By using thermal storage, the power plants can produce electricity in a plannable way - even at night.

A decisive advantage associated with the solar thermal power plants, when compared to the other forms of renewable energy, is the possibility to produce electricity even when there is no sunshine.

The parabolic trough power plants stand for their ability to store solar thermal energy and dispatch it when it’s needed most. As a result, thermal energy storage (TES) allows parabolic trough power plants to achieve higher annual capacity factors—from 25% without thermal storage up to 70% or more with it.

Heat storage systems absorb part of the heat produced in the solar field during the day. The turbine produces electricity using this heat during the night, or when the sky is overcast.

Parabolic trough thermal energy storage technology includes: storage systems, molten-salt heat transfer fluid and storage media.

Considering the utility of such systems, rapid strides have been made in this arena.

Late last year, the thermal energy storage system of Europe’s first parabolic trough power plant, Andasol 1, was put into operation. The use of what has been described as the world’s biggest molten salt thermal energy storage in any industrial plant to date by Solar Millenium enables a plannable plant operation even when the sun isn’t shining.

Significantly, this method is gaining in popularity, as it shows signs of going beyond Europe.

The likes of Eygpt and Israel have already witnessed the setting up of CSP plants, relying on hybridisation and  heat storage systems.

The fact that companies within these regions are working on a mix of indigenous and more established concepts (or ones which have been implemeneted in a market like Europe) has resulted in a mixed viewpoint as far as storage is concerned.

For instance, in case of Solar Millenium, with parabolic trough power plants, it is applying a proven technology to solar power generation, leveraging it into a completely new dimension by using energy storage systems. In case of Andasol 1, the heat generated in the solar field is to be stored in a molten mixture of 60% sodium nitrate and 40% potassium nitrate. Both substances are used in food production as preservatives and are also used as fertilizer. The storage tank consists of two, 14-meter high tanks with a diameter of 36 meters and a capacity of 28,500 tons of molten salt. During the pumping process from the ‘cold' to the ‘hot' tank, the molten salt absorbs additional heat at an outlet temperature of approx. 280° C, thereby reaching a temperature of 380°C. A fully loaded storage system can keep the turbine in operation for 7.5 hours, which means almost 24-hour operation of the power plant in summertime. Molten salt has been used in different industrial applications for about 60 years to date, e.g. in galvanizing.

A couple of months ago, the company also started construction of the first parabolic trough solar field in Egypt.

“The Solar Millennium group is already investing in Research & Development projects for new thermal storages,” said its spokesperson Sven Moormann. “In Europe, the group is integrated in R&D projects together with other companies and DLR, especially in Southern Spain (Plataforma Solar de Almeria). In the US, the Solar Millennium group is going to test a new salt mixture which is working in a wider temperature range than the salt mixture used in the Andasol projects. This R&D project is co-financed by the DOE (USA). Further R&D programmes should give high priority especially to HVDC grids which will accelerate the deployment of CSP plants.”

As far as the plant in Egypt is concerned, the hybrid power plant, which is located in Kuraymat south of Cairo, uses both natural gas and solar-thermal energy.

On the challenges in setting up such a plant, Moormann said, “The most ambitious challenge is the integration of the solar field into the waste heat recovery system of the turbine unit (gas and steam).”

According to Moormann, in general, the main factor to achieve optimal results is the system integration from solar heat into the combined cycle (CC).

“On one hand, the thermal energy from the solar field can be converted into electricity with an higher efficiency in comparison to a solar only rankine cycle as the heat from the flue gas allows a higher temperature level for the water steam cycle, the downside is that the steam turbine operates at partial load efficiency when the sun is not available. That optimisation is the most challenging design task. One positive side effect is the increase of capacity during high radiation periods due to the solar contribution. The capacity of “normal” CC plants is going down at high ambient temperatures when energy is needed to operate air conditioning systems,” said Moormann.

Challenges

A company like AORA Solar, which recently bagged the first licence for a solar thermal energy plant in Israel, sees several challenges as far as storage is concerned.

“Storage is an interesting option, but we think that it will take much effort to develop viable high-temperature storage. Also, it needs larger solar field to collect power, because you can collect solar energy only in daytime for use round-the-clock. Presently the share of solar power in the grid is small, and we think all the power can contribute to consumption in daytime. When solar becomes a more dominant part of the power mix, storage will certainly have to be addressed as a high priority,” says AORA’s chief technology officer Pinchas Doron . 

Commenting on the availability of and problem-free, cost-effective thermal storage systems to achieve full sustainability concerning the environment, Gianluca Gigliucci, head of renewable energy research, Enel told CSPToday.com: “Energy storage is a key aspect to achieve predictable and stable solar plant power production under variable sun rradiation. Hybridisation with natural gas boilers does not represent a general solution, due to natural gas practical unavailability at highly irradiated sites and to the strategic requirement of building self-sustaining renewable energy plants independent of fossil fuel supply (i.e. non subject to fossil fuel unavailability and variable prices).”

Therefore low cost storage technologies are indeed a key aspect for CSP plant exploitation and should be regarded as one of the key R&D topics, as their availability would allow reliable power production forecasting, avoiding instability problems for existing grids in case of high fraction of renewable energy production and allowing higher investment proficiency thanks to energy dispatching on actual peak hours along the whole year, added Gigliucci.

CSP plants in hybrid configuration 

For his part, Doron considers hybridising solar with alternative fuels to be an extremely promising approach.

“Using renewable fuels for the non-solar operation allows for round the clock green power supply. For example, integrating our system with waste treatment plants that would generate the fuel to run our turbine can address both, power supply and waste disposal issues. The modular structure of our plant provides the flexibility that will enable its integration with any size waste treatment facility,” said Doron.

He added: “We do not integrate into existing combined cycles which are typically large-scale plants located away from the end users, but rather think it is more efficient to use combined heat and power (CHP) which makes best use of our near-the-client capability.  We do not look just at stand-alone units – though we are probably the single (or maybe almost-single) CSP option for stand-alone. In grid-connected mode we can provide power whenever it is needed thanks to the hybrid operation capability.”

According to Enel’s Gigliucci, CSP plants in hybrid configuration with natural gas combined cycles are not intended to use combined cycles more efficiently.

“In fact, the opposite applies, as the use of large scale existing combined cycle allow high efficiency solar energy conversion into electricity thanks to the use of large scale high efficiency steam turbines and high efficiency water pre-heating, as well as investment cost reduction due to utilisation of common plant components. Hybrid plants with existing combined cycles will therefore constitute a very effective mean to increase CSP plant penetration even in case of sites that would not allow investment proficiency for stand-alone plants,” said Gigliucci.

Increasing reliance on alternative sources of energy

Renewable energies are being criticised for not being able to cover the base load (being available all the time), as wind and sun offered by nature are not constantly available.

“Solar thermal power plants have one great advantage due to the integration of thermal storage: their electricity generation can be provided in a planned manner and thus contributes to stabilise the power grid. With our technology, the plannable generation of electricity is made possible not only by thermal storage, but also by a combination of several energy sources in hybrid operation, e.g. natural gas and solar energy. Firing systems, e.g. natural gas burners, can support a plannable electricity generation. With the hybrid solution solar thermal power plants can produce electricity round the clock,” said Moormann.

“Our goal is to offer plannable electricity (round the clock) fully on a regenerative basis,” said Moormann.

Even as new technology companies may have varied opinion regarding storage and hybridising, it is clear that there wont be any dearth of action in the time to come in this arena as the significance of solar-thermal power generation in India, China, the Middle East and North Africa is also growing. And at the same time, taking into consideration the growth scenarios for gas-fired plants in developing countries, it seems it is worthwhile to invest into the integrated solar combined cycle plants,  because every ton of CO2, which is avoided by the solar field, working as a fuel saver, is reducing the risks in climate change.