90% Efficient heating and cooling system

New York University recently unveiled a heating and cooling system that it says is 90 percent efficient.

New York University is in the final phases of opening a power plant that provides electricity for its lights, elevators and computers and steam for heating and cooling water. The new plant is nearly 90 percent efficient, meaning it gets almost three times as much useful energy out of a unit of fuel as a typical utility power plant does. And its carbon dioxide output is 23 percent smaller than that of N.Y.U.'s old system.

Call it "collateral benefit." For the United States, reductions in carbon dioxide emissions are actually turning out to be side-effects of other economic changes. N.Y.U. has been generating its own electricity since the 1970's and is one of the largest entities doing so in Manhattan. Over the years it has used a variety of technologies, including giant diesel engines that burn oil to turn generators, with the exhaust heating water into steam.

But in the last few years, air pollution regulators have told the university that it must reduce emissions of conventional pollutants, like those that cut smog, from its power plant. N.Y.U.'s solution was to switch to natural gas, which has fewer conventional pollutants but also happens to have less carbon dioxide content per unit of energy than oil does, and to then use each bit of energy four times.

This nifty diagram explains the system. First the gas is burned in a gas turbine, a device that resembles a jet engine, to turn a generator. Then the waste heat is used to boil water into steam, which turns another turbine, to make more electricity. That system, called a combined cycle generator (it has a gas cycle and a steam cycle) is in common use today for providing electricity to the grid.

But normal utility practice is to take the steam leaving the steam turbine and use cooling water to condense it back into water in preparation for another trip through a heat exchanger to pick up more heat from the gas turbine exhaust to be boiled back into steam. Under ideal conditions, that can produce electricity with 60 percent of the energy value of the original natural gas.

At N.Y.U., the steam that leaves the steam turbine, having lost most of its energy, is used to heat water. In the summer it is used to turn a second steam turbine that runs a chiller that makes cold water for air conditioning. It replaces a 600-horsepower electric motor that would otherwise be powered by burning more natural gas.