The Other Solar Power

There is a method of generating electricity from solar energy that you may have never heard of. Rather than converting sunlight directly into electricity using a photovoltaic panel, concentrated solar power uses sunlight to heat water which, in turn, drives a turbine to generate electricity. How concentrated solar power works and whether it is a good method of generating electricity are questions I hope to answer for you.

Concentrated solar power (CSP) is actually an older technology than photovoltaic panels. In fact, the single largest existing solar generating plant, measured by generating capacity (470 MW), is based on CSP, and the first solar generating plant, built in Italy in 1968*, was based on CSP. These plants use a huge array of mirrors, covering square kilometers of area, to collect and focus sunlight onto water. At the input, liquid water enters and goes through the area where the sunlight is focused. As a result of the intense heat generated by the concentrated sunlight, the water turns into steam at the output. The steam continues through a complex of pipes to a turbine where it turns the blades of the turbine and generates electricity in much the same way as turbines at other types of power plants.

One of the advantages of a CSP plant is that its construction can be very simple**. No special materials are required for the collectors, which are just arrays of mirrors. Each array contains multiple mirrors and must be aimed at a receiver. The receiver itself is made of a material that absorbs and transfers heat efficiently to the water. For one CSP approach (the so-called “power tower”; see image below), these receivers may be up to 40 square feet in size. Furthermore, using steam to drive a turbine is a well-established technology, presenting no particular complications, and driving well over half of all the generating plants in the world (not just CSP).

Power tower
Example of a power tower concentrating solar generator. Image credit: afloresm

So why, if CSP is the picture of simplicity, do we not hear more about it? In fact, Spain has perhaps the largest total installed solar power generating capacity through its numerous CSP installations. There is also a newer project to build one of the world’s largest CSP facilities near existing CSP installations in the Mojave desert. These do not come without an environmental impact, however, from covering so much of the desert with mirrors and needing to use large quantities of water. Water use associated with CSP generation is a particular concern in the desert where water conservation is already a big issue. Below, I have summarized Table 2 from this study of CSP water consumption (pdf), which compares how much water different power plant technologies consume.

Type of GenerationType of CoolingGallons of Water Required per MW hour
Nuclear and CoalRecirculating400 - 750
Natural GasRecirculating200
CSP (power tower)Recirculating500 - 750
CSP (parabolic trough)Recirculating800
CSP (Stirling engine)Mirror Washing20
Data is from a Department of Energy Report to Congress (pdf) titled "Concentrating Solar Power Commercial Application Study: Reducing Water Consumption of Concentrating Solar Power Electricity Generation".

Water is used for two purposes in CSP: the first is for the generation of steam while the second is the much more mundane need to wash the panels so they can still reflect sunlight at high levels. What is interesting here is that water-usage for CSP generation is largely competitive with other generation methods that are already used.

Ultimately, it is possible to reduce the water use of CSP generation if hybrid dry-wet cooling systems (pdf) are adopted, or if water is replaced by other transfer fluids like molten salt. Circulating molten salt through a CSP system would not only minimize the amount of water needed, but it could offer significant energy storage benefits (I will leave the details of energy storage for a future post).

So why would we choose CSP for power generation? I will answer this question in two ways. If we are comparing to all generation types, CSP offers an emissions-free alternative to coal- or natural gas-fired power plants–hence mitigating climate change–with an ability to keep costs reasonable. Should we propose it instead of photovoltaic solar power generation? Here I think it should not be a stark choice: CSP seems well-suited for large-scale installations because materials are not expensive and are easily replaced. Because PV panels often contain exotic materials which can degrade in performance over time, PV may be more difficult to turn into a large installation. On the other hand, it is a lot easier to install PV panels atop individual buildings, like your home, so that power generation is more distributed.

* I have not found a primary source for this information other than the one linked through Wikipedia.
** There are multiple types of CSP, that I do not discuss in detail here, but which I will cover in a future post.

Note: I benefited in the preparation of this post from a seminar given by Julius Yellowhair of Sandia National Laboratories at the University of Arizona on January 27th, 2012.
Pascal Mickelson 25 February 2012
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