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Air source heat pumps and their role in the Swedish energy system
Itai Danielski*,a, Morgan Frölinga
a Ecotechnology, Department of Engineering and Sustainable Development, Mid Sweden University,
83125 Östersund, Sweden
* Corresponding author: firstname.lastname@example.org, Tel: +46 (0)63-165416,
Newly produced air source heat pumps can provide heat energy from outdoor air at temperature as low
as -20°C. As a result they could be utilized during most days of the year even in the cold Nordic
climates. The drawback of air source heat pumps is the reduction in efficiency as the outdoor air
become colder, resulting in lower heat supply in times when it is most needed. Despite its inverse
relationship between efficiency and outdoor temperature, air source heat pumps were installed in
57000 detached houses in Sweden during 2010 alone, which is 3% of the total detached houses stock.
That makes air source heat pumps the most sold heating technology for detached houses in Sweden
during 2010, 1.6 times more than the number of installations of ground source heat pump and 3 times
more than the number of connections of detached houses to district heating during the same year.
Similar trends can be found in other Nordic countries.
This study compares the use of an air source heat pump with other existing commercial technologies
in detached houses and analyzes the impacts on primary energy use, on final energy use, on electricity
production and on costs benefits for house owners. It was found that converting existing electric
heated Swedish detaches houses to district heating with biomass based CHP or bed-rock heat pump
could reduce the use of resources, which could benefit Sweden as a society. Converting electric heated
Swedish detaches houses to district heating or pellets stove could reduce power demand and level out
the power demand load curve. That would benefit utilities of power supply as it could secure power
supply. However cost effectiveness in one of most important drivers for house owners of detached
houses to choose energy efficiency measures. For that reason house owners may most likely benefit by
the installation of air-source heat pumps.
Large share of the Swedish residential building were built during the 1960s and 1970s with
pick of new constructed units ending during the oil crisis in 1973 (Statistics Sweden 2012a).
Until the oil crisis fossil fuel and electricity prices were relatively low, and energy
conservation in buildings was not highly prioritized. Oil and electricity were widely used as
energy source for space and domestic water heating. With higher fossil prices, many of the
detached houses converted to biomass and heat pumps. District heating networks were
established providing, currently, space and domestic hot water for 92% of the multifamily
dwellings and only for 27% of the total 1.9 million detached buildings. The detached houses
stock has the highest final energy use in the service sector for space and water heating (The
Swedish Energy Agency 2011a) and four fold higher electricity use in comparison to
multifamily dwellings (Statistics Sweden 2012). Electricity is still the most common form of
energy used for heating and hot water in detached buildings. Gustavsson and Joelsson (2010)
found that the choice of end use energy carrier have a greater influence on the primary energy
savings than energy conservation measures done on the thermal envelope of the buildings. In
addition, the energy conservation measures were less cost effective when converting to more
energy efficient heating system.
Heat pump were available since the 70s but they got their large breakthrough only during
2005 (Nowacki 2007) and were installed mainly in detached houses. About 46% of the
detached houses in Sweden has some sort of heat pump installed (The Swedish energy agency
2011b). The most common type of heat pump is the air source heat pump, which include
mainly air-to-air, air-to-water heat pumps. Since 2005 the selling of air-source heat pumps has
accelerated (Nowacki 2007) and reached 57,000 households during 2010, which make it the
most sold heating technology in detached houses in Sweden.
Air source heat pumps are consider being one main reason for the large reduction in the
average specific final energy use of the entire detached house stock in Sweden; from 170
kWh/(m2 year) during 1977 to 140 kWh/(m2 year) today (The Swedish energy agency 2011).
However air source heat pumps have major drawback, they provide less heat when it is
needed the most, i.e. when the outdoor temperature decreases. During those cold periods,
supplement heat from other sources is needed to maintain comfort indoor conditions, in most
cases by resistance heaters. Larsson et.al (2006) study the electricity consumption in 437
detached houses and concluded that the impact of detached houses on the Swedish peak
power production is significant. It may increase the power production needed by an additional
1 GW in a 20 year cold winter in comparison to normal year.
In this study the impact of the air source heat pump, installed in Swedish detached houses
built in the 70s, is analyzed by several parameters: the final energy use, the primary energy
use, its cost effectiveness and the impact on the energy system in Sweden as a whole.
2.1. Case study
The case study is an existing detached house built in 1974. It has two stories and a total heated
floor area of 115 m2 heated by electric resistance heaters. It has a inclined roof with ceramic
tiles that consist of 150 mm mineral wool between wooden beams above particle boards
panels with U-value 0.29 W/(m2 K). The external walls consist of 16 mm gypsum board,
moisture protection sheet, 120 mm mineral wool between wooden beams, and 20 mm wood
panel with total u-value of 0.33 W/(m2 K). The ground floor consists of 15 mm oak boarding
on 20 mm particle board above 110mm mineral wool laid on 200 mm concrete plate and 150
mm macadam and have U-value of 0.2 W/(m2 K). All the windows and two of the three
external doors are double glazed with a total area of 23.7 m2 and U-value of 2.7 W/(m2 K).
The indoor temperature is assumed to be constant 20°C. The yearly final energy use for
household electricity and domestic water heating are assumed to be 3348 and 3074 kWh/year
2.2. Technologies and efficiencies
The COP and heating output of the air source heat pump were based on the test results done
by the Swedish energy agency (2009a) for few outdoor temperatures and compressor output
conditions. The results were extrapolated linearly to the entire outdoor temperature range as
illustrated in Fig. 1.
Fig. 1. Data source the Swedish energy agency (The Swedish energy agency 2009)
The air-source heat pump was compared with several commercial technologies, which
includes: electric resistance heaters, pellets stove, bed-rock heat pump and district heating.
The efficiency of the pellets stove was assumed to be 90% (The Swedish energy agency
2009b). The COP of the bed-rock heat pump was assumed to be 2.6 (The Swedish energy
A dynamic method was used to calculate the primary energy used by a district heating power
plant, which include the interaction between the supply and demand sides. The method as
well as the reference district heat production system is described in Gustavsson et.al. (2011).
The value of cogenerated electricity was calculated using the subtraction method. Where the
cogenerated electricity was considered as a by-product and assumes to replace an equivalent
amount of electricity produced in a marginal power plant. The marginal power plant assumed
to be a coal steam turbine (CST) power plant with 46% efficiency. The distribution losses for
district heat and electricity to the building were assumed to be 7% and 11% respectively. The
primary energy losses for production of coal and biomass were assumed as 10% and 4%
respectively. The electricity and heat used in pellets production were assumed to be 12% and
4% of the total energy embodied in the pellets (Nyström, Nilsson et al. 2011) and assume to
be produced in the marginal power plant and in a standalone boiler with 90% efficiency
The power load demand of the different technologies was compared to the Swedish power
demand load that was constructed by hourly data received from the Swedish national grid
(2010a) for year 2010.
2.3. Simulation program
The VIP-Energy software (Strusoft 2011) was used to simulate the final energy use in the case
study. VIP-Energy is a commercial dynamic energy balance simulation program that
calculates the energy performance of buildings hour by hour. The software was validated by
IEABESTEST, ASHRAE-BESTEST and CEN-15265. The case study was simulated in four
different Swedish cities representing different Nordic climate conditions as listed in Table 1.
The climate data obtained from the Swedish Meteorological and Hydrological Institute for
Table 1. Climate scenarios year 2012. Source: The Swedish Meteorological and Hydrological Institute (SMHI)
-40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0
Outdoor tempperature °C
Space heating demand Max heat supply COP
Copressor load 50%
Compressor load 100%
Climate scenarios (cities):
Average outdoor temperature
Average daily global solar radiation kWh/(m2 day)
Average wind speed [m/s]
2.4. Economy and prices
In this study average values were use for the costs of energy, i.e., electricity, district heating
and pellets. The prices for energy systems were obtained by different suppliers and assume to
be representative. It is important to note that in reality prices are not uniform and could
change with time, by location and differ among suppliers. Large price differences could be
found between the values used in this work and real cases but these assumed to be few. The
study aims to analyse the driver forces and trends in the Swedish market. The results apply to
the prices that are used in this study and should represent the situations in most cases in
The total yearly cost was calculated by the sum of the yearly costs for installation, equipment,
maintenance (Table 2) and energy costs (Table 3). Eq.1 calculates the yearly cost for
installation and equipment (A) by multiplying the total costs for installation and equipment
(P) by the capital recovery factor with interest rate (i) of 5% and the expected life time of the
products in years (n).
𝐴=𝑃 ∗ 𝑖∗(1+𝑖)𝑛
Table 2. Prices for equipment, installation and maintenance of different heating technologies
Bed-rock heat pump
Air-source heat pump
Water based radiatora
a Price per unit
Table 3. Prices for different energy carriers
a Depends on the energy tax and power output. Prices are for one year contract (Statistics Sweden
b Depends on max power output (Statistics Sweden 2012b).
c (District Heating in Sweden 2011)
Fig. 2 illustrates the power demand of the case study with different heat sources technologies
and with different outdoor temperatures. Air-source heat pump is the most sensitive to
variations in outdoor temperatures. The power demand increases exponentially with colder
outdoor temperatures until it reaches the power demand of resistance heater. Installing air-
source heat pump in detached houses that are heated by resistance heaters will reduce the final
energy use but the peak electricity load may remain unaffected.
With connection to the district heating, the peak power demand is the lowest among the
different heat source technologies. The variations in power demand are relatively low and are
the result of daily variations in household electricity. The amount of electricity, which is co-
generated together with the heat at the CHP plant, can be higher than the household power
demand load resulting in periods with positive balance of power production. This is not
shown in Fig. 2.
The bed-rock heat pump is assumed to cover 100% of the heating demand. Some bed-rock
heat pumps may be dimensioned to provide up to 95% (Larsson and Bröms 2007). That will
result with higher power demand during low outdoor temperatures if the peak heating load is
covered by direct electricity heating, which is usually the case.
Fig. 2. The electricity production load that is needed to provide similar indoor conditions by different heat source
technologies and for different outdoor temperatures in the case study building.
The Swedish power demand curve, illustrated in Fig. 3, was constructed by hourly data
received from the Swedish national grid (2010a) for year 2010. Year 2010 was relatively cold
year but similar trends are found in previous years as well. The Swedish power demand is
sensitive to outdoor temperatures with peak power during the cold periods as a result of
electricity heating for space and water heating in the service sector, with the detached houses
as the major contributor. According to Fig. 2, resistance heaters and air-source heat pumps
may have the largest contribution to the peak load demand. The sensitivity of the industry
power demand to variations in outdoor temperatures is marginal (Börgesson, Doorman et al.
The differences between the electricity production and electricity demand in Fig. 3 shows that
Sweden is a net exporter of electricity during low power demand. During high power demand,
Sweden has deficit in electricity production that need to be covered by import. Hydro power
is the most important regulator between high and low power demand followed by the co-
generated electricity produced in CHP plants.
Fig. 3. Electricity load demand curve in Sweden and electricity production by technology for year 2010 (Source of
data: The Swedish national grid (2010)).
Converting from the case study building heating system from resistance heaters to air-source
heat pump found to reduce the primary energy by 30% to 35% as illustrated in Fig. 4. A pellet
stove heating system use slightly more primary energy than air-source heat pump. District
heating from CHP production and bed-rock heat pump provide the lowest primary energy use;
30% to 37% lower than air-source heat pump.
Fig. 4. The primary energy use for different heat source technologies and in different climate conditions
Fig. 5 illustrates the yearly costs of installing and using each heat source technology with and
without the Swedish ROT tax. The ROT tax provides 50% tax return on the price of
installation and reparation in private houses. The ROT tax favours large installations as bed-
rock heat pump and connection to the district heating network. However in both cases air-
source heat pumps have the lowest yearly costs for all climates conditions.
The price of electricity includes an energy tax that is lower in north of Sweden with its colder
climate as listed in Table 2. Low rate energy tax benefits technologies that consume electricity
for heating as heat pumps and resistance heaters. The lower energy tax rate allows bed-rock
heat pump to be cost effective as air source heat pump in climate with annual average outdoor
-2.0 0.0 2.0 4.0 6.0 8.0
Average outdoor temperature [°C]
RH Bed-rock HP Air source HP Pellets Stove DH
temperatures below 0°C. In warmer climates, and higher energy tax, the yearly costs of bed-
rock heat pump are higher and district heating may be more comparative. According to Nair
et al. (2010) economy is generally the main reason for energy efficiency measures among
private house owners. The end user economy is thus most probably an important driver for
the increasing number of installation of air-source heat pumps in Sweden.
Fig. 5. The yearly costs including material, installation, maintenance, and energy costs for different heat source
technologies and climate conditions with and without the Swedish ROT tax.
Air source heat pumps have the lower investment cost among the heat source technologies
and lower yearly investment cost per kWh saved yearly in comparison to bed-rock heat
pumps as illustrates in Fig. 8.
Fig. 6. A comparison of the cost of each unit of final energy saved between air-source and bed-rock heat pumps in
different climate conditions.
Air-source heat pumps were found to be cost effective even if they are installed in detached
houses that are already connected to the district heating network. The energy costs in detached
houses that are connected to the district heating could be reduced by 1500 to 4000 SEK,
depending on the climate conditions, if air source heat pump would be installed in addition to
the district heating.
-2.0 0.0 2.0 4.0 6.0 8.0
Average outdoor temperature [°C]
Bed-rock heat pump Air source heat pump
Fig. 7. The yearly costs including material, installation, maintenance, and energy costs for heat supplied from the
district heat and heat supplied from both the district heating and air-source heat pump.
Air-source heat pumps provides 35% lower final energy use in comparison to resistance
heaters (RH), district heating (DH) and pellets stove (PS) as illustrated in Fig. 7. Bed-rock
heat pumps result with even lower final energy use because it covers the total heating demand
including domestic water heating.
Fig. 8. Final energy use by different heat technology sources and in different climate conditions
Heat pumps result with lower final energy use than resistance heaters, pellets stove or district
heating. Lower final energy is an advantage for labelling in building. The Nordic Ecolabelled
building provides up to 10 points by reducing the final energy below a certain level, while the
minimum amount of points needed to acquire the labelling is 9 out of a total of 22 points that
could be gained (Nordic Ecolabelling 2009). Since 2009, an energy declaration is needed for
existing detached houses before they are sold, which record the final energy use of the
dwelling and give suggestion for cost effective energy efficiency measures. Gustavsson and
-2.0 0.0 2.0 4.0 6.0 8.0
Average outdoor temperature [°C]
District heating District heating + Air source HP
-2.0 0.0 2.0 4.0 6.0 8.0
Average outdoor temperature [°C]
RH, DH, PS Bed-rock HP Air source HP
Joelsson (2007) found that energy conservation measures were less cost effective when
converting to more efficient heating system. The installation costs of air source heat pump are
relatively lower than the costs of energy efficiency measures done on the thermal envelop of
the dwelling and therefore may be favourable by detached house owners.
Air source heat pumps have lower installation costs and are more cost effective than
resistance heaters, bed-rock heat pumps, pellets stove and district heating. Air-source heat
pumps were found to be cost effective even if they are installed in detached houses that are
already connected to the district heating network. Cost effectiveness may be the main driver
for the increasing number of installation of air-source heat pumps in Sweden. In addition, the
installation of air source heat pumps is relative simple and quick.
However the efficiency of the air-source heat pumps is reduced with decreasing outdoor
temperature. At low outdoor temperatures additional heat source is needed. In most cases
direct electricity heating is used, which could be built-in in the heat pump itself or by external
resistance heaters. The use of air-source heat pump and direct electricity heating for peak
heating load increases the power demand exponentially as the outdoor temperature decreases.
District heating provide more uniform power load demand throughout the year followed by
pellet stove and bed-rock heat pump.
The Swedish power demand is sensitive to outdoor temperatures with peak power during the
cold periods as a result of electricity heating for space and water heating with the detached
houses as the major contributor. Sweden export electricity through the Nord pool spot market
during low power demand when electricity prices are usually low and import electricity
during high power demand with higher prices. During the coldest days in 2010 the spot price
of electricity reached a new record of 14 SEK/kWh, which had impact both on house owners
and the industries (The Swedish national grid 2010b). Hydro power is used to regulate
between periods of low and high power demand. However hydro power is not sufficient
during peak power demand and fossil fuel based electricity is used from domestic production
and import. Installing air-source heat pump in detached houses with resistance heaters reduces
the final energy use but will maintain the high peak power demand during the cold days.
A conversion from resistance heaters to district heating in detached houses will reduce power
demand and increase production of co-generated electricity from the CHP plant during peak
power demand. Therefore increasing the number of district heated detached houses could
reduce the dependency on imported electricity. In addition, replacing imported fossil fuel
based electricity from Denmark, Poland and Germany by biomass based CHP production may
assist to realize the decision of the Swedish government to break the fossil fuels dependency
of the building sector until 2020.
The Swedish ROT tax, which provides 50% tax return on the price of installation and
reparation in private houses, increase the cost effectiveness of large installations as district
heating and bed-rock heat pump. Higher energy tax increases the cost effectiveness of the
district heating in relation to air-source heat pumps. However, for district heating or bed-rock
heat pump to be more cost effective than air-source heat pump additional policies are needed.
Converting electric heated Swedish detaches houses to district heating with biomass based
CHP or bed-rock heat pump could reduce the use of resources, which could benefit Sweden as
a society. Converting electric heated Swedish detaches houses to district heating or pellets
stove could reduce power demand and level the power demand load curve. That would benefit
the power utilities as it would be easier to meet the power demand and secure power supply.
However cost effectiveness in one of most important drivers for house owners of detached
houses to choose energy efficiency measures. For that reason house owners may benefit the
most by the installation of air-source heat pump.
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