Heating, ventilating, and airconditioning (HVAC) systems have an important role to play in green buildings since many of the green building factors are directly or indirectly affected by the performance of the HVAC systems. HVAC accounts for nearly 50-60 per cent of the energy used in commercial buildings in India. The HVAC system for green building shall be designed to reduce energy consumption while maintaining the interior conditions at a comfortable level to keep occupant’s health and productivity. HVAC system design should not only meet the standard on energy front but beat the standard codes like Energy Conservation Building Codes (ECBC), India and American Society of Heating, and Refrigerating and Air Conditioning Engineers (ASHRAE) standards to achieve higher level of green building rating.
Consequently, any commercial building has the potential to realise significant savings by improving its control of HVAC operations and improving the efficiency of the system it uses. An integrated and holistic design process beginning at a project’s inception is required to optimise the HVAC design, HVAC commissioning and operation for green buildings.
Designing for Energy Efficiency: Through Building Simulation
The systems in air-conditioned buildings are so dynamic and interactive that it becomes very difficult to analyse the benefit of an Energy Conservation Measure with respect to other using the traditional method of calculations using various formulae. For example: any reduction in lighting load not only reduces the lighting consumption but also reduces the heat load and hence provides energy savings in HVAC. As we look for a glass with higher light transmittance, the solar co-efficient of the glass also relatively increases consequently leading to higher thermal load.
That’s where energy simulation comes in as a Design Assistance tool. There are various simulation software available in the market today – Visual DOE, EQuest, Energy Plus, IES etc, to name a few.
Energy Efficient Design Measures to reduce Heat Load and increase energy efficiency of the Building
Solar passive techniques: Incorporation of solar passive techniques in a building design helps to minimise load on conventional systems such as heating, cooling, ventilation and light. There are considerable opportunities for significant energy savings through efficient, integrated design, especially at the building envelope. An integrated and efficient building envelope with appropriate window, glazing design, insulated walls and roof can not only reduce the energy and operating costs of a facility, but can also reduce the size and cost of the HVAC system.
Building orientation: Building orientation has a major effect on solar gain. Proper orientation can significantly reduce the cooling load and improve energy efficiency. In tropical climate like India long facades of buildings oriented towards North-South are preferred. East and West receive maximum solar radiation during summer. In predominantly cold regions, also North-South long facades are advisable, as South orientation receives maximum intensity of solar radiation in winter months.
Proper Shading: All the elements of a building are vulnerable to heat gains. Proper shading is therefore a very important aspect in solar passive building design. It is observed using software simulations that, shading of roof, walls and windows have considerable potential in reducing the cooling energy consumption.
Window Wall Ratio: Window Wall Ratio is the ratio of vertical fenestration area to gross exterior wall area. Gross exterior wall area is measured horizontally from the exterior surface; it is measured vertically from the top of the floor to the bottom of the roof. The cooling energy demand increases with increase in window wall ratio. The optimum WWR recommended is 20-30 per cent with daylight integration
Building Envelope: Building Envelope consists of roof, fenestration, glazing and the exterior walls. A thermally insulated building envelope involves using materials that reduce heat transfer by conduction, radiation and convection. It helps cut down the heat load in the building, downsizing the air conditioning equipment required to cool the spaces and hence, results in lowering of the capital costs for the project. Thermal performance of fenestration, wall and roof assembly depends on Solar Heat Gain Coefficient (SHGC) and that of glazing depends on the solar energy transmittance through the glazing (measured by U-value). Lower the SHGC and U values, better are the thermal insulating properties of the wall, roof, fenestration and glass.
Energy Efficient HVAC Systems & Equipment
Following strategies can be adopted in the HVAC system to meet green buildings requirements:
Selection of Chiller
All the major HVAC equipment like Chillers and Packaged AC units are procured based on their energy performance rating to beat the energy rating specified in ECBC and ASHRAE Standards.
Variable Speed Drives for Pumps, Fans and Compressors
Pump and fan capacities can be reduced and energy saved by using variable speed drives to control their speed. Reductions in both peak and off-peak energy costs can be obtained by using variable speed drives on pumps, fans and compressors that operate at varying loads. They pay off better if the systems they are applied to operate at part load for relatively long hours. Variable speed pumping can dramatically increase energy savings, particularly, when it is combined with demand-based pressure resent controls. Variable speed drives on pumps/fans provide a soft start, extending equipment life. Variable speed systems are quieter than constant speed systems
Dedicated Outdoor Air Systems: A DOAS uses a separate air handler to condition the outdoor air before delivering it directly to the occupied spaces. While a DOAS can be applied in any design, it is the most beneficial in a facility with multiple spaces with differing ventilation needs. It reduces a building’s energy use when compared to mixed air systems that requires over ventilation of some spaces. It allows the designer to decouple the latent load from the sensible load, hence providing more accurate space humidity control.
Supply Air System Control: Using Variable Air Volume boxes and dedicated individual control for 50 per cent of the occupied people or all closed cabins in the occupied area will become mandatory to ensure human comfort and energy saving benefit.
Demand Control Ventilation: In order to maintain the IAQ of the occupied area, it is required to install a fresh air system which controls, measures, monitors continuously the flow of fresh air (Outdoor air) supplied to the occupied area. CO2 sensor can be used to measure or control the per person ventilation rate and, in turn, allow the designer to introduce a ventilation demand control strategy.
Apart from these standard measures following new and innovative technologies in HVAC could be used to enhance energy efficiency in green buildings.
Air to Air Heat Recovery System: A heat exchange enthalpy wheels can be used in comfort application, where energy in the exhaust stream would otherwise be wasted. Energy recovery is most economical when there are large temperature differences between the airstreams, the source of supply is close to the exhaust, and they are both relatively constant throughout the year. With a total energy wheel, the humidification costs may be reduced in cold weather and dehumidification costs may be lowered in warm weather.
Chilled Beams: Chilled beams do not require a secondary fan so they are inherently more energy efficient than fan coil units, their main air terminal device rivals. On top of this, chilled beams use higher chilled water flow temperatures than fan coil units (around 14ºC), which means there is a significant part of the year when chillers do not need to be working and free cooling is available. The net result of the above, the chilled beam systems always lower energy consumption and operating costs.
Thermal Storage System for Cooling: Consider a thermal storage system when designing your chiller plant. With a thermal storage system, the idea is to run chiller equipment off-peak and store cooled water or ice, then draw on this cooling during the peak times of the day. These systems take one of three forms: chilled water, ice or a salt-water hybrid of both—called a eutectic system. Specifying which system is based on the availability of space for storage media, cooling load profile, rate schedule and current equipment.
Gas Fired Chillers: Chilled water systems that use fuel types other than electricity can help offset high electricity prices, whether those high prices are caused by consumption or demand charges. Absorption chillers use thermal energy (rather than electricity) to produce chilled water. This type of system can be thought of when natural gas prices are significantly lower than electric prices. The other option is to go for gas based captive power engine to produce electricity and the waste heat from the gas engine can be used to generate chilled water through heat recovery VAM chillers.
Control cooling tower fans by sensing ambient wet bulb temperature
Control cooling tower fans by sensing ambient wet bulb (wb) temperature. Adjust the set point for an approach of about 2-degree F (controller will measure outside wb and adjust set point to 2-degree F warmer).
Commissioning and O&M
Commissioning is arguably the most critical aspect of the sustainable development cycle as it ensures that a building is handed over to operate safely, efficiently and in accordance with how the owner intended. Commissioning has come to be recognised as the most important process needed for design, construction and operation and maintenance of HVAC system to realise owners true project intent and establish energy saving and sustainable operation and management of the building system.
The commissioning process define commissioning as: “A quality-focused process for enhancing the delivery of a project. The process focuses upon verifying and documenting that all of the commissioned systems and assemblies are planned, designed, installed, tested, operated and maintained to meet the owner’s project requirements.”
Commissioning ensures building quality by using design review and on-site verification to help maximise energy efficiency, environmental health, and occupant comfort. The process improves indoor air quality by ensuring building components work correctly and that plans are implemented efficiently and effectively. Commissioning also confirms that maintenance plans, O&M manuals, and training procedures are correct and in-place for maintenance staff to follow. It is also a mandatory requirement to hire Commissioning Authority for achieving Green Certification for your project
Building Control or Automation systems play an important role in the operation of a building and determine whether many of the green design aspects included in the original plan function as intended. Control systems are at the core of building performance. Control system has artificial intelligence built in. Key lies in ‘Managing the Building Automation System’.
Operations and Maintenance plays very critical role in ensuring energy efficiency in the building. It is recognised as an important tool for achieving operational excellence for a facility. It has significant implications on cost, reliability of systems, safety, good ambience in work space. Asset value is enhanced. Good documentation, manpower training, eye for details, housekeeping, routine audits are important to sustain best management practices on operations and maintenance of Facility.
HVAC engineers can play a crucial role in environmental design by being the technical/analytical resource for the team, and by encouraging the development of more rigorous assessment tools that are appropriate, practical and friendly, and defensible. To make a difference for the green design goal, they should push architects to design better envelopes, encourage HVAC decisions be based on life-cycle costing, insert their influence earlier in the process and support the proper commissioning of buildings. They should also try to educate building owners or developers the value of a green or sustainable design so that the green process can be carried out successfully.
Green building approach enables building owners and managers to reduce energy consumption, improve the work environment, and reduce the environmental impacts of building operations. If the building can reduce operating costs, increase occupant productivity, and decrease health complaints, as well as be environmentally responsible, it is a green building. If green means happier tenants and healthier occupancy rates, more building owners would want to incorporate these technologies to gain a powerful market advantage. HVAC and building designers are responsible for bringing this into reality and contributing to the green revolution.