Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

Looking into Solar Thermal power systems.

with 8 comments

Atomic Insights has an interesting recent post asking some pertinent questions about solar thermal energy systems:

What are the steam cycle parameters? What is the overall thermal efficiency?

What is the cooling medium for your condensers?

How much water will the plant consume per unit of power?

Are the mirrors steered so that they track the sun?

What is the installation cost per unit of energy produced each year?

These are good questions – they’re worth asking. I’m very interested in learning the answers to these questions too – so I’ve done a little bit of, well, Google-ing (it seems unfair to call it “research” when it’s so fast and easy, doesn’t it?) and found some interesting information from solar thermal manufacturers. Admittedly, much of what I’ve found doesn’t really come as a surprise.

The Abengoa Solar corporation has several large-scale solar thermal systems in operation and on the drawing board, including this 280 MW (nameplate) plant which will be built near Phoenix, Arizona.

The schematic diagram on that page clearly shows that a fairly conventional water-based cooling tower is proposed as the basis of the condenser heatsink.

The Arizona system is based on the 50 MW Solnova 1 installation, in Spain. This installation does include a mechanical single-axle drive mechanism for steering the trough collectors.

Solnova 1 has a design power rating of 50 MW. Based on the local solar resource, the plant is predicted to deliver 114.6 GWh of clean energy per year.

That’s a capacity factor of 26%.

Here’s what Adams had to say about the thermal efficiency expected from such a system:

Based on my back of the envelope computations it appears that the steam conditions will be roughly equivalent to those found in the second generation nuclear plants operating today. That implies a thermal efficiency of about 33%, and a condenser cooling water requirement that is comparable to a nuclear power plant on a per unit power basis.

Here’s what the company says:

At peak conditions, the plant converts available solar radiation into heat at an efficiency near 57%. Combined with the efficiency of the steam cycle, the overall plant efficiency is approximately 19%.

The efficiency of the steam cycle based on the manufacturer’s official claims, then, must be 19% divided by 57%, or 33%.

Well, that’s really all that needs to be said on that question!

In terms of efficiency of the energy conversion in these solar-thermal systems, there’s nothing particular special about them – the efficiency, and therefore the condenser water cooling requirement, is comparable to any other typical Rankine-cycle steam power plant.

You’ll sometimes hear this argument about water consumption put forward by the anti-nuclear-power set. It uses so much water, they say. The fact is, the condenser cooling requirements for a typical Rankine-cycle steam power system are all pretty much comparable, per unit of electrical energy output, irrespective of what the heat source is – the heat source might be solar thermal, it might be nuclear fission, it might be coal, it might be oil – it doesn’t matter!

The laws of thermodynamics certainly don’t show any prejudice against nuclear fission heat sources, or against solar thermal heat sources, or anything else.

Some power plants, particularly the common coal-fired power plants, can achieve higher temperatures – and higher efficiency – utilizing supercritical water as the working fluid. A supercritical coal-fired plant, for example, might be able to achieve improved efficiency – and therefore a reduced condenser cooling water demand per unit electrical energy output – compared to a non-supercritical nuclear generating unit. However, the same concept can be applied to nuclear generating units, too. Supercritical light-water nuclear power systems are under serious development.

(Note that that does not mean supercritical in the nuclear physics sense of that word!)

Now, how much will it cost?

Estimated build cost for the Solana project: 1 billion dollars.

Nameplate capacity of 280 MWe. Since it’s using molten-salt thermal energy storage, it’s fair to expect a capacity factor that is superior to the Solnova 1 installation discussed above. But, of course, they just have to be difficult, and not provide any mention of the actual capacity factor expected (or the actual energy output per year), and instead only providing this difficult “supply energy to n homes” stuff.

Solnova 1: 50 MW / 25,700 = 1946 W of nameplate capacity per home

Solana: 280 MW / 70,000 = 4000 W of capacity per home.

Obviously there’s something missing here – the efficient thermal-storage Solana installation should be expected to require less capacity for a given number of homes supplied, but instead it requires twice as much. I guess they assume, maybe, that Americans are going to use more electricity than the people of Spain?

Unfortunately, at this point, without some basis to make reasonable estimates of capacity factor for the thermal-storage solar thermal plant, it’s almost impossible to make any meaningful comparison of the cost.

Almost impossible. We could make the most unrealistically conservative, optimistic, renewable-energy-is-infalliable estimate conceivable. We could estimate, just for argument’s sake, that a thermal-storage solar plant like the Solana facility has a capacity factor of 100%.

Then, the installation cost comes to 3.6 billion dollars per gigawatt of “real” average power output. It’s proportionately higher if you factor in some realistic capacity factor.

And finally, here’s something else that’s interesting – but perhaps not too surprising.

That is enough to supply 25,700 homes and to reduce CO2 emissions by more than 31,200 tons per year. To supplement power generation under conditions of low solar radiation, Solnova 1 is equipped to burn natural gas. This can be used to deliver 12-15% of the plant output.

Emphasis is mine. Obviously this clean, green, greenhouse gas emissions free energy is clearly so much more preferable to any kind of nuclear energy. I’m sure Amory Lovins would be proud of them.

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8 Responses

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  1. I don’t know. Sometimes I think I’m missing some large piece of the puzzle, on these projects. I mean these things don’t get designed by idiots, or built by idiots. They must have had the same education that I had, taken the same basic thermodynamics that I did, and to the best of my knowledge, the fundamentals haven’t changed in the last 40 years, in fact they were as I remembered them two years ago when I retired from full time work as a chemist. How in the name of science can these people not know that they are not going to meet the broader objectives of these projects when it so crystal-clear to me? Hell I’ve bailed out of projects that had a better chance of working than some of these renewable energy ones, because I was sure they were going to fail and take me down with them.

    What are they smoking?


    April 25, 2008 at 4:49 pm

  2. Eh … These things are mostly scams, when you come down to it. I mean, they wouldn’t be if we lived in a world where energy is far more expensive and far less available than the one that we live in today (not that I would want to live in such a world).

    Then again, it has never failed to amaze me how many people working as engineers don’t really know the fundamentals. It wasn’t that long ago (about 5-10 years ago) that I was teaching young engineering students. It was scary to discover how little they know of what I consider to be fundamental concepts — and these were the “bright” students!

    Be afraid for the future. Unless something changes soon (and it might, with more students coming to western Universities from places like China and India, where the students are hungry to learn), we’re in for some rough times. Rigor is not dead, but it’s on life support in many fields these days.


    April 25, 2008 at 5:22 pm

  3. What would you guys have to say if I told you that there have been solar thermal plants (CSP) delivering base-load electricity in California for 20 years already? Google SEGS (solar Energy Generating Systems). Spain has some too!
    Back to school maybe? :-)

    Eric Mair

    April 29, 2008 at 9:04 pm

  4. Nobody ever said that there isn’t solar thermal energy already in use, Eric – quite the opposite.

    Spain has some too? Really, is that so? I can only conclude that you didn’t even read my post.

    The SEGS facilities in the Mojave desert are not baseload generators, they are designed as peaking units, and what’s more, they are backed up by a good old environmentally destructive fossil fuel fired boiler on cloudy days.

    All the SEGS installations have a total nameplate capacity of 310 MW – with a capacity factor of 26%, which we’ll assume, the same as Solnova I, that’s 706.5 GWh of energy generated in a given year – that’s only about 9 percent of the energy output of a typical nuclear power plant (or coal fired generator), with a 1 GW nameplate capacity and a 90% capacity factor. That’s pretty small indeed – and this is one of the biggest solar energy installations in the world.

    How much did it cost to construct these facilities?

    The FPL promotional fact sheet for SEGS, here, gives the 310 MW figure quoted above, and explains how it is a peaking unit, backed up by a natural gas fired boiler.

    Luke Weston

    June 6, 2008 at 5:50 pm

  5. My concern is the quantity and quality of water required to support the Rankine cycle in solar thermal power generating systems.
    All the locations of existing and proposed plants are in desert areas, which means (when I went to school at least) – little water.
    I see two effects:
    1. Piping water long distances to these plants increases both the base and operating costs.
    2. The South-West already has a critical shortage of water resources and a steep competition
    for the available supply. This has both environmental and long term cost impacts.

    None of the “get solar thermal energy fast” schemes seem to address the water issue.
    Any comments.

    Paul Martin Smith

    July 3, 2008 at 11:01 pm

  6. “Then, the installation cost comes to 3.6 billion dollars per gigawatt of “real” average power output.”

    Anyone care to estimate what a GW of nuclear power costs nowadays (and please include waste disposal and decommissioning). Costs of the finnish plant in Olkiluoto (1.6 GW) now stand at about 7 billion dollars, and further cost overruns are inevitable. Of course, these costs do not include fuel, and more importantly, waste disposal and decommissioning. Furthermore, AREVA was so keen on building a new plant that it’s making a huge loss with these numbers anyway (paid by their gullible or powerless customers).

    Jan van Beilen

    October 2, 2008 at 7:36 am

  7. Even with the cost overruns – for this first-of-a-kind plant – the most pessimistic estimates I’ve seen put the cost at somewhere between 5 and 6 billion – I’ll say 6 billion to be conservative. What are your references for the cost?

    The cost of nuclear energy is dominated by the capital construction cost – fuel cost, decommissioning and handling used fuel are in fact very small relative to the capital cost. Here we have just considered the construction cost, as with the solar energy plant, which similarly has its costs dominated by the cost of construction.

    If Olkiluoto-3 does cost $6 billion, for 1.6 GW of capacity, and the solar thermal plant as mentioned above generates energy at a 95% capacity factor, then they are expected, in fact, to be effectively equal in terms of capital costs.

    Luke Weston

    October 3, 2008 at 2:18 am

  8. Here’s some “from the hip” numbers that I’ve been able to put together for photovoltaic power. The power density of sunlight is about 1.4 kW/square Meter.
    The efficiency of the most advanced solar cells is <50%
    The future largest solar power plant planned will be in Deming NM. It will be 3200 Acres (5-square miles) and produce 300 MW or 300 MW x 8hrs (only works in sunlight) x 320days = 770 GWatt-hrs./yr.
    Palo Verde Nuclear Generating Station sits on 4,000 acres, a large amount of which is unused. So the size is about the same as the Deming Solar plant. PVNGS produces about 4,000 MW, or 4,000 MW x 24 hrs x 320days = 30,000 GWatt-hrs./yr.
    If we normalize to 8 hour days due to the limitations of solar, a solar plant would have to be in the neighborhood of thirteen times the size of a nuclear plant to deliver the same power during daylight hours on a clear day AND we would still need a power source of comparable size for the remaining 16 hours each day. As you can see, even if we can increase the efficiency up to 100%, photo-solar just can’t do the job on a large scale. This doesn’t even address the environmental impact of the loss of sunlight on the surface of the earth.

    Regarding the new APS Solana plant, it’s supposed to cost $1B to build and will generate 280 MW. Considering that one of PVNGS units generates 1400 MW, that would put Solana very conservatively at $5B with NO cost overruns for construction to generate an equivalent amount of power. Figuring that it only can generate about 1/3 of the time that PVNGS does and giving them the benefit of 100% conversion efficiency in their molten-salt storage facility, that would put construction at $15B for comparable output. And again, unless you overbuild to store power for the time you can’t generate, you need another power source to generate when there’s no sun.

    Dave H

    January 6, 2009 at 2:57 pm

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