SOLAR POWER

A utility-scale solar plant flowers in the California desert

1 August 2010



When completed, the 400 MW Ivanpah solar power complex will almost double the amount of commercial solar thermal electricity produced in the US today.


BrightSource Energy is currently developing its first solar power facility in the Mojave Desert. Known as the Ivanpah Solar Power Complex it will be located on three distinct collection sites in the basin formed by the Ivanpah dry lake, a six square mile area owned by the Bureau of Land Management approximately 50 miles northwest of Needles, California, and about five miles from the California-Nevada border. At nearly 400 MW the complex will be the largest of its kind in the world, and the first commercial solar tower power plant project to break ground in the USA.

Construction is scheduled to begin in the second half of 2010, following a permitting review by the California Energy Commission and the Department of Interior’s Bureau of Land Management (BLM). The first plant is scheduled to come online in mid-2012. When completed, it will almost double the amount of commercial solar thermal electricity produced in the US today. It is expected to achieve an overall thermal efficiency of 18% and a capacity factor of 30%.

The plan envisages three concentrating solar thermal power plants with shared facilities on federal land managed by the Bureau of Land Management. It is to be constructed in three phases: two 100 MW phases (known as Ivanpah 1 and Ivanpah 2) and a 200 MW phase, Ivanpah 3. The three plants are collectively referred to as the Ivanpah Solar Electric Generating System (ISEGS).

The project will provide power under a 1300 MW contract with Southern California Edison and a 1310 MW contract with Pacific Gas and Electric company, contingent on the fruition of Brightsource plans to build 14 solar power plants in the US Southwest by 2016.

The thermal power plants are based on distributed power tower and heliostat mirror technology, in which fields of these mirrors focus solar energy on power tower receivers near the centre of each heliostat array.

The project is expected to create 1000 jobs at the peak of construction, and 86 permanent jobs. Its lifetime total economic benefit is exxpected to top $3 billion. The environmental benefits are routine – lower CO2 emissions, reduction in water usage - but include one non-routine item. The individual mirrors are placed on metal poles that are driven into the ground, reducing the need for extensive land grading and using far fewer concrete pads.

Relocation, relocation

BrightSource had originally planned to develop its solar farm at a remote 2076 hectare location known as Broadwell Dry Lake, but in September last year, facing intense opposition from local environmentalists and politicians, it opted to walk away from the original project and relocate on a less contentious site.

One of the major issues centred on a threatened wildlife species, the desert tortoise, whose habitat would have been seriously curtailed by the development, and a formidable campaign was worked up on behalf of this animal. The conflict does underscore one of renewable technology’s weaknesses as it evolves from a largely idealistic enterprise into a big business. There is an inherent conflict between conservation in general and renewables, however green, which take up a lot of land. It is certainly not the case that conservationists and green energy enthusiasts are always on the same side.

BrightSource has not officially commented but it seems to have concluded that fighting the opposition from environmentalists and powerful politicians, including Senator Dianne Feinstein (Democrat, California) was not worth the time and money. Instead, the company decided to move to one of the 19 other pieces of acreage it had already secured for development from the BLM.

BrightSource intended originally to build a 440 MW complex and in that plan each 100 MW+ site (50000 heliostats) required approximately 1.3 square miles and three tower receivers and arrays; the 200 MW site would have required approximately 2.5 square miles and 4 tower receivers and arrays. The total area required for all three phases would, including the administration building/operations and maintenance building and substations, be approximately 3400 acres, or 5.3 square miles.

In February BrightSource submitted a new design for the project. This mitigation proposal, filed with the California Energy Commission and the BLM reduced the project's size from its original 440 to 392 MW. The new proposal, although not yet formally permitted, was developed in response to concerns over the environmental and wildlife impacts of the project, and has brought Ivanpah a step closer to being the first solar thermal power plant permitted and constructed in California in nearly two decades.

The alternative design would reduce the footprint of the third Ivanpah plant by 23 % and of the overall site by 12%, reduce anticipated desert tortoise relocations by 15 %, avoid the area with the highest rare plant density, reduce the tower count from ten to three and reduce the maximum number of heliostats by about 40000.

Background

Although this is the company’s first commercial solar construction project, its personnel have, it says, amassed nearly three decades of experience in designing, building, and operating the world’s largest solar energy plants, mainly in California in the 80s and 90s. The company is currently developing more than 4 GW of solar power projects in southwestern states of the USA.

A pilot unit at the company’s Solar Energy Development Centre in Israel’s Negev Desert is now fully operational, demonstrating BrightSource’s Luz Power Tower (LPT) 550 technology which produces very high temperature steam from solar energy.

Financing

Brightsource Energy announced in May this year that it is had received financing from an investment fund, the California State Teachers Retirement System, and from Alstom, among others, to the tune of for $150 million. It was also offered, in February, a $1.37 billion loan guarantee from the US Department of Energy, part of the DOE’s Title XVII loan guarantee programme, which was started in 2005 under the Energy Policy Act. It was the largest such loan awarded by the DOE at the time. The company will use this government backing to secure financing to support construction of Ivanpah, as well as five other projects in the US southwest it has planned.

BrightSource has partnered with Bechtel as the EPC contractor for Ivanpah. In addition, Bechtel Enterprises, the company’s project development and financing arm, will become an equity investor in all of the Ivanpah solar power plants.

Brightsource investors include Morgan Stanley, VantagePoint Venture Partners, JP Morgan, Google, BP Alternative Energy, DFJ, and Chevron Technology Ventures. Recently, Alstom added itself to that list with a $55 million investment that has made it the company’s largest shareholder and marks a significant move by Alstom into the renewables business, with a view to future orders for power block supply and EPC contracting.

Project overview

The complex consists of three separate plants to be built in phases between 2010 and 2013, and employing the BrightSource Luz Power Tower (LPT) technology.

In each solar plant, one Rankine-cycle reheat steam turbine receives live steam from the solar boiler and reheat steam from one solar reheater located in the power block at the top of the 130 metre tower. In the original design additional heliostats were located to focus on a separate reheat tower, but that concept was abandoned during the course of the re-design.

Each plant also includes a partial-load natural gas-fired auxiliary steam boiler, which would be used for thermal input to the turbine during the morning start-up cycle to assist the plant in coming up to operating temperature more quickly. The boiler would also be operated during transient cloudy conditions, in order to maintain the turbine on-line and ready to resume production from solar thermal input. As steam condensation would be effected by an air cooled unit, water consumption would be mainly that used for washing the heliostats. Auxiliary equipment at each plant includes feed water heaters, a deaerator, an emergency diesel generator, and a diesel fire pump.

The solar field and power generation equipment will be run only during daylight hours – there is no storage facility.

How LPT works

Thousands of mirrors track the sun in both vertical and horizontal aspects of its path and reflect the sunlight to a boiler that sits at the top of a tower. Heat from the concentrated sunlight is transferred directly to water circulating inside the boiler which heats it to 550°C creating superheated steam. This temperature is on a par with the highest achieved by any system in this industry and backs the claim that this arrangement achieves the highest operating efficiency. This highly energetic steam is then piped from the boiler to a standard turbogenerator set.

To conserve water in this desert environment the steam is condensed by air cooling in a closed-loop with the boiler, minimising water consumption.

BrightSource Energy’s decision to use a power tower design stems from its engineering team’s experience designing and building the nine solar electric generating stations (SEGS) in California between 1984 and 1990. These nine plants still represent more than 80 percent of the commercial solar thermal energy produced in the US today. While built durably, the SEGS plants’ trough technology has efficiency and cost limitations, but these were solved during the evolution to the LPT system. Its principle benefits are said to be its higher capacity factor – more megawatt hours produced per megawatt of installed power equipment – and lower capital costs owing to the avoidance of concrete foundations, and less piping and cabling.

Heliostats

The LPT 550 heliostats consist of two flat-glass mirrors, a support structure and a tracking system. The mirrors are mounted onto the pylon and track the sun in two dimensions. Each heliostat yields a reflecting surface of 7.04 square m. They are arranged in arcs around the solar boiler towers asymmetrically.

BrightSource’s smaller, flat mirrors are simpler to manufacture, and cost less to install than the parabolic mirrors used in solar troughs. The heliostats are highly accurate and should have over 35 years of longevity with practically zero maintenance – mainly cleaning. The ability to follow the sun in two dimensions enables the power system to track a greater percentage of the sun’s energy and achieve a much higher efficiency. Each heliostat is individually installed and controlled with optimisation software, resulting in greater flexibility in site configuration, and requiring much less site preparation.

Control system

The aiming control system and the layout of the solar fields are optimally designed to maximise steam output usng optimisation software to instruct the solar field controller where each heliostat should aim to maximize solar energy collection and output. This patent pending software system accounts for the light flux intensity and distribution required on the boiler’s receiver, and various other conditions such as sun radiation, wind, air pressure and the number of heliostats available for tracking. When computing the optimal aiming policy, the aiming control system factors in the differences between heliostats with respect to their tracking accuracy, the intensity of the beam they reflect (both of these factors are dependent mainly on the distance to the receiver), the shape of the beam and other relevant aspects.

Tower and boiler

The receiver is a conventional steam boiler made by Riley-Babcock. It is positioned at the top of the tower and converts the concentrated energy it receives directly into superheated steam. The boiler tubes are coated with a material that maximises energy absorbance. The boiler has steam generation, superheat and reheat sections and is designed to generate superheated steam at 550 °C and 160 bar.

Power block

The LPT 550 power block design is based on a standard steam powered generation facility. The first of the three towers incorporates a 123 MW Siemens SST-900 reheat industrial steam turbo-generator set specially adapted to meet solar technology requirements and offers high efficiency under varying operating conditions. Today’s most efficient steam turbo-generators are designed for 550 °C. The LPT 550 system is designed specifically for this steam temperature, thereby maximising the solar plant’s overall efficiency. It is intended to match the steam conditions found in a conventional fossil-fired plant, which gives the most flexible operation and allows for load swings and frequent starting and stopping. The turbine, the largest fully solar-powered steam turbine supplied anywhere to date, will be manufactured in Sweden, and the generator in Germany. the air cooled condenser reduces water usage by 90%.

Transmission

Ivanpah will be connected to the Southern California Edison grid through upgrades to SCE's 115 kV line passing through the site. Upgrades would include a new 220/115 kV breaker-and-a-half substation between the Ivanpah 1 and 2 project sites. The existing 115 kV transmission line from the El Dorado substation would be replaced with a double-circuit 220 kV overhead line connected to the new substation. Power from Ivanpah will be transmitted at 115 kV to the substation.


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