It's not so much extracting energy as it is transforming heat into useful electricity through performing work. If your goal was to heat homes and businesses directly, you'd use a heat exchanger and a fluid with a high heat capacity to distribute the heat. But because you want energy in a different form, you're pretty much left to the Rankine cycle if you want high efficiencies between your conversion from heat to electricity. The reason water is typically used as the working fluid in most Rankine cycles is because it is non-toxic, cheap/abundant and has a high heat of vaporisation amongst other thermodynamically favourable properties. There are other working fluids, for instance for an Organic Rankine Cycle (ORC), such as toluene, but an ORC is better suited for lower-temperature waste heat and the like, and toluene for instance is nasty stuff you'd prefer not to use.