The Sterling engine was invented about 150 years ago, and they have been around for a long time. However, it's not easy to build a good one. NASA spent a lot of money, more or less fruitlessly. There are simpler hot air engines (I have a patent on one) which will work on any heat source, including sunlight. Any heat engine is limited by the ratio of input and exhaust temperatures (absolute, of course), so thermal efficiency will be low. However, if the heat source is cheap enough, such engines can be practical.
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