Six energy-saving tips for offices and commercial premises
By Ir Dr Eddy W T LAU
On 14 May 2015, the Government released the "Energy Saving Plan for Hong Kong's Built Environment", which sets out the plan to reduce energy consumption from 2015 to beyond 2025. According to the document, Hong Kong's commercial sector accounted for the largest share (42%) of energy end uses, followed by transport, residential and industrial sectors. The document also points out that the bulk of energy reduction in Hong Kong is expected to come from existing buildings, especially commercial buildings. This article points out some commonly ignored office/commercial energy waste and proposes six simple but effective ways to save energy in offices and commercial premises.
Occupants' behaviour can have a big effect on energy bills. Believe it or not, in many offices, objects (like adhesive tape, paper, etc) are placed in the way of air-conditioning (AC) diffusers. As a result, energy is unnecessarily wasted in overcoming friction or counteracting obstructions.
These obstructions may be there to reduce overcooling, but this is not a proper way to solve the problem, the common causes of which include oversized AC system, improper selection or location of diffusers, as well as a mismatch between air flow rate and diffuser capacity. To combat these problems, there are three useful design techniques or principles to follow, namely:
- the supply air flow rate should match the space cooling load; no oversupply should be allowed
- diffusers should never blow directly towards occupants
- adjustable air vanes should preferably be used in supply air grilles to provide flexibility in ventilation direction
Normally, chilled air is directed to blow horizontally along the ceiling, allowing room air to be entrained into the stream as it travels. A proper design should ensure that the air flow rate allocated to each diffuser matches its design capacity. Too high a flow rate will cause gusty discomfort whereas too low a flow rate would cause "cold air dumping". The key is to engage a qualified AC engineer to carry out AC system design regardless of the size of a project. Where equipment oversizing is observed, adjustment, replacement or even decommissioning of equipment should be considered.
Another typical phenomenon is occupants having portable fans at their workstations. This is the other extreme of the problem: the AC system is undersized. While desk fans can solve the problem, it is not uncommon to see occupants leave their fans on when they leave the office for lunch or after work. Fortunately, there is a new way to eliminate this problem. There are two kinds of ceiling fans on the market that are suitable for offices/commercial premises. One is a rotating louvre fan (Figure 1). There is a hood at the back of this fan which can be connected to the AC supply air duct. In this way, the fan acts as an extended-range diffuser that adds pressure to the supply air stream and causes it to sweep through a wider area. In cases where ceiling cassette AC units are used, the addition of these fans can enable a single AC unit to serve a wider area, thus reducing the number of AC units required. Since the blow angle and fan speed are adjustable via a handheld remote, the fans can also enhance individual environmental control, just like the adjustable AC diffusers on air-conditioned buses.
Another kind of fans that have gradually gained popularity are high-volume, low-speed (HVLS) fans (Figure 2). Unlike rotating louvre fans, HVLS fans have a larger diameter, blow in a fixed direction and require a higher headroom (2.8 m) for installation. It can be applied to lobbies, restaurants, assembly halls, and even office areas provided that partition screens are not too high. These devices have a low-friction aerofoil design, adopt energy-efficient DC motors, and have a wide range of selectable speeds (six to seven steps are common). For example, a 1,500 mm diameter fan consumes only about 10 W to 60 W, depending on the speed settings. When these fans are operating, the temperature set point of the AC system can be adjusted higher, thus reducing the AC energy bill. In an office setting, the fans can also be used when workers are working overtime at night or during weekends.
Desk lamps are another source of energy waste during lunch hour or after work. To prevent energy waste from task lighting when occupants forget to turn off these lights, there should be a master switch for non-computer devices such as printers, desk fans and desk lamps. During lunch hour or after work, this master switch can be switched off, thus preventing any equipment from continuing to operate. A master switch can also be used to cut off the power supply to lighting, AC, Wi-Fi routers, photocopiers, TV sets, water dispensers and even drink vending machines (provided that the products will not spoil). Computers can also be set to automatically shut down at night.
Exit signs are usually kept on around the clock, even when nobody is inside the premises. In the old days, these devices typically utilised T8 fluorescent tubes rated at 10-18 W. A more sensible option nowadays is to replace them with LED exit signs that consume about 3-5 W.
When it is time to carry out batch replacement of lamps, the opportunity should be taken to evaluate whether there is a business case to upgrade the lamp type from conventional ones to LEDs. For example, a 600 mm long T8 LED tube (designed for replacing fluorescent tubes) will consume just 10 W and will last for 30,000 to 40,000 hours. It costs about a hundred Hong Kong dollars but it saves you half the operating cost while lasting twice to three times longer. However, when fluorescent lamps are replaced with LED tubes, certain modification works have to be carried out on the light fittings, such as bypassing the ballast and taking out the starters. The installation technicians also have to pay attention to whether the LED tubes are designed as single-ended or double-ended products, as the required wiring modification works would be different. Modification works for double-ended tubes are simpler, but in terms of installation safety, single-ended tubes are usually better as there is no risk of an electric shock arising from touching both ends of an installed single-ended tube.
Besides retrofitting light fittings, nowadays an energy retrofit can also be done on fan coil units (FCUs). Brushless DC motors with permanent magnet, which are more energy-efficient than AC induction motors, were previously applied to small devices like computer disc drives and machine tools but are now used for fans and AC equipment as well. Recently, the application of DC motors to existing FCUs was made possible by a kind of retrofit kit. This kit is commercially available and is intended to allow users to replace the motors of an FCU without replacing the fans and the cooling coil. The author has tried this retrofit kit on a number of 800 CFM FCUs. The average energy savings at low, medium and high fan speed settings are found to be 64%, 47% and 24% respectively. Another advantage of re-motoring is that, with the use of electronically commutated motors, speed control becomes step-less. This can be linked to room temperature control to provide a variable-air-volume algorithm to reduce fan energy.
Hong Kong has now got a long-term energy saving plan for the coming ten years and beyond. As engineers, we can do a lot to help Hong Kong achieve its energy saving target. Let's join hands to make our offices and commercial premises greener and more energy-efficient.
About the author: Ir Dr Eddy W T Lau is a building services engineer who had worked in the maintenance and retrofit fields for over 17 years. He is now the head of green labelling at Hong Kong Green Building Council. He can be contacted at email@example.com.
Figure 1: A rotating louvre fan
Figure 2: A high-volume, low-speed fan