How energy efficient are Dhaka’s buildings?
Residential buildings are the second most energy consuming structures in Bangladesh (after the industry sector), consuming 30.5% energy
In 2008, two researchers, Tahmina Ahsan and Orjan Svane from the Royal Institute of Technology, Sweden, chose a residential building in Dhaka's Lalmatia for a survey on energy efficient design features in tropical climates.
The building they chose was in an upper-income neighbourhood, located at the heart of the city. It had Mohammadpur and Sher-e-Bangla Nagar to the north, Raja Bazar to the east, and Dhanmondi and Rayerbazar to the south. It represented upper middle-income households who have a minimum of one air conditioner.
The typical six-story multi-unit residential building had 15 households, out of which seven were surveyed by the researchers. The seven apartments had nine air conditioners and 237 artificial lights.
In an average summer month, it was seen that the apartments used an average of 40% energy on cooling, and the average energy used for air conditioners as a percentage of the total cooling energy alone was 24%.
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In fact, in households that had two ACs, half of the energy they used was consumed through them.
In the last few decades, the haphazard construction of concrete buildings has destroyed the skyline of Dhaka. There is no straight line on the horizon. As a result, while some can afford ample lights and air, others cannot. And the cityscape has been destroyed so badly that there is no way we can fix it now.
At the end of the month, the units that had no air conditioners had a total energy consumption of 450-500 kWh, while the air-conditioned units used as far as 1762 kWh.
They used an average of 40% energy on lighting. While Compact Fluorescent Light (CFL), or energy saving light, is energy efficient and consumes 80% less electricity than incandescent light, the apartments used 51 energy saving light bulbs among the 237 lights.
The researchers explained that the building exterior was exposed to sunlight from morning until dusk, especially the eastern and western facades.
"The temperature starts to rise at 10 am and the outer envelope of the building starts to get warmer, creating a temperature gradient across the thickness of the facade. As a result, heat is conducted through the inefficiently designed building envelope, causing a rise in the inside temperature. As this stored heat cannot be dissipated outside due to inadequate cross ventilation and placement of openings, the inhabitants of the building swelter in the high temperature", they said in the paper.
As a result, the use of HVAC (Heating Ventilation and Air Conditioning) increased, especially in the afternoon and at night.
Another such survey was executed in 2019 in Dhaka's Mohammadpur area by Rakibul Hasan and Jhumana Akter, who analysed the energy performance of a residential building. They found out that the residents of the building consumed 90% of the energy by electricity and 10% by fuel.
Also, most of the electricity was consumed by the HVAC system (about 82%). Annually, the building emitted 4.62 tons of carbon dioxide.
"In summer, from May to September, the most heat was gained through walls, windows and roofs. The additional heat had to be removed for thermal comfort of the inhabitants. While improvement of wall insulation, roof, and thermal property of windows using shading devices could be beneficial, the residents chose electric cooling devices instead," they mentioned in the paper.
A couple of years ago, SREDA (Sustainable and Renewable Energy Development Authority) and the power division of the ministry of power, energy and mineral resources of Bangladesh published the 'Energy Efficiency and Conservation Master Plan up to 2030' report.
According to the paper, residential buildings are the second most energy consuming structures in Bangladesh (after the industry sector), consuming 30.5% energy.
One of the reports by SREDA says that in the last decade, the number of apartment units in Dhaka has increased by almost 600%. The increase in demand for new buildings, mainly in the residential sector, shows the potential impact of energy and water use.
The report also points out some examples of poor building designs that lead to higher energy and water consumption. The air conditioning units are not regulated, and lighting systems are not designed for energy efficiency.
Some buildings have excessive lights installed with no daylight control. Electric lighting generates heat, which leads to more air conditioning loads in buildings.
Water fittings (taps, flushes) are not water efficient and lead to high water consumption.
Rising temperatures add further stress to the building sector in the form of higher energy demand and consumption in private and public buildings, as well as increasing GHG (greenhouse gas) emissions.
Public buildings (non-residential buildings) are no less, as they account for most of the energy consumption in the commercial sector.
According to data from April 2017, the total construction area of public buildings is 14.27 million square metres. In commercial buildings, nearly 50% of the total energy is consumed by ACs and 10-30% by lighting systems.
A 2021 report on energy efficiency in public buildings in Bangladesh says, roughly 122 central air-conditioners and 23,247 split air conditioners are installed in different public buildings, mostly in Dhaka and other cities.
We spoke with Nurunnahar Mili, an urban planner and a member of BIP (Bangladesh Institute of Planners), about this.
"In the last few decades, the haphazard construction of concrete buildings has destroyed the skyline of Dhaka. There is no straight line on the horizon. As a result, while some can afford ample lights and air, others cannot. And the cityscape has been destroyed so badly that there is no way we can fix it now," she said.
According to Mili, architects have had innovative ideas and designs that are adaptive to the climate of Dhaka. "It's the consumers who need to understand that just because an American building looks beautiful, doesn't mean it would be good in this hot and humid climate. Instead of copying western designs, we should be welcoming climate adaptive structures," she said.
To address these issues, many studies have focused on creating energy efficient buildings with high levels of comfort by adopting passive strategies, suitable building materials, smart building automation systems, advanced building information technologies, etc.
While policymakers are speaking of regulations, architects and designers are thinking of design features for energy efficient buildings and structures.
Four architectural design factors for energy efficiency
A building "has to be able to breathe without artificial aids," Bangladeshi architect Marina Tabassum rightfully said this while talking about her Agha Khan award winning project, the Bait-Ur-Rouf Mosque. This gets us to the question: What do we really mean when we talk about a building being energy efficient?
According to the Development and Land Use Policy Manual for Australia, the objective of energy efficient buildings is to improve the comfort levels of the occupants by reducing energy use for heating, cooling and lighting. The United Nations defines energy efficient buildings to have minimum levels of energy inputs.
An energy efficient building is "a well-designed building that maintains the best environment for human habitation while minimising the cost of energy," according to Tahmina Ahsan and Orjan Svane.
The two researchers focused on four design factors of a building that impact its total energy consumption - the building exterior (facade), roof, window and sun shade over them.
While working on the residential buildings in Lalmatia, they saw that residential buildings in Dhaka have 125 mm thick external walls made of brick, to make most of the floor area and to reduce construction costs. But these thin walls absorb more heat.
"Older buildings had thicker walls ranging from 250 mm to 500mm. With the advent of multi-unit residential buildings due to increasing pressure on land and structural systems, these thick walls were replaced with 125 mm walls," they mentioned.
According to them, the use of thicker construction on east and west external walls can reduce solar radiation heat gain. The cooling load can be reduced by 7-10 % when the thickness of the external wall is doubled.
The roof of the building receives most of the solar radiation, and its shading is not easy. The heat entering the building structure through the roof is the major cause of discomfort in case of non air-conditioned buildings or the major load in air-conditioned buildings. "Using hollow clay tiles (HCT) on the roofs can save 18%-30% of the energy used in an air conditioned building", Tahmina and Orjan stated.
In a 2020 research paper published in Solar Energy, researchers talked about how a wall to window ratio of 50% can cut down the demand for artificial lighting by over 15%, and provide more comfortable indoor conditions.
With windows for light ventilation, proper solar shading is also necessary. "On walls, the heat from the direct sunlight warms the outer surfaces, and that heat can transfer to the inside. However, the impact is significantly higher when the sunray enters the room through a glazed window or door. This direct sunlight will increase the temperature of the room. Hence, you should try to avoid this condition in the tropics", the researchers said.
With glass windows or doors, the heat from the sun can easily pass through the glass. Once inside, that heat remains in the room. It is difficult for it to pass back out through the glass.
This is where proper solar shading plays its role. For example, the sun rises in the east and sets in the west. Because of this, the sun's angle is low as it climbs up from or dips down to the horizon.
Therefore, vertical sunshading devices like fins, vertical louvres or even trees are more suitable. Horizontal sun shading will have little effect unless it is extremely deep and extends far from the house.
However, in their analysis, Tahmina and Orjan saw that in typical residential buildings of Dhaka, shading devices are either absent or their sizes are much smaller than the recommended ones.
The researchers showed that if the building were to adopt energy efficient features i.e., a thicker facade, roof made of hollow tiles, windows with solar shading, then the energy used for cooling of the surveyed flats would be reduced by 64%.
The SREDA report on energy efficiency in public buildings in Bangladesh talks about strategic approaches to reduce energy consumption.
The measures can be grouped into three strategies to reduce climate impact of cooling in buildings: avoid, shift and improve.
The first step should always be to avoid cooling loads by adapting the building design to the climate conditions. The second strategic approach is to use renewable energies to meet the electricity demand in order to avoid GHG emissions.
In the last approach, the climate impact can be reduced by improving the cooling appliances, for instance, through more energy-efficient units or the use of natural refrigerants or coolants.
The Programme for Energy Efficiency in Buildings (PEEB) and the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) recommend an improved building design in hot climates like Bangladesh that can reduce the massive need for space cooling.
The recommendations include:
- East to west oriented buildings along the main path of the sun that expose only smaller facades to high solar radiation.
- Better air circulation through larger distances between buildings and a green surrounding area.
- Most openings (doors, windows, and vents) should face north or south to reduce sun exposure.
- The walls of air conditioned buildings should be airtight and light to mid-weight with thermal insulation. Materials such as fibreglass insulation, rammed earth walls, ceramic tiles (floor) and stones can be used.
- For open structures without air conditioning, light walls that have many openings and vents near the floor and ceiling are great, because this creates a pressure difference that causes air circulation and constant ventilation.
- Light-coloured roofs with a reflective coating, that have thermal insulation, are good. Green roofs are even better.
