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

Posts Tagged ‘water use of energy systems

Looking into Solar Thermal power systems.

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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.