What I'm going to do here is briefly describe a power cycle.
That is a cycle where we use high temperature heat to generate work.
Now this is essentially the basis of the power plants that generate most of the electricity,
whether they're burning coal or natural gas or some petroleum product.
The cycle for large scale, commercial processes can have additional complications but the
concept is the same.
And the idea is that we have some high temperature source of energy burning coal.
And we use that to evaporate a liquid.
So what we're going to feed to this boiler is a high pressure liquid.
The high pressure is important because we want this to boil at high temperature.
The efficiency is better if this is at high temperature.
So we come with the liquid and we leave with the superheated vapor.
So we've boiled it and we've raised the temperature above the boiling point so this has a high
enthalpy.
We then run this superheated vapor through a turbine and the idea of the turbine is that
we're going to get work out.
We are then going to leave with a lower temperature, a lower pressure, but it's still a vapor.
It may have some quality, meaning there's some fraction of liquid vapor en-trained in
the vapor but this change in enthalpy is where the work comes from.
This is the whole objective is to get some work out.
But to complete the cycles we have to take this vapor and condense is and what we're
doing here is make it a liquid so we can raise it back to high pressure.
So we ended with a liquid, we are at low pressure, because we left the turbine at low pressure.
We have to remove quite a bit of heat.
This low temperature reservoir is something like a river or a lake or a cooling tower
where we're removing enough energy to make a liquid.
And we want a liquid here because we'll have to put energy in to raise the pressure back
up so we can get something that will boil at high temperature and we can complete the
cycle.
It's much easier, it takes much less energy to pump a liquid to high pressure than to
pump a gas.
You have a very small volume change.
This is a small number, absolute value, compared to this, which we hope, absolute value is
a big number.
So we go around the cycle and some fraction of this high temperature source is converted
to work.
And we can talk about the efficiency of such a process as the net work, so let's add these
two terms, the work from the turbine, here, and the work, so this is from the pump.
The work out is going to be negative, work in is going to be positive, and we divide
this by the heat added.
So you can look at this efficiency as the heat added, that's what we have to pay for,
and the net work out is what we can sell.
So some fraction of this energy, for example a typical fraction might be .4, 40% of our
heat, 40% of the energy in the coal we can convert to net work.
So some fraction of the work of course from the turbine has to go to operate this pump.
This is the cycle that's used large scale, some variations of it, for essentially all
electricity generated.
We're boiling a liquid and we're taking advantage that it doesn't take much energy to raise
the pressure of a liquid to complete the cycle.