The ORC method

Author: Jan Akander; Kth; []

Keywords: ;

Abstract: The ORC Method (Optimised RC-networks) provides a means ofmodelling one- or multidimensional heat transfer in buildingcomponents, in this context within building simulationenvironments. The methodology is shown, primarily applied toheat transfer in multilayer building components. For multilayerbuilding components, the analytical thermal performance isknown, given layer thickness and material properties. The aimof the ORC Method is to optimise the values of the thermalresistances and heat capacities of an RC-model such as to givemodel performance a good agreement with the analyticalperformance, for a wide range of frequencies. The optimisationprocedure is made in the frequency domain, where the over-alldeviation between model and analytical frequency response, interms of admittance and dynamic transmittance, is minimised. Itis shown that ORC's are effective in terms of accuracy andcomputational time in comparison to finite difference modelswhen used in building simulations, in this case with IDA/ICE.An ORC configuration of five mass nodes has been found to modelbuilding components in Nordic countries well, within theapplication of thermal comfort and energy requirementsimulations.Simple RC-networks, such as the surface heat capacity andthe simple R-C-configuration are not appropriate for detailedbuilding simulation. However, these can be used as basis fordefining the effective heat capacity of a building component.An approximate method is suggested on how to determine theeffective heat capacity without the use of complex numbers.This entity can be calculated on basis of layer thickness andmaterial properties with the help of two time constants. Theapproximate method can give inaccuracies corresponding to20%.In-situ measurements have been carried out in anexperimental building with the purpose of establishing theeffective heat capacity of external building components thatare subjected to normal thermal conditions. The auxiliary wallmethod was practised and the building was subjected toexcitation with radiators. In a comparison, there werediscrepancies between analytical and measured effective heatcapacities. It was found that high-frequency discrepancies wereto a large extent caused by the heat flux sensors.Low-frequency discrepancies are explained by the fact that theexterior climate contained other frequencies than those assumedin the interior climate.Key words: Building component, building simulation, heattransfer, thermal performance, frequency response, RC-network,finite difference model.

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