At a lower cost than thermal storage, CSP-Biomass hybridization promises to deliver 100 percent renewable energy, around the clock. But it comes with its own set of sustainability issues.

By Rajesh Chhabara

In order to commercialise CSP as a dispatchable energy, industry has developed hybrid models that use natural gas or fossil fuel in combination with the solar resource. But in view of long-term scarcity and related price volatility of fossil fuels, this approach provides only a medium-term solution.

Instead, industry players are now examining how CSP can be hybridised with biomass energy to achieve 'round-the-clock, 100 percent renewable energy.

Biomass consists of agricultural and forestry wastes and by-products, which can be utilized as fuel for producing energy. Examples include forest slash, urban wood waste and lumber waste, and agricultural waste such as orchard trimmings, fruit pits, nut shells and rice hulls.

“If the biomass fuel is available, hybrids are certainly feasible.  Extending the use of the power block beyond solar hours by using biomass will certainly help to reduce costs, as well as provide the benefit of 24-7 operation,” says Craig Tyner, senior vice president of engineering at e-Solar.

Valerio Fernandez, operations manager at Abengoa Solar, agrees that CSP-biomass hybridization is a very interesting option to help meet the peak demand during daytime on cloudy days.

“We can ensure a stable supply to the grid by using a renewable resource such as biomass together with a CSP plant,” says Fernandez.

Apart from improving the profitability of a CSP plant by ensuring day-night production, biomass hybridization also meets a key demand of environmental NGOs, such as Greenpeace, who advocate 100 percent energy from renewable resources.

California Biomass Energy Alliance, an association of biomass-fuelled power plants in California, says that using biomass for producing energy also reduces carbon emissions.

According to the association, if left to their various alternate fates, the biomass materials would emit substantially higher levels of greenhouse gases as a result of their biodegradation in landfills or in the forests, or from their open-burning in agricultural fields or in the forests.

World’s first CSP-Biomass hybrid plant

A proposal to build the world’s first CSP-Biomass hybrid plant in Fresno County in California is already under review by the California Energy Commission.

According to the Commission, the project consists of two hybrid design solar thermal electric generating plants, comprising a solar field and biomass facility for each plant and will produce up to a nominal 106.8 megawatts net of renewable energy.

Known as San Joaquin Solar 1 and San Joaquin Solar 2, the plants will be owned and operated by San Joaquin LLC, the US subsidiary of Portugal-based Martifer Renewables Electricity.

Pacific Gas and Electric Company, the California utility, has already signed a deal with San Joaquin to purchase power from the plants. If approved by the Commission, the plants will become operational by 2011.

The biomass unit of the plant will use orchard trimmings, agricultural waste and cow manure - all of which are available in plenty in the area - as feedstock. The success of the plant will trigger more CSP-Biomass hybrid plants in California, which set a target in 2006 to have 20 percent of the renewable electricity from biomass by 2010 and maintaining this level through 2020.

Abundant biomass resource availability within California makes CSP-Biomass hybridization more feasible. California produces an estimated 47 million bone dry tonnes of biomass annually, according to the California Energy Commission.

Grupo Ibereolica, a Madrid-based renewable energy utility, says it has plans for a future upgrade to biomass hybridization of the two 50 MW CSP solar thermal power plants it is building in Seville and Badajoz in Spain, in order to increase production significantly during the night. Grupo Ibereolica is building these plants in partnership with Inveravante, another Spanish firm with interests in renewable energy.

Given the interest, companies have started developing CSP-Biomass hybrid technologies. For example, California-based Power Smith Group has developed a CSP-Biomass hybrid model. The model eliminates the need for costly solar thermal storage as it does not require heat transfer fluid and associated equipment.

“Cost and carbon emissions savings are achieved by operating the biomass at reduced rate during the day with full solar operation.  As the sun sets, the biomass boiler operation is increased to compensate for declining solar,” explains Robert Emery, the founder of Power Smith Group and a former Luz project manager. He adds that the carbon savings vary depending on the amount of fossil fuel that is offset.

Emery says that the hybrid is fully integrated and uses common equipment lowering operation and capital costs.  The hybrid solar field consists only of solar collectors, water injection piping and control system integration.

Availability of feedstock is a critical factor for the feasibility of a CSP-Biomass hybrid plant. “CSP works significantly better in desert environments, where biomass is less abundant. Thus finding sites with good resources of both solar and biomass may limit the application,” cautions Tyner.

Biomass supply a potential problem

Accurately estimating long term supply of biomass for the plant also poses challenges. Removing agricultural residue for biomass also has concerns of sustainability.

A Roadmap for the development of biomass in California, a 2006 draft report by California Energy Commission, highlights that agronomic effects such as loss of organic material and nutrients from soils, soil carbon and nitrogen cycles, water- and wind-borne soil erosion, irrigation water infiltration into soils and habitat, and effects of weed control need to be considered in relation to crop residue removal.

The report adds that sustainable agricultural residue removal rates need to be determined and should be part of the agricultural reside best practices determination.

Developers of CSP-Biomass hybrid plants will need to factor these challenges in their planning.

Technologically, CSP-Biomass hybridization still needs to be tested at large and commercial stage. The first few plants therefore will be closely watched to fully understand the potential.