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January 15, 2014 -
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Today’s tip from Building Operating Management comes from Daniel H. Nall of Flack + Kurtz. One major opportunity to reduce HVAC energy use is to improve the efficiency of the cooling sources.
There is a basic conflict between optimizing the efficiency of the distribution systems and optimizing the efficiency of the heating and cooling source equipment.
When it comes to cooling sources, the frequent need for dehumidification introduces a complicating element. The usual temperature setpoints of a chilled water system are often set by the fact that the dew point temperature of the ideal comfort condition (75 degrees F temperature, 50 percent relative humidity) is 55 degrees F. Maintaining that dew point in the space would typically require a supply air dew point temperature of 53 degrees F. Producing that temperature at a cooling coil typically would require an entering chilled water temperature at the coil of 45 to 46 degrees F, and a supply-chilled water temperature from the chiller of 44 degrees F. For an optimized all-air HVAC system, reduction of air transport energy is usually more effective than reduction of chilled water production energy, so selection of a coil with a 6 degrees F approach temperature should be accompanied by 44 degrees F chilled water and 51 degrees F temperature off the coil, reducing the fan volume and energy by 10 percent.
Minimization of cooling energy can also be achieved by reduction of the temperature at which the refrigeration device rejects the heat that it has removed from the conditioned space. Utilization of evaporative heat rejection with cooling coils or evaporative condensers is one way of achieving this end. Rejection of heat-to-surface water features such as ponds or rivers is another method. With either of these evaporative heat rejection methods, resetting the condenser water set point in response to non-design exterior conditions results in additional savings.
Another strategy for enhanced energy efficiency is rejection of refrigeration heat to the ground itself, although this method usually requires accompanying heat extraction from the ground. The ground-coupled heat pump is a popular strategy for enhanced energy efficiency, although failure to maintain a balance of heat rejected into the ground during cooling cycle and heat extracted from the ground during the heating cycle can result in long-term temperature drift in the soil surrounding the ground heat exchanger. Many systems have failed or delivered poor energy performance, over the years, due to this imbalance. Well-designed and balanced systems can deliver significant energy savings compared with air source heat pumps or cooling systems with evaporative heat rejection.