Return Temperature in DH as Key Parameter for Energy Management

Transcription

1 Interntionl OPEN ACCESS Journl Of Modern Engineering Reserch (IJMER) Return Temperture in DH s Key Prmeter for Energy Mngement Normunds Tlcis 1, Egīls Dzelzītis 2, Agnese Līckrstiņ 2 *JSC Rīgs siltums ** Deprtment of Het, Gs nd Wter Technology, Fculty of Civil engineering, Rig Technicl University, Ltvi Corresponding Author: Normunds Tlcis ABSTRACT: Decresed supply temperture of district heting networks gives numer of system s dvntges, like incresed overll efficiency, possiility to use renewle energy resources nd decresed het loss from the district heting network. To ensure the pproprite het supply to the customers nd void lrge pipeline dimeters, it is importnt to keep the temperture difference s lrge s possile. Therefore the return temperture of the district heting network should lso e decresed. When supply nd return tempertures re lowered, the district heting network is more sensitive to the temperture difference fults, mking fluctutions in the temperture grphs. Due to this fct it is importnt to predict ehvior of district heting system s temperture difference, depending on the outdoor temperture. KEYWORDS: District heting, temperture difference, supply temperture Dte of Sumission: Dte of cceptnce: I. INTRODUCTION The ltest type of district heting systems clled 4 th genertion is chrcterised y eing prt of the long term infrstructure plnning, leding to smrt het distriution network with low supply, return tempertures nd integrted renewle sources [2].Tody the district heting system represents the high efficiency nd environmentlly friendly het nd hot wter supply to the highly populted res. The key spect of the system is to supply otherwise wsted het to the customers, y using the het distriution network of pipes [1]. According to [3] the shre of the district heting (DH) in the EU het demnd covers pproximtely 13% nd hs tendency to increse y the trnsition to low-co2 energy system, utilising renewle energy resources nd operted t lower supply nd return tempertures. The decrese of network tempertures is considered s one of the key fctors to increse the efficiency of the new DH systems [3]. Lowering the supply temperture of the DH network gives severl dvntges [2, 4], e.g.: incresed het recovery from the energy production units like oilers nd CHP-plnts; utilistion of renewle energy resources; s well s incresed electricl output from CHP-plnts. As it is shown in [3] if supply temperture is lowered from 0 C to 0 C nd the return temperture is lowered from 0 C to 0 C, the efficiency of het production increses y pproximtely % in solr therml plnts nd % in het plnts operted y het pumps. Recent studies [4-5] hve indicted tht even reltively low supply tempertures (slightly ove 0 C) cn meet consumers demnds for Centrl nd Northern Europen countries. The decresing of the het supply temperture in district heting network llows chieving lower het losses nd higher efficiency of the entire system. II. LITERATURE REVIEW The supply nd return tempertures in district heting systems ply crucil role in efficiency of such systems. According to [3] district heting systems cn e divided into 4 groups, depending on the supply temperture. High temperture DH systems re chrcterised y the temperture etween 0 0 C nd 1 0 C nd re used very rndom. The most commonly used systems re so-clled medium temperture DH, hving the IJMER ISSN: Vol. 8 Iss. 7 July 18 88

2 Return Temperture In DH As Key Prmeter For Energy Mngement supply tempertures etween 65 0 Cnd 95 0 C. Further decrese of the supply temperture fesile nd resonle ut it requires dditionl investments into district heting wter (DHW) systems due to the eventul grow of Legionell cteri. Decresed supply tempertures of the DH networks require lrger network dimeters, nd result in higher pumping costs in order to ensure the sufficient flow. To void tht, it is lso necessry to lower the return temperture nd keep the temperture difference etween supply nd return flows s lrge s possile [4]. During the heting seson, the district heting supply temperture closely correltes with the outdoor temperture in order to provide the optiml comfort conditions for the inhitnts. Johnsson et l. [6] reports the strong correltion etween mesured outdoor temperture nd the het lod. As it ws noticed in [4], during the heting seson the spce heting domintes in the uilding s het demnd, resulting into temperture difference etween supply nd return tempertures inversely proportionl to outdoor temperture. As soon s the spce heting loses its dominting role, no generl correltion etween temperture difference nd outdoor temperture is oserved. Theoreticlly the supply nd return tempertures of the DH network should fluctute only due to fluctutions of customers demnd. In relity the tempertures of DH network re influenced y oth the chnges in customers demnd nd the temperture difference fults [4]. Very frequently the temperture difference fults would led to incresed return temperture of DH network nd results into incresed supply temperture [4], s the temperture difference etween the supply nd return temperture should e kept constnt. Due to this fct it is very importnt to predict the pttern of the temperture difference of the DH network, thus llowing to estimte nd exclude the temperture difference fults in much shorter time. The difference etween supply nd return tempertures T usully vries during the yer. Empiricl study in [4] showed tht correltion exists etween temperture difference T nd outdoor temperture T out during heting seson in Denmrk. The trend line showing the decrese of T s T out increses is presented in [4] for the cse T out < 0 C. For lrger vlues of the outdoor temperture the dt re scttered over wide rnge of vlues (from 0 0 C to 0 C) nd no correltion exists eyond T out > 0 C. Since there re some differences in heting systems opertion in Denmrk nd Ltvi, the study hs een conducted with the ojective to otin the reltionship etween temperture difference T nd outdoor temperture T out for heting sttions in Rig, Ltvi. III. RIGA DISTRICT HEATING SYSTEM The district heting system in Rig city is operted y the JSC Rīgs siltums [7]. The compny provides het for spce heting nd hot wter from severl oiler plnts nd heting sttions locted in Rig city. The two lrgest heting sttions TEC1 nd TEC2 hve een selected for the study for two heting sesons 15/16 nd 16/17, respectively. The instlled het cpcity t the heting plnt TEC1 reches 490 MW, while the heting sttion TEC2 is the lrgest cogenertion plnt in Ltvi, hving the instlled het cpcity of MW including the hot wter oilers. The heting seson in Rig city lsts from Octoer until the end of April. During the heting seson of 15/16 the mount of het produced in TEC1 nd TEC2 reched 782 GWh nd 1154 GWh correspondingly. The produced mount of het during the next heting seson hs incresed due to the low outside temperture nd extended heting seson in the spring of the yer 17, reching 842 GWh for TEC1 nd 1289 GWh for TEC2. It should e noted tht the connected het lod for the heting re of TEC1 nd TEC2 is 9 MW t the winter temperture of C. The totl length of DH network in Rig city is 0 km, while the het supplied from TEC1 nd TEC2 is delivered y 557 km long network with the totl volume of m 3, ensuring the het for 5515 customers. Both networks re interconnected nd re le to provide het for the sme heting re or they cn e split in order to ensure het supply into two seprte regions. In order to mesure the het flow in the network, the mesuring devices re instlled t the end of min pipelines. The mesuring devices consist of flow meter, integrtor nd temperture sensors for supply nd return flows. The flow meter is instlled on the supply pipe nd it genertes impulses proportionl to the wter flow. The integrtor mkes the dt processing nd collecting. The temperture difference is clculted nd multiplied y the wter flow nd correction coefficient in order to tke into ccount the density of the wter nd its het content. Fig.1 shows very similr pttern of the temperture grphs for supply nd return tempertures in correltion of the ctul outside tempertures for oth heting sttions TEC1 nd TEC2 for the heting sesons 15/16 nd 16/17. The significnt increse of the supply temperture is required for the outside temperture elow 3 0 C, Fig1 nd Fig2. IJMER ISSN: Vol. 8 Iss. 7 July 18 89

3 Return Temperture In DH As Key Prmeter For Energy Mngement Figure 1: Supply nd return tempertures in correltion of the ctul outside tempertures from TEC1 for the heting sesons 15/16 () nd 16/17 (). Similr trend hs een oserved for the heting network from TEC2, see Fig 2. The temperture ptterns slightly devite. It is in correltion with the ide (GADD) tht every cse hs slightly different nd specific temperture grph. Figure 2: Supply nd return tempertures in correltion of the ctul outside tempertures from TEC2 for the heting sesons 15/16 () nd 16/17 (). The verge supply temperture during the heting seson depends on outdoor temperture nd reches pproximtely 72 0 C, while the return temperture is t 41 0 C. In order to evlute the correltion etween supply nd return temperture difference nd outdoor temperture, the clcultions hve een performed y using the softwre progrm Mtl. The results for the heting sttion TEC1, see Fig 3, show the strong correltion etween supply nd return temperture difference T nd outdoor temperture T out, when the outside temperture is elow 3 0 C. Gdd nd Werner [4] noticed tht in Denmrk strong correltion exists etween temperture difference T nd the outdoor temperture T out in the rnge of - 0 C< T out < 0 C, ut for higher outdoor tempertures T is prcticlly independent on T out. As it is shown in [1], for T out > 0 C the temperture difference temperture difference is widely scttered in the intervl (0 0 C, 0 C). Similr trend hs lso een oserved in DH network in Rig. The min difference from the dt presented in [4] is tht the correltion IJMER ISSN: Vol. 8 Iss. 7 July 18 90

4 Return Temperture In DH As Key Prmeter For Energy Mngement etween temperture difference nd outdoor temperture ecomes independent lredy when the outdoor temperture exceeds 3 0 C Figure 3: ( T) versus outdoor temperture (T out ) for heting sttion TEC1 in Rig during one yer 15/16() nd 16/17(). Since the correltion etween T nd T out exists, regression nlysis hs een performed y fitting the dt set for heting sesons in 15/16 nd 16/17, using second degree polynomil. The clcultion results re shown in Fig. 4 nd Fig. 5, where the est-fit second-degree polynomil nd the corresponding dt points re plotted from oth heting sttions. R 2 = R 2 = Figure 4: The est-fit second-degree polynomil nd dt points from the smple for the heting sttion TEC1 for heting seson 15/16() nd 16/17(). R 2 =0.8 R 2 = Figure 5: The est-fit second-degree polynomil nd dt points from the smple for the heting sttion TEC2 for heting seson 15/16() nd 16/17(). The nlysis shows tht the second-degree polynomil fits ll the dt well, s the coefficient of determintion vries from to The corresponding p-vlues reported in Mtl re smller thn -4 IJMER ISSN: Vol. 8 Iss. 7 July 18 91

5 Return Temperture In DH As Key Prmeter For Energy Mngement in ll the considered cses. Almost ll dt points in Figs. 4, nd 5 lie etween the upper nd lower ounds of the 95% confidence intervls. IV. CONCLUSION The regression nlysis used to evlute the DH network supply nd return wter temperture difference on the outdoor tempertures show tht the curves ove nd elow the grphs of the second-degree polynomil in Figs. 4 nd 5 correspond to 95% confidence intervl for the min predicted vlues. As cn e seen from Figs. 4 nd 5, the typicl difference etween the upper nd lower confidence intervl limit is out 9 0 C. The 95% confidence intervl estimtes to provide much smller uncertinty thn the estimtes sed on three stndrd devitions from the trend line [4]. As suggested in [4], such grphs cn e used in order to identify fults in district heting systems, s the loction of n oserved dt point outside the confidence intervl limits my indicte on fult in the DH network system. The otined nlysis is one of the first steps towrds the online fult nlysis of the DH network. There is potentil of decresing the supply nd return tempertures of the DH network in Rig city nd trnsform the network into more efficient nd sustinle one. REFERENCES [1]. S. Werner, District Heting nd Cooling in Sweden, Energy 126, 17, [2]. M. Rämä, K. Sipilä, Trnsition to Low Temperture Distriution in Existing Systems, The 15th Interntionl Symposium on District Heting nd Cooling, Energy Procedi 116, 17, [3]. D. Østergrd, S. Svendsen, Spce Heting with Ultr-Low-Temperture District Heting Cse Study of Four Single-Fmily Houses from the 19s, The 15th Interntionl Symposium on District Heting nd Cooling, Energy Procedi 116, 17, [4]. H. Gdd, S. Werner, Achieving Low Return Tempertures from District Heting Susttions, Applied Energy 136, 14, [5]. Guidelines for Low-Temperture District Heting: A Deliverle in the Project EUDP -II: Full-Scle Demonstrtion of Low- Temperture District Heting in Existing Buildings, April 14. [6]. C. Johnsson, M. Bergkvist, D. Geysen, O. De Somer, N.Lvesson, D. Vnhoudt, Opertionl Demnd Forecsting in District Heting Systems Using Ensemles of Online Mchine Lerning Algorithms, Energy Procedi 116, 17, [7]. N.Tlcis, A.Līckrstiņ, E.Dzelzītis, The Oservtion of Trgets Achieved during District Heting Development in Rig City, Modern Environmentl Science nd Engineering 3(4), 17, Normunds Tlcis1, " Return Temperture in DH s Key Prmeter for Energy Mngement. Interntionl Journl Of Modern Engineering Reserch (IJMER), vol. 08, no. 07, 18, pp IJMER ISSN: Vol. 8 Iss. 7 July 18 92