Mainstreaming high performance commercial building HVAC – stage 3 report

20 Jun 2019

This report presents a summary of the findings and activities of Stage 3 of the LCL-CRC research project RP 1033 “High Performance Commercial Building HVAC”.
This study utilised the knowledge gained form Stages 1&2 and examined the opportunities for further improving the HVAC design of a high-performance commercial office building (referred to as the case study building investigated in Stage 2). The focus of this study is air handling systems as it is a major component of HVAC energy consumption and the energy consumption is significantly influenced by design practice. Hence, substantial opportunities for fan energy improvement identified in Stage 2 such as optimizing duct sizes and air face velocity of coil or air filter were further analysed to reduce the pressure drop of air handling plants. This approach has the potential for significant carbon emission reductions.
This was done by investigating various design/ sizing methods for a large and a small air handling system in the case study building in terms of system energy use and cost. The results showed that the optimum duct sizing method is based on a duct pressure gradient that varies between 0.4 and 0.8 Pa/m. Further, it was found that the AHU filters should be cleaned once the filter pressure drop exceeds 100 Pa. Moreover, an optimum air face velocity of 1.30 m/s was determined for selecting AHU coils and filters.
Subsequently, a validated IES model for the case study building and its associated high efficiency HVAC design (the case study HVAC design) was utilised to model two other HVAC designs (the improved HVAC design and the BCA compliant HVAC design) for the same building. The BCA compliant HVAC design is a design which just complies with the minimum HVAC energy requirements of the BCA. However, the improved HVAC design was created by developing the air handling systems of the case study HVAC design based on the adoption of the above methods for optimizing duct and designing coils/filters. Eventually, all IES models were simulated and the results were compared. It was identified that the fan power consumption of AHU’s for the improved design is less than the usage of the same fans for the case study and BCA compliant HVAC design. Particularly, significant fan power savings occur in the large AHU’s which have maximum fan power consumption of greater than 10 kW.
Further, it was found that the total electrical energy consumption of AHU fans for the improved HVAC design is 29% and 45% less than the case study and BCA compliant HVAC design respectively. Finally, it was determined that the total electrical energy consumption of HVAC systems for the improved HVAC model is 13% and 24% less than the same metric for the case study and BCA compliant HVAC model respectively.
In conclusion several strategies were found to be technically and economically cost effective in reducing the energy requirements of AHUs for a large commercial building HVAC system.
Duct systems were sized by utilizing an optimized duct sizing method – lowering the pressure gradient design criteria from 1 Pa/m to 0.4 - 0.8 Pa/m.
Even larger energy savings (~40%) with only a slight increase in overall costs (~2%) were achieved for the largest AHU investigated with a pressure gradient design criterion of 0.2 – 0.4 Pa/m.
More efficient fittings (fitting with lower loss coefficient factors) were utilized. For example, utilising bends with turning vanes instead of normal bends.
The optimal maximum AHU filter pressure drop was found to be 100 Pa, which requires regular cleaning of filters to maintain this figure – a 100 Pa reduction from typical operational practice.
The optimal air face velocity of AHU coils and filters was found to be 1.3 m/s – significantly lower than the AIRAH recommendation (2.25 m/s for cooling coils and 3.5 m/s for heating coils).
A pressure drop of 25 Pa was assumed for VAV boxes based on the recommendation from REHVA.
A pressure drop of 20 Pa was used for AHU outside air louvres considering larger and more efficient louvres.
Overall – these design strategies produced air handling systems with pressure drops on average 500 Pa lower than typical systems complying with the Building Code of Australia and industry recommended design rules. If such high efficiency air handling designs were adopted across all Australian non-residential buildings this approach would deliver CO2 emissions reductions of 1.6 MT per annuum and financial savings of $255 M per annum.

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