La Fontenette, Geneva

A centralized wastewater heat pump with a complementary gas boiler, for a high performance multifamily buildings complex, with heat distribution via a dedicated district heating system.

Lessons learned 

  • Use of innovative heat pump source (wastewater) in combination with fossil fuels needs careful cooperation of stakeholders regarding the system’s regulation and its optimization.
  • Heat pump covers a high share of the heat production with this local heat source.

Key facts

BuildingHeat Pump and Source
LocationGeneva, SwitzerlandNumber of HP1
Construction2015-2020Installed power200 kWth
Heat distributionunderfloor heatingOperation modebivalent
Heated area30.440 m²Heat sourcewastewater
No. of apartments120 (total),
17 per building (avg.)
Level of insulationHigh performance
Heating sytemDomestic Hot Water
Heat demand22 kWh / (m²a)DHW demand43 kWh / (m²a)
Heating
temperature
35 °CMax. temperature63 °C
SH share37%DHW share63%
Type of systemcentral
Other information
Add. heat productionGas boiler (600kW)COP
PV installation210 kWpSPF heat pump3
Investment costsCHF 91.- / m²
Heat Cost (LCOE)CHF 10.-/ (m²a)

While heat pump systems combined with district heating could reduce the CO2 emissions of Geneva’s heating sector, it is crucial to know and control their performance in real condition of use.

We monitored this new low energy multifamily buildings complex for 3 years.

SH demand and DHW supply are higher than planned/normed values. For SH that can be explained by operation conditions which differ from the norm (higher indoor temperatures, window openings), and DHW supply is certainly high, but coherent with a benchmark on other buildings in Geneva.

Description of the technical concept

Wastewater from the 7 buildings is collected in a common tank (37 m3), which contains: a filtration system to retain and remove the solid materials; as well as an immersed heat exchanger (FEKA system). HP evaporator is connected to this submerged heat exchanger via a glycol-water circuit.

The heat produced by the HP is transferred to a buffer stock. Additional heat is provided by a condensing gas boiler located downstream of the buffer stock. A motorized valve regulates the primary flow through the buffer stock (DH return). Heat is produced in a centralized boiler room and is distributed to the buildings via two heating networks.

DH supply temperature is set to meet SH needs (max. 40°C), and raises above 60°C several times a day to enable DHW production and storage in the buildings. This is done to improve heat production performance (mainly for the HP) and limit distribution losses.

Heat production operates according to the following two regimes:

  • HP production: this mode is used when the HP and its stock are able to meet the DH demand (in terms of power and temperature).
  • Gas boiler production (with HP isolated from distribution): this mode is used when HP and its stock can no longer meet DH demand (in terms of power and temperature). In this case, HP is isolated from the boiler and DH to prevent the boiler from heating the stock via DH return. HP then operates in a closed loop, to recharge the buffer stock, and stops when reaching its maximum setpoint temperature.

Final report: CALLEGARI, Simon Augustin et al. (2021). La Fontenette – « Les Auréa » : Analyse technique et sociale d’un complexe d’immeubles HBM de haut standard énergétique, équipé d’une PAC centralisée sur eaux usées.
Url: https://archive-ouverte.unige.ch/unige:149374

CH010 La Fontenette