The water pressure is there, so you might as well use it.

That's the business plan of Zeropex, a Norwegian company that produces pico- or micro-hydro generators. The company has created a device that harvests the excess pressure from water running through municipal water plants to run a turbine and create electricity. Water agencies -- along with heavy-duty private sector water consumers like oil refineries and agribusiness -- have to pressurize water to purify it or move it. Currently, most municipal water agencies use pressure reduction valves to reduce the water pressure before sending it along.

"We're basically replacing pressure reduction valves with a turbine," said CEO Tor Ersdal during a presentation at Nordic Green II. The company's current turbine can generate up to 80 kilowatts of power. A second-generation turbine in pilot testing at the moment can generate up to 300 kilowatts. At 300 kilowatts, the payback on the machine is just under two years, he said. The electricity created by the turbine can be fed into the grid or consumed locally.

The turbine can also be placed in desalination plants. Reverse osmosis desalination requires pressurizing water to high levels, an energy-intensive process. Converting the pressure from waste streams into power that can be fed back into the process ideally could reduce the cost of desalination.

Rentricity and a few other companies have been trying to popularize these sorts of devices for years. The difficulty has been the state of technology and the customer base. In short, the early turbines didn't work as well as advertised and municipal water districts are notoriously conservative. The picture, however, is changing. Michael Russell, a Cabinet Secretary in Scotland, recently told us that his country is investing in pico hydro. (He was on a swing through the West to celebrate John Muir Day.)

The cost is also coming down, said Ersdal. Zeropex's current product costs about $2 to $2.5 per watt in terms of capital expeses. The company has a calculator that will estimate an individual's costs--each site differs. It only requires about six to twelve hours of maintenance a year.

So what are the other six forms of water power?

1. Hydroelectric Dams. The only commercially widespread type of water power, but it likely has peaked. You don't see a lot of available rivers these days. Side note: Norway gets nearly all of its power from hydroelectric dams.

2. Wave Power. Still in testing.

3. Tidal Power. Still in testing.

4. Osmotic pressure gradients. Statkraft recently kicked off a test with Energy Recovery in Norway.

5. Ocean Thermal Energy Conversion. Like tidal and wave, OTEC has been understood for years. The challenge has been harvesting it.

6. Aquatic Air Conditioning. Local Host, another Norwegian company, has come up with a way to cool data centers with chilly fjord waters. Hawaii will also build an aquatic air conditioner that will serve a wide swath of downtown Honolulu -- more on that in a coming story.

Nuclear, coal and manysolarthermal processes also rely on water as an integral component. Coal plants heat water, turn it into steam, and then exploit the steam to turn a turbine. But water in those cases is effectively a medium -- different liquids technically could be substituted -- so I don't count them. And if you know of more, let me know. I may have missed some.