cooling_load_estimation_20131002.ppt

1、Cooling Load Estimation same as space cooling load if space temperature is kept constant without fluctuation.,3,Terms,Further illustration of difference between heat gain and cooling load:,4,Terms,Sensible heat: energy leading to change in space air temperature with no change in moisture content. La

2、tent heat: energy related to moisture change without change in dry bulb temperature. Cooling coil load: rate of energy removal at cooling coil including the space cooling load, ventilation load and any distribution system heat gain.,5,Terms,6,Total Cooling Load,Addition of individual space cooling l

3、oad components of a zone at a given time Load profile: variation of load with time Calculation needed for different hours of a day and for different months of a year to arrive at the peak value of a zone,7,Total Cooling Load,Peak load: used to determine supply air flow rate for a zone; summation of

4、peak loads of various zones used to ascertain the volume flow rate of a constant air volume system Block load: max. summation of cooling loads of zones occurring at the same time; used to determine supply air flow rate of a variable air volume system Block load of a building determines the max. tota

5、l cooling requirement,8,Heat Gain/Cooling Load Components,External, internal and infiltration loads.,Top floor of a multi-storey building,9,CLTD/SCL/CLF Method,Simplified version of transfer function method, manageable manual calculation. CLTD: cooling load temperature difference SCL: solar cooling

6、load CLF: cooling load factor Thermal storage effect taken into account by these 3 factors in calculation.,10,Important Design Parameters,Outdoor design conditions Summer Code of Practice for Energy Efficiency of Building Services Installation(BEC 2012, EMSD) Max. dry bulb temperature: 35oC Max. wet

7、 bulb temperature: 29oC ASD Projects Dry bulb temperature: 33oC Relative Humidity: 66% Winter CoP for Building Energy Code (BEC 2012, EMSD) Minimum dry bulb temperature: 7 oC ASD Projects Dry bulb temperature: 10oC Relative Humidity : 40%,11,Important Design Parameters,Indoor design conditions therm

8、al comfort or industrial process requirements taking also into account energy consumption factors affecting thermal comfort dry bulb temperature relative humidity air movement (velocity) clothing activity level various thermal comfort charts relating these factors together, e.g. ASHRAE,12,Important

9、Design Parameters,Indoor design conditions (contd) CoP for Building Energy Code (EMSD) for offices Summer( for office and classroom) Min. dry bulb temperature: 23 oC Min. relative humidity: 50 % Winter: ( for applications other than hotel) Max. dry bulb temperature: 22 oC Max. relative humidity: 50

10、% ASD Projects Summer( for general office ) Dry bulb temperature: 25.5 oC Relative humidity: 50-60 % Winter: ( for general office) Dry bulb temperature: 20 oC Relative humidity: 50 %,13,Cooling Load Component,Fenestration: glazed aperture in building envelope Cooling load: conduction heat gain due t

11、o temperature difference & solar heat gain due to solar radiation Cooling load due to solar heat gain affected by solar radiation intensity, type of glazing and shading thermal storage of building combined effect of solar heat gain and thermal storage given by Solar Cooling Load (SCL in W/m2),14,Coo

12、ling Load Component: Conduction Heat Gain through roofs, walls & glass,Cooling load due to conduction heat gain evaluated by qcond, g = U A (CLTD) (Unit:W) U: overall heat transfer coefficient of roof, wall or glass in W/m2 K (thermal property dependent) A: area of roof, wall or glass, m2 CLTD: cool

13、ing load temperature difference, roof, wall or glass,15,Cooling Load Component: Conduction Heat Gain through roofs, walls & glass,Cooling load due to conduction heat gain (contd) Examples of CLTD values for given indoor and outdoor temperatures for conduction through glass Corrected values be obtain

14、ed by CLTDc = CLTD + (25.5 - tr) + (tm - 29.4)oC tr = indoor temp., oC tm = mean outdoor temp., oC = max. outdoor temp. - (daily range)/2,16,Cooling Load Component: Conduction Heat Gain through roofs, walls & glass,Cooling load due to conduction be evaluated by qcond = U A (CLTDc) CLTD values depend

15、s on wall/roof / window no, orientation, latitude and time Examples for wall no. 4 at 40o North Latitude,17,Cooling Load Component: Internal Partition,Cooling load be estimated simply by considering the temperature difference between the adjacent space and the room qcond, p = U A (tb - tr),18,Coolin

16、g Load Component: Solar Heat Gain,Cooling load due to solar heat gain (contd) SCL values for double strength glass (a reference glass) available from design handbook conversion of SCL values for actual fenestration used by Shading Coefficient (SC) cooling load due to solar radiation obtained by qrad

17、, g = A (SC) (SCL) SC values available from handbook for various types of fenestration (glass and shading),19,Cooling Load Component: Solar Heat Gain,Cooling load due to solar heat gain (contd) SCL values depends on latitude, time, orientation and zone type (A, B, C and D) of the space Examples of S

18、CL: 40o North Latitude, Zone Type B at selected solar time 11 hour to 18 hour Examples of zone type,20,Cooling Load Component: Solar Heat Gain,Cooling load due to solar heat gain (contd) Examples of SC If fenestration partially shaded, solar heat gain through sunlit and shaded parts be evaluated and

19、 added,21,Cooling Load Component: Lighting,Cooling load be estimated using qel = W Ful Fal (CLFel), W where W = total light wattage (rating of lamps), W Ful = lighting use factor Fal = lighting allowance factor (1.18 to 1.3) CLFel = lighting cooling load factor Examples of CLF,22,Cooling Load Compon

20、ent: Lighting,CLF = 1 if air conditioning is off at night or where the lights are on continuously non-stop: thermal storage effect cannot be made use of for reduction of peak cooling requirement,23,Cooling Load Component: Equipment,Cooling load estimated by qe = P (Fl,e) (Fu,e) (CLFe) / P = Power ra

21、ting of equipment, W Fl,e = Load factor (fraction of rated load delivered) Fu,e = Use factor = Motor efficiency CLFe = Cooling load factor for equipment CLFs for equipment be obtained similar to those for lighting CLF = 1 when air conditioning is off or with 24-hour equipment operation,24,Cooling Lo

22、ad Component: Occupants,People generate sensible and latent heats depending on activities Examples of sensible and latent heat gains for offices(Based on 24 room dry-bulb temperature) Sensible cooling load be estimated using qo, s = N (SHGo) (CLFo), W N = no. of occupants SHGo = sensible heat gain p

23、er occupant, W CLFo = cooling load factor for occupant,25,Cooling Load Component: Occupants,Sensible cooling load (contd) CLF depends on zone type and pattern of occupation Examples of CLF for a space of zone type B CLF = 1 if air conditioning system is off at night or with 24-hour occupancy or with

24、 high occupant density,26,Cooling Load Component: Occupants,Latent cooling load be estimated by qo, l = N (LHGo), W LHGo is the latent heat gain per occupant, W no storage of moisture is assumed and CLF = 1,27,Cooling Load Component: Ventilation & Infiltration,Outdoor air must be introduced to venti

25、late conditioned spaces. ASHRAE Standard 622001 recommends minimum ventilation rates for most common applications.,28,Cooling Load Component: Ventilation & Infiltration,Infiltration of air through gaps in building due to wind pressure and temperature difference Infiltration rate Vif estimated by air

26、 change rate method: no. of air change per hour Vif (m3/s) = (no. of air change per hour) (room volume in m3) / 3600 Cooling load due to ventilation & infiltration includes sensible and latent heats,29,Cooling Load Component: Infiltration,Outdoor air to be cooled and dehumidified from outdoor condit

27、ion to room air condition Sensible cooling load (associated with temperature reduction) estimated by qif,s = 1.23 V (to - tr) Latent cooling load (associated with moisture removal) estimated by ql = 3010 V(wo - wr) Total cooling load qt = 1.20 V(ho-hi),30,Rules of Thumb (for HK),31,Rules of Thumb (contd),32,OTTV,Definition Building (Energy Efficiency) Regulat