HANGING food above cool water in a well used to be one of the most popular ways to keep it cool in summer, and so did blocks of ice for ice boxes in the days before refrigerators that ran on electricity.

Now that old approach - using the constant temperature of underground water - is again being used today for air conditioning.

The idea of the small well has been enlarged to more than 10,000 square meters in northern Shanghai's Baoshan District.

In an energy conservation center in Baoshan, you can see a line of round holes in the wall of the conference hall.

It's air conditioning for the Shanghai International Energy Conservation and Environmental Protection Park.

It blows out warm air in winter, cool air in summer.

This new environmentally friendly air conditioning consumes little electricity (some equipment uses electricity) and does not generate pollution.

The secret lies under the 3,500-square-meter lawn in front of the building, according to Ye Ying, director of the Economic Committee of Baoshan District and a research fellow at the China Academy of Management Science.

Just a few months ago, the area was bare earth. More than 200 small connecting wells, 6-10 centimeters in diameter, were being drilled to a depth of around 85 meters.

One end of a U-shaped tube goes into one hole and comes up through another hole. The tubes are filled with water and heated in the earth.

At the surface, the tubes are connected with a compressor, a condenser, an evaporator and other equipment that raises and distributes warm water and air.

Everything is linked to rooms in the building through other pipes.

Together this represents one of the most environmentally friendly air-conditioning systems in the world ?? a geothermal heat pump, or geo-exchange system.

A geothermal heat pumping system draws energy from the ground where the temperature is stable between 10-16 degrees Celsius, and warms air for rooms.

Despite the name "geothermal heat pump," the energy comes from ground that is heated by the sun, not the earth's core that is far deeper. The local climate determines the temperature of the ground.

"The temperature of underground water and soil won't be influenced by changing weather," says Ye. "Even though it is below zero centigrade on the surface, the soil in the deep area keeps warm. So we borrow some zero-cost heat from the earth."

The tubes buried in the earth under the lawn are kept warm and the water inside is recycled.

Clean surface water of normal temperature flows to the deep layer of the earth through the tubes, exchanges heat with the soil there and travels back to the surface again.

The water in the heat exchange system is used to create warmer air that is blown into rooms at comfortable temperatures in summer and winter.

The geothermal heat pump at this energy conservation park (Phase 1) provides heat for structures within the 19,000-square- meter area, including the conference center, exhibition center and an office building.

It provides cold water of 7-12 centigrade in summer and warm water of 40-45 centigrade in winter to the air-conditioning system, which helps keep the temperature of the air-conditioned rooms below 26 centigrade in summer and above 18 centigrade in winter.

Regardless of the weather above ground, the system works effectively.

"Ordinary household air conditioners may not work when the temperature is below freezing, but a geothermal system can still keep the room at about 20 degrees Celsius and prevent freezing," says Ye.

The engineers added a cooling tower, usually absent in other geothermal heat pumping system. With the cooling tower, says Ye, people can control the temperature of the air sent to the room as accurately as a regular thermostat.

Inserting all those U-tubes into the ground took around three months of hard work, but it was worthwhile, considering the energy-saving and environmental protection of the air-conditioning system.

What is now the International Energy Conservation and Environmental Protection Park used to be one of the biggest energy-consuming and air-polluting enterprises in Shanghai ?? the Shanghai Ferroalloy Factory.

The factory was closed in 2006 and converted into the energy-saving and environmental protection park in late 2007.

Phase 1 was completed in October, including the air-conditioning system.

Apart from efficient warming and cooling, the geothermal heat pump is a big energy saver.

This kind of air conditioning can save more than 50 percent in electric bills, compared with standard one, according to Ye.

That saves more than 300,000 yuan (US$44,000) in electricity bills for the park each year, says Ye.

Since no coal or oil is burned in the heating process, no carbon dioxide or other greenhouse gases are produced.

An ordinary air conditioner may only work properly for about 10 years, but the geothermal heat pump system is infinitely renewable energy; the system life itself is at least 25 years for the equipment, while the underground loop lasts for more than 50 years, maintenance free.

Though installation is costly, the investment can be recouped in savings on electric fees and lack of maintenance. It is not yet used in individual households in China but in the future "green" residential developments could use it.

The geothermal heat pump technology appeared in the Western world in the 1930s.

And it has been widely used in major construction since the 1970s and arrived in China around 10 years ago.

It was first widely used in oil fields in China.

The Baoshan District park is the first structure to install the geothermal exchange pumping system in Shanghai.

Ye says the system will be later recommended for hospitals and schools that have high demand for air conditioning.

There's still a big problem in popularizing geothermal exchange heating in Shanghai because of limited space.

As tubes are buried underground, it can only be applied in areas of lawn or trees and shrubs with shallow roots.

So if you can't spare a lawn, it's not for you.

Renewable energy

Geothermal power

This is energy generated from heat stored in the earth, or a collection of absorbed heat derived from underground. The most common type of geothermal power plants are closed-cycle operations with virtually no greenhouse gas emissions. Geothermal power supplies less than 1 percent of the world's energy in 2008, but it is much more widely used in countries such as Iceland (with a volcanic geology) and the Philippines.

Solar power

Solar radiation can be collected in panels of solar cells and converted into electricity for heating, cooling, powering equipment and industrial use.

Wind power

Wind turbines convert wind energy into electricity using wind turbines. It is favored by many environmentalists as an alternative to fossil fuels, as it is plentiful, renewable, available almost everywhere, clean and produces low emissions.

Although the wind produces only 1 percent of the world's electricity now, it is growing rapidly. It is popular in Denmark, Spain, and Portugal.

Tidal power

This form of hydropower converts the energy of the tides into electricity or other forms of power. Although not yet widely used, tidal power has great potential. Tides are more predictable than wind energy and solar power.

Wave power

Waves on the ocean's surface transport energy that can be converted to electricity. Another inexhaustible energy source, it is not yet a commercial technology.

Biomass power

Biomass, a renewable energy source, are organic materials that can be used for fuel or industrial production. Plant and animal waste generate methane gas that can be used for heating and cooking. It includes plants grown to generate biofuel.