In recent years, the HVAC industry has witnessed the evolution of three distinct approaches to advancing energy efficiency. For decades, the industry focused on improving full-load efficiency before the current shift to part-load efficiency standards for equipment. Today, new part-load rating methods better account for how equipment operates at off-design conditions when loads vary hour by hour. The next stage in the evolution of building energy efficiency will focus on whole building efficiency for facilities.
This first installment of a two-part series will examine the evolution of equipment-efficiency standards from full to part load and their relationship to whole-building efficiency. The second article will look at how HVAC technologies are advancing to modulate capacity to match variations in building loads, a major factor in improving whole-building energy performance.
Continued Energy Savings Predicted
The energy performance of buildings is expected to improve significantly in the decades ahead. From 1980 to 2009, for every percent of growth in U.S. commercial building space, primary energy consumption grew by 1.19%. The U.S. Energy Information Administration estimates, however, that from 2009 to 2035, every percent of growth in space will increase energy consumption by only 0.79% — a 33% improvement in energy savings.
That raises the question: How will those predicted energy savings be obtained?
Considering that approximately 40% of the energy in commercial buildings is consumed by HVAC equipment, it's reasonable to conclude that mechanical system efficiency will have to improve substantially to achieve those results. Consequently, HVAC equipment designers must look for new solutions to old challenges. To improve the efficiency of mechanical equipment, system designers face the perennial thermodynamics problem: how to move heat from one place to another using the least amount of energy.
Inside a building, heat is generated by people, processes, equipment, and lighting — factors that constitute the internal load. Outside, the climate and thermal performance of the building's exterior — including the amount of insulation, number of windows, and whether the building is north or south facing — comprise the external load. A building designer can reduce the load by improving the building envelope and cut energy consumption by employing mechanical equipment with the flexibility to modulate capacity at lower loads.
Multiple factors constitute a building’s heat load.
In some facilities, however, HVAC systems operate at full capacity and are simply switched on and off as cooling is needed. Accordingly, over the last few decades, system designers have improved the full-load efficiency of equipment. In fact, since 1980, average chiller full-load efficiency has improved more than 35%, despite the adoption of less efficient refrigerants. Advances in refrigeration compressors, heat exchangers, cooling towers, fans, and pump motors have achieved significant improvements when the system is running at 90 to 100 percent of its designed capacity.
The trouble is, most buildings experience internal and external loads that vary throughout the day. Consequently, buildings with systems optimized only for full-load operation are wasting energy when the loads fall below 90%.
Subscription users only!
Subscribers are able to view the whole article. Please register/subscribe (it's free and easy) to read all articles.