J. Cent. South Univ. (2017) 24: 1663-1670

DOI: 10.1007/s11771-017-3572-z

Exergy analysis of operation process of district heating systems

JIE Peng-fei(介鹏飞)¹, KONG Xiang-fei(孔祥飞)², LI Jing(李静)³

1. School of Mechanical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China;

2. School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300401, China;

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: The nature of the exergy consumption of district heating (DH) systems can not be explained clearly using the first law of thermodynamics. Exergy analysis method was used. A case study based on a DH system in Inner Mongolia, China, was carried out. The impact of operating parameters and design parameters on the energy quality of circulating water and exergy losses of DH systems during heat distribution was revealed. Results show that the energy quality of circulating water and exergy losses of DH systems during heat distribution could be reduced by decreasing the indoor temperature or increasing radiator areas. Compared with other factors, the outdoor temperature and indoor temperature have a greater impact on the energy quality of circulating water, exergy losses of circulating water, and total exergy losses during heat distribution. When the outdoor temperature varied by 10.00%, the average variation rates of such parameters were 85.12%, 90.02%, and 64.60%, respectively. When the outdoor temperature was 273.00 K and indoor temperature varied by 50.00%, the average variation rates of such parameters were 83.88%, 99.34% and 32.87%, respectively. It can be observed that the energy quality and exergy losses of DH systems can be reduced in the operation process.

Key words: energy quality coefficient; exergy loss rate; design parameters; operating parameters; energy conservation measures

1 Introduction

District heating (DH) systems have been widely used in many countries due to their advantages. Firstly, compared with decentralized heating systems, more energy can be saved because of the higher energy efficiency of DH systems. Secondly, DH systems can improve thermal comfort. Thirdly, DH systems can improve labor productivity. Fourthly, plenty of land can be saved by using DH systems. Moreover, some renewable energy resources, such as soar energy, biomass fuels, geothermal energy, and wind energy can also be used during heat production [1-4]. This significantly reduces the fossil fuel consumption and greenhouse gas emission. Therefore, heating areas and heating consumption are both growing rapidly. In many countries, the energy used in space heating accounted for about 40% of the total energy consumed in the residential sector [5]. In European Union countries, the average share of space heating in the residential sector was 57%. Especially, in Greece, the energy consumption for heating accounted for 60.9% of that in the residential sector [6, 7]. In 2012, the areas of buildings connected to DH systems were more than 9.54 m². The energy consumption for DH in towns in the north of China was about 171 million tons of standard coal which accounted for 24.7% of the total building energy consumption [8, 9].

Various studies were conducted in order to reduce the energy consumption for DH. Energy analysis method and exergy analysis method were mainly used [10-14]. Energy analysis method is based on the first law of thermodynamics. Using the energy analysis method, the conversion, losses, and utilization of energy can be revealed from the quantity perspective. Thus, only the energy quantity is considered in the energy analysis method. According to the energy analysis method, some energy conservation measures which can be taken are as follows:

1) The air tightness and thermal performance of the building envelope can be improved to reduce the heat demand of buildings.

2) The thermal performance of pipes can be improved to reduce heat losses during heat distribution.

The energy efficiency of heat sources should be improved to reduce the energy consumption for heat production.

3) However, the exergy consumption can not be revealed profoundly using the energy analysis method [15-17]. With the development of DH systems, more attention was paid to the reduction of the available work during energy consumption process [18-21]. Therefore, the exergy analysis method was applied widely. The exergy analysis method is based on the first law of thermodynamics and the second law of thermodynamics. Both the quantity and quality of the energy used in DH systems are analyzed in the exergy analysis method. The conversion, losses and utilization of the energy can be revealed using the exergy analysis method [22]. It is necessary to use this method to analyze how much exergy is consumed in each stage of DH systems. The exergy analysis method also enables us to reveal the reason for the exergy losses. Thus, we need to find effective method to reduce the exergy losses, which create conditions for using various low-temperature heat sources.

Exergy analysis of DH systems was carried out in some previous studies. DOVJAK et al [23] used the energy analysis method and exergy analysis method to analyze the energy use in space heating in Slovenian. The exergy consumption rate from boiler to building envelope was calculated to analyze how much exergy is consumed in each stage. They suggested further improving the thermal performance of the building envelope together with boiler efficiency to effectively save energy and obtain better thermal comfort. LI and SVENDSEN [24] studied the exergy losses of a low-temperature DH system with reduced supply water temperature and return water temperature. Results showed that the exergy efficiency increased with the decrease of the supply water temperature and return water temperature. It was found that the quality match between heat demand and heat supply should increase. YILDIZ and [11] conducted the exergy analysis of the whole process of space heating in an office. Three kinds of heat sources were considered in the analysis. It was found that the largest exergy losses occurred in the combustion process when boilers were used as heat source. et al [13] analyzed the exergy losses occurring in the DH network of a university campus. Results showed that supply water temperature and return water temperature were the most important factor affecting the exergy losses during heat distribution. WANG [25] used equivalent electricity calculation method to analyze the energy quality of each stage of DH systems, including heat sources, heat exchange stations, radiators, and building envelopes. Further energy conservation measures were proposed based on the equivalent electricity of each part. On the other hand, some previous studies focused on the exergy analysis of geothermal DH systems. OZGENER [26] used exergy flow diagrams to illustrate the exergy destructions in two local Turkish geothermal DH systems. Results showed that exergy destructions occur mainly due to pump losses, heat exchanger losses, and hot water distribution losses. [27] conducted exergy analysis of a geothermal DH system in Afyon, Turkey. Exergy flow diagram showed that the largest exergy losses occurred in heat exchangers and then in reinjection wells.

There remain two main problems in previous study. Firstly, only design cases or typical cases were considered in the exergy analysis of DH systems. Few studies focused on the whole operation process. Secondly, design parameters were often used instead of operating parameters in the exergy analysis. Thus previous studies were mainly based on design parameters. It should be noted that operating parameters differ from design parameters during most of the heating season. Therefore, it is very necessary to analyze the relationship between operating parameters and exergy losses of DH systems. Moreover, the impact of variations of operating parameters on the exergy losses of DH systems should also be clearly revealed.

The present study differs from previous study due to the fact that the exergy analysis of the operation process of DH system was conducted. A DH system in Inner Mongolia, China, was used as the basis of the case study. The impact of operating parameters and design parameters on the quantity and quality of the energy required for the DH system during heat distribution was revealed. Such parameters included mass flow rate (MFR), outdoor temperature, indoor temperature, and radiator area. Finally, effective measures were suggested to reduce the energy quality of circulating water and exergy losses of DH systems during heat distribution.

2 Methodology

According to the energy conservation law, the total amount of energy remains unchanged in the process of conversion. Various forms of energy are different in quality. Even though two kinds of energy have the same form, the quality may be different due to the difference of the specific parameters. Exergy is the maximum available work which can be obtained from an energy source. It can be regarded as the standard to evaluate the energy quality.

2.1 Energy quality coefficient

The energy quality coefficient can be defined as [22]

                                     (1)

where λ is the energy quality coefficient; E_x is the amount of exergy (kJ); E is the amount of energy (kJ).

The energy quality coefficient refers to the amount of exergy contained in per unit amount of energy. As for electricity, it can be completely converted into available work, so its energy quality coefficient is 1. The energy quality coefficient of other forms of energy should be calculated according to the maximum available work obtained from the energy source. According to Ref. [22], the energy quality coefficient of the circulating water of DH systems can be expressed as

                         (2)

where λ_w is the energy quality coefficient of circulating water, T₀ is the environmental temperature (K); T_g is the supply water temperature (K); T_h is the return water temperature (K).

2.2 Exergy loss rate

Exergy is consumed in DH systems. Exergy losses per one second is called exergy loss rate. It can be defined as [28, 29]

                                   (3)

where e_x is the exergy loss rate (kW).

The sketch of exergy utilization for heat distribution is shown in Fig. 1. As can be seen in Fig. 1, e_xt is the total exergy loss rate during heat distribution, e_xw is the exergy loss rate of circulating water, and e_xe is the exergy loss rate of electricity. As for DH systems, the total exergy loss rate during heat distribution mainly includes that of circulating water and electricity. Also, the more the exergy losses which maintain the stable operation of DH systems, the more the available work required for DH systems. Therefore, the exergy loss rate was used to evaluate the impact of the operating parameters and design parameters on the energy quality and available work required for DH systems during heat distribution.

Fig. 1 Sketch of exergy utilization for heat distribution

3 Results

A DH system located in Inner Mongolia, China was used as the basis of a case study to analyze exergy losses in the operation process. Only the heat demand caused by space heating was considered in this DH system. Hot water is used to transfer heat from heat sources to end users through radiators installed in rooms. The details of this DH system are as follows: Its heating area is 15137 m². The design supply water temperature and return water temperature (T_g/T_h) is 358.00/293.00 K. The design heating load index is 45.00 W/m², so the design heating load is 681.17 kW. The design radiator areas are 1547.51 m². The design MFR is 23.43 t/h. The design indoor temperature for space heating is 291.00 K. The design outdoor temperature for space heating is 256.20 K [30].

The supply water temperature, return water temperature, MFR, and other operating parameters can be controlled and measured using data monitoring devices. The real-time operating data is shown in Table 1. As can be seen in Table1, the actual MFR is 3.15 times of the design value, which results in the reduction of the difference between supply water temperature and return water temperature. This is very common in DH systems [31]. The operating mode of the DH systems is different from the design mode. The impact of operating parameters and design parameters on the energy quality and available work required for the DH system during heat distribution was studied below.

Table 1 Real-time operating parameters of selected DH system

3.1 MFR

In DH systems, the indoor temperature is always uneven due to the hydraulic imbalance. Increasing the MFR can effectively relieve the impact of hydraulic imbalance on indoor temperature. Also, the variations of MFR have an impact on the exergy losses of the energy required for the stable operation of the selected DH system. The relative MFR ratio is defined as

                                   (4)

where is the relative MFR ratio; G_op is the operating MFR (t/h); G_de is the design MFR (t/h).

The indoor temperature and radiator areas were assumed to be the design value. The relative MFR ratio () varied from 1.00 to 3.00. Then, the corresponding variations of e_xt, e_xw and λ_w versus different relative MFR ratio are described in Figs. 2-4. The outdoor temperature is 256.20 K, 263.00 K, and 273.00 K, respectively. As can be seen in Figs. 2 to 4, λ_w and e_xw both remain nearly unchanged when the relative MFR ratio increases. This relates to the fact that the supply water temperature decreases with the increase of MFR, while the return temperature increases in this process. The average water temperature remains unchanged. But the increase of MFR leads to the increase of the pump electrical energy consumption, which results in the increase of the total exergy loss rate (e_xt) of the energy required for the stable operation of the DH system.

.

Fig. 2 Impact of relative MFR ratio on exergy loss of DH system (Outdoor temperature is 256.20 K)

Fig. 3 Impact of relative MFR ratio on exergy loss of DH system (Outdoor temperature is 263.00 K)

Fig. 4 Impact of relative MFR ratio on exergy loss of DH system (Outdoor temperature is 273.00 K)

3.2 Outdoor temperature

The indoor temperature, radiator areas, and MFR were assumed to be the design value. So the pump electrical energy consumption also remained unchanged. The outdoor temperature increased from 256.20 K to 282.20 K. Then, the corresponding variations of e_xt, e_xw and λ_w versus different outdoor temperatures are described in Fig. 5. As can be seen in Fig. 5, λ_w decreases as the outdoor temperature increases. This is because when the MFR, indoor temperature, and radiator areas are constant, the supply water temperature, return water temperature, and average water temperature all decrease with the increase of the outdoor temperature. The heat demand also decreases with the increase of the outdoor temperature. Both effects of head demand and energy quality coefficient of circulating water lead to the decrease of e_xw and e_xt.

Fig. 5 Impact of outdoor temperature on exergy loss of DH system

3.3 Indoor temperature

If the heat supply is greater than the heat demand, the actual indoor temperature will be higher than the required indoor temperature. This is very common in some DH systems which are lack of regulating devices. The relative indoor temperature ratio is defined as

                                    (5)

where is the relative indoor temperature ratio; t_nc is the actual indoor temperature (K); t_nd is the design indoor temperature (K).

The radiator areas were assumed to be the design value. And the MFR was assumed to be the operating value (see Table 1). The relative indoor temperature ratio increased from 1.00 to 1.50. Then, the corresponding variations of e_xt, e_xw and λ_w versus different relative indoor temperature ratios are described in Figs. 6-8. The outdoor temperature is 256.20 K, 263.00 K, and 273.00 K respectively. As can be seen in Figs. 6-8, λ_w increases with the increase of the indoor temperature. This is because when the radiator areas, outdoor temperature, and MFR are constant, the supply water temperature, return water temperature, and average water temperature all increase with the increase of the indoor temperature. The heat demand also increases with the increase of the indoor temperature. Both effects of heat demand and energy quality coefficient of circulating water lead to the increase of e_xw and e_xt.

Fig. 6 Impact of indoor temperature on exergy losses of DH system (Outdoor temperature is 256.20 K)

Fig. 7 Impact of indoor temperature on exergy losses of DH system (Outdoor temperature is 263.00 K)

Fig. 8 Impact of indoor temperature on exergy losses of DH system (Outdoor temperature is 273.00 K)

3.4 Radiator areas

Radiator areas have an important impact on the exergy losses of energy required for the DH system. The relative radiator area ratio is defined as

                                   (6)

where is the relative radiator area ratio; F_rc is the actual radiator area (m²); F_rd is the design radiator area (m²).

The indoor temperature was assumed to be the design value. And the MFR was assumed to be the operating value (see Table 1). The relative radiator area ratioincreased from 1.00 to 3.00. Then, the corresponding variations of e_xt, e_xw and λ_w versus different relative radiator area ratios are described in Figs. 9-11. As can be seen in Figs. 9-11, λ_w decreases with the increase of radiator areas. This is because when the indoor temperature, outdoor temperature, and MFR are constant, the supply water temperature and return water temperature both decrease with the increase of radiator areas. The decrease of λ_w leads to the decrease of e_xw and e_xt. The decreasing speed of λ_w, e_xw and e_xt becomes slowly with the increase of radiator areas. When radiator areas increase to a certain extent, the decrease of the exergy losses caused by further increase of radiator areas is not obvious.

Fig. 9 Impact of radiator areas on exergy loss of DH system (Outdoor temperature is 256.20 K)

3.5 Sensitivity analysis

Fig. 10 Impact of radiator areas on exergy loss of DH system (Outdoor temperature is 263.00 K)

Fig. 11 Impact of radiator areas on exergy loss of DH system (Outdoor temperature is 273.00 K)

It can be observed that the energy quality and available work required for DH systems during heat distribution are influenced by some related parameters, such as MFR, outdoor temperature, indoor temperature, and radiator areas. Sensitivity analysis method was widely used to study the impact of sensitivity factors on research objectives [32-34]. One-factor sensitivity analysis method was used to study the impact of such parameters on the energy quality and available work required for the selected DH system. In the process of sensitivity analysis, one factor changed while other factors remain constant. Then e_xt, e_xw and λ_w changed with the variations of the sensitivity factor. When the sensitivity factor varies by 50%, the average variation rate of e_xt, e_xw and λ_w are shown in Table 2. Similarly, when the outdoor temperature varied by 10.00%, the average variation rates of e_xt, e_xw and λ_w were 85.12%, 90.02%, and 64.60% respectively. It can be seen that compared with the MFR and radiator areas, the outdoor temperature and indoor temperature have a greater impact on e_xt, e_xw and λ_w. The reason for the above relates to the fact that the supply water temperature, return water temperature and average water temperature all change significantly with the variations of the outdoor temperature and indoor temperature. Especially for the indoor temperature, when the outdoor temperature is 273.00 K, the average variation rate of of e_xt, e_xw and λ_w were 83.88%, 99.34%, and 32.87%, respectively.

Table 2 Sensitivity analysis results

The energy and exergy consumption both decrease with the reduction of the indoor temperature. Therefore, effective measures should be taken to ensure that the actual indoor temperature is controlled based on the required value. The quantity and quality match between the heat demand and heat supply should be increased.

3.6 Comparison with other results

In previous study, the design case or typical case of DH systems were considered in the exergy analysis. However, the exergy analysis of the operation process of DH systems was conducted. In Refs. [23, 25], results showed that the exergy consumption rate of boilers was the largest in the four stages of space heating. And it was suggested that low-temperature heat sources should be used. In this work, the impact of MFR, outdoor temperature, indoor temperature, and radiator areas on the energy quality and available work required for DH systems during heat distribution was investigated. Results showed that the outdoor temperature and indoor temperature had a greater impact on e_xt, e_xw and λ_w. It was suggested that the indoor temperature should be controlled based on the required value. Avoiding overheating is beneficial for reducing the energy and exergy consumption of DH systems. Also, results showed that the exergy losses can be reduced by increasing radiator areas. Suggestions were given to increase the quantity and quality match between heat supply and heat demand, which was similar to that in Ref. [24].

4 Conclusions

The impact of MFR, outdoor temperature, indoor temperature, and radiator areas on the energy quality of circulating water and exergy losses of the selected DH system during heat distribution was revealed. Firstly, results showed that MFR had no impact on the energy quality and exergy losses of the circulating water in DH systems. But the pump electrical energy consumption increased with the increase of MFR. As a result, the total exergy losses during heat distribution increased in this process. Also, the e_xt, e_xw and λ_w all decreased with the increase of outdoor temperature or radiator areas. On the contrary, when indoor temperature increased, such parameters all increased.

Sensitivity analysis showed that compared with MFR and radiator areas, outdoor temperature and indoor temperature had a greater impact on the e_xt, e_xw and λ_w. When the outdoor temperature varied by 10.00%, the average variation rate, of e_xt, e_xw and λ_w were 85.12%, 90.02% and 64.60%, respectively. When the outdoor temperature was 273.00 K and indoor temperature varied by 50.00%, the average variation rates of e_xt, e_xw and λ_w were 83.88%, 99.34% and 32.87%, respectively.

It was found that the energy quality of circulating water and exergy losses of DH systems during heat distribution could be reduced by decreasing the indoor temperature or increasing radiator areas. Also, when the outdoor temperature increases, the energy quality and exergy losses decrease. Furthermore, it was suggested that various low-temperature heat sources could be used in such cases.

In the next study, the exergy analysis of heat sources in the process of operation should be conducted. Moreover, more effective measures should be taken to further increase the quantity and quality match between the heat demand and heat supply in the future.

Acknowledgements

The research presented was also supported by “Building Energy Research Center of Tsinghua University” and “Beijing Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering”, which are hereby greatly acknowledged. We would like to thank Dr. Craig Larson for his useful correction of the English language use. We acknowledge the reviewers for their help. We also acknowledge the authors of the references.

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(Edited by DENG Lü-xiang)

Cite this article as: JIE Peng-fei, KONG Xiang-fei, LI Jing. Exergy analysis of operation process of district heating systems [J]. Journal of Central South University, 2017, 24(7): 1663-1670. DOI: 10.1007/s11771-017-3572-z.

Foundation item: Project(51408184) supported by the National Natural Science Foundation of China; Project (15JCQNJC07800) supported by Tianjin Natural Science Foundation, China; Project (YQ2014005) supported by Excellent Young Foundation of Hebei Educational Committee, China

Received date: 2016-07-25; Accepted date: 2016-10-17

Corresponding author: KONG Xiang-fei, PhD, Lecture; Tel: +86-22-60435787; E-mail: xfkong@hebut.edu.cn