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 Post subject: Making HTR-PM more efficientPosted: Jul 28, 2012 10:57 pm

Joined: Apr 23, 2011 8:47 pm
Posts: 104
First, let’s compare different energy power efficiency.
See image
Source for HTR-PM 40%
“driving a single steam turbine at about 40% thermal efficiency.”
Note: there are better sources to view HTR-PM but this site showed 40%
http://nextbigfuture.com/2008/06/worlds ... -high.html
We can add on to this by
“The 250ºC engine will convert industrial exhaust heat to power, converting the energy from a temperature differential of 230 degrees.
This engine will operate at 25% efficiency
The expander will produce power with an efficiency of 24%. Coupled with a 250ºC heat engine, the system will produce power at 49% electric efficiency
http://www.rgpsystems.com/technologies.html
Note that expander performs at 40%. Therefore systems should produce more then 40%
Question: What is the outlet temperature of steam? Is it 200C* 20% efficiency , 100C* 13% efficiency.
Purpose: To make nuclear power as efficient as fossil power. NOW not 20 years from now.
If exhaust steam was 200 C*, nuclear power efficiency would be at least 55%
Inlet core temperature is at 200C*

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Last edited by Wilson on Jul 29, 2012 1:01 am, edited 1 time in total.
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 Post subject: Re: Making HTR-PM more efficientPosted: Jul 29, 2012 12:30 am

Joined: Apr 23, 2011 8:47 pm
Posts: 104
"Main feed-water temperature, 205 C*"
Nuclear Safety and Simulation, Vol. 2, Number 2, June 2011

Looks like this could hit 55% efficiency

NOW, not 20 years from now.

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 Post subject: Re: Making HTR-PM more efficientPosted: Jul 29, 2012 6:49 pm

Joined: Apr 23, 2011 8:47 pm
Posts: 104
Problem here is these stirling engines are piston, not turbines. Small heat to power systems. 1 megawatt
We need to have a turbine system that is as efficient as rgpsystems stirling engine.
At 205 C*
http://www.rgpsystems.com/technologies.html

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 Post subject: Re: Making HTR-PM more efficientPosted: Jul 30, 2012 1:22 pm

Joined: Dec 14, 2006 1:01 pm
Posts: 379
I applaud your inventiveness, but this ground seems well-plowed to me.

The regenerative people have a viable system for industrial waste heat, but most power plants already use that heat. Most steam power systems have a rejection temperature just a bit higher than a hot summer day, say 35-40C. It's difficult to extract more energy from this.

Combined cycles can work. The most popular modern combined cycle is for natural gas, which combines a gas turbine as a topping cycle with a steam cogeneration system as a bottoming cycle. This combination reaches 60% in the GE H-frame turbine, but that's possible in large part because the cycle starts at a 5,000C combustion temperature, and everything else is well-tuned.

My favorite lost technology uses magnetohydrodynamic generators as a topping cycle for coal. This is completely developed (there's more than 70 years of continuous development history, ending with a successful prototype run by the U.S. DOE), it works -fine-, adds 17% on top of a coal plant's 42%, and costs only slightly more than a combined-cycle natural gas power plant. That is, they are uneconomic.

The problem with bottoming cycles is that the equipment gets very large, very quickly, for only a small return of energy. The real trick is not whether a bottoming cycle is possible, but rather how to construct an immense heat engine on the cheap. There are two schemes I've seen that have real promise:

One is the solar updraft tower: http://en.wikipedia.org/wiki/Solar_updraft_tower, which has been successfully prototyped several times. This could easily be goosed with the waste heat of a conventional power plant.

Another version of this cleverly eliminates the expensive part, the chimney, substituting an artificial vortex, a low-density tornado-in-reverse: http://en.wikipedia.org/wiki/Vortex_engine The inventor clearly intended this weird thing to act as a bottoming cycle for a conventional power plant. Either nuclear or coal would work, because the heat engine is so huge that it could use 35C heat on a cold day. I think a full-size vortex engine would be something to see, and its big advantage is that it just adds power-generation equipment and a bit of land to the existing heat rejection equipment that a power plant already needs. Vortex engines aren't in use because nobody wants to take the risk of building the first full-sized one. It's less-risky to make the conventional part of the power-plant 15% larger.

The 55% closed-cycle regenerating gas turbines advocated by various people in this duscussion group are difficult only because prototypes cost a half billion dollars. Potchefstroon University in S. Africa built a twenty-megawatt prototype. The expense hit when they needed to scale it up 10x, and this is what killed the S. African pebble-bed reactor program.

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 Post subject: Re: Making HTR-PM more efficientPosted: Jul 30, 2012 11:25 pm

Joined: Apr 23, 2011 8:47 pm
Posts: 104
Thank you for your input rgvandewalker
What dos this mean?
Steam generator inlet water 205C*
Outlet superheated steam temp 570C*
From status of HTR-PM

Dos that not imply that steam is not cooled down to 35-40C* as you say but is only cooled down to 205C*?

Also, where I live, Calgary, a utility company is installing a AD NGCC 800 MW power plant and they are planning to sell the waste heat on top of that. I have emailed them to see what temperature this waste heat would be at but have not had a reply from them.

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 Post subject: Re: Making HTR-PM more efficientPosted: Sep 09, 2012 2:03 pm

Joined: Dec 14, 2006 1:01 pm
Posts: 379
Facts are wonderful. The likely rejection temperature for a district heating combined-cycle will be just below the bubble-formation temperature of water. Bubbles would erode the pumps. Maybe 95C?

Perhaps there's a preheat heat exchanger in there somewhere to get the steam inlet up to 205? 30C is a minimum for an outfall. Outfall waste-heat heat exchangers often emit at higher temperatures to reduce the size of the equipment, or the pumping energy, or the electricity needed for the fans. A high temperature outfall could be a real opportunity. I would worry about the details. For example:

Steam generator inlet water is often preheated, for example by reverse-flow heat exchangers warmed by the condenser water before routing the condenser water to the outfall heat exchanger (if environmental cooling water is used). If the condenser water is run to a bottoming cycle, more fuel might be needed to heat the steam generator inlet water. If the fuel needed is enough, it might actually increase power costs and pollution. It's a trade-off between the efficiency of the preheat heat exchanger (and its sunk costs), the fuel costs, and the value of the extra power from the bottoming cycle, and the capital costs and plant-shutdown costs (lost revenue, missed bond payments, etc.) of installing the new equipment.

The sunk preheater and shutdown costs might make a bottoming cycle uneconomic to add to an old plant even if it could be economically built into a new plant of the same type.

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 Post subject: Re: Making HTR-PM more efficientPosted: Sep 09, 2012 3:26 pm

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
The steam is cooled down to 30-40 C. But it is usually preheated before it goes into the boiler or reactor. This is done with steam bled off from the various turbine stages, sent to a HX where it heats up the incoming feed water. The feed water, initially at 30-40 C would be increased to a quite high temperature, usually 200-350 C. This increases the final temperature at which you get the steam/supercritical water, all things being equal (you can only transfer so much heat so having the cold feedwater at a higher temperature makes for hotter steam). It took a long time for me to get this.

A HTR-PM with efficient steam cycle is very difficult, because the steam cycle must operate at much higher pressures than the helium reactor coolant to be efficient. Ideally you want the supercritical Rankine turbines. These operate at more than 240 atmospheres. So there's a potential driver to put steam in the reactor. That's bad because steam rapidly corrodes the graphite. The attraction with a direct helium cycle is that you avoid all these problems. You also avoid some of the complexities of the Rankine cycle and get a simpler, once through, direct, helium Brayton cycle. However Brayton cycles are not actually that efficient per degree temp difference, because they need a lot of power to run the compressor. The point about Brayton is that you can use a higher temperature to offset that penalty. Then you have a higher temperature, but simpler, heat engine than the Rankine steam cycle.

The Carnot efficiency for a 250 C hot, 20 C cold, heat engine, is 44%. So it is not possible to get to 49% with this temperature, not even theoretically. The 49% is probably referring to combined cycle efficiency, with the first heat engine efficiency added in. Which is not very good since state of the art large CCGTs are 60% today. But maybe it's apples and oranges if the application is industrial heat cogen.

Rule of thumb. Usually an efficient heat engine can extract 60 to 80% of the theoretical. So 26 to 35% is the best hoped for achievable for a 44% carnot. For a small heat engine you'll never get the 80%, only the biggest most efficient cycles get those. So the company's claim of 25% efficiency with 250 C heat source sounds reasonable.

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