Active and Passive Ventilation Strategies for a High Performance Building Design Presented at : VENTCONF 2011 : First edition Conference organized by Delhi Chapter of ISHRAE (Indian Society of Heating, Refrigerating and Air-Condition) on the subject of ventilation. http://www.dcishrae.org/ventconf/about-us.html
Sana Salman ( S ust ai nab i l i t y C onsul t ant , A E C O M, India Pvt. Ltd)
S hik ha s ha rm a
A s his h Ra k he ja
( S ust ai nab i l i t y C onsul t ant , A E C O M, India Pvt. Ltd)
R egi onal Managi ng Di rect or. A E C O M Indi a, Bui l di ng Ent e r pr i ses.
Active and Passive Ventilation Strategies for a High Performance Building Design Introduction The story of evolution of mankind is synonymous with its desire to understand and win over the might of Mother Nature. In its pursuit to live comfortably, the evolution began from taking shelter in caves thereafter graduating to temporary wood or stone construction settlements along the rivers, further advancing to permanent low rise stone buildings and finally to cement/steel sky scrapers. It is important to note that all along this journey, evolution was the key wherein human beings slowly understood best techniques to harness abundant offerings of nature. It is only after the industrial revolution, the progression fast tracked and the world became energy intensive. This brought in change of lifestyles and along with it and process requirements. The current so called modern dwellings have the ability to fight the natural elements for enhancing human comfort. The classic example can be seen in air-conditioned indoor spaces which have become a necessity. However, while designing these cool buildings, both architects and engineers focus on matching the cooling demand through energy guzzling air-conditioners rather than harnessing natural elements to minimize the air-conditioning load without compromising on thermal comfort. Today’s trend of dependency on fossil fuels to gain comfort may not be applicable in future as they are depleting at a fast rate. There is a need to find alternate methods to achieve the comfort and what is better than understanding how our ancestors were able to achieve and remold what we already knew without disturbing the ecology. This article focuses on passive design strategies that coupled with active techniques can help in significantly reducing the energy demand of a building and also its carbon footprint. Understanding Passive Design? For designing a climate responsive, environment friendly and sustainable building, incorporating passive design is a necessary ingredient. It is important to note that word “Passive” does not only refer to environment but also to techniques that can be based on harnessing & enhancing nature’s offerings. It is a clear link between the nature and the human comfort to ensure that the interiors remain comfortable for the occupants through different climatic zones. Thus passive is a relation between nature and built form. This practice of using natural element to gain comfort has been practiced since the evolution and can be remolded in modern time to use in various ways. Passive cooling systems rely on natural heat sinks to remove excess heat from a building. They derive cooling directly from evaporation, convection & radiation without using electrical energy. All strategies rely on diurnal changes in temperature and relative humidity. The applicability of each system depends upon the prevailing climatic conditions India and history of passive design From the evolution of human race, these elements of nature have been controlled to gain comfort in different ways. India being a large country with different climatic zones has its own history of incorporating passive features in building structures for various climatic zones which is today known as Vastu Shastra i.e. "science of construction" & "architecture") which is a traditional Hindu system of design based on directional alignments.
Looking at the history of Indian traditional buildings one can determine how these elements have played an important role in developing architecture for different climatic zones. It is important to note that a combination of both passive design features coupled with ventilation strategies can help in providing an optimum design. Passive Design Strategies:Before discussing the ventilation strategies it is important to understand the other passive elements of building design as only a combination of both can help in providing comfortable environment to occupants and also cutting down on energy use. A partial application may not provide desired results and hence it is important to understand the following concepts Orientation: Building orientation is the most simple and primary step for a well designed building which brings in ample daylight while keeping out the direct solar radiation from entering the building. This helps in lowering down indoor temperatures and also need for supplementary cooling is reduced. The ideal orientation of the building in hot and dry climates is to have longer axis oriented East/West Proper orientation can help increase or decrease the internal heat gains by upto 5% Courtyards: With the harsh prevailing climate of the tropics, the common design feature in the traditional homes is the courtyards. The buildings with courtyards offer substantial potential for utilizing passive strategies for enhancing indoor thermal comfort.
Wipro Technologies building designed by D&D in Gurgaon has a compact building envelope opening out with greater transparency towards courtyard
Wipro Technologies in Gurgaon, has taken its design inspired by a traditional inward looking haveli plan.
Over the years, courtyards are being designed incorporating other elements such as water bodies and vegetation taking in account wind pattern and local vegetation. The prevailing wind can be directed into the building which is thus pre-cooled to reduce the surrounding ambient temperature. In addition, vegetation further enhances thermal comfort by softening the harsh summer sun penetrating inside the building. Thus a lower temperature micro climate around the development not only helps in minimizing heat gain in occupied spaces but also reduces ventilation load (due to outdoor air addition) in a building. This strategy can help in reduction of up-to 2-3% in building energy demand.
Insulating Building envelope:Insulation is one of the important and effective means of energy conservation which works silently and discretely. Insulation is provided essentially on walls and roofs of the buildings since most of the heat transfer happens through these surfaces. It improves building envelope performance by minimizing heat loss and heat gains through its surfaces. Before insulating surfaces one should ensure that they're properly air sealed and moisture controlled in order to have comfortable indoor environment quality.
Graph showing effect of Wall & Roof insulation over Annual Energy Consumption
The insulation thus lowers down indoor temperatures and presents an opportunity to designers for applying passive & active ventilation strategies to further enhance the indoor thermal comfort. Optimum insulation for External Walls & roof can result up to 5% of energy savings over conventional building. Window Wall ratio (WWR) Glazing is the key element of built form for providing light, ventilation and views. It can enhance the appearance of a building and connect with the outdoor spaces. Glazing has significant impact on the energy efficiency of the building envelope. Poorly designed windows, skylights or any other glazed surface can make the indoor environment uncomfortable for the occupants round the year. It can over heat or over cool the dwelling in summers and winters respectively. But if well designed they can help maintain year round comfort thereby reducing the demand for artificial cooling or heating While choosing the glazing and the glazed area for the built form, the designer needs to design in harmony with climatic condition. It is important to have an efficient Wall to Window ratio (WWR) to have ample daylight and ventilation without over exposing building to the sunlight. High performance modern buildings have achieved typically up to 2% energy reduction for every 10% reduction in glazing area and thereby presenting an opportunity of enhancing thermal comfort with the help of ventilation strategies rather than active cooling techniques. Shading Devices:Shading the building especially during summer months can reduce the temperature and improve occupant comfort. The efficient shading can block up to 90% heat from the direct sun. If the shading devices are well designed it not only reduces summer heat gain but can also prevent glare thus providing comfortable working conditions. Shading methods can range from operable, to fixed and from metals to more natural like plants. For the best results, designer may choose methods of shading that would allow maximum daylight while
preventing unwanted heat gains. Plants should be selected that allows filtered light inside the building. Shading devices should have high reflectance values. The use of shading devices can help in reducing up to 5% internal energy demands. Natural ventilation: Natural ventilation depends solely on air movement to cool the occupants by collecting prevailing winds and replacing the warm air inside with fresh cool air. The building should be designed in a way that it captures natural winds and should be able to eject warm spent air outside the building. Since the natural wind cannot be scheduled, it is the ability of the designer to enhance natural ventilation using stack vent, operable windows, skylights, monitors and jaali screens. With development of advanced innovative techniques, new methods of ventilations such as turbo ventilator and solar chimneys are being provided on the roof of the building for efficient ventilation. These work on the principle of stack air effect but with higher air change rate. Better results can be achieved with combination of conventional and innovative passive techniques which will work best for the climatic zone where building is situated. In the project below (Refer Figures 1 & 2), the architect and the engineer worked together in eliminating the expensive air-conditioning for CFD Model to analyze External Wind directions common areas, corridors & passages without compromising thermal comfort. The building has been oriented in a manner that cold air over the swimming pool is drawn into corrdiors & common spaces. The spent air is expelled out of building from atrium & tall stacks which have turbo ventilators for enhancing air flow rate. The air pattern and its movement have been studied using CFD techniques and computer simulation tools for achieving nearly 20 degree temperature reduction over peak ambient conditions. Evaporative cooling Evaporative cooling is based on the principle of thermodynamics wherein the indoor air temperature is lowered by evaporating water. The evaporative cooling can be categorized broadly into Direct or indirect evaporative cooling Direct evaporative cooling is used in semi open spaces such as courtyards. The water body is placed in the direction of wind and as the wind blows over the water body, it immediately cools the surrounding thus reducing the ambient temperatures directly. Innovative techniques have been developed to cool larger spaces based on this technology. Traditionally, the desert coolers (with air blowing over wet khus pads), were effective means of achieving cooling in homes. However, due to lack of control over Relative Humidity, these have lost ground to air conditioners. However, it is important to note that evaporative coolers work well in dry ambient climates like Nagpur, Jaipur, and Hyderabad and have significantly lower power consumption over compressor based air conditioners. Alternatively, the cooling can also be achieved by indirect methods wherein air and water are not allowed to come in physical contact but a simple heat exchange is achieved.
‘Sheetal Minars’ strategically located in the wind direction for CII - Sohrabji Green Business Centre in Hyderabad. It is the first LEED Platinum building in India designed by Karan Grover Associates.
The first LEED Platinum building of India in Hyderabad namely CII Godrej Green Business Centre boasts of “Sheetal Minars” or “Wind Towers”. These Towers have high thermal mass which is cooled down by spraying water in night time. During the day time, outdoor hot air is drawn through this pre-cooled tall structure for meeting the ventilation requirement of building as per ASHRAE Standard 62.1. Since the ambient air achieves lower temperature without significant gain in latent load, hence it helped in reducing the overall air-conditioning load and also provided free
cooling opportunities during shoulder months. Similarly, the third LEED Platinum Building of India namely “Wipro Office Building in Gurgaon” employs indirect cooling methodology for reducing ventilation air temperature. The outdoor air is passed over coil carrying water from cooling tower to reduce the temperature before injecting into AHU room. The cooling effectiveness of evaporative machines have been further enhanced by using combination of Direct & Indirect techniques and they are now gaining acceptance in market for achieving amazingly low indoor temperatures especially in hot dry climate/seasons. However, the limiting factor in adoption of this technology is availability of soft water and high maintenance. Variants of these techniques are currently being employed on larger projects to create a favorable micro climate. The common examples are ultra fine misters etc. One of the community projects in India currently under design is considering using combination of pipes carrying water from adjacent lake laid under the open pathways along with misters and shading devices to achieve comfortable environment for occupants even during peak summers.
Air side Economizers or Free Cooling Most parts of India are blessed with four clear seasons and mild winter temperatures. This provides an excellent opportunity to meet the cooling demand of building with minimum or no utilization of energy consuming compressor based cooling systems. Spectral Corporate Office at Noida which is a LEED Platinum Building relies heavily on free cooling option for reducing annual electricity bills. Each Air Handling Unit (AHU) in the building has combination of supply and relief air fans with motorized dampers in outdoor/return/exhaust air streams. Thus, for nearly four months in the year (during low ambient temperature conditions), building operates on free cooling based on enthalpy control thereby eliminating use of chillers. In addition, the economizer can also help in reducing “pull-down” load of a building during start up conditions in morning by automatically flushing the indoor spaces with outdoor cool air in early morning (even during peak summers) and thereby lowering the power consumption. High performance buildings have achieved upto 3% reduction in overall energy demand by employing this technique. One of the project under design is also attempting utilizing indoor swimming pool on building terrace with operable insulated roof. At night, the roof is opened and the water loses of heat by radiation to the night sky. Early in the morning, the insulating roof panels are replaced to protect the water from the heat of the day and solar radiation.
The water temperature thus remains relatively cool throughout the day thereby lowering the temperature of the space below. Geothermal Cooling One of the popular concepts of passive cooling is to utilize the constant temperature of the earth as an exchange medium (to reject building heat) instead of the outside air. Since the earth air temperature is constant round the year (at approximately 5 m depth below the surface) and thus the ground temperature is warmer than ambient air during winters and cooler than ambient air during summers, hence Geothermal harnesses this phenomenon by exchanging heat with the earth through a ground to water/air heat exchanger. Thus it helps in reducing indoor heating/cooling demand and also hot water generation for small scale buildings like homes & commercial establishments. Earth Air Tunnel’s (EAT) are effective medium for generating low temperature air using earth as heat sink. The tunnel is created by burying long length of hume pipe (either in garden outside or below the basement structure) wherein ambient air is sucked from one end and depending on the ambient temperature; air gets either cooled in summer or heated up in winter. This cooled/heated air is then supplied to the regularly non-occupied areas (like cafeteria, corridor etc.) in the building for meeting space cooling/heating demand and can provide thermal comfort to the building occupants. Some of the areas of concern include possible risk of thermal equilibrium with ground temperature and also preventing growth of micro-organisms or reptiles in dark humid tunnels. The Earth air tunnels are operating successfully in various parts of country including TERI retreat at Gurgaon.
Teri Retreat, Gurgaon : Space Conditioning through Earth Air Tunnels
Another variant includes the principle of thermal mass storage wherein Labyrinths are created in basement of building or under foundation by placing small concrete blocks. Outdoor air is pumped through the Labyrinth's cells at night, thereby cooling the concrete walls. By day, air is gently pumped through the cells whereby the air is cooled in turn by the concrete walls. The labyrinth thus acts as thermal batteries which stores the cool air during night and uses the air during the day to reduce the energy loads. The difference in temperature depends upon the usage of the building and the outside temperature. Towards a Net Zero Future The “Eco Commercial Building” (ECB) in Greater Noida is a Net-Zero Energy project initiated by Bayer Climate Program initiative launched in 2007. The office building spread over ground & first floors with an area of 9600sqft consists of work stations, cabins, meeting rooms, support areas etc. and is aiming to achieve LEED Platinum rating under LEED India New Construction (NC). Through ECB, India is in the league of countries that can boast of Net Zero Buildings. Some of the project highlights include: •First Net Zero Energy Building in India.
•Up to 42% savings in primary energy utilization (compared with the standard building in India) through implementation of passive strategies and further implementing active measures associated with the improved HVAC and electrical power supply system. •100% solar and thus emission-free on-site energy generation through the use of a 54 kW photovoltaic system spread over a total surface area of approx. 450 m². This onsite photovoltaic electricity generation saves 100 % in Energy costs over the LEED® mandated ASHRAE 90.1-2004 baseline. •Employing Chilled Beams for radiative cooling thereby eliminating the fan energy from the system. •Zero Water Discharge whereby entire sewage generated on-site is recycled and treated water is used for makeup in cooling towers, flushing & landscape purposes. •The project not only aims at being LEED Platinum rated building but also to be counted amongst the “greenest in world”. Since the on-site power generation is primarily through solar energy, hence it presents a limited option due to small roof area where the solar cells can be housed. Thus, the incorporation of passive strategies play an important role to lower the overall annual energy demand thereby reducing the capacity of solar power cells. Energy Passive Strategies adopted for the ECB project are as follows: • Both walls and roof of building are heavily insulated thus bringing down the heat gain from these elements to be negligible. In the External Wall construction, the insulation materials are: 150mmthk Autoclaved Aerated Concrete Blocks (AAC), Fly Ash block work and 75mmthk Polyurethane Foam (PUF). For the roof, the materials are: 75mmthk PUF insulation, 75mm Fly ash and 100mmthk mineral wool. Careful selection of insulation materials for wall and roof construction resulted in a cumulative annual savings of nearly 6% over a conventional building. • A window to wall ratio (WWR) of 30% ensures maximum day lighting potential with minimum solar heat gains. The location of the windows is optimized by using computer modeling tool using software’s like Ecotect, Radiance. Double insulated high performance glass with a thermal conductivity of 0.27 Btu/ hr.ft²°F has been employed to balance daylight, views and minimize energy demands. The building roof has been designed with a projection on all the glazing for all the facades of the building to cut off heat and provide glare free light. By incorporating this strategy annual energy savings of nearly 2.1% were achieved over a conventional office building. For ECB, computer simulation was conducted using software like Equest and Energy Plus to create year round AC load profile that helped in better understanding of the operating Conditions. Building Thermal calculations were made to size and select mechanical equipment and to evaluate & assess energy performance of the building. This further led in identifying the most cost effective energy efficient measures for the project. The analysis resulted in the following ventilation strategies to be adopted:
India's first Net Zero Building (ECB) : Energy Simulation conducted by Spectral Sustainability Group (SSG) part of Spectral Services Consultants, An AECOM Company.
Dedicated Outdoor Air System (DOAS) & Demand Control Ventilation (DCV) – DOAS was selected for the project to cater to the entire latent load in the building. Dry outdoor ventilation air is supplied through an externally mounted air handling unit which dehumidifies the air before supplying to occupied space. This dry outdoor air also acts as primary air to the chilled beams. The design ventilation rate is 30% additional over ASHRAE standard 62.1-2004 in accordance with the LEED guidelines that enhance the Indoor Air Quality within the building and provide occupant comfort. Air quality is monitored by installing CO2 sensors in various spaces, thereby achieving the concept of ‘outdoor air on demand’ reducing the operating costs and cooling load. Incorporating this strategy in the design, led to an annual savings of over 6.5% as compared to a conventional office building. Heat Recovery – Heat recovery units provide an option of achieving waste energy from building exhausts. Dehumidified cold exhaust air from toilets and space is collected in each services core, which enters one side of the rotating wheel and drying the desiccant coating. This cool and dry part of the wheel then rotates into the outdoor air stream where it absorbs heat and gains humidity from the incoming ventilation air before it is cooled to room temperature in AHU room. The energy recover wheel helped to reduce the ventilation AC load by over 60%, thereby minimizing operating energy as well as the sizing of the air conditioning equipment. This further led to an annual savings of 1.2% over a conventional office building. Conclusion As it can be seen from above discussions, the emphasis on high performance buildings is to design in tandem with nature duly harnessing the potential as well as capitalizing on conventional wisdoms. The Building Owners and Designers have unique challenge to look for innovative design solutions that are best suited to their budget as well as buildings. The Integrated Process of design (IPD) is the best way wherein Owners/ Architect/Consultants/Manufacturers/Contractors sit across the table to jointly optimize building design using collective wisdom. Any High Performance Building cannot ignore passive techniques which can reduce energy demand upto 40% over conventional buildings in India. Passive strategies can be most efficient ways to obtain results which are sustainable in their output. Since ventilation is a very important aspect for a comfortable living and a large amount of money is invested in installing Air conditioning systems and good amount for operating them. They can be reduced greatly If all the professions of the industry encourage using passive strategies in their design. With the increasing awareness of energy consumption measures many clients are inquisitive about the amount of energy savings achieved through these efficient measure and in order to quantify the results we at SSG perform simulation with the help of software’s like COOL/T, IES, PHDC and Optivent along with our regular simulation tools like Equest and Ecotect.