Excerpt
Table of Content
Resumen (Spanish)
Abstract (English)
Kurzfassung (German)
List of Figures
List of Tables
Nomenclature
1. Introduction
1.1. Motivation
1.2. Aim
1.3. Objectives
2. Fundamentals of Biomass and Geothermal Energy Systems
2.1. Thermodynamics of Power Plants
2.2. State of the Art of Geothermal Energy Systems
2.3. State of the Art of Biomass Energy Systems
3. Combination of Geothermal and Biomass Energy Systems
3.1. Geothermal Part
3.2. Biomass Part
3.3. Thermodynamic Proposals
3.4. Economic Considerations
3.5. Risk Management
3.6 .Existing Hybrid Power Plants
4. Conclusion and Recommendation
5. Further Research
References
Acknowledgements
Academically I thank following institutions and persons for their intellectual and infrastructural support during the preparation of this w ork:
- FundacionChile (Santiago de Chile, Chile) Exceptionally Senora Ana Maria Ruz y Senor Oscar Coustasse for the opportunity to gather experiences in Chile and for the confidence they put into me.
- De Montfort University Leicester (Leicester, UK) Primarily Dr Andy Wright for supervision and the IESD staff for equipping me with academic skills necessary to undertake own research,
- GeothermalCenterBochum (Bochum, Germany)
Particularly Prof. Dr. rer. nat. Rolf Bracke and Prof. Dr. Gerrit Höfker for
confidence, review and supervision, Dipl.-Ing. Katja Winkler for informational support and review,
Personally I thank my family, in particular my parents, Uwe and Sonja, and my sister, Kathrin, for their never-ending trust in me and their interest in my work. Moreover my last but not least thanks goes to my friends as they created the indispensable world besides research for me.
Thank you.
Bastian Görke
Resumen (Spanish)
Gracias a la tecnologia hibrida, basada en calor geotérmico y biomasa, la generacion de electricidad es posible de forma eficiente, aun de fuentes de entalpias bajas. Hasta ahora ésto era solamente posible por procesos de centrales energéticas binarios con eficiencias reducidas. Calentando el agua del ciclo de la central energética con calor geotérmico y un calentador de biomasa, un Ranike-Ciclo produce electricidad con eficiencias ideales de 34%. El consumo de biomasa, ademâs, se reduce por 6%.
Aunque este proceso es termodinamicamente deseable, sufre problemas economicas. Para una central energética hibrida casi todas las partes de una central geotérmica y una central energética de biomasa son necesarias. Las expensas altas iniciales solamente pueden ser ahhorradas por ahorros de gastos de combustibles y eficiencias mas altas.
Una exploracion fallada normalmente hay que cesar el projecto geotérmico pero con tecnologia hibrida eso no es necesario. Ahora este deposito geotérmico puede ser usado para generar electricidad eficientemente y sosteniblemente.
Ademas la central energética hibrida esta mas independiente de fluctuaciones en facilitation y precios de biomasa porque el porcentaje de biomasa de la alimentation de energia esta reducido.
A pesarde este conocimiento inicial y basico, una investigation mas profunda, referiendo a los resultados ya explorados, es necesario y una lista con proposiciones esta presentado al final del trabajo.
Abstract (English)
This research draws conclusions from existing biomass and geothermal electricity generation systems and combines these to a hybrid plant concept which is subsequently evaluated thermodynamically, economically and in concerns of risk management.
Using a hybrid pow er plant concept based on geothermal and biomass energy input, low enthalpy geothermal reservoirs can be utilised more efficiently for electricity generation then it was up to now possible with ordinary binary cycles. By passing the geothermal fluid through a heat exchanger to preheat the biomass boiler feedwater a classical Rankine-Cycle generates electricity with an ideal efficiency of 34%. Due to the geothermal contribution biomass is saved by the amount of 6%.
This thermodynamically desirable process, however, raises economic problems as all components of conventional biomass plants as well as the source-side components of geothermal power plants become necessary. Costs for these parts accumulating at the development’s beginning only are compensated by reduced fuel costs and higher efficiencies.
For geothermal exploration failure and the underachievement of the geothermal reservoir the proposed hybrid technology opens up an additional alternative. Having to have to abandon projects with inadequate qualities in former times it is now possible to produce electricity efficiently and sustainably by the new hybrid power plant. Moreover, the dependence on the fluctuation in terms of biomass quality and prices decrease as the contribution of biomass itself is reduced.
Although delivering first findings concerning these hybrid plants, this paper highlights the future research demand and suggests a list of worthwhile research topics.
Kurzfassung (German)
Mittels der hybriden Kraftwerkstechnik basierend auf geothermischer Wärme und Biomasse können Niedertemperatur-Geothermiereservoirs effizienter, als bislang durch binäre Kraftwerksprozesse möglich, zur Stromerzeugung genutzt werden. Wird das geothermische Medium zur Vorwärmung des Speisewassers eines Biomasseboilers verwendet, kann mittels eines angeschlossenen Rankine Kreislaufprozesses Elektrizität mit einem idealen Wirkungsgrad von 34% bereitgestellt werden. Durch den Einsatz der Geothermie reduziert sich der Biomasseverbrauch um etwa 6%.
Dieser thermodynamisch durchaus erstrebenswerte Prozess leidet allerdings unter wirtschaftlichen Gesichtspunkten, werden doch für ein hybrides Kraftwerk sowohl alle Komponenten eines konventionellen Biomassekraftwerkes als auch sämtliche quellseitigen Komponenten eines geothermischen Kraftwerks nötig. Diesen zu Projektanfang anfallenden erhöhten Investitionskosten und entsprechenden Zinseffekte wirken lediglich leicht verringerte Brennstoffkosten und eine gesteigerte Effizienz entgegen.
Bei einem Fehlschlagen von geothermischen Explorationen und einer daraus resultierenden unbefriedigten Erwartungshaltung an das geothermische Reservoir eröffnet die hybride Kraftwerkstechnik eine Alternative und erweitert den Entscheidungshorizont. Einer bislang notwendigen Aufgabe der geothermischen Erschließung steht mit der hybriden Technologie die Möglichkeit gegenüber nachhaltig und effizient Elektrizität bereitzustellen. Das hybride Kraftwerk ist ebenfalls unabhängiger von preislichen und quantitativen Schwankungen in der Biomassebereitstellungskette, da der Anteil der Biomasse an der gesamten Energiezufuhr reduziert wird.
Aufbauend auf diese ersten Erkenntnisse über hybriden Kraftwerkstypen wird auf weiteren Forschungsbedarf hingewiesen und eine Liste lohnenswerter Themen bereitgestellt.
List of Figures
Figure 1 Energy conversion path of a biomass power plant from the source to electricity (according to Kaltschmitt and Hartmann, 2001)
Figure 2 Energy conversion path of a geothermal power plant from the source to the final
product (according to Kaltschmitt et al. 1999)
Figure 3 T-s-diagram of the ideal Rankine-Cycle (Moran et al., 2002)
Figure 4 Schematic chart of the Rankine-Cycle with pump, boiler, turbine, and condenser. Shown are energy Transfers Q and W over the system’s boundaries (Moran et al., 2002)
Figure 5 T-s-Diagram including qin, qout and wout
Figure 6 The effect of low ering the condenser pressure in the T-s-Diagram
Figure 7 The effect of superheating the steam in the T-s-Diagram
Figure 8 The effect of increasing the boiler pressure in the T-s-Diagram to low er TL
Figure 9 The effect of reheating in the Diagram to increase TH
Figure 10 The effect of preheating the boiler feedwater in the T-s-Diagram
Figure 11 Simplified diagram of a dry steam pow er plant with back-pressure turbine (Forsha and Nichols,1997)
Figure 12 Simplified diagram of a dry steam pow er plant with condensing turbine (Forsha and Nichosl, 1997)
Figure 13 Simplified chart of a single flash power plant with condensing turbine ( Forsha and Nichols, 1997)
Figure 14 Simplified diagram of a double flash power plant with condensing turbine (Darling, 2006)
Figure 15 Simplified diagram of a Very Low Pressure (VLP) Flash Plant as proposed by Yamada (2000)
Figure 16 Simplified diagram of a single binary pow er plant based on the Organic- Rankine-Cycle (Rafferty, 2000)
Figure 17 Simplified diagram of a dual binary pow er plant based on the Organic-Rankine- Cycle (Kanoglu, 2002)
Figure 18 Simplified diagram of a flash combined cycle power plant (Shokouhmand and Atashkadi, 1997)
Figure 19 From the raw material to the product "electricity", a diagram of biomass conversion paths (Yoshida et al., 2003)
Figure 20 Simplified diagram of electricity generation based on direct combustion of solid biomass (Heinloth, 1997)
Figure 21 Simplified diagram of a biomass fired Organic-Rankine-Cycle (Heinimö et al., 2005)
Figure 22 Simplified diagram of electricity generation with a Stirling engine powered with biomass (Bauen et al. 2005)
Figure 23 Simplified diagram of a internal combustion gas turbine fuelled with Syngas
from biomass Gasification (Jurado et al. 2003)
Figure 24 Simplified diagram of a externally biomass fired gas turbine (Cocco et al., 2006)
Figure 25 Summary of relevant temperature ranges of power plant technologies
Figure 26 Influence of the thermal efficiency v on the mass flow m (Ah = 3000kJ/kg) 32 Figure 27 Influence of the thermal efficiency v on the power output W (Ah = 3000kJ/kg)
Figure 28 Comparison of the enthalpies h involved in biomass plants and geothermal resources
Figure 29 T-s-Diagram for proposal 1 with direct integration of the geothermal fluid preheating the boiler's feedw ater
Figure 30 Simplified diagram of a hybrid power plant according to proposal 1
Figure 31 Sankey-Diagram of proposal 1 's process showing the energy flows according to table 3
Figure 32 T-s diagram of the cyclic process of proposal2
Figure 33 Simplified chart of the proposal two hybrid plant
Figure 34 The Influence of increased geothermal energy input on biomass input
Figure 35 Efficiencies of proposal 2 with and without the geothermal energy input
Figure 36 Proportional breakdown of geothermal capital costs based on Hance (2005) and Stefânsson (2002)
Figure 37 Relation of electricity production costs and geothermal resource temperature (source: Kaltschmitt et al., 2003)
Figure 38 Proportionate costs of pow er from biomass, adding up to 0.037€/kWh for whole forest biomass and 0.040€/kWh for agricultural residue for optimum size plants (Kumar et al., 2003)
Figure 39 Influence of geothermal energy input on fuel costs
Figure 40 Assessment of risks for geothermal projects (reproduced from Barnett et al., 2006)
Figure 41 Diagram of the hybrid power plant Wendel, USA
Figure 42 T-s diagram of the hybrid power plant Wendel, USA
Figure 43 Procedural diagram of the projected hybrid power plant in Neuried-Altenheim, Germany
List of Tables
Table 1 Summary of parameters characterising different technologies that serve for electricity generation from geothermal energy sources
Table 2 Thermodynamic states and properties of proposal 1
Table 3 Energy flows from and to the process of proposal 1
Table 4 Concentrations and boiling temperatures of exemplary compounds of geothermal fluids (UNU, 2003)
Table 5 Thermodynamic states and properties of proposal 2
Table 6 Biomass fuel costs
Table 7 Financial data on geothermal and biomass power plants
Nomenclature
(in order of first appearance)
Q,W heat and work flow [kJ]
q,w specific heat and work flow rates [kJ/kg]
KE,PE kinetic and gravitational potential energy [kJ]
U, u internal energy [kJ], specific internal energy [kJ/kg]
dE change of energy content [kJ]
v velocity [m/s]
gz gravitational force [kJ]
h specific Enthalpy [kJ/kg]
p pressure [bar]
v specific volume [m3/kg]
T temperature [K], [°C]
V efficiency [-]
S entropy [kJ/K]
x steam quality [kg/kg]
Subscripts
in transferred to the cycle
out transferred from the cycle
cv control volume
i,e properties of mass entering (i) or leaving (e) a control volume
1,2,3 ... allocation of properties to a state in a cycle
s fictive state for the calculation of isentropic efficiencies th thermal
el electrical
L referring to a cold reservoir
H referring to a hot reservoir
net netto
ges all
regen regenerative
t turbine
p pump
geo geothermal
bm biomass
Introduction
Current energy consumption and energy conversion heavily rely on the combustion of finite fossil fuels, such as oil, coal or natural gas. Their contribution exceeds 80% of total global primary production (IEA, 2005). Apart from heat, electricity is the dominating product of energy conversion as it is easily transportable over long distances and needed for various applications. The global energy demand, including electricity, is projected to steadily rise (Sarensen, 2004). Industry, transportation and other commercial or private sectors contribute significantly with their energy demand to the depletion of fossil energy resources and endanger future energy supply security.
The anthropogenic exploitation of fuel resources and the combustion of these fuels release carbon dioxide that was stored over millions of years in the ground causing an increasing atmospheric carbon dioxide concentration anticipated to cause changes in the global climate (UNFCCC, 2006).
Acknowledging the finiteness of resources and the effects of climate change, alternative solutions for the global energy supply are investigated. These are summarised under the term of “renewable energies”. Prominently hydrological, solar and wind power have been developed and are successfully applied in large scale projects. Further renewable energies from, for example, geothermal resources, wave and tidal potentials or biomass, however, are in theirs infancy. Hydrogen is regarded, additionally, to be the future storage medium for energy and to take over a huge part of future energy supply.
Although success have been achieved the exploitation of renewable energy resources, such as wind, solar radiation or geothermal heat, often reveal low efficiencies and embodies further problems, such as high production costs and high rates of risks (Köhler 2005).
Therefore investigation started recently to combine renewable energies with conventional systems to achieve advantageous effects with these hybrid plants. The co-firing of biomass in conventional coal power plants achieves reduction in carbon emission and fosters the local agriculture and forestry. Moreover the combination of high temperature hydrogen fuel cells with micro gas turbines is believed to convert energy very efficiently and therefore is currently underdevelopment (DLR, 2006).
Two renewable energies with considerable potential to supply significant shares of the future electricity demand are found in biomass and geothermal heat (Barnett et al. 2006). As evidences of hybrid plants combining these tw o renewable energy resources have not been found, this paper investigates thermodynamic, economic and risk considerations of these hybrid power plants.
1.1 Motivation
During a study visit in Santiago de Chile a cooperation with a local innovation company was set up. This innovation agency executes, amongst others, renewable energy and electricity projects and has gained considerable experiences in biomass developments in different regions of Chile. A new business segment is sighted in the fields of geothermal electricity projects. First endeavours on behalf of the agency found a geothermal source with temperatures below 120°C in Curacautin in the IX. Region in the south of Chile. However, as current technologies are thought to be too complex and inefficient to remotely generate electricity form such sources (as outlined in chapter 3.1) the idea was created to combine this low enthalpy geothermal resource with a biomass resource which is also available in the same Chilean region.
1.2 Aim
The aim of this research paper is to show whether hybrid plants utilising geothermal and biomass resources are viable in terms of thermodynamics, economics and what other implications they might have in terms of risk management of each original plant type. The results of this study are thought to enable the Chilean agency and other developers to gain insight into the topic of geothermal and biomass hybrid plants. Furthermore the basis for further research is anticipated to be delivered by this work.
1.3 Objectives
In order to work tow ards its aim this paper considers the following objectives:
- to summarise available processes for electricity generation from geothermal sources,
- to summarise available processes for electricity generation from biomass sources,
- to investigate possible combinations of existing technologies to create a hybrid approach utilising geothermal and biomass energy,
- to thermodynamically evaluate the hybrid concepts,
- to show effects on economics,
- to indicate potential for risk management in project development,
- and finally to indicate further worthwhile research in the field of biomass and geothermal hybrid power plants.
Executing one objective after another can be interpreted as the sequence or methodology the work is conducted with in this paper. As a starting point for combination the current technologies of geothermal and biomass electricity production in relation to the fundamentals of thermodynamics are chosen. By finding a thermodynamically viable electricity focused hybrid power cycle the investigation of economic and risk considerations subsequently can be adequately adjusted. All objectives primarily concern the generation of pow er. The supply of heat is principally not considered.
2. Fundamentals of Biomass and Geothermal Energy Systems
2.1 Thermodynamics of Power Plants
The purpose of power plants is the conversion of other forms of energy into electric energy. Therefore they are subjected to the science of thermodynamics.
2.1.1 Energy Conversion Paths
Different types of technologies utilise various kinds of energy to accomplish the generation of electricity. The energy conversion paths of biomass and geothermal power plants are respectively outlined in figure 1 and figure 2.
Abbildung in dieser Leseprobe nicht enthalten
Figure 1 Energy conversion path of a biomass power plant from the source to electricity (according to Kaltschmitt and Hartmann, 2001)
Figure 2 Energy conversion path of a geothermal pow er plant from the source to electricity (according to Kaltschmitt et al. 1999)
The solar irradiation arriving at the earth surface, either on landmass or on the oceans, embodies solar energy which is converted by the process of photosynthesis to chemical energy. This chemical energy is stored in the organic matter of the earth’s living organisms, such as plants and animals. By various processes, such as outlined in chapter 2.3, this stored energy can be converted into heat energy which is transferred to the power plant’s operation cycle. Subsequently the heat energy is converted to kinetic energy by the turbine. Finally this kinetic energy is transformed into electric energy by the plant’s generator. Figure 1 outlines the energy conversion path in biomass power plants.
The energy conversion path of geothermal power plants starts with distinctive energies. The chemical energy that is converted by radioactive decay in the inner of the earth into heat energy delivers up to 70% of the input energy (Kaltschmitt et al., 1999). The remaining 30% are taken from the earth stored heat energy that is transported by convection and conduction to the earth’s outer layers. Fed into the power plant’s cycle this heat is converted into kinetic energy before electric energy is generated. figure 2 indicates this path.
2.1.2 Introduction to thermodynamic cycles
The operation cycle in a common power plant is thermodynamically denoted power cycle and has the purpose to convert an input of energy into power, i.e. electricity. Power cycles can be distinguished according to their working medium. If gas circulates through the cycle it is a gas cycle and if steam circulates it is a steam cycle. As outlined in chapters 2.2 and 2.3 most of the efficient technologies of geothermal and biomass pow er plants utilise steam as w orking fluid, except gas turbines. Therefore this paper concentrates on steam cycles.
Steam is a common w orking medium as it is of low cost, widely available, non-hazardous and in the superheated state of high enthalpy. Applications in low er temperature ranges, such as the binary cycles, described in chapter 2.2, however, uses other fluids to vaporise, such as organic substances.
Another criterion to distinguish power cycles is whether they are closed or open systems. The system with back-pressure turbine shown in figure 11 can be defined as an open system, or control volume, as it disposes the expanded steam to the environment. Fresh steam is taken from the geothermal reservoir and fed into the process. Compared to the open cycle in figure 11 the system in figure 16 is a closed system. After expanding in the turbine the working fluid is conditioned to its starting state and re-circulated.
As energy in form of heat and w ork is added to the power cycles, concerned here, from the exterior, these cycles present external combustion engines. Gas turbines, as described in 2.3, however, are internal combustion engines, as the energy conversion process occurs within the boundaries of the system.
For external combustion engines with a closed steam cycle a specific designed technology is established. Invented in the late 19th century by the Scottish engineer William John Macquorn Rankine the Rankine-Cycle is today’s standard cycle of power cycles operating with steam. The same cycle is sometimes named Clausius-Rankine- Cycle as the German physicist Rudolf Clausius found results similar to Rankine’s.
Abbildung in dieser Leseprobe nicht enthalten
Figure 3 T-s-diagram of the ideal Rankine-Cycle (Moran et al., 2002)
Figure 4 Schematic chart of the Rankine-Cycle with pump, boiler, turbine, and condenser. Shown are energy Transfers Q and W over the system’s boundaries (Moran et al., 2002)
The ideal Rankine-Cycle is outlined in figure 3 and figure 4. As it is an ideal cycle it is assumed that no internal irreversibilities are involved. The cycle consists of the following four processes:
Process from state 4 to state 1
Liquid water of state 4 is compressed isentropically, i.e. without an increase in entropy, to the operating pressure of the boiler and the turbine. As the pressure of the fluid is increased the fluid’s temperature increases slightly as well. The compression is accomplished by the pow er input of Win.
Process from state 1 to state 2
The pressurised fluid of state 1 enters the boiler where its enthalpy is increased. This causes the fluid to vaporise and to reach the superheated state 2. In the Rankine-Cycle this process ideally occurs isobarically, i.e. at constant pressure. The energy input is denoted Qin and can be extracted from energy sources such as biomass or geothermal fluids.
Process from state 2 to state 3
Entering the turbine the superheated fluid expands isentropically and propels the turbine’s shaft causing the generator to produce electricity. The power output of the generator is denoted Wout. By increasing its volume the superheated fluid experiences a drop in pressure and temperature. In most cases the fluid relapses to saturated vapour of high quality and reaches state 3.
Process from state 3 to state 4
The high quality saturated vapour of state 3 enters the condenser where heat is transferred to the system’s surroundings causing an isobaric decrease of the vapour’s enthalpy by the amount of Qout. The medium leaves the condenser as a liquid of state 4 and enters the pump, completing the cycle.
2.1.3 Modelling of power cycles
In order to achieve a simplification and to keep complications in a manageable range the systems of power cycles are idealised. As various devices, such as pump, boiler, turbine, condenser, pipes, valves and measuring units, are involved in an actual power cycle, idealisation is necessary to gain insight into processes und foster understanding of the cycles’ behaviour.
Therefore it is common to assume that no internal irreversibilities are involved. The presence of friction and the lack of time to reach the equilibrium state are ignored in the ideal modelling of power cycles.
For steam cycles a typical Rankine-Cycle illustrated in figure 3 is the ideal cycle. Processes are either isobaric or isentropic. Both do not correspond to reality as friction, for example, causes pressure drops and increases in entropy.
Models in this paper assume idealisation by removing internal irreversible processes as well. Idealisations and simplifications that are applied are:
(1) Friction does not exist and therefore is not concerned in the models. This results in isobaric processes for condenser and boiler processes and fluid transport through pipes. How ever, in this paper friction is concerned in the processes operated by the turbine and the pump. Isentropic efficiencies are set to n=0.85 (Vijayaraghavan and Goswami, 2004).
(2) A process from one state of the fluid to another is assumed to occur in quasiequilibrium. States do not change with time.
(3) Devices, such as pipes and valves, and all outer surfaces are insulated in a way that no heat transfer occurs to the system’s surroundings.
(4) Changes in the kinetic and the gravitational potential energy are not significant and therefore negligible.
These idealisations and simplifications allow an insight into processes and evaluate the behaviour of systems when its parameters are changed. However, the loss of accuracy is the expense of these assumptions. Consequently cycle modelling is likely to present tendencies. Results of quality characteristic can be output by models. Nonetheless, quantitative results of ideal cycle models are not representative for real cycles.
2.1.4 Energy Analysis of Power Cycles
The Rankine-Cycle as a whole is thermodynamically a closed system as no mass flow occurs through its system boundaries. Referring to the idealisation this paper assumes, the equation of the energy balance for closed systems gets reduced from its original form in equation (1) to equation (2).
Abbildung in dieser Leseprobe nicht enthalten
The heat transfer Q has two components. As outlined in figure 4, Qin takes the heat transfer to the cycle into account and Qout the heat transfer from the system to its environment. Work enters the cycle by the amount Win and leaves the cycle by Wout. As it is common to relate equation (2) to the mass flow within the cycle the equation becomes
Abbildung in dieser Leseprobe nicht enthalten
Although the power cycle as a whole is a closed system, the evaluation of single devices, such as boiler or turbine, is subjected to the energy balance for control volumes. Assuming that the control volume operates at steady-state and that due to the idealisation of the system the difference of the internal energies equals the difference of its enthalpies equation (4) is simplified to equation (5).
Abbildung in dieser Leseprobe nicht enthalten
For each device of the ideal Rankine-Cycle equation (5) can be expressed. Indices relate to figure 4.
Abbildung in dieser Leseprobe nicht enthalten
Relating mass flow, efficiency and power output of a power cycle to each other
Abbildung in dieser Leseprobe nicht enthalten
In the analysis of power cycles enthalpies play a significant role. Enthalpy accounts for the ‘heat content’ of a fluid in a specific state in kJ/kg (Crebe and Hoffmann, 1996). According to equation (12) it is the sum of internal energy u and the product of pressure p and specific volume v.
Abbildung in dieser Leseprobe nicht enthalten
Although idealisations are assumed for a reasonable cycle analysis an isentropic efficiency is attached to the pump and the turbine of the cycle. This kind of efficiency accounts for the internal irreversibilities that occur in the pump and turbine. These processes lead to an increase in entropy of the fluid (Moran et al., 2002). In the T-s- diagram this results in slopes of the process lines.
Abbildung in dieser Leseprobe nicht enthalten
The ratio of the enthalpy decrease of a process with an isentropic efficiency of nt/p (h2-h-i) and the decrease of an isentropic process (h2s-h-i) is defined as isentropic efficiency and outlined in equation (13).
A basic consideration of a power cycle always is its efficiency. As the cycle’s purpose is the conversion of other forms of energy into w ork, the ratio of accomplished power output and the necessary energy input, named thermal efficiency, is of significance.
The maximal thermal efficiency that any cycle can operate with only depends on the temperatures of the hot and cold reservoir it is working in between. In thermodynamic terms a reservoir is an infinite energy source or sink, which does not change its properties when energy is extracted or added. Geothermal reservoirs are considered to be thermodynamic reservoirs as w ell due to their size and inertia. The French mathematician Nicolas Léonard Sadi Carnot calculated the maximum efficiency according to equation (14). It is denoted Carnot-Efficiency.
As the maximal efficiency nth,camot only is a function of the temperature TL of the energy sink and the temperature TH of the energy source the actual design of the cycle evaluated has no significance. Therefore the Carnot efficiency can be applied as a benchmark any cycle can be compared against. All cycle efficiencies must be below the Carnot-Efficiency (Moran, 2000). The actual thermal efficiency nth can be calculated for each specific cycle. For the ideal Rankine-Cycle this value is determined by equation (15).
By transposing the defining equation for entropy, equation (16), equation (17) is produced which helps to evaluate thermal efficiency in a graphical way.
Abbildung in dieser Leseprobe nicht enthalten
The area under the cycle’s curve corresponds, according to equation (17), with the energy transfer from or to the cycle. The area s1-1-2-s2 represents the energy input qin to the cycle by the boiler. By the condenser energy of the amount qout, displayed by the area s1- 4-3-s2, is disposed to the environment. According to equation (10) the area 4-1-2-3 is consistent with the work output of a cycle wout. The ratio of the area 4-1-2-3 to area s1-1- 2-s2 is the thermal efficiency shown in a graphical way. Efforts of increasing a cycle’s efficiency have to result in the increase of the area 4-1-2-3.
Abbildung in dieser Leseprobe nicht enthalten
Figure 5 T-s-Diagram including qin, qout and wout
The determining of the cycles' state properties in this paper has been supported by the utilisation of two thermodynamic programs. These are:
- “Interactive Thermodynamics v2.0” (Moran et al., 2000)
- “FluidEXL Graphics” [Microsoft Excel Add-In]
2.1.5 Standard improvements of the Rankine-Cycle
The basic idea of any improvement of the ideal Rankine-Cycle has to foster the concept delivered by Carnot and his formulation of the efficiency in equation (14). Therefore an increase of the temperature TH of the fluid passing the boiler and a decrease of the temperature TL at which the heat of the fluid is disposed to the environment by the condenser have to be focused. Standard improvements of this kind are demonstrated in the diagrams from figure 6 to figure 9.
Abbildung in dieser Leseprobe nicht enthalten
Figure 6 The effect of lowering the condenser pressure in the T-s- Diagram
Figure 7 The effect of superheating the steam in the T-s-Diagram
Lowering of the condenser pressure (lowers TL)
The working fluid passing through the condenser exists at saturation pressure and is cooled until it reaches it saturated liquid state at constant temperature. By reducing the pressure in the condenser the saturation temperature of the working fluid decreases as well. For that reason the condenser pressure is held well below atmospheric pressure in today’s power cycles. Subsequently heat is rejected to the environment at lower average temperatures in the condenser. TL in equation (14) decreases. The result is a gain in wout shown in figure 6 by increased cycle efficiency according to Carnot.
Superheating the steam to higher temperatures (increases TH)
Thermal efficiency of a cycle can be improved as well by superheating the steam. By the increase of the heat supply (area under 2-2' in figure 7) to the working fluid the shaded area in figure 7 is gained as wout. Even though the cycle requires more energy input the efficiency is increased as heat is supplied to the working fluid at higher average temperatures.
Abbildung in dieser Leseprobe nicht enthalten
Figure 8 The effect of increasing the boiler pressure in the T-s-Diagram to Diagram to lower TL
Figure 9 The effect of reheating in the increase TH
Increasing of the boiler pressure (increases TH)
Increasing the working fluid’s pressure when it passes through the heat exchanger of the boiler changes the cycle’s appearance in the T-s-Diagram according to figure 8. Although causing a decrease in wout at one part of the cycle thermal efficiency is increased as the gain in wout by this process is predominant.
Reheating the turbine exhaust for second stage process
Modifying the ideal Rankine-Cycle to an ideal reheat Rankine-Cycle increases efficiencies. After leaving the first turbine stage the steam enters a further heat exchanger and is reheated again to the level it had at the inlet of the first turbine stage. An expansion in a second stage turbine occurs. The pressure drop from the inlet of the first turbine to the condenser pressure equals the pressure drop in the ideal Rankine-Cycle. The corresponding diagram is shown in figure 9.
Regenerative preheating of boiler feedwater
A fifth alternative of the ideal Rankine-Cycle influencing its performance is given by a regenerative preheating of the boiler feedwater. Before entering the boiler the cycle’s fluid passes an additional heat exchanger, takes up regenerative energy and enters the boiler at a higher temperature. Although this causes adverse effects on the boiler design the gain in “free” energy overweighs. Indicated in figure 10 no change of overall efficiency occurs. However, as the regenerative part qinregen, taken, for example, from renewable energy resources, decreases the amount qinges- qin,regen which is supplied to the cycle in non-regenerative, conventional manner. Therefore the efficiency in terms of conventional energy input increases.
Abbildung in dieser Leseprobe nicht enthalten
Figure 10 The effect of preheating the boiler feedw ater in the T-s-Diagram
2.2 State of the Art of Geothermal Energy Systems
2.2.1 Introduction to Geothermal Energy Systems
The term geothermal energy describes all exploitable energy existing below the earth surface regardless of its sources (Dickson and Fanelli, 2003). Apart from the solar energy that only influences the earth crust in the upper 30 m the heat within the earth is mainly made up by two components.
With the genesis of the earth heat was produced by gravitational forces. This heat today still is stored within the earth and is steadily released from the high temperature core of the earth. It gradually conducts from the inner to the outer of the earth (Heinloth, 1997).
The second and most important component is the heat generated by the decay of radioactive material. Stored in the earth, isotopes, such as Uranium (U , U ) or Thorium (Th232), decay under the release of heat. This process primarily occurs in the earth’s crust (Kaltschmitt et al., 1997).
The production of heat within the earth causes a heat flow from the inside to the outside. Its value was investigated to be between 60 and 65 mW/m2 (Kaltschmitt et al., 1997). Stone is a excellent heat storage as 1m3 of it stores an energy equivalent of 9kg of coal, assuming a temperature change of 100°C (Heinloth, 1997). With increasing depth the temperature of the earth rises reaching . Although depending on the specific characteristics of the geological matter in each case a gradient of 30 K/km can be used as a rough point of reference (Dickson and Fanelli, 2003). Attractive geothermal systems with gradients in excess of 30 K/km can be localised around geological plate margins. Geothermal energy is steadily available.
Geothermal systems are classified according to the enthalpy the geothermal fluid can be extracted with from its deposit. As enthalpy is more or less proportional to temperature classifications are often given in °C. Today’s classification is dominated by the limits of Axelsson and Gunnlaugsson (quoted in Dickson and Fanelli, 2003) which distinguish between low enthalpy and high enthalpy resources with temperatures <=190°C and >190°C, respectively. However, according Hochstein (1990) or Benderitter and Cormy (1990), this paper defines geothermal resource of low enthalpy quality as resources with fluid temperature not in excess of 120°C.
The stored energy within the earth can be harvested by the help of w ells that are drilled to the necessary levels of depth. The fluids accessible by this method can be used directly or indirectly. The direct utilisation of geothermal heat comprises balneology, industrial supply and space heating. The indirect utilisation focuses on the power generation. The latter is concentrated on by this paper as global electricity demand is rising and power is a form of energy that is easily and over long distances transportable, unlike heat itself (Dickson and Fanelli, 2003).
2.2.2 Existing Geothermal Electricity Generation Technologies
Various technologies to convert the heat energy of geothermal sources into electricity have been developed and still are under development. The subsurface heat exchanging open or closed systems to gain the extract the geothermal heat from the ground are not considered.
Dry Steam Power Plants
An aquifer coinciding with geological anomalies produces a fluid of high enthalpy. These reservoirs are exploited by drilling w ells which deliver the steam to the surface. “The Geysers” field in the United States of America, for example, produces with w ells of 1500 - 2900 m depth a dry steam with a pressure of 30 bar and an enthalpy of 2800 kJ/kg (Zahoransky, 2001).
Due to its conditions the steam can be expanded directly in a steam turbine. Downstream of the turbine a condenser cools the expanded steam and holds pressure below the atmospheric level. The condensed fluid is re-injected to the geothermal reservoir.
Schematic diagrams of dry steam power plants are outlined in figure 11 and figure 12. As temperature differences of incoming geothermal steam and ambient air is significant the Carnot-Efficiency can be calculated to exceed 30% (assumed ambient temperature of 25°C and turbine inlet temperature of 180°C).
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Figure 11 Simplified diagram of a dry steam power plant with backpressure turbine (Forsha and Nichols,1997)
Power plants based on the dry steam technology were the first facilities utilised to generate electricity from geothermal sources. In 1904 Prince Piero Ginori Conti built the first commercial power plant only fuelled by geothermal steam in Ladorello, Italy. Being upgraded with the time the plant today still uses the same geothermal reservoirs (Rau, 1978).
Flash Power Plants
Flash Power plants are applied when a combination of steam and hot water or only hot water can be pumped from the geothermal well. In comparison to the dry steam process, however, the brine is not used directly. Coming with high pressure from the well the w ater is sprayed into a flash tank which is held at a lower pressure. This causes that a portion of the brine rapidly vaporises, or “flashes”, into steam and water. After being expanded in the turbine the steam is condensed and re-injected to the geothermal reservoir together with the water that did not flash in the flash stage.
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Figure 13 Simplified chart of a single flash power plant with condensing turbine (Forsha and Nichols, 1997)
A second flash stage can be attached if the enthalpy of the geothermal fluid is high enough. In this case the water leaving the first flash tank is directed to a second tank of even lower pressure. Here another portion “flashes” and produces more steam of a lower pressure that subsequently enters together with the high pressure steam a dual-entry turbine. The additional extraction of heat from the geothermal fluid increases the electrical net output of double flash plants by up to 30% compared to single flash plants at the same flow rate (Darling, 2006). Plant sizes of flashing plants easily reach 20-60 MWel. According to the number of flashing stages these processes are denoted “single flash” or “double flash”.
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Figure 14 Simplified diagram of a double flash power plant with condensing turbine (Darling, 2006)
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