On the error of calculation of heat gains through walls by methods using constant decrement factor and time lag values

C. R. Ruivo, P. M. Ferreira, Daniel Cardoso Vaz

Research output: Contribution to journalArticle

21 Citations (Scopus)

Abstract

A transient heat transfer model was developed to numerically predict the thermal behaviour of the external walls of a room under realistic outdoor conditions. The excitation is not simply sinusoidal even though it is considered to have daily periodicity. The numerical model is based on the finite difference method and handles one-dimensional heat conduction through multilayered walls. The external boundary condition is described with the sol-air temperature concept. The temperatures of indoor air and of other internal surfaces in the room are assumed be equal and constant. The numerical results were used to calculate values of the decrement factor and time lag of several walls. The calculation followed two methods, in which these parameters are assumed constant, distinguished by the temperature evolution used: the sol-air or the wall’s outer surface. Additionally, the inner surface temperature is used in both methods. The walls investigated range from low to high mass construction, face towards various directions and have light or dark coloured sunlit outer surfaces. The results show that both simplified methods have poor accuracy in a significant number of cases. Moreover, it was found that the wall’s azimuth significantly affects the time lag.
Original languageEnglish
Pages (from-to)252–261
JournalEnergy and Buildings
Volume60
DOIs
Publication statusPublished - May 2013

Keywords

  • Multilayer wall
  • Transient heat transfer
  • Decrement factor
  • Time lag

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