ASHRAE Journal - August 2022 - 56

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
Reaching New Heights
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE
At the time of writing, I have just returned home from the 15th Gustav Lorentzen
conference, which was held (for the second time) in the professor's home town of
Trondheim, Norway. One of the most striking features of the conference (which was
excellent in many ways) was the amount of research being conducted into the challenge
of delivering high temperature heat from heat pumps.
On the face of it this should come as no surprise.
Firstly, the demand for heat pumps is increasing dramatically
due to the high cost and environmental challenges
of using traditional fuels. Secondly, the need for
heat pumps is also increasing: the decreasing number
of alternative ways to deliver heat to a process, an office
or a home, which leaves heat pumps as an outstandingly
good idea for the next generation of heating systems.
However, we need to be careful about definitions.
What, in this context, does " high " mean? What would
be acceptable from an efficiency point of
view and what does efficiency mean in
this context? How does our definition of
" high " color our thinking about acceptable
efficiency?
Taking these questions in reverse order:
engineers have struggled for over 150 years
with the concept of heating efficiency
when applied to heat pumps because the
heat delivered is usually higher than the calorific input
in the form of electricity, so the traditional definition of
heating efficiency will return a figure greater than 100%.
This gaping plot-hole can be masked by using the term
" coefficient of performance, " but this can still be problematic.
People are happier with the idea of the " heat
advantage " as an analogy with the mechanical advantage
afforded by a gear wheel or a lever. The question of what
level of heat advantage is required to make the system
economical depends on how much you pay per kWh for
electricity compared to how much you pay in equivalent
units for an alternative fuel, whether it is gas, coal, oil or
something else. If the alternative is electric heat, the heat
pump wins every time.
Standard heat pumps raise temperatures available
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ASHRAE JOURNAL ashrae.o rg
A U G UST 2022
from a source to about 140°F (60°C) which is useful
in many washdown applications and is increasingly
relevant to home heating. " High temperature " used to
refer to delivery temperatures up to 194°F (90°C), but
this is not sufficient to serve many processes, particularly
those using steam. A new generation of systems is
coming, offering temperatures up to 356°F (180°C), but
with heat pumps the heat advantage is determined by
the temperature lift.
Buster couldn't figure out where to put
the hay in.
One challenge evident in many of the conference
papers is that clients tend to ask for systems
that replicate what they already have. The
old system has steam at 150 psi (10 bar) so
the new system needs to match that. Often
this is not necessary and an advantage can
be gained by optimizing the process of heat
pump heating rather than fuel combustion.
Some of the technologies discussed at the
Gustav Lorentzen conference included the
use of water as a refrigerant, use of a combined absorption/compression
system, use of butane in a single stage
or cascade system and use of transcritical carbon dioxide.
Each of them has strengths and weaknesses but the
common theme was the need to learn new ways to work
with new technology. I liken that to your great-grandfather
replacing his horse with a newfangled automobile
and having to get used to the fact that he didn't fuel the
vehicle with hay or take it to the blacksmith to be reshod.
So heat pumps are reaching new heights in terms of
temperature; higher than previously achieved by conventional
heat pumps, but perhaps not as high as the
old heights delivered by fossil fuel.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in
Glasgow, UK.
http://ashrae.org
ASHRAE Journal - August 2022

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