Article
Reference
Valorisation thermique des eaux profondes lacustres : le réseau genevois GLN et quelques considérations générales sur ces
systèmes
FAESSLER, Jérôme, et al.
Abstract
L'énergie thermique des lacs est de plus en plus valorisée, principalement pour répondre aux demandes croissantes de climatisation des bâtiments. A partir du retour d'expérience réalisé sur le système GLN à Genève, les effets de telles infrastructures sur le milieu lacustre ont été étudiés et les impacts ont été évalués comme très faibles. La généralisation de tels systèmes est principalement limitée par la demande de climatisation qui restera restreinte sous nos climats, plutôt que par les limites physiques de la ressource. De plus, la comparaison des impacts sur l'environnement avec la filière énergétique traditionnellement utilisée (électricité thermique et groupe de froid) se montre favorable à ce type de système. Une utilisation correcte de cette ressource garantissant des impacts aussi faibles que possible nécessite une meilleure connaissance scientifique du système lacustre lui-même.
FAESSLER, Jérôme, et al. Valorisation thermique des eaux profondes lacustres : le réseau genevois GLN et quelques considérations générales sur ces systèmes. Archives des Sciences, 2012, vol. 65, p. 215-228
Available at:
http://archive-ouverte.unige.ch/unige:28925
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Thermal valorisation of deep lake water: the Geneva GLN network and some general considerations on these systems
Jérôme FAESSLER, Pierre HOLLMULLER, Bernard LACHAL and Pierre-Alain VIQUERAT
Université de Genève, Institut des sciences de l’environnement & Section des Sciences de la Terre et de l’Environnement
Original paper: FAESSLER, Jérôme, et al. Valorisation thermique des eaux profondes lacustres : le réseau genevois GLN et quelques considérations générales sur ces systèmes. Archives des Sciences, 2012, vol. 65, p. 215-228. Available at: http://archive-ouverte.unige.ch/unige:28925
Abstract
There is an increased use of thermal energy from lakes, mainly due to the growing demand of air conditioning of buildings. Based on the study of the GLN system in Geneva, the effects of such facilities on the lake environment are studied and the impacts assessed 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. In addition, the comparison of environmental impacts with traditional cooling (thermal electricity and chillers) is in favour of this type of system.
Appropriate use of this resource to ensure the lowest possible impacts requires a better scientific understanding of the lake system itself.
Keywords: lake, thermal use, network, air conditioning, environmental impact Introduction
The use of the thermal energy contained in the lakes, which comes essentially from its interaction with the sun and the atmosphere, is recent. Examples include the use of lake water as a cold source for heat pumps or as a sink for waste heat from air conditioning systems (Faessler et al. 2009a).
Newly, large-scale thermal network infrastructures using the "cold" resource of deep lake layers are developed at various points around the world. Thus, the EPFL and Cornell University (New York State) respectively use the "cold" of the deep layers of Lake Geneva and Lake Cayuga to cool their own buildings and evacuate the heat from their activities (datacenter, lab, ...).
The most advanced system is that of Toronto (Viquerat 2012), which supplies the entire city centre with cooling (total cooling network capacity of 250 MW, of which 60 MW comes from Lake Ontario).
In addition to its size, an originality of this system is the synergy of "material" and "energy" uses since the calories drawn from the users of the thermal network are used to de-ice the drinking water, pumped at low temperature (4°C in summer, due to a pumping depth of 83 m). The Toronto system is
"multi-resource", since in addition to the contribution of the ribbon-type "lake" cooling, a
"conventional" cooling plant covers peak demand.
The GLN (Geneva Lake Nation) cooling network was initiated in the early 2000s by Merck-Serono during the construction of their new building in the Sécheron district on the right bank of Lake Geneva;
it was followed by a larger project, managed by the Services Industriels de Genève (SIG), to meet the air-conditioning needs of many international organisations in the vicinity (see Fig. 1). Based on the deep layers of Lake Geneva, it is now operational since 2009 and aims first and foremost at covering the air-conditioning requirements.
A total of 4000 m³/h of water (2700 for GLN and 1300 for Merck Serono) are pumped at a depth of 37 m. A 2.5 km long, existing but decommissioned pipeline, in good condition, carries the water to the pumping station located on the shore. From there, a 6 km long network distributes the fresh water to the users and collects the heated water, which is finally rejected into the surface layers of the lake at a depth of 4.5 m. The lake is thus a cold source in its deepest part and a heat sink on the surface, naturally warm in the summer. The system is also active in winter, when it operates very differently and preferably in a closed loop. Heat gains mainly from data centres tend to counterbalance the extraction of heat for heating of buildings via heat pumps. The role of the lake is then to maintain the loop temperature within operating conditions of the connected systems, injecting lake water only when necessary. In addition to eliminating possible impacts on the environment, closed-loop operation allows for lower electricity consumption, because it is no longer necessary to overcome the 75 m difference in level between the lake and the highest point of the circuit.
Fig. 1. Geneva-Lake-Nation network (GLN).
The characteristics of the GLN system are:
• The cooling network is a single-resource system, so there is no possible correction of the temperature of the lake resource, except at the level of each customer after the heat exchanger;
• It is not dedicated to a single large customer, so all the cold must be distributed to a large number of customers (commercial network);
• Almost all of these customers are already equipped with a conventional air-conditioning system that has been in operation for many years. These decentralized systems that are kept in complement of the GLN network. Waste heat from cooling units is also evacuated into the GLN network;
• In winter, the waste heat from the data centres is recovered to supply the heat pumps and the system makes minimum use of the lake.
The main data from the GLN project are presented in Table 1 (Viquerat 2012).
Table 1. GLN characteristics.
These innovative aspects allowed the GLN project to be chosen as a pilot system of the European Concerto programme (TETRAENER project, 2005-2010) and to benefit from funds not only for the implementation of the infrastructures but also for research and evaluation concerning the following points:
• Resource
• Effects and impacts of discharge
• Energy efficiency of the system
• Connectability of existing buildings
• Cost-effectiveness of the system.
In this article, and based on this feedback, we will focus mainly on the "sustainable" potential of the thermal resource of the deep layer of a lake such as Lake Geneva and its ability to contribute to the solution of the energy problem. To do this, we will first describe the main results of GLN's environmental assessment and explore the maximum sustainable potential of the lake.
In a second phase, we will establish the total cooling demand at the canton level and estimate which part of it can be used if we take into account the various constraints related to physics, economy and territorial aspects such as the density of the cold demand and the possibility of setting up such a network. Finally, more general considerations on competing energy sources will enable us to highlight the main stakes and challenges for a significant use of the lake as a thermal resource.
Feedback on the GLN system
An assessment of the effects and impacts on the lake was carried out as part of the European TETRAENER project and was the subject of a doctoral thesis (Viquerat 2012), of which only the main conclusions will be repeated here.
Thermal regimes of the Petit-Lac
In summer, the upper layer of the lake (epilimnion) heats up while the lower layer (hypolimnion) remains cold. The thermal gradient is located between these two layers, in the metalimnion. This strong gradient disappears during the winter. Fig. 2 illustrates this dynamic during the year 2008 for the water column located above the GLN pumping site. However, the summer stratification may be temporarily disrupted, mainly during sudden outdoor temperature drops (depressurization episodes), resulting in a sudden and brief rise in temperature at the hypolimnion level (local drop in the thermocline level).
Resource
For the GLN system, the resource consists of the hypolimic waters of the Petit-Lac (southern part of the lake, next to the City of Geneva), pumped from a specific point. Measurements taken at the network's pumping site (37 m deep) show that the temperature stays mostly between 5 and 10°C (Fig.
2). Low and stable in winter (5-7°C), it increases starting from spring and reaches an average temperature between 7 and 10.5°C in autumn, the season during which the temperature variation is the widest (several peaks).
Fig. 2. Petit-Lac temperature at the GLN pumping station; hourly data 2008 (Viquerat 2012).
These temperature rises occur mainly when the outside temperature is below 15°C and the cooling demand is low (see Fig. 3). In summer, when there is a high demand for air-conditioning, the resource is more stable and only on rare occasions does it exceed 10°C.
Fig. 3. GLN resource temperature as a function of outdoor temperature; 2008 hourly data (Viquerat 2012).
Fig. 4 shows the relation between the power demand and the outdoor temperature, measured between April 2010 and March 2011, at a time when about half of the rated power was connected.
Fig. 4. Signature of the network's cooling capacity; hourly data from April 1, 2010 to March 31, 2011 (Viquerat 2012).
It should further be stressed that above thermal fluctuations are site-dependent and linked to overall lake-wide phenomena such as seiche and external forcing (Viquerat 2012; Le Thi et al. 2012). A slightly deeper water intake (-45 m) such as in the GLN analogous but smaller system developed at Versoix shows great stability (Fig. 5). One understands better the importance of a good knowledge of the resource and especially the currentology and internal movements of the lake, studies for which F.-A.
Forel was a great precursor.
Fig. 5: Temperature of the GLN (37 m deep) and Versoix (45 m deep) pumps; hourly data 2007 (Viquerat 2012).
Environmental effects and impacts Effects due to pumping
The suction strainer was designed to prevent living organisms from being sucked in. The maximum suction speed has been limited to 21 cm/s, so that small fish (< 8 mm) can escape. In order to avoid
"biological" fouling of the pipes, the pumped water is chlorinated with an injected dose between 0.3
and 0.4 mg Cl2/l. A control system stabilizes the residual chlorine concentration in the rejected water to a value lower than 0.05 mg Cl2/l.
Thermal effects of rejection and potential impacts
Concerning thermal effects and impacts, several distinctions must be made:
• summer conditions, with a highly thermally stratified lake and thermal discharges that are always positive, versus winter conditions, where the lake is destratified and discharges are positive or negative depending on the cooling and heating demands of the connected buildings;
• rather local effects, such as the plume of discharge, versus global effects, such as the depletion of the resource by excessive extraction;
• thermal effects, expressed as temperature or flow differentials, directly and instantaneously measurable or calculable, versus the impacts on the affected natural ecosystem (fauna, flora) resulting from these effects, which are multifactorial, much more difficult to define and longer to evaluate.
Globally, during the stratified summer period, a hydrothermal system will transfer water from the deep layers to the superficial layers; the warmed discharged water will still be colder than the receiving environment. There will therefore be a localized cooling ("plume") in the surface layer, which will spread out in the direction of the current while descending towards the colder and denser zones. As a result, the intermediate layer (metalimnion) volume increases and the lower layer, the resource, decreases. Thus, in a three layer-system such as this one, the lake thermal energy will expand mainly through increase in the "warm" intermediate volume to the detriment of the cold lower volume.
Locally, the cooling observed near the discharge (2 m downstream) is frequent throughout the warm period, with an amplitude of 2 to 5°C, as shown in Fig. 6 for a typical summer week. The temperature difference of the water columns located at the discharge and at the reference point, established a few hundred meters upstream, was measured at different depths. The thermal influence of the discharge is effective only on the layers below 4 m depth, and is all the stronger as the temperature stratification in the upper layer is low. As soon as the discharge power exceeds about 2 MW (10% of the rated power of the system), the temperature drop of the receiving medium remains stable as can be seen in Fig. 7.
Fig. 6: Thermal effects of discharge at different depths relative to a reference column a few hundred meters upstream;
typical week in 2010, hourly data (Viquerat 2012).
Fig. 7: Cooling at 2.25 m downstream of the discharge for 3 layers located at 1.3 and 4.5 m depth with respect to a reference column located a few hundred meters upstream; hourly data April-July 2010 (Viquerat 2012).
The cooling effect of the layers is totally absent at a depth of 1m, very little present at 3m and very marked at 4.5m. Farther from the discharge, within a radius of 50 m, the induced thermal effects are most of the time masked by the spontaneous thermal fluctuation of the medium, even if cooling of up to 3-4°C can occasionally be measured but rare.
In winter, the role of the lake is to maintain the loop temperature at the operating conditions of the connected systems, typically above 6°C for heat pumps to avoid the risk of freezing and below 12°C to supply the data centres, whose exhaust system is sized for such a temperature. If there is too much heat extraction and the heat exchange with the ground along the route is insufficient, a supply of lake water to the loop will bring it up to about 6°C, and the discharge will then be a little colder, about 2°C.
In the opposite case, where the excess heat input in the closed loop results in a temperature that is too high, a supply of lake water will bring it down to the range desired by the cooling needs, so that the corresponding discharge temperature will be higher than that of the lake and close to 10°C. The situation is therefore more complex than in summer and will depend mainly on the ratio between the cooling and heating demand, as the volume of water exchanged with the lake is reduced. For now, GLN's case study indicates a much lower solicitation of the system and therefore of the lake in winter, but this conclusion cannot be generalized to systems for which winter use would be more important.
Chemical effects
The chemical quality of the water was also intensively monitored, as the potential effects of the system on the natural dynamics of parameters fundamental to the equilibrium of the lake ecosystem remained unknown: local modification of pH, remobilization of phosphorus transferred from the deep layers to the surface layers, residual active chlorine and turbidity variation. These measurements were carried out monthly (April to October) from 2006 to 2010, on the water flowing through the system (pumping, pumping station and discharge), as well as in the lake (discharge area and reference).
Although the chemical quality of the water is sometimes affected, it remains most of the time within the legal or recommended limits.
Impacts on the current ecosystem
Based on the monitoring of macrophytes and benthic macrofauna (GREN 2010) as well as on legislation related to ichthyofauna (LEaux 1991; OEaux 1998), the potential impacts on these organisms were judged to be low overall. Only the benthic macrofauna is likely to be affected, but its monitoring has not revealed any perceptible impact. The potential impact of summer cooling is a priori very low on the ichthyofauna and macrophytes: the discharge site is a pike spawning area, but the spawning period (February to May) has passed and juveniles are no longer as sensitive to a rapid change in temperature as they were just after spawning (Bruslé et al. 2001).
Regional hydrothermal potential
Beyond the local effects linked to particular thermal discharges, the intensification of the hydrothermal sector requires a discussion on the scale of the regional hydrological system.
The available "cold" resource is very important: the volume of water in the Petit-Lac located at a depth of less than 40 m is already worth 0.95 km³ (Viquerat 2012); with 1000 hours of full load operation in summer, this represents a possible rated flow rate of 1 million m³/hour or a power of 5 GW for 5 K of temperature increase between the extraction and the discharge. These power levels are one to two orders of magnitude higher than the regional demand, and take into account only a small part of Lake Geneva. In winter, the lake is destratified and the energy available is even higher.
In terms of environmental impacts, the maximum usable quantity possible without damaging the lake in a detrimental way (a term that should be specified) is very difficult to estimate, but is certainly important (Viquerat 2012). Before embarking on such an estimation, which would require the study of the "uses-effects-impacts" chain and the definition of a maximum admissible response from the environment, we propose to take into consideration two other aspects that put this type of approach into perspective:
i. A realistic limit of the regional demand which could be met by such systems, their future evolution and the relevance of the use of hydrothermal systems to fulfil them, a point that will also be addressed in the next chapter for cooling. Indeed, this type of system is on the one hand mainly valued in summer and, on the other hand, the thermal effects in winter are more poorly ascertained, but probably less important.
ii. Impacts on other aquatic systems avoided thanks to the substitution of a large quantity of electricity (thermal or radioactive pollution from nuclear power plants, thermal impacts on the Rhone and Lake Geneva from the turbining of reservoirs upstream of the lake, change in the regime of rivers downstream of these reservoirs, etc.), a point that will be outlined later.
Potential for the cold valorisation of lake water
As an example of estimation of the regional potential for the valorisation of the freshness of the water of the Lake, we will develop the case of the canton of Geneva, which should represent a small half of the total potential of valorisation around Lake Geneva.
Cooling demand in the Canton of Geneva
The evaluation of the cooling demand at the level of the Canton of Geneva was carried out based on summary sheets of cooling requests authorisations submitted to the Cantonal Energy Service for the period 1980-2009 (i.e. since the introduction of the procedure). These applications cover cooling requests for comfort as well as for data centers, excluding mobile devices. They do not include commercial and industrial cooling systems.
The available data are the year of the request, the air-conditioned surface, the thermal power, the electrical power, the estimated annual electrical consumption and the address of the installation.
When data is missing, the overall values (at the level of the entire park) were reconstructed by statistical analysis (Hollmuller et al. 2011). In addition, data concerning the name of the owner and/or the type of buildings to be cooled made it possible to assign (as systematically as possible) a typology of use to each facility.
It should be pointed out that the data from authorizations do not necessarily correspond to the field reality, given that: (i) it is well known that a large number of installations (in principle small in size) are installed without authorization; (ii) the data transmitted at the time of the request for authorization does not necessarily correspond to the resulting system; (iii) the database does not include installations prior to 1980, nor the decommissioning of installations that have become obsolete.
For the city centre, analysed in more detail, some of these inconsistencies were revealed by comparison with field data in our possession (Faessler et al. 2011). In addition, a quality control of data entry was performed on all installations above 1000 kWth and on a random selection of 50 installations below 1000 kWth, for which the error rate was about 5%, mainly related to the addresses of the installations (Tschopp 2011).
At the global level, the analysis of this database allows the following observations (Fig. 8 and table 2):
Fig. 8: Evolution of air-conditioning requests in the Canton of Geneva, cumulative values.
Table 2. Air conditioning requests by type of use, cumulative period 1980-2009, Canton of Geneva and zoom on the zone of influence of the future hydrothermal network of the city center. Ninst: number of installations ; Pth: thermal power ; Pel:
electrical power ; Eel: electrical energy; Sclim: air-conditioned surface
• In 2009, the 911 listed installations represent a rated thermal capacity of 272 MWth. In electrical terms, it corresponds to it a rated power of 91 MWel (or 18% of the peak power at the canton level) and an estimated annual consumption of 225 GWhel.
• This situation is the result of a steady growth in the number of requests since 1980, with an acceleration from the beginning of the 2000s (note that the particular peak of the year 2000 comes from two authorizations for large datacenters of more than 15 MWth each). Thus, over the entire period, the average growth is 9.4 MWth/year (3.9 MWth/year over the period 1980- 1999, 14.1 MWth over the period 2000-2009).
• The cooled surface area represents around 1.8 million m2, i.e. a little more than 10% of the total surface area of non-residential premises in the canton.
• Only 5% of the requests concern installations of more than 1000 kWth, but their cumulated power represents more than half of the total conceded power. At the other end of the scale, more than half of the requests concern installations of less than 100 kWth, whose cumulated power represents less than 10% of the total power.
These results have been confirmed by the analysis of the correlation between the electrical load curve of the Canton of Geneva and the weather temperature, in the summer period, in order to extract the component related to comfort air conditioning (Hollmuller et al. 2011). The results show that the electrical power related to comfort air conditioning is currently around 50 to 60 MWel (depending on whether the analysis is based on average or peak daily power), with an increasing trend of around 2 MWel per year. This order of magnitude is consistent with the value of 63 MWel obtained through cooling permits (subtracting the contribution of data centers, whose power demand depends only slightly on the outside temperature). While errors certainly remain on a case-by-case basis of the installations, the statistical results that can be extracted from the database of cooling authorizations thus seem robust, at least at the level of comfort cooling.
Potential for connection to future hydrothermal networks
Thanks to its insertion in the geo-referenced information system of the Geneva territory, the above database is used to evaluate the cooling demand that can be substituted by the two hydrothermal networks under project (Hollmuller et al. 2011). We will focus here on the urban network, which should irrigate the entire city center on both sides of the lake (Fig. 9). The statistical analysis of the air- conditioning permits for the area in question gives the following results:
• The area in question represents less than 2% of the territory of the Canton (4.6 million m2), but houses 29%of the surface area of non-residential premises (4.9 million m2).
• Similarly, 29% of the Canton's air-conditioning power is concentrated there (78 MWth), on 26% of the air-conditioned surface area (1.8 million m2). This represents an air conditioning rate for non-residential premises of 9.4%, very close to the Canton's value.
• Thus, for all types of use, the nominal air-conditioning power of this area is on average about 18 Wth per m2 of territory.
• Finally, the evolution of authorization requests over the period 1980 - 2009 follows the same dynamics as for the Canton as a whole (Fig. 8), with growth in two phases: 1.3 MWth/year until 2000, 3.4 MWth/year after 2000 (for an average growth of 2.8 MWth/year over the 30 years).
Based on the above installed capacity and growth rates (and with no change in the built-up area), the following overall trends for downtown air conditioning demand over a 20- to 40-year time horizon are obtained:
• With a low growth rate of 1.3 MWth/year, demand would reach about 104 MWth in 2030 (13%
of non-residential premises), respectively 130 MWth in 2050 (16%).
• With the current growth rate of 3.4 MWth/year, it would be about 146 MWth in 2030 (18% of non-residential premises), respectively 214 MWth in 2050 (26%).
• Even with a hypothetical accelerated growth, we would certainly remain well below the air conditioning saturation of non-residential spaces by 2050 (all the more so since it would then be a question of considering an evolution of the logistic function type).
Fig. 9 Permissible air-conditioning thermal capacity in the city center, by sub-areas, as well as area of influence of the urban hydrothermal network project (black line).
In terms of actual potential for connection to the future hydrothermal network, this gross demand must however be filtered by a certain number of technical, economic and organizational constraints.
First of all, we are dealing with commercial and multi-user network, on which connections will be made based on canvassing potential customers, with pricing and green image issues at stake. This issue is all
the more crucial since in most cases it involves replacing the use of existing cooling facilities. In this respect, and as for the canton as a whole, the very numerous installations of less than 100 kWth, which are economically unattractive, represent less than 10% of the total air conditioning power, while installations of more than 1000 kWth represent about half of this power. While it is therefore essential to ensure the connection of large customers before the start of the project, it is illusory to want to connect the entire air-conditioning demand of the area.
Moreover, with a view to the development of renewable energies at reasonable cost, full coverage of peak air-conditioning power is not necessarily recommended. Indeed, and as for most renewable energies which are capital intensive, a reduced infrastructure makes it possible to cover the essential part of the basic demand, the peaks being able to be covered by a traditional back-up system (this all the more so as cooling machines are already present at the customers' premises). To illustrate this point, we cite the example of one of the large customers of the GLN network (3.72 MWth peak / 2.15 GWhth annual), for which the analysis of the load curve shows that the ribbon power of the datacenter (130 kWth) represents only 3% of the peak power, but already covers 40% of the annual energy demand! Even including a share of comfort air conditioning, more than 80% of the cooling energy can be covered with 50% of the peak power.
In conclusion, we can estimate the potential recoverable by lake hydrothermal systems in the medium term for the canton of Geneva in a range of 50 to 100 MWth, i.e. a nominal flow of about 10,000 to 20,000 m³/h or 3 to 6 m³/s, far from the depletion flow of the deep layer (50 to 100 times less than the estimate made above for the Petit-Lac alone). In comparison, the Rhone outflow has an average annual flow of 250 m³/s.
Comparison with the classic cold production process
In order to reposition GLN-type devices in the energy system, two simplified flow analyses were carried out during the summer on:
• A "classic" system of thermal electricity production and use of cooling units, with performance data from real measurements (Santamouris 2004; Adnot 1999; Faessler 2009b),
• A system directly using the deep waters of the lake, based on observations made on the GLN system (Viquerat 2012)
Represented graphically (Sankey diagram in Fig. 10), these flows make it possible to compare the energy efficiency of traditional chillers with networked systems by fixing the heat discharged from the building identical in both cases (equal performance).
Thanks to the direct use of a natural resource, excellent rates of energy efficiency are achieved with lake cooling, resulting in an overall electricity consumption for cooling networks that is ten times lower (compared to conventional cooling units with the same performance). Overall, heat emissions to the environment (air or water) are more than double in the case of cooling units, with different distributions between local and non-local emissions. This is mainly due to thermal power plants for the production of electricity, which discharge large quantities of non-recoverable heat into the environment.
The production of electricity via dams is not dealt with here, but would imply a proper assessment of the impacts on watercourses of peak daytime electricity consumption, given the specific needs of air conditioning. In the same way, and in view of the seasonality of comfort air-conditioning needs, the
development of heat-power coupling in the Swiss energy mix would not bring any advantage for these cooling services (temporal incompatibility).
At the level of local discharges, the effects on the air are quantitatively more important with conventional systems but could appear qualitatively less serious for the environment than discharges into the lake. In reality, two advantages of GLN-type systems contradict this second assertion:
• On the one hand, the removal of air-cooled towers from the cooling units avoids the risk of development of legionella, the bacteria responsible for Legionnaire's disease (Viquerat 2012), as well as the negative impact in terms of noise and urban climate.
• On the other hand, the thermal pollution caused to the lake by a GLN type system is of a very different nature. It should imply minor impacts if one confines oneself to a low use of the available resource.
Fig. 10: Simplified heat rejection balance for a standard cooling unit (top) and a lake water system (bottom) with equal performance (building exhaust heat set at 10 units).
Conflicts with the rational use of energy
The large-scale development of this type of network raises the question of possible competition with energy saving objectives and in particular, whether these networks encourage the deployment of initially unwanted air conditioning. In addition, the heavy investments made for this type of infrastructure may detract from other energy conservation programs or building envelope improvements. To evaluate these questions on the level of air-conditioning performance, two approaches are possible: pragmatic or idealistic.
In Geneva, in spite of a legal limitation of centralized air conditioning - need to provide proof of need - since 1986 a growth of 10 MWth/year has been observed over the last 30 years, even 14 MWth/year over the last 10 years, as shown above and in particular for large consumers.
The pragmatic approach would therefore be to accompany this continuous development by implementing cooling networks of a certain size, allowing the connection of large consumers, the use of a local renewable resource and a concrete reduction in electricity consumption. Moreover, when connecting buildings to a cooling network, internal improvements related to actions on building regulation are necessary (Mermoud et al. 2008). If GLN-type networks are developed, additional investments in energy savings will then be more effective in reducing the needs of unconnected buildings or in insulation to reduce winter energy consumption.
As for the idealistic approach, it would consist in wanting to invest all the available financial means in the renovation of buildings, in order to reduce the need for air-conditioning. The actors involved are then no longer the same and include the building owners. Although preferable in principle, this option comes up against the difficulty of convincing each owner to make the necessary investments, and remains without any possible action on the cooling needs of data centres or conference or show rooms.
In any case, the development of lake hydrothermal networks does not exclude, on the contrary, the implementation of energy efficiency measures in buildings.
Conclusions
A less carbon-intensive and more local energy system primarily results in the use of energy present in the local environment, inevitably causing local impacts. It is no longer a question of protecting the local environment against external aggressions but of integrating it into the industrial system. The notion of
"environmental protection" must be overcome for others, such as "sustainable development" or
"industrial ecology". This is a profound shift away from the dominant vision of the natural sciences, which will imply an interdisciplinary approach to these questions and a decompartmentalization of the state services that deal with them.
In this respect, the demand and therefore the medium-term potential for cooling buildings by lake hydrothermal systems is, for the canton of Geneva, well below the lake's resource. For a generalized use of heating via heat pumps, the situation is a little more complex and will depend mainly on the ratio between cooling and heating demands. GLN's experience feedback indicates for the moment a much lower winter load on the system and therefore on the lake, but this conclusion cannot be generalized to systems where winter use would be more marked.
Correct use of this resource to ensure the lowest possible impacts requires a better scientific knowledge of the lake system itself, since it serves both as a source and a heat sink. Conversely, the scientific monitoring and evaluation of a system of thermal use of lake water is an opportunity to deepen knowledge of these systems for the benefit of all.
Acknowledgements
Part of this work was carried out in the framework of the European project Concerto "TetraEner", Integrated Project FP6 Priority 6.1 "Sustainable Energy System". It was also co-financed in the framework of the Partnership signed between the University of Geneva and the Services Industriels de Genève.
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