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165.6 156.4 147.3 138.0 0 20 40 60 80 100 120 140 160 180 0% 15% 30% 45% kWh/m 2.year

% Reduction on Occupancy and Equipment Load

BEI Chiller Energy Index Fan Energy Index

Case Description

Base Case Building operates at design condition of 10 m2 _{per person and 15 W/m}2 _{of equipment load}

Case 1 Actual operating occupant and equipment load is reduced by 15% from Base Case. _{Air flow rate of CAV system based on peak load design condition.} Case 2 Actual operating occupant and equipment load is reduced by 30% from Base Case._{Air flow rate of CAV system based on peak load design condition.} Case 3 Actual operating Occupant and equipment load reduced by 45% from Base Case._{Air flow rate of CAV system based on peak load design condition.} Case 4 Actual operating Occupant and equipment load reduced by 45% from Base Case. _{Air flow rate reduced to match reduced actual operating occupant and equipment load.}

146 | Building Energy Efficiency Technical Guideline For Active Design

Figure 7.5 below displays the effect of reducing the supply air flow rate of a CAV system according to the actual cooling load in the building. A very significant building energy reduction of 8% is achieved, reducing the building energy index (BEI) from 138 to 126 kWh/m2_{.year, saving}

approximately RM155,000 per year on this simulation building model of 17 floors and a GFA of 38,400m2_.

fIGURE 7.5 | PERfORMANCE Of A CAv SYSTEM AT REDUCED SUPPLY AIR fLOW RATE TO MATCH ACTUAL BUILDING SENSIBLE LOAD AT PART LOAD

It was interesting to note that in the simulated model, a reduction of building occupants and equipment by 45% allowed a reduction of 20% of the supply air flow rate. Modelling the reduction of 20% of the air flow rate reduces the fan energy by 45%. The fan energy reduction is from a combined effect of a reduction in total pressure loss and a lower air flow rate (this double reduction effect is known as the “Fan Affinity Law”). The fan energy reduction also leads to a further reduction of 2% in chiller energy consumption due to the lower heat energy produced by the fan. The compounding effects of reducing the supply air flow rate by 20% in this model lead to a very significant total building energy reduction of 8%.

Based on this result, it is recommended to provide CAV system with a variable speed drive (VSD) for all major AHUs in a building, especially for buildings where there is a possibility that the actual building occupancy may be significantly lower than the design assumptions. This would make it possible for an energy manager (or facility manager or a commissioning agent) of the building to fine-tune the AHU system based on the actual operating heat load on site. A reduction of the supply air flow rate in a CAV system may require rebalancing work to be conducted again on the duct network to ensure that the supply air is evenly distributed. The need to rebalance the air flow from diffusers may be minimised by providing a duct network design that would cater for minor changes in air flow rate in a CAV system.

8% reduction 2% reduction 45% reduction 0 20 40 60 80 100 120 140 150

BEI Chiller Energy Index Fan Energy Index

Base Design Condition Revised Design

(reduce fan peak air flow rate)

Building Energy Efficiency Technical Guideline for Active Design | 147

A Variable Air Volume (VAV) system is a system where the design supply air temperature is fixed. If the temperature in the space supplied by the VAV system is higher than the set point temperature by the sensor (room is hot), the supply air volume is increased via a control system consisting of a VAV box and variable speed drive (VSD) on the motor for the fan. A VAV box is basically a motorised damper with a temperature sensor. If it senses the space temperature is higher than the set point temperature, it will signal the motorised damper to open to allow more supply air into the cooled space. The opening of the damper will reduce the pressure in the supply duct. A pressure sensor located in the duct system will detect the pressure drop and will signal the VSD to ramp up the motor to increase the fan speed to maintain the pressure in the ducting system. The reduction of fan speed during part load scenarios reduces fan power consumption in the building, increasing the building’s energy efficiency.

Static Pressure Reset

In addition, during actual operation, the implementation of a static pressure reset of a VAV system will increase efficiency by reducing the static pressure set point when the building is running at part load. A static pressure reset reduces the fan static pressure at part load by allowing the VAV boxes to be fully open for low restriction of air flow. The successful implementation of a static pressure reset is highly dependent on the controller logic and the location of the pressure sensor in the duct network. In some cases it may require more than one pressure sensor to be installed. Designers are recommended to seek further design and installation tips to implement this successfully from ASHRAE journals and publications such as “Increasing Efficiency with VAV System Static Pressure Setpoint Reset” by Steven T. Taylor, 2007.

It is becoming common these days to have VAV systems specified for new buildings in Malaysia because of the keen interest in energy efficiency. A VAV system may be designed with a couple of VAV boxes in the ducting system to provide comfort control of East and West zones or may have many VAV boxes to provide comfort control of individual (or group of) rooms. These VAV boxes improve the control and comfort condition of the air-conditioning system by ensuring that each zone is maintained at the right air-temperature even at different times of the day. A VAV system will have lower energy consumption than a CAV system because it allows the fan to run at lower speed (and with a static pressure reset, lower pressure too) depending on the cooling load of the building.

However, the use of multiple VAV boxes, temperature sensors, pressure sensors and motorised dampers in a VAV system also increases the possibility of equipment failure in comparison to a simple CAV system. Finally, a VAV system will have a higher cost of implementation than a CAV system.

VARIABLE AIR VOLUME