Mathematical Modelling Of Passive Cooling In Buildings

Abstract

This thesis presents mathematical models for thermal energy storage in low energy buildings. Two different cooling systems are considered. One uses a ventilated slab, the other a packed bed. The ventilated slab design is incorporated in a large commercial building in Harare, and uses concrete floor and ceiling slabs with an air gap in between to store coolth. The site engineer provided measured temperature data for various positions in the building, sufficient to identify the performance of the building. Because the measurements indicate an unexpected outcome, the instrumentation was carefully tested and proved to be correct. A mathematical model for the design is proposed and a comparison is made of three numerical methods in order to establish the most suitable. The computer code allows for time varying fan speeds and convective heat transfer coefficient, and slab inlet temperatures from measured data. Measured outlet temperatures are compared with the model predictions; there

is good agreement between the trends but some differences. These were investigated through varying certain parameters. The cooling system at the Harare International School uses a packed bed for storing night coolth to be used later for day-time air conditioning. This is described and a mathematical model stated which includes heat dispersion in the fluid and heat loss to the ground surrounding the bed. A numerical method of solution is outlined and the results are compared with measured data at the outlet of the bed both using the measured inlet temperature. A good agreement of trends is seen. The differences are examined through sensitivity analyses for both the convective heat transfer coefficient and air velocity. A parametric study for heat

storage with materials and bed size is given.