Concentrated Solar Power (CSP)

Concentrated Solar Power (CSP)

The basic idea behind Concentrated Solar Power (CSP) is dead simple, especially if you liked to start fires with a magnifying glass when you were a kid:  Focus a lot of sunlight to gather heat, and use this heat to generate electricity.

There are several ways to do this.  A simple way, to get our mental juices flowing, would be to use the heat source to boil water to generate steam, and then use a steam engine to drive a generator to produce electricity.  While simple, this approach turns out to have a few problems, e.g., needing a water supply in the middle of a desert and the fact that steam is not a good energy storage medium.  Thus, sometimes this approach is used as a “back end” of a more complicated system, where one fluid is heated to a very high temperature, and this hot fluid is used to create steam for a steam engine.  This is discussed more below.

A vastly better approach is to focus the sun’s rays to create a very hot liquid, and to use it to heat a Stirling Engine to generate electricity.  The very hot liquid stays external to the Stirling Engine.  There is a nice article on Stirling Engines in Wikipedia here. Like steam engines, they are external combustion engines as opposed to internal combustion engines as are gasoline and diesel engines.  Here’s a picture from the Wikipedia article.  Since WordPress doesn’t support motion in gif files, you’ll have to click on the picture to see motion.

Stirling Engine

For this simple Stirling Engine, there is only one cylinder, hot at one end and cold at the other. A loose fitting displacer shunts the internal fluid between the hot and cold ends of the cylinder. A power piston at the end of the cylinder drives the flywheel, which in turn drives the generator.  In our case, our external hot fluid replaces the external “fire” in this picture.

Let’s break a CSP system down a little more.  There are several ways to “focus” the sunlight.  First there are three dimensional parabolic dishes that focus the light essentially onto the hot side of the Stirling Engine.  Here are some images taken from Google’s image collection (Click on them to see their original source.):

Above there is a single CSP with one Stirling Engine at the focal point, and below an array of such CSPs.

Note in this array there are multiple Stirling Engines. While more expensive, this is goodness, because the array keeps working in the presence of even multiple engine failures.

Another approach is for movable mirrors (heliostats) to reflect the sunlight to a “point” on a tower.  The mirrors are usually rather flat parabolic mirrors so that their focal point is quite far away – near the top of the tower.

In this tower/heliostat approach, the external fluid is heated at the top of the tower and piped down to a Stirling Engine or to a steam engine.  After it has been used for heating, the cooled external fluid is piped back to the tower to be reheated.

A second form of CSP uses a two dimensional parabolic trough:

Along the line of parabolic foci is a thermal collector pipe that not only serves as an “oven” for the heated fluid, but it also transports the heated fluid to a central generating facility.  Below is such a facility in Abu Dhabi, at one of the world’s largest CSP plants.

A third way to focus sunlight is with a Fresnel lens or mirror.  Recall these are used by many CPV systems as well.  In the lens case the thermal collector pipe is below the lens.  For Fresnel mirrors, it is above the mirrors. Here’s a photo:

The company Areva, has a fourth combination-type of setup.  Here are a couple photos from their web site:

The blue trough is a parabolic trough focusing the light onto the overhead “compact linear Fresnel reflector” which in turn focuses the light onto a thermal collector pipe.  Here’s a close-up photo of that:

To maximize efficiency, all CSP systems need tracking so that they “point” towards the sun.  The trough and Fresnel systems need only single axis tracking, while dual axis tracking is needed for tower/heliostat CSPs and also for parabolic dish arrays.

One potential problem CSP systems have is glare.  A system under construction in Cloncurry, Australia was scrapped in November 2010 due to concerns about reflective glare in an urban environment.

The next part of a CSP system is the external heating fluid, which is typically some sort of liquid salt, although other materials including super-heated steam and graphite have been used. The molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate, commonly called saltpeter. New studies show that calcium nitrate could be included in the salts mixture to reduce costs and with technical benefits. This salt melts at 220 °C (430 °F) and is kept liquid at 290 °C (550 °F) in an insulated storage tank.   The main feature of liquid salt is that it can be heated to very high temperatures, in excess of 500 °C, and at this heat, it can be used to drive a Stirling Engine or generate steam for many hours after the sun goes down.  This time of course depends on the initial temperature, the quantity of liquid, the insulation, and the efficiency of the Stirling or steam engine used.  Typically current systems continue to generate significant amounts of electricity for 7 to 8 hours after the CSP stops heating the salt.  This isn’t quite enough for 24 hour operation, and some utility companies set up “hybrid” systems where the load at night is met with a gas fired generator.

These systems are improving, and the 19.9 MW CSP system in Andasol, Spain generated 24 hours of continuous electricity in July 2011 using a molten salt storage system.  This design should average 20 hours per day of electrical power generation, hitting 24 hours on particularly sunny summer days.  This plant has a field of 2650 heliostats that focus sunlight on a central tower. Here’s a photo:

Future posts will compare CSP systems with CPV systems for efficiency and cost effectiveness.

-gayn

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