The bin method to investigate the effect of climate conditions on the
SCOP of air source heat pumps: the Italian case
EUGENIA ROSSI DI SCHIO, VINCENZO BALLERINI, PAOLO VALDISERRI
Department of Industrial Engineering
Alma Mater Studiorum – Università di Bologna
Viale Risorgimento 2, I-40136 Bologna
ITALY
Abstract: The paper investigates the seasonal performances of electric air-to-water heat pumps specifically
related to climatic conditions of the place where the heat pump is installed. The analysis is carried out using easily
available weather data for some Italian towns differently located, and using the bin-method proposed by UNI/TS
11300-4. Two different types of heat pumps (on-off and inverter-driven variable speed compressor) are
considered and comparisons between different types of heat pumps and different places of installation are
performed. The analysis shows that the climate of the installation place is the most important factor that affects
seasonal indexes of heat pumps; moreover, as expected, inverter heat pumps better perform than on-off ones.
Key-Words: bin method, air-source heat pump, decarbonization, climate conditions
Received: July 22, 2021. Revised: March 13, 2022. Accepted: April 17, 2022. Published: May 7, 2022.
1 Introduction
It was Lord Kelvin who first proposed a practical heat
pump system, or “heat multiplier” [1]. Nowadays,
there is a need to increase the reduction of greenhouse
gas emissions from space heating and cooling in
order to meet decarbonization goals. For this reason,
heat pumps are becoming increasingly utilized all
around the world [2].
In a recent report dealing with decarbonizing space
heating by employing air source heat pumps
(ASHPs) [3], the authors conclude that ASHPs are
cost-competitive today in places where the climate is
mild and that, “with climate policies consistent with
rapid decarbonization and reasonably foreseeable
technological progress, air-source heat pumps are
the low-cost option for typical residential buildings
across much of the US by the mid-2030s.” Duicu’s
study [4] reported the advantages of using heat
pumps as a renewable source of energy.
Different studies are also related to the employment
of ground sources heat pumps [5]; in those
applications, the performance of the entire heating
and cooling system is significantly affected by soil
thermal properties [6] and the circuit arrangement of
the ground heat exchanger [7].
Regarding ASHP, in the recent literature, the effects
of climate conditions are investigated by Mouzeviris
and Papakostas [8] with reference to Greece. The
authors study the seasonal coefficient of performance
in heating (SCOP) of 100 different ASHPs from 12
manufacturers, in a range of heat pumps’ thermal
capacity up to 50 kW. They conclude that the
seasonal performance of the various models
examined is affected by the heating capacity, the
local climate, the supply water temperature, the
compressor’s technology, and the control system.
Pospisil et al. [9] evaluated the real seasonal
coefficient of performance (SCOP) of air-to-water
heat pumps operating in Central Europe. The aim of
their study is the identification of a possible increase
in SCOP of the air-to-water heat pump with a
predictive regulation, mainly employing the ASHP
during the warmer period of the day with the addition
of an accumulation system.
The seasonal performance of electric air-to-water
heat pumps is also investigated in [10], by comparing
an on-off heat pump with an inverter-driven variable
speed compressor. Reference is made to a heat pump
for domestic space heating installed in Bologna, and
the bin method proposed by UNI/TS 11300-4 [11] is
employed in order to investigate the effect of external
temperature.
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ASHPs are also characterized by the frost formation
on the external heat exchanger during winter
operation. In a recent study [12] the effect of real
climate data on the seasonal coefficient of
performance of ASHPs are investigated considering
three different heat pump systems coupled with the
same building located in three different Italian
municipalities. The authors conclude that the
electrical energy absorbed by the HP considering
defrost is higher with respect to the case with no
defrost every year mainly in the localities where the
average relative humidity is higher.
In Italy, the recent introduction of Italy’s new
Superbonus [13], providing a 110% tax discount, can
be seen as a very ambitious heat pump installation
scheme to date. In [14], a general analysis of the
Italian energy system is performed, focusing on the
possible energy, environmental, and economic
effects that the utilization of individual heat pumps
for winter heating can produce.
In the present study, the authors investigate electric
air-to-water heat pump performances specifically
related to climatic conditions of the place where the
heat pump is installed. The analysis is carried out
using easily available weather data for a lot of Italian
towns and using the bin method proposed by UNI/TS
11300-4. Two different types of heat pumps (on-off
and inverter-driven variable speed compressor) and
Fig. 1 - Hourly mean air temperature distributions, year 2019
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five Italian towns characterized by different weather
conditions were considered (Milano, Bologna,
Genova, Livigno and Ventimiglia). The main aims
are the evaluation of seasonal performances of the
heat pumps related to the weather of the place where
the thermal machine is placed and to establish
comparisons between different types of heat pumps
and different places of installation.
2 Setting of the analysis
The analysis is carried out using the bin-method
proposed by UNI/TS 11300-4, and on account of
weather data easily available for a lot of Italian
towns. In detail, we refer to five Italian towns
characterized by different weather conditions
(Milano, Bologna, Genova, Livigno and Ventimiglia)
and we consider different types of heat pumps (on-
off and inverter-driven variable speed compressors).
Technical data related to the heat pumps under
investigation, such as the maximum output thermal
power and COP (Coefficient Of Performance), are
obtained from the product datasheets for different
outdoor air temperatures and for different water
delivery temperatures.
The most important parameter for the analysis is the
mean outdoor air temperature of the place where the
heat pump (HP) is installed. The hourly mean air
temperature te for the five towns previously
mentioned are obtained from ARPA (Regional
Environmental Protection Agency) website of the
specific Italian region (ARPA Lombardia for Milano
and Livigno, ARPAE for Bologna and ARPAL for
Genova and Ventimiglia). Downloaded data have
been further processed to obtain exploitable
parameters for the analysis. Temperature data are
obtained for the five cities with reference to the year
2019; moreover, for Bologna, Livigno, and Milano
data for years 2017 and 2018 are used as well, in
order to determine seasonal performance variations
from one heating season to another.
Values of hourly mean air temperature are shown in
Fig. 1. An initial examination restricted to January
2019 underlines that Genova and Ventimiglia have a
similar climate with oscillation of around 8°C;
Bologna and Milano have a similar climate instead,
but with oscillation of around 3°C, and finally
Livigno has a cold climate, with an average monthly
air temperature of -11°C.
2.1 Methodology
Attention is paid to two electric air-to-water heat
pumps, one on-off HP and the other one an inverter-
driven variable speed compressor HP, combined with
auxiliary electric resistances (mono-energetic
bivalent operating mode heating system). The
auxiliary heat source (electric resistances) switches
on only for high thermal demand, typically when the
outdoor air temperature is low, close to design
temperature tdes.
Seasonal performance indexes analyzed are SCOPon
and SCOPnet. SCOPnet is defined as the ratio between
thermal energy supplied by the heat pump Qth,HP and
the electric energy input to the heat pump EEin,HP:
(1)
In SCOPon performance index also the auxiliary
energy related to the electric resistances for heating
is considered, thus giving:
(2)
where Qth,BU is the thermal energy supplied by the
resistances and EEin,BU is the electric energy input to
the auxiliary resistances.
We assume that the thermal power requested by the
building to the heating system linearly decreases for
increasing values of the outdoor air temperature and
that no power is requested for te ≥16°C. Energy
analysis is set up for the same two HPs virtually
placed in the five towns mentioned above. Since the
heat pump power output depends on the outdoor air
temperature, we can state that generally, the power
output of the heat pump installed in a place with a
warm climate (Genova, Ventimiglia) is higher than
the power output of the same heat pump installed in
a place with cold climate (Livigno). To meet the
sizing limit of the machine, we may assume a
different power demanded by the building to the
heating system, which varies from one place to
another.
Table 1 – Design temperature, power requested to the heating
system, and power of the auxiliary resistances.
City
tdes (°C)
Thermal power
requested (kW)
Auxiliary
power (kW)
Livigno
9
5
2
Bologna
-5
7
2
Milano
-5
7
2
Genova
0
8
3
Ventimiglia
0
8
3
The buildings in Ventimiglia and Genova request the
maximum power whilst Livigno the minimum, as
shown in Table 1, where tdes is the design temperature
for the heating system. COP also depends on the
capacity ratio CR, defined as the ratio between the
thermal power supplied by the HP at a specific te and
the maximum thermal power that the HP can supply
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at the same te. In the analysis, hot water delivery
temperature is set constant and equal to 35°C, thus
allowing the COP to vary only on te and CR. We
obtain the COPD values (at CR=1) and the thermal
power supplied for different te, from the HP
datasheets, as shown in Table 2, both for the on-off
and inverter heat pump considered.
Table 2 – Data from datasheet of on-off and inverter heat pump.
ON-OFF HP
INVERTER HP
te (°C)
Maximum
power (kW)
COPD
Maximum
power (kW)
COPD
-7
5.09
2.51
5.43
2.86
2
6.15
3.25
5.67
2.96
7
8.67
4.09
8.06
3.92
12
10.24
4.26
9.23
4.37
15
10.83
4.31
9.74
4.56
The heat pump will operate at partial load (below its
maximum rated capacity) over a long period during
the heating season. This aspect affects the COP and
is taken into account by introducing the coefficient
fCOP , according to UNI EN 14825 [15] defined as
(3)
In Eq. (3) CC is a degradation coefficient and in
absence of further data from heat pump datasheet,
CC =0.9. With reference to the two HPs considered,
punctual values of fCOP have been provided by
manufacturer and COP at part load (COPPL) is given
by
(4)
In Fig. 2 the COP of the on-off heat pump is reported
versus of te and CR. Since the COP variations
strongly affect the seasonal performance both a
monthly and daily analysis would be inaccurate since
a great variation of te may occur during the day as
well.
Fig. 2 - COP for the on-off HP vs te and CR
A correct analysis should be carried out considering
every hour of the heating season. Indeed, according
to UNI/TS 11300-4 the “bin method” will be
employed, i.e. measured hourly mean outdoor air
temperature values replace the normal distribution of
the outdoor air temperature proposed by the same
norm. The regulation introduces the concept of “bin
hours”, defined as the number of hours of the heating
season in which there is a te included in a defined
temperature range called bin, i.e. is a temperature
interval of 1K centered on an integer value.
In Table 3 the heating period, the climatic zone, and
heating degree days (HDD) according to Italian law
DPR 412/1993, are reported for the five Italian
mentioned municipalities.
Table 3 – HDD, climatic zone and heating system operational
period.
City
Climatic
zone
Heating
Degree Days
Heating system
operational period
Ventimiglia (IM)
C
1119
15 Nov - 31 Mar
Genova (GE)
D
1435
1 Nov - 15 Apr
Bologna (BO)
E
2259
15 Oct - 15 Apr
Milano (MI)
E
2404
15 Oct - 15 Apr
Livigno (SO)
F
4648
365 days
Weather data downloaded from ARPA websites have
been revised in relation to the heating period shown
in Table 3 to obtain bin hours for every town
considered. In Fig. 3, bin hours obtained for Bologna,
years 2017, 2018, and 2019 are reported. Bin hours
distributions have been obtained for Ventimiglia,
Milano and Livigno as well. For example, Fig. 3
shows that, for Bologna, 213 bin hours at 14°C
occurred during 2019: it means that for 213 hours
during the mentioned heating season, the hourly
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mean outdoor air temperature was in the range 13.5-
14.5°C.
Fig. 3 - Bin hours for Bologna, year 2017, 2018 and 2019.
The behavior of the heat pumps is analyzed by
employing spreadsheets, where the main items were:
• Mean hourly outdoor air temperature te (°C)
• Number of hours of heating season hb such that
the specific outdoor air temperature is within a fixed
range (bin hours)
• Thermal power Pth,REQ (kW) and energy Qth,REQ
(kWh) requested by the building for the heating
system
• Maximum thermal power supplied by heat pump
Pth,maxmHP (kW)
• Electric power Pin,HP (kW), and energy EEin,HP
(kWh) to the heat pump
• Energy EEin,BU (kWh) to auxiliary electrical
resistances
• COP, evaluated by eq. (4)
• Thermal energy output from heat pump Qth,HP
(kWh) and thermal energy output from auxiliary
electrical resistances Qth,BU (kWh).
In Table 4 there is an extract from the spreadsheet set
up for the analysis. Seasonal indexes are calculated
by employing eqs. (1) and (2) and as a result of the
summation of the thermal and electrical energies
from the spreadsheet shown in Table 4.
2.2 Influence of heat pump sizing on
performances
The correct design and sizing of the heat pump have
an important influence on its performance. Usually,
the heat pump is sized in order to partially (but not
totally) supply the thermal request, especially for on-
off HPs. Thereby the machine is sized to work for a
long time with a high value of COP. The residual part
of the thermal request is supplied by an auxiliary
generator (in this case auxiliary electrical resistances)
that are switched on only whenever the HP is unable
to supply the thermal request of the building. In the
scenarios of the present analysis, for every
considered place there is a value of the power
requested by the building to the heating system that
maximizes seasonal index SCOPon. For example for
Bologna, on-off HP and the year 2017 the maximum
value of SCOPon is 3.44 for a heating system (heat
pump and auxiliary electric resistances) sized to
supply a thermal power requested by the building of
7kW (Fig. 4). In Table 1 the maximum power
requested from the building is reported as the value
that maximizes SCOPon for the considered town.
Table 4 – Extract from spreadsheet, scenario: Livigno, inverter HP, year 2019.
te
hb
Pth,REQ
Qth,REQ
Pth,max,HP
Pin.HP
COPPL
Qth,HP
Qth,BU
EEin,HP
EEin,BU
°C
h
kW
kWh
kW
kW
-
kWh
kWh
kWh
kWh
-5
371
3,00
1113
5,55
0,79
3,78
1109
0
293
0
-4
402
2,86
1149
5,56
0,76
3,76
1149
0
306
0
-3
439
2,71
1192
5,53
0,73
3,72
1194
0
320
0
-2
451
2,57
1160
5,49
0,70
3,69
1164
0
316
0
-1
413
2,43
1003
5,49
0,67
3,64
1008
0
277
0
0
370
2,29
846
5,48
0,63
3,62
844
0
233
0
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Fig 4 - SCOPon for on-off HP in Bologna.
3 Results
The seasonal indexes are shown in Table 5. As
already underlined, with reference to Bologna,
Milano and Livigno the analysis covers the years
2017, 2018 and 2019, while for Genova and
Ventimiglia it refers to the year 2019.
Table 5 shows that seasonal indexes vary a lot from
one place of installation of the heat pump to another.
For example, for Livigno SCOPon value is 3.36 and
SCOPnet 3.42 for inverter heat pump (year 2019);
with reference to the same inverter HP, for
Ventimiglia SCOPon is 4.70 and SCOPnet is 4.71. The
air-to-water heat pumps perform better when
installed in a warm climate and worse if installed in
a cold climate, as expected from the COP trend. This
comparison shows that the climate of the place where
the HP is installed strongly reflects on seasonal
indexes, showing increasing up to 45% between
Livigno and Ventimiglia. The comparison also shows
that the variation is more relevant for on-off HP.
Results in Table 5 underline also the influence of heat
pump typology on SCOP: inverter heat pump better
behaves in all the considered scenarios and the
maximum increase on seasonal indexes is observed
for machines installed in a colder or warmer climate
(the increase of SCOPnet related to an inverter heat
pump for Livigno, Genova and Ventimiglia is at least
20% larger than SCOPnet of on-off heat pump). For
an intermediate climate (like Bologna, Milano)
SCOP variations are approximately 10%.
Lastly, analysis is made considering different heating
seasons for the same town. Results show that
percentage increases (or decreases) of seasonal
indexes are lower than the previous comparison
showed above (less than 6%). SCOP variations are
larger for the inverter heat pump instead of on-off
heat pump for all places considered in this
comparison (Bologna, Milano and Livigno), and the
smallest SCOPon and SCOPnet variations are
observed for Livigno instead of Bologna and Milano:
for example for the inverter HP in Milano SCOPon for
year 2017 is 3.55 and for year 2019 is 3.85, and that
shows an increase on SCOPon of approximatively 8%
between year 2017 and 2019. For the same inverter,
HP placed in Livigno SCOPon increase is 1.8%
between 2017 and 2019.
In conclusion, we found that the climate of the
installation place is the most important factor that
affects seasonal indexes of HP (SCOP variation up to
45%); heat pump type affects SCOP with variation
up to 27% instead, and finally, also the heating season
affects the seasonal performances of the heat
pump/heating system, but less than 10%.
Table 5 – Seasonal indexes for Bologna, Milano, Livigno (years 2017, 2018, 2019) and
for Genova and Ventimiglia (year 2019).
ON-OFF HP
INVERTER HP
Place
Year
SCOPon
SCOPnet
SCOPon
SCOPnet
Bologna
2017
3.44
3.47
3.87
3.92
Bologna
2018
3.39
3.45
3.81
3.90
Bologna
2019
3.51
3.51
4.00
4.01
Milano
2017
3.27
3.38
3.55
3.72
Milano
2018
3.37
3.42
3.73
3.81
Milano
2019
3.37
3.44
3.74
3.85
Livigno
2017
2.63
2.67
3.30
3.38
Livigno
2018
2.68
2.71
3.36
3.41
Livigno
2019
2.67
2.69
3.36
3.42
Genova
2019
3.77
3.79
4.50
4.56
Ventimiglia
2019
3.86
3.86
4.70
4.71
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4 Conclusion
The performances of electric air-to-water heat pumps
have been investigated with reference to the climatic
conditions of the installation place. We focused on
five different Italian municipalities, and, in order to
evaluate the weather conditions, the bin-method
proposed by UNI/TS 11300-4 is employed. Two
different types of heat pumps (on-off and inverter-
driven variable speed compressor) are compared. The
analysis shows that
• the climate of the installation place is the most
important factor which affects seasonal indexes of
HP;
• heat pump type affects SCOP as well, and inverter
HP better performs than on-off HP;
• also the heating season affects the seasonal
performances of the heat pump/heating system, but is
less important than the previous ones.
Since important SCOP variations occur depending on
climatic conditions, also with reference to differently
behaving hears considered for heating season, further
investigations may expand the number of years for
the analysis alongside dynamic simulations [16].
Moreover, further investigations of the present
analysis may include the influence of the relative
humidity of external air on heat pump performances,
as other authors suggest [17].
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