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The 150MW Noor III CSP tower, Morocco

Storage gives boost to solar energy

Image credit: PA

The latest concentrated solar power projects, combined with energy storage, promise to make a significant contribution to the renewable energy effort.

Concentrated solar technology systems use mirrors or lenses with tracking systems to focus a large area of sunlight onto a small area. These innovations, however, are far from new. In fact, legend has it, in 213 BC, Greek engineer Archimedes had set fire to Roman vessels attacking his home city with a giant mirror focusing the Sun’s rays on the invaders.

Today, they are used to provide industrial process heating or cooling, with excess thermal energy stored to generate electricity on demand. Quite the opposite to reducing ships to ashes, by far.

Concentrated solar power (CSP) may not be a new technology, but its modern development has been held back somewhat by an initial high cost of capital, then the global slowdown after 2008 leading to lack of finance. CSP remained expensive compared to fossil fuels. This effectively meant that focus within the renewables sector was directed elsewhere in terms of both finance and government support and structures.

Nevertheless, some countries have tried hard to create a sustainable CSP industry; notably Spain and the United States as well as Morocco and some Gulf countries, Saudi Arabia in particular. In all these countries, there have been subsidisation programmes supported by governments to make sure that CSP plant operators had what they needed to be viable. Over 70 concentrated solar-thermal power plant projects were built between 2005 and 2012. Further, the cumulative installed capacity of CSP in the world increased from 1,092MW in 2010 to 5,597MW in 2018 at a CAGR of 22.5 per cent, according to GlobalData Power Database.

Indeed, it is during those past few years that the cost of raising the capital to build and maintain plants has reduced massively and economies of scale have been created, further reducing costs. In addition, there have been ongoing research and development and technological advancements in the actual solar field. This means that the cost of equipment such as mirrors, reflectors and collectors fell; engineering, procurement and construction (EPC) has come down too.

A third factor, that the energy generated by CSP could not be stored and saved for later, also hindered the initial development of this industry. But again with research and development, reliable and long-lived storage capability is now viable and CSP looks to be gaining prominence as a reliable, stable and cost-competitive energy source.

The importance of being able to store the energy created is that it turns CSP from an instant source of power to one that is stable, scalable and reliable. The method is thermal storage and it effectively helps to retain the solar heat generated during daylight hours and then convert it to electricity when needed. Once converted to electricity the power can join the grid and be dispatched to wherever it is needed.

It works essentially by the sun heating fluid, which is then stored in a huge metal tank. The energy can be stored in molten salts (at about 565°C) or in a heat transfer fluid (at about 400°C). Once the stored thermal energy is required, the liquid can be sent to a heat exchanger where the heat is extracted and then used to boil water. This in turn creates steam to run a steam turbine, much like earlier power plants that use up a fuel, such as natural gas, coal, or nuclear, to rotate a massive turbine to create energy.

Once the molten salt has had the heat taken out it can be stored elsewhere then sent back to the tower to be heated again by the sunlight. And because molten salts are very effective when it comes to retaining heat, losing only 1 per cent of heat per day, then it is possible to  store, and then top up, this thermal energy for months.

In practical terms, it is much more workable to use the stored energy on a daily basis after sundown to create a stable and reliable daily flow of energy to meet daily peak demand. In simple terms, storage needs to allow for enough energy to be stored and then electricity generated after sundown and to be able to be scaled up for peak usage times; when people are cooking, for example.

Molten salt thermal energy storage can be heated and cooled daily like this for at least 30 years.

The importance of all of this is demonstrated by the figures; of the 5.6GW active CSP capacity by the end of 2018, around 2.6 GW is with energy storage and around 3GW is without storage. In contrast, of the total CSP projects under various stages of development, 95.8 per cent of the upcoming capacity has storage. Only 4.2 per cent of the under-development CSP capacity is without storage, according to GlobalData Power Database.

A majority of the active CSP projects with storage have a thermal storage capacity in the range of 6-10 hours. In the case of the ‘under-development’ capacity, 62.8 per cent is with storage of 10-13 hours and 14 per cent has over 13-hour storage, the GlobalData Power Database records.

These figures are encouraging in that not only do CSP plans have the ability to provide stable and reliable power 24/7, but they can also use the longer storage capability of CSP to reduce generation costs overall.

Morocco is a good example of a long-lasting and sustainable CSP industry. It initially launched two ground-breaking standalone CSP projects: Noor I and Noor II & III. The 150MW Noor III CSP tower exceeded its performance targets on output and storage integration in the first few months of operation. Now Morocco plans to develop two hybrid CSP/PV solar plants at Noor Midelt, with a gross CSP capacity (with storage) of 150-190MW for each plant.

South Africa has been active too. In 2016, its Bokpoort CSP plant supplied power round the clock for a total of 14 days. During this time, the capacity of the plant was 66 per cent. This proved that CSP could replace coal for night-time power and reduce dependence on coal-fired power and avoid blackouts. Operating at full capacity, the plant was judged to be able to run for 9.3 hours solid.  

Meanwhile, the Dubai Electricity and Water Authority recently issued a Request for Qualification for developers to build and operate the 900MW fifth phase of the Mohammed bin Rashid Al Maktoum Solar Park, the largest single-site solar energy project in the world based on an independent power producer model. This phase will use photovoltaic solar panels. The fourth phase was awarded to a consortium of Chinese Shanghai Electric and Saudi Arabia’s ACWA Power and will use three technologies: 600MW from a parabolic basin complex, 100MW from a concentrated solar tower, and 250MW from photovoltaic panels. The solar park aims to have a planned total production capacity of 5,000MW by 2030.

In Australia, companies are offering CSP with thermal storage to fill the country’s peak electricity gap; 15-16GW of coal capacity is due to retire within 15-20 years.

In a time where sustainable energy is a priority, CSP with storage could well make a significant contribution to the world’s overall energy mix. The next challenge will be how to best integrate it into the overall electricity grid.

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