Innovative methods of storing thermal energy come to the fore

By Ben Sampson of Professional Engineering


After years of being taken for granted, are we finally beginning to feel the heat?

Last week the government closed its consultation on the renewable heat incentive, a scheme it plans to start in April 2011. Its premise is almost identical to the feed-in tariff, which was launched this month. But instead of encouraging people to generate their own electricity, the renewable heat incentive will encourage people to generate their own heat.

Renewable Energy - Thermal storage

Where the feed-in tariff aims to encourage technology such as solar photovoltaics and wind turbines by financially rewarding them per kilowatt hour, the renewable heat incentive (RHI) aims to encourage the development and uptake of technology such as ground source heat pumps and biomass boilers.

The tariff will be a world first in supporting renewable heat technology, says the government. Certainly, the scheme marks a change from the previously dismissive attitude the government took towards heat as part of the energy mix. Over the last decade energy policy, legislation and research funding has targeted mainly electricity generation and use. That heat is now gaining currency in political circles is because of a growing awareness of its role in energy efficiency and storage.

Storing Thermal Energy from power stations for district heating

The Technology Strategy Board, the Carbon Trust and the Energy Technologies Institute (ETI) have big plans for the technology. The ETI announced a feasibility study in January into creating giant heat stores near power stations.

With “interseasonal heat storage” the stores would capture heat normally lost from the power stations and discharge it via district heating when needed.

Professor Brian Norton, who is director of the Dublin Energy Lab at Dublin University and runs the European Union’s research network on heat storage, says that interseasonal heat storage on a large scale is technically feasible but there may be economic considerations against it.

Charging the heat reservoir results in thermal stratification – layers of different temperatures within the store – and discharge of heat does not take place until thermal equilibrium is reached, which could take several years, says Norton. He adds: “The insulation would be a huge cost. Then there are the economics of developing the controls and instrumentation. It would also be very location-specific. Storage competes with fossil fuel energy and the economics of storage have to stack up to be viable compared to fossil fuels.

“The important part is the interaction of the storage material with the system – how it behaves with the pattern of the loads and inputs.”

Norton is effusive about the state of heat energy storage research and says there are numerous projects and organisations across Europe engaged in the area. Researchers are using computer simulations to improve their understanding of heat in the context of storage and transfer materials and devices. Engineers have to do a lot of calculations to work out the optimum size of the store, the temperature, and the best material for the job, he says. For example, storage media for a system in a building would require an ambient temperature and slightly above, whereas at solar power stations you would want to store at a higher temperature with sodium.

A great deal of research deals with “phase change” materials for heat energy storage and transfer, materials such as molten salt, fatty acids and liquid sodium. The main direction of research here is to reduce the mass of stores. There are also many novel applications in development, such as using phase change as a cooling medium or in water desalination plants. “Or you might want to create a thermal delay, store energy and then discharge it at a particular rate,” says Norton.

Interseasonal Heat Transfer for on-site renewable energy

Not all interseasonal thermal energy stores have to be massive, and a few companies in the UK offer services to design and build heat stores for buildings. Since 2006, ICAX has installed several interseasonal thermal stores in new buildings. Its latest project, a community centre in Merton, south London, was completed last month.

According to Edward Thompson, director for ICAX, the architects of the community centre initially considered a biomass boiler. However, there were problems, such as the delivery of biomass, storage of fuel, personnel for the boiler, and the disposal of the ash. “It was more expensive than people thought,” he says. Once designed, the architects realised that cooling the structure would be a bigger problem than heating it.

They specified an Interseasonal Heat Transfer system which collects heat from the sun and the air-conditioning units. The collected heat is pumped into a “thermal bank” in the ground and stored and pumped back into the building when required.

“It’s extremely efficient and so simple that sometimes people don’t believe it works,” says Thompson. “The building is unusual in London because it has no gas supply or boiler. We’re confident that the system will supply sufficient heat throughout the winter. We’ve done it before and we’ll do it again.”

Each ICAX system has been tailored to a specific building because every building has different thermal properties. Engineers at ICAX use CFD software to model the heat flows in the thermal bank. “Thermal modelling is critical to the installation of the system, in the building and in the thermal bank. The bank has to be the right size,” says Thompson.

Thermal energy stores are being touted as a way of filling the intermittency hole an increased dependence on renewable energy may leave us with in the future. Solar energy collectors, tidal and wave energy collectors, wind turbines, all share an engineering challenge in that their power output varies according to environmental factors – the strength of the wind, the times of tides, the amount of sunshine – that do not match the requirements of society. Although a future “smart” grid may cope with this alongside a constant baseload, many experts believe an effective way of storing energy, more practical than pumped hydro, is needed.

Pumped heat electrical energy storage

Cambridge start-up Isentropic has developed what it calls pumped heat electrical energy storage (PHES), which uses gravel as its phase change material. Jonathan Howes, Isentropic’s technical director, says: “PHES has the potential to dominate the large-scale electrical storage market.

“It provides a cost-effective solution to the intermittency problems of renewable energy sources. Moreover, it has no geographical constraints, is compact, can demonstrate a round-trip efficiency of 72%-80% and has the lowest costs of any storage technology.”

The system consists of two silos, one at 500°C and one at -150°C, containing gravel. The machine compresses or expands air to either of the two temperatures and the air passes through the two piles of gravel where it gives up its heat or cold to the gravel. In order to regenerate the electricity, the cycle is simply reversed. The temperature difference is used to run the machine as a heat engine.

Because gravel is such a cheap and readily available material, the company claims the cost can be kept very low – $55/kWh and $10/kWh at scale.

The Technology Strategy Board is interested and selected the firm to represent promising clean technology companies on a trade trip to the US recently. Isentropic is in talks with a sponsor company to build a demonstrator plant.

Heat drives cyclic adsorption process

At the opposite end of the spectrum, Professor Bob Critoph from the University of Warwick and his spin-out Sorption Energy, are working to miniaturise a thermal process and exploit it efficiently.

Based on 20 years of research, the firm is several years away from its first commercial product – a domestic boiler that promises to be up to 40% more efficient than conventional boilers. The company is targeting the automotive sector and air-conditioning industry and has a Carbon Trust loan for R&D. “The technology can utilise the waste heat of the engine for the car’s air-conditioning,” says Critoph. “It’s basically a heat-operated compressor.”

The adsorption process at the heart of Sorption’s technology uses active carbon instead of a mechanical compressor and is driven by heat rather than mechanics. The system uses natural refrigerants such as water or ammonia.

Molecules of the refrigerant accumulate on the active carbon, creating a film on the surfaces of the micropores within the carbon. The process differs from absorption, where a substance diffuses into a liquid to form a solution. An adsorbent will adsorb the refrigerant vapour when cooled and desorb (release) it when heated.

According to the company, adsorption heat pumps can be driven by waste heat or solar energy and are cyclic in operation. They require a condenser, expansion valve and evaporator similar to those used in conventional compressor-driven systems. However, the compressor is replaced by the adsorption system.

The main challenge has been squeezing the technology into a small footprint. “It’s engineering the technology to make it small enough and low enough cost to make it commercial,” says Critoph.

“There are people out there doing fuel cells, CHP and various heat pumps for this market. From a consumer point of view, they just use less gas. The main driver for the technology is the government’s stance on global warming.”

With schemes like the renewable heat incentive and more renewables coming online, it’s likely we will see more thermal energy devices and stores, big and small, soon.