The concept of adding solar thermal energy to fossil power plants is gaining momentum.

Such projects involve adding steam generated by a solar thermal field to a conventional fossil fuel-powered steam cycle, either to offset some of the coal or natural gas required to generate electric power or to boost overall plant power output.

In the last few months, a series of studies/ projects have been announced related to such hybrid technology.

Recently, Florida Power & Light Company broke ground on FPL’s Martin Next Generation Solar Energy Center, termed as the world’s first hybrid solar energy plant and the first utility-scale solar facility in Florida. In another development, the Electric Power Research Institute (EPRI) is currently leading two projects to help electric power companies add solar energy to fossil-fueled electric power plants.

Currently, 27 states in the U.S. have enacted renewable portfolio standard (RPS) policies. Some include specific mandates that a percentage of the renewables requirement be met with solar energy. However, most current solar applications are not cost-competitive with other power generating options.

According to EPRI, using solar to augment coal or natural gas is potentially the lowest-cost option for adding solar power to the generation fleet, as it utilises existing plant assets. And because the highest-intensity solar energy typically is within a few hours of peak summer loads, it makes solar augmented steam cycles a particularly attractive renewable energy option.

The projects initiated by EPRI will provide a conceptual design study and two detailed case studies. Design options to retrofit existing plants will be analysed and new plant design options will be identified. The study will examine the potential use of solar-thermal technology at a pair of coal-fired power stations, in New Mexico and North Carolina.

In the case of FPL, the solar facility will consist of approximately 180,000 mirrors over roughly 500 acres of land at the existing FPL Martin Plant location.

A significant feature of the facility, which is scheduled to be fully operational in 2010, is the implementation of solar thermal technology, which is projected to decrease fossil-fuel usage by approximately 41 billion cubic feet of natural gas and more than 600,000 barrels of oil. It will provide enough power to serve about 11,000 homes. Over 30 years, the solar facility is predicted to prevent the emissions of more than 2.75 million tons of greenhouse gases.

Another US utility, Pacific Gas and Electric Company, signed a power purchase agreement with Martifer Renewables Electricity for 106 MW of power from a similar set-up last year. Martifer is combining solar thermal technology with biofuels to produce energy. The project, which will be located in California’s Central Valley near Coalinga, will source the biofuel from a combination of locally-produced agricultural wastes, green wastes and livestock manure.

“A hybrid energy center… is a great way to create around-the-clock renewable energy generation that rivals the reliability provided by more conventional power resources,” Hal LaFlash, Director of Emerging Clean Technology Policy, Pacific Gas and Electric Companytells CSPToday. 

In the emerging markets, AORA Solar, which recently became the first company in Israel to obtain a licence for a solar-thermal power plant, is constructing its first hybrid solar power station on a half-acre (0.2 hectare) plot in Israel’s Negev desert.

Generating electricity “around the clock”

Since solar fields feed their heat energy into a conventional steam turbine, they can be integrated with ease into the relatively clean new generation natural-gas-fired combined- cycle power plants. It is also possible to retrofit existing conventional steam power plants with parabolic trough solar fields as an additional solar steam generator.

Hybrid technology means improved utilisation of the turbines and thus optimal operation of the entire power plant block, and therefore favourable power prices based on a mixed calculation. And in comparison to molten salt storage technology, the use of auxiliary heating with fossil fuels can provide a more cost-effective buffering of fluctuations in solar radiation.

AORA has used a gas turbine in its hybrid design that can handle super-high temperatures and then work off external fuel when sunlight is unable to produce the necessary heat.

“[By] integrating our system with waste treatment plants that generate the fuel to run our turbine [we] can address both, power supply and waste disposal issues. The modular structure of our plant provides the flexibility that will enable its integration with any size waste treatment facility,” AORA’s chief technology officer Pinchas Doron told CSPToday.com.

The power conversion unit (PCU) of AORA’s system comprises a solarised micro gas turbine fed by a unique, advanced high temperature receiver that can heat air to meet the turbine requirements. The company has tailored the patented receiver to integrate with a state-of-the-art gas turbine.

Integrated solar combined-cycle system

Among some of the established players, Abengoa Solar is currently associated with what it describes as the world’s first Integrated Solar Combined Cycle or ISCC plant, which is being constructed in Algeria. Abengoa Solar is providing the design and will act as the technician of the solar field (In ISCCS power plants, parabolic trough fields are combined with modern gas-fired combined-cycle power plants).

Due to its maturity and proven track record, the solar technology used in ISCC plants are typically trough systems. An ISCC is very similar to a conventional combined cycle. The primary difference is that heat from a solar field supplements the exhaust heat used to run the steam turbine.

According to Abengoa, the solar combined cycle hybrid plants are supported by the attractive configuration offered by conventional plants with combined cycle.

A conventional combined cycle plant is formed by a gas turbine, a heat retriever, and a steam turbine. In the case of the ISCC solar hybrid plant, solar energy is used as auxiliary energy that will allow increasing cycle’s yield, whilst reducing emissions.

The operation of a solar combined hybrid plant is similar to the one of a conventional combined cycle plant. The fuel (preferably natural gas) is burned generally on a combustion chamber of a gas turbine. The heat coming from the solar field is added to escape gases that direct to the heat retriever, resulting on an increase of steam generation and, consequently, an increase on electricity production on the steam turbine.

To install solar combined cycle hybrid plants, Abengoa highlights certain requirements, such as:

1) Topography: the site needs to be level, preferably with a slope less than 1%

2)  Irradiation: the direct normal insolation (DNI) should be a high as possible

3) Water Availability: water is needed for cooling in the power block

4) Electric Transmission: electric lines and transmission capacity are needed to convey solar power from the plant to the consumer

Costs

As per the estimates of SunsLab, formed after combing the CSP departments of two national laboratories — Sandia National Laboratories in Albuquerque, New Mexico, and the National Renewable Energy Laboratory in Golden, Colorado, CSP technologies currently offer the lowest-cost solar electricity for large-scale power generation (10 megawatt-electric and above).

Current technologies cost $2–$3 per watt. This results in a cost of solar power of 9¢–12¢ per kilowatt-hour. New innovative hybrid systems that combine large concentrating solar power plants with conventional natural gas combined cycle or coal plants can reduce costs to $1.5 per watt and drive the cost of solar power to below 8¢ per kilowatt hour.

Overall, such projects exemplify the potential within states like California to utilise diverse renewable energy resources to meet respective climate goals.

However, there are several factors which need to be considered. For example, according to LaFlash, wind in California frequently has its maximum output at night, which makes it a good complement to peak-hour solar.  The challenge in integrating the two is finding an area with both strong solar and wind resources without subjecting the solar equipment (especially mirrors) to excessive wind loading. The other issue is devising a curtailment protocol for those times when wind and solar produce at the same time.

Also, the distance to the solar field should not be far off otherwise long-distance heat transfer would be costly. Plus, the support from regulatory bodies, too, is paramount. Even those plants with the right sun and space would not move forward until governments put a firm price on carbon emissions from coal.