Solar-Thermal Power: A Shining Example of Clean Energy

Nevada Solar One sits on 400 acres of desert land 25 miles southwest of Hoover Dam. And while the solar plant generates 69 MW compared to the nameplate capacity of about 2080 MW for the hydro-electric plant, both projects are brilliant feats of engineering. Nevada Solar One cost $266 million and when it began generating power in 2007, it was one of the most advanced solar thermal projects built to date.

Bob Cable, plant manager at Nevada Solar One, started with the project at its very earliest stages, working in 2001 with the team that wrote a proposal to get the project with NV Energy. He then worked on the engineering, reviewed potential contractors, and at one point during construction, even became the manager of the site. He has been the plant manager since it first started running in 2007, and is justifiably proud of the facility that provides power to 40,000 homes.

Nevada Solar One uses proprietary technology to track the sun’s location and concentrate its rays during peak demand hours. Eight hundred parabolic trough concentrators (Figure 1) with 192,000 mirrors concentrate the sun’s rays onto 19,200 receiver tubes placed at the focal axis of the troughs. These receiver tubes contain synthetic oil, a heat transfer fluid (HTF), which is a good fit for this application.

The receiver tubes in this plant are made of a new borosilicate glass with the same thermal expansion coefficient as steel. The result is that the receivers can handle the changes in temperature that occur as cool Nevada desert nights quickly become hot desert days. The special design also allows for 96% of the length of the tube to capture solar radiation. At the time the plant was built, these design improvements increased the receivers' overall efficiency by 2% over other receivers.

There are 100 loops of the solar arrays in the field (Figure 2) and each loop is in parallel with itself. The cold fluid coming out of the power block feeds each of these loops via a 24-inch header that feeds into the smaller loops with 3-inch diameter piping. For each of the loops, there are two gate valves (one each on the inlet and outlet) to isolate the loop and a manual globe valve (on the inlet) to control the flow.

“Most of the isolation valves are definitely gate valves, and we do have some globe valves where we need flow control.” He continued, “We also have large control valves (Figure 3) that control the flow going out to the solar fields. Some of the plants have been built since this one and use variable speed pumps to move the fluid. We don’t have variable speed pumps, so we rely very heavily on the control valves.”

From early May through August, the plant produces at full capacity unless there are clouds in the area. “This plant has a north-south rotational axis, so the profile of the energy is heavily weighted in the summer. We make four to five times as much power on an average summer day as on an average winter day, but that’s good, since that’s when we need the power. Those air conditioners are running at full capacity all summer.”

Because Las Vegas enjoys 292 average days of sun every year, Nevada Solar One has all the fuel it can use. Cities like Phoenix, Arizona and El Paso, TX also enjoy more than 250 days of sunshine per year, and there are at least 13 cities with sunshine more than 75% of the time. With this renewable resource so readily available, there’s no doubt that solar thermal technology could be one of the most powerful options for many more bustling U.S. cities with an insatiable thirst for air conditioning.

Kate Kunkel is senior editor of VALVE Magazine. Reach her at kkunkel@vma.org