Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 2 (13)
ISSN 1843-6188
GLOBAL SURVEY OF GEOTHERMAL POWER PRODUCTION.
ENERGY, ENVIRONMENTAL AND ECONOMIC ANALYSIS
OF HIGH ENTHALPY TECHNOLOGIES
R. RAMOS, M.E. MONDÉJAR, J.J. SEGOVIA, M.C. MARTÍN, M.A. VILLAMAÑÁN and C.R. CHAMORRO
Grupo de Termodinámica y Calibración (TERMOCAL), Dpto. Ingeniería Energética y Fluidomecánica, Escuela de
Ingenierías Industriales, Universidad de Valladolid, Paseo del Cauce, 59, E-47011-Valladolid, Spain
E-mail: cescha@eis.uva.es
Flash power plants
Flash steam power plants are used when a mixture of
steam and liquid is produced at the wellhead of the
hydrothermal reservoir. This type of reservoir is most
common than vapor-dominant reservoirs. In single–flash
power plants mixture from the wellhead is separated into
different phases in a flash vessel and vapor is sent to the
turbine-generator unit. Design of single-flash power
plants may be improved if liquid-phase goes through an
additional flash process to obtain more steam at a lower
pressure. Double-flash power plants obtain more power
output from the same geothermal reservoir than singleflash power plants, usually a 15-25 % more.
Binary plants
When water temperature in the geothermal reservoir is
lower than 150 ºC appropriate technology is binary cycle.
In binary plants working fluid, other than geothermal
fluid, undergoes a closed cycle. Working fluid receives
energy from geothermal fluid through a heat-exchanger
unit and evaporates, expands in the turbine, condenses
and a pump returns it to the heat exchanger. Most binary
plants are based on Rankine or Kalina cycles.
Other technologies
Besides these traditional techniques are other interesting
possibilities that increase global efficiency of geothermal
power plants, like combined cycle and hybrid
technologies with fossil fuels or with other renewable
energies.
Abstract: This communication presents world current status
of geothermal power production. Geothermal energy has
been used to produce electricity for over 100 years. Today
world installed electric generation capacity is more than
10000 MW. Most of the geothermal power plants use the
scarce high enthalpy hydrothermal resources. In this
communication an analysis from energy, environmental and
economic points of view of the high temperature (high
enthalpy) geothermal power plants technologies is presented.
Given some reservoir characteristics, optimization process
allows to determine the best thermodynamic parameters for
plant operation. For a given reference economic frame a
financial analysis has been performed.
Keywords: Geothermal energy, Geothermal power production
1. INTRODUCTION
Electric generation from geothermal sources has a long
history of more than one hundred years. In 1904, in
Larderello (Italy), Prince P. G. Conti set up the first
device able to produce electricity from a geothermal
steam well. Ten years later, in 1914, in the same location,
a 250 kW turbo-alternator was the first commercial
geothermal power system connected to the electric grid.
Since those historic days several countries have
developed geothermal power plants using the relatively
scarce high-temperature hydro-geothermal reservoirs.
Electric energy produced from geothermal resources is
only a small percentage, less than 0.5 % of world´s
electricity needs, but emerging exploitation technologies
will allow a significant increase of the contribution of this
renewable energy source in a near future.
1.2 World current status of geothermal power production
Geothermal power plants based in the above mentioned
technologies are running in at least 24 countries in the
world. Nowadays installed power capacity exceeds 10000
MWe, and annual energy produced is near 60000 GWh
(2008 GIA Annual Report [1]). Table 1 shows installed
capacity and annual electricity generated by country in
2008. Figure 1 shows the evolution of world installed
capacity since 1975 (Bertani [2], 2008 GIA Annual
Report [1]). Two periods with a bigger slope may be
observed in this graph. The first, from 1975 to 1980, due
to the effect of 1973 oil crisis in energy markets, and the
second one, from 2005 to our days, due to the recent
increasing interest in geothermal energy. Rising trend of
this last period is confirmed by the large number of
1.1 Geothermal power generating technologies
In general terms three main technologies are in use today
to produce electricity from geothermal reservoirs. These
well established technologies are dry steam, flash and
binary plants. Technology used is directly related with the
state of the fluid and its temperature as it comes from the
well of the geothermal reservoir.
Dry steam power plants
Dry steam power plants use high temperature, vapordominant, hydrothermal reservoirs. Steam from the
well passes through the turbine-generator unit and
produces electricity.
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 2 (13)
ISSN 1843-6188
geothermal power plants, mainly binary-cycle type, under
construction, or planned, in several countries.
Electricity generated is a better indicator of the great
interest of geothermal energy than installed capacity since
capacity factor of geothermal power plants is higher than
other types of power plants, and by far the highest of all
renewable. Capacity factor is defined as the actual
electricity generated for a period of time divided by the
energy the plant would have produced at full nominal
capacity for the same time. For geothermal power plants
capacity factor can reach values as high as 90 %, against
much lower values for other renewable energies: hydro
(20 - 70 %), biomass (25 - 80 %), wind (20 – 30 %), solar
photovoltaic (8 – 20 %), solar thermal electricity (20 – 35
%) or tidal (20 – 30 %) (Fridleifsson, [3]).
Table 1. Geothermal power installed capacity and annual
electricity generated by country and total in 2008.
Country
Australia
Austria
China
Costa Rica
El Salvador
Ethiopia
France
Germany
Guatemala
Iceland
Indonesia
Italy
Japan
Kenya
Mexico
New Zealand
Nicaragua
Papua N.Guinea
Philippines
Portugal
Russia
Thailand
Turkey
USA
TOTAL
Installed
capacity
MWe
0.1
1.1
27.8
162.5
204.2
7.3
14.7
6.6
53.0
575.0
992.0
810.5
535.3
128.8
958.0
632.0
87.4
56.0
1970.0
23.0
79.0
0.3
38.0
3040.0
10405.0
Electricity
generated
GWh/yr
2
3
96
1145
967
not available
2
18
212
4000
6085
5181
3064
1088
7056
3962
271
17
9419
90
85
2
105
15000
57957
1.3 The future of geothermal electricity
High-temperature hydrothermal reservoirs are only a
small part of the huge amount of energy stored in the
Earth’s crust. Moderate-temperature hydrothermal
reservoirs, by far more abundant, may be exploited
through binary cycle power plants. This technology will
become the most important contribution to global
geothermal power production in a near future.
In the long term, Enhanced (or Engineered) Geothermal
Systems (EGS) will allow to exploit Hot Dry Rock
(HDR) reservoirs. This will provide almost unlimited
energy, since almost any geological site at a convenient
depth could be considered as a reservoir. In 2006
Massachusetts Institute of Technology (MIT) published a
report on the future of geothermal power. The main
conclusion was that EGS could produce 100 GWe, only
in the U.S., by 2050, if a reasonable investment was
guaranteed (MIT [4]).
2. HIGH ENTHALPY HYDRO-GEOTHERMAL
POWER SYSTEMS
Dry steam and flash power plants, which use the scarce
high-temperature
(high
enthalpy)
hydrothermal
reservoirs, represent today more than 90 % of the total
installed capacity. Binary power plants, despite being the
more numerous, account for less than a 10 % of the total
installed capacity, due to their small power per unit (less
than 4 MWe by average). Table 2 shows number of
units, total installed capacity and average size by plant
type at 2007 (DiPippo [5]).
Table 2. World power plant distribution by plant type
(2007 data).
Installed
Average
Capacity
size
Plant type
Units
MWe
MWe
Dry steam (DS)
Single flash (1F)
Double-flash (2F)
Other (mainly binary)
Figure 1. Evolution of installed geothermal power
generation since 1975.
It must be pointed out that in some of the countries
mentioned in Table 1 more than 10 % of national power
generation capacity is geothermal, and in some of these
cases more than 20 % of electricity consumed in the
country comes from this renewable source.
61
159
69
214
2471
4015
2191
834
40.5
25.3
31.8
3.9
To study high enthalpy hydrothermal power systems,
Dry steam (DS), Single-flash (1F) and Double-flash
(2F), from energy, environmental and economic points
of view, we have defined three model power plants
based on average size for actual geothermal power plants
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 2 (13)
of each technology, and comparable reservoir conditions.
Regarding reservoir characteristics we have assumed that
saturated water at the wellhead has a temperature of
200 ºC (saturation pressure of 15.55 bar), for the three
models, and is saturated liquid for the 1F and 2F model
plants, and saturated vapor for the DS model plant.
Specific exergy, e, for water streams at the wellhead are
calculated by:
e = h – h0 – T0·(s– s0)
Table 4. Operating parameters of the three model plants
(1)
where h, s are water specific enthalpy and entropy at
wellhead temperature and pressure, and h0, s0 are water
specific enthalpy and entropy at ambient conditions T0
and P0 (T0 = 15 ºC and P0 = 1 bar).
Mass flow, ṁ, at the wellhead is assumed to be 500 kg/s
for the 1F and 2F models. To allow comparisons
between the three models proposed even though DS
power plant uses a qualitatively better resource, with a
much higher specific exergy value, a smaller mass flow
rate (96.90 kg/s) has been assumed for the DS power
plant. This value has been calculated to make equal the
exergy flow, Ė, of the three streams at wellhead:
Ė = ·e
ISSN 1843-6188
Item
DS
1F
2F
Wellhead T (ºC)
200
200
200
Wellhead P (bar)
15.55
15.5
15.55
Steam flow (kg/s)
96.90
-
-
First flash T (ºC)
-
120.4
145.8
First flash P (bar)
-
2.015
4.24
1F Steam flow (kg/s)
-
78.74
56.04
Second flash T (ºC)
-
-
95.3
Second flash P (bar)
-
-
0.86
2F Steam flow (kg/s)
-
-
97.98
Condenser T (ºC)
50
50
50
Condenser P (bar)
0.124
0.124
0.124
Isentropic efficiency for turbines has been corrected
according to the Baumann rule to take into account the
fact that turbines for the three defined models will operate
always in the wet region. Assuming the dry turbine
isentropic efficiency to be constant ηtd = 0.85, the
isentropic efficiency for a turbine operating with wet
steam, ηtw, is given by:
(2)
ηtw = ηtd·(x1+x2)/2
Water properties are calculated from the IAPWS
Formulation for the Thermodynamic Properties of
Ordinary Water Substance for General and Scientific
Use [6]. Table 3 summarizes main parameters defining
reservoir characteristics at wellhead for these three
model plants.
where x1 and x2 are vapor quality at turbine inlet and
outlet. In studied cases steam enters the turbine as
saturated vapor, and x1 is always 1. Mass flow rate for the
turbine in the DS plant is the same that comes from the
reservoir. For 1F and 2F model plants steam flow is
calculated from operating conditions in the flash
separators. For the 2F model plant two independent
turbines have been considered: one high pressure turbine
(HPT) for the steam produced in the first flash, and one
low pressure turbine (LPT) for the saturated vapor
resulting from the mixture of steam produced in the
second flash with the exhaust of the HPT. Results for the
energy analysis are presented in Table 5.
Table 3. Reservoir characteristics at wellhead for the three
defined model geothermal power plants
Item
DS
1F
2F
Water temperature (ºC)
200
200
200
State
sat. vap.
sat. liq.
sat. liq.
Specific exergy (kJ/kg)
940.76
182.33
182.33
Mass flow rate (kg/s)
96.90
500
500
Exergy flow (MW)
91.16
91.16
91.16
(3)
Table 5. Energy analysis of the three model plants.
Item
ηtw
Steam flow (kg/s)
HPT ηtw
HPT Steam flow (kg/s)
LPT ηtw
LPT Steam flow (kg/s)
Turbine power (MW)
Auxiliary consumption
3. RESULTS AND DISCUSSION
The three model plants defined above have been studied
from energy, environmental and economical points of view.
3.1 Energy and environmental analysis
Wellhead temperature and pressure are equal for the three
model plants. In 1F and 2F model plants, the pressure
(and corresponding saturation temperature) for flash
separation has been optimized to obtain maximum power
output in the turbines. Results for such optimization
process are shown in table 4. Condenser pressure in all
cases is assumed to be 12.4 kPa (condensing steam
temperature of 50 ºC).
50
DS
1F
2F
0.783
0.810
-
96.90
78.74
-
-
-
0.822
-
-
56.04
-
-
0.793
-
-
97.98
55.59
26.92
34.25
(% of turbine production)
5
7
8
Plant power (MW)
Exergy efficiency
Capacity factor (%)
Elect. Gen. (GWh/yr)
52.81
25.04
31.51
0.579
0.275
0.346
90
90
90
416.49
197.21
248.50
Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 2 (13)
ISSN 1843-6188
Energy consumptions for auxiliary equipment in the
plants have been estimated to be a 5% of the turbine
production for the DS model plant, a 7% for the 1F plant
and 8% for the 2F model plant. For the estimation of
electricity generated per year (GWh/yr) a capacity factor
of 90% has been assumed.
The net annual CO2 emission reduction is estimated to be
159122 tons of CO2 for the DS plant, 75345 tons of CO2
for the 1F plant, and 103777 tons of CO2 for the 2F plant.
Table 8. Operating parameters of the three model plants
Item
DS
1F
2F
659406.7
223036.9
378932.7
SPP (yr)
4.9
8.8
6.0
IRR (%)
11.6
3.0
8.0
COE ($/MWh)
29.58
48.23
32.39
NPV (k$)
4. CONCLUSIONS
3.2 Cost, financial and sensitivity/risk analysis
In order to make economic comparisons independent on
specific location characteristics some assumptions have
been made to estimate initial cost of geothermal power
plant and other economical parameters for these three
model plants. Total initial cost is partly proportional to
installed power capacity, plus a fixed amount for steam
field exploration and exploitation. Operation and
Maintenance (O&M) cost per year is assumed to be
proportional to energy production, with different
coefficients for each model plant (7 $/MWh for DS, 7.5
$/MWh for 1F and 8 $/MWh for 2F). Resulting values
are resumed in Table 6.
It is not surprising that the defined model for Dry-Steam
geothermal power plant has the best values from energy,
environmental and economic points of view. Net power is
more than twice than 1F model plant, and twice is also
the CO2 reduction. Cost of energy is as low as
2.96 c$/kWh. Simple Payback Period is less than 5 years,
and the Internal Rate of Return is bigger than 11 %.
Even when we have assumed that exergy flow from the well
is the same for the three models, it is clear that DS reservoir is
much better than 1F and 2F reservoirs, and that can be seen in
final value of exergy efficiency for the three models (0.579
for DS, 0.275 for 1F and 0.346 for 2F). Unfortunately, it is
also clear that vapor-dominant reservoirs are less frequent in
nature than liquid-dominant ones.
More interesting is the comparison between 1F and 2F
proposed models, as they use identical reservoir
characteristics. Introducing a second flash separator, and
modifying operating parameters for the whole plant,
results in an increment of installed power capacity of
more than 25 % (from 25.04 MW to 31.51 MW). Simple
Payback Period is reduced from 8.8 to 6 years, and
Internal Rate of Return increases from 3 % to 8 %. Cost
of energy is reduced from 4.82 c$/kWh for the 1F model
plant to 3.24 c$/kWh for the 2F model plant.
Table 6. Economical parameters assumed for the three
model plants
Item
Total initial cost (k$)
O&M (k$/yr)
Project life (yr)
DS
1F
2F
171335.0
146023.4
156330.8
2915.4
1479.0
1988.0
30
30
30
Interest rates used in the economical analysis are
summarized in Table 7.
Table 7. Interest rates and other economical parameters
Item
Fuel cost escalation rate (%)
Inflation rate (%)
Debt ratio (%)
Debt interest rate (%)
Debt term (yr)
Electricity export rate ($/MWh)
Electricity export escalation rate (%)
Value
4.5
2
100
6
20
91.3
2.1
5. AKNOWLEDGMENTS
Support for this work came from the Junta de Castilla y
León GR152.
6. REFERENCES
[1]
[2]
[3]
Technical and economical parameters resumed in Tables
4, 5, 6 and 7 are used as input in the RETScreen Clean
Energy Project Analysis Software (RETScreen [7]). This
software, developed by the Natural Resources Canada
(NRCan) Department, evaluates the energy production
and performs the environmental, as well as the
economical, financial and sensitivity/risk analysis of
energy projects. Feasibility of the three model plants is
evaluated in terms of the Net Present Value (NPV),
Simple Payback Period (SPP), Internal Rate of Return
(IRR) and Cost of Energy (COE). Values obtained for
these parameters are shown in Table 8.
[4]
[5]
[6]
[7]
51
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RETScreen Clean Energy Project Analysis Software,
http://www.retscreen.net