The garbage men of the city of Hamburg/Germany are showering with water that is heated by the waste heat of a server room. The key to this ecologically and economically favorable solution is a large heat accumulator with a volume of 30,000 liters.

Stadtreinigung Hamburg (SRH) is the company responsible for the waste collection and the street cleaning of the German harbor city. The surplus heat is produced by its servers that run the computers that manage customer data and its financial accounting. Almost constantly 35 kW of cooling load have to be removed from the data center. Until two years ago, this waste heat had been emitted completely unused into the atmosphere over a conventional air conditioning.

Since a large-scale renovation in 2009, a newly installed heat pump cools the ambient air in the data center to 16° C using a brine circuit. This leads to a room temperature of 21° C. The circuit is cooled from 14° C to 10° C. The gained energy continuously heats up 30,000 liters of water in a huge heat storage on a different floor of the same building. The heating water reaches a temperature of up to 65° C. The flow-through principle of a heat exchanger heats a tank of reserve freshwater to supply the building's 55° C hot water.

Most of the hot water is consumed daily between 1 and 3 pm by the 500 employees of the waste collection, street cleaning and repair shops, when 68 showers are in full swing. While the heat storage is supplied 24 hours a day with waste heat from the server room, the hot water is discharged in just two hours. This situation calls for a relatively large heat storage. The installation location is not easily accessible, so the heat stored in an accumulator with a diameter of 4.40 m and a height of 2.80 m had to be mounted onsite. This was made possible through a modular design, offered by the manufacturer Haase GRP Engineering (Großröhrsdorf/Germany).

The renewal of the heating system helped SRH to save 500,000 kWh last year, thereby avoiding 81 tons of CO2. Dominik Iskenius-Eggers, the engineer of Stadtreinigung Hamburg who was in charge of the project from the start, brings to the point, how successful the investment was: "With this example, the frequently used argument 'ecology doesn’t pay' has been thoroughly disproved. Our hot water needs can be met throughout the year on the data center, so that the heating network can be turned off completely in the summer.”

The cooling of the server room is now more reliable and resilient, for the former cooling system has been preserved and can be used as a back-up. It turns on, for example, when the water tank can’t take more heat, or when the complete system would fail. SRH's Climate Protection Officer, Dr. Stefan Lübben adds: "The savings were much higher than we had expected. This is the reason, why the previous calculations with a payback period of ten years were reduced to about three years.”