Review of technologies for hybrid uses of
geothermal and additional RES
Activity 3.2 in WP 3
Coordinated by: UM
Index
INDEX
2
SUMMARY
3
1. BINARY CYCLES
1.1. Organic Rankine Cycle (ORC) process
1.2. Kalina process
4
5
6
2. GEOTHERMAL SOURCE AND SOLAR ENERGY
2.1. Solar-geothermal hybrid plant based on an ORC
2.2. Micro CHP systems operating with geothermal and solar energy
7
7
9
3. GEOTHERMAL SOURCE AND BIOMASS
3.1. Technologies
12
12
4. GEOTHERMAL SOURCE AND NATURAL GAS AND BIOGAS
4.1. Technologies
17
17
5. GEOTHERMAL SOURCE AND WASTE HEAT
5.1. Technologies
20
20
REFERENCES
23
LIST OF FIGURES
26
2
Summary
Low-enthalpy geothermal resource, as the name suggests, is a source of relatively lowtemperature geothermal water. According to various authors as well as a variety of the limit to
which the temperature geothermal resource is low-enthalpy resource. We will assume that the
temperature is up to 150°C.
Further in this report is presented the technology that could be used for the electricity generation
from low-enthalpy geothermal resource with an additional renewable energy source such as solar,
biomass, biogas and waste heat. The second energy source is mainly used for medium or steam
additional heating. These renewable energy sources are chosen because of most efficient use in
the area of South-eastern Europe.
3
1. Binary cycles
Binary cycles are characteristic for use of low-temperature geothermal energy. In binary cycle
geothermal power plants we are pumping hot water from a geothermal well, through a heat
exchanger. The cooler water is reinjected to the underground reservoir. For a working medium we
use organic substance (for ORC process) or a mixture of ammonia and water (Kalina process).
These substances have a low boiling point. The greatest advantage of these processes is that can
be used geothermal water with a temperature less than 90°C in the shallow layers of the earth,
which means that we do not need expensive deep wells. With binary cycles we also increase the
efficiency of the steam system, this to seize as much power [1].
Both processes are exploitable geothermal source to vaporize the working fluid that evaporates at
lower temperatures in heat exchanger. The working fluid in a state of superheated vapour
expands through the turbine and then in the condenser liquefies. After the condenser there is a
pump which pumps the medium back to the heat exchanger and thereby conclude the circular
process. Scheme of the devices in the binary thermodynamic process is shown in Fig. 1 [1].
steam turbine
with generator
5
D
3
4
outlet
G
A
heat
exchanger
geothermal
source
evaporator
inlet
B
2
pump
working medium –
binary thermodynamic
process
1
condenser
A
Fig. 1: Scheme of the devices in the binary thermodynamic process.
Thermodynamic process is shown in Fig. 2.
4
Temperature (°C)
171 °C
C
Process diagram
175
5
150
turbine
4
D
100
3
B
A
0
2
pump
50
1
21 °C
250
500
750
1000
1250
1500
1750
Enthalpy (kJ/kg)
Fig. 2: Binary thermodynamic process diagram
Used geothermal water is reinjected into the aquifer [1].
1.1. Organic Rankine Cycle (ORC) process
ORC process uses as working medium hydrocarbons, for example R114 –
dichlortetrafluorinemethane (C2Cl2F4), perfluorinepenthane (C5F12), isobutane (C4H10). Process
scheme is shown in Fig. 3 [1].
5
Fig. 3: ORC process
Geothermal source is used for the vaporization of the working medium in the evaporator, which is
in the state of superheated steam and then expanded in a turbine, then liquified in the condenser.
The condenser has to be cooled with water or air, but we can also remove the heat for the
example heat pump and additionally take advantage of the available heat. Working fluid from the
condenser is pumped back into the preheater. Own energy use for circulation pump is relatively
large [1].
1.2. Kalina process
Kalina process is similar to the ORC process, except that the working medium is a mixture of water
(85 %) and ammonia (15 %). The working medium is evaporated passing through the evaporator,
where on the secondary side is the geothermal source. The steam produced powers a steam
turbine. The scheme of the process is in Fig. 4. In the Kalina process is used dual-mixture as a
working medium which has the advantage that it does not have a specific evaporating
temperature, but the whole area (Fig. 5) which allows a better heat exchange [1].
6
Fig. 4: Kalina process
Temperature
geothermal
source
binary
mixture
working medium (NH3 – H2O)
saturated water vapor –
evaporation at a constant
temperature
H2O
NH3 – H2O
Heat transfer
Fig. 5: Evaporation area of the Kalina process
2. Geothermal source and solar energy
In the beginning there is presented solar-geothermal hybrid plant based on an Organic Rankine
Cycle (ORC). In the second part, two micro CHP systems operating with geothermal and solar
energy are described.
2.1. Solar-geothermal hybrid plant based on an ORC
This solar-geothermal integration for electricity generation was proposed by Lentz and Almanza
(2006a,b) for possible application in Mexico. The plant which was proposed is based on a single or
double flash cycle, where steam turbine expands the steam produced in flash separators from the
geothermal brine. There were suggested different configurations to enhance steam production
with solar energy (but all based on the Direct Steam Generation – DSG), where the geothermal
brine is evaporated by flowing directly in the solar collectors. The problem is aggressive species in
most geothermal brines, such as CO2, H2S, SO2, HCl or NH3 [2].
Reference cycle definition and on-design performance
Binary units (based on ORCs) are becoming increasingly common. In Fig. 6 the supercritical
regenerative ORC is shown. As working fluid was chosen R134a because it is a widely used,
nontoxic and nonflammable fluid. Assumed temperature from the geothermal brine is 150°C,
waste brine is injected at not less than 70°C to minimize cooling of the geothermal reservoir. The
efficiency of the ideal process, i.e. the efficiency of the Lorenz cycle receiving heat between 150°C
and 70°C and discharging heat at ambient temperature (Di Pippo, 2004), is equal to 24,5 %. The
second-law efficiency of the cycle at on-design conditions is 55,5 % [2].
Fig. 6: Schematic of the assumed supercritical regenerative ORC and its representation on the temperatureentropy plane. [2]
7
Heat exchanger profiles are displayed on a temperature-heat plane in Fig. 7.
Fig. 7: Temperature-heat diagram of the ORC plant heat exchangers. Heat is expressed as the percentage of
the heat entering the cycle from geothermal brine. [2]
Off-design performance
An additional heat exchanger placed after the main heat exchanger of the base plant was taken
into consideration. This additional heat exchanger allows the mass flow rate and/or the maximum
temperature of the R134a to increase. The off-design analysis is a general thermodynamic
assessment of the effects of an additional heat source (derived from for example solar field) on
the ORC cycle. In Fig. 8 schematic of the hybrid configuration for off-design calculations is shown
[2].
8
Fig. 8: Schematic of the hybrid configuration used for off-design calculations and its representation on the
temperature-entropy plane. [2]
Plant should be operated to achieve the highest conversion efficiency of the additional heat
available. Optimal combination of max temperature and mass flow rate can be identified for any
value of the additional heat. The temperature-heat diagram of the plant heat exchangers for two
specific off-design conditions with different maximum cycle temperatures is shown in Fig. 9.
9
Fig. 9: Temperature-heat diagram of the plant heat exchangers for two specific off-design conditions with
different maximum cycle temperatures. Heat is expressed as the percentage of the heat entering the cycle
from both geothermal and solar source. [2]
2.2. Micro CHP systems operating with geothermal and solar energy
The performances of the geothermal ORC plants were measured in terms of first and second law
efficiencies. Energy efficiency is usually in the range 5 – 15 % while exergy efficiency is typically in
the range 20 – 54 %. Below are described two innovative CHP ORC systems powered by lowtemperature geothermal resource (about 90°C) and solar energy captured by solar collectors. The
reference system is designed to produce 50 kWe. The electrical efficiency of the cycle is expected
to be low (around 9 %) [3].
Single-pressure configuration with heat transfer fluid
Organic fluid coming from the condenser is preheated in a heat exchanger (geothermal heater) by
a geothermal flow rate at 363 K. Then the organic fluid is heated up to the 420 K passing through a
heat exchanger connected to an Evacuated Tube Collectors (ETCs) field. After producing
mechanical work in the turbine, the organic fluid has still high temperature and enthalpy values.
The organic fluid is then fist cooled from superheated conditions down to saturated vapour
conditions at variable temperature (DSH). The DSH can be connected to a domestic heating
system. Organic fluid is then circulated to the geothermal heater to start the cycle again.
Described process is shown in Fig. 10 [3].
Fig. 10: Single-pressure geothermal/solar ORC layout. [3]
The thermodynamic cycle for R134a as a working fluid is shown in the T-s diagram in the Fig. 11.
Fig. 11: Thermodynamic cycle for R134a- [3]
Dual-pressure system with direct-steam collectors, using two different solar collectors
The organic fluid is directly heated by the ETC and then by the Parabolic Trough Collectors (PTCs)
fields. The ETCs are used only for liquid preheating and phase transition with a recirculation loop
producing vapour. A vessel is used for separating the saturated vapour phase, part of which is
directed to the low-pressure turbine (LPT). An appropriate amount of the saturated liquid phase in
the separation vessel is compressed and directed to the high-pressure loop using PTCs. After LPT
10
the organic fluid is cooled in De-Super Heater (DSH) and condensed to start the cycle again. The
system is shown in Fig. 12 [3].
Fig. 12: Dual-pressure geothermal/solar ORC layout. [3]
11
In all solar power systems, the solar filed represents the main cost item, accounting for about 80 %
of the electricity cost, which leaves the solar source still uncompetitive with fossil fuels for power
generation, however integration with a solar field could improve the attractiveness of the many
low-enthalpy geothermal sources [2].
3. Geothermal source and biomass
3.1. Technologies
Hybrid technologies for coupling geothermal and biomass sources are in early development
stages. The development of hybrid biomass-geothermal systems therefore, the use of combined
technologies, is a research priority in Europe (Alakangas et al., 2012). To date, little information is
available on implemented plants and on the theoretical aspects. Presented herein is the available
information for operating plants followed by the description of plants that have been proposed in
the literature.
The first ever installed hybrid geothermal-biomass plant is located in Wendel, California (USA) and
is still operating. In this plant, called Honey Lake Power, geothermal heat is used for drying wood
chips and for preheating fluid for a 35.5 MW biomass unit (Lund et al., 2010 and Bertani, 2012).
This steam cycle power plant uses 22 kg/s geothermal fluid at 118°C (Böszörményi & Böszörményi,
2003) and it contributes to about 1.5 MWe of the total plant output (Sifford, 2000). The final
report (GeoProducts Corporation, 1988) of the project on the hybrid plant gives additional
information. Additional electrical energy is produced by a binary cycle due to isobutene heating.
This heating process is first provided by a geothermal fluid heat exchanger and then by a heat
exchanger receiving extraction steam from the biomass boiler. The working fluid feeds the turbine
(hydrocarbon expander) that is connected to a generator that finally produces energy. The
vaporized working fluid is transferred to a condenser for cooling and successively returned to the
geothermal heat exchange. In Austria in the area of Neukirchen an der Vöckla a combined use of
geothermal and biomass energy sources has been operating since 2012. The district heating of the
town comes from the new geothermal scheme and a biomass plant. The geothermal scheme
consists in a deep well (downhole temperature of 105°C). The energy from the hot treated water
is extracted through a heat exchanger and the heat is then sent along a 1 km pipeline which
operates a biomass heating plant and a district heating system. The biomass plant uses local
timber (RAG, n.a.). Another combined system for district heating will be operating in Hungary. The
combined system will be composed by a biomass boiler fed with wood chips (with plant capacity
of 1.2 MW) and a geothermal well (with a water temperature of 60-65°C) combined with a heat
exchanger (Kontra, 2011).
Over the last years several different plants coupling geothermal and biomass energy sources have
been suggested and hereafter a brief description of the proposed technology is presented. Most of
the studies have proposed the utilization of Organic Rankine Cycle (ORC) technology.
Kohl and Speck (2004) presented a system in which energy transfer is achieved utilizing three heat
exchangers: a pre-heater, a steam generator and a steam super-heater (Fig. 13). For the two
former heat exchangers the energy source is geothermal whereas for the latter biomass is the
most likely source to be used.
12
Fig. 13: Configuration of the proposed hybrid geothermal-biomass plant (taken from Kohl & Speck, 2004).
The authors tested the proposed hybrid system using ‘water vapor’, hexane and isobutene as
working fluids (ORC) for the production of electrical energy.
Also Bombarda et al., (2010) proposed a hybrid geothermal-biomass plant based on ORC
technology (see Fig. 14). They considered a geothermal source with temperatures ranging from
110 to 150°C.
13
Fig. 14: Configuration of the proposed hybrid geothermal-biomass plant (taken from Bombarda et al.,
2010).
To transfer heat between the biomass-fired furnace and the power unit synthetic oil was selected
(the assigned loop temperature is the temperature common for biomass plants). Furthermore, to
test the proposed hybrid plant, data for the biomass source was taken from a plant located in the
Molasse basin (Germany) that generates electrical and thermal energy through the year and
according to seasonal demand, respectively. A cascade cycle was selected with octamethyltrisiloxane (MDM), regenerative, and R245fa as working fluids for the upper and lower cycle,
respectively. The results highlighted a higher net power production compared to separate plants.
Borsukiewicz-Gozdur (2010) assessed two hypothetical hybrid geothermal-biomass systems. For
both systems the assumed geothermal water had low temperatures (80-120°C) with a constant
flow rate (30 kg/s). The first plant consists in a dual-fluid- hybrid power plant (DFH) formed by an
upper and lower cycle (Fig. 15).
Fig. 15: Configuration of the dual-fluid hybrid geothermal-biomass plant (taken from Borsukiewicz-Gozdur,
2010).
In the upper cycle (Hirn cycle) the working fluid (water) is pre-heated and vaporized in a biomass
boiler (u4 to u1), and subsequently directed to a steam turbine (u1 to u2) where the fluid expands
and performs work. Then the fluid is condensed in a condenser-vaporizer exchanger (u2 to u3) and
subsequently is moved to the biomass boiler by the circulating pump that pressurizes the fluid to
the apt value (u3 to u4). In the lower cycle (ORC), connected to the upper one by the condenservaporizer, after preheating using geothermal sources, the working fluid evaporates in the
condenser-vaporizer (b5 to b1) and then is directed to the vapor turbine (b1 to b2) that is linked to
an electrical energy generator. Subsequently the working fluid is directed to the condenser (b2 to
b3), circulation group (b3 to b4) and, finally, to the organic liquid heater (b4 to b5). The working
fluids that were tested are: R365 mfc-water, R245fa-water and R236fa-water.
The second hybrid plant consists of a single-fluid hybrid power plant (HYB) (Fig. 16).
Fig. 16: Configuration of the single-fluid hybrid geothermal-biomass plant (taken from BorsukiewiczGozdur, 2010).
This hybrid plant is fed by combustion of biomass in the upper part and a geothermal source is
used to preheat the working fluid. For this proposed hybrid cyclohexane and water were tested as
14
working fluid. The author highlights the advantage of the combined use of geothermal and
biomass against only geothermal sources and that the best option, in the case the general purpose
is to use the least share of biomass derived energy, will be using a dual-fluid-hybrid power plant
with R236fa as working fluid.
A technical and economic analysis of a proposed hybrid geothermal-biomass system for district
heating was carried out for Cornell University (Ithaca, NY, USA) (Lukawski et al., 2013). In Fig. 17
the schematic of the coupled energy system is shown. Low-temperature geothermal resource is
considered.
15
Fig. 17: Configuration of the hybrid geothermal-biomass system. The ORC unit is not included in the 'DH'
scenario. Lines are in red for hot supply water and in blue for cold return water (taken from Lukawski et al.,
2013).
The researchers evaluated two design scenarios: (i) the geothermal source was applied only for
district heating (‘DH’) and (ii) the geothermal source was used for the combined generation of
heat (for district heating) and electrical energy through an Organic Rankine Cycle (ORC) plant (‘DHORC’). The EGS reservoir and the district heating (DH) network are hydronically divided by a heat
exchanger. Furthermore, a backup steam-to-water heat exchanger was inserted. In both scenarios,
a boiler fed with torrefied biomass was used to produce the additional heat for the DH when
needed. Heating and domestic hot water needs are fulfilled by the DH water supply pumped to
building substations, from which, roughly the 30% of return DH water is cascaded to greenhouse
substations. The cycle is closed by pumping the return water to the central heat exchanger.
In the proposed DH pressurized liquid water at temperatures ranging from 90°C to 140°C is used.
The temperature of the DH supply can be lowered as in this case study it was constrained by the
existing space heating system. Substations of the proposed system use plate and frame water-towater heat exchangers. The hybrid system also has water circulation pumps and roughly 20% of
the pipeline network is formed by pre-insulated pipes. In the greenhouses a soil heating system
with a fluid temperature ranging from 30 to 60°C was considered. The greenhouses network
consisted in polyethylene tubes and a finned coil air heater. Furthermore, the ORC unit consisted
in two low capacity ORC power plants operating only during low heat demand conditions. For the
ORC unit, the researchers considered 25 different fluids and selected R32 fluid.
In conclusion it must be emphasized that research is ongoing. For example, Enel Green Power is
focusing in particular on geothermal in combination with biomass and solar energy and the
“European technology platform: Renewable heating and cooling” has a Focus group specifically for
renewable energy hybrid systems.
16
4. Geothermal source and natural gas and biogas
4.1. Technologies
The geothermal energy and natural gas electricity production is one of the best combinations
regarding CO2 emissions (global warming effects). The electricity production with combination of
geothermal and gas energy is very useful also regarding gas emission (Fig. 18). This term is
especially important when we wish to produce electricity near big cities. Unfortunately, in
scientific literature is not possibly to find many projects or plans, how to build geothermal and
natural gas power stations in combination.
Fig. 18: The CO2 emissions regarding electricity production. [4]
In principle it is possible to use processes as it as described in combination of geothermal with
biomass, and solar energy, in this case is the added energy gas or biogas energy (source 2 in Fig.
19).
Fig. 19: Principle of application of natural gas with geothermal energy.
It is possible to combine natural gas-biogas and geothermal energy in many ways. One suggested
way shown by MAN [5]. MAN combined gas engines with ORC process. In this way we could
combine heat from exhaust gases with geothermal energy in ORC process.
17
Fig. 20: Two-stroke, low-speed diesel engine of MAN B&W design in combined cycle. [5]
Cornell University [5] owns a state of the art natural gas combined cycle CHP plant, which has
been fully operational since 2009. The objective in this study was to show an efficient heat
production and distribution system which uses only geothermal, gas and biomass resources (Fig.
21) to cover demand of a fraction of the Cornell campus.
18
Fig. 21: Actual system of cogeneration. [5]
Fig. 22: Plans how to improve the system with adding geothermal energy. [5]
19
5. Geothermal source and waste heat
5.1. Technologies
Waste heat is energy discharged in the form of heat from the system (machine, facility, bio-system
etc.) undergoing some kind of energy transformation. Waste heat is not targeted or desirable
energy output, but sometimes it is necessary byproduct, such as in power plants operating by
Rankine cycle (heat of condensation, or latent internal energy of exhaust flue gasses). There are a
variety of waste heat sources with very different properties (geometry, heat capacity, temperature
etc.), and those properties are determining regarding the possibilities for waste heat recovery.
Nowadays the most actively researched option for waste heat recovery is the conversion of lowgrade waste heat (heat sources with low specific exergy) to electricity utilizing Organic Rankine
cycle [7-10], Kalina cycle [11, 12] or some other methods [13]. Low-grade waste heat has the
similar temperature range to low-temperature geothermal source, hence the way to make
electricity out of those heat sources is quite similar as well [14-16].
Theoretically, if waste heat source is available, surplus waste heat can be used in different lowtemperature geothermal energy systems, which include use in spas, heating and cooling systems,
aquaculture, agriculture, snow melting, low wattage power generation, in order to improve their
efficiency and/or power output. Specific solution is dependent on the properties of lowtemperature geothermal source, waste heat source, the purpose of the system and cost
effectiveness analysis.
Other possible way of using waste heat in low-geothermal applications is for “recharging” of the
geothermal source.
For example, all systems for indirect use of geothermal energy can operate with additional heat
source. If available, waste heat can be regarded as additional heat source. Depending on the
properties of the low-temperature geothermal source and waste heat source, there are many
different possibilities. Fig. 23 shows typical and simplified schematics of the heating system with
indirect use of geothermal source and additional heat source (waste heat). These systems can be
effectively used in agriculture or for greenhouses [17].
20
Fig. 23: Heating system with indirect use of geothermal source and additional heat source
In low-temperature geothermal binary plants utilizing ORC (Fig. 24), Kalina cycle, etc. waste heat
from some other process can be used in different ways, such as preheating or superheating, in
order to improve system efficiency or power output. If on the other hand ORC plant is primarily
using low-grade waste heat, surplus low-temperature geothermal source can be used in order to
provide steady heat supply to the system (as waste heat may not be always available or in the
same quantity), hence enabling easier optimization of the system and increase of its efficiency. In
those kind of applications one should be aware of low specific exergy of low-temperature ground
source and low-temperature waste heat.
Fig. 24: Organic Rankine cycle
21
Other group of applications, which is fairly common, refers to “recharging” of low-temperature
geothermal sources with waste heat. Basic principle can be explained on systems using ground
source (low-temperature geothermal) heat pump for heating and possibly cooling, which are
commonly called geoexchange systems. These systems account for around half of all geothermal
use [18]. In the heating period soil is used as a heat source (low-temperature geothermal), and in
the cooling period, it can be used as a heat sink1 (Fig. 25). In that way temperature degradation
rate of low-temperature geothermal source during its lifecycle is reduced, which has positive
effect on heat pump COP and system cost effectiveness in whole.
22
Fig. 25: a) Geoexchange system in the heating mode; b) Geoexchange system in the cooling mode
There are some other solutions similar to this concept, like hybrid energy system that includes a
fuel cell and geothermal heat pump, that improves the efficiency of a ground-source heat pump
by using low-grade heat from the fuel cell [19]. The system takes the waste heat from the fuel cell
and incorporates the waste heat into the geothermal heat exchange system. The fuel cell provides
electricity to the geothermal system and the digital control unit, allowing the system to operate
off the grid. The fuel cell can also provide electrical power to the building. The temperature of the
fuel cell is regulated by a digital control unit. Fuel cells are more efficient than generators.
However, fuel cells typically waste about 50% of the input energy in the form of heat. In the
heating mode, the waste heat from the fuel cell is pumped into the building for heating purposes.
In the cooling mode, the waste heat is pumped into the ground where it is absorbed by the earth.
This technology can improve the efficiency of the heating and cooling systems in the building and
also has applications in remote or backup power generation systems for homes and commercial
buildings.
1
Cooling can be passive or by the means of a heat pump. Either way, the heat removed from the cooled space is
stored in the ground, thus recovering portion of the energy used from the low-temperature geothermal source during
the heating period.
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Kohl, T., & Speck, R. (2004). Electricity production by geothermal hybrid – plants in lowentalpy areas. Twenty-Ninth Workshop on Geothermal Reservoir Engineering Stanford
University, Stanford, California, Januray 26-28. Retrieved from http://www.geothermalenergy.org/pdf/IGAstandard/SGW/2004/Kohl.pdf
Kontra, J. (2011). Construction of an Innovative Geothermal and Biomass-based Heating
Plant in Vácrátót. 3rd IEEE International Symposium on Exploitation of Renewable Energy
Sources - March 11-12, 2011, Subotica, Serbia (pp. 15–16).
Lukawski, M. Z., Vilaetis, K., Gkogka, L., & Beckers, K. F. (2013). A proposed hybrid
geothermal - natural gas - biomass energy system for Cornell University. Technical and
economic assessment of retrofitting a low - temperature geothermal district heating
system and heat cascading solutions. Retrieved from
https://pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2013/Lukawski.pdf
Lund, J. W., Gawell, K., Boyd, T. L., & Jennejohn, D. (2010). The United States of America
country update 2010. Proceedings, Thirty-Fifth Workshop on Geothermal Reservoir
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Sifford, A. (2000). Geothermal electric power production in the United States: a survey and
update for 1995-1999. World Geothermal Congress 2000 Kyushu - Tohoku, Japan, May 28 June 10, 2000 (Vol. 2). Retrieved from http://www.geothermalenergy.org/pdf/IGAstandard/WGC/2000/R0243.PDF
List of Figures
Fig. 1: Scheme of the devices in the binary thermodynamic process.
4
Fig. 2: Binary thermodynamic process diagram
5
Fig. 3: ORC process
5
Fig. 4: Kalina process
6
Fig. 5: Evaporation area of the Kalina process
6
Fig. 6: Schematic of the assumed supercritical regenerative ORC and its representation on the
temperature-entropy plane. [2]
7
Fig. 7: Temperature-heat diagram of the ORC plant heat exchangers. Heat is expressed as the
percentage of the heat entering the cycle from geothermal brine. [2]
8
Fig. 8: Schematic of the hybrid configuration used for off-design calculations and its representation
on the temperature-entropy plane. [2]
8
Fig. 9: Temperature-heat diagram of the plant heat exchangers for two specific off-design
conditions with different maximum cycle temperatures. Heat is expressed as the percentage
of the heat entering the cycle from both geothermal and solar source. [2]
9
Fig. 10: Single-pressure geothermal/solar ORC layout. [3]
10
Fig. 11: Thermodynamic cycle for R134a- [3]
10
Fig. 12: Dual-pressure geothermal/solar ORC layout. [3]
11
Fig. 13: Configuration of the proposed hybrid geothermal-biomass plant (taken from Kohl & Speck,
2004).
13
Fig. 14: Configuration of the proposed hybrid geothermal-biomass plant (taken from Bombarda et
al., 2010).
13
Fig. 15: Configuration of the dual-fluid hybrid geothermal-biomass plant (taken from BorsukiewiczGozdur, 2010).
14
Fig. 16: Configuration of the single-fluid hybrid geothermal-biomass plant (taken from
Borsukiewicz-Gozdur, 2010).
14
Fig. 17: Configuration of the hybrid geothermal-biomass system. The ORC unit is not included in
the 'DH' scenario. Lines are in red for hot supply water and in blue for cold return water
(taken from Lukawski et al., 2013).
15
Fig. 18: The CO2 emissions regarding electricity production. [4]
17
Fig. 19: Principle of application of natural gas with geothermal energy.
17
Fig. 20: Two-stroke, low-speed diesel engine of MAN B&W design in combined cycle. [5]
18
Fig. 21: Actual system of cogeneration. [5]
18
Fig. 22: Plans how to improve the system with adding geothermal energy. [5]
19
Fig. 23: Heating system with indirect use of geothermal source and additional heat source
21
Fig. 24: Organic Rankine cycle
21
Fig. 25: a) Geoexchange system in the heating mode; b) Geoexchange system in the cooling mode
22
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