How much water per kilowatt hour does a solar thermal plant need?

There is a lot of excitement these days for solar energy plants, with some particular amount of breathless enthusiasm for solar thermal power plants that use concentrating mirrors to add heat to a transfer fluid that is then used to create steam. Some of the technology’s supporters even claim that this kind of system, since it allows some power production after sundown, helps to eliminate the intermittent nature of solar energy.

Most reporters on the topic simply repeat the sales claims of the project sponsors and do not work to explain the technical details enough to help me answer some of the questions that I have about how the technology is really going to work, what are its cost implications, and what are its short and long term environmental impacts. All large scale energy systems require tough, fair questioning and explanation; they all impose some risks and all cost a lot of money. In the case of solar thermal plants, there are huge federal, state and local taxpayer subsidies involved along with the best hidden subsidy that money can buy – a mandate from the government that forces utilities to purchase power from the installations at prices that are well above market rates.

Even my friends at NEI Nuclear Notes recently provided a report titled The Snap Together Energy Plant that begs a lot of questions about the level of knowledge that solar thermal promoters have about inherent challenges of steam systems. Here is link to a blog with some reasonably good discussion about solar thermal plants Nevada Solar One Pictures That discussion still leaves me with some unanswered questions; perhaps some of you can help me come up with some answers. Some of the questions should be very familiar to many nukes – we deal with them all the time ourselves.

Here is a list of questions that I think need some good answers before I get too excited about solar thermal power plants, even ones sponsored by one of my favorite technology companies:

• What are the steam cycle parameters? What is the overall thermal efficiency?
• Do the answers to the first two questions change during the course of the day as the sun’s incidence angle changes?
• Who will be supplying the high pressure piping and large pressure vessels?
• What is the cooling medium for your condensers?
• How much water will the plant consume per unit of power?
• How often do the mirrors need to be cleaned?
• Is the plant in an area that receives much snowfall?
• Are the mirrors steered so that they track the sun?
• What is the installation cost per unit of energy produced each year?
• What technological improvements indicate that operations and maintenance costs will be any lower than the costs that prevented Solar One (1982-1986) and Solar Two (1996-1999) from producing economically competitive power? (Solar Power Tower technical description)
• Why did it take 9 years to convert Solar One into Solar Two? (I think I know the answer, but have not been successful in my searches this morning for the details of the story.)

Note: Solar thermal plants are destined for installation in hot, dry climates, but they are also steam plants that have rather modest steam conditions. 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.

Obviously, that is not something that should cause an “all stop” on its own, but it is certainly a question that must be addressed, just like it is for any other thermal plant that plans to put itself in a place where water is precious.