ANALYSIS OF 30 UNDERGROUND THERMAL ENERGY STORAGE SYSTEMS FOR BUILDING HEATING AND COOLING AND DISTRICT HEATING

Transcription

1 ANALYSIS OF 3 UNDERGROUND THERMAL ENERGY STORAGE SYSTEMS FOR BUILDING HEATING AND COOLING AND DISTRICT HEATING M. Philippe (A,C), D. Marchio (B), S. Hagspiel (B), P. Riederer (C), V. Partenay (C,A) Corresponding author: M.Philippe@brgm.fr (A) BRGM Geosciences for a sustainable Earth 3 avenue Claude Guillemin BP ORLEANS CEDEX 2 FRANCE (B) Ecole des Mines de Paris Center for Energy and Processes 6 boulevard Saint Michel PARIS CEDEX 6 FRANCE (C) CSTB Centre Scientifique et Technique du Bâtiment Department of Energy, Health and Environment / Renewable energy group 29 route des Lucioles BP SOPHIA ANTIPOLIS CEDEX ABSTRACT About 3 different underground thermal energy storage projects around the world have been compared with the aim to determine the best practices and the applicability in a French context. The following information has been gathered and analyzed: - design of the storage - sizing of equipments - global costs, energy performance and CO 2 emissions Additionally to an inventory of referenced projects, enquiries have been carried out with stakeholders, designers or operators. The results of these enquiries are classified and analyzed by categories: - storage types: ATES or BTES - production: with or without auxiliary heating - building type: residential, office buildings - storage recharge: solar active recharge or use of reversible systems for heating and cooling By analyzing these design and operation data, some trends have been obtained allowing to quickly estimate the design of UTES systems by this empirical approach.

2 1. BACKGROUND In France, heating and cooling of buildings represent about 34 % of the final energy consumption and 18.5 % of the CO 2 emissions. To satisfy these energy requirements with an acceptable efficiency while considering that the building can alternatively (or simultaneously for some types of buildings) consume or produce heat, one solution is to store the heat in the underground. In this underground storage thermal losses are minimized since the temperature level of the storage volume is close to that of the surrounding ground and thanks to the constant temperature of the soil during the year below a depth of about 1 m. The French GEOBAT consortium (CSTB, BRGM, CEP, LOCIE) is carrying out a project on the use of underground thermal energy storage (UTES) for heating and cooling of buildings in France. Several projects had been carried out in France until the early 8 s but this research had not been continued after post oil crisis. In other European and North American countries UTES systems have still been installed and a high level in their design and operation has been reached. The GEOBAT project aims in facilitating the introduction of UTES in France. This goal shall be reached with a global approach analyzing this technology from technical, economical, regulatory and practical points of view and suggesting solutions that are appropriated to the French specificities. The project is divided into four main parts: benchmarking of existing UTES systems, with a main focus on BTES, detailed analysis of these systems, estimation of the potential in France, proposal of appropriated solutions for French typical buildings. In order to obtain technical and economical data of different UTES projects around the world, enquiry templates have been prepared and sent to the designers of the energy systems. The main analysis of this paper will be the comparison between the data of some representative borehole thermal energy storage (BTES) projects around the world. A methodology to analyze the data and to check the coherency of the values is developed. Comparisons of the data are then made in particular concerning sizing, energy performance and economical criteria of the different projects. With these enquiry data, trends are obtained in order to quickly size a storage volume or the necessary power of the heating systems for typical buildings. 2. PROJECTS REFERENCING AND ENQUIRY RESULTS Detailed enquiry templates have been prepared requiring data related to the geology of the ground storage, heat pump characteristics, ground heat exchanger geometry as well as operation and investments costs of the plant. Before inquiry, data have been collected through papers and publications to facilitate the answer. An overview of the key figures of the different projects is presented in Table 1. The projects are classified in a table following these categories: - with auxiliary heating or cooling systems and solar collectors (green) - with auxiliary heating or cooling systems and without solar collectors (orange) - with solar collectors and without auxiliary heating or cooling systems (yellow) - without solar collectors and without auxiliary heating or cooling systems (blue) In each category, the projects are also split by a bold line among the type of building (office or residential).

3 N = Location Country Type of building Heated floor Refrigerating power Number of Active length of Heating power (kw) area (m²) (kw) BHE the BHE (m) 1 Langen Germany Office Lucerne Switzerland Office Neckarsulm Germany District heating Crailsheim Germany District heating Truro Canada Prison Lugano Switzerland Single family house no cooling Sopot Poland Hospital (3 with the boiler) Münster Germany Office Ulm Germany Office Athens Greece Office Lincoln USA School (181 with the boiler) Melle Belgium Office 27 5 (2 with the boiler) 378 (12 with the refrigerating machine) Attenkirchen Germany District heating Aachen Germany Office Donaueschingen Germany Office Schöffengrund Germany Office no cooling Stuttgart Germany Office Markham Canada School Pylaia Greece Town Hall Wollerau Switzerland Office Stadl Paura Austria Office Onamia USA School ,2 23 Vestal USA Office , ,2 24 York County USA Office , Oslo Norway Office, hotel Setubal Portugal University Sabadell Spain Office Montreal Canada Store Lancaster USA Restaurant Wysox USA Hotel ,1 3 91,4 Table 1 : Extract of enquiry results of 3 projects The coherency of the different data has been checked through typical values of sizing. Energy requirements The heat and cooling demands have been compared against each other, the values of the different projects are plotted in Figure 1. Energy [kwh/m²] 3 25 Heating Cooling Installation number Figure 1 : Energy demands for the different projects These values are typical for buildings: between 2 and 28 kwh/m² for heating and between 1 and 1 kwh/m² for cooling. Values lower than 5 kwh/m² for heating represent new buildings with high energy standards. For some installations, the energy needs are unknown or have not been obtained within the inquiry. In that case, a rough estimation is proposed to complete the data base and to expand the comparison panel. Assuming the nominal heating power equal to the maximal thermal losses, it is proposed to evaluate the missing heating demands using the degree days method : - calculation of the heat loss coefficient U loss (in W/m²K): Pheating U loss = (T T )S set basis

4 with: S: heated area [m²] T set : indoor setpoint temperature (18 C) T basis : outdoor design temperature, given in tables of the ASHRAE Handbook [1] for each town [ C] - calculation of the annual thermal losses (in Wh/m²): E heating = 24 DD with: DD a Tnh : the annual degree days in T nh basis, given by an online software provided by Weather Underground [2]. T nh : maximum external temperature for heating (assumed to be 12 C for office buildings and 16 C for residential applications) The data base has been updated using these calculations. Figure 2 shows the completed energy demands for the different installations. The evaluated values are coherent with the given values. Energy [kwh/m²] 3 25 Tnh a U loss Heating Cooling Installation number Figure 2 : Energy demands for the different projects after completion by the degree days method The heating demands of the project number 28 are not calculated because the nominal heating power of this building is unknown. Sizing of the borehole heat exchangers The specific power of the borehole heat exchangers (in W/m) of all projects is plotted in Figure 3. It is obtained from the nominal power at the evaporator side of the heat pump, equal COP 1 to Pground = Pheating (with P ground on the evaporator side and P heating on the condenser COP side in case of space heating). This power is divided by the total length of borehole. Specific power (W/m) Installation number Figure 3 : Specific power of the borehole heat exchangers

5 These values can be compared with the values of the German guideline VDI 464 [3]. This guideline, developed for residential applications with single boreholes, gives values between 2 and 1 W/m depending on the thermal conductivity of the soil and the operation time of the heat pump. The diversity of the values is in particular due to the diversity of climates with average outdoor temperature and then very different underground temperatures. Definition of harmonised system layouts The different BTES projets are classified in 4 categories depending on the conception of the system (Recharge of the BTES storage by solar collectors, use of auxiliary heating systems or not), as shown in Table 1 by the different colors. For each type of project, a simplified layout is drawn (from a real and representative plant) to illustrate the operation of the system. Project n =2: Crailsheim Project n =8: Münster Project n =13: Attenkirchen Project n =2: Pylaia Figure 4 : Diagrams of installations of each category (with solar collectors or not, with auxiliary heating/cooling system or not) As can be seen in the examples, there is not a general rule for system conception with borehole thermal energy storage. Different combinations are possible and the choice of the configuration depends on several parameters such as building type, geology of the soil, cost optimization, CO 2 content in the electrical energy

6 3. COMPARISON OF THE DATA SOME TRENDS With the data obtained through the enquiries, some values are compared and trends are searched. Performances of the heating and cooling systems In Figure 5, the annual coefficients of performance in heating and cooling mode are plotted (only for the projects where these figures were given). Annual COP 7 6 Heating Cooling Installation number Figure 5 : Annual coefficients of performance of the different installations The values of annual coefficients of performance for heating and cooling are quite good (about 4 for the majority of the projects). Some performances (not represented in Figure 5) are better in cooling mode thanks to the operation of the majority of these systems in direct ground cooling (8 installations: 1, 5, 9, 12, 14, 15, 17, 21). The performances of these systems are not plotted in Figure 6 because the annual electrical consumptions of the feed pumps are unknown and are assumed to influence significantly the COP. Sizing of a borefield (total length of the heat exchangers) In Figure 6, the nominal heating or cooling power (the maximum figure of both has been chosen for the plot, depending mainly on building type and climate) is plotted with the corresponding total length of the heat exchangers for the installations of power lower than 1 MW. Power (kw) 1 Max(Heating Power, Cooling Power) (kw) Total length (m) Figure 6 : Maximum power (for heating or cooling) as function of the total length of boreholes

7 The linear regression line in Figure 6 allows to evaluate the sizing of a BTES and to estimate its costs (which depend strongly on the total length of boreholes). The following equation is given to evaluate the total length of boreholes compatible with Figure 3: P [ W ] = L [ m] max total This rule gives not a design value but a first idea of the total length of boreholes (the next step is to define the distance between boreholes and thus the storage volume) which can help the designer to evaluate the interest of a such BTES project considering the necessary heating or cooling power. Using the simple ratio 7 /m (average value in France), it is possible to have an idea of the investment. 4. PROSPECTS AND FUTURE WORKS This first part of GEOBAT project provided some elements of comparison between BTES projects. Four different typical schemes have been identified, the average coefficient of performance of the projects is around 4 and an estimated value of about 55 W/m has been obtained for sizing (as a starting point for further considerations). The main difficulty related to the comparison of the different projects is due to the diversity of the given information. It could be of common interest for the research community to agree on a standardized information template to be filled in for every European BTES projects with the most important data of the installations. Such an enquiry template is proposed in appendix A, it concerns general data, heating system, underground storage and properties of the soil. Appendix A: Enquiry template for BTES projects General Data Heating/cooling system Underground storage Properties of the soil Location Type of building Heated floor area Energy demands (for heating and cooling) Heating nominal power Cooling nominal power Part of auxiliary heating or cooling systems Nominal coefficients of performance (COP) Nominal Energy efficiency ratio (EER) Seasonal performance factor (SPF) for heating and cooling Number and type of boreholes Length of each borehole Spacing between the boreholes Geometry of the storage (square, rectangle, circle ) Temperature of storage (at the center of the storage volume, at the end of the storage period each year) Annual stored energy Annual draw-off energy Thermal conductivity Thermal capacity Acknowledgments: The authors want to thank French Agency for Energy and Environment (ADEME) for financial support of this study.

8 References [1] ASHRAE Handbook: Fundamentals, 1997 [2] Internet website of Weather Underground: [3] Verein Deutscher Ingenieure: Guideline VDI 464 Thermal Use of the Underground, September 21 [4] Operating Experiences with Commercial Ground-Source Heat Pump Systems, ASHRAE, 1998 [5] Rafraîchissement par geocooling: Bases pour un manuel de dimensionnement, Pierre Hollmuller, Bernard Lachal et Daniel Pahud, Université de Genève, 25. [6] Internet website of the european project Groundreach : [7] Monitoring and Data Analysis of two Low Energy Office Building with a Thermo-Active Building System (TABS), D. Kalz, J. Pfafferott and S. Herkel, 4 th European Conference on Energy Performance & Indoor Climate in Buildings, Lyon, 26. [8] Solar district heating with seasonal storage in Attenkirchen, M. Reuss, W. Beuth, M. Schmidt, W. Schoelkopf, ECOSTOCK Conference, 26. [9] Der Erdsonden-Wärmespeicher in Crailsheim, D. Bauer, W. Heidemann, H. Müller-Steinhagen, 17. Symposium Thermische Solarenergie, Kloster Banz, Bad Staffelstein, 27 [1] Das Low Energy Office der Deutschen Flugsicherung in Langen mit geothermischer Wärme/Kälte- Speicherung, E. Mands, B. Sanner, M. Sauer und W. Seidinger, UbeG. [11] Monitoring Results and Operational Experiences for a Central Solar District Heating System with Borehole Thermal Energy Store in Neckarsulm, J. Nussbicker, W. Heidemann, H. Müller-Steinhagen, ECOSTOCK Conference, 26. [12] Bau und Betrieb des Erdsonden-Wärmespeichers in Neckarsulm-Amorbach, J. Nussbicker, D. Mangold, W. Heidemann, H. Müller-Steinhagen, 24. [13] Ground Heat Exchanger of Pylaia - Energy Evaluation after the First Year of Operation, N. Kyriakis, A. Michopoulos, K. Pattas, 22. [14] Etude pilote pour le stockage diffusif des bâtiments du centre D4 de la suva a Root (Lucerne) Analyse de 2 tests de réponse géothermique et intégration du stockage dans le système, Programme Stockage de chaleur, D. Pahud, 21.