Energy Saving Air Conditioning

energy saving air conditioning

Energy Saving Air Conditioning

The Hong Kong Government issued advice to building owners and operators encouraging setting the air conditioning thermostats to 25.5 Deg C. Of course the audience for generic advice was the general public, the broad message air temperature of 25.5 deg C (dry bulb temperature) is comfortable.  However, Hong Kong is located in sub-tropic climate region with ambient RH (relative humidity) up to 98% in the summer months.

The most popular piece of air moving equipment is the ubiquitous fan coil unit, controlled with a simple dry bulb  thermostat and has no humidity control, therefore the only way for the user to try and lower the relative humidity is lowering the air temperature.

Air Conditioning Energy Design

Any engineer worth his salt will tell you that just quoting the air (dry bulb) temperature is worthless information, it only tells half the story. For example would you like to work in air temperature 25.5 deg C & 90% RH? or perhaps 25.5 deg C & 10% RH? No, both are incredibly uncomfortable for humans, we prefer a range of humidity between 50-60% RH.

Comparative Conditions

Engineering the built environment means understanding the complex human physiology, and key to designing human comfort. So how can we use human physiology to lower operating costs? We are more sensitive to changing environment, therefore if you walk into a building from hot and humid conditions, let’s say 35 deg C and 90% RH, into a space at 30 deg C & 50% RH, only 5 degrees cooler, we immediately feel significantly cooler than the 5 degrees would indicate (of course we can’t use this technique for continuous occupancy spaces).

Air Conditioning Equipment

The operation of the air conditioning equipment is another key factor for building energy efficiency.  Our weather changes throughout the day, and year. However, often we find that the air conditioning system is never adjusted. The system commissioned on day one, and never adjusted or tuned until there is a problem.Automatic controls are set to provide chilled water at 7 deg C, irrespective of the external ambient conditions, and chilled return water temperature.  Many building systems also suffer from low delta T (temperature difference between the supply and return chilled water temperature) and don’t realise they have problems.

Over the years consulting engineers have been chained to their desks, pushed far from the oversight of building operation. Once upon a time, air conditioning and other plant rooms were clean and tidy, maintained in a near spotless condition by the operator was trained or perhaps brainwashed is a better term, to keep the plant room clean. The reason is simple dust and dirt let to accumulate will infer with mechanical equipment, including moving parts, gears, bearings, or block air flow over cooling fins necessary for efficient equipment cooling.

by John Herbert
energy consultant
EnergyLAB limited

About John A. Herbert, REA, energy consultant, BEAM Pro

John is director of energyLAB energy consultants firm, he has with more than 30 years engineering experience gained across three continents. He is Registered Energy Assessor, a member of HKAEE. Read his blog at http://www.johnherbert.hk
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