Tiêu đề Taking a Cue from Trees Passive Cooling Building Envelopes based on Evapotranspiration_Energize '23.pdf ✅

Taking a Cue from Trees: Passive Cooling Building Envelopes based on Evapotranspiration Monish Siripurapu1 , Srimayee Krishna1 , Pranjal Maheshwari1 1Ant Studio Pvt Ltd, Delhi, India contact: [email protected] [email protected] Keywords: Passive Cooling, Climate Responsive Facades, Building Facade, Biophilic, Envelope Cooling Abstract The buildings of today suffer from uncontrolled heat gain, creating an urgent need for cooling. In hot climates like India, a growing economy with a rising per capita income is leading to a significant rise in cooling demand. This demand is currently met by harmful synthetic refrigerants, contributors of 10% of global CO2 emissions [1]; leading to the creation of ‘urban heat islands’: cities are at least 3°C hotter than their surroundings [2]. The use of passive cooling strategies is the first step in reducing the energy demand for cooling. Adoption of climate-appropriate building envelopes has the potential to reduce cooling demand in India by 20% by 2037-38 [3]. Trees are the ‘coolers’ in nature. Inspired by the process of evapotranspiration in trees, the research is aimed at exploring and designing passive cooling building envelopes using evapotranspiration and assessing its impact on the Comfort Levels and Operational Cooling Loads of the building. Highlights ● The rising cooling demand of the planet can be eased using passive cooling techniques. ● Passive cooling techniques using building façade system can relieve indoor air temperature by more than 5 ͦC for 33% of the year. ● Reductions upto 33.69% in peak cooling loads were observed using further improvements in envelope specification. ● Naturally Ventilated Mode enhanced with Evaporative Cooling was able to provide a comfortable environment for a significant portion (40.2%) of the year. Introduction In India, a country experiencing hot climates, the growing economy and increasing per capita income are driving the need for cooling solutions. This demand is primarily driven by the construction industry, particularly for space cooling in buildings. The India Cooling Action Plan (ICAP), formulated by the Ministry of Environment, Forest and Climate Change, projects that the cooling demand in the building sector will increase by 11-fold over the next two decades [3].To effectively manage this demand, a balanced approach is recommended.This involves advocating for the adoption of energy-efficient cooling systems, integrating sustainable design practices that utilize passive cooling techniques in buildings. Space Cooling: Increasing demand for RACs and the ‘Cooling Paradox’ Despite the current lower penetration of air-conditioning compared to the global average [3], the need for cooling solutions is expected to rise. Currently, a significant portion of the cooling demand is met by active refrigeration and air conditioning (RACs), which primarily rely on synthetic refrigerants. Although efforts are underway to develop and test less harmful refrigerants, many industries still rely on harmful ones, contributing to the issue of global warming. The International Energy Agency (IEA) reports that RAC technologies account for 10% of global CO2 emissions. Consequently, the release of gases from air conditioning refrigerants leads to the trapping of considerable heat, adversely affecting the microclimate of cities. A city with a population of one million is, on average, 3°C hotter than its surrounding areas [4]. The use of RACs for indoor cooling exacerbates the outdoor heat, creating a cycle that intensifies the demand for indoor cooling, thus giving rise to what is known as the "cooling paradox." Need for Passive Cooling Strategies in India Building Envelopes, made of Roofs and facades, face high heat and solar radiation. This causes thermal transmission that affects indoor spaces negatively in tropical regions where temperatures are already too hot. [5]
ACs are harmful to the environment and use a lot of energy. For instance, in Indian homes, ACs account for 20-40% of the electricity consumption [6]. Hence, it is important to lower the cooling energy demand in buildings while maintaining thermal comfort. Traditionally in India, buildings were designed in consideration to the environmental context which channelized air and sun into the building interiors in a way that reduced heat gain and increased the thermal comfort [7]. Passive cooling strategies can help achieve this by reducing the direct heat gain into the buildings. According to ICAP, using building envelopes that suit the climate can cut down the cooling energy demand by 20% by 2037-38 [3]. Towards Alternative Cooling Techniques: Inspired by Nature, Guided by Tradition The temperature under a tree is often 10-12° C lower than the surroundings [2]. This difference in temperature is the combined result of the phenomenon of ventilation, shading, and evapotranspiration. The stomata regulate the exchange of CO2 and water between the plants and the atmosphere. A large amount of water evaporates from the canopy; a process that uses an equivalent amount of heat energy from the surrounding, thus cooling it. Figure 1 Lower Temperatures Below the Tree (source: Author) Passive cooling systems based on the principle of evapotranspiration can be very effective for providing thermal comfort in building interiors, especially in hot-and-dry climatic regions such as India. Such cooling systems can be realized as an additional ‘second skin’ that can also be attached to the existing building facade to provide thermal comfort through passive means. Research Questions ● Can building envelopes be inspired from the natural process of trees? What if buildings had a second skin that acts like the foliage of the tree? ● What is the impact of such building envelopes on the Cooling Load of buildings? ● What is the impact of such building envelopes on the Thermal Comfort of the occupants? ● How can passive design strategies effectively reduce the energy demand for cooling in buildings in hot and dry regions like India? Methods The proposed solution is realized as a second skin facade, called the ‘Aerofoils’ made with an assembly of porous material modules.Water is circulated through this system which cools the passing air through the principle of evaporative cooling. After evaluating different materials, terracotta was found to be effective in both retaining and releasing water through capillary action. Various module shapes and setups were tested at different scales for real-world use, leading to the chosen model for simulation and testing. The main goal was to achieve the best performance at reasonable costs. Figure 2 Part plan of the system (source: Author)
Each module in the system is designed in the shape of an aerofoil made of terracotta to imitate evapotranspiration of trees. It stores water which gradually evaporates through its surface to facilitate evaporative cooling as air flows between two modules. The distance between two Aerofoil tiles is 60mm at the narrower Water compartment 13 junctions and approximately 100 mm at the wider aperture (variable according to the angle of the foils). The aerofoil shape is assembled to create a nozzle effect that creates a pressure difference to allow better airflow The analysis is primarily to evaluate the potential of façade based evaporative cooling for providing comfort in Naturally ventilated or Hybrid Air-conditioned modes of operation. The following case has been proposed for the simulation study: Project Type: Residential Location: Hyderabad, Telangana Climate: Composite Climate (ECBC 2017) Built up Area: 2000 sqft. Facade orientation: Aerofoils proposed on West and South Facades Figure 3 Front Elevation of the Residential Building (source: Author) Figure 4 Bird-Eye View of the Residential Building (source: Author) Hyderabad has a composite climate as per ECBC 2017 [11]. The maximum temperature peaks around 42 ͦC and average summer daytime temperatures rise above 33 ͦC. The climate observes high diurnal variation of 9.8 ͦC on average, which ranges up to 13 ͦ C in peak summers on May. This offers a great potential for night time natural ventilation with adequate Solar protection required in the day time. Along with this, average relative Humidity is at 63%. Night time Relative Humidity is higher than 60% on average while during the day it drops to an average of 42%. Correlating this to Wet-bulb depression indicates a good potential for Evaporative cooling with average WBD of 10 ͦC in peak summers. Upon close investigation, a feasibility of evaporative cooling, as described in the image below is assessed for those hours where a temperature reduction of at least 5 ͦC can be attained. 33% of time of the year (approx. 2931 hours) passive cooling strategies such as Aerofoil facade have the potential to reduce the outdoor air by more than 5 ͦC. The maximum effectiveness of the Aerofoiil facade is in April when the average reduction is 8 ͦC from outdoor DBT.
Figure 5 Evaporative Cooling Potential For the assessment of the applied strategies, the building design was simplified and modeled in Rhino 3D and analysed with the use of Energyplus on Openstudio and Honeybee (Ladybug tools). Figure 6 Simplified Building Model used for Simulation The input parameters for the simulation model are as follows: Construction Details Table 1 Construction Assemblies used in the Project Assembly Description Value Wall 230mm Brick Wall with 13mm plaster U value – 2.3 W/m2K Roof 150mm RCC slab with 50mm Tiling + Screed U value – 2.9 W/m2K Pitched Roof Terracotta Roof Shingles U value – 3.2 W/m2K Glass Regular Single Clear Glass U value – 5.8 W/m2K, SHGC – 0.8 Internal Loads and Schedules. Internal loads were modelled as per ASHRAE 90.1.2016 [12] reference for Multifamily residences. Certain modifications were made based on the modelers experience and discussions with the prospective clients. Table 2 Project Internal Loads Bedrooms Circulation Living / Dining Occupant density (ppl/m2) 0.1 0.02 0.2 Operation Through: 31 Dec, Until: 05:00, 1.00, Until: 06:00, 0.80, Until: 08:00, 0.75, Until: 09:00, 0.50, Until: 15:00, 0.43, Until: 18:00, 0.50, Until: 21:00, 0.75, Until: 24:00, 0.80, Through: 31 Dec, Until: 07:00, 0, Until: 08:00, 0.5, Until: 09:00, 1, Until: 10:00, 0.5, Until: 17:00, 0, Until: 18:00, 0.25, Until: 19:00, 0.5, Until: 20:00, 0.75, Until: 22:00, 1, Until: 23:00, 0.75, Until: 24:00, 0.25, Through: 31 Dec, Until: 06:00, 0, Until: 07:00, 0.25, Until: 10:00, 1, Until: 11:00, 0.25, Until: 17:00, 0, Until: 19:00, 0.5, Until: 21:00, 1, Until: 22:00, 0.3, Until: 24:00, 0, Equipment Power density (W/m2) 7 3.5 12 Lighting Powe (W/m2) 10.5 10.5 10.5 Cooling Setpoint 24C 26C 24C Window Operation Same as occupancy