A HeatPump is a system which extracts Energy from a temperature gradient. It's basically a fridge, but in reverse. The high temperature is used to vapourise some material (the refrigerant in the case of a fridge, which is not an entirely appropriate name in this case). This is under pressure, which then drives a turbine. At the other side the refrigerant is condensed and recycled. Different refrigerants can be selected to cope with different temperature ranges.
The are other systems for extracting Energy from a temperature gradient, including devices like a ThermoCouple. There are now also solid state coolant units; you should be able to turn these backwards and use them as Energy extractors.
Regardless of exactly how you do it, Energy can be extracted from a temperature gradient. The variety of technologies available mean that you can do this for either small or large gradients, of many different volumes.