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Name: Chuck
Status: educator
Grade: 12+
Location: MA
Country: USA
Date: Winter 2011-2012


Question:
How does Rentricity work? This is a new concept for converting the lost energy of lowering the pressure on a flow of water to mechanical energy through a turbine



Replies:
Chuck

This article will explain it.

http://www.bloomberg.com/news/2011-06-29/rentricity-uses-pressure-in-city-pipes-to-make-electricity.html

In essence, excess water pressure, that is water pressure that has to be reduced in this case for municipal water treatment, is used to spin turbines that spin generators that produce electricity.

Sincere regards, Mike Stewart


Rentricity is the name of a company, not a scientific principle. Basically, they are using water pressure to drive a turbine to generate electricity (just like a hydroelectric dam, but much smaller scale). Instead of falling river water (like at a hydroelectric dam), they are using pressurized water that enters a water treatment plant. It turns out, the water entering the plant is too high pressure, and valves are needed to lower the pressure. Instead of using the valves (which wastes the energy contained in the water due to the pressure), turbines were added, which converts the energy to electricity. Hope this helps, Burr Zimmerman


I looked at the web site. It is no big deal, not something that could save the national energy balance. These guys just noticed that there was some wasted energy in certain rare situations and decided to try to make a little money recovering some of it.

In some cities or towns, the city water tank is up a hill 300+ feet above the tap-water users. instead of on a tower only 100-200feet up.

Normally on flat land one builds a water tower, and builds it only as high as needed to generate the desired 60-100psi water pressure.

But if your town is next to a mountain, one might think to save money by building a tank sitting right on the ground at some small flat place well up the slope. Much less structural steel is needed that way, and money is saved.

But the available flat spot you build on might be higher than 300 feet, and so the water pressure down in the town would be 150psi+, which is frightfully forceful coming out of the tap and hard on the plumbing too. So then you have to put in the way a pressure regulator which is a valve that opens only just enough to let out 100psi to the water users. Regulators are usually easy and cheap to build, but they inherently waste energy by making water push its way through a half-open valve. (The water company or department had to pay for that energy to fill the tank.) The ambitious engineer is often bothered by this and thinks maybe there is money to be saved or made by doing something fancier than a valve. That something is the turbine(s) used in Rentricity. Some valves in series and parallel are needed too, because a) turbines need servicing, and you might as well let the town have water while you are doing it. b) turbines cannot ever block the water completely, so the town does need a turn-off valve, In fact, a turbine can be a pretty poor match to the available power-flow in the water-stream if the water-usage rate is highly variable. I wonder how adaptive these turbines are to a range of flow-rates. Having several small turbines in parallel, and only valving open the optimum number of them, is probably one way to adapt.

Anyway, the turbine turns a generator shaft to make some electricity, which can be sold back into the power grid or used to power a single site such as a factory.

Jim Swenson



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