Earlier this year, on October 27th, Ausra commissioned their first solar thermal pilot plant, a 5.0 megawatt facility located in Kimberlina, just north of Bakersfield, California. It is the first solar thermal power facility to be commissioned in California in over 20 years – significantly, the approximately 350 megawatts of solar thermal power installed back in the 1980′s are all still operating.
There are three basic types of utility scale solar thermal power, all of which have strengths and weaknesses and all of which currently compete to become the most cost effective version. Bright Source Energy has been working on an improved “power tower” design, where a field of two-axis tracking mirrors (which can be installed on single poles placed on unimproved ground) reflect sunlight onto a single boiler. The advantages of this design are less site preparation, no transfer fluid plumbing required in the solar field, and a much hotter boiler which can make condensation and reuse of the steam on the backside of the turbine more cost-effective. Bright Source commissioned a 1.5 megawatt pilot plant in Israel earlier in 2008, and has projects in the pipeline in California.
Acciona Energy’s 64 megawatt plant, located near the Hoover Dam in Nevada and commissioned earlier this year, uses the “parabolic trough” design, where the solar field consists of single-axis trough-shaped parabolic tracking mirrors that turn from east to west each day, reflecting sunlight onto a tube containing heat transfer fluid that runs lengthwise at the focal point above each trough. This heated fluid is collected from the solar field and fed to a central steam turbine.
Ausra’s design, unlike the well-established power tower and parabolic trough designs, combines attributes from each of them, and adds a few unique features. Similar to the parabolic trough design, Ausra runs heat transfer fluid into the solar field into linear heat collection tubes located at the focal point of the reflectors. But the reflectors are only slightly concave, and the heat collection tubes are located further above the reflectors. This innovation allows one heat collection tube to sit at the focal point of several mirrors, allowing higher temperatures and greatly reducing the amount of plumbing required to go into the field.
|Two lines from Ausra’s Kimberlina solar field – note how several mirrors
share one collector tube, which is positioned well above the collectors.
One of the surest ways to determine which design is most efficient is to compare prices. Another interesting factor is the amount of land required, since concern over land consumption is a common criticism of utility scale solar power. It is probably too early to determine Bright Source’s costs or land consumption per megawatt, since their pilot plant is relatively small, but data is in on Acciona and Ausra.
Acciona’s Nevada Solar One plant cost $260 million and produces 64 megawatts at full output, a cost of $4.1 million per megawatt. Ausra’s Kimberlina plant cost $15 million and produces 5.0 megawatts, a cost of $3.0 million per megawatt. Ausra’s planned Carrizo plant, intended to produce 177 megawatts at a cost of $500 million, is estimated to come in at $2.8 million per megawatt. For comparison, Optisolar’s utility scale Sarnia solar field in Ontario, Canada, using thin-film photovoltaics, cost $300 million and produces 40 megawatts at full output, at a cost of 7.5 million per megawatt. Of course, when calculating cost per kilowatt hour, you have to take into account the “capacity factor,” the full-output-equivalent hours per year divided by the total hours in a year. By this measurement, Acciona’s desert plant has a capacity factor of 23%. Ausra’s estimated capacity factor, in the only slightly less hot and sunny southern San Joaquin Valley is between 18% and 22%. One can only imagine the capacity factor – not disclosed – for Optisolar’s plant located in Canada is significantly less than this.
In terms of land consumption, the comparisons are also interesting. Ausra’s Kimberlina pilot plant, at 10 acres and 5.0 megawatts, generates 0.5 megawatts per acre, or 320 megawatts per square mile. Their Carrizo plant will consume about 550 acres, generating 177 megawatts, which means at scale their space efficiency estimate drops to 206 megawatts per square mile. But that is actually very good by comparison.
Acciona’s Nevada Solar One plant consumes 400 acres and generates 64 megawatts – equating to only 102 megawatts per square mile. And Optisolar’s thin-film solar field consumes 902 acres to generate 40 megawatts, a paltry 27 megawatts per square mile.
When you consider megawatt-hours per year per square mile, the capacity factor comes into play. It is unlikely you will get more than about 1,200 full-sun-equivalent hours per year in Ontario, Canada, which equates to a capacity factor for Optisolar’s Sarnia field of 14%, which in-turn equates to 34,000 megawatt-hours per square mile per year. By contrast, Acciona’s Nevada Solar One, with 2,000 full-sun-equivalent hours per year can generate 204,000 megawatt-hours per year per square mile, and Ausra’s Kimberlina plant, at a slightly lower 1,800 full-sun-equivalent hours per year, but a much higher output of 320 megawatts per square mile in full sun, can generate an impressive 576,000 megawatt-hours per square mile per year. Ausra’s planned Carrizo plant, at full scale, projects a somewhat lower 206 megawatts per square mile in full sun, but that still equates to 370,000 megawatt-hours per square mile per year.
Without going into cost per kilowatt-hour derivations in this analysis, it is probably fair to say utility scale solar thermal plants such as Ausra is developing have a chance to compete with conventional energy on a level playing field – given they are already coming in at $3.0 million per megawatt and have capacity factors exceeding 20%. A thin-film energy plant at scale, at $7.5 million per megawatt, and a capacity factor that is likely lower than 15%, is going to have a much tougher challenge to commercially compete with conventional energy. And the fact that Ausra’s Carrizo solar field, megawatt-hour vs. megawatt-hour, will consume literally ten times less land than Optisolar’s Sarnia solar field, should not be lost on anyone considering desirable options for utility scale solar development. With solar energy, where you build it – and what technology you employ – dramatically impacts the costs and the land consumption necessary.
In a recent email received from Ausra’s Chief Development officer, Rob Morgan, he states “Ausra’s core technology is the most land use efficient solar technology in operation today.” The facts would seem to bear this out, when comparing parabolic trough and thin film technology to Ausra’s novel hybrid design. It will be interesting to see how Bright Source’s power tower technology compares, when that data becomes available.
Some Related Posts: Optisolar’s Thin Film, Bright Source’s Power Tower, Photovoltaic vs. Thermal, Utility Scale Photovoltaics, Acciona’s Nevada Solar One, Ausra’s Solar Thermal Power