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Re: [Phys-l] Efficiency problem



< http://www.jcmiras.net/jcm/item/93/ >
indicates that electric generators are up to 97% efficient (at converting mechanical energy to electrical energy) and
< http://www.halfbakery.com/idea/Highest_20Efficiency_20Electric_20Motor >
indicates that electric motors (used to pump the water) are about 95% efficient.
So if you need to deliver (.95)(.97)100kWh = 92 kWh to a customer, you could do it by sending 92 kWh from your local power plant directly to the customer, or you could take 100 kWh from your local power plant, use it to pump some water uphill, then let the water flow downhill through a generator to produce your 92 kWh of energy which you then send directly to the customer. So your local power plant has to produce an extra 8 kWh of energy for every 92 kWh delivered to the customor via the pumping scheme. This is a conservative estimate of the loss in that it doesn't take into account the conversion of mechanical energy into thermal energy in the pumping process. So, I would tend to side with your student. On the face of it, the scheme is not "green." On the other hand, if your local power plant is a hydroelectric plant, and by smoothing out the demand on it by means of the pumping scheme one is able to avoid the need for the construction of a coal-fired plant to meet the demand, one could argue that it is greener than grass.

On a related note: There is a company near here that sells multi-ton flywheels that further smooth out the demand on a plant even after the day/night disparities have been ironed out. The devices are designed to contiuously monitor consumption, and provide power to the grid whenever consumption is above a certain level, and store energy from the grid in the form of the kinetic energy of the flywheel whenever the consumption is below a certain level. See:
< http://www.beaconpower.com/ >

________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of Peter Schoch
Sent: Fri 5/28/2010 4:49 PM
To: Forum for Physics Educators
Subject: [Phys-l] Efficiency problem



I've had a former student return with a question that I could use some help with...

In his community, a developer wants to convert an old quarry that is now a man-made lake into a power generation scheme. The developer would excavate under the quarry. Then, during the day, they'd let the water run from the "lake" down into the holding tanks underneath, in the process turning a generator and creating electricity. Then, at night they'd pump the water back up to the "lake" when the electricity was cheaper. In this way they'd generate electricity and the claim is that this is "green" electricity.

The student came to me because he remembered (yes, I did jump for joy that someone remembered something I had presented them as part of the class!) that no process could have an efficiency of 1, so his thought was that this process is not "green" as advertised. He thought that due to this loss, this process would ultimately consume more electricity than it would generate and that it was just a "shell game" for making money.

Well, my first inclination is to agree with him. However, I did tell him that he was trying to apply an efficiency concept from Thermodynamics to electricity generation. I assured him I'd do a bit of digging and get back to him. Well, I can't seem to find how I might calculate the efficiency for this process and how to determine if it is truly "green" or not -- thus being a way to make money but not be green.

Any suggestions and/or help would be appreciated,
Peter


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