Applied Thermal Engineering 27 (2007) 1750–1755 www.elsevier.com/locate/apthermeng

Coupling of geothermal heat pumps with thermal solar collectors Valentin Trillat-Berdal *, Bernard Souyri, Gilbert Achard LOCIE – Laboratoire Optimisation de la Conception et Inge´nierie de l’Environnement – ESIGEC, Universite´ de Savoie, Savoie Technolac, 73376 Le Bourget du Lac, France Received 20 January 2006; accepted 27 July 2006 Available online 26 September 2006

Abstract The study discussed relates to the design and development of a process consisting of combining a reversible geothermal heat pump with thermal solar collectors for building heating and cooling and the production of domestic hot water. The proposed process, called GEOSOL, has been installed in a 180 m2 private residence in 2004. This installation is the subject of long-term experimental follow-up to analyse the energy-related behavior of the installation at all times of the year. In addition, different configurations of this combined system (geothermal heat pump and thermal solar collectors) have been defined and will be simulated numerically using TRNSYS software. A comparative analysis of these different alternative versions will be conducted to determine the best configuration(s) of the GEOSOL process in terms of energy, economical and environmental performances. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ground-coupled heat pump; Borehole heat exchanger; Ground heat depletion; Thermal solar collectors

1. Introduction In recent years, under the impetus of a number of parties (politicians, associations, energy sectors, etc.), the renewable energy market has been growing. However, while in the public services sector, it can be seen that accounting for environmental factors is frequently a proactive act, motivation levels remain low in the general public. Nevertheless, there are energy solutions that are suitable for both sectors. In this way, the use of incident solar energy via thermal solar collectors and energy stored in the superficial layers of the soil (less than one hundred metres deep) via borehole heat exchangers (BHE) offer clear potential in terms of valorization both for private residences or small public buildings due to the suitability of the diffuse nature of these energies for scattered settlements, and for large public buildings, provided that the soil surface is compatible. *

Corresponding author. Tel.: +33 4 79 75 88 21; fax: +33 4 79 75 81 44. E-mail address: [email protected] (V. TrillatBerdal). 1359-4311/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2006.07.022

In this context, Agence De l’Environnement et de la Maıˆtrise de l’Energie (French Agency for Energy and the Environment) is contributing to the funding of a research and development project proposed by the laboratory LOCIE (Laboratoire Optimisation de la Conception et Inge´nierie de l’Environnement – Design optimisation and environmental environment engineering laboratory) de l’Ecole Supe´rieure d’Inge´nieurs de Chambe´ry de l’universite´ de Savoie (Engineers’College of Chambe´ry, University of Savoie) and three industrial partners: CIAT(1), Eco’Alternative(2) and CLIPSOL(3) which are respectively represented by Eric Auzenet (Research Engineer, [email protected]), Pierre-Albert Watier, (Director, [email protected]) and Philippe Papillon (Research Engineer, [email protected]). (1) CIAT – Av. Jean Falconnier, BP 14 01350 Culoz France (2) Eco’Alternative – 251, route de la Serraz ZI de la Plaisse, 73375 Le Bourget du Lac France (3) CLIPSOL – Parc d’Activite´s Economiques Les Combaruches 73100 Aix-les-Bains, France

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Nomenclature BHE borehole heat exchanger COP coefficient of performance HDPE high-density polyethylene

GCHP ground coupled heat pump SPF seasonal performance factor PSD plancher solaire direct (floor heating circuit)

The project set up, known as GEOSOL, consists of designing a system based on the combination of a geothermal heat pump and rooftop thermal solar collectors for optimum valorization of the incident solar energy, and thus contribute to improving energy efficiency and reducing the greenhouse effect in the dwelling. In fact, it should be possible to achieve improved energy integration between the renewable sources available and residences thermal requirements, while guaranteeing a satisfactory level of comfort and quality of use under all circumstances. The geothermal heat pump is used for heating and also for cooling a building via low-temperature delivery systems. The solar collectors are used to complete the system in three areas: preheating the domestic hot water, heating dwelling (via a direct solar floor option [1] or via doping of the temperature level of the cooling source) and thermal recharging of the soil during excess solar production periods. Therefore, it will be possible to recharge the soil in the summer with the heat pump in cooling mode and the solar collectors to maintain a satisfactory long-term performance coefficient of the system. The coupling of solar collectors to underground can also help to reduce the length of borehole [2] which can contributes to decrease the cost of the system. Renewable energies offer great prospects as demonstrated by the study of another combined heat pump –

thermal solar collectors system which has been the subject of a real site experiment in Lugano since 2000 [3]. 2. Principle of low-temperature geothermics Low-temperature geothermics are based on the use of the heat contained in the soil via embedded heat exchangers and heat pumps. The heat pumps increase the natural temperature of the soil (between 7 and 12 °C in France) to values of the order of 40 °C. They are generally combined with low-temperature delivery systems like floor heating to achieve the ‘‘Coefficient of Performance’’ or the COP of more than 3. They can also be reversible (heating mode or cooling mode) and thus be used for cooling in the summer. Heat is extracted from the soil by means of closed loop systems, which exchange heat with the soil via a fluid (water–antifreeze mixture) circulating in high-density polyethylene (HDPE) tubes. There are two types of exchangers: vertical exchangers and horizontal exchangers. Our study only relates to BHE (Fig. 1), as it makes it possible to make use of underground temperature stability which helps maintain a satisfactory COP value, unlike horizontal exchangers which are embedded at depths between 0.80 and 1.50 m, and in which the performances are directly related to local climatic conditions [4–6]. In

to heat heatpump pump Text

SOIL SOIL

FILLING FILLI NG MATERIAL MA TERIAL (ciment bentonite (ciment bentonitemixture mixture or sand) LIQUID COOLANT COOLANT (water anda (water and antifreeze) ntifreeze)

Tsoil

BOREHOLE between550 between 0 and and150 150 mm deep deep between 10 between 10and and2020 cmcm diameter diameter SINGLE U-PIPES SINGLE U-PIPESHEAT HEAT EXCHAN GER consisting EXCHANGER consistingof of HDPEtube HDPE tube

BOTTOM VIEW BOTTOM VI EW OF OF BOREHOLE EXCHANGER EXCHANGER WI TH A SINGLE U-PIPES U-PIPES

Fig. 1. Schematic of the concept of borehole heat exchanger.

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addition, the soil surface area mobilised by BHE’s is very low compared to horizontal ground heat exchangers. However, it should be pointed out that the cost of boreholes represents the major drawback of the BHE system. 3. Thermal depletion The term ‘‘thermal depletion’’ refers to the decrease in the average soil temperature in the vicinity of the BHE (distance of less than 10 m). This discharge is generated by extracting heat from the soil. In Europe, the utilization of BHE increases, especially in Germany, Austria and Switzerland. For example, only in Switzerland, more than 30 000 ground coupling heat pumps (GCHP) using boreholes have been installed [7] and an Estimate of Ozgener [8] indicates that there are over 140 000 GCHP in the USA. Nevertheless, the system of geothermal heat pumps is a relatively recent concept, and in spite of a number of operational systems already high in some countries, it can still be interesting to study the ground temperature decrease in the long-term, especially for small systems (little number of borehole) since they are most widespread, particularly for single-family houses. Our project represents the ideal opportunity to study this temperature decrease in more detail, both in theoretical terms using various publications or simulations carried out with TRNSYS software [9] and experimental terms through the installation on a private residence. Eugster and Rybach [10] conducted a series of measurements on a single, coaxial, 105 m deep BHE used to heat a single family house located in Zurich (Switzerland). The ground temperature at a distance of one metre from the BHE cooled down in the first two years of use. After ten years of use, a new stable thermal equilibrium is established between BHE and ground, at temperatures which are some 2 °C lower than originally. However, it is important to note that this study was conducted on a single BHE, resulting in relatively high natural ground recovery due to the infinite volume of ground around the BHE. Thermal ground

recovery is particularly recommended in the case of high concentrations of vertical exchangers on the same plot of land [11]. The time to reach a complete thermal recovery depends on how long the BHE has been operational. Principally, the recovery period equals nearly the operation period [10]. We also studied the thermal discharge from the ground with TRNSYS using type 557 modelling double-U BHE. Two BHE’s coupled with a heat pump were simulated with the following assumptions : heating required 6 months per year, inlet floor temperature and flow rate equal to 30 °C and 2000 kg/h respectively, heat pump operation 11 h per day and heat extraction from two boreholes 90 m long at a distance of 10 m. This simulation is relatively simplistic since it defines an inlet floor temperature equal to 30 °C throughout the total heating season, but it gives an initial indication of ground temperature variations. Fig. 2 shows that the thermal depletion from the ground is relatively rapid during the first years of heat extraction, and then decreases at an increasingly slower rate, but never achieves a steady state, even after 20 years of operation. The ground temperature difference between the end of the first heating season and after 20 years of operation is of the order of 2 °C. Therefore, these results confirm those obtained by Eugster and Rybach [10]. We can therefore conclude that ground temperature decrease is possible more or less in the long term, to a varied extent depending on soil characteristics, initial ground temperature, moisture content, building loads, borehole spacing, borehole fill material, etc. If this thermal depletion of the soil is too extensive, it may result in a decrease in the seasonal performance factor (SPF), which may render its long-term operation uncertain and insufficient longevity of geothermal heat pump systems. Our study aims to remedy this problem by finding a balance between thermal depletion and recovery in ground. This will be enabled by thermal ground regeneration via solar collectors during excess solar production periods and heat pump operation in cooling mode when the building is being cooled.

Average soil temperature (°C)

15 14 13 12 11 10 9 8 7 6 5 0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 Time in years

Fig. 2. Variations in average soil temperature in the vicinity of a borehole heat exchanger.

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4. GEOSOL project overview 4.1. Description of process The GEOSOL process (Fig. 3) is being testing under real conditions in a 180 m2 single family house (Fig. 4) at St Jean d’Arvey in Savoie (France) since the autumn of 2004. The main characteristics of the installation are as follows: two double-U borehole heat exchangers 90 m long; 12 m2 of rooftop thermal solar collectors, this surface area is oversized with respect to the domestic hot water requirements alone so that the excess solar energy production is routed to the floor heating or to the boreholes to favour thermal ground recovery; a reversible heat pump with an heating power of 15.5 kW (CIAT Aure´a ILA Z60); a 500 l combined solar/electrical hot water tank; a low-temperature floor distribution system; the water–antifreeze mixture (35% glycol solution, antifreeze to 18 °C) circulates throughout the installation. It is important to note that the circulation pumps only operate sequentially. Therefore, their electricity consumption is minimized and, in any case, less than or equal to solutions comprising circulation pumps coupled with three-way valves, which generate higher maintenance costs. A regulation system splits the operation of the installation into three main operating sequences described in the section below.

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The measurement system set up (circuit temperatures, hydraulic system flowrates, power consumption of the heat pump, . . .) enables precise tracking of energy flows (energy extracted from ground (heating mode) or injected in ground (cooling mode), solar energy available, energy consumptions for domestic hot water production, for heating and cooling mode). This data will be used to validate the models developed with TRNSYS, and subsequently to optimise the design and operation of the GEOSOL process.

Fig. 4. View of house used for the experiment.

Fig. 3. Principle diagram of GEOSOL process.

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4.2. Description of operating sequences During the first possible operating sequence, the solar collectors are involved in domestic hot water production and, in addition, the geothermal heat pump meets the building’s heating or cooling requirements. During the second possible sequence, the solar energy produced is injected directly into the low-temperature heating floor (direct solar floor heating principle) or, if the temperature level is insufficient, it may be used to supply the heat pump cooling source to improve its COP. Finally, the third possible operating sequence consists of recharging the soil via solar collectors or meeting the building’s cooling requirements by the reversible heat pump.

5. Theoretical approach TRNSYS software is widely used by the international scientific community in the field of energy applications and is based on interconnectable models known as ‘‘TYPEs’’. The TYPEs correspond to the modelling of the components of the system studied (BHE system, thermal solar collectors, etc.) or specific functions (weather file reading, etc.). The software includes a basic component library, but it is also possible to create new components or make changes to those that already exist. The modelling will make it possible to conduct a theoretical study on the various alternative versions of the GEOSOL process (see Table 1 below), in order to determine the best process configuration(s) representing the optimum technical and economical solution guaranteeing the commercial distribution of the system. This analysis will enable a competitive positioning of the installations under study with respect to conventional heating installations. So as to obtain a progressive comparative approach, initially, a geothermal heat pump only, which will serve as a reference for all the other alternative versions in the study, will be envisaged. On the other hand, the direct solar floor system only will also be taken into consideration so as obtain, with the above system, two diametrically opposed basic solutions, since one only uses energy from the Table 1 Overview of the various alternative versions studied Technical solution

Heating

Domestic hot water

Heat pump reference solution Intermediate solution GEOSOL solution

Heat pump

Electrical hot water system

Heat pump

Solar reference solution

Direct solar floor heating and energy supply

Independent solar hot water system and electrical supply Oversized independent solar hot water system and electrical supply Solar energy and electrical supply

Heat pump

ground, while the other only uses energy supplied by thermal solar collectors. In order to tend towards a solution where the combination of the two above systems is optimised, the following two intermediate solutions will be envisaged: – Intermediate solution: consists of juxtaposing a geothermal heat pump for heating and an independent domestic solar hot water system, with each component being designed in a completely independent manner. – GEOSOL solution: consists of a combination of a geothermal heat pump and thermal solar collectors for heating and domestic hot water production. Compared to the intermediate solution, the solar collectors circuit is connected to the floor heating circuit (PSD) and the BHE system in order to recharge the ground during periods of excess solar production. The purpose of the modelling will also be to study the various operating sequences of the GEOSOL process in detail. The purpose of simulating these sequences is to determine whether their respective operating time is significant and to evaluate their influence on the installation’s annual energy and environmental balance. In each sequence, sensitivity studies will be conducted to analyse the influence of the different parameters. Other sequences liable to improve the installation’s performance will also be envisaged and studied. 6. Conclusions The combination of renewable energies such as thermal solar energy and geothermal energy in a single system should make it possible to meet heating, cooling and hot water requirements, while guaranteeing a satisfactory level of comfort and quality of use under all circumstances. The project developed is indicative of the determination of the industrial partners working together to design new energy systems in line with current technical, economical and environmental constraints, aiming to reduce greenhouse gas emissions and, more generally, have sustainable operating system. The end purpose of the GEOSOL project is to offer an alternative technical solution which helps reduce operating costs compared to those generated by conventional solutions using fossil energies. In addition, our solution initially devised for private dwellings may be extended to collective dwellings and the tertiary sector. For these two types of application, thermal solar collectors could help reduce the number of boreholes and the investment cost of the installation. Acknowledgements This study was financially supported by the APS (Assemble´e des Pays de Savoie) and by the ADEME (Agence franc¸aise de l’Environnement et de la Maıˆtrise

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de l’Energie) and PUCA (Plan Urbanisme Construction et Architecture) within the scope of proposals for ‘‘Energy, environmental and sanitary quality: prepare the Building of 2010’’. References [1] P. Papillon, Contribution a` l’ame´lioration de la technique du plancher solaire direct – Analyse de la solution ‘‘dalles minces’’ et gestion optimise´e du chauffage d’appoint, MS Thesis in Civil Engineering and Housing Science, University of Savoie, 1992. [2] A.D. Chiasson, C. Yavuzturk, Assesment of the viability of hybrid geothermal heat pump systems with solar thermal collectors, ASHRAE Transactions 109 (2) (2003) 487–500. [3] D. Pahud, B. Lachal, Misure di un impianto di riscaldamenton con sonde geothermiche a Lugano, intermediate report, Energy research programme commissioned by the Federal Energy Board, Switzerland, 2002. [4] M. Reuss, B. Sanner, Design of closed loop heat exchangers, International Summer School on Direct Application of Geothermal Energy (2000) 147–156.

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[5] Dossier Ge´othermie, Revue bimestrielle Syste`mes solaires – L’observateur des e´nergies renouvelables, no. 148, 2002, pp. 20–60. [6] E. Bose, D. Smith, D. Spitler, Advances in ground source heat pump systems an international overview, in: Proceedings of the Seventh International Energy Agency Heat Pump Conference, 1:313:314, Beijing, 2002. [7] Presentation of low-temperature geothermics. Available from: