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Abundant, undeveloped, and essentially ‘free’ thermal energy exists in the form of low-temperature water from sources such as geothermal hot water, oceanic and fresh water thermal gradients, and industrial waste heat. Recovery of useful mechanical and electrical energy from these vast and inexpensive resources has been hampered by lack of effective and economical thermal energy conversion systems (heat engines). A research team at McDonnell Douglas Astronautics Company (MDAC) is developing low-temperature heat engines which use Joule effect solid state materials as the working media. These materials, when heated, contract with substantially greater force than is required to stretch them at lower temperatures, and the force difference is translated into work. Heat engines built by using one of these materials, Nitinol—a nickel titanium alloy—have operated successfully at temperature differences as low as 5°C.