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Efficiency evaluation of hydropower station operation: A case study of Longyangxia station in the Yellow River, China
Accepted Manuscript Efficiency Evaluation of Hydropower Station Operation: A Case Study of Longyangxia Station in the Yellow River, China
Jianxia Chang, Yunyun Li, Meng Yuan, Yimin Wang PII:
S0360-5442(17)31043-5
DOI:
10.1016/j.energy.2017.06.049
Reference:
EGY 11057
To appear in:
Energy
Received Date:
16 January 2016
Revised Date:
11 May 2017
Accepted Date:
09 June 2017
Please cite this article as: Jianxia Chang, Yunyun Li, Meng Yuan, Yimin Wang, Efficiency Evaluation of Hydropower Station Operation: A Case Study of Longyangxia Station in the Yellow River, China, Energy (2017), doi: 10.1016/j.energy.2017.06.049
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ACCEPTED MANUSCRIPT Efficiency Evaluation of Hydropower Station Operation: A Case Study of Longyangxia Station in the Yellow River, China Jianxia Chang *, Yunyun Li, Meng Yuan, Yimin Wang State Key Laboratory Base of Eco-hydraulic Engineering in Arid Area, Xi’an University of Technology, Xi’an 710048, China *Corresponding author. [email protected]
Abstract: Hydropower plays a major role in the Chinese electricity generation industry. It is of significant importance to perform efficiency evaluation of the economic operation in a power station, which can help promote better water resources management and create more benefits for power network. Therefore, the main goal of this study is to establish a set of evaluation indexes and investigate the evaluation method to assess the economic operation in a power station. Based on the operation chart, three evaluation indexes including the Relative Water Consumption Rate (RC), the Relative Hydropower Utilization Rate (RU), and the Relative Hydropower Utilization Increasing Rate (RI) were proposed. The Longyangxia power station, as the biggest peak regulation source in the Northwest Power Network in China and the largest reservoir in the Yellow River Basin, was selected to perform an efficiency evaluation of economic operation. The result showed that the economic operation of this power station was not reasonable with RC>1, RU<1 and RI<0. And the rationality of the evaluation result was illustrated through analyzing the actual operation situation of the Longyangxia reservoir. Keywords: Efficiency evaluation; Economic operation; Evaluation indexes; 1
ACCEPTED MANUSCRIPT Evaluation method
1. Introduction Over the past several decades, a huge electric energy demand has accompanied with the process of rapid industrialization, modernization, and urbanization in China (Lin et al., 2012). Especially in the past three decades, the yearly electricity generation has increased 15-fold from 351.4TWh in 1983 to 5347.4TWh in 2013(Chen et al., 2015). According to the International Energy Agency (IEA), the electricity demand will continue growing at an annual rate of 2.5% by 2030, and approximately 16% of electricity is produced by hydropower (IEA, 2012). Typically, compared with traditional fossil fuel (thermal power) and other green energy resources (wind power), hydropower is a sustainable source of green energy with relative less air pollutants and greenhouse emissions (Zheng et al., 2015). It is mature in development, reliable and flexible in operation, easy to maintain and financially competitive (Ma H et al., 2009; Liang et al., 2013; Liu et al., 2014; Nikoofard et al., 2015; EI-Agouz SA and Kabeel AE., 2015). With these advantages, hydropower in China, a country that has the most severe environmental pollution, has been actively pursued with significant achievement in recent years (Huang et al., 2009; Hong et al., 2013; Zeng et al., 2013; Hennig et al., 2013). Its gross installed hydropower capacity has reached approximately 280 GW in 2013, exceeding the cumulative values of the Canada, Brazil and USA (IHA, 2013). Thus, hydroelectric power is an inevitable source in meeting China's growing electricity needs. The significance of hydroelectric power indicates that the efficiency evaluation of economic operation in a hydropower 2
ACCEPTED MANUSCRIPT plant is essential to the management of hydropower generation, for which directly influences the total economic benefits in the power network. However, due to the lack of systematic methods to evaluate operation efficiency, a lot of electric energy is being wasted in actual production. According to statistics, owing to unreasonable regulation modes such as low head operation, the annual average loss of electricity was up to 1.5 billion kWh (Chang et at., 2006).Therefore, it is an urgent need to perform efficiency evaluation of hydropower plant operation to enhance the overall benefits to the power network. Few attempts have been made in relation to this topic in China. International researches regarding hydropower efficiency have been carried out in succession in recent years (Jha and Shrestha, 2006; Barros and Peypoch, 2007; Barros, 2008; Minville et al., 2009; Moeini and Afshar., 2011; Inglesi-Lotz and Blignaut.,2014). Kahraman et al. (2009) performed efficiency evaluation of Keban hydropower plant experimentally and numerically in Turkey by using thermodynamic analysis. The results show that thermodynamic method was an effective and easy method for the determination of turbine efficiency in a hydraulic power plant. Sözen et al. (2012) conducted electricity generation of ten hydropower plants through Data Envelopment Analysis (DEA) and window analysis (WA) in Turkey, and defined and pursued two efficiency indexes based on production and energy unit cost performance. The results revealed the operating conditions of Turkey’s forthcoming hydropower plants. Wang et al. (2014) assessed the efficiency of hydropower generation in Canada from comprehensive viewpoints of electricity generating capability using the TOPSIS (the Technique for 3
ACCEPTED MANUSCRIPT Order Preference by Similarity to Ideal Solution) method. The results mainly revealed that the overall efficiency of hydropower generation in Canada experienced a downward trend from 2005 to 2011, following an improvement in 2012. Past related studies mainly made critical achievements in taking the value of electricity generation as a decisive index to assess hydropower efficiency. However, it may not be reasonable to only use the electricity generation as a decisive index to evaluate the efficiency of hydropower operation, since electricity generation is not only related to the reservoir regulation mode, but also largely depends on natural inflow. For example, electricity generation is sure to be different in the wet year and dry years. This will lead to a change of the power plant output. Therefore, the main difference of current study from previous studies is that we assess the hydropower plant operation efficiency not only based on the amount of electricity generation, but also based on the operation mode with the operation chart, which has removed the impact of natural inflow variation on the efficiency evaluation because it has the same constraints and running conditions as the evaluation instance. Furthermore, water utilization rate is another important factor to assess hydropower plant operation efficiency, which should be highly considered when performing efficiency evaluation of hydropower station under different operation modes in a certain inflow condition. Hence, the main scientific motivation behind the work is to propose a set of evaluation indexes (not only including the electricity generation, but also considering the variation of water utilization rate) based on the operation chart to assess the hydropower plant operation efficiency, which
4
ACCEPTED MANUSCRIPT can be more objective and effective to reflect the complexity operation characteristics of hydropower plant. The objectives of this paper are as follows: 1) to propose a set of evaluation indexes of hydropower station operation; 2) to investigate the evaluation method for a multiyear regulation reservoir; and 3) to illustrate the rationality of the evaluation results and give some suggestions for improving efficiency economic operation. The present study would play a timely role and expand the research breadth in this field from a sustainable perspective.
2. Methods 2.1. Evaluation indexes Economic operation of a hydropower station mainly contains two aspects. One is to decrease water consumption by raising or maintaining the water head as high as possible. The other is to make full use of water resources by avoiding surplus water as much as possible. Thus, Water Consumption Rate and Hydropower Utilization Rate can be selected as indexes to evaluate the reservoir operation from the perspective of energy-saving and optimal conditions, respectively. There would be no comparability to use absolute value, since regulation performance, water head, and type of units vary from power station to power station. Therefore, a relative value is used for evaluation, and three evaluation indexes, including Relative Water Consumption Rate (RC), Relative Hydropower Utilization Rate (RU), and Relative Hydropower Utilization Increasing Rate (RI), are finally selected. Their calculation formulas are shown below. 5
ACCEPTED MANUSCRIPT (1) RC index RC (Relative Water Consumption Rate) is applied to access the rationality of the reservoir operation from the perspective of energy saving. Before calculating RC, a variable of Water Consumption Rate (A) is defined as A
W 100% E
(1)
Then
RC
A0 AK
(2)
Where W is the water used for power generation (m3). E is the hydropower electricity generation (kWh). A0 and AK represent the actual and checking hydropower station water consumption rates, respectively. If RC<1, the energy-saving operation in a hydropower plant is reasonable. (2) RU and RI indexes RU (Relative Hydropower Utilization Rate) and RI (Relative Hydropower Utilization Increasing Rate) indexes are also employed to evaluate the rationality of optimal operation and they are defined as follows:
RU RI
E0 100% Ek
(3)
E0 Ek 100% Ek
(4)
Where E0 represents actual power generation (kWh). Ek represents checking power generation. If RU>1 and RI>0, the optimal operation in a hydropower plant is reasonable. Moreover, the larger the index values are, the better the effect of an optimal operation 6
ACCEPTED MANUSCRIPT will be. 2.2. Determination of checking power generation We need to determine the checking power generation for the computation of the evaluation indexes, and the value of which should be only related to the hydropower station operation mode. The objective of the hydropower station operation is to realize optimal operation, which has the characteristics of high dimensionality, strong coupling, uncertainty, etc. (Mandal and Chakraborty, 2009; Jiang et al., 2014; Chang et al., 2010). The operator needs to consider not only the hydraulic and electrical connection between the upstream and downstream reservoirs, but also a lot of other constraints. Although many optimal methods are applied in the reservoir operation (Huang et al., 2002; Ma et al., 2013; Mukand et al., 2012; Chang et al., 2013; Zhu et al., 2014), it is very difficult to adopt optimal decision making for a real-time reservoir operation due to the uncertainties of the predicted inflow. To pursue better optimization or a satisfactory solution, it is very important to make reasonable operation rules (Bolouri-Yazdeli et al., 2014; Chou and Wu, 2015). Reservoir operation rules are logical or mathematical equations that take system variables into account to calculate water release based on the inflow and water capacity values, and these rules could guide operation management. The operation rules are usually predefined and presented in the form of charts, tables or functions. In this paper, checking power generation was obtained by using the reservoir operation chart, which mainly consists of different guide curves and corresponding operation zones, and is usually obtained through a reverse calculation using the hydrological runoff data of typical years. 7
ACCEPTED MANUSCRIPT 3. Case study for multi-year regulation reservoir 3.1. Description of the Longyangxia hydropower station The Yellow River, draining an area of 7.95 × 105 km2, crosses “the three major stairs” of the macroscopic landform structure of China, i.e., the Qinghai-Tibet Plateau, the Loess Plateau and the North China Plain, and empties into the Pacific Ocean. The Yellow River flows north, turns south and then bends east for a total of 5464 km before debouching into the ocean. The Yellow River region is one of the most important basins in China, which is the largest water supply source in northwest and north China. It undertakes the water supply task of a population of about 140 million (more than 50 large and medium-sized urban cities), and has heavy agriculture irrigation task of an area of 1.6 × 105 km2 (accounting for about 15% of the whole country). Hence, the water resources development and utilization of the Yellow River Basin plays a vital role in the sustainable economic and social development of china. There are 24 reservoirs scattered widely in the Yellow River basin with storage capacities exceeding 1.0×108 m3, in which four major reservoirs along the mainstream are the most influential: the Longyangxia, Liujiaxia, Sanmenxia, and Xiaolangdi reservoirs. The Longyangxia reservoir, the biggest peak regulation source of the Northwest Power Network in China, is the largest reservoir in the Yellow River Basin. It was constructed in 1985, and the storage capacity and dam height are 247×108 m3 and 178 m, respectively. In addition, there have another six cascade reservoirs in the upper Yellow River: Lijiaxia, Liujiaxia, Yanguoxia, Bapanxia, Daxia, and Qingtongxia (Fig.1). Their main operation characteristics are listed in Table 1. Since the completion of 8
ACCEPTED MANUSCRIPT Longyangxia reservoir, it has operated jointly with Liujiaxia reservoir and other five runoff reservoirs to supply water to meet the demands of municipal, industrial, irrigation, and power generation in the north-west electricity network. Therefore, the operation efficiency of the Longyangxia reservoir directly influences the total power generation of cascade reservoirs in the upstream of the Yellow River. However, due to the lack of systematic methods or index systems to quantify the efficiency of the economic operation in the Longyangxia Power Station, a large amount of waterpower has been wasted, resulting in enormous economic loss to the cascade reservoirs and even to the Northwest Power Network. Therefore, research on efficiency evaluation is an urgent need for the Longyangxia power station and it has practical significance and application value to improve the entire power generation of the Yellow River cascade hydropower stations. In this paper, the operation process of Longyangxia power station in the year of 2001 is taken as an example to illustrate whether its economic operation is reasonable. Fig.1 Locations of seven cascade reservoirs in the Yellow River Table 1 Operation characteristics of seven cascade reservoirs 3.2. Data sources The datasets used for the efficiency evaluation of hydropower stations operation mainly include two categories: basic characteristics and actual operation conditions of the power plant. Basic characteristics data include reservoir operation chart, the curves of water-level as a function of storage-capacity and downstream water level as a function of releasing discharge. The operation chart used in this study is the cascade 9
ACCEPTED MANUSCRIPT operation chart of the upper Yellow River (Fig.2). From Fig.2, it can be easily observed that if the water level in the reservoir is below line A, neither the water supply nor power generation can meet the basic needs. If the water level in the reservoir is between line A and line B, the power generation can be increased by increasing output, while the water supply can’t be satisfied. If the water level in the reservoir is between line B and line E, the water supply can’t be satisfied, but the power generation can meet with the normal standard with guaranteed output. If the water level in the reservoir is between line B/E and line C, both the water supply and power generation can meet with the normal needs. If the water level in the reservoir is between line C and the normal water level, not only the water supply can be satisfied, but also the power plant can obtain more electricity by increasing output. The curves of water level versus storage-capacity (Fig.3) and downstream water level versus releasing discharge (Fig.4) of the Longyangxia reservoir are provided by the Yellow River Conservancy Commission (YRCC). The practical operation conditions include monthly inflow, releasing discharge, surplus water, initial water level, and hydropower generation of the Longyangxia reservoir, which is also provided by the YRCC. Fig.2 The cascade operation chart of the upper Yellow River Fig.3 The curve of water level versus storage capacity Fig.4 The curve of downstream water level versus releasing discharge 3.3. Evaluation steps As for the Longyangxia reservoir, there are two aspects that should be taken into 10
ACCEPTED MANUSCRIPT account specially when carrying out an efficiency evaluation. One is that the seven cascade reservoirs should jointly operate to meet the demand of municipal, industrial, irrigation water supply, power generation, and ecological purposes. When calculating the checking generation, the cascade operation chart of seven reservoirs instead of the single Longyangxia reservoir is used. Hence, the output of Longyangxia reservoir in each month ( N L,i ) will have to be further calculated (step1).The other is that the Longyangxia reservoir is a multi-year regulating reservoir, whose year-end level is not a dead level and it is also different with different reservoir operation modes, leading to different values of water storage at the end of year. During evaluation, the actual reservoir year-end water level may be different from that of the calculated values through the operation chart. Hence, it is not reasonable to take the calculated power generation as the checking one. To overcome this limitation, the difference between the calculated and the actual water storage at the end of year should be converted into the corresponding hydropower generation ( ELk ). Then the checking value ( ELk ) can be determined (step 2). Based on step1 and step 2, the values of the three evaluation indexes can be finally obtained (step 3). Step 1: Calculating the output of Longyangxia reservoir in each month ( N L,i )
N L,i is obtained by a trial and error method through MATLAB programming and the calculation flow chart is shown in Fig. 5.The detailed procedures are described as follows: ① Cascade output ( NTotal,i )
NTotal,i can be directly obtained from the operation chart of cascade reservoirs and 11
ACCEPTED MANUSCRIPT the function is presented as
NTotal,i =f ( Z1,i )
(5)
Where Z1,i represents the water level of the Longyangxia reservoir at the beginning of month i ( i = 1, 2,3,...,12 ). ② Output of the Liujiaxia( N 3,i ), Yanguoxia( N 4,i ), Bapanxia( N 5,i ), Daxia( N6,i ), and Qingtongxia( N7,i ) reservoirs The releasing discharge of the Liujiaxia reservoir ( Q3,outi ) is determined by the monthly water demand at Lanzhou section ( QLZ,i ), which is shown in Fig.6. As the Yanguoxia, Bapanxia, Daxia, and Qingtongxia are daily regulation power stations, their releasing discharge are the same as that of the Liujiaxia reservoir. The output formulas are expressed as
N 3,i K 3Q3,iout H 3,i N 4,i K 4 Q4,iout H 4,i N 5,i K 5Q5,iout H 5,i
（6）
N6,i K6 Q6,iout H 6,i N7,i K7 Q7,iout H7,i
N 3-7,i = N 3,i + N 4,i + N 5,i + N6,i + N7,i
（7）
Where Q4,outi , Q5,outi , Q6,outi , Q7,outi (m³/s)represent the releasing discharge of the Yanguoxia, Bapanxia, Daxia, and Qingtongxi in month i , respectively. K 3 , K 4 , K 5 ,
K 6 , K 7 represent the output coefficients of the Yanguoxia, Bapanxia, Daxia, and Qingtongxi, respectively. H 3,i , H 4,i , H 5,i , H 6,i , H 7,i (m) represent the water heads of the Yanguoxia, Bapanxia, Daxia, and Qingtongxi in month i , respectively. N 3-7,i (kW) represents the total output of five reservoirs in the month i (i.e., Liujiaxia,Yanguoxia, Bapanxia, Daxia, and Qingtongxia) 12
ACCEPTED MANUSCRIPT ③ Output of the Longyangxia( N1,i )and Lijiaxia ( N 2,i ) reservoirs Likewise, being daily regulation power station, the releasing discharge of the Lijiaxia reservoir ( Q2,outi ) is equal to that of the Longyangxia reservoir ( Q1,outi ).Here, is obtained based on the inflow of the Liujiaxia reservoir ( Q3,ini ) which is an Q1,out i arbitrary hypothetical value. N1,i and N 2,i are calculated by Eqs. (8). Q1,iout Q2,iout N1,i K1Q1,iout H 1,i
（8）
N 2,i K 2Q2,iout H 2,i
Where K1 and K 2 represent the output coefficients of the Longyangxia and Lijiaxia reservoir, respectively. H1,i (m) and H 2,i (m) represent the water head of the Longyangxia and Lijiaxia reservoir in month i , respectively. ④ Determining the output of Longyangxia reservoir in each month( N L,i ) Before determining the value of N L,i , an intermediate variable ( N1,i' ) is introduced to verify the accuracy of Q3,ini ' N1,i NTotal ,i N 2,i N 3-7,i
(9)
If N1,i' = N1,i , the hypothesis of Q3,ini is reasonable and N L,i = N1,i . Fig. 5 Output calculating flow chart of the Longyangxia reservoir ( N L,i ) Fig.6 Monthly water demand at Lanzhou section Step 2: Determining the checking hydropower generation of Longyangxia reservoir (
ELk ) ① Conversion hydropower generation ( ELK ) The value of ELK , which is directly related to the accuracy of ELk , must be calculated by using a reasonable approach. Four approaches, therefore, are put forward 13
ACCEPTED MANUSCRIPT to get the value of ELK , and based on the results of these values, the final value of
ELK is then determined. The detailed methods are showed below: 1 Method 1: Based on the water storage change in each month, ELK is defined
as: 12
1 ELK K1 (WLo,i -WLc,i ) H 1.i
(10)
i 1
Method 2: Based on the actual water consumption rate in month i ( qLo,i , m³ / kW), 2 is defined as: ELK
WLo,i -WLc,i qLo,i i 1 12
2 ELK
(11)
Method 3: Based on the calculation water consumption rate in month i ( qLc,i , m³ / kW), ELk3 is defined as:
WLo,i -WLc,i qLc,i i 1 12
3 ELk
(12)
Method 4: Based on the average water head ( H L ), ELk4 is defined as: 12
ELk4 K1 (WLo,i -WLc,i ) H L
(13)
i 1
Then,
ELk =
1 ELk +ELk2 +ELk3 +ELk4 4
(14)
Where WLo,i (m³) and WLc,i (m³) represent the actual and calculated releasing water storage values of the Longyangxia reservoir in the month i , respectively. ② Checking hydropower generation ( ELk )
The checking electric energy can be calculated by Eqs. (15).
14
ACCEPTED MANUSCRIPT 12
ELC K1 N L,i H 1,i i 1
(15)
ELK ELC ELk Step 3: Calculating the evaluation indexes On the basis of step 1 and step 2, the three evaluation indexes can be finally calculated by Eq. (16):
RC
A0 WLo / ELo AK WLc / ELc
RU
E0 E 100% Lo 100% Ek ELK
RI
(16)
E0 Ek E ELK 100% Lo 100% Ek ELK
Where WLo and WLc represent the actual and calculated annual water consumption values of the Longyangxia reservoir in 2001, respectively. EL0 , ELc , ELK represent the actual, checking and calculated annual hydropower generation of the Longyangxia reservoir in 2001, respectively.
4. Results and discussion In this paper, the operation process of Longyangxia hydropower station in 2001 is evaluated. The reservoir water level at the beginning of the operation is 2585m, and Table 2 shows actual water level and electric energy after each period. According to the valuation step 1, the cascade output for each month ( NTotal,i ) is firstly obtained, and the output of the Longyangxia reservoir for each month ( N L,i ) in 2001 is then calculated (Fig.7). Also, the results of calculation running condition are shown in Table 3. From Table 3, big differences can be seen between the actual and calculated results, which indicates that the calculated results can’t be directly used to evaluate the 15
ACCEPTED MANUSCRIPT operation efficiency of the Longyangxia reservoir. Taking the yearly begin and end water level as an example, which is shown in Fig.8, the actual and calculated values have same begin water level of 2565.91m. While the calculated year-end water level is 2566.83m, which is 9.44m higher than the actual value (2557.39 m). In an evaluation system, the basic condition is that the evaluated objectives should have same situations. Hence, the difference between the calculated and actual yearly end water levels should be taken into account when evaluating whether the operation efficiency of the Longyangxia power plant is reasonable. Therefore, to evaluate the actual running condition more fairly, 25.01 108 m3 water (the difference between the actual and calculated water storage of the Longyangxia reservoir) should be converted to the corresponding hydropower, which is then used as an intermediate variable to obtain the checking electric energy when evaluating the operation efficiency of the Longyangxia power plant. Hence, through the evaluation step 2, the values of conversion electric energy and the checking electric energy are calculated and the results are shown in Fig.9. Table 2 Actual running condition of the Longyangxia reservoir in 2001 Fig.7 Monthly output of the Longyangxia reservoir and the cascade output of seven reservoirs in 2001 Fig. 8 Actual and calculation water level in the Longyangxia reservoir of 2001 Fig. 9 Results of the conversion electric energy and the checking electric energy Table 3 The comparison results between actual and calculated running conditions of the Longyangxia reservoir in 2001 16
ACCEPTED MANUSCRIPT Based on the results of step1 and step 2, the evaluation indexes are finally obtained by step 3. The results of the main evaluation values and the efficiency evaluation of economic operation in the Longyangxia power plant are shown in Table 4. From it, we can see that the value of RC is 1.2, revealing that the energy-saving operation in the Longyangxia hydropower plant is not reasonable. Furthermore, the values of RC and RI are 98.72% and -1.26%, respectively, and both of them do not meet the assessment standard. The optimal operation in the Longyangxia hydropower plant, therefore, is also not reasonable or economic. Table 4 Efficiency evaluation results of economic operation in the Longyangxia power station The results of the three evaluation indexes indicate that the economic operation in the Longyangxia hydropower plant is not encouraging, which can be verified by a mixture of reasons. Among them, the following three aspects are the main driving factors that lead to the low efficiency of the economic operation in the Longyangxia hydropower plant. (1)The actual operation process of the Longyangxia reservoir was not consistent with the operation chart (Fig.10).As the Longyangxia reservoir plays an important role on peak regulation and frequency modulation of Northwest Power Network, a certain amount of electric energy has to be lost to ensure the stable and safe operation of the power network. Hence, the benefited cascade reservoirs in the downstream should give some economical compensation to the Longyangxia hydropower plant for its sacrifice for the overall interests of the whole power network. 17
ACCEPTED MANUSCRIPT (2) To pursue immediate interests in the Longyangxia hydropower plant, the electric energy in the current year was forced to be generated as much as possible. However, large amount of power generation in the hydropower plant leads to low yearend level, which not only decreases the hydropower utilization increasing rate, but also has negative impacts on reservoir operation in subsequent years, such as high water consumption rates and low generation benefits. (3) Acting as an excellent multi-year regulating reservoir, the Longyangxia reservoir undertakes a heavy compensation task, which not only includes the electric power compensation for the downstream cascade hydropower stations, but also contains the water compensation in the lower Yellow River. Although reasonable compensation has been taken into consideration in the cascade operation chart, the leaders sometimes have to add additional compensation tasks to cope with unexpected practical problems. For example, long-distance water divisions of the Longyangxia reservoir has been made for several times to compensate the lower Yellow River for urgent water shortage, which in turn caused loss of electric energy in the Longyangxia power plant. Fig.10 Monthly process of calculation and actual releasing discharge
5. Suggestions for increasing the efficiency economic operation of the Longyangxia power plant The efficiency of station for economic operation can be improved through operational changes to the Longyangxia reservoir. Improvements of runoff forecast 18
ACCEPTED MANUSCRIPT accuracy, water use and supply benefits through optimal water resources allocation are considered as the most fundamental solutions to solve the problem, namely, exploring reasonable operation rules for the Longyangxia reservoir. Here are four important suggestions: 1. During flood season period, the idle units should be operated coordinately, and the power generation and flood control should be taken into prior consideration. 2. During dry season period, moderate increase of the output can improve the power generation. And the reservoir profit, such as power generation and water supply, should be also harmonized with ecological target regulation. 3. The decreasing mode of reservoir water level is closely associated with water resource utilization, and should be therefore given special attentions (e.g., setting flexible sluice gates at the dam crest) to mitigate the negative impact on reservoir operation. 4. The high tailrace level, which is easy to lead to backwater problems and has large influence on power generation, should be solved by taking some measures such as expanding discharge channel.
6. Conclusions It is an urgent need to establish a set of evaluation index and investigate the evaluation method to assess the efficiency of the economic operation in a power plant, for which is the key link in the electric system. In this study, efficiency evaluation method with three evaluation indexes including Relative Water Consumption Rate 19
ACCEPTED MANUSCRIPT (RC), Relative Hydropower Utilization Rate (RU), and Relative Hydropower Utilization Increasing Rate (RI) were proposed based on operation chart. A muti-year regulation reservoir (Longyangxia power plant) was taken as an example to apply the proposed evaluation indexes, and the rationality of evaluation results was verified by analyzing the actual operation process of the Longyangxia reservoir. And four constructive suggestions were given to increase the hydropower plant operation efficiency. The main conclusions are as follows: (1) Based on the operation data of 2001, three evaluation values of RC, RU and RI were 1.20, 98.72% and -1.26%, respectively, which indicated that the economic operation of the Longyangxia power plant for this year was neither reasonable nor encouraging. (2) Three actual operation situations of the Longyangixia power station, mainly including the inconsistent of the actual operation process with that in the operation chart, being forced to be generated as much as possible in pursuit of immediate interests; and undertaking heavy power compensation and water compensation tasks for downstream cascade hydropower stations, verified the rationality of evaluation results. Hence the proposed indexes in this study are reasonable, and can be used to evaluate other hydropower plant operation efficiency objectively and accurately. (3) Aimed at the low operation efficiency of the Longyangxia power plant, four constructive countermeasures were proposed to increase the efficiency of power station for economical operation through operational changes to the Longyangxia reservoir. Given the important role of hydropower stations in China but limited attention 20
ACCEPTED MANUSCRIPT being paid attention to their efficiency evaluation, the present work attempted to fill the knowledge gap on this subject by putting forward the evaluation indexes and evaluation method. The results of this research are expected to contribute to an efficient decisionmaking system for hydropower station economic operation and water resources management.
Acknowledgements This research was supported by the Natural Science Foundation of China (51679189) and the Key Innovation Group of Science and Technology of Shaanxi (2012KCT-10). Sincere gratitude is extended to the editor and the anonymous reviewers for their professional comments and corrections.
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ACCEPTED MANUSCRIPT IEA. World Energy Outlook 2012 International Hydropower Association. IHA Hydropower Report 2013 Inglesi-Lotz R, Blignaut JN. Improving the electricity efficiency in South Africa through a benchmark-and-trade system. Renew. Sust. Energy Rev. 2014; 30:833840. Jha DK, Shrestha R. Measuring efficiency of hydropower plants in Nepal using data envelopment analysis. IEEE Trans. Power Syst 2006; 21:1502-1511. Jiang ZQ, Ji CM, Sun P, Wang LP, Zhang Yk. Total output operation chart optimization of cascade reservoirs and its application. Energy Conversion and Management 2014; 88: 296-306. Kahraman G, Yucel HL, F.Oztop H. Evaluation of energy efficiency using thermodynamics analysis in a hydropower plant: A case study. Renewable Energy 2009; 34:1458-1465. Lin B, Wu Y, Zhang L. Electricity saving potential of the power generation industry in China. Energy 2012; 40:307–16. Liang X, Wang Z, Zhou Z, Huang Z, Zhou J, Cen K. Up-to-date life cycle assessment And comparison study of clean coal power generation technologies in China. J Clean Prod 2013; 39:24-31. Liu L, Zong H, Zhao E, Chen C, Wang J. Can China realize its car bone mission reduction goal in 2020: from the perspective of thermal power development. Appl Energy 2014; 124: 199-212. Ma C, Lian JJ, Wang JN Short-term optimal operation of Three-gorge and Gezhouba 23
ACCEPTED MANUSCRIPT cascade hydropower stations in non-flood season with operation rules from data mining. Energy Convers Manage 2013; 65: 616-27. Ma H, Oxley L, Gibson J. China's energy situation in the new millennium. Renew Sustain Energy Rev 2009; 13: 1781-1799. Mandal KK, Chakraborty N. Short-term combined economic emission scheduling of hydrothermal power systems with cascaded reservoirs using differential evolution. Energy Convers Manage 2009; 50: 97-104. Minville M, Brissette F, Krau S, Lecont R. Adaptation to climate change in the management of a Canadian water-resources system exploited for hydropower. Water Resour. Manag 2009; 23: 2965-2986. Moeini R, Afshar MHM. Arc-based constrained ant colony optimization algorithms for the optimal solution of hydropower reservoir operation problems. Can. J. Civ. Eng 2011; 38: 811-824. Mukand SB, Chien ND, Mullick MRA, Umamahesh VN. Operation of a hydropower system considering environmental flow requirements: a case study in La Nga river basin, Vietnam. J. Hydro-Environ. Res 2012; 6: 63–73. Nikoofard S, Ugursal V, Beausoleil-Morrison L. Economic Analysis of Energy Upgrades Based on Tolerable Capital Cost. J. Energy Eng 2015; 141. Sözen A, Alp I, Kilinc C. Efficiency assessment of the hydro-power plants in Turkey by using Data Envelopment Analysis. Renewable Energy 2012; 46: 192-202. Wang B, Nistor I, Murty T, Wei YM. Efficiency assessment of hydroelectric power plants in Canada: A multi criteria decision making approach. Energy Economics 24
ACCEPTED MANUSCRIPT 2014; 46: 112-121. Zeng M, Li C, Zhou L. Progress and prospective on the police system of renewable energy in China. Renew Sustain Energy Rev 2013; 20: 36-44. Zhu XP, Zhang C, Yin JX, Zhou HC, Jiang YZ. Optimization of water diversion based on reservoir operating rules-a case study of the Biliu River reservoir, china. Journal of Hydrologic Engineering 2014; 19:441-421. Zheng TF, Qiang MS, Chen WC, Xia BQ. An externality evaluation model for hydropower projects: A case study of the Three Gorges Project. Energy 2016; 108:74-85.
25
ACCEPTED MANUSCRIPT
Fig.1 Locations of seven cascade reservoirs in the Yellow River
26
ACCEPTED MANUSCRIPT
Fig.2 The cascade operation chart of the upper Yellow River
27
ACCEPTED MANUSCRIPT
Water level (m)
2610 2600 2590 2580 2570 2560 2550 2540 2530 2520 2510 2500 0
50
100 150 200 Storage capacity (108m³)
250
300
Fig.3 The curve of water level versus storage capacity
Downstream water level(m)
2464 2462 2460 2458 2456 2454 2452 2450 2448 2446 2444 0
1000
2000
3000
4000
5000
6000
7000
Releasing discharge(m³/s)
Fig.4 The curve of downstream water level versus releasing discharge
28
ACCEPTED MANUSCRIPT
Fig. 5 Output calculating flow chart of the Longyangxia reservoir ( N L,i )
1200
Runoff(m³/S)
1100 1000 900 800
800
750
750
800
750
750
600 400 200 0
0 1
0 2
0 3
0 4
5
6
7
Month
8
9
10
Fig.6 Monthly water demand at Lanzhou section
29
11
12
ACCEPTED MANUSCRIPT
Fig.7 Monthly output of the Longyangxia reservoir and the cascade output of seven reservoirs in 2001
2568
Begin of the year
Water level (m)
End of the year 2564
2560
2556
2552 actual
calculation
Fig. 8 Actual and calculation water level in the Longyangxia reservoir of 2001
30
ACCEPTED MANUSCRIPT
Fig. 9 Results of the conversion electric energy and the checking electric energy
700
Calculation releasing discharge
Actual releasing discharge
Dischage(m³/s)
600 500 400 300 200 100 0 1
2
3
4
5
6 7 Month
8
9
10
11
12
Fig.10 Monthly process of calculation and actual releasing discharge
31
ACCEPTED MANUSCRIPT Table 1 Operation characteristics of seven cascade reservoirs Reservoir
Total storage (108m3)
Normal level (m)
Dead level (m)
Installed capacity (MW)
Output power coefficient
Longyangxia Lijiaxia Liujiaxia Yanguoxia Bapanxia Daxia Qingtongxia
247 16.5 57 2.2 0.49 0.9 5.65
2600 2180 1735 1619 1578 1480 1156
2530 2178 1696 1618 1576
1280 2000 1160 396 18 30 30.2
8.3 8.3 8.3 7.5 8.3 7.6 8.3
Table 2 Actual running condition of the Longyangxia reservoir in 2001 month
1 2 3 4 5 6 7 8 9 10 11 12 Total
Inflow (m³/s)
Releasing discharge (m³/s)
Releasing water (108m³)
Surplus water (108m³)
Begin
End
Electric energy （108KWh）
144 168 190 323 470 804 586 537 769 755 378 198 /
438 493 456 449 574 582 578 575 439 383 631 594 /
11.52 12.97 11.99 11.81 15.10 15.31 15.20 15.12 11.55 10.07 16.60 15.62 162.85
0 0 0 0.01 0.1 0 0 0 0 0 0 0.01 0.02
2565.91 2565.04 2560.06 2557.26 2555.94 2554.94 2557.15 2557.24 2556.84 2560.21 2563.96 2561.52 /
2565.04 2560.06 2557.26 2555.94 2554.94 2557.15 2557.24 2556.84 2560.21 2563.96 2561.52 2557.39 /
3.03 3.01 3.04 2.78 3.64 3.59 3.76 3.73 2.73 2.51 4.25 4.03 40.12
32
Water level (m)
Table 3 The comparison results between actual and calculated running conditions of the Longyangxia reservoir in 2001 Month
Inflow
1 2 3 4 5 6 7 8 9 10 11 12 Tatal
144 168 190 323 470 804 586 537 769 755 378 198 5322
Releasing discharge (m³/s) actual calculation 438 493 456 449 574 582 578 575 439 383 631 594 6192
480 520 575 400 300 345 460 465 485 405 390 415 5240
Releasing water (108m³) actual calculation 11.52 12.97 11.99 11.81 15.10 15.31 15.20 15.12 11.55 10.07 16.60 15.62 162.8
12.62 13.68 15.12 10.52 7.89 9.07 12.10 12.23 12.76 10.65 10.26 10.91 137.8
Surplus water (108m³) actual calculation 0 0 0 0.01 0.1 0 0 0 0 0 0.01 0.22
0 0 0 0 0 0 0 0 0 0 0 0 0
Water level (begin) (m) actual calculation 2565.91 2565.04 2560.06 2557.26 2555.94 2554.94 2557.15 2557.24 2556.84 2560.21 2563.96 2561.52 /
33
2565.91 2562.64 2559.49 2555.26 2554.45 2556.31 2561.05 2562.28 2562.98 2565.65 2569.06 2568.95 /
Water level (end) (m) actual calculation 2565.04 2560.06 2557.26 2555.94 2554.94 2557.15 2557.27 2556.84 2560.21 2563.96 2561.52 2557.39
2562.64 2559.49 2555.26 2554.45 2556.31 2561.05 2562.28 2562.98 2565.65 2569.06 2568.95 2566.83
Electric energy （108KWh） actual calculation 3.03 3.01 3.04 2.78 3.64 3.59 3.76 3.73 2.72 2.51 4.25 4.03 40.12
3.27 3.10 3.64 2.42 1.91 2.17 3.06 3.11 3.19 2.84 2.69 2.93 34.36
ACCEPTED MANUSCRIPT Table 4 Efficiency evaluation results of economic operation in the Longyangxia power station
WLo
WLc
ELo
ELc
ELK
(108m³)
(108m³)
(108KWh)
(108KWh)
(108KWh)
162.85
137.85
40.12
34.36
40.64
34
RC
1.20
RU
RI
(%)
(%)
98.72
-1.26
ACCEPTED MANUSCRIPT 1. Based on the operation chart, a set of evaluation indexes was established. 2. The evaluation method for a multi-year regulation reservoir station was investigated. 3. The economic operation of the Longyangxia station was not reasonable. 4. Constructive suggestions were given to increase the operation efficiency.
Jianxia Chang, Yunyun Li, Meng Yuan, Yimin Wang PII:
S0360-5442(17)31043-5
DOI:
10.1016/j.energy.2017.06.049
Reference:
EGY 11057
To appear in:
Energy
Received Date:
16 January 2016
Revised Date:
11 May 2017
Accepted Date:
09 June 2017
Please cite this article as: Jianxia Chang, Yunyun Li, Meng Yuan, Yimin Wang, Efficiency Evaluation of Hydropower Station Operation: A Case Study of Longyangxia Station in the Yellow River, China, Energy (2017), doi: 10.1016/j.energy.2017.06.049
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ACCEPTED MANUSCRIPT Efficiency Evaluation of Hydropower Station Operation: A Case Study of Longyangxia Station in the Yellow River, China Jianxia Chang *, Yunyun Li, Meng Yuan, Yimin Wang State Key Laboratory Base of Eco-hydraulic Engineering in Arid Area, Xi’an University of Technology, Xi’an 710048, China *Corresponding author. [email protected]
Abstract: Hydropower plays a major role in the Chinese electricity generation industry. It is of significant importance to perform efficiency evaluation of the economic operation in a power station, which can help promote better water resources management and create more benefits for power network. Therefore, the main goal of this study is to establish a set of evaluation indexes and investigate the evaluation method to assess the economic operation in a power station. Based on the operation chart, three evaluation indexes including the Relative Water Consumption Rate (RC), the Relative Hydropower Utilization Rate (RU), and the Relative Hydropower Utilization Increasing Rate (RI) were proposed. The Longyangxia power station, as the biggest peak regulation source in the Northwest Power Network in China and the largest reservoir in the Yellow River Basin, was selected to perform an efficiency evaluation of economic operation. The result showed that the economic operation of this power station was not reasonable with RC>1, RU<1 and RI<0. And the rationality of the evaluation result was illustrated through analyzing the actual operation situation of the Longyangxia reservoir. Keywords: Efficiency evaluation; Economic operation; Evaluation indexes; 1
ACCEPTED MANUSCRIPT Evaluation method
1. Introduction Over the past several decades, a huge electric energy demand has accompanied with the process of rapid industrialization, modernization, and urbanization in China (Lin et al., 2012). Especially in the past three decades, the yearly electricity generation has increased 15-fold from 351.4TWh in 1983 to 5347.4TWh in 2013(Chen et al., 2015). According to the International Energy Agency (IEA), the electricity demand will continue growing at an annual rate of 2.5% by 2030, and approximately 16% of electricity is produced by hydropower (IEA, 2012). Typically, compared with traditional fossil fuel (thermal power) and other green energy resources (wind power), hydropower is a sustainable source of green energy with relative less air pollutants and greenhouse emissions (Zheng et al., 2015). It is mature in development, reliable and flexible in operation, easy to maintain and financially competitive (Ma H et al., 2009; Liang et al., 2013; Liu et al., 2014; Nikoofard et al., 2015; EI-Agouz SA and Kabeel AE., 2015). With these advantages, hydropower in China, a country that has the most severe environmental pollution, has been actively pursued with significant achievement in recent years (Huang et al., 2009; Hong et al., 2013; Zeng et al., 2013; Hennig et al., 2013). Its gross installed hydropower capacity has reached approximately 280 GW in 2013, exceeding the cumulative values of the Canada, Brazil and USA (IHA, 2013). Thus, hydroelectric power is an inevitable source in meeting China's growing electricity needs. The significance of hydroelectric power indicates that the efficiency evaluation of economic operation in a hydropower 2
ACCEPTED MANUSCRIPT plant is essential to the management of hydropower generation, for which directly influences the total economic benefits in the power network. However, due to the lack of systematic methods to evaluate operation efficiency, a lot of electric energy is being wasted in actual production. According to statistics, owing to unreasonable regulation modes such as low head operation, the annual average loss of electricity was up to 1.5 billion kWh (Chang et at., 2006).Therefore, it is an urgent need to perform efficiency evaluation of hydropower plant operation to enhance the overall benefits to the power network. Few attempts have been made in relation to this topic in China. International researches regarding hydropower efficiency have been carried out in succession in recent years (Jha and Shrestha, 2006; Barros and Peypoch, 2007; Barros, 2008; Minville et al., 2009; Moeini and Afshar., 2011; Inglesi-Lotz and Blignaut.,2014). Kahraman et al. (2009) performed efficiency evaluation of Keban hydropower plant experimentally and numerically in Turkey by using thermodynamic analysis. The results show that thermodynamic method was an effective and easy method for the determination of turbine efficiency in a hydraulic power plant. Sözen et al. (2012) conducted electricity generation of ten hydropower plants through Data Envelopment Analysis (DEA) and window analysis (WA) in Turkey, and defined and pursued two efficiency indexes based on production and energy unit cost performance. The results revealed the operating conditions of Turkey’s forthcoming hydropower plants. Wang et al. (2014) assessed the efficiency of hydropower generation in Canada from comprehensive viewpoints of electricity generating capability using the TOPSIS (the Technique for 3
ACCEPTED MANUSCRIPT Order Preference by Similarity to Ideal Solution) method. The results mainly revealed that the overall efficiency of hydropower generation in Canada experienced a downward trend from 2005 to 2011, following an improvement in 2012. Past related studies mainly made critical achievements in taking the value of electricity generation as a decisive index to assess hydropower efficiency. However, it may not be reasonable to only use the electricity generation as a decisive index to evaluate the efficiency of hydropower operation, since electricity generation is not only related to the reservoir regulation mode, but also largely depends on natural inflow. For example, electricity generation is sure to be different in the wet year and dry years. This will lead to a change of the power plant output. Therefore, the main difference of current study from previous studies is that we assess the hydropower plant operation efficiency not only based on the amount of electricity generation, but also based on the operation mode with the operation chart, which has removed the impact of natural inflow variation on the efficiency evaluation because it has the same constraints and running conditions as the evaluation instance. Furthermore, water utilization rate is another important factor to assess hydropower plant operation efficiency, which should be highly considered when performing efficiency evaluation of hydropower station under different operation modes in a certain inflow condition. Hence, the main scientific motivation behind the work is to propose a set of evaluation indexes (not only including the electricity generation, but also considering the variation of water utilization rate) based on the operation chart to assess the hydropower plant operation efficiency, which
4
ACCEPTED MANUSCRIPT can be more objective and effective to reflect the complexity operation characteristics of hydropower plant. The objectives of this paper are as follows: 1) to propose a set of evaluation indexes of hydropower station operation; 2) to investigate the evaluation method for a multiyear regulation reservoir; and 3) to illustrate the rationality of the evaluation results and give some suggestions for improving efficiency economic operation. The present study would play a timely role and expand the research breadth in this field from a sustainable perspective.
2. Methods 2.1. Evaluation indexes Economic operation of a hydropower station mainly contains two aspects. One is to decrease water consumption by raising or maintaining the water head as high as possible. The other is to make full use of water resources by avoiding surplus water as much as possible. Thus, Water Consumption Rate and Hydropower Utilization Rate can be selected as indexes to evaluate the reservoir operation from the perspective of energy-saving and optimal conditions, respectively. There would be no comparability to use absolute value, since regulation performance, water head, and type of units vary from power station to power station. Therefore, a relative value is used for evaluation, and three evaluation indexes, including Relative Water Consumption Rate (RC), Relative Hydropower Utilization Rate (RU), and Relative Hydropower Utilization Increasing Rate (RI), are finally selected. Their calculation formulas are shown below. 5
ACCEPTED MANUSCRIPT (1) RC index RC (Relative Water Consumption Rate) is applied to access the rationality of the reservoir operation from the perspective of energy saving. Before calculating RC, a variable of Water Consumption Rate (A) is defined as A
W 100% E
(1)
Then
RC
A0 AK
(2)
Where W is the water used for power generation (m3). E is the hydropower electricity generation (kWh). A0 and AK represent the actual and checking hydropower station water consumption rates, respectively. If RC<1, the energy-saving operation in a hydropower plant is reasonable. (2) RU and RI indexes RU (Relative Hydropower Utilization Rate) and RI (Relative Hydropower Utilization Increasing Rate) indexes are also employed to evaluate the rationality of optimal operation and they are defined as follows:
RU RI
E0 100% Ek
(3)
E0 Ek 100% Ek
(4)
Where E0 represents actual power generation (kWh). Ek represents checking power generation. If RU>1 and RI>0, the optimal operation in a hydropower plant is reasonable. Moreover, the larger the index values are, the better the effect of an optimal operation 6
ACCEPTED MANUSCRIPT will be. 2.2. Determination of checking power generation We need to determine the checking power generation for the computation of the evaluation indexes, and the value of which should be only related to the hydropower station operation mode. The objective of the hydropower station operation is to realize optimal operation, which has the characteristics of high dimensionality, strong coupling, uncertainty, etc. (Mandal and Chakraborty, 2009; Jiang et al., 2014; Chang et al., 2010). The operator needs to consider not only the hydraulic and electrical connection between the upstream and downstream reservoirs, but also a lot of other constraints. Although many optimal methods are applied in the reservoir operation (Huang et al., 2002; Ma et al., 2013; Mukand et al., 2012; Chang et al., 2013; Zhu et al., 2014), it is very difficult to adopt optimal decision making for a real-time reservoir operation due to the uncertainties of the predicted inflow. To pursue better optimization or a satisfactory solution, it is very important to make reasonable operation rules (Bolouri-Yazdeli et al., 2014; Chou and Wu, 2015). Reservoir operation rules are logical or mathematical equations that take system variables into account to calculate water release based on the inflow and water capacity values, and these rules could guide operation management. The operation rules are usually predefined and presented in the form of charts, tables or functions. In this paper, checking power generation was obtained by using the reservoir operation chart, which mainly consists of different guide curves and corresponding operation zones, and is usually obtained through a reverse calculation using the hydrological runoff data of typical years. 7
ACCEPTED MANUSCRIPT 3. Case study for multi-year regulation reservoir 3.1. Description of the Longyangxia hydropower station The Yellow River, draining an area of 7.95 × 105 km2, crosses “the three major stairs” of the macroscopic landform structure of China, i.e., the Qinghai-Tibet Plateau, the Loess Plateau and the North China Plain, and empties into the Pacific Ocean. The Yellow River flows north, turns south and then bends east for a total of 5464 km before debouching into the ocean. The Yellow River region is one of the most important basins in China, which is the largest water supply source in northwest and north China. It undertakes the water supply task of a population of about 140 million (more than 50 large and medium-sized urban cities), and has heavy agriculture irrigation task of an area of 1.6 × 105 km2 (accounting for about 15% of the whole country). Hence, the water resources development and utilization of the Yellow River Basin plays a vital role in the sustainable economic and social development of china. There are 24 reservoirs scattered widely in the Yellow River basin with storage capacities exceeding 1.0×108 m3, in which four major reservoirs along the mainstream are the most influential: the Longyangxia, Liujiaxia, Sanmenxia, and Xiaolangdi reservoirs. The Longyangxia reservoir, the biggest peak regulation source of the Northwest Power Network in China, is the largest reservoir in the Yellow River Basin. It was constructed in 1985, and the storage capacity and dam height are 247×108 m3 and 178 m, respectively. In addition, there have another six cascade reservoirs in the upper Yellow River: Lijiaxia, Liujiaxia, Yanguoxia, Bapanxia, Daxia, and Qingtongxia (Fig.1). Their main operation characteristics are listed in Table 1. Since the completion of 8
ACCEPTED MANUSCRIPT Longyangxia reservoir, it has operated jointly with Liujiaxia reservoir and other five runoff reservoirs to supply water to meet the demands of municipal, industrial, irrigation, and power generation in the north-west electricity network. Therefore, the operation efficiency of the Longyangxia reservoir directly influences the total power generation of cascade reservoirs in the upstream of the Yellow River. However, due to the lack of systematic methods or index systems to quantify the efficiency of the economic operation in the Longyangxia Power Station, a large amount of waterpower has been wasted, resulting in enormous economic loss to the cascade reservoirs and even to the Northwest Power Network. Therefore, research on efficiency evaluation is an urgent need for the Longyangxia power station and it has practical significance and application value to improve the entire power generation of the Yellow River cascade hydropower stations. In this paper, the operation process of Longyangxia power station in the year of 2001 is taken as an example to illustrate whether its economic operation is reasonable. Fig.1 Locations of seven cascade reservoirs in the Yellow River Table 1 Operation characteristics of seven cascade reservoirs 3.2. Data sources The datasets used for the efficiency evaluation of hydropower stations operation mainly include two categories: basic characteristics and actual operation conditions of the power plant. Basic characteristics data include reservoir operation chart, the curves of water-level as a function of storage-capacity and downstream water level as a function of releasing discharge. The operation chart used in this study is the cascade 9
ACCEPTED MANUSCRIPT operation chart of the upper Yellow River (Fig.2). From Fig.2, it can be easily observed that if the water level in the reservoir is below line A, neither the water supply nor power generation can meet the basic needs. If the water level in the reservoir is between line A and line B, the power generation can be increased by increasing output, while the water supply can’t be satisfied. If the water level in the reservoir is between line B and line E, the water supply can’t be satisfied, but the power generation can meet with the normal standard with guaranteed output. If the water level in the reservoir is between line B/E and line C, both the water supply and power generation can meet with the normal needs. If the water level in the reservoir is between line C and the normal water level, not only the water supply can be satisfied, but also the power plant can obtain more electricity by increasing output. The curves of water level versus storage-capacity (Fig.3) and downstream water level versus releasing discharge (Fig.4) of the Longyangxia reservoir are provided by the Yellow River Conservancy Commission (YRCC). The practical operation conditions include monthly inflow, releasing discharge, surplus water, initial water level, and hydropower generation of the Longyangxia reservoir, which is also provided by the YRCC. Fig.2 The cascade operation chart of the upper Yellow River Fig.3 The curve of water level versus storage capacity Fig.4 The curve of downstream water level versus releasing discharge 3.3. Evaluation steps As for the Longyangxia reservoir, there are two aspects that should be taken into 10
ACCEPTED MANUSCRIPT account specially when carrying out an efficiency evaluation. One is that the seven cascade reservoirs should jointly operate to meet the demand of municipal, industrial, irrigation water supply, power generation, and ecological purposes. When calculating the checking generation, the cascade operation chart of seven reservoirs instead of the single Longyangxia reservoir is used. Hence, the output of Longyangxia reservoir in each month ( N L,i ) will have to be further calculated (step1).The other is that the Longyangxia reservoir is a multi-year regulating reservoir, whose year-end level is not a dead level and it is also different with different reservoir operation modes, leading to different values of water storage at the end of year. During evaluation, the actual reservoir year-end water level may be different from that of the calculated values through the operation chart. Hence, it is not reasonable to take the calculated power generation as the checking one. To overcome this limitation, the difference between the calculated and the actual water storage at the end of year should be converted into the corresponding hydropower generation ( ELk ). Then the checking value ( ELk ) can be determined (step 2). Based on step1 and step 2, the values of the three evaluation indexes can be finally obtained (step 3). Step 1: Calculating the output of Longyangxia reservoir in each month ( N L,i )
N L,i is obtained by a trial and error method through MATLAB programming and the calculation flow chart is shown in Fig. 5.The detailed procedures are described as follows: ① Cascade output ( NTotal,i )
NTotal,i can be directly obtained from the operation chart of cascade reservoirs and 11
ACCEPTED MANUSCRIPT the function is presented as
NTotal,i =f ( Z1,i )
(5)
Where Z1,i represents the water level of the Longyangxia reservoir at the beginning of month i ( i = 1, 2,3,...,12 ). ② Output of the Liujiaxia( N 3,i ), Yanguoxia( N 4,i ), Bapanxia( N 5,i ), Daxia( N6,i ), and Qingtongxia( N7,i ) reservoirs The releasing discharge of the Liujiaxia reservoir ( Q3,outi ) is determined by the monthly water demand at Lanzhou section ( QLZ,i ), which is shown in Fig.6. As the Yanguoxia, Bapanxia, Daxia, and Qingtongxia are daily regulation power stations, their releasing discharge are the same as that of the Liujiaxia reservoir. The output formulas are expressed as
N 3,i K 3Q3,iout H 3,i N 4,i K 4 Q4,iout H 4,i N 5,i K 5Q5,iout H 5,i
（6）
N6,i K6 Q6,iout H 6,i N7,i K7 Q7,iout H7,i
N 3-7,i = N 3,i + N 4,i + N 5,i + N6,i + N7,i
（7）
Where Q4,outi , Q5,outi , Q6,outi , Q7,outi (m³/s)represent the releasing discharge of the Yanguoxia, Bapanxia, Daxia, and Qingtongxi in month i , respectively. K 3 , K 4 , K 5 ,
K 6 , K 7 represent the output coefficients of the Yanguoxia, Bapanxia, Daxia, and Qingtongxi, respectively. H 3,i , H 4,i , H 5,i , H 6,i , H 7,i (m) represent the water heads of the Yanguoxia, Bapanxia, Daxia, and Qingtongxi in month i , respectively. N 3-7,i (kW) represents the total output of five reservoirs in the month i (i.e., Liujiaxia,Yanguoxia, Bapanxia, Daxia, and Qingtongxia) 12
ACCEPTED MANUSCRIPT ③ Output of the Longyangxia( N1,i )and Lijiaxia ( N 2,i ) reservoirs Likewise, being daily regulation power station, the releasing discharge of the Lijiaxia reservoir ( Q2,outi ) is equal to that of the Longyangxia reservoir ( Q1,outi ).Here, is obtained based on the inflow of the Liujiaxia reservoir ( Q3,ini ) which is an Q1,out i arbitrary hypothetical value. N1,i and N 2,i are calculated by Eqs. (8). Q1,iout Q2,iout N1,i K1Q1,iout H 1,i
（8）
N 2,i K 2Q2,iout H 2,i
Where K1 and K 2 represent the output coefficients of the Longyangxia and Lijiaxia reservoir, respectively. H1,i (m) and H 2,i (m) represent the water head of the Longyangxia and Lijiaxia reservoir in month i , respectively. ④ Determining the output of Longyangxia reservoir in each month( N L,i ) Before determining the value of N L,i , an intermediate variable ( N1,i' ) is introduced to verify the accuracy of Q3,ini ' N1,i NTotal ,i N 2,i N 3-7,i
(9)
If N1,i' = N1,i , the hypothesis of Q3,ini is reasonable and N L,i = N1,i . Fig. 5 Output calculating flow chart of the Longyangxia reservoir ( N L,i ) Fig.6 Monthly water demand at Lanzhou section Step 2: Determining the checking hydropower generation of Longyangxia reservoir (
ELk ) ① Conversion hydropower generation ( ELK ) The value of ELK , which is directly related to the accuracy of ELk , must be calculated by using a reasonable approach. Four approaches, therefore, are put forward 13
ACCEPTED MANUSCRIPT to get the value of ELK , and based on the results of these values, the final value of
ELK is then determined. The detailed methods are showed below: 1 Method 1: Based on the water storage change in each month, ELK is defined
as: 12
1 ELK K1 (WLo,i -WLc,i ) H 1.i
(10)
i 1
Method 2: Based on the actual water consumption rate in month i ( qLo,i , m³ / kW), 2 is defined as: ELK
WLo,i -WLc,i qLo,i i 1 12
2 ELK
(11)
Method 3: Based on the calculation water consumption rate in month i ( qLc,i , m³ / kW), ELk3 is defined as:
WLo,i -WLc,i qLc,i i 1 12
3 ELk
(12)
Method 4: Based on the average water head ( H L ), ELk4 is defined as: 12
ELk4 K1 (WLo,i -WLc,i ) H L
(13)
i 1
Then,
ELk =
1 ELk +ELk2 +ELk3 +ELk4 4
(14)
Where WLo,i (m³) and WLc,i (m³) represent the actual and calculated releasing water storage values of the Longyangxia reservoir in the month i , respectively. ② Checking hydropower generation ( ELk )
The checking electric energy can be calculated by Eqs. (15).
14
ACCEPTED MANUSCRIPT 12
ELC K1 N L,i H 1,i i 1
(15)
ELK ELC ELk Step 3: Calculating the evaluation indexes On the basis of step 1 and step 2, the three evaluation indexes can be finally calculated by Eq. (16):
RC
A0 WLo / ELo AK WLc / ELc
RU
E0 E 100% Lo 100% Ek ELK
RI
(16)
E0 Ek E ELK 100% Lo 100% Ek ELK
Where WLo and WLc represent the actual and calculated annual water consumption values of the Longyangxia reservoir in 2001, respectively. EL0 , ELc , ELK represent the actual, checking and calculated annual hydropower generation of the Longyangxia reservoir in 2001, respectively.
4. Results and discussion In this paper, the operation process of Longyangxia hydropower station in 2001 is evaluated. The reservoir water level at the beginning of the operation is 2585m, and Table 2 shows actual water level and electric energy after each period. According to the valuation step 1, the cascade output for each month ( NTotal,i ) is firstly obtained, and the output of the Longyangxia reservoir for each month ( N L,i ) in 2001 is then calculated (Fig.7). Also, the results of calculation running condition are shown in Table 3. From Table 3, big differences can be seen between the actual and calculated results, which indicates that the calculated results can’t be directly used to evaluate the 15
ACCEPTED MANUSCRIPT operation efficiency of the Longyangxia reservoir. Taking the yearly begin and end water level as an example, which is shown in Fig.8, the actual and calculated values have same begin water level of 2565.91m. While the calculated year-end water level is 2566.83m, which is 9.44m higher than the actual value (2557.39 m). In an evaluation system, the basic condition is that the evaluated objectives should have same situations. Hence, the difference between the calculated and actual yearly end water levels should be taken into account when evaluating whether the operation efficiency of the Longyangxia power plant is reasonable. Therefore, to evaluate the actual running condition more fairly, 25.01 108 m3 water (the difference between the actual and calculated water storage of the Longyangxia reservoir) should be converted to the corresponding hydropower, which is then used as an intermediate variable to obtain the checking electric energy when evaluating the operation efficiency of the Longyangxia power plant. Hence, through the evaluation step 2, the values of conversion electric energy and the checking electric energy are calculated and the results are shown in Fig.9. Table 2 Actual running condition of the Longyangxia reservoir in 2001 Fig.7 Monthly output of the Longyangxia reservoir and the cascade output of seven reservoirs in 2001 Fig. 8 Actual and calculation water level in the Longyangxia reservoir of 2001 Fig. 9 Results of the conversion electric energy and the checking electric energy Table 3 The comparison results between actual and calculated running conditions of the Longyangxia reservoir in 2001 16
ACCEPTED MANUSCRIPT Based on the results of step1 and step 2, the evaluation indexes are finally obtained by step 3. The results of the main evaluation values and the efficiency evaluation of economic operation in the Longyangxia power plant are shown in Table 4. From it, we can see that the value of RC is 1.2, revealing that the energy-saving operation in the Longyangxia hydropower plant is not reasonable. Furthermore, the values of RC and RI are 98.72% and -1.26%, respectively, and both of them do not meet the assessment standard. The optimal operation in the Longyangxia hydropower plant, therefore, is also not reasonable or economic. Table 4 Efficiency evaluation results of economic operation in the Longyangxia power station The results of the three evaluation indexes indicate that the economic operation in the Longyangxia hydropower plant is not encouraging, which can be verified by a mixture of reasons. Among them, the following three aspects are the main driving factors that lead to the low efficiency of the economic operation in the Longyangxia hydropower plant. (1)The actual operation process of the Longyangxia reservoir was not consistent with the operation chart (Fig.10).As the Longyangxia reservoir plays an important role on peak regulation and frequency modulation of Northwest Power Network, a certain amount of electric energy has to be lost to ensure the stable and safe operation of the power network. Hence, the benefited cascade reservoirs in the downstream should give some economical compensation to the Longyangxia hydropower plant for its sacrifice for the overall interests of the whole power network. 17
ACCEPTED MANUSCRIPT (2) To pursue immediate interests in the Longyangxia hydropower plant, the electric energy in the current year was forced to be generated as much as possible. However, large amount of power generation in the hydropower plant leads to low yearend level, which not only decreases the hydropower utilization increasing rate, but also has negative impacts on reservoir operation in subsequent years, such as high water consumption rates and low generation benefits. (3) Acting as an excellent multi-year regulating reservoir, the Longyangxia reservoir undertakes a heavy compensation task, which not only includes the electric power compensation for the downstream cascade hydropower stations, but also contains the water compensation in the lower Yellow River. Although reasonable compensation has been taken into consideration in the cascade operation chart, the leaders sometimes have to add additional compensation tasks to cope with unexpected practical problems. For example, long-distance water divisions of the Longyangxia reservoir has been made for several times to compensate the lower Yellow River for urgent water shortage, which in turn caused loss of electric energy in the Longyangxia power plant. Fig.10 Monthly process of calculation and actual releasing discharge
5. Suggestions for increasing the efficiency economic operation of the Longyangxia power plant The efficiency of station for economic operation can be improved through operational changes to the Longyangxia reservoir. Improvements of runoff forecast 18
ACCEPTED MANUSCRIPT accuracy, water use and supply benefits through optimal water resources allocation are considered as the most fundamental solutions to solve the problem, namely, exploring reasonable operation rules for the Longyangxia reservoir. Here are four important suggestions: 1. During flood season period, the idle units should be operated coordinately, and the power generation and flood control should be taken into prior consideration. 2. During dry season period, moderate increase of the output can improve the power generation. And the reservoir profit, such as power generation and water supply, should be also harmonized with ecological target regulation. 3. The decreasing mode of reservoir water level is closely associated with water resource utilization, and should be therefore given special attentions (e.g., setting flexible sluice gates at the dam crest) to mitigate the negative impact on reservoir operation. 4. The high tailrace level, which is easy to lead to backwater problems and has large influence on power generation, should be solved by taking some measures such as expanding discharge channel.
6. Conclusions It is an urgent need to establish a set of evaluation index and investigate the evaluation method to assess the efficiency of the economic operation in a power plant, for which is the key link in the electric system. In this study, efficiency evaluation method with three evaluation indexes including Relative Water Consumption Rate 19
ACCEPTED MANUSCRIPT (RC), Relative Hydropower Utilization Rate (RU), and Relative Hydropower Utilization Increasing Rate (RI) were proposed based on operation chart. A muti-year regulation reservoir (Longyangxia power plant) was taken as an example to apply the proposed evaluation indexes, and the rationality of evaluation results was verified by analyzing the actual operation process of the Longyangxia reservoir. And four constructive suggestions were given to increase the hydropower plant operation efficiency. The main conclusions are as follows: (1) Based on the operation data of 2001, three evaluation values of RC, RU and RI were 1.20, 98.72% and -1.26%, respectively, which indicated that the economic operation of the Longyangxia power plant for this year was neither reasonable nor encouraging. (2) Three actual operation situations of the Longyangixia power station, mainly including the inconsistent of the actual operation process with that in the operation chart, being forced to be generated as much as possible in pursuit of immediate interests; and undertaking heavy power compensation and water compensation tasks for downstream cascade hydropower stations, verified the rationality of evaluation results. Hence the proposed indexes in this study are reasonable, and can be used to evaluate other hydropower plant operation efficiency objectively and accurately. (3) Aimed at the low operation efficiency of the Longyangxia power plant, four constructive countermeasures were proposed to increase the efficiency of power station for economical operation through operational changes to the Longyangxia reservoir. Given the important role of hydropower stations in China but limited attention 20
ACCEPTED MANUSCRIPT being paid attention to their efficiency evaluation, the present work attempted to fill the knowledge gap on this subject by putting forward the evaluation indexes and evaluation method. The results of this research are expected to contribute to an efficient decisionmaking system for hydropower station economic operation and water resources management.
Acknowledgements This research was supported by the Natural Science Foundation of China (51679189) and the Key Innovation Group of Science and Technology of Shaanxi (2012KCT-10). Sincere gratitude is extended to the editor and the anonymous reviewers for their professional comments and corrections.
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25
ACCEPTED MANUSCRIPT
Fig.1 Locations of seven cascade reservoirs in the Yellow River
26
ACCEPTED MANUSCRIPT
Fig.2 The cascade operation chart of the upper Yellow River
27
ACCEPTED MANUSCRIPT
Water level (m)
2610 2600 2590 2580 2570 2560 2550 2540 2530 2520 2510 2500 0
50
100 150 200 Storage capacity (108m³)
250
300
Fig.3 The curve of water level versus storage capacity
Downstream water level(m)
2464 2462 2460 2458 2456 2454 2452 2450 2448 2446 2444 0
1000
2000
3000
4000
5000
6000
7000
Releasing discharge(m³/s)
Fig.4 The curve of downstream water level versus releasing discharge
28
ACCEPTED MANUSCRIPT
Fig. 5 Output calculating flow chart of the Longyangxia reservoir ( N L,i )
1200
Runoff(m³/S)
1100 1000 900 800
800
750
750
800
750
750
600 400 200 0
0 1
0 2
0 3
0 4
5
6
7
Month
8
9
10
Fig.6 Monthly water demand at Lanzhou section
29
11
12
ACCEPTED MANUSCRIPT
Fig.7 Monthly output of the Longyangxia reservoir and the cascade output of seven reservoirs in 2001
2568
Begin of the year
Water level (m)
End of the year 2564
2560
2556
2552 actual
calculation
Fig. 8 Actual and calculation water level in the Longyangxia reservoir of 2001
30
ACCEPTED MANUSCRIPT
Fig. 9 Results of the conversion electric energy and the checking electric energy
700
Calculation releasing discharge
Actual releasing discharge
Dischage(m³/s)
600 500 400 300 200 100 0 1
2
3
4
5
6 7 Month
8
9
10
11
12
Fig.10 Monthly process of calculation and actual releasing discharge
31
ACCEPTED MANUSCRIPT Table 1 Operation characteristics of seven cascade reservoirs Reservoir
Total storage (108m3)
Normal level (m)
Dead level (m)
Installed capacity (MW)
Output power coefficient
Longyangxia Lijiaxia Liujiaxia Yanguoxia Bapanxia Daxia Qingtongxia
247 16.5 57 2.2 0.49 0.9 5.65
2600 2180 1735 1619 1578 1480 1156
2530 2178 1696 1618 1576
1280 2000 1160 396 18 30 30.2
8.3 8.3 8.3 7.5 8.3 7.6 8.3
Table 2 Actual running condition of the Longyangxia reservoir in 2001 month
1 2 3 4 5 6 7 8 9 10 11 12 Total
Inflow (m³/s)
Releasing discharge (m³/s)
Releasing water (108m³)
Surplus water (108m³)
Begin
End
Electric energy （108KWh）
144 168 190 323 470 804 586 537 769 755 378 198 /
438 493 456 449 574 582 578 575 439 383 631 594 /
11.52 12.97 11.99 11.81 15.10 15.31 15.20 15.12 11.55 10.07 16.60 15.62 162.85
0 0 0 0.01 0.1 0 0 0 0 0 0 0.01 0.02
2565.91 2565.04 2560.06 2557.26 2555.94 2554.94 2557.15 2557.24 2556.84 2560.21 2563.96 2561.52 /
2565.04 2560.06 2557.26 2555.94 2554.94 2557.15 2557.24 2556.84 2560.21 2563.96 2561.52 2557.39 /
3.03 3.01 3.04 2.78 3.64 3.59 3.76 3.73 2.73 2.51 4.25 4.03 40.12
32
Water level (m)
Table 3 The comparison results between actual and calculated running conditions of the Longyangxia reservoir in 2001 Month
Inflow
1 2 3 4 5 6 7 8 9 10 11 12 Tatal
144 168 190 323 470 804 586 537 769 755 378 198 5322
Releasing discharge (m³/s) actual calculation 438 493 456 449 574 582 578 575 439 383 631 594 6192
480 520 575 400 300 345 460 465 485 405 390 415 5240
Releasing water (108m³) actual calculation 11.52 12.97 11.99 11.81 15.10 15.31 15.20 15.12 11.55 10.07 16.60 15.62 162.8
12.62 13.68 15.12 10.52 7.89 9.07 12.10 12.23 12.76 10.65 10.26 10.91 137.8
Surplus water (108m³) actual calculation 0 0 0 0.01 0.1 0 0 0 0 0 0.01 0.22
0 0 0 0 0 0 0 0 0 0 0 0 0
Water level (begin) (m) actual calculation 2565.91 2565.04 2560.06 2557.26 2555.94 2554.94 2557.15 2557.24 2556.84 2560.21 2563.96 2561.52 /
33
2565.91 2562.64 2559.49 2555.26 2554.45 2556.31 2561.05 2562.28 2562.98 2565.65 2569.06 2568.95 /
Water level (end) (m) actual calculation 2565.04 2560.06 2557.26 2555.94 2554.94 2557.15 2557.27 2556.84 2560.21 2563.96 2561.52 2557.39
2562.64 2559.49 2555.26 2554.45 2556.31 2561.05 2562.28 2562.98 2565.65 2569.06 2568.95 2566.83
Electric energy （108KWh） actual calculation 3.03 3.01 3.04 2.78 3.64 3.59 3.76 3.73 2.72 2.51 4.25 4.03 40.12
3.27 3.10 3.64 2.42 1.91 2.17 3.06 3.11 3.19 2.84 2.69 2.93 34.36
ACCEPTED MANUSCRIPT Table 4 Efficiency evaluation results of economic operation in the Longyangxia power station
WLo
WLc
ELo
ELc
ELK
(108m³)
(108m³)
(108KWh)
(108KWh)
(108KWh)
162.85
137.85
40.12
34.36
40.64
34
RC
1.20
RU
RI
(%)
(%)
98.72
-1.26
ACCEPTED MANUSCRIPT 1. Based on the operation chart, a set of evaluation indexes was established. 2. The evaluation method for a multi-year regulation reservoir station was investigated. 3. The economic operation of the Longyangxia station was not reasonable. 4. Constructive suggestions were given to increase the operation efficiency.