Heat Pump Metrics
The UK
Government’s Green Homes Grant scheme (GOV.UK, 2020) may have had some success
in that it motivated some home owners to think about how they would make energy
improvements to their homes if they could find a suitable contractor and
received a grant. One of the measures
included in the scheme was the installation of a heat pump, of either the
ground or air source type. Within the scheme these devices were classified as low
carbon heat measures, and were subject to additional requirements. One was that
a “minimum level of insulation is recommended to ensure the proper design and
operation of the relevant technology in line with relevant standards” and
another that all “heat pump systems must have a minimum Seasonal Performance
Factor (SPF) of 2.5.” Ofgem (2021) defines the SPF as “a measure of the
operating performance of an electric heat pump heating system over a year. It
is the ratio of the heat delivered to the total electrical energy supplied over
the year.” Ofgem explains that heat pumps need electricity to power their
compressors and that for maximum efficiency a property “should be well
insulated and have a low temperature heating system, such as underfloor heating
or low temperature radiators”. This
enables the heat pump to use less electricity and produce a high SPF. The requirements for insulation level and
seasonal performance factor referred to above are therefore not independent;
the question of what is a “minimum level of insulation” for any particular home
needs further consideration.
The Centre
for Alternative Technology provides guidance on heating systems which use heat
pumps (CAT, 2021). It points out that heat pumps should ideally deliver low
temperature heat in conjunction with underfloor heating, but notes that “adding
this to an existing house is often not feasible” and that for satisfactory
operation “decent levels of insulation” are needed, with home retrofit reaching
“a level better than current UK Building Regulations.”
The report “Energy
efficiency of housing in England and Wales” (ONS, 2020) compares the CO2
emissions and energy costs of new and existing dwellings in England and Wales
in the financial year ending 2019, stating that “the median estimated CO2
emissions for existing houses were equivalent to the emissions of more than two
new houses combined”, and that “the median estimated energy cost per year for
an existing house … was more than twice as much as that estimated for a new
house”. While this might give some idea of the typical level of required
improvement implied above by the CAT, we should bear in mind that the average
size of newly built houses has decreased in every decade from the 1970s (LABC, 2019).
Energy
performance standards in relation to building area are mentioned in a status
report on the implementation of the Energy Performance of Buildings Directive
(EPBD, 2018), where the current requirements for a semi-detached building are
stated as 117 kWh/m2/year. This can be contrasted with the Passivhaus standard
of 15kWh/m2/year and the EnerPHit standard of 25kWh/m2/year for space heating
and cooling (UK Alternative Energy, 2020). Quantifying the energy performance
of a building by dividing its annual energy use by its floor area arguably
leaves much unsaid: the building may not be occupied throughout the year; its
occupants may choose a higher or lower room temperature; the building may be
located in a warm or cold area.
This view
seems to be supported by Fairey and Goldstein (2016) in their attempt to
clarify the metrics used to describe building efficiency: “A very commonly used
metric for building energy performance is the energy use index (EUI), defined
as the energy consumption per unit conditioned floor area”. (Conditioned floor
area is described elsewhere (Law Insider, 2021) as “the total floor area of the
dwelling, excluding: a) floor area that is not fully enclosed; b) bathrooms
(but not ensuites) and laundries, with a ventilation opening; and voids, store
rooms, garages and carparks.”) Fairey
and Goldstein note the “very wide variance between the energy use per square
meter of different buildings of the same type in the same country or even the
same city”, stating that the “range of variation in energy use per unit floor
area is 4.5 to 1 between the top and bottom 5 percentile for offices” in New
York City. They list the reasons for this variation in commercial properties,
and some of these factors can be applied to domestic dwellings, particularly variation
in occupancy, and variation in the demands of occupants with respect to
temperature, humidity and ventilation.
In view of
the variables related to occupancy which can affect the energy use index of a
building, it would seem prudent to treat with caution any claim that its EUI
value alone defines whether or not its heating needs can be met by a heat pump.
The floor area of a building is one of many factors used by Li et al., (2020)
to ‘normalize’ a building’s energy use, but they suggest that the number of
occupants in an office building, the number of beds in a hospital, the occupancy
rate in a hotel, weather conditions and degree days all have their place as
normalizing factors according to the purpose and situation of a building. Degree
days are used in an attempt to take into account not only weather conditions
throughout the year, but also the temperature threshold (or thresholds) chosen
to activate a building’s heating and cooling systems. Heating degree days
(HDDs) apply when the outside temperature is low enough for the heating system
to operate and cooling degree days (CDDs) when it is warm enough to activate
the cooling system. “One HDD means that the temperature conditions outside the
building were equivalent to being below a defined threshold comfort temperature
inside the building by one degree for one day” (Wikipedia, 2020). This article
points out that the threshold temperature may be defined differently in
different countries or regions. From a knowledge of the total heating and
cooling energy used in a building and the total number of degree days in a
year, a measure of the building’s thermal performance can be calculated which
is arguably more useful than the simple figure of energy consumption per unit
conditioned floor area (EUI) referred to above, and the heat loss performance
of a building taking into account the number of degree days has been termed its
specific heat loss rate.
A more comprehensive approach is to use the idea of thermal comfort rather than rely
on temperature alone. A thesis by Vatougiou (2020) refers to work on thermal
comfort which identifies six factors, apart from individual adaptation. These
are indoor air temperature, mean radiant temperature, air velocity, relative
humidity, metabolic rate and clothing insulation. Some of these factors are
reflected by various standards for assessing thermal comfort in domestic
buildings: for example a guide from the Chartered Institution of Building
Services Engineers tabulates metabolic rate and clothing insulation alongside
room type and recommended temperature range, and the World Health Organization
recognises that the minimum temperatures which a heating system should be able
to maintain in cold weather vary from room to room (21°C for a living room but only 16°C for a kitchen, when the outside
temperature is 0°C). One of the methods of expressing
thermal comfort is the Predicted Mean Vote (PMV) scale, which ranges from -3 (cold)
through 0 (neutral) to +3 (hot). Vatougiou uses this scale in presenting
conclusions on the applicability of air to water heat pumps (AWHPs) in
retrofitting a wide range of simulated housing stock in the North-East region
of England. Only a few of the 756 house archetypes modelled achieved an average
PMV “within the comfort band” of -0.7 to +0.7. These were mainly “highly
insulated semi-detached and mid-terrace houses with floor area lower than ~160
m2 as well as medium-insulated mid-terrace houses with floor area lower than
~72 m2”. Across the range the seasonal performance factor (SPF) of the heat
pump itself was found to be around 3.0, but
the use of supplementary electric heating reduced the overall SPF of the
heating system to a range of 2.0 to 2.5.
Appendix B of
the thesis includes work presented in 2018 on the same simulated housing stock,
representing almost a million real houses. The objective here was primarily to
investigate the effectiveness of an AWHP system in terms of energy use and
heating capacity. Thermal comfort is not mentioned, but the work showed that
only 482 of the 756 simulated housing archetypes were eligible for an AWHP
retrofit with their existing levels of thermal insulation. The systems modelled
all used the same type of heat pump and followed “the common practice in domestic
retrofit applications” of integrating the heat pump with the existing
distribution system, which typically employs high temperature radiators, rather
than using underfloor heating or large low temperature radiators. Results
showed that 284 of the simulated house archetypes were under-heated for at
least 300 hours during the heating season of October to April inclusive. The
simulated housing stock is tabulated against several criteria including floor
area and the thermal transmittance or U-value of exposed walls. Approximately
245 of the simulated houses had exposed wall U-values greater than 1.5 W/(m2⋅K), 85 had less than 0.5 W/(m2⋅K) and the remainder fell between
these two figures.
In view of the difficulty of ascertaining accurate U-values for the structure of existing buildings, the several factors involved in thermal comfort, and the issues of building use and climate referred to earlier, it is easy to see why it is tempting to fall back on a metric based solely on the Energy Use Index when deciding whether a heat pump would be suitable for a particular home. It also seems possible that ignoring the wider range of issues could lead to an expensive mistake.
References
CAT, 2021, “Heat pumps”, Centre for Alternative Technology, online, accessed 28 April 2021
https://cat.org.uk/info-resources/free-information-service/energy/heat-pumps/
EPBD, 2018,
“EPBD implementation in the United Kingdom – England”, online, accessed 28
April 2021
https://epbd-ca.eu/wp-content/uploads/2019/05/CA-EPBD-IV-UK-England-2018.pdf
Fairey, P.,
and Goldstein, D., 2016, “Metrics for Energy Efficient Buildings: How Do We
Measure Efficiency?” 016 ACEEE Summer Study on Energy Efficiency in Buildings,
online, accessed 29 April 2021
http://fsec.ucf.edu/en/publications/pdf/fsec-rr-664-16.pdf
GOV.UK,
2020, Green Homes Grant, online, accessed 27 April 2021
https://www.gov.uk/guidance/apply-for-the-green-homes-grant-scheme
LABC, 2019,
“What is the average house size in the UK?” Local Authority Building Control,
online, accessed 28 April 2021
https://www.labcwarranty.co.uk/blog/are-britain-s-houses-getting-smaller-new-data/
Law Insider,
2021, “Conditioned floor area definition” Law Insider, online, accessed 28
April 2021
https://www.lawinsider.com/dictionary/conditioned-floor-area
Li, H., et
al., 2020, “System-level key performance indicators for building performance
evaluation”, Lawrence Berkeley National Laboratory, 2020, online, accessed 27
April 2021
https://escholarship.org/content/qt417235ks/qt417235ks.pdf
and
https://www.sciencedirect.com/science/article/abs/pii/S0378778819324326
ONS, 2020, “Energy
efficiency of housing in England and Wales”, Office for National Statistics, EnerPHit
Retrofits, online, accessed 28 April 2021
Ofgem, 2021,
“Seasonal Performance Factor (SPF)”, Ofgem, online, accessed 4 May 2021
https://www.ofgem.gov.uk/key-term-explained/seasonal-performance-factor-spf
UK
Alternative Energy, 2020, “EnerPHit Retrofits”, online, accessed 28 April 2021
Vatougiou,
P., 2020, “The heating performance of air-to-waterheat-pumps in the retrofit of
domestic building stock”, Doctoral Thesis, Loughborough University, online, accessed 26 March 2021,
Wikipedia,
2020, “Heating degree day”, online, accessed 27 April 2021
Comments
Post a Comment