Energy and Exergy Evaluations of a Combined Heat and Power System with a High Back-Pressure Turbine under Full Operating Conditions

Shifei Zhao,Weishu Wang,Zhihua Ge

doi:10.3390/en13174484

Abstract

High back-pressure technology is a promising method for the waste heat recovery of exhaust steams in combined heat and power systems. In this research, a 300 MW coal-fired subcritical combined heat and power system was selected as the reference system, and modeled in EBSILON professional. Then, energy-based and exergy-based performances of the high back-pressure system and traditional combined heat and power system were compared under full operating conditions. Moreover, a novel exergy-based evaluation method, which considers the energy level of the heating supply, was proposed and applied to evaluate the two systems. Results show that: In design conditions, both the heating capacity and power output of the high back-pressure system were higher than those of the extraction condensing system, which led to 17.67% and 33.21% increments of the gross thermal efficiency and generation efficiency, respectively. Compared with the extraction condensing system, the exergy efficiencies of the high back-pressure system were 7.04–8.21% higher. According to the novel exergy-based evaluation, the exergy efficiencies for the generation of the high back-pressure system and extraction condensing system were 46.48% and 41.22%, respectively. This paper provides references for the thermodynamic performance evaluation of the combined heat and power system.

Highlights

Combined heat and power (CHP) is a well-proven energy-saving technology for fossil energies [1].By the end of 2016, the gross installed capacity of CHP reached 755.2 GW and kept incrementing by 2.8%per year [2]
Some of the leakages are recovered to the gland heater (GH), while the others are sent to the seal system of the low-pressure turbine (LPT)
It can be seen that the energy-based evaluation methods show similar trends, which are that ηg,CHP and ηe,CHP will increase with the increment of the heat load when the live steam remains the same

Summary

Introduction

Combined heat and power (CHP) is a well-proven energy-saving technology for fossil energies [1].By the end of 2016, the gross installed capacity of CHP reached 755.2 GW and kept incrementing by 2.8%per year [2]. In China, CHP is widely used in the coal-fired power plants due to the coal-dominated resource structure [3]. The total installed capacity was 435 GW in 2018, accounting for 39.4% of the coal-fired power plants. In the traditional CHP system, the steam for heating is usually extracted between the medium-pressure turbine and low-pressure turbine. The heat loss of exhaust steam is large and hard to recover due to its low energy level. In the HBP system, with the improvement of the back-pressure of the low-pressure turbine, the temperature of the exhaust steam is largely increased. The exhaust steam can heat the thermal network water, which leads to nearly zero heat loss of the turbine [5]. Due to Energies 2019, 13, 4484; doi:10.3390/en13174484 www.mdpi.com/journal/energies

Methods

Results

Conclusion