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Solar Rising
by Rob Rogan, Senior Vice President, Americas at eSolar
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Solar power is often offered as one of the most promising forms of renewable energy. How effective can solar power be, and how close is the solar power industry to being able to deliver on the promise of the technology?

Solar energy is the least utilized source of energy, even among other renewable energy sources such as wind and geothermal, despite that worldwide concentrated solar power (CSP) capacity is forecast to increase nearly 18-fold in the next five years from its current 588-MW potential to around 10.5 GW. Amongst this tremendous potential to deliver inexpensive, pervasive power at peak, adoption of solar power has increased only in recent years.

To understand this slow pace of deployment, one needs to look only at the perceived barriers to entry: The energy is too expensive to deploy; the construction and permitting process is too lengthy; and the investment needed to build new transmission lines is too great.

In recent years, photovoltaic (PV) solar has had an easier time integrating into our energy mix and avoiding these barriers. Thin film PV panels are lower in cost and have been used in utility-scale solar power, but their low efficiency leads to higher installed system cost. Smaller PV solar installations were easier to get off the ground at first. In contrast, CSP is becoming more attractive for large-scale solar energy needs.
eSolar heliostat field
CSP is used as a utility-scale resource and can scale economically. CSP is already the least expensive and most efficient form of solar power for utility-scale providers in areas with favorable incentives and ample solar resources of at least 2,000 kW per hour of annual direct normal irradiation per square meter. In the solar-rich, U.S. Southwest on 63,000 square miles, more than 1,100 quadrillion British thermal units of solar radiation reach the earth each year. Consequently, concentrating solar thermal power stands to make the biggest impact on U.S. utilities' power generation portfolios.

With solar subsidies in place and increased consumer awareness of the benefits of solar energy, there is finally the opportunity for solar to play a major role in the United States' power generation mix. We are seeing the emergence of solar thermal companies that have seized old industry challenges (cost, transmission lines, permitting) as opportunities to forge transformative, disruptive solutions.

As the first tower CSP plants come online in the United States in more than 20 years, which models will rise to the top? Four central challenges face the industry that we at eSolar think will distinguish companies that can effectively deliver value to customers from those that cannot:
  • Price
  • Speed of deployment
  • Scalability, and
  • Grid impact.
Leading solar thermal companies understand this and are moving to address these obstacles, thereby advancing the industry.

Concentrating Solar Technologies
There are five main concentrating technologies: trough, compact linear Fresnel reflector (CLFR), dish Stirling engine, concentrating PV (CPV) and tower.

Trough technology. The trough is the most mature concentrating solar technology with commercial demonstrations since the 1980s and total installed capacity of more than 550 MW. It uses a one-axis tracking system to concentrate sunlight onto tubes, inside which synthetic oil runs and then through a heat exchanger converts water into steam that consequently goes through the steam turbine.

The concentration level achieved by the optics system is low because it uses single-axis tracking, hence the maximum temperature achieved is low, driving down the efficiency of the plant. The main advantage of this technology is its maturity. Its main disadvantage is the relatively higher cost and, at times, limited supply of materials.

CLFR technolology. In CLFR technology, mirrors track the sun and focus on a tube where synthetic oil gets heated and goes to heat exchangers to produce steam, or direct steam generation can take place directly in the tubes. Unlike in trough technology, the tubes are fixed in place, but the tracking field is still one-dimensional and the concentration level is low (thus having lower maximum temperature and lower operating efficiency necessitating the use of water cooling).

Stirling engine technology. The dish Stirling engine uses a dual-axis tracking system in a tracking dish to focus the sunlight on a Stirling engine. Each dish system produces between 10 to 25 kW and is considered the highest efficiency solar thermal solution available. This technology, however, foregoes some of the most established benefits for concentrating solar thermal in utility-scale applications. Practically, it offers minimal economies of scale for large-scale projects, making it more suitable for distributed power applications under 10 MW. The dish engine technology neither provides storage nor can be used to hybridize a traditional steam power plant.

CPV technology. Another variation of concentrating solar energy is CPV technology, in which light is concentrated on semiconductors that convert light energy directly to electricity. Several companies using high-efficiency solar cells adopted this concept recently. Solar Systems of Australia uses mirrors to focus light on an array of semiconductors, and the California-based Amonix system uses Fresnel lenses to focus light on individual solar cells grouped in series and parallel.

Tower technology. Tower technology uses either direct steam generation or molten salt technology. Both approaches have been demonstrated in Solar One in the 1980s and Solar Two (using molten salt) in the 1990s in California. Today, however, there are only 35 MW of commercial installation worldwide. The latest demonstration is Spain's PS-10 and PS-20 projects. The tower system uses dual-axis tracking mirrors called heliostats to concentrate sunlight on a central receiver atop a tower. Because of the dual-axis tracking, higher temperatures can be achieved, hence higher efficiency. Tower technology has the potential of being the lowest-cost. The PS-10 represents typical tower technology, which uses large mirrors-each mirror is 120 square meters — that are mounted using poles that are dug into the ground with large motors and drives to rotate the heliostats and track the sun.

The eSolar Solution
eSolar developed its technology with the key hurdles to solar power plant deployment in mind. eSolar's approach, which starts with a 46-MW generating unit on 160 acres that can be replicated to scale to more than 500 MW, was designed to meet the barriers to entry while delivering a range of options to a variety of power producers, from large utilities and power providers to smaller, regional utilities. Uniform design and a smaller minimum footprint give the company its competitive advantages in price, scalability and speed and facilitate grid integration. Companies able to innovate around existing deployment hurdles will be best positioned to serve utilities by offering easily scalable, cost-effective solar technology.

The eSolar approach leverages prefabricated parts produced in high volumes on pre-existing manufacturing lines, which can quickly scale to meet future increase in demand, and therefore do not require construction of new production facilities. Furthermore, prefabricated parts can be easily assembled in the field with minimal labor and, with the use of a proprietary system for heliostat calibration and tracking, the overall capital cost of a plant as well as the cost of operations is reduced further.

eSolar Case Study: Sierra SunTower
eSolar's Sierra SunTower power plant in Lancaster, Calif., demonstrates a new blueprint for global solar deployment and is the only operating CSP tower in the United States. The facility uses 24,000 mirrors across 20 acres, reflecting sunlight onto two thermal receivers from which steam is directed through a refurbished 1947 GE turbine. Sierra SunTower provides 5 MW of clean, renewable energy to some 4,000 Southern California Edison households.

The development is the first of many planned in the southwestern United States for the company, which has signed power purchase agreements totaling 429 MW with three utilities in California and New Mexico.

During the 12 months of construction, the project created more than 300 temporary jobs. In operation, Sierra SunTower employs 21 permanent staff and continues to contribute to the economic growth of the local community and was commended by California Gov. Arnold Schwarzenegger.

"California's energy and environmental leadership are advancing carbon-free, cost-effective energy that can be used around the world," Schwarzenegger said.

Further lessening the plant's environmental impact, Sierra SunTower is on private land. eSolar's smaller footprint allows it to use private land exclusively for the development of its plants, which in turn mitigates the environmental concerns that come with development on pristine desert land and permits the company to site its plants closer to existing transmission lines.

As demonstrated by the flipping of the switch on eSolar's Sierra SunTower, the solar industry is gaining momentum and showing its potential to be a leading contributor in the United States' energy mix. None of the technologies discussed are without their pros and cons, but the way to move forward is to wholly understand the perceived limitations and work with these barriers. Know when to forgo expensive materials for improved technology; know how to improve business processes to speed up production; understand the complexity of permitting and siting and consider re-positioning.

With technologies already in place and strong public support, the solar companies that move forward with the most innovative, flexible and strategic blueprints will rise to the top. We look forward to our bright solar future — a landscape dotted with unique models of solar success and a freedom from fossil fuels.

Author
Rob Rogan, Senior Vice President, Americas at eSolar

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