Modern solar thermal systems are capable of producing large quantities of clean energy. One of the most impressive commercial scale projects is just outside Las Vegas, Nevada, near the home of one of the most impressive hydro plants in the country.
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.
“In the late 1980s and early 1990s, there were nine plants built like this. They were similar in design, but this project benefitted from the experience of the core of people who had worked on the earlier projects,” said Cable. “This is the first utility size project that was built during that period, and it has a lot of improvements because that team learned lessons from older projects and applied them to this project.”
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.
When the plant fires up in the morning, the HTF is about 400°F. A motor-operated valve opens to direct the fluid, bypassing it around the heat exchangers so that it can circulate through the solar field and get up to the temperature that matches that of the fluid that has been bottled up in the steam generators (~550°F). At this point the valve is closed and the transfer fluid is redirected back toward the heat exchangers and steam production begins. Within two hours the plant will achieve full load (75 MW gross/69 MW net).
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.
The valves used for this high gain solar (hGS) application have relatively low pressure requirements, around 550 psi, but those used in the steam system have 1200 psi pressure requirements. “We use about every valve in the business,” said Cable. “Most of them are not super specialty, not stainless. The plant utilizes a mixture of primarily cast and forged valves. But the packing is always graphite, and we don’t use Teflon, because it does not hold up well in this application.
“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.”
None of the valves in the solar field are motor operated, but most of the valves of the HTF system within the Power Block are motor operated. Only the isolation valves in this area are manually operated. There are discharge/suction valves for each of 5 HTF pumps, (Figure 4) and they are motor operated.
Once the HTF passes by the heat exchangers, the system operates basically like any other steam-generated system (Figure 5); the turbine spins and generates electricity. The water for this system comes from the local municipal supply. Boulder City, NV, which owns the land on which the plant is located and also provides the water. Incoming raw water is treated and demineralized for use in the steam and condensate loop or feed directly to the cooling tower. The plant utilizes wet cooling to reduce turbine exhaust steam into condensate. Like most plants, cooling tower water is recycled through the process until a concentration threshold is reached and then blown down to evaporation ponds.
Valves associated with the demineralization process and storage are a mix of stainless steel and CPVC construction as well as a mix of gate, globe and butterfly, and they are hand, pneumatic and electric motor operated. The primary valve material used throughout the plant is carbon steel. (Figure 6)
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.”
What happens when the clouds roll in? “Well, we don’t have a lot of storage in the sense that some other, newer plants do. We do not have the salt storage that some plants have,” said Cable. “It was a matter of funding at the time, but this plant has a lot of piping and a buffer tank that holds an allotment of the hot fluids. This system allows us to stay online in the event of a cloud for about an hour.” (Figure 7) The monsoon season, which lasts from July to September, can also have an impact on power production, but generally individual storm systems arrive and depart quickly, disrupting power production for only a short time.
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.