Double-flash Plant Research Articles

Improved Layouts and Performance of Single- and Double-Flash Steam Geothermal Plants Generated by the Heatsep Method

Abstract Single- and double-flash steam power plants are commonly used in the utilization of high enthalpy liquid-dominated geothermal resources. In these plants, the expansion line in the wet steam region results in significant penalties of turbine isentropic efficiency and power output. Accordingly, the “self-superheating” and “interstage heating” plant modifications have been recently proposed in the literature, where the saturated steam at turbine inlet is superheated by using the heat of the geothermal liquid, which is cooled before the flashing process. In this study, the aforementioned and additional new flash steam plant layouts are generated by using a systematic method, called Heatsep, for the optimum design of energy systems. All the thermal connections between consecutive basic plant components are “cut” to let these temperatures vary and in turn generate additional hot and cold streams, which are combined to enhance the overall performance of the system. It is demonstrated that the single-flash plant with self-superheating is simply obtained by cutting two out of five thermal links. In the double-flash plant, the higher number of components allows for a higher number of thermal cuts and heat integration options. Unlike the existing literature, the maximum power output is not constrained by a predefined heat transfer network. The optimization results show that the maximum power output of the novel single- and double-flash steam plants exceeds by 5.5–9.2% and 3.9–7.7% the maximum attainable by the corresponding traditional plants without internal heat integration.

Modern geothermal power: Binary cycle geothermal power plants

In the second part of the review of modern geothermal power plant technologies and equipment, a role, a usage scale, and features of application of binary cycle plants in the geothermal economy are considered. Data on the use of low-boiling fluids, their impact on thermal parameters and performance of geothermal binary power units are presented. A retrospective of the use of various low-boiling fluids in industrial binary power units in the world since 1965 is shown. It is noted that the current generating capacity of binary power units running on hydrocarbons is equal to approximately 82.7% of the total installed capacity of all the binary power units in the world. At the same time over the past 5 years, the total installed capacity of geothermal binary power units in 25 countries increased by more than 50%, reaching nearly 1800 MW (hereinafter electric power is indicated), by 2015. A vast majority of the existing binary power plants recovers heat of geothermal fluid in the range of 100–200°C. Binary cycle power plants have an average unit capacity of 6.3 MW, 30.4 MW at single-flash power plants, 37.4 MW at double-flash plants, and 45.4 MW at power plants working on superheated steam. The largest binary cycle geothermal power plants (GeoPP) with an installed capacity of over 60 MW are in operation in the United States and the Philippines. In most cases, binary plants are involved in the production process together with a steam cycle. Requirements to the fluid ensuring safety, reliability, and efficiency of binary power plants using heat of geothermal fluid are determined, and differences and features of their technological processes are shown. Application of binary cycle plants in the technological process of combined GeoPPs makes it possible to recover geothermal fluid more efficiently. Features and advantages of binary cycle plants using multiple fluids, including a Kalina Cycle, are analyzed. Technical characteristics of binary cycle plants produced by various manufacturers are considered, and data on the Russian pilot binary geothermal power unit in the Pauzhetskaya GeoPP is provided. Expediency of the use of binary cycle plants for autonomous power supply and energy extension of existing GeoPPs without drilling extra wells and in flowsheets of newly designed combined GeoPPs are noted.

Life cycle inventory of a flash geothermal combined heat and power plant located in Iceland

This paper presents a life cycle inventory (LCI) describing the material and energy demands for constructing and operating a geothermal combined heat and power (GCHP) plant as well as direct emissions of gases, waste water, and waste heat. The data are based on a newly constructed GCHP plant in Iceland, representing the design of both single flash (SF) and double flash (DF) power plants that currently produce the majority of electricity from geothermal plants worldwide. Primary data were collected for the construction, operation, and maintenance of a GCHP plant. As the design and operation of geothermal flash power plants is site-specific due to the different nature of geothermal resources, a method of scaling data to a site specific parameter is proposed to make the LCI available as representative secondary data for such plants. These parameters along with other data identified as site-specific serve as the minimum data to be collected for adjusting the presented data to represent other flash power plants with or without combined heat production. The construction stage dominates the material burdens for the electricity and heat production. For the life cycle of electricity, it includes 80 % of diesel fuel use (whereof 96 % originates from well drilling), while 99 % of groundwater is used during the operational stage. The use and composition of geothermal fluid is site-specific but accounts for all direct emissions from the electricity production. The main materials in terms of mass used for the construction of the GCHP plant are water, diesel, steel, cement, asphalt, bentonite, and silica flour. Mineral wool and aluminum were also among the main material contributors. Material and energy burdens per functional unit are generally higher for a SF plant compared with DF plants. For heat production, 1.7 MJ of waste heat from power generation is used to produce 1 MJ of usable heat. By presenting LCI data scaled with site-specific parameters, the flexibility of its use is increased as secondary data. However, the collection of primary data for the composition of geothermal fluid and values for site specific parameters is always required to represent local conditions. Thus, the LCI for Hellisheiði GCHP can be regarded as representative data for electricity and heat from geothermal flash power plants.

Geothermal double-flash plant with interstage reheating: An updated and expanded thermal and exergetic analysis and optimization

The concept of a double-flash geothermal power plant with an interstage reheater is updated and extended. The conceptual plant holds a thermodynamic performance advantage over a conventional double-flash plant over the range of reservoir temperatures studied from 220 to 320°C, with the advantage increasing as the temperature increases. An exergy assessment is performed on the whole plant with a detailed examination of the reheater. The effect on performance of the terminal temperature difference in the reheater is found to be small. The trend in performance of the new plant is determined as a function of rising wellhead quality in the event that the reservoir begins to “dry out” yielding higher steam fractions at the wellhead. The potential for silica scaling in the lower-temperature components is examined and found to be of concern. The exact optimized separator and flash temperatures are compared to the simple rule-of-thumb, the “equal-temperature-split” rule, which turns out to be quite accurate. A conventional double-flash plant using a turbine designed to remove moisture after each stage is also examined and found to be theoretically roughly equal to the conceptual plant in terms of power performance. The extra power that can be generated by a reheat plant is monetized to compare with the extra cost involved in the more complex plant relative to a conventional double-flash plant. The projected payback is relatively short indicating that the proposed plant should be both thermodynamically and economically worthwhile on condition that some means of controlling silica precipitation is adopted.

Modeling, Simulation, Control, and Optimization of a Geothermal Power Plant

The main results of the simulation study of a double-flash geothermal plant, including a steam-CO/sub 2/ reboiler-based separation process, are presented. Dynamic models of two-phase, H/sub 2/O-CO/sub 2/ components are derived, and their use for dynamic simulation and operating point optimization is discussed. Control system issues are then considered. On one hand, the simulator is used as an aid to design and test standard level and pressure control loops, whose actual performance would otherwise be difficult to predict due to the high degree of interconnection of the plant components. On the other hand, the control concept for the innovative part of the plant is discussed, recognizing that the control problem and the operating point optimization problems have to be tackled simultaneously. A simple, yet effective, solution for this problem is proposed.