Ivanpah tower plant's receiver. Picture credits: BrightSource.

By Jason Deign

It is enough to brighten the day of the saddest of CSP fans. The International Energy Agency (IEA) says sunshine could be the biggest source of electricity on earth by 2050, with PV supplying 16% of demand and CSP adding a further 11%.

“Combined, these solar technologies could prevent the emission of more than 6 billion tonnes of carbon dioxide per year by 2050,” said the Agency last month as it unveiled two roadmaps to achieve a sun-powered future.

“That is more than all current energy-related CO2 emissions from the United States or almost all of the direct emissions from the transport sector worldwide today.”

However, added the IEA: “The two reports do not represent a forecast.

“As with other IEA technology roadmaps, they detail the expected technology improvement targets and the policy actions required to achieve that vision by 2050, highlighting priority actions and milestones for governments, research and industry stakeholders.”

The roadmaps include key assumptions about the cost of solar technologies, willingness to combat climate change and constraints on other low-carbon options such as nuclear. “It’s a model result,” explains Cédric Philibert, of the IEA’s renewable energy division.

“In this scenario, that aims at halving global energy-related CO2 emissions, both nuclear power and carbon capture and storage are artificially constrained to reflect any possible barriers to their development.

“Under these assumptions, solar power ends up as the largest source of power. This results from a combination of cost and potential considerations in each country.”

Energy storage

So what does a world where 11% of electricity comes from CSP look like? The most critical point, says the IEA, is that thermal energy storage plays a crucial role. “It is absolutely vital for CSP to have storage in order to reach the level described by the IEA,” says Philibert.

“The higher cost of solar thermal electricity, by comparison to PV, would be less justified otherwise. Greater inertia [and] possibilities for hybridisation with fossil fuels have a value, but would not be enough to scale up CSP to such high levels.”

Storage would give CSP the ability to generate electricity after sunset, when PV power vanishes, the IEA believes. With 16% of electricity generation coming from PV, this after-hours supply is likely to be highly valued, Philibert says.

In terms of technology, the CSP roadmap recognises the value of recent innovations such as power towers and molten salt storage. “All CSP technologies will likely play a role in achieving an 11% level, and all have significant room for improvement,” comments Philibert.

“However, with the need for several hours storage, molten salt towers using the storage medium as a heat transfer fluid with very easy gravitation draining may represent the most effective design with current status of technologies.”

Other areas that the CSP industry might want to explore include new heat transfer fluids, storage devices, supercritical turbines, collectors and receivers, he says.

The IEA also sees greater potential for convergence of CSP with PV, capturing the electric and thermal potential of sunshine for greater efficiency. Companies such as Emcore are known to be studying such approaches, but how realistic is the rest of the IEA’s vision?

“Our roadmaps do not offer forecasts,” Philibert repeats. “They don't show what will happen, they show what should happen if we were serious about climate change.”

Current pipeline

In addition, even though 11% seems a long way off today’s level of solar thermal generation, he says: “In the scenario CSP really takes off after 2025. This has no direct connection with the current pipeline of projects, which is only indirectly relevant.”

Nevertheless, there are a number of factors that make the IEA’s roadmap seem optimistic, to say the least.

For example, while it is true that carbon capture and storage has so far failed to take off, nuclear energy is enjoying something of a resurgence worldwide, with more than 70 new plants currently under construction. That is the highest level in more than 20 years.

Perhaps more importantly, although there is much political rhetoric around carbon reduction in Europe and America, the power requirements of emerging economies such as China and India mean the objective of halving emissions by 2050 seems as far away as ever.

But for Dr Gianluca Ambrosetti, head of research at the CSP systems developer Airlight Energy, the main failing of the roadmap was perhaps that it lacked vision.

While the IEA was keen to showcase the potential of power towers with molten salt storage, for example, Airlight is already commercialising a parabolic trough technology with an air and gravel-bed heat store that costs just 1% or 2% of the plant, instead of 10% or more.

“I was disappointed,” Ambrosetti says. “We have a long way to go. If there was to be a roadmap, I would like for it to be more inspiring.”