Energy / Usually, “green” and “cheap” don’t go together. In the case of the first hot city in the world to have its heart cooled by seawater pumped from the deep–that’s Honolulu–building owners will save money in several different ways and green-house emissions will be cut, according to an environmental impact statement published this fall.

The technology is already used in Toronto and Stockholm to balance temperatures inside large buildings, notably to cool areas with computer servers and telephone exchanges during the summer months. But, says William Mahlum, president of Honolulu Seawater Air Conditioning LLC, which is undertaking the $240-million project, “This is the first time it will be used to cool a warm-weather city center.” The company, owned by investors in Hawaii, Sweden and Minnesota, is managed by Renewable Energy Innovations, a unit of Ever-Green Energy Co. of St. Paul, Minn., which runs that city’s heating and cooling systems.

Groundbreaking is expected next summer and the first 40 buildings are expected to come online in late 2012. Another five will be added the following year.

Once the system is up and running, predicts Mahlum, it should inspire tropical coastal cities around the world to harness the technology. “All you need is a steep enough coastal gradient and concentrated demand,” he says.

The system will save its clients about 20 percent in cooling costs, the company says; and by reducing power use by 77 million kilowatt-hours/year, or 75 percent of the present consumption, it will cut carbon dioxide emissions by 84,000 tons a year. It will also reduce the use of refrigerants and save 260 million gallons per year of drinking water.

And finally, it will end the dumping into city sewers of 84 million gallons per year of used cooling-tower water, which contains a host of toxic chemicals, according to Ingvar Larsson, Honolulu Seawater’s Vice President of Engineering.

Mahlum says the seawater cooling system, will reduce its customers’ cost for air conditioning from the start. Over time, the savings will grow if the price of oil increases.

“Most green energy projects focus on generating clean electrons,” says Jeff Mikulina, head of the Blue Planet Foundation, whose goal is to promote clean energy. “The beauty of this one is that it avoid electrons altogether by tapping into a vast local resource.”

Here’s how it works: a five-foot-wide pipe extends four miles out to sea to a depth of 1,700 feet, brings in 44,000 gallons per minute of water at 45 degrees F. Once ashore, the water goes through a conventionally powered chiller that brings it down to exactly 44 degrees F, then loops through a heat exchanger with a closed-circuit freshwater system and is released back into the sea at a depth of 200 feet at 56 degrees–the natural temperature at that depth.

Meanwhile, the cooled freshwater makes its way to the buildings’ air conditioning units and cools the air propelled by fans over the coils. This allows the building to turn off the energy-hungry compressor that previously chilled the coil and the cooling towers.

Hawaii sees a surge of power demand in the middle of the day driven by air conditioning, so the project saves not only power, but premium power, notes Mikulina. “This project will help relieve some strain on the power grid.”

Worried that it warms up the ocean? Not at all, says Larsson. “If you look at the heat we emit in both the ocean and the atmosphere, it’s 40 percent less than a conventional air conditioning system, and of course by cutting greenhouse gases, we slow global warming.”

Peter Rosegg, a Hawaiian Electric Company spokesman, has nothing but praise for the project. The system will reduce electricity generation in Hawaii by only one percent–one year’s worth of growth in demand. But Rosegg says, “One percent may not seem like much, but this is an important one percent because the downtown area has banks and medical centers that require very reliable power. They can’t afford dips and curls, and seawater cooling is much more even than, say, wind and power, which go up and down a lot.”

The next logical step would be Waikiki, where more than one-third of the power use is taken up by air conditioning, he says.

Honolulu Seawater Air Conditioning officials confirm that once the project is finished, they expect to create another unit in Waikiki, with its own pipe into the ocean. This would take another five years.

It’s not surprising this first came about in Hawaii. It was on the Big Island that in 1974, the Natural Energy Laboratory of Hawaii Authority did the first U.S. tests of Ocean Thermal Energy Conversion (OTEC) to see if temperature differences between the deep and the surface could be turned into electricity economically. The answer was: not yet, but as a sideline, the laboratory created the world’s first air conditioning system using cold seawater pumped in from the deep.

That system is still working.

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