SolarCity, Geneva
A solar assisted heat pump in combination with unglazed solar collectors for a new mutlifamily building complex in Geneva, Switzerland.

Lessons learned
- Excellent system reliability.
- A single heat distribution circuit with decentralized DHW storage which does not allow for solar preheating and thus deteriorates the potential of direct solar heat production.
- A large part of the heat is produced at high temperature (60°C) for DHW production, decreasing the expected Seasonal Performance Factor (SPF).
Key facts
Building | Heat Pump and Source | ||
Location | Geneva, Switzerland | Number of HP | 1 |
Construction | 2010 | Installed power | 30 kWth |
Heat distribution | underfloor heating | Operation mode | monoenergetic |
Heated area | 927 m² (one block) | Heat source | unglazed solar collectors |
No. of apartments | – | ||
Level of insulation | high performance | ||
Heating sytem | Domestic Hot Water | ||
Heat demand | 19 kWh / (m²a) | DHW demand | 48 kWh / (m²a) |
Heating temperature | 35 °C | Max. temperature | 60 °C |
SH share | 28 % | DHW share | 72% |
back-up heat production | Direct electricity | Type of system | decentral |
Circulation system | No | ||
Other information | |||
HP share | 80% | COP | – |
Direct solar heat share | 19% | SPF | 2.7 |
Back-up heat share | 1% | ||
Solar collectors | 116 m² |
This case study concerns a coupled solar and HP system which was implemented in 2010 in a new housing complex, called SolarCity, located in Geneva (Switzerland).
The complex is composed of 4 buildings, each subdivided in 2 or 3 blocks, for a total of 10 blocks. The buildings present a high thermal performance envelope and a total living surface of 9’552 m².
This case study concern only one of the 10 existing buildings blocks, which are all equipped with their own identical and independent heat production system.
The results show a very low SH demand for Switzerland and an unusually high DHW consumption, which can partly explain the relatively low HP SPF.
Description of the technical concept

The energy concept consists of solar collectors that can be used for direct solar heat production, via a heat exchanger, but are also the heat source of the HP (they are directly connected to the evaporator). Hence, when there is no solar radiation, the solar collectors work as a heat absorber on ambient air.
For each building block, there is: a 30 kWth heat pump; 116 m2 of unglazed solar collectors; 2 x 3’000 L of water for centralized heat storage with an electric rod in the storage tank in case of HP failure.
A specificity of the system consists in a single distribution circuit to the flats, so that SH (floor heating) and DHW cannot be supplied simultaneously and therefore are supplied alternatively. Each flat is therefore equipped with a 300 L DHW tank. DHW distribution has priority over SH distribution, which means that when one of the 300 L tanks is at a temperature below 40°C, the system switches automatically to DHW mode and rises the temperature of all the 300 L tanks up to 60°C.
The system has 4 main operating modes, with the following priorities: (i) Direct solar heat production for SH or DHW (bypassing the HP), the surplus being used to charge the heat storage; (ii) Storage discharge, which is activated when the solar production does not reach the required distribution temperature; (iii) Activation of the HP when the storage temperature is below the required distribution temperature, with surplus production used to charge the heat storage; (iv) Direct electric heating, which is activated in case of HP failure (in particular when the evaporator temperature drops below -20°C).
In summer, the system can also be used for night cooling, by activating the floor distribution circuit and dissipating the heat in the solar collectors.
Final report: DE SOUSA FRAGA, Carolina (2017). Heat pump systems for multifamily buildings: which resource for what demand? Thesis, University of Geneva.
Url: https://archive-ouverte.unige.ch/unige:94939
CH013 SolarCity