Performance and Optimum for a Ground-Coupled Liquid Loop Heat Recovery Ventilation System

Transcription

1 urdue University urdue e-us International Refrieration and Air Conditionin Conference School of Mechanical Enineerin 006 erformance and Optimum for a Ground-Coupled Liquid Loop Ht Recovery Ventilation System Yasu Zhou Donhua University er Fahlén Chalmers University of Technoloy Torjoern Lindholm Chalmers University of Technoloy Follow this and additional works at: Zhou, Yasu; Fahlén, er; and Lindholm, Torjoern, "erformance and Optimum for a Ground-Coupled Liquid Loop Ht Recovery Ventilation System" (006). International Refrieration and Air Conditionin Conference. aper This document has een made availale throuh urdue e-us, a service of the urdue University Liraries. lse contact epus@purdue.edu for additional information. Complete proceedins may e acquired in print and on CD-ROM directly from the Ray W. Herrick Laoratories at Herrick/Events/orderlit.html

2 Donhua R07, ae ERFORMANCE AND OTIMUM FOR A GROUND-COULED LIQUID LOO HEAT RECOVERY VENTILATION SYSTEM Yasu ZHOU, er FAHLEN and Torjörn LINDHOLM University, Department of Buildin Environment and Equipment Enineerin, Shanhai 0005, China zhouys@dhu.edu.cn Chalmers University of Technoloy, Department of Buildin Services Enineerin, SE-4 96, Gothenur, Sweden ABSTRACT Ground-source ht pump systems can e desined to include liquid-loop Ht Recovery Ventilation (HRV), free coolin and recharin of the orehole collector. The HRV-system uses an exhaust-air coil to warm rine that has een prehted y a orehole ht exchaner. The warm rine then hts a supply-air coil and returns to the orehole. The performance of the exhaust-air and supply-air coils has an influence on the HRV system efficiency. This paper presents a mathematical model to investiate how the rine flow rate and the allocation ratio etween the exhaust and supply-air coils affect the ht recovery efficiency.. INTRODUCTION Ground-coupled ht pumps are commonly used in Swedish residential htin systems. The most popular ht source is a vertical orehole and there are now more than 30,000 systems in operation. In the oreholes, temperature will slowly drop with operatin time and hence CO of the ht pump as well as the ht extraction will drop. To mitiate this situation, a deeper orehole is usually the most cost-effective solution for a new installation. However, recharin the orehole may e a etter method to prevent a lon-term deradation of performance and even improve on the initial results. Recharin can e used for oth new and existin installations and will compente for the extracted enery and resultin lon-term temperature drop. Most Swedish houses have mechanical ventilation and exhaust-air is a fsile ht source availale all the yr. The effect of recharin the orehole to improve the performance of a round-coupled ht pump system has een examined y experimental [Fahlen, 00] and theoretical resrch [Claesson et al., 985]. A round-coupled ht recovery ventilation system includes three ht exchanes (see fiure ): two coils are connected with the orehole ht exchaner. The loop rine was warmed y the exhaust coil, and then warm the supply coil and finally ein prehted y the orehole ht exchaner. Hence it is complicated to determine the operatin conditions for maximum ht recovery efficiency. This paper set up a mathematical model to etter understand the characteristics of a round-coupled HRV system. The model can also assist in calculatin the overall ht recovery efficiency and in findin the optimal rine flow rate. Finally, a model will e helpful in the srch for the optimal ht transfer allocation ratio etween the exhaust and supply coils. An optimal comination implies the maximum ht recovery efficiency.. MATHEMATICAL MODEL In a round-coupled liquid-loop HRV system the rine extracts ht from the exhaust coil to ht the supply coil. After ein cooled y the supply-air, the rine is prehted y the orehole ht exchaner. Usin the effectiveness- Ntu method, and considerin the enery alance, a set of equations can e written to descrie ht transfer in the round-coupled HRV system. International Refrieration and Air Conditionin Conference at urdue, July 7-0, 006

3 R07, ae Exhaust -air coil Supply -air coil t m. t V t. V t t. V t t Borehole t Fiure : A round-coupled liquid-loop HRV system The exhaust-air coil C & V& ρ a c p, a [W/K] () Q C ( t t ) [W] () Q C ( tm t ) [W] (3) Q & ε Q&,max [W] (4) Q C ( t t ) [W] (5), max,min Ntu,min [-] (6),min ε f ( Ntu, ) [-] (7),max The supply-air coil C & V& ρ a c p, a [W/K] (8) Q C ( t t ) [W] (9) Q C ( tm t ) [W] (0) Q & ε Q&,max [W] () Q C ( t t ) [W] (), max,min m Ntu,min [-] (3),min ε f ( Ntu, ) [-] (4),max The orehole ht exchaner & V& ρ c [W/K] (5) C p, International Refrieration and Air Conditionin Conference at urdue, July 7-0, 006

4 R07, ae 3 Q ( ) C t t [W] (6) Q ( t t ) [W] (7) Ntu [-] (8) ε exp( Ntu ) [-] (9) t t ε [-] (0) t t Equation () defines the ht recovery efficiency of a HRV system, which is applicale also to a round-coupled system: t t η [-] () t t Oviously, the types and sizes of the three ht exchaners, their ht transfer capailities (U A)BB, (U A)BB and (U A)BB, the air and rine flow rates V &, V &, and V &, as well as the operational temperatures tbb, tbb and tbb, will affect the system ht recovery efficiency. In order to achieve the maximum ht recovery efficiency, there are two optimisin tasks. The first is to decide on the est allocation of the individual ht transfer capacities (U A)BB, (U A)BB and (U A)BB within a fixed total capacity (i.e. a fixed first cost). The second is to determine the optimal rine flow rate V & in the loop. To simplify the investiation of the efficiency of a HRV system with three ht exchaners, we define the followin three dimensionless parameters: m U A n U A Cr [-] () where (U A) is the total overall ht transfer coefficient of the exhaust coil and the supply coil, i.e. U A +. Hence, the ht recovery efficiency η of the round-coupled HRV system is related to the parameters m, n, CBrB, tbb, tbb, and tbb. Unfortunately, it is difficult to express the efficiency η explicitly as a function of these parameters. Therefore, a numerical analysis was made usin the prorammin platform EES [EES, 005]. 3. NUMERICAL ANALYSIS For a traditional liquid-coupled HRV system with two-coil, alanced ht capacity flow rates C & provide the maximum thermal efficiency [Ior et al., 003] with equal ht transfer capailities of the exhaust and supply coils. For this round-coupled HRV system with three ht exchaners, what relationship etween three ht exchaners can we et the maximum thermal efficiency? For investiatin the characteristics, we chose an existin installation [Fahlen, 00] with an exhaust-air coil for recharin of a orehole collector as a case study. In this, we investiated the effects of varyin the parameters that affect the thermal efficiency of the descried three-ht exchaner HRV system. 3. Conditions of the case study International Refrieration and Air Conditionin Conference at urdue, July 7-0, 006

5 R07, ae 4 A mathematical model ased on the previous discussion was used to analyse the ht recovery efficiency of a round-coupled HRV system in a sinle-family house [Fahlen, 00]. The house has an exhaust-air-to-rine coil which is a sinle-pass, cross-counter-flow ht exchaner with oth fluids unmixed. And the house is also equipped with an identical supply-air coil. The rine-side of the coils is connected to a round ht exchaner in a vertical orehole with L 60 m, DBhB 5 mm, and DBoB 33.4 mm (a U-tue made of polyethylene). The rine is a 30 % ethanol-water mixture. In the calculations the followin data, larely ased on msurements, are used: thermal - - conductivity of the edrock λ B B 3.5 W m K, undistured round temperature tbb 6.5 C, exhaust-air temperature tb B0 C, and air flow rate V & V & 3 65 m /h. 3.. Optimisin rine flow rate Usin the data of 3. as input to the model, as well as assumin outdoor air temperature tb B -0 C, the ht recovery efficiency of the round-coupled HRV system was calculated. Ground orehole ht exchane transfer capaility (U A)BB is time dependent, so the efficiency for different rine flow rates was calculated after one hour of operation. Fiure a shows that there is an optimal rine flow rate ivin a maximum ht recovery efficiency of the system. The ht recovery efficiency η decrses rapidly when the rine flow rate oes elow the optimum, ut it is much less sensitive for flow rates exceedin the optimal value. The optimal flow rate is desinated CBr,optB. In the current example, CBr,optB.5 (see fiure a). From calculation result, also ettin m (U A)BB/(U A) 0.5, and n (U A)BB/(U A) a) ) η Cr, opt 0, 0, 0 0 C r,opt C r C /C η C r,opt,9,8,7,6,5,4,3,, 0, 0,7 m(u A) /(U A) Fiure : The optimal rine flow rate (a) for m 0.5 and n 0.68, and the effect () of the allocation ratio m of (U A) on optimal rine flow rate for n 0.68, tb B -0 C. Maintainin the overall U A and the other input data of the case study constant, the allocation ratio etween the exhaust coil and the supply coil can e chaned. This implies chanin the dimensionless parameter m to study the effects on system performance. Fiure illustrates the resultin relation etween CBrB,BoptB and the dimensionless parameter m. The diaram shows that the optimal flow rate CBrB,BoptB depends to some extent on the allocation ratio m. The optimal rine flow rate decrses with incrsin exhaust-air coil ht transfer surface. The effect on CBrB,BoptB, however, is wk Optimisin allocation ratio etween the exhaust coil and the supply coil The relative size of (U A)B Band (U A)BB will affect the performance of the HRV system. The question is how to allocate the ht transfer capacity to ch of the two coils to achieve maximum efficiency. Assumin that the total overall ht transfer coefficient (U A) is fixed, fiure 3a shows that there is a maximum ht recovery efficiency of the system when the allocation ratio m is chaned. For ch allocation ratio m, the system operates at the optimal rine flow rate. Fiure 3a indicates that the optimal allocation ratio is mbopt B This mns that (U A)BB is rouhly one third of the total overall ht transfer capacity (U A), i.e. when (U A)B B /3 (U A) or (U A)B B / (U A)BB, the system provides a maximum ht recovery efficiency. International Refrieration and Air Conditionin Conference at urdue, July 7-0, 006

6 R07, ae 5 Fiure 3 shows the effect of the ratio n of (U A)BB and (U A) on mboptb. In the rane n 0. to n, mboptb only chanes from 0.36 to 0.3. The averae value of mboptb is 0.33 and thus mboptb /3. Therefore, in a round-coupled HRV system, the exhaust-air coil size should e half of that supply-air coil size which provides the maximum ht recovery efficiency. A rson for allocatin more capacity to the supply coil is that this coil handles the total capacity of the round and exhaust-air coils. a) ) η m ax 5 η Cr,opt m opt 0, m opt 0,7 m(u A) /(U A) 0, 0, 0, 0,7 0,8 0,9 n(u A) /(U A) Fiure 3: The effect (a) of the allocation ratio m of U A on ht recovery efficiency for n 0.68 and the effect () of the ratio n of (U A)BB and U A on the optimal allocation ratio mboptb. Outdoor temperature tb B -0 C Effect of outdoor temperature on the HRV system performance The outdoor-air temperature determines the thermal load of the exhaust and supply coils and hence affects the ht recovery efficiency of the round-coupled HRV system. Fiure 4a shows that the efficiency incrses when the outdoor temperature decrses. The diaram provides information at four different ratios n of (U A)BB and U A. The round coil transfer capacity (U A)BB was fixed at the value of the case-study and the total capacity of the air coils was chaned. The ratio n mns U A (U A)BB, and n 0. mns U A 0 (U A)BB. As shown y fiure 4a, the larer U A is and the lower the outdoor temperature is, the more enery is transferred to the supply air in a roundcoupled HRV system. At low outdoor temperatures, there will e an incrsed enefit form the round-coil as the rine temperature after the supply coil drops to a level that is lower than that of the round. a) 0,9 ),5 ηmax 0,8 0,7 n C r,opt,5 n , t [ C] t [ C] Fiure 4: The effect of outdoor temperature on ht recovery efficiency (a) for m /3 and on optimal rine flow rate () for m /3. Fiure 4 shows that the optimal rine flow rate will incrse with a decrsin outdoor temperature. The diaram also illustrates that for different ratios n, the outdoor temperature influence on the optimal rine flow rate will e International Refrieration and Air Conditionin Conference at urdue, July 7-0, 006

7 R07, ae 6 different. The larer U A is, the less is the optimal rine flow rate affected y the outdoor temperature. For example, fiure 4 indicates that for n 0., i.e. U A 0 (U A)BB, the optimal rine flow rate may e kept approximately constant durin the whole htin sson. Hence a constant speed rine pump can e used and still provide a maximum efficiency Effect of the ht transfer capacity of the air and round coils on ht recovery efficiency Improvin ch ht exchaner performance must improve the total HRV system performance. The question is which one is more efficient to improve the system performance. Fiure 6a shows, when keepin the me (U A)BB as in the case-study the ht recovery efficiency is improved y chanin the U A. With an incrsed U A, see curve in fiure 5a, efficiency incrses, and the maximum efficiency is very sharp. Deviatin from the optimal flow rate, the efficiency decrses rapidly. On the other hand, with a decrse of U A, see curve 3, the efficiency decrses and the efficiency curve is flat which implies that the maximum ht recovery efficiency is not very sharp. a) 0,8 ) 0,8 0,7 0,7 η η 3 3 0, , C r C /C C r C /C Fiure 5: The effect of U A (a) with (U A)B B (U A)BB,Bcase-studyB and (U A)BB () with U AB B (U A)Bcase-studyB on the ht recovery efficiency and optimal flow rate for m /3 and tb B -0 C. Desinations in the diarams: a) : U A 3 (U A)Bcase-study, B: data from case-study, 3: U A /3 (U A)Bcase-studyB; ) : (U A)B B 3 (U A)B,case-studyB, : data from case-study, 3: (U A)BB /3 (U A)B,case-studyB Fiure 5 shows that when keepin the me U A as in the case-study, chanin (U A)BB improves the ht recovery efficiency. However, compared with the effect of U A in fiure 6a, the effect of (U A)BB is wk. Curve in fiure 5 indicates that even incrsin (U A)BB to three times the value of the case-study, the efficiency only improves a little. This indicates that the effect of U A on the ht recovery efficiency is larer than the effect of (U A)BB with the prerequisites of the current study. If the round coil is excluded, the maximum efficiency oes down from 58.4 % to 48.6 % with equal ht transfer capailities of the exhaust and supply coils (U A)B B (U A)BB and alanced ht capacity flow rates CBrB. Compared to fiure 3a, which shows that the maximum efficiency is 58.4 %, the roundcoupled HRV system could make the ht recovery efficiency incrse 0 % when keepin the me U A as in the case study. This is without the extra enefit of not needin a defrost sequence Brine inlet temperature to the exhaust coil To maintain a hih ht transfer rate, condensed water vapour must not freeze on the exhaust air coil. This can e avoided if the rine temperature enterin the exhaust coil is hiher than zero (0 C). In a round-coupled HRV system, the rine temperature enterin the exhaust coil is pre-hted y the orehole ht exchaner. Fiure 6 shows that the rine temperature decrses with the outdoor temperature. However, it never oes elow zero (0 C) in the rane of outdoor temperatures of fiure 6 and hence there will e no frostin and defrostin will not e needed for the round-coupled HRV system. Experience from conventional recuperative HRV systems has shown that in the est of circumstances, the efficiency drops y at lst 0 % when defrostin ecomes necesry. The roundcoupled system ecomes correspondinly more favourale durin that part of the yr when ht recovery and prehtin of supply air is needed the most. International Refrieration and Air Conditionin Conference at urdue, July 7-0, 006

8 CBrB capacity R07, ae 7 7 6,5 t [ C] 6 5,5 5 4, t [ C] Fiure 6: The rine inlet temperature at the exhaust coil for n 0.68 and m /3. 4. CONCLUSIONS A round-coupled liquid loop HRV system, with three ht exchaners, has an optimal allocation ratio etween the exhaust coil and the supply coil and the optimal rine flow rate that provides the maximum ht recovery efficiency. The optimal allocation ratio will depend on the specific conditions of the uildin. In the current case-study, it was determined that the ht transfer capaility of the exhaust air coil should e half that of the supply air coil, i.e. (U A)BB/ (U A)BB. The optimal rine flow rate varies with the outdoor temperature and the ratio of (U A)BB and U A. A larer optimal rine flow rate will e needed for a lower outdoor temperature. The larer U A is, the less the outdoor temperature affects the optimal rine flow rate. When U A is larer than ten times (U A)BB, it is possile to operate with a constant optimal rine flow rate for the entire htin sson. The performance of the exhaust-air and supply-air coils has a stron influence on the possiility to improve the efficiency of a round-coupled HRV system. The larer the value of U A is and the lower the outdoor temperature is, the more enery can e transferred to the supply air. Furthermore, ecause of the orehole ht exchaner, the rine temperature enterin the exhaust coil never oes elow zero (0 C) in optimal operation. Hence, contrary to the case of conventional liquid-loop HRV systems, defrostin will not e needed for the round-coupled HRV system. This can make the round-coupled system over 0 % more efficient than a conventional recuperative system at low outdoor temperatures. Also, the efficiency incrses when the outdoor temperature decrses. This is a desirale characteristic in most uildins since normally the efficiency has to e turned down anyway at risin outdoor temperatures due excess ht from internal loads. ACKNOWLEDGEMENTS I would like to thank the China State Scholarship Fund for supportin a one-yr visit as a uest resrcher in Buildin Services Enineerin at Chalmers University of Technoloy. NOMENCLATURE C & ht capacity flow rate ( V& ρ c p ) [W K- ] rate ratio ( C r / ) [-] c - - specific ht capacity [J k K ] m overall ht transfer coefficient ratio (m (U A)BB/(U A)) n overall ht transfer coefficient ratio (n (U A)BB/(U A)) Ntu the numer of ht transfer units [-] Q & ht transfer rate [W] International Refrieration and Air Conditionin Conference at urdue, July 7-0, 006

9 R07, ae 8 t U A V & Celsius temperature [ C] - ht exchaner transfer capaility [W K ] 3 - volumetric flow rate [m s ] Greek symols ε ht exchaner effectiveness [-] η round-coupled ht recovery efficiency [-] - - λ thermal conductivity, [W m K ] 3 ρ density [k m ] Suscripts rine h orehole m middle parameter etween exhaust coil and supply coil exhaust-air or exhaust-air coil round supply-air or supply-air coil Areviations EES Enineerin Equation Solver HRV Ht Recovery Ventilator REFERENCES Claesson, J., Eftrin, B., Eskilson,., Hellstrom, G., 985, Ground ht - A source-ook for thermal analysis - art III: Natural ht sources (in Swedish), Swedish Council for Buildin Resrch, BFR-report T8: 985, Ch.0, Stockholm, Sweden. EES 005.Software: Enineerin Equation Solver V th Fahlén,., 00, Ground-source ht pumps Recharin of oreholes y exhaust-air coils, 7 IEA Ht ump Conference, vol., p , Beijin, China. Ior Balen, etc Analysis of the coil enery recovery loop system. International Journal of Enery Resrch 7: International Refrieration and Air Conditionin Conference at urdue, July 7-0, 006