（節(jié)選）外文翻譯----地?zé)衢W蒸發(fā)電廠的熱經(jīng)濟(jì)優(yōu)化（英文）

1、Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010 1 Thermoeconomic Optimization of Geothermal Flash Steam Power Plants Mehdi Zeyghami End of Poonak-e-Bakhtari Blvd., Shahrke Ghods, P.O.BOX: 1

2、4665/517 Tehran\IRAN MZeyghami@nri.ac.ir Keywords: Thermoeconomic modeling, single flash, doubles flash, optimization. ABSTRACT Flash steam power plants are relatively a common method used to convert the geothermal ene

3、rgy into electricity when production wells produce a mixture of steam and liquid in a geothermal system. In comparison to the single flash system, the double flash steam power plant generates more power from geother

4、mal fluid at the same conditions. However, electricity generation costs are higher for double flash plants. A thermoeconomic optimization model is presented in this paper as the generation costs of single and double

5、flash steam power plants. By considering the pressures in the separator and flash vessel (only in a double flash plant) as the independent variables in the objective function and using numerical search methods (Golden

6、 Ratio and Nelder-Mead ), the minimum power generation cost was calculated at different geofluid conditions for either single or double flash steam power plants. The analysis was carried out for plant sizes from 5 to

7、 150 MW. The economy of scale was taken into account for Investment costs and Operation & Maintenance costs. The results for minimum generation cost and net power output using single and double flash geothermal p

8、ower plants is presented for different geofluid temperatures and flow rates. 1. INTRODUCTION Geothermal energy is the natural heat energy within the earth. This energy can be recovered in specific areas in hot water

9、 or steam. Hydrothermal systems are natural geothermal sources in which water is heated in contact with hot rocks beneath layers of the earth's surface and turns to steam (vapor dominated systems) or hot water (li

10、quid dominated systems) (El-Wakil, 1985). In hydrothermal systems, it is possible to extract the hot geothermal fluid by drilling wells to suitable depths in proper places. The flow could either be natural flow under

11、 its own pressure or pumped to the surface. In liquid dominated systems, pressure drop in the well turns some liquids into vapor, which results in a low quality, two phase mixture at the wellhead. Using flash steam po

12、wer plants is one of the common ways to convert the extracted heat to mechanical power and electricity. Single flash steam power plants are often the first power plants installed at newly developed liquid dominated g

13、eothermal fields (Dipippo 2007). The term “single flash” indicates that geothermal fluid undergoes one flashing process in the system. Flashing is a process of lowering the geofluid pressure below the saturation pres

14、sure corresponding to the fluid temperature to transform pressurized liquid to a mixture of liquid and vapor (Dipippo 2007). In order to achieve a fluid mixture with higher quality and steam flow in the turbine in a s

15、ingle flash power plant, the geothermal fluid is subjected to a constant enthalpy throttling process. After the separation of liquid and vapor phases in a separator, the vapor is used to drive a steam turbine and gene

16、rate mechanical power. The liquid portion of the flow is reinjected to the source via injection wells. Due to high temperature and relatively low quality in the separator, the exiting brine has a high flow rate and wo

17、rking potential compared to the steam used to drive the turbine (El-Wakil, 1985). In order to use this wasted energy in a double flash power plant, the liquid exiting the separator is run through a second flashing pro

18、cess (flash vessel and separator). The resulting low pressure steam is sent to a lower stage of the turbine or another steam turbine. At the same geothermal fluid conditions, a double flash steam plant can produce 15

19、-25 % more power output than a single flash steam plant. However, the plant is more complex, more costly, and requires more maintenance (Dipippo 2007). Flash steam power plants account for a significant share of all

20、geothermal power plants in the world. According to the available data presented by Dipippo (2007), approximately 32% of geothermal plants in the world have a single flash design, and approximately 14% have a double fl

21、ash design. In a single flash power plant, separator pressure has a significant effect on the amount of power generated from extracted geothermal fluid and the performance of the cycle. For specific fluid conditions a

22、t the wellhead, higher separator pressures (resulting from higher flashing pressure) result in higher pressure steam leaving the separator. Therefore, steam at the turbine inlet would have higher working potential. H

23、owever, by reducing the flashing pressure (and the dependent separator pressure) increases the mixture quality in the separator, which results in higher steam flow rates at the turbine inlet. However, the specific av

24、ailable energy (exergy) of the steam flow would decline. Thus, separator pressure is a key design parameter in single flash power plants and has a major influence on power generation system performance. In double flas

25、h power plants, the flashing pressure should also be considered as an important design parameter. For evaluation of flash steam power plants, design parameters should be selected precisely to achieve the maximum powe

26、r generation at the lowest generation cost. The aim of the present work is to introduce a thermoeconomic model for geothermal flash steam power plants. Optimization of design parameters for single and double flash po

27、wer plants based on this model makes it possible to compare utilization of these plants in similar conditions. To achieve this goal, net power output from flash steam power plants was calculated at different geofluid

28、 temperatures by using thermodynamic equilibriums and cycle specifications. After considering economic factors, the thermoeconomic objective function was defined as the generation cost of the plant. Then, the app

29、ropriate design variable values that minimized the objective function were calculated by means of a numerical Zeyghami 3 2.2 Double Flash System A schematic of a double flash power plant and its T-s diagram are given i

30、n Figures 3 and 4, respectively. In a double flash power plant, brine flows from the separator (4) to a flash vessel, which is a secondary low pressure separator. In the flash vessel (5), saturated liquid is re- flash

31、ed at lower pressure, and the steam portion of the flow is admitted to a lower stage of the turbine. Remaining liquid from the flash vessel (7) is reinjected into the ground. The rest of the cycle is the same as a sin

32、gle flash power plant. The exiting brine from the plant has lower available energy, resulting in reduced losses in the energy conversion process. Using thermodynamic equilibriums and the conservation laws of mass and

33、 energy, Dipippo (2007) has declared the calculation method for the analysis of fluid characteristics in different parts of the plant. This subject won't be discussed further in this paper. Considering fluid condi

34、tions at different points of the system as known parameters, it is possible to calculate the performance characteristics of the plant. Figure 3. Simplified schematic flow diagram for a double flash power plant Figure

35、 4. Temperature-entropy diagram for a double flash power plant Assuming 5% parasitic power requirements, the gross power from steam turbine and the net generated power from the plant by are given in Equations 5 and 6

36、(Note: the D subscript denotes a double flash system): )] 10 ( ) 9 ( [ ) 9 ()] 8 ( ) 3 ( [ ) 3 ( .D D DD D D D T h h mh h m W? ×+ ? × =(5) ] [ 95 . 0 . . D T D net W W × =(6) The utilization factor for a

37、double flash power plant is expressed in Equation 7: D inD net D EW UF.. =(7) )] ) 1 ( ( ) ) 1 ( [() 1 (0 0 0. s s T h hm ED DD D in ? ? ?× =(8) Similar to the single flash system, it is assumed that the fluid con

38、ditions at the wellhead are those of a saturated liquid, and the condensing pressure is considered to be constant at 12.3 kPa. Therefore, the only remaining design variables are the separator pressure and the flash ve

39、ssel pressure. 2. THERMOECONOMIC OBJECTIVE FUNCTION In order to compare the utilization of single and double flash power plants to generate power from a geothermal field, the thermodynamic objective function has been

40、 established to be the generation cost of the plant. The generation cost function is the total annual cost of the plant divided by the mean annual energy output of the power plant. The aim of the optimization procedur

41、e is to minimize the objective function for each system. The generation cost is given in Equation 9: H WC GFnetTot × =(9) where net W and H are the mean annual output of the plant and the plant working hours each

42、 year. Tot C is the total annualized cost of the plant, which includes the amortized investment cost and annual Operation & Maintenance costs, as shown in Equation 10: M O A Tot C C C & + =(10) For calculating

43、 power generation cost, the economy of scale was considered (i.e. larger production capacities are less expensive per kW than smaller ones). Based on the work of Sanyal (2005), the specific O&M costs for geotherma

44、l power plants approximately range from 2.0 US¢/kWh for a 5 MW plant to 1.4 US¢/kWh for a 150 MW plant. Assuming an exponential decline in specific O&M cost ( m o c & ) in US¢/kWh with plant cap

45、acity (P) in MW, we have: ) 5 ( 0025 . 0 & 0 . 2 ? ? × = P m o e c(11) And the annual O&M cost is: H W c C net m o M O × × = & &(12) Based on information presented by Enthingh and McVeigh