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Don’t forget heat storage

One of the biggest challenges and opportunities for the energy industry is the continuation of research and development in the storage and management of thermal energy.

This should be seen as an essential aspect of energy generation which promises to increase the efficiency of existing systems and also maximise the use of intermittent energy supply side technology.

Looking at existing energy systems, at a macro scale, centralised large scale electricity generation in the UK is currently limited with frustrating system efficiencies of 35 to 55 per cent - simply because low grade heat is not used and is dispersed to the atmosphere. At a micro scale, buildings have inherent thermal mass and active plant systems which are not always optimised and forced to run at low efficiencies at part load.

Large scale heat storage

Taking a step back, large scale heat storage connected to inefficient electrical generation would be a potentially huge efficiency gain. The challenge is to connect supply with demand. Many UK power stations are situated in isolated locations away from highly populated areas so there would need to be a huge infrastructure investment to bring the two together. Additionally, heat demand is mainly seasonal apart from process and domestic hot water loads while electricity demand is more constant throughout the year.

So, is there potential for seasonal heat storage using waste heat from power stations? Heat could be stored in various ways and at different scales – for example, locally adjacent to or under a building or in larger geological formations.

At the building scale it is important to analyse the complete system inclusive of buildings structure, finishes and other passive elements to help minimise net heating and cooling demands. More recently, mechanical engineering and plumbing engineers have started to engage more closely with architects and other design team members to ensure this occurs, but input is needed early in the design process – at the massing and orientation optioneering phase and not just from RIBA Stage C onwards.

Project team’s should look for opportunities for diurnal and seasonal heat storage options by considering building mass, foundations, the integration of phase change materials and use of other closed or open loop ground energy systems

Temporary storage

At a more micro scale, there are various ways to apply diurnal or temporary storage, either through the use of thermal storage connected to Combined Heat and Power, or through optimising the passive design using thermal mass. The latter is an approach that is being debated continually as thermal mass is not always a preference in buildings which require rapid heating or cooling, or with low occupancy. Additionally, active thermal mass in buildings could be an unwanted feature with average temperatures due to increase significantly over the next few decades.

Buro Happold has completed and is currently working on a number of underground thermal energy storage systems for building projects which look to store heat from numerous sources.

Lower temperature heat storage is inherent in most ground energy systems where heat is abstracted in heating mode, and rejected to the ground in cooling mode. Certain systems, such as aquifer and borehole thermal energy storage, promise to optimise this by ensuring heat and “coolth” are stored purposefully.

There are two further applications that are being explored with the concurrent installation of CHP and solar thermal systems in some buildings. Extending the operating hours and improving the efficiency of CHP is made possible by rejecting heat in periods of low demand in the ground.

Conventional thermal stores generally only make short term heat storage possible. Peak solar thermal output is intermittent and seasonal so to maximise heat generation it is also possible to reject heat to the ground to help balance heating dominated ground systems or simply to improve efficiencies of heat pumps.

With the ongoing ramp up in renewable and low carbon technology it is imperative that we continue to maximise efficiency and heat storage at every scale. Heat storage promises to play a large part in reducing carbon emissions, as well as capital and operating costs, but will require interdisciplinary approaches to move it beyond convention.

James Dickinson is project leader at Buro Happold

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