Data Centres: Heat Reuse for a Low Carbon Future

Data centres are significant energy users, and their impact on the global energy supply and the environment is an issue. Removing the heat produced during operation is a constant challenge that is becoming more complicated with new targets on efficiency and carbon emission reductions being set by governments and local authorities around the world. 

These drivers include building regulations and local planning laws. Several countries have halted local data centre development due to their perceived impact on energy use and carbon emissions. In addition, data centre clients are also increasingly concerned with the carbon footprint of their data use. They are therefore focused on working with providers in this field who can demonstrate low-carbon credentials. 

Overall, data centre developers and operators are under increasing pressure to provide robust, fault-free services while reducing energy use and emissions - a challenging balancing act. 

Measuring data centre efficiency 

There has been a great deal of focus on selecting cooling technologies to match building requirements while meeting energy-reduction targets. However, there is another way to approach this challenge: to view data centres as sources of heat energy that can be applied to other buildings. Developments in heat pumps (which use low-grade heat as their energy source) and heat networks are making this scenario more achievable. It’s a method that has been adopted across northern Europe. 

By shifting the focus away from cooling and onto the reuse of heat energy, data centres have the potential to be a key factor in decarbonising heat.

Data centre efficiency is commonly measured by power usage effectiveness (PUE). This is the ratio of the total energy used by a data centre (including IT load, cooling, lighting and other electrical systems) to the energy delivered to the computing equipment. The ideal PUE is therefore 1.0 However, the ability to continuously improve PUE is reaching its practical limits. 

The Uptime Institute 2022 global survey highlights that the average PUE in 2022 was 1.55. The number in 2007 was 2.5, showing a significant improvement during that time. In 2021, the average PUE was 1.57, and the Institute notes that on energy efficiency, ‘progress has slowed’.

Alongside PUE, the Energy Reuse Factor (ERF) is set to become a vital consideration for the design and operation of data centres. ERF is a measure of the amount of reused energy divided by the total amount of electrical energy supplied to a data centre. A new standard ISO/IEC 30134-6:2021 Information technology - data centres key performance indicators - Part 6*: Energy Reuse Factor (ERF) provides a method for data centres to measure their performance in this area. 

The standard defines reused energy as: “The utilisation of energy used in the data centre for an alternate purpose outside the data centre boundary. Energy ejected to the environment does not constitute reused energy.” 

Microsoft estimates that it is possible to achieve an ERF range of up to 69% during winter months and 86% in the summer. Mitigating factors include the type of cooling system used in a data centre and ambient temperatures. 

Recognising heat’s potential

The World Economic Forum (WEF) has recognised the significance of tapping into the heat produced by data centres. It estimates that the data centre heating market could be worth US$2.5bn by 2025. 

More importantly, data centres can help other buildings, including homes, become more sustainable by providing an alternative heat source. Leading data centre developers and owners are embracing the benefits of heat reuse, particularly in the Nordic region. 

Several approaches to heat recovery can be applied, depending on a data centre’s heat output and location. It is important to analyse the local heating load requirements in order to optimise the use of waste heat from a data centre. For example, it may not always be possible to connect a data centre to a heat network, but there are options for reusing rejected heat on-site. 

Simultaneous heating and cooling chillers are particularly useful in data centres where there are coincidental heating and cooling loads. The heat extracted from areas that require cooling can provide heating to occupied spaces in or close to the data centre, such as offices. It can also boost the temperature of hot water to reduce the load on chillers that supply hot water for washrooms, showers or on-site restaurants. 

It is also possible to use a dedicated water source heat pump that uses the condenser water or return chiller water from the data centre cooling system as its energy source. This is an excellent approach for large, water-cooled chiller applications that improves the performance of large-capacity chillers and dedicated plants, enhancing the ROI of capital expenditure and improving long-term performance. One of the primary benefits of using heat in this way is that no gas boiler is required since the system provides cooling and space/hot water heating.

How Mitsubishi Electric can help

Mitsubishi Electric has been involved in data centre waste heat use in heat networks across the world, and our experience has given us some useful insights. 

Our first piece of advice when considering linking the data centre to a new or existing heat network is to ensure that there is a need for the waste heat at a reasonable distance from the data centre - or that there is an existing heat network that can use extra capacity. A successful match of data centre heat output and local heating requirements is what designers will look for when setting out these projects. 

Buildings which are close to the data centre, such as nearby offices or public buildings may not have high heat requirements. However, heat networks which supply domestic customers have higher and more predictable heat demand profiles. Buildings such as hospitals, schools and leisure centres are also sources of heat demand to consider. 

Heat mapping is an important part of this, with governments undertaking extensive research to identify areas where there is significant potential for heat networks. It’s then vital to understand what the cooling demand of the data centre is across the year, and to size and specify cooling equipment. The ideal solution is a water-to-water heat pump, or a heat pump chiller. The heat output of the heat pump can then be calculated to establish the annual heat output profile. 

It will be helpful to consider future data centre growth and how that could impact cooling requirements, as well as considering client goals for carbon footprint reduction. Consideration must be given to the embodied and operational carbon of equipment. Embodied carbon in products such as heat pumps is impacted by the type of refrigerant used. 

For instance, hydrofluoroolefins (HFOs) have the lowest GWP (Global Warming Potential) on the market and are used in several Mitsubishi Electric heat pump options.

Framing the data centre sector as part of the solution for our decarbonised future, rather than simply an energy user, has clear benefits for future development and growth. However, heat networks are a relatively new approach to low carbon heat, so it is vital to work with partners who have experience of these projects at scale. 

At Mitsubishi Electric, we have been working with data centre partners for decades and can bring unique insights into heat recovery and heat pumps from global projects.

* ISO/IEC 30134-6:2021 Information technology - data centres key performance indicators -Part 6: Energy Reuse Factor (ERF). Read more here.

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