Thermoeconomic design of an earth to air heat exchanger used to preheat ventilation air in low energy buildings

Transcription

1 Recent Researches n Energy, Envronment, Entrepreneurshp, Innovaton Thermoeconomc desgn of an earth to ar heat exchanger used to preheat ventlaton ar n low energy buldngs GABRIELA-ELENA VLAD, CONSTANTIN IONESCU, HORIA NECULA, ADRIAN BADEA Power Engneerng Faculty, Energy Generaton and Use Department POLITEHNICA Unversty of Bucharest Splaul IndependenŃe 313, Bucharest, ROMANIA gab_ev@yahoo.com Abstract: Geothermal energy s one of the renewable sources of energy that can generate electrcty and heat wth a low mpact on the envronment. An mportant percent of the prmary energy consumpton s used to provde buldngs heatng and coolng. Thermal energy demand for buldngs s drven by populaton growth and clmatc condtons. For these reasons new solutons had to be found n the dwellngs sector and the passve house concept has proved to be very effcent. A passve house can be equpped wth geothermal systems such as heat pumps and earth to ar heat exchangers. The paper presents the descrpton and the desgn of an earth to ar heat exchanger nstalled to preheat and cool the ventlaton ar for a passve house bult wthn Poltehnca Unversty of Bucharest. Key-Words: geothermal energy, earth to ar heat exchanger, energy consumpton reducton, passve house, heat recovery, heat transfer, thermoeconomc optmzaton 1 Introducton The depleton of the classc fuels reserves and more ntense utlzaton of the renewable energes led to the development of new technologes. Geothermal energy s classfed as a renewable source of energy for electrcty producton and buldngs heatng. Nowadays, n Europe the buldng sector s responsble for about 40% of the total prmary energy consumpton. One nterestng soluton for payable energy reducton s the utlzaton of the great thermal potental of the ground. The ntensty of the thermal energy ncreases wth depth. At m depth, superfcal layers of the earth contan large quanttes of heat. An earth to ar heat exchanger s a surface geothermal system able to explot these resources. A passve house, whch s a low energy house, has to fulfll two mportant requrements: heatng energy consumpton must be lower than 15 kwh/m 2 /year and total prmary energy consumpton must be under 120 kwh/m 2 /year [1]. These condtons can be accomplshed only by a combnaton between a specal type of archtecture, a south orentaton, a compact shape, hgh tghtness and a HVAC system based on renewable sources of energy (heat pumps, solar collectors, hot water tanks, fan-cols unts, earth to ar heat exchangers). Energy demand for space heatng of ths type of buldng has been reduced by ca. 80% compared wth conventonal buldngs [2]. The buldng costs of a passve house do not exceed the costs of a usual house by more than 16% [3]. In Germany, where the Passve House Insttute from Darmstadt s conductng research n ths feld, over 7,000 unts were bult [4]. Other European countres such as Sweden, France and Austra developed ther projects wth the help of German experence. In Europe, the exstence of over 20,000 passve houses by 2010 s estmated [5] and ther number s growng. The effcency of an earth to ar heat exchanger (EAHX) system depends on the clmate of the locaton. For contnental areas lke Romana, the dfference between the sol temperature and the outsde ar temperature s hgher than 10 C. Ths fact makes the EAHX the sutable soluton for preheatng the ventlaton ar, durng wnter and coolng t durng the summer. In European countres, the earth to ar heat exchanger (EAHX) s also known as earth tubes, earth coolng tubes, earth warmng tubes or put canaden. Even f the passve house concept was formalzed 15 years ago by the Passve House Insttute from Darmstadt, n Romana t s stll new. A new project s developed by the Poltehnca Unversty of Bucharest. Two passve houses havng a surface of 180 m 2 each are under constructon n the campus of the unversty. One of these houses wll be equpped wth an earth to ar heat exchanger, used to preheat/cool the ventlaton ar. ISBN:

2 Recent Researches n Energy, Envronment, Entrepreneurshp, Innovaton 2 Descrpton of the Earth to Ar Heat Exchanger (EAHX) An earth to ar heat exchanger s a system whch ncreases the nteror temperature of the buldng durng wnter and t s also able to decrease t durng summer, by usng the thermal potental of the ground. Throughout a year, the outsde ar temperature n Bucharest ranges from -15 C to 40 C, whle sol temperature has a lower range, only from 4 C to 18 C. The ground contans heat that can be captured and transported through the ppes nsde the buldng. There are two dfferent types of earth to ar heat exchangers: the frst one wth the ar flowng nsde the tube and a second one wth brne. A brne-ground heat exchanger operates n the same mode as the tradtonal type and t s another possblty to preheat the ventlaton ar n polluted areas. The water crculatng nsde the tube usng a pump absorbs heat from the ground and then t enters a water-ar heat exchanger. Insde the heat exchanger, the energy accumulated by water s delvered to the ventlaton ar. For an EAHX of ths type, the length of the thermal channel s usually half of the ar volume flow [6]. The nvestgated earth to ar to ar heat exchanger s placed near one of the two passve houses of the Poltehnca Unversty of Bucharest. Durng the wnter, the ar passes through the underground collector by usng a fan. In ths way, the low temperature of the ar whch enters the house s ncreased. Durng summer, the system uses the coolness of the ground to lower the temperature of the fresh ar. The qualty of the fresh ar depends on the nlet ar locaton. The exteror ar ntake wll be placed at 1 m heght on a 10 cm concrete socle away from the car parkng areas. Other mportant elements such as a lattce, a hat and a flter protect the earth channels from nsects, brds, rodents or dust. The ground thermal channel represents the most mportant part of the system because t assures the energy change between sol and ar. The tube should be placed at 1-2 m depth to reduce the nfluence of the outsde envronment as much as possble [7]. The crtera for choosng the ppes referred to mechancal strength, durablty, tghtness, thermal conductvty and mpermeablty. The collector network can be made up of one ground loop or several tubes connected n parallel [8]. The confguraton of the network depends on the sze of the avalable surface. The walls of the ppes are treated n order to reduce mcrobal growth that could pollute the fresh ar. Studes concernng ar polluton were conducted and they showed that the concentraton of a possble harmful growth of bactera could even be decreased due to the ar flowng [8]. The range of materals used to make the thermal tubes s qute large: concrete, sandstone, cast ron, polyethylene or polypropylene [8]. The materal chosen for the ppe of the analyzed EAHX s a type of polyethylene wth a hgh thermal conductvty. Ths materal fulflls the envronmental requests, t does not release toxc gas and t has low roughness. Radoactve gas exsts n hgher or lower concentratons n any type of sol and n any area. The rsk of lung cancer ncreases for the people lvng n the buldngs located n hgh concentraton radon (radoactve gas) areas. The nvestgated EAHX s tght enough and the radon does not get nsde. Whle the ar s passng through the tube, water vapors condense on the nner surface, turnng nto small drops. The condensaton process s mportant and t s taken nto consderaton for two reasons: f the water remans nsde the channels, a perturbaton of arflow appears and the qualty of the ar s affected by growth of bactera and fung. There are two possbltes to remove the condensed water. For a buldng wth a basement, the condensed water s collected by a sphon placed at the lowest pont of the nstallaton. A brne to ar heat exchanger also needs a sphon to remove the drops of water. The scheme presented n fg. 1 s typcal for a house wthout a basement lke "Poltehnca house". A condensaton tower was bult at the lowest pont of the earth to ar heat exchanger, close to the wall. The condensed mosture s removed by a condensaton pump. Ar ntake Condensaton tower fresh ar nlet exhausted ar heat recovery unt (HRV) 1-3% EAHX ar extracted from buldng preheated ar Fg. 1: Method used to remove condensed mosture Fg. 2: Scheme of the EAHX-HRV system ISBN:

3 Recent Researches n Energy, Envronment, Entrepreneurshp, Innovaton For ths type of buldng the underground ppe s nstalled at an angle of 1-3% so that the condensate could flow to the place where s collected. Snce the house s ventlated wth fresh ar, the heatng load durng wnter becomes hgher. To reduce energy consumpton for heatng, the ar s preheated n two steps: frst n the ground heat exchanger and second n a heat recovery unt (HRV) whch can save even more than 90% from the energy of exhausted ar (fg. 2). The heat recovery ventlaton system conssts n a plate heat exchanger and two fans. The fans are dentcal, synchronzed so that the basc ventlaton condtons to be fulflled. Insde the PHE, the exhausted ar transfer heat to the supply ar. The two ar streams must be equal and n counter-flow. The utlzaton of a HRV system brngs energy gans and mproves also the qualty of the nteror ar. 3 Theoretcal model of the EAHX Ths work presents the algorthm for calculaton of optmum value for the length of the ppe ntegrated n an EAHX. The model s dvded n two parts. The frst part of the algorthm s based on a common method ncluded n many applcatons used n the heat exchangers feld. The parameters from the followng numercal applcaton are characterstc for the studed earth to ar heat exchanger. All the nput parameters used for theoretcal study are presented n table 1. A hgh densty polyethylene ppe was selected as thermal tube. The ppe s bured below the freezng pont of the ground, n our case at 2 m deep. Table 1: Input parameters of the theoretcal model Dmenson / property Symbol Value Unt Ppe characterstcs Wall tghtness δ 7,8 mm Ppe dameter (exteror) D e 200 mm Wall roughness r 1, mm Thermal conductvty λ 0,28 W/(m. K) Volumetrc flow rate V a 165 m 3 /h Sol temperature t s 5 ºC Tube nlet ar temperature t a, -15 ºC We made the assumpton that the sol temperature, t s, s constant durng heatng perod. Its value dependng on depth s obtaned from a dagram specfc for each clmate zone. The consdered volumetrc flow rate of the ar was obtaned followng the ventlaton needs of the house, accordng to a 0.5 changes/hour. The tube nlet ar temperature, the same as the outsde temperature, s set at the mnmum calculaton temperature of -15 C, typcal for the month wth the lowest outsde ar temperatures. The value of the tube outlet ar temperature (t a,o ) s found by usng the expresson of the tube thermal effcency: t = t a, o a, ε (1) s t t a, Ths s a very mportant nput parameter and t takes nto consderaton both the ar temperatures and the sol temperature. The frst step of the algorthm s to settle ts value at 0.85 (the recommended average value for the HRV used n passve houses). To evaluate the heat exchange rate, we must frst determne the thermophyscal propertes of the ar n the tube, at the temperature average value. The convectve heat transfer coeffcent α s calculated as a functon of the ar thermal conductvty (λ a ) and the nner ppe dameter (D ), wth the classcal expresson: α Nu λ D a a = [W/(m 2.K)] (2), ISBN:

4 Recent Researches n Energy, Envronment, Entrepreneurshp, Innovaton where the Nusselt number of the ar (Nu a ) s done by the Gnelnsk expresson [9], as a functon of the Reynolds and Prandtl numbers: Nua 0,8 0,4 0,0214(Rea 100) Pra = (3) The velocty of the ar n typcal applcatons usually ranges between 1 and 3 m/s. In ths case t s about 1,72 m/s and assures a turbulent flow. The next step, whch should be followed to determne the heat transfer between sol and ar, s to calculate the lnear thermal resstances R cd, R cv : 1 De 1 Rcd = ln, Rcv = [m.k/w] (4) 2πλ D πdα R cd s the thermal resstance due to conducton heat transfer between the nner surface of the ppe and ts outer surface; R cv s the thermal resstance specfc to the convectve heat transfer between the ppe wall (ts nner surface) and the ar flowng nsde; If the values of the thermal resstances are known, the overall heat transfer coeffcent can be estmated: [ D( R + R )] 1 k π [W/(m 2.K)] (5) s = cd cv The heat flux delvered by the ground s the heat gan by the ar (or heat loss durng summer): Q & = k S t = c m& t t ) (6) s m pa a ( a, o a, where t m s the logarthmc mean temperature dfference between ar and sol, c pa s the ar specfc heat and m& a s mass flow rate of ar. From equaton (6), the heat transfer surface (S) s obtaned and also the ppe length derved from t: S L= [m] (7) π D In the second part of the work, the thermal effcency of the ppe vares between 0.5 and Whle the ground and ar temperature at the EAHX nlet are mantaned constant, the ar temperature at EAHX outlet changes accordng to the thermal effcency of the ppe. The ppe length ranges from 11.5 m to 76 m as ts thermal effcency rses (fg. 3). As t was expected, an earth to ar heat exchanger s more effcent f the length of the tube s longer. The ar temperature at the EAHX outlet s growng wth the ncreasng of the ppe length as t can be seen n fg. 4. Ppe length, L (m) ,50 0,55 0,60 0,65 0,70 0,75 0,80 0,85 0,90 0,95 0,99 Thermal effcency, ε Fg. 3: The nfluence of the thermal effcency on the ppe length EAHX outlet ar temperature, ta,o (ºC) Ppe length, L (m) Fg. 4: The nfluence of the ppe length on the ar temperature at the EAHX outlet The temperature of the preheated fresh ar from the ppe outlet s rased by an HRV system descrbed above. The outgong exhausted ar (t a,e ) has 20 o C and t contans a substantal amount of heat that s transferred to the fresh ar n the HRV system. The temperature at the HRV unt outlet depends on ts thermal effcency. Accordng to the nformaton delvered by the manufacturer of the utlzed heat recovery unt, the thermal effcency s of 91% [10]. From the thermal effcency equaton: ISBN:

5 Recent Researches n Energy, Envronment, Entrepreneurshp, Innovaton = t t R a, o ε R, (8) ta, e ta, o the temperature of the ar at the heat recovery unt outlet (t R ) s obtaned. Not only the outlet temperature of the EAHX depends on the ppe length but also the temperature at the outlet of the HRV unt. The ar temperature reaches hgher values because the heat transferred from the ground ncreases wth the tube length. Fg. 5 llustrates the varaton of the ar temperature at the HRV unt outlet accordng to the tube length. To meet the nteror comfort condtons, the fresh ar s further heated by electrcal resstances. HRV outlet ar temperature, t R (ºC) 18,6 18,4 18,2 18,0 17,8 17,6 17, Ppe length, L (m) Fg. 5: The nfluence of the ppe length on the temperature at HRV unt outlet Annual expenses crteron, Z (EUR/year) ,50 0,55 0,60 0,65 0,70 0,75 0,80 0,85 0,90 0,95 1,00 Thermal effcency, ε Fg. 6: Varaton of the annual expenses as a functon of the tube thermal effcency In order to fnd the optmum length of the tube, the followng annual expenses crteron s used: Z = c I + C [ /year] (9) where: c s the economcal effcency coeffcent, I represents the nvestment cost and C s the annual operatng costs. The nvestment refers to drllng costs and the acqurng cost of the EAHX. The operatng cost takes nto account the power consumpton of the ar fan, the HRV unt ventlators and the electrcal resstances. The pont where the EAHX operates optmally s the one correspondng to the lowest value of the annual expenses. As the fg. 6 presents, ths value s reached for a tube thermal effcency of about 90%. An optmal ppe length of 38 m corresponds to ths pont. Consderng the calculated length of the tube and the avalable land surface, the confguraton of the EAHX ppe from the passve house from the Poltehnca Unversty of Bucharest s the one presented n the fg. 7. N Condensaton dran Ar ntake House Fg. 7: Confguraton of the EAHX tube from the Poltehnca house (a) Fg. 8: Installaton of the EAHX tube a) man sde wth fresh ar ntake; b) short sde wth condensaton dran and house connecton (b) To beneft of the heat regeneraton of the ground provded by sun, the man sde of the tube s bured on the east sde of the buldng n an unshaded area. In the fg. 8a a pcture from the nstallaton of the man sde of the ISBN:

6 Recent Researches n Energy, Envronment, Entrepreneurshp, Innovaton EAHX s presented. In the background the ar ntake can bee seen. The fg. 8b presents the short sde of the ground heat exchanger wth the house connecton n the foreground and the condensaton dran n the center vew. The future work ntends to study the varaton of ar temperature at the EAHX ext and the constructon of a forecast chart for the predcton of the addtonal heat need, dependng on clmatc condtons durng the year. 4 Concluson Insde the campus of Poltehnca Unversty of Bucharest two low energy houses whch are ntended for passve house certfcaton, are under constructon. To reduce the prmary energy consumpton for heatng and coolng the ventlaton ar, one of these houses s equpped wth an earth to ar heat exchanger (EAHX). An algorthm for the EAHX tube length szng s purposed. The frst part of ths work serves n szng the geothermal system, startng wth the tube parameters, the earth and outsde ar temperature. For proper szng of the tube length, the extreme values for the two temperatures were consdered. The calculaton tool conssts n usual equatons typcal for a heat exchanger szng. In the second part, economc aspects are taken nto consderaton. The EAHX s a complex system whch has to be consdered as a whole. After preheatng n the EAHX tube, the ventlaton ar gets to the satsfactory temperature due to the other two systems connected to ths devce: a heat recovery unt and electrcal resstances. In order to apply the economcal crteron, all energy consumptons are taken nto account. The calculaton reveals that not only the outlet EAHX ar temperature depends on the ppe length but also the electrcal energy consumpton of the ar fan, the HRV unt and the electrcal resstances. The optmzaton calculaton n order to determne the EAHX tube length was developed as a functon of the tube thermal effcency. As the thermal effcency ncreases, the tube length has an exponental ncrease. Ths evoluton has an mportant nfluence to the ar temperature at the EAHX outlet that regsters a sgnfcant growth. The ar temperature at the heat recovery unt outlet has also a slght ncrease. The pont where the EAHX optmally operates s the one correspondng to the lowest value of the annual expenses. Ths value s reached for a tube thermal effcency of about 90%. An optmal ppe length of 38 m corresponds to ths pont. Takng nto account the calculated length of the tube and the avalable land surface, a confguraton n a sngle loop was chosen. The man sde of the tube s bured on the east sde of the buldng n an unshaded area, to beneft of the ground heat regeneraton provded by sun. Acknowledgements The work has been co-funded by the Sectoral Operatonal Programme Human Resources Development of the Romanan Mnstry of Labour, Famly and Socal Protecton through the Fnancal Agreement POSDRU/89/1.5/S/62557 References [1] Kaan H.F., Boer B.J., Passve Houses: Achevable Concepts for Low CO 2 Housng, ISES Conference 2005, Orlando, USA, September 2005 [2] Fest W., Schneders J., Dorer V., Haas A., Re-nventng ar heatng: Convenent and comfortable wthn the frame of the Passve House concept, Energy and Buldngs, vol. 37, ssue 11, 2005, pp [3] Audenaert A., De Cleynb S.H., Vankerckhove B., Economc analyss of passve houses and lowenergy houses compared wth standard houses, Energy Polcy, vol. 36, 2008, pp [4] Antonova A., Passve house for Latva - Energy effcency and techncal-economc aspects, Lund Unversty, Sweden, MoS Thess, 2010 [5] Kolle J., The Passve House: Green Wthout Gzmos, Fne Homebuldng, no. 210, March 2010, pp [6] Schwatzenberger L., Mason passve et pompe therme à chaleur, S.A.E.P, France, 2008 [7] Flordes G., Kalogrou S., Ground heat exchangers A revew of systems models and applcatons, Renewable Energy, vol. 32, 2007, pp [8] Loyau F., Put canaden et ventlaton a basse énerge, I Inedte, France, 2009 [9] Benkert F., Hedt D., Schöler D., Calculaton tool for earth heat exchangers GAEA, Proceedngs Buldng Smulaton '97, Ffth Internatonal IBPSA Conference, Prague, Czech Republc, September 8-10, 2007 [10] PAUL Wärmerückgewnnung, Compact Mechancal Ventlaton Heat Recovery Unt Focus 200, Product nformaton, 2010 ISBN: