July 19, 2018

All-Electric HVAC Heat Pumps – Are They Right for Your Operation?

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Heat Pump Background

As decarbonization trends have accelerated over the last few years, across many industries, the demand for heat pumps has increased. “Heat pump” has become a common, everyday, term, no longer confined to the residential HVAC world. As long as battery buses have been on the market, users have been asking HVAC suppliers for heat pumps.  These requests often came from users in moderate climates who were familiar with the use of heat pumps in their home HVAC systems.

As battery buses continue their ascent in the marketplace, the heat pump conversation becomes more relevant every day.  In some cases, the energy required to heat a bus can be as much as twice that needed to air condition a bus. Therefore, the idea behind incorporating a heat pump is that the amount of energy required to heat a bus utilizing a heat pump is greatly reduced compared to traditional electric resistance heaters. This, then, allows more of the energy in the bus’s batteries to be directed to the bus’s propulsion, and not to the heating system, thereby extending the range of the bus.

While heat pumps are, in fact, more efficient than electric resistance heat, a heat pump has limitations that the user must be aware of.  The heat pump can only act as the primary heat source during moderate ambient conditions. Most climates, especially those that experience extreme conditions, still require a secondary heat source for optimum passenger comfort.

Heat Pump Application Basics

Heat pumps use an electric HVAC unit’s refrigeration system to extract energy from the ambient air and move that energy through the refrigeration system, to the inside of the bus, warming the cabin. The size and selection of the HVAC system and subcomponents, as well as ambient conditions, determine how much heat the heat pump can produce. The use of heat pumps will likely be most popular in moderate climates where the ambient temperature does not get too cold. A heat pump can be used in colder climates, as well, but its utilization will be diminished during colder periods.

Heat pumps will also be required to go into defrost mode periodically. This is due to the fact that the outdoor heat exchanger coil (traditionally called the condenser coil) actually turns into the evaporator coil during heating mode. When ambient conditions are right, condensate will form, and potentially frost, on the outdoor heat exchanger coil. The heat pump will engage a defrost cycle to melt the frost before returning to heating mode. While the heat pump is in defrost mode, heat from the heat pump will be temporarily unavailable. 

Heat Pump Design, Cost and Service Considerations Versus Non-Heat Pumps

Design

  • Incorporating heat pump functionality into an HVAC system requires a few additional refrigeration components.
  • The outdoor heat exchanger coil (traditionally called the condenser coil) will be tube and fin type (in lieu of microchannel type) to accommodate the freezing and thawing of condensate.
  • Due to the added refrigeration components and different outdoor coil type, the heat pump will be heavier.

Cost

  • Due to the added refrigeration components, a heat pump will have a higher first cost. In many climates, a secondary heat source on the bus is still recommended for heating in extreme conditions.

Service

  • Due to the added refrigeration components, there are a few more components to service and/or replace upon a failure.
  • The unit’s outdoor fans (traditionally called condenser fans) and compressor will see additional run time due to their operation in heating mode.

Conclusion

Heat pumps deliver heat more efficiently than traditional electric resistance heat, which can lead to less battery power being used, and extended bus range.

The user must understand a heat pump’s performance limitations and trade-offs when comparing to other HVAC systems and heating sources.

Please contact your Thermo King representative to discuss heat pumps, application considerations, and determine if they are right for your bus.