Energy consumption is increasing dramatically with the explosive population growth, urbanization, industrialization, and technical advancement. This fast-upward situation brings about some vital issues such as energy crisis and environmental contamination. As the energy demand is still dominated by fossil energy, solving the contradiction between fossil energy consumption and environmental pollution can be achieved mainly in two ways: growing alternative energy resources, especially renewable ones, and improving the energy utilization efficiencies of fossil fuels [1
]. The latter is a cheaper, faster and easier way to realize this task.
For improving the energy utilization efficiencies of fossil fuels, the thermodynamic performances of equipment and systems need to be evaluated. Usually, the thermodynamic performances of equipment and systems are investigated by energy standards based on the first law of thermodynamics. In the past several decades, exergy analysis has been developed based on the second law of thermodynamics and proved as an effective way to evaluate the thermodynamic performances of systems as it can not only detect the causes, location, and magnitudes of irreversibility in the system but also highlight the improvement potential of components [2
]. These points are the major difference between exergy analysis and energy analysis. Therefore, performing exergy and energy analyses together for a system can provide an all-round depiction of the system characteristics.
Energy and exergy analyses are widely used and have been applied to many fields such as thermal power plants [3
], hybrid energy systems [8
], polygeneration systems [12
], the manufacturing sector [17
], the food processing industry [20
] and district heating systems [24
] to explore better energy production, transmission, and utilization solutions and determine the links to improvement. Among them, combined heating and power (CHP) is one of the important ways to optimize energy utilization, which has considerable advantages, including high efficiency in use and conversion of primary energy, improving the economy of fuels, creating competitive conditions for thermal power market and reducing greenhouse gas emissions. In 2018, CHP accounted for nearly 80% in the global urban heating market [28
]. In China, CHP is mainly used to meet heating demand. It’s expected that by the end of 2020, the central heating rate of CHP in large and medium-sized cities in North China will be more than 60% [29
]. CHP has been developed to a certain scale, but it is far from meeting the actual demand. Thus, many researchers are spurred to carry out performance analysis and efficiency optimization for CHP and relevant systems to further improve their efficiency and satisfy the growing heating demand. For an existing CHP system, Ahmadi et al. [14
] performed energy, exergy and environmental analyses, and it was revealed that the boilers have the highest exergy destruction, followed by the steam pressure reduction stations and turbo-generators. They recommended that, instead of the boilers, a gas turbine may perform better, and a heat recovery steam generator may be also needed for improvement. Heberle and Brüggemann [30
] were interested in the exergy performance of a geothermal power plant. They found that, compared with a power generation, the exergy efficiency of the plant with a CHP technology could be greatly improved. Kanoglu and Dincer [31
] conducted energy and exergy analyses for many building CHP systems and investigated how the energy and exergy efficiencies changed with the fluctuation of water temperature, steam pressure and some other parameters. The results indicated that systems with diesel and geothermal have an advantage in exergy efficiency over those with steam and gas turbines. Wang and Yang [32
] proposed a hybrid combined cooling, heating and power (CCHP) system based on using biomass and solar energy and analyzed their complementarity to promote the energy efficiency of the system. It was revealed that the biomass subsystem contributed to the primary energy ratio and exergy efficiency of the total system more than the solar subsystem. Al-Sulaiman et al. [33
] evaluated the exergy level of a CCHP system and found that the main exergy destruction occurred in the biomass combustor and the organic Rankine cycle evaporator. The energy and exergy efficiencies were both increased by using CCHP when compared with only using power generation, and as the temperature and pressure varied, the heating-cogeneration and CCHP cases varies less in the exergy analysis than the power generation and cooling-cogeneration ones. Khaljani et al. [34
] performed thermodynamic assessment of a CHP cycle and revealed that the combustion chamber was the main exergy destruction component, and the heat recovery steam generator stand the next, then is gas turbine. They also carried out the exergo-economic analysis for the cycle and found that the overall exergo-economic factor was 10.59%.
It is clear that many efforts and trials have been made in improving and innovating CHP systems by energy, exergy and economic analyses. However, as the feedwater pump, induced draft fan and desulfurization circulating pump are important auxiliary equipment in CHP systems, their power consumption represents a huge burden for heating plants. With stricter rules and regulations about emissions, the power consumption of induced draft fan and desulfurization circulating pump will escalate. Thus, researchers have been further motivated to look for ways to innovate the CHP system. Considering some thermal power plants use the small turbines to drive the feedwater pump and induced draft fan, it may be feasible to develop a CHP system using turbines to drive pumps and induced draft fans for heating plants. In this case, the trigeneration of work, heat, and power can be realized to promote energy utilization and the contradiction between the growing heat demand and surplus power can also be relieved. However, to the authors’ knowledge, few in any studies of such a CHP system have been reported, nor further performance analysis.
In this paper, a CHP system using turbines to drive a feedwater pump, desulfurization circulating pump and induced draft fans was proposed, and the energy, exergy and economic performances were determined based on the system’s operational data to figure out the breakthrough points for energy conservation, which will lay the foundations for future optimization studies.