Underground Thermal Energy Storage for Efficient Heating and Cooling of Buildings Marcel Hendriks1,3, Aart Snijders1, Nick Boid2 1

IFTech International BV, de Gewanten 8, 6836 EB Arnhem, Netherlands 2 IFTech Ltd, 62-68 Rosebery Avenue, London EC1R 4RR, UK 3 IFTec GeoEnergía SL, C/Doctor Esquerdo 10 4º centro, 28002 Madrid, Spain

Abstract. Underground Thermal Energy Storage (UTES) systems and Ground Source Heat Pump (GSHP) systems use the underground for exchange of thermal energy (heat and “cold”) for efficient heating and cooling of buildings. The application of GSHP systems is based on the natural ground temperature. The GSHP extracts heat from the ground in winter and injects heat into the ground in summer. The application of UTES systems is based on the storage of heat and “cold” in the underground for later use. The stored thermal energy can be used for direct heating or cooling, but can also be used in combination with a heat pump. In general two types of UTES can be distinguished: ATES (Aquifer Thermal Energy Storage) and BTES (Borehole Thermal Energy Storage). GSHP and UTES systems are applied in various European countries. In some countries these systems are already considered as a standard design option for heating and cooling. In other countries the application of the technologies is quite recent. The application of GSHP’s , ATES and BTES is quite different for the various countries considered in this paper (Belgium, Denmark, Germany, Netherlands, Spain, Sweden and the UK) . Some of these differences can be explained by climatological conditions and underground conditions. However, the presence of clear energy efficiency targets for buildings and the availability of the GSHP and storage technologies on the market seem to be the major explanation for these differences. Keywords: Efficient heating and cooling, ATES, BTES, Energy saving, Reduction in Carbon Dioxide Emission

1

Introduction

Underground Thermal Energy Storage (UTES) refers to the use of the ground for storage and exchange of heat and “cold” for the purpose of providing efficient heating and cooling for buildings. It has been demonstrated as a viable heating and cooling system for residential, commercial and institutional buildings throughout Europe and North America. UTES has a wide range of applications for efficient heating and cooling of buildings. In the Netherlands over 500 projects have been realised in which UTES is applied. In other countries, like the United Kingdom and Spain, the implementation of UTES for heating and cooling of buildings is new. However, because of the need for energy saving and reduction in CO2 emissions the interest is growing, resulting in the realization of the first projects in these countries. In this paper we will present the status of UTES in various European countries with some examples of typical applications.

2

GSHP and UTES Technologies

2.1

Ground source Heat Pumps

A ground source heat pump (GSHP) is a heat pump that uses the ground as either a heat source, when operating in heating mode, or a heat sink, when operating in cooling mode. For the exchange of thermal energy the GSHP is connected to the ground with a loop. The most common connection is a closed loop, existing of U-tubes of high density polyethylene inserted into boreholes of 50 to 200 meters deep. A less common design is the direct use of water from an aquifer (often called an open-loop system). One or several wells supply the water necessary for a GSHP application, a similar number of wells would be used to inject the water. The application of a GSHP system is based on the natural ground temperature. The GSHP extracts heat from the ground in winter and injects heat into the ground in summer. 2.2

Underground Thermal Energy Storage

Whereas a GSHP extracts or injects heat, is UTES based on the storage of heat and “cold” in the underground for later use. In most cases UTES is applied as a seasonal storage. The stored energy can be used for direct heating or cooling, but it can also be used in combination with a heat pump. In general two types of UTES can be distinguished: 1) ATES: Aquifer Thermal Energy Storage 2) BTES: Borehole Thermal Energy Storage 2.2.1 ATES An ATES system is a large open-loop system optimized and operated to realize seasonal thermal storage, i.e. by reversing extraction and injection wells seasonally. The principle is shown in Figure 1. In summer, groundwater is extracted from the cold well(s) and used for cooling purposes. The warmed up water is injected in the warm well(s). In winter the process is reversed. Water is pumped from the warm well(s) and applied as a heat source, e.g. as low temperature heat source for a heat pump. The heat pump supplies (part of) the heating. The chilled groundwater is then injected into the cold well(s) again. With ATES no groundwater is discharged. All the water extracted from one well is reinjected in another well. This means that there is no net extraction of groundwater from the soil, which minimizes negative impacts on the environment. ATES systems require that relatively high well yields can be obtained on site. Because of this the applicability depends strongly on site-specific hydrogeological conditions.

Fig. 1. Principle of ATES. 2.2.2 BTES A BTES system consists of a radial, circular array of boreholes resembling standard drilled wells. Rather than penetrate the aquifer as in the ATES system, BTES is closed loop and after drilling, a plastic pipe with a “U” bend at the bottom is inserted down the borehole. To provide good thermal contact with the surrounding soil, the borehole is then filled with a high thermal conductivity grouting material. The principle is shown in Figure 2 en Figure 3. During winter the borehole heat exchanger is used for extraction of heat from the ground, e.g. as heat source for a heat pump. While the circuit water passes through the heat pump the temperature of the water cools down. The chilled circuit water is returned in the borehole heat exchanger and the ‘cold-energy’ is stored in the ground. In summer the flow in the BTES system is reversed. The stored cold is extracted and passed through a heat exchanger providing direct cooling to the building. When necessary the (reversible) heat pump can be put in use as peak load chiller as support in periods of peak cooling demand. The store circuit water will pick up energy from the building and thus be raised in temperature. This water, the temperature of which is higher than the ground temperature, will be returned in the borehole heat exchanger where the ‘warm energy’ is stored in the ground around the boreholes for the next heating season. Closed-loop BTES systems depend less on site-specific hydrogeologic conditions than ATES systems and are better suited for areas where relatively high well yields are not obtainable.

-

Heating

+

-

HE

Flow of heat

+

HP

HP HE

Cooling

HE

HE

Flow of heat

Fig. 2. Principle of BTES (HP = heat pump / HE = heat exchanger).

Fig. 3. Aerial view of BTES field and side view of single borehole.

3

Applications and experiences in various European countries

GSHP and UTES systems are applied in various European countries. In some countries these systems are already considered as a standard design option for heating and cooling. In other countries the application of the technologies is quite recent. Between the various countries applying GSHP and UTES systems already, there are significant differences in the number and type of applications (see Table 1). Table 1: Implementation of GSHP and UTES systems in various European countries

GSHP ATES BTES

3.1

Belgium

Denmark

Germany

Netherlands

Spain

Sweden

United Kingdom

•• •• •

••• •• -

•••• • •••

••• •••• ••

• -

•••• ••• ••••

•• • •

• ••

few applications

••• ••••

many applications

some applications

very many applications

Belgium

The acceptance of using underground thermal energy storage for applications where heating and cooling is required, is slowly forcing a way in Belgium without being a “booming” market. More than ten ATES systems are in operation. All large scale (> 500 kWcooling) and most of them are located in the Campine (region of Flanders). The applications are mainly related to combined cooling and heating of office buildings and hospitals. Due to the hydrogeological limitations, the most populated regions and cities of Belgium are not suitable for ATES. In this regions BTES could be applied. The interest in BTES applications is slightly growing with several feasibility studies underway and a few realized projects [1].

Photo 1: “Zonnige Kempen”, Westerlo (Belgium). Social housing project with BTES in combination with solar panels and asphalt collector

GSHP systems are applied for heating of single family houses all over the country. The number of installed heat pumps is growing. In 2000 less than 400 heat pumps were installed. In 2005 the number was increased to over 2,600 . 3.2

Denmark

By the end of 2007, over 1,000 GSHP’s will be operational in Denmark, as well as about 25 groundwater cooling projects [2]. As far as known, no BTES projects will be operational in Denmark by that time. The majority of the groundwater cooling projects provide direct cooling to industrial applications. In general, the warm groundwater is reinjected into the aquifer without thermal balancing. Recently, there is a growing interest in the application of ATES for the heating and cooling of buildings. The first project of this kind was operational by the end of 2007. The major reason for this increasing interest is the introduction of the European Energy Performance Directive for Buildings. Lack of awareness is considered to be the major bottleneck to the application of GSHP’s and UTES technologies in Denmark. 3.3

Germany

Only a few ATES projects have been installed in Germany. The number of BTES projects, however, amounts to several hundreds. The major application is for heating and combined heating and cooling of small commercial and residential buildings (BTES capacity in the range of 50 - 500 kW). A few BTES projects are applied for seasonal storage of solar heat at relatively high temperatures (60 - 90º C). The total number of GSHP applications in Germany is about 90.000 - 100.000. Heating and combined heating and cooling of single family homes are the major applications of this technology. About 15% of the GSHP systems are open loop systems, 85% are closed loop systems, either with vertical U-tubes (60 - 70% of the closed loop systems) or horizontal closed loop systems. As can be seen from Figure 4 [3], there is a strong relationship between the (increase of) the average annual oil price (blue line/right axis) and the number of GSHP systems installed per year (bar diagram/left axis). The high demand for GSHP systems in 2006 has resulted in a shortfall of drilling capacity in that year.

90 Quellen: Ölpreis: www.tecson.de Wärmepumpen: GtV, BWP

25.000

75

20.000

60

15.000

45

10.000

30

5.000

15

jährlicher Zuwachs an Wärmepumpen

2010

Jahr

2005

2000

1995

1990

1985

1980

1975

1970

1965

0 1960

0

durchschnittlicher Rohölpreis in US$ / barrel

Installierte Wärmepumpen pro Jahr

30.000

Jahresdurchschnitt des Rohölpreises

Fig. 4. Correlation between GSHP’s installed per year and average crude oil price [3]

3.4

The Netherlands

In the Netherlands, UTES started to be implemented in the early eighties. In first instance the objective was to store solar energy for space heating in winter. In the first project (commissioned in 1983) vertical soil heat exchangers were used (BTES application). Given the good experience with aquifer storage in later projects and the fact that in the Netherlands aquifers can be found almost everywhere, in particular the application of ATES has been further developed in the Netherlands. In 2005 the number of registered ATES projects was 537 [4]. In almost every major city a number of ATES projects are in operation. The aim of most ATES projects is to store cold in winter for cooling in summer. In general, cooling is direct, that is to say without using a chiller. In most projects the cooling capacity supplied from storage lies between 500 kWt and 2000 kWt. This means that by applying cold storage these projects economise on a large chiller [5]. Until 2000 most ATES applications were for individual buildings like offices and hospitals. However, since about 2000 ATES also started to be applied as a central (collective) system for a number of buildings, mixed developments and housing projects. At present several utility companies are offering their clients to supply heating and cooling with ATES based district heating and cooling systems, whereby the system is owned and managed by the utility.

600 2005

550 500

Agricultural

450

Industry

400

Housing Commercial

350

Hospitals

300

Offices

250

2003

0%

10%

20%

30%

40%

50%

200 150 100 50 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Total number

Annual number

Fig. 5. Number of ATES projects in the Netherlands and distribution per application area. There are several BTES project realized in the Netherlands. Most of them are for collective systems for housing projects. GSHP systems are mainly applied for heating or combined heating and cooling of single family houses or small buildings. For 2005 the estimated total number of installed heat pumps is between 30,000 and 31,000 [3]. About one third of these heat pumps are GSHP’s. Photo 2: “Oostelijke Handelskade”, Amsterdam (NL). Collective heat pump and ATES system for heating and cooling of mixed use development (passengers terminal, hotel, arts centre, offices and apartments).

3.5

Spain

In Spain the GSHP and UTES technology are very new technologies. Since a few years GSGP´s are starting to be implemented for heating and cooling. Several projects have been realized, mainly for small scale residential buildings and most of them in Bask Country and Catalonia. However, all over Spain a growing interest in renewable energies and the use of the underground for exchange of thermal energy is developing. It is expected that within a few years GSHP´s and UTES application will contribute to more efficient heating and cooling systems in the Spanish building industry. 3.6

Sweden

Sweden probably shows the highest density of GSHP systems for single family homes in the world: over 275.000 installed systems by the end of 2004. About 99% of the GSHP systems are closed loop systems, mainly vertical U-tube systems (heating only) [6]. The number of large scale ground coupled heat pump systems was about 2.000 by the end of 2004. These systems range from 100 kW to more than 5 MW and can be subdivided as following: • • •

large scale GSHP systems, mainly for apartment blocks and small housing developments; over 200 BTES systems, mainly for combined heating and cooling of commercial and residential buildings; over 50 ATES systems, mainly for combined heating and cooling of larger commercial buildings and mixed developments.

The major obstacles for the implementation of ATES and BTES systems are lack of awareness of the technologies, lack of experience with engineering and drilling companies, and the complexity of the permit procedure (ATES systems). 3.6

United Kingdom

In the United Kingdom (UK) GSHP application started in the early 90’s. By 2005 there were approximately 500 GSHP installations in operation across the UK. With an estimated increase over the last two years of 60%, this results in approximately 800 GSHP installations in the UK by 2007. The majority of these installations are monodirectional GSHP systems and are small scale (100 kW have been installed recently. There is only one known ATES system installed to date in the UK. The system is for a residential development in West London and has a storage capacity of 250 kW. The system was installed in 2006 [7]. By the end of 2007, there were a number of larger scale (>500 kW) ATES and BTES systems under development, and the level of interest

in UTES application is increasing. This is to a large extent attributable to recent sustainability requirements for larger scale new developments and retrofits. Photo 3: “Westway Beacons”, London (UK). Collective ATES system for the heating and cooling of 130 apartments.

So far, the heavy bias towards GSHP installations is due to the fact that the UTES technology is only just starting to enter into the UK market and thus is regarded as a “new” technology. The availability of suitable aquifers varies significantly in the UK and therefore certain areas are suitable for ATES systems and others areas are more favourable to closed loop BTES systems. London, the South East, Birmingham, Liverpool and East Anglia are examples of areas where open loop GSHP or ATES systems are viable. In the UK, the Environment Agency (EA) is the government body which regulates the groundwater industry. Any larger scale open loop ground source heating and/or cooling system has to go through the EA permitting procedure. The EA is becoming increasingly worried about net heating or cooling effects on the ground of GSHP´s and is therefore in favour of ground coupled systems like ATES and BTES, creating a thermal balance annually.

4

Conclusions

The application of GSHP’s , ATES and BTES is quite different for the various countries considered in this paper. Some of these differences can be explained by climatological conditions (combined heating and cooling is more favourable for the application than heating only) and underground conditions (ATES versus BTES application). However, the presence of clear energy efficiency targets for buildings and the availability of the GSHP and storage technologies on the market seem to be the major explanation for these differences.

References 1. Desmedt, J. Hoes, H. Van Baal J.: Status of Underground Thermal Energy Storage in Belgium. Ecostock 2006, Stockton, New Jersey (2006) 2. Sørensen, S.: Personal communication. EnOpSol, Hellerup (2007) 3. Reuβ, M.: Techniken der Oberflächennahen Geothermie. In: Oberflächennahen Geothermie OTTI, Freising (2007) 19-21 4. Centraal Bureau voor de Statistiek (National Bureau of Statistics): Duurzame Energie in Nederland 2005, Voorburg/Heerlen (2006) 5. Snijders, A.L: Lessons from 150 aquifer storage projects. Proceedings Terrastock, Stuttgart, Germany (2000). 6. Andersson, O.: GSHP Systems in Sweden. International Working Conference “Experience with Ground Source Heat Pumps”, SenterNovem Utrecht (2005) 7. Kennet, S: Trailblazing on the Westway. In: Building service journal 05/06, London (2006) 38-40