Themeninfo I/2013
A compact guide to energy research
Electrically driven
heat pumps
Latest results from research and field tests
A service from FIZ Karlsruhe
2
BINE-Themeninfo I/2013
Straight to the point
Electric heat pumps have become firmly established in the German heating market. In
recent years they have gained an eight to ten per cent share of the market. Approximately
every fourth new building is heated with a heat pump. Whereas until a few years ago
ground source heat pumps were still sold the most, the sales figures have shifted in
recent years in favour of air source heat pumps. Carefully planned, heat pump systems
compare very economically with other heating systems, whereby their higher procurement costs are offset by their lower energy and operating costs. Heat pumps are also
competitive in terms of their ecological balance. This situation is continually improving
with the growing proportion of renewable energies used for supplying electricity.
The success enjoyed by heat pumps is a result of intensive research. The efficiency and
reliability of the systems has been decisively increased and climate-harming working
fluids have been replaced with much more environmentally-friendly refrigerants.
However, despite the high level of development, there are good reasons for continuing
research with new focal areas:
For example, higher insulation standards require efficient heat pump systems with a
particularly small output. There is also increasing demand for systems that enable
double use for heating and cooling. Increased efficiency and cost reductions can be
achieved not just through detailed improvements to the systems but also by optimising
the system components. The use of new, natural working fluids will also further reduce
greenhouse gas emissions.
Some manufacturers also have thermally driven absorption and adsorption heat pumps
available, which in particular can replace boilers and condensing boilers in buildings
that already have gas-fired heating systems. However, the technology is not yet as
advanced as electric heat pumps and currently only represents a fringe area of the
market – although there is much to suggest that this situation will change in future.
The efficiency of heat pumps depends much more on the operating conditions than other
heating systems. Their annual performance factor, as a measure of energy efficiency, can
therefore vary significantly from building to building. Until now insufficient concrete data
has been available from practice and the measurement results from test rigs are only of
limited information value for actual operation. A recently concluded monitoring project
is providing new data. These serve as a base to derive concrete recommendations for
planning, installing and operating heat pump systems. The results of this research
project form the focus of this BINE-Themeninfo brochure.
Your BINE editorial team
[email protected]
Imprint
ISSN
1610 - 8302
Publisher
FIZ Karlsruhe GmbH · Leibniz Institute
for Information Infrastructure
Hermann-von-Helmholtz-Platz 1
76344 Eggenstein-Leopoldshafen
Germany
Authors
Simon Braungardt
Danny Günther
Marek Miara
Jeannette Wapler
Fraunhofer Institute
for Solar Energy Systems (FhG-ISE)
Werner Weßing
E.ON Ruhrgas AG
Editor
Dr Franz Meyer
Cover image
Viessmann Werke, Allendorf
Copyright
Contents
Text and illustrations from this
publication can only be used if
permission has been granted by
the BINE editorial team.
We would be delighted to hear from you.
3 Performance of heat pumps
5 In practice: Single-family home with electric heat pump
5 En passant: Bridge technology
9 In practice: School building with thermal heat pump
10 Efficiency under real conditions
16 Capacity-controlled heat pumps
17 In portrait: German and Swiss design consultants report
18 Solar-assisted heat pumps
20 Heat pumps in the grid
Kaiserstraße 185-197, 53113 Bonn
Phone +49 0228 92379-0
Fax +49 0228 92379-29
[email protected]
www.bine.info
BINE-Themeninfo I/2013
3
Fig. 1 Heat pump test rig
Source: FhG-ISE
Performance of heat pumps
Research institutes, manufacturers and operators are measuring the efficiency of heat pump systems during actual operation. Although the same characteristic values are being used,
the results can only be compared with one other to a limited
extent since the balance boundaries and analysis methods
often differ considerably.
The coefficient of performance for heat pumps can in
principle be determined for any possible steady operation
conditions. Since the coefficient of performance considerably depends on the operating conditions, in particular
the temperatures, it should only ever be specified and
considered in relation to the operation conditions.
Dependence of the heating capacity and coefficient
of performance on the temperature
The anticlockwise Carnot cycle provides an ideal reference
cycle for comparing heat pump processes. With the Car-
Fig. 2 Coefficient of
performance and heating
capacity of a brine-water
heat pump in accordance
with the brine input
temperature and the
heating circuit (HC)
output temperature.
Source: FhG-ISE
10
10
8
8
6
6
4
4
2
2
0
–5
0
5
10
Brine input temperature [°C]
HC supply temperature 35 °C
15
20
25
0
30
HC supply temperature 55 °C
Coefficient of performance COP
The coefficient of performance is determined on test rigs
with defined boundary conditions. For example, the
B0/W35 operating point in accordance with EN 14511 is
used as the rated standard operating point for brine-water
heat pumps. This describes the operation with a brine
temperature of 0 °C/-3 °C (input / output) and a heating
circuit temperature of 35 °C/30 °C (output / input). However, the coefficient of performance is still to some extent
specified according to the previously applicable standard,
EN 255, whereby the brine-water heat pumps are measured with the same brine temperatures but with heating
circuit temperatures of 35 °C/25 °C. When calculating the
coefficient of performance in accordance with the standards, not only is the electrical power consumed by the
compressor taken into account but also the electrical
power consumed by the source pump and heating circuit
pump in order to overcome internal pressure losses.
Heating capacity [kW]
In the European project “SEasonal PErformance factor
and MOnitoring for heat pump systems in the building
sector (SEPEMO-Build)”, installation and analysis methods
for field measurements for heat pump systems are being developed that can be used as guidelines. In order
to assess the efficiency of heat pumps, i.e. to determine
their cost-benefit ratio, the coefficient of performance
and the seasonal performance factor are the decisive
characteristic values.
• The coefficient of performance (COP) is determined
in the steady state, i.e. under constant operating
conditions. It indicates the ratio of the heating
capacity to the electrical power consumption
electrical power of the heat pump.
• The seasonal performance factor (SPF) describes the
ratio of the provided thermal energy to the consumed
electrical energy over a longer period of time
(e.g. one year).
4
BINE-Themeninfo I/2013
Monitoring projects from the Fraunhofer Institute for Solar Energy Systems
No. of systems
WP EffizienzApprox. 100 Partners
Duration
BMWi funding,
10/2005 to 09/2010
7 HP manufacturers,
2 energy suppliers
WP im BestandApprox. 70
E.ON Energie AG WP MonitorApprox. 10012 HP manufacturers, EnBW 10/2006 to 12/2009
12/2009 to 05/2013
Fig. 3 Source: FhG-ISE
not cycle, the efficiency is only dependent on the upper
temperature TU and the lower temperature TL between
which the cycle runs (COP_Carnot = To /( To – TL ); T in K).
Even if the coefficient of performance for a heat pump is
considerably lower, its temperature dependence is still
largely comparable with the reference cycle. The respective evaporation and condensation temperatures are
therefore decisive for the efficiency of heat pumps. By way
of example, Fig. 2 shows the coefficient of performance
for a brine-water heat pump relative to the brine input
temperature and the heating circuit (HC) output temperature. The coefficient of performance increases with
a reduction in the temperature difference, i.e. with an
increasing source temperature or a reduction of the sink
temperature.
Fig. 4 System
boundaries for determining the seasonal
performance factor
Source: FhG-ISE
An increase in the evaporation temperature or a reduction
in the condensation temperature causes the coefficient
of performance to improve by between 1.5 and 4 % per
kelvin during normal heat pump operation (operating
points with frosting of air-cooled evaporators are not
taken into account here). The amount by which the efficiency changes partly depends on the thermodynamic
interrelationships: a temperature change of 1 K has a
greater effect with a small temperature lift than with a
larger temperature lift, and a change in the evaporation
HPHS
Underfloor heating
temperature alters the efficiency to a greater extent than
the same change to the condensation temperature.
However, process-based aspects also have an influence.
The heating and cooling capacity of – non capacity controlled – heat pumps is also strongly dependent on the
temperature, and increases with a greater evaporation
temperature and lowers with a greater condensation
temperature (Fig. 2).
Balance boundaries in heat pump systems
When determining the seasonal performance factor for
a heat pump system, different system boundaries can
be defined. By way of example, Fig. 4 depicts three possible system boundaries for a heat pump system that,
with the help of a horizontal ground heat exchanger,
uses the ground as a heat source and provides heat for
space and domestic water heating:
The “narrowest” system boundary (HP) only includes the
energy required by the heat pump unit (compressor, control system and, if required, an oil sump heating system
for the compressor). If the heat source circuit’s ventilator,
brine or well pump is also included in the balancing scope
with supplementary electrical heating when installled
this is described as a heat pump system (HPS). When
balancing both the HP and the HPS, the thermal energy
is determined directly behind the heat pump and/or the
electrical back-up heater. When considering the efficiency
of the entire heat pump heating system (HPHS), only the
effective energy – i.e. behind the storage systems – is
taken into account. In this case the charge pumps are
also incorporated into the calculation as loads.
Heating storage tank
HPS
Hot water
HP
Cold water line
Horizontal
ground heat
exchangers
Heat
pump
DHW tank
Parameters influencing the seasonal performance
factor of heat pumps
Because the temperature has a considerable impact on
the efficiency, the annual performance factor for a heat
pump is substantially determined by the temperature
level on the heat source and heat sink side. There are a
diverse range of factors that influence the operating
temperatures, whereby it is not just the field of application
of the heat pump that plays an important role but also
the planning, installation, commissioning and operating
phases.
BINE-Themeninfo I/2013
In practice
En passant
Single-family home with electric HP
In a new single-family home with 127 m2 of heated
building area and a low heating energy requirement,
a heat pump is used for heating the building and for
providing domestic hot water heating. The device,
which is placed outside, has a heating capacity of
7.5 kW (with A2/W35 as per EN 14511) and uses the
external air as the heat source. Supplementary electric
heating elements, which are installed in both the DHW
and space heating storage tanks, operate in accordance
with individually set parameters. The rooms are heated
using underfloor and wall heating. The towel rail in
the bathroom is also operated with the low heating
circuit temperature. This building is being investigated
as part of the “WP Monitor” (HP monitor) project.
From May 2011 to April 2012, the system achieved an
annual performance factor of 3.3, which is a relatively
high value when compared with the other results in the
“WP Effizienz” monitoring project. The heating circuit
was operated at average temperatures of 35 °C/28 °C.
The DHW storage tank was charged at average tempe­
ratures of 47 °C/42 °C. 84 % of the heat provided was
used for space heating and 16 % to supply DHW. The
heat pump was operated for 2,100 hours during the
year. The two electric back-up heaters were rarely in
operation so that less than 1 % of the heating
requirement for space heating and DHW was
provided using these electric back-up heaters.
Profile: Single-family home
Building type Single-family home
Construction/installation year 2010
Heated building area 127 m2
Energy building class
Low-energy house
Type of heat pump
Electrically driven heat pump
Heating capacity 7.5 kW (with A2/W35 as per EN 14511)
COP
3.7 (with A2/W35 as per EN 14511)
RefrigerantR404A
Application area
Space and DHW heating
Heat source system Air-source – placed outside
Storage system 200-l buffer storage tank
300-l domestic hot water tank
Heating distribution system Underfloor, wall heating,
towel rail
Heating circuit temperature Supply: 38 °C / Return: 33 °C
Operation type
Single energy source
Fig. 5 Source: FhG-ISE
Buffer storage tank (BST)
Underfloor
heating
BST heating element
Heat pump
Domestic hot water
tank (DHWT)
Hot water
(HW)
Air as heat source
HW heating element
Cold water line
Fig. 6 System diagram. Source: FhG-ISE
Fig. 7 Geothermally
heated bridge in
Berkenthin.
Source: H.S.W.
Ingenieurbüro Rostock
Bridge technology
The road surfaces of bridges, particularly in geographically
unfavourable locations such as above water, in gorges, shady areas
and low-lying areas, ice up considerably more quickly in winter than
road sections with direct contact with the ground. This considerably
increases the risk of accidents and forces the winter gritting services
to begin operations at a much earlier point in time, even if the
general road conditions do not require this. Automatic de-icing spray
systems can provide help, but they are controversial for ecological
and economic reasons.
One solution is to use shallow geothermal energy, which has been
tested for the very first time in Germany as part of a pilot project
when resurfacing the Berkenthin road bridge across the Elbe-Lübeck
Canal. A heating coil has been embedded in the asphalt surface of
the bridge. Besides heating the road surface in winter it is also
possible to cool the asphalt in summer, which thus reduces the risk
of rutting and lengthens the service life of the surface material. The
road surface is heated and cooled via a heat pump that is connected
to a single well system.
Fig. 8 Road surface heating prevents black ice.
Source: H.S.W. Ingenieurbüro Rostock
5
6
BINE-Themeninfo I/2013
Fig. 9 Storage and distribution system in a logistics centre.
Source: Bosch Thermotechnik GmbH, Wetzlar
• The field of application of a heat pump is limited to a
certain extent by the choice of heat pump technology
(e.g. the type of heat source). In addition there are
boundary conditions and limits in terms of the required
sink temperatures: for example there are relevant
differences between their use in old buildings that
have not been refurbished and in new buildings. In
new buildings with underfloor heating, the heating
operation differs considerably from the operation for
domestic hot water heating.
• Through their choice and size of heating system,
design engineers determine the required heating circuit
temperatures within the framework provided by the
heating requirements and the spatial conditions.
• Careful installation, professional commissioning and
controlled operation help to maintain the planned
operating temperatures and adapt to any deviating
requirements in practice. For example, a non-adjusted
heating curve could mean that the system is operated
with heating circuit temperatures that are higher than
required. An unfavourable positioning of the storage
temperature sensors can cause the storage tanks to
be incorrectly charged, particularly with combined
Fig. 11 Greenhouse gas
emissions
(without taking
refrigerant losses
into account) as well
as the primary energy
requirement for heat
pumps and gas-fired
condensing boilers.
Source: FhG-ISE
Ground source heat pump
Air source heat pump
Fig. 10 Heat pumps in an apartment building in Augsburg. Source: Bundesverband Wärmepumpe e. V., Berlin
storage tanks: the heat pump then generates more
energy at the high domestic hot water temperature
level than is required. Not completely closing 3-way
vents and missing check valves can cause undesired
discharging of the domestic hot water storage tank.
In addition to aspects that influence the operating temperature, the auxiliary energy also, of course, has to be
taken into account.
Assessment of heat pumps
The coefficients of performance of heat pumps enable different heat pumps from various manufacturers to be compared with one another; of course under the assumption
that the coefficients of performance have been determined
under the same boundary conditions. Likewise, a comparison of the results from different field tests is only possible to a limited extent if they have not used precisely
the same balance boundaries and analysis methods. In
addition to the issue as to where the system boundaries
were defined, there are also other aspects that are relevant.
Annual performance factor: 4.9 ...... 3.9 .......................... 2.6
Annual performance factor: 3,4 ...... 2,9 ............................ 2,1
Annual utilisation rate: 108 % ... 96 % ...... 85 %
Condensing boiler
(natural gas)
50
100
150
200
0
Greenhouse gas emissions in g CO2 equivalent per kWh heat energy
Ground source heat pump
Air source heat pump
250
300
350
Annual Performance factor: 4.9 ...... 3.9 ....................... 2.6
Annual Performance factor: 3.4 ...... 2.9 ........................ 2.1
Annual utilisation rate: 108 % ... 96 % ..... 85 %
Condensing boiler
(natural gas)
0
0,2
0,4
Primary energy requirement
0,6
0,8
1,0
1,2
1,4
1,6
BINE-Themeninfo I/2013
Fig. 12 HP storage system with measurement technology.
Source: Vaillant Deutschland GmbH & Co. KG, Remscheid
For example, when calculating the seasonal performance
factor it makes a difference whether unused heating energy is taken into account that was produced in summer
as a result of the system or as a result of faulty operation.
It is also only possible to compare the same balancing
periods with one another (e.g. one year).
In classifying the seasonal performance factor information, not only do the balance boundaries and the balance periods need to be specified but also the type of
heating source, the application area (e.g. building standard, heating systems, ratio of the heating requirement
to the domestic hot water requirement) and the operating
temperatures. Quite often only the supply temperatures
are specified as operating temperatures in the heating
circuit. However, these are not the only ones that are
decisive for the condensation temperature. The return
temperature also has an impact.
This can be clearly seen when observing the course of
temperatures in a condenser. For example, there is a
greater condensation temperature at 35/30 than at 35/25.
This is also shown by measurements made by the NTB
Buchs testing centre in Switzerland. Standard-based
measurements according to EN 14511 and EN 255 were
conducted for 13 air-water heat pumps and 19 brine-water
heat pumps, whereby the COP for air source heat pumps
with a heating circuit temperature of 35/30 was on average
7 % lower than with 35/25, and brine source heat pumps
were on average 6 % lower. This therefore shows that
the supply and return temperatures (or alternatively one
temperature and the spread) should always be specified.
If it is easier to only use one operating temperature in
the heating circuit when comparing several systems, it
is better to use the mean heating circuit temperature
than the heating circuit supply temperature.
Fig. 13 Underfloor heating in a construction project in Duisburg.
Source: Thomas Lienemeyer, Mülheim a. d. Ruhr
Fig. 11 compares the primary energy requirement for heat
pumps and gas-fired condensing boilers for different
annual performance factors and annual utilisation rates.
The spectrum of annual performance factors is based
on the “WP Effizienz” (HP efficiency) and “WP im Bestand”
(HP in existing buildings) monitoring projects. The data
for the condensing boilers was determined using 60 gasfired condensing boilers as part of the “Felduntersuchung:
Betriebsverhalten von Heizungsanlagen mit Gas-Brenn­
wertkesseln” (Field test: Operating behaviour of heating systems with gas-fired condensing boilers) project.
The underlying specific primary energy factors for electrical energy (2.35) and natural gas (1.12) reflect the respective final energy generated in Germany in 2010. The
comparison shows that, with correct configuration and
operation, heat pumps can save considerable primary
energy compared with gas-fired condensing boilers. For
example, ground source heat pumps with a annual performance factor of 3.9 (mean value in the “WP Effizienz”
project) save 49 % of the primary energy in comparison
with gas-fired condensing boilers with an annual utilisation rate of 96 %. And air source heat pumps with an
annual performance factor of 2.9 (mean value in the
“WP Effizienz” project) even save 32 %. However, this also
shows that – depending on the efficiency of the reference
systems – only very slight or no savings can be made
with heat pumps with very low annual performance factors.
Assessment of heat pumps in comparison with
other heat generators
The greenhouse gases generated by the final energy requirements for heat pumps and condensing boilers are
depicted in Fig. 11. In comparison with condensing boilers, heat pumps show a similar potential for saving
greenhouse gas emission when generating heat as they
do with primary energy. For example, ground source
and air source heat pumps with respective annual performance factors of 3.9 and 2.9 achieve reductions in
greenhouse gas emissions during operation that amount
to 43 % and 23 % respectively. However, greenhouse gas
emissions that are caused by refrigerant losses were
not taken into account in these values.
Different characteristic values can be taken into consideration when directly comparing heat pumps with other
energy generators, e.g. gas-fired condensing boilers. The
following section describes the primary energy requirement needed for space heating and domestic hot water
heating and the resulting greenhouse gas emissions.
Refrigerant emissions can occur along the entire process
chain, ranging from the manufacture and application
(i.e. in this case during the heat pump operation) to the
disposal. The contribution made by the refrigerant to
greenhouse gas emissions as a result of heating with a
heat pump therefore depends on the respective refrig-
7
8
BINE-Themeninfo I/2013
Fig. 14 Installation of ground source pumps in a new-build scheme in Mülheim a. d. Ruhr.
Source: Thomas Lienemeyer, Mülheim a. d. Ruhr
erant losses and the type of refrigerant used. The German Federal Environment Agency specifies a value of
2.5 % of the refrigerant charge per year for refrigerant
losses from heat pumps used for heating in Germany.
This value represents a mean value and covers both
“gradual” emissions as well as emissions caused during
servicing and accidents, and is based on an average
service life expectancy of 15 years. The disposal is
based on an average recycling rate of 70 %.
How this effects greenhouse gas emissions shall be
shown more specifically using the example of a standard air source heat pump for two different refrigerants.
It is assumed that a building has a 160-m² living space with
a specific annual heating requirement of 70 kWh/m2
p.a. for space heating and 17 kWh/m2 p.a. for domestic
hot water heating. The heat pump contains 3 kg of refrigerant and the heating capacity amounts to 7.5 kW
with an annual performance factor of 2.9.
If the R407C refrigerant is deployed, which is the refrigerant most widely used in new heat pumps deployed for
heating (global warming potential (GWP) value = 1.774),
the greenhouse gas emissions caused by the refrigerant
losses increase by 9 %. They increase by 19 % if the rarely
applied R404a refrigerant is used. R404a has the highest
GWP value (3.922) of all hydrofluorocarbons deployed
in heat pumps used for heating.
This shows that, when the assumed refrigerant losses
are also taken into consideration, heat pumps only have
8 % to 16 % less greenhouse gas emissions than gas-fired
condensing boilers.
Current coefficients of performance of heat pumps
The quality label for heat pumps awarded by the European Heat Pump Association (EHPA) certifies the quality
of heat pumps based on technical, design and servicespecific quality guidelines, whereby heat pumps used
for heating purposes have to achieve sufficient minimum values for the coefficients of performance (measured according to EN 14511). Many of the devices
available on the market achieve these threshold values
(Fig. 15).
Another reference value for classifying the efficiency of
heat pumps is provided by the threshold values for the
annual performance factor, which are used for subsidising heat pumps. The current funding guidelines from
the German Federal Office of Economics and Export
Control (BAFA) stipulate that only heat pumps used in
existing buildings (constructed before 2009) may be
subsidised. The calculated annual performance factors
must achieve values of 3.8 for brine-water heat pumps
and 3.5 for air-water heat pumps.
Coefficients of performance (COP) of heat pumps at the rated standard operating point as per EN 14511
Type
Operating point Minimum COP value for EHPA quality label
COP of heat pumps
on the market
Brine-waterB0/W35
4.34.0 … 5.0
Water-waterW10/W35
5.15.0 … 6.0 (6.5)
Air-waterA2/W35
3.13.0 … 4.0 (4.4)
Fig. 15 Source: FhG-ISE
BINE-Themeninfo I/2013
In practice
School building
with thermal heat pump
The heating system in the school building,
which was built in 1907 in Plaidt, underwent
refurbishment in 2010, whereby two electric heat
pumps with the ground as the heat source were
replaced with a newly developed gas-absorption
heat pump. This also uses the ground as the
heat source. The 1,500-litre buffer storage
tank was not replaced. To cover peak loads,
a gas-fired condensing boiler has been installed
instead of the electric heating element. The heat
is transferred to the building via radiator-based
heating. Hydraulic balancing was carried out to
ensure that the heat, which is generated highly
efficiently, can continue to be distributed
effectively and to keep the level of the heating
circuit supply temperature as low as possible.
Fig. 16 The new gas absorption heat pump for the school building in Plaidt.
Source: E.ON Energie AG, Munich
Gas-fired
condensing
boiler
Heating
HM
Buffer
storage
tank
~1,500 l
HM
VGas~2.7m³/h
Results
During the 2011 / 2012 heating season, the gas
heat pump achieved an annual utilisation rate of
137.5 %. The annual utilisation rate for gas heat
pumps is the ratio of the heat volume released
by the gas heat pump to the heat volume added
from the natural gas and is therefore not directly
comparable with the annual utilisation rates
generally specified for condensing boilers.
The system was operated for 2.335 hours during
the year.
Savings
According to measurements made across
a heating season, replacing the old electric
heat pumps with a new, natural gas-based
absorption heat pump/gas-fired condensing
boiler system has enabled energy costs
savings of 39 % and a CO2 reduction of 44 % for
the overall system (GHP, central heating). This
is shown by the billing figures from the local
energy supplier.
Adjustable
HC control
Pel
VBrine ~3,000 l/h HM
Gas
heat
pump
HM
VHZ ~3.000 l/h
Borehole heat
exchanger
Fig. 17 System diagram for the pump system of the school building.
6,000
[kg CO2/p.a.]
[€/p.a.]
Source: E.ON Energie AG, Munich
5,000
Profile: School in Plaidt
25,000
20,000
4,000
15,000
3,000
10,000
2,000
5,000
1,000
0
EHP
GHP
0
EHP
GHP
Fig. 18 Comparison of the costs and CO2 emissions per year between electric heat
pumps (EHP) and gas heat pumps (GHP). Source: E.ON Energie AG, Munich
Building type School
Year of construction 1907
Installation year 2010
Heated building area 840 m2
Energy building class Old building
Type of heat pump
Gas absorption heat pump
Application areaHeating
Heat source/sink Ground (borehole heat
exchanger 12 * 100 m)
Heating distribution system Radiator-based heating
Design heating temperature Supply: 70 °C / Return: 50 °C
Operation type
Single energy source (natural gas)
Refrigerant R717
Supplementary Gas-fired condensing
heating system boiler (50 kW)
Fig. 19
Source: E.ON Energie AG, Munich
9
10
BINE-Themeninfo I/2013
Fig. 20
1 Lorem
Measuring
ipsum dolor sit
amet,
a heatconsectetuer
pump as partadipisof the
cing Effizienz”
“WP
elit. Aenean
project.
commodo
ligula eget
Source:
FhG-ISE
dolor. Aenean
massa.
Efficiency under real conditions
In three broad-based monitoring projects, the Fraunhofer Institute for Solar Energy Systems
has investigated the efficiency of electrically driven heat pump systems for space heating and
domestic hot water heating in old and new buildings. These field tests have not only determined
the efficiency achieved in actual operation but have also investigated the operating conditions
and analysed the system behaviour. This was used for assessing the respective systems and
for determining potential for optimisation.
The monitoring projects are based on recording heat
and electrical energies as well as volume flows and temperatures at one-minute intervals, and also include the
daily retrieval of remote data and its storage and evaluation at the institute. The generated thermal energy is
balanced directly after the heat pump – separately for
the space heating and domestic hot water heating. To
calculate the seasonal performance factor, the thermal
energy is divided by the energy input of the electrical
components. This takes into account the compressor
and control system of the heat pump as well as the motor
of the heat source circuit’s fan, brine or well pump and
the electrical back-up heater..
As part of the “WP Effizienz” project, Fraunhofer ISE
measured around 100 heat pumps in newly built singlefamily homes that had a specific heating consumption
between 30 and 150 kWh/m2 p.a. In a similar project
(“WP im Bestand”), Fraunhofer ISE investigated heat
pumps in existing buildings that had not been or only
partly refurbished. Here around 70 systems were measured that had been installed as a replacement for oilfired boilers.
Seasonal cycle of performance factors
The efficiency that a heat pump can achieve in operation
is largely determined by the temperature level on the
heat source and heat sink side. The smaller the difference between the heat source and the heat sink, the
higher the efficiency of the heat pump. This correlation
becomes clear when examining the monthly performance
factors for the “WP Effizienz” project.
Fig. 22 depicts the supply temperatures for the heat
sink circuit (separated for space heating and domestic
water heating) and the monthly performance factors
during the course of a year (July 2009 to June 2010) as a
mean value for all evaluated ground-to-water heat
pumps. It can be clearly seen how the monthly performance factors fluctuate across the course of the year.
These reflect the changing operating conditions. Due to
the almost exclusively used radiant heating, the supply
temperatures for space heating achieve an average of
36 °C, whereas the hot water operation achieves an average value of 51 °C.
During the course of the year, the ratio of heating energy
that is respectively provided for space heating and domestic water heating varies in accordance with the
heating demand for the particular months. During the
core heating season, an average of around 10 % was
used for hot water heating in the systems investigated,
which is a rather low value for new buildings. The
weighted mean value of the heat sink temperature is
therefore close to the temperature for space heating in
the winter months and close to the temperature for domestic hot water generation during the summer months.
The source temperatures (Fig. 29) also show seasonal
variations. However, the higher brine temperatures in
the summer months cannot “compensate” for the high
sink temperatures. In addition to the higher temperature
lift in the summer months, the increased proportion of
electrical energy used by the control systems as a result
of the short operating times of the heat pump also has
a negative effect on the seasonal performance factor.
The mean monthly performance factors in July/August
amount to just over 3.0 and increase at the beginning of
BINE-Themeninfo I/2013
Comparison of the monitoring projects
Research project
WP Effizienz
WP Monitor
Project partners and E.ON Energie AG
project funding
WP im Bestand
7 HP manufacturers,
2 energy suppliers, BMWi
12 HP manufacturers,
EnBW
Project duration
10/2006 – 12/2009 10/ 2005 – 09/ 2010 12/ 2009 – 05/ 2013
Rated HP heating capacity
(min … mean … max)
5 kW … 14 kW … 37 kW
6 kW … 9 kW … 17 kW
5 kW … 9 kW … 17 kW
Ground source heating
3656
47
Air source heating
3418
34
Heating distribution system
Surface heating: 3%
Surface heating: 94%
Surface heating: 88%
Radiators: 71%Radiators: 1%Radiators: 4%
Combined: 26%Combined: 5%Combined: 8%
in the four combinations comprising old and new buildings and air and ground as the heat source. As described above, the ground source heat pump systems
in the “WP Effizienz” project were operated with an average annual performance factor of 3.9, whereas the
ground source heat pump systems in the “WP im Bestand”
The investigated air-to-water heat pumps are also installed
in buildings that on average show a similarly high proportion of space heating with low heating circuit (HC)
temperatures. This means that there are comparable
conditions on the heat sink side (Fig. 22). However, there
is a considerable difference in the monthly performance
factors compared with ground-to-water heat pumps.
Whereas with ground source heat pumps the heating
seasons can be clearly recognised by the greater monthly
performance factors, it is particularly during the winter
months that the air source heat pumps work most inefficiently, since only low source temperatures are available
to the heat pumps. For example, the lowest average
monthly performance factor (2.6) is in January – which
correlates with the outside temperature. Although the
air temperatures are considerably higher in the summer
months, the weighted mean sink temperatures are also
higher due to the high share of domestic hot water. The
highest monthly performance factors can therefore be
found in the transitional periods (November 2009 and
May 2010 with 3.2). When the outside temperature is
slightly below the heating limit, the average heat sink
temperatures are lower than in summer due to the low
temperatures for the space heating operation, whereas
the outside temperatures are still relatively high. As with
ground-to-water pumps, the “standby consumption” of
the heat pumps in the summer months with shorter operating times has a greater (and therefore negative) effect
on the monthly performance factor than in the months
with longer operating times. The average annual performance factor is 2.9.
Fig. 22 Average seasonal performance factors (SPF) for all systems.
Above: Air source heating, below: Ground source heating.
Source: FhG-ISE
Comparison of old and new buildings
Fig. 25 compares the respectively achieved annual performance factors for the two projects. The diagram shows
the average annual performance factors determined
across the described evaluation periods for all systems
4.5
56
4.0
49
35
3.0
28
2.5
21
2.0
14
1.5
1.0
[seasonal performance factor]
42
3.5
7
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
2009/2010
0
4.5
56
4.0
49
3.5
42
35
3.0
28
2.5
21
2.0
14
1.5
1.0
7
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
2009/2010
seasonal performance factor
HC supply temperature
0
DHW supply temperature
Weighted average supply temperature for space and DHW heating
Proportion of heating/domestic hot water energy
[°C]
[seasonal performance factor]
the heating season to values above 4.0. During this
period the heat pump benefits from the still regenerated ground and the lower average sink temperatures.
During the core heating season, the monthly performance factors drop slightly to below 4.0. Considering
the whole year, the average annual performance factor
(SPF) was 3.9.
[°C]
Fig. 21 Source: FhG-ISE
11
12
BINE-Themeninfo I/2013
Fig. 23 Schematic showing an air-water heat pump.
Source: Fig. 23 – 24 Bundesverband Wärmepumpe e. V., Berlin
Fig. 25 Bandwidths
for the annual performance factors determined in the field test for
ground source and air
source heat pumps in
new and old buildings.
Source: FhG-ISE
Fig. 24 Schematic showing
a ground-water heat pump
project achieved an average annual performance factor
of 3.3. This reflects the installed heat distribution systems:
the new buildings are almost exclusively equipped with
underfloor heating whereas the space heating in most
of the old buildings is provided using radiators. With an
average of 54 °C, the heating supply temperatures in
the old buildings were almost 20 K above those in the
new buildings. By trend, the same difference between
the two projects is also shown with the measured air
source heat pumps: whereas an average annual performance factor of 2.9 was achieved in the new buildings,
the average annual performance factor with the old
buildings was 2.6.
the core heating season as a result of a generously
sized borehole heat exchanger. In the case of the old
buildings, the ground source system with the lowest efficiency achieved an annual performance factor of 2.2
and the system with the highest efficiency, which is located in a comprehensively refurbished building with
underfloor heating and is not used for domestic water
heating, has an annual performance factor of 4.7. The
annual performance factors for the air source heat
pumps cover roughly the same range with both the existing and new buildings and extend from 2.1 to 3.3 and
from 2.3 to 3.4 respectively.
The examination of the mean values only reveals the
general tendency of the differences between the two
projects. Within each project, however, there are some
considerable differences in the annual performance
factors of the individual systems. For example, although
three quarters of the ground source heat pumps in the
“WP Effizienz” project achieved annual performance
factors in the range of +/– 10 % around the project mean
value, individual values for the annual performance factors ranged between 3.0 and 5.2. The highest value was
achieved by a system that on the one hand was operated
with below average sink temperatures and on the other
hand also maintained high source temperatures during
Use of electrical back-up heaters
Old buildings (no: 34)
2.6
New buildings (no: 18)
2.9
Old buildings (no: 36)
3.3
New buildings (no: 56)
3.9
1.0
1.5
2.0
2.5
Annual Performance factor
3.0
3.5
4.0
4.5
5.0
5.5
Average PF for air source heat pumps
Bandwidths
Average PF for ground source heat pumps
Extreme values
Fortunately, the use of electrical back-up heaters is very
low in most of the measured systems. The results from the
“WP Effizienz” project for the year 07/2009 – 06/2010
are presented here. In the case of ground source systems, the majority of the 56 plants did not activate the
electrical back-up heaters at all. And only around 10 %
of the systems provided more than 1 % of the thermal
energy for space heating and domestic water heating
with an electrical back-up heater. The detailed evaluation
of the activities of the electric back-up heaters indicates
that these activities were at least partly due to a deliberate
use of the electric back-up heaters to provide support
while drying out the building, unfavourable parameterisation of the control systems or short-term heat pump
failures.
As expected, air source heat pump systems provided a
larger share of the heat with electrical back-up heating.
Here around 40 % of the 18 systems provided more than
1 % of the thermal energy with electric back-up heaters.
However, with the exception of one system, less than
5 % of the heat was provided by electric back-up heaters.
Whereas with ground source systems the heating element
activity does hardly correlate with the outside temperatures, this correlation is evident in most air source heat
pumps. In almost all investigated systems, activity of
back-up heaters was detected during the coldest
months. Some systems activated the electric back-up
heater also during the hot summer months. With extremely high outside temperatures, operational conditions can lie outside the characteristic diagram of the
BINE-Themeninfo I/2013
Source: FhG-ISE
Fig. 27 Test rig.
Source: FhG-ISE
compressor. Then, the heat pump is not activated and the
electric back-up heater takes over the hot water heating.
Electrical energy consumption for fans and pumps
The components on the heat source side, i.e. pump or fan,
accounted for a very similar share of energy consumption
for both heat sources .The ground source heat pumps
required an average of 5.9 % of the heat pump’s electrical
energy consumption (without electric back-up heaters)
to drive the source pump. The bandwidth for the individual systems was quite evenly spread in a range between
2 and 10 %; in one case the share amounted to 12 %. The
reasons for this are diverse and range from the different
sizing of the source systems and operating outside of
the design flow rates to different pump efficiencies at
the respective operating point.
Fig. 28 Online visualisation of operating data.
Source: FhG-ISE
Supply and return temperature [°C]
Fig. 26 Installed measurement technology.
13
25
2007/08
2008/09
2009/10
20
15
10
5
0
–5
May
Jan
Mar
Nov
Jul
Sep
May
Jan
Mar
Nov
Jul
Sep
May
Jan
Mar
Nov
Sep
Jul
– 10
With air source heat pumps, the fans required 6.7 % on
average. The bandwidth for the individual systems is
comparable to the source pump for ground source systems and ranges from 2 to 11 %.
External temperature (source: DWD)
Borehole heat exchangers
Horizontal ground heat exchangers
Brine as a heat source
In the measured ground source heat pump systems,
borehole heat exchangers or horizontal ground heat exchangers were mostly used as heat source systems.
Special solutions (such as geothermal energy baskets)
were the exception.
The systems with borehole heat exchangers show different characteristics in terms of the seasonal course of
the brine temperatures than systems with horizontal
ground heat exchangers. In Fig. 29, the average courses
of the supply and return temperatures for all evaluated
systems are depicted separately for these two types of
heat source systems. In addition, the external temperature
is depicted as a mean value of the different locations in
Germany. The diagram shows that systems with borehole
heat exchangers have on average considerably less
fluctuating brine temperatures during the course of the
year than systems with horizontal ground heat exchangers: the ground output temperature of the systems with
borehole heat exchangers – here shown as the supply
temperature – dropped to 4 °C during the core heating
season and increased in the summer months to 13 °C.
With the systems with horizontal ground heat exchangers,
the brine supply temperature dropped at the beginning
of the heating season to below the corresponding temperature for systems with borehole heat exchangers. It
then exceeded this temperature once again from June
when the upper earth layers heated up. This is reflected
in the seasonal performance factors. In the summer
months and at the beginning of the heating season, the
systems with horizontal ground heat exchangers work
on average with higher seasonal performance factors
than the systems with borehole heat exchangers. The
effect reverses in the core heating season and in spring.
The greater provision of energy in winter compared with
summer meant that the average annual performance
factor for the systems with borehole heat exchangers
was higher than the average annual performance factor
for systems with horizontal ground heat exchangers.
When viewed across the balancing period, the heat
Fig. 29 Brine supply
and return temperatures
averaged over the
evaluated systems.
Source: FhG-ISE
14
BINE-Themeninfo I/2013
Efficiency versus effectiveness
When assessing heat pumps, the focus is normally
on the seasonal performance factor. However, is this
characteristic value always the right parameter for
assessing heat pump systems? The efficiency of the
heat generation does not indicate whether the heat
was appropriately generated and how much electrical
energy was used. This is where the term effectiveness
helps, which encompasses the actual goal, namely
saving heating and primary energy.
To clarify this aspect, two systems from the “WP Effizienz” monitoring project will be presented, which are
demonstrated in Fig. 30. The first example shows a
ground source heat pump system that achieved an
annual performance factor of 4.2. It provided a specific
heat energy of 100 kW/m2 p.a. for space heating in
the building and 21 kW/m2 p.a. for heating domestic
water. This results in a specific primary energy consumption of 74 kWh/m2 p.a. The second example
shows an air-water heat pump in a better-insulated
building; the heat energy consumption was only
around half with 47 kWh/m2 p.a. Owing to the heat
source used, the efficiency of the heat pump with 3.3
is considerably lower (although above average in
the context of the measured air source heat pump
systems). However, there was a roughly 30% lower
specific primary energy consumption (57 kWh/m2 p.a.)
for covering the energy requirement for space heating
and domestic water heating. It can therefore be determined that in the first example an efficient heat
pump is working in a – from an energy viewpoint –
less effective overall system, whereas in the second
example there is a more effective overall system despite the lower efficiency of the heat pump.
This observation is also relevant when evaluating heat
pump systems in increasingly energy-efficient newbuild schemes that have surface heating systems
such as underfloor and wall heating. The large difference in heat sink temperatures during the heat
pumps’ space heating and domestic water heating
operation means that the ratio of the space heating
to the hot water requirement is increasingly affecting
the efficiency of the heat pumps. A heat pump in a
building built to the passive house standard works
for almost half of its operating time to produce hot
water, i.e. at an operating point that only allows low
efficiency. That has a negative effect on the annual
performance factor. However, the heat pump system
requires less electrical energy than a building equipped with the same heat pump, space heating and
domestic hot water system but which has greater
heating energy requirements as a result of its lower
insulation standard.
Ground-water heat pump
3,706 kWh p.a. electrical energy
Heating energy consumption
100 kWh/(m² p.a.)
APF = 4.2
12,980 kWh p.a. heating energy
12,009 kWh p.a.
environmental energy
2,735 kWh p.a. DHW heating
Primary energy consumption
74 kWh/(m² p.a.)
* Primary energy factor = 2,6
Air-water heat pump
3,520 p.a. electrical energy
Heating energy consumption
47 kWh/(m² p.a.)
APF = 3.3
7,511 kWh p.a. heating energy
7,956 p.a.
environmental energy
3,965 kWh p.a. DHW heating
Primary energy consumption*
57 kWh/(m² p.a.)
* Primary energy factor = 2,6
Fig. 30 Energy balance of two heat pump systems for space heating and domestic water heating (balance year: 2009).
Source: FhG-ISE
pumps with borehole heat exchangers were operated
with an average supply temperature of 7 °C and the
systems with horizontal ground heat exchangers were
operated with a supply temperature of just under 4 °C.
The average return temperature for the heat pumps was
almost 4 K beneath the supply temperature.
The brine temperatures that occur during operation are
dependent on a diverse range of factors. In the case of
heat pumps with borehole heat exchangers, a comparison
of the lowest weekly average temperatures for brine
during the observation period from April 2009 to March
2010 shows that the individual systems differ by up to 8 K.
With the “least favourable” system, the weekly average
temperature for the supply (return) dropped during operation to 1 °C (–3 °C); with the “most favourable” system,
on the other hand, it only dropped to 9 °C (5 °C).
When designing borehole heat exchangers in singlefamily homes, a parameter which is often used is the
BINE-Themeninfo I/2013
Fig. 31 Monitoring the building services equipment with an App.
Source: Viessmann Werke GmbH & Co. KG, Allendorf
Fig. 32 Prototype of a lamellar heat exchanger.
Source: FhG-ISE
specific extraction capacity, which is the extraction capacity relative to the overall length of the borehole heat
exchanger(s). The relevant guidelines (VDI 4640) stipulate three general reference values in accordance with
the ground used by the heat pumps for space heating
and domestic water heating: 20 W/m with poor subsoil,
50 W/m with normal bedrock and water-saturated sediment as well as 70 W/m for bedrock with high thermal
conductivity. Fig. 33 depicts the measured brine return
temperatures for individual systems relative to the respective average specific extraction capacity (determined
from the measurement values for the annual energy extraction and the running period as well as the total
length of the borehole heat exchangers). The bandwidth
of the measured average extraction capacities ranges
between 30 W/m and 64 W/m. On the one hand this
shows that the systems with the highest brine temperatures in the heating season were operated with rather
low extraction capacities. However, this also clearly shows
the wide bandwidth of temperatures for those systems
that had the same specific extraction capacities. It makes
clear that in addition to the specific length of the respective borehole heat exchanger system, other factors
also have a considerable influence such as the subsoil
properties (mean temperature of the undisturbed soil,
soil quality, occurrence of ground water), the borehole
heat exchanger (in particular the filling) and the operation
(flow regime, operational duration). Because no information about the ground was available in the project,
the respective effects of the influencing factors cannot
be investigated in more detail.
In contrast to the previous two projects, measurements
are also being conducted on ground source heat pump
systems that use a direct evaporator instead of a heat
source circuit filled with a water-glycol mixture. In addition, capacity-controlled heat pumps are also now being investigated in line with market developments. The
partners in the “WP Monitor” project include twelve
German and Austrian manufacturers and an energy
supplier.
“WP Monitor” is the title of a current field test that is being
conducted on heat pumps by Fraunhofer ISE. Around
100 heat pumps are being measured in the project,
whereby around 50 systems are being taken over from
the “WP Effizienz” project. The extension of the monitoring period makes it possible to generate a comprehensive database for long-term investigations. Measurement data can be collected across a period of up to
six heating seasons, which for example improves the
possibilities when investigating the ground as a heat
source.
Fig. 33 Brine return temperature (minimum weekly average temperature)
and average specific extraction capacity for 27 heat pump systems in the
observation period from April 2009 to March 2010 in the “WP Effizienz” project.
Source: FhG-ISE
Min. brine return temperature (weekly average) [°C]
“WP Monitor”: Monitoring project
with online display of measurement data
All those interested in heat pumps have been given the
possibility to follow the operation of real heat pump
systems. For this purpose the project homepage (www.
wp-monitor.ise.fraunhofer.de/english/index/index.html)
provides free access to the measurement data for anonymised heat pump systems. In addition to a description of the systems, all measurement parameters are
shown as daily values and the most important energy and
seasonal performance factors are shown as monthly
values.
6
5
4
3
2
1
0
–1
–2
–3
–4
20
30
40
Specific extraction capacity [W/m]
50
60
70
15
16
BINE-Themeninfo I/2013
Fig. 34 Scroll compressor.
Source: Viessmann Werke
GmbH & Co. KG, Allendorf
Capacity-controlled heat pumps
The heating capacity of heat pumps with on-off controlled compressors
increases with rising heat source temperatures. Particularly air source heat pumps
achieve the highest heating capacities when they are not actually needed. Here, the
number of cycles increases while operation times decreases, which is non-beneficial
for such a heating system in terms of lifetime and efficiency. One possibility to
counter this is to use capacity-controlled compressors.
The current state-of-the-art technology in the output range
for single-and two-family houses is the so-called scroll
compressor, which has almost completely replaced the
reciprocating compressor. With the scroll technology, the
compression takes place between two meshed spirals.
Irrespective of the type of compressor, until now machines
have been mostly used whose compressor capacity
only depends on the pressure or temperature level on
the evaporator and condenser side respectively. With
these on-off controlled compressors there is a direct
correlation between the heat source temperature and
the heating capacity. Particularly with air source heat
pumps, this means that the high heating capacity is
provided precisely when the space heating requirement
is low. The heating capacity of the heat pump and the
heat load of the building only correspond at the design
point. For the operation this means an increase in the
cycles with increasing external temperatures. Numerous
start-ups will reduce the compressor’s durability. At the
same time, short cycle times cause longer operation under
start-up conditions, which reduces the efficiency.
In order to ensure continuous operation, heating buffer
storage systems can be used or minimum running or
resting periods can be set in the control system for the
compressor. Furthermore, the monovalent use of heat
pumps with an unregulated heating capacity (i.e. systems
with single heat generators) is more disadvantageous
than with conventional heating systems, particularly
when external air is used as the heat source. This would
suggest the use of an auxiliary system to cover peak
loads. In general this is realised by equipping heat
pumps with electric back-up heaters that are inefficient
in primary energy terms. It is possible to regulate the heat
load by on-off controlled heat pumps using capacity
stages or a rack of compressors.
There are a diverse range of possibilities for actively
matching the compressor’s heating capacity to the heat
load. Control systems that require intervention in the
cooling circuit and involve losses (e.g. suction pressure
or hot gas bypass control systems) play very little role in
practice; the building sector actually uses variable-speed
control or Digital Scroll™ technology to adjust heating
capacity.
The almost exclusively used variable-speed control systems are also known as inverters and have already been
successfully used for many years in air-conditioning
technology. Using power electronics, a rotation speed
corresponding to the current heat requirement is set in
the drive motor. The great advantage of the inverter
technology is the high COP in the partial load range. The
disadvantage of this technology is the continual energy
required by the electronics.
The Digital Scroll™ compressor works according to the
same basic principle as the on-off controlled scroll compressor. The difference is that the Digital Scroll™ can
also be operated in the so-called “unloaded” or “idle”
state, whereby the motor continues to work at constant
speed while the orbital scroll is disengaged. The compressor capacity is a result of the ratio of the running
time in the unloaded state to the duration of a heat
pump operation cycle. The disadvantage of this technology is that the capacity can only be modulated by
losing efficiency.
BINE-Themeninfo I/2013
Im Portrait
Refrigerants
In the past, ozone layer-depleting refrigerants have
been mostly replaced with hydrofluorocarbons (HFCs),
which, however, still harm the climate. For this reason,
natural refrigerants with low global warming potential
are becoming increasingly significant. For example,
propane (R290) and propylene (R1270) could replace
HCFCs and HFCs in heat pumps since they have similar
thermodynamic properties. Their flammability, however,
means that they are subject to several safety-related
requirements, such as regarding the positioning and
ventilation of the systems, the components used and
possible safety devices. In Germany there are only a
few heat pumps with these refrigerants on the market.
This is partly due to the product liability risk borne
by the component manufacturers as a result of partly
unclear legal provisions combined with the low sales
potential. Their area of application is mostly limited
to heat pumps used for heating domestic water with
a refrigerant charge of less than 150 g and externally
placed air-water heat pumps. Significant research
activities are carried out with the aim ofminimising
the inner volume of components and to introduce
new safety components. The BMU research project
“Ersatz fluorierter Treibhausgase durch natürliche
Kältemittel” (Replacement of fluorinated greenhouse
gases with natural refrigerants) supports the market
introduction of natural refrigerants.
From an ecological and safety point of view, carbon
dioxide (CO2 ) is an almost ideal refrigerant. It is
cheap, chemically inert, neither toxic nor flammable
and has compared to almost all hydrofluorocarbons
a negligible global warming potential. However, it
requires considerable operating pressures that place
particular requirements on technical components
such as compressors and heat exchangers. A unique
thermodynamic feature is its low critical temperature
of 31 °C. With high sink temperatures the refrigerant
no longer condenses but cools with a “strongly sliding”
temperature (isobaric). CO2 is therefore particularly
suitable when there is a large temperature spread
between the supply and return line. Examples of this
include domestic water heating as well as simultaneous
hot water heating and low-temperature heating via
two heat exchangers in series and air heating systems.
One manufacturer currently offers CO2 heat pumps
with a small heating capacity (5 and 9 kW) on the
German market. Small heat pumps that would be
suitable for the passive house sectors are not yet
available.
Researchers at TU Braunschweig have investigated the
heat supply for zero-energy and passive houses.
CO2 heat pumps with a small heating capacity and a
connected stratified storage tank demonstrated potential
energy savings relative to conventional heat pumps.
Refrigerant research is also investigating new
materials and material mixtures for cooling-based
applications. The “Low-GWP-Kältemittel“ (Low-GHP
refrigerant) project, which is being conducted by
the Institute of Air Handling and Refrigeration (ILK)
in Dresden with funding from the German Federal
Ministry of Economics and Technology, shows
promising results for a propane-CO2 mixture.
German and Swiss design
consultants report
Rüdiger Grimm
Managing partner of geoENERGIE Konzept GmbH –
an internationally active specialist design office
for geothermal energy, which was founded
in January 2007
Around 25,000 ground source heat pumps are currently installed
each year in Germany. In order to achieve efficient and sustainable
operation, a smooth interaction between the building services technology, geothermal engineers and drilling technology is imperative – it is
only when this tallies that the system will show a correspondingly high
annual performance factor. However, the operating data is not even
known for many systems. This means that planning, installation and
operating errors are therefore not even recognised, making them
difficult to alleviate when problems occur.
Therefore a substantial focus of our daily work as planning consultants
for shallow geothermal energy is concerned with advising customers
about implementing monitoring systems at an early stage. This can
occur at different processing levels and ranges from the simple
recording of the heating and electricity requirements and determining
the annual performance factor to detailed online monitoring with data
transferred at one-minute intervals.
At any rate it should be noted that official requirements stipulated in
water law-related authorisations, such as for example the submission
of annual balances or the regular measurement of subsoil temperatures,
do not represent excessive demands but ultimately benefit the user.
Peter Hubacher
Owner of the Hubacher Engineering
office in Engelburg, Switzerland
Despite intensive research, the efficiency of heat pump systems has
still not reached the desired level. In Switzerland, field monitoring
has been conducted for more than 16 years. The researchers opted at
the beginning for a simple recording system so that the budget could
cover as large a range of systems as possible. The monitoring of field
systems was intended to close the initial gaps in knowledge. That has
proved worthwhile, since the initially very modest results regarding
the efficiency of small heat pumps triggered a competitive process
that has positively influenced the development of heat pumps.
Whereas the annual performance factor APF for air-water heat pumps
was initially around 2.6, today 3.0 or even higher APF values can be
expected. With brine-water heat pumps, the value has increased from
around 3.4 to 4.0–4.5.
The field monitoring has provided the manufacturing companies
and installers with a diverse range of experience values and operating
information, which, with a sample size of 280 systems, is also statistically
secured. The most important aspects concern not just mean values
for the APF, the degree of utilisation, susceptibility to malfunctions,
operating behaviour and running times as well as the heat production
and electricity consumption but also suitable hydraulic systems and
the maintenance and servicing costs. Such research projects make it
easier to find answers to issues relating to energy policy and electricity
consumption, including concerning electricity grid loads.
17
18
BINE-Themeninfo I/2013
Fig. 35 Heat pumps
and solar energy
can be combined.
Source: obs/Schüco
International KG
Solar-assisted heat pumps
In many cases, heat pump systems can be successfully
combined with solar thermal systems so that solar thermal
heat exchanger can be made smaller depends among
other things on the relative reduction in the heat removal
and the existence of groundwater flows.
energy can be used to meet a large proportion of the hot
water requirements in summer and part of the heating load
during transitional periods. Alternatively, the efficiency of
heat pumps increases significantly when the temperature
of the heat source is increased with solar thermal energy.
Solar thermal systems are frequently designed in residential buildings so that the collector surface area and
storage tank meet around 60% of the annual hot water
requirement. Larger sized systems can also be used for
providing space heating support. The remaining energy
required for domestic water heating and space heating
is provided by a further heat generator, for example a gas
boiler or a heat pump. The solar collector is connected
to a storage tank via a solar circuit.
“Conventional” incorporation of solar thermal systems
Fig. 37 shows a system configuration that combines a
solar thermal system with a heat pump. The solar thermal system charges a domestic hot water storage tank
that – whenever required – is also heated by the heat
pump. In this example the space heating is directly and
exclusively provided by the heat pump, i.e. without a
heat storage tank.
It is important that the heat pump control system prioritises the solar heat generation. The high seasonal performance factor of the solar thermal system means that
the electrical energy requirement lowers and the system
efficiency increases. The improvement in efficiency and
the operational cost savings depend on many parameters
such as the solar irradiance, the type of main components
and their sizing. With systems that use ground source
heat pumps, the solar thermal system reduces the heat
absorbed from the ground in summer. The extent to which
this reduces the drop in ground temperature during the
heating operation as a result of this, or the extent to
which the borehole heat exchanger or horizontal ground
Solar thermal energy as a heat source for heat pumps
A further approach is to integrate the solar thermal system
on the source side of the heat pump so that the solar
thermal energy is either the sole heat source for the
heat pump or provides supplementary heat. A growing
number of systems have been available on the market
for several years now that sometimes differ only to a
slight extent but are also sometimes fundamentally different from one another. The main differences are in
terms of the following aspects:
• Heat source: Type and sizing of the heat source(s)
• Heat storage system: Is a storage tank connected on
the heat source side? Which type of storage tank is used?
• Collector type: At which temperature level is the solar
thermal system available?
• Incorporation: In addition to being incorporated on
the source side, is the solar thermal energy also
utilised on the sink side, i.e. for directly heating
domestic hot water and water for space heating?
Three examples from the diverse range of existing system
concepts are described below.
Fig. 38 shows a system with a ground source heat pump
that incorporates unglazed collectors to provide heat
cost-effective at a low temperature level. The solar heat is
exclusively injected on the source side. The solar thermal
energy stored in summer can be used for regenerating
the ground. This stabilises the heat source against any
possible, unforeseeable increases in the heat removed
and (slightly) increases the heat source temperature of
the heat pump.
Fig. 39 shows a system that exclusively uses the solar
thermal system as the heat source for the heat pump. The
flat plate collector gives precedence to directly heating
the domestic hot water storage tank. If the solar thermal
energy generated surpasses the requirements or the
generated temperatures are too low, the solar heat is
fed into a buffer storage tank that is used as the heat
BINE-Themeninfo I/2013
19
DHW tank
Cold water
Ground
heat exchanger
Heat pump
Heat pump
Heat pump
Solar collector
Heating circuit
Solar collector
DHW
Heating circuit
DHW
Solar collector
Heating circuit
DHW tank DHW
DHW tank
Ground
heat exchanger
Ground
Cold water
Cold water
DHW tank
Fig. 37 System diagram
forexchanger
a heat pump system with a flat plate collector
heat
Fig. 36 Solar storage tank for heat pumps.
Source: Bundesverband Wärmepumpe e. V., Berlin
connected
Ground to the domestic hot water storage tank. Cold water
heat exchanger
Heating circuit
Heat pump
DHW
Heating circuit
DHW
Heat pump
source for the heat pump. The buffer storage tank ensures
that the solar thermal energy can be used at a later
stage following its generation.
Fig. 40 illustrates a system concept with an air source
heat pump and a flat plate collector. The solar thermal
energy principally heats a domestic hot water storage
tank. If the generated temperature is not sufficient, the
solar thermal energy is directly used as an additional
heat source for the heat pump during the heat pump
operation. This increases the source temperature of the
heat pump, which for air source heat pumps is only very
low on cold days during the heating season.
Ground
External air
heat exchanger
Heating circuit
DHW tank
DHW
Heating circuit
Heat pump
DHW tank DHW
Cold water
Heat pump
Ground
heat exchanger
DHW tank
Field measurements and simulation studies show improvements in the efficiency when using solar thermal
systems on the heat sink side (“standard system”). Their
incorporation on the heat source side requires a sophisticated, robust control concept; the added value needs
to be considered in a more differentiated way. Until now
there have been no comprehensive analyses of the different concepts with different application areas and
boundary conditions. Whereas relevant guidelines exist
for heat pumps and solar collectors, no corresponding
standards are available for combined systems. But established simulation-based test methods of solar thermal
heat generation systems were already extended to cover
heat pumps as auxiliary heating equipment.
The “Solar and Heat Pump Systems” project is being organised by the International Energy Agency (IEA) and lasts
from January 2010 until December 2013. It is concerned
with comparing solar thermal heat pump systems,
whereby among other things it is developing evaluation
parameters and methods for investigating the systems
using simulation methods, laboratory measurements
and field tests. This joint project is also incorporating
the results from a project conducted by the University of
Stuttgart and funded by the German Federal Ministry for
the Environment, Nature Conservation and Nuclear Safety,
which is concerned with developing performance tests
and ecologically evaluating combined solar heat pump
systems (HPSol).
Solar collector
Solar collector
Solar collector
Cold water
DHW tank
Cold water
Fig. 38 System diagram for a heat pump system with an unglazed solar Solar collector
collector that feeds solar heat to the ground or directly
to the
heat
pump
Cold
water
Heating
circuit
as a second heat source.
Solar collector
Ground
DHW
Heat pump
Heating circuit
heat exchanger
DHW
Heat pump
Heating circuit
DHW tank DHW
External air
Heating circuit
Heat pump
External air
System assessment
Solar collector
Heat pump
Solar collector
Solar collector
DHW tank Cold
DHW
water
DHW tank Cold water
DHW tank
Buffer storage tank
Ground
heat exchanger
Heating circuit
Heat pump
Cold water
Solar collector
Cold water
DHW
Solar collector
Heating
circuit
Fig. 39 System diagram for a heat pump
system
with flat plate collectors for heating
the domestic hot water storage tank and as a general heatDHW
source for the heat pump.
Heat pump
DHW tank
Buffer storage tank
Heat pump
Solar collector
Heating circuit
DHW tank
Cold
water
DHW
Cold water
External air
Buffer storage tank
DHW tank
Cold water
Fig. 40 System diagram for a system with an air source heat pump and
solar collector for directly heating domestic hot water and as a second heat source.
Source: Fig. 37 – 40 FhG-ISE
Solar collector
Heating circuit
Heat pump
DHW
BINE-Themeninfo I/2013
Heat pumps in the grid
The German federal government is aiming to increase the share of electricity from
renewable sources to 35% by 2020, whereby the driving technologies are fluctuating
energy generators such as wind power and photovoltaics. Active load control of heat
pumps as part of an intelligent supply system can help to synchronise the energy
supply and demand. This utilises the storage capability provided by heating and
domestic hot water storage tanks and the building mass.
At the end of 2011, the electrical connected load of all heat pumps installed in Germany
amounted to approximately 1.5 GW. In extreme cases, these heat pump systems could
use 36 GWh of electrical energy per day. By way of comparison, the current capacity of
the pumped storage power plants installed in Germany amounts to around 40 GWh.
According to the German heat pump association (Bundesverband Wärmepumpe), the
electrically connected load could increase to 4.4 GW by 2020. In 2012, the electricity
generated by wind energy ranged between 0 and 24 GW, whereby there was a considerable
seasonal dependence in terms of the energy fed into the grid. The same applies to
photovoltaic energy, whose peak of 22 GW in the same period occurred, as would be
expected, at midday. Due to their storage capacity, heat pump heating systems can only
be used for buffering on a daily basis. The general contribution made by heat pumps is
closely linked to the potential to shift operating times. Here the energy supply companies
need to develop price concepts and set a corresponding price signal. In addition, the
control strategies for the heat pump operation need to be adapted.
The tests conducted by Fraunhofer ISE have confirmed that only a few of the heat
pumps investigated are fully loaded at temperatures in the design range. In addition to
the design, a role is also played by the type of operation, the control and parameterisation
as well as the storage capacity in the heating system and building. This raises questions
concerning the comfort limits for the residents. The greater the controllable temperature
range, the greater the flexibility with which heat pumps can be used as a controllable
load. Another issue that has not yet been clarified is how electricity-led operation affects
the efficiency of heat pump heating systems. Deviations from the exclusively heat-led
operation can affect, for example, the system temperatures, storage losses or the
compressor running times. Another aspect that still needs to be clarified in this respect
is the design of the communication and control capability. A practical solution which,
however, can only be implemented in the long term, is the development of a new
information and communications technology (ICT) infrastructure for bidirectional
communication, such as part of the dissemination of smart meters. At the same time,
each heat pump needs to be equipped with a corresponding communication gateway
that is ideally uniform.
Project organisation
German Federal Ministry of Economics
and Technology (BMWi)
11019 Berlin
Germany
Project Management Organisation Jülich
Forschungszentrum Jülich GmbH
52425 Jülich
Germany
Project number
0327401A
0327393B
0327841A, B
Contact · Info
Questions regarding this
Themeninfo brochure?
We will be pleased to help you:
0228 92379-44
Further information on this topic
is available from BINE Information
Service or at www.bine.info.
More from BINE Information Service
>> Cooling with heat. BINE-Projektinfo brochure 07/2012
>> Geothermal heating for railway points. BINE-Projektinfo brochure 12/10
>> Using geothermal energy in office buildings. BINE-Projektinfo brochure 07/10
>> G
round-coupled heat pumps for new buildings. BINE-Projektinfo brochure 03/10
Links and literature (in German)
>> w
ww.wp-effizienz.ise.fraunhofer.de | www.wp-monitor.ise.fraunhofer.de |
www.wp-im-gebaeudebestand.de
>> Bongs, C.; Günther, D.; Helmling, S. u. a.: Wärmepumpen. Heizen – Kühlen –
Umweltenergie nutzen. FIZ Karlsruhe. BINE Informationsdienst, Bonn (Hrsg.).
Stuttgart: Fraunhofer IRB Verl., 2013. ca. 160 S., ISBN 978-3-8167-9046-4,
BINE-Fachbuch, 29,80 Euro (Print); 29,80 Euro (E-book), www.baufachinformation.de
Concept and design: iserundschmidt GmbH, Bonn – Berlin, Germany · Layout: KERSTIN CONRADI · Mediengestaltung, Berlin, Germany
20