Sustainable
Geothermal
Power
A preliminary study on the sustainable use
of a geothermal power plant
Title: SUSTAINABLE GEOTHERMAL POWER; A preliminary study on the sustainable use of a geothermal power plant in the built environment Author: ing. R. van Pruissen (#0611059) rvanpruissen@gmail.com Education: Eindhoven University of Technology Department of the Built Environment Unit Building Physics and Systems Master Building Services Graduation: prof.dr.ir. J.L.M. Hensen committee Department of Built Environment Building Physics and Systems ir. P. Hoes Department of Built Environment Building Physics and Systems dr.ir. M.F.M. Speetjens Department of Mechanical Engineering Energy Technology Date: 12 May 2014
A
CKNOWLEDGEMENTS
This report is the result of the final project of my master study Building Services at Eindhoven University of Technology. In the relatively uncommon subject of geothermal energy, I found an interesting research topic involving both mechanical engineering (my bachelor study) and Building Services. During this research, I did not only learn a lot about geothermal energy in the built environment, but I also learnt a lot about myself and the people surrounding me.
I would like to express my gratitude to the many people who have assisted me along the way in any way. The following people I wish to thank explicitly. I would like to thank prof.dr.ir. Jan Hensen for his pragmatic advice and for accepting me in his professional and enthusiastic research group at a time when I needed it the most. Also, I would like to thank dr.ir. Michel Speetjens for his perseverance in providing me with valuable advice, especially on the topics of geothermal energy and system modelling. Many thanks to ir. Pieter‐Jan Hoes, with whom I determined the general research approach and who provided me with daily guidance in meetings that were both helpful and enjoyable. I would also like to thank dr.ir. Rinus van Houten and ir. Gert Boxem, who are not members of the final committee, but provided me with guidance and advice in the first part of this research.
Last but not least, I would like to thank my friends and family for providing me with a comforting environment and the necessary distractions, without which I would not have been able to get over this final hurdle. Especially Martine, who had to sit through a lot of complaining whenever the going got tough, patiently waited during the many study weekends and assisted me wherever she could.
Rick van Pruissen May, 2014
S
UMMARY
Although geothermal energy is considered a promising sustainable energy source, little is known about the sustainable use of a geothermal power plant in the built environment. Results from the research described herein, show that geothermal power could provide a considerable contribution to the sustainable energy supply of a built environment like the TU/e Science Park. A literature study into the concept of geothermal energy in general, and more specifically the use of geothermal energy in the built environment was performed. This literature study showed that geothermal energy is a promising sustainable energy source with a vast potential. Using the concept of enhanced geothermal systems (EGS), the number of possible locations for geothermal energy in the Netherlands can be greatly increased. Globally, the main applications of geothermal energy in the built environment are space heating, space cooling and electricity generation.
A computer model of an EGS system was used to assess the performance of geothermal energy in the built environment for an operational period of 40 years. With this model, 18 scenarios were compared, based on 3 energy demand projections and 6 energy supply variants for the Eindhoven University of Technology campus (TU/e Science Park).
The results of this performance assessment show that it is feasible to use an enhanced geothermal system to sustainably meet a considerable part of the energy demand of a built environment like the TU/e Science Park. The geothermal source could provide a valuable contribution to the total sustainable energy supply of an environment like the TU/e Science Park, especially in the first 20‐30 years of operation. The highest electricity production can be achieved when the geothermal source is used for electricity only. If a reduction in CO2 emission
between 30 and 40 % is required, a geothermal power plant is the best option. For a reduction between 55 and 70 %, both geothermal electricity combined with geothermal heating, and geothermal electricity with other renewables would be a good choice. The latter provides the option of increasing the CO2 emission reduction to approximately 87 % when geothermal
heating is added. However, for a fully sustainable energy supply to the TU/e Science Park in terms of energy use and CO2 emission, the energy demand should be reduced considerably and
a combination of several sustainable sources would be required. The generic model created in this research provides a tool to investigate the sustainable performance of geothermal energy for other combinations of energy supply and demand.
T
ABLE OF CONTENTS
Acknowledgements ... i Summary ... iii List of symbols and abbreviations ... vii 1 Introduc on ... 1 1.1 Geothermal energy ... 1 1.2 Research aim and questions ... 1 1.3 Research methodology ... 2 1.4 Outline ... 2 2 Geothermal energy and the built environment ... 3 2.1 Geothermal energy ... 3 2.2 Applications in the built environment ... 10 2.3 Concluding remarks ... 20 3 Enhanced Geothermal System model ... 23 3.1 Description of the used EGS model ... 23 3.2 Performance indicators ... 32 3.3 Model verification ... 37 4 Use case: TU/e Science Park ... 39 4.1 The campus ... 39 4.2 Current energy balance ... 39 4.3 Future and trends ... 43 4.4 Energy scenarios ... 47 4.5 Concluding remarks ... 51 5 Use case performance ... 53 5.1 Geothermal system ... 53 5.2 The built environment ... 60 6 Conclusions and future work ... 69 6.1 Conclusions ... 69 6.2 Future work ... 71 Bibliography ... 73 Appendices ... 77
L
IST OF SYMBOLS AND ABBREVIATIONS
Symbol Unit Description
C [J/K] Total heat capacity cp [J/kg.K] Specific heat capacity
D [m] Reservoir centre diameter dm [m] Well diameter used in the model d [m] Actual well diameter
ε [m] Well roughness
ηcarnot [‐] Carnot efficiency
fd [‐] Darcy friction factor
frec [‐] Geothermal reservoir recovery factor g [m/s²] Gravitational constant
H [J] Enthalpy
h [m] Well depth
Iwell [MPa.s/l] Well impedance per well
κ D Permeability
λ [W/m.K] Thermal conductivity L [m] Total model length Lres [m] Reservoir length
μ [Pa.s] Dynamic viscosity n [‐] Number of wells
ΔP [Pa] Pressure difference
Pel [kWe] Electric power Pelev [Pa] Well elevation head Pth [kWth] Thermal power
Re [‐] Reynolds number
ρ [kg/m³] Density
τ [yr] Characteristic time constant
Θ [‐] Volume fraction T [°C] Temperature
Tlow [K] Lowest temperature in the system
Thigh [K] Highest temperature in the system
Tsink [K] Temperature of the heat sink
t [s] Time
t* [yr] Reservoir break‐through time u [m3/s∙m2] Fluid (darcy) velocity field
U [J]
Internal energy of a system
V
gf [m³/s] Geofluid volume flow
Vr [m³] Reservoir volume
W [m] Total model width V [km³] Reservoir volume
ε [m] Well roughness
Tinit [°C] Initial reservoir temperature
Te [°C] Average temperature of the environment
Abbreviation Description Abbreviation Description
avg Average inj Injection
dem Demand prim Primary energy
el Electric / Electricity prod Production
eq Equivalent r Reservoir
g Granite ret Return
gf Geofluid ts Thermosyphon
ht Heat / Heating wh Wellhead
1 I
NTRODUCTION
Possibly the most important question of the 21st century is how to meet our energy demand. With rapidly developing countries like China and India, the amount of energy needed worldwide will keep on rising in the coming decades while fossil fuels, which constitute a major part of the global energy supply, are running out [1]. Additionally, the use of these fossil fuels has a large environmental impact because of harmful emissions and inherent risks. In
appendix I the problems associated with conventional energy sources are described in more
detail. To meet our energy demand in the future and sustain our way of living, other energy
sources are needed [2].
1.1 Geothermal
energy
Geothermal energy is a promising sustainable energy source [3, 4]. Geothermal energy literally means: the heat contained within the earth. The term ‘geothermal energy’ however, more commonly refers to the part of this thermal energy that could be recovered and exploited by man [5]. Apart from its vast energy potential, other advantages of geothermal energy include: continuous availability (high capacity factor), base load capability without storage/buffer, small footprint and low emissions [3]. This combination of characteristics makes it an interesting addition to the energy mix.
Recent technological developments in geothermal systems have made it possible to enhance existing rock volumes at depth, enabling the creation of reservoirs and drastically increasing the amount of possible locations for geothermal systems [3]. These so‐called enhanced geothermal systems (EGS) are still in the research phase, although the first commercial EGS plant went into operation in 2007 in Landau, Germany [6]. Main research topics of EGS are raising the flow rate while maintaining sufficient heat exchange surface area, and long‐term operability and sustainability [3].
1.2 Research
aim and questions
This research is a preliminary study into the operational feasibility of geothermal energy in the Netherlands. More specifically, the relation between the sustainability of an enhanced geothermal source and its implementation in the built environment is investigated. The aim of the research presented in this report is to answer the following question: What is the sustainable performance of an enhanced geothermal system that is used to meet the energy demand of a built environment like the TU/e Science Park? To answer the main research question, the following sub questions are formulated: 1. How is geothermal energy currently used in the built environment?
a) What are the main principles and characteristics of geothermal energy? b) What is the energy potential of geothermal energy?
1.3 Research methodology
2. How should an enhanced geothermal system be modelled to assess its long‐term
performance?
a) What are common modelling approaches and how do they compare?
b) What are the important modelling parameters of a geothermal source?
3. What is the performance of a geothermal system meeting the energy demand of TU/e
Science Park?
a) What is the energy demand of the TU/e Science Park?
b) How can this demand be met with geothermal energy?
c) What are relevant performance indicators of a geothermal system?
1.3 Research
methodology
A literature study is performed on the topics of geothermal energy and energy in the built
environment. The first part of the study focusses on the main principles of geothermal energy, its characterising properties and its sustainability. The study on geothermal energy in the built environment is performed to identify and characterise the possible applications of geothermal energy in the built environment.
As the sustainability of a geothermal source depends on a large number of variables, often interdependent and transient, an analytical approach is impossible. A generic computer model is developed which calculates the performance of a geothermal system based on a set of parameters defining the geothermal source and its energy load. A two‐dimensional (2D) transient model of a geothermal source is developed in COMSOL Multiphysics, which approximates the long‐term behaviour of a geothermal source for a given energy demand profile. The model is based on the enhanced geothermal source on research site Bad Urach, Germany. The generic nature of this model allows for easy application to other parameter sets. With the simulation results, the energy output is calculated and the system performance is expressed in several performance indicators, to make comparison possible.
The TU/e Science Park (the Eindhoven University of Technology campus) serves as a use case for a built environment. The energy demand of this campus is characterised based on available measuring data. The general properties of the campus and its energy use are investigated to allow for future comparison with other environments.
1.4 Outline
This report is organised as follows. In Chapter 2, a literature study on the topics of geothermal energy and energy in the built environment is presented. A theoretical model is then created in Chapter 3. In Chapter 4, the TU/e Science Park is defined as a use case. The performance of the use case is assessed in Chapter 5, where the results of the simulations are presented. Finally, in Chapter 6 the results are discussed and conclusions are drawn to answer the research questions.2 G
EOTHERMAL ENERGY AND
THE BUILT ENVIRONMENT
Assessing an energy source and its sustainability requires a good understanding of the general principles of the source, the load and the conversion. For this reason, the subjects of
geothermal energy (§2.1) and energy demand in the built environment (§2.2) are investigated. The results of this investigation are used for further research in Chapter 3.
2.1 Geothermal
energy
Geothermal energy has been used for a long time; usage for bathing, washing and cooking dates back to prehistory [7]. Since the first use of geothermal energy, the techniques have evolved from the simple usage of heat from geysers, to completely man‐made systems at depths of several kilometres producing electricity and heat for large industries and urban areas [5]. For decades, geothermal energy has played a major role in the energy supply in several countries around the world. Typically, current applications of geothermal energy occur in geographically favourable locations, i.e. places with a high geothermal temperature gradient and naturally occurring underground reservoirs.
To get familiarised with the subject and determine the characteristics and parameters of a geothermal source, the subject is investigated in this paragraph. The main topics are the basic principles of a geothermal plant (§2.1.1) and the potential of geothermal energy (§2.1.2). For a better understanding of the origins of geothermal energy and its vast potential, an introduction to the earth’s composition and plate tectonics can be found in appendix II. 2.1.1 Basic principles A geothermal system is used to extract the heat from the earth and consists of wells, pumps, a geothermal reservoir and a heat exchanger. The basic principle of such a system is as follows: the geofluid (water/steam) is pumped from one or more wells, energy is transferred to a secondary distribution fluid in a heat exchanger and the cooled geofluid is re‐injected into the ground in another well. This principle is illustrated in Figure 1. In some situations, the geofluid is used directly, without a secondary distribution system.
In an ideal system, all water re‐injected into the ground travels through a permeable layer to the production well(s), creating a closed loop. In reality, some of this water might be lost. This should be avoided to minimise operational cost and environmental burden. Research on the Soultz‐sous‐Forêts (France) test site has shown that geothermal systems without fluid loss are possible. The part of the permeable layer used to exchange heat is called the geothermal reservoir. The injected water is cooler than this layer and is heated along the way as it passes through the reservoir.
Thereservoir
A good geothermal reservoir has a sufficiently high output temperature and permeability, and is able to sustain these properties for decades. These properties determine the technical potential
2.1 Geothermal energy
of a geothermal system and with that its possible applications (see §2.2). Both location and depth of the boreholes have a large influence on these properties.
Figure 1: The basic principle of a geothermal system with typical dimensions. The geofluid is
pumped up from the left well, travels through a heat exchanger where it heats the secondary fluid
and is re‐injected in the right well [based on: http://www.aardwarmtedenhaag.nl/].
The temperature of the earth rises with increasing depth and the rate at which it does, the geothermal gradient, depends on the location. Down to accessible depths (<10 km), this geothermal gradient is 25‐30 °C/km on average. Also varying with depth and location is the composition of underground materials and with it its porosity and permeability. Porosity is the fraction between the empty volume (the pores) and the material volume. The extent to which these pores are interconnected is called permeability (κ). Related to the permeability is the hydraulic conductivity, which also takes into account the dynamic viscosity of the geofluid. The SI‐unit for permeability is m². A more common unit for permeability in aquifer or ground water flow is darcy (D), which is approximately 10‐12 m². In Table 1, the hydraulic conductivity and permeability of several common crust materials are shown.
Table 1: Hydraulic conductivity and permeability of common materials found in the earth's crust [8]
For the geofluid to be able to flow from the injection well to the production well, the reservoir must be permeable. A high permeability (see Table 1) will require less pump energy for the same
energy) of a geothermal system, a high permeability is required. A high permeability however does not necessarily result in a good reservoir e.g. a large opening or crack in the reservoir material can cause ‘short‐circuiting’ of the wells, limiting the source capacity. [3] The properties of a geothermal reservoir change over time. Several processes contribute to this among which: thermal contraction by cooling, thermal fracturing and deposition of minerals (scaling) [9]. A typical flow for an engineered system is 50‐100 l/s per well [3, 10]. The hydraulic performance of a geothermal well is often expressed as its productivity index, the flow rate per unit pressure drop, or its inverse, well impedance. For a certain reservoir, the well impedance is inversely proportional to the reservoir permeability, i.e. the higher the permeability the lower the impedance and vice versa. Typical well impedances for an EGS source (see next paragraph) lie in the range 0.15‐0.25 MPa.s/l [3]. These figures are expected to decrease as the technology matures.
Reservoirtypes
Four types of geothermal energy are usually distinguished: hydrothermal, geo‐pressed, magmatic and enhanced. The hydrothermal type of reservoir is currently the only one that is used commercially. This type of reservoir is a permeable sedimentary layer between two impermeable layers in the crust containing (hot) water. Figure 2 shows a graphical representation of hydrothermal reservoir and its surroundings. In this figure, there is only one well, because the water in the reservoir gets naturally recharged. A reinjection well is often used to minimise the risk on seismic events and because natural recharge is generally a slow process and impedes commercial production rates. Also, to minimise the environmental impact, legislation often requires reinjection for mass balance.
Figure 2: A graphical representation of an ideal geothermal system
[http://www.geothermal‐energy.org/].
Besides the common hydrothermal reservoirs, other types of reservoirs can be used or created. A geo‐pressed reservoir is a hot‐water aquifer like a hydrothermal reservoir but under high pressure, often containing dissolved methane. The use of this type of reservoir is often problematic due to the dissolved methane and contaminants in the heat exchanger impeding functioning. Another type is the magmatic reservoir. Geothermal energy from magma uses the hot molten rock at 700‐1200 °C to heat the water. This type of resource is in a very early stage of development and only applicable in regions with a high geothermal gradient, e.g., in Iceland [5].
2.1 Geothermal energy
The fourth and most promising type of reservoir is Enhanced Geothermal Systems (EGS) [3]. EGS is a concept in which a geothermal reservoir is created in volumes of rock with insufficient porosity and/or permeability. Figure 4 shows a schematic representation of a typical EGS system. Ideally, a closed loop is created between the production and reinjection well with a large heat‐exchanging surface and a high permeability. This is achieved by fracturing deep rock formations. Stimulated volumes of up to 3 km³ can be created [3]. The geometry of the created reservoir depends on the initial stress field in the rock and the applied stimulation. The concept of EGS was first demonstrated in the 1970’s at a research site in Fenton Hill, New Mexico. An on‐ going European research project was started in 1987 in Soultz (France) to explore the possibilities of EGS for electricity generation and heating. Many similar research projects are currently being conducted around the world [3]. The main technology‐related topic of research is limiting the short circuiting between the reinjection and production well as this causes an output temperature/capacity drop for the system. Another (related) main topic is the long‐term operability and sustainability of an EGS‐source. [3] In the research projects mentioned above, techniques are tested to limit or even avoid these problems. Other research topics include: site surveying, better understanding of the role of major pre‐existing faults in well flow, measuring down‐hole parameters and the prediction of scaling or deposition [3, 11].
Location
Finding a suitable location for geothermal energy can be as simple as looking for the presence of a geyser, a boiling mud pot or a steaming pool. This is how a location was determined for the early projects (the Geysers in California or Larderello in Italy). Because these natural surface manifestations of geothermal energy are not very common on a global scale, techniques have been developed to determine possible geothermal locations. In the Netherlands, a lot of information of the underground is freely available. This includes datasets with soil characteristics, drill records, surface features and drill cores from earlier drilling, for example from the exploration of oilfields. In Figure 3 an example is given for the available information in the Netherlands; a map of the geothermal potential for space heating and a map of the temperature at 5000 m depth. Please note that these maps do not directly translate to the applicability of geothermal energy.
a) b)
Figure 4: A schematic representation of an Enhanced Geothermal System (EGS) with
typical values indicated for the Netherlands [13].
2.1 Geothermal energy
In general, a first estimation of the applicability of geothermal energy is made by analysing available geological data [14]. After choosing a suitable location, based on the available geologic information, the location will be investigated more thoroughly. These investigations provide a general sense of the site potential although the final temperature, mass flow and sustainability of the well are not known until the actual well has been drilled and tested.
2.1.2 Sustainable energy potential
Resourcebase
The global geothermal energy resource base consists of energy from the formation of the earth (accretion energy), radioactive decay in the crust (mainly continental) and absorption of solar energy [14]. Energy flows constantly from the earth to the atmosphere. With an average of 65 mW/m² the global terrestrial heat flow is estimated to be 4.4*1013 W [15]. The total heat content of the earth’s crust is estimated to be 5.4*109 EJ [5]. This illustrates the vast amount of geothermal energy that is present. Of course, due to technical and economic reasons not all this energy can be harvested. It was estimated in the year 2000 that the potential that would become technically possible to acquire in the next 40‐50 years is approximately 5,000 EJ/yr, about 10 times the current annual demand [3].
The utilisation of geothermal energy often benefits from the geographically varying geothermal gradients, i.e. places exist where higher temperatures exist closer to the surface than in other places. These so‐called hot spots exist due to several geologic processes. For example, near tectonic plate boundaries relatively hot material emerges from the deeper parts of the earth (mantle), and locally increases the temperature of the crust (see Appendix I). In addition, the build‐up of the lithosphere (thin spots in the crust for example), the geothermal gradient varies. An example of a high gradient area is Iceland with gradients varying from 50 to 150 °C/km [12].
Heatbalance
The heat balance of a system in general, is dictated by the first law of thermodynamics. If mechanical work and reservoir regeneration are ignored, the first law is as follows:
dU
H
in( )
t
H
out( )
t
V
gf
gfc
p gf,
T
prod( )
t
T
inj(t)
V
r
rc
p r,dT
r avg,( )
t
dt
dt
(1)
With:
U Internal energy of a system [J]
H Enthalpy [J]
V
gf Geofluid volume flow [m³/s]
ρgf Density of the produced geofluid [kg/m³]
cp,gf Specific heat capacity of the geofluid [J/kg.K]
Tprod Temperature of the produced geofluid [°C]
Tinj Temperature of the injected geofluid [°C]
Vr Reservoir volume [m³]
ρr Density of the reservoir [kg/m³]
cp,r Heat capacity of the reservoir [J/kg.K]
Tr,avg Average reservoir temperature [K]
For a geothermal reservoir, the internal energy is contained in the rock/sediment and fluid. The heat fluxes over the boundary of geothermal reservoirs are: the production and resupply of the
In EGS reservoirs, advection of geothermal brine from and to the environment is often very low or even non‐existent. In the reservoir itself, heat transport occurs by convection in the geofluid and conduction in the (porous) reservoir material. Figure 5 illustrates the heat flows in a typical hydrothermal system and in an EGS system. The lack of advection from its surroundings makes the EGS reservoirs dependent on conduction for energy supply to the reservoir (regeneration). Due to the low thermal diffusivity of surrounding rock materials, this conduction is a process with a large time constant [12].
Figure 5: An illustration of the heat flows in a typical hydrothermal system (left) and in an EGS (right). Note the
lack of advection in the EGS reservoir. Both shapes are arbitrarily chosen and differ to indicate their variety.
Sustainabilityandoperation
With respect to environmental impact, the terms ‘sustainable’ (determined by the way a resource is used) and ‘renewable’ (the nature of a resource) are often confused. The sustainability of a geothermal system depends on the initial heat and fluid in place and the rate of their usage and regeneration [5, 11], i.e. the net amount of heat and fluid extraction should be lower than or equal to the influx from the material surrounding for truly sustainable production [9]. In practice, for economic and technical reasons, energy extraction from sedimentary reservoirs often exceeds resupply considerably. This is commonly called ‘excessive production’ and will eventually lead to reservoir depletion [11]. Geothermal energy from hydrothermal sources is generally classified as a renewable source; the energy removed from the source is continuously replaced on time scales similar to those required for energy removal of typical societal systems [11].
In a more general sense, the term ‘sustainable’ refers to a commonly used definition by the Brundtland commission: “...meeting the needs of the present without compromising the ability of future generations to meet their own needs” [16]. In this context, for sustainable development the use of a specific geothermal source does not have to be fully sustainable, as long as a suitable replacement can be found after depletion [17]. In addition, when geothermal energy is considered on a larger scale, a certain overall production level could be sustained by creating new systems when others are depleted.
When using energy from a geothermal reservoir, the temperature of this reservoir will inherently drop. This process causes the sustainability of geothermal energy to be subject of debate. When the reservoir cools down during production, the sustainability of a geothermal source could be limited. It is also possible that a new equilibrium is reached at a lower temperature, i.e. the lower temperature and pressure cause a higher influx of energy, possibly in
2.2 Applications in the built environment
balance with the heat extraction rate [11]. A reservoir is considered depleted when the production temperature or flow becomes insufficient to supply the required amount of energy. At this point, the reservoir might still contain a considerable amount of energy. The recovery factor of the reservoir is often used to describe the efficiency at which the energy content of the reservoir was used during its lifetime and is defined as the extracted energy divided by the total energy content. For EGS reservoirs, with a volume greater than 0.1 km³, recovery factors typically lie in the range of 0.4‐0.5, after a lifetime of 30 years [18]. A study by Sanyal and Butler suggests that the net generation profile, i.e. the development of the net electricity output, is a more appropriate criterion [18]. Another performance indicator of a geothermal power plant, is the pump load factor, i.e. the fraction of the gross electricity produced that is used for the pumps.
When a reservoir is depleted, the exploitation is stopped and the reservoir temperature, fluid and pressure, will eventually regenerate itself. Reservoir regeneration is an asymptotic process that happens on various time scales, depending on the reservoir type, size, production system, the extraction rate and properties of the reservoir [11].
In the case of an EGS reservoir (and confined hydrothermal reservoirs), regeneration happens through conduction only, making excessive production almost inevitable for economical production [19]. This type of reservoir relies primarily on the heat initially stored in the reservoir and surrounding material (heat content). It should therefore strictly be considered as a finite energy source [19]. It has been shown for EGS sources that a lower production rate does not necessarily mean that the total gains of the source will decrease in the same period. In a study performed by Sanyal and Butler in 2005, the total net (pump energy subtracted) electricity generation of an EGS power plant with 5 production wells was simulated for a high (500 kg/s) and low (126 kg/s) flow rate over a period of 30 years. Both cases yielded a similar amount of electricity, respectively 245 and 250 MWeyears. However, the source with the low flow rate is
still usable after 30 years production, while the high flow source stopped producing after 20 years because of an insufficient production temperature, making the low flow‐rate case more sustainable [18]. Two effects contribute to a higher overall performance of the geothermal system with a lower flow rate: when the flow rate is lowered, the pump energy will also be reduced exponentially, [20] and the production temperature will decline at a lower rate, causing the conversion efficiency (see §2.2.2) to remain higher [11]. These observations demonstrate the importance of a good operational strategy for the sustainable exploitation of any geothermal source and EGS in particular.
2.2 Applications in the built environment
With a total of 10 installed geothermal systems, geothermal energy in general, is a relatively unknown energy source in the Netherlands [21]. The main application for geothermal energy in the Netherlands is green houses. The current application of geothermal energy in the built environment in the Netherlands is limited to one district heating project in the city of The Hague.
applications for geothermal energy in the built environment are: space heating (direct use) and electricity generation. Another potentially interesting application is space cooling. For all these applications the conversion, distribution and usage as well as the possibility of combining these applications, will be discussed in the following paragraphs.
2.2.1 Space heating (direct use)
The term ‘direct use’ refers to the usage of geothermal energy without energy conversions. One of the most obvious and common direct uses of geothermal resources in the built environment is space heating, which is logical because no conversions (with inherent losses) are needed. This makes the system principle uncomplicated: the hot geofluid runs through a heat exchanger and part of the geothermal energy is transferred to the relatively cold transport medium of a district heating system (typically water).
Figure 6: Direct usage with the use of a heat exchanger ‐ the relatively hot geofluid (Th,in) runs through
the heat exchanger and part of the geothermal energy (Q) is transferred from the hot geofluid
to the relatively cold fluid (Tc,in) in for instance the district heating system [10].
Geothermal heat is one of the most attractive renewables with respect to environmental burden and base load capability [10]. The first municipal district heating system using geothermal energy was set up in Reykjavik, Iceland in 1930. Today, about 90 % of the heating demand of Iceland is covered by geothermal energy [22]. Large‐scale district heating systems using geothermal energy have been built in many countries [23]. Refer to Appendix III for further statistics.
Distribution
The distribution of district heat is typically realised with a closed loop network with water transporting the energy to the end‐users. As transportation of energy causes losses, the place with energy demand should be close to the geothermal energy plant to acquire the highest possible system efficiency. In other words, the thermal load density should be as high as possible close to the source. This is particularly the case with thermal energy transport [5]. High‐ rise buildings increase the thermal load density when compared to a typical residential area. The distribution of residential district heat is characterised by supply temperatures between 50‐90 °C and return temperatures between 30‐70 °C [12].
Another consideration to be made is the demand profile on several timescales. Combining different types of users (industry, offices, etc.) in the heating network might be necessary to optimise the demand profile for the geothermal heating plant. On a daily scale, many homes require a very small amount of heating energy during the day in the heating season, while the demand rises quickly after office hours. For offices the opposite applies, i.e. the highest demand
2.2 Applications in the built environment
Figure 7: Lindal diagram showing numerous applications of geothermal
occurs during the day. On a seasonal scale, heating demand varies greatly between the highest demand in the winter and a low demand during summer because of the changing ambient temperature. On a scale of decades, slower processes like a changing climate or improved thermal insulation of buildings, influence the heating demand.
Figure 8: Space heat demand profile relative to the ambient temperature [10].
A similar (direct) use of geothermal energy is industrial heating; this includes greenhouses, industrial processes and aquaculture. In the Netherlands, 4 projects have been realised for the heating of greenhouses [21]. In greenhouses, the geothermal energy is used to heat the interior of the greenhouse to create optimum conditions for the crops to grow. In practice, geothermal energy is mainly used for greenhouses without lighting, as CHP (Combined Heating and Power) solutions with fossil or biofuels are more interesting when electricity is needed. Another advantage of CHP is that it has CO2 as a waste product, which can be used to maintain an
optimum CO2 level in the greenhouse. The usage for industrial processes is comparable to that
of space heating, i.e. the energy is used directly to heat a medium. The difference lies in the supply temperature needed for the specific process, which ranges from 30oC for aquaculture to more than 180oC for the paper industry [5]. Another big difference is that industrial processes have a relatively constant heat demand. Industrial processes in which geothermal energy is already used, can be seen in the Lindal diagram in Figure 7.
2.2.2 Electricity
Electricity generation is another common use of geothermal energy in the built environment. Many countries have a small part of their total electricity demand covered by geothermal power. The first geothermal power plant in the world is located in Larderello Italy and has been producing electricity since 1912. The total installed capacity for geothermal power in the world was 10,715 MWe in 2010 and the total amount of energy generated (67,246 GWhe) accounted
for 0.32 % of the world’s total demand (21,248 TWhe [24]) in that year [25, 26]. With about 25 %
of its electricity demand covered by geothermal power, Iceland is the largest geothermal electricity producer in the world [22]. In some developing countries, geothermal power makes a substantial contribution to the total. For example in the Philippines, it represents 21 % of the total electricity supply, making it the second largest geothermal power producer in the world. More information about geothermal electricity generation can be found in appendix IV.
2.2 Applications in the built environment
Conversion
Three main types of geothermal power plants exist: dry steam, flash and binary. In dry steam and flash plants the geofluid is used directly and in binary plants a secondary energy medium is used. Two main categories of geothermal electricity generation are generally distinguished: high enthalpy and low enthalpy. High enthalpy generation is similar to traditional Rankine steam power plants using fossil fuels or nuclear energy: steam is expanded, powering one or more turbines which drive a generator. The difference between conventional steam plants and high enthalpy geothermal plants lies in the fact that the latter typically uses a temperature between 150 °C and 250 °C [23], while temperatures can reach 620 °C for traditional steam powered plants (current metallurgical maximum) [27].
The theoretical maximum conversion efficiency of heat into work, the Carnot efficiency, illustrates the value of a high temperature resource. low sink gf 1 1 ( )
carnot high T T T T t (2) With:ηcarnot Carnot efficiency [‐]
Tlow Lowest temperature in the system [K]
Thigh Highest temperature in the system [K]
Tsink Temperature of the heat sink [K]
Tgf Temperature of the produced geofluid [K]
If a sink at 10 °C is assumed, the Carnot efficiencies for traditional power plants (620 °C) and high enthalpy geothermal plants (250 °C) are respectively 68.3 % and 45.9 %. This demonstrates that the theoretical efficiency of a high enthalpy plant is much lower than that of conventional steam power plant designs. In practice, this efficiency ranges from 10 to 17 % for currently running high enthalpy geothermal electricity generation plants [23] while modern traditional power plants reach values of 34 % for nuclear and 40 % for fossil fuelled steam plants [27]. The practical efficiency of a high enthalpy plant can be estimated by multiplying its Carnot efficiency with a ‘utilisation factor’ of 0.45 [18]. This utilisation factor is the ratio between and actual work potential and theoretical work potential. Sanyal and Butler investigated the average electricity generation of an EGS source with an average initial temperature of 210 °C over a period of 30 years. They found that this is roughly 26(±5%) MWe per cubic kilometre of stimulated volume,
regardless of well arrangement, fracture spacing and permeability [18].
Due to local legislation and contamination in the geofluid, it is not always possible to directly use the geofluid in the turbine (flashing). In this case binary systems are used in which the geofluid is separated from the working fluid by a heat exchanger. When using a binary plant design, the working fluid does not necessarily have to be water/steam as it is contained in a closed circuit. Using other working fluids than water is particularly useful when using a low enthalpy geothermal source. When the temperature of the geofluid is lower than 150 °C, it is difficult to efficiently run a flash power plant and often a binary plant with an alternative working fluid (with a lower boiling point) is used. If an organic working fluid like isobutene or isopentane is used, the used cycle is commonly referred to as Organic Rankine Cycle (ORC). In this cycle heat from the geothermal source vaporises the working fluid in a heat exchanger in a secondary
type of fluid in the secondary circuit determines the required minimum and maximum temperature of the geothermal energy supplied. The maximum temperature depends on the stability of the fluid and the minimum temperature depends on the economical size of the heat exchanger to vaporise the fluid. The relatively low temperature results in a Carnot efficiency of 32.9 % with a geothermal source at 423 K (150 °C) and an assumed sink temperature of 283 K (10 °C). A more realistic value of the plant efficiency would be 10‐13 % for these conditions [3, 10]. In Figure 9 several measured efficiencies of existing ORC power plants around the world are shown [28]. A relatively new binary system is the Kalina cycle. In this cycle a water‐ammonia mixture is used as a working fluid. Typically in a 3:7 ratio, depending on required boiling temperature or the available source temperature. The main advantage of a Kalina cycle compared to an ORC cycle is a variable evaporation (and condensing) temperature which can be fitted to the falling temperature of a geothermal source and the possibly changing temperature of the sink, continuously optimising system efficiency [29]. Because of better recuperation and the possibility to vary the composition of the mixture, the Kalina system is more efficient (20‐40 %) than the conventional ORC systems, especially when varying supply and sink temperatures are to be expected. Figure 10 shows the average specific resource utilisation of different cycles suitable for geothermal power production, i.e. the of energy flow needed to produce 1 MWe of
electricity at a certain geofluid temperature. In the low enthalpy range, the curve of the Kalina cycle lies considerably lower than the ORC curve, illustrating its higher efficiency. Other advantages of using the water‐ammonia mixture instead of more common organic working fluids are: it does not deplete ozone, it has no global warming potential, it is well known from its use in absorption chillers and it is relatively cheap. In the near future, the cost of a Kalina cycle plant is expected to be lower (up to 30 % for low enthalpy sources) than conventional Rankine plants [30]. The main disadvantage of this technique is the increased heat exchange; approximately 25 % more heat exchanging surface is needed compared with conventional Rankine cycles, increasing the initial investment costs. Another disadvantage is the added complexity in operation and maintenance, which increases exploitation costs [31].
Distribution
As electrical power grids are widely developed in many countries [12], the necessity for an additional distribution infrastructure for this application of geothermal energy is limited. The only requirement is that the power plant has to be connected to the grid.
2.2 Applications in the built environment
Figure 9: Practical conversion efficiencies of existing ORC systems [32].
Figure 10: Specific average resource utilisation of different cycles suitable for geothermal power production [32].
2.2.3 Space cooling
Space cooling is another possible application for geothermal heat in the built environment. Similar to direct use, the generated cold can be used for district space cooling and for industry loads.
A well‐established technique to convert heat into cold is absorption cooling [4, 12]. Absorption chillers are very common in situations where heat is relatively cheap and a cooling load is present. This is the most applicable technique for cooling using geothermal energy [12].When applying absorption refrigeration systems a few considerations should be noted. Absorption chillers are much more expensive than vapour‐compression systems in terms of investment as well as service, because they are quite uncommon and relatively complex. They occupy more space and when a suitable natural heat sink in the form of surface water is not present, a cooling tower is needed which is larger than the condenser of a similar vapour‐compression system [27]. Alternative heat‐driven refrigeration systems exist (e.g. adsorption, metal‐hydrates, zeolites, etc.), but these are relatively uncommon. Therefore, these alternative systems will not be considered in this report, as they are not very likely to be economically and practically applicable.
Conversion
In Figure 11, a schematic representation of an ammonia‐water absorption chiller is shown that illustrates the working principle of an absorption chiller.
Figure 11: A schematic representation of an ammonia‐water absorption chiller [adapted from 35].
When compared to a typical compressor type chiller the left half of the schematic above is similar. The basic principle is also the same: refrigerant is evaporated in the evaporator which extracts heat from its environment, the vapour is compressed and condenses in the condenser rejecting heat to the environment. A pressure valve separates the high‐ and low‐pressure part of the system. The difference with a compression chiller lies in the compressor part which is thermally driven (by geothermal energy) instead of mechanically. Water is the transporting medium and ammonia is the refrigerant in this example. The compressor part (dotted line) works as follows [27]:
2.2 Applications in the built environment
The refrigerant vapour (NH3 in the picture) leaves the evaporator and enters the
absorber where it is exothermically absorbed by a relatively cool ammonia‐water mixture at a relatively low pressure. This mixture is cooled by a cooling source as was discussed in last paragraph.
The mixture is then pumped to the generator, increasing the pressure.
In the generator, the mixture is heated by geothermal energy, evaporating the ammonia again which has a lower boiling point than the water, still at a relatively high pressure. The high pressure ammonia continues to the condenser and the water is separated in the rectifier returning it to the generator.
The solution of ammonia and water (hot and high pressure) is passed through a regenerator to transfer heat to the mixture leaving the absorber, is throttled and returned to the absorber.
Ammonia based systems can provide output temperatures from ‐40 °C to 20 °C [4]. This makes them suitable for most cooling needs, including air‐conditioning, process cooling and freezing applications. Other common combinations of refrigerant and transport medium include water‐ lithium bromide (LiBr) and water‐lithium chloride (LiCl). Both the LiBr and LiCl systems are unsuitable for low temperature cooling because of the risk of crystallising water in the system [4, 12].
The conversion efficiency of absorption cooling is commonly expressed in a coefficient of performance (COP). The COP of an absorption cooling system is defined as follows [12]:
COP
cool
coolgen pump gen
Q
Q
Q
W
Q
(3)
With:
Qcool Heat extracted by the chiller [W]
Qgen Heat input to the generator (geothermal energy) [W]
Wpump Mechanical work by the circulation pump [W]
In a multistage arrangement, two or more absorption chillers are combined to allow for a better overall conversion efficiency. A two‐stage absorption chiller heated with steam has a typical COP of 1.2. A single stage absorption chiller on water of 90 °C typically achieves a COP of 0.7 [4]. In comparison with vapour compression cycles, the pump energy is negligible, i.e. approximately 5‐8 % of the refrigeration capacity [12]. The energy extracted (Qcool) and the energy supplied
(Qgen) depend strongly on the various temperatures present in the system. According to Ziegler and Alefeld, the COP of an absorption chiller can be approximated by the following formula [33]: supply sink , sink supply , , , ( ) ( ) COP 1 ln ( (t)) ( ) ( ) ( ) h out h in h out h in T t T T t T T T t T t T t (4) With:
Tsupply Cold supply temperature [K]
Tsink Heat sink temperature [K]
Th,in Heat source supply temperature [K]
Th,out Heat source return temperature [K]
In Figure 12, this formula is illustrated for a heat source supply temperature of 80 °C, 100 °C and 120 °C and a constant sink temperature of 31 °C and a constant average chill provision temperature of 9 °C.
Figure 12: The COP of absorption chillers with an assumed chill provision
temperature of 9 °C and average heat sink temperature of 31 °C [12].
Distribution
For the distribution of cold, two options exist: a central cooling plant provides cooling power to a dedicated cooling network or local cooling plants use the heat from a district heating network [12]. The advantage of the latter is the absence the need for an additional supply infrastructure. As with direct use, a high energy demand density is also preferable for a high system efficiency. In contrast with space heating demand, the highest space cooling demand occurs in the summer, when ambient temperatures are high. The opposite behaviour of demand in relation to ambient temperature might make these two applications a good candidate for combined application (see §2.2.1).
2.2.4 Combined application
By combining applications, the utilisation of the available energy at the production well can increased [12]. Two factors contribute to this:
1. Increased use of potential: the ratio between energy used and energy produced can be higher (or the return temperature lower), if applications with different temperature requirements are used in (partially) serial setup.
2. Load levelling: When the demand of a certain application drops, other applications might still have a demand for which the geothermal energy can be used.
An example of increased use of potential is using the geofluid returning from a geothermal power plant, for heating purposes in de vicinity. The temperature of the geofluid leaving an ORC cycle is often still suitable for a variety of heating applications. This setup results in a lower temperature of the reinjected geofluid. The relatively low temperature requirement for heating purposes (see Figure 7), often makes it technically possible to use it as a secondary application. To reach the required temperature, the primary geofluid might still be needed.
Combining space heating with space cooling is an example of load levelling. When the primary heat demand for space heating gets lower, the primary heat demand for space cooling increases
2.3 Concluding remarks
as both are related to the ambient temperature [12]. In an ideal case, the sum of the primary energy demand for both the heating demand and the cooling demand equals the geothermal energy supply. This ideal case is illustrated in Figure 13.
Figure 13: An idealised graph of load levelling by combining geothermal space
heating and absorption cooling with geothermal energy [12].
2.3 Concluding
remarks
Geothermal energy is a promising energy source for the built environment. Hydrothermal reservoirs are used in several locations around the world, mainly near geothermal anomalies. In the near future, Enhanced Geothermal Systems are expected to become technically viable, significantly increasing the spatial applicability of geothermal energy. For this reason, this type of reservoir is chosen as subject for further research in this report.
The most important parameters for the performance of a geothermal reservoir are: initial temperature, permeability and heat capacity. Because these parameters vary geographically, they translate into location, depth and volume of the reservoir. In the case of an EGS the degree of reservoir fracturing determines its permeability.
Because an EGS reservoir cannot be operated both fully sustainably and economically, the reservoir is used as a finite energy source, i.e. due to excessive production, the reservoir is cooled to a point where further use is not economical. After abandonment, the reservoir temperature will slowly recover under heat conduction from the environment. However, sustainability should be considered in a broader perspective: an EGS reservoir is able to provide clean energy for decades without the disadvantages of conventional energy sources and a combination of multiple reservoirs could make it a fully sustainable energy source. The amount of years an EGS reservoir can be used depends heavily on the operational strategy. Lowering flow does not necessarily decrease total gains and is likely to increase sustainability. Relevant performance indicators for sustainability are investigated in Chapter 5.
In the built environment, geothermal energy can be used for heating, cooling and electricity generation. Because of distribution losses, the heating and cooling load should be located close to the geothermal system. Distance between source and demand is of less importance with geothermal electricity, i.e. transportation of electricity is relatively efficient. Combining applications might increase system performance although possibly lower/no demand periods might also be preferable to allow for regeneration during operation.
2.3 Concluding remarks
3 E
NHANCED
G
EOTHERMAL
S
YSTEM MODEL
This chapter describes the model that is used for the performance assessment of an enhanced geothermal energy source for energy supply in the built environment. A computer model is created in COMSOL Multiphysics, which is used to investigate the performance of an EGS for the scenarios described in Chapter 4. Using Matlab, the output of these simulations is analysed and expressed in performance indicators. This general approach is illustrated in Figure 14.
Figure 14: A schematic representation of the general approach used for the simulations.
In §3.1 the modelling of the enhanced geothermal system is described. In §3.2 the analysis of the output data from the EGS model is explained and in §3.3 the model output is verified.
3.1 Description of the used EGS model
The performance of an EGS reservoir involves several coupled physical and geochemical processes, including heat and mass transfer, pressure distribution, deformation and scaling. Due to the large number of parameters involved to define geometry, hydraulics, heat transfer and material stresses, the only viable approach to modelling a geothermal system, is to use numerical methods [34].
Numerical simulation with a computer model requires a good balance between the required level of detail and the computing power available. The required level of detail depends on the aim of the simulations. A model concept is typically chosen depending on the specific properties of the assessed reservoir, the required level of detail and the available computing power [34, 35]. Several modelling concepts exist for porous and fractured media. In order of increasing detail, these modelling concepts are [36]:
Continuum / Equivalent Porous Medium (EPM): the whole reservoir volume is assumed to have the same properties and equivalent values are calculated to characterise them. Although limited in the detail it provides, this approach has been shown to give good results for the gross behaviour of the geothermal system [35] and is widely accepted for a coarse approximation of the behaviour of a geothermal source [8].
Multi continuum: both fractures and rock are characterised separately by continuum models. Multiple models can be used for multiple fracture size groups.
3.1 Description of the used EGS model
Discrete fracture model: the geometry of the fractures is modelled and no fluid flow (hence no heat advection) is assumed in the surrounding material.
Combinations of both: the most realistic modelling concept is a combination of the discrete fracture model combined with a continuum model for fluid flow through the porous surrounding material.
The most common concepts for different domain (reservoir) classifications are shown in Figure 15.
Figure 15: Modelling concepts for the description of porous fractured media [36].
A finite element computer model of an EGS is used to calculate the production temperature and flow of the source and the temperature distribution in the source and its surroundings. The finite element software suite Comsol Multiphysics version 4.2 is used to generate this model and perform simulations. As sustainability is of a slow nature by definition, i.e. measured on a timescale of decades, a high level of detail is unnecessary for the research at hand. Combined with the absence of detailed statistical fracture data and the limited computing power available, this observation leads to the choice of an Equivalent Porous Medium (EPM) approach. As this approach assumes homogeneity and isotropy in the reservoir, a two‐dimensional cross section of the reservoir is assumed to be representative for the whole reservoir. A transient, two‐ dimensional (2‐D) model is created of the horizontal cross section (parallel to the earth’s surface) of an enhanced geothermal reservoir and the surrounding material. Based on the fact that several processes can both improve or deteriorate the reservoir properties, they are assumed to remain the same during the simulation period.
As described in §2.1, a high enthalpy geothermal reservoir is needed for electricity generation. For this reason, an enhanced geothermal reservoir is assumed at a depth of 5000 m. For the city of Eindhoven, the temperature at this depth is roughly 185 °C (see Figure 3b). It is assumed that an EGS at this depth and location will be possible in the near future.
