There exists a concept in Thermodynamics, which describes theoretical limits in the efficiency of all possible heat-engines. This principle states, that if we have a heat-source and a heat-sink, each has an absolute temperature. The ratio between these temperatures defines the highest-possible output of free energy from a heat-engine, as well as the lowest-possible consumption of free energy by a refrigeration-device.
The principle is based on the axiomatic assumption, that there exist no perpetual-motion machines, which simply convert ambient heat into free energy. If we could connect a heat-engine to an air-conditioner, and if these limits could be exceeded, we would have such a perpetual-motion machine.
This also explains why in practice, air-conditioners, refrigerators and heat-pumps can transfer heat from a colder source to a warmer sink, with the energy in heat far-exceeding the electricity consumed. They are all examples in which the ratio of absolute temperatures is close to 1.0 . Actually, what matters is the ratio of the temperatures of the working-fluid in each case, which is actually more oblique than the ratio for air temperatures, because heat-exchangers are never perfectly efficient. And the working-fluids used tend to be similar, because the temperatures at which those systems are designed to work, are also similar.
This also implies that if we wanted to build a heat-engine that uses small temperature-differences to generate electricity, large reserves of heat would be needed as a source, and sent to the sink, before even small amounts of electricity result – which might sometimes be available – but which constrains the system, regardless of what type of heat-engine is used.
Well, in Industrial Power Generation, the temperatures which the heat-source can be run at, depend firstly on what type of fuel is burning, but also depend on the range of temperatures at which water will boil. At 1 atmosphere of pressure, water only boils at 100⁰C, which is also 373K, while the external temperature tends to be around 273-300K .
Actually, by keeping the water boiling at much higher pressures, its boiling-point can also be increased. But it is generally not boosted beyond 200⁰C , which corresponds to about 473K . And so, according to basic principles, no power station based on water and steam, can be more than 50% efficient.
(Edit 05/12/2017 : Additionally, my late father, who was a professional Engineer, used to tell me, that something prevents a steam turbine from being more than 50% efficient. But, this is not a subject I know about, even though it would additionally limit the maximum efficiency of steam-turbine-based power-stations, to approximately 25%. )
In theory, if we could operate our heat-engine at 1000K, and its heat-sink still at 300K, we could achieve efficiencies closer to 70%. Mind you, that that point our heat-source might resemble a lightbulb, more than what we are used to, but this would still obey the rules of Thermodynamics.
My only point being, that the use of water, and its associated boiling-points, is an arbitrary decision. There is no magical reason why we must use it. We could use vaporous sodium if we knew how to work with it safely.
If one breaks out, a sodium-fire is a nasty hazard, much more dangerous in its nature than wood or oil-fires already are.
(Edit 05/12/2017 : )
One detail which I should mention, is that Engineers partially bypass this limit in efficiency, in steam-powered power-stations, by using Superheated Steam. The idea is that merely by increasing the operating pressure, they can only increase its boiling-point so far. But they can add a section to the boiler, which continues to heat the steam after it has finished boiling, thus causing it to expand more, and causing its temperature to get closer to that of the burning fuel.
Oddly enough, in real boiler-designs, the waste-gas from the flame only reaches the superheater, after it has passed through the boiler -proper, so that even this superheated steam is not as hot as the actual flame. But I was told that more energy is invested in boiling the steam, than is invested in superheating it. But, superheating the steam increases efficiency.
Older power-station designs, dating back to the 1950s and before, did not superheat the steam, and are thus referred to as Saturated Steam designs. They have drawbacks that go beyond low efficiency.
And I believe that in a nuclear power station, the steam generators also have a superheater-stage, after the actual steam-conversion stage. Logically, in this case the inner loop flows in the reverse direction of the outer loop, thus reaching the superheater-stage first, in the top section of the steam-generator units, so that the temperature coming out of the superheater, belonging to the outer loop, can get close to the temperature with which inner-loop water leaves the reactor.
(Edit 05/13/2017 : )
To paraphrase the word ‘oblique’ above:
If we were running an air-conditioner, and the air-temperatures were the same, then the fluid in the condenser (which is expelling heat) would still be warmer, than it is in the evaporator (which seems to be producing cold). This is one reason why an air-conditioner will continue drawing power, even to transfer heat under such (easy) conditions.
But then, if the condenser (hot-side) air-temperature starts to become significantly warmer than the evaporator (cold-side), then the difference in fluid-temperatures only becomes greater, as it would on a very warm day.
Either way, the compressor of refrigeration systems works against the resulting difference in fluid-pressures.