Looking for practical storage solutions for an electricity grid dominated by renewables, we have already examined gravity, pressure, and momentum as ways to capture and hold energy for later use. Next on our list is heat.

It is helpful to store heat in the transition to net zero as society uses a lot of it. About three-quarters of the energy demand of buildings in the UK is for space and water heating. So companies like Sunamp, Tepeo, and Caldera have developed heat batteries you can charge with renewable electricity, heat pumps, or waste heat from any available source, releasing the heat when you most need it. Process heat is required in many industries and again heat batteries can make use of any source of heat available on site, storing it until required.

However, despite the many applications of thermal energy stores to deliver heat when and where required. I want to talk about using heat to store excess renewable electricity at grid scale until it can be converted back into electricity when the demand is there.

There are two main approaches:

• Phase change – using the transition from solid to liquid or liquid to gas as the energy store
• Pumped heat energy storage – creating a temperature difference between two stores and using the gradient to generate power

Before exploring these options, there is one more application of heat storage that needs mentioning. Concentrated solar power stations use mirrors to concentrate sunshine onto tubes carrying a heat transfer fluid, or onto a central tower with a receiver mounted at the top. The heat collected from the sun is used to generate steam to power a turbine and generator. These work best in areas with high levels of solar radiation and can only generate power when the sun is shining. A way around this is to store heat in a molten salt at a high temperature. This allows steam to continue to be generated in the dark and means the facility can produce electricity 24 hours a day. The Noor complex near Ouarzazate in Morocco is a 582 MW solar power plant. 510 MW of concentrated solar power, and 72 MW of conventional PV. The concentrated solar power part of the complex uses molten salt technology to store spare heat and can keep generating power for up to 7 hours at night or when clouds block the sun.

Noor Concentrated Solar Power Station at Ouarzazate, Morocco

This is not really taking spare electricity and storing it as heat until needed as electricity again, but it is a storage technology that enables solar power plants to keep delivering when the sun is not shining. It all helps to create a net zero electricity grid.

It’s just a phase!

Using phase changes to store energy is a very interesting strategy. During a phase change (solid to liquid or liquid to gas), the temperature stays the same until the change is complete. In an iced drink, the temperature of the whole drink stays close to 0°C until all the ice has melted. So, if you are capturing or releasing energy, the temperature of the storage medium is pretty stable. Easier to design and engineer than a system that involves big temperature swings.

Highview Power is storing energy in liquid air. Spare renewable electricity drives a refrigeration plant to produce liquid air at -196°C. This is all standard technology for producing industrial liquified gases. To recover the energy, the liquid air evaporates, releasing enormous volumes of gas that can drive a turbine.

Cryogenic Energy Storage from Highview Power

It’s a bit more complicated than that if you want good efficiency. As with compressed air energy storage, the air heats up when compressed to liquefy it, and cools as it expands. This restricts the round-trip efficiency to about 60%. But, with a source of waste cold or waste heat, you can drive the efficiency towards 100%. Plenty of industrial processes have waste heat and cold, so co-locating your storage system with other industrial plant is efficient.

These cryogenic energy storage systems don’t need mountain lakes, big shafts, or large underground caverns. Built from standard industrial process components, you can put them anywhere there is excess renewable electricity.

After a few years’ successful experience with a demonstrator project, Highview Power is developing a 50MW/300MWh storage system at the Trafford Energy Park near Manchester, UK and a 200MW/2.5GWh system in Yorkshire. A 300MWh system is under development in the Canary Islands and they are exploring optiond in Australia to go with that country’s drive for solar and wind energy.

Skipping several phase changes considered for energy storage, we move to the other end of the temperature scale. The Australian company 1414 Degrees uses abundant silicon as its storage medium. Silicon melts at 1414°C, hence the name of the company, and in the transition between liquid and solid can store over 1 MWh/m³. An impressive energy density. The stored heat is used to produce steam to drive a turbine and generate electricity. The round-trip efficiency is around 90%.

Just like cryogenic energy storage, you don’t need a special geography or geology, and the plant can be installed wherever you have abundant renewable energy and the capacity to generate and use electricity. 1414 Degrees are currently developing with partners a large solar generating plant in South Australia. This will be a hybrid system using both batteries and thermal storage to support a 70MW PV farm. Concentrated solar power could be added later.

Degrees of separation

Pumped heat energy storage systems (PHES) use electricity to create a temperature difference between two heat stores. Electricity drives a heat pump that cools one store and heats the other, operating exactly like a domestic heat pump that extracts heat from the outside air and delivers it into a home. Then run the system in reverse to produce mechanical power and drive a generator.

General Layout of a Pumped Heat Energy Store

In a typical design, the heat stores are two insulated steel tanks containing crushed rock gravel. You compress a working gas such as argon using renewable electricity heating it up. The gas filters through the hot store, giving up its heat to the gravel. It is still pressurised, but now at ambient temperature. Allow it to expand, cooling it down. The cold gas filters through the cold store, chilling the gravel and warming back up to ambient. Repeat the cycle, charging the hot and cold stores. Then reverse the process to get the electricity back. Argon at ambient temperature flows through the cold store, cooling the gas down and warming the store. Compression heats the gas back to ambient. It flows through the hot store, heating the gas and cooling the store. You then have hot compressed gas that can expand to drive a turbine, generating electricity.

Hot and Cold Stores for the Pumped Heat Energy Storage Demonstrator at Newcastle University

Newcastle University, UK has a grid connected demonstration plant of this type rated at 150kW/600kWh. The temperature difference between the two stores is over 600°C.

Other configurations are possible, and Malta Inc are collaborating with Siemens to design and construct PHES systems using molten salt as the hot store and anti-freeze as the cold store with air as the working fluid. Well established, non-toxic and safe materials. They are designing 100MW /1GWh systems.

Round trip efficiency for PHES is about 60%. Not as high as some other options, but the capital costs should be much lower, and you have a simple design using low-cost components with a long life, low operating costs and freedom of location. I can find no fully commercial installations yet, but a lot of R&D interest. Commercial products may not be far away.

Using heat to store renewable electricity you get:

• Low capital costs
• You don’t need special geography or geology – you can locate it where you want
• Long operating life
• Simple systems using industry standard components
• Readily scalable
• Opportunity to integrate with other industries

Next up – Direct storage of electricity

Previous articles:

• Renewable Energy and Storage – the Options
• Electricity Storage Options 2 – Using Gravity
• Electricity Storage Options 3 – Springs and Tops