Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)22

Low Temperature Heat Recovery for
Geothermal Binary Plant
NORIHIRO FUKUDA*1

NORITO KATSUKI*2

KAN OGATA*3

SHOJIRO SAITO*4NOBUYUKI HOSHI*5

KOJI IWAI*6

The geothermal power plant business of Mitsubishi Heavy Industries, Ltd. (MHI) provides
small-capacity binary power generation systems in addition to conventional large-capacity flash
power generation systems in order to respond to the needs of all markets and customers. Thisbinary power generation system can be applied to biomass plant or exhaust heat recovery. This
paper presents the advantages and thermal system characteristics of binary systems, as well as thetechnologies of Turboden s.r.l., which is a group company of MHI and Italian binary power
generation system manufacturers that has delivered roughly 250 binary systems, mainly forbiomass plants.

|1. Introduction
It is known that the first use of geothermal energy for power generation occurred in
Larderello, Italy, in 1904, where naturally emerging superheated steam was used for
0.75-horsepower power generation. Since then, geothermal power generation has been introduced
around the world as a nature-friendly energy source independent of fossil fuel, and its power
generation capacity has reached approximately 11 GW around the world as of 2011(1). In Japan, on
the other hand, the number of geothermal power plants has not increased since the start ofoperations at the Hachijojima Geothermal Power plant in 1999 although the capacity reachedapproximately 500 MW in the 1990’s. One of several possible causes for this is the environmental
burden of large-scale geothermal power generation facilities (such as land preparation and changes
in environment and scenery because of geothermal steam discharge) on geothermal reservoir areas
suitable for power generation, many of which exist in national parks.
Recently, vigorous discussions on this issue have been conducted, and a notification that
permits the development of “binary power generation that involves small-scale geothermal
development work and has little influence on scenery and the landscape or uses existing hot springwater” in national parks (class II, class III, and common areas) was published with conditions inMarch 2012(2). In addition, responding to the fact that the Feed-in Tariff Scheme for Renewable
Energy launched in Japan in 2012 sets the purchase price from geothermal power generation of lessthan 15 MW at 42 Yen, domestic demand for small-scale binary power generation facilities is
expected to grow in the future. The following contents outline binary power generation systems andpresent MHI’s efforts therefor.

|2. Features of geothermal power generation
2.1

Geothermal heat source

Geothermal heat emerges from the ground and is considered to originate from decay heatgenerated by radioactive materials inside the earth. The strength of its huge amount of energy can*1

Chief Staff Manager, Nagasaki Research & Development Center, Technology & Innovation Headquarters*2

Nagasaki Research & Development Center, Technology & Innovation Headquarters*3

Senior Manager, Nagasaki Research & Development Center, Technology & Innovation Headquarters*4

Senior Manager, Power Systems Project Management Division,*5

Chief Staff Manager, Power Systems Project Management Division,*6

Manager, Power Systems Project Management Division,

Engineering Headquarters, Power Systems Division
Engineering Headquarters, Power Systems Division

Engineering Headquarters, Power Systems DivisionMitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)23

be observed only in a small proportion of the earth’s surface, such as in erupting volcanoes.
However, human beings cannot use the energy directly. Therefore it is used indirectly throughwater (groundwater). Groundwater, which originates from water soaking into the ground, is heatedunderground by geothermal heat. Groundwater existing at relatively shallower depths blows out of
the surface, and is used directly as hot springs in some cases.
When groundwater reaches deeper depths (around 1,000 m or deeper), some of it is confined
under a hard stratum (referred to as a cap rock) and stays there at high temperatures and under highpressure. This is referred to as a geothermal reservoir. Areas suited for geothermal power
generation are selected depending on the capacity of the geothermal reservoir and its accessibility
from the surface. In this way, the possibility of typical geothermal power generation depends on theavailability of a geothermal heat source and an appropriate water circulation cycle. Therefore
searching for and the development of areas suitable for geothermal power generation requires muchtime, and it is a business accompanied by risks. In recent years, the research and development of
hot dry rock geothermal power generation, which creates a geothermal reservoir artificially by
injecting water from the surface, has been advanced.

2.2

Flash geothermal power generation system

Geothermal water that is brought to the surface by excavation can be classified largely into
steam-dominated and hot water-dominated type, depending on the proportion of steam and hot
water. In the case of steam-dominated type, a steam turbine can be driven almost directly by thesteam because evaporation completes in the course of rising to the surface. Therefore simpler
systems can be used, as seen in earlier geothermal power generation systems. However, such a
convenient geothermal source is rare. Most geothermal power generation plants in service aroundthe world are the hot water-dominated type.
Figure 1(a) shows a schematic system diagram of a flash geothermal power generation plant
as one of the typical power generation system using hot water-dominated geothermal heat.
Geothermal power generation technology using a hot water-dominated-type production well wasdeveloped in New Zealand for the first time and then actually applied to the Wairakei Geothermal
Power Generation Plant in 1958. Following this success, MHI started the development of ageothermal power generation system using a hot water-dominated-type production well jointly with
Kyushu Electric Power Co., Inc. In this development, the foundation of material selection was laidthrough chemical analyses of geothermal fluid and material tests of main equipment such as
turbines in a geothermal steam atmosphere.

Figure 1

System diagram of typical geothermal power generation systems

In addition, basic data for equipment design was accumulated through analyses andverification tests of the characteristics of the main equipment necessary for a geothermal power
generation plant such as separators, direct contact condensers, two-phase flow transportation
piping, and gas extractors. Based on the data, the Otake Power Plant (output 11 MW) of Kyushu
Electric Power Co., Inc. was designed and constructed, and started operation in 1967 as the first hot
water-dominated-type geothermal power generation plant in Japan. Since then, MHI has produced
103 geothermal power generation plants, with a total output of 3108 MW, in 13 countries including
Japan, while developing and applying various technologies for the improvement of performance,
economic efficiency and reliability.

2.3

Binary geothermal power generation system
Figure 1(b) shows a typical system diagram of a binary geothermal power generation plant.

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)24

The binary geothermal power generation system is an indirect power generation system that
introduces geothermal fluid into the heat exchanger to evaporate the working fluid in the secondary
cycle to drive the turbine. The main characteristics of the binary geothermal power generation
system are described in Table 1 and the following additional explanations.
Table 1
Item
FeatureWorking Fluid
Unit capacity
Turbine corrosion
Scaling(countermeasure)
Cooling system

Characteristics of geothermal power generation system

Flash cycle- High power output- High efficiency (double flash)- Geothermal steam- Non-flammability
Up to hundreds of MW (highcapacity power generation plant)- Countermeasures necessary- First stage nozzle of turbine(turbine cleaning equipment)- Injection line (pH adjustment)- Wet cooling tower- White smoke rises

Binary cycle- 100% injection possible- Chlorofluorocarbon, hydrocarbon, etc.
- Necessary to pay attention to global warming andozone depletion (chlorofluorocarbons) andflammability (hydrocarbon)- Up to tens of MW (low capacity)- Countermeasures unnecessary- Low grade materials can be used
- Heat transfer surface of preheater and injection
line (pH adjustment)- Air-cooled condenser- No white smoke rises(1) Full reduction of geothermal fluid: This system returns geothermal fluid coming out of theground back underground. Therefore the mass balance within the area is maintained. In
addition, this system has little influence on the surrounding environment such as forests, aswell as on scenery and the landscape, because white smoke (steam), which is considered thesymbol of geothermal power generation plant, does not rise into the air.
(2) Working fluid: Typical working fluid poses a risk to the environment. Two often-usedworking fluids include an alternative chlorofluorocarbon that requires consideration of its
ozone depletion and global warming potential, and a hydrocarbon working fluid that
requires appropriate fire extinguisher l equipment. In this case, the system needs to havespecial measures against leakage.
(3) Countermeasures to corrosion and scale: In the case of a flash power generation system,
geothermal fluid (steam, hot water, and non-condensing gas) passes through almost all
system components including the turbine, and therefore all of them require measures against
corrosion or non-condensing gas depending on the characteristics of the geothermal fluid. In
the case of a binary power generation system, on the other hand, heat is transferred from thegeothermal steam and water to the working fluid by the heat exchanger, and therefore
basically only the upstream system of the heat exchanger requires measures to deal withgeothermal fluid. However, scaling of heat transfer tubes of the heat exchanger has a direct
effect on performance and therefore requires attention.
(4) Cooling system: Generally many of areas suitable for a geothermal power generation plant
have difficulty in the availability of cooling water for turbine condensate. In the case of aflash power generation system, condensed water of geothermal steam can be used as
makeup water for cooling water. In the case of a binary power generation system, on theother hand, this method cannot be used, and therefore an air-cooled condenser, which istypically larger than a water-cooled one, is used. As a result, the air-cooled condenser may
be the largest structure among the components of the binary power generation system.
Therefore appropriate design in consideration of the initial introduction cost and power ofauxiliary equipment (i.e., the cooling fan of the air-cooled condenser) is required.
(5) Low volume flow rate: When a low boiling point working fluid is used for the secondary
cycle, the condensate pressure can be set higher than that of the steam. This allows for alower volume flow rate and more compact turbine design.
As described above, a binary power generation system has both advantages anddisadvantages. There is no conclusive definition of whether a flash or binary power generation
system is better. What is important is to study each site individually in terms of economic
efficiency and environmental friendliness before making the final determination of the system to be
introduced.

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)25|3. Mechanism of binary power generation system
The word “binary” means “two.” The binary power generation system is a system that uses
secondary fluid, in contrast to the flash power generation system that directly uses geothermal
fluid. The secondary fluid used for a binary power generation system is typically a low boilingpoint working fluid, in most cases an organic compound such as alternative chlorofluorocarbon.
Therefore the secondary cycle is typically referred to as the ORC (Organic Rankin Cycle). The
following contents outline the mechanism and features of the ORC.

3.1

Working fluid for ORC

In principle, any material having vapor pressure that can be commonly handled over theoperation temperature range (heat source temperature to atmospheric temperature) can be used as
the working fluid for the ORC. In fact, however, there are limitations from various aspects, andtherefore practically applicable working fluid are limited. A working fluid to be used needs to be
selected in consideration of the following major factors.
(1) Cycle characteristics (power generation efficiency)(2) Ease of handling (including safety, stability and environmental friendliness)(3) Economic efficiency
The following sections describe the selection policy of a working fluid from the perspectiveof the cycle characteristics. However, even when there is a material with favorable cycle
characteristics, it cannot be selected in terms of long-term operation if the ease of handling or
economic efficiency is ignored. It is necessary to make a final determination of a working fluid
taking all of the factors described above – in addition to the cycle characteristics – into
consideration in a balanced manner.

3.2

ORC characteristics

For an evaluation of the cycle characteristics, the use of a T-h diagram is an easily
comprehensible method. Figure 2 shows an example of a simplified heat source side heat exchange
model. In this example, the heat source is assumed to have only sensible heat and no latent heat for
simplification. In the case of geothermal power generation system, hot water including no steam
corresponds to such a heat source. In cases other than a geothermal heat source (e.g., plant exhaust
heat recovery), exhaust heat recovery from exhaust gas corresponds to such a heat source. Figure 3
compares T-h diagrams of butane and water, for example.

Figure 2

Heat exchanger model of ORC

Figure 3 T-h diagram examples of butane and water

The horizontal axis represents the heat amount (W) obtained by multiplying the specificenthalpy of each fluid (J/kg) by the flow rate (kg/s). However, the flow rate of the heat source

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)26

varies in each case because it is obtained by heat balancing for adjustment of the heat exchangeamount. In a heat exchange process, the temperature of the heat source fluid decreases along withthe release of heat. However, the cycle outlet temperature is determined by the boiling starttemperature of the cycle fluid, which acts as a constraint (pinch point). As shown by geometricconsiderations, if fluid having a high ratio of latent heat to entire heat exchange amount, such as
water, is used, the heat source outlet temperature is high, and therefore the fluid is discharged
without sufficient heat exchange. Typically, the larger the gap between the operation temperature(pressure) and the critical temperature (pressure) of the working fluid ratio becomes, the more thelatent heat ratio increases.
Figure 4 shows the calculation results of the ratio of latent heat with the assumption that thecondensate temperature in the ORC is 35°C. In addition to isobutane, normal pentane and R245fa,
which are typically used as ORC working fluid, water is shown for comparison in this figure. All ofthese working fluid show the characteristics that the ratio of latent heat sharply drops to zeroaround the critical temperature.

Figure 4

Ratio of latent heat in ORC

The ratio of latent heat is defined as (latent heat) / (sensible heat + latent heat)

The above description is based on the assumption that a simple sensible heat source is used,
and that the ratio of latent heat of the ORC can be set with comparative flexibility. On the other
hand, in the case of an actual geothermal heat source, some of which emerge as a two-phase flowof steam and hot water due to high bottom hole temperature, it is necessary to match the ratio oflatent heat of the ORC with the ratio of latent heat of the heat source. In addition, some geothermalheat sources cause higher silica concentration in hot water, resulting in the constraint of theinjection of temperature for the suppression of scaling. In this way, constraint conditions varysignificantly depending on the characteristics of the geothermal heat source. Therefore optimizationaccording to each heat source is important.

3.3

Exergy loss in heat exchange processThis section considers exergy loss occurring in the heat exchange process as one of thefactors that influence the economic efficiency of an ORC, using the heat exchange model shown in
Figure 2. Heat amounts released by the heat source side and received by the heat-receiving side arerepresented by the following formulas using entropy. (Hereinafter entropy is a relative value andentropy for the lowest temperature of each fluid is defined as zero.)
S1(Heat release side)Q1 = ∫ T1dS

(1)(Heat-receiving side)Q 2 = ∫ T2 dS

(2)

0
S2
0

where the suffix “1” represents the heat source side, the suffix “2” represents the ORC side,
Q (kW) is the heat amount, G (kg/s) is flow rate, T (K) is temperature, and S (kJ/sK) is entropy(defined by multiplying specific entropy by the flow rate).
Figure 5 shows the heat exchange process schematically. Q1 in the formula (1) isgeometrically equal to the area surrounded by the horizontal lines S=0 and S=S1, the operation
temperature curve, and the horizontal axis (absolute zero temperature). If there is no heat releaseloss, Q1 is equal to Q2. And S1 is smaller than S2 considering that T1 is larger than T2. When thegraphs of the two formulas are shown in an overlapped manner, S2 protrudes to the right (towardMitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)27

the higher entropy side) so that the two shaded areas have the same area, as shown in Figure 5(b).
This intuitively represents the increase of entropy in the heat exchange process. Considering theentire ORC, the increases of entropy at the turbine, the condenser, etc., are added to this, and thenfinally the heat is released to the outside of system.

Figure 5 Schematic diagram of energy loss
In graph (b), energy loss corresponding to entropy increment (S1 - S2) occurs

This serial cycle can be explained thermodynamically as the following. In the heat transfer
process from a high temperature heat source to a low temperature heat source (temperature of TL(K)), only the portion corresponding to TL(S2 - S1) of physical energy that can be ideally drawn (ina reversible manner) is converted to thermal energy (i.e., released) as an increase of entropy. This is
the so-called exergy loss. As for the heat exchanger model shown in Figure 5, it is necessary for the
reduction of exergy loss to decrease the temperature difference between the heat release side andthe heat-receiving side. To decrease the temperature difference, however, a larger heat transfer areaof the heat exchanger is required, which leads to increased cost. For the design of a binary cycle, it
is important to determine the temperature profile appropriately while balancing exergy loss and thecost of each piece of equipment.

3.4

Wetness of turbine outlet

Characteristics with respect to wetness of the turbine outlet differ significantly between waterand a low boiling point working fluid. Figure 6 shows the Rankin cycle (T-s diagram) of water (a)and normal pentane (b) as an example. This figure defines the high operation temperature as 120°C
and the low operation temperature as 35°C. In this figure, turbine loss is considered as zero forsimplification. The adiabatic expansion curve of the actual cycle protrudes slightly to the right, andis not a vertical line. The lower part of water’s T-s diagram on the steam side (the low temperatureside of the steam saturation line) protrudes largely to the right, and enters into a wet area after
adiabatic expansion. It is well known that this causes harmful effects such as efficiency degradationand erosion due to liquid droplets. In contrast, that of the working fluid (b) represented by pentane
enters into a dry area and seems to be favorable. However, the high degree of superheat at the
turbine outlet causes much waste of the heat amount to the condenser. In such cases, a feed liquidheater is sometimes added in order to recover the heat amount corresponding to the superheat,
depending on the conditions of the heat source.

Figure 6 Wetness of turbine outlet

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3.5

Points of concern for design of binary power generation system

The above describes the typical characteristics of an ORCe, but there are some points ofconcern for application to a geothermal binary power generation system.
(1) When silica supersaturation occurs due to the characteristics of geothermal water, it isnecessary to be aware of piping blockages and performance deterioration of the heatexchanger caused by scaling. Scaling speed varies significantly depending on the silica
concentration in hot water, pH and temperature. Therefore it is important to take measures
to prevent scaling as necessary, such as the management of pH (including the injection of
an inhibitor) and the control of injection temperature for underground injection. In this case,the cycle outlet temperature is sometimes constrained as described above. Therefore thedesign of an ORC needs to take this into consideration.
(2) It is necessary to pay sufficient attention to the corrosion of heat exchangers, which act as
contact points with the environment, in particular on the heat source side (i.e., the preheaterand evaporator), because they come into contact with geothermal water. The characteristics
of geothermal water vary considerably depending on the site. Therefore it is necessary to
sufficiently study the characteristics of the hot water to select the appropriate material, also
in consideration of the cost. The components of an ORC, other than the heat exchangers,
have no possibility to come into contact with geothermal water. However, they may besurrounded by corrosive gas such as hydrogen sulfide depending on the location. In suchcases, it is necessary to take measures against corrosion according to the gas concentration.

|4. Activity in development of geothermal binary system
MHI has been working on the commercialization of geothermal binary systems jointly with
Turboden s.r.l., an Italian manufacturer specializing in ORC, in order to meet the market needs inJapan as quickly as possible. Turboden s.r.l., founded in 1980, has past records of around 250 ORC
systems delivered mainly for biomass palnts. In recent years, they have delivered several
geothermal binary power generation plants (maximum output 5.6 MW) in Europe as shown in
Table 2. Among them, the 1000kW plant in Austria has been running stably for more than 10 years
since its delivery, demonstrating the reliability of the system.
Table 2
Country
Austria
FranceGermany
Italy
Germany
Germany
Germany

Delivery records of Turboden s.r.l. geothermal power generation systems
Power generation capacity
1000kW
1500kW
200kW
500kW
5600kW
5600kW
5000kW

Remarks
Operation launched in 2001; Cogeneration
Operation launched in 2008; Dedicated to power generation
Operation launched in 2009; Cogeneration
Operation launched in 2012; Supercritical (prototype)Operation launched in 2013; Dedicated to power generation
Operation launched in 2013; Dedicated to power generation
Operation launched in 2013; Cogeneration

Figure 7 Schematic diagram of geothermal binary structure

Figure 7 schematically shows the structure of the geothermal binary system. Part of the heatsource can be used for local heat supply. Air-cooled type, water-cooled type, or a combination of

Mitsubishi Heavy Industries Technical Review Vol. 51 No. 1 (March 2014)29

the two can be selected for the design of the condensate system. Figure 8 shows a general view ofthe existing binary power generation plant in Germany as an example. This plant is a 5.6MW
power generation plant using a hot water heat source.

Figure 8

Existing binary plant in Germany|5. Conclusion
MHI has mainly promoted the commercialization of flash geothermal power generation
plants as activities in the geothermal power generation field. However, actions related to binary
geothermal power generation plants have been limited to the accumulation of basic technologies
and the development of a prototype. In the future, MHI will develop products that can effectively
utilize low temperature heat sources previously unused for power generation and realize
community-based geothermal power generation plants, and prepare many applications such asincreasing the output of existing geothermal power generation plants jointly with Turboden s.r.l., tocontribute to the effective utilization of renewable energy and local revitalization.

References
1. Bertani, R., Geothermal Power Generation in the world 2005-2010 update report, Geothermics Vol. 41(2012) pp. 1-29
2. Ministry of the Environment, No. 20327001 “Treatment of Geothermal Power Generation System
Development in National Parks and Semi-national Parks” (2012)
http://www.env.go.jp/press/press.php?serial=15019