CSP Tower vs. Trough comparison
So far there are >500MW of operational CSP worldwide with just over 100MW in Europe (all in Spain). Currently, the longest period of operating experience using CSP technology is that gained from the 354MW “trough” plants built by Luz International in the Mojave Desert in the USA in the 1980s and 1990s. The key members of BrightSource Energy’s commercial and technological management team today were all part of the original Luz team, developing, financing, building and operating these plants. This unrivalled experience and expertise has given BrightSource an important competitive advantage in the design of their tower technology today and in winning contracts in the US.
BrightSource’s team identified early that there were inherent limitations in the trough technology, which include the operational temperature, water requirements and high-cost of custom-made components. This meant that trough technology never developed a learning curve as steep as that achieved by solar PV in recent years.
Applying their in-depth knowledge of CSP plant design and operations, BrightSource’s team conceived a new solar tower technology that can drive down costs sufficiently to compete with conventional energy technologies. Tower technology allows operation at higher temperatures and pressures, thus increasing operational efficiency, while water requirements are reduced by 90% and all components are standardised, providing significant scale economies.
In tower design, thousands of individual heliostats (mirrors on mounting poles with tracking capability) focus the sun rays directly onto a boiler, which sits on top of a tower, to produce steam. Trough design consists of series of parabolic trough mirrors that concentrate the sunlight to a central pipe running across the focal point of the trough. The pipe usually contains a transfer fluid, which is then used to produce steam through a heat exchanger. In both cases, steam is turned to electricity by the means of a steam turbine.
Trough design has a solid track record of reliable performance while existing tower projects consist mainly of pilot and test installations. However, improvements in tracking software and boiler monitoring methods have allowed tower design to become commercially available and with lower costs than troughs. The advantages of tower design over that of parabolic troughs are listed below:
- Can be built over more rough terrain as each heliostat’s position is independent to its neighbouring heliostats.
- Has lower parasitic losses in comparison to trough which employs many kilometres of pipes to transfer the heating fluid, resulting in heat losses.
- Can reach much higher temperatures and pressures (more than 150bar pressure and more than 500 degrees temperature), leading to greater efficiency.
- Has one less energy transfer step as it produces steam directly and not through a heat exchanger, thereby minimising energy loss and maximising plant efficiency.
- The flat mirrors used in heliostats are cheaper than parabolic trough mirrors.
The only disadvantage of tower as compared to trough design is that towers have limited commercial experience with 97% of existing solar thermal power plants being troughs. Despite the lack of commercial experience, most signed new capacity announced, especially in the United States, is utilising tower design due to the lower cost. Hence, within a few years, solar power towers will have accumulated a sizable trackrecord to make the technology as bankable as trough designs.
Of all tower technology suppliers only BrightSource Energy has secured PPAs on a large scale (2.6GW to date). This is a reflection of the confidence in BrightSource Energy’s design and engineering team that large US utilities are showing.