能源總線系統(tǒng)多熱源混水和熱回收過程變分析

2016-10-25 04:10王培培龍惟定

制冷學(xué)報(bào) 2016年4期

王培培　龍惟定

(1同濟(jì)大學(xué) 機(jī)械與能源工程學(xué)院　上?！?00092; 2 同濟(jì)大學(xué) 中德工程學(xué)院　上海　200092)

王培培1龍惟定2

(1同濟(jì)大學(xué) 機(jī)械與能源工程學(xué)院上海200092; 2 同濟(jì)大學(xué) 中德工程學(xué)院上海200092)

在能源互聯(lián)網(wǎng)時(shí)代，區(qū)域供冷供熱系統(tǒng)將由原本單一形式的熱源向多種形式熱源并存轉(zhuǎn)變，尤其是可再生能源和未利用能源。不同形式不同品位的熱源集成必將引起系統(tǒng)能量變化。能源總線系統(tǒng)是集成化規(guī)?；瘧?yīng)用區(qū)域內(nèi)可再生能源及未利用能源的多源多用戶能源系統(tǒng)。本文針對(duì)能源總線系統(tǒng)相對(duì)常規(guī)分散系統(tǒng)而言特有的多源多用戶特征進(jìn)行系統(tǒng)混水和熱回收過程的變分析，將能源總線系統(tǒng)抽象為一系列工作在高溫?zé)嵩春偷蜏責(zé)嵩粗g的勞倫茲循環(huán)的集成，通過建立能源總線系統(tǒng)與常規(guī)分散系統(tǒng)的理想熱力學(xué)模型，找到能源總線系統(tǒng)混水和熱回收過程變的規(guī)律及影響因素。結(jié)果表明：系統(tǒng)的變化與各子系統(tǒng)低溫?zé)嵩催M(jìn)出口溫度、高溫?zé)嵩催M(jìn)口溫度以及高低溫?zé)嵩促|(zhì)量流量比相關(guān)，不同的設(shè)計(jì)參數(shù)會(huì)導(dǎo)致混水過程能量發(fā)生增加或者減小，亦或不變。通過分析得到熱回收過程影響源側(cè)總線熱量變化的相關(guān)參數(shù)并找到熱量變化規(guī)律，并得到最佳總線供水溫度TEBS1的確定方法。

能源總線系統(tǒng)；能源規(guī)劃；勞倫茲模型；熱回收；

1 能源總線系統(tǒng)理想熱力學(xué)模型

能源總線系統(tǒng)實(shí)質(zhì)是由一系列制冷制熱循環(huán)系統(tǒng)集合而成的冷熱量交換系統(tǒng)。將實(shí)際制冷制熱循環(huán)系統(tǒng)抽象為工作于一定高溫?zé)嵩春偷蜏責(zé)嵩粗g的逆卡諾循環(huán)。考慮熱源流體與冷源流體進(jìn)出口溫度會(huì)隨著吸熱放熱過程而改變，理想循環(huán)的熱源都具有一定的比熱容(也不是無限大)，因此，將能源總線系統(tǒng)視為工作在一系列變溫?zé)嵩唇M成的理想循環(huán)(即勞倫茲循環(huán))中的冷熱量交換系統(tǒng)。

對(duì)夏季制冷工況進(jìn)行理論分析。理想能源總線系統(tǒng)夏季循環(huán)可視為工作在總線的高溫?zé)嵩碩EBSm和不同低溫?zé)嵩碩lmi之間的一系列勞倫茲循環(huán)。能源總線系統(tǒng)與常規(guī)分散能源系統(tǒng)的理想熱力學(xué)模型的對(duì)比，見圖1、圖2。其中，總線中水的高溫?zé)嵩碩EBSm為多源混合熱源，即由不同高溫?zé)嵩碩hmi組合而成。多源混合過程存在流體摻混，流體摻混過程帶來的能量變化直接影響能源總線系統(tǒng)運(yùn)行和設(shè)計(jì)，下面對(duì)熱源混合過程中能量在數(shù)量與質(zhì)量上的變化進(jìn)行分析。

圖1 能源總線系統(tǒng)制冷工況理想熱力學(xué)模型Fig.1 Ideal thermodynamic model of the energy bus system in cooling mode

圖2 常規(guī)系統(tǒng)制冷工況理想熱力學(xué)模型(基準(zhǔn)模型)Fig.2 Ideal thermodynamic model of the conventional system in cooling mode

2 能源總線系統(tǒng)多源混水過程變計(jì)算

2.1 EBS制冷循環(huán)能量平衡計(jì)算模型

夏季制冷循環(huán)的能量平衡計(jì)算模型見圖3。設(shè)定：區(qū)域i夏季制冷循環(huán)過程低溫?zé)嵩礈囟扔蒚li1變化到 Tli2，常規(guī)系統(tǒng)區(qū)域i的高溫?zé)嵩礈囟扔蒚hi1變化到Thi2，能源總線系統(tǒng)熱源溫度由TEBS1變化到TEBS2。系統(tǒng)運(yùn)行環(huán)境溫度T0。則有:

圖3 制冷循環(huán)能量平衡模型Fig.3 Refrigeration cycle energy balance model

QNi+Wi=QKi

(1)

(2)

(3)

(4)

勞倫茲循環(huán)溫度之間的關(guān)系為[17]:

(5)

(6)

(7)

式中:C1、C2分別為兩個(gè)熱源流體的比熱容；Tlm、Thm、TEBSm分別為低溫?zé)嵩础⒏邷責(zé)嵩匆约翱偩€熱源的熱力學(xué)平均溫度。

根據(jù)高溫?zé)嵩磁c低溫?zé)嵩礈囟扔?jì)算常規(guī)系統(tǒng)與能源總線系統(tǒng)勞倫茲循環(huán)制冷系數(shù)ε1：

(8)

(9)

圖4 制冷循環(huán)平衡模型Fig.4 Exergy balance model of refrigeration cycle

ExQNi+∑Li=Wi+ExQKi

(10)

(11)

(12)

(13)

(14)

(15)

3 EBS制冷工況多源混水過程影響變主要影響因素分析

已知 Th21≠Th11，當(dāng)Th21=Th11時(shí)，源側(cè)溫度相同，不存在混合變化，即ΔW1=0 。令Th21>Th11，即(Th21-Th11)>0，則有：

根據(jù)公式(15)分析能量相對(duì)變化ΔW1%的正負(fù)與大小，令n=2。

ΔW1%=

(16)

選定變量為τ1、τ2、φ1，分析公式(16)，設(shè)定Tl11=Tl21=273+12=285K，Tl12=Tl22=273+7=280K，Th11=293K，Th21=305K，即兩個(gè)系統(tǒng)制冷循環(huán)低溫?zé)嵩催M(jìn)出口溫度Tl1、Tl2相同時(shí)，分析不同φ1,τ1,φ2,τ2條件下，源側(cè)進(jìn)水溫度293 K和305 K混合后系統(tǒng)循環(huán)相對(duì)功耗變化量，此時(shí)的ΔW1%是φ1,τ1,τ2的函數(shù)(φ2=1-φ1)，即ΔW1%=f(φ1,τ1,τ2)。

設(shè)定τ1、τ2的變化范圍為[0.8,1.0]，φ1的變化范圍為[0.1,0.9]。

計(jì)算結(jié)果見圖5。圖中可以看出，ΔW1%max=3.71%，ΔW1%min=-3.63%。功率變化趨勢證明前面對(duì)于能量損失定性分析的正確。

圖5 制冷工況高溫?zé)嵩磦?cè)混水過程能量相對(duì)變化(切片φ1)Fig.5 Relative energy change in water mixing of high temperature heat source side in cooling mode (Cut φ1)

表1 制冷工況混水能量相對(duì)變化極值函數(shù)擬合結(jié)果

(17)

4 EBS熱回收過程功率變化及最佳供水溫度分析

設(shè)定能源總線系統(tǒng)總線熱源溫度由TEBS1變化到TEBS2i或TEBS2j；有n1個(gè)區(qū)域進(jìn)行制冷循環(huán)，低溫?zé)嵩礈囟确謩e由Tli1變化到 Tli2，常規(guī)系統(tǒng)區(qū)域i的高溫?zé)嵩礈囟扔蒚hi1變化到Thi2；有n2個(gè)區(qū)域進(jìn)行制熱循環(huán)，高溫?zé)嵩礈囟确謩e由Thj1變化到Thj2，常規(guī)系統(tǒng)區(qū)域j的低溫?zé)嵩礈囟扔蒚lj1變化到 Tlj2；系統(tǒng)運(yùn)行環(huán)境溫度T0；設(shè)定高低溫?zé)嵩戳黧w比熱C相等。設(shè)定外界溫度T0，此時(shí)采用常規(guī)熱泵系統(tǒng)的i區(qū)域中，Thi1=T0；采用常規(guī)熱泵系統(tǒng)的j區(qū)域中，Tlj1=T0。QNi、QKj為各區(qū)域冷熱負(fù)荷，設(shè)定δ為全部區(qū)域總冷、熱負(fù)荷之比。

同樣設(shè)定全部i區(qū)域 Tli1=285 K，Tli2=280 K，τi=1，全部j區(qū)域Thj2=318K，Thj1=313K，τj=1?？梢缘玫?

能源總線系統(tǒng)能耗WEBS與總線供水溫度TEBS1、區(qū)域冷、熱負(fù)荷比例δ相關(guān)聯(lián)，見圖6。當(dāng)確定了區(qū)域的總冷、熱負(fù)荷比例δ之后，對(duì)應(yīng)室外溫度T0，可以從圖中確定更節(jié)約輸入功率的總線水溫度TEBS1。

δ>0.88時(shí)，總線放熱量大于吸熱量，WEBS隨著TEBS1升高而增加；δ=0.88時(shí)，總線內(nèi)冷熱平衡，WEBS與TEBS1大小不相關(guān)；δ<0.88時(shí)，總線吸熱量大于放熱量，WEBS隨著TEBS1升高而減少。

圖6 供水溫度和區(qū)域冷、熱負(fù)荷比例對(duì)EBS能耗的影響Fig.6 Influence of supply water temperature and ratio of regional cold load and heat load on EBS energy consumption

5 結(jié)論

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About the corresponding author

Wang Peipei, female, Ph. D., School of Mechanical Engineering, Tongji University, +86 13916459060, E-mail: wwwangwangaaa@163.com. Research fields: district energy planning, low carbon building and energy-saving building.

Analyses of Exergy Change in Multi Heat Source Water Mixing and Heat Recovery Process of Energy Bus System

Wang Peipei1Long Weiding2

(1. School of Mechanical Engineering, Tongji University, Shanghai, 200092, China; 2. College of Engineering in Germany, Tongji University, Shanghai, 200092, China)

In the internet era of energy, district heating and cooling system will change from a single form of energy to the various forms of energy, especially renewable energy and untapped energy. Integration of different grades of heat sources will cause system energy change. The energy bus system is a multi-source and multi-user thermal energy system that can make integration of renewable energy sources or untapped energy sources in large scale for district heating and cooling. This paper focus on analysis of exergy change in multi-source water mixing and heat recovering process of energy bus system comparing with the conventional system. The energy bus system can be modeled as integration of a series of Lorenz cycles. Through the theoretical analysis of the ideal thermodynamic model of energy bus systems and conventional distributed systems, exergy change law and its influencing factors in multi-source water mixing process and heat recovering process of energy bus system are analyzed. The results show that the inlet and outlet temperatures of each subsystem low temperature heat sources、inlet temperatures of high temperature heat sources、the mass flow ratio of low temperature heat sources to high temperature heat sources affect exergy change of multi-source system. Different design parameters can cause the energy to increase or decrease, or unchanged. In this paper, heat recovering process of energy bus system is also analyzed theoretically, and the related parameters which affect the heat change of the bus side are obtained and the heat variation law is found, also the way to get the optimum bus water temperatureTEBS1.

energy bus system; energy planning; Lorenz model; heat recovery; exergy

0253- 4339(2016) 04- 0106- 06

10.3969/j.issn.0253- 4339.2016.04.106

2016年3月16日

TU831