hot water heaters pumping hot water through a radiant heat system buried in a large concrete foundation. That foundation can continue to radiate pleasant heat for days. You can read about thermal mass and its uses in an earlier post here.
Conclusion
There are a number of other technologies in the works for energy storage. Hydrogen is a prime example. I’m not going into those here as I wanted to limit myself to current common technology. You might have noticed that batteries are still the best answer for onsite residential power storage. I find this a little scary considering how little improvement there has been in this area over the last few decades. In fact most improvements in battery life have far more to do with greater efficiencies of the devices being powered then the batteries themselves. I hope this means there is some major break through on the horizon because I can only imagine that our need for good batteries is going to increase.
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