Energy Assessment of a Combined Cycle Power Plant through Empirical and Computational Approaches: A Case Study ^†

Abstract

Energy management on the demand side is an important practice through which to address the challenge of energy shortage. In Pakistan, power plants have no specific energy management practice and a detail energy audit is normally observed as a one-time estimation that does not give significant information. In this study, an energy audit of a combined-cycle gas turbine power station was conducted and empirical data were compared with those obtained through a model developed in ASPEN, a simulation software that forecasts process performance. Next, an optimization tool was used to modify the ASPEN results and a comparison was drawn to estimate the amount of energy saved. It was found that compressor power consumption can be decreased up to 14.68% by increasing the temperature of compressed air from 320.2 °C to 423.79 °C for gas turbines. The output of gas turbines can be enhanced up to 13.5% and 21.4% with modelled and optimized data, respectively, using a multistage air compressor and multistage expansion. The calculated efficiency of the steam turbine was found to be 30.4%, which is 27.61% less than that of its designed efficiency. Steam turbine efficiency can be increased by 5% using a variable-speed water pump, leading to an estimated energy-saving potential of 8–9%. The combustion efficiency of gas turbines is not only important for higher turbine power output but also for better steam generation through heat-recovery steam generators in case of combined-cycle operations. The overall steam turbine efficiency is estimated to have increased by 19.27%, leading to a 12.68% improvement in combined efficiency.

1. Introduction

The world’s energy requirements heavily depend upon use of non-renewable sources for power generation, especially in developing countries. Presently, around 80% of electricity in Pakistan is produced using fossil fuel-fired power plants [1]. High fuel prices, gas supply shortages, and the poor conversion efficiency of existing plants not only cause higher electricity costs but also create a gap between supply and demand. The inefficient combustion of fossil fuels adversely affects the environment. Therefore, there is always space to make amendments to power plants for the efficient and effective utilization of fossil fuels. In order to assess the improvement potential and to identify the major energy-consuming subcomponents of a system, detailed energy audits are widely applied. The types of energy audit depend upon many factors, including the size and type of plant, the possibility of energy saving, and cost reduction [2]. Different types of power plants are currently operating in Pakistan, and out of these, combined-cycle power plants (CCPP) possess good thermal efficiency and low carbon dioxide emissions. Energy audits are underrated in Pakistan due to their high initial cost, seemingly low-performance gains, and the unfamiliar benefits in the long term. Secondly, a lack of knowledge about the expenses of energy audits and energy-reduction aids in plant operation are obstacles to the commercialization of energy-efficiency methods [3]. In this study, an energy audit was performed for a 147 MW combined-cycle thermal power station and the results were compared with those of obtained through a simulation of the same site using ASPEN software.

2. Methodology

In In the current study, a combined-cycle Thermal Power Station of 147 MW located in Faisalabad, operated by Northern Power Generation Company Limited (NPGCL), Pakistan, was selected to estimate its energy-saving potential. The plant comprises four gas turbines, each of 25 MW, and one steam turbine of 47 MW. The plant was divided into two parts, named topping and bottoming cycle. Operating parameters for the compressor, gas turbine, and steam turbine used during the energy audit were set. The operating efficiency of the gas turbine was calculated using Equation (1) [4]

$${\eta }_{th}=\frac{\left(860\right)\left(W_{out}\right)}{H_i}$$

(1)

where η_th is thermal efficiency of the turbine, W_out stands for load of gas turbine (MW), and H_i is the heat input (Kcal/h).

For the modeling of the combined-cycle power plant, ASPEN HYSYS was used, as shown in Figure 1. The second main part of the bottoming cycle was the application of Heat Recovery Steam Generated (HRSG) equipment. An indirect auditing method was applied on the HRSG to obtain its losses and efficiency. Equation (2) was used for calculating the surface losses [5]:

$$L \mathit{surface}=0.548[\{{(\frac{Ts}{55.55})}^4-{(\frac{Ta}{55.55})}^4\}+1.957{(Ts-Ta)}^{1.25}\ast \sqrt{\frac{(196.85Vm+68.9}{68.9}}$$

(2)

The efficiency of the combined cycle can be estimated by using following equation:

$${\eta }_{cc}=\frac{\left(Egt-Ecomp\right)+\left(Est-\frac{Epump}{0.6}\right)}{mNG\ast \left[LHV\right]NG}$$

(3)

3. Results and Discussion

The calculated efficiency of the steam turbine was found to be 30.4%, which is 27.61% less than its designed efficiency (42%). This was due to the reduced steam pressure (39.5 bar) in the condenser, which should be at an optimized value of 47.92 bar. This reduced pressure could be have been due to the decreased temperature of the steam condensation, which subsequently increased the temperature difference in the process. The efficiency could be enhanced by using the feed-water heating system and by adding a preheater in the steam turbine. The types of flow and feed water pump also exert an impact on the turbine efficiency. The plant under consideration used throttle valve and a constant-speed feed-water pump to control the flow rate instead of using a variable-speed water pump with adjustable frequency motors which can increase the turbine efficiency by 5%. Therefore, by taking into account this measurement, the energy saving potential was estimated to be 8–9% which was close to the design efficiency.

Based on the audited, modelled and optimized values of the different parameters in

Table 1. Comparison of results obtained from optimization, simulation and audit of combine cycle.

Parameters	Parameter Symbol	Unit	Optimized Results	ASPEN Results	Audit Results
Exhaust flue gases move toward HRSG	Mass flow	Ton/h	180.2	180.2	171
	Pressure	Bar	8.021	8.53	9.625
	Temperature	C	830.21	867.54	924.34
HRSG	Duty	MW	25.87	33.15	42.87
Steam Stream	Temperature	C	500	505	513
-	Pressure	Bar	47.92	43.7	39.5
Turbine	Isentropic efficiency	%	75	75	73.6
	Polytropic efficiency	%	75	72.64	69.76
	Power output	MW	46.04	43.47	40
Condenser	Duty	MW	95.7	103	110
Pump	Pressure difference		50	50.1	51.4
	Duty	MW	1.075	1.252	1.5
	Adiabatic efficiency	%	75	75	75
Overall efficiency	-	%	32.26	33.14	30.4
Combined efficiency	-	%	43.87	40.23	38.93

, there are some energy-saving approaches to improving the efficiency of the steam turbine and combined cycle. For the optimized values, by decreasing the temperature and pressure of the exhaust flue gases up to 830.21 °C and 8.021 bar while maintaining the temperature and pressure of the inlet flue gases at 500 °C and 47.92 bar, the turbine output power was found to be 46.04 MW (15% more than the existing power). High energy consumption by HRSG (77.33% efficiency) can be improved by optimizing the pressure level and mass flow ratio. Although the condenser and pump are working in better conditions, by reducing their power consumption to optimum levels i.e., up to 13% and 28% of the existing values (audited results) respectively, the overall turbine efficiency would increase by 19.27%, leading to a 12.68% improvement in combined efficiency.

Moreover, the comparative outcomes of the optimized and modelled (ASPEN) for the various main parameters (compressor power, gas flow, turbines outputs, pump power, overall and combined efficiencies) of the system are shown in Figure 2. It can be observed that the modelled values are close to the optimized. The combustion efficiency of the gas turbine is not only important for higher turbine power output but also for better steam generation through the Heat Recovery Steam Generator (HRSG) in combined-cycle operations.

4. Conclusions

This study was conducted to estimate the energy-saving potential and evaluate the energy performance of a combined-cycle power plant using empirical and computational techniques. A detailed energy audit was conducted and optimized operating conditions were found by developing its simulated model in ASPEN HYSYS. It was found that the performance of the power plant was affected by the gas turbine, air compressor and heat recovery steam generator (HRSG), where the compressors consumed more than 60% of the energy produced by the turbine and 14% of the energy consumed by the HRSG. This article provides a feasibility study through which to establish and quantify the cost of various energy inputs to and flows within a power plant in a given period.