L6: Annual energy demand. Model of building. Energy balance. Methods for energy use calculations. Hand calculations. Helena Bülow-Hübe 1

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1 Smple methods for the calculaton of annual energy use Helena Bülow-Hübe Components Model of buldng Heat balance of room ar Convecton at nner surfaces Long-wave radaton exchange between nner surfaces Heat flows through wndows Heat flows n walls and slabs Convecton at exteror surfaces Long-wave radaton exchange between exteror surfaces Solar and sky radaton Clmatc data, shadowng, absorpton n exteror surfaces, transmsson and absorpton n glazed parts, dstrbuton nsde rooms Solar radaton Heatng (elec., ol, dstrct heat,...) Smoke, gases Sewage water Energy balance Free heat from people, lamps, whte goods etc. Ventlaton Ar leakage Transmsson towards ground Transmsson towards ar Methods for energy use calculatons Hand calculatons: Degree-days or degree-hours Smple energy calculatons on computers: Steady-state calculaton, manly monthly or per day or per hour Advanced energy smulatons on computers: Dynamc smulatons hour by hour or even shorter tme-steps What s the am of the calculaton? Hand calculatons To show that the buldng code requrements are met To estmate the annual energy use Estmate temperatures, comfort Calculate peak heatng and coolng loads to desgn heatng and coolng systems Pros Gves an approxmate energy demand Peak heat load can be easly estmated transparent, prncples become clear Cons Less accurate Dstrbuton over the year? Indoor temperatures are not estmated Comfort cannot be evaluated Helena Bülow-Hübe 1

2 Smple computer calculatons To thnk about Pros Gves an approxmate energy demand Often consders solar radaton Dstrbuton over the year s shown, often monthly Cons Does not consder thermal storage Less transparent, prncples hdden Indoor temperatures are not calculated Solar radaton calculaton can be very smplfed Even f the program s user-frendly, t mght not be approprate for your study. Thnk about whch buldng physcal phenomena that need to be modelled as accurately as possble, and choose program thereafter! Advanced energy smulatons Pros Gve a lot of results regardng heatng, coolng, ndoor and surface temperatures etc. Consders solar radaton, thermal storage, free heat, etc. Advanced buldngs can be smulated, for example offces, hghly glazed buldngs, buldngs wth lots of HVAC nstallatons etc. Cons Demands much more knowledge about the systems to be smulated, and about the program tself Longer learnng tmes Less transparent, prncples hdden Examples of programs Steady-state BKL-metoden ENORM NESA-Blanz tool (acc. EN 832) Isover Energy Enloss (SMHI) BV 2 Dynamcal VIP+ IDA-ICE Derob-LTH/ParaSol Bsm2000 TRNSYS ESP-R EnergyPlus Estmaton of annual energy demand Transmsson losses Ventlaton losses Free heat Solar gans DHW (domestc hot water) Helena Bülow-Hübe 2

3 Transmsson losses P = U A ( T T ) o t [ ] = 2 W m K [W] The U-value, U, decdes how much heat, q, that flows through a buldng component per unt of tme The area, A, of that buldng component thus also decdes the total transmsson loss Heat flows from areas wth hgher temperatures (normally ndoors, T ) to areas wth lower temperatures (normally outdoors,t o ). Thus, the larger the temperature dfference the larger the heat flow The lower the U-value the better! W 2 m K FTX control (F)rom and (T)o ar wth heat recovery (X) Prncpskss av kanaldragnng 2- planslägenhet Solbyn Dalby Thermal brdges P =ψ L ( T T ) o t [W] Prncple for heat recovery wth a FTX-system W m K [ W] = m K ψ (Ps), lnear U-value, measure of the sze of the thermal brdge per metre of the constructon The length, L, of the thermal brdge, T = ndoor temperature, T o = outdoor temperature Ventlaton losses wthout heat recovery Ventlaton losses wth heat recovery P v = ( ncontrolled + nleakage ) V ρ c p ( T To ) [W] The energy demand wth ar-to-ar heat recovery s reduced to: [ ] = 1 3 kg Ws m K W s m kg K v ( n 1 ) + n ) 0,33 V ( T T ) [W] P = η control ( leakage o The ar exchange, n, (ach/h) tmes the ar volume, V (m³), decdes how much heat s needed to heat the ar from outdoor to ndoor temperature The ar exchange usually conssts of a controlled part (by exhaust fan) n controlled, plus a leakage part (nfltraton/ exfltraton), n leakage The densty of ar, ρ, s 1,2 kg/m 3 The specfc heat of ar, c p, s 1000 J/kgK Pv = 0,33 n V ( T Tu ) [W] where n = the number or ar exchanges per hour (h -1 ) (controlled flow and leakage flow) η = energy effcency of the heat recovery (Note! Only the energy n the controlled flow can be recovered) V = ventlated volume (nner volume of buldng) (m 3 ) T, T o = nner and outer temperature Helena Bülow-Hübe 3

4 Q Specfc losses (W/K) ( n (1 ) + n ) [W/K] = U A + ψ L + 0,33 control leak V t+ v η Descrbes the total energy demand per degree of temperature dfference between ndoors and outdoors It s very good to make a table of the specfc losses wth all ndvdual buldng components lsted separately. A very good way to quckly dentfy the largest thermal leakages and where energy-effcency mprovements wll gve the larges beneft. Balance temperature, t b Heatng power demand (Wh/h) Outdoor temperature ( C) t b Maxmum heatng demand (W) P t+ v dm = Qt + v, DUT ( T 20) The maxmum heatng demand decdes the sze of your heatng system Shall be calculated for a dmensonng (desgn day) outdoor temperature, e.g. DUT 20, the lowest outdoor temperature that statstcally occurs once every 20 years. (The buldngs heat capacty also decdes the DUT temperature) Thermal brdge losses should be ncluded n the specfc losses Degree-hours The sum of the temperature dfferences between ndoor and outdoor ar tmes the tme durng whch ths dfference exsts : G = ( T T o ) dt = ( T T o ) Δ t year [ Kh] T s sometmes replaced by T b (the balance temperature). Ths s the outdoor temperature when the heatng season of the buldng starts. Ths s one way of takng care of nternal loads already when the degree-hours are estmated (mplctly). Note! There are several very smlar methods of defnng the balance temperature, and thus the degree-hours Energy losses of a whole year? Sum up all the momentary losses for every hour wth a heatng demand The total temperature dfference (out-n) for every hour wth a heatng demand must be calculated (the degree-hours) The balance temperature of the buldng must be guessed Transformaton losses n the heat dstrbuton system must be added afterwards Temperatur ( C) Stockholm 1988 ΔT G=ΣΔT Tute Tnne -15 Tgräns -20 Tmmar (h) Hours =To =T =Tb Helena Bülow-Hübe 4

5 Degree-hours Smplest assumpton: G = (T -T o,year_average )*24h*365days If T = 20 C then for Malmö: 8 C G = 105 k Ch Växjö: 6 C G = 123 k Ch Umeå: 3 C G = 149 k Ch Estmate the losses Plan, secton & facade areas and volymes Pck U-values for roof, wall, wndow,.. Choose ndoor temperature and ste degree-hours Now transmsson losses can be estmated E Total heat loss For a whole year the transmsson and ventlaton losses become: t+ v = Qt + v G = ( U A + Ψ L + 0,33 ( ncont (1 ) + nleak ) V ) G [kwh] η W K [ kwh] = kkh where Q t+v = specfc losses (W/K) G = number of degree-hours, (kkh/år) Choose ventlaton rates and, f any, heat recovery and effcency η Same degree-hours as before Now the ventlaton losses can be estmated Add the losses: These shall now be balanced aganst nternally generated heat, solar radaton and delvered energy Energy balance Ventlaton levels gans losses Bought or delvered Solar gan Free heat (people, equp.) Transmsson Ventlaton (ncl leakage) Common assumptons: controlled ventlaton: 0.5 ach/h or mnmum 0.35 l/s,m² floor area Infltraton/exfltraton: ach/h Heat recovery n FTX-system: (50-), 65-85% Helena Bülow-Hübe 5

6 Estmate the gans Make a reasonable assumpton of free heat from people and equpment Make a reasonable assumpton of solar gan utlsaton The remanng part must be delvered from the heatng system. Add system losses and the need for bought energy for space heatng can be estmated Normal assumptons about energy use n households free heat or nternal gans from people and equpm. (household elec.) Depends on the number of peope and total lvng space Accordng EN832 a value of 5 W/m² s allowed f the member country have no other requrements. For houses wth energy-effcent whte goods a value of 3-4 W/m² can be assumed Body heat = Kroppsvärme Solar gans through wndows Wndows n general ncrease the buldngs space heatng demand. Only f the U-value s low and the wndow s orentated correctly, can the net energy balance be postve Solar gans vary dependng on orentaton and g-value of the glazng (Solar Heat Gan Coeffcent SHGC = g) Normally, only a small fracton of the solar gans can be utlsed, the rest gves overheatng Helena Bülow-Hübe 6

7 E Net energy balance of wndows Karlsson s wndow equaton net = E sol E trans = A ( g S ( t ) U G( t )) = solar gans where E energy transport through the wndow (kwh/år) A wndow (or glazng) area (m²) g annual average of g (total solar energy transmttance) (-) S total rradaton towards the wndow summed up to the buldngs balance temperature, t b (kwh/m²,a), U U-value of wndow (or glazed part) (W/m²K) G degree-hours summed up to the buldngs balance temperature, t b (k h). b b Ack. solnstrålnng (kwh/m²) Gradtmmar (k Ch) Söder Öster Väster Norr Horsontellt G Stockholm Utetemperatur ( C) Älvkarleby data (lat 61 N) Per square metre of wndow we can wrte South wndows: East wndows: West wndows: North wndows: E = g 465 U 127 E = g 324 U 127 E = g 234 U 127 E = g 130 U 127 Calculated for a balance temperature of 13 C and an ndoor temperature of 20 C Ack. solnstrålnng (kwh/m²) Gradtmmar (k Ch) Söder Öster Väster Norr Horsontellt G(tb) 388 kwh/m² 100 Lund Utetemperatur ( C) Solar gans of wndows: The Karlsson method Calculate the solar radaton through wndows for all hours of the year when there s a heatng demand Use the gans part of Karlsson s equaton for the glazed part of the wndow Note that the transmsson losses through the wndows has often been estmated earler, n the ΣUA calculaton of the buldng s total transmsson losses. DHW demand Make an assumpton of the annual hot water use Estmate the demand of heatng ths amount of cold water to hot water of a sutable temperature, add stand-by losses of storage tank, and converson losses (f any) Can the demand be reduced further? Helena Bülow-Hübe 7

8 Hot water producton The water flow, V, decdes how much energy s needed to warm the ncomng cold water T n to the desred outlet temperature T out. By settng Tout to the hghest temperature n the boler/tank, you can account for the ppe losses to the faucets. Densty of water, ρ, 1000 kg/m 3 Specfc heat of water, c p, 4180 J/kgK If you nput the annual hot water use n m³, the annual energy use becomes: P E dhw dhw = V& ρ c p ( T Tn ) out 3 m kg Ws [ W] = K 3 s m kg K = 1,16 V ( Tn Tout ) [kwh] [W] Summary: heatng balance for a sngle famly house Gans Internal heat from people and equp. (20-25%) Solar gans (20-25%) Heatng system (50-60%) Losses transmsson losses (approx 60 % wthout heat recovery) ventlaton losses (approx 40 % wthout heat recovery) Tank losses The storage tank losses exst as long as the water tank s kept warm and can be about W for a normal household. Ths gves an annual loss of about 500 kwh: E loss 60W 24h 365days = = 525kWh 1000 There may also be other losses between the burner and the storage tank (.e smoke gases) The need for bought energy? Calculated space heatng demand + losses n converson/dstrbuton losses Calculated energy demand for hot water producton Household electrcty Operatonal electrcty (fans, pumps) Total energy demand for hot water Normal use: approx l/person, day Consdered hgh compared to many other European countres, l/pers s good Resultng energy demand for DHW can be approx kwh/a for a famly of 2+2 Reference case Stockholm: 3150 kwh DHWdemand kwh tank losses (60W) + other losses 919 kwh (η=80%). Sum 4600 kwh or 31 kwh/m²a for a house of 150 m² Helena Bülow-Hübe 8