Investigations on the Energy Efficiency of Stratified Air Distribution Systems with Different Diffuser Layouts

sustainability

Article Investigations on the Energy Efﬁciency of Stratiﬁed Air Distribution Systems with Different Diffuser Layouts

Yuanda Cheng 1,*, Jinming Yang 1,*, Zhenyu Du 1 and Jinqing Peng 2 1 College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China; [email protected] 2 College of Civil Engineering, Hunan University, Changsha 410082, China; [email protected] * Correspondence: [email protected] (Y.C.); [email protected] (J.Y.); Tel.: +86-351-317-3586 (Y.C.)

Academic Editor: Marc A. Rosen Received: 27 May 2016; Accepted: 27 July 2016; Published: 30 July 2016 Abstract: This paper investigated the inﬂuence of diffuser layouts on the energy performance of stratiﬁed air distribution systems (STRAD). The energy saving potentials of STRAD systems are theoretically analyzed. The cooling coil load of a STRAD system is proportion to the return air temperature, while inversely proportional to the exhaust air temperature. Based on that, numerical studies are conducted for the applications of STRAD systems in three typical building space types. Two evaluation indices are developed to assess the energy performance of STRAD systems. The simulation results demonstrated that further energy saving could be achieved by keeping the exhaust grille at ceiling level and decreasing the height of return grille. Therefore, in order to optimize the energy saving capacity of STRAD systems, the return grille is recommended to be located as low as possible, whilst paying special attention on the “short-circuit” of cold supply air. Furthermore, when the STRAD system is applied in large space buildings with a big horizontal span, supply diffusers should be distributed surrounding the occupied zone as uniformly as possible, while avoiding installing return diffusers at exterior walls.

Keywords: stratiﬁed air distribution; diffuser layout; energy saving; building space types

1. Introduction In recent years, the stratified air distribution (STRAD) system has been widely developed, due to its better ventilation efficiency and energy saving capacity, in comparison with traditional mixing ventilation systems [1–3]. In a STRAD system, the cold fresh air is supplied directly to the occupied zone and flows up due to thermal buoyancy, which resulted in a large vertical temperature stratification in the room air [4]. As a result of that, convective heat sources at or above the stratification height will rise up and exit the space without mixing into the lower zone, which is beneficial to the energy savings of air conditioning systems. Based on CFD simulations, Awbi [5] pointed out that when cooling a room, the ventilation effectiveness for heat distribution in a displacement ventilation system is almost twice of the value in a mixing ventilation system. A number of studies [6,7] identified that the significant energy benefit of STRAD systems is mainly generated by high cooling air temperature, which extended the economizer or free-cooling capabilities of HVAC systems. With extensive studies, Xu and Niu [8] concluded that the energy saving potentials for an STRAD system derived from three factors: an extended free cooling time, a reduced ventilation load, and increased coefficients of performance (COP) for chillers. Because of the large indoor thermal stratification, the layout of exhaust/return grilles plays an important role in the energy performance of STRAD systems, which is different with that in a

Sustainability 2016, 8, 732; doi:10.3390/su8080732 www.mdpi.com/journal/sustainability Sustainability 2016, 8, 732 2 of 13 well-mixed ventilation system [9,10]. Lam and Chan investigated the temperature distribution and air movement in a gymnasium with a stratified air-conditioning system [11]. The numerical results revealed that the exhaust position greatly affected the thermal stratification of indoor air, and then significantly influenced the annual cooling load of the STRAD system. Experimental studies conducted by Awad et al. in a chamber with a stratified ventilation system also indicated that the exhaust location directly influenced the stratification height, which consequently affected the cooling load of air conditioning units [12]. Filler [13] pointed out as well if return-air grilles were placed close to building’s perimeter walls, convective heat gain occurring there could be removed directly by the return air before it entered into the occupied zone, which was benefit to save energy for the systems. These studies certified the great influence of exhaust/return grilles on the energy saving performance of STRAD systems. However, such limited research offers little insight in the diffuser layout designs of STRAD systems. In addition, in most previous research of STRAD systems, the exhaust and return grilles were combined together and installed at ceiling level. It was proposed that, with separated locations of return and exhaust grilles, additional energy savings was able to be achieved for STRAD systems, which would be much more pronounced for large spaces with high ceilings [14]. Therefore, the influence of diffuser layout on energy saving performance of STRAD systems, especially for STRAD systems with separated locations of return and exhaust grilles, are required to be further investigated. In this paper, energy saving potentials of STRAD systems obtained by splitting locations of return and exhaust grilles are theoretically studied and clearly clarified first. Based on that, the effect of diffuser layout on the energy saving capacity of STRAD systems are numerically investigated. Since space type is one of the most important parameters that affecting the performance of STRAD systems [15,16], three typical buildings with different ceiling heights are studied in this paper, namely a small office, terraced classroom, and terminal building of an airport. With extensive simulation results, the effects of diffuser layouts on the energy performance of STRAD systems are illustrated and corresponding design principles are proposed for the application of STRAD systems in different building spaces.

2. Energy Saving Principle for STRAD Systems In a well-designed mixing ventilation system, locations of return and exhaust inlets have no significant influence on the cooling coil load, since the room temperature is uniform. Different with that, due to the obvious thermal stratification of indoor air, locations of return and exhaust grilles in a STRAD system will have great effect on the cooling coil load. As shown in Figure1, the cooling coil load in a STRAD system can be expressed as following equation, which is suitable for STRAD systems with different configurations:

. . Qcoil´STRAD “ Cp ˆ mr ˆ ptr ´ tsq ` Cp ˆ mo ˆ pto ´ tsq (1)

. . where, mr (kg/s) is the return air flow rate and mo (kg/s) is the outdoor air flow rate. tr is the return ˝ ˝ air temperature ( C), to is the outdoor air temperature ( C), and ts is the supply air temperature (˝C). Equation (1) indicates that the cooling coil load in a STRAD system is directly dependent on the return air temperature. The lower the return air temperature, the smaller the cooling coil load. This is consistent with the findings of a series experimental studies presented by Olivieri and Singh [5]. They found that as the return air temperature decreased, less capacity of the cooling unit is required by the system. On the other hand, once the space heat source arrangement is fixed, the space cooling load QSpace is a constant. According to the space energy balance analysis, the space cooling load can be calculated as: . . QSpace “ Cp ˆ mr ˆ ptr ´ tsq ` Cp ˆ me ˆ pte ´ tsq (2) ˝ . where, te is the exhaust air temperature ( C) and me (kg/s) is the exhaust air flow rate. Equation (2) indicates that an inversely proportional relationship exists between the return air temperature and Sustainability 2016, 8, 732 3 of 13

where, te is the exhaust air temperature (°С) and me (kg/s) is the exhaust air flow rate. Equation (2) indicates that an inversely proportional relationship exists between the return air temperature and Sustainability 2016, 8, 732 3 of 13 the exhaust air temperature. Furthermore, with some further algebra operations, Equation (2) can be written as: the exhaust air temperature. Furthermore, with some further algebra operations, Equation (2) can be C m ()() t  t  Q  C  m  t  t (3) written as: p r r s Space p e e s . . Substitute Equation C(3)p ˆintomr Equationˆ ptr ´ t s(1),q “ theQSpace cooling´ C pcoilˆ m loade ˆ pofte STRAD´ tsq systems could also be(3) expressedSubstitute as: Equation (3) into Equation (1), the cooling coil load of STRAD systems could also be expressed as: Qcoil- STRAD Q Space  C p  m e. () t e  t o (4) Qcoil´STRAD “ QSpace ´ Cp ˆ me ˆ pte ´ toq (4) Equation (4) reveals obviously that in order to decrease the return air temperature and improve Equation (4) reveals obviously that in order to decrease the return air temperature and improve the energy saving capacity of the cooling unit, the exhaust air temperature should be increased as the energy saving capacity of the cooling unit, the exhaust air temperature should be increased as high high as possible. Since the vertically thermal stratification is remarkable in a well-designed STRAD as possible. Since the vertically thermal stratiﬁcation is remarkable in a well-designed STRAD system, system, it is possible to obtain higher exhaust air temperature and lower return air temperature by it is possible to obtain higher exhaust air temperature and lower return air temperature by splitting the splitting the locations of return and exhaust grilles. As shown in Figure 1b, by keeping the exhaust locations of return and exhaust grilles. As shown in Figure1b, by keeping the exhaust grilles at ceiling grilles at ceiling level, while installing the return grilles at the lower cool zone, much more heat can level,be discharged while installing by exhaust the return air directly, grilles compared at the lower with cool that zone, in Figure much 1a more. Consequently, heat can be the discharged exhaust air by exhausttemperature air directly, is increased compared and withthe cooling that in coil Figure load1a. would Consequently, be further the reduced. exhaust This air istemperature partially inis increasedagreement and with the the cooling viewpoints coil load of wouldGorton be and further Bagheri reduced. [6]. They This supposed is partially that in when agreement the height with of the viewpointsreturn grilles of Gortonvaried from and Bagheriupper zone [6]. to They lower supposed zone, the that cooling when load the decreased height of returnand the grilles decrement varied fromwas upper dependent zone onto lower building zone, construction the cooling characteristics, load decreased load and characteristics, the decrement and was the dependent upper zone on buildingtemperature. construction characteristics, load characteristics, and the upper zone temperature.

(a)

(b)

FigureFigure 1. 1.Schematic Schematic diagrams diagrams of of ((aa)) STRADSTRAD systemsystem with combined combined exhaust/return exhaust/return grilles grilles;; (b ()b STRAD) STRAD systemsystem with with separate separate locations locations of ofexhaust/return exhaust/return grilles grilles.

3. Numerical Study on the Effect of Diffuser Layout 3. Numerical Study on the Effect of Diffuser Layout

3.1. Physical Models Geometrical models for three typical space types, namely the small ofﬁce, terraced classroom, and terminal building, were built up separately. As shown in Figure2, the space heat sources in the Sustainability 2016, 8, 732 4 of 13

3.1. Physical Models Sustainability 2016, 8, 732 4 of 13 Geometrical models for three typical space types, namely the small office, terraced classroom, and terminal building, were built up separately. As shown in Figure 2, the space heat sources in the small office office room include two detailed thermal manikins manikins,, two two computers computers,, and and a a miniature miniature printer, printer, with a total heat load of 1020 W. TheThe roomroom isis conditionedconditioned by stratifiedstratified air distribution system with floorfloor-mounted-mounted supply diffusers. The The supply supply and and return return air air parameters, parameters, including including the the air air flow flow rate rate,, the supply air air temperature, temperature, an andd diffuser diffuser locations locations in in different different simulation simulation cases cases are are listed listed in in Table Table 11.. What should be noted is that the height of return grille is decreased from ceiling level in Case 1 1-1-1 to floorfloor level in Case 1-7.1-7.

Figure 2. Geometrical model for typical small ofﬁce office room. room. 1—Computer;1—Computer; 2—Printer;2—Printer; 3—Thermal3—Thermal manikin; 4 4—Supply—Supply diffuser; 5 5—Lighting;—Lighting; 6 6—Exhaust—Exhaust grille; 7 7—Exterior—Exterior wall; 8 8—Exterior—Exterior window; 99—Return—Return grille grille..

TTableable 1 1.. Different simulation cases for the small ofﬁce.office.

Simulation Cases Hr 1/m Vs 2/L·s−1 Ts 3/°C Vr 4/L·s−1 1 2 ´1 3 ˝ 4 ´1 SimulationCase Cases1-1 Hr 2.6/m Vs /L¨s Ts / C Vr /L¨s CaseCase 1-1 1-2 2.62.3 CaseCase 1-2 1-3 2.32.0 Case 1-3 2.0 CaseCase 1-4 1-4 1.71.7 120120 18 18 100 100 CaseCase 1-5 1-5 1.31.3 CaseCase 1-6 1-6 0.80.8 CaseCase 1-7 1-7 0.30.3 1 2 1 HrHr—Height—Height of of return inlet inlet (the (the height height is from is from floor floor level tolevel the bottomto the bottom edge of theedge return of the grille), return m; grille),Vs—The m; supply air flow rate of the stratified air distribution system, L/s; 3 Ts—The supply air temperature of the 2 3 Vsstratified—The air supply distribution air flow system, rate˝C; of4 Vr—The the stratified return air air flow distribution rate of the stratifiedsystem, airL/s distribution; Ts—The system, supply L/s. air temperature of the stratified air distribution system, °C; 4 Vr—The return air flow rate of the stratified airFigure distribution3a presents system the, L/s. geometrical information of a large terraced classroom, which refers to an existing classroom in a university campus. The height from ceiling to floor of the classroom varies Figure 3a presents the geometrical information of a large terraced classroom, which refers to an from 5 m at 1st row to 3 m at 10th row. For the purpose of a more comfortable thermal environment, existing classroom in a university campus. The height from ceiling to floor of the classroom varies a hypothetical STRAD system with air simultaneously supplied from diffusers located at floor-level, from 5 m at 1st row to 3 m at 10th row. For the purpose of a more comfortable thermal environment, terrace, and desk edge was designed, as indicated in Figure3b [ 17]. Its notable that part of the air is a hypothetical STRAD system with air simultaneously supplied from diffusers located at floor-level, exhausted from three grilles located at ceiling-level, while the rest four return grilles are distributed on terrace, and desk edge was designed, as indicated in Figure 3b [17]. Its notable that part of the air is surrounding walls at middle-level. To simply the geometry model and improve the computational exhausted from three grilles located at ceiling-level, while the rest four return grilles are distributed efficient, simple human model is adopted to represent most of occupants, as shown in Figure3b. on surrounding walls at middle-level. To simply the geometry model and improve the computational There are 130 seats arranged in the classroom and the total space thermal load, including the occupants efficient, simple human model is adopted to represent most of occupants, as shown in Figure 3b. and lamps, is 10.6 KW. Simulation cases with different diffuser layouts are presented in Table2. Sustainability 2016, 8, 732 5 of 13

ThereSustainability are 130 2016 seats, 8, 732 arranged in the classroom and the total space thermal load, including5 of 13 the occupants and lamps, is 10.6 KW. Simulation cases with different diffuser layouts are presented in There are 130 seats arranged in the classroom and the total space thermal load, including the SustainabilityTable 2. 2016, 8, 732 5 of 13 occupants and lamps, is 10.6 KW. Simulation cases with different diffuser layouts are presented in Table 2.

FigureFigure 3.3. (a) Schematic diagram of the classroomclassroom modelmodel inin thethe simulationsimulation cases;cases; (b)) SideSide view view ofof differentdifferentFigure air3.air ( supplyasupply) Schematic locations.locations diagram. of the classroom model in the simulation cases; (b) Side view of different air supply locations. TableTable 2.2. SimulationSimulation casescases withwith differentdifferent returnreturn grillegrille height. height Table 2. Simulation cases with different return grille height ´1 −1 ˝ ´−11 SimulationSimulation Cases Cases Hr/mHr/m Vs/LV¨ss/L·s TsT/ sC/°C VVr/Lr/L·s¨s Simulation Cases Hr/m Vs/L·s−1 Ts/°C Vr/L·s−1 CaseCase 2-1 2-1 2.8 2.8 Case 2-1 2.8 CaseCase 2-2 2-2 2.0 2.0 Case 2-2 2.0 565565 1818 206206 CaseCase 2-3 2-3 1.1 1.1 565 18 206 CaseCaseCase 2-4 2 2-4-3 0.3 0.31.1 Case 2-4 0.3 Note:Note: Meanings Meanings of ofparameters parameters listed listed inin thisthis table table are are same same as thatas that in Table in Table1. 1. Note: Meanings of parameters listed in this table are same as that in Table 1. Figure 4 is the geometry model of the terminal building. Considering the computational resource FigureFigure4 is 4 theis the geometry geometry model model of of the the terminal terminal building.building. Considering the the computational computational resource resource limitations, only a middle section of an elongated waiting hall with a dimension of 24.9 m (W) × 11.2 limitations,limitations, only only aa middlemiddle section section of of an an elongated elongated waiting waiting hall hallwith witha dimension a dimension of 24.9 ofm 24.9(W) × m 11.2 (W) m (L) × 8 m (H) in a terminal building is investigated. There are 128 seats provided for occupants ˆ m11.2 (L) m × 8 (L) m ˆ(H)8 min a (H) terminal in a terminal building buildingis investigated. is investigated. There are 128 There seats are provided 128 seats for provided occupants for occupantswaitingwaiting for waitingfor departure. departure. for departure. Typical Typical under Typicalunder floorfloor under air ﬂoor distribution air distribution systemsystem system forfor larg larg fore espaces largespaces spaces is is adopted. adopted. is adopted. As As Asindicatedindicated indicated in in Figure in Figure Figure 4, 4, the4 the, the air air airis is supplied issupplied supplied fromfrom from diffuser diffuser 1,1, whichwhich 1, which isis locatedlocated is located at at the the at bottom thebottom bottom of of the the of air theair- - air-conditioningconditioningconditioning unit, unit, unit, and and and returned returned returned from from from middlemiddle middle height height grilles grilles ofof diffuserdiffuser of diffuser 2. 2. To To 2. investigate Toinvestigate investigate the the impact the impact impact of of ofdiffuserdiffuser diffuser locations locations locations on on onventi venti ventilationlationlation performance, performance, performance, DiffuserDiffuser Diffuser 33 andand 3 and DiffuserDiffuser Diffuser 4 4 are are 4 assumed are assumed assumed to to be be to located belocated located on on onsideside side walls, walls, walls, and and and used used used as as asadditional additional additional returnreturn return inletsinlets inlets and and supply supply outletsoutlets outlets,, ,separately. separately. separately. The The The diffuser diffuser layouts layouts 2 inin in differentdifferent different simulationsimulation simulation casescases cases are are are listed listed listed in inin Table TableTable3 . 3. The The total totaltotal heat heatheat gain gain gain of of of the the the spacespace space isis is 85.485.4 85.4 W/m W/m2 of2 of floor ﬂoorfloor areaareaarea andand and thethe the supplysupply supply airair air temperaturetemperature temperature is isis constant constantconstant atat 1818 ˝°CC inin differentdifferent cases. cases.

Figure 4. Geometry model of the terminal building. 1—Diffuser grilles; 2—Diffuser grilles; 3—Diffuser grilles; 4—Diffuser grilles; 5—Air conditioning units; 6—Moving sidewalk; 7—Information desk; 8—Entrance; 9—Occupants; 10—Lighting panels; 11—Skylights; 12—Exhaust grilles; 13—Facilities; 14—Facades. Sustainability 2016, 8, 732 6 of 13

Figure 4. Geometry model of the terminal building. 1—Diffuser grilles; 2—Diffuser grilles; 3— Diffuser grilles; 4—Diffuser grilles; 5—Air conditioning units; 6—Moving sidewalk; 7—Information Sustainabilitydesk; 82016—Entrance, 8, 732 ; 9—Occupants; 10—Lighting panels; 11—Skylights; 12—Exhaust grilles; 13—6 of 13 Facilities; 14—Facades. Table 3. The diffuser layouts in different simulation cases. Table 3. The diffuser layouts in different simulation cases.

´1 ´1 SimulationSimulation Case SupplySupply Grille Grille ReturnReturn Grille Grille Vs/LV¨ss/L·s−1 Vr/LV¨rs/L·s−1 CaseCase 3 3-1-1 11 2 2 CaseCase 3 3-2-2 11 4 4 CaseCase 3 3-3-3 11 & & 3 3 2 2 21302130 15551555 Case 3-4 1 & 3 4 CaseCase 3 3-5-4 11 & & 3 3 2 & 44 CaseNote: 3- Meanings5 of parameters1 & 3 listed in this table2 & are 4 same as that in Table1. Note: Meanings of parameters listed in this table are same as that in Table 1.

3.2. Grid Generation and Numerical Models The correctnesscorrectness and and accuracy accuracy of simulation of simulation results results depend depend highly on highly grid quality. on grid Grid quality. generation Grid takesgeneration up most takes of up the most time and of the human time labor and human during labor the whole during simulation the whole process, simulation especially process, for buildingsespecially withfor buildings complex conﬁgurations.with complex configurations. According to the According geometrical to the characteristics, geometrical characteristics, computational domainscomputational of the domains three typicalof the three building typical spaces building are spaces respectively are respectively divided divided into several into several parts parts with non-conformalwith non-conformal grids. grids. Cuboids Cuboids enclosing enclosing adetailed a detailed human human body body are are meshed meshed withwith unstructured grids and regions close to body surface areare reﬁned,refined, as presentedpresented inin FigureFigure5 5.. While,While, thethe remainingremaining building spaces are discretized with structured gridsgrids toto achieveachieve betterbetter gridgrid quality.quality.

Figure 5. ((a)) Grids Grids distribution distribution around around the head; ( b) Mesh generation of the small ofﬁce.office.

Meshes of of the the three three typical typical building building models models are are all generated all generated by Gambit by Gambit 2.4 (Fluent 2.4 (Fluent,, Inc., New Inc., NewYork, York,NY, USA NY,). USA). The total The mesh total number mesh number of the small of the office, small the office, terraced the terracedclassroom classroom and the terminal and the terminalbuilding buildingare respectively are respectively about 2.1 about million, 2.1 2.3 million, million 2.3, million,and 1.4 million. and 1.4 million.The grid The sensitivity grid sensitivity of each ofmodel each is model analyzed is analyzed in detail. in For detail. instance, For instance, two mesh two generation mesh generation schemes schemes with different with different mesh space, mesh space,namely namely grid space grid 1 space and grid 1 and space grid 2, space were 2,adopted were adopted for the small for the office small model. office The model. total The cell totalnumber cell numberof these oftwo these schemes two schemes are 2,504,433 are 2,504,433 and 2,119,414 and 2,119,414,, respectively. respectively. As shown As shown in Figure in Figure 6, heat6, heat flux flux of ofeach each body body segments segments in in the the two two cases cases with with different different grid grid spaces spaces are are almost almost the the samesame.. Thus, Thus, it is considered that gird space 2 is reasonablereasonable andand practicablepracticable toto bebe usedused inin thethe simulations.simulations. Sustainability 2016, 8, 732 7 of 13 Sustainability 2016, 8, 732 7 of 13

Figure 6. Heat fl ﬂuxux of body segments in cases with different mesh spaces.spaces.

Correct simulation simulation of of airflow airflow in a in space a space depends depends on proper on specifications proper specifications of boundary of conditions boundary andconditions selection and of selection turbulence of turbulence model. However, model. anHowever, ideal turbulence an ideal turbulence model, which model, is suitable which foris suitable the not fullyfor the developed not fully turbulentdeveloped flow turbulent around flow a human around body a human sitting inbody almost sitting stagnant in almost ambient stagnant air, is ambient difficult toair, select. is difficult Therefore, to select. in this Therefore, study, the standard in this study,k ´ ε model the standard is adopted k  as a model compromise, is adopted because as it a iscompromise, capable of reproducingbecause it is thecapable thermal of reproduc plume structureing the andthermal the associatedplume stru thermalcture and stratification the associated [18]. Thethermal y+ ofstratification detailed human [18]. The body y+ surfaceof detailed was human about body 1 and surface an enhanced was about wall 1 and function an enhanced was used wall to calculatefunction was the heatusedtransfer to calculate between the heat human transfer body between and the human environment. body and While the environment. for all wall surfaces While infor the all terminal wall surfaces building in the model, terminal an average building y+ model, of 45 was an achieved average y+ and of the 45 standard was achieved wall function and the wasstandard adopted. wall function was adopted. All thermophysical properties are assumed to be constant except for density, which is treated with the the Boussinesq Boussinesq model. model. The The NavierNavier-Stokes-Stokes equations were solved with a commercial coded Fluent 15.0 15.0 based based on on a afinite finite-volume-volume method. method. The Theconvection convection terms terms are discretized are discretized by second by second order orderupwind upwind scheme scheme and the and diffusion the diffusion term by term central by central differences differences and with and withsecond second-order-order accuracy. accuracy. The TheSIMPLE SIMPLE algorith algorithmm is adopted is adopted as aspressure pressure-velocity-velocity coupling coupling method. method. The The convergence convergence criteria criteria are such thatthat thethe residualsresiduals areare setset to to 10 10´−44 for thethe continuitycontinuity andand momentummomentum equationsequations andand 1010´−77 for the energy equation, which are rigorous qualitative measurements for convergence [[19].19]. It is worthwhile to note note that that the the radiant radiant heat heat transfer transfer is calculated is calculated by bythe thediscrete discrete ordinates ordinates (Do) (Do) radiation radiation model. model. This Thismodel model solves solves the radiative the radiative transfer transfer equation equation (RTE) (RTE) for a forfinite a finitenumber number of discrete of discrete solid solidangles, angles, each eachassociated associated with witha vector a vector direction direction fixed fixed in inthe the global global Cartesian Cartesian system. system. The The numerical numerical model model was validated by an experimental study, the agreement between the measured and modeled temperature profilesprofiles inin aa climateclimate chamberchamber withwith smallsmall spacespace waswas acceptableacceptable [[20].20].

3.3. Boundary Conditi Conditionsons In this study, thethe supplysupply airair inletsinlets andand thethe returnreturn airair outletsoutlets are setset asas velocityvelocity inlets, while the exhaust airair outletsoutlets areare set set as as pressure-outlets. pressure-outlets The. The supply supply and and return return air air velocities velocities in differentin different cases cases are calculatedare calculated as following: as following: Vi νi “ Vi (5)  i  1000Ai (5) 1000Ai where, νi is the supply or return air velocity (m/s); Vi is the total supply or return air ﬂowrate (L/s), 2 andwhereAi, is i theis the corresponding supply or return area air of grillevelocity openings (m/s); V (mi is). the Heat total gains supply in the or return threetypical air flowrate spaces (L/s) are, 2 different.and Ai is Detailed the corresponding boundary settings area of ofgrille heat openings sources in (m different). Heat simulationgains in the cases three are typical listed space in Tables are4. Whatdifferent. should Detailed be noted boundary is that settings the clothing of heat resistance sources in is different considered simulation as 0.5 clo,cases when are listed ﬁxed in surface Table 4. What should be noted is that the clothing resistance is considered as 0.5 clo, when fixed surface temperature is set for the detailed human body. All of building enclosures are set as adiabatic wall boundaries, expect those specially listed in Table 4. Sustainability 2016, 8, 732 8 of 13 temperature is set for the detailed human body. All of building enclosures are set as adiabatic wall boundaries, expect those specially listed in Table4.

Table 4. Boundary settings of heat sources in different simulation cases.

Simulation Model Boundary Settings of Heat Sources Computers 113.6 W/m2; Lightings 100 W/m2; Fixed heat flux: Small office Printer 151.5 W/m2; Exterior wall/windows 38.9 W/m2 Fixed temperature: Detailed human body 307 K Fixed heat flux: Lightings 70.8 W/m2; Simple human body 40.3 W/m2 Terraced classroom Fixed temperature: Detailed human body 307 K Moving sidewalk 103 W/m2; Facilities 33.9 W/m2; Terminal building Fixed heat flux Lighting 76.2 W/m2; Exterior walls 29 W/m2; Skylights 19 W/m2; Simple human body 40.3 W/m2

4. Simulation Results

4.1. Evaluation Indices Currently, there is still a lack of generally recognized assessment indices for the energy saving potential of STRAD systems. In this paper, two indices, namely the absolute energy saving potential ∆Qcoil and the relative energy saving potential ∆Qcoil´R are deﬁned, to evaluate the energy performance of STRAD systems in different cases. The absolute energy saving potential refers to the amount of energy saved directly for the cooling coil of STRAD systems compared with the cooling coil load of mixing ventilation system for the same conditions. It can be calculated as following:

∆Qcoil “ Qcoil´MV ´ Qcoil´STRAD (6) where, Qcoil´MV and Qcoil´STRAD are respectively the cooling coil load of mixing ventilation systems and stratiﬁed air distribution systems for the same building space. For a well-designed mixing ventilation system, Qcoil´MV is comprised by two sections, the space cooling load and the ventilation cooling load. While the relative energy saving potential ∆Qcoil´R indicates the proportion of energy saved for the cooling coil to the total space cooling load, which is expressed as:

∆Qcoil ∆Qcoil´R “ (7) QSpace

4.2. Simulation Results

4.2.1. Small Office Energy savings for STRAD systems with different diffuser locations in the small office are presented in Table5. It can be found that as the height of return grille decreased from ceiling level to floor level, the return air temperature decreased while the exhaust air temperature increased. The main reason is that as the height of return grille decreased, more convective heat flow with thermal plumes from occupied zone to upper zone of the room and then expelled directly by exhaust air. Correspondingly, space heat removed by return air and then contributed to the cooling coil was reduced. Hence, the energy saving capacity is most prominent in Case 1-7, in which up to 17.3% of the space cooling load was saved for the cooling coil. However, special attention should be paid on the “short-circuit” of cold supply air. As indicated in Figure7, when the return grille is located at lower level and close to the supply diffusers in Case 1-7, part of the cold supply air was returned directly before mixed with the indoor air. As a result of that, less cold air was entrained by thermal plumes and Sustainability 2016, 8, 732 9 of 13

Table 5. Energy saving for STRAD systems with different diffuser locations in the small office.

1 2 Simulation Cases Hr/m Tr /°C Te /°C Qcoil /W QQcoil/ Space /% 1-1 2.6 25.1 24.8 -4.8 -0.5% 1-2 2.3 24.9 25.1 2.4 0.2% 1-3 2.0 24.8 26.1 27 2.6% 1-4 1.7 24.6 27.0 48 4.7% 1-5 1.3 24.2 28.8 92 8.9% 1-6 0.8 23.6 30.9 142 14.0% 1-7 0.3 22.5 32.3 176 17.3%

1 Tr—The return air temperature, °C; 2 Te—The exhaust air temperature, °C.

The energy saving performance of the STRAD system in Case 1-1 was worst and the absolute energy saving potential Qcoil was even negative, which means the cooling coil load of the STRAD system was even larger than that in a mixing ventilation system. That was because in Case 1-1, partial Sustainabilitywarm air in2016 the, 8, 732upper zone was discharged from the return grille installed at ceiling level to9 ofthe 13 cooling coil, and greatly increased the cooling coil load. As obviously shown in Figure 8, due to the induced function of return grille, most warm air of the thermal plume generated from human body an overheated thermal problem was caused by warm air temperature at the upper zone, as revealed in wasSustainability mixed 2016 with, 8 ,return 732 air and then discharged to the cooling coil. 9 of 13 Figure7. Table 5. Energy saving for STRAD systems with different diffuser locations in the small office.

The energyFigure saving 7. Temperature performance and of velocity the STRAD distributionsdistributions system atat in PlanePlane Case XX ==1 - 1.51 was m in worst Case 1-7.1 and-7. the absolute energy saving potential Qcoil was even negative, which means the cooling coil load of the STRAD Table 5. Energy saving for STRAD systems with different diffuser locations in the small ofﬁce. system was even larger than that in a mixing ventilation system. That was because in Case 1-1, partial warm air in the upper zone was discharged1 ˝from the return2 ˝ grille installed at ceiling level to the Simulation Cases Hr/m Tr / C Te / C ∆Qcoil/W ∆Qcoil{QSpace/% cooling coil, and greatly increased the cooling coil load. As obviously shown in Figure 8, due to the 1-1 2.6 25.1 24.8 ´4.8 ´0.5% induced function of return grille, most warm air of the thermal plume generated from human body 1-2 2.3 24.9 25.1 2.4 0.2% was mixed with return1-3 air and then 2.0 discharged 24.8 to the cooling 26.1 coil. 27 2.6% 1-4 1.7 24.6 27.0 48 4.7% 1-5 1.3 24.2 28.8 92 8.9% 1-6 0.8 23.6 30.9 142 14.0% 1-7 0.3 22.5 32.3 176 17.3% 1 ˝ 2 ˝ Tr—The return air temperature, C; Te—The exhaust air temperature, C. Figure 8. Temperature and velocity distributions at Plane X = 1.5 m in Case 1-1. The energy saving performance of the STRAD system in Case 1-1 was worst and the absolute 4.2.2. Terraced Classroom energy saving potential ∆Qcoil was even negative, which means the cooling coil load of the STRAD systemThe was space even height larger of terraced than that classroom in a mixing is higher ventilation than that system. in the Thatsmall wasoffice, because which inis benefi Casecial 1-1, partialto the warm full development air in the upper of thermal zone was plume dischargeds. Therefore, from the much return more grille convective installed at heat ceiling is levelable toto the be coolingextracted coil, by and thermal greatly plume increased from thethe coolingoccupied coil zone load. to As the obviously upper zone, shown and in Figure then discharged8, due to the to inducedoutdoors function with exhaust of return air. grille,Corresponding most warm to airthat, of the the energy thermal saving plume capacity generated of fromSTRAD human system body in wasterraced mixed classroom withFigure return is 7.generally Temperature air and thenbetter and discharged than velocity that indistributions to the the small cooling office,at Plane coil. as X presented = 1.5 m in Casein Table 1-7. 6. From Table 6, it also can be found that as the height of return grill decreased, the return air temperature decreased

Figure 8. Temperature and velocityvelocity distributionsdistributions atat PlanePlane XX == 1.5 m in Case 1-1.1-1.

4.2.2. Terraced Classroom 4.2.2. Terraced Classroom The space height of terraced classroom is higher than that in the small office, which is beneficial The space height of terraced classroom is higher than that in the small office, which is beneficial to to the full development of thermal plumes. Therefore, much more convective heat is able to be the full development of thermal plumes. Therefore, much more convective heat is able to be extracted extracted by thermal plume from the occupied zone to the upper zone, and then discharged to outdoors with exhaust air. Corresponding to that, the energy saving capacity of STRAD system in terraced classroom is generally better than that in the small office, as presented in Table 6. From Table 6, it also can be found that as the height of return grill decreased, the return air temperature decreased Sustainability 2016, 8, 732 10 of 13 by thermal plume from the occupied zone to the upper zone, and then discharged to outdoors with exhaust air. Corresponding to that, the energy saving capacity of STRAD system in terraced classroom Sustainabilityis generally 2016 better, 8, 732 than that in the small office, as presented in Table6. From Table6, it also can10 of be 13 found that as the height of return grill decreased, the return air temperature decreased while the whileexhaust the air exhaust temperature air temperature increased. increased. Consequently, Consequently, the absolute the energy absolute saving energy potential saving and potential the relative and theenergy relative saving energy potential saving are potential both obviously are both improved. obviously improved.

TableTable 6 6.. EnergyEnergy saving for STRAD systems with di differentfferent return grille heights in the lecture theatre.theatre.

Simulation Hr/ ˝ ˝ Q QQcoil/ Space Simulation Cases Hr/m TrT/°r/СC TTee//°СC ∆Qcoilcoil/W/W ∆Qcoil{QSpace/% /% Cases m 2-12-1 4.8 25.9 25.9 25.8 25.8 786 786 14.8% 14.8% 2-2 4 25 25.9 830 15.6% 2-32-2 3.14 25 24.6 25.9 26.2 830 961 15.6% 18.1% 2-42-3 2.33.1 24.6 23.9 26.2 26.6 961 1136 18.1% 21.4% 2-4 Note: Meanings2.3 of variables23.9 are the26.6 same as that in1136 Table 5. 21.4% Note: Meanings of variables are the same as that in Table 5.

Figure9 9 presents presents the the thermal thermal comfort comfort for for the the numerical numerical thermal thermal manikin manikin located located at the at ninththe ninth row rowin different in different cases, cases, evaluated evaluated by using by using the UC the BerkeleyUC Berkeley thermal thermal comfort comfort model model [21]. [21]. It indicates It indic thatates thatthermal thermal comfort comfort for the for thermal the thermal manikin manikin was was greatly greatly affected affected by the by height the height of return of return grille, grille, which which was wasespecially especially obvious obvious at head at head level. level. As the As height the height of return of return grille grille located located at the at back the back wall decreasedwall decreased from fromceiling ceiling level tolevel ﬂoor to level, floor thelevel, induced the indu functionced function of the return of the grille return led grille a portion led a of portion the cold of supply the cold air supplyreturn toair the return AHU to directly, the AHU and directly, thus the and head thus experienced the head experienced less cold airstreams. less cold airstreams. Meanwhile, Meanwhile, the head thelevel head was levelalways was exposed always to exposed the thermal to the plumes thermal ﬂowing plumes from flow fronting rows from of front occupants rows which of occupants moved whichto the warmermoved to side the as warmer the height side ofas returnthe height grille of decreased. return grille Therefore, decreased. the Therefore, local thermal the local environment thermal environmentfor the head level for the was head deteriorative level was indeteriorative Case 2-4. in Case 2-4.

Figure 9. ThermalThermal comfort comfort for for local local body body parts parts and and whole whole body body with with different different heights heights of return return grilles grilles..

4.2.3. Terminal Building Table7 7 isis thethe energyenergy savings of of STRAD STRAD system system with with different different diffuser diffuser layouts, layouts, when when applied applied in thein the terminal terminal building. building. The The results results reveal reveal that that the the energy energy saving saving capacity capacity of of STRAD STRAD system system was greatly enhanced by instal installingling additional supply diffusers at side walls. The relative energy saving potential waswas increasedincreased fromfrom 16.8% 16.8% in in Case Case 3-1 3- to1 to 20.8% 20.8% in in Case Case 3-3. 3- The3. The main main reason reason is that is that in Case in Case 3-3, 3heat-3, heat transfer transfer between between the the cold cold supply supply air air and and heat heat sources sources in in the the occupied occupied zonezone waswas muchmuch more full than that in Case 3-1, because of the reversed air flow formed by the collision of the two supply airstreams with opposite directions, as shown in Figure 10. In consequence, much more convective heat was contained by thermal plume and flow to upper zone in Case 3-3, in comparison with that in Case 3-1, and thus the energy saving capacity in Case 3-3 was greatly improved. Sustainability 2016, 8, 732 11 of 13

Table 7. Energy saving for STRAD systems with different diffuser locations in the terminal building.

Simulation Qcoil Tr/°С Te/°С QQcoil/ Space /% Cases /W 3-1 24.9 30.5 4094 16.8% 3-2 25 30.4 4020 16.5% 3-3 24.7 31.8 5047 20.8% 3-4 24.9 30.6 4176 17.2% 3-5 24.9 30.7 4273 17.6% Note: Meanings of variables are the same as that in Table 5.

In addition, Figure 10 also reveals that in Case 3-3 a smaller thermal stratification in the occupied zone was achieved, due to the enhanced turbulence intensity, which is beneficial to promoting the thermal environment. However, it is notable that the local draft risk for the collision region of supply airstreams was also increased. As shown in Figure 11, the draft risk values in Case 3-3 was larger than those in Case 3-1 for the collision region of supply airstreams as marked for the collision region of supply airstreams, and approached the limit. The thermal environment for this region is verified to be poorer in Case 3-3 than in Case 3-1. As shown in Figure 12, the PPD values in this region in Case Sustainability3-3 are greater2016, 8than, 732 10, which is the maximum value allowed in ASHRAE Standard [1]. 11 of 13 Different from that, the energy saving capacity of STRAD systems is slightly weaker by installing an additional return diffuser at side walls. As shown in Table 7, the relative energy saving potential fullin Case than 3 that-4 is inreduced Case 3-1, by because3.6%, in comparison of the reversed with air that ﬂow in Case formed 3-3. by The the main collision reason of is the that two thermal supply airstreamsplumes along with with opposite exterior directions, walls are asimpeded shown due in Figure to the induced10. In consequence, function of return much grilles more convectivelocated at heatside waswalls. contained Thus, partial by thermal warm plumeair flow ands back ﬂow to to the upper occupied zone zone in Case which 3-3, then in comparison increases the with cooling that in Casecoil load. 3-1, and thus the energy saving capacity in Case 3-3 was greatly improved.

(a)

(b)

FigureFigure 10.10. The temperature and and velocity velocity distributions distributions at at Plane Plane X X= 5.6 = 5.6 m min in(a) ( aCase) Case 3-1 3-1 and and (b) (Caseb) Case 3-3. 3-3.

Table 7. Energy saving for STRAD systems with different diffuser locations in the terminal building.

˝ ˝ Simulation Cases Tr/ C Te/ C ∆Qcoil/W ∆Qcoil{QSpace/% 3-1 24.9 30.5 4094 16.8% 3-2 25 30.4 4020 16.5% 3-3 24.7 31.8 5047 20.8% 3-4 24.9 30.6 4176 17.2% 3-5 24.9 30.7 4273 17.6% Note: Meanings of variables are the same as that in Table5.

In addition, Figure 10 also reveals that in Case 3-3 a smaller thermal stratification in the occupied zone was achieved, due to the enhanced turbulence intensity, which is beneficial to promoting the thermal environment. However, it is notable that the local draft risk for the collision region of supply airstreams was also increased. As shown in Figure 11, the draft risk values in Case 3-3 was larger than those in Case 3-1 for the collision region of supply airstreams as marked for the collision region of supply airstreams, and approached the limit. The thermal environment for this region is verified to be poorer in Case 3-3 than in Case 3-1. As shown in Figure 12, the PPD values in this region in Case 3-3 are greater than 10, which is the maximum value allowed in ASHRAE Standard [1]. Different from that, the energy saving capacity of STRAD systems is slightly weaker by installing an additional return diffuser at side walls. As shown in Table7, the relative energy saving potential in Case 3-4 is reduced by 3.6%, in comparison with that in Case 3-3. The main reason is that thermal plumes along with exterior walls are impeded due to the induced function of return grilles located at side walls. Thus, partial warm air flows back to the occupied zone which then increases the cooling coil load. Sustainability 2016, 8, 732 12 of 13 Sustainability 2016, 8, 732 12 of 13 Sustainability 2016, 8, 732 12 of 13

(a) (b) (a) (b) Figure 11. The draft risk values at Plane X = 5.6 m in Case 1 and Case 3. (a) Case 3-1 and (b) Case 3-3 FigureFigure 11. 11.The The draft draft risk risk values values at at Plane Plane X X = = 5.6 5.6 m m in in Case Case 1 1 and and Case Case 3. 3. ( a(a)) Case Case 3-1 3-1 andand ((bb)) CaseCase 3 3-3.-3

(a) (b) (a) (b) Figure 12. PPD values at Plane Z = 0.6 m in Case 1 and Case 3. (a) Case 3-1 and (b) Case 3-3 FigureFigure 12. 12.PPD PPD values values at at Plane Plane Z Z = = 0.6 0.6 m m in in Case Case 1 1 and and Case Case 3. 3. ( a(a)) Case Case 3-13-1 andand ((b) Case 3 3-3.-3 5. Discussion and Conclusions 5. Discussion and Conclusions 5. Discussion and Conclusions This paper investigated the effect of diffuser layouts on the energy performance of STRAD This paper investigated the effect of diffuser layouts on the energy performance of STRAD systems.This The paper theoretical investigated analysis the indicates effect of that diffuser the cooling layouts coil on load the energyof STRAD performance systems is ofassociate STRADd systems. The theoretical analysis indicates that the cooling coil load of STRAD systems is associated withsystems., but The not theoretical totally dependent analysis indicates on, the that space the cooling cooling coilload. load The of cooling STRAD coilsystems load is is associated directly with, but not totally dependent on, the space cooling load. The cooling coil load is directly proportionwith, but notal totallyto the dependent return air on, temperat the spaceure, cooling while load. inversely The cooling proportion coil loadal is directlyto the proportional exhaust air proportional to the return air temperature, while inversely proportional to the exhaust air temperature.to the return airThe temperature, higher the exhaust while inversely air temperature, proportional the tolower the exhaustthe return air air temperature. temperature The and higher the temperature. The higher the exhaust air temperature, the lower the return air temperature and the smallerthe exhaust the cooling air temperature, coil load. theTwo lower evaluation the return indices air, temperaturenamely the absolute and the energy smaller saving the cooling potential coil smaller the cooling coil load. Two evaluation indices, namely the absolute energy saving potential andload. the Two relative evaluation energy indices, saving namelypotential the, are absolute developed energy in this saving paper, potential which andcould the be relative used to energyassess and the relative energy saving potential, are developed in this paper, which could be used to assess thesaving energy potential, performance are developed of STRAD in thispaper, systems which with could different be used todesigns. assess the The energy numerical performance study the energy performance of STRAD systems with different designs. The numerical study demonstratedof STRAD systems that by with decre differentasing the designs. return Thegrille numerical height while study keeping demonstrated the exhaust that grille by decreasing at ceiling demonstrated that by decreasing the return grille height while keeping the exhaust grille at ceiling level,the return the exhaust grille height air temperature while keeping increased the exhaust and the grillereturn at air ceiling temperature level, the decreased. exhaust airCorresponding temperature level, the exhaust air temperature increased and the return air temperature decreased. Corresponding toincreased that, the and energy the returnefficiency air temperatureof STRAD systems decreased. is able Corresponding to be further improved, to that, the which energy is efficiencymuch more of to that, the energy efficiency of STRAD systems is able to be further improved, which is much more prominentSTRAD systems for building is able to space be furthers with improved, high space which height is muchs. Based more on prominent the simulation for building results, spaces it is prominent for building spaces with high space heights. Based on the simulation results, it is recommendedwith high space that heights., for buildings Based on the with simulation small space results,s, the it return is recommended grille should that, be for located buildings at a withlow recommended that, for buildings with small spaces, the return grille should be located at a low occupiedsmall spaces, zone, the in returnto obtain grille further should energy be located saving for at a the low cooling occupied coil. zone, Special in attention to obtain should further be energy paid occupied zone, in to obtain further energy saving for the cooling coil. Special attention should be paid onsaving the “short for the-circuit” cooling of coil. cold Special supply attention air to avoid should an overheat be paid oning the phenomenon “short-circuit” of the of coldspace. supply For spaces air to on the “short-circuit” of cold supply air to avoid an overheating phenomenon of the space. For spaces withavoid large an overheating horizontal span, phenomenon the supply of thediffuser space. is Forsuggested spaces withto be largeinstalled horizontal uniformly span, surroundi the supplyng with large horizontal span, the supply diffuser is suggested to be installed uniformly surrounding thediffuser occupied is suggested zone to toimprove be installed the energy uniformly performance surrounding of STRAD the occupied systems. zone Whereas, to improve installing the energy the the occupied zone to improve the energy performance of STRAD systems. Whereas, installing the returnperformance diffuser of at STRAD exterior systems. walls should Whereas, be avoided, installing since the the return development diffuser at of exterior thermal walls plume shoulds along be return diffuser at exterior walls should be avoided, since the development of thermal plumes along withavoided, exterior since walls the development would be deteriorated of thermal by plumes the induced along with function exterior of return walls woulddiffuser. be What deteriorated is notable by with exterior walls would be deteriorated by the induced function of return diffuser. What is notable inthe this induced paper functionis that only of return the energy diffuser. efficiency What isof notable STRAD in systems this paper was is discussed. that only theFor energya well- efficiencydesigned in this paper is that only the energy efficiency of STRAD systems was discussed. For a well-designed STRADof STRAD system, systems satisfying was discussed. performance For a in well-designed thermal comfort STRAD and system, indoor air satisfying quality performance should also be in STRAD system, satisfying performance in thermal comfort and indoor air quality should also be achievedthermal comfort simultaneo andusly. indoor air quality should also be achieved simultaneously. achieved simultaneously. Acknowledgments: The authors appreciated the financial support from the National Natural Science Foundation Acknowledgments: The authors appreciated the financial support from the National Natural Science ofAcknowledgments: China (No. 51408391). The authors appreciated the financial support from the National Natural Science Foundation of China (No. 51408391). Foundation of China (No. 51408391). Sustainability 2016, 8, 732 13 of 13

Author Contributions: Yuanda Cheng is responsible for most of the work, including the numerical simulations and the analysis based on results, Yuanda Cheng and Jinming Yang designed the study and revised the manuscript, and Zhenyu Du and Jinqing Peng provided valuable academic suggestions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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