about geothermal energy

1. Geothermal energy conversion
Leda Gerber
August 19, 2009
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2. 1 Introduction and history
Geothermal energy is a renewable source of energy that present many advantages: it does not
depend on the climatic conditions, nor on the seasons or the time of the day, it emits very few
air pollutants and it has a huge potential. Its various levels of temperature and depth makes
it suitable for a wide range of applications: electricity production, district heating, cooling, hot
water production and heat for industrial uses. Today, the worldwide use of geothermal resources
vary from shallow resources that have a temperature of around 25C and a depth of a few hundred
meters, which are used for domestic heating by the mean of heat pumps, to deep resources at a
few kilometers of depth that have a temperature of more than 200C, which can be used for the
production of electricity. Uses of geothermal energy are generally divided in two categories: direct
use and electricity production. In 2005, it was estimated that the installed thermal capacity for
direct uses of geothermal energy was around 28’268 MWth [12], and that the worldwide installed
capacity for geothermal electricity production was around 8’930 MWe [3].
Since this course is specifically about the conversion of geothermal energy and not about
direct uses, it will mainly concentrate on the production of electricity, which requires specific
technologies. The use of geothermal heat pumps will also not be treated here, since another
chapter of the energy conversion course specifically deals with heat pumps.
Electricity from geothermal steam was first commercially produced in 1913 at Larderello, in
Italy. Some other projects took place in the following decades, notably in Japan and in California,
but geothermal power generation really started to expand in the 1950’s, and has been growing
steadily since, as shown by figure 1.
Figure 1: Evolution of worldwide installed capacity for geothermal electricity production (source:
Barbier, 2002 [2])
Historically, electricity production from geothermal energy has been strongly developed in
countries and places having an easy and therefore cheap access to geothermal resources. These
places present generally particular geological conditions: a high thermal gradient and special
features such as hot springs or geothermal steam fields. Such locations are generally close to
the tectonic plate boundaries or to a magma hot spot. However, with the development of
new technologies and the growing need of indigenous, renewable and environmentally-friendly
energy sources, many other countries show interest in accessing to unconventional geothermal
resources for commercial electricity production. Hot Dry Rock (HDR) resources, also known as
Enhanced Geothermal Systems (EGS), are a promising option for a more spread and an increased
geothermal power production capacity in the next decades [20, 13].
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3. 2 Thermodynamic principles
Conversion of energy from geothermal resources deals with the conversion of heat that is available
from hot water coming from a geothermal well to electricity, generally using a thermodynamic
cycle. An important characteristic of the geothermal resources is that the temperature level is
different among the different geothermal resources, and this determines the heat that is avail-
able from a geothermal resource. This temperature level is generally function of the depth
when the location presents no special geological conditions, and an average geothermal gradient
of 3.1C/100m can be taken to estimate the temperature level in function of the depth of the
geothermal well [7]. Then, once the temperature level and the flow rate from the geothermal
well are known, the available heat is calculated by eq. 1:
Q̇ = ṁcp(Tin − Tout) (1)
where ṁ is the mass flow rate of the geofluid coming from the geothermal well, cp is the specific
heat of water, Tin is the inlet temperature of the geofluid and Tout is the outlet temperature of
the geofluid, or reinjection temperature.
This heat available can be used to calculate, for an existing cycle producing electricity from
a geothermal resource, what is the energy efficiency, or the first law efficiency, of this cycle. The
energy efficiency of a geothermal power plant producing electricity is given by eq. 2:
ηe =
Ė−
Q̇+
(2)
where Ė−
is the net electricity produced by the cycle, after removing the parasitic loads, and Q̇+
is the heat available from the geothermal resource. In the case of geothermal resources, since the
temperature is rather low compared to conventional cycles producing electricity using fossil fuels,
the energy efficiency will then inherently be low, because of the Carnot factor. Therefore, it is
also important to use the exergy efficiency to conduct an accurate performance assessment of a
geothermal power plant [6]. The exergy available, or maximal potential work, in the geothermal
resource is calculated by eq. 3:
Ėx = Q̇ ∗ (1 −
Ta
Tlm,geo
) (3)
where Q̇ is the heat available from the geofluid, Ta is the temperature of the cold source used
for electricity production, usually air or river water, and Tlm,geo is the log-mean temperature
difference of the geofluid, calculated by eq. 4:
Tlm =
Tin − Tout
ln( Tin
Tout
)
(4)
where Tin is the inlet temperature of the geofluid, Tout is the outlet temperature, or reinjection
temperature, of the geofluid. We use this equation to calculate the temperature of the hot source,
because the geofluid is not at a constant temperature and is cooling down as its heat is taken out
of it for electricity production. The effect of the temperature of the geofluid and its reinjection
temperature on the Carnot factor and the exergy available can be seen on figure 2. The shaded
areas in figure 2 represent the total exergy available from the geofluid.
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4. Figure 2: Carnot factor of a geofluid and its exergy available for different inlet and reinjection
temperatures (source: OFEN, 2007 [13])
The exergy efficiency of a cycle producing exclusively electricity from a geothermal resource
can be calculated by eq. 5:
ηex =
Ė−
Ė+
x
(5)
In the case the geothermal power plant is a cogeneration power plant, and is therefore pro-
ducing district heating or heat for other purpose in addition to electricity production, the eq. 2
and 5 have to be adapted to take into account the additional amount of the geothermal resource
that is valorized in the form of heat. The energy efficiency becomes:
ηe =
Ė−
+ Q̇−
DH
Q̇+
(6)
And the exergy efficiency becomes:
ηex =
Ė−
+ Q̇−
DH ∗ ( Ta
Tlm,DH
)
Ė+
x
(7)
3 Technology
This chapter gives an overview of all the different cycles that can be used for the production of
electricity from geothermal resources, presenting their working principle, the important aspects
for their design and the possible issues and drawbacks. Possibilities of cogeneration for district
heating are also considered.
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5. 3.1 Dry steam power plants
There are very few geothermal fields where the resource is in the form of dry steam and can
directly be used to drive a turbine to produce electricity. In most of the cases, the pressure and
temperature of the geothermal resource to exploit make it to be in the form of a mixture of
liquid and vapor or a liquid-dominated resource. Therefore, more complex cycles are required
to convert their available heat to electricity. However, despite the limited locations where dry
steam is directly produced, dry steam power plants represented, in 2007, 26% of the worldwide
installed geothermal power capacity, and 12% of the geothermal power plants units [7]. Much of
this production is parted between the two main geothermal steam fields in the world: Larderello
in Italy, and the Geysers in California.
3.2 Single-flash steam power plants
For most of the geothermal resources, the geofluid is a liquid-dominated resource or a mixture of
liquid and vapor. Therefore, if one wants to directly use this resource in steam form to drive a
turbine to produce electricity, it is necessary to separate the vapor phase from the liquid phase.
The thermodynamic principle of the operation is displayed at figure 3, and the schematic process
can be seen at figure 4: a liquid-vapor separator, or flash drum, is placed before the turbine,
and the steam is used to drive the turbine to produce power while the liquid phase goes directly
for reinjection in the geothermal reservoir. A condensation below atmospheric pressure allows
increasing the efficiency of the cycle. Since a part of the geofluid is being lost in the atmosphere
through the cooling towers used for condensation, water make-up can be necessary to avoid
pressure drop in the geothermal reservoir.
Figure 3: Thermodynamics of a single-flash steam process on a T-s diagram
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6. Figure 4: Schematic representation of a single-flash steam power plant
Single-flash steam cycles can be used to produce power from liquid-dominated or liquid-
vapor mixture geothermal resources, having a temperature over 150C. Below this temperature,
the production of power directly using the geofluid is not economically advantageous. In 2007,
single-flash steam power plants represented 42% of the worldwide installed geothermal power
capacity, and 32% of the geothermal power plants units [7].
For a single-flash steam cycle, the operating pressure of the flash drum is a parameter to be
optimized: the higher the pressure, the higher is the specific power output per unit of steam, but
the lower is the total steam flow rate passing through the turbine.
Some operating issues with the flash steam cycles can occur due to the physico-chemical
characteristics of the resource. First, dissolved minerals contained in the geofluid can precipitate
during phase separation and cause clogging of the system. Therefore, regular cleaning is required,
which results in relatively high maintenance costs. Then, the geofluid can have a high content
in gases such as carbon dioxide or hydrogen sulphide. These gases pass along with the steam
in the turbine and eventually arrive to the condenser, but they are non-condensable. If they
accumulate, they cause pressure increase in the condenser and decrease the efficiency of the
system. They can also cause corrosion to the turbine if they return back to it. Therefore, a gas
removal system, generally using ejection devices, can be required, and the non-condensable gases
are released to the atmosphere. It should be noted that this penalizes the efficiency of the cycle.
In order to increase the utilization of the resource and therefore the energy and exergy effi-
ciencies, single-flash steam power plants can be designed to provide also district heating, if there
is a demand to satisfy. This is done by adding a heat exchanger at the liquid part of the resource,
as shown at figure 5. This allows valorizing the liquid part of the resource, which is otherwise
not used and just re-injected.
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7. Figure 5: Schematic representation of a single-flash steam power plant with cogeneration
It should be noticed that the temperature of the liquid part of the geothermal resource is still
rather high after the separation, and that this is efficient to provide high-temperature district
heating around 80-90C, or any other industrial heat demand having a similar temperature level
requirement.
3.3 Double-flash steam power plants
The double-flash steam geothermal power plant is an improvement of the single-flash steam power
plant. Compared to single-flash systems, double-flash systems can produce 15-25% more power
output for the same geothermal resource. The principle is explained at figure 6 and 7. The
difference is that the liquid part remaining after the first separation is flashed a second time at
a lower pressure, which produces additional steam that is either used in a lower pressure turbine
to produce additional power or directly injected in the same turbine at this lower pressure level.
Figure 6: Thermodynamics of a double-flash steam process on a T-s diagram
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8. Figure 7: Schematic representation of a double-flash steam power plant
Double-flash steam power plants can be selected for the conversion to electricity of geothermal
resources having similar conditions than the ones that are used in single-flash steam power plants.
In 2007, double-flash power plants represented 23% of the worldwide installed geothermal power
capacity and 14% of the geothermal power plants units [7].
The optimal design of a double-flash steam power plant is more complex than for a single-
flash steam power plant. Indeed, it is not only the pressure of the 1st flash drum that requires
to be optimized, but also the pressure of the 2nd one.
The operating issues in the case of a double-flash steam power plant are the same than the
ones linked to the operation of a single-flash: potential clogging caused by mineral precipitation
in the flash separators and issues linked with the possible accumulation of non-condensable gases
in the condenser, which can require to also install a gas removal system.
It is also possible to have more than two flashing stages in the power plants, and a few triple-
flash steam plants have been built. However, this results in a really complex design and high
investments costs.
Double-flash steam power plants can also be used for cogeneration in the same way than
single-flash steam power plants, by adding a heat exchanger at the liquid part remaining after
the second flashing.
3.4 Binary power plants - ORC
Binary cycles refer to geothermal power plants where the heat from the geofluid is transferred
to another fluid that is then expanded to produce power and working in a closed cycle. These
geothermal power plants work therefore on the same principle than conventional fossil-fueled or
nuclear power plants, except that the working fluid is not water but another type of fluid. Indeed,
the temperature level of the geothermal resources is too low to use the water as the working fluid
in an efficient way, and other types of fluids are selected.
Binary cycles are generally used when a flash cycle can not be used. This can happen either
when the temperature of the geofluid is too low, below 150C, or when the physico-chemical
quality of the resource is too bad to build a flash steam plant because of mineral precipitation
issues. ORCs can be used from temperatures going from 70C up to 300C [13].
It has to be noticed that binary cycles is not a technology restricted to geothermal resources,
but has important applications for waste heat recovery from industrial processes, generally at
low temperature.
Organic Rankine Cycles (ORCs) are the most important category of binary cycles used for
geothermal applications. The working fluid used is an organic fluid. Figures 8 and 9 show the
thermodynamics of two possible ORC process, with and without superheating. Figure 10 displays
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9. the working principle of an ORC. The working fluid is preheated in a liquid state, evaporated,
sometimes superheated, the vapor is then expanded in a turbine, condensed and pumped at the
higher pressure to start again the cycle.
Figure 8: Thermodynamics of an ORC process with superheating on a T-s diagram
Figure 9: Thermodynamics of an ORC process without superheating on a T-s diagram
Figure 10: Schematic representation of an ORC power plant
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10. Since the working fluid at the outlet of the turbine is generally still in a superheated vapor
state, additional heat is available from the process, and instead of sending this heat to the
condenser, it can be used to preheat a part of the liquid working fluid. This is done by adding a
heat exchanger, called a recuperator, between the turbine and the condenser, as shown in figure
11 below.
Figure 11: Schematic representation of an ORC power plant with recuperator
Though ORCs represented just 4 % in 2007 of the worldwide installed geothermal power
capacity, they represented 32% of the installed geothermal power-producing units [7].
An important aspect when designing an ORC is the selection of an appropriate working
fluid for the temperature level of the geothermal resource. This is done by looking at the
thermodynamic properties of the potential working fluids, such as critical temperatures and
pressures, molar weight, but also the shape of the saturation curve, which can either be normal
or retrograde. Figure 12 displays possible working fluids that can be used for ORCs in geothermal
applications.
Figure 12: T-s diagrams of the different potential working fluids that can be used in ORCs for
geothermal applications (source: OFEN, 2007 [13])
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11. Other important design aspects for ORCs are the higher and lower operating pressures of the
system. Unlike a flash steam cycle, the condensing pressure is higher than atmospheric pressure.
The operating issues linked with the direct use of the geofluid occuring in the case of a flash
system do not occur in the case of an ORC, and maintenance is therefore easier. Also for the
same reason, and since full resource is reinjected, water make-up might not be necessary, except
in the case the geothermal reservoir is artificially engineered such as in the case of an EGS. An
other operating advantage of the ORC is that the condensing pressure is higher than atmospheric
pressure, and there is therefore no risk of atmospheric air accidental inlet.
However, there is generally higher exergy losses in an ORC than in a flash cycle, because of
the minimal temperature difference at the heat exchange between the geofluid and the organic
fluid, and because of the evaporation of the working fluid. The investment costs will also be
higher for an ORC than in the case of a flash steam plant because of the more complex design
and additional process equipment required. Another issue is related to the safety aspects, since
organic fluids are flammable.
Cogeneration when there is a district heating demand is also possible with an ORC. Depending
on the temperature level required by the district heating, the design of the cycle and the place of
the heat exchanger differ. For a high temperature district heating, around 80-90C, the district
heating heat exchanger operates in parallel of the ORC, as shown by figure 13. In the case of
a low temperature district heating, around 50-70C, a bleeding is done at the turbine to provide
this low-temperature heat, as shown by figure 14.
Figure 13: Schematic representation of an ORC power plant with cogeneration for high-
temperature district heating
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12. Figure 14: Schematic representation of an ORC power plant with cogeneration for low-
temperature district heating
It is important to underline that in both cases there is anyway a trade-off between the
electricity production and the heat production.
3.5 Binary power plants - Kalina cycles
Kalina cycle is a particular type of binary cycle which uses a mixture of water and ammonia as
the working fluid. The advantage is that a mixture of fluids does not evaporate and condense at
constant temperature. Therefore, it introduces a glide in the vaporization profile which reduces
the exergy losses when the heat is transferred from the geofluid to the working fluid, as it can
be seen at figure 15. This theoretically allows producing 30% more power than with an ORC
for a similar geothermal resource [14]. The advantage of the mixture of water and ammonia is
also that its behaviour is well known since it has been used for long in refrigeration applications.
It is also a cheap working fluid compared to the organic fluids. A drawback is the toxicity of
ammonia, which can be problematic in case leakages occur.
Figure 15: T-Q diagram of a Kalina cycle for geothermal power generation (source: Zamfirescu
et al, 2008 [21])
Kalina cycle is a patented system, and figures 16 and 17 show two types of commercialized
designs for geothermal applications. Figure 16 shows a design which is suitable for geothermal
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13. sources at low-temperature, below 120C, using a separator [14]. Figure 17 shows a design which
is suitable for geothermal sources at high-temperature, above 120C up to more than 200C [14].
Figure 16: Schematic representation of a low-temperature Kalina cycle KCS34
Figure 17: Schematic representation of a higher temperature Kalina cycle KCS11
An important parameter of the Kalina cycle is the concentration of ammonia in the mixture.
An advantage is that it can be adapted in case the temperature of the geofluid changes with
time. The higher and lower operating pressures of the system, and the splitting factors, are also
other important parameters to be optimized.
The main issue with the Kalina cycle is that it is currently still a non-mature technology,
and there is therefore a lack of experience and practical operation. Only one Kalina cycle
for geothermal application has been built for commercial operation up to now. A comparison
conducted concluded that it had an exergy efficiency similar to the ORC technology [6]. Therefore
some important improvement is required to reach the promised theoretical efficiency.
Regarding investment costs, these will generally be higher for a Kalina cycle than for an
ORC, though in a similar range.
Cogeneration is also possible with a Kalina cycle, in the same way than it is for an ORC.
3.6 Flash-binary power plants
Flash steam power plants and binary power plants can also be combined together to improve
the utilization efficiency of a geothermal resource. The principle is to use a binary cycle as a
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14. bottoming cycle of a flash steam cycle to produce additional electricity at a lower temperature
from the separated liquid part of the resource. This is illustrated by a schematic example at
figure 18, showing a bottoming simple ORC for a single-flash steam power plant.
Figure 18: Schematic representation of a bottoming ORC for a single-flash steam power plant
Flash-binary power plants represented, in 2007, 4% of the worldwide installed geothermal
power capacity, and 9% of the installed geothermal power-producing units [7].
While such systems allow improving the utilization of a geothermal resource, they result
however in significantly higher investment costs, and an economic survey should be conducted
to find out if the investment of an ORC to produce additional power is worth.
3.7 Hybrid systems
There is also the possibility to use geothermal resources in synergy with other energy resources.
The option the most often mentioned is to use geothermal resource in conjunction with a con-
ventional fossil fuel power plant [5, 10]. There are two possibilities: either use the exhaust gases
of the fossil fuel power plant to superheat the geothermal steam prior this one enters the turbine,
or to use the geothermal resource do the pre-heating in the fossil fuel power plant. The first
possibility is illustrated by the figure 19 below, showing the integration of a flash-steam cycle in
a gas turbine, where the exhaust gases of the gas turbine are used to superheat the geothermal
steam before it enters the turbine.
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15. Figure 19: Schematic representation of the topping of a single-flash steam cycle by a gas turbine
For the moment, just one commercial hybrid power plant has been built and used to produce
power [7].
4 Thermo-economic performances
The energy and exergy efficiencies of the technologies presented above can be calculated by
using eq. 2 to 7. For the economic performances, the investment costs of the power plant, the
operating costs of the power plant and the levelized cost of produced electricity can be considered.
It should be noted that in the case of geothermal energy, the investment costs are generally more
determining than the operating costs since:
• the "fuel" (e.g. the geothermal hot water) is free, unlike in the case of conventional fossil
fuel power plants.
• the investment costs include not only the process equipment of the power plant, but also
the exploration phase and the drilling of the wells, which can become a high part of these
investment costs, especially for accessing to deep geothermal resources (OFEN 2007).
Table 4 presents a summary of the thermo-economic performances of the technologies pre-
sented at the above chapter, including their typical size ranges [20, 13, 7]. For the Kalina cycle,
since there is only one cycle in operation, the efficiencies in the table are the ones calculated for
this cycle only [6, 15]. For the production costs of electricity displayed here [11], it should be
noted that these are only indicative values, since it can depend heavily on the site conditions,
which influe on the costs of drilling for the wells and on the size of the installation.
Flash ORC Kalina
Size range in MW 3-110 0.1-10 2
Energy efficiency in % 10-20 5-15 12
Exergy efficiency in % <75 <50 45
Investment costs plant in $/kW 750-1950 1500-2500 n.a.
Maintenance costs in $/kW/yr 100 n.a. n.a.
Cost of electricity production in $/kWh 0.02-0.03 0.03-0.05 n.a.
Table 1: Thermo-economic performances for different types of geothermal power plants
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16. 5 Applications
The following chapter presents a few examples of commercial application of geothermal electricity
production, with their associated characteristics, efficiencies and costs. They are presented at
table 2. For the Beowawe double-flash steam power plant, there are two operating pressures,
which are the two pressures of the two successive flash drums.
Guanacaste Beowawe Svartsengi Husavik
Type Single-flash Double-flash ORC Kalina
Fluid geo. steam geo. steam isopentane 82%NH3/18%H2O
Size [MW] 55 16.7 1 2
Source T [C] 230 215 103 121
Exergy eff. 29.5% 46.7% 35% 45%
Cost [$/kW] n.a. 1900 2400 975
Op. pres. [bar] 6 4.21/0.93 6.2 38.8
Cond. pres. [bar] 0.123 0.044 n.a. 5.4
Year 1994 1985 n.a. 2000
Country Costa Rica USA Iceland Iceland
Table 2: Examples of geothermal power plants used for commercial power production
6 Perspectives
Though in this chapter mainly technological improvement are presented, it should be noted also
that an important potential for a better utilization of geothermal resources for electricity produc-
tion lies in the valorization of the important amount of waste heat, either by cogeneration or even
polygeneration. Cascaded use of the resource for energy demands having different temperature
levels should therefore be considered [9]. The possibilities of seasonal storage of heat at shallow
depth with the use of heat pumps is also a promising possibility [16, 18].
Another potential of improvement in the field of geothermal energy lies in the possible utiliza-
tion of other resources than the conventional resources, at higher depth or with higher tempera-
tures or pressures, such as hot dry rock, geopressure resources or magma energy. This is however
not the goal of this course to present them in details, and more information can be found in the
literature [20, 13, 7].
The following chapter presents therefore the improvements that could be brought to existing
technologies for the conversion of geothermal resources to electricity, or the promising technolo-
gies that have not yet been commercialized and that are still at the stage of research.
6.1 Advanced ORC
There are three main possible ways to improve the efficiency of the basic ORC, mainly by reducing
the exergy losses during the evaporation of the fluid:
• dual-pressure ORC
• fluid mixture ORC
• supercritical ORC
The working principle of these possibilities is briefly presented here.
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17. In the case of a dual-pressure ORC, the idea is to add a second pressure level for evaporation,
in order to reduce the exergy losses caused at the evaporation of the working fluid, and to
match better the curve of the heat available from the geofluid [7, 4]. The theoretical principle is
explained at figure 20 below, where the reduction of the exergy losses at the evaporation can be
seen.
Figure 20: Schematic thermodynamic comparison of a simple ORC with a dual-pressure ORC
The dual-pressure ORC results in a much more complex design of the heat exchanger network,
and a possible design for a dual-pressure ORC is displayed at figure 21 below.
Figure 21: Schematic representation of a possible dual-pressure ORC power plant
The dual-pressure ORC is also refered to as cascaded ORC, 2-stage ORC or 2-pressure level
ORC.
Another possibility to reduce also the exergy losses occuring during the evaporation of the
working fluid is to use a mixture of two different working fluids. Indeed, a mixture does not
evaporate at constant temperature and allows therefore a better match with the curve of available
heat from the geofluid [7, 1]. This is illustrated by figure 22 below.
17

18. Figure 22: Schematic thermodynamic comparison of a simple ORC with a fluid mixture ORC
The composition of the mixture requires to be chosen in function of the temperature of the
geofluid. It has the advantage that it can be adapted if the temperature of the geofluid changes
with time. However, the behaviour of organic mixtures is not well-known.
A last possibility to increase the efficiency of an ORC by avoiding an evaporation is to have a
cycle operating at supercritical conditions, by having a higher operating pressure which is higher
than the critical pressure [17, 8]. A theoretical thermodynamic cycle is displayed at figure 23.
Figure 23: Thermodynamics of a supercritical ORC process on a T-s diagram
Though the efficiency is improved compared to a conventional ORC, a supercritical cycle
requires thicker pipes and heat exchangers because of the high operating pressure, which makes
the investment costs of such a cycle much higher than for a conventional cycle.
6.2 Trilateral cycles
Another type of possible cycle that can be used for the conversion of geothermal resources to
electricity is the trilateral cycle. Indeed, the ideal cycle for the conversion of geothermal resources
has a triangular shape. Such a cycle, displayed at figure 24, is achievable in theory if a liquid
is preheated and then expanded from saturated liquid state to a mixture of liquid and vapor to
produce power, then condensed and pumped back to start again the cycle. The difference with
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19. a conventional ORC lies in the expansion device, which must be able to expand not vapor like a
turbine but a mixture of liquid and vapor [21, 19].
Figure 24: Thermodynamics of a trilateral process on a T-s diagram
The most important feature of the trilateral cycle is the expansion device, which has saturated
liquid at the inlet and expands a mixture of liquid and vapor. Screw-type expanders can be used
for this purpose. However, the expanders currently available have bad isentropic efficiencies, in
the range of 60%, and more improvement in their design is required before trilateral cycles can
be used for commercial geothermal applications [6].
6.3 Thermo-electric devices
This family of technologies do not use thermodynamic cycles to convert heat from geothermal
resources in electricity, but make a direct conversion of this heat to electricity by using the
Seebeck effect, or thermo-electric effect. This relates to the electric current that is created
between two semi-conductors placed between two electrodes when the two electrodes are at
different temperatures. This is illustrated by figure 25.
Figure 25: Schematic representation of a thermo-electric device using the Seebeck effect
The efficiency of such devices is however in average 25% lower compared to other systems
such as ORCs and their costs are higher per unit of power produced. Therefore, thermo-electric
devices are at the moment not suitable to be selected for industrial power production from
geothermal resources [13].
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20. 7 Bibliography and further readings
In addition to the references listed at the end of this document, some interesting information can
be found at the following websites:
• Societe Suisse pour la Geothermie:
http://www.geothermie.ch
• Centre de Recherche en geothermie Neuchatel:
http://www.crege.ch/news.php
• International Geothermal Association:
http://www.geothermal-energy.org/geo/geoenergy.php
• Animated Flowsheet of the Hellisheidi Geothermal Power Plant, Iceland:
http://www.or.is/flash/framl/index.html
References
[1] Gianfranco Angelino and Piero Colonna DiPaliano. Multicomponent working fluids for
Organic Rankine cycles (ORCs). Energy, 23(6):449–463, 1998.
[2] Enrico Barbier. Geothermal energy technology and current status: an overview. Renewable
and Sustainable Energy Reviews, 6:3–65, 2002.
[3] Ruggero Bertani. World geothermal power generation in the period 2001-2005. Geothermics,
34:651–690, 2005.
[4] Lucien Y. Bronicki. Advanced Power Cycles for Enhancing Geothermal Sustainability. IEEE
Power and Energy Society 2008 General Meeting: Conversion and Delivery of Electrical
Energy in the 21st Century, 2008.
[5] Matthias Bruhn. Hybrid geothermal-fossil electricity generation from low enthalpy geother-
mal resources: geothermal feedwater preheating in conventional power plants. Energy,
27:329–346, 2002.
[6] Ronald DiPippo. Second Law assessment of binary plants generating power from low-
temperature geothermal fluids. Geothermics, 33:565–586, 2004.
[7] Ronald DiPippo. Geothermal Power Plants - Principles, Applications, Case Studies and
Environmental Impact. Elsevier, 2008.
[8] Zhaolin Gu and Haruki Sato. Performance of supercritical cycles for geothermal binary
design. Energy Conversion and Management, 43:961–971, 2002.
[9] Jon S. Gudmundsson, Derek H. Freeston, and Paul J. Lienau. Lindal Diagram. Geothermal
Resources Council Transactions, 9, 1985.
[10] T. Kohl, R. Speck, and A. Steinfeld. Using Geothermal Hybrid Plants for Electricity Pro-
duction From Enhanced Geothermal Systems. Geothermal Resources Council Transactions,
26:315–318, 2002.
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21. [11] John W. Lund. World Status of Geothermal Energy Use Overview 1995-1999, 1999.
[12] John W. Lund, Derek H. Freeston, and Tonya L. Boyd. Direct Application of geothermal
energy, 2005 Worldwide review. Geothermics, 34:691–727, 2005.
[13] Rudolf Minder, Joachim Kodel, Karl-Heinz Schadle, Kathrin Ramsel, Luc Girardin, and
Francois Marechal. Energy conversion processes for the use of geothermal heat. Technical
report, Swiss Federal Office of Energy, 2007.
[14] Henry Mlcak. Kalina Cycle Concepts for Low Temperature Geothermal. Geothermal Re-
sources Council Transactions, 26:707–713, 2002.
[15] Henry Mlcak, Mark Mirolli, Hreinn Hjartarson, and Marshall Ralph. Notes from the North:
A Report on the Debut Year of the 2 MW Kalina Cycle Geothermal Power Plant in Husavik,
Iceland. Geothermal Resources Council Transactions, 26:715–718, 2002.
[16] Joachim Poppei, Peter Seibt, and Dirk Fischer. Recent examples for the utilisation of
geothermal aquifers for heat or cold storage or improvement of the reservoir conditions by
heat injection (storage and combined production/storage projects in Germany). In Proceed-
ings, Twenty-Third Workshop on Geothermal Reservoir Engineering, 1998.
[17] Bahaa Saleh, Gerald Koglbauer, Martin Wendland, and Johann Fischer. Working fluids for
low-temperature organic Rankine cycles. Energy, 32:1210–1221, 2007.
[18] Burkhard Sanner, Constantine Karytsas, Dimitrios Mendrinos, and Ladislaus Rybach. Cur-
rent status of ground source heat pumps and underground thermal energy storage in Europe.
Geothermics, 32:579–588, 2003.
[19] Ian K. Smith, Nikola Stosic, and Ahmed Kovacevic. Screw Expanders Increase Output
and Decrease the Cost of Geothermal Binary Power Plant Systems. Geothermal Resources
Council Transactions, 29:787–794, 2005.
[20] Jefferson W. Tester, Brian J. Anderson, Anthony S. Batchelor, David D. Blackwell, Ronald
DiPippo, Elisabeth M. Drake, John Garnish, Livesay Bill, Michal C. Moore, Kenneth
Nichols, Susan Petty, M. Nafi Toksoz, and Ralph W. Veatch Jr. The future of geother-
mal energy - Impact of Enhanced Geothermal Systems (EGS) on the United States in the
21st Century. Technical report, Massachusetts Institute of Technology, 2006.
[21] Calin Zamfirescu and Ibrahim Dincer. Thermodynamic analysis of a novel ammonia-water
trilateral Rankine cycle. Thermochimica Acta, 477:7–15, 2008.
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