By Steve D. Johnson
This is the second installment in this ongoing series about zero-emission buses and their relationship with HVAC. The first installment was about the conflict with specifications that still forced HVAC suppliers to build over-sized systems while battery bus builders sought to get the best range. Passenger comfort was part of that discussion as it will be here. It seems that 150 miles or something north of 100 miles has emerged as the ideal range for a battery bus. The HVAC is now the second largest load on the battery, right behind the traction motors. There is not much that can be done about the traction motors. The bus has to roll. But, if the HVAC was not drawing power from the battery, it could roll a lot farther. And therein lies the dilemma. When the bus isn’t rolling, which is the case 50 percent of the time in stop-and-go traffic, the HVAC is still drawing power. Get stuck in an unpredicted traffic back up and the bus may not have enough power to make it home. The HVAC would be the first to go. But, what if the bus had a full passenger load? They would not be happy.
Prior to making any proposed changes to passenger comfort, we should hear from both passengers and transit operators. The first proposals were about providing less air conditioning, or giving complete control of the HVAC system to the bus power management system, meaning it could be turned off at will, regardless of the temperature. Passengers have become accustomed to comfortable cabin temperatures. They will complain if that’s not the case and the blame will be placed on the HVAC. Transit operators will not dispatch a bus with a faulty HVAC. Hot bus complaints usually result in the bus being taken off route. Costly for the operator, and a bad experience for the passenger. So what can we do?
We do what agencies have always done, take a practical approach when trying to solve a problem or introduce new technology. Despite the fever pitch interest in zero-emission vehicle adoption, this will take some time. Most agencies are using grant money to conduct trials with a small number of buses. Here are a few things I’ve learned from talking with operators.
• Range claims based on test track data are not reliable or realistic in some cases. What matters is the range in actual transit duty.
• Range is not very predictable. One bus may have a range of over 100 miles, while another bus of the same make gets only 50 miles. Agencies want to understand the variables and be able to predict range based on routes and sub-component power consumption.
• They all understand that the HVAC is a major consumer of the available power but none have asked for less cooling. They want the bus OEMS to work with the other sub-component suppliers to provide more efficient systems.
Improved efficiency from the HVAC system does not necessarily mean less cooling. It means the same amount, or an adequate amount of cooling using less power. There are some things that can be done that could have an immediate impact.
• Decide on a reasonable specification for all-electric HVAC. A specification that does not force us to build systems with excess capacity that will never be used in normal duty. We can the size the systems smaller, take out weight and manage the power more efficiently.
• The HVAC’s primary responsibility is to remove heat. If less heat were introduced into the bus, the HVAC would use less power. We know that you can reduce the cooling demand by up to 20 percent with a better insulated bus.
These two are low-hanging fruit that can be harvested now. Beyond that, how do we get all of the sub-systems, each bringing the most efficient solution working together to conserve and manage power? There are several “Think Tanks” working with the Transits and their work should result in better power management strategies. Based on what I have learned so far, it seems that the ideal range will vary and be route specific. Once the operator understands that the bus will only go 50 miles on a one hundred mile route, he/she will know exactly where to put the charging station. While this may not sound overly complicated, it will take some time to work through.
Steve D. Johnson, Sr. serves as product marketing manager, Bus HVAC, at Thermo King, Minneapolis, MN. Thermo King is a world leader in transport temperature control systems for buses. Thermo King also manufactures auxiliary power units, which dramatically reduce engine idling. All Thermo King products are backed by a nationwide dealer network. Visit www.thermoking.com.