It towers into the sky on the edge of the banking district in downtown Frankfurt am Main: the tower of the former headquarters of the European Central Bank, which moved into a new building a few years ago. Like a hotel, a data center has now found its place on the seventh floor of the skyscraper. It could be accommodated on the old bank office floor. The room with the high computers is operated by the Dresden company “Cloud&Heat”, for which Stefan Reckling works.
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“With the cabinets you see behind me, the whole building is heated and hot water all year round.”
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Around 500 tons of CO2 are saved each year in the former headquarters of the ECB alone. But it shouldn’t stop there, stresses Ingmar Kohl. The industrial engineer manages the so-called “district heating department” at Mainova AG. The company is one of the largest regional energy suppliers in Germany with around one million customers. Mainova is 75 percent owned by the City of Frankfurt.
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Cooling water as a carrier for waste heat
In the future, the company also wants to use the waste heat from data centers on a larger scale in its district heating network, which is around 300 kilometers long. The carrier for the waste heat is the cooling water from the data centers, which is heated up by the waste heat. However, the approximately 30 degree warm water must first be increased to at least 90 degrees.
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“And that is also the temperature level that we will need in the foreseeable future. This temperature difference now has to be bridged, and heat pumps really come into their own here,” explains Ingmar Kohl.
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The heat pumps then heat the water from the data centers in such a way that it can be fed into the district heating pipes at 90 degrees or more without there being any loss of temperature. The heat pumps are best operated with electricity from renewable energies, according to Ingmar Kohl.
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“You can turn one kilowatt of electricity into three, sometimes four, five or six kilowatt hours of heat. And if that happens using renewable electricity, then of course it’s a great thing for the climate.”
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60 data centers in the Frankfurt am Main region
There are already around 60 data centers in the Frankfurt am Main region. Next to London, Rhein-Main is the most important data processing location in Europe and currently has the second-biggest growth prospects in the Europe-Middle East-Africa world region.
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Taken together, all of the data centers in Frankfurt am Main consume more energy than Frankfurt Airport. So the potential for using waste heat is huge.
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“We also examined the waste heat potential here. Incidentally, there is also a very good basis from the city of Frankfurt with the waste heat register,” says Ingmar Kohl from the municipal energy supplier Mainova.
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But in order to use the waste heat potential of the data centers, the computer has to be switched from air to water cooling. Because with air cooling, the heated air is usually released into the outside air. The heated water, in turn, can be used directly to heat rooms. But when it comes to water cooling, data center operators are often hesitant.
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“Water and electricity are not entirely compatible. That’s why I can’t blame it at all,” says Ingmar Kohl.
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Water cooling plays important role
“For example, we have something called high-temperature liquid cooling. We can cool the computers with liquid flowing through them. In the past, computers were always air-cooled. But air cooling is very, very inefficient. And with this liquid cooling, even with this high-temperature variant, we are allowed to go into these computers with up to 40 degrees of coolant. And then the liquid might come out at up to 50 degrees and we just have to bring it down to 40 degrees. And in Hamburg, here on our roof, it has never been more than 38.5 degrees. That means we can achieve the temperature drop passively.”
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The hot liquid is simply pumped onto the roof, the ambient air is brought in via slowly rotating fan blades and the water is thus cooled down to the prescribed 40 degrees and less and then runs back into the computer, says Thomas Ludwig. This means that as little waste heat as possible is released into the environment. The result is an almost closed cooling circuit, says Thomas Ludwig.
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“This method saves on the otherwise very energy-hungry cooling systems. Such a cooling system to cool this IT can easily add another 30 to 50 percent to the electricity consumption of the computers, so that the electricity costs are 1.3 to 1.5 as high. And with this high-temperature liquid cooling, the additional effort for cooling is maybe 4 percent.”
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Water heat is used inside the house
In the data center in the former ECB building in downtown Frankfurt, however, the warm water is not cooled on the roof, but used directly inside the building to heat other office floors or the hotel. Also a good solution, according to Ingmar Kohl from Mainova.
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“If there are local opportunities to use your own potential here in terms of energy efficiency, then that can only be viewed very positively as an additional measure. If you manage to reduce your own heat output with such measures, then that is also a very positive contribution to the heat transition here in Frankfurt.”
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A few kilometers west of Frankfurt city center – still in the urban area of the Main metropolis. This is where the new Westville district is being built, which is to be supplied with the waste heat from a neighboring data center. The district includes 1,300 apartments for around 3,000 people. There are already projects of this magnitude in Scandinavia, but the Westville project is unique in Germany. It is also being developed by Mainova AG.
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“By including waste heat from data centers in our network,” says Heidi Göbel-Stahl. She is responsible for heat at the municipal energy supplier as a sales manager. Her colleague Ingmar Kohl once again underlines the importance of this project, which is unique in Germany, for an environmentally friendly heating mix in the Main metropolis.
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“One simply notices there that the connection that was created there, between the district heating on the one hand and then the waste heat and the heat pumps on the other hand, that this was the most sensible measure at the point.”
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According to Ingmar Kohl, a purely decentralized concept such as that used in the old headquarters of the European Central Bank or the German Climate Computing Center would not have been feasible in the Westville development area in Frankfurt. From his point of view, the heat transition in the big city needs a mix of ideas and technology.
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