How does the steam turbine work in a solar thermal power plant?

[Solibus]

(English is not my native language so I apologize for the rest of the post)

Hi everyone!

I'm an engineering student and I'm currently working on a solar thermal project. I'm trying to model a concentrating solar power (CSP) plant according to DNI (Direct Normal Incidence) change during a day, in the objective of knowing the electricity generation of the plant.

As a first step, I would like to model it with an ideal Rankine cycle:

• 1-2 Isentropic compression in a pump
• 2-3 Constant pressure heat addition in a boiler
• 3-4 Isentropic expansion in a turbine
• 4-1 Constant pressure heat rejection in a condenser

But I'm still trying to understand how does the steam turbine work in a solar thermal power plant. Right now, if I have to follow the Rankine cycle, I would think that the inlet temperature and pressure, and the outlet pressure are all steady parameters. And probably the steam mass flow rate decreases if the DNI decreases as well. So the turbine will rotate more slowly and generate less electricity. Can you tell me if it actually happens like this?

But can I find somewhere on internet some charts with for example the steam inlet parameters and it will show me the electricity production of the generator and the steam outlet parameters?

Thank you for your help.

 

[rock.freak667]

But I'm still trying to understand how does the steam turbine work in a solar thermal power plant. Right now, if I have to follow the Rankine cycle, I would think that the inlet temperature and pressure, and the outlet pressure are all steady parameters. And probably the steam mass flow rate decreases if the DNI decreases as well. So the turbine will rotate more slowly and generate less electricity. Can you tell me if it actually happens like this?

Your solar plant would work in the same manner as a normal plant. Except that instead of a boiler providing heat to make steam, your solar cells would provide the energy to convert the water to steam.

Are you given some parameters to work with? Such as how much electrical energy do you plan to produce, what steam flow rate you would like to use, etc.

But can I find somewhere on internet some charts with for example the steam inlet parameters and it will show me the electricity production of the generator and the steam outlet parameters?

I don't know if you will find a chart like that since the output from the turbine will depend on design and isentropic efficiencies which vary with turbine design.

 

[Danger]

I can't say anything about your question, Solibus, but I do want to assure you of one thing. You express yourself immaculately in English, better than a huge percentage of people born into this language. Don't ever apologize for your wordage again, lest I seek ye out and smack ye with a salmon.

 

[Solibus]

Thank you for your answers and from now on I will be careful not to be smacked with a salmon!

I would like to run a 1 MW power plant using only the energy from the sun. I don't really know what steam flow rate I would like to use (so if you have some interesting resource about this topic I'm interested in more information), but according to some similar thermal power plants, the inlet parameters of the turbine may be a temperature of 400 °C and a pressure of 2.5 MPa.

Before your answer, I thought the turbine inlet temperature and pressure are always constant. So depending on the available energy from the sun, the mass flow rate of steam will not be the same. Thus, the electrical energy production will also vary during the day.

According to your point of view, a solar thermal power plant is a binary system. The power plant will start working when the solar field will produce enough energy to run the turbine. So in this case, the turbine inlet temperature, pressure and flow rate will be always constant. But in the solar field, the mass flow rate of the heat transfer fluid will vary depending on the energy available from the sun and when enough energy is produced, the steam boiler will start working. Am I right?

Thank you for your help.

 

[SteamKing]

If you are going to use your turbine for electric power generation, it would be ideal that steam with the proper inlet temp. and pressure be furnished at a fairly constant rate, otherwise the power grid for which the turbine is generating electricity would see instability in the amount of current being generated, which is hard on electrical devices.

The attached brochure for Elliott turbine gives you a range of machines to look at. You can see the inlet conditions for the various units and a chart of steaming rates (the amount of steam consumed by the turbine to produce the design output is given at the top of page 4.

http://www.elliott-turbo.com/Files/Admin/Literature/tg.pdf

 

[jim hardy]

Your English is fine and your physics seems fine, too.

Indeed the energy you collect will be variable throughout the day.

The first thought is -Your cycle efficiency is better at higher temperatures so you'll want to vary flow to match how much water you can boil into steam at your chosen temperature.
BUT steamflow is set by the turbine throttle valves, so there's a throttling process at turbine inlet. That cools the steam before it gets to the blades.
My employer experimented with with sliding boiler pressure and wide open throttle valves on superheat units .

Since this is a simulation, experiment and see what falls out of the thermodynamics?

A friend of mine works at this solar-hybrid plant

http://www.fpl.com/environment/solar/images/martin.JPG
It uses solar collectors to preheat water for a conventional boiler. Every BTU from the sun is one they didn't have to buy from the gas company.
Solar collectors collect visible but re-radiate infrared hence don't do very well above ~400 degrees F. That's only about 250 psi, so they get better return on investment by using it as a preheater for a higher pressure cycle.
http://www.nytimes.com/2010/03/05/business/05solar.html?_r=2&Your little 1mw 250 psi plant would be a lot of fun.
Guess I didn't really tell you anything about your question...

But in the solar field, the mass flow rate of the heat transfer fluid will vary depending on the energy available from the sun and when enough energy is produced, the steam boiler will start working. Am I right?

Yes.

I thought the turbine inlet temperature and pressure are always constant.

As designer that is your choice..

So depending on the available energy from the sun, the mass flow rate of steam will not be the same. Thus, the electrical energy production will also vary during the day.

Right.

 

[sanka]

You also need to consider your condenser. As it is a CSP plant, I assume there is little water avaliable for wet-cooling and instead an ACC will be used. If this is the case, the ambient temperature of the cooling air will largely govern the condenser temperature & pressure and consequently determine the exit conditions of your turbine.

For example, if the ambient temp increases dramatically (as it would in a desert location during the afternoon), the condenser temp & pressure will also increase. This means that the pressure at the steam turbine exit will be higher and thus, the work extracted from the turbine will be less.

Just something to consider!

 

[Solibus]

Thank you for all your explanations and sorry for the late reply.

Jim Hardy, is there any general formula to determine the output power depending on the mass flow rate of the steam? I would like to estimate the electrical energy generation in a direct steam generating power plant working only with the sun's energy.

Sanka, can you tell me more about the exit conditions of the turbine? If the ambient temperature increases dramatically, I wonder whether the work extracted from the turbine is less with an air cooled condenser or also with a wet cooling tower? Do you know some resources online about this topic?

Thank you for your help.

 

[jim hardy]

Thank you for all your explanations and sorry for the late reply.

Jim Hardy, is there any general formula to determine the output power depending on the mass flow rate of the steam? I would like to estimate the electrical energy generation in a direct steam generating power plant working only with the sun's energy.

.

Energy Out = (Inlet Specific Enthalpy - Outlet Specific Enthalpy) * Mass Flow
so you must define your inlet and outlet steam conditions.

Here's a handy calculator that'll let you play with the numbers.
It let's you define turbine inlet and exhaust conditions so you'll see the benefits of a condenser, and of superheat.

http://www4.eere.energy.gov/manufacturing/tech_deployment/amo_steam_tool/equipTurbineHave fun !

 

[sanka]

Thank you for all your explanations and sorry for the late reply.


Sanka, can you tell me more about the exit conditions of the turbine? If the ambient temperature increases dramatically, I wonder whether the work extracted from the turbine is less with an air cooled condenser or also with a wet cooling tower? Do you know some resources online about this topic?

Thank you for your help.

Solibus,

The exit conditions of your turbine are determined by the inlet conditions of your condenser. That is why condensers usually operate in sub-atmospheric conditions (i.e. a vacuum). This means there is a lower pressure in your condenser, giving a lower pressure at the exit of the steam turbine, thus resulting in a lager enthalpy drop through the turbine. The lower the pressure of your condenser, the more work will be extracted from the steam in the turbine.

The issue with an ACC is that the condenser conditions are determined by the cooling fluid, which is the ambient air. As such, on hot days (or in desert locations where the ambient is high), the minimum pressure which can be achieved in the condenser is the steam saturation pressure corresponding to the steam saturation temperature accordingto the ambient temperature. And this is an ideal scenario, when in reality you will have other contributing factors such as air leaks, condensate resistance, etc. which increase the pressure/temperature in the condenser even more! If the pressure in your condenser is relatively high, you will have a small(er) enthalpy drop through your turbine, resulting in less plant output. Not good.

To answer your question regarding ACCs vs wet-cooling, wet-cooling is generally preferable due to the fact that it uses water as a cooling medium, whose temperature remains relatively constant. Thus lower condenser pressures/temperatures can be achieved. Also, the thermal conductivity of water is a lot higher than air so in short, less surface area is needed to remove the heat from the steam, leading to reduced capital costs. Good!

Hope you found some of that useful.

 

[jim hardy]

Solibus,The issue with an ACC is that the condenser conditions are determined by the cooling fluid, which is the ambient air. As such, on hot days (or in desert locations where the ambient is high), the minimum pressure which can be achieved in the condenser is the steam saturation pressure corresponding to the steam saturation temperature accordingto the ambient temperature...

...
To answer your question regarding ACCs vs wet-cooling, wet-cooling is generally preferable due to the fact that it uses water as a cooling medium, whose temperature remains relatively constant. ...

I'll add one small point to Sanka's excellent reply:

In a dry desert situation, water cooling with evaporation can bring condenser temperature down to about the dewpoint which in someplace like Phoenix could be tens of degrees lower than ambient.
Palo Verde nuke plant uses Phoenix's waste water.
http://www.azein.gov/azein/Shared%20Resources/PVNGS%20Media%20Kit/Water%20Reclamation%20Facility_brochure.pdf

We experimented with a small cooling tower in S Florida. It's just too humid there to be worthwhile, might as well use ocean water.

Water evaporated from Palo Verde 's cooling towers, courtesy this guy: http://skywalker.cochise.edu/wellerr/month-photo/january2011.htm

That looks to be a very humid day in the desert - look how low are the cloud bottoms.

 

[Solibus]

Thank you again for all your very interesting explanation. However, it also worries me a little, because the exit conditions of the turbine or the inlet conditions of the condenser seems pretty hard to estimate.

I know the relation of the energy out, but in order to get the power output you need the isentropic efficiency and the generator efficiency. The generator efficiency is not a problem, you can assume a value of 95%. But the isentropic efficiency pose a problem, because this value will vary a lot depending on the mass flow rate of the steam. I can't find any relation to determine it.

I summarize your information in order to know if I well understood. In the case of an ACC, the minimum inlet pressure which can be ideally achieved in the condenser is the steam saturation pressure corresponding to the steam saturation temperature according to the ambient temperature. But in reality, the pressure is even more, so the temperature is also higher.
For example, if the ambient temperature is 20°C (295.55 K), we obtain a saturated vapor pressure of 0.57171 MPa. So the inlet pressure of the condenser will be even higher. But is it theoretically possible to estimate this pressure? Perhaps, there are some efficiencies approximation for different types of condensers.

In the case of a wet cooling system, the water temperature remains relatively constant and this temperature is never too high. Thus pressures/temperatures in the condenser are also relatively steady and also never too high.
Jim Hardy in you last post when you wrote water cooling with evaporation can bring condenser temperature down to about the dew point, are you talking about the condenser inlet temperature or about the outlet temperature of condensate water?

 

[sanka]

Thank you again for all your very interesting explanation. However, it also worries me a little, because the exit conditions of the turbine or the inlet conditions of the condenser seems pretty hard to estimate.

I summarize your information in order to know if I well understood. In the case of an ACC, the minimum inlet pressure which can be ideally achieved in the condenser is the steam saturation pressure corresponding to the steam saturation temperature according to the ambient temperature. But in reality, the pressure is even more, so the temperature is also higher.
For example, if the ambient temperature is 20°C (295.55 K), we obtain a saturated vapor pressure of 0.57171 MPa. So the inlet pressure of the condenser will be even higher. But is it theoretically possible to estimate this pressure? Perhaps, there are some efficiencies approximation for different types of condensers.

In the case of a wet cooling system, the water temperature remains relatively constant and this temperature is never too high. Thus pressures/temperatures in the condenser are also relatively steady and also never too high.
Jim Hardy in you last post when you wrote water cooling with evaporation can bring condenser temperature down to about the dew point, are you talking about the condenser inlet temperature or about the outlet temperature of condensate water?

No need to worry really, just a question of how accurate your analysis needs to be. Let me elaborate a little. You say it is hard to estimate the turbine exit conditions. I guess it is difficult because of the way the ambient varies during the course of a day, varies depending on location, etc. and how this effects your condenser. However, for a robust analysis, you should just assume some mean (average) ambient value based on a location representative of your CSP plant. Based on this mean value, you can estimate the condenser temperature and consequently the saturation pressure based on the estimated temperature. This will give you the exit conditions of your ST (steam turbine).

Regarding your question about calculating the condenser pressure based on ambient, I have not found anything in the literature that addresses this. I assume this sort of analysis has been done before but only on a proprietary basis (so it is kept out of the public domain) by ACC manufacturers. So to estimate your condenser temperature based on ambient, I would advise you to assume the condenser temperature to be a few degrees (celcius) higher than the ambient. This will always be the case in reality as there will be thermal resistances from the steam core to the ambient air. Be aware though, that guessing your condenser temperature is a very crude method of determining your ST exit conditions but for a simplified analysis it will be satisfactory as the differences in power out from your ST based on a few degrees error in your condenser will be small compared to the actual output from your turbine.

For example if you have a 50MW ST and you assume a condenser temperature that is a few degrees higher than nominal, you might get an output of 49.7MW which isn't a massive difference overall but I suppose it all comes down to a question of accuracy!

 

[jim hardy]

Dewpoint would be the lower limit on the condenser's heatsink. Your turbine exhaust will be a few degrees higher than that because of its own internal pressure and temperature drops.
This author places that difference in the range of 5 to 10 degrees F.

http://www.ndwr.state.nv.us/hearings/past/springetal/browseabledocs/exhibits%5CSNWA%20Exhibits/SNWA_Exh_123_Micheletti%20and%20Burns,%202002.pdf

Here's a tabulation of local dewpoint readings:
http://www.wrcc.dri.edu/htmlfiles/westcomp.dp.html


and an introductory article. I think you're past needing them, but the pictures are fun:
http://tpowertec.com/coolingsystems.htm
http://me.emu.edu.tr/haybar/ME446/Chapter5.pdf

 

[Solibus]

Thank you again for your messages.

@ Jim Hardy: are you talking about this extract:
"The difference in cold water and inlet air wet-bulb temperatures is known as the “cooling approach”. Most cooling towers are designed for an approach between 5 and 10 °F; for power plant cooling towers, the approach is generally 8 °F."

But the author doesn't talk about the turbine exhaust temperature in this extract. Am I right?

Actually I'm a little bit lost now. I thought for a long time that the cold water in the wet cooling tower had almost always the same temperature. My explanation was pretty simple, a wet cooling tower consumes a lot of of water, so the cold water is always renewed and probably this makeup water comes from buried pipes. They are buried because the temperature of the ground is almost constant throughout the year and in summer, this temperature is definitely lower than the ambient temperature which is good for the cooling efficiency.

Ok I was wrong, I probably overestimated the water make-up ratio. Hence, the cold water temperature is approximately 8°F higher than the inlet air wet-bulb temperature.

But in this case, the condenser temperature has to be even higher than this temperature, because it's physically impossible to reach the temperature of the cold water. I mean a condenser is a heat exchanger and it's not perfect. Am I wrong?

 

[SteamKing]

The coolant flow into the condenser must have a lower temperature than the exhaust steam in order for heat transfer to occur from the steam to the coolant. This is thermodynamics 101. The heat rejected by the condenser is largely the latent heat of vaporization of the steam, which allows the steam to change phase from vapor to liquid.

 

[Solibus]

Steamking: thank you for trying to help. I did a diagram to show my problem. In this case, the turbine is cooled down thanks to a wet cooling tower.



According to the paper: Emerging issues and needs in power plant cooling systems, the cooling approach which is the difference in cold water and inlet air wet-bulb temperature is between 5°F and 10°F, the approach is generally 8°F (=4.5°C).

If the ambient temperature is 28°C with a relative humidity of 45%. The wet-bulb temperature will be 19°C. Thus the cold water temperature will be approximately 24°C (temperature at the point 4).
The exit cooling water will be higher (so the temperature at the point 5 is maybe 34°C). Then, the turbine exhaust is even higher (so the temperature at the point 2 is maybe 39°C).

I will try with some values. Thank you for you help.

 

[marciokoko]

Solibus

I just read this thread and I am very interested in getting in touch with you. I am actually wanting to start a project about this matter. Could you contact me @nrgea

I have the idea of setting up a small project, I guess 1MW is more than ok for my purposes. My question is, the steam turbine is part of it. The collectors are another part and the actual container with the water to be steamed is the other. Do you guys know of an online resource to start learning about the design of the actual plant?