Prediction of CLTD values for cases with two daily indoor-temperature set points

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Abstract

The cooling load of sunlit walls and flat roofs can be predicted by simplified methods, for example, by using the well known cooling load temperature difference method from ASHRAE (CLTD method). Its application requires the use of reference hourly CLTD values, which are available in tables for different orientations, as well as a procedure to correct the values in order to take into account the influence of indoor and outdoor temperatures, external surface colour of the envelope, latitude and month. This paper describes a model that allows the prediction of the dynamic thermal behaviour of the external building envelope. The model is numerically solved by means of the finite difference method, while assuming one-dimensional heat transfer under periodic heat flow. Three walls and a roof of portuguese construction are simulated. Two daily indoor-temperature set points are considered instead of the constant set point usually assumed when using simplified methods. CLTD values are calculated using the heat transfer rate at the internal face of the envelope, predicted by the model. Instantaneous conversion of the heat gain in cooling load is assumed. Hourly CLTD values are presented for the walls and for the roof considering the two daily set points for two different applications: residential and labour buildings scenarios. They are also compared with the tabulated values of ASHRAE, which are valid only when a single set point temperature is adopted. In both applications the maxima values of CLTD are higher than the maximum value for the single set point case, meaning that the cooling load is greater when two set points are adopted. However, the average values are lower, meaning that the control strategy with two set points reduces the amount of energy that must be removed by the HVAC system. The simulation software, developed in MATLAB, is a user-friendly tool that easily provides CLTD values for any construction envelope, as long as the heat flow can be assumed one-dimensional.
Original languageUnknown
Title of host publicationCongreso Iberoamericano de Ciencias y Técnicas del Frío
Pages1-10
Publication statusPublished - 1 Jan 2012
EventCYTEF-2012: VI Congreso Ibérico y VI Congreso Iberoamericano de Ciencias y Técnicas del Frío -
Duration: 1 Jan 2012 → …

Conference

ConferenceCYTEF-2012: VI Congreso Ibérico y VI Congreso Iberoamericano de Ciencias y Técnicas del Frío
Period1/01/12 → …

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