Controlling building energy use by Overall Thermal Transfer Value (OTTV)

Overall Thermal Transfer Value (OTTV) is used as a control parameter for building energy use in the Hong Kong Special Administrative Region (HKSAR). However, there are several approaches to determining the OTTV that produce quite different results. Four different methods for calculating OTTV are briefly reviewed and their formulation clarified in this paper. They are (1) the equations derived by Chow and Chan through extensive energy simulations, (2) the ASHRAE 90A-80 method, (3) the one recommended in the Hong Kong Code of Practice (HKCOP) published by the HKSAR Government, and (4) the basic definition of OTTV using data generated from computer simulations by TRACE 600. The OTTV of building envelopes is analyzed by studying the energy use of single-zone buildings with volumes varying from 4000 to 40,000 m3. Results of OTTV calculated from the four methods are compared and discussed. Method (1) is the simplest and easiest one to apply, and the results are found to be close to those simulated by computer programs. Finally, new approaches to total building energy control are described for consideration in the next revision of building energy codes.AeEnvelope area, including roof (m2)AfFloor area (m2)ArGross area of external walls (m2)AwGross area of roof (m2)topTotal system operating time (h)hiInternal surface film thermal transmittance (W m-2K-1)hoExternal surface film thermal transmittance (W m-2K-1)k Thermal conductivity (W m-1K-1)QcAnnual heat energy gain through building envelope (MWh)R Thermal resistance (m2K W-1)SC Shading coefficient, SC for window, SCsfor skylightSF Solar factor, SF for vertical surface, SFrfor horizontal surface (W m-2)TDEQEquivalent temperature difference, TDEQfor wall, TDEQrfor roof (K)U U-value, Uffor fenestration, Urfor roof, Usfor skylight, Uwfor wall (W m-2K-1)VsSpace volume (m3)α Absorptivity, αwfor wall, αrfor roofΣ SummationΔT Temperature difference between indoor and outdoor conditions (K)ASHRAE American Society of Heating, Refrigerating and Air-conditioning EngineersESM External Shading MultiplierOTTV Overall Thermal Transfer Value (W m-2)OTTV90wOTTV for wall, calculated by Eq. (5) (W m-2)OTTV90rOTTV for roof, calculated by Eq. (6) (W m-2)OTTV90eOTTV for envelope, calculated by Eq. (7) (W m-2)OTTVaOTTV calculated using Eq. (2) (W m-2)OTTVbSummer OTTV, calculated using Eq. (3) (W m-2)OTTVcOTTV for curtain-walling, calculated using Eq. (4) (W m-2)OTTVHKwOTTV for wall, calculated by Eq. (8) (W m-2)OTTVHKrOTTV for roof, calculated by Eq. (9) (W m-2)OTTVHKeOTTV for envelope, calculated by Eq. (10) (W m-2)OTTVTRACEOTTV simulated by TRACE 600 (W m-2)SSR Skylight-to-Roof RatioWWR Window-to-Wall Ratio.