April 1, 2009

Fossil Fuels + Solar Energy = The Future of Electricity Generation

Dave Ugolini and Dr. Justin Zachary, Bechtel Power Corp., and Hyung Joon Park, Bechtel Enterprises

Renewable energy, though still accounting for a comparatively small portion of overall supply, generates a larger portion of the world’s electricity each year. Combining many of the available solar energy conversion technologies with conventional fossil-fueled technologies could reduce fuel costs while simultaneously helping utilities that are struggling to meet their renewable portfolio goals.

Renewable energy technologies face two near-term deployment hurdles when compared to traditional forms of power generation. First, their initial capital cost typically is much higher on a dollars per installed kilowatt basis, and that first cost is only partially compensated for by lower operation and maintenance (O&M) and fuel costs. This is especially true when today’s higher project costs are compared to those of conventionally fueled projects installed a decade or more ago.

The other important issue is dispatchability. There are few renewable options available to a dispatcher on a still, overcast day when the public demands electricity. Fast-acting gas turbines will have the advantage over renewable energy supplies when instantaneous matching of supply with demand is required — at least until some form of energy storage mitigates the intermittent nature of renewable energy sources. However, progress to commercialize large-scale energy storage technologies has been evolutionary, rather than revolutionary, and many technical and cost issues are yet to be resolved.

So why not take the best of both power generation technologies and integrate renewable power sources, such as concentrating solar thermal power (CSP), with existing or new combined cycles or conventional steam plants? The resulting hybrid plant will increase power or reduce fossil fuel consumption (justifying the high capital costs), mitigate the intermittent nature of most renewable technologies, remain dispatchable, and help many utilities with large fossil plant investments meet their renewable energy mandates.

The Best of Both Worlds

Conventional gas-fired combined-cycle plants represent perhaps 40% of the installed power generation resources in the U.S., yet they produce much less than half of the electricity sold. These plants uniformly have very high thermal efficiency and the smallest carbon footprint of any fossil-fueled generation technology, but the steeply rising cost of natural gas has pushed these plants into unfamiliar territory, where they operate as cycling rather than baseload plants. In other words, a typical combined-cycle plant is suitable for including in an integrated solar combined cycle (ISCC) configuration, where the solar energy portion of the plant can provide additional power at peak demand. We explore the solar power options for conventional steam plants later in this article.

The conversion of a combined-cycle plant to an ISCC begins with adding an additional source of heat, such as solar energy, to reduce natural gas consumption and thereby improve overall plant efficiency.

There are other advantages of an ISCC, even when compared with standalone CSP-inspired plant designs. (See POWER, December 2007 for a review of the Nevada Solar One CSP plant.) For example, the ISCC uses existing components (such as steam generators, steam turbine, and condensing system) that reduce the installation cost of a typical CSP system. Also, the potential for generation is increased because the steam turbine would be already synchronized to the grid when the solar energy contribution is added, thus avoiding lost generation during start-up. Another key advantage is gained during rising ambient temperatures, when gas turbine performance steadily drops. Operation of the solar energy portion of the ISCC compensates for that loss in efficiency and electricity production and improves the plant’s part-load performance.

Combining solar energy with conventional coal-fired plants is also possible in regions with reasonably good solar conditions. For these plants, where the steam pressures and temperatures are higher than for ISCC, the type of solar conversion technology used (Fresnel, parabolic trough, or tower) will dictate how solar is integrated into the plant.

Finally, don’t discount the possibility of hybridizing conventional plants with other, even multiple, forms of renewable energy such as biomass and wind. Our discussion of ISCC illustrates a single development path electric utilities could follow to efficiently and inexpensively bring multiple forms of renewable energy online in short order. Many other options are available, depending on the design of existing plants and their location particulars.