Thermal power plant – water circulation

When we talk about power plants, we talk about cheapest possible way to produce electric energy. Thermal power plants, no matter whether we talk about coal, oil, gas or nuclear power, are among the cheapest kilowatt-hours producers.

When you take a look at a power plant you sometimes see very large structures – cooling towers. These are enormously big and probably very costly so I assure you they are essential for power plant operation. Cooling towers are heat exchangers (cooling cells) between the plant and the atmosphere. Its purpose is to deposit heat into the atmosphere (the cold reservoir). All thermal power plants need a way to deposit their remaining heat into some sort of cold reservoir. Sometimes plants use nearby river or lake as a cold reservoir or even our homes that we heat during winter.

Thermodynamics teach us that if we want to make some mechanical energy out of heat (to run electric generators) there must be a heat flow between hot and cold reservoir – and only a part of this energy can be transformed into mechanical form.

Let’s take a closer look. The steam in the boiler (steam generator) is at very high temperature and pressure. It is then guided through pipes to the first stage of the steam turbine. The first stage is designed to work using high-pressure steam. It is a quite small unit, but very powerful. At its exit the steam has a smaller temperature and a smaller pressure but a larger volume (the mechanical energy can only be produced if we let the volume to expand). The steam is then guided into the second stage steam turbine.

The second stage works on medium pressures. It is considerably larger than the first stage. Of course, at the exit of the second stage the stem has further reduced temperature and pressure and even larger volume. The steam is then guided into the third, low-pressure stage.

The low-pressure stage is big like a house. We could build fourth stage to get some more mechanical power but it will have to be incredibly huge (stages tend to grow exponentially) and so, I believe, it would probably generate more expenses than profit. At the end of third stage we have huge volume of relatively cold steam at low pressure.

What do we do with this steam? We do not let it out in the atmosphere because of environmental reasons and because of economical reasons. Instead, we return it back into the boiler for reheating. But of course, prior to that, we cool it further in cooling cells (like these enormous cooling towers). Why?

As the steam runs through turbine stages it transfers some of its energy into the turbine shaft and so it cools down. This is approximately an adiabatic process. If we would now immediately compress (also an adiabatic process) the steam back into the boiler, we would return to the very beginning point and no net mechanical work would be produced. What I wanted to say is that the same amount of mechanical energy would be needed for the compression as it was extracted in turbines.

Therefore, after it passes through turbines, we must cool the steam further down in our cooling cells. As we cool the steam further (at the same pressure – an isobaric process) its volume will drop. Now, when we cooled it enough, we can compress it back into the boiler. Note that as we now have a smaller volume of it, we will need somewhat less energy to do this compression and therefore produced net mechanical work for the whole plant will be positive.

The P-V diagram on the right shows how does the steam moves in an imagined power plant (without condensation). The extracted mechanical work is aproximately equal to the area inside the curve. On the left, we see what happens if we don't employ cooling cels.

True, the compressed steam that we return to the boiler will have somewhat lower temperature than it had at the very beginning and so we will have to re-heat it, but this is what we use our fuel for, anyway (natural gas, oil, coal, uranium).

Now the charming element… If in our cooling cells we cool the steam down below its boiling point it will condense into a liquid – the water. The volume of the water is many, many times smaller than the volume of the steam. Therefore, to compress the water to the pressure inside the boiler we need only a fraction of energy that we would need if we compress the steam directly. It is really beneficial if we can condense the steam prior its return into the boiler.

But it is not easy to condense so much steam. To condense the steam you must remove lot of the latent heat and this is why thermal power plants need to have cooling cells so huge. These cooling cells are expensive to build, but they make it possible for a power plant to operate efficiently.

Danijel Gorupec, 2006

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