HomeDecentralized EnergyCogeneration CHPThe benefits of CHP for data centres

The benefits of CHP for data centres

Data centres are an excellent potential market for CHP, but uptake has been slow to date. Tildy Bayar spoke with MTU Onsite Energy’s Christian Mueller about the issues involved.

Power consumption in data centres is rising

Credit: Texas Department of Information Resources

Data centres use a lot of power. In the US, estimates of the power consumed by data centres range between 1.5% and 2% of the nation’s total electricity consumption. In Europe, data centres’ electricity usage is expected to reach 104 TWh by 2020. Globally, according to a recent report by management consultancy McKinsey, the data centre sector consumes around 2% of the world’s primary electricity supply.

Much of this power – up to 50% in some cases – is used for continuous cooling. In addition to environmental concerns, their need for power and cooling 24/7 makes data centres an excellent potential market for combined heat and power (CHP) applications around the world. However, to date CHP uptake in the sector has been slow, with data centre operators called ‘late adopters’ compared to other commercial users and CHP referred to as an ‘under-utilized’ solution in this particular sector.

The reasons for this are fairly clear. Data centre operators’ business depends on keeping their servers up and running, and thus all data centres already feature one or more backup power systems, most normally in the form of double connections with major power supply lines, as well as both uninterruptible power supplies (UPS) and diesel gensets. Installing another power system may simply not be seen as either necessary or economically viable.

Christian Mueller, a sales manager with MTU Onsite Energy, a Minnesota-based company in the US, says his firm does ‘a lot of data centre work’ – primarily supplying standby diesel gensets to large companies. ‘The cogeneration aspect is fairly new, and there are not many data centres which actually use that aspect,’ he adds. ‘It will take some time to reach a level of market acceptance.’

However, he says MTU has seen a change in the market as big IT firms such as Apple and Google become more concerned about being environmentally friendly and stepping up their investment in renewable technologies. In these cases, cogeneration becomes more attractive because the firms can market its green credentials, he adds. And where the bigger companies lead, others will follow.

Not a substitute for standby

A data centre typically features two layers of backup power, Mueller explains. UPS battery systems provide uninterruptible power for the first crucial seconds until the diesel generators start up in, typically, 10 seconds. However, he notes, ‘cogeneration gas engines typically take a few minutes to start and reach full load, therefore are not really suitable as a quick-start standby system.’ A CHP system can ‘only complement’ a data centre’s existing backup power arrangement, constituting an extra layer of security which operators might be reluctant to install.

The question these operators ask, Mueller says, is: ‘Is it worthwhile to invest all that money in a cogeneration system – am I going to get the return on investment necessary to install it?’ But an equally important question, he suggests, is: ‘Can I use a cogeneration system to save money in generating my own electricity and cooling?’

Available figures suggest that the answer to both questions is ‘yes’. A CHP system can significantly reduce a data centre’s expenditure on power, with a typical ROI of four to five years. It can provide cooling through an absorption chiller utilizing the waste heat from the gas engine – which will substantially reduce electricity demand when compared with electrically-powered cooling systems – and it offers environmental benefits through reduced emissions.

‘A cogeneration system is installed primarily for the economic benefits as a highly efficient method to generate power and useful thermal energy, and then also has a backup aspect to it,’ Mueller says. During a prolonged power outage, for example due to a storm, a data centre operator ‘could continue to run a cogen system, taking some load off the diesel generators and conserving diesel fuel,’ he explains. ‘You then have not only two standby systems [UPS and diesel gensets] but a third, the gas generator.’

‘We’ve seen a lot of interest in data centres recently. Some was towards a “save the diesel standby generators” mindset… That didn’t work out,’ he says, ‘as the startup time of a large gas engine cannot be reduced to a level comparable to a diesel generator set. The resulting need for additional UPS systems to provide uninterrupted power proved cost-prohibitive.’

In a recent project, MTU installed a CHP system in a large data centre and was ‘able to market that we now have an additional layer of backup,’ Mueller says. The project was particularly challenging, he adds. The data centre customer did not begin by requesting CHP but looked into a number of different technologies, including fuel cells, to meet its needs for on-site power generation.

‘It was really interesting to accompany the customer from having this new idea, wanting to do something different, to the end where a cogen system with a gas engine was what was chosen’, Mueller says. Among the challenging aspects of the project, he noted ‘finding what is the best technology which fits, examining the storage of liquefied or compressed gas on-site, and finding the best way to integrate with existing standby systems’.

Just chill

Mueller adds that, while the cooling ability of CHP in other facilities, such as hotels, is ‘one aspect, but not the only one’, a data centre’s primary use for thermal energy is for cooling. Data centres do not need much heating because ‘almost all energy put into a data centre in the form of electricity is then converted in processors, computers and data racks to heat. The only thermal need a data centre has is to get rid of all that heat,’ says Mueller – and this is true even for data centres located in cold climates.

A data centre CHP installation

Credit: MTU Onsite Energy

A conventional data centre, he explains, will be grid-connected. Approximately 25%-35% of a data centre’s electric needs are for cooling, which will be provided by electric chillers or compression refrigeration systems. A CHP installation at a data centre will use an absorption chiller to produce cold water from the engine cooling water and exhaust heat (exactly how it works ‘depends on the design of the chiller,’ Mueller says), with the chilled water then run through the facility in pipes.

Sizing CHP systems for data centres

Mueller notes that sizing a CHP system correctly can have a significant impact on reliability and corresponding uptime, both critical aspects for data centres. According to Mueller, a 50%-100% operating load range is acceptable for a gas-based cogeneration system. However, a load between 80% and 100% is where CHP systems achieve maximum efficiency. A data centre’s load can fluctuate, and if this fluctuation is wide the system’s sizing becomes crucial in order to accommodate it. Careful load matching is key to correct sizing, says Mueller.

In addition, CHP systems depend on the ability to work with a network of peripheral equipment such as heat recovery systems, cooling equipment, ventilation and electrical, as well as control systems. Each of these components must also be sized correctly, Mueller notes, or the CHP unit will not operate optimally.

When asked whether MTU’s CHP system is packaged, Mueller replies: ‘Yes and no.’ The system is not a typical, standardized packaged solution, he notes – and here again, sizing is a crucial issue.

‘A cogen system has to be sized, designed and built according to the customer’s and site’s needs,’ he says. ‘You have to also consider the fuel, the ambient conditions, all of these factors and then build a customized package.

‘I wouldn’t say cogen is a standardized product with a one-size-fits-all approach,’ he adds. ‘For these sites you typically look to install in a building rather than in an outside enclosure; if you go into a building there’s a bit more space.’ Space constraints for cogeneration installations can vary greatly – for example, data centres in tight urban areas will be very conscious of their footprint. However, most larger data centres are located in non-urban areas which offer more space for the installation.

To size a CHP system in the correct way, he says, ‘you have to look at the thermal and electrical demands of the facility and how they vary throughout the day and year. The aim is to find a solution which enables you to run a cogeneration system at as high a load as possible for as much time as possible, and ideally 24/7.

‘You have to size the unit so there is always enough electricity or thermal demand, cooling or heating, so it can run continuously throughout the year,’ Mueller says. ‘Typically, a customer looks at peaks – they will note that the maximum demand is so high – and asks for a unit that meets that. We go back and say we should size more toward baseload, in line with the consistent demand throughout the year rather than the peak, so the system can run throughout the year at a very high load factor. There is a greater risk of over-sizing than under-sizing.’

In the US, he says, ‘most customers think of the genset as a backup power system, so it has to provide for the maximum demand of the facility. For the CHP unit, though, we can make it a little bit smaller to increase the average load factor.’

Integrating even a correctly-sized CHP unit with other systems on-site can also be challenging. The unit must play well with, e.g., UPSs, diesel generators and switching equipment, as well as other technologies designed to enhance reliability.

The need for maintenance

Another reason the decision to go with CHP can be complex for data centre operators is the need for scheduled maintenance. In contrast to diesel standby generators, cogeneration systems run continuously and therefore require more frequent service and maintenance at regular, recommended intervals. Visual inspections and routine systems checks must be performed, hoses replaced, parts updated on a schedule, filters changed. Mueller notes that keeping spare parts handy is a fundamental part of maintaining the system, and that these parts must be stored close to the unit.

And, of course, there is the human element. ‘Typical [data centre] operators are probably not initially familiar with this type of technology,’ says Mueller. So, when MTU installs a unit, it offers in-house training for the operator because, ‘at the end of the day, they will be the one on-site every day and will be the first responder.’ Training courses are offered on the engine’s mechanical and electrical components and control system.

CHP must play well with generator sets

Credit: MTU Onsite Energy

The operator will be alerted to any problems or scheduled maintenance by the condition monitoring system, but operators can also ‘be trained to identify situations where parts need to be changed,’ Mueller says. At the next level, MTU’s distribution service network can handle more complex maintenance. In addition, MTU can troubleshoot many issues remotely through well-tested web-based access.

Long-term thinking

For a data centre operator, installing CHP ‘involves detailed planning and is not a “decide and install the next day” thing,’ says Mueller. A variety of factors will come into play when determining economic viability, such as the costs of gas and electricity – ‘The higher electricity costs go, the more attractive cogeneration systems become,’ he says. But these external factors are, of course, ‘hard to predict’.

Changing energy efficiency regulations will also affect a data centre operator’s decision on CHP. Mueller sees today’s stricter rules as ‘a benefit’ that can encourage uptake, but he adds that ‘you would prefer a cogen system to also be profitable without them, because you can never count on them 100%’ – especially in the US in an election year, when an incoming administration could easily overturn existing rules. And, of course, the rules differ from state to state. ‘In states like New York,’ Mueller says, ‘with the NYSERDA programme, they are focused on promoting CHP and the state has tremendous incentives which can cover between 50% and 80% of your cogeneration system if you qualify’. In general, though, he confirms that incentives for cogeneration are greater in Europe than they are in the US.

In Europe, there is a voluntary energy efficiency initiative for data centres, managed by the European Commission’s Joint Research Centre. The Commission says this Code of Conduct was created in response to the need to reduce the environmental, economic and energy supply security impacts of data centres’ growing power consumption. According to the Commission, 220 data centres have requested Participant status and 184 have been approved.

Participating data centres must commit to an initial energy audit designed to identify major energy-saving opportunities; submit an Action Plan and a timetable by which they will implement it; and allow regular energy monitoring.


Data centres can benefit greatly from CHP systems using trigeneration. Through increased efficiency of on-site power generation and cooling through absorption chillers using waste heat from the CHP, data centres can reduce costs of operation and greenhouse gas emissions.

In addition, reliability and continuous uptime are enhanced by adding an additional on-site power generation resource next to standby systems.

Christian Mueller is Sales Manager at MTU Onsite Energyà‚ www.cogen.mtuonsiteenergy.com