THE ENERGY SAVING POTENTIAL OF MEMBRANE-BASED ENTHALPY RECOVERY IN VAV SYSTEMS FOR COMMERCIAL OFFICE BUILDINGS

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1 SiBuild Fift National Conference of IBPSA-USA THE ENERGY SAVING POTENTIAL OF MEMBRANE-BASED ENTHALPY RECOVERY IN VAV SYSTEMS FOR COMMERCIAL OFFICE BUILDINGS Liping Wang, Pilip Haves, and Jon Bresears 2 Lawrence Berkeley National Lab, Berkeley, CA 2 Arcitectural Applications, Portland, OR ABSTRACT A design tool to evaluate te eat and ass transfer effectiveness and pressure drop of a ebrane-based entalpy excanger was developed and ten used to optiize te configuration of an entalpy excanger for iniu pressure drop and axiu eat recovery effectiveness. Siulation was used in a paraetric study to investigate te energy saving potential of te entalpy recovery syste. Te case witout energy recovery and air side econoizer was used as a baseline. Two coparison cases for te ipleentation of entalpy recovery wit and witout air side econoizer were siulated in EnergyPlus. A case using a desiccant weel for energy recovery was also investigated for coparison purposes. Te siulation results sow significant energy saving benefits fro applying a low pressure drop, ig effectiveness entalpy excanger in two US cities representing a range of uid cliates. Te sensitivity of te energy savings potential to pressure drop and eat and ass transfer effectivenesses is also presented. INTRODUCTION Entalpy excangers transfer sensible and latent eat between exaust air and outdoor ventilation air to reduce eating and cooling loads due to ecanical ventilation in HVAC systes. Entalpy recovery as significant benefits for building types requiring ig ventilation rates to dilute indoor air pollutants, and/or tose cliates were outdoor air ventilation loads accounts for a large fraction of total cooling loads. Mebrane-based entalpy recovery eploys a pereable ebrane to enable bot eat and ass transfer between exaust air and outdoor air. Using a large surface area, wit low air flow rate results in a ig effectiveness and a sall increase in fan power. Tere ave been a few recent studies to investigate te perforance of ebrane-based entalpy recovery. Zang and Jiang (999) conducted experients and nuerical analysis to investigate te perforance of ebrane-based entalpy recovery. Min and Su (200) developed a ateatical odel to predict te teral and ydraulic perforance of a ebrane-based energy recovery ventilator, and investigated te relationsip between entalpy effectiveness and cannel eigt and pressure drop. Min and Su (20) investigated te effects of air teperature and uidity on te perforance of a ebrane-based energy recovery ventilator operating in bot ot and cold conditions. Nasif, et al. (200) assessed te teral perforance of a ebrane-based entalpy excanger. Teperature and uidity ratio easureents were carried out to deterine te sensible and latent effectivenesses. Te perforance of an HVAC syste coupled to te ebrane-based entalpy excanger was siulated and te results sowed savings of up to 8% in annual energy consuption in a uid cliate. Te purposes of te work reported ere were ) to develop a tool for paraetric evaluation of various configurations of entalpy excanger, and 2) to evaluate te energy saving potential of ig perforance entalpy recovery caracterized by low pressure drop and ig effectiveness wen used wit a conventional variable-air-volue (VAV) HVAC syste. TOOL FOR HEAT AND MASS TRANSFER ACROSS A WATER-PERMEABLE MEMBRANE A spreadseet-based tool for calculating pressure drop and effectiveness for ebrane-based entalpy recovery was developed to conduct paraetric studies of various entalpy recovery configurations. Te tool was designed to identify ig perforance configurations, caracterized by ig eat and ass transfer effectivenesses and low pressure drop. Te tool is capable of calculating effectiveness and pressure drop for bot cross-flow and counter flow geoetries, eac wit a defined nuber of parallel ebrane layers. Te tool inputs define te excanger geoetry, te properties of te ebrane and te 60

2 SiBuild Fift National Conference of IBPSA-USA teral properties of air. Bot flat and pleated configurations can be studied using te tool; it was found tat flat plate configuration yielded a ore favorable trade-off between effectiveness and pressure drop. Te tool uses epirical correlations for eat and ass transfer coefficients and pressure drop relationsips listed in ASHRAE (2009), togeter wit te standard effectiveness- relationsips for te counterflow and cross-flow excanger configurations. Te sceatic diagra of te ebrane-based entalpy recovery is sown in Figure. It is assued tat air flow is uniforly distributed inside eac cannel. Outdoor Air Relief Air After ER Figure Sceatic diagra of ebrane-based entalpy recovery excanger (OA-outdoor air, ERentalpy recovery) Te calculation of effectiveness assues equal ass flow rates and equal capacity rates on eac side of te excanger. Te eat excanger effectiveness for cross flow wit equal capaity rates and bot streas unixed is: e exp Te eat excanger effectiveness for counter flow wit equal capaity rates is: () (2) Te ass excange effectiveness is analagous te eat excange effectiveness. Te ass excange effectiveness for cross-flow wit equal ass flow rates and bot streas unixed is: e exp and for counter flow is: 0.78 OA After ER Relief Air 0.22 (3) (4) were and are te Nuber of Transfer Units, defined by Eq. 5 and Eq. 6, for eat and ass transfer respectively. C, defined by Eq. 7, is te capacity rate of eiter air strea, is te air ass flow rate (kg/s) and c p (J/(kgK)) is te specific eat. U A/ C U A/ C c p Te total eat transfer coefficient U (W/ 2 K) is: U s e (5) (6) (7) (8) were s (W/ 2 K) and e (W/ 2 K) are te convective eat transfer coefficients for outdoor air strea and relief air strea, respectively, δ () is te tickness of te ebrane and λ (W/K) is its teral conductivity. Te total ass transfer coefficient is: U k s r k e (9) were k s (/s) and k e (/s) are te convective ass transfer coefficients for te outdoor air strea and te relief air strea, r (s/) is te te ass transfer resistance of te ebrane. Te convective eat transfer coefficient (W/ 2 K) and convective ass transfer coefficient k (/s) can be expressed in ters of te Nusselt nuber and te Serwood nuber (analogous to te Nusselt nuber) respectively. Te Nusselt nuber is: Nu D / and te Serwood nuber is: S kd / D v () (2) were D () is te ydraulic diaeter and D v ( 2 /s) is te diffusivity of water vapor. Te results were exained over a broad range of flow regies. However, te design strategy adopted for te ebrane-based entalpy recovery excanger was to aintain operating conditions witin te lainar flow regie in order to iniize te ratio of te pressure drop to te eat and ass transfer. In regular-saped cannels, lainar flow occurs wen te Reynolds nuber, Re, is less tan 2300, transitional flow occurs wen Re is in te range and turbulent flow occurs wen Re > Te relationsips (Eqs. 3 6

3 SiBuild Fift National Conference of IBPSA-USA and 4) used to calculate te Nusselt and Serwood nubers apply to partially developed lainar flow and are taken fro ASHRAE (2009) ( D / L) Re Pr Nu 3.66 (3) 2 / [( D / L)Re Pr] 0.065( D / L)Re Sc S [( D / L)Re Sc] 2 / 3 (4) were L is te lengt of te flow cannel. For flat plate configurations, te reasons for using te relationsips for partially developed lainar flow are as following. ) Te flow pattern witin te entrance lengt of te cannel is not fully developed. 2) Te relationsips in ASHRAE (2009) to calculate te Nusselt and Serwood nubers for developed lainar flow are valid for unifor surface teperature or unifor eat flux; owever, tese unifor boundary conditions do not apply in te designs considered ere. EVALUATION OF CONFIGURATIONS To identify te optiu eat excanger configuration, paraetric studies for flat plate ebranes and pleated ebranes was copleted for various featured paraeters including aspect ratio, energy recovery area per odule, nuber of layers, nuber of pleats (for pleated configurations) or gap diension (for flat plate configurations, te separation distance between unifor ebrane layers). A counterflow configuration was assued in all cases. Unless stated oterwise, te value of te water vapor pereance of e ebrane used in te study was fixed. As sown in Figure 2, wen te nuber of layers is increased, te effectiveness increases and pressure drop decreases for flat configuration. Tis is also true for pleated configuration. SAVINGS ASSESSMENT METHODOLOGY Buildings in two cliates in te United States were siulated to investigate te benefits of te tecnology under consideration. Miai, FL (DOE cliate zone: A) as a tropical onsoon cliate wit ot and uid suers and sort, war winters. Atlanta, GA (DOE cliate zone: 3A) as a uid subtropical cliate wit ot uid suers and ild winters. Entalpy recovery was ipleented in te EnergyPlus odel (EnergyPlus, a, b) for te DOE Bencark Mid-size Coercial Office Building (Coercial Reference Buildings, 20). Tis odel is widely used to assess and copare te ipact of various energy efficiency easures and systes. Te building, illustrated in Figure 3, as tree stories, 5 teral zones and a floor area of 4, Te HVAC syste is a packaged VAV syste wit direct expansion (DX) cooling coils, gas-fired eating coils, supply fans and VAV terinal units wit electric reeat. Configurations wit and witout econoizers were siulated. Tree different sizes of entalpy excanger were ipleented. Teir effectivenesses are sown in Table and L, M and H denote low effectiveness, ediu effectiveness, and ig effectiveness, respectively. Figure 2 Te effects of nuber of layers on te effectiveness and pressure drop for te flat configuration wit aspect ratio =, noralized energy recovery area per layer=.2 2/ (L/s air), gap diension=3 Figure 3 Geoetry of te DOE bencark id-size coercial office building Table Entalpy recovery effectiveness L M H 75% 00% 75% 00% 75% 00% Sensible Latent

4 SiBuild Fift National Conference of IBPSA-USA EVALUATION OF ENERGY SAVING POTENTIAL FOR CONVENTIONAL VAV SYSTEMS Six different cases were investigated for conventional VAV syste. Case : Baseline: no entalpy recovery, no econoizer A fixed iniu outdoor air flow rate is provided during occupied periods to aintain acceptable indoor air quality. A diagra of te conventional VAV syste wit te entalpy recovery ventilation is sown in Figure 4. Te entalpy excanger is connected to te air side econoizer to pre-condition te outdoor supply air at ties wen te difference between te return air entalpy and te required supply air entalpy is less tan te difference between outside air entalpy and te required supply air entalpy. Case 2: Air side econoizer, no entalpy recovery Te syste includes an outdoor air econoizer, wic can odulate te outside air fraction daper to provide free cooling wen te outdoor air conditions are favorable. Case 3: Entalpy recovery, no econoizer Te optiized recovery tecnology pre-conditions a fraction of te outside air wen te return air entalpy is ore favorable wenever HVAC syste is operating. Case 4: Entalpy recovery and air-side econoizer Te entalpy recovery ventilator is fully bypassed wen te air-side econoizer is in free cooling ode and wen te outdoor air condition is ore favorable tan tat of te return air, even wen te outside air flow rate is at its iniu value. Wen not in free cooling ode, a variable fraction of te outdoor air strea by-passes te entalpy excanger in order to eet te set-point of te supply air outlet teperature. In te EnergyPlus odel, te Energy Manageent Syste facility is used to integrate te control of te entalpy excange bypass wit te control of te econoizer. Case 5: Desiccant weel entalpy recovery, no econoizer Case 5 is siilar to Case 3 except tat a desiccant weel is used in place of a ebrane-based entalpy excanger, resulting in a substantially iger pressure drop (~25Pa). Case 6: Desiccant weel entalpy recovery and air-side econoizer Case 6 is siilar to Case 4 except tat a desiccant weel is used in place of a ebrane-based entalpy excanger, resulting in a substantially iger pressure drop (~25Pa). Figure 4 Syste diagra for conventional VAV syste integrated wit energy recovery (optiized syste, OA-outdoor air, EA-return air, ER-entalpy recovery, HC-eating coil, CC-cooling coil) Table 2 suarizes te energy savings predicted for Cases 2-4, relative to te baseline, in ters of HVAC energy use (energy consuption for doestic ot water is not included). Te HVAC syste includes DX cooling, a direct-fired gas eating coil and electric reeat. In Miai cliate, te entalpy excanger cobined wit an outdoor air econoizer provides significant savings (-7%, depending on te effectiveness) wile te savings due to te ipleentation of te econoizer are very sall (%). In te Atlanta cliate, Case 4 (coupled syste) provides te greatest energy savings (7%); te energy savings due to te ipleentation of te econoizer is about 6% as tere are ore ours of airside econoizer use in Atlanta tan in Miai; wile Case 3 (wit entalpy recovery and witout econoizer) provides lower energy savings as an energy penalty is introduced wen entalpy is excanged between te outside air and te return air wen te outdoor conditions are favorable for free cooling. As expected, te energy savings potential increases wen te effectiveness of te excanger is increased. However, weter to select te excanger wit te igest effectiveness needs to be judged on a case-by-case basis, based on a cost/benefit analysis. 63

5 SiBuild Fift National Conference of IBPSA-USA Table 2 Annual HVAC energy savings for various HVAC syste options absolute savings per unit floor area and percentage of te Case baseline. L,M and H refer to ig, ediu and low entalpy recovery effectiveness, as defined in Table. Econ = econoizer, Recov = entalpy recovery. Description CASE 2 CASE 3L CASE 3M CASE 3H CASE 4L CASE 4M CASE 4H Econ only Recov only Recov only Recov only Recov + Econ Recov + Econ Recov + Econ Miai (kj/2) Miai (%) % 0% 3% 5% % 4% 7% Atlanta (kj/ 2 ) Atlanta (%) 6% 2% 3% 3% 5% 6% 7% Energy savings potential for desiccant weels assessed using EnergyPlus siulations are listed in Table 3. A coercial product was selected based on te required outdoor air flow rate and conventional sizing practice. Te latent and sensible effectivenesses and pressure drop for use in te EnergyPlus odel were obtained fro te anufacturer s catalog listed in Table 4. Te savings due to te desiccant weels are significantly less tan for ebrane-based entalpy recovery due to te iger pressure drop. Figures 5 and 6 sow te process lines for te base case and te case wit ediu effectiveness entalpy recovery plotted on te psycroetric cart for te design condition in Miai cliate. To avoid te overlapping of labels for eac point, te teperature rise across te supply fan (~0.5K) is not sown. (Note tat te design condition is suc tat an econoizer, if one were installed, would be in iniu outside ode and so te process lines are independent of weter an econoizer is present.) Figure 5 sows te case witout eat recovery. Te required aount of outdoor air (OA) for ventilation is ixed wit return air (RA). Ten te ixed air (MA) is cooled and deuidified to te supply air condition (SA); te required reduction in te specific entalpy of te supply air is 24.9kJ/kg. Figure 6 sows te process wit entalpy recovery; te outdoor air is pre-cooled by te entalpy excanger, and te required reduction in te specific entalpy of te supply air is 7.9kJ/kg less tan in te base case. Table 3 Energy savings for desiccant weel systes CASE 5 CASE 6 Desiccant weel only Desiccant weel + econoizer Miai (kj/2) Miai (%) 2% 3% Atlanta (kj/ 2 ) Atlanta (%) -% 3% Table 4 Perforance data for desiccant weel 00% Heating Condition 75% Heating Condition 00% Cooling Condition 75% Cooling Condition Sensible Effectiveness 80% 72% 83% 77% 80% 72% 83% 77% Latent Effectiveness Pressure drop (Pa) 25 64

6 SiBuild Fift National Conference of IBPSA-USA Figure 5 Process witout energy recovery and econoizer (OA-outdoor air, MA-ixed air, RA- return air, ROroo air, SA- supply air) Figure 6 Process wit optiized syste coupled wit air-side econoizer (OA-outdoor air, AfterER-outlet of entalpy excanger, MA-ixed air, RA- return air, RO-roo air, SA- supply air) 65

7 SiBuild Fift National Conference of IBPSA-USA To illustrate te perforance of te entalpy excanger, tie series plots of key variables are presented for te suer design day for Miai cliate. Figures 7 and 8 sow ourly teperature and uidity ratio profiles for te outdoor air, te processed outdoor air after entalpy recovery and te relief air. It can be seen tat sensible eat and water vapor are transferred fro te outdoor air to te relief air wen building is occupied and outdoor air conditions are not suitable for econoizer operation. Te ourly fan energy consuption profiles for systes wit and witout entalpy recovery are presented and copared in Figure 9. A sligt fan energy penalty can be identified wen te ebranebased entalpy recovery is on. Te benefits of energy consuptions for HVAC end use can be observed fro Figures 0. Te ourly values of te COP ratio, defined as te ratio of COP for syste wit entalpy recovery to COP for syste witout entalpy recovery, are presented in Figure. Te COP can be increased by as uc as 26% troug te use of te optiized entalpy recovery tecnology. Figure 9 Fan energy consuption profiles for te suer design day (7/2) Figure 0 HVAC source energy consuption profile for te suer design day (7/2) Figure 7 Teperature profile for te suer design day (7/2) Figure COP ratios for te suer design day (7/2) Figure 8 Huidity ratio profile for te suer design day (7/2) 66

8 SiBuild Fift National Conference of IBPSA-USA CONCLUSION A siple design tool was set up to assess te perforance of ebrane based eat recovery for various configurations. Te case of entalpy recovery cobined wit an air side econoizer provides te greatest energy benefit of te various test cases - 7% reduction in annual HVAC energy consuption in a conventional coercial office building in Miai and Atlanta. Te tecnology raises te syste COP by up to 26% over te course of a typical suer design day for Miai cliate. ACKNOWLEDGEMENTS Tis work was supported by te Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Tecnology, State and Counity Progras of te U.S. Departent of Energy under Contract No. DE-AC02-05CH23 and by te U.S. Departent of Energy Advanced Researc Projects Agency - Energy. REFERENCES ASHRAE ASHRAE Handbook: Fundaentals, Atlanta, GA Coercial Reference Buildings 20 ttp:// l_initiative/reference_buildings.tl. EnergyPlus a ttp:// EnergyPlus b ttp://apps.eere.energy.gov/buildings/energyplus/ energyplus_testing.cf. Min, J., Su, M. 200 Perforance analysis of a ebrane-based energy recovery ventilator :Effects of eebrane spacing and tickness on te ventilator perforance. Applied Teral Engineering 30: Min, J., Su, M. 20 Perforance analysis of a ebrane-based energy recovery ventilator: Effects of outdoor air state. Applied Teral Engineering 3: Nasif, M., Al-Waked, R., Morrison, G., Benia, M. 200 Mebrane eat excanger in HVAC energy recovery systes, syste energy analysis. Energy and Buildings 42: Zang, L. Z., Jiang, Y. 999 Heat and ass transfer in a ebrane-based energy recovery ventilator. Journal of Mebrane Science 63: