Going Electric

Where would you expect to find an all-electric air conditioner? Let's start with the central air or window unit in your home. How about the units on the roof of the building where you work, or the subway or electric trolley car that brought you to work? Yes, all of these are electric air conditioning applications, different only in the size of the components, the packaging and the methods used to supply the electrical power

So what are your chances of boarding a transit bus that is climatically controlled by an all-electric HVAC system? Maybe better than you think and improving all the time. Why? Considerable effort and resources are being devoted to discovering alternative fuels and drive systems that reduce emissions harmful to the earth's atmosphere. A pure battery-powered bus or a hybrid bus (combination of battery and diesel engine) uses accessories that are not dependent on the bus engine to operate the compressor and fan motors. In fact, every major transit bus OEM has or is planning to develop a hybrid bus.

On the forefront of this transit trend, Thermo King has been involved in five electric A/C unit applications, three for North American Transit. The most notable, and the one which represents the best application so far, is the Advanced Technology Transit Bus (ATTB) project. In 1992, the Federal Transportation Administration funded this project to develop a transit bus that applied new and advanced technologies for OEMs to adopt and apply in the transit industry. Northrop Grumman, which was awarded the contract, asked Thermo King to supply an all-electric HVAC system for the five prototypes.

For Thermo King the ATTB project became a benchmark in three different areas: project management, concept to market cycle time and designing to an exact specification. When the bus OEM and HVAC supplier agree on the specifications and design, the end result is a system that performs to expectations and does not require extensive modification or re-engineering.

"There are certain performance characteristics of the REH unit used on the ATTB that would not be present if the OEM (Northrop Grumman) had not allowed us to design to specifications," said Steve Johnson, North America product marketing manager. For example, Johnson explained that the original specs called for the noise level not to exceed 68dba anywhere in the bus, a unit weight limit of 400 pounds and a required capacity of 100,000 BTU/hr at the standard rated conditions. However, the low noise level and high capacity could not be accomplished given the weight limit of only 400 pounds. Because a high cooling capacity and low noise are critical in the transit industry, Northrop adjusted the specification to allow for more weight. When the A/C system was paired with the bus and tested, all of the performance requirements were achieved and expectations were exceeded.

The success of the ATTB was due in part to some unique and advanced design concepts:

• The modular power train can be completely removed and reinstalled in less than one hour.
• The bus shell was made of a lightweight composite material that reduced the axle weight by 10,000 pounds when compared to a typical 40-foot transit bus.
• The composite frame, with its superior heat transfer, kept the weight of the REH HVAC unit down. The unit could have easily been 100 pounds more if more capacity was needed.

Compared with a similar roofmount system and belt-driven compressor, the ATTB pulldown time was 30 percent faster. In addition to its low U factor, the bus has ducts sized and constructed to deliver large amounts of air, reducing the pulldown time. The chart labeled "Houston Pulldown" shows the pulldown took only 20 minutes of the alloted 30 minutes called for in the specification.

Thermo King Engineering Manager Don Nielsen said, "We have tested a lot of units on many different buses, and we have never achieved Houston Pulldown this quickly on a 40-foot bus. One reason for the superior performance is that the HVAC system capacity is not dependent on engine RPM. Full capacity is available during the initial pulldown where it's needed most."

An all-electric HVAC system offers many other advantages:

• Its one-piece, semi-hermetic design allows the unit to be evacuated and charged with R407c refrigerant at the factory.
• Expensive field installation is eliminated along with the possibility for contamination and other installation errors, because the compressors are part of the unit.
• Hoses, belts and copper re not needed to connect the unit, reducing the number of parts and opportunities for failures.
• Less energy is required to operate the electric system for the same capacity. At the standard rated conditions of 95/80/67, the REH requires 14 horsepower for a capacity of 95,000 BTU/hr while a conventional system needs 21 horsepower at a compressor speed of 2,600 RPM for the same capacity.

Even though the initial cost of the electric system is slightly more, the elimination of parts, increased reliability and lower power consumption will significantly reduce operating costs over the life of the unit.

Before bus OEMs and Transit Authorities commit to the widespread use of electric and hybrid vehicles, advancements in technology must demonstrate that the total system is cost-effective as well as environmentally friendly. However, the future is coming sooner than we think, so don't be surprised if you find yourself going electric in the not too distant future.