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Effective evaluation on the thermal performance of envelope plays an important role towards the reduc-tion of energy consumption for space cooling and heating. In order to calculate the energy consumptionfor cooling and heating and assess the whole energy efficiency of envelop designs, a new evaluation indexon energy and thermal performance for office building envelop (EETPO) is put forward. Three cities ofShenyang, Wuhan and Guangzhou in China are selected for EETPO analysis, which represent the coldzone, hot summer cold winter zone and hot summer warm winter zone, respectively. The regressionequations between EETPO and energy use for cooling/heating are studied in three cities, illustrationsindicate that the regression lines fit extremely well and the algorithm is accurate and simple. Accordingto the compulsory indices stipulated by standard (GB50189-2005), the maximum allowable values ofEETPO are determined in three cities, the maximum EETPOc in cooling period is 1.750 W/m3 K in Wuhanand 1.733 W/m3 K in Guangzhou, the maximum EETPOh in heating period is 0.200 W/m3 K in Shenyangand 0.414 W/m3 K in Wuhan. This index and energy use calculation method can help designers to eval-uate the whole energy and thermal performance of the proposed envelopes and analyze energy savingeffects for different energy conservation measures.29859
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1. IntroductionThe energy demand in China is rapidly growing due to theincreasing population, urbanization and the improvement of livingstandards. China’s buildings sector currently accounts for 27.6% oftotal energy use and is projected to increase to 35% by 2020 [1,2].There is a growing concern about energy consumption in buildingsin China, especially for the office buildings. The energy consump-tion of cooling and heating accounts for a large proportion of thetotal energy use in office buildings, surveys showed that the pro-portions are 30–60% in July and August for 4 typical office buildingsin Hong Kong [3], 24–54% for 105 office buildings in Beijing city [4],the average proportion is 34.3% for 198 high rise office buildingsin Shenzhen city [5], and 44.0% for 3 government office buildingsin Wuhan city [6]. The proportion is projected to increase year byyear, so energy conservation for space cooling and heating of officebuildings has become an important part of national energy strategy. Building envelopes are the interface between indoor and theoutdoor environment which affect the indoor heat gain and heatloss. Data shows that 20–50% of the cooling and heating energyconsumption is caused by the envelope [7], evaluation on energyand thermal performance of building envelope plays an importantrole towards the reduction of energy consumption for space heat-ing and cooling. In response to the continuous increase in energyuse of office building sector, the government introduced legislativecontrol of the envelope designs for office buildings, the nationalstandards for energy efficiency named “design standard for energyefficiency for public buildings (GB50189-2005)” [7] and “design codefor office building (JGJ67-2006)” [8] were continuously launched. Thecontexts about the envelope designs of office building are identicalin the two standards. Two types of evaluation indices on energy andthermal performance of envelopes are included, one is compulsoryindex, and the other is annual energy consumption index based onenergy simulation tools. Compulsory index gives a set of alternatecomponent packages concerning different climatic characteristicsfor the standard to determine the design criteria of building enve-lope components, such as the fenestration and opaque envelope,maximum allowable heat transfer coefficient and shading coeffi-cient can be selected from the alternate component packages withrespect to different areas of fenestration, the maximum allowable heat transfer coefficient can be selected with respect to differentbuilding shape coefficients (such as severe cold and cold zones).This method provides a simple and easy means of compliance,and allows some design flexibility by providing trade-offs betweensome factors within fenestration and exterior walls/roof [9]. If thethermal performance of the building envelope cannot meet thecompulsory requirements, the annual energy consumption indexbased on energy simulation tools is used to assess the energyefficiency. This approach stipulates the maximum allowable heat-ing/cooling energy consumption of a building in comparison to abaseline case based on energy simulation tools. The baseline casediffers for different climate zones in China which is classified bysevere cold and cold zone, hot summer cold winter zone, and hotsummer warm winter zone. The energy consumption of proposedbuilding must be lower than the standard energy consumption ofbaseline case. This method provides even greater flexibility of enve-lope design for the designers to achieve overall building energyefficiency, it is suitable for projects with innovative designs orthose intended to use energy simulation programs to determinealternatives.Obviously, there is a lack of effective performance index in Chi-nese standards, which focuses on the minimum requirement of thewhole envelope thermal performance. Performance index allowsdesigners greater flexibility than the compulsory index in select-ing variables and tends to encourage more innovative buildingdesigns. The typical performance indices include Overall Ther-mal Transfer Value (OTTV), Perimeter Annual Load (PLA), Envloadindex, and so on. The overall thermal transfer value (OTTV) con-cept was introduced in the ASHRAE Standard 90A-1980 by theAmerican Society of Heating, Refrigerating and Air-ConditioningEngineers [10], the OTTV is a measure of average heat gain trans-ferring into a building through the building envelope, it can beadopted for comparison of thermal performance among differentbuilding designs [11], two equations for calculating OTTV of exter-nal walls (OTTVw) and of the roof (OTTVr) were recommended bythe ASHRAE Standard 90A-1980 and the Hong Kong Code of Practice[12]. Some Southeast Asian Nations like Malaysia [13], Philippines[14] and Thailand [15] also introduced this index in building energycodes. PLA proposed by Japan government is the total annual cool-ing and heating load in perimeter of building per unit floor areas,the perimeter of building refers to the perimeter spaces within 5 mof exterior wall and the whole top floor. It includes the heat conduc-tion through envelope caused by indoor and outdoor temperaturedifference, solar radiation heat gain, fresh air load and indoor heatgain [16]. The Envload index is the cooling requirement of per unitbuilding area which is used in Taiwan from 1990s [17], it is a regres-sion equation involved the parts of heat transfer through envelope,solar radiation heat gain and indoor heat gain. Two meteorologi-cal variables and three architectural design variables are included[18]. For the existed performance indices, the trade-offs of thesedesign variables between fenestration and exterior walls/roof arepossible, however, all of them are incapable of revealing the effectof building shape coefficient on energy consumption.There is an urgent need to develop an appropriate performanceindex in the China context for the evaluation of the whole ther-mal performance of office building envelope and the prediction ofenergy use at the early stage of architectural design. The object ofthis study is to provide a new index for the evaluation on energyand thermal performance of office building envelope named EETPO.EETPO index is defined as the average heat-transfer rate throughbuilding envelope per unit building volume and per unit indoor andoutdoor temperature difference during cooling or heating period. Itregards the envelope as a system and focuses on the whole thermalperformance of the building envelope, the most advantage is thatthe effect of building shape coefficient on the energy efficiency ofthe envelope can be reflected. Three cities of Shenyang, Wuhan and Guangzhou located at thenorthern, central and southern China are selected for EETPO anal-ysis, they represent the severe cold zone, hot summer cold winterzone and hot summer warm winter zone, respectively. The correla-tions between EETPO and electricity consumption for cooling andheating of the selected cities are studied, and the maximum allow-able values when just meeting the compulsory indices of standard(GB50189-2005) are calculated. The results have important mean-ings to carry out the effective evaluation on energy and thermalperformance of office building envelope and achieve the sustain-able building envelope designs.2. MethodThe heat gain through the envelope include four parts, the heattransfers of walls, roof and windows caused by indoor and outdoortemperature difference, and the solar radiation heat gain throughthe windows. So the thermal performance of envelope is decided bythe heat transfer rate and solar radiation heat gain through enve-lope, the average rate of heat gain in cooling period (Qc, W) and heatloss in heating period (Qh, W) caused by building envelope can becalculated by:n where, k is the heat transfer coefficient (W/(m2/K)); f is area (m2); TD is the equivalent temperature difference between outdoorand indoor conditions considering the effect of solar radiation (K).The first term and second term in Eq. (1) are the average ratesof heat gain through wall (Q,w) and roof (Qf) in cooling period,respectively, and those in Eq. (2) are the average rates in heat-ing period; taking Q,w for example, the value is equal to the heatgain absorbed by the external finish of wall which can be expressedas: Qw = [hout (tout − tw) + I − Qlw] • f , here, hout is the convectiveheat transfer coefficient (W/(m2/K)), tout and tw are the outdoortemperature and the wall surface temperature (◦C), respectively, is the absorption coefficient of solar radiation, I is the solar radia-tion intensity(W/m2), Qlw is the difference of long wave radiationbetween the external surface and the surrounding objects (W/m2),it can be ignored for vertical surfaces, such as the walls; the aver-age rate of heat gain of wall or roof is mainly affected by the solarradiation and the outdoor temperature, it can be calculated bysimulation program, then with the given areas and heat transfercoefficients, the equivalent temperature differences of different ori-entations and horizontal roof can be back-calculated by equation: TD = Q/(k × f ). T is the indoor and outdoor temperature differ-ence (K); q is the standard solar heat gain factor, which is the solarheat gain through a 3 mm normal glass per unit area (W/m2); SC isthe overall shading coefficient of external window (SC = SCg × SCs);SCg is the shading coefficient of glazing; SCS is the shading coef-ficient of shading system; n, m are the numbers of external wallsand windows, respectively; subscripts w, r, g mean external wall,roof and external window, respectively; subscript i represents the different orientations; subscripts c, h stand for cooling period andheating period.The average heat transfer coefficient (kavg, W/(m2/K)) can beused to calculate the total heat transfer through the envelope,which depends on the properties of the wall/roof/window and theirsurface characteristics, it is equal to the ratio of total heat transferthrough envelope to the corresponding surface area, and it can becalculated by follows:kavg = ni=1awikwifwi+ ar kr fr + mi=1kgifgi fr + ni=1fwi+ mi=1fgi (3)Considering the effect of building’s shape coefficient, heat trans-fer volume coefficient (kV, W/(m3/K)) and solar heat gain volumefactor (qV, W/m3) are defined as the following equations:kV = kavg × B = ni=1awikwifwi+ ar kr fr + mi=1kgifgi V(4)qV = mi=1qi× SCi× fgiV(5)where, B is the shape coefficient (B = fr + ni=1fwi+ mi=1fgi /V,m−1), V is the building volume (m3); a is equivalent temperaturedifference coefficient which is the ratio of TD to T; Therefore, thecooling electricity consumption (Ec, kW h) and heating electricityconsumption (Eh, kW h) caused by envelope can be written as thefollowings:Ec = Es − E1 = Qc × cEER× 10−3(6)Eh = Ew − E2 = Qh × h × 10−3(7)where Es and Ew are the total electricity consumptions for cool-ing and heating in cooling season and heating season, respectively(kW h); E1, E2 are the electricity consumptions for cooling and heat-ing cased by base loads (air exchange and indoor heat gain such asoccupants, equipment, lighting etc.), respectively; Qc is the aver-age rate of heat gain in cooling period (W), Qh is the average rateof heat loss in heating period (W); c and h are the cooling andheating hours, respectively (h), EER and are the seasonal energyefficiency ratio for cooling and heating system, respectively. EETPO(W/(m3 K)) is defined as the heat transfer rate through buildingenvelope pided by building volume and temperature differencebetween outdoor and indoor conditions, four parts are includedwhich are the heat transfers through the wall, roof and windowcaused by indoor and outdoor temperature difference, and the solarradiation heat gain through the window, respectively. The evalu-ation on energy and thermal performance of envelope in coolingperiod EETPOc and that in heating period EETPOh can be calculatedby the following equations: The total electricity consumption per unit volume (includingthe envelope load and the base loads) in the cooling and heatingperiods can be calculated by the EETPOc and EETPOc as the followsaccording to Eqs. (6) and (7):EsV= Tc × cEER× 10−3× EETPc + E1V= b1 × EETPc + E1V(10)EwV= Th × h × 10−3× EETPh + E2V= b2 × EETPh + E2V(11)where b1, b2 are the gradients and indicate the electricity use dueto building envelope load for per unit increase in the EETPOc andEETPOh; E1/V and E2/V are the electricity consumptions of baseloads per unit building volume in cooling period and heating period,respectively; if the densities of base loads (air exchange and indoorheat gain such as such as occupies, equipment, lighting etc.) aresupposed to be fixed values during cooling and heating periods,E1/V and E2/V are the constants.EETPO index is a performance index which is used to evalu-ate the whole energy and thermal performance of office buildingenvelope. It regards the whole envelope as a system instead ofstanding on points of each component of building envelope. Besidesmeteorologic parameter, the geometric parameters and thermalparameters of the building envelope are included, such as the shapecoefficient of building, the areas of roof/wall/window, the heattransfer coefficients of external wall/roof/window, the solar radia-tion absorptance (SRA) of external wall/roof, the shading coefficientof the window, and so on; the EETPO also can be expressed by thewindow-to-wall area ratio (WWR). The meteorological parametersincluded in the expression of EETPO are determined based on thecooling and heating periods.3. Calculation of EETPO parametersEETPO index includes four types of parameters; the first typeis the geometry parameter of envelope, including the area of eachcomponent (WWR), the building shape coefficient and the build-ing volume; the second type is the thermal parameter, such as theheat transfer coefficients of external wall, roof and window, theSRA of external wall and roof, the shading coefficient of the win-dow; the third type is the climate parameter based on the coolingand heating periods, including cooling/heating hours, the indoorand outdoor temperature difference, standard solar heat gain factorand equivalent temperature difference; the fourth type is relatedto energy efficiency of the cooling/heating equipment. Parametersof equivalent temperature difference TD, standard solar heat gainfactor q and shading coefficient of shading system SCs referred bythe EETPO can be back-calculated by the energy simulation toolduring cooling and heating periods.3.1. Simulation program eQUESTeQUEST is an easy building energy analysis program which pro-vides high quality results by combining a building creation wizard,an energy efficiency measure wizard and a graphical results displaymodule [19]. The simulation engine within eQUEST is derived fromthe latest official version of DOE-2, but extends and expands DOE-2’s capabilities in several important ways, including interactiveoperation, dynamic and intelligent defaults, and improvements tonumerous long-standing shortcomings in DOE-2 that have limitedits use by mainstream designers and buildings professionals [20].Study on the comparison of the annual cooling and heating electric-ity consumption using DOE-2 and harmonic reaction analysis haddemonstrated that the results calculated by the two methods arethe same; this result demonstrates that DOE-2 is suitable to ana-lyze envelope dynamic thermal performance and annual energy摘要