Concrete is becoming even more important, as now it can store solar energy

Masdar Institute and a Norwegian company have joined forces to show how the traditional building material can be used to store concentrated solar energy.

There are few substances more important to construction than concrete.

Typically produced by mixing cement, sand, water and crushed rock, it has been used in various guises for thousands of years, notably by the Romans, who made use of it to fashion, among many other things, the celebrated dome on the Pantheon in Rome.

In modern times it continues to change the way the world looks, creating everything from multi-storey car parks to dams, from driveways to bridges, from breakwaters to the structure of the Burj Khalifa.

As if all of these applications were not enough, it is now being used in one of the most cutting-edge industries: solar thermal energy.

EnergyNest, a Norwegian company, has joined forces with Abu Dhabi’s Masdar Institute of Science and Technology to demonstrate how concrete can be used to store heat in solar power plants, a function often carried out by molten salt.

EnergyNest’s system has been installed at Masdar with the aim of demonstrating that the technology works and with the hope that it could be used by concentrated solar power (CSP) plants.

“We’re in negotiation with companies that want to implement this into their system. We’re ready to do that today. This is the whole point of the Masdar Institute project – to show the world that here you have a cost-effective alternative to molten salt that is scalable and that has low cost over time with regards to operation and maintenance,” said Jon Bergan, EnergyNest’s project manager during the construction of the plant.

“That’s our objective – to establish a universally applicable energy storage solution that can be deployed pretty much anywhere in the world.”

The principle behind CSP is simple: mirrors or lenses concentrate sunlight, creating thermal energy that is turned by heat into steam, which drives a turbine and generates electricity.

But because the Sun only shines during the day, the thermal energy often has to be stored for many hours and discharged when it is needed.

Batteries are often considered too expensive to use on a large scale so, instead, the heat is frequently used to warm up salt, which releases its thermal energy at night.

At a single plant tens of thousands of tonnes of molten salt could be heated up each day.

This method is becoming more widely used, such as in the Noor-Ouarzazate plant, which is being developed on the edge of the Sahara in Morocco.

The largest solar plant in Africa, its first phase was recently completed and when it reaches its eventual installed capacity of 500 megawatt hours (MWh), it will provide enough power for more than one million homes. The molten salt here is heated to nearly 400°C by the Sun.

As an alternative, EnergyNest’s technology instead uses a form of concrete with improved thermal properties called Heatcrete, developed with a German company, HeidelbergCement. Its composition is commercially confidential, although Mr Bergan said it contained significant quantities of a rock called quartzite.

“It has the ability to contain a large amount of heat. You can bring the heat into the system or extract the thermal energy within a certain amount of time. This is very important. This is how the storage media differentiates itself,” said Mr Bergan, EnergyNest’s supply chain director.

The model system being tested at Masdar consists of concrete columns in which heat exchanger pipes are embedded. A heat transfer fluid, its temperature raised up to 393°C by the Sun, flows into the system and heats up the concrete.

At the end of the day, the flow is reversed, discharging the heat.

Mr Bergan said the system retained heat to about the same extent as a comparable molten salt system, and the outgoing liquid was hot enough to produce steam that drives turbines and generates electricity. Water desalination could also be integrated into the system.

EnergyNest has been charging the Masdar Institute system for seven hours and discharging it for the same length of time.

So far, the system is performing as well as or better than simulations suggested it would.

“This is very encouraging. We’re getting higher heat capability than we thought,” Mr Bergan said.

The company hopes that its system could also be deployed when it is useful to store large amounts of fluctuating renewable electricity. The electricity can be converted into heat, then later back into electricity or heat.

This could prove useful for integrating more renewable energy on to electricity grids, ensuring that a steady supply of power that otherwise was not be possible is provided.

Although the Masdar work involves CSP, EnergyNest said its technology had “broad application value” since it could be used in a number of sectors. The system can be adjusted for whatever size is needed because the configuration of pipes and concrete blocks is fully modular.

There are other potential methods of heat storage, including with rocks, a technique being researched by Frank Dinter, a professor in the solar thermal energy research group at Stellenbosch University in South Africa.

Having long held an interest in the field, in 1991 he co-edited a book, Thermal Energy Storage for Commercial Applications.

The method he is researching involves rocks that have been broken up to a diameter of about 5 or 6 centimetres.

Prof Dinter’s university holds a patent to the technology.

Solar radiation is used to produce hot air that then heats up the rocks. The rocks release the hot air later on and, through a heat exchanger, produce steam that drives a turbine to create electricity.

So will it be salt, concrete or rocks that end up as the heat storage medium of choice in future? Mr Bergan predicted that a variety of methods would be used.

“There’s no one storage technology that will be applicable across the board, he said.

“You will see many different storage technologies catering to specific functions.

“I don’t think there will be one technology that takes it all.

“It’s a matter of finding a technology that’s simple, that’s scalable and that enables various forms of energy distribution. This where our storage is unique.”

Daniel Bardsley is a UK-based freelance journalist and former reporter at The National