Biomass hybrids rival thermal storage for base-load power

Concentrated solar power developers are increasingly turning to solar thermal and biomass power plan

By Jason Deign in Barcelona

Could the blueprint for future concentrated solar power plants be sitting in Thailand? The TRESERT project unveiled by Solarlite at the Thai School of Renewable Energy Technology (SERT), Naresuan University, certainly holds a lot of potential, for two reasons.

The first is that it could potentially provide round-the-clock carbon emissions-free power. Nothing too unusual there; so can Gemasolar and a host of planned concentrated solar power (CSP) plants. But the second is it can operate for 24 hours a day without the need for an expensive heat storage sink.

Instead, the plant, which is bankrolled by the Thai government, combines Solarlite SL 2300 and SL 4600 parabolic trough collectors for steam generation with a backup biomass boiler to deliver 500 kW of thermal output and 50 kW of electrical power from a low-temperature turbine.

The developers note that the primary reason for hybridising CSP with biomass in this project is not to provide dispatchable base-load power, but to keep the operating temperature stable for the delivery of decentralised electricity, hot water and air conditioning.

“Projects like TRESERT are the right approach to finding a long-term solution to energy problems in the rural areas of Thailand,” said Solarlite’s chief executive and founder Dr Joachim Krüger in a press statement.

Nevertheless, this CSP-biomass hybrid concept is catching on elsewhere precisely because it overcomes the intermittency problems of solar thermal power without the need for storage, which can add significantly to project costs.

Says Alison Mason, director of marketing at the parabolic trough maker SkyFuel: “When an economical source of biogas is available, then a high return on investment can be expected for a CSP-biogas hybrid plant. Both technologies share the power cycle equipment.”

SkyFuel last August unveiled a 50 MW hybrid plant project in Coremas, Northeastern Brazil, in conjunction with Braxenergy, a Brazilian hydro, biomass and biogas project developer.

Grid stability

The Coremas plant will use CSP for electricity generation during the day and switch to burning coconut waste at night or when it is cloudy. “Hybridization eliminates all grid stability issues,” Mason points out.

“The CSP-biogas hybrid can reach continuous or base-load status in operation.”

She adds that SkyFuel is working with project developers in the USA, Turkey, Chile, Brazil and Italy on hybrid projects, and is collaborating with turbine manufacturers on a standard system design to improve the economics of small plants.

Other CSP players are similarly keen on the potential of hybrid solar thermal-biomass plants.

Aora Solar of Spain, for example, is commercialising a small-scale plant model combining 600 to 700 square-metre heliostat fields and a 35-metre tower with a conventional gas or bio-fuel-powered micro turbine to deliver 100 kW of electrical power.

One of the operating modes it proposes, is to use excess solar power to supply hot water for a bio-digestor that can convert urban, agricultural, distillery and food waste to biogas during the day, and then use the gas to drive the turbine when there is no sunlight.

The company estimates it requires about five tonnes of biomass per day to produce the 45 cubic metres of biogas an hour that a 100 kW plant would need to operate in hybrid mode.

Aora Solar’s design is in operation in Israel, awaiting certification in Spain and being proposed for a research park in Arizona, USA, with a price tag of USD$500,000 per plant and a construction time of three months.

Hybrid plants

Like TRESERT, it seems suited for decentralised or off-grid applications. And, indeed, there is a question mark over whether such hybrid plants could be made large enough to make a significant contribution to the grid.

Jenny Chase, solar insight manager at Bloomberg New Energy Finance, says: “It’s an interesting concept, perhaps very difficult to scale.”

Mason of SkyFuel counters that: “These projects are very scalable. The optimum is driven by local incentives and permitting requirements, availability of bio-feed stocks, land and financing.” 

She adds: “Most hybrid projects are in the 5 MW to 50 MW range; however, there is no technical limit to how large they could grow.”

The fact remains, however, that while sunlight (when available) is plentiful and free, availability of biomass can depend on how much land locally is needed for food production, and large amounts of biological waste are not usually found in the desert locations best for CSP.

Furthermore, when it comes to project development there is potentially an issue of how CSP and biomass hybrids stack up commercially against alternative sources of power, including traditional gas-powered plants.

Mason states that even with their cost advantage over CSP and storage plants, solar thermal-biomass hybrids usually still require government-backed market incentives.

Whether that makes them the best bet for renewable energy generation will thus depend very much on local market conditions. In many cases, says Chase, they “will be in competition with a cheaper and more modular PV and battery system.”      

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Image credit: California Energy Commission