Thermal solar moves into the mainstream

The construction of thermal solar power plants introduces a new source of energy

By David Appleyard

CSP (Concentrating solar thermal power) has long been viewed favourably by the wholesale energy sector. Now, advances in technology – including the vital thermal storage capacity that enables solar variability to be decoupled from a plant’s output – have succeeded in converting that sentiment into reality as a series of major new projects are commissioned. However, there is still some ground to cover before the technology achieves its potential. One crucial area is the development of industry International Standard.

Ivanpah solar electric generating station - a tower receiving system (Photo: NREL) Ivanpah solar electric generating station - a tower receiving system (Photo: NREL)

Not that recent

Some 150 years after the French mathematician Augustin Mouchot demonstrated steam generation from concentrating solar energy, the use of solar energy steam generators connected to fairly standard conventional power islands – a steam turbine and generator – is a technology that is now becoming increasingly popular.

Indeed, while the various designs of solar collector may present some novelties, CSP installations share many common traits with their fossil-fired cousins. It is perhaps for this reason that CSP has attracted the interest not only of utility companies keen to expand their renewable portfolios, but also of original equipment manufacturers which have traditionally supplied the utility market.

Many technologies, but one aim: heat

CSP comprises a range of technologies that are used to collect and concentrate sunlight, turning it in to medium to high temperature heat. This heat may then be used to generate electricity in a conventional way using a steam turbine or a Stirling engine, or used in other applications, for example supplying process heat. With the exception of dish-Stirling systems in CSP power plants, the solar energy is typically absorbed by a heat transfer fluid, such as oil or molten salts, which is then passed through a heat exchanger and its associated steam circuit.

The mirror systems used in CSP plants are either linear or point-focusing systems. Linear systems typically concentrate the solar radiation by about one hundred times and achieve working temperatures of up to 550°C. Point systems can achieve far higher concentrations, more than a thousand times, and consequently can reach far higher temperatures, with 1 000°C or more possible.

There are four main types of commercial CSP technologies in operation today. Linear systems include Fresnel lensing and the far more common parabolic trough types. Point concentrating systems include parabolic dish-type systems, typically used with a Stirling engine, or the more common central or tower receiver systems.

Global potential

Regions with a relatively high level of annual insolation, such as southern Europe, the south western USA, the Middle East and North Africa, India, China and South Africa, for example, are prime locations for the development of CSP.

The world’s first commercial CSP facility, the PS10 development by Abengoa Solar, which uses a central tower receiver, is located near Seville in Spain and was commissioned in 2007. PS10 produces saturated steam, has a peak output of some 11 MW and is one of a series of CSP developments in the region that are expected to generate around 300 MW once completed.

Rapid developments

Giving an indication of how far the technology has progressed in just a few short years, in March 2013 the world’s largest CSP project, Shams 1 in Abu Dhabi in the United Arab Emirates, was commissioned with a capacity of 100 MW. Shams 1 is a parabolic trough-type installation which was developed by Masdar, the state-owned renewable energy company, together with partners Total SA and Abengoa.

The project cost USD 600 million and took three years to build, featuring more than 258 000 mirrors mounted on 768 tracking collectors. Shams 1 features an additional gas-fired booster that heats the steam further, dramatically increasing the efficiency of the thermal cycle. The project also includes a dry-cooling system that significantly reduces water consumption.

Shams has already been eclipsed by a US project, which began full commercial operations in February 2014, high in California’s Mojave Desert. Supplying a peak of some 392 MW of additional renewable energy capacity to the grid, the Ivanpah project, a joint project by NRG Energy Inc., Google and BrightSource Energy, comprises three central receiver type units with a trio of 140-metre high towers absorbing the radiation reflected from the surrounding solar field of 173 500 heliostats. Ivanpah accounts for nearly 30% of all solar thermal energy currently operational in the US. Developing and commissioning the project required some USD 2,2 billion of investment.

Commenting on the project, Tom Doyle, president of NRG Solar said: “We see Ivanpah changing the energy landscape by proving that utility-scale solar is not only possible, but incredibly beneficial to both the economy and in how we produce and consume energy”.

The largest parabolic trough CSP plant currently operating is the 280 MW Solana installation in Arizona. Featuring molten salt energy storage it can operate for 6 hours without any sunshine, boosting plant capacity to 41%.

Thermal storage makes all the difference

One of the most significant advantages CSP has over other solar energy technologies is its ability to partially decouple plant output from solar insolation using energy storage. Unlike electrical energy, thermal energy is relatively easy to store and a range of solutions have been explored to improve the efficiency and energy density of storage media. These technologies use a secondary material or fluid – such as water, steam, concrete, graphite, molten salts or phase change materials – which absorb the collected heat energy before reversing the process when required and transferring it back to the working fluid.

Commercial-scale projects have advanced this technology sufficiently to be able to generate electricity throughout the hours of darkness, when using sufficient heat storage capacity. Two commercial storage technologies are currently in use: steam storage in pressurised vessels and molten salt storage using insulated tanks. For example, Torresol Energy’s 20 MW Gemasolar project was the world’s first commercial-scale plant to use a central tower receiver and molten salt heat storage. The storage capacity enabled the plant to operate for some 15 hours without any solar input.

Needless to say, since the Gemasolar project was commissioned, storage technologies have also progressed, a point emphasised by Elisa Prieto, director of strategy of Abengoa Solar and an Expert with IEC TC (Technical Committee) 117: Solar thermal electric plants. Abengoa is a leading developer of CSP installations, covering both parabolic and central receiver technology.

24/7 capability

Prieto refers to a new CSP project planned for the Minera El Tesoro copper mine in Chile’s Atacama Desert which will be operating 24 hours a day, seven days a week. “It’s a plant with 17 hours of storage which will allow the plant to operate around the clock with a really high capacity factor, close to 90%,” says Prieto.

In October 2013 Abengoa Solar was awarded a contract under a South African government tender for the Xina One plant, a 100 MW parabolic trough type project with 5 hours of storage capacity.

Prieto emphases this fundamental difference between CSP and PV (photovoltaic) solar systems, saying: “The key to these technologies is that everyone understands that they're different products. PV is a cheap way to get electricity but it is also an intermittent source of energy. There is another part of the mix that you need which is the part that gives you security of supply.”

The ability to supply dispatchable power is what sets CSP apart, explains Prieto.

Stavros Tassos, Solar Energy Team Leader at engineering consultancy firm Mott MacDonald, also highlights these distinctive properties of CSP, saying: “The integration of thermal energy storage to CSP, coupled with CSP’s adaptability to a range of capacities, is one of the technology’s main differentiators. It is one that pushes the sector forward as utilities and grid operators would favour the more dispatchable and flexible renewable energy plant. Developers can use the resource/generation decoupling capabilities that storage affords to maximise revenue generation where time of day tariffs exist. National incentive programmes for CSP are increasingly asking for storage to be included, pushing the industry towards integrating and optimising storage design”.

In addition, Tassos stresses “CSP’s potential for hybridisation through integration with existing fossil fuelled power plants. Integrated solar combined cycle or solar augmentation in coal-fired plants is currently an area being looked at very closely by utilities and likely to further fuel the CSP industry in the near future”.

Advancing CSP technology

Tassos argues that CSP is still a sector where innovation is prominent. “We regularly see important incremental innovations across virtually all technological areas of the solar field as well as with processes. These have the effect of progressively reducing losses and improving efficiencies and availability, ultimately achieving cost reductions or LCOE (levelized cost of energy) improvements, which is the main focus in the CSP industry at the moment,” he says.

Indeed, Prieto acknowledges that CSP is still a relatively new technology with limited operational experience in comparison with some other renewable energy technologies, saying: “Right now we have more experience in operations than anyone else, and that’s good, but we still need to learn a lot and I’m sure that we can improve a lot. We’re going to see how these plants become old and how to improve the strategy of operations for every stage of the life of the plant”.

She continues: “The challenge is to keep advancing technology so that we are able to reduce the costs more and more. Our ambition is that by 2020 we’ll be competitive with combined cycle steam, taking into account the cost of their CO2 emissions. In terms of technology, we’re investing a lot improving our storage, because storage is central to dispatchability”.

Among a number of important technology challenges, Tassos identifies developing higher temperature thermal storage systems and improving molten salt mixtures to have a lower freezing point (while maintaining stability at higher temperatures), hence reducing energy costs for maintaining temperature when the systems are not operating.

Tassos also highlights key technology trends, including highly-efficient superheated to supercritical steam plants, alternative heat transfer fluids and larger apertures in parabolic trough and Fresnel collectors.

Developing industry International Standards

Another major trend identified by both Tassos and Prieto is the burgeoning development of industry Standards for the emergent CSP sector.

As Tassos points out, CSP is in the relatively early stages of global development and industry Standards could provide a foundation upon which to develop new technologies and enhance existing practices. “This could also provide additional comfort to potential investors and lenders, reducing barriers to bankability and subsequently accelerating market penetration”, he says. Tassos continues: “As Standards generally reflect the best experience of the industry, they constitute an important basis for improving the credibility of new products, assisting in the development and implementation of novel technical solutions”.

He argues that the main effort in the early stages of standardization should be placed on elements such as terminology, optical and thermal characterisation of new collectors, performance testing and modelling and environmental and safety requirements.

For example, among other activities, IEC TC 117 is currently running three ad hoc groups related to CSP Standards development, considering themes such as systems and components and energy storage. As for IEC TC 120: Electrical Energy Storage (EES) Systems, it includes thermal storage in its scope, but "only from the electricity exchange point of view".

Prieto also flags up the advantages of developing a comprehensive system of Standards, saying: “In a very global world, where tenders are international, those people who are organising tenders ― they’re usually governments ― need to be sure that the requirements they are asking for are met and the only way they can do that is through Standards”.

She concludes: “CSP is a very promising industry; we have a huge market ahead. We need to make an effort and the effort should be based on technology, so we should keep diminishing costs thanks to technology ― and Standards will help a lot.”

Ivanpah solar electric generating station - a tower receiving system (Photo: NREL) Ivanpah solar electric generating station - a tower receiving system (Photo: NREL)
CSP dish Stirling system CSP dish Stirling system
Parabolic trough solar collectors at Kramer Junction in the Mojave desert in California (Photo: Desertec-UK) Parabolic trough solar collectors at Kramer Junction in the Mojave desert in California (Photo: Desertec-UK)