July 19, 2018

All-Electric HVAC Efficiency


The term “efficiency” has been applied very broadly when it comes to all-electric HVAC, specifically on new battery buses. In layman’s terms, efficiency can be defined as the amount of power being input (in kilowatts [Kw]) versus the amount of power being used to achieve a defined output in Kw. If the output of the HVAC system needs to be 18 Kw, how many Kw is required to achieve that output? The method that requires the least input to get the desired output is the most efficient. This article will try to define efficiency within several contexts in which the term is used. Here are a few scenarios to consider:

Performance Specifications: Customers continue to ask for specific pull down performance when they write specifications for all-electric HVAC systems. Take a look at the pulldown chart below and assume that these are three different HVAC systems. Is the system that pulls down the fastest the most efficient? The answer is yes if the goal was to achieve a pre-determined temperature in a pre-determined amount of time.

Power Consumption: What if you changed the parameters of the pull down and took time out of the equation? Would you then only look at the power consumption required for pull down to a predetermined temperature? The bar graph shows three different results for power consumption during the same pulldown. Is the system that uses the least amount of power the most efficient?  The answer is yes if power conservation and power management is your pre-determined objective.

Cabin Temperature Maintenance: This is also a requirement in the performance specifications. Users typically want the cabin temperature to be maintained between 68 and 72 degrees Fahrenheit on a fully-loaded bus with normal door openings. If two different systems were tested, and one could achieve these results using 30% less energy, which one is the most efficient? The answer is obvious in this case.

Unsubstantiated Claims: These three scenarios clearly illustrate the ambiguity that can be associated with making broad claims of better efficiency. We advise you to question any claims of improved efficiency without comparative data. This data should represent actual testing, under the same conditions, for the same amount of cooling. The integrity of the data is best when it comes directly from fleets. All of Thermo King’s all-electric HVAC systems use R407C, the most efficient refrigerant for large bus HVAC. It has been demonstrated in actual fleet operations that R407C provides an 8% reduction in energy consumption for the same amount of cooling. This is not an unsubstantiated claim!

Conclusion: These efficiency claims surfaced with the coming of the battery bus, where efficiency is defined by the range of the bus. Customers should be reminded that we are still early in this journey, and that the sacrifices for extended range have not been clearly defined. For example, you can shut off the HVAC all together and get 40-50 miles in additional range from the battery. While that is not feasible, it is the responsibility of all HVAC suppliers to show the kilowatt per hour energy consumption under several specific conditions like ambient temperature and desired interior temperature to start with. As with previous new technologies, the gains from other operational efficiencies will become apparent as the buses are operated in actual transit duty.