Concentrated Solar Power and energy storage

Based on a Discussion Webinar held Friday 1st February 2008

Concentrated Solar Power (CSP) is not a new technology. A CSP plant in the California Desert has been functioning effectively for 15 years. Until recently, however, this technology has been overshadowed by photovoltaic solar energy. This has started to change as CSP begins to attract increasing attention. In Spain, approximately 500 MW of CSP units are in the development phase and grid access has been demanded for another 4,200 MW of CSP projects.

The Spanish Transmission System Operator (TSO) Red Eléctrica de España (REE) declared that those units should be ‘dispatchable,’ which means that they will need some kind of energy storage to compensate for their intermittency. What is the best storage technology to combine with CSP? Leonardo Energy broached this as a side-subject during the discussion webinar on Fuel Cells on 1st February 2008. The following presents a short introduction to CSP and a few key points that were tackled during the discussion webinar.

CSP in all its variants

Concentrated Solar Power (CSP) uses mirrors to concentrate the sun's rays on a pipe or vessel. Three main types of CSP exist: central receptors, parabolic troughs, and parabolic discs (see figure.) In the first two types, heat is collected in a primary working fluid and then transferred to water to generate steam, powering a conventional steam turbine. In the parabolic disc technology, a Stirling engine is located in the focus of the disc. This is a more modular technology, but is currently still in the development phase.

 

In some countries, CSP is coupled with a Combined Cycle Power Plant using natural gas. The solar field generates steam at 370°C, which is then further heated in the Heat Recovery Steam Generator of the Combined Cycle. This makes a lot of sense from a thermodynamic point of view.

A promising technology for the future is a CSP plant with direct steam generation. In such a plant, the steam is directly generated from the parabolic collectors, without the intermediate stage of a heat exchanger. This should present a genuine technological breakthrough since it would decrease the investment cost and increase plant efficiency.

CSP in Spain and the rest of the world

In regions with intensive sunshine, CSP has an enormous potential. According to ESTELA, the European Association of the CSP sector, 62,000 MW of CSP could be constructed in Southern Europe by 2030. In North Africa, which is an excellent location for CSP, electricity connections crossing the Mediterranean Sea are being considered for transporting the energy from CSP units in the Sahara to Europe.

Up until now, however, Spain has been the only stable market for CSP, although the US might follow soon. Approximately 500 MW of CSP units are in the development phase in Spain and grid access has been demanded for an additional 4,200 MW of CSP projects. Arizona Power Service (USA) recently announced the construction of the biggest CSP plant in the world with a capacity of 280 MW, located 100 km south-east of Phoenix. The Spanish engineering and construction firm Abengoa, specialized in CSP, is in charge of designing and building this plant. The plant will go into operation by 2011.

An intermittent renewable source

The power output of a CSP unit is intermittent, since it works only during daylight. It is, however, a lot more regular and predictable than the output of other renewable energy systems such as wind turbines. Moreover, it can partly be used for compensating the midday peak loads that occur from air-conditioning systems during the hot summer months. By combining CSP with energy storage, it becomes a dispatchable type of generation, but this implies of course a certain loss in efficiency.

Another way to make a CSP plant more dispatchable and reliable is to build a hybrid installation using make-up fuel (such as natural gas). This possibility is not considered by the promotion scheme set-up in Spain where installations have to comply with a number of conditions to benefit from feed-in tariffs. The decision to set a higher ‘hybridising rate’ versus providing more storage capacity needs to be analysed further to determine the optimal balance between both.

CSP units in Spain have to be dispatchable

Red Eléctrica de España (REE), the Spanish TSO, was apprehensive regarding the fact that large number of CSP units combined with an ever-growing amount of wind generation could make their grid uncontrollable. They therefore declared that CSP units should be ‘dispatchable’ or their access to the grid could be denied. This means that they need to include an energy storage capacity equivalent to four hours of full load operation of the solar field. The CSP units should also provide predictability of 90% for the coming 24 hours and 95% for the coming six hours.

The concerns of REE may well be justified and their demands achievable from a technical point of view, but a few remarks are in order:

  • Providing energy storage is a major investment and one that was not taken into account when the Spanish Energy Ministry defined the feed-in tariffs for CSP (around 00.25€/kWh)
  • It could be seen as unfair competition with wind energy projects, which don’t have to include storage facilities.
  • At grid level a broader choice of balancing tools are available than at local level, which allows benchmarking and choosing the most efficient one. In the near future these tools could include demand side management, for instance by controlling local cogeneration units or the heating systems of well insulated houses.

How can energy from CSP units be stored?

CSP energy can be stored at two different stages.

  1. Heat can be stored before generating electricity. Currently the most viable option seems to be melted salts at about 400ºC. Research is being carried out on high temperature melted salts (up to 580°C) and concrete (up to 500°C).

  2. The energy can be stored after electricity generation. In this case, various types of storage technologies are available: batteries (redox flow or sodium-sulphur), hydrogen (combining hydrolysis to produce hydrogen with fuel cells to convert it back to electricity), and pumped storage.

All those storage types can be co-located with the CSP plant except for pumped storage, and all can be located remotely, except for heat storage.

It is difficult to predict which system would have the largest overall efficiency (dispatchable electrical energy/captured solar heat.) A simulation model is required for these kinds of assessments. Investment issues should also be addressed. A thermal storage capacity of four hours increases the investment for a CSP unit by around 1.5-2 M€/MWe.

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