05 July 2001

Engineers feel the heat

by Bob Felton

Improved monitoring, instrumentation could reduce energy use 30%.

Commercial buildings consume about 15% of the U.S.'s energy, at an annual cost of $85 billion, according to a report from Lawrence Berkeley National Laboratory. Further, the report said, "Half of this consumption is wasted, compared to what is cost-effectively achievable."

With the economy idling, the nation's generating capacity aging, and energy prices arcing skyward, ensuring efficient energy usage is claiming increased attention from engineers. Operators won't be able to reduce energy consumption in a systematic fashion, however, until there is substantial improvement in the tools used to monitor energy consumption, the report said

"Buildings do not perform as well in practice as is anticipated during design," the authors wrote. "There are many reasons for this, including improper equipment selection and installation, lack of rigorous commissioning and proper maintenance, and poor feedback on operational performance, including energy performance. Few tools are available to the on-site engineer to address these problems, which results in excess energy use and higher operations and maintenance costs. A related problem is that existing energy management and control systems (EMCSs) used to monitor building performance are becoming more complex over time and are difficult for the average operator to understand. Building operators typically have only monthly utility bills to track how much energy is used, and EMCSs have limited data collection, archival, and visualization capabilities and have few techniques to extract relevant information from the data."

The researchers deployed a prototype Information Monitoring and Diagnostic System (IMDS) in a 100,000-square-foot office building in San Francisco. There are 57 sensors distributed outdoors and throughout the facility, each of which reports each minute. The data is archived for later analysis using a variety of graphical and analytical tools.

Savings surprises

At the end of one year, the researchers found several critical building control problems they hadn't known existed, which resulted in improved operator understanding of the in-place system, reduction of occupant complaints, and extended equipment life.

Additionally, "the reduction in labor-hours required to operate the building as a result of the IMDS is worth about $20,000 per year. These savings would pay for the IMDS in about five years. The operations staff expects to see even greater savings as they proceed with a control retrofit planned using the IMDS. The research team presented the operations staff with a series of recommendations for actions to achieve additional energy savings, some of which they will adopt over the next year. Overall, they expect to reduce energy use by more than 20% over the next year, saving more than $30,000 per year."

Those predictions may seem unrealistic because, disappointingly, the researchers said they did not achieve "the 15% energy savings from building tune-up activities that are often possible with such technology. However, the IMDS has been used to solve control problems and improve overall comfort."

When one retrofits the existing system with the IMDS, however, the researchers project savings of 1.84 x 10⁶ kBtu.

IMDS trained by operator

Once the IMDS was installed, thanks to Internet accessibility built into the system, researchers evaluated the data at will. Further, they interviewed the operators every two weeks to determine how they had been using the system and to jointly assess the performance of the system.

"One of the concepts of the IMDS," the report explained, "is that rather than trying to detect specific faults, the system was specified to monitor performance in areas of the building that were performing poorly.

"The intent was to tell building engineers: 'There is something wrong with subsystem X,' rather than, 'The front bearing on pump 6 is failing.' The concept behind IMDS was that once building operators are aware of a problem in a specific subsystem, they are reasonably good at tracking it down.

"The principal advantage of using human-based diagnostics is that one can immediately generate useful detectors for potential efficiency increases. By partnering with actual building operators, we can progress in an orderly fashion toward increased specificity without running the risk of missing 'big picture' issues."

The operators stayed close to the data. The "team had the specific objective of interfacing an analysis prototype with the real data and so provide a means by which the building operators could investigate the data, have access to benchmark data sets and readily available plots.

"A demonstration system implemented in Matlab has a rudimentary capability to learn from real data. The idea was that the operators would make some specification such as, 'On such and such a date and time, X was fixed,' and the automated system would learn by example and create a test to determine whether data presented to the system indicated that X was fixed or faulty."

Ideally, consistent usage will "train" the IMDS to recognize and respond automatically to energy-related problems.

"Sensor accuracy, reliability, and data acquisition sampling rate," the researchers observed, "have been a major issue in the project. The results of the fault detection work indicate that the ability of the IMDS to measure cooling load and chiller power to 1% accuracy with a sampling interval of 1 minute permits the detection of faults that might otherwise go undetected. The measurement of power and thermal loads at the 1% level is also desirable for performance contracts, since profits can be significantly affected by undetected degradations in performance at the 1% level."

The complete report is available at www.energy.ca.gov. IT