heat transfer coefficient, W/ q heat per unit area, W/m 2 heat, W time, s

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1 ESL-IC DEALING WITH BIG CIRCULATION FLOW RATE, SMALL TEMPERATURE DIFFERENCE BASED ON VERIFIED DYNAMIC MODEL SIMULATIONS OF A HOT WATER DISTRICT HEATING SYSTEM Li Lian Zhong, Senior Sales Consltant, Danfoss Atomatic Controls Management (Shanghai Co.,Lt, Anshan, China ABSTRACT Dynamic moels of an inirect hot water istrict heating system were evelope base on the first principle of thermoynamics. The ieal moel was verifie by sing measre operational ata. The ieal an verifie moels were applie to obtain an analyze the system characteristics sch as the tenency of the isse relating to big circlation flow rate, small temperatre ifference. From the simlate an analyze reslts, it is realize that the electrical pmping cost col be significantly rece e to the over circlation water mass flow rate. It is also shown that by applying proper spply water temperatre set point in the control system, the zone air temperatre can be atomatically ajste with enogh accracy. The major reasons an soltions for the water mass flow rate an the temperatre ifference problem have been given in the last section. KF heat transfer coefficient, W/ q heat per nit area, W/m Q t heat, W time, s T temperatre, TD temperatre ifference, 3 Sbscripts control signal factors 1, primary, seconary system act arg b en ex actal average boiler esign enclosre exchanger KEYWORDS District heating system; ynamic moels; simlations; pmping cost; reasons an sggestions; NOMENCLATURE c specific heat, J/Kg or a factor relate to heat transfer coefficient test of heaters f h int n o fel heater internal nmber of HES otsie C controller or thermal capacity, J/ r retrn e error E electricity consmption, W f factor F area, m G mass flow rate, Kg/s HV heating vale, J/Kg k p proportional gain s sp sols v spply set point solar raiation from soth sie verify w, wi water, water in seconary system for each HES z zone k i integral gain Proceeings of the 14th International Conference for Enhance Biling Operations, Beijing, China, September 14-17, 14

2 ESL-IC stations, heaters as three parts with control strategy configration. 1 INTRODUCTION De to the most share of energy consmption for istrict heating (DH in constrction fiel, the energy efficiency, energy savings an environmental protection play very important rles these ays. Therefore, focsing on the crrent sitations combining with ftre soltions has been attracting researchers to evelop applications. For example in the existing isses in DH systems, the operation moe: big circlation flow rate, small temperatre ifference col be fon in many DH systems, this phenomenon is mainly reslte from both esign an operation phases. In esign stage, esigners wish to ensre more safety e to the responsibility; they sally try to increase safety factors from the heating loa calclation, terminal selection, pipe imension, pmp selection an the eqipment for the heat sorce as well. This in fact will case over sizing of the overall system configration. On the other han, in the system operation stage, some operators an managers also have the same thinking by increasing the operational parameters to overcome the problems reslte from ncomfortable zone air temperatre an hopeless system control an ajstment. Some researchers an experts have fon the scenarios in system operation, an escribe the reasons case it with wors. In orer to sty the sitations incling the reasons, the reslts an the eep soltions, a ynamic moel of a typical an real inirect istrict heating (IDH system was chosen for this research to assist the analyses an avices for recing the pmping cost from with qantificationally an qalitatively. Base on this motivation, an actal IDH system with heate floor area.44mm was selecte as an objective. The system has been renovate in 13 by sing Danfoss high-technologies an procts an operate in heating perio. In the IDH system, there are 4 hightemperatre hot water boilers installe in the heat sorce, an heat exchange stations (HES in the overall system with 9 heat exchange nits in total. The terminals were installe by raiator. The serve bilings were bilt in as economic bilings. The simplifie system iagram is plotte in Figre 1 with aggregate boilers, heat exchange Figre 1. Simplifie Diagram With Control Configration DYNAMIC MODEL DEVELOPMENT.1 Ieal Dynamic Moel The esign parameters of inoor an otoor air temperatres, spply an retrn water temperatres in the primary an seconary systems, water mass flow rates in the primary an seconary systems, an heating loa inex are given as 18, - 6, 11, 7, 8, 6, 47T/h, 91T/h, W/m respectively. These parameters are se for the evelopment of the mathematical moel. As known that IDH systems are very complicate systems when consiering ynamic processes. To avoi the complexity, certain conitions are assme: the heat losses an the makep water losses from the pipe network are ignore withot affecting the system responses too mch e to limite amonts actally. In aition, several physical parameters are aggregate sch as thermal capacity, overall heat transfer rate of HES, raiator an biling enclosre. By applying for the first principle of thermoynamics, state variables are taken into accont for the simplifie ieal ynamic moel, an the ynamic eqations are expresse as follows. Boiler moel: ( Cb f G f HV[ 1( T ( 3( ] cw1g1 ( 1 T T r b b b (1 Proceeings of the 14th International Conference for Enhance Biling Operations, Beijing, China, September 14-17, 14

3 ESL-IC In Eqation (1, the net heat store in the boiler boy is eqal to the ifference of fel combstion an the heat transporte from the primary network in the heat sorce. Note that the boiler efficiency is state as nonlinear property base on experience. Heat exchanger moel: C C ex1 r1 s 1 cw1g 1 ( r1 fexkfex{[( s r1 r ][ln( ] } T ( Ts c G c G r1 ( ex w 1 1 b r1 w s r (3 In Eqations ( an (3, the net heat store to the heat exchangers in both sies are consiere as the heat transferre from the primary system, the heat exchanger itself an the seconary system. The heat exchangers are selecte as plate type, an the temperatre ifference between the primary an seconary sies is compte by sing logarithmic mean temperatre ifference metho. Raiator moel: r ( Tz Cz cw G q F q F f KF s r sols sols int en en z o ( In Eqations (, the net heat store in the zone air can be expresse by the ifferences between the heat obtaine from heat inpts (the seconary system, solar raiation an the internal heat gains an the heat otpts from the biling enclosre. For simplicity, the solar raiation is consiere by soth sie of the bilings only, an the internal gains are assme by 3.6W/m as maximm vale. In smmary, the simplifie ieal IDH system ynamic moel consists of ynamic eqations.. Properties Analysis From The Ieal Moel Simlations Design vales of the network are set to the ieal ynamic moel for simlations. With the esigne fel consmption an otsie air temperatre as inpts, the ynamic responses are isplaye in Figre (. From this figre it can be seen that the otpts present ientical as the esign vales, meaning correct relationships among the inpts an the otpts Ch T r s r (1 c cwg s r fhkfh ( z T (4 De to the non-linear characteristic of raiators, the heat transfer from the terminals is calclate by sing exponential moel. In Eqation (4, the net heat store in the heaters is the ifference between the heat inpt from the seconary system an the heat otpt from the terminals. Room moel: Temperatre ( C Tr1 Ts Tr Tz Time (h Figre. Dynamic Responses In Ieal Conitions The system properties col be obtaine by simlating with the ieal ynamic moel, which the metho is entitle as open loop tests. When the solar raiation an the internal heat gains are set to be zero, an the zone air temperatre maintains 18, the Proceeings of the 14th International Conference for Enhance Biling Operations, Beijing, China, September 14-17, 14

4 ESL-IC simlation reslts are aresse in Figre 3 associate with the changes of water mass flow rates in the primary an seconary systems an ifferent otsie air temperatre. Only one parameter changes while the other was consiere as ieal conition in the simlation. As shown in Figre 3(a, it can be observe that, when the water mass flow rate in the primary system varies from lower to over esign vale, the temperatre ifference (TD in the primary sie will change from higher to lower vales, an the TDs in the seconary system are almost ientical; by observing the sitation in the seconary sie plotte in Figre 3(b, it has the same tenency compare with the Figre 3(a. In aition, the varying rate in the lower otsie air temperatre is faster than that in the higher otsie air temperatre. fel an manally control of the fel fee to the boiler(s, the actal fel consmption is reveale a bit e-centrality. Note that the heat transfer areas of the HESs an the raiators are oversize an the water flow rates in the primary an seconary systems are rnning with excess of esign vales in the real system operation. Operation Data Ts1, C Tr1, C Twarg, C Gcoal,T/Day Ts1v, C Tr1v, C Twargv, C Gcoalv,T/Day Temperatre Difference ( C changes TD1,1=.8 TD,1=.8 TD1,1=1 TD,1=1 TD1,1=1. TD,1=1. Temperatre Difference ( C changes TD1,=.7 TD,=.7 TD1,=1 TD,=1 TD1,=1.3 TD,=1.3 TD1,=1.6 TD,= Otsie Air Temperatre ( C Figre 4. Verifie Moel Responses With Operational Data.4 Properties Analysis From The Verifie Moel Simlations (a Otsie Air Temp. ( C (b Otsie Air Temp. ( C Figre 3. System Properties With The Changes Of Different Parameters (TD an refer to the temperatre ifference an water mass flow rate control signal respectively.3 Verifie Dynamic Moel Becase the evelope ynamic moel is sitable for ieal conitions only, it shol be verifie to reach enogh accracy for simlations with actal conitions. A trial an error metho is applie for obtaining the factors base on actal ata from the IDH operation in an represente by f, f, f,, ] [1.1,1.,1,1.3,1.4] [ h en 1 ex. The responses of the water temperatres an the fel consmption with ifferent otsie air temperatre from the verifie moel are shown in Figre 4. Comparing the reslts (lines from the verifie moel with the measre ata (ots, this verifie moel col be tilize for system simlations an preictions. De to the heating vale changes of the Base on the verifie moel, the factors [ fex, fh, fen] [1.1,1.,1] of the IDH system are fixe when the system was implemente. De to the fact of over circlation flow in system operation, the water mass flow rate control signals of the primary an seconary systems are set between 1 an 1.4, an the simlate ata is state in Figre. From Figre (a, the TD changes epening on the otsie air temperatre an the ratio of the water mass flow rate control signal in the primary system. Increasing water mass flow rate in the primary system by 1%, the TD will be ecrease by.~3. for the esign otsie air temperatre, an it reces faster in lower otsie air temperatre than that in higher otsie temperatre. On the other han, in orer to maintain the esign zone air temperatre, the water mass flow rates in the seconary (Figre (a an primary (Figre (b systems shol be the same accoring to the simlation reslts epening on which sie of water mass flow rate changes. Proceeings of the 14th International Conference for Enhance Biling Operations, Beijing, China, September 14-17, 14

5 ESL-IC Temperatre Difference ( C changes TD1,1=1 TD,1=1 TD1,1=1. TD,1=1. TD1,1=1.4 TD,1= (a Otsie Air Temp. ( C Temperatre Difference ( C changes TD1,=1 TD,=1 TD1,=1. TD,=1. TD1,=1.4 TD,= (b Otsie Air Temp. ( C Figre. Water Temperatre Difference With The Changes Of Otsie Air Temperatre An Circlation Flow Rate To this en, no matter which sie of water mass flow rate changes, the TD will be rece when the water mass flow rate is increase, an the other TD in the IDH system oes not change too mch for keeping the same inoor air temperatre. Accoring to the verifie massage from the verifie ynamic moel, the control signals of the water mass flow rate in the primary an seconary systems are 1=1.3 an =1.4 respectively. With these sitations, a control system configration (Figre 1 is sggeste to maintain the zone air temperatre by setting point temperatre as. In the selecte IDH system operation, the spply water temperatre from the heat sorce has been monitore manally (fel controller Cf relating to the otsie an insie air temperatres, solar raiation an win spee; the heat balance has been reglate base on the average water temperatre in the seconary system by ajsting the water mass flow rate (1 of each HES in the primary system; an the water mass flow rate into the bilings has been controlle by sing self-active flow control valves with certain settings (. However, in this paper for simplicity, the control signals of 1 an are set to be 1.3 an 1.4 respectively, meaning over mass flow rates of circlation water comparing with the esign vales (1=1, =1, an only Cf is taken into accont for this simlation. 3 PUMPING COST BASED ON THE CHANGE OF WATER MASS FLOW RATE The pmp affinity law has escribe the relationship between the water mass flow rate an the electrical power consmption of the circlation pmp for the overall pipe network in DH systems. To compare with the pmping cost in the esign case, the relationship can be expresse in Eqation (6 as below: A typical PI controller C f is se to reglate the spply water temperatre from the heat sorce, an the fel control signal f is formlate an compte in the following algorithm: f k p bsp k b t i bsp b (7 Eact E ( act 3 (6 Accoring to this eqation, the power consme will vary rapily. In another wor, for instance, if the circlation water mass flow rate is ecrease by 1%, the electrical pmping cost will be ecrease by 7% approximately. The set point of the spply water temperatre from the boiler is given as a fnction of otsie an insie air temperatres, solar raiation an internal heat gains, an expresse in Eqation (8 below: T bsp f T T, Q, (8 ( o, z sol Qint 4 TWO DAYS OPERATION WITH DIFFERENT WATER MASS FLOW RATE SETTINGS In orer to obtain the system ynamic responses, the otsie air temperatre, the solar raiation, the internal heat gains se in the Proceeings of the 14th International Conference for Enhance Biling Operations, Beijing, China, September 14-17, 14

6 ESL-IC simlations are presente in Figres 6 (c an (. A esign system ( 1=1 an =1 an an actal system ( 1=1.3 an =1.4 are simlate for comparison. As shown in Figres 6(a an 6(b, the spply water temperatres from the heat sorce an the HES are almost ientical becase of the same spply water temperatre set points from the heat sorce. The retrn water temperatres from the HES an the terminals in actal case are higher than those in the esign case. The reason behin is that the over mass flow rate are operate by the primary an seconary systems; an also by oing it, the TDs in the primary an seconary systems are lower by 6-8 an 3-4 respectively than those in the esign case, which means that more electrical power is reqire to overcome the circlating water resistance. As seen in Figre 6(c, by reglating the spply water temperatre from the boiler with the sggeste control strategy, the zone air temperatre ranges within ±.1 in the actal case compare with that from 18.9 to 19.3 in the esign system. The ifference between the zone air temperatres is reslte from its set point of the spply water temperatre from the heat sorce, an it shol be set an monitore accoring to actal operation strategy. The accracy achieve is obtaine from the compensation of the aitional heat gains an the feeback of the measre zone air temperatre se to the control system. In aition, by comparing actal system with the esign system, the pmping cost in actal system has consme abot 4% more electricity in operation. Temperatre ( C (a Time (h Temperatre ( C 1-1 Tz Tz act To (c Time (h Tr1 TD1 act Tr1 act TD1 act Temperatre ( C Aitional Gains (W/m (b Time (h 1 qsols qint* ( Time (h Figre 6. Two Days Operation With C f Controller Ts Tr TD Ts act Tr act TD act SUGGESTIONS OF SOLVING THE ISSUE OF BIG CIRCULATION FLOW RATE, SMALL TEMPERATURE DIFFERENCE The major reasons case by the isse of big circlation flow rate, small temperatre ifference col be smmarize as: (1 The plan: final planning has been implemente in one step; ( The esign: the safety factor is neee to be taken into accont, bt it oes not mean that each heat transfer process an eqipment selection is reqire to mltiply a factor more than 1; (3 The operation: the settings of the water mass flow rate in both pipe network sies shol be calclate scientifically instea of experience only for each heat season; on the other han, in orer to improve thermal comfort level an cover p the isse of ajstment an control, the water mass flow rate is increase artificially; (4 Sitable set points: wrong setting points of DH systems not only reslts in heat an hyralic nbalance, bt also leas to increasing the water mass flow rate easily to conceal the contraictory. De to the reasons escribe above, the soltions to solve this isse col be taken into accont accoringly: (1 The implementation of DH systems shol track an follow the overall plan an the actal evelopment in certain time span; ( To be carefl of consiering safety factors in esign process; otherwise, it will increase the investment an the operational expense of DH systems greatly; (3 From the simlation reslts, it is not necessary to increase the water mass flow rates to reach the esign vales of zone air temperatre set points; on the opposite, it is bigger potential way for operational energy saving by recing the flow rates; (4 To rece the pmping cost in operation, the water mass flow rate in both primary an seconary systems shol be ecrease accoring to the operation strategy ajste for each heating season, an combine with sitable set points of the control system an eqipment; Proceeings of the 14th International Conference for Enhance Biling Operations, Beijing, China, September 14-17, 14

7 ESL-IC ( To flfill the heat an hyralic balances, the set points of the overall DH systems incling the heat amont an the spply water temperatre from the heat sorce an the water mass flow rates in both hyralic networks nee to be calclate base on preiction of heating loa reqirement an symmetrically optimal process for each heat perio. 6 CONCLUSIONS By applying for the first principle of thermoynamics, ieal an verifie ynamic moels have been evelope for an actal IDH system, an se for obtaining system characteristics an control strategy investigation. From the simlation reslts, it can be seen that the TD has been ecrease associate with the increase of water mass flow rate. In the actal case, the pmping cost is consme more than 4% comparing with that in the esign conition. Also, the simlation has shown that the zone air temperatre col be reglate more accracy with sitable set point of the control system. The isse of the circlation flow rate an temperatre ifference has been iscsse, analyze an the soltions are sggeste consiering how to eal with it. [6] Tingyao Chen. A methoology for thermal analysis an preictive control of biing envelope heating system. Concoria University; 1997, p Biography Form - Please retrn to icebo@esl.tam.e Name of Paper: Dealing with big circlation flow rate, small temperatre ifference base on verifie ynamic moel simlations of a hot water istrict heating system Athor s Name(s: Li Lian Zhong Name of Presenter: Li Lian Zhong Presenter s Organization: Danfoss Atomatic Controls Management (Shanghai Co.,Lt Presenter s Location: Liao Ning, An Shan Presenters Aress: Qianshan District, Himin Street, 1# Presenter s Telephone: Fax: Presenter s lilianzhong@anfoss.com Biography to be se to introce presenter at conference session REFERENCES [1] Lianzhong, L., an Zaheerin, M. 4. A Control strategy for energy optimal operation of a istrict heating system. International Jornal of Energy Research,Vol.: [] Lianzhong, L., an Zaheerin, M. 8. Dynamic moelling an fzzy agmente PI control of a high-rise biling hot water heating system. Energy for sstainable evelopment, the jornal of the international energy initiative. Vol. XII(: 49-. [3] Felgner, F., Claera, R., Merz, R., an Litz, L. 3. Moeling thermal biling ynamics with moelica. Proceeings of the 4th MATHMOD Conference; Vienna. [4] 李连众, 丁传国. 7. 间接连接区域供热系统动态模型及控制策略仿真. 区域供热. 第 期. [] Zhilong, Z. 1. Temperatre control strategies for raiant floor heating systems. Concoria University Proceeings of the 14th International Conference for Enhance Biling Operations, Beijing, China, September 14-17, 14