ASHRAE Journal - January 2023 - 56

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
The Year of the Elevator
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE
My New Year's resolution is to stop using the term " heat pump " and instead, wherever
possible, to refer to a device that takes heat from a low temperature to a high temperature
as a " heat elevator. "
In my very first column for this magazine ( " The
Temperature Lift, " ASHRAE Journal, March 2012) I used
the analogy of lifting a piano uphill to illustrate the
challenge of lifting heat. You can roll the piano down to
a convenient place but then you need to work hard to
raise it up to another convenient place where you can
then roll it down into its required location. It strikes me
that you wouldn't talk about " pumping " the piano up
the hill, so " heat elevator " seems a more apt name for
the temperature raising device.
The confusion created by the misuse of " pump " in this
context is matched, or possibly even surpassed, by the
idea that the heating efficiency of such
a system can be more than 100%. For
some reason many people, particularly
mechanical engineers, struggle with
this notion. They feel that an efficiency
ought to be a measure of what was actually
achieved compared to the theoretical
maximum that might have been gained
and so by definition must be less than
100%. Electrical engineers, however, seem to have no
such problem. For them efficiency is a measure of output
divided by input. In the case of heating efficiency it
is perfectly feasible to deliver more heat in a heat elevator
than would have been created by putting the electrical
input through a resistance heater. In fact, since the
work input is added to the heat that is being elevated
and contributes to the heating effect delivered at high
temperature it ought to be obvious to all that 100% efficiency
is the minimum that can be achieved by a heat
elevator, not the maximum.
To get this point across I like to use another analogy:
comparing the heating efficiency of a heat elevator to
the mechanical advantage afforded by a lever, pulley
56
ASHRAE JOURNAL ashrae.o rg
J A N U A RY 2023
Who wants a heat elevator?
or gear mechanism. With these it is perfectly normal to
put in a certain force and achieve a higher force (albeit
acting over a shorter distance) on the other side of the
device. Describing the heating coefficient of performance
or the heating efficiency as a " heat advantage "
gets over the mechanical engineers' objection by speaking
their language.
Of course the most beneficial application of a heat elevator
is the situation where someone has a need for the
heat being delivered and someone else can benefit from
the cooling effect created by the removal of heat from
the low temperature source. Problems arise in calculating
a performance metric in this case. If
the cooling coefficient of performance
(COP) is 3 and the heating advantage is
400%, does that mean that there is a system
COP of 7? It's all heat transfer, but
adding the two effects together like this
is like adding apples and bananas-it's
all fruit but not the same. Another way
to think of it is that the cooling effect is
like someone digging a hole in the ground. They are very
interested in the length, width and depth of the hole,
but they don't really care about the pile of sand that they
are creating in the process. On the other hand someone
building a fabulous sand sculpture doesn't really care
where the sand came from but they are totally focussed
on the size, shape and form of the pile that is being created.
Of course it makes no sense to add the hole in the
ground and the sand sculpture together, just as adding
the cooling effect and the heating effect together is
meaningless.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in
Glasgow, UK.
ISTOCK PHOTO: PIKUSISI STUDIO

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