Discussion report of the joint IOR and CIBSE ASHRAE Group debate session on
Thursday 10th January 2013 on “Heat Pumps or Passive Cooling - the most
effective low carbon solution for building cooling” by Becci Taylor and David
Butler FInstR. Chaired by Andy Pearson, FInstR (President) and Graeme
Maidment FInstR (President-Elect):
Graham Maidment congratulated both speakers on their excellent presentations and
asked for questions from the floor.
Steve Connolly said that the use of reversible refrigeration systems was not recognised
under the Renewable Heat Incentive Scheme and the government would not support it.
David Butler in reply said that the Renewable Heat Incentive was for the provision of
heat to buildings, which could include ground to water heat pumps, and they were also
looking at the possibility of including air to water heat pumps. He understood that at the
present time DECC had decided not to include air to air heat pumps, which were reversible
air conditioners. The reason they seemed to be giving was the difficulty in actually
quantifying the amount of heat delivered by an air to air heat pump system. The RHI paid
back on the basis of per kWh of heat generated using renewable energy technology, but the
difficulty was that that heat had to be metered. One could do that relatively easily with a
water based system but with an air to air heat pump system you were delivering warm air
and it was physically quite difficult to measure the quantity of heat being delivered, which
was where the stumbling block was at present and the reason why DECC had not included
that technology in the RHI.
Steve Connolly also argued that it was an obsolete technology to have HFC based heat
pumps bearing in mind the proposed revised F Gas Regulations and the cap and phase down
of HFC refrigerants.
David Butler said that he agreed with that comment, but this technology had also been
used in the past with R22, R12 and R502 and shifted to HFCs when these were phased out.
He felt that it would also shift to other refrigerant fluids when it was necessary.
Andy Pearson said that Becci had referred to a school in Ghana which produced a very
satisfactory comfort environment with passive cooling, whereas David suggested it was very
difficult in London with the heat island effect to produce passive cooling. So who was right?
Becci Taylor stated firstly that the school in Ghana had the big advantage of having no
electricity and so the only internal heat gains were from the students - and they had quite a
flexible wardrobe! She said she used that example because it was extreme, but it did teach
them some lessons about keeping the internal heat gains as low as possible, particularly in
regard to the way they approached heating and lighting, was definitely an opportunity.
Obviously the utility of buildings in Ghana compared with London were quite different but
the point about small buildings was valid because there was a lot of opportunity there and
they tended to be less likely to be used around the clock whereas larger buildings were
more of a challenge.
David Butler said he partly agreed with Becci but countered by saying there were quite
a few small buildings in southern England, such as the Tesco Express type shop, and it would
be very difficult to undertake night cooling in buildings like that which were intensively
operational round the clock. However he agreed that there were a number of small
commercial offices not necessarily operating 24/7 and in those cases night cooling was more
feasible than in the example he had given of the all night convenience store. Therefore there
would be some buildings where it was more feasible than in others.
Becci Taylor added that the example of Tesco Express was quite interesting because
with a building like that it seemed that there might be an opportunity to go right back to the
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beginning and completely rethink the way convenience stores worked and make them much
more open to passive cooling in the summer.
Robert Heap thought both presenters might agree that the idea of the passive design of
buildings was very old. If they went right back to the medieval palaces of the Moors and the
Moguls, they had some very good ideas. The idea of mechanical refrigeration in the form of
a heat pump or cooling had been around for less than 100 years and yet it had more or less
taken over the job, so there had to be a reason for this. He suggested that certainly as far as
the UK was concerned, where there were high land prices and high labour costs, the ideal
solutions relating to passive design were just not affordable in nearly every case.
David Butler said he agreed with Robert Heap. In the UK and also in parts of the
Middle East there were very high land prices due to demographic pressures and the
movement of populations to large cities. This was worse in the UK because of the lack of
land for building and the effect that that had on land values. The way that buildings were
used in this country had almost pushed us to a mechanical means of providing a comfortable
environment in those buildings. This technology had proved to be highly effective and in the
100 years since the start of mechanical refrigeration the advances in technology had been
immense, perhaps only matched by the innovations in automotive engineering, and in our
modern world we would find it very difficult to live without that kind of technology.
Becci Taylor repeated the point she had made earlier that it was a continuum. She was
a building services engineer and she had nothing against refrigeration but there was no
reason why, just because the mechanical technology was available, they should be lazy. It was
about the design process and mechanical cooling was one of the tools in that design process,
but not the only one.
John Morley said he thought the debate was about the future, not the present, and felt
they should be discussing buildings that were about to be built. If they looked at the trends
for the future, basically cities were becoming more densely populated, which essentially
meant that buildings were going to get taller. Therefore every effort would have to be made
to make these buildings more comfortable because people would be living and working in
them 24 hours a day in the future.
Taking as an example the Gherkin in London, which was a very tall building, his
understanding was that it was designed to be as passive as possible, with air flows being
guided through the building, and so on. Could that work to the exclusion of
electromechanical refrigeration, he asked.
Becci Taylor in reply said that for that type of building the most likely solution would be
a mixed-mode solution. The danger was that designers often just thought about power
whereas they should be thinking about energy and time. She was working on buildings at
present which were about to be built and some of the most interesting things they were
doing was looking at diversity, control systems and how you could turn things off when you
did not need them. What really mattered was the overall energy being used in the building.
David Butler added that one of the problems with mixed mode was that one always had
to have a cooling system that could meet the peak load. So although in moderate weather
conditions one might be able to make use of natural ventilation, which may be sufficient to
undertake the internal cooling, one still had to have a full size cooling system plus all the
related heat transport infrastructure, chilled water and ducted air systems. Then you got
into the question of the actual energy involved in the manufacture, installation and
maintenance of those mechanical systems. You might be able to operate the building for
some of the time on a natural basis and run the mechanical system in peak and warm
weather. Essentially you would have two systems, which may not be as sustainable as it
initially appeared.
Becci Taylor said that that could be analysed and tested. The real answer was that they
should think about the solution in terms of the life cycle and make a decision based on that.
John Crawford stated that he had visited a few of these passive buildings and if they
were low density like the BRE building the conditions were not too bad, but when one
looked at offices with dozens of people in them they tended to have poor air quality and
Proc. Inst. R. 2012-13. 4-10
frankly be rather smelly. If it was in the middle of winter you could not have open windows
and this was a problem with offices which were not air conditioned. It was very difficult to
have high density occupation and passive ventilation.
Becci Taylor commented that passive cooling was not the same as passive ventilation
and a little bit of help from a fan was all right.
David Butler added that he had visited one of the buildings mentioned at BRE and there
was an issue with ventilation. The engineers had tried to provide a moderately deep plan
because that was the way one obtained an economical use of the footprint of the building;
the difficulty there was getting enough ventilation and natural air into the core area, which
probably worked better in summer than in winter. Perhaps what was needed was a kind of
mixed load system and a mechanical ventilation system, but one then ran the risk of
providing lots more systems than perhaps would be needed with a mechanical system alone.
Chris Dickson said he was a bit surprised that phased change materials had not been
mentioned. Was there any appetite for this new technology, he asked.
David Butler replied that phase change materials had been used in buildings to assist
with passive cooling and in fact they had just undertaken a trial at BRE, but they still worked
on the premise that they stored and released heat. The idea was that they absorbed heat
during the day and released heat at night and so generally they were used with a night
cooling system. However the issue remained that one still needed to get cooled outdoor air
into the building across or through the phase change material in order to get a good heat
exchange and to get that heat out in order to resolidify the phase change material in
readiness for the following day to absorb heat again. In practical terms it was quite a
challenge to do that economically in terms of the amount of energy that phase change
materials could store. The simplest phase change material was ice, of course, and ice
thermal storage had been used for decades; it was used widely in parts of North America,
but probably more so for load shifting purposes than to aid passive cooling.
Chris Dickson asked if the test carried out had not proved very successful.
David Butler replied that there had been a very moderate effect but he would not say it
could be a replacement in general for mechanical cooling.
Becci Taylor agreed with David: it offered an opportunity but it did not necessarily solve
all the problems.
Steve Connolly, referring to phase change solutions and the earlier question on heat
pumps, said he was surprised there was not agreement because surely the answer was to
use the waste refrigeration from the heat pump to freeze the phase change materials instead
of throwing waste heat away. If the heat pump was throwing out heat from one side and
refrigeration and coldness from the other side, why not use the waste coldness to freeze up
a phase change solution and use no extra energy, he asked.
David Butler replied that in most of the buildings they were discussing, cooling was the
major problem. If the heating season was relatively short he suspected that to make that
kind of system work then during the heating season one would have to use the outdoor unit
that was working as an evaporator to solidify phase change material, but an enormous
volume in weight of phase change material would be needed to make such a scheme
worthwhile. He doubted whether it would be viable and physically possible to make it
worthwhile considering the amount of engineering that would have to be carried out to take
the evaporator through some sort of phase change bank.
Steve Connolly replied that it was being done now. If one wanted to save up energy for
months on end then a large system would be needed, but if it was just to bridge from the
morning to the afternoon or the night to the day it was easy to do.
David Butler said that to an extent it was being done with a different type of heat pump
and in fact in Holland they had ground source heat pumps that used the ground as a heat
sink and heat source. They could take heat out of the ground and put heat back into the
ground depending on requirements. However he suspected that for a lot of the buildings in
city centres, in relation to the amount of energy that would be needed to store and take out
and the amount of ground volume needed to undertake that, the cost would be high and the
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energy balance very marginal. It was worth looking at and it had been used before but he
would challenge whether it has been shown to be cost effective because if it had, why was it
not being used more.
Becci Taylor said that they did do that quite a lot, usually in the ground, and on a
smaller scale they used heat recovery if there were buildings with a high cooling demand that
also had a hot water demand, where they could recover the heat from the heat pump
cooling. Adding phase change materials was simply another dimension to that system, but
heat pumps were used in buildings to try and reduce energy consumption by moving heating
and cooling around to where it was useful and it was fairly common technology.
Kevin Yearley referring to phase change material, said that they had developed a system
that used the phase change material (PCM). They froze it at night using night time cooling
and drew cool air into the building over the PCM, freezing the PCM and providing night time
cooling for the building. It had been quite effective and they had a working system which did
it that way round. There was enough drop even in cities to freeze enough PCM to cool the
offices the following day.
David Hodge stated that both arguments appeared to be focused more on smaller
buildings or owner/occupier type buildings. Did the presenters think the technology was
adaptable for large-scale high density buildings, he asked.
David Butler agreed that relatively small reversible heat pump technology was not
suitable for very large multi-storey buildings of the type seen at Canary Wharf. Traditionally
those buildings would generally have either chilled air or probably a chilled water based air
conditioning system. However, the primary source of cooling was a chiller, which in a sense
was still a heat pump. Therefore rather than using the refrigerants as the heat transfer
medium one would be using an intermediate coolant and chilled water in most cases, which
from a practical and cost point of view was probably more appropriate for that kind of
building. That would be a perfectly reasonable engineering approach to that situation. The
average heat pump technology still had an advantage for the smaller scale buildings but for
very large buildings central plant air conditioning and the economies of scale associated with
that made a lot of sense.
Becci Taylor said that from her perspective it was a matter of how forward thinking and
open minded one wanted to be at the beginning of the process. There were examples of
large office buildings that were usually owner-occupied, but if one started from the outset
with the aim to have some element of passivity one could dispense with the fan cooled units
and just have chilled slabs and have opening windows in perimeter offices. There were lots
of things that could be done and in her opinion it would be a more pleasant environment
with a degree of user control.
David Hodge commented that that would be more suitable for a fairly low density
occupation, whereas at the present time they were seeing a trend for high density
occupation. There was the potential for internal gains due to the change in technology, LED
lighting, and so on, but were the proposals suitable for high density large-scale development,
he wondered.
Becci Taylor felt that the precise opportunities for each building needed to be studied at
the beginning and if one removed as far as possible the internal gains then there was the
opportunity to look at these kinds of things. Part of it would come down to the size of the
floor plate, but there were new ways of looking at office buildings where not as much
cooling was needed as in the past. A lot of it was about setting the brief and reducing the
internal gains.
Robin Williams) said that he loved passive buildings and whenever he entered one that
was working properly it was a little bit like a living organism and it had character. The
problem came when the people using such a building did not understand how it worked and
did not adapt very easily to the change of use.
David Butler in reply said that several years ago they had visited a building operated by a
large organisation in the financial services sector and they had a mixed mode approach, with
chilled beams on warm days and open windows on other days. Basically they had to send
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little messages to people’s screens telling them whether it was an open window day or a
mechanical cooling day, when they should keep their windows closed. It was very difficult to
educate people as to when it was appropriate to open a window and when it was not
because if a mechanical cooling system was working one would not want to open windows
and certainly not if it was a chilled beam system because of the risk of condensation, and so
on. The simpler the building the more likely it was that the users would do the right thing.
The building concerned had a large floor plate and a traditional work station type set up and
they were pushing the technology in that situation, but most of the buildings in the UK were
not air conditioned and most people found them quite satisfactory by opening the windows
when they felt too warm. They may still be too warm but the psychological benefit of being
able to control their situation by opening a window was quite often sufficient for people to
feel the environment was acceptable.
The downside of that was where there was a large floor plate with lots of people sharing
an office space because very quickly you could get the situation where one person did not
like a draft and wanted the window closed and another person wanted it left open. So for
simple buildings with low occupancy, yes, but for large buildings with high occupation, it was
not impossible but much more difficult.
Becci Taylor concurred with that statement.
David Cowan asked the two speakers to comment on the feasibility of using ground
source cooling as an alternative to no cooling, that is, with no refrigeration system and a
ground source heat pump.
David Butler stated that it was feasible in certain locations. The BRE building did have a
borehole based cooling system but it was very rarely used. It was installed as a safeguard
and the fact that it was rarely used meant it was not maintained and had now fallen into
disuse. Portcullis House, where most MPs had their offices, utilised ground cooling and
ground water in a similar way to the BRE building, but he suggested that in very densely
occupied city centre areas one would very quickly run out of cool ground. Ground cooling
was largely the result of seasonal changes in weather, so the ground was cooled in the
winter and warmed in the summer by sunshine. It would work in a large open area where
there were wide spaces between buildings but in Canary Wharf it might be a significant
challenge.
Becci Taylor added that essentially if one was not going to use any refrigeration one was
limited by the cooled energy under the building, but there were opportunities to improve
that. There was a certain amount of continuum and one could install a heat pump that
would be used when extra heating was needed, but sometimes there would be enough heat
from the ground. Equally if one was to use a heat pump for heating in the winter then one
would be dumping cold into the ground that could be used in the summer, so the seasonal
energy transfer offered an opportunity as well, which was particularly relevant in our
country compared with others.
Graeme Maidment thanked those present for the excellent thought-provoking
questions and asked each speaker to recap their arguments and state why it was so
important that they should win the debate.
Becci Taylor, in her concluding remarks, said the main point she wished to make was
that heat pumps should not be used as an excuse not to design things properly and the
whole of the design team, including the clients as well, should engage at the beginning of the
design process and look at all the opportunities and consider all the constraints, working
closely with the architects and engineers to come up with a solution which would use the
least energy. This may well include an element of passive cooling and that should come first,
but heat pumps should not make us lazy.
David Butler agreed with Becci that mechanical cooling could make us lazy and it should
not be an excuse not to intelligently design buildings. The analogy was the car: once we
owned one we tended to use it for journeys where in the past we would have walked. The
same applied to building services: where there was air conditioning we tended to use it
more than perhaps we needed to and it also led to lazy building design. We had seen over
Proc. Inst. R. 2012-13. 4-13
several decades the development of large numbers of prestige high-rise office buildings,
which essentially were glass boxes, with all the attendant problems of excessive heat gains
and solar heat gains generally, and the difficulties in dealing with that and sizing the cooling
system and making it big enough. But why not design a more sensible building which did not
admit so much solar heat when it was not required?
However, he said it was clear that in the UK and in southern England especially, in urban
and city areas, the utilisation of space was such that we had to use buildings intensively in
terms of floor area use and time use, with the expansion of 24/7 working. In those
situations mechanical cooling was virtually unavoidable. For the right situation reversible DX
heat pumps were an ideal technology and even air cooled chillers would be appropriate for
certain types of building. He felt that it they were going to find it very difficult to dispense
with mechanical cooling in the future.
Graeme Maidment asked those present to vote on the question: Heat pumps or
passive cooling as the most effective low carbon solution for the cooling of buildings.
Passive cooling – 23 For
Heat pumps – 14 For
He concluded that from the debate it was clear that they were both important and had a
role to play for modern building design and there probably was a need for both in a mixed
design. He asked who would have voted for that option and this was nearly everyone on a
show of hands.
In closing the meeting, Graeme thanked those present for making it an extremely
interesting debate evening.
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