In a previous blog I discussed three very different types of solar energy electricity generation; Photo-voltaic (PV), Concentrated Solar Power (CSP) and Solar Updraft chimneys. This blog will deal with a branch of CSP that hasn’t received much attention and hasn’t yet been commercially deployed on a large scale. This could be referred to as non-thermal CSP.
CSP involves using lenses or mirrors to concentrate solar energy in order to generate high temperatures. A simple demonstration of CSP would be the use of a magnifying class to singe a piece of paper on a bright summer day (to this day hours of sunlight are often still measured in exactly this way using a Campbell-Stokes recorder).
The most common reason for using CSP for electricity generation is to heat a fluid to a temperature much higher than 100 degrees Celsius. This fluid is then circulated through a water reservoir using a heat exchange system in order to generate steam to drive a turbine. More then 1GW of capacity is installed worldwide using this type of CSP plant, predominantly in Spain and the United States.
There are significant advantages that come with this configuration, particularly when it includes molten salt Thermal Energy Storage (TES) as most of the newer plants do. The steam boilers and generator act to filter out any short term variability in received radiation that is caused by passing clouds. The TES can allow the facility to generate electricity through the afternoon and evening, which allows the supply to match demand. Finally, by providing a secondary non-renewable source such as natural gas to heat the boilers these plants can act as “dual-fuel” generators able to produce electricity around the clock. This capability has been implemented at the SEGS facility in California.
It is worth noting that although the SEGS facility has provided reliable power for more than 20 years it had a rough start. The developer, Luz International, despite enjoying the benefit of generous tax breaks and a guaranteed and significantly higher-than-market price for the electricity it produced, went bankrupt shortly after completing the ninth plant in the complex. Commercializing a new electrical generating technology (as CSP was in 1984) is extremely difficult for a private sector company that is under constant pressure to deliver short term profitability. I would argue that it is almost impossible.
The need to provide a steam generator for CSP plants significantly increases the cost and complexity of the facility. For example, the 280 MW Solana CSP plant being built in Arizona will cost more than $1.4 billion, or about $5/watt compared to installed PV costs of $2-$3/watt. As with any solar power installation the actual amount of energy generated on average is much less then the peak/rated power so costs/watt are even greater. For example, the SEGS plant realizes a capacity factor of about 21% so that the capital cost/GW-Hour is several times that of an equivalent natural gas-fired plant or nuclear plant. Even so, the ability to generate power well past sunset and the less variable output as compared to PV make thermal CSP a very compelling technology. A group of scientists and politicians have even suggested that a network of CSP plants in North Africa could provide a significant portion of energy requirements for Europe.
There are several companies that have tried or are trying to make non-thermal CSP work on a commercial scale. The first variation of this approach is to combine a parabolic mirror assembly with a Stirling engine which is able to convert heat directly into electricity. Although Stirling engines have been around for a very long time it is only recently that material science techniques have been able to create a compact Stirling engine that is reliable in a production environment and can be mass produced for a reasonable cost.
Stirling Energy Systems (SES) developed a 25 KW unit and constructed a demonstration project consisting of 60 such units near Phoenix, Arizona. Unfortunately, despite hundreds of millions of dollars of private investment and technology support from Sandia National Laboratory, SES declared bankruptcy on September 30, 2011.
Another company using Stirling engines and parabolic mirrors is Infinia Corporation. Their Powerdish units, smaller in size than those manufactured by SES are designed using a less complicated architecture offering greater reliability and attractive manufacturing costs. A large deployment of these units is underway at the Tooele Army Depot in Utah. This 1.5 MW installation will provide a test bed for the technology, which has the potential to be widely deployed in many parts of the world.
The major advantages of Stirling engine CSP are significantly greater efficiency than currently available PV units, ease of installation even on rough terrain, and flexibility in terms of size of the facility. These systems also require very little water to operate using only a small amount to clean the parabolic mirrors from time to time. This is an important advantage as CSP systems are typically installed in arid or desert areas.
Another type of non-thermal CSP installation has been developed by an Australian company. Like the SES and Infinia Systems these products make use of parabolic mirrors and dual axis sun tracking to maximum the concentration of solar energy throughout the day. The Solar Systems units make use of high efficiency, triple junction photo-voltaic cells to generate electricity. These systems offer the same advantages in terms of ease of installation and flexible size as the Infinia Powerdish. Solar Systems also claims that it will be relatively easy and inexpensive to upgrade the small photo-voltaic cells as triple-junction technology improves.
Do I really believe that non-thermal CSP units such as those produced by Infinia Corporation and Solar Systems can make a significant contribution when it comes to developing a sustainable energy future? To be honest I don’t know. But I believe that every promising technology needs to be supported and investigated fully in order to assess and ultimately deploy the optimal mix of generating assets.
We cannot afford to have more companies such as Luz and Stirling Energy Systems go bankrupt while at the same time subsidizing mass deployment of PV and wind generation facilities which are notoriously unreliable.