# Why don't we use hot water instead of steam to drive the turbines?

I guess if we do this then, the size of the turbines could become smaller. The condensor's size also might be reduced. But the catch over here is the energy density of hot water is less then that of steam. This could be overcome by increasing the amount of water sent in.

• Have you done any comparative calculations? You do have a set of steam tables to hand? Jul 25, 2019 at 14:32
• This is not an exact duplicate question, but the answer has been given before here: engineering.stackexchange.com/questions/28254/… Basically, these systems rely on the large energy changes that occur as a result of changing phase from a liquid to a gas (and vice versa). Running on entirely liquid (hot water) has the same inherent problems as entirely gas (hot air). Jul 25, 2019 at 15:21
• How would you pump the water through a turbine, using only heat? In fact hot water in a turbine would not be much different from cold water, which works very well in hydroelectric power generation. Jul 25, 2019 at 15:27
• @JonathanRSwift steam can hold more energy than air... Jul 25, 2019 at 15:44
• Thank you, everyone, for helping me out. I am satisfied Jul 25, 2019 at 16:15

Most people will probably say to "Look at a steam table". While they're not wrong, I think what you're looking for is a less textbook answer, and to understand the abstract concept better.

Lets look at what happens when you run cold water through a turbine, such as in a hydraulic dam power plant. The water in this case, an incompressible fluid, has a static head pressure created by the head-height of the water. Hydraulic power plant turbines, placed well below the top of the water height, utilize this head pressure to generate electricity. Now, what mechanism is used to generate that electricity? Is it heat? Does the water entering a hydraulic turbine come out of the other end colder? No, it certainly doesn't. So what mechanism is used? Is it the velocity of the water? Is it the pressure of the water?

Now, lets transfer over to a steam turbine. Don't worry too much about the type for right now - just think about a turbine blade using steam as the working fluid. What is driving the actual motion of the turbine blades? Is it heat? Well, we do know that the fluid changes temperature as it travels through the turbine.... but is that what is pushing those blades around, physically imparting the working fluid energy and creating a torque force on the blades? How does heat "push" the turbine? Hmm, doesn't make much sense does it... What other forms of potential energy are there, though? Kinetic? Sure! Oh, and pressure! Pressure is there too! Well shoot, which one is it? is it pressure or is it kinetic energy (velocity)?

The answer is: it depends on the turbine design. Via Wikipedia:

Impulse turbines change the direction of flow of a high velocity fluid or gas jet. The resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. There is no pressure change of the fluid or gas in the turbine blades (the moving blades), as in the case of a steam or gas turbine, all the pressure drop takes place in the stationary blades (the nozzles). Before reaching the turbine, the fluid's pressure head is changed to velocity head by accelerating the fluid with a nozzle.

Reaction turbines develop torque by reacting to the gas or fluid's pressure or mass. The pressure of the gas or fluid changes as it passes through the turbine rotor blades.[3] A pressure casement is needed to contain the working fluid as it acts on the turbine stage(s) or the turbine must be fully immersed in the fluid flow (such as with wind turbines). The casing contains and directs the working fluid

So - We know the working mechanism now behind a turbine. It's either Kinetic (velocity component), pressure (reactionary, mass or simply pressure component), or some combination of the two energies. So again we ask, Why don't we use hot water instead of steam to drive the turbines?

Well, if we're going to run hot water through a turbine, it has to have either a velocity or pressure potential energy. Where are we going to get that energy? Probably a pump right? But if we're using a pump to produce velocity or pressure head (kinetic or pressure component).... won't we just be using the same amount of energy to create that potential energy? We wouldn't be gaining anything! it would take just as much energy to pressurize/speed up the water as we would get in the form of useful work, except we would lose much to friction! That doesn't seem like it would work well... Well what about steam? Why would steam work better?

If we take that same hot water, use that pump to pressure it up, but then ALSO heat up the water to the point it turns into steam, then we just added more energy in the form of heat, but that heat was able to turn the water into steam, which is a volume increase of more than 1000x! Now remember that we have this fluid in an enclosed space, so volume is fixed. What happens instead? we produce either a pressure component or we produce a velocity component (or some of both). Hey, that's what we need to turn the turbine!

So, if we only utilized a pump and just pumped hot water around in circles, we wouldn't gain anything because we lack an additional pressure component or velocity component. The reason we utilize steam is that we can take that heat component that isn't able to turn the turbine in its current form and we change it into a pressure component or velocity component, or a bit of both. Now, if you're feeling ambitious, you could pull up a P-V diagram of a Rankine cycle and see if you can draw what the alternative path would be if you never boiled the water into steam.

*disclaimer: there may be some details when it comes to the actual calculations that don't quite translate to what I've described here. If I made a mistake just point it out and I'll fix. Thanks :)

The way I like to think about it is what source of energy drives the process. The function of a steam turbine is effectively to translate chemical energy into mechanical energy. Heat is generated by the burning of some fossil fuel that then is used to heat up water. That water/steam is under a confined condition so that when the temperature increases, the volume is not allowed to increase at a comparable rate so pressure increases. That pressure is then released in such a way that it converts some of the pressure into kinetic energy. That kinetic energy is then imparted onto the turbine and converted into kinetic energy in the turbine. The turbine then converts the kinetic energy into usable electrical energy via a generator.

The start point of the chain is the chemical energy. The trick is converting the chemical energy into mechanical then into electrical. By heating the water to steam the steam at temp will have pressure and the pressure is stored mechanical energy, which is available to use. Heated water only gets stored thermal energy because pressure doesn't go up with temp for liquid water unless you're in one of the parts of the steam table where you're super heated and at extreme pressure so the water is technically not gaseous yet.