Elsevier

Energy

Volume 175, 15 May 2019, Pages 891-899
Energy

Performance maximization of a solar aided power generation (SAPG) plant with a direct air-cooled condenser in power-boosting mode

https://doi.org/10.1016/j.energy.2019.03.133Get rights and content

Highlights

  • The impacts of the solar input on turbine exit pressure are revealed.

  • The optimal (turbine) exit pressure value under a certain condition is obtained.

  • Four proposed operating strategies of the air-cooled condenser are analyzed.

  • A sample daily operation procedure to demonstrate the operation is shown.

Abstract

Solar aided (coal-fired) power generation (SAPG) technology has been approved to be an efficient way to use solar energy for power generation. However, most solar-rich areas are often short of cooling water supply, especially in China. The air-cooled condenser could alleviate this issue effectively for power plants in these areas. In this paper, a solar aided power generation with direct air-cool condenser (SAPG + ACC) plant is studied and optimized to maximize its performance. Due to the addition of solar heat in an SAPG plant, the exhaust steam flow rate leaving the turbine changes, especially in power-boost mode, which affects the turbine exit pressure and deviates the condenser from its designed operating condition. To achieve maximum net power output, the correlation among the optimum (turbine) exit pressure (OEP), the solar heat input and the ambient temperature, is obtained in this study. The annual plant's performances with four proposed operating strategies of the air-cooled condenser are compared and analyzed. The results show that for an SAPG + ACC plant, the operating Strategy.3, i.e. operated with the hourly OEP is the optimal one, resulting the plant has annual solar-to-electricity efficiency (SEE) of 17.82% and its levelized cost of electricity (LCOE) is of 0.09 $/kWh.

Introduction

Nowadays, coal still plays a significant role in the worldwide energy mix. In 2016, 3,945,678 GWh electricity in China comes from coal, which accounted for 66% of the electricity generation mix [1]. The massive use of fossil energy has brought serious environmental problems, such as air pollution, climate change and global warming. The combustion of coal contributes the most to acid rain and air pollution, and has been connected with global warming. Two major approaches to relieving this tension between environment and energy demand are improving energy efficiency and expanding the share of renewable energy.

Solar Aided Power Generation (SAPG) has been proved an efficient way to utilize solar energy for power generation purpose [2,3], where solar heat serves as a substitution of the extraction steam to preheat the feedwater in a regenerative Rankin power station.

Most of the current studies on SAPG systems focus on designs of systems [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]], operation modes [2,7,11,[15], [16], [17], [18]], economic analysis [9,11,[19], [20], [21], [22], [23], [24]] and evaluation method [[25], [26], [27], [28], [29], [30]]. In terms of the design of the system, many studies have focused on optimization of integration schemes. Zoschak and Wu [3] discussed the performance of SAPG system with different replacements and solar collector types under different operating conditions. Using the THERMOFLEX software, Popov [8] compared three types of arrangements: (1) replacing low-pressure heaters; (2) replacing high-pressure heaters; and (3) replacing high-pressure heaters and economizer. Eric et al. [5,6,13]evaluated SAPG system integrated with different power plants at various temperature levels under design condition. The impact of power station capacity was studied in Ref. [9] as well as the optimization method of both solar aperture area and energy storage capacity [[10], [11], [12]]. In terms of operation modes, the operation can be categorized into either a power-boosting (PB) mode and/or a fuel-saving (FS) mode [7]. Hou and Wu analyzed the annual performance of an SAPG plant operated in FS mode [10]. Zhang et al. [17] analyzed the simulated performance of an SAPG plant in FS mode. Eric et al. demonstrated an operation model mixing PB and FS modes in Ref. [16]. Based on the operation modes, both the annual performance and the off-design performance were analyzed [11]. Based on a sophisticated model of SAPG plants, a remedial control measure was developed to stabilized operation in FS mode [18]. In the research into economic analysis, the economic efficiency of different solar integration approaches was calculated [9,11,[19], [20], [21], [22]] and the optimal solar collector field was found through this analysis. A method is derived for evaluating the thermo-economic performance by Lei [24]. In the field of the evaluation method, Hou and Xu et al. [26] proposed an evaluation method of SAPG systems based on the exergy analysis and five common evaluation methods of solar contribution rate are analyzed and compared by Zhu [28]. In addition, the 4E method was also analyzed [25], thermo-economic structural theory and life cycle assessment were used for evaluation [27,30].

These studies overlooked the impact of the condenser on the SAPG plant. In an SAPG plant, the exhaust steam flow rate changes, due to the addition of the solar energy, which would have an impact on the performance of the condenser. Especially, an SAPG plant is most likely to be located in arid environments where water is scarce, the issue of water consumption is heightened. Air-cooled condenser could effectively alleviate this issue without using water, but its performance is even more sensitive to the change of the exhaust steam flow rate than that of a water cooled condenser.

In a coal-fired power plant, the performance of its condenser determines the turbine exit pressure and thus directly affect the performance of the plant. For a power plant operated under a certain condition (a certain weather condition and a certain turbine operating condition), there is an optimum (turbine) exit pressure (OEP) existing. Few previous research looking at this issue i.e. determining OEP under a certain condition for an SAPG plant, in which steam flow rate changes with the solar radiation changes.

In this study, a case study of a 600 MW SAPG with air-cooled condenser (SAPG + ACC) plant is analyzed and optimized in terms of OEP to improve its performance. The correlation among the OEP, the solar heat input and the ambient temperature for such a plant is developed. If the plant was operated with the OEP, it would achieve the highest annual SEE and the lowest LCOE.

Section snippets

System description

Fig. 1 shows a schematic diagram of an SAPG + ACC system. The SAPG + ACC plant consists of 4 key subsystems: the solar field subsystem, the boiler subsystem, the turbine subsystem and the air-cooled condenser subsystem.

In the plant, all high-pressure extraction steams are assumed to be replaced by solar thermal energy carried by a heat transferring fluid (HTF) to preheat the feedwater through the HTF/water exchanger. The saved extraction steam can then continue expanding in the turbine to

Performance simulation model of air-cooled condenser

To achieve the aim of this research which is to maximize the performance of an SAPG + ACC plant, a simulation model of the plant is necessary. In our previous work [9], the main components, other than the air-cooled condenser, of such a plant had been established and verified. In the current study, the focus is to develop a simulation model for the air-cooled condenser.

The study case

A 600 MW direct air-cooled coal-fired plant (N600–16.67/538/538) located in Yulin, China (38.30°N, 109.77°E) was selected as the study case. The key parameters of the original plants in the 100% turbine heat acceptance condition are given in Table .1. The hourly DNI and the ambient temperature of a typical year in Yulin are used and shown in Fig. 6. The plant was assumed to be operated in PB mode and following the 9th scenario (S1 VT-LH) in Ref. [14].

The air-cooled condenser consists of 640

Conclusion

The solar aide power generation with air-cooled condenser (SAPG + ACC) plant is more acceptable in the area where cooling water is scarce. In an SAPG + ACC plant, especially when operated in Pb mode, the exhaust steam flow varies with the solar heat load, thus affecting the exit pressure and the plant's performance.

In this study, the impacts of the solar heat input on the turbine exit pressure and the plant's performance are revealed. The correlation among the OEP, the ambient temperature and

Acknowledgments

This work was supported by the National Basic Research Program of China (2015CB251505), State Grid Science and Technology Program (SGJN0000ASJS1700136) and the China Scholarship Council (No. 201806730031). The authors would also like to acknowledge with great thanks to Phoenix support team, University of Adelaide, for supporting this simulation work.

References (34)

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