Effects of thermal mass, window size and night-time ventilation on peak indoor temperature in the warmhumid climate of Kumasi, Ghana
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Date
2012-06-15
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Abstract
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
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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
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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.
Description
A Thesis submitted to the Department of Building Technology, Kwame
Nkrumah University of Science and Technology
In partial fulfilment of the requirements for the degree of
DOCTOR OF PHILOSOPHY.