FAQs

Do towers have a cost/performance advantage over troughs?

Yes. Based on BrightSource’s own analyses as well as those in independent, externally published sources, the levelized cost of electricity from a tower system will be between 30% to 40% lower than with a trough system. The cost/performance advantage of tower systems is based on five key contributing factors:

 
• More efficient production of steam from solar radiation due to two-axis tracking
 
• More efficient generation of electricity from steam due to higher temperature steam production
 
• Less ‘parasitic’ energy usage for plant operation due to reduced movement of thermal mass
 
• Higher capacity factor – more megawatt hours produced per megawatt of installed power equipment
 
• Lower capital costs due to commodity-based inputs, no concrete foundations, and fewer pipes and cabling

What are the main advantages of solar power tower systems?

There are three primary advantages of tower systems over parabolic trough systems:

 
• Significantly lower cost of producing electricity
 
• Ease of implementation
 
• More positive environmental impact

How does a tower system produce steam more efficiently?

Parabolic trough systems lose a relatively large proportion of heat, with about two-thirds of the losses occurring at the heat-collecting pipes in the troughs themselves and the remainder in the long pipes distributing the oil throughout the solar field. More energy is lost when reflected sunlight must pass through an evacuated glass tube in order to reach the heat-collecting pipe.

Tower systems have much lower heat losses because their heat-collecting pipes are concentrated in the receiver and not dispersed around the solar field.

Other factors are related to the geometry of the mirrors and their targets. For example, the mirrors in a tower system receive sunlight at a more advantageous angle than parabolic trough mirrors because they track the sun on two axes (i.e., in three dimensions) rather than on only one axis. The tracking advantage is particularly important when the sun is relatively low in the sky, such as in winter, or even in the early and late daylight hours at other times of the year. This means that a larger proportion of sunlight is reflected and ultimately utilized for electricity on a yearly basis.

How does a solar power tower system work and how is it different from parabolic trough systems?

In a solar power tower system, computer-controlled mirrors track the position of the sun to reflect light onto a ‘central receiver’ or boiler sitting atop a tower. The boiler, containing water, is designed to be heated from the outside to produce superheated pressurized steam. The steam is then transported to a traditional steam turbine generator to produce electricity.

By contrast, parabolic trough systems use synthetic oil as an intermediate ‘heat-transfer fluid’ to absorb heat, which is then pumped through heat-collecting pipes mounted in the focus of parabolic trough-shaped mirrors.  The pipes pass through a heat exchanger to generate steam, which drives a turbine generator to produce electricity.

What is ‘parasitic’ energy usage and why do tower systems use less?

Parasitic energy is how much electricity the plant itself uses. For example, the pumps and motors of a solar field or receiver are examples of parasitic energy.  The biggest use of parasitic energy in a parabolic trough plant is to pump the synthetic oil throughout the heat-collecting pipes throughout the field.

Tower systems avoid this costly expenditure of energy simply by not circulating fluid – water – in the solar field. The water/steam circulation pump in a central receiver requires far less electricity, and as a result total parasitic energy usage in a tower system is at least 50% less than in a comparable trough plant.

Typical parasitic energy values (including all solar field and heat exchange systems, the power block and balance of plant) are 12% to 14% of electricity produced for parabolic trough systems and 5% to 6% for a solar power tower plant.

 

How does the “capacity factor” make tower systems more economical?

The capacity factor of a power plant is simply the number of hours of electricity it produces divided by the number of hours in a year.

During the winter, the poor angle of the sun onto horizontal troughs lowers system performance.  But because the tower’s solar field can provide adequate electricity throughout the year, towers have a higher capacity factor.

Furthermore, a tower system can be designed to work at peak output levels for more hours over the course of the year, simply by adding inexpensive heliostats to an existing array of tower, receiver and power equipment.  In contrast, the investment in trough plants is more evenly distributed throughout the solar field, and the raising of capacity factor is far more costly.

Why is generation of electricity from steam more efficient in a tower system?

New generations of turbines can convert supercritical steam to electricity at efficiencies of more than 50%.  BrightSource’s tower systems take advantage of the most efficient steam turbine generators, and the company’s initial projects in California are rated at 540°C to 560°C and 140 to 160 bar with a net cycle efficiency of 40%.  Future projects are planned to operate in the supercritical range of temperatures and pressures, with steam-to-electricity efficiency reaching 50%.

Trough systems, on the other hand, cannot make use of the same advances in turbine technology to increase the efficiency of electricity generation because the synthetic oils used for heat collection are limited to temperatures of about 390°C. Based on currently available information, turbines serving parabolic trough systems are generally around 36% efficient.

How do the capital costs of towers and troughs compare?

Towers have a unit capital cost advantage over troughs, which can be broken down into four distinct elements:

 
• Glass:  Flat glass mirrors are less expensive than curved glass mirrors.
 
• Structural steel:  Tower heliostats are mounted singly or in pairs, creating a low wind load and therefore requiring far less structural steel per square meter of mirror.
 
• Pipes:  A tower system contains far fewer heat-collecting pipes in its boiler because of the higher sunlight concentration ratios. Furthermore, tower piping is installed only at the central tower and not distributed throughout the field. In addition, trough systems require kilometers of header pipes for distribution of cold and hot oil to and from the working collector assemblies.
 
• Civil works:  Trough assemblies require sizable concrete foundations, and trenching and cabling throughout the solar field to bring power to the drive motors. The compact heliostats in a BrightSource tower systems do not require foundations and use minimal cabling.

BrightSource says that power tower systems are actually easier to implement than parabolic trough systems. Why is this?

First, tower technology has surpassed solar plant topographic limitations: trough systems require extremely flat terrain with grades limited to <1%, while tower systems can be sited on terrain with grades of up to 5%.

Second, tower technology does not face as many barriers in terms of field equipment. There are fewer manufacturers of curved glass appropriate for trough mirrors than manufacturers of simple flat glass mirrors. Furthermore, there are, at present, only two manufacturers of the specialized heat-collecting pipes used in parabolic trough systems.

Third, the potential adverse environmental impacts of trough systems often require more intensive environmental scrutiny and longer permitting processes.

Aren’t all solar thermal technologies the same in terms of environmental benefits?

Some solar thermal technology components can have adverse environmental impacts of their own.  BrightSource decided to employ a more expensive dry cooling approach in its power plants, which greatly reduces the load on local water resources, including transportation and disposal of waste.

In addition, the potential for introducing hazardous materials into the environment is greatly reduced when the working fluid in a solar thermal system is water/steam and not the synthetic oil of trough systems, which is known to present issues in terms of hazardous waste, spill cleanup and fire hazards.