The prodigal sun: can solar thermal technology become a bridge to renewables?

18 November 2015 (Last Updated November 18th, 2015 18:30)

Solar thermal energy that heats transfer fluids in traditional coal-fired plants has the potential to provide a bridge from fossil fuels to renewables. Julian Turner analyses a new scientific study into the process that calls for changes to policy to encourage greater adoption of renewable energy.

The prodigal sun: can solar thermal technology become a bridge to renewables?

Power plant India

Ask almost anyone about the main barriers to the widespread adoption of renewable energy and they will likely cite issues such as cost (to both governments and consumers), reliability (can nascent technologies be scaled up sufficiently to satisfy growing demand?) and globalisation (why should Western nations invest in renewables when China is busy building new coal-fired power plants?).

The problem of perception is rarely mentioned, however, perhaps because it is more esoteric and therefore less easily quantifiable. Put simply, most informed people now agree that de-carbonising the global environment is a priority and that there is no silver bullet when it comes to energy policy.

But, having grown used to plentiful cheap energy from fossil fuels, many struggle to conceive of a greener future powered by disparate sources, everything from nuclear to biomass, wind to solar.

How, for example, can industries remain competitive while simultaneously weaning themselves off dirty fuels, and will the costs be passed on to consumers? To renewable advocates, the answer lies in bridge fuels, low-carbon alternatives that help industry facilitate the transition to cleaner energy.

One such solution, the integration of solar thermal energy (STE) into the heat cycle of existing fossil fuel power plants, offers efficient solar-to-electric conversion without the need for plant upgrades.

Hot topic: solar thermal energy vs carbon capture and storage

Solar thermal plants harness the power of the sun to generate heat or electricity. Coupled with a thermal storage strategy, they can deliver baseload electricity through the existing power grid.

Nature, even when easy to understand, can be incredibly difficult to copy.

According to a new study published in the scientific journal Nature Climate Change, solar thermal heat injected into conventional power plants can offset fossil fuel combustion using existing technology. ‘Solar-aided’ facilities also offer better efficiencies than conventionally deployed solar thermal power plants.

By heating transfer fluids to a benchmark of 400°C using solar energy, it is possible to reduce fossil fuel consumption by up to 50% in plants that employ a type of steam-operated heat engine based on the Rankine cycle, the process whereby heat is converted into mechanical work to create electricity.

"Integrating solar thermal systems into Rankine-cycle power plants can be done with minimal modification to the existing infrastructure," the paper states. "This presents an opportunity to introduce these technologies into the commercial space incrementally, to allow engineers to build familiarity with the systems before phasing out fossil-fuel energy with solar electricity.

"We suggest that such projects be encouraged by extending the same subsidy/incentives to the solar thermal fraction of a ‘solar-aided’ plant that would be offered to a conventionally deployed solar thermal power plant of similar capacity," the study continues. "Such a policy would prepare the ground for an incremental solar thermal takeover of fossil-fuel power plants."

The paper, titled ‘Solar thermal technologies as a bridge from fossil fuels to renewables’, also argues that retrofitting coal-fired power plants in this manner reduces greenhouse gas emissions. To achieve this, they must operate in the "fuel-saving" mode, where solar-heat supplants that generated by coal-firing.

To significantly displace use of coal, thermal storage technologies must be deployed to store solar heat gathered during sunlit hours for use after sun-down or during times of cloud cover.

The paper shows that solar thermal integration is also more economically attractive than carbon-capture technology for the same greenhouse-gas mitigation. The Intergovernmental Panel on Climate Change (IPCC) estimates CCS can increase the fuel needs of a coal-fired plant by up to 40%, and the overall cost of energy generated from purpose-built facilities by as much as 91%.

Breaking the cycle: the ‘one-plant-one-fuel’ paradigm and government subsidies

The study not only promotes solar thermal energy as a cost-effective bridge to renewables; it also recommends key changes to global energy policy designed to incentivise solar-aided power plants.

"A major thrust of the policy change that we recommend is to break away from the ‘one-plant-one-fuel’ paradigm."

"A major thrust of the policy change that we recommend is to break away from the ‘one-plant-one-fuel’ paradigm," it states. "This paradigm is implicit in practically every nation’s renewables policy; for example, policies of the US and Spain actually prescribe an upper limit to the fossil-fuel co-firing permissible in a solar thermal power plant and then reward the entire output of the plant – a policy that excludes the minority solar thermal fraction in a solar-aided fossil-fuel power plant.

"Our argument is that, with the solar-aided power plant paradigm, the conventional power infrastructure can actually prepare the way for development and deployment of renewable power."

The study’s authors – Vishwanath Haily Dalvi, Sudhir V. Panse and Jyeshtharaj B. Joshi – -reject calls for the mothballing of coal-fired power plants as unworkable, and argue that while an excellent case can be made for pricing or taxing CO2 emissions, the strategy is likely to be politically untenable and result in electricity and fuel prices that disproportionately burden the poor. Instead, they propose to incentivise renewables using funds raised by progressive taxation.

"Policymakers should drop the insistence on majority solar fractions in solar thermal power plants and instead offer the same incentives to the (initially minority) solar thermal fraction of solar-aided power plants that would be offered to the corresponding, conventionally deployed solar thermal power plant – including the favourable feed-in tariffs," the reports states.

"Solar thermal power production is indispensable for a renewable future but has not seen the support from policymakers, entrepreneurs or researchers that solar photovoltaics has enjoyed," the paper concludes. "The time is ripe for a revolution in the field of solar thermal power production… perhaps this small policy change will help kick-start that revolution."

A bridge too far? Modularity and investment in solar thermal energy

With the renewables renaissance under pressure from a new era of cheap hydrocarbons provided by the US shale gas boom, can co-opted STE technology gain real traction as a bridge to clean energy?

The Green Deal has been scrapped, the regulations designed to make zero-carbon homes the standard have been stripped away.

Examples of synergetic solar-aided fossil-fuelled power plants include the Martin Next Generation Solar Energy Centre in Florida, US (installed solar capacity corresponds to 2% of the 3.8GW plant), Kogan Creek Solar Boost in Australia (6% of 750 MW) and ISC C Kuraymat in Egypt (15% of 140MW).

Despite these notable successes, and similar projects such as the Solar Electricity Generation System (SEGS) systems in the Californian desert, the problem of perception is once again an issue in that the majority of global policymakers still view the solar thermal model as too risky and too expensive.

"The main complaints stem from the lack of modularity because solar thermal plants (as conventionally deployed) need to install tens of megawatts of capacity before production can start: requiring much more sophisticated project management and better access to finance than an equivalent photovoltaics installation," the paper published in Nature Climate Change states.

However, with thermal-power plant integration comes increased modularity, making solar thermal technology a much more attractive option for both power plant operators and potential investors.

"With thermal power-plant integration, solar thermal becomes modular, with the fossil-fuel-fired side ready to take up any slack in the solar side," the paper confirms. "Capacity can, therefore, be added incrementally depending on availability of land, solar resource, funds and so on.

"Also, entire sections of the solar field can be shut down without violating contract obligations. The reliability of the integrated system will also enable greater access to finance and the ability to recover from setbacks will increase developers’ appetite for experimentation and innovation.

"Another major advantage resulting from modularity is that it allows deployment of solar thermal technologies for power production in regions with limited land and/or direct normal irradiance and
using collectors – for example, vacuum tubes used in solar hot-water heaters – that normally would not be considered useful for power production.

"Deployed in this manner, solar thermal technologies are unique among renewables in that they present businesses traditionally involved with coal power with an evolutionary mechanism (that is, the business benefits from each solar increment) to transition to renewables in a way that leverages their expertise, capital and connections."