Not to get off topic, but steam engines still drive the whole world; whether nuclear or gas or coal fuel, or concentrated solar power, steam turbines are how h

Not to get off topic, but steam engines still drive the whole world; whether nuclear or gas or coal fuel, or concentrated solar power, steam turbines are how heat is turned into motion to make electricity. Modern ones operate very close to the theoretical limit of efficiency (around 40% [1]). The great advance James Watt made in the late 18th century, was a technique to recapture heat from the exhaust and raise practical efficiencies to several percent, with the remainder of the theoretical limit eeked out progressively over the following century. One of the ways that limit is approached is to use the largest temperature difference the materials can stand; steam for large turbines is hot. The exhaust is still steam, but no longer hot enough to drive things efficiently. Indeed, the cooling loop of many plants run steam-hot, just not superheated-hot. Another common district heating approach (used in NYC) is a small direct gas-fired turbine driving an electric generator, with steam cooling. Combined generation is the use of that waste heat, for other heating purposes, and can raise total system efficiency. Similarly, Iceland uses a lot of geothermal heating; some directly but much is waste heat from geothermal-electric plants. Since the geothermal taps are not that hot, an enormous volume of fluid is moved and the cold side needs to be quite cool; the output is barely-hot water after going through the cycle. But there's a lot of it. So much so, that they have under-sidewalk heating in Reykjavik and outdoor pools in winter. Most places just throw such heat away without making further use of it. It's traditionally been cheaper (or at least easier/simpler) to distribute and burn fossil fuels than to try and recapture and redistribute low intensity heat. The pipes are indeed heat-leaky, despite usually being heavily insulated. [1] https://en.m.wikipedia.org/wiki/Carnot%27s_theorem_(thermodynamics) https://en.m.wikipedia.org/wiki/Carnot%27s_theorem_(thermody...