What is the estimated efficiency of geothermal power plants

8.6 Closing remarks

In this chapter, a binary geothermal power plant (GES) technology with an air-cooled organic Rankine cycle (ORC) was introduced. According to the first and second laws of thermodynamics for a binary GPP with double pressure, the mass, energy, and exergy balance equations were shown. As a case study, the flow characteristics and components of an existing plant were given. Exergy analyses were made according to the Second Law of Thermodynamics using data collected from the plant under real operating conditions. As a result of the exergy analysis applied to the plant, it was calculated that the highest exergy destruction occurs in reinjection as well as the condenser, vaporizer, and pumps. The total exergy input to the system is 53.8 MW, and 75.8% of it caused exergy destruction by the components. 23.7% of them were sent to reinjection, and its 0.5% were released to nature with NCGs. According to the results of the analysis, the total exergy production in the system was 21 MW, and the exergy efficiency of the system was 39.1%. As a result of the exergy analysis of the plant, the highest exergy destruction occurs in the condensers (Cond_1 and Cond_2). When the components with the highest exergy destruction were ranked, it occurred in condenser 2 (Cond_2), vaporizer 1 (Vap_1), condenser 1 (Cond_1), and the feed pumps (F_Pump_1 and F_Pump_2). Their values were calculated as 4.6%, 4.6%, 3.7%, 3.6%, and 3.6% of the total exergy input, respectively. In this order, the components that need improvement are determined.

1.2.4 Binary-organic Rankine cycle and Kalina cycle power plants

The binary geothermal power plant (B-GPP) generates electrical energy from a secondary separated process. Preheating of the working fluid is involved and heat is lost upon contacting the geothermal fluid [10]. Geothermal resources with a temperature range of 20–150°C [25] or 85–170°C [27] are well suited for binary configurations [25]. A higher temperature range provide thermal stability of the working fluid while the lower temperatures are more feasible in terms of technoeconomic and financial factors. Further, the impacts of corrosion and scaling are not apparent at high temperature, as there is no contact between the power generation equipment and the geofluid. Under a conventional Rankine cycle, there is the functionality of the secondary fluid (working fluid) in the binary system [28], and the binary cycle is identified as the Organic Rankine Cycle (ORC) due to the organic nature of the working fluid.

Binary power plants are versatile and the functionality of the power plant is decided by the secondary cycle. Different types of configurations of binary power plants are in operation, including B-GPP using an ORC with an internal heat exchanger (IHE), B-GPP with a regenerative ORC, and B-GPP with a regenerative ORC using IHE [29]. In 1982, Kalina patented a variation in B-GPP [30]. The working fluid used in the Kalina cycle consists of water and ammonia and can be used in different compositions to suit various configurations [31]. A thermal efficiency of 30%–40% is achievable and is considered more efficient than that of an ordinary B-GPP [28].

A closed loop of a thermodynamic Rankine Cycle used for the energy conversion system of a basic binary geothermal power plant is shown in Fig. 1.7. Harvesting the geothermal fluid via the production wells (PW) and then transporting it through various primary cycle components necessitates that pumping systems are deployed. The scouring and erosion of pipes and tubes can be prevented by extracting sand from the geofluid by employing sand removers (SR). Finally, an evaporator (E) and a preheater (pH) are used for continuous fluid flow while the geothermal fluid is reinjected into the reservoir near the injection well (IW) by using an injection pump (IP).

Regarding the working cycle, two heating-boiling procedures are included in the working fluid. The PH is the location of the boiling point of the working fluid. Upon contact of PH with E, the working fluid becomes a saturated vapor. This results in the expansion and condensation of the working fluid in the turbine. The working fluid is then returned to the evaporator, thereby concluding the loop process and beginning the process again [31]. To prevent steam eruption and calcite scaling within the pipes, monitoring the geothermal fluid above the flash pressure point is necessary [13]. Thus, the low-temperature geothermal resources are enabled by this binary-type energy conversion setup and by relying upon specialized highlights to attain amazingly high plant performances [32].

Fig. 1.8 presents the pressure-enthalpy of a binary geothermal power plant. At state 1, the working fluid at the saturated vapor point accesses the turbine and facilitated the expansion and production of work, and this marks the beginning of the thermodynamic process. Based on the work produced, electrical energy is generated by the generator. At state 2, after the expansion process of the turbine, the temperature and pressure of the saturated vapor reduce. At state 3, a temperature reduction occurs as the working steam-fluid enters the condenser, and this eventually culminates into fluid condensation. The application of cooled water from the air-cooled tower initiates the cooling process of the working fluid between states 3 and 4. The transformation of the working fluid state into a saturated liquid is achieved by this cooling phase. At states 5 and 6, the working saturated liquid-fluid is pumped back to the preheater and evaporator, respectively. At state 1, the emergence of the working fluid as saturated vapor initiates the process repeatedly [14, 15].

Regarding the condenser, the turbine, and the feed pump, the flash and dry-steam plants share similar thermodynamic analyses. The equation for the analysis of a binary geothermal power plant is captured in Table 1.5. The turbine expansion process work generated is computed in Eq. (1.37) while the isentropic turbine efficiency (ηt) is described in Eq. (1.38). Similarly, in Eqs. (1.39), (1.40), the power of the turbine (Wṫ) as well as the power of the generator (Wė) are evaluated as ṁwf connotes the mass flow rate of the working fluid and ηg is the efficiency of the generator. Qċ is the description of the working fluid heat discarded due to cooling during the condensation process. Eq. (1.42) presents the power transferred to the working fluid from the feed pump (Wṗ). A steady flow, well-insulated PH and E as well as insignificant potential and kinetic energy are the three propositions useful in analyzing the heat exchange process [15]. Eq. (1.43) is administered to the thermodynamic system where a symbolizes the geothermal fluid inlet while b represents the geothermal fluid after E and c after PH.

Table 1.5. Thermodynamic equations for binary geothermal power plants [15].

The analysis of geothermal fluid and the working fluid evaporation heat transfer rate is shown in Eq. (1.46). The known brine inlet temperature is represented as Ta; Tb is obtained from the pinch-point temperature (minimum temperature difference between two fluids supplied by the manufacturer) and the known T5. Lastly, Eqs. (1.48)–(1.50) give the performance assessment parameters of the cycle. Based on the input of thermal power (Q̇PH/E) and the thermal power rejected (Qċ ), Eq. (1.48) is the presentation of the thermal efficiency of the entire cycle (ηth) [15, 19].

During the design process of a B-GPP, choosing the working fluid is critical and entails considering the geofluid and working fluid thermodynamics characteristics as well as safety, health, and the effect on the environment [15]. The economy and the efficiency of B-GPP are described by the working fluid adoption [33]. Table 1.6 shows different working fluids and explicitly explains how working fluid critical temperatures (CT) and critical pressures (CP) are extremely lower in contrast to water. Different contemporary binary technologies have emerged, promoting advancement at higher performances via the flexible adoption of a secondary cycle in B-GPP [24]. Studies by DiPippo [15] and Valdimarsson [34] describe various other binary configurations, including the dual-pressure binary cycle, the dual-fluid binary cycle, the Kalina binary cycles, and regenerative ORC.

Table 1.6. Working fluids commonly used in binary geothermal plants [15].

To adopt ORC-GPP, different methods have been investigated regarding working fluid. A work by Quoilin [35] put forward an approach for the selection of the working fluid and an expansion process in the same system. For any ORC process, the working fluid and expansion mechanism are adopted by the application of this method. Mikielewicz and Mikielewicz [25] studied 20 working fluids for an ORC and concluded that R123 and R141b possess the most suitability for small-scale operations. Regarding the adoption of the best applicable working fluid for an ORC, extensive indicators are provided in [35], namely thermodynamic performance, isentropic saturation vapor curve, high vapor density, low viscosity, high conductivity, evaporating pressure, condensing gauge pressure, high-temperature stability, melting point, low ozone-depleting potential, low greenhouse warming potential, availability, and low cost. Astolfi [36] conducted a comprehensive study of binary ORC power plants focusing on harvesting low-medium temperature geothermal sources [37]. The above authors examined 54 working fluids in six dissimilar cycle configurations and concluded that the optimal fluid is decafluorobutane at a low temperature of 120°C while at a higher temperature of 180°C, R236ea has been the optimum fluid.

Heat Transfer Coefficients

A critical problem in binary geothermal power plant design is the requirement for tremendous amounts of heat transfer at relatively small temperature differences. A large portion of all heat transfer occurs in either the boiling or condensing of the fluid. Therefore, high boiling and condensing coefficients become desirable. However, it must be remembered that to boil the fluid, heat flows from water to a film of dirt, thru the dirt film, thru a conducting wall, possibly thru another dirt film, and finally to the boiling fluid. In the condenser the five steps are reversed. Therefore, the boiling or condensing coefficients of the fluid are not the only factor affecting heat exchanger costs.

14.4.1 Holistic design approach—theoretical considerations

The design of the subsystems in geothermal binary power plants depends on different site-specific influences and parameters. In order to design reliable and efficient geothermal binary power plants it is therefore important to integrate these different characteristics in an overall or holistic design approach. The most important aspects or differences of a holistic design approach compared to the separate design of each subsystem are:

An optimum geothermal fluid flow rate: The gross power which can be generated from a geothermal resource linearly increases with increasing geothermal fluid flow rate. However, the considerations above have shown that increasing the flow rate at a site also results in an over-proportional increase in power consumption of the fluid production system (Fig. 14.3). This means that an optimum geothermal fluid flow rate exists for which the net power provision of a geothermal binary power plant reaches its maximum. Assuming a specific plant setup on the surface, the optimum flow rate for a site with lower reservoir productivity is therefore lower. Referring to different surface plants setups, the optimum flow rate is increasing with a more efficient and better utilisation of the geothermal heat such as with more efficient (and reliable) binary units or the supply of the residual heat in the geothermal fluid after the heat transfer to the binary unit.

An optimum working fluid: The selection of the working fluid enables the adaptation of the binary conversion cycle to the characteristics of the geothermal heat source. This is due to different shapes of the dew-point curve and different evaporation characteristics which can be realised with different media and mixtures. The choice of a suitable working fluid is therefore an important aspect in designing geothermal binary power plants. A suitable working fluid must allow reliable operation (e.g. thermally stable in the long-term, compatible with other materials used in the binary cycle), a high conversion efficiency and a good utilisation (i.e. the cooling of the geothermal fluid) of the geothermal heat. Due to the relatively large waste heat amount in geothermal binary power plants, also a selection of the working fluid according to the heat sink and operation characteristics must be considered.

An optimum evaporation temperature: The evaporation temperature contrarily influences the conversion cycle efficiency and the utilisation of the geothermal heat. Hence, an optimum evaporation temperature exists for which the power output reaches a maximum. Regarding the design of the evaporation also annually varying ambient conditions which might influence the condensation temperature must be considered. If a geothermal binary power plant should also supply heat in serial connection to the binary power unit, the evaporation temperature also depends on the temperature which is required at the outlet of the binary unit in order to provide a certain supply temperature. The outlet temperature of the geothermal fluid can be increased with higher evaporation temperatures. Another possibility is internal heat recuperation in case a dry working fluid is used.

An optimum condensation temperature: The gross power output of a binary unit is increasing for decreasing condensation temperatures due to the increasing enthalpy difference in the expansion machine. However, also the requirements for the recooling are increasing with decreasing condensation temperatures. This is because lower temperatures must either be realised by lower cooling sink temperatures such as in case of once-through cooling systems. At many sites, where once-through cooling is not an option, lower condensation temperatures can only be realised by a larger auxiliary power input to the fans of wet cooling towers or air coolers. The correlation between condensation temperature and auxiliary power demand is determined by the recooling system, its performance and the ambient conditions. Therefore, regarding the net electricity production, also an optimum condensation temperature does exist. It must be considered that the ambient conditions can significantly vary during the year. Regarding the relatively large waste heat amounts in geothermal binary power plants, the technical use of the waste heat (e.g. conventional cogeneration) can also reduce the demand for recooling.

16.3.1 Description of the system

This study focused on an air-cooled binary GPP with an organic Rankine cycle (ORC). The simple schematic design diagram of the GPP is shown in Fig. 16.3. The main components of the GPP consist of generators, turbines, condensers, evaporators (vaporizers, preheaters, recuperator), and pumps. The heat source of the power plant is medium-temperature brine. The brine is extracted from production wells at 165–170°C, 10–14 bar pressure, and 1100–1700 ton/h of mass flow rate. The brine is then sent to the separator and separated as steam at 165°C, 10 bar, and 30 ton/h and as a liquid at 165°C, 10 bar, and 1600 ton/h. The plant has two different ORCs with high pressure (13 bar and 570 ton/h n-pentane) and low pressure (7 bar and 700 ton/h n-pentane). As seen in Fig. 16.3, the geothermal brine is the heat source of two different ORCs. However, geothermal steam is only sent to the vaporizer on the low-pressure side as an additional heat source. The path followed by the brine liquid is vaporizer I, vaporizer II, preheater II, and preheater I. It is finally taken into the cooling pool at 85°C and 6 bar. It is then pumped into the reinjection well at 70°C by mixing with the condensed liquid, which leaves vaporizer II. There are certain differences between the two high-pressure and low-pressure ORCs: (i) the fluid pressures (13 and 7 bar) and temperatures (150°C and 120°C) are different, (ii) the recuperator is only used in the high-pressure ORC, and (iii) in addition to geothermal fluid, geothermal steam is supplied to the vaporizer of the low-pressure ORC. In high- and low-pressure ORCs, the pentane is cooled by air-cooled condensing units. Besides, turbines are directly connected to the generator via a single shaft. Here, the speed of the shaft is balanced with injection valves on the turbines. The electricity produced by the generator first meets the electricity need of the main and auxiliary components, such as the fans and pumps of the power plant, and the remaining part is transferred to the network.

Geothermal power

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11.6.2.4 Results and discussion

The exergy flow diagram is given in Fig. 11.47 shows that 64.5% of the exergy entering the plant is lost. The remaining 35.5% is converted to electricity, 18.1% of which is used for parasitic loads in the plant. The exergy efficiency of the plant is 34.2% based on the exergy input to the isopentane Rankine cycles (i.e., the exergy decreases in the brine in the vaporizer and preheater) and 29.1% based on the exergy input to the plant (i.e., the brine exergy at the Level I vaporizer inlet) (Table 11.9).

Bodvarsson and Eggers [51] report exergy efficiencies of single- and double-flash cycles to be 38.7% and 49.0%, respectively, based on a 250°C resource water temperature and a 40°C sink temperature. Both values are significantly greater than the exergy efficiency calculated for the binary plant analyzed here. This is expected since additional exergy destruction occurs during heat exchange between the geothermal and working fluids in binary plants. DiPippo and Marcille [52] calculate the exergy efficiency of an actual binary power plant using a 140°C resource and a 10°C sink to be 20% and 33.5% based on the exergy input to the plant and the Rankine cycle, respectively. Kanoglu and Cengel [48] report exergy efficiencies of 22.6% and 34.8% based on the exergy input to the plant and the Rankine cycle, respectively, for a binary geothermal power plant with a 158°C resource and 3°C sink.

Because they use low-temperature resources, geothermal power plants generally have low energy efficiencies. Here, the plant energy efficiency is 5.8% based on the energy input to the plant and 8.9% based on energy input to the isopentane Rankine cycles. This means that more than 90% of the energy of the brine is discarded as waste.

The results suggest that geothermal resources are best used for direct heating applications such as district heating instead of power generation when economically feasible. For power generation systems where used brine is reinjected back into the ground at a relatively high temperature, cogeneration in conjunction with district heating may be advantageous. The energy flow diagram in Fig. 11.48 shows that 35.2% of the brine energy is reinjected, 57.8% is rejected in the condenser, and the remainder is converted to power. These data provide little information on how the performance can be improved, highlighting the value of exergy analysis.

The primary exergy losses in the plant are associated with vaporizer-preheater losses, turbine-pump losses, brine reinjection, and condenser losses, which represent 13.0%, 13.9%, 14.8%, and 22.6% of the brine exergy input, respectively (Fig. 11.47). The exergy efficiencies of the Level I and II vaporizer-preheaters are 87% and 83%, respectively. These values are high, indicating efficient heat exchange operations. In binary geothermal power plants, heat exchangers are important components and their individual performances affect considerably overall plant performance. The exergy efficiency of the vaporizer is significantly greater than that of the preheater, mainly because the average temperature difference between the brine and the working fluid is smaller in the vaporizer than in the preheater.

The exergy efficiencies of the turbines in Levels I and II are 75% and 70%, respectively. These efficiencies indicate that the performance of the turbines can be somewhat improved. This observation is confirmed by the relatively low turbine isentropic efficiencies (in the range of 65–70%) listed in Table 11.9. That a reasonable margin for improvement exists can be seen by considering a recently built binary geothermal power plant, which has a turbine with an exergy efficiency of over 80% [49]. The pumps seem to be performing efficiently.

The exergy efficiencies of the condensers are in the range of 30%, making them the least efficient components in the plant. This is primarily due to the high average temperature difference between the isopentane and the cooling air. The brine is reinjected back into the ground at about 65°C. In at least one binary plant using a resource at about 160°C, the brine is reinjected at temperatures above 90°C [49]. Compared to this, the percent exergy loss associated with the brine reinjection is low in this plant. It is noted that condenser efficiencies are often difficult to define and interpret since the objective of a condenser is to reject heat rather than create a product.

For binary geothermal power plants using air as the cooling medium, the condenser temperature varies with the ambient air temperature, which fluctuates throughout the year and even through the day. As a result, power output decreases by up to 50% from winter to summer [48]. Consequently, the exergy destruction rates and percentages vary temporally as well as spatially, this effect being most noticeable in the condenser.

15.2.3 Noncondensable gases

Working fluids in geothermal power stations contain impurities in the form of gases that do not condense at the same temperature as the working fluid. Thus, when the working fluid condenses, these impurities remain in the gaseous state causing “vapor locks” in the flow channels and are detrimental to system performance, particularly in surface condensers. In flash-steam systems, common noncondensable gases found in geothermal fluids include CO2 and H2S. Impurities found in organic fluids used in binary geothermal power plants vary with the working fluid. When first assembled, the piping and equipment will contain gases, usually air and water vapor, and it is important to evacuate the entire system before operation.

For low-pressure working fluids, where the operating pressure of the condenser is less than ambient pressure, even slight leaks can be a continuing source of noncondensables. In such cases, a purge system that automatically and safely expels noncondensable gases may be used, such as vacuum pumps and jet ejectors.

When present, noncondensable gases collect on the high pressure side of the system and raise the condensing pressure above that corresponding to the temperature at which the working fluid is actually condensing. This results in decreased power from the turbine, increased parasitic power consumption, and reduced heat rejection rates. The excess pressure is caused by the partial pressure of the noncondensable gas. These gases form a resistance film over some of the condensing surface, thus lowering the heat transfer coefficient. The actual impact of noncondensable gases is difficult to characterize if they tend to accumulate in confined areas far from the heat transfer surface. In these cases, a relatively large amount of noncondensables may be tolerated. One way to account for noncondensables is to treat them as a fouling resistance to be discussed in the subsections that follow.

8.1 Introduction

Binary cycle geothermal power plants are the closest in thermodynamic principle to conventional fossil or nuclear plants in that the working fluid undergoes an actual closed cycle. The working fluid, chosen for its appropriate thermodynamic properties, receives heat from the geofluid, evaporates, expands through a prime-mover, condenses, and is returned to the evaporator by means of a feedpump.

The first geothermal binary power plant was put into operation at Paratunka near the city of Petropavlovsk on Russia's Kamchatka peninsula in 1967 [1]. It was rated at 670 kW and served a small village and some farms with both electricity and heat for use in greenhouses. It ran successfully for many years, proving the concept of binary plants as we know them today.

At the birth of the commercial geothermal power age in 1912 at Larderello, Italy, a so-called “indirect cycle” was adopted for a 250 kW plant. The geothermal steam from wells was too contaminated with dissolved gases and minerals to be sent directly to a steam turbine so it was passed through a heat exchanger where it boiled clean water that then drove the turbine. This allowed the use of standard materials for the turbine components and permitted the minerals to be recovered from the steam condensate [2].

Today binary plants are the most widely used type of geothermal power plant with 155 units in operation in July 2004, generating 274 MW of power in 16 countries. They constitute 33% of all geothermal units in operation but generate only 3% of the total power. Thus, the average power rating per unit is small, only 1.8 MW/unit, but units with ratings of 7–10 MW are coming into use with advanced cycle design. Several binary units recently have been added to existing flash-steam plants to recover more power from hot, waste brine. See Appendix A for more statistics.

11.4.2 The multipressure radial outflow turbine

The multipressure ROT allows multiple admissions and/or extractions in the same machine.

Multiple extractions give the opportunity to perform more efficient cogenerative cycles, because only a part of the working fluid is used as a thermal input for utilities, while the other part can continue its expansion for providing electricity.

On the other hand, an interesting application of multiple admissions is in geothermal power plants. It is well documented that binary geothermal power plants have the possibility to be developed as multipressure level cycle systems and for many geothermal resources; a system designed in this way can deliver a significant increase in the power output, in comparison to a single pressure level system (Di Pippo, 2005). These cycle configurations enable the recovery of heat from the geothermal source by vaporizing the organic fluid at multiple pressure and usually reduce both the thermal irreversibility associated with such heat exchange and increase the cycle efficiency. The concept recalls the combined cycles.

As represented in Figs. 11.18 and 11.19, a single pressure level cycle is intrinsically simpler and usually less expensive than a multipressure level system. However, the advantage in the efficiency offered by the latter gives a favorable payback, thus making it the preferred option in the geothermal field.

Multipressure level geothermal binary systems require at least one turbine per pressure level to expand the organic vapor, and these turbines are normally installed with an overhung configuration (Di Pippo, 2005).

The ROT gives the possibility to employ a different solution.

In order to expand multipressure flows in the same axial turbine, it would be necessary to enlarge the space between the disks, allowing a second flow to be inserted to mix with the primary flow; the mixed flow would then expand in the remaining stages.

As a consequence, the shaft length of the axial turbine must increase, resulting in several rotor dynamic problems for the overhung configuration, as the distance between the center of gravity of the turbine disks and the bearings consequently increases. However, it does not appear possible to design such a turbine. Having a single-disk/single stage configuration, the radial inflow turbine does not allow multiple pressure entries.

A different scenario is instead provided by the ROT; its unique single-disk/multistage configuration makes it possible to enlarge the spacing between the stages (Figs 11.20 and 11.21), allowing a low pressure flow to enter the turbine, whilst still maintaining an overhung configuration without any negative rotor dynamic consequences.

The ROT can therefore allow multiple pressure levels cycles to be expanded over a single turbine.

The major advantages related to this configuration are the techno-economical savings associated with a lower number of turbines to be installed per plant. This minimizes the plant overall costs, and reduces the amount of rotating equipment, spare parts and maintenance. Furthermore the plant layout is more compact, reducing the amount of foundations, control equipment, and control complexity.