In Petaluma, California, the United States Coast Guard training facility has a 200,000 gallon pool. While trainees may have to rescue each other from sharks or play underwater cribbage--hey, I've never been through Coast Guard training--they don't have to do it in cold water. The USCG keeps the pool at a comfortable 80 degrees. Maintaining 200,000 gallons of water at that temperature, though, takes a staggering amount of energy. That's why the facility just recently installed a solar pool heating system. Solar pool heating uses solar thermal technology, an efficient and cost-effective heat transfer process.