Passive design for commercial buildings - Part 2: Active systems for passive design
September 29, 2021 By Anthony Broatch
Passive design for commercial buildings
Passive design of buildings is a practice that utilises natural energy to heat, cool, ventilate, and light a building. Active design relies on mechanical heating, ventilation and air-conditioning (HVAC) systems and internal lighting to produce the same results.
Additional challenges are faced when implementing passive design for commercial buildings in comparison to residential designs.
If you're interested in the principles of passive design and the challenges faced in implementing passive design in commercial buildings, check out our first article in the serries Passive design for commercial buildings - Part 1: Achieving passive design
Active Systems That Complement Passive Design
Certain active systems are required or beneficial for optimal temperature, humidity, and ventilation control in commercial buildings that incorporate passive design principles. These systems are either required to facilitate the operation of automatic natural ventilation systems, ensure all systems in the building are controlled properly or are required due to the challenges faced in implementing passive design in commercial buildings described in part 1 of this article. These systems include:
Heat recovery systems
Heat recovery systems should be added to all passive buildings. These systems introduce outdoor air and remove exhaust air without losing all of the heating or cooling energy in the space by the use of a heat exchanger. A heat recovery system bypass should also be provided wherever these systems are installed. This is because at certain times of the year you want to utilize cold air outside to cool the space without passing it through a heat exchanger.
Figure 3: Heat recovery system
Automatic control of louvres and opening windows
If outdoor air is introduced via openings windows or louvres these openings should be automatically controlled. These openings should be controlled on enthalpy control (the energy of the air) as opposed to temperature control and should be interlinked with any active HVAC systems to ensure that they do not interfere with their operation. This precise control can consider the effect of the natural ventilation on the entire building and can help provide continuous comfortable internal conditions if designed well.
Energy efficient HVAC systems
Energy efficient HVAC systems should be looked at where the passive design principles cannot meet the required internal temperatures. These systems should be designed based on outputs from thermal models and should be carefully selected with consideration for carbon emissions and energy efficiency. As a general rule heat pump air-conditioning systems, including chillers, should be looked at as an energy efficient option for heating and cooling a building. Heat Pump systems work by transferring heat from outside to inside for heating and vice versa for cooling. The energy required to transfer heat is significantly less than required to create heat from electric resistance. This is how heat pump systems can produce significantly more heat energy out than electrical energy in. Energy efficient HVAC systems should also incorporate the following design features.
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Night purge systems: A “night purge” pre-cools the building/ high thermal mass elements within the building at night using cold outdoor air. These high thermal mass elements continue to cool the building during the day reducing the operating hours of the HVAC system. This system feature works perfectly with passive building as passive buildings are already designed with thermal mass elements to heat the building.
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Free cooling systems: “free cooling” is where a HVAC system supplies cold air from the outside to cool the building when possible/required. These systems perform the same function as opening windows for cooling; however the air is filtered and the air is introduced in a more controlled manner. This type of system should also be controlled based on enthalpy.
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Occupancy sensors should be used to stop HVAC systems from running when spaces are not occupied. Alternatively these sensors can be used to reduce the amount of heating or cooling the systems provide while unoccupied to prevent spaces getting too hot or cold. Switching off or reducing the output of HVAC systems when not required greatly reduces energy consumption.
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Controlling outdoor air fan speed based on carbon dioxide levels in a space as opposed to running fans at a constant speed.
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Systems that provide simultaneous heating and cooling can be very efficient systems when designed and controlled well. Good examples of these are VRV/VRF systems with heat recovery. 4-pipe chillers should only be installed where required as they can be complex and prone to issues if the system is not designed correctly.
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Selecting heat pumps and chillers with variable speed fans and compressors so that the unit can modulate its capacity based on the required heating and cooling demand.
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Installing variable speed pumps that ramp their speed up or down based on demand.
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Optimizing chilled water systems. This involves considering the combined efficiency of all components in a system as opposed to a single component. Changes that form part of a chilled water system optimization strategy include chilled water rest, condenser water reset, pump control, fan control, etc.
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Regular maintenance plans to ensure systems are not working against blocked filters, strainers, etc.
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Operating systems on time clocks so they only operate when required.
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Continuous commissioning plans to make sure that all components are working at the correct levels and producing the optimal internal conditions.
Occupancy and daylight
Occupancy and daylight sensors should be provided for internal lighting systems to prevent lights from running unnecessarily. Lighting should also be zoned so areas that get natural light can be turned off independently.
Solar hot water
Solar hot water (different to photovoltaic solar panels) is a good way to get free water heating, there is no need for batteries as the hot water is stored in your hot water cylinder.
Energy efficient hot water heating
An alternative to using solar hot water is to use high efficiency hot water heat pump systems. They work on the same principles as the HVAC system heat pumps described above.
Photovoltaics solar panels
Photovoltaics solar panels and wind turbines should only be installed if there is a satisfactory payback period. The majority of the power in New Zealand is renewable so, in general, it is no more sustainable to install local solar panels and wind turbines when centralised visions of those systems already exist.
Waste heat from refrigeration systems
Waste heat from refrigeration systems (chiller or cooling rooms) or other heat producing process should be utilised where possible to provide a free constant source of heat.
Conclusion
For commercial buildings, once the first principles of passive design have been implemented and allowances have been made for the site shortfalls, thermal modeling should be carried out to determine how the building performs. Once the thermal modeling is complete the results should be reviewed to determine where modifications need to be made and where active systems need to be implemented to achieve an optimal building design.
Further Reading
Sustainability Certification
Sustainability certifications like Greenstar, Homestar, or NABERSNZ can provide you with a benchmark to see how your building performs against other similar buildings.
Government initiatives
Read the current government initiatives on climate change here.