In Minnesota, arenas in St. Cloud, Paynesville, Austin and five other cities already use geothermal refrigeration systems to good effect. They'll be joined by a two-rink complex, the Burnsville Ice Arena, by October 2010 if all goes well.

In an arena geothermal refrigeration, the temperature-tempering qualities of the Earth are accessed. The temperature below the frost line is relatively constant, around 50 degrees. Loops of tubes are placed deep under the frost line, and a mixture of coolant and water is circulated. A heat pump extracts the heat, leaving behind a coolant cold enough to create ice for the arena.

The green appeal to geothermal refrigeration systems: they use far less coolant than traditional arena refrigeration units. In Burnsville, the current refrigeration unit -- albeit an ancient 37-year-old unit -- uses between 10,000 and 12,000 pounds of Freon. The new geothermal refrigeration unit will between 120 and 260 pounds of Freon.

Retrofitting the two sheets will not be cheap: the total cost of the project is expected to be $4.3 million, with $360,000 coming through the recent federal stimulus funding. (The rest of the funding is also in place for the whole project, with $5 million already bonded for the project.)

With the old unit requiring replacement -- it worked far beyond its promised 25-year life expectancy -- the choice was between a new refrigeration unit costing between $1.5 million and $2 million or the more expensive geothermal refrigeration system. The costs on energy savings will be enough to pay the difference: the unit is expected to save at least $105,000 annually in energy costs for 25 years, a total savings of $2.625 million. Maintenance costs are expected to be lower on the geothermal refrigeration system as well, and if the heat generated by the unit can be used elsewhere in the building or in nearby City Hall, energy costs incurred by the city can be lowered further.