A Systems Approach for Total Cooling Design

I have long advocated for the “Whole Building Design” approach, it has been an uphill struggle without a doubt. The renewed interest in green building has certainly increased awareness of this important skill. Now more help is at hand the Whole Building Design Guide (http://www.wbdg.org). It is published by the National Institute of Building Sciences (USA) so is naturally it is biased towards the USA market, however it will save us acolytes tremendous effort in the longer term.

The whole building design approach is really simple. If designers conceptualise buildings without considering energy costs from day one, that building will surely become an energy hog. The WBD (Whole Building Design) approach means thinking about the whole building impacts simultaneously.  A simple example, if a west facing glazing is shaded, reduce or eliminated, both the initial capital cost, and operating cost for the cooling plant will be reduced.  Since 63% of Hong Kong’s carbon footprint, and 90% of all the electricity generated is attributed to buildings, the opportunities for improvement are obvious.

The hidden beauty is that the principle is equally applicable to other sectors, including process, industry, and even cooling systems. And the latter is one area where the WBDG has overlooked an opportunity to apply whole system design approach for cooling systems.

Too often, building codes and energy codes only specify COP (coefficient of performance) for chiller plant, yet it is one part of the cooling system cycle. In the diagram below, each circle represents a heat exchange process.

kelcroft designConsider all the electrical power consumed for every heat exchange process, and divide by the total cooling capacity gives us a common metric kilowatts per ton (Kw/Ton) defining the whole cooling system efficiency.

The whole system includes all the electrical power used by:

  1. motors driving fans in the AHU (Air Handling Units) and other air moving equipment
  2. motors driving the chilled water pumps
  3. motors powering the chiller compressor
  4. motors driving the condenser water pumps
  5. motors driving fans in the cooling tower

With the focus elsewhere many cooling systems operate inefficiency in a range between 1.0-1.2 Kw/TR, whereas an efficient system would operate nearer 0.6-0.70 Kw/TR.

energyLAB limited Hong Kong

The question is where is your system operating?  If your cooling system is operating in the red, the good news is you have opportunities for improvement.

John A. Herbert
Consultant
Kelcroft E&M Limited

helping lower the cost of doing business in Asia