- Email: [email protected]
Section 4 Electric power production from geothermal energy
Geotherrnics, Vol. 14, No. 2/3, pp. 157
163, 1985.
0375
Printed in Great Britain.
6505/85 $3.00 + 0.00 Pergamon Press Ltd. .~c~ 1985 CNR.
UN Seminar on the Utilization of Geothermal Energ~ for Electric Power Production and Space Heating. Florence 1984
SECTION ELECTRIC
POWER
PRODUCTION
4
FROM
GEOTHERMAL
ENERGY
General Report prepared by M. Guglielminetti, AGIP S.p.A., Milan, appointed General Rapporteur by the Government of Italy R. 104
INTRODUCTION Electricity-generating capacity from geothermal resources has risen from 1100 MW in 1974 to 3400 MW by the end of 1983. This corresponds to a yearly rate of increase of about 13% that is substantially higher than the rate of electric energy demand. Although in the energy balance of the world, geothermal energy still represents a minor asset, in a few countries such as Philippines, E1 Salvador, Nicaragua and Kenya this resource provides a significant contribution to the total electric generation and, at the same time, plays an important role in the global economic development of the above countries. Moreover, the intensive activities of geothermal exploration throughout the last decade have led to the discovery and assessment of many new geothermal fields, which are now ready for immediate development. The implementation of these development projects has to confront the presently difficult energy market, characterized by uncertainty or even reduction in prices and by an increase in the demand at a rate much lower than expected. These conditions, definitely less favourable than in the past, require a marked improvement in the technology of exploitation of geothermal resources: in fact, such technology, although already well advanced, must further raise its standard of efficiency and at the same time reduce costs, in order to maintain a competitive level of technical and economic validity. Multipurpose projects, such as electric energy generation associated with salt water desalination, can offer, in certain cases, very interesting opportunities for geothermal development. The results of the experiments carried out at Fenton Hill in New Mexico (U.S.A.) make it possible to foresee that, in the near future, an advanced technology for the exploitation of hot dry rock will be available and reliable, and that it will be possible to exploit large amounts of energy from these man-made geothermal reservoirs with positive economic results. These evolutions could create the conditions for the execution of more ambitious projects such as the exploitation of the earth magma. Although interesting and attractive, the prospect must be studied in detail and new technologies must be identified before taking it into consideration. The studies and research which are being carried out should be further pursued in the future, with special emphasis on the following fields: efficiency of traditional processes for energy transformation; - - introduction of new processes for energy transformation; - - special projects; corrosion, scale and waste disposal management; advanced design, system operation and costs. -
-
-
-
-
-
T R A D I T I O N A L PROCESSES FOR E N E R G Y T R A N S F O R M A T I O N Under this category, we consider power-plants equipped with back pressure or condensing 157
158
M. Guglielminetti
turbines, utilizing geothermal steam either directly produced by dry fields or obtained through flashing, single or double, from fluids in water-dominated fields. The main steps of the process consist of: - - fluids conveyance and steam separation; conversion of thermal energy in the steam into electrical energy; systems of steam condensation and gas extraction for the condensing turbines. The widespread use of two-phase conveyance and the concentration of steam separation equipment into a limited number of units, each serving as many wells as economically feasible, introduced important elements of progress into rationalizing field management. Severe problems are, on the contrary, encountered in the application of multiple flash of residual water, after the first steam separation. Such problems are mainly related to scaling effects, due to reinjection of water at a temperature of about 170°C. The efficiency of energy transformation in the turbines has achieved a high standard, since the relative technology is essentially identical to the one applied in conventional steam plants. Particular attention is paid to material selection and to the devices for mist elimination which are subject to continual improvement. Special design criteria are sometimes sought to obtain high flexibility of turbine operation. Interesting solutions have already been studied in the past, such as the combination of a back pressure turbine, to be installed at the beginning of field exploitation, especially in the case of a high content of gas in the steam, to which a condensing section can subsequently be coupled on the same axis; or the possibility of modifying the blading of the turbine in such a way as to be able to change the steam pressure in very large fields without changing the rated capacity. Minson et al. (R.74) show that the turbines installed in the Tongonan field can generate 37.5 MW both if steam from the second separation at 1.14 bar is available, and when it is fed only by steam at about 5.6 bar. Mertoglu (R.36) mentions experimental turbines, designed in Turkey, which are capable of operating under very different conditions. The modern direct contact condensers can achieve an accentuated vacuum by means of an optimal distribution of water volumes and flow, and by means of an effective fractioning of water drops. The corresponding energy can be conveniently exploited if the gas extraction system is more efficient than the traditional steam ejectors. The application of mechanic compressors, used in the past in the presence of a high content of incondensable gases (percentage of some units in weight), should be extended to plants with moderate gas content as well. Should the gas content in the steam be very high, roughly about 5% in weight, a new and efficient solution to be considered is the combination of back pressure steam turbine unit - atmospheric, surface condensers - - binary cycle unit. This latter unit will be examined further -
-
-
-
on.
The circulating water systems for the cooling of the condensers normally include a wet-type cooling tower. This part of the plant is rather costly since the cooling water is made up of geothermal steam condensated with dissolved gases, in particular CO: and H,S, and various salts, and becomes aggressive on contact with the oxygen in the air inside the cooling tower and, therefore, requires corrosion resistant materials. The improvement in the choice of the materials with the use of chemical products for pipes, tower filling, etc., may in the future reduce plant and operating costs, so avoiding also the chemical treatment of the circulating water. One particular operating problem of the cooling tower is microbiological activity, encouraged by the sulphur in the H,,S, and the corrosion w.hich it causes. This interesting matter is dealt with later. Back pressure portable units are frequently used now during the initial experimental period
G e o t h e r m a l Electric P o w e r Production
159
of exploitation of a newly discovered field. The limited efficiency of this type of turbine (about one half as compared to the condensing type) is counterbalanced by the low cost, the rapidity of installation and the amount of information on reservoir characteristics and evolution that can be drawn, before the full-scale condensing turbine is installed. Two papers show the solutions adopted for very different projects. The first, presented by Jaud and Lam6the (R.77), describes a prototype power-plant of small capacity (4200 kW), constructed by Electricit6 de France on the island of Guadeloupe; the second, presented by Minson et al. (R.74), refers to the 112 MW Tongonan power-plant on Leyte Island of the Philippine archipelago. The small condensing power-plant located in Guadeloupe is fed by two-pressure geothermal steam of 6 bar and 1 bar. Since it is located on the beach, sea water is utilized for cooling the barometric, direct contact-type condenser. The same water, discharged by the condenser, is mixed with the residual geothermal waste water coming from the second steam separation and is discharged back into the sea at a temperature lower than 45°C in order to limit the effects of thermal pollution. Obviously, this prototype solution is expensive, since it requires an investment of 3800 F / k W (470 US$/kW), which results in a total cost of the energy produced of 0.67 F / k W h (about 84 US mills/kWh), equivalent to the cost of the energy produced by diesel units. This power plant, on the other hand, has the advantage of exploiting local resources for a small, isolated electric system, such as that of an island. The paper presented by Minson et al. (R.74) contains a detailed description of the design of the 112 MW power-plant in the field of Tongonan, Philippines, and gives concrete evidence of some of the points described above. The paper is mainly descriptive, but does put forward some design criteria which deserve mention as examples of good engineering practice: - - rationalization of the fluid collection system, which uses extended two-phase transmission piping and centralizes separation facilities; - - search for a high degree of reliability, necessary to feed the copper smelter which is the main energy user. This criterion appears useful in general, considering the serious economic problems arising from planned and forced outage, which diminish the exploitation of the geothermal resources; - - flexibility of production in order to adapt generation to the fluctuating loads through regulation of the total well output and, in particular, remote control of the valves at the head of six wells, which make it possible to exclude them from the fluid collection system without loss of steam into the atmosphere. The project confirms the difficult solution of the problems relative to the reinjection of residual waters and, in particular, reinjection at relatively low temperatures. In fact, the possibility of the flashing of water with the second separation of the steam at low pressure (1.14 bar at entry into the turbine) is foreseen; however, its utilization must be confirmed by tests which show the technical feasibility of reinjection at 100°C. Furthermore, the possibility is foreseen of using two thermal ponds to regulate water disposal in the case of a short period of emergency when there are problems in the reinjection system. There is no paper dealing systematically with design methods for optimizing the geothermal power-plant, taking into consideration the entire geothermal system including the geothermal reservoir, production wells, fluid collection system, power generation and reinjection. There are numerous and important interdependences between the various components which influence both the choice of the main parameters such as steam pressure, and the optimization of the single components. The result of achieving highest efficiency with lowest costs can only be reached with an integrated design which is based on a widespread use of mathematical models for the reservoir and special calculation programmes to determine the parameters of the process in the plants.
M. Gu~lielminelti
160
NEW PROCESSES FOR ENERGY T R A N S F O R M A T I O N The main processes at present under development are based on "binary cycles" and "total flow prime movers".
Binao' cycles Binary cycles use a secondary fluid of organic nature to generate power. This fluid is evaporated by geothermal heat through a heat exchanger; it then expands in a turbine and is condensed, releasing heat to a cooling system, to be finally pumped again in the evaporator. In recent years, several experimental plants of this type have been successfully designed in Europe (particularly in Italy), U.S.A. and Japan. Similar applications have been subsequently introduced in fields not related to geothermics, e.g. the recovery of waste heat from the discharge of industrial processes. Such technology can be, therefore, considered as adequately advanced and its potential applications are to be evaluated under the following conditions: - - geotherntal fluids with salt or gas content such as to involve severe problems with traditional processes; geothermal fluids with relatively low enthalpy and high specific heat consumption, An important field of application for binary cycles has already been mentioned, that is, for a suitable exploitation of geothermal steam with high gas content. We are convinced that, in these cases, the binary cycle applied to a back pressure steam turbine exhaust represents a practical and effective solution to improve the economic feasibility of the exploication of these fields. Small-capacity plants can be easily installed in a package module, complete with accessories. This solution would allow a substantial cost reduction, coupled with high efficiency, and it might prove convenient either for electrical systems of small capacity or for geothermal resources of limited size and temperature. In spite of the great interest aroused by this technology and the quantity of experiments. unfortunately no written accounts have been presented. The results of the experimental unit operation could, in fact, supply valuable information for future full-scale industrial plants. -
-
Total Jlow prime movers Numerous research programs on the application of total flow prime movers are under' way; although these programs have brought interesting results, the economic validity of the method still remains to be proven. The concept of the total flow prime mover is based on the possibility of expanding simultaneously water and steam through a nozzle, thereby exploiting the related energy in a turbine. The theoretical advantages rest on the elimination of the energy losses, which are associated with the conventional method of flashing and steam separation. The practical adoption has been inhibited by the expected difficulties which stem basically from the widely different thermodynamic properies of steam and hot water. These difficulties appear from the total flow nozzle project, thus leading Ryley (R.65) to carry out an appraisal of some aspects of the analysis of the wet steam nozzle. The paper looks critically at the model previously identified by various experts, examines in detail some of the assumptions made and tries to identify the limitations of the model and to clarify some of the basic problems encountered. The conclusion indicates that the droplet size to which the liquid can be atomized is of paramount importance, and that future research should be towards increasing the dispersion of the entry liquid and doing so with the minimum expenditure of energy. Sebestybn (R.87) describes research carried out in Hungary, starting from 1971, on two-phase turbines and illustrates the axial flow-type turbine and accessory equipment projects. The project is based both on theoretical studies and experiments which made it possible to test and
Geothermal Electric Power Production
161
improve important equipment components. The schemes foresee two-phase turbines for exploiting expansion energy during the flash and the successive separation of steam which is conveyed to conventional steam turbines. According to the calculations carried out on the assumption that 5 bar, 152°C hot water would be available, the increase in the output of twophase turbines is more than 30%. The author states that the greatest problems are hazards caused by mineral salt deposits which are even more troublesome than those connected with erosion. Considering the large increase in thermodymanic transformation efficiency, the abovementioned research should be complemented by the implementation of experimental projects, aimed at proving the technical and economic feasibility of industrial plants.
SPECIAL PROJECTS H o t dry rocks (HDR) The basic idea in extracting energy from hot dry rock is to form a man-made geothermal reservoir by drilling into high-temperature, low-permeability rock. A circulating loop is then formed by connecting a second drillhole to the first by hydraulic fracturing and forcing water to sweep heat from the rock in the fractured region between the wellbores. The hot water produced at this surface may be used for generating electricity, space heating or other direct uses.
The characteristics of the H D R projects, the mining and plant problems, the results attained by the systematic approach to reservoir development adopted in the U.S.A., the enhancement of performance of the first 60 kW plant already installed using corrosion and monitoring instruments and, finally, the scaled-up design for new commercial size power plants - - which illustrates the technical and economic feasibility of a 75 MW electric power plant - - are well described in the papers by Shannon et al. (R.63, abstract only) and Murphy et al. (R.62). Research and experiments are performed at Fenton Hill (New Mexico, U.S.A.). The project develops by stages: stage I, which consisted of the construction of a 60 kW research size power plant and has been in operation for more than two years; stage 2, which is the first step in developing commercial-size reservoirs and includes the creation of a man-made reservoir capable of supplying a 35 MW thermal power, corresponding to an electric power of 6.5 MW. The wells of this stage have already been completed. By utilizing various reservoir patterns of stage 2 it is possible to extend the project up to 75 MW. Murphy's paper emphasizes that most investments and operating costs are intended for the implementation and maintenance of the reservoir. In order to have as economic a project as possible, it is necessary that the costs of the wells for a commercial plant should not exceed those of oil and gas wells, i.e. they must not include " l e a r n i n g " costs as the preceding experimental stages. The optimum solution for the energy conversion equipment is to adopt the binary cycle which has the advantage, a m o n g others, of not dissipating the water required in great quantities to feed the reservoir. The thermal conversion efficiency is approximately 15%. The total project costs are estimated at 2300 U S $ / k W and the generating cost at 42 US mills/kWh approximately. These values are very interesting and renew confidence in this new technology for the exploitation of earth heat. Multipurpose utilization Sometimes the exploitation of geothermal resources presents serious difficulties due to manifold constraints.
162
M. Guglielminetti
This is the case on an island with various different requirements, very changeable and rather modest; meeting these requirements separately does not repay, of course, the costs of the geothermal plants. For the island of Vulcano in Italy, coupling electric power generation with desalination of the water for domestic use makes it possible - - by suitably regulating water production - - to meet the island's requirements through the continuous and optimum exploitation of the geothermal resources. The proposed scheme provides for the desalination of the sea water, reaching a daily production of 4200 m 3 of potable water, and the installation of a 5 M W power plant (R.14, abstract only). C O R R O S I O N , SCALE A N D WASTE D I S P O S A L M A N A G E M E N T Corrosion tests carried out in the past in many geothermal fields gave consistent results, that brought to the formulation of standard criteria for the selection of materials. It can be observed that the extended operation of the plants has confirmed the overall validity of these criteria; particular problems are still encountered in specific components of the plants, such as steel subjected to elevated stress or concrete in the cooling towers affected by micro-organisms attack. In spite of the high level of knowledge already reached in the field of corrosion, some margin of improvement still exists, and at any rate it is recommended to keep as a standard practice the execution of corrosion tests in new fields, both under aerated and unaerated conditions of the geothermal fluids. Tacchino and others (R.6) illustrate an up-to-date method that uses modern instruments for corrosion tests, which has been adopted in San Martino power plant at Larderello, Italy. The fluids of this field are particularly aggressive and likely to create new problems, in spite of the long experience gained with the fluids of nearby reservoirs. On the basis of the analysis of test results, optimization criteria for turbine materials are proposed. Citernesi et al. (R.18) have made a detailed and excellent study of corrosion phenomena in concrete and iron and also of failures in the cooling towers, caused by micro-organisms that feed on the chemicals of geothermal fluids. The study of these complex phenomena requires great competence in microbiology and material corrosion. Shannon et al. (R.63, abstract only) carried out corrosion and sealing tests on the 60 kW binary cycle experimental power plant for the hot dry rock project mentioned previously. The tests for an innovatory plant are essential to control the feasibility of future full-scale projects. Their monitoring methods for detecting operational problems are extremely interesting. Serious problems and constraints, many of which are still unsolved, are related to scalin~ phenomena. Silica scaling, which may occur in residual water at temperatures below 170°C, discourages the use of double flash at a near atmospheric pressure due to the risk of silica deposition in the wells and in the reservoir during reinjection. A better knowledge of the kinematics of incrustation and of inhibitor additives might allow an adequate management of relatively cold fluids and bring about a higher efficiency of exploitation. In carbonate-rich waters, CO2 may be released at f,lashing point, followed by calcite deposition, either in the production wells or in the surface conveyance pipes. These problems are usually solved by means of traditional technologies, by shutting down the wells and plants and cleaning them mechanically. Interesting experiments are now under way on the use of inhibitor complexes or in-hole pumps that maintain a high pressure and prevent flashing. The successful results of these studies would constitute a major breakthrough in the technology of geothermal exploitation. Finally, reinjection has now become a routine solution for the disposal of waste water, mainly because of environmental reasons. The effects of reinjection on field behaviour depend on many factors and must be evaluated on a case-to-case basis. Comprehensive reservoir
Geothermal Electric Power Production
163
engineering studies are needed to compile the available data, define common criteria of interpretation, estimate the long-term effects and establish guidelines for the implementation and monitoring of reinjection under different circumstances. A D V A N C E D DESIGN, SYSTEM O P E R A T I O N A N D COST In addition to the already mentioned process and materials selection, major improvements can be achieved through a proper design of plant components. Standardizing components, keeping up to date with the evolving technology, periodically revising the design of details and selection of the corresponding material, can substantially enhance the reliability of the plants and simplify their operation and maintenance, while reducing at the same time the relative costs. A m o n g the minor components that could benefit from advanced design we might mention the equipment directly in contact with geothermal fluids, such as valves, separators, steam traps, gas extractors, primary elements of measuring systems; electrical equipment; measuring, regulation and control equipment, in contact with ambient air polluted by hydrogen sulphide. Automation of plants can be extended by making larger use of remote-control up to the point of installing complete, unmanned units, thus drastically reducing operation costs. For isolated areas served by small electrical systems, a cheap energy supply achieved through cost reductions would constitute an important incentive for industrial development and social growth. Additional opportunities for saving costs have to be evaluated wherever electrical generation can be combined with water desalination or with other direct uses of heat, for example in industries for food conservation, mining processes and space heating. LIST OF R E P O R T S In this report, prepared with M. Cozzini, the followingpapers submitted to the Secretariat up to 16 March 1984have been taken into account: EP/SEM.9/R.6
Material selection for advanced geothermal machineries. G. Tacchino and G. Cerisola (Italy) (E).
Abstract only. EP/SEM.9/R.18 Microbiological aspects of concrete and iron deterioration in geothermal power-plants. U. Citernesi, G. Benvenuti and G. C. Ferrara (italy) (E). EP/SEM.9/R.62 Economics of a conceptual 75 M W hot dry rock geothermal electric power-station. H. Murphy, R. Drake, J. Tester and G. Zyvoloski (U.S.A.) (E). EP/SEM.9/R.65 A critical appraisal o f some aspects o f the analysis o f the wet steam nozzle as used in total flow machines. D. J. Ryley(U.K.)(E). EP/SEM.9/R.72 Magma-paradox and electric power. R. J. Varney (U.S.A.) (E). EP/SEM.9/R.74 Tongonan I geothermal power development, Philippines. A. A. C. Minson, J. A. Kivell and T. J. Fry (New Zealand) (E). EP/SEM.9/R.77 The Bouillante geothermal power-plant. Guadeloupe. P. Jaud and D. Lam~the (France) (F). EP/SEM.9/R.87 Design o f two-phase turbines and geothermal electric power-plants in Hungary. I. Sebesty~n (Hungary) (E). (E) Original in English. (F) Original in French.
163, 1985.
0375
Printed in Great Britain.
6505/85 $3.00 + 0.00 Pergamon Press Ltd. .~c~ 1985 CNR.
UN Seminar on the Utilization of Geothermal Energ~ for Electric Power Production and Space Heating. Florence 1984
SECTION ELECTRIC
POWER
PRODUCTION
4
FROM
GEOTHERMAL
ENERGY
General Report prepared by M. Guglielminetti, AGIP S.p.A., Milan, appointed General Rapporteur by the Government of Italy R. 104
INTRODUCTION Electricity-generating capacity from geothermal resources has risen from 1100 MW in 1974 to 3400 MW by the end of 1983. This corresponds to a yearly rate of increase of about 13% that is substantially higher than the rate of electric energy demand. Although in the energy balance of the world, geothermal energy still represents a minor asset, in a few countries such as Philippines, E1 Salvador, Nicaragua and Kenya this resource provides a significant contribution to the total electric generation and, at the same time, plays an important role in the global economic development of the above countries. Moreover, the intensive activities of geothermal exploration throughout the last decade have led to the discovery and assessment of many new geothermal fields, which are now ready for immediate development. The implementation of these development projects has to confront the presently difficult energy market, characterized by uncertainty or even reduction in prices and by an increase in the demand at a rate much lower than expected. These conditions, definitely less favourable than in the past, require a marked improvement in the technology of exploitation of geothermal resources: in fact, such technology, although already well advanced, must further raise its standard of efficiency and at the same time reduce costs, in order to maintain a competitive level of technical and economic validity. Multipurpose projects, such as electric energy generation associated with salt water desalination, can offer, in certain cases, very interesting opportunities for geothermal development. The results of the experiments carried out at Fenton Hill in New Mexico (U.S.A.) make it possible to foresee that, in the near future, an advanced technology for the exploitation of hot dry rock will be available and reliable, and that it will be possible to exploit large amounts of energy from these man-made geothermal reservoirs with positive economic results. These evolutions could create the conditions for the execution of more ambitious projects such as the exploitation of the earth magma. Although interesting and attractive, the prospect must be studied in detail and new technologies must be identified before taking it into consideration. The studies and research which are being carried out should be further pursued in the future, with special emphasis on the following fields: efficiency of traditional processes for energy transformation; - - introduction of new processes for energy transformation; - - special projects; corrosion, scale and waste disposal management; advanced design, system operation and costs. -
-
-
-
-
-
T R A D I T I O N A L PROCESSES FOR E N E R G Y T R A N S F O R M A T I O N Under this category, we consider power-plants equipped with back pressure or condensing 157
158
M. Guglielminetti
turbines, utilizing geothermal steam either directly produced by dry fields or obtained through flashing, single or double, from fluids in water-dominated fields. The main steps of the process consist of: - - fluids conveyance and steam separation; conversion of thermal energy in the steam into electrical energy; systems of steam condensation and gas extraction for the condensing turbines. The widespread use of two-phase conveyance and the concentration of steam separation equipment into a limited number of units, each serving as many wells as economically feasible, introduced important elements of progress into rationalizing field management. Severe problems are, on the contrary, encountered in the application of multiple flash of residual water, after the first steam separation. Such problems are mainly related to scaling effects, due to reinjection of water at a temperature of about 170°C. The efficiency of energy transformation in the turbines has achieved a high standard, since the relative technology is essentially identical to the one applied in conventional steam plants. Particular attention is paid to material selection and to the devices for mist elimination which are subject to continual improvement. Special design criteria are sometimes sought to obtain high flexibility of turbine operation. Interesting solutions have already been studied in the past, such as the combination of a back pressure turbine, to be installed at the beginning of field exploitation, especially in the case of a high content of gas in the steam, to which a condensing section can subsequently be coupled on the same axis; or the possibility of modifying the blading of the turbine in such a way as to be able to change the steam pressure in very large fields without changing the rated capacity. Minson et al. (R.74) show that the turbines installed in the Tongonan field can generate 37.5 MW both if steam from the second separation at 1.14 bar is available, and when it is fed only by steam at about 5.6 bar. Mertoglu (R.36) mentions experimental turbines, designed in Turkey, which are capable of operating under very different conditions. The modern direct contact condensers can achieve an accentuated vacuum by means of an optimal distribution of water volumes and flow, and by means of an effective fractioning of water drops. The corresponding energy can be conveniently exploited if the gas extraction system is more efficient than the traditional steam ejectors. The application of mechanic compressors, used in the past in the presence of a high content of incondensable gases (percentage of some units in weight), should be extended to plants with moderate gas content as well. Should the gas content in the steam be very high, roughly about 5% in weight, a new and efficient solution to be considered is the combination of back pressure steam turbine unit - atmospheric, surface condensers - - binary cycle unit. This latter unit will be examined further -
-
-
-
on.
The circulating water systems for the cooling of the condensers normally include a wet-type cooling tower. This part of the plant is rather costly since the cooling water is made up of geothermal steam condensated with dissolved gases, in particular CO: and H,S, and various salts, and becomes aggressive on contact with the oxygen in the air inside the cooling tower and, therefore, requires corrosion resistant materials. The improvement in the choice of the materials with the use of chemical products for pipes, tower filling, etc., may in the future reduce plant and operating costs, so avoiding also the chemical treatment of the circulating water. One particular operating problem of the cooling tower is microbiological activity, encouraged by the sulphur in the H,,S, and the corrosion w.hich it causes. This interesting matter is dealt with later. Back pressure portable units are frequently used now during the initial experimental period
G e o t h e r m a l Electric P o w e r Production
159
of exploitation of a newly discovered field. The limited efficiency of this type of turbine (about one half as compared to the condensing type) is counterbalanced by the low cost, the rapidity of installation and the amount of information on reservoir characteristics and evolution that can be drawn, before the full-scale condensing turbine is installed. Two papers show the solutions adopted for very different projects. The first, presented by Jaud and Lam6the (R.77), describes a prototype power-plant of small capacity (4200 kW), constructed by Electricit6 de France on the island of Guadeloupe; the second, presented by Minson et al. (R.74), refers to the 112 MW Tongonan power-plant on Leyte Island of the Philippine archipelago. The small condensing power-plant located in Guadeloupe is fed by two-pressure geothermal steam of 6 bar and 1 bar. Since it is located on the beach, sea water is utilized for cooling the barometric, direct contact-type condenser. The same water, discharged by the condenser, is mixed with the residual geothermal waste water coming from the second steam separation and is discharged back into the sea at a temperature lower than 45°C in order to limit the effects of thermal pollution. Obviously, this prototype solution is expensive, since it requires an investment of 3800 F / k W (470 US$/kW), which results in a total cost of the energy produced of 0.67 F / k W h (about 84 US mills/kWh), equivalent to the cost of the energy produced by diesel units. This power plant, on the other hand, has the advantage of exploiting local resources for a small, isolated electric system, such as that of an island. The paper presented by Minson et al. (R.74) contains a detailed description of the design of the 112 MW power-plant in the field of Tongonan, Philippines, and gives concrete evidence of some of the points described above. The paper is mainly descriptive, but does put forward some design criteria which deserve mention as examples of good engineering practice: - - rationalization of the fluid collection system, which uses extended two-phase transmission piping and centralizes separation facilities; - - search for a high degree of reliability, necessary to feed the copper smelter which is the main energy user. This criterion appears useful in general, considering the serious economic problems arising from planned and forced outage, which diminish the exploitation of the geothermal resources; - - flexibility of production in order to adapt generation to the fluctuating loads through regulation of the total well output and, in particular, remote control of the valves at the head of six wells, which make it possible to exclude them from the fluid collection system without loss of steam into the atmosphere. The project confirms the difficult solution of the problems relative to the reinjection of residual waters and, in particular, reinjection at relatively low temperatures. In fact, the possibility of the flashing of water with the second separation of the steam at low pressure (1.14 bar at entry into the turbine) is foreseen; however, its utilization must be confirmed by tests which show the technical feasibility of reinjection at 100°C. Furthermore, the possibility is foreseen of using two thermal ponds to regulate water disposal in the case of a short period of emergency when there are problems in the reinjection system. There is no paper dealing systematically with design methods for optimizing the geothermal power-plant, taking into consideration the entire geothermal system including the geothermal reservoir, production wells, fluid collection system, power generation and reinjection. There are numerous and important interdependences between the various components which influence both the choice of the main parameters such as steam pressure, and the optimization of the single components. The result of achieving highest efficiency with lowest costs can only be reached with an integrated design which is based on a widespread use of mathematical models for the reservoir and special calculation programmes to determine the parameters of the process in the plants.
M. Gu~lielminelti
160
NEW PROCESSES FOR ENERGY T R A N S F O R M A T I O N The main processes at present under development are based on "binary cycles" and "total flow prime movers".
Binao' cycles Binary cycles use a secondary fluid of organic nature to generate power. This fluid is evaporated by geothermal heat through a heat exchanger; it then expands in a turbine and is condensed, releasing heat to a cooling system, to be finally pumped again in the evaporator. In recent years, several experimental plants of this type have been successfully designed in Europe (particularly in Italy), U.S.A. and Japan. Similar applications have been subsequently introduced in fields not related to geothermics, e.g. the recovery of waste heat from the discharge of industrial processes. Such technology can be, therefore, considered as adequately advanced and its potential applications are to be evaluated under the following conditions: - - geotherntal fluids with salt or gas content such as to involve severe problems with traditional processes; geothermal fluids with relatively low enthalpy and high specific heat consumption, An important field of application for binary cycles has already been mentioned, that is, for a suitable exploitation of geothermal steam with high gas content. We are convinced that, in these cases, the binary cycle applied to a back pressure steam turbine exhaust represents a practical and effective solution to improve the economic feasibility of the exploication of these fields. Small-capacity plants can be easily installed in a package module, complete with accessories. This solution would allow a substantial cost reduction, coupled with high efficiency, and it might prove convenient either for electrical systems of small capacity or for geothermal resources of limited size and temperature. In spite of the great interest aroused by this technology and the quantity of experiments. unfortunately no written accounts have been presented. The results of the experimental unit operation could, in fact, supply valuable information for future full-scale industrial plants. -
-
Total Jlow prime movers Numerous research programs on the application of total flow prime movers are under' way; although these programs have brought interesting results, the economic validity of the method still remains to be proven. The concept of the total flow prime mover is based on the possibility of expanding simultaneously water and steam through a nozzle, thereby exploiting the related energy in a turbine. The theoretical advantages rest on the elimination of the energy losses, which are associated with the conventional method of flashing and steam separation. The practical adoption has been inhibited by the expected difficulties which stem basically from the widely different thermodynamic properies of steam and hot water. These difficulties appear from the total flow nozzle project, thus leading Ryley (R.65) to carry out an appraisal of some aspects of the analysis of the wet steam nozzle. The paper looks critically at the model previously identified by various experts, examines in detail some of the assumptions made and tries to identify the limitations of the model and to clarify some of the basic problems encountered. The conclusion indicates that the droplet size to which the liquid can be atomized is of paramount importance, and that future research should be towards increasing the dispersion of the entry liquid and doing so with the minimum expenditure of energy. Sebestybn (R.87) describes research carried out in Hungary, starting from 1971, on two-phase turbines and illustrates the axial flow-type turbine and accessory equipment projects. The project is based both on theoretical studies and experiments which made it possible to test and
Geothermal Electric Power Production
161
improve important equipment components. The schemes foresee two-phase turbines for exploiting expansion energy during the flash and the successive separation of steam which is conveyed to conventional steam turbines. According to the calculations carried out on the assumption that 5 bar, 152°C hot water would be available, the increase in the output of twophase turbines is more than 30%. The author states that the greatest problems are hazards caused by mineral salt deposits which are even more troublesome than those connected with erosion. Considering the large increase in thermodymanic transformation efficiency, the abovementioned research should be complemented by the implementation of experimental projects, aimed at proving the technical and economic feasibility of industrial plants.
SPECIAL PROJECTS H o t dry rocks (HDR) The basic idea in extracting energy from hot dry rock is to form a man-made geothermal reservoir by drilling into high-temperature, low-permeability rock. A circulating loop is then formed by connecting a second drillhole to the first by hydraulic fracturing and forcing water to sweep heat from the rock in the fractured region between the wellbores. The hot water produced at this surface may be used for generating electricity, space heating or other direct uses.
The characteristics of the H D R projects, the mining and plant problems, the results attained by the systematic approach to reservoir development adopted in the U.S.A., the enhancement of performance of the first 60 kW plant already installed using corrosion and monitoring instruments and, finally, the scaled-up design for new commercial size power plants - - which illustrates the technical and economic feasibility of a 75 MW electric power plant - - are well described in the papers by Shannon et al. (R.63, abstract only) and Murphy et al. (R.62). Research and experiments are performed at Fenton Hill (New Mexico, U.S.A.). The project develops by stages: stage I, which consisted of the construction of a 60 kW research size power plant and has been in operation for more than two years; stage 2, which is the first step in developing commercial-size reservoirs and includes the creation of a man-made reservoir capable of supplying a 35 MW thermal power, corresponding to an electric power of 6.5 MW. The wells of this stage have already been completed. By utilizing various reservoir patterns of stage 2 it is possible to extend the project up to 75 MW. Murphy's paper emphasizes that most investments and operating costs are intended for the implementation and maintenance of the reservoir. In order to have as economic a project as possible, it is necessary that the costs of the wells for a commercial plant should not exceed those of oil and gas wells, i.e. they must not include " l e a r n i n g " costs as the preceding experimental stages. The optimum solution for the energy conversion equipment is to adopt the binary cycle which has the advantage, a m o n g others, of not dissipating the water required in great quantities to feed the reservoir. The thermal conversion efficiency is approximately 15%. The total project costs are estimated at 2300 U S $ / k W and the generating cost at 42 US mills/kWh approximately. These values are very interesting and renew confidence in this new technology for the exploitation of earth heat. Multipurpose utilization Sometimes the exploitation of geothermal resources presents serious difficulties due to manifold constraints.
162
M. Guglielminetti
This is the case on an island with various different requirements, very changeable and rather modest; meeting these requirements separately does not repay, of course, the costs of the geothermal plants. For the island of Vulcano in Italy, coupling electric power generation with desalination of the water for domestic use makes it possible - - by suitably regulating water production - - to meet the island's requirements through the continuous and optimum exploitation of the geothermal resources. The proposed scheme provides for the desalination of the sea water, reaching a daily production of 4200 m 3 of potable water, and the installation of a 5 M W power plant (R.14, abstract only). C O R R O S I O N , SCALE A N D WASTE D I S P O S A L M A N A G E M E N T Corrosion tests carried out in the past in many geothermal fields gave consistent results, that brought to the formulation of standard criteria for the selection of materials. It can be observed that the extended operation of the plants has confirmed the overall validity of these criteria; particular problems are still encountered in specific components of the plants, such as steel subjected to elevated stress or concrete in the cooling towers affected by micro-organisms attack. In spite of the high level of knowledge already reached in the field of corrosion, some margin of improvement still exists, and at any rate it is recommended to keep as a standard practice the execution of corrosion tests in new fields, both under aerated and unaerated conditions of the geothermal fluids. Tacchino and others (R.6) illustrate an up-to-date method that uses modern instruments for corrosion tests, which has been adopted in San Martino power plant at Larderello, Italy. The fluids of this field are particularly aggressive and likely to create new problems, in spite of the long experience gained with the fluids of nearby reservoirs. On the basis of the analysis of test results, optimization criteria for turbine materials are proposed. Citernesi et al. (R.18) have made a detailed and excellent study of corrosion phenomena in concrete and iron and also of failures in the cooling towers, caused by micro-organisms that feed on the chemicals of geothermal fluids. The study of these complex phenomena requires great competence in microbiology and material corrosion. Shannon et al. (R.63, abstract only) carried out corrosion and sealing tests on the 60 kW binary cycle experimental power plant for the hot dry rock project mentioned previously. The tests for an innovatory plant are essential to control the feasibility of future full-scale projects. Their monitoring methods for detecting operational problems are extremely interesting. Serious problems and constraints, many of which are still unsolved, are related to scalin~ phenomena. Silica scaling, which may occur in residual water at temperatures below 170°C, discourages the use of double flash at a near atmospheric pressure due to the risk of silica deposition in the wells and in the reservoir during reinjection. A better knowledge of the kinematics of incrustation and of inhibitor additives might allow an adequate management of relatively cold fluids and bring about a higher efficiency of exploitation. In carbonate-rich waters, CO2 may be released at f,lashing point, followed by calcite deposition, either in the production wells or in the surface conveyance pipes. These problems are usually solved by means of traditional technologies, by shutting down the wells and plants and cleaning them mechanically. Interesting experiments are now under way on the use of inhibitor complexes or in-hole pumps that maintain a high pressure and prevent flashing. The successful results of these studies would constitute a major breakthrough in the technology of geothermal exploitation. Finally, reinjection has now become a routine solution for the disposal of waste water, mainly because of environmental reasons. The effects of reinjection on field behaviour depend on many factors and must be evaluated on a case-to-case basis. Comprehensive reservoir
Geothermal Electric Power Production
163
engineering studies are needed to compile the available data, define common criteria of interpretation, estimate the long-term effects and establish guidelines for the implementation and monitoring of reinjection under different circumstances. A D V A N C E D DESIGN, SYSTEM O P E R A T I O N A N D COST In addition to the already mentioned process and materials selection, major improvements can be achieved through a proper design of plant components. Standardizing components, keeping up to date with the evolving technology, periodically revising the design of details and selection of the corresponding material, can substantially enhance the reliability of the plants and simplify their operation and maintenance, while reducing at the same time the relative costs. A m o n g the minor components that could benefit from advanced design we might mention the equipment directly in contact with geothermal fluids, such as valves, separators, steam traps, gas extractors, primary elements of measuring systems; electrical equipment; measuring, regulation and control equipment, in contact with ambient air polluted by hydrogen sulphide. Automation of plants can be extended by making larger use of remote-control up to the point of installing complete, unmanned units, thus drastically reducing operation costs. For isolated areas served by small electrical systems, a cheap energy supply achieved through cost reductions would constitute an important incentive for industrial development and social growth. Additional opportunities for saving costs have to be evaluated wherever electrical generation can be combined with water desalination or with other direct uses of heat, for example in industries for food conservation, mining processes and space heating. LIST OF R E P O R T S In this report, prepared with M. Cozzini, the followingpapers submitted to the Secretariat up to 16 March 1984have been taken into account: EP/SEM.9/R.6
Material selection for advanced geothermal machineries. G. Tacchino and G. Cerisola (Italy) (E).
Abstract only. EP/SEM.9/R.18 Microbiological aspects of concrete and iron deterioration in geothermal power-plants. U. Citernesi, G. Benvenuti and G. C. Ferrara (italy) (E). EP/SEM.9/R.62 Economics of a conceptual 75 M W hot dry rock geothermal electric power-station. H. Murphy, R. Drake, J. Tester and G. Zyvoloski (U.S.A.) (E). EP/SEM.9/R.65 A critical appraisal o f some aspects o f the analysis o f the wet steam nozzle as used in total flow machines. D. J. Ryley(U.K.)(E). EP/SEM.9/R.72 Magma-paradox and electric power. R. J. Varney (U.S.A.) (E). EP/SEM.9/R.74 Tongonan I geothermal power development, Philippines. A. A. C. Minson, J. A. Kivell and T. J. Fry (New Zealand) (E). EP/SEM.9/R.77 The Bouillante geothermal power-plant. Guadeloupe. P. Jaud and D. Lam~the (France) (F). EP/SEM.9/R.87 Design o f two-phase turbines and geothermal electric power-plants in Hungary. I. Sebesty~n (Hungary) (E). (E) Original in English. (F) Original in French.