Cold district heating at FGZ Zürich, Switzerland

‘Familienheim-Genossenschaft Zürich’ (FGZ) is a nonprofit housing cooperative owning and operating more than 2,200 mainly residential buildings (185,000 m² heated floor area) for about 5,300 inhabitants forming a city quarter on its own within Zürich with a heat demand of 35 GWh/a for space heating and domestic hot water.

FGZ decided on a master plan regarding the further development and stepwise refurbishment of the FGZ district and its buildings to reduce the heat demand to 13 GWh/a in 2050. Therewith complementary, a new heat supply concept was developed by experts from the Zürich based company ‘anex Ingenieure AG’ for energetic crosslinking of low temperature heat sources, borehole thermal energy storages (BTES) and central heating plants with heat pumps using a cold district heating system (CDH) with a temperature level < 28 °C. This replaces the priory used oil and gas boilers.

The construction started in 2011 and the CDH was commissioned in 2014 and has been successfully in operation since then. An enlargement project to connect further central heating plants, borehole storages and a second waste heat supplier was finished in 2019. In future, the whole city quarter will be supplied by the CDH.

The CDH grid consists of a flow- and a return line made of uninsulated pipes and uses water as heat transfer medium. Currently it has a linear topology (Figure 79, blue line) but will be a ring with linear connections to heat sources, BTES and central heating plants after the final construction stage (Figure 79, red line). There is no central pumping and predefined flow direction respectively as in conventional district heating systems. Individual pumps in each central heating plant and at feed-in points generate the individually required mass flow. The BTES within the system are used to compensate short term as well as seasonal mismatches of thermal supply.

Each central heating plant connects the ULTDH with a district heating sub-grid using high efficient large scale heat pumps to provide the required temperature level of each sub-grid while using the CDH as heat source (approx. 80% of the heat is provided with 68 °C, 20% with 40 °C). The annual COP ranges between 4 and 5 (including pumping). 

The temperature level in the CDH grid ranges between 4 and 28 °C and is mainly depending on the charging status of the BTES and the time of the year respectively. The heat is provided by the cooling load of two large data centres. The CDH allows free-cooling of these data centres and thus reducing their cooling efforts.

General plant configuration:

•  Ultra-low temperature district heating grid (primary grid)
•  Two low temperature heat sources (data centres) · total heat capacity of approx. 4.5 MW
• 2 BTES with 332 boreholes, each with 250 m depth (450 boreholes planned for 2050)
• 6 central heating plants (10 in 2050) connecting the CDH with WDH sub-grids for heat distribution to buildings (existing secondary district heating grids) · Heat pumps with total capacity 4.8  MW (8.8 MW in 2050) · Total heat capacity incl. peak load coverage 7.2 MW (12 MW in 2050)