ASHRAE Journal - February 2015 - 49

COLUMN REFRIGERATION APPLICATIONS
Andy Pearson

Big Heat Pumps
Justifying heat pumps for domestic use can be tricky, particularly in older buildings
and in colder climates. Operating efficiency is critical to the financial case and small
systems operating at cold source temperatures with a need for high temperature hot
water are a tough proposition. However, there is an alternative way to achieve the
necessary efficiency, using a principle that is universally understood and regularly
applied in all walks of life. It's called "economies of scale."

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The principle is much the same as the math that makes infrastructure investment could easily be paid back over
cooking one meal for 20 people cheaper than cooking 20 a 10-year cycle from the profits of the district heating
meals for one, or that makes transporting commuters in scheme, giving a significantly better rate of return than
groups, on trains or buses, better than providing indiany government bond will offer while at the same time
vidual transport for each one. Industrial-sized compres- investing for a long-term benefit that will pay back many
sors are typically 10% to 20% more efficient than small
times over.
domestic ones because of economies of scale inherent
Where networks already exist, but big heat pumps
in their construction; a large cylinder built to the same
have not yet been deployed, the economics are even
tolerances as a small one has much lower losses because more attractive. Use of big heat pumps can greatly
the gaps are the same size but the internal volume is
reduce carbon emissions, particularly if the heat
much larger.
network is coal or diesel fired. In an
What's on the other channels?
Diversity adds a further improvement. If
economy such as Norway where the
a system serves one home it may operate
majority of electrical generation is from
on low load for much of the year and spend
hydro power, or France where it is mainly
a disproportionate amount of time heatnuclear, the carbon emissions from such
ing up and cooling down as it switches on
a scheme would be virtually zero. Best of
and off. However, if a big heat pump serves
all for other countries as they clean up
5,000 homes, then many of the variances are smoothed
their generating stations, switching from coal to gas
out and the system runs at a more constant condition. If and from gas to renewable, the heat network just gets
the central plant has several modules then the variation cleaner and cleaner.
in load that can be achieved efficiently is even greater.
The key thing to understand about big heat pumps is
The downside is that a central distribution network is
that the rules of thumb for operating conditions and
required, and this can be expensive to retrofit, but that
performance that are well understood for domestic
wasn't an impossible hurdle for the widespread use
heat pumps cannot be applied to large systems. Big
of electricity, gas, mains water, drainage or even cable
heat pumps reach much higher temperatures more
TV, so there is still hope for some joined up thinking to
efficiently so that heating a water pipe network to 196°F
prevail.
(90°C) from a source at 46°F (8°C) with a heating coeffiThis is where government subsidies could be really
cient of performance of 3.0 becomes possible. Of course,
powerful-encouraging communities to lay in the
if the source is warmer and the delivery temperature
infrastructure required for the delivery of more effiis lower the performance is even better. The one thing
cient heating on a neighborhood scale in order to reap
missing from these systems is the sight and sound of the
the carbon reduction benefits of these economies of
open log fire. Perhaps every installation should include a
scale for generations to come. This subsidy system
flat screen TV in a brick surround.
Andy Pearson, Ph.D., C.Eng., is group engineering director at Star Refrigeration in Glasgow, UK.
would not even need long term support. The cost of the
FEBRUARY 2015

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