Tower of power (baseload solar)

[OKIsItJustMe]

http://www.theengineer.co.uk/Articles/303825/Tower+of+power.htm

Tower of power

Published: 03 January 2008 03:30 PM
Source: The Engineer Online

Hamilton Sundstrand and the US Renewables Group are to commercialise a concentrated solar power tower technology and corresponding molten salt storage system developed by United Technologies' Rocketdyne division through a new company known as SolarReserve.

The power tower system itself uses a large field of mirrors to concentrate sunlight onto the top of a tower, where a receiver sits. This heats molten salt flowing through the receiver. Then, the salt's heat is used to generate electricity through a conventional steam generator.

Molten salt retains heat efficiently, so it can be stored for days before being converted into electricity. That means electricity can be produced on cloudy days or even several hours after sunset.

'Due to the unique ability of the product to store the energy it captures, this system will function like a conventional hydroelectric power plant, but with several advantages. We will have the capability to store the sun's energy and release it on demand. This product is more predictable than water reserves, the supply is free and inexhaustible, and the environmental impact is essentially zero,' said Lee Bailey, managing director of the US Renewables Group.

...

[DCKit]

1. THIS technology could be the Apollo program of renewable energy...

from Florida to California and north as far as Colorado: The one thing we could begin now and get off imported oil within ten years. Betcha the cost of the Iraq invasion, the ruination of the U.S. economy and the devaluation of the dollar would have paid for it many times over. And, unlike our current situation, we'd have had something to show for the investment.

Thanks Dick and Georgie.

[Dead_Parrot]

2. Could be useful

Although "several hours after sunset" suggests that calling it "baseload" is bit optimistic.

[AlecBGreen]

3. Thats the million dollar question

"Molten salt retains heat efficiently, so it can be stored for days ..."

HOW efficient? Lets see some real numbers. If it retains, say, 80% of its heat after a full day, Id be impressed. Until they produce real data for us to evaluate, its just another "fluff" piece, eye-candy for the greens.

[Dead_Parrot]

4. Well, I see no reason why it couldn't manage 80 or 90%

But I think it goes down to how much it keeps hot.

With apologies for the mixed units: If it's holding around 22GJ of heat energy when the sun goes down and can convert it at 50% eff., that's 3GWh of juice: if that drops to 18GWh after a day, it's still 2.5GWh. But you'd need 4 or 5 1GW plants to get you through the night - more, if it's going to be overcast.

Some really-real numbers (and costs) would be a lot more useful than my guesswork, but it's not entirely without merit. Yet.

[TheWraith]

17. The efficiency of heat retention isn't the biggest problem.

It's converting from heat to electricity, which is ridiculously lossy.

I'm afraid, like almost everything about the "solar revolution," this is so much puff. We've got to quit chasing solar. The only "renewable" technologies with any kind of serious power production are hydro, and to a lesser extent wind.

[OKIsItJustMe]

18. An interesting attitude

I'm not sure exactly what you mean by "hydro." If you mean conventional hydroelectric, there's not a lot of development to be done there.

http://www1.eere.energy.gov/windandhydro/hydro_potential.html

Hydropower Resource Potential

DOE has completed a resource assessment for 49 states (no report was generated for Delaware because of scarce resources). The completed work has identified 5,677 sites in the United States with undeveloped capacity of about 30,000 MW. By comparison, today there is about 80,000 MW of hydroelectric generating plants in the United States.

...

Tidal and/or wave power are tantalizing, but we don't have much experience with them.

Wind power is going great guns.


You complain about the efficiency of concentrating solar as compared to "hydro." Why?
http://www.energylan.sandia.gov/sunlab/faqs.htm

...
Question: Do concentrating solar plants require a lot of land?

Answer: Relatively speaking, no. Consider the Hoover Dam. Lake Mead covers nearly 250 square miles. A Concentrating Solar Power system occupying only 10-20 square miles of land could generate as much power on an annual basis as the Hoover Dam did last year. Considering the land required for mining, concentrating solar power plants also use less land than coal power plants.

...

[OKIsItJustMe]

5. Solar Two

http://www.wipp.energy.gov/science/energy/powertower.htm

...

What is the Status of Power Tower Technology?

Power towers enjoy the benefits of two successful, large-scale demonstration plants. The 10-MW Solar One plant near Barstow, CA, demonstrated the viability of power towers, producing over 38 million kilowatt-hours of electricity during its operation from 1982 to 1988. The Solar Two plant was a retrofit of Solar One to demonstrate the advantages of molten salt for heat transfer and thermal storage. Utilizing its highly efficient molten-salt energy storage system, Solar Two successfully demonstrated efficient collection of solar energy and dispatch of electricity, including the ability to routinely produce electricity during cloudy weather and at night. In one demonstration, it delivered power to the grid 24 hours per day for nearly 7 straight days before cloudy weather interrupted operation.

The successful conclusion of Solar Two sparked worldwide interest in power towers. As Solar Two completed operations, an international consortium, led by U. S. industry including Bechtel and Boeing (with technical support from Sandia National Laboratories), formed to pursue power tower plants worldwide, especially in Spain (where special solar premiums make the technology cost-effective), but also in Egypt, Morocco, and Italy. Their first commercial power tower plant is planned to be four times the size of Solar Two (about 40 MW equivalent, utilizing storage to power a 15MW turbine up to 24 hours per day).

This industry is also actively pursuing opportunities to build a similar plant in our desert Southwest, where a 30 to 50 MW plant would take advantage of the Spanish design and production capacity to reduce costs, while providing much needed peaking capacity for the Western grid. The first such plant would cost in the range of $100M and produce power for about 15¢/kWh. While still somewhat higher in cost than conventional technologies in the peaking market, the cost differential could be made up with modest green power subsidies and political support, jump-starting this technology on a path to 7¢/kWh power with the economies of scale and engineering improvements of the first few plants. It would, at that point, provide clean power as economically as more conventional technologies.

...

http://www.osti.gov/bridge/servlets/purl/751185-e05IMD/webviewable/751185.pdf

[OKIsItJustMe]

6. More on Solar Two

http://www.azsolarcenter.com/links/faqs/solar.pdf

[OKIsItJustMe]

8. Solar Tres

http://en.wikipedia.org/wiki/Solar_Tres_Power_Tower

[phantom power]

7. Are there any figures that describe the energy storage density for molten salt?

As in, "we can store X kilowatt-hours worth of energy per Y metrics tons of molten salt."

To build a baseload plant of say, one gigawatt, you roughly need 4 gigawatts of solar collector, and 3 gigawatts worth of molten salt energy storage. I have to assume that's "rather a lot" of molten salt, but it would be useful to know a number.

Because we all thrive on numerics, here in E/E.

[OKIsItJustMe]

9. Solar Tres specifications answer your question

http://en.wikipedia.org/wiki/Solar_Tres_Power_Tower

...
A larger thermal energy storage system, storing 6,250 T of molten nitrate salt (16 hours, 600 MWh).
...

Let's call it 10 T/MHh.

[phantom power]

10. Aha. Then, for a gigawatt-class baseload installation...

I'll assume we want to store 18 gigawatt-hours (using the usual 25% loading figure). At 10,000 T per GWh, we get 180,000 T of molten salt. The density of molten salt is about 1.6 grams per cubic centimeter (1.6 T per 1,000 L = 1m^3). So, that would occupy a volume of 112 thousand cubic meters (112 million liters). A cube of molten salt 48 meters on a side. (And hot. At some temperature > 800C)

Also, I'm assuming sodium chloride. And metric tons.

[OKIsItJustMe]

11. I wouldn't assume you need 18 hours of storage

Remember, power usage is not constant; peak demand is during the daylight hours. (When, by coincidence, available power is at its peak.)

http://www.azsolarcenter.com/links/faqs/solar.pdf

... Solar Two was the world’s largest power tower, producing 10 MW of electricity with enough thermal storage to operate the turbine for three hours at full capacity.

...

Solar Two achieved the following major breakthroughs in utility-scale solar power technology. It:

• Routinely produced electricity during cloudy weather and at night,
• Delivered power to the utility grid 24 hours/day for 7 straight days, and
• Demonstrated a 97% energy-storage efficiency.

...

As for the salt:
http://www.azsolarcenter.com/links/faqs/solar.pdf

...

The salt storage medium is a common fertilizer,a mixture of 60% sodium nitrate and 40% potassiumnitrate. It melts at 220°C and is always molten in the "cold" storage tank. Molten salt is used because it is inexpensive and provides for efficient storage (99%); it is liquid at atmospheric pressure and its "hot" operating temperature perfectly matches the needs of today’s high-pressure and high-temperature steam turbines. The molten salt is safe since it is nonflammable and nontoxic.

...

[phantom power]

12. Or, you might want to assume more

if you are engineering for N consecutive cloudy days.

[OKIsItJustMe]

13. Well... let's make a bold assumption...

Let's assume (just for the sake of argument) that the people who've actually done the research have more knowledge about it than either of us looking on from the sidelines. ;-)

http://www.azsolarcenter.com/links/faqs/solar.pdf

By integrating 13 hours of thermal storage into the design, a power tower can operate around the clock. Other solar technologies (like photovoltaics) do not have cost-effective energy storage and, thus, only produce electricity during daytime periods.

(This is the description of an illustrative graph. You may want to check it out.)

[phantom power]

14. I don't assume that. Because they aren't answering the same questions I'm asking.

I have never seen any purveyor of a solar technology commit to a claim of the form "if we deploy this much storage, we can guarantee X megawatts, 24/7, for up to N cloudy days, with probability 99.99%"

Saying "we can operate around the clock." tells us nothing about whether they can operate at a constant rate around the clock, nor does it tell us whether they are immune to a string of cloudy weather, nor for how many days of cloudy weather.

It wouldn't surprise me if somebody has done those calculations, but I haven't seen them. It's probably not an easy calculation to do, since you have to take a lot of random variables into account, and any answer you come up with will by necessity come with a probability attached to it. (which is fine, because all forms of energy come with an up-time probability attached to them)

I would like to reiterate that I don't post these nit-picky questions because I want to knee-cap renewable energy. I post them because (a) I'm truly curious, and (b) because those are exactly the questions that an electric utility would absolutely need the answers to, in order to make any serious plans for incorporating a significant amount of renewable energy. So far, people have gotten away with waving their hands at these questions because solar and wind have accounted for an insignificant amount of energy, and even if they dropped off altogether, nobody was counting on them anyway. As they deploy more and more of this stuff, renewable energy vendors are going to have to get down and dirty, and start addressing the really hard problems.

And, to grind my other axe, we are all going to find out what the actual economic and environmental costs really are, which I believe will be a surprise to most people. I predict that in 20 years, many people will find themselves feeling betrayed and angry from decades of claims about "free" and "clean" energy. Assuming that we aren't otherwise occupied by running for our lives because we chose to burn coal instead.

[OKIsItJustMe]

15. Have you read the papers I've cited?

It's not as if the folks at Sandia threw darts at a dartboard or something like that.

[phantom power]

16. No, but I'll read them if you think it will help.

Probably not today. I've got a proposal deadline coming down on me like a freight train. It sometimes occurs to me that I should really, really consider spending more time doing what my employers assume I'm doing.