Day 8 – Helmets on! GLN: Thermal heating & cooling

Written by Flor Mitchell

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GLN is a project coordinated by SIG, the canton´s provider of energy and other services (more in their website or Attila`s post) which supplies heat and cooling facilities to 25 companies, organizations and schools in the Sécheron district such as ONU, College Sismondi, Campus Biotech, Hotel InterContinental.

I – Energy context in Geneva

Geneva canton, with a population of 482.545, every year consumes an average of around 38.000 TJ[1] of energy (not taking into account the solid fuels). In the last years, approximately a third of it is electricity, and the rest is fossil fuels used directly as a source of energy. But from the data it is also observable that there is a tendency to electrification and less carbon intensive fossil fuels (natural gas). At the same time, of all electricity provided, 89% is from renewable sources (hydroelectric, solar and biomass) and 11% from natural gas.

[1] Bilan des livraisons d’énergie aux consommateurs finals, depuis 1980. Office Cantonal de la Statisque (Link)

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This is the result of the SIG vision: “to reduce a dependency on natural resources, to lessen impact on the environment and to ensure steady economic growth”.

Due to its location Geneva has big potential to produce electricity and heat from renewable sources.

Thanks to its topography and high levels of annual rainfall, Geneva has ideal conditions for the utilization of hydropower. Right now, SIG uses three hydroelectric power stations on the Rhone River (Verbois, Chancy-Pougny and Seujet) and one on the Arve (Vessy) and they provide more than 20% of all electricity used by Genevans[1]. Though the discussion still persists on whether hydropower is a clean source of energy, it indubitably contributes to the decarbonisation of the energy system.

Solar energy, can be used with the help of solar collectors for heat production (hot water and auxiliary heating), and through the use of photovoltaic systems for electricity production. In the first case, if all existing buildings were to be optimally improved in terms of energy efficiency, it would be possible to meet the heating requirements of all Switzerland’s households through the use of solar collectors. While with photovoltaic systems by 2050 it would be possible to meet around 20 percent of the current level of electricity demand in Switzerland[2]. And that’s why SIG has made a sound investment supporting this technology.

Geneva’s local geology means that there is excellent potential for geothermal energy. Underground resources can be divided into three categories: shallow, medium or deep and could cover a significant two-thirds of the canton’s heating needs and part of its electricity consumption. At shallow depths (50m to 400m), rocks in the earth have a constant, year-round temperature of 12°C to 25°C. Installing geothermic probes or pumping water into underground groundwater systems, together with heat pumps, represent significant potential for generating heating solutions. At medium depths (from approximately 400m to 3,000m), the underground temperature is between 25°C and 100°C. Water that circulates in these geological layers or in fault lines can be pumped and used directly in thermal installations, for urban heating or heating greenhouses. At much deeper levels (between 3,000m and 6,000m), rocks can reach temperatures of almost 200°C. This energy, when captured, can produce heat and also electricity. Deep geothermal heating presents enormous energy potential, but considerable investment is required to develop this sector and improve our knowledge of what lies this deep underground[3].

II – Genève Lac Nation

The thermal Genève Lac Nation (GLN) project is based on this kind of energy source, but instead of using the constant temperature of the earth, it takes advantage of the same phenomenon but in the deep waters of the Lake Leman.

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The project now supplies water that can be used by the participating buildings as heat sink to the climate control systems. Mainly used for air conditioning systems in conference rooms and data centers, it also provides heating in newly-constructed buildings via the installation of high-performance heat pumps. To guarantee the efficiency of the system as a total, buildings applying for heating services are audited before being incorporated.

Many different organizations take part of the project in various ways. SIG is coordinating them all but also financed it together with the federal state. On the other hand, University of Geneva is measuring the system and studying it. And as it was said before, 25 institutions use its services. The project is of interest to many actors because of the possibility of replicating it in other neighborhoods and cities.

III – How does GLN works?

Diagram of the complete system (You’ll have to forgive my little graphic design skills, there’s also a nice video here)

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  1. Water flows into a water deposit through a pipe 40 meters deep, where water is around 6/7 ºC all year round. No pumping system is used in this instance to diminish environmental impact. Especially for flora and fauna that could be dragged into the pipes due to aspiration systems.
  2. Pumps distribute the cold water to all the participating buildings.

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  1. Buildings with need of cool air for their rooms use heat exchangers to use the cold water as a heat sink. The hot air coming from their rooms becomes colder, warming up the water flowing in the GLN pipes. Thanks to the heat exchanger, the water from the lake never mixes with the water from the building facilities.
  2. Buildings equipped with high efficient heat pumps, can use the water from the lake as a heat source even when its temperature it’s not so high, cooling the water. Thus, they can supply the heating facilities of their rooms. This technology also keeps the water from both systems separated; therefore it can be pumped again into the lake.
  3. On its way back to the lake, water flow is used to generate electricity with a turbine. That energy complements the energy supply from the electric line increasing the system efficiency.

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  1. If at the end of the circuit there is still residual heat, the water flows into a closed loop and starts its way through the buildings with heat pumps again.
  2. Finally water it`s pumped into the lake again at a depth of 5 meters, where the water is around 3ºC in winter and 12ºC in summer. Since in winter water is mostly used for heating purposes the outgoing water is colder than when it comes in, while in summer, the water is used to cool down the buildings, so water going out from the system is hotter than the incoming one. Thereby the difference between the outgoing water and the lake is minimized.

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All the system is monitored to identify problems and improvements. Even the performance of the buildings facilities connected to GLN is closely followed to keep efficiency as high as possible.

IV – What next?

With the success of GLN, SIG proposed to expand it with a new project: Genilac. This new venture while supply with water for cooling or heating purposes downtown Geneva by 2021 and the airport by 2022.

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V – Environmental impact

Basing on the study of the GLN system in Geneva, the effects of such facilities on the lake environment are studied and the impacts assesses as very low. The generalization of such systems is mainly limited by the cooling demand, which should remain limited in the Swiss climate, rather than by the physical limits of the resource[1].

This harnessing of water resources would bring down the temperature of the lake less than 0.2 ºC, which would have little effect on the body of water[2].

Anyway, more research must be done to determine the long term impact this kind of projects may have on the ecosystem.

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[1] Valorisation thermique des eaux profondes lacustres: le réseau genevois GLN et quelques considérations générales sur ces systèmes. Jérôme FAESSLER, Pierre HOLLMULLER, Bernard LACHAL and Pierre-Alain VIQUERAT.


[1] SIG, Renewable Energy, Hydraulic. Link

[2] Swiss Federal Office of Energy (SFOE).

[3] SIG, Renewable Energy, Geothermal. Link


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