Effects of thermal mass, window size and night-time ventilation on peak indoor temperature in the warmhumid climate of Kumasi, Ghana

There is lack of empirical data and practical advice on thermal performance of building envelope materials used in Ghana in figures readily appreciated by building designers, prospective builders and facility managers. There is predominant use of low mass sandcrete blocks and adoption of universal building designs with extensive use of glazing that is characterized by high solar and conductive heat gains. Relatively lower night-time outdoor air temperatures are not harnessed to contribute in maintaining thermal comfort in buildings. This research aimed to advance knowledge in passive cooling of buildings in warm-humid climates by exploring the integration of passive and low energy cooling techniques in building design in Ghana to enhance thermal comfort and reduce energy use for space cooling. Adopting building performance simulation and experimental approaches, the effects of thermal mass, window size and night-time ventilation on peak indoor air temperature (PIAT) were evaluated using three variables: (1) the maximum temperatures, (2) the temperature difference ratio (TDR) and (3) the percentage of overheated hours. Following the simulations, experimental cells were designed and constructed based on the specifications of the best performing simulation models. Measured data from the experiments were used to validate the simulated results employing both graphical and statistical analyses. From the study it was observed that an increase in thermal mass by changing to materials of higher densities led to a reduction in peak indoor air temperature. Baked bricks (BB) and concrete (CONC) reduced peak indoor air temperature (PIAT) below that of solid sandcrete blocks (SSB) by 0.7°C and 3°C respectively. Increased thermal mass also led to an increase in the number of hours of delay of PIAT occurrence after of peak outdoor air temperature (POAT), with SSB, BB and CONC having delays of 2, 3 and 5 hours respectively. From the analysis, the study also revealed that reduction in window size lead to a reduction in PIAT. Even though the model with no windows exhibited the best performance, windows are important sources of natural light and views of nature and outdoor environment that should not be completely eliminated. From the study, activation of night-time ventilation at a rate of up to 10ACH, PIAT was reduced for all the thermal masses of various window to floor ratios tested. Concrete resulted in a decrease of PIAT of between 0.17°C and 0.19°C below that of the corresponding none night-time ventilated concrete models. With a ventilation rate of 10ACH, BB obtained a reduction on PIAT temperature of between 0.32°C and 0.04°C below that of the baked bricks VI models with no night-time ventilation. Solid sandcrete blocks with ventilation rates of 10ACH obtained a reduction on PIAT temperature of between 0.02°C and 0.05°C below the Solid sandcrete blocks models with no night-time ventilation. Increases of ventilation rates to 20ACH and 30ACH observed reduction in PIAT of average of below 0.06°C, 0.009°C and 0.008°C for concrete, baked bricks and solid sandcrete blocks respectively. Even though the combined effects of thermal mass, window size and night-time ventilation maintained PIAT below that of the control model, they were all above the mean outdoor air temperature. Concrete, baked bricks and solid sandcrete blocks maintained average temperatures of 2.5°C, 4.8°C and 5.5°C respectively above the mean outdoor air temperature. With a reference temperature of 29°C, expected reduction of the overheated hours varied between 35% and 39%, 37% and 39% and 36% to 42% for concrete, BB and SSB respectively. From the analysis, the thermo-physical properties of materials specified in the simulation program were found to be consistent with that of the local materials tested in this research. A high level of prediction accuracy was obtained in predicting the effects of thermal mass, window size and night-time ventilation with the E+ simulation program. The corresponding root mean square difference (r2) were 0.82 and 0.83 between predicted and measured data observed for mean air temperature and mean radiant temperature respectively. Coefficient of variance of root mean square error (CV[RMSE]) of 14.75% and 16.80% between predicted and measured data observed for mean air temperature and mean radiant temperature respectively. The study has made a number of contributions to the body of knowledge on passive and low energy cooling techniques regarding the development of Ghana. A significant contribution of this research to the body of knowledge is the provision of empirical evidence with respect to peak indoor air temperature drop to support the assertion that heavy thermal mass can improve the thermal performance of buildings in Ghana. Until this research, this assertion had not been supported by any empirical study from research findings in Ghana. Calibrated simulation models are used in retrofit analysis for improvement in the thermal performance of buildings. Therefore another significant contribution of this research to the body of knowledge is the achievement of a validated simulation model for the mode of building design and construction in Ghana. This provides a reference point for future calibrated simulation studies in Ghana. Another significant contribution of this research to the body of knowledge is the provision of sufficient evidence to confirm that thermo-physical properties VII of basic materials used for construction in Ghana are consistent with that used in international standard building simulation programs. Until this research there had been a widely held assertion that most building simulation programs are developed with thermo-physical properties of materials used in temperate developed countries and are not consistent with localized conditions as that of Ghana. Keywords: Buildings envelop, Thermal mass, Night-time ventilation, Peak indoor temperature, passive cooling techniques, Warm-humid climates.

Source: KNUST