Concentrating Solar Thermal Energy and its Uses

by Roger Molina, P.E., The M8Group

The worldwide solar energy industry has grown quickly during the past few years and combined with research and development has become competitive with modern fossil fuel technologies in electric power and thermal energy facilities. 

 

Design of concentrating solar thermal (CST) energy technology varies among companies, but the objective is the same: Harness energy from the sun and use it to convert water into steam energy for various uses.

For example, one technology, heliostat, uses mirrors that track the position of the sun for maximum energy harvest, then radiates the concentrated energy to a receiver, or boiler, installed on top of a support structure 180 feet to 400 feet high, where water or molten salt is heated to produce high-pressure, high-temperature steam. This is sometimes called tower technology.

Other CST technologies use large mirrors called troughs that transfer heat from the sun along a large pipe containing water or oil to produce steam.

Depending on the industry application, the steam can be used to generate electricity in a steam turbine generator, provide mechanical power to drive rotating equipment or heat fluids used in manufacturing.

Site Issues

CST installations, however, are not without serious challenges. They require large land areas with high solar intensity like the southwestern United States, Middle East, Africa, India, southwestern China, Chile and Australia.

In addition, a greenfield project is generally in a remote, isolated location—far from load centers—that lacks supporting infrastructures such as electric transmission lines, water supply systems, wastewater treatment plants, roads and highways. These infrastructures are expensive and when built for dedicated use might make the undertaking impractical if the project scale is not large enough to bear the external costs.

Because of the large land area required for construction, obtaining permits to construct and operate is difficult, especially for sites on pristine land. In California, permitting for large power projects might take 24 months or longer before one could shovel dirt at the site.

Immediate Impact

CST uses encompass industries including electric power generation, oil extraction, chemical processes, manufacturing and refineries. Countries with limited freshwater resources for domestic use also could use solar thermal energy to produce potable water through desalination.

This technology would reduce fossil fuel use immediately by repowering or retrofitting existing plants with solar energy systems. These projects appear to be attractive investments because the infrastructure required to support a solar facility is already in place, reducing capital outlay while simplifying permitting and shortening construction.

The integration of solar thermal energy systems to existing facilities also significantly reduces operating and maintenance costs, thus enhancing capital payoff.

There are hundreds of simple-cycle (peaking units) and combined-cycle power plant installations representing opportunities for fast deployment of CST energy facilities. Some plants were installed in the ’70s and ’80s and are due for major overhauls—excellent timing for investigating the feasibility of using solar energy for repowering steam turbine generators. 

CSTs can supplement or replace fossil fuel used in duct-fired, heat-recovery steam generators or as an additional steam supply resource to the steam turbine for increased output during daytime peak demand periods.

One project of this kind is the February 2009 bid solicitation from the Sacramento Municipal Utility District to supply, by a thermal energy (steam sales) purchase agreement, 200,000 #/hr of 700 Psig and 675 F steam for injection into the steam system of the combined-cycle, 500-MW Cosumnes Power Plant. The plant was placed in commercial operation in February 2006. Other power generating facilities in the southwestern United States are being evaluated for similar application using the concepts shown in Figures 1 and 2.

There are also numerous opportunities in industries where thermal, mechanical and electrical energy are used extensively. As an example, a steam turbine-driven high-pressure feed pump can be installed for daytime operation to augment electric motor-driven pumps in a desalination plant using RO Membrane technology (see Figure 3) or provide a combination of electric power through the use of backpressure turbine generator and thermal energy in a thermal distillation process (see Figure 4).

The technology is also suitable for use in recovering oil from tar sand, in thermal-enhanced oil recovery and in absorption cooling systems.

Cloud Cover

The development of solar thermal facilities in the country will accelerate once President Barack Obama’s commitment to reduce imported fossil fuel through renewable energy becomes a reality—with favorable new regulations or policies.

The worldwide financial woes, however, will take a long time to turn around, and this dire situation combined with dropping natural gas and oil prices have created difficulties in attracting money from investors.

Construction of solar thermal energy facilities is inevitable. When will private capital start flowing to support this fledgling business? There are many unknowns in the equation amidst the economic turmoil. We must wait to see how the situation will evolve.

Author

Roger Molina may be reached at m8group@gmail.com and 925-324-4773.

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The Clarion Energy Content Team is made up of editors from various publications, including POWERGRID International, Power Engineering, Renewable Energy World, Hydro Review, Smart Energy International, and Power Engineering International. Contact the content lead for this publication at Jennifer.Runyon@ClarionEvents.com.

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