How advancing standards and equipment can improve building energy performance
This article is Part 1 of a two-part series on part-load efficiency. Increased emphasis on part-load efficiency is contributing to whole-building efficiency.
- Understand the evolution of equipment efficiency standards from full to part load and their relationship to whole-building efficiency.
- Examine how HVAC technologies are advancing to modulate capacity to match variations in building loads, a major factor in improving whole-building energy performance.
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 by the hour. The next stage in the evolution of building energy efficiency will focus on whole-building efficiency for facilities.
Given the progress to date, part one of this article will examine the evolution of equipment efficiency standards from full to part load and their relationship to whole-building efficiency. Part two 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.
From full- to part-load equipment efficiency: a major factor in improving building performance
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 faces north or south-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.
In some facilities, however, HVAC systems operate at full capacity and are simply switched on and off as cooling is needed. Accordingly, over the past 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 compressors, heat exchangers, cooling towers, fans, and pump motors have achieved significant improvements when the system is running at 90% to 100% 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%.
Because off-design conditions can occur as much as 99% of the time, a lot of energy can be wasted by systems optimized for full-load operation. To save that energy, systems must be designed for part-load operation. At lower loads, cooling capacity can be reduced or "turned down." Methods that can be used to turn down capacity exist today, such as employing manifold compressors, reducing fixed-speed compressor power consumption at part load, and using variable-speed compressors, fans, and pumps. These technologies will be discussed in part two of this article.
Meanwhile, ASHRAE develops test methods and efficiency minimums, and the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) develops the performance rating for an application. Their work together influences the design of rooftop units, chillers, and other air conditioning applications prominent in the U.S. Similarly, the International Ground Source Heat Pump Association (IGSHPA) has led the way in providing a complete set of standards for ground-source heat pump systems.
Although significant energy savings can be obtained by applying advanced technologies and system designs, the amount of savings depends on the specific technologies and the particular application.
Consequently, technical committees for some industries are developing their own building efficiency standards to encourage the use of equipment optimized for part-load operation.
For example, data centers are recognized for using substantially more energy than office buildings. Because data centers run 24/7 at varying loads, systems optimized for part-load operation can realize energy-cost reductions of 30% or more. These facility standards use a unique measure of power-usage effectiveness (PUE) for the facility, which takes into account new HVAC technologies, the part-load efficiency of systems, and various cooling strategies. PUE simplifies cost budgeting and estimating data center infrastructure efficiency (DCiE). Both measures were developed by The Green Grid, an association of information technology professionals aiming to raise data center energy efficiency.
This indicates how whole-building efficiency and technology standards are evolving together. New standards are being created and existing standards are being revised for different applications within an energy efficiency ecosystem. The goal is to improve equipment efficiency, as well as the equipment rating method, to more closely reflect real-world operation. As equipment standards define particular test methods used within the HVAC industry, these developments influence the definition of relevant regulatory requirements and energy mandates in the public sector.
As a microcosm, the data center industry shows equipment standards and a whole-building approach are co-evolving. Likewise, other private and public sectors are taking complementary paths that are improving equipment efficiency standards.
Part-load efficiency standards are evolving
The ASHRAE 90.1 Standard establishes minimum efficiency values for equipment. Compared to the ASHRAE 90-1975, the improved ASHRAE 90.1 has lowered annual chiller energy consumption as much as 59%. The efficiency of commercial rooftop units has improved 45%. An additional 30% to 45% in energy savings is expected with the adoption of ASHRAE 90.1-2013.
These developments are not happening in isolation. In California, Title 24 Building Energy Efficiency Standards for Non-Residential Buildings sometimes mirrors and sometimes modifies ASHRAE 90.1. From 1998 to 2008, Title 24 increased required chiller efficiency by 25%. A similar increase in chiller efficiency was mandated by the Title 24-2013.
These changes coincided with changes to ASHRAE 90.1 itself.
In 2003, ASHRAE adopted a part-load efficiency rating method known as the Integrated Part Load Value (IPLV) described in ANSI/AHRI Standard 550-590. IPLV was developed by AHRI to rate the performance of systems capable of capacity modulation at part loads. IPLV rates a system's cooling efficiency over a hypothetical season rather than at a full-load point at the worst time of the year.
Used to rate chillers, IPLV is calculated using four operating points. Each point is weighted by the amount of time a chiller spends at that load point: 1% of the time at 100% load, 42% of time at 75%, 45% of time at 50% load, and 12% of time at 25% load.
For rooftop units, a different rating method is used: the Integrated Energy Efficiency Ratio (IEER). This method is described in ANSI/AHRI Standard 340-360. Similar to IPLV, four weighted load points are used: 100% (full load), 75% of full load, 50%, and 25%-each of which is multiplied by a weighted value (0.020, 0.617, 0.238, and 0.125, respectively).
Over the years, the part-load IPLV rating method-and more recently, IEER-has undergone significant improvement, but the HVAC industry recognizes that shortcomings remain. Consequently, new test methodologies are under consideration. The short-term goal is to develop standards that include more rating points and more complex mathematics to model performance over a continuum of conditions.
The AHRI technical committee on compressors, for example, continues to investigate new mathematical polynomials to model variable-speed compressor performance curves. Finding better-fitting polynomials will more accurately describe what compressors can do. Equipment designers can then plug these expressions into simulation programs to get a better idea of how a system will perform in different conditions.
However, using multifactor formulas to rate a system using two fixed-speed compressors can yield the same result as a variable-speed system, but will miss the full energy-savings potential with variable speed technology. Also, the formulas don't have the flexibility to account for changing building loads encountered in different climate zones.
While AHRI and ASHRAE ponder these issues, further improvements in equipment efficiency are being spurred by other public initiatives:
- In 2014, the U.S. Department of Energy (DOE) Notice of Proposed Rulemaking proposed further developments that would lower commercial rooftop air conditioner energy use by about 30%. This proposal would save 3.5 trillion kWh over 30 years, achieving the largest national energy reductions of any standard ever issued by DOE.
- The Consortium for Energy Efficiency is promoting high-efficiency equipment specifications and quality installations by using an elevated tier rating system based on SEER, IEER, EER, and HSPF metrics.