Heating and cooling Swiss Re buildings using energy from the lake

Nearly half of all energy consumption in Switzerland is for heating buildings and water. Add to that the energy used for air conditioning, especially in summer. Swiss Re Next is designed to require far less energy for heating and cooling than did the previous building. Its ideal lakeside location permits using carbon-neutral energy from Lake Zurich, both for cooling in summer and heating in winter. This is made possible by heat pumps, which will work even if Lake Zurich should freeze over again.

Zurich last experienced a "Seegfrörni" in the winter of 1962/1963, a local dialect term referring to a period when the lake was frozen solid. Even in conditions as extreme as those, however, the temperature at the bottom of the lake never drops below +4°C. At a depth of some 20 metres and roughly 500 metres offshore, where we catch the water to generate the energy for Swiss Re Next, the water temperature of Lake Zurich averages between 10°C and 15°C, in winter as in summer. This makes Lake Zurich water particularly well suited to use in heat pumps.

Same concept as a refrigerator

You, too, come into contact with an electric heat pump at least once a day. Heat (or thermal) pump technology for buildings works much like the fridge you have in your home. Whereas a fridge works by removing heat from its compartments and bleeding it to the surrounding space, a heat pump extracts heat from its outside environment (such as air, soil or water) and transfers it to a building in the form of energy. This energy is then used for heating the building and supplying hot water.

In the Swiss Re buildings on Mythenquai, the energy used for heating comes from lake water which is piped to the lake water pumping plant (Seewasserzentrale) and from there is supplied to the heat pumps installed in the different buildings. Exactly how this process works is shown in the diagram below. In summer, it is the same process but in reverse. Even in the warmest season, however, the water 20 metres below the lake surface is rarely more than 18°C, making it ready for use in cooling buildings without it first needing to be circulated through the heat pump system. This process is known as freecooling and delivers far better energy efficiency than do conventional cooling systems.

Sixty percent less

Thanks to its very efficient shell along with the heat pumps and state-of-the-art building utility systems, Swiss Re Next will use nearly 60% less energy than did the previous (significantly smaller) office building. Swiss Re is not the only company to use Lake Zurich as an energy reservoir. Already, there are three lake water supply networks in Zurich (Escherwiese, Fraumünster and Falkenstrasse) providing the various buildings with thermal energy. Combined, they reduce CO2 emissions along Zurich's lower lake basin by some 1 100 tonnes a year.

The heat pump cycle in winter:

Photo Gallery: Heating and cooling Swiss Re buildings using energy from the lake