Thermal storage batteries

PCM thermal battery connected to a domestic hot water system

Now that the UK has officially left the EU and the government has such a large majority in parliament, our industry should expect some rapid changes and firm decisions to support the decarbonisation of heat. The Department of Business, Energy and Industrial Strategy (BEIS) is now focusing its resources on tackling climate change and will be publishing its low-carbon roadmap for our industry in the summer of 2020.

The heat pump heat demonstrator project will be commencing in mid 2020, in which the government will be investing £16.5 million into the installation of 750 heat pumps into consumer homes to look at everything from performance to consumer behaviour; a similar project is currently being investigated with hydrogen boiler technology. The heat demonstrator project will also be looking to test near-to-market technologies to see how they may improve the efficiency and adaptability of heat pump technology. One such technology is the use of thermal energy batteries to increase the amount of domestic hot water available in an equivalent space to that occupied by a traditional domestic hot water cylinder.

How can thermal storage play an important role in the decarbonisation of heat?

One of the major challenges that government and industry face in their quest to decarbonise heat is being able to match low-carbon fuel from its generation directly to the consumer’s heating or hot water load. If the UK is to meet its tough 2050 emissions targets then it will require some form of infrastructure change that may include the mass deployment of green hydrogen through the existing gas network combined with the electrification of heat. However, the deployment of electricity to heat our homes will not come without challenges due to the seasonal nature of producing renewable electricity from wind, solar and hydro sources. One such solution to this obstacle could be the use of seasonal storage technology that can be deployed to capture green electricity through the use of low-cost off-peak energy tariffs and turning it into thermal energy, while off-peak is likely to be defined as when the wind blows or the sun is shining. This article aims to look at the current water storage thermal technology on the market today and to compare them against some of the new disruptive solutions appearing on the horizon such as phase-change materials.


Domestic hot water storage thermal energy batteries

The majority of us link the term battery to those types that are used to store electricity. However, in this article we will be referring to a battery as a thermal energy battery; a physical structure used for the purpose of storing and releasing thermal energy. In essence, a domestic hot water cylinder is a form of battery, as it stores energy temporarily in a volume of water to be used at a later date for domestic hot water. Depending on the size of the hot water cylinder, it defines how much water you are able to draw off at the required temperature at any given flow rate. This is typically stored at 60°C and the effectiveness of different DHW cylinders may be calculated using the methodology within BS EN 12831 Part 3. In simple terms, before an installer specifies a hot water cylinder into the consumer’s home, it is critical for them to understand the actual consumer’s hot water usage patterns to meet their needs.

Simple thermal stores or buffer tanks

A simple thermal store or buffer tank is quite common in log gasification boiler installations where a large volume of water is used to maintain the efficiency of the biomass boiler during its combustion phase by avoiding the issues experienced by boilers with low turndown ratios. However, more advanced biomass boilers have a higher turndown ratio due to their ability to modulate down as low as a modern natural gas boiler. Buffer tanks are also used in combination with heat pumps to ensure a minimum volume of water is present within the heating system at times when the heating load is low. This helps to prevent the heat pump from short cycling, maintaining a minimum flow rate through the heat pump when heating zones are turned off (figure 1, simple buffer tank in a heat pump system). However, more modern heat pumps have an ability to operate without a buffer tank with the use of components such as variable compressors.

Phase-change material (PCM)

A PCM thermal battery incorporates a material with a high latent heat capacity at narrow temperature ranges which can achieve high energy densities compared to water. These types of materials melt and solidify at very specific narrow temperature ranges and are defined as phase-change materials (PCM). The two most common materials used are both inorganic and organic such as parrafin wax and salt water mixtures and additives. Figure 2 demonstrates the considerable increase of usable energy when comparing 200kg of a phase-change material within its phase-change temperature range and 200 litres of DHW. During this phase-change period, the outlet hot water temperature would be maintained at the desired DHW temperature of 40°C until the PCM has completed its phase change between a liquid and a solid; however, the issue that a future product designer will face is the inherent low conductivity of the PCM itself, which requires a specialist internal heat exchanger to charge and discharge energy at a sufficient rate (figure 3, depiction of a PCM store with integral heat exchanger). The amount of theoretical stored energy within 200kg of a specific PCM, if fully discharged, equates to around 490 litres of water delivered at 40°C. However, further advancement into various PCM mixtures that increases the thermal conductivities is moving at pace.


Compared to a traditional DHW cylinder, a PCM thermal battery avoids the need for a G3 building regulations certificate and eliminates legionella growth that would normally present a risk within a stored domestic hot water cylinder. As we move towards the mandating of low flow temperature heating systems with a maximum flow temperature of 55°C, this type of technological innovation could help minimise the risk of legionella growth.

Be prepared!

How can the installer prepare for the potential rapid changes industry faces in the future?

  • Be a member of the CIPHE to be connected with forthcoming legislation and to help us shape a future that works for both the installer and consumer;
  • Carry out consistent continued professional development to stay ahead of your competition and expand your knowledge of new technology;
  • Start to look at your customers’ real patterns and behaviours – how they use their heating and hot water – before you specify a heating and hot water system;
  • Understand how to correctly size a domestic hot water system that meets the needs of your customers.

This article first appeared in the Mar/Apr 2020 issue of P&H Engineering, the magazine for members of the CIPHE. Find out how to join here.

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