Turbine that extracts useful heat by condensing steam and utilizing heat of vaporization?

Question

Most engine cycles involve a turbine followed by a condenser and pump to recirculate liquid water to a heat source and the cycle repeats. Is it possible to extract more work from the steam by condensing it in the turbine itself and using the heat of vaporization of steam? Is there any scientific literature or documentation on it?

Answer 1

"Condensing" turbines do exist, but not in the way that you describe. To get the maximum differential pressure across a turbine, which extracts maximum work from the steam flowing through the turbine, it is possible to build a closed system which contains only steam and no air. When a condenser is placed at the discharge of such a turbine, the exhaust steam from the turbine can be condensed under vacuum conditions.

What isn't done, and is avoided by careful design of the process, is installing a turbine which experiences steam condensing into water droplets on the blades of the turbine. For the speeds that turbines operate, such conditions are quite detrimental to turbine blades, with water droplet impingement causing wear and erosion.

Answer 2

The simple answer is the turbine won’t turn if you try to use the enthalpy (heat) of the condensing process. That’s because the condensing process occurs at constant (low) pressure and therefore there will be no pressure difference (net force) to drive the turbine.

Another thing is turbine blades don’t like to get wet. It makes them corrode. This is why the steam exiting the turbine is usually either saturated steam or high quality (I.e. low moisture) steam.

Hope this helps

Answer 3

Most engine cycles involve a turbine followed by a condenser and pump to recirculate liquid water to a heat source and the cycle repeats.

Let's have a look at such a heat engine, the turbine of the new nuclear power plant OL3. The wet steam that is coming from the low-pressure turbine and going to the condenser has the following design parameter values. $$\begin{align} p_1&=0.0289\ \mathrm{bar}\\ h_1&=2277.4\ \mathrm{kJ/kg}\\ T_1&=23.46\ \mathrm{^\circ C}\\ x_1&=0.8911 \end{align}$$ (Note that actually the steam has already started to condense at this point. By mass, only $89.11\ \%$ are still steam, the other $10.89\ \%$ are already liquid. Steam turbines of most nuclear power plants are operated with saturated steam; i.e. the steam is not superheated. In this case, the steam entering the high-pressure turbine may already contain up to $0.25\ \%$ liquid droplets. This is important for the design of the main steam system and the turbine since liquid droplets could damage the blades of turbine since the turbine is turning with a speed of $1500\ \mathrm{min^{-1}}$ and the last stage blades have a diameter of $6.72\ \mathrm m$; thus, their tips are moving with a speed of more than $500\ \mathrm{m/s}$. Furthermore, this means that the condensing water has to be drained from the individual stages of the turbine and liquid water has to be separated from the wet steam before entering the low-pressure turbine. Your suggested change of the operating principle would drastically change the design of the last expansion stage of the low-pressure turbine since it would have to handle a flow rate of about $1200\ \mathrm{kg/s}$ of liquid (!) water.)

Coming back to the real example: After passing through the condenser, the (now liquid) water has the following design parameter values: $$\begin{align} p_2&=0.0289\ \mathrm{bar}\\ h_2&=98.4\ \mathrm{kJ/kg}\\ T_2&=23.46\ \mathrm{^\circ C}\\ \end{align}$$ Note that (approximately) temperature and pressure have not changed. In the condenser, steam (that was already in equilibrium with liquid water) has been completely condensed at constant temperature and pressure.

Is it possible to extract more work from the steam by condensing it in the turbine itself and using the heat of vaporization of steam?

So essentially what you want to do is to utilize the difference between $ h_1=2277.4\ \mathrm{kJ/kg}$ and $h_2=98.4\ \mathrm{kJ/kg}$ in order to do some additional work.

Just looking at the first law of thermodynamics (i.e. conservation of energy), this should actually be possible. However, this process would violate the second law of thermodynamics. Remember that $T_1=T_2$, i.e. the wet steam entering the condenser and the water leaving the condenser are already in a thermal equilibrium. According to Carnot’s principle, there is no heat available that could be converted to work; i.e. the efficiency of this step is zero. Your suggested design change could be considered a perpetual motion machine of the second kind (a machine which spontaneously converts thermal energy into mechanical work).