Inverse Problem Method to Optimize Cascade Heat Exchange Network in Central Heating System

Inverse Problem Method to Optimize Cascade Heat Exchange Network in Central Heating System

Yin Zhang (Sichuan University, China), Yinping Zhang (Tsinghua University, China) and Xin Wang (Tsinghua University, China)
Copyright: © 2020 |Pages: 21
DOI: 10.4018/IJEOE.2020070105

Abstract

In central heating systems, heat is often transferred from heat source to users by the heat network where several heat exchangers are installed at the heat source, substations, and terminals. In this article, the mathematical optimization model of the cascade heat exchange network with three-stage heat exchangers in a series is established. The aim is to maximize the cold fluid temperature for a given hot fluid temperature and overall heating capacity, the optimal heat exchange area distribution, and the medium fluid flow rates are determined through an inverse problem and variation method. The results show that the heat exchange areas should be distributed equally for each heat exchanger. It also indicates that in order to improve the thermal performance of the whole system, more heat exchange areas should be allocated to the heat exchanger where flow rate difference between two fluids is relatively small.
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Introduction

A heat exchanger is a device used to transfer heat between two or more fluids. They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The corresponding heat exchange networks have been playing an important role in various engineering fields involving energy production, transfer, conversion and terminal utilization (Bergles, 1997). With the rapid industrialization and modernization over the recent two decades, the global total energy consumption has grown by 49% (Zhang, et al., 2015). Therein, buildings account for more than 30% of the total energy consumption and the percentage keeps increasing due to the rapid urbanization (Luis et al., 2008; Yilmaz et al., 2016). During the same period, building energy consumption in China has increased with an average annual rate of over 10% (Cai et al., 2009).

Building energy usage mainly derives from the so-called heating ventilation and air conditioning (HVAC) systems (Mihai & Zmeureanu, 2017). According to the latest statistics analysis, urban heating in northern China accounts for about 40% of total building energy consumption (Zhang et al., 2015). Hence, improving the energy efficiency of heating systems is of high significance in building energy saving (Roy & Ghosh, 2017). Due to high energy and economic efficiency, low pollution emissions and high energy supply safety and reliability, central heating constitutes the main system form for space heating in northern China (Mago et al. 2009). In particular, combined heating and power (CHP) systems, also known as co-generation systems, show great potentials of applications in central heating systems, which has caused more and more attentions during recent years (Barisal et al., 2017). Cascade heat exchange networks containing several heat exchangers in series are also widely used in central heating systems to deliver heat from source to terminal users, since thermal power plants are often far away from residential buildings (Li et al., 2011). Generally speaking, heat is often transferred from heat source to users by two water circulating networks (i.e., primary and secondary network), and three kinds of heat exchangers installed at heat source, substations and terminal users respectively (Li et al., 2011).

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