Simulation - based exploration of the thermal performance of selected multi-storey office buildings in Accra, Ghana

The lack of empirical data and practical advice on thermal performance and efficient use of energy in buildings are gradually becoming a burden to the country. Amidst the recent advancement in the usage of curtain walls for office buildings, high consumption of energy and poor thermal comfort issues have become dominant. Given the warm-humid climatic characteristics of Ghana, energy needs for cooling of office buildings represent an increasing burden on the environment and the economy. In many instances, the building design is not supported by a detailed analysis and evaluation of thermally relevant features as well as options related to orientation, envelope, glazing ratio, shading devices, and thermal mass. Thus, design decision making is not sufficiently informed by relevant expertise pertaining to energy efficient building design methods and technologies. By adopting subjective thermal comfort models, building performance simulation and experimental approaches, this research aimed at advancing knowledge on how thermal comfort conditions could be enhanced and energy-use reduced in Ghanaian office buildings. In this context, the research had the following objectives: i. To assess occupants view of their indoor thermal comfort conditions within the selected buildings. ii. To determine the thermal comfort conditions of the indoor environment of the selected buildings. iii. To identify energy reduction strategies for the indoor conditions of the selected buildings based on validated models. vi | P a g e iv. To identify overheating reduction strategies (passive case-no air conditioners) of the selected buildings. Adopting a case study research strategy, a number of data collection methods were employed. Both quantitative and qualitative data were collected and analysed in line with a framework designed to examine and extract relevant materials in relation to the research questions. The research database included 4 multi-storey office buildings (Ridge Towers [R.T.], World Trade Centre [W.T.C.], Premier Towers [P.T.], and Heritage Towers [H.T.]) and 195 occupants‘ questionnaires. From May, 2012 to April, 2013, indoor and outdoor climatic conditions (mainly temperature and relative humidity) were monitored, using data loggers. To evaluate the existing indoor climatic conditions, measured air temperature and relative humidity values were plotted in the psychrometric and bioclimatic charts. A survey (questionnaire) of 195 occupants was conducted to record their views on indoor environment, installed systems and energy use. At a general level, the study provided insight into the character of occupants within the buildings, their general views and concerns regarding energy use and thermal comfort. It provided evidence of how the buildings could be made comfortable by means of the psychrometric and bioclimatic charts. Significantly, the study showed how building cooling loads could be reduced to the minimum whiles providing comfortable indoors passively. On the psychrometric chart, the monthly hourly temperature and relative humidity values for R.T. and P.T. were within the comfort zone. All the values for W.T.C. were outside the comfort zone whiles H.T. had the months of January, May, June and July inside the comfort zone. This vii | P a g e suggests that R.T. and P.T. could operate passively all year round while H.T. could do that within certain months of the year. W.T.C. was found to be very uncomfortable all year round. Plotting both W.T.C. and H.T. values on the bioclimatic charts, 7 months were within the comfort zone of Olgyay‘s chart in the H.T. building. This gave an indication of five uncomfortable months for the same building. Givoni‘s chart suggests that W.T.C. could be made comfortable by means of comfort ventilation, conventional dehumidication and air-conditioning for the various months. Olgyay‘s chart suggested that air velocity of between 0.1m/s to 1m/s could improve the comfort conditions of the spaces within the W.T.C. In the H.T., Givoni‘s chart recommends high thermal mass and comfort ventilation whiles Olgyay‘s chart proposes air velocity of 0.1m/s to get the other 6 months within the comfort zone. Additionally, the analysed data from the questionnaire among others showed that the three most important parameters for occupants‘ satisfaction in the office spaces studied were air quality, thermal comfort and fire safety. Again, the respondents (occupants) were interested in receiving training on the effective and efficient operation of building systems, which could help increase satisfaction, comfort and reduce energy performance of buildings. The calibrated simulation results suggested that measures regarding building fabric and controls could improve buildings‘ energy performance. Particularly, careful combinations of improvement measures (such as efficient glazing, thermal mass, façade insulation, night ventilation, efficient electrical lighting, form and orientation) have a significant potential to reduce buildings' cooling loads (31% – 49%) in the climatic context of Accra. viii | P a g e When the buildings operated passively, the alternative improvement scenarios considerably reduced the mean overheating in the offices up to about 2.9K, depending on the reference overheating temperature assumption. Though not exactly identical, there is a clear correspondence between the ranking of the scenarios in view of lower cooling demands ( active building mode) and lower overheating tendency ( passive operation mode ). A significant contribution of this research to the body of knowledge is the provision of empirical evidence with respect to improvement of thermal performance in multi-storey office buildings in Accra, Ghana. Until the current research, the above assertion had not been supported by any empirical study within the localized climate of the capital city of Ghana. Another significant contribution of this research to the body of knowledge is the provision of sufficient evidence to confirm that the procedure for the determination of the comfort zone on the psychrometric chart could be adjusted for tropical climates where people are generally adapted to higher relative humidity and moderate temperatures. Validated simulation models are used in retrofit analysis for improvement in the thermal performance of buildings. Therefore another significant contribution of this research is the achievement of validated simulation models for energy assessment in multi-storey office buildings in Ghana. This provides a reference point for future validated simulation studies involving multi storey office buildings with curtain walls in Ghana. Moreover, this study is the first of its kind in the climatic context of Accra, Ghana

Source: KNUST