MACS Monthly Newsletter - 2022 - OCT4

Figure 6: Dual Loop SLS, Cabin Cooling with Condensing via a Low Temperature Radiator
So, for reference, an electrical resistance heater
has a COP = 1, meaning that for each kilowatt of required
heating power, one kilowatt of battery power
is used. With the DX system, each required kilowatt
of heating can be achieved with as little as ¼ of a
kilowatt of power into the compressor (or a COP =
4). Over most of the operating range of a typical vehicle
however, the COP will vary between about 2
and 4 and differs slightly between a DX and SLS system.
And, at very low ambient temperatures it becomes
more difficult to extract heat from the colder
ambient air and the COP can drop to one or below
depending on the refrigerant properties, however,
this is a smaller portion of the total yearly heating
need. Thus, overall, the heat pump provides a significant
battery energy savings and thereby increases
cold weather driving range when compared to direct
resistance heating.
One thermal system advantage and simplification
that arises from the dual loop SLS is that
the basic
refrigerant system operates the same whether
a heating, cooling or simultaneously heating and
cooling mode is required. This functionality is enabled
via the use of the two coolant loops, one
which provides coolant heated by the WCC and
one which provides coolant cooled by the chiller.
To manage the thermal demands of the vehicle, the
coolant loops are interconnected via hoses, valving
and pumps to the other vehicle components requiring
thermal management.
One important and desirable feature of heat
pumps is the fact that they can increase their operOctober
2022
4
ating efficiency, or COP, by collecting and using so
called " waste " or excess heat available in the system.
Moreover, this waste heat recovery is a natural
extension of the SLS architecture. Also, since
this recovery involves removing the available heat
from the various potential sources in the propulsion
system and delivering that heat to the low or
evaporating side of the refrigerant system, the coolant
loop can effectively perform this function. Some
additional heat exchangers or coolant flow options
and additional system controls may be required to
accomplish the recovery function. In addition to collecting
the waste heat, the system extracts input energy
from the ambient air for the heat pump which,
in a dual loop SLS, is enable via the LTR loop.
More Heat Pump Basics
A thermal management system operating as a
heat pump allows the refrigerant system to take low
temperature energy from the ambient air or other
sources, pump that energy to a higher temperature
and deliver it as useful heat.
Additionally, the electricity that drives the refrigerant
compressor adds to the heat available. This
happens by way of the electrical power flowing into
the compressor being transferred to the refrigerant
along with increasing its pressure and, in turn, this
adds to the desired heating output as the refrigerant
condenses. Moreover, since when operating as a
heat pump heating is the desired output, the added
compressor power makes the COP in the heat pump
mode higher than the A/C mode when each system
is operating against similar temperature differencMACS
Service Reports
Tim Craig
MACS Monthly Newsletter - 2022

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