Performance enhancement of earth air tunnel heat exchanger using evaporative cooling

Transcription

1 *Corresponding uthor: Performnce enhncement of erth ir tunnel het exchnger using evportive cooling... Viks Bnsl nd Jyotirmy Mthur * Mechnicl Engineering Deprtment, Mlviy Ntionl Institute of Technology, Jipur , Indi... Abstrct A therml model hs been developed to investigte the potentil of using the storge cpcity of the ground for cooling with the help of n erth to ir het exchnger (EAHE) system integrted with evportive cooler. Prmetric studies performed for the EAHE coupled with the evportive cooler illustrte the effects of buried pipe length, pipe dimeter, volumetric flow rte of ir, number of pipes nd surfce-to-volume (S/V) rtio on the outlet temperture of the EAHE. An nlyticl solution hs been derived by considering the fundmentl eqution of energy, het trnsfer nd psychrometry, for predicting the temperture t the outlet of EAHE. The results of the EAHE coupled with evportive cooling re compred with tht of EAHE without evportive cooling for different S/V rtio nd bypss fctor. It is observed tht the length of the EAHE pipe is reduced significntly s much s 93.5% for obtining desired temperture t the outlet of the EAHE by the integrtion of evportive cooling with EAHE. Reduction in the length of buried pipe is lso noted with decrese in bypss fctor of evportor cooler. Keywords: erth ir tunnel; evportive cooling; surfce-to-volume rtio; bypss fctor Received 21 Mrch 2008; revised 8 April 2009; ccepted 26 My INTRODUCTION As the energy demnd for the cooling of buildings is incresing for the lst few yers, the chievement of indoor therml comfort while minimizing energy consumption in buildings is key im in most countries. There is rising interest for cooling systems bsed on renewble sources of energy. In generl, most people feel comfortble indoors when the temperture is between 228C nd 278C nd reltive humidity is within the rnge of 40 60%. Becuse of the high therml inerti of the soil, the temperture fluctutions t the ground surfce re ttenuted deeper in the ground. Further, time-lg occurs between the temperture fluctutions t the surfce nd in the ground. Therefore, t sufficient depth, the ground temperture is lower thn the outside temperture in summer. It is well-known fct tht t depth of bout 3 6 m, the erth provides very stble therml environment. The temperture t such depth is round 268C, which is the nnul men temperture of the loction. This very stble therml environment cn be coupled to buildings with the help of n erth-ir-pipe system to crete humn therml comfort conditions inside when mbient ir is drwn through buried pipes, the ir is cooled in summer nd heted in winter, before it is used for ventiltion. Thus, erth-to-ir het exchngers (EAHE) cn fulfill both purposes demnded bove: ( pre-) heting in winter nd (pre-) cooling in summer. Cooling the outdoor ir through buried pipes by mens of n EAHE hs been known for mny yers to hve potentil for incresing building s demnd. As spce-cooling technology utilizing nturl energy, EAHE systems hve ttrcted incresing interest for energy conservtion. Bnsl et l. [1] evluted lrge erth-ir tunnel system ment to provide therml comfort inside whole building complex t one of the hospitls in Indi. Goswmi nd Biseli [2] demonstrted n open loop, underground ir tunnel system s well s n indirect ir tunnel system to improve the coefficient of performnce of het pump or refrigertion system. Singh [3] developed mthemticl model to optimize for mximum het flux removl from the room by the system. Mihlkkou et l. [4] presented prmetricl model with vrying prmeters such s pipe length, pipe rdius, velocity of the ir inside the tube nd depth of the buried pipe below erth surfce. Sntmouris et l. [5] investigted the impct of different ground surfce boundry conditions on the efficiency of single nd multiple prllel erth-to-ir het exchnger Interntionl Journl of Low-Crbon Technologies 2009, 4, # The Author Published by Oxford University Press. All rights reserved. For Permissions, plese emil: journls.permissions@oxfordjournls.org doi: /ijlct/ctp017 Advnce Access Publiction 24 June Downloded from

2 Performnce enhncement of erth ir tunnel het exchnger system. Thnu et l. [6] developed n erth-ir-pipe system t Gulmohr frm house, Gurgon, Indi to monitor the temperture nd reltive humidity t suction nd delivery points of the erth-ir-pipe system. Kumr et l. [7] developed numericl model to predict the effects of ground temperture grdient, surfce conditions, moisture content nd vrious design spects of erth-ir tunnel. Hollmuller nd Bernrd [8] described finite differences numericl model for buried pipe systems, ccounting for sensible s well s for ltent het exchnges, so s for fully three-dimensionl het diffusion in soil nd flexible border conditions. Ghosl et l. [9] developed therml model to investigte the performnce of EAHE integrted with green house. Lee nd Strnd [10] conducted prmetric nlysis to investigte the effect of pipe rdius, pipe length, ir flow rte nd pipe depth on the overll performnce of the erth tube under vrious conditions. This pper describes the performnce enhncement of the erth ir tunnel het exchnger when it is integrted with the evportive cooler. Mthemticl modelling of the combined system hs been presented, together with prmetric nlysis considering chnge in bypss fctor of evportive cooling stge nd vrition in surfce-to-volume rtio (S/V) of the tunnel. 2 DESCRIPTION OF THE MODEL The EAHE system presented in this model is integrted with n evportive cooler t the inlet of the EAHE system. Therefore, cooling is crried out in two stges. The stges re s follows: Evportive cooler EAHE system The schemtic of both EAHE without evportive cooling nd EAHE integrted with evportive cooler re shown in Figure 1 nd b, respectively. 2.1 EAHE system The EAHE system presented in this pper ws modelled s two coupled het-trnsfer processes, nmely convection het trnsfer between ir flowing in the pipe nd the inner surfce of the pipe, nd conduction het trnsfer between the outer surfce of the pipe nd the soil environment. A circulr ground cooling pipe is modelled s cross-flow het exchnger with one fluid unmixed (i.e. ir). An externl therml resistnce ws provided by surrounding concentric cylinder of erth of rbitrry thickness, which ws exposed to n undisturbed subsoil temperture s boundry condition. In this work, the thickness of the nnulus is tken s being equl to the rdius of the pipe s shown in Figure 2. In order to develop simple nlyticl model, the following ssumptions re mde: (1) The soil surrounding the pipe is isotropic with homogenous therml conductivity in ll ground strt. (2) The therml resistnce of the pipe mteril is negligible (thickness of the pipe is very smll). (3) The surfce temperture of the ground cn be pproximted to the mbient ir temperture, which equls the inlet ir temperture. (4) The pipe is of uniform circulr cross-section. (5) The therml effect of soil surrounding the pipe is negligible fter distnce r from the pipe outer surfce, where r is the pipe rdius. (6) Diffusion of moisture from soil to ir flowing through the buried pipe is negligible. The model is bsed on the fct tht the erth s temperture t lrger (3 m) depths remins constnt, nd is equl to n ll-yer verge vlue of soil ir temperture. For clculting the length of the pipe for pre-defined outlet temperture, the model proposed by Al-Ajmi et l. [11] hs been used for developing the lgorithm s given below: Stedy-stte nlysis gives the therml resistnce (R s ) of the soil nnulus s: R s ¼ ln r 1 r ð1þ 2pLk The therml resistnce (R c ) owing to convection het trnsfer between ir in the pipe nd the pipe inner surfce my be Figure 1. Schemtic digrm of () EAHE without evportive cooling. (b) EAHE with evportive cooling. Interntionl Journl of Low-Crbon Technologies 2009, 4, Downloded from

3 V. Bnsl nd J. Mthur The overll coefficient of het trnsfer is defined by: U ¼ 1 R tot ð11þ For pipe of infinite length, the fluid (unmixed) through constnt temperture (T pipe surfce ¼ T 1 ); the effectiveness of the EAHE cn be defined s: Figure 2. Erth ir het exchnger (EAHE) system with the lyer is shown in cross section. expressed s: where R c ¼ 1 2prLh h ¼ Nu k ir ð3þ d Here, therml conductivity of the ir (k ir ) in Eqution (5) is clculted using Eqution (4): k ir ¼ 0:02442 þð10 4 ð0:6992t ÞÞ The convective het trnsfer coefficient h in Eqution (3) bove is function of Reynolds number Re; nd Nusselt number Nu, where Re ¼ Vd ð5þ v Here, the kinemtic viscosity of ir n in Eqution (5) is clculted using Eqution (6): v ¼ 10 4 ð0:1335 þ 0: T Þ The Nusselt number for flow inside pipe is given by: ð2þ ð4þ ð6þ Nu ¼ 4:36 if Re, 2300 ð7þ Turbulent flow in circulr pipe for the rnges 0.5 Pr 2000 nd 2300, Re, is given by: ð f =8ÞðRe 1000ÞPr Nu ¼ 1 þ 12:7ð f =8Þ 1=2 ðpr 2=3 1Þ ð8þ where 1 ¼ 1 e UA= _mc p ð12þ _m ¼ r pd2 V 4 The definition of temperture effectiveness 1 is given by: 1 ¼ T T 2 T T 1 ð13þ ð14þ 2.2 Evportive cooler As the ir psses through the evportive cooler, evportive cooling of the ir tkes plce. Evportive cooling is bsed on simple principle: As wter evportes, the ltent het of vporiztion is bsorbed from the surrounding ir. As result, the ir is cooled during the process s shown in Figure 3. The evportive cooling process is shown on psychrometric chrt in Figure 4. Hot, dry ir t Stte 1 enters the evportive cooler, where it is spryed with liquid wter. Prt of the wter evportes during this process by bsorbing het from the irstrems. As result, the temperture of the irstrems decreses nd its humidity increses (Stte 2). In the limiting cse, the ir leves the evportive cooler sturted t Stte 2 0. This is the lowest temperture tht cn be chieved by this process. Prcticlly, this lowest temperture cnnot be ttined becuse for this the wter pd through which the ir must pss should hve very fine structure becuse of which lrge pressure drop will tke plce. Hence, to void this, the wter pd should hve corser structure which results in bypssing of some ir through the wter pd without ny cooling effect. Therefore, the mbient ir which is delivered to the cooler is cooled for criticl fctor. This fctor is termed Here, f is the friction coefficient for smooth pipes nd is determined using the reltionship: f ¼ð0:79 ln Re 1:64Þ 2 ð9þ The totl therml resistnce R tot between pipe ir nd surrounding soil of the EAHE system my then be determined from: R tot ¼ R s þ R c ð10þ Figure 3. Evportive cooler. 152 Interntionl Journl of Low-Crbon Technologies 2009, 4, Downloded from

4 Performnce enhncement of erth ir tunnel het exchnger negligible. Then the mss nd energy blnces for the dibtic mixing of two irstrems reduce to: Mss of dry ir : m 1 þ m 2 ¼ m 3 ð16þ Mss of wter vpour : v 1 m 1 þ v 2 m 2 ¼ v 3 m 3 ð17þ Energy : m 1 h 1 þ m 2 h 2 ¼ m 3 h 3 ð18þ Eliminting m 3 from the bove reltions, we hve m 1 m 2 ¼ v 2 v 3 v 3 v 1 ¼ h 2 h 3 h 3 h 1 ð19þ Figure 4. Evportive cooling process on psychrometric chrt. Using expression (19), we cn esily get the condition of the ir fter dibtic mixing. This ir is then fed to the EAHE. The properties of the ir which is obtined fter the dibtic mixing re shown in Tbles 1 nd Input prmeters In the foregoing nlysis, volumetric flow rte of ir through the EAHE remins constnt while the dimeter of the tunnel vries for different numbers of pipes (n), so tht the effect of rtio of surfce re per unit length vilble for het exchnge to volumetric flow rte (S/V) cn be studied. In this nlysis, it is ssumed tht cross-sectionl re of the first pipe remins the sme for ggregte cross-sectionl re of the cse in which there is n number of pipes (n ¼ 2, 3, 4, 5, 6 nd 8), tht is n p 4 d2 n ¼ p 4 d2 ð20þ Figure 5. Adibtic mixing process on psychrometric chrt. s Bypss fctor which is defined s follows: b ¼ T 2 T 1 T 2 0 T 1 ð15þ In evportive cooling process the het trnsfer between the irstrem nd the surroundings is usully negligible. Therefore, the evportive cooling process follows line of constnt wet-bulb temperture on the psychrometric chrt. Since the constnt wet bulb temperture lines lmost coincide with the constnt-enthlpy lines, the enthlpy of the irstrem cn lso be ssumed to remin constnt. The ir which comes in contct with the wter pd of the evportive cooler is gin mixed with the bypss ir s shown in the psychrometric chrt in Figure 5. During this process negligible het trnsfer tkes plce; therefore this process is clled dibtic mixing. This process involves no work interctions, nd the chnges in kinetic nd potentil energies re nd volumetric flow rte of ir through first pipe remins the sme for ggregte volumetric flow rte of the cse in which there is n number of pipes (n ¼ 2, 3, 4, 5, 6 nd 8). Tht is nq n ¼ Q ð21þ In these cses it is ssumed tht seprtion between the pipes is kept such tht the het trnsfer owing to individul pipe is not ffecting the performnce of the others becuse spce vilble for buril of pipe is enough. Using these ssumptions, dt re tbulted in Tble 3. The length of the tunnel for the cse in which there is no evportive cooling is tking plce is found with the help of expressions explined in Section 3.1 nd the input prmeters which re shown in Tble 3. The soil temperture is equl to 258C, which is the verge nnul temperture of Jipur ( N, E). The exit temperture from the EAHE is ssumed to be 308C. The length of the tunnel when EAHE is coupled with evportive cooling is clculted using expressions explined in Sections 3.1 nd 3.2. In this cse, the length of the tunnel is found for different vlues of bypss fctor nd S/V rtio s shown in Tble 3. Interntionl Journl of Low-Crbon Technologies 2009, 4, Downloded from

5 V. Bnsl nd J. Mthur Tble 1. Dry bulb temperture (DBT) of ir t the inlet of tunnel (fter dibtic mixing). b DBT of ir t the inlet of tunnel for different WBT t the inlet of evportor cooler (8C) Dry bulb temperture t the inlet of evportor cooler (8C) b, bypss fctor; WBT, wet bulb temperture of the ir. These conditions correspond to reltive humidity greter thn 60%l therefore not modelled s explined erlier. Tble 2. Reltive humidity of the ir t the inlet of tunnel (fter dibtic mixing). b Reltive humidity of ir t the inlet of tunnel for different WBT t the inlet of evportor cooler (%) Dry bulb temperture t the inlet of evportor cooler (8C) b, bypss fctor; WBT, wet bulb temperture of the ir These conditions correspond to reltive humidity greter thn 60% therefore not modelled s explined erlier. 3 RESULTS AND DISCUSSIONS In this pper, the length of the buried pipe is found for both the cses, viz. without evportive cooling nd with evportive cooling. This nlysis studies the effect of S/V rtio on the performnce of the EAHE. The effect of bypss fctor on the performnce of the EAHE is lso studied (Figure 6). 3.1 Length of ech pipe required for obtining n exit temperture of 308C without evportive cooling When EAHE is not integrted with the evportive cooler, the length of the ech buried pipe is found for different S/V rtio. S/V rtio is vried by chnging the number of pipes nd their dimeters. Length of ech pipe required for different climtic 154 Interntionl Journl of Low-Crbon Technologies 2009, 4, Downloded from

6 Performnce enhncement of erth ir tunnel het exchnger Tble 3. Input prmeters for performnce evlution of erth to ir het exchnger (EAHE). System No. of pipes (n) n ¼ 1 n ¼ 2 n ¼ 3 n ¼ 4 n ¼ 5 n ¼ 6 n ¼ 8 Pipe dimeter (m) Men velocity of ir (m/s) Volumetric flow rte through ech pipe (m 3 /s) Soil temperture (8C) Density of ir (kg/m 3 ) Specific het of ir (J/kg 8C) Therml conductivity of soil (W/m 8C) Bypss fctor Exit temperture through EAHE (8C) S/V rtio Figure 6. Different cses of EAHE with S/V, number of pipes nd dimeter for flow rte of ir ¼ 1m 3 /s. Figure 7. Length of pipe required for obtining exit temperture of 308C for different number of pipes (without evportive cooling) nd different inlet conditions t evportor cooler. Figure 8. Length of pipe required for obtining exit temperture of 308C for different bypss fctor. conditions re shown in Figure 7. It is observed tht s the number of pipe increses, the length of ech pipe required for the sme cooling effect is reduced. 3.2 Effect of bypss fctor In Figure 8, it is shown tht s the bypss fctor of the evportor cooler is incresed, the length of the buried pipe Interntionl Journl of Low-Crbon Technologies 2009, 4, Downloded from

7 V. Bnsl nd J. Mthur required for obtining the sme exit temperture from the EAHE increses, i.e. the performnce of the system deteriortes. The vlue of bypss fctor equl to unity corresponds to the cse in which EAHE is not integrted with the evportive cooler. This is observed when the reduction in length of buried pipe owing to evportive cooling is bout 75.9% in the cse when dry bulb temperture (DBT) nd wet bulb temperture (WBT) of the ir t the inlet of the evportor cooler re 458C nd 258C, respectively. The vlue of the sme is bout 77.6% when DBT is 508C nd WBT ¼ 258C. This is lso observed tht reduction in the length of the buried pipe is high when bypss fctor decreses from 0.5 to 0.2 becuse in these cses more ir comes in contct with the wter pds of the evportor cooler nd more cooling of ir tkes plce in the evportor cooler. 3.3 Effect of S/V rtio The curve between S/V rtio nd length of ech pipe of the EAHE for different climtic condition nd for the bypss fctor 0.2, 0.3, 0.4 nd 0.5 re shown in Figure 9 d, respectively. This is evident from the curves tht s the S/V rtio increses, the length of the buried pipe required for obtining the sme temperture t the exit of EAHE decreses. The S/V rtio is chnged by vrying the number of pipes nd their dimeters. As the number of pipe increses, the length of spce required for the buril of pipe decreses becuse for obtining the sme temperture t the exit of EAHE, the length of ech pipe decreses significntly. 3.4 Combined effect of bypss fctor nd S/V rtio In Figure 9 d, combined effect of bypss fctor nd S/V rtio is shown. This is observed tht for sme S/V rtio, the performnce of the system is better for smll vlues of bypss fctor since low bypss fctor mens more cooling of ir tkes plce in evportor cooler, therefore, length of the pipe required is less. This conclusion cn lso be mde tht s the S/V rtio increses (i.e. s the vlue of n increses), the length Figure 9. Length of ech pipe required for obtining exit temperture of 308C for different surfce-to-volume rtio nd different inlet conditions: () for b ¼ 0.2; (b) for b ¼ 0.3; (c) for b ¼ 0.4; (d) for b ¼ Interntionl Journl of Low-Crbon Technologies 2009, 4, Downloded from

8 Performnce enhncement of erth ir tunnel het exchnger of buried pipe decreses. It is cler from the plot tht this decrement is inverse exponentil. 4 CONCLUSIONS The results of mthemticl modelling of EAHE coupled with evportive cooling show tht for given dimeter of pipe, volumetric flow rte, set of inlet nd outlet tempertures owing to coupling of the evportive cooler with EAHE, there is decrese in the length of the buried pipe. This decrese is found to be in the rnge of % of length. The results of mthemticl modeling lso show tht besides depending upon the inlet conditions, the decrese in length lso depends upon the bypss fctor of evportive cooler. This is lso observed tht reduction in the length of the buried pipe decreses by % when bypss fctor decreses from 0.2 to 0.5. The results of mthemticl modelling for given inlet nd exit condition for EAHE lso show tht s the S/V rtio increses (i.e. s the vlue of n increses), the required length of ech buried pipe decreses s much s by 82.5% for chnge in S/V rtio from to The benefit of employing more number of pipes cn especilly be useful in loctions where spce for ccommodting required length of pipes is not vilble. NOMENCLATURE b bypss fctor C p specific het t constnt pressure of ir (J/kg K) DBT dry bulb temperture of the ir d pipe dimeter (m) d n pipe dimeter (m) for n-th pipe EAHE erth ir tunnel het exchnger f friction fctor for smooth pipe h convective film coefficient (W/m 2 8C) h 1 enthlpy of ir which is cooled by evportive cooler h 2 enthlpy of bypss ir h 3 enthlpy of ir fter dibtic mixing m 3 k soil therml conductivity (W/m 8C) k ir therml conductivity of ir (W/m 8C) L pipe length (m) ṁ mss flow rte of ir (kg/h) m 1 mss of dry ir which is cooled by evportive cooler m 2 mss of bypss dry ir m 3 mss of dry ir fter dibtic mixing NTU no. of trnsfer units Nu Nusselt number n no. of pipes P tmospheric pressure (P) Pr Prndtl number Q volumetric flow rte (m 3 /s) Q n volumetric flow rte (m 3 /s) through n-th pipe R c therml resistnce between ir nd pipe inner surfce (m 2 8C/W) Re Reynolds number R s therml resistnce of erth nnulus (m 2 8C/W) R tot totl therml resistnce of erth nnulus (m 2 8C/W) r pipe rdius (m) r 1 rdius of cylinder denoting thickness of soil surrounding pipe (m) T temperture of the ir delivered to EAHE fter dibtic mixing (8C) T db1 dry bulb temperture t the inlet of evportor cooler (8C) T outlet exit ir temperture delivered through the pipe outlet (8C) T wb1 wet bulb temperture t the inlet of evportor cooler (8C) T 1 temperture t the inlet of cooler (8C) T 2 temperture t the outlet of evportor cooler (8C) T 2 0 lowest temperture tht cn be chieved by evportive cooling U overll conductnce of tube (W/m 2 8C) V verge velocity of ir (m/s) v kinemtic viscosity of ir (m 2 /s) WBT wet bulb temperture of the ir 1 effectiveness of EAHE system v 1 specific humidity of ir which is cooled by evportive cooler v 2 specific humidity of by pss ir v 3 specific humidity of ir fter dibtic mixing r density of ir (kg/m 3 ) REFERENCES [1] Bnsl NK, Sodh MS, Singh SP, Shrm AK, Ashvini K. Evlution of n erth-ir tunnel system for cooling/heting of hospitl complex. Build Environ 1985;20: [2] Goswmi DY, Biseli KM. Use of underground ir tunnels for heting nd cooling griculturl nd residentil buildings. Fct Sheet EES 78: A Series of the Florid Energy Extension Service, Florid Coopertive Extension Service, Institute of Food nd Agriculturl Sciences, University of Florid, [3] Singh SP. Optimiztion of erth-ir tunnel system for spce cooling. Energy Convers Mnge 1994;35: [4] Mihlkkou G, Sntmouris M, Asimkopoulos D, Tselepidki I. Prmetric prediction of the buried pipes cooling potentil for pssive cooling pplictions. Solr Energy 1995;55: [5] Sntmouris M, Mihlkkou G, Asimkopoulos D, Lewis JO. On the ppliction of the energy blnce eqution to predict ground temperture profiles. Solr Energy 1997;60: [6] Thnu NM, Swhney RL, Khre RN, Buddhi D. An experimentl study of the therml performnce of n erth-ir-pipe system in single pss mode. Solr Energy 2001;71: Interntionl Journl of Low-Crbon Technologies 2009, 4, Downloded from

9 V. Bnsl nd J. Mthur [7] Kumr R, Rmesh S, Kushik SC. Performnce evlution nd energy conservtion potentil of erth-ir-tunnel system coupled with non-ir-conditioned building. Building Environ 2003;38: [8] Hollmuller P, Bernrd L. Buried pipe systems with sensible nd ltent het exchnge: vlidtion of numericl simultion ginst nlyticl solution nd long-term monitoring. In: Ninth Conference of Interntionl Building Performnce Simultion Assocition, Ecole Polytechnique de Montrél, Cnd, 2005, pp [9] Ghosl MK, Tiwri GN, Ds DK, Pndey KP. Modeling nd comprtive therml performnce of ground ir collector nd erth ir het exchnger for heting of greenhouse. Energy Buildings 2005;37: [10] Lee KH, Strnd RK. Implementtion of n erth tube system into EnergyPlus progrm, in: Proceedings of the SimBuild 2006 Conference, Boston MA, USA, [11] Al-Ajmi F, Lovedy DL, Hnby V. The cooling potentil of erth ir het exchngersfor domestic buildings in desert climte. Building Environ 2006;41: Interntionl Journl of Low-Crbon Technologies 2009, 4, Downloded from