Hot-water tanks are one of the best-known thermal energy storage technologies and are fully commercialised. They are already widely used at a building scale, in combination with electrical or solar thermal water heating systems, to store water over several hours from when it is heated – such as at night when electricity is cheaper or during the day when the sun is shining – until it is needed. In the future, larger versions could be combined with heat pumps.

At an even bigger scale, hot-water storage can also be used in conjunction with district heating (DH) systems, when heat is provided from combined heat and power (CHP), biomass boilers and/or large-scale solar water heating. 

Using thermal stores or accumulators would allow the CHPDH operator to optimise the fuel utilisation and load factor of a district energy scheme by generating electricity during peak periods and storing any excess heat, which can subsequently be distributed when demand is high. The storage efficiency could be further improved by designing to ensure optimum stratification of water in the tank.

According to the Royal Academy of Engineering, storing heat is easier and cheaper than storing electricity but is not as cheap as storing oil or coal. 

A tank of water 5m x 5m x 2m deep, which could be built in the basement of a traditionally built family house, could store enough heat to warm the living space for a month in winter, or longer at milder times of the year. If recharged by solar water heaters and/or off-peak electricity, such a system would be able to match the heating needs of a house with the availability of low-carbon-intensity supplies.

For a district heating scheme, a well-insulated tank of similar area to a public swimming pool could provide the capacity needed for several weeks’ storage of heat. Owing to economies of scale, large tanks are cheaper per unit volume of storage and the heat losses are lower. 

In Scandinavia, almost all cities and towns have large, pressure-less storage tanks (operating to 95°C). The largest of these, in Odense, Denmark, is a 75,000m³ tank at the Fynsværket CHP plant. 

Some heat-transmission systems have semi-pressurised tanks – for example, the 
two 23,000m³ vessels at Avedøre CHP plant, operating at up to 120°C.

Tank technology in Denmark has been combined with landfill know-how to store solar heat at temperatures up to 85°C from summer to winter in a more cost-effective way. The largest such operation in Denmark is a 75,000m³ pit store at Marstal, but facilities two-to-three times this size are under development. 

For these stores, the economy of scale, and further development, reduce costs significantly. The all-inclusive cost of storage by this method, from summer to winter (one load cycle), is between £15 and £20 per MWh.

Other methods of storage contain heat at a lower temperature, which cannot be used directly for heating, but can use a heat pump. Larger volumes and longer periods – up to months – can be achieved by storing hot (or cold) water underground at a modest temperature. Naturally occurring aquifers – such as a sand, sandstone or chalk layer – are most frequently used. Groundwater is extracted from the layer and then reinjected at a different temperature level at a separate location nearby.

There are also various projects worldwide that use underground storage in boreholes. Vertical heat exchangers are inserted into the ground and thermal energy is then stored in the clay, sand or rock. Boreholes are often used to store solar heat in summer for the space heating of houses or offices.

Another alternative is cavern or pit storage, in which large water reservoirs are created in the subsoil to serve as thermal energy storage systems. These technologies are technically feasible, but application is still limited because of their high investment costs.

In summary, sensible heat systems are favoured because engineers have many years’ experience of them and they are relatively cheap. Furthermore, the economies of scale provide an opportunity, but only in combination with large low-temperature heat loads, from industry and district heating systems.

On the downside, they offer low energy density, so large volumes/masses are required and efficiency is low because of heat losses. Costs are also high for small-scale storage.

PHASE-CHANGING MATERIALS: LATENT HEAT STORAGE  

To overcome the disadvantages of the smaller-scale sensible heat storage (SHS), the possibilities of phase-change materials are being explored. Such chemical compounds can include inorganic salts – such as sodium sulphate and its hydrates – or organic materials – including paraffins and beeswax – that absorb heat and undergo a phase transition at a particular temperature, for example, dissolution or melting. On cooling, the reverse phase transition occurs, for example, crystallisation or freezing, and heat is released. Phase-change materials (PCMs) are classified as latent heat storage (LHS) units.

Initially, solid-liquid PCMs behave like SHS materials; their temperature rises as they absorb heat. Unlike conventional SHS, however, when PCMs reach the temperature at which they change phase  – their melting temperature – they absorb large amounts of heat at an almost constant temperature. 

The PCM continues to absorb heat without a significant rise in temperature until all the material is transformed to the liquid phase. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat.

Many PCMs are available in any required temperature range from -5 up to 190°C. They can store from five to 14 times more heat per unit volume than conventional materials such as water, masonry or rock.

PCMs can be incorporated into containers as a standalone store or be included in building materials, such as wall panels, thereby storing solar energy during the day and releasing it during the cold night. Suitable PCMs would ideally meet several criteria, including the ability to release and absorb large amounts of energy when freezing and melting, having a fixed and clearly determined phase-change temperature, remaining stable and delivering reproducible behaviour over many freeze/melt cycles, and 
being non-hazardous.

PCMs are of interest because they offer high energy density, so smaller volumes/masses are required. They are relatively cheap and can deliver heat 
over a range of temperatures, depending on materials.

On the downside, they are not suitable for long-term storage owing to the inevitable heat losses to the surroundings. And reproducible performance over multiple heating/cooling cycles can be compromised by effects such as incongruent melting of salt hydrates. Salt hydrates can also cause corrosion of components, and organic-based PCMs may be flammable.

CHEMICAL REACTION SYSTEMS

Thermochemical storage is a new and potentially promising concept that consists of systems that use reversible physicochemical sorption phenomena to store energy. 

On heating, water – or another volatile component – is desorbed from the material and is then stored separately. This is an endothermic process, often referred to as ‘charging’ or ‘activation’ of material. On recombining the desorbed component with the activated material, an exothermic process occurs. 

Energy can therefore be stored in the activated material for extended periods with negligible thermal losses. This makes the technology attractive for long-term seasonal storage of heat. Energy densities are also higher than for SHS and LHS systems.

Thermochemical storage systems include low-cost crystalline or amorphous silica-based porous materials and their composites – often impregnated with hygroscopic inorganic salt hydrates; zeolites; metal hydroxides and carbonates; and micro-porous aluminophosphates. 

Requirements for large-scale applications include a charging temperature below 140°C, an energy density above 250kWh/m³, and resistance to material degradation. Furthermore, 
the total storage density, which includes all the components (in particular tanks and heat exchangers), is sometimes barely above that of water because of 
the space required for assembly of these components. 

The economics of this approach are still uncertain, but there is undoubtedly potential for research and development to improve performance and reduce costs through mass production.

In summary, chemical reaction systems have high energy density, so smaller volumes/masses are required. They represent long-term storage options with low heat losses.

However, energy densities are compromised by the space required by ancillary components. In addition, potential corrosion problems are associated with the use of salt hydrates. Chemical reaction systems are also a relatively immature technology. 

The content of this article came from a report – Energy Storage: The Missing Link in the UK’s Energy Commitments – published by the IMechE, available for download here: www.imeche.org/knowledge/themes/energy/energy-storage