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Comparison and analysis of thermal efficiency and exergy efficiency in energy systems by case study
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
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Energy Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online atatwww.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy (2018) 000–000 161–168 EnergyProcedia Procedia153 00 (2017) www.elsevier.com/locate/procedia
5th International Conference on Energy and Environment Research, ICEER 2018 5th International Conference on Energy and Environment Research, ICEER 2018
a a
Comparison and analysis of thermal efficiency and exergy Comparison and analysis of thermal efficiency and exergy efficiency in energy systems by Heating case study The 15th International Symposium on District and Cooling efficiency in energy systems by case study a Youyuan Shaoaa, Hanmin Xiaoaa, Baiman Chenaa,the Simin Huang , Frank G.F. Qina,a,* Assessing the feasibility of using heat demand-outdoor a Youyuan Shao , Hanmin Xiao , Baiman Chen , Simin Huang , Frank G.F. Qin * Guangdong Provincial Key Laboratory of Distributed Energy Systems, School of Chemical Engineering and Enenrgy Technology, Dongguan temperature functionUnversity for Energy a long-term district heat demand forecast Guangdong Provincial Key Laboratory of Distributed Systems,Dongguan School of 523808,China Chemical Engineering and Enenrgy Technology, Dongguan of Technology, Unversity of Technology, Dongguan 523808,China
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Abstract Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques Environnement - IMTinherent Atlantique, 4 rue AlfredinKastler, Nantes, This paper analyzes thermal efficiency, exergy et efficiency, and their relationship energy44300 systems. TwoFrance typical thermal This paper analyzes thermal efficiency, exergy efficiency, and theirItinherent relationship in efficiency energy systems. Two equipment, steam boiler and steam turbine, are used for case study. shows that the exergy may not be typical ideally thermal high in steam boiler and have steama turbine, are used for caseItstudy. It showstothat the exergy efficiency may not be high in aequipment, process even though it may high thermal efficiency. is insufficient evaluate a system’s energy saving byideally using thermal a process even it maya have a high thermal efficiency. It is insufficient to evaluate a system’s energy saving byenergies, using thermal efficiency only.though Therefore comprehensive analysis of thermaland exergyefficiency should be adopted, and with Abstract efficiency only.should Therefore a comprehensive analysis andenergy exergyefficiency should system be adopted, and energies, with different grade, be utilized in cascade to achieveofanthermaloptimized saving for a thermal or equipment. different grade, should be utilized in cascade to achieve optimized energyindex saving for a thermal system or equipment. Keywords: thermal efficiency; exergy efficiency; energyan system; evaluation District heating areexergy commonly addressed in system; the literature as one of the most effective solutions for decreasing the Keywords: thermalnetworks efficiency; efficiency; energy evaluation index greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat © 2018 The Authors. Published by Elsevier Ltd. ©sales. 2018 Due The Authors. Authors. Published by Elsevier Elsevier Ltd. and building renovation policies, heat demand in the future could decrease, to the changed climate conditions © 2018 The Published by Ltd. This isisan access article under the This anopen open access article under theCC CCBY-NC-ND BY-NC-NDlicense license(https://creativecommons.org/licenses/by-nc-nd/4.0/) (https://creativecommons.org/licenses/by-nc-nd/4.0/) prolonging the investment return period. This is an and openpeer-review access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection under responsibility of the scientific committee of International Conference on Energy and Selection and peer-review under responsibility of the scientific committee of the the–5th 5th International Conference on Energy and The main scope of this paper is to assess the feasibility of using the heat demand outdoor temperature function on for Energy heat demand Selection and peer-review under responsibility of the scientific committee of the 5th International Conference and Environment Research, ICEER 2018. Environment Research, ICEER 2018. forecast. The district ICEER of Alvalade, Environment Research, 2018. located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Keywords: thermal efficiency; exergy efficiency; energy system; evaluation index renovation scenarios wereexergy developed (shallow, deep). Keywords: thermal efficiency; efficiency; energy intermediate, system; evaluation indexTo estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications Nomenclature (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Nomenclature scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). b--Standard coal consumption for power generation (kg/(kW·h)); The value ofcoal slope coefficient increased ongeneration average within the range of 3.8% up to 8% per decade, that corresponds to the b--Standard consumption for power (kg/(kW·h)); COP of performance of a LiBr refrigerator; -- Coefficient decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and COPe----Coefficient Coefficient ofperformance performanceofofana LiBr refrigerator; COP electrical refrigerator; renovation scenariosofconsidered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the COP e-- Coefficient of performance of an electrical refrigerator; coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.
*©Corresponding author. Published Tel.: +86 0769 22862619;Ltd. 2017 The Authors. by Elsevier * Corresponding author. Tel.: +86 0769 22862619; E-mail address: [email protected] Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and E-mail address: [email protected]
Cooling.
1 1
1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2018 Thearticle Authors. Published by Elsevier Ltd. This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is anand open access article under the CC BY-NC-ND licensecommittee (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the scientific of the 5th International Conference on Energy and Environment Selection and peer-review Research, ICEER 2018. under responsibility of the scientific committee of the 5th International Conference on Energy and Environment Research, ICEER 2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 5th International Conference on Energy and Environment Research, ICEER 2018. 10.1016/j.egypro.2018.10.081
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Youyuan Shao al. / Energy Procedia 153 000–000 (2018) 161–168 Youyuan Shao et al. / et Energy Procedia 00 (2018)
CPf-- Specific heat of natural gas [kcal/(Nm3.0C]; CPair-- Specific heat of the air [kcal/(Nm3.0C]; ebc-- Energy consumption per unit of product during the studied period (tce); ejc-- Energy consumption per unit of product during the base period (tce); Ex – Proess exergy (kJ); EinF -- Total exergy of fuel consumed by a CCHP system (kJ); EoutC-- Cooling exergy output from a CCHP system (kJ); EoutH-- Heating exergy output from a CCHP system (kJ); EoutP -- Work exergy output from a CCHP system (kJ); F-- Biomass gas flow (kg/s); hw-- Water enthalpy (kJ); hS-- Steam enthalpy (kJ); HL-- Low natural gas calorific value (MJ/Nm3); H-- High natural gas calorific value (MJ/Nm3); H0 -- Enthalpy in the ambient condition (kJ); Pe-- Power generation capacity (kW); P0-- Environmental pressure (kPa); Qout --output energy; Qin --input energy; Qc-- Refrigeration capacity (kW); Qh -- Heating capacity (kW); S -- Entropy in the working condition (kJ/K); S0 -- Entropy in the ambient condition (kJ/K); T0 -- Environmental temperature (K); Vair-- Air flow (kg/s); Vg-- Smoke amount (Nm3/Nm3fuel); W-- work input to the heat pump; VG -- Flue gas flow (kg/s); VH2O -- water flow (kg/s); C -- Coefficient of cooling exergy; F -- Coefficient of fuel exergy; H -- Coefficient of heat exergy; P -- Coefficient of work exergy;
-- thermal efficiency;
b --
Boiler efficiency; Power generation efficiency of power plant; e -- Efficiency of gas turbine; e x -- Exergy efficiency; h -- Efficiency of heat recovery. ce --
1. Introduction For a long time, people have been exploring better methods for energy system analysis [1-4]. As we know, the evaluation index of energy analysis is mainly thermal efficiency, which is based on the first law of thermodynamics, known as energy conservation, so it is also known as the First law efficiency[5-7]. It correlates the quantity between energies, but it does not show the quality changes in the process of energy transfer and/or conversion. Therefore, using the first law efficiency (i.e. the thermal efficiency) only is inadequate for analyzing and assessing the thermal process of an equipment. The evaluation index of exergy analysis is known as exergy efficiency, which includes consideration of both the first- and the second-law of thermodynamics, and known as the second law efficiency. It not only correlates the quantity of energy, but also indicates the quality and effective utilization of energy [8-11]. It is more accurate,
Youyuan Shao et al. / Energy Procedia 153 (2018) 161–168 Youyuan Shao et al. / Energy Procedia 00 (2018) 000–000
163 3
more appropriate and more comprehensive in describing the perfection of the process or equipment. Exergy efficiency is also used to measure the degree of irreversibility of processes. 2. Thermal efficiency [12~15] For a given process or device, the ratio of effective output energy (Qout) to input energy (Qin) is known as the thermal efficiency, which is dimensionless and is expressed in percentage.
=
Qout 100% Qin
(1)
In energy systems, there are specific definitions of thermal efficiency for different facilities, such as boiler thermal efficiency, heat pump thermal efficiency, power generation efficiency, coal consumption rate of power generation, total heat efficiency of cogeneration, as shown in Table 1. Table 1. Thermal Efficiency Definitions Employed in Energy Systems Items Boiler efficiency
Heat pump efficiency
=
(1)Refrigeration
COP =
(2)Heating
=
Power generation efficiency
Standard coal consumption for power generation (kg/(kW·h))
Expression
Qh 100% F HL Qc 100% W
Qh 100% W 3600 Pe = F HL
(1)Direct energy balance
b=
(2)Indirect energy balance
b=
Bs Pe 3600 H L
Qh + Qc + Pe Total thermal efficiency of CCHP = 100% system F HL
The definitions listed in Table 1 indicate that the general thermal efficiency and the specific thermal efficiency definitions are all based on the principle of energy conservation, which reflects the quantities among energies, but does not account for changes in the quality or grade of energy during the processes of energy transfer and conversion. Therefore, using only the first law efficiency to evaluate the energy efficiency of a CCHP system is often inconsistent with actual conditions, and may even be contradictory to actual conditions, such as in the calculation of the energy utilization of afterburning and multi-energy inputs, and for CCHP multivariate flow systems. Therefore, thermal efficiency is not sufficient to provide a comprehensive and systematic evaluation of the energy efficiency of distributed CCHP systems. 3. Exergy efficiency ( ex ) As we known energy has dual attributes of quantity and quality. Exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with the surrounding. In other words, exergy is the
Youyuan Shao et al. / Energy Procedia 153 (2018) 161–168 Youyuan Shao et al. / Energy Procedia 00 (2018) 000–000
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energy that is available to be used. For a steady-state open system with ambient temperature To (K), exergy in the working condition (kJ) is defined as the sum of the enthalpy difference and the entropy difference in the working and standard conditions:
( H − H 0 ) − T0 ( S − S0 )
Ex =
(2)
The characteristics of exergy present definite advantages for expressing the inequivalence of heat and power in the grade. The concept of exergy lays a foundation for combining the first and second laws of thermodynamics, and presents a new direction for determining the effective use of heat energy and energy-saving technologies. Exergy efficiency is the main indicator employed in exergy analysis, and represents the exergy balance, which is also denoted as the second law efficiency. The exergy efficiency is defined as the ratio of the total exergy output (Eout) from a system to the total exergy input (Ein), as follows. ex =
Eout Eout C + Eout H + Eout P = Ein EinF
Q + H QoutH + P Pout = C outC F QinF
(3)
From Eq. (3) reveals the inequivalence of work and energy in thermodynamics. The exergy efficiency is thermodynamically more accurate than efficiency indicators based solely on the first law of thermodynamics because it accounts for the thermodynamic inequivalence of the energy of cooling or heating and work. In addition, exergy efficiency takes into account the equivalence ratio of the energy of cooling or heating to work, and proportionally reduces the effective cooling or heating energy output. Therefore, it can express the degree of efficiency in processes and equipment more accurately, more appropriately, and more comprehensively than efficiency indicators based solely on the first law of thermodynamics. Additionally, we note that the heat generation to electricity generation ratio and the ESR can only reflect the energy changes of the system. However, adopting the exergy efficiency for system evaluation provides appropriate evaluation results irrespective of the afterburning change of system. 4. Case analysis [16] Energy conservation analysis and exergy analysis are used to calculate and analyze the thermal efficiency and exergy efficiency of two thermal equipment in this section, and the results are compared. Take the natural gas boiler for steam production as an example. Based on the different steam quality of the boiler, two types of conditions are adopted: Boiler I: Output steam pressure: 3.5 MPa (a), steam temperature: 243C, superheated high pressure steam. Boiler II: Output steam pressure: 0.3 MPa (a), steam temperature: 133.5C, saturated low pressure steam.
Figure 1 Energy flow diaogram in and out of the boiler
Youyuan Shao et al. / Energy Procedia 153 (2018) 161–168 Youyuan Shao et al. / Energy Procedia 00 (2018) 000–000
4.1. Boiler I The materials and energy entering and leaving the boiler are shown in Figure 1. The parameters of natural gas boilers are shown in Table 2. Table 2. Boiler Parts Parameters Name
Parameter
Environment temperature
t0 = 250 C , T0 = 298.150 C
Water At 250C and 1atm
H 0 = 25.1kcal / kg
Water enthalpy
= S0 Water entropy
0.0876kcal / (kg K )
tw = 1050 C ， pw = 3.5Mpa hw = 105.7 kcal / kg Water enthalpy = Sw 0.3249kcal / ( kg K ) Water entropy 0
At 105 C and 3.5Mpa(a)
Water flow
VH 2O = 14.6kg / ( Nm3fuel)
Steam
tV = 2430 C ， PV = 3.5Mpa （a）
At 2430C and 3.5Mpa(a)
hs = 670.2kcal / kg
Steam enthalpy
Ss Steam entropy=
1.465kcal / (kg K )
Fuel Low natural gas calorific value High natural gas calorific value Natural gas supply temperature
H L = 9000kcal / Nm3
H H = 9990kcal / Nm3 t f = 650 C
= Cpf Specific heat of natural gas
1.5kcal / ( Nm3 0 C )
Air Supply temperature Required air volume
t0 = 25 0C
Vair = 13Nm3 / ( Nm3fuel) （1atm）
= C pair Specific heat of the air
0.31kcal / ( Nm3 0 C )
Smoke exhaust 0
Exhaust gas temperature t0 = 25 C = CPfuel 0.27 kcal / ( Nm3 0 C) Specific heat of smoke（1atm） Smoke amount
Vg = 14.5 Nm3 / ( Nm3fuel)
Calculation and analysis of boiler input side energy is shown in Table 3. Table 3. Input energy of boilers Content
Thermal efficiency method
Exergy efficiency method
①Heat of combustion
H L = 9000
Chemical exergy of fuel
② Sensible heat of the fuel
Q f = C Pf t f
Exergy input from the fuel
= 97.5
E f = 0.95 H H = 9490.5
T E= CPf ( t f −t0 ) − T0 ln f fs T0 = 3.5
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Youyuan al. / Energy Procedia 153000–000 (2018) 161–168 Youyuan Shao etShao al. / et Energy Procedia 00 (2018)
166 6
③ Sensible heat of the air
Exergy input from the air
Qair = Vair C Pair t air
Eair = 0
= 100.75
H H 2O = VH 2O hH 2O
④ Sensible heat of supply water
Exergy input from supply water
(
Tatal
)
EH 2O= VH 2O hH 2o − h0 − T0 ( Sw − S0 )
= 1543.22
= 144
QBinlet = 10741.47
EBinlet = 9638.0
Calculation of the Energy on the Output Side of the Boiler is shown in Figure 1. Table 4. Calculation of the Energy on the Output Side of the Boiler Content
Energy conservation method
⑤Enthalpy value of generating steam
H s = VH 2O hs
⑥Enthalpy of the smoke
H g = Vg CPg t g
⑦Loss (incomplete combustion, irreversible heat transfer, etc.)
Q = QBinlet − QBoutlet Loss
= 9784.92
= 783
= 173.55
Exergy efficiency method
Exergy of generating steam
= Es VH 2O ( hs − h0 ) − T0 ( S s − S 0 ) = 2930.76
Exergy of the smoke
T = Eg Vg CPg ( t g − t0 ) − T0 ln g T0 = 120.66 Loss of exergy
= EH 2O
E
i ,inlet
− Ei ,outlet
= 6586.58
The thermal efficiency and exergy efficiency of the boiler can be calculated from the calculation results of Table 3 and table 4. The difference between the two is more than 2.1 times. 4.2. Boiler II Now, the calculation is still based on the input of 1Nm3 natural gas to the boiler. In order to simplify the calculation process, the thermal budget of the system is no longer calculated, but its efficiency is directly calculated. The parameters of natural gas boiler are as follows: Ambient temperature, air, fuel, and smoke exhaust: The parameters are the same as those of the above natural gas boiler I. Water supply: 0 Temperature: tw = 105 C , Pressure: pw = 0.3Mpa (a) , Flow: Steam generation:
t = 132.80 C
p = 0.3Mp (a)
a , Pressure g , Flow Temperature g The two kinds of efficiency calculation results of the boiler are shown in Table 5.
。
Table 5. Efficiency of Boiler Content Primary energy input (payment side) Produced steam (revenue side)
Energy conservation method Low calorific value H L = 9000
H s = QH 2O
(hS − hw )
= 809.4
Exergy efficiency method Chemical exergy of fuel
E f = 0.95 H H = 9490.5
Eb =Exergy for stream – exergy for supply water =2480.5
Youyuan Shao et al. / Energy Procedia 153 (2018) 161–168 Youyuan Shao et al. / Energy Procedia 00 (2018) 000–000
Efficiency
= h, B Q= 90.0% g / HL
167 7
= E , A E= 26.1% b / Ef
From the data shown in the table, it can be seen that the exergy rates are very different in the case of similar thermal efficiencies of the boilers. The reason is that the boiler has a high ability to do work so that the exergy rate is higher when the boiler produce high-temperature and high-pressure superheated steam, while when the boiler produces lowpressure saturated steam, the ability to continue to do work is poor, so the exergy rate of is low. 5. Remark and conclusions The indexes of thermodynamic performance commonly used at the present for evaluating the energy efficiency of distributed CCHP systems mainly include thermal efficiency, primary energy ratio (PER), and energy saving rate (ESR) [17]. The shortcoming of this index system is that it ignores the grade of energy. The thermal efficiency and exergy efficiency are compared in this paper by a case study in which a superheated high pressure steam boiler and a saturated low pressure steam boiler are used as examples. It is found that high thermal efficiency may not result in high exergy efficiency. It is inadequate to determine the level of efficiency of the thermal system by using thermal efficiency only, and the exergy efficiency in the process should also be considered. Low exergy efficiency represents large energy irreversible loss. Therefore in order to improve the process performance, it is necessary to improve the energy efficiency by improving the incomplete combustion or the irreversible loss of heat transfer in the process. The primary energy, electrical energy, etc. are all high-grade energy. It should be avoided using high-grade energy to convert to the low-grade energy, like low temperature heat, etc., and energy should be rationally distributed to achieve cascade utilization in order to receive relatively good energy-saving effect. Acknowledgements This work was supported by the Special Fund for Science and Technology Development of Guangdong Province in 2017 (Grant No. 2017A010104014), the National Natural Science Foundation of China (Grant Nos. 51176036 and 21376052), and the National Basic Research Program of China (Grant Nos. 2010CB227306 and 2016YFD04003032). References [1] Li Chao.Comparison and analysis of boiler thermal effciency based on GB10184-88 and GB10184-2015[J].Boiler manufacturing, 2018 (1):112. [2] Ding Cong.Energy efficiency analysis and improvement measures of WNS gas fired boiler[J].Chemical engineering & equipment,2018,2:235236. [3] Li Xia.The effciency of enenrgy utilization evaluation index sysytem and applied research of China [D]. China university of geosciences, 2013, 5. [4] Liu Meng,Zhen Danxing.Research on indicator system of enenrgy saving ratio for distributed enenrgy sysytem of combined cooling heating and power[J]. Standard science, 2013, 11: 50-53. [5] Lin Rumou,Guo Dong,Jin Hongguang,Sui Jun.A new evaluation criterion of distribyted energy systems for CCHP-the energy cascade utilzation effciency[J].Gas turbine technology,2010,23(1):1-10. [6] Yang Donghua.Analysis of thermal engineering problems by exergy [J]. Engineering thermophysics, 1981, 2 (1):1-7. [7] Chang Guangsheng. Exergy effciency and thermal efficiency [J].Beijing institute of light industry, 1983, 1 (1):21-26. [8] Guo Yangfei,Wu Qin,Cheng Lin,Huang He,Gao Song.Efficiency analysis model of integrated energy system based on the exergy Efficiency [J].Renewable energy Resources,2017,35(9):1387-1394. [9] Liu Qiang,Duan Yuanyuan.Exergy ananlysis for thermal power system of a 600MW supercritical power unit[J].Proceesings of the CSEE, 2010, 30(32):8-12. [10] Wang Jiaxuan,Zhan Shufang.Exergy method and application in power plant [M]. Beijing:Electric press,1993:26-184. [11] Liu Yanpeng,Zhong Beijing,Li Shaohua.Exergy effciency ananlysis on large-scal circulating fluidized bed boilers[J].Thermal power generation, 2015, 44(8):33-48. [12] Chen Lan, Zhao Tai,Gong Duo. Analysis of energy saving rate in summer cooling mode of distributed energy system [J]. Energy conservation, 2015, 391(4):26-29. [13] DL/T 904-2004[M.] Calculation method of technical and economic index of thermal power plant. [14] GB 10148-1988[M.]Specification for performance test of power plant boilers. [15] DL/T 606.3-2006[M.]Guide for thermal balance of thermal power plant. [16] Liu Zhiqing,Sa Weihua,Ji Guoliang. Analysis of thermal efficiency and price efficiency of several of the thermal power system [J]. Central air conditioning market, 2014, 12: 88-93.
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[17] Li Yong,Wang Yanhong,Zhang Bingwen.Correction of Boiler Thermal Efficiency and Standard Coal Consumption Rate Based on different Standards[J].Chinese Society of Power Engineering.
ScienceDirect ScienceDirect
Energy Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online atatwww.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy (2018) 000–000 161–168 EnergyProcedia Procedia153 00 (2017) www.elsevier.com/locate/procedia
5th International Conference on Energy and Environment Research, ICEER 2018 5th International Conference on Energy and Environment Research, ICEER 2018
a a
Comparison and analysis of thermal efficiency and exergy Comparison and analysis of thermal efficiency and exergy efficiency in energy systems by Heating case study The 15th International Symposium on District and Cooling efficiency in energy systems by case study a Youyuan Shaoaa, Hanmin Xiaoaa, Baiman Chenaa,the Simin Huang , Frank G.F. Qina,a,* Assessing the feasibility of using heat demand-outdoor a Youyuan Shao , Hanmin Xiao , Baiman Chen , Simin Huang , Frank G.F. Qin * Guangdong Provincial Key Laboratory of Distributed Energy Systems, School of Chemical Engineering and Enenrgy Technology, Dongguan temperature functionUnversity for Energy a long-term district heat demand forecast Guangdong Provincial Key Laboratory of Distributed Systems,Dongguan School of 523808,China Chemical Engineering and Enenrgy Technology, Dongguan of Technology, Unversity of Technology, Dongguan 523808,China
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Abstract Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques Environnement - IMTinherent Atlantique, 4 rue AlfredinKastler, Nantes, This paper analyzes thermal efficiency, exergy et efficiency, and their relationship energy44300 systems. TwoFrance typical thermal This paper analyzes thermal efficiency, exergy efficiency, and theirItinherent relationship in efficiency energy systems. Two equipment, steam boiler and steam turbine, are used for case study. shows that the exergy may not be typical ideally thermal high in steam boiler and have steama turbine, are used for caseItstudy. It showstothat the exergy efficiency may not be high in aequipment, process even though it may high thermal efficiency. is insufficient evaluate a system’s energy saving byideally using thermal a process even it maya have a high thermal efficiency. It is insufficient to evaluate a system’s energy saving byenergies, using thermal efficiency only.though Therefore comprehensive analysis of thermaland exergyefficiency should be adopted, and with Abstract efficiency only.should Therefore a comprehensive analysis andenergy exergyefficiency should system be adopted, and energies, with different grade, be utilized in cascade to achieveofanthermaloptimized saving for a thermal or equipment. different grade, should be utilized in cascade to achieve optimized energyindex saving for a thermal system or equipment. Keywords: thermal efficiency; exergy efficiency; energyan system; evaluation District heating areexergy commonly addressed in system; the literature as one of the most effective solutions for decreasing the Keywords: thermalnetworks efficiency; efficiency; energy evaluation index greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat © 2018 The Authors. Published by Elsevier Ltd. ©sales. 2018 Due The Authors. Authors. Published by Elsevier Elsevier Ltd. and building renovation policies, heat demand in the future could decrease, to the changed climate conditions © 2018 The Published by Ltd. This isisan access article under the This anopen open access article under theCC CCBY-NC-ND BY-NC-NDlicense license(https://creativecommons.org/licenses/by-nc-nd/4.0/) (https://creativecommons.org/licenses/by-nc-nd/4.0/) prolonging the investment return period. This is an and openpeer-review access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection under responsibility of the scientific committee of International Conference on Energy and Selection and peer-review under responsibility of the scientific committee of the the–5th 5th International Conference on Energy and The main scope of this paper is to assess the feasibility of using the heat demand outdoor temperature function on for Energy heat demand Selection and peer-review under responsibility of the scientific committee of the 5th International Conference and Environment Research, ICEER 2018. Environment Research, ICEER 2018. forecast. The district ICEER of Alvalade, Environment Research, 2018. located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Keywords: thermal efficiency; exergy efficiency; energy system; evaluation index renovation scenarios wereexergy developed (shallow, deep). Keywords: thermal efficiency; efficiency; energy intermediate, system; evaluation indexTo estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications Nomenclature (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Nomenclature scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). b--Standard coal consumption for power generation (kg/(kW·h)); The value ofcoal slope coefficient increased ongeneration average within the range of 3.8% up to 8% per decade, that corresponds to the b--Standard consumption for power (kg/(kW·h)); COP of performance of a LiBr refrigerator; -- Coefficient decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and COPe----Coefficient Coefficient ofperformance performanceofofana LiBr refrigerator; COP electrical refrigerator; renovation scenariosofconsidered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the COP e-- Coefficient of performance of an electrical refrigerator; coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.
*©Corresponding author. Published Tel.: +86 0769 22862619;Ltd. 2017 The Authors. by Elsevier * Corresponding author. Tel.: +86 0769 22862619; E-mail address: [email protected] Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and E-mail address: [email protected]
Cooling.
1 1
1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2018 Thearticle Authors. Published by Elsevier Ltd. This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is anand open access article under the CC BY-NC-ND licensecommittee (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the scientific of the 5th International Conference on Energy and Environment Selection and peer-review Research, ICEER 2018. under responsibility of the scientific committee of the 5th International Conference on Energy and Environment Research, ICEER 2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 5th International Conference on Energy and Environment Research, ICEER 2018. 10.1016/j.egypro.2018.10.081
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Youyuan Shao al. / Energy Procedia 153 000–000 (2018) 161–168 Youyuan Shao et al. / et Energy Procedia 00 (2018)
CPf-- Specific heat of natural gas [kcal/(Nm3.0C]; CPair-- Specific heat of the air [kcal/(Nm3.0C]; ebc-- Energy consumption per unit of product during the studied period (tce); ejc-- Energy consumption per unit of product during the base period (tce); Ex – Proess exergy (kJ); EinF -- Total exergy of fuel consumed by a CCHP system (kJ); EoutC-- Cooling exergy output from a CCHP system (kJ); EoutH-- Heating exergy output from a CCHP system (kJ); EoutP -- Work exergy output from a CCHP system (kJ); F-- Biomass gas flow (kg/s); hw-- Water enthalpy (kJ); hS-- Steam enthalpy (kJ); HL-- Low natural gas calorific value (MJ/Nm3); H-- High natural gas calorific value (MJ/Nm3); H0 -- Enthalpy in the ambient condition (kJ); Pe-- Power generation capacity (kW); P0-- Environmental pressure (kPa); Qout --output energy; Qin --input energy; Qc-- Refrigeration capacity (kW); Qh -- Heating capacity (kW); S -- Entropy in the working condition (kJ/K); S0 -- Entropy in the ambient condition (kJ/K); T0 -- Environmental temperature (K); Vair-- Air flow (kg/s); Vg-- Smoke amount (Nm3/Nm3fuel); W-- work input to the heat pump; VG -- Flue gas flow (kg/s); VH2O -- water flow (kg/s); C -- Coefficient of cooling exergy; F -- Coefficient of fuel exergy; H -- Coefficient of heat exergy; P -- Coefficient of work exergy;
-- thermal efficiency;
b --
Boiler efficiency; Power generation efficiency of power plant; e -- Efficiency of gas turbine; e x -- Exergy efficiency; h -- Efficiency of heat recovery. ce --
1. Introduction For a long time, people have been exploring better methods for energy system analysis [1-4]. As we know, the evaluation index of energy analysis is mainly thermal efficiency, which is based on the first law of thermodynamics, known as energy conservation, so it is also known as the First law efficiency[5-7]. It correlates the quantity between energies, but it does not show the quality changes in the process of energy transfer and/or conversion. Therefore, using the first law efficiency (i.e. the thermal efficiency) only is inadequate for analyzing and assessing the thermal process of an equipment. The evaluation index of exergy analysis is known as exergy efficiency, which includes consideration of both the first- and the second-law of thermodynamics, and known as the second law efficiency. It not only correlates the quantity of energy, but also indicates the quality and effective utilization of energy [8-11]. It is more accurate,
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more appropriate and more comprehensive in describing the perfection of the process or equipment. Exergy efficiency is also used to measure the degree of irreversibility of processes. 2. Thermal efficiency [12~15] For a given process or device, the ratio of effective output energy (Qout) to input energy (Qin) is known as the thermal efficiency, which is dimensionless and is expressed in percentage.
=
Qout 100% Qin
(1)
In energy systems, there are specific definitions of thermal efficiency for different facilities, such as boiler thermal efficiency, heat pump thermal efficiency, power generation efficiency, coal consumption rate of power generation, total heat efficiency of cogeneration, as shown in Table 1. Table 1. Thermal Efficiency Definitions Employed in Energy Systems Items Boiler efficiency
Heat pump efficiency
=
(1)Refrigeration
COP =
(2)Heating
=
Power generation efficiency
Standard coal consumption for power generation (kg/(kW·h))
Expression
Qh 100% F HL Qc 100% W
Qh 100% W 3600 Pe = F HL
(1)Direct energy balance
b=
(2)Indirect energy balance
b=
Bs Pe 3600 H L
Qh + Qc + Pe Total thermal efficiency of CCHP = 100% system F HL
The definitions listed in Table 1 indicate that the general thermal efficiency and the specific thermal efficiency definitions are all based on the principle of energy conservation, which reflects the quantities among energies, but does not account for changes in the quality or grade of energy during the processes of energy transfer and conversion. Therefore, using only the first law efficiency to evaluate the energy efficiency of a CCHP system is often inconsistent with actual conditions, and may even be contradictory to actual conditions, such as in the calculation of the energy utilization of afterburning and multi-energy inputs, and for CCHP multivariate flow systems. Therefore, thermal efficiency is not sufficient to provide a comprehensive and systematic evaluation of the energy efficiency of distributed CCHP systems. 3. Exergy efficiency ( ex ) As we known energy has dual attributes of quantity and quality. Exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with the surrounding. In other words, exergy is the
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energy that is available to be used. For a steady-state open system with ambient temperature To (K), exergy in the working condition (kJ) is defined as the sum of the enthalpy difference and the entropy difference in the working and standard conditions:
( H − H 0 ) − T0 ( S − S0 )
Ex =
(2)
The characteristics of exergy present definite advantages for expressing the inequivalence of heat and power in the grade. The concept of exergy lays a foundation for combining the first and second laws of thermodynamics, and presents a new direction for determining the effective use of heat energy and energy-saving technologies. Exergy efficiency is the main indicator employed in exergy analysis, and represents the exergy balance, which is also denoted as the second law efficiency. The exergy efficiency is defined as the ratio of the total exergy output (Eout) from a system to the total exergy input (Ein), as follows. ex =
Eout Eout C + Eout H + Eout P = Ein EinF
Q + H QoutH + P Pout = C outC F QinF
(3)
From Eq. (3) reveals the inequivalence of work and energy in thermodynamics. The exergy efficiency is thermodynamically more accurate than efficiency indicators based solely on the first law of thermodynamics because it accounts for the thermodynamic inequivalence of the energy of cooling or heating and work. In addition, exergy efficiency takes into account the equivalence ratio of the energy of cooling or heating to work, and proportionally reduces the effective cooling or heating energy output. Therefore, it can express the degree of efficiency in processes and equipment more accurately, more appropriately, and more comprehensively than efficiency indicators based solely on the first law of thermodynamics. Additionally, we note that the heat generation to electricity generation ratio and the ESR can only reflect the energy changes of the system. However, adopting the exergy efficiency for system evaluation provides appropriate evaluation results irrespective of the afterburning change of system. 4. Case analysis [16] Energy conservation analysis and exergy analysis are used to calculate and analyze the thermal efficiency and exergy efficiency of two thermal equipment in this section, and the results are compared. Take the natural gas boiler for steam production as an example. Based on the different steam quality of the boiler, two types of conditions are adopted: Boiler I: Output steam pressure: 3.5 MPa (a), steam temperature: 243C, superheated high pressure steam. Boiler II: Output steam pressure: 0.3 MPa (a), steam temperature: 133.5C, saturated low pressure steam.
Figure 1 Energy flow diaogram in and out of the boiler
Youyuan Shao et al. / Energy Procedia 153 (2018) 161–168 Youyuan Shao et al. / Energy Procedia 00 (2018) 000–000
4.1. Boiler I The materials and energy entering and leaving the boiler are shown in Figure 1. The parameters of natural gas boilers are shown in Table 2. Table 2. Boiler Parts Parameters Name
Parameter
Environment temperature
t0 = 250 C , T0 = 298.150 C
Water At 250C and 1atm
H 0 = 25.1kcal / kg
Water enthalpy
= S0 Water entropy
0.0876kcal / (kg K )
tw = 1050 C ， pw = 3.5Mpa hw = 105.7 kcal / kg Water enthalpy = Sw 0.3249kcal / ( kg K ) Water entropy 0
At 105 C and 3.5Mpa(a)
Water flow
VH 2O = 14.6kg / ( Nm3fuel)
Steam
tV = 2430 C ， PV = 3.5Mpa （a）
At 2430C and 3.5Mpa(a)
hs = 670.2kcal / kg
Steam enthalpy
Ss Steam entropy=
1.465kcal / (kg K )
Fuel Low natural gas calorific value High natural gas calorific value Natural gas supply temperature
H L = 9000kcal / Nm3
H H = 9990kcal / Nm3 t f = 650 C
= Cpf Specific heat of natural gas
1.5kcal / ( Nm3 0 C )
Air Supply temperature Required air volume
t0 = 25 0C
Vair = 13Nm3 / ( Nm3fuel) （1atm）
= C pair Specific heat of the air
0.31kcal / ( Nm3 0 C )
Smoke exhaust 0
Exhaust gas temperature t0 = 25 C = CPfuel 0.27 kcal / ( Nm3 0 C) Specific heat of smoke（1atm） Smoke amount
Vg = 14.5 Nm3 / ( Nm3fuel)
Calculation and analysis of boiler input side energy is shown in Table 3. Table 3. Input energy of boilers Content
Thermal efficiency method
Exergy efficiency method
①Heat of combustion
H L = 9000
Chemical exergy of fuel
② Sensible heat of the fuel
Q f = C Pf t f
Exergy input from the fuel
= 97.5
E f = 0.95 H H = 9490.5
T E= CPf ( t f −t0 ) − T0 ln f fs T0 = 3.5
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③ Sensible heat of the air
Exergy input from the air
Qair = Vair C Pair t air
Eair = 0
= 100.75
H H 2O = VH 2O hH 2O
④ Sensible heat of supply water
Exergy input from supply water
(
Tatal
)
EH 2O= VH 2O hH 2o − h0 − T0 ( Sw − S0 )
= 1543.22
= 144
QBinlet = 10741.47
EBinlet = 9638.0
Calculation of the Energy on the Output Side of the Boiler is shown in Figure 1. Table 4. Calculation of the Energy on the Output Side of the Boiler Content
Energy conservation method
⑤Enthalpy value of generating steam
H s = VH 2O hs
⑥Enthalpy of the smoke
H g = Vg CPg t g
⑦Loss (incomplete combustion, irreversible heat transfer, etc.)
Q = QBinlet − QBoutlet Loss
= 9784.92
= 783
= 173.55
Exergy efficiency method
Exergy of generating steam
= Es VH 2O ( hs − h0 ) − T0 ( S s − S 0 ) = 2930.76
Exergy of the smoke
T = Eg Vg CPg ( t g − t0 ) − T0 ln g T0 = 120.66 Loss of exergy
= EH 2O
E
i ,inlet
− Ei ,outlet
= 6586.58
The thermal efficiency and exergy efficiency of the boiler can be calculated from the calculation results of Table 3 and table 4. The difference between the two is more than 2.1 times. 4.2. Boiler II Now, the calculation is still based on the input of 1Nm3 natural gas to the boiler. In order to simplify the calculation process, the thermal budget of the system is no longer calculated, but its efficiency is directly calculated. The parameters of natural gas boiler are as follows: Ambient temperature, air, fuel, and smoke exhaust: The parameters are the same as those of the above natural gas boiler I. Water supply: 0 Temperature: tw = 105 C , Pressure: pw = 0.3Mpa (a) , Flow: Steam generation:
t = 132.80 C
p = 0.3Mp (a)
a , Pressure g , Flow Temperature g The two kinds of efficiency calculation results of the boiler are shown in Table 5.
。
Table 5. Efficiency of Boiler Content Primary energy input (payment side) Produced steam (revenue side)
Energy conservation method Low calorific value H L = 9000
H s = QH 2O
(hS − hw )
= 809.4
Exergy efficiency method Chemical exergy of fuel
E f = 0.95 H H = 9490.5
Eb =Exergy for stream – exergy for supply water =2480.5
Youyuan Shao et al. / Energy Procedia 153 (2018) 161–168 Youyuan Shao et al. / Energy Procedia 00 (2018) 000–000
Efficiency
= h, B Q= 90.0% g / HL
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= E , A E= 26.1% b / Ef
From the data shown in the table, it can be seen that the exergy rates are very different in the case of similar thermal efficiencies of the boilers. The reason is that the boiler has a high ability to do work so that the exergy rate is higher when the boiler produce high-temperature and high-pressure superheated steam, while when the boiler produces lowpressure saturated steam, the ability to continue to do work is poor, so the exergy rate of is low. 5. Remark and conclusions The indexes of thermodynamic performance commonly used at the present for evaluating the energy efficiency of distributed CCHP systems mainly include thermal efficiency, primary energy ratio (PER), and energy saving rate (ESR) [17]. The shortcoming of this index system is that it ignores the grade of energy. The thermal efficiency and exergy efficiency are compared in this paper by a case study in which a superheated high pressure steam boiler and a saturated low pressure steam boiler are used as examples. It is found that high thermal efficiency may not result in high exergy efficiency. It is inadequate to determine the level of efficiency of the thermal system by using thermal efficiency only, and the exergy efficiency in the process should also be considered. Low exergy efficiency represents large energy irreversible loss. Therefore in order to improve the process performance, it is necessary to improve the energy efficiency by improving the incomplete combustion or the irreversible loss of heat transfer in the process. The primary energy, electrical energy, etc. are all high-grade energy. It should be avoided using high-grade energy to convert to the low-grade energy, like low temperature heat, etc., and energy should be rationally distributed to achieve cascade utilization in order to receive relatively good energy-saving effect. Acknowledgements This work was supported by the Special Fund for Science and Technology Development of Guangdong Province in 2017 (Grant No. 2017A010104014), the National Natural Science Foundation of China (Grant Nos. 51176036 and 21376052), and the National Basic Research Program of China (Grant Nos. 2010CB227306 and 2016YFD04003032). References [1] Li Chao.Comparison and analysis of boiler thermal effciency based on GB10184-88 and GB10184-2015[J].Boiler manufacturing, 2018 (1):112. [2] Ding Cong.Energy efficiency analysis and improvement measures of WNS gas fired boiler[J].Chemical engineering & equipment,2018,2:235236. [3] Li Xia.The effciency of enenrgy utilization evaluation index sysytem and applied research of China [D]. China university of geosciences, 2013, 5. [4] Liu Meng,Zhen Danxing.Research on indicator system of enenrgy saving ratio for distributed enenrgy sysytem of combined cooling heating and power[J]. Standard science, 2013, 11: 50-53. [5] Lin Rumou,Guo Dong,Jin Hongguang,Sui Jun.A new evaluation criterion of distribyted energy systems for CCHP-the energy cascade utilzation effciency[J].Gas turbine technology,2010,23(1):1-10. [6] Yang Donghua.Analysis of thermal engineering problems by exergy [J]. Engineering thermophysics, 1981, 2 (1):1-7. [7] Chang Guangsheng. Exergy effciency and thermal efficiency [J].Beijing institute of light industry, 1983, 1 (1):21-26. [8] Guo Yangfei,Wu Qin,Cheng Lin,Huang He,Gao Song.Efficiency analysis model of integrated energy system based on the exergy Efficiency [J].Renewable energy Resources,2017,35(9):1387-1394. [9] Liu Qiang,Duan Yuanyuan.Exergy ananlysis for thermal power system of a 600MW supercritical power unit[J].Proceesings of the CSEE, 2010, 30(32):8-12. [10] Wang Jiaxuan,Zhan Shufang.Exergy method and application in power plant [M]. Beijing:Electric press,1993:26-184. [11] Liu Yanpeng,Zhong Beijing,Li Shaohua.Exergy effciency ananlysis on large-scal circulating fluidized bed boilers[J].Thermal power generation, 2015, 44(8):33-48. [12] Chen Lan, Zhao Tai,Gong Duo. Analysis of energy saving rate in summer cooling mode of distributed energy system [J]. Energy conservation, 2015, 391(4):26-29. [13] DL/T 904-2004[M.] Calculation method of technical and economic index of thermal power plant. [14] GB 10148-1988[M.]Specification for performance test of power plant boilers. [15] DL/T 606.3-2006[M.]Guide for thermal balance of thermal power plant. [16] Liu Zhiqing,Sa Weihua,Ji Guoliang. Analysis of thermal efficiency and price efficiency of several of the thermal power system [J]. Central air conditioning market, 2014, 12: 88-93.
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