Thermographic inspection and quality assurance of energy conservation procedures for electric buses
Electric buses are one of the solutions for improving air quality in our cities. Many states are adopting “no new diesel bus” policies, thus increasing the pressure to develop alternative vehicles. The fledgling electric vehicle technology suffers from acceptance problems by major transit authorities due primarily to limited travel range from each battery charge. Utilizing electric buses in the Northeast has the added problem of maintaining an adequate cabin temperature without the availability of waste heat from the inefficiency of a diesel motor.
Heating the passenger cabin of an electric bus with an electric resistant heater draws from the batteries’ stored energy, thereby making less energy available to propel the vehicle. The net effect of this is to further reduce the already modest range of these vehicles. Effective energy conservation design can play an important role in allowing electric vehicles to provide comfortable and practical transit services. Infrared Thermography, in conjunction with pressurized air-leakage testing, has proven to be an excellent diagnostics tool for developing energy-efficient bus designs, as well as a valuable performance evaluation method.
This paper is based on tests performed on several Advanced Vehicle Systems, Inc. (AVS) electric buses. This research was performed under grants from the Northeast Alternative Vehicle Consortium (NAVC) and Defense Advanced Research Projects Agency (DARPA), two entities with an interest in advancing electric vehicle technology. This work demonstrates the thermographic methods used and the improved real-world performance of a retrofitted electric bus. Further, work in support of a new bus design standard and test methods to evaluate design performance are presented. As a result of this research, thermography, pressurized air-leakage testing, and fog-analysis techniques have been developed for use by electric bus manufacturers.
Background of Electric Bus Project
Electric buses are different from conventional diesel-powered, mass-transit buses in that they must operate with little or no fossil fuel emissions; therefore, they have no diesel engine to provide waste heat for heating the passenger cabin. Transit authorities are faced with mandated reductions of tailpipe emissions requiring them to find alternatives such as electric vehicles. The U.S. government is supporting accelerated efforts to develop new technologies that will make this “clean” vehicle a practical choice for mass-transit use.
The travel range of an electric vehicle is the key to general acceptance and widespread use. The maximum possible distance from a single battery charge is dependent on a number of factors. Most researchers are currently focusing only on new technologies that will improve the efficiency of the vehicle’s drive motors, controls, and the storage capacity of the batteries. The amount of energy required to keep the passenger compartment at a reasonably comfortable temperature is commonly overlooked. Heating and defogger systems can require significant amounts of electrical energy. As the range of electric buses is limited by the energy storage capacity of the batteries, it is important to reduce or eliminate any unnecessary loads on these resources while still providing comfort and safety.
The design of early electric buses, originally used in California, had little planning for severe climates. As northern New England cities attempt to move into low-emission transportation systems, this lack of preparation for cold weather has resulted in many failed attempts to use electric buses. The challenges presented by New England’s cold include: new infrastructure requirements such as battery charging terminals, propane storage and handling, additional operator training, and the difficulty in planning routes with the constraints of limited range. The additional problem of inadequate heating and defrosting capabilities has made most northern bus operators reluctant to buy into this technology, or has forced those who have tried to abandon their efforts.
The Portland Transit Authority is one northern New England entity that has made the commitment to addressing air- quality problems by using electric buses as a part of their in-city fleet. Not only are these buses clean-transportation technology, they are used for shuttling commuters to perimeter lots, thereby eliminating inner-city traffic. The first two electric buses purchased by the Portland Transit Authority were successfully in service until the cold weather rendered them impractical for both driver and rider. Even with the manufacturer installed propane heaters, it was impossible to get the buses above forty degrees when the outside temperature was below freezing. Safety was also in question as defroster performance was unable to provide adequate driver visibility.
To address the effects of cold weather, the EVermont project, a Vermont-based public-private partnership actively working on electric vehicle cold-climate research, was contacted by NAVC and the Portland Transit Authority to see if the results of their research could be applied to Portland’s problem1. As the first step in a complete energy-management review, EVermont sub-contracted with H. C. Fennell, Inc. to do a full-scale audit including infrared and pressurized air-leakage testing. This work with the Portland Transit Authority became part of Phase I of a two-part research project that led to the development of an advanced fleet of buses designed and manufactured specifically for cold-weather sites. The research, development, and manufacturing portions of the grant are complete, with follow-up testing to be completed in the spring of 1998. This work will be referred to as Phase II2.
H. C. Fennell had worked with EVermont on previous projects where infrared and pressurized testing was used to improve cabin thermal management for an electric car. This project determined the possible increase in range resulting from reducing cabin heat and defroster energy. Work was done with battery thermal management as well. FOAM-TECH, a division of H. C. Fennell, Inc. which specializes in high-tech insulation and air-leakage and vapor control systems for specialty applications, performed the retrofit work indicated by the analysis of the thermal envelope.
The initial use of the infrared technology in the diagnostics phase has evolved into a method for diagnosing and quality assurance testing which utilizes a number of tools to obtain the maximum information about the thermodynamics of the bus’s shell. These tools include: infrared scans, pressurized air-leakage testing, and fog-leakage analysis. As the project progressed, many of these tests were performed simultaneously to achieve significantly better data. By integrating the test procedures, each diagnosis was supported or enhanced in some way by the other(s). Following is a detailed account of the analysis of thermal envelopes in electric buses for northern climates utilizing non-destructive testing techniques.
Cold weather places two major energy demands on vehicles. One of these claims is heating the cabin, and the other is maintaining a clear windshield. The two Portland buses were fitted with an RV-style propane heating system (30,400 BTU output) by the manufacturer. These units should have been adequate to heat a twenty-two foot bus in a cold climate; however, because the bus’s shell was not properly insulated and had such a high infiltration rate, the heater could not maintain acceptable comfort levels. In addition to the problems with the thermal shell, the heater’s warm-air distribution system was extremely ineffective.
Temperature monitoring data showed that the capacity of the heater could not be fully utilized. Because the air flow was restricted through the ducts and outlet diffusers, the unit would shut off due to overheating. This meant that the distribution fans ran constantly because the thermostat was never satisfied. Infrared tests on one of the two original Portland/AVS buses showed that the heater compartment and duct chases were quite hot; yet, very little heat was being delivered to the passengers, especially in the front of the bus.
Operation of the bus in cold weather documented that the second problem facing the northern operators was visibility. The fan, resistance heater, and distribution system for the defroster system were all undersized; consequently, icing and fogging became a major safety concern. During stationary indoor tests of the bus}s shell, thermal imaging showed that only a small area in the lower quadrants of the left and right side of the windshield was being warmed; this indicated the cause of the defogger’s ineffectiveness. To remedy this, larger fans and greater heater capacity were incorporated into the design of the Phase II buses.
Once the EVermont team was able to optimize the performance of the heating and defogging systems, the total amount of energy the bus would require for uses other than moving became dependent on the thermal efficiency of the shell. Therefore, the next step was to obtain diagnostic information on the passenger cabin itself. Two non-destructive procedures were used to identify the locations with the greatest conductive losses and to quantify the air leakage. The infrared test4 (I.S.I. Model 91 Videotherm Camera with video recorder) readily identified the relative loss and gain areas in the shell of the bus. Because this was an indoor test with only the heater fan and stack-effect pressures to create air flow, only the gross air- leakage areas were visible.
For the second test, the bus was pressurized. This was done to enhance the infrared scan and to simulate the forces which would be acting on the bus when it was in motion. This particular test proved to be marginally beneficial because the heat from the bus’s propane furnace was unable to maintain the bus interior temperature high enough - relative to the bus exterior temperature (delta T) for the problem areas to show up on the scan.
It turned out that the air leaks were so significant that the rapid air changes quickly dissipated all of the available heat in the bus without warming the surfaces around the large openings. Consequently, the first round of infrared imaging could not easily locate the sites of minimal air leakage. Initial air-leakage tests4 were able to verify that the leakage rate was relatively high for a structure of this size. Stack effect and ground effects were generally assumed to be of little concern in these small buses that move at slow speeds and stop frequently. It was further theorized that the front of the bus had cold air coming in, and the rear of the bus allowed the conditioned air to escape as it moved. Because of the initial difficulty in locating the leakage areas, a fog machine (Rosco Model 1500 Fog Machine w/remote) was used in combination with the pressurization fan (Minneapolis “Duct Blaster” model with digital meter) to locate and prioritize the leakage areas. The protocols for the individual and combined test procedures are included in the documentation of the NAVC Project4.
The pressurized fog test4 showed that the largest leakage areas were in the front and rear of the buses. The front dashboard area had numerous penetrations for the controls and mechanical system maintenance access panels, while the rear compartment had openings for major electrical and heating system components. A summary of the test procedures used for each bus, as well as the procedural sequence followed, is included in Appendix A.
Based on the results of this preliminary assessment, a first round of repairs were made on one bus. The modifications were limited to work that could be performed without dismantling the vehicle. Areas indicated by the above tests as high priority were remedied with non-invasive repairs or performed on “out of sight” surfaces (i.e., the bottom of the bus or in the rear controls compartment, etc.). The heat loss areas were insulated and/or air sealed with a combination of rigid and sprayed polyurethane foam insulation. Following these energy upgrades, both the modified and unmodified buses were outfitted with thermocouples to enable data collection for a comparative study performed with the buses in normal operations.
This first round of repairs reduced the air leakage by approximately 80% (almost 35% of the total thermal shell losses), and many of the previously un-insulated areas were retrofitted with an additional R-7 of rigid-foam insulation to reduce conductive losses as shown in Appendix B Phase I Bus Insulation Modifications. While the windows were still half of the total thermal shell losses after the bus was modified, they were not addressed for reasons of cost. The other major un-insulated area that could not be addressed was the bus’s frame (about 20% of the losses).
The infrared scan showed dramatically that the frame compromised the thermal envelope. Unfortunately, the thin wall construction and bus size constraints prohibited the installation of a thermal break on the extensive steel-space frame. Modeling showed that the addition of just a half inch of rigid foam insulation over the steel frame would provide a much higher R-value than the “average” R-value achieved with the current combination of the un-insulated steel and the two-inch foam insulation between the framing members. Insulating the frame or replacing major portions of the frame with lighter weight energy efficient materials was one of the recommendations for the Phase II buses.
Despite the fact that the windows and tubular frame could not be addressed in these buses, the initial repairs made a dramatic improvement. The Portland Transit Authority agreed that the modified bus was ready for regular winter service. Testimonial responses from the drivers and the passengers, as well as the test data, indicated a significant improvement. The cabin was easily maintained at a comfortable temperature, and the length of time that the door was left open increased, indicating that the driver was no longer cold.
Even on the coldest days with one of the two small Espar D3LC kerosene heaters (10,000 BTU output) out of service, the bus was now easily kept at a comfortable temperature. A second air-leakage test was then performed on the bus while it was in motion. This test quantified the air leakage while the bus was on route, further expanding the baseline information for the upcoming Phase II bus research grant.
The modified bus maintained a fifty degree minimum temperature inside the bus with no added heat, while the outside temperature was consistently forty degrees or below during the three hour test run. Bus speeds up to forty miles per hour were measured with wind velocities as high as fifteen miles per hour. The temperature was maintained from heat stored originally in the thermal mass; body heat from the driver and the four research passengers; and internal heat gain from the batteries, electric drive, and controls. This demonstrated the zero-emission goal of the technology.
A second set of infrared tests were performed after the bus was modified. Because the bus could now achieve and maintain an elevated interior temperature, more detailed information about the air leakage in the bus shell could be developed. Because the air leakage was now limited to much smaller penetrations and the amount of overall energy loss was significantly reduced, patterns of flow through and within the shell of the bus were now visible. Conductance images were also clearer as the temperature difference could now be maintained at higher levels, even during tests where the air pressurization fan was used to enhance the infrared testing.
Infrared images showed that the bus was actually gaining heat through the un-insulated battery compartment walls. As a result, these walls were left un-insulated for northern climates and noted as design considerations for buses requiring air conditioning in southern latitudes. In later combined testing4, an infrared R-value meter (Omega Infrared Pyrometer and Thermal Radiation Meter) was used to record the surface temperature and conductance of different parts of the body shell (floor, wall insulation, window, frame, etc.). To identify the physical composition of the parts being tested, the infrared imaging capabilities of the ISI Model 91 were used in combination with this meter. This data was used to verify or refine calculated values for modeling the original baseline buses.
After the first bus was successfully placed back in daily service, the second of the two original Portland buses was retrofitted using the same procedure developed for the first bus. Subsequent Phase II baseline data collection tests determined that this bus did not perform as well as the first. During the simultaneous infrared and pressurized fog tests, problem areas near maintenance access points and other locations not anticipated from prior experiences with the first bus were easily located. The problem areas were then sealed and insulated, bringing the two buses to the same level of performance. This proved that no two buses are identical, and that simply following a prescribed insulation and sealing procedure would not guarantee a minimum level of performance. It was recommended that the above test procedure be performed as a standard part of the bus manufacturers’ quality assurance procedures.
The first series of diagnostic evaluations and the retrofitted energy conservation measures of the two existing Portland buses (Phase I) became part of the baseline information for the next NAVC/DARPA Cold-Weather Electric Bus Project (Phase II). This second grant will integrate what was learned in Phase I into a new industry-wide manufacturing specification for electric buses that will be manufactured to perform well in cold weather. The second fleet, currently in production, will also be tested and compared to the earlier Phase I buses. Because of H. C. Fennell’s role in the Phase I work and the importance of the thermal envelope as a major component of the energy management of these buses, he continued as a partner with EVermont and others in the second grant.
In order to expand the baseline information for the modeling needed in Phase II, additional testing was done on the two original Portland buses. The only testing or data collection done after the initial retrofit work was the temperature monitoring and testimonials collected from the bus operators. New scans and air-leakage tests provided before and after baseline performance comparisons. This information would be used in a computer model and as benchmarks for forecasting the limits of the thermal improvements. Criteria for the NAVC model were determined by the adaptation of the California ZEV standards to the electric bus, the practical necessities of the bus operators, the defogger requirements, and environmental and passenger data from the Transit Authorities who would be purchasing the Phase II buses.
In the original infrared thermal scans and air-leakage tests, only gross conductance and infiltration areas could be identified. The improved buses provided much clearer information about the thermal shell. This indicated that it is important to do multiple tests; first, to identify the areas requiring the initial improvements, then, as the bus gets tighter and better insulated, the results become more specific. The technology can give an overall sense of the problem, but it isn’t until the improvements are focused on the final details that the full potential of the procedures is reached.
For example, in this project, the leakage area in the early Phase I tests was literally measurable in square feet, where the area that was sealed in the final quality assurance procedures totaled a few square inches. Until the initial large holes were closed, there had not been enough pressure drop across the smaller cracks and holes to make them detectable. As the bus performance was improved, combination test results were more precise. Once the problems could be individually identified (fans, windows, un-insulated floor areas, etc.), they could be isolated and temporarily covered or “masked out.” By doing this to the heat leakage areas one at a time, a value could be established for each potential improvement.
The information about the magnitude of potential system improvements for specific leakage sites was later used in a list or “matrix” of bus components and systems. Every thermal shell component selection from this matrix had an associated energy usage value based on the surface area and the heat conductance coefficient of the component’s materials or the related energy loss from air leakage through the component. The matrix was designed to create a “flexible” manufacturing specification for buses that would be used in cold-weather climates.
This specification allows bus manufacturers to make choices from a menu of available systems and shell components. For example, the roof was one component group. There were several ways to manufacture a roof listed in that group. An example of a system group would be the defogger system. Any of the options could be chosen from the matrix, but the total of the values associated with the choices from all the component and system groups had to equal less than the maximum total energy usage available. The limit of the energy available was the total energy stored in the batteries plus the energy provided by the occupants (a minimum of one was assumed). This energy had to supply the following:
The combined values of all of the materials and systems selected by the manufacturers from this list provided a total “UA” (energy usage) for the bus design. It was this total energy usage number that had to meet the minimum standards established by the computer model. To pass the pre-delivery tests, the manufacturer will be required to perform the same real-world quality assurance tests as those developed in the research and development work above. (See Appendix C which is an excerpt from the Matrix showing the shell component groups and the other systems. At the bottom are the goals or standards that must not be exceeded by the total of the combined component energy usage values. Note that the total UA of the sample configuration components passes the test - 201 vs. the 203 maximum allowed).
During the development of the specification matrix, the Chittendon County Transit Authority (CCTA) contracted to purchase a new AVS bus for use in Burlington, VT. They specified that it be manufactured to meet the new NAVC Project standards established for the upcoming Phase II test fleet. While this bus was not a part of the grant work, it served as a beta test of the NAVC Cold-Weather Project Model. After the CCTA bus was manufactured, it was tested at AVS to verify that it met the matrix standards. Initially, the bus did not pass, but the pressurization test procedures became a reliable quality assurance tool for the manufacturer. The manufacturer’s technicians met the required values by installing gaskets and caulking as directed by the air-leakage test procedure. The completed bus was delivered to Burlington in the fall of 1997 and has been used successfully through the 97-98 winter months. The CCTA has had no complaints about the heating or defrosting systems, and the bus’s range has proved adequate for the forty-mile transit route their first electric bus is serving.
After the Cold-Weather Model and Specification2 was completed, the Portland Transit Authority purchased a third, early-style (Phase I) bus from another New England Transit Authority. By using the testing and quality assurance procedures discussed above, it was possible to retrofit this bus to meet these newly established standards. While the infrared images and temperature readouts showed the locations of conductive and major air-leakage heat losses before and after the earlier round of repairs, the pressurization test combined with the fog generator allowed the sealing of air leaks down to hairline cracks and pinholes in the body shell and undercarriage.
Using this combined test during the repairs, while the technicians did the actual insulation and air sealing, made the work efficient and cost effective. The fog was easily visible at the leakage sites. Once the large areas had been addressed, the pressure was increased and the small detail areas were sealed as the fog was streaming out. This combination of testing and quality assurance procedures produced a dramatic visual tool that was evident at ambient temperatures. The improvements were quantifiable by monitoring the pressurization flow rates as the air-sealing efforts progressed. These procedures are low-tech, low-cost and are a great training and quality assurance tool for factory workers untrained in weatherization techniques. The addition of the fog analysis to the pressurization test saves the quality assurance team a significant amount of time in locating and prioritizing leakage so that minimum standards can be met as quickly as possible.
The ability to retrofit this earlier style bus to new bus standards is evidence that future buses can be more thermally efficient and have increased ranges from currently available battery and drive motor systems. Improved materials and designs, and the prescribed quality assurance procedures can provide comfortable, energy efficient buses for use in New England and other cold-weather climates.
The utilization of infrared thermal imaging and temperature sensing tools for the emergent electric bus market has proven to be an appropriate application of this technology. The infrared thermal imaging was used very successfully in the preliminary assessment of the Portland buses to locate large, uninsulated areas and major air leaks. The testing clearly helped to demonstrate the feasibility of making zero fuel vehicles useful for conventional mass-transit operations.
Infrared imaging and R-Value analysis techniques, alone and in conjunction with other diagnostic tools, aided in the follow-up assessments to establish accurate baseline and modeling data for the Northeast Cabin Thermal Management Research Project. This project has also served to illustrate the value of infrared non-destructive testing methods for use in quality assurance procedures for electric bus manufacturers and transit authorities.
The testing procedures revealed that the usefulness and specificity of the information gathered by infrared techniques was enhanced when used in conjunction with other diagnostic tools. The project also showed that the infrared techniques must be used in the proper sequence with other tasks to arrive at the intricate and detailed information - the ultimate goal of the procedures. When the thoroughness required for these methods was appreciated, the full potential of infrared technology was achieved.
NAVC, MJ Bradley & Associates, “Northeast Electric Bus Assessment”, Case Studies and Analysis, Portland ME Case Study, March 1997
NAVC, EVermont, “Modification of Greater Portland Transit District Battery - Powered Electric Bus”, The Vermont Electric Vehicle Demonstration Project, February 1997
NAVC, “Cabin Thermal Management Project, Interim report”, Northeast Electric Bus Technology Demonstration Project, June 1997
H. C. Fennell, “Cabin Thermal Management Project”, Procedural outlines, Combined Infrared and pressurization testing, Air leakage pressurization testing (ALP), Combined ALP and fog testing, October 1997
California Code of Regulations, Title 13, Section 1960, “Zero Emission Vehicle (ZEV) Standards”, Zero Fuel Guidelines