September 26, 2011 | 5
By, Brent Stephens and Joshua Rhodes
Just like check-ups with your doctor can evaluate your health and spot early warning signs of future problems, energy audits can evaluate your home for inefficiencies and find ways to reduce ongoing energy waste. A detailed energy audit can find culprits responsible for very noticeable problems, such as high energy bills or uncomfortable rooms, and can also find hidden problems, such as air leaks in the envelope, duct leakage, and low air-conditioner airflow or refrigerant levels. A good energy auditor will be able to recommend improvements that could be made to stop current and future problems and ultimately help you save on your energy bills.
Several cities in the U.S. have recently mandated energy audits on homes that are are sold or renovated, from Berkeley, California to Burlington, Vermont. The same now goes here in Austin, where the Energy Conservation Audit and Disclosure (ECAD) ordinance requires that buildings receive an energy audit before they are sold so that potential buyers can be made aware of that building’s future energy performance. The goal of the ECAD ordinance is to improve the energy efficiency of the entire Austin building stock, so it applies to both residential (single- and multi-family) and commercial buildings. Certified auditors conduct the energy audits and give a report to Austin Energy, the local municipally owned electric utility, which then gives the information to prospective buyers. So not only does a prospective buyer now know if the countertops are granite or the floors are hardwood, they will also know the ducts are and how old and inefficient the air-conditioner is. Thus, the system is designed to use market forces to increase the energy efficiency of the building stock by giving people better information and relying on prospective buyers to choose efficiency over waste. In the end, buyers and sellers should be motivated to ensure they are dealing with a competitive and efficient product, while the City of Austin is motivated by building improvements having the potential to reduce peak electric demand, which can avoid capital costs of new plant acquisitions for Austin Energy (which, being publicly owned, is funded by taxpayers in Austin).
So, just how inefficient are existing homes? We recently published an article in the journal Energy and Buildings investigating that very question. We reviewed data from almost 5000 energy audits of single-family detached homes that were performed in Austin between 2009 and 2010 as part of the ECAD ordinance. We characterized the homes and their air-conditioning systems in four ways that affect both energy consumption (at the level) and peak demand (at the utility level), in terms of 1) air-conditioner efficiency, 2) air-conditioner oversizing, 3) duct leakage, and 4) measured air-conditioner capacity relative to rated capacity. We focused on air-conditioning systems because air-conditioning is usually the largest consumer of energy in a Texas home, where 7.7 million households (both single-family and multi-family units) use approximately 43 TWh of electricity for air-conditioning annually especially in the summer when the percentage of load on the ERCOT (Electric Reliability Council of Texas) electric grid attributed to residential users more than doubles from spring to summer.
What we found generally agrees with other studies: residential buildings are often inefficient and tremendous energy and peak power savings could be achieved by alleviating some common symptoms. For example, Figure 1 shows the distribution of the rated Energy Efficiency Ratios, or EER, of air-conditioners in homes in the database (an EER is a measure of the cooling output of an air-conditioner, in BTUs, divided by the electric energy input required to run the equipment, in Watt-hours). For reference, the ENERGY STAR program (a joint venture between the US Department of Energy and the US Environmental Protection Agency) requires a minimum of EER 12, and the upper end of equipment available on the market today has an EER of about 14.
Figure 1. Distribution of air-conditioner efficiency in single-family homes in Austin, Texas
None of the almost 5000 homes in the audit database had an EER 14 air-conditioning unit and only about 14% had an EER 12 unit, which means that a whopping 86% of air-conditioning units in Austin could be upgraded to meet minimum ENERGY STAR requirements and achieve significant savings. In fact, we estimate that approximately 70% of homes in the database could save at least 25% in cooling energy by upgrading their air-conditioners to EER 14. At the utility scale, we estimate that about 17-18% of peak demand in Austin is likely attributed to the operation of single-family residential air-conditioners, and if every system was upgraded to EER 14, peak demand would decrease by about 205 MW, or 8% overall. Austin would be 30% of the way to meeting its peak demand reduction goals for 2020, as outlined in their Climate Protection Plan.
In the paper, we also estimate that about 30% of systems in the audit database were oversized relative to their required design capacity, which has additional peak power consequences. We also estimate that sealing duct leaks and servicing air-conditioning units to tweak them to function at their nominal capacity could save the average homeowner approximately 18% and 20% in cooling energy, respectively. Taken together, these improvements could make a significant and lasting difference in both energy consumption and peak demand.
Finally, although we found some helpful information in the audit database, we also found some shortcomings in some of the audit procedures. For example, windows are major drivers of energy usage, especially in the hot climate of Texas, but unfortunately the audit procedures don’t require auditors to note the location or area of windows. Also, air leakage testing is not required, but testing for air leakage is a common audit tool that can be performed quite easily with calibrated fans (often referred to as “blower doors”). Additionally, duct leakage measurements could be improved to isolate leaks on either the supply side or the return side, or to isolate leaks to the interior or the exterior. Finally, airflow rates through the HVAC systems were not measured in the most accurate way and refrigerant charge levels at the outdoor units were not measured. These improvements to the audit procedures would be helpful additions and would allow for a better and more accurate understanding of the link between these common system issues and overall energy performance.
Photo and Graph Credit:
About the Authors:
Brent Stephens is a PhD candidate in Civil, Architectural and Environmental Engineering at the University of Texas at Austin, where he is working to develop novel methods for measuring the fate and transport of indoor pollutants in buildings. He received his B.S. in Civil Engineering from Tennessee Technological University in 2007 and his M.S. in Environmental and Water Resources Engineering from UT-Austin in 2009. He is currently a member of a National Science Foundation IGERT program in Indoor Environmental Science and Engineering (http://www.caee.utexas.edu/igert/).
Joshua Rhodes is a Ph.D. candidate in Civil, Architectural, and Environmental Engineering at the University of Texas at Austin. His current research is in the area of residential smart grid applications, including system-level applications of energy efficiency and distributed generation. He enjoys mountain biking and rock climbing, and will get up early to make it happen before class. (http://www.webberenergygroup.com/people)