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Danish Gas Technology Centre • Dr. Neergaards Vej 5B • DK-2970 Hørsholm • Tlf. +45 2016 9600 • Fax +45 4516 11 99 • www.dgc.dk
Hybrid heating systems and smart grid System design and operation – market status
Project report
April 2013
Hybrid heating systems and smart grid
System design and operation - market status
Mikael Näslund
Danish Gas Technology Centre
Hørsholm 2013
Title : Hybrid heating systems and smart grid
Report
category : Project report
Author : Mikael Näslund
Date of issue : 15.04.2013
Copyright : Dansk Gasteknisk Center a/s
File number : 737-36; H:\737\36 Hybridsystemer\Rapport\Hybridrapport final.docx
Project name : Hybridsystemer til gas og el i samspil med smart grid
ISBN : 978-87-7795-361-3
DGC-report 1
Table of contents Page
Summary ......................................................................................................................................... 2
1 Introduction .................................................................................................................................. 4
1.1 Hybrid system definition ....................................................................................................... 7
1.2 Early work and appliances ..................................................................................................... 8
2 Current hybrid system appliances ................................................................................................ 9
2.1 Products on the market .......................................................................................................... 9
2.1.1 Buderus .................................................................................................................. 10
2.1.2 Daalderop .............................................................................................................. 11
2.1.3 Vaillant .................................................................................................................. 12
2.1.4 Junkers/Bosch ........................................................................................................ 13
2.1.5 Viessmann ............................................................................................................. 14
2.1.6 Glow-worm ........................................................................................................... 15
2.1.7 MHG ...................................................................................................................... 16
2.1.8 Carrier .................................................................................................................... 17
2.2 Summary of hybrid systems ................................................................................................ 18
3 Hybrid model and calculations .................................................................................................. 20
3.1 Calculation parameters ........................................................................................................ 21
3.2 Gas boiler operates as peak load – capacity control ............................................................ 24
3.3 Price controlled operation .................................................................................................... 25
3.4 Energy costs for the hybrid system ...................................................................................... 30
4 Conclusions ................................................................................................................................ 32
References ..................................................................................................................................... 34
DGC-report 2
Summary
In future smart grids, where gas and electricity grids are interacting, appli-
ances that use both gas and electricity are highly interesting. New gas appli-
ances for space heating fit into the smart-grid concept. These appliances
operate on the consumer level and do not directly require active involve-
ment from the inhabitants or changes in their daily energy consumption pat-
tern. The gas-based technologies suitable for smart-grid integration are hy-
brid systems and micro cogeneration.
Hybrid systems have the possibility to switch between gas and electricity for
the space heating while micro cogeneration produces both heat and electrici-
ty. Both technologies offer the possibility to be externally controlled and
used for grid balance and thus maximizing the efficient use of renewable
electricity. The gas grid acts as support and back-up for intermittent renew-
able energy with these gas appliances.
This study focuses on hybrid systems consisting of a condensing gas boiler
and a small air-to-water heat pump. A market survey of product status 2012
is presented and simulations of hybrid system annual performance in Danish
installations are presented.
The marketed hybrid systems can be divided into three basic designs: gas
boiler and heat pump in separate cabinets, gas boiler and heat pump inte-
grated in a common cabinet and heat pump and control systems that can be
added to existing gas boilers.
The annual performance calculations show that the gas consumption for an
annual space heating demand of 20,000 kWh and 2,000 kWh hot water de-
mand will be reduced to the 5,000-10,000 kWh range including hot water
production. The exact number depends on heat pump size, COP and energy
costs. The cost for gas and electricity for the consumer is likely to be re-
duced by up to 15-20 % with the current Danish energy prices and a reason-
ably efficient and sized hybrid system. The investment and maintenance
costs for the consumer are not known today due to the fact that hybrid sys-
tems are only at a market introduction stage.
DGC-report 3
The advantages for the gas and electricity utilities lie mainly in the possibil-
ity of using the hybrid systems as part of a smart grid where gas and elec-
tricity interact. Renewable electricity can be used in the best possible way.
The hybrid systems can be switched over to gas operation when for example
the wind power output is low and the option is expensive power generation
with high CO2 emissions.
DGC-report 4
1 Introduction
The Smart Grids European Technology Platform1 defines a smart grid as “A
Smart Grid is an electricity network that can intelligently integrate the ac-
tions of all users connected to it – generators, consumers and those that do
both – in order to efficiently deliver sustainable, economic and secure elec-
tricity supplies.” A graphical representation of EU smart grid projects is
shown in Figure 12. This report describes a study that deals with a technolo-
gy that can be characterized either as home application or integrated system.
The map clearly shows that investments in smart meters are by far the most
common action. The gas grid is also more and more considered as an im-
portant part of the smart grid concept, since using the gas grid is the most
efficient way of storing energy.
Figure 1 Overview EU smart grid projects
1 http://www.smartgrids.eu/
2 http://ec.europa.eu/dgs/jrc/index.cfm?id=1410&obj_id=13670&dt_code=NWS&lang=en
DGC-report 5
New gas appliances for heating include gas and solar energy, gas heat
pumps, micro cogeneration and hybrid systems. In this study a hybrid sys-
tem is defined as a gas boiler and an electric heat pump in an integrated op-
eration. Gas and solar energy and gas heat pumps reduce the gas consump-
tion due to the use of renewable energy. Hybrid systems and micro cogener-
ation can be described as follows regarding gas consumption.
Hybrid systems use less gas energy at the consumer site. Renewable
energy from the heat source is used in the heat pump. Electricity
may also be renewable. In the future also renewable gas such as up-
graded biogas may be distributed in the gas grid.
Micro cogeneration increases gas consumption at the consumer site.
Grid electricity is replaced by on-site generated electricity. The gain
is reduced primary energy consumption since the micro cogeneration
is assumed to have a better overall fuel utilization than centralized
power plants. Early micro cogeneration appliances use either Stirling
engines or small internal combustion engines. The electric efficiency
ranges from 10 % to 25 %. In the future fuel cells with higher elec-
tric efficiency are supposed to be used.
Hybrid systems may be a part of the future smart grid concept where the
electricity and gas grids are interacting in order to use renewable electricity
as efficiently as possible. In this report a hybrid system for single-family
houses is investigated.
This report deals with small-scale, downstream heating appliances that may
be part of the smart-grid concept. These appliances are described as dual-
fuel appliances in the EU Commission Task Force for Smart Grids [1]. The
following appliances are suggested in the EU document as dual-fuel or dual-
output appliances in a smart gas grid:
A heat pump providing heat for base load and a condensing gas boil-
er for peak loads and often hot water production as well
Condensing gas boiler with electrically heated storage tank
Micro cogeneration
DGC-report 6
DGC has worked with all of these topics. This study concerns the first item.
Electric heaters as supplement to existing gas boilers have been studied in
[2]. In this study an electric 2 kW heater was directly connected to the boiler
return line. No storage facility was used.
Finally, DGC has a long experience in cogeneration technologies. For ex-
ample, for third-party testing, certification and field test evaluation in the
Danish micro cogeneration program DGC is aiming at developing and
demonstrating Danish gas-fired fuel-cell micro cogeneration units for sin-
gle-family houses3.
The potential interruptible power demand when using hybrid systems can be
illustrated as follows. Assume that hybrid systems consisting of an electric
heat pump and a condensing boiler become a major replacement in the cur-
rent Danish gas heating market. In this example we assume 100,000 installa-
tions, corresponding to one third of the gas-heated single-family house pop-
ulation. The heat pump in the hybrid system has a compressor input of 1
kW. The COP at the temperatures 7/354 is 4.25. The power demand that can
be replaced by gas heating as a function of the outdoor temperature then
becomes as shown in Figure 2. The two dotted curves represent the total
space heating demand in 100,000 houses with 10,000 and 20,000 kWh an-
nual heating demand. The solid curves show the electricity demand for the
heat pumps in the hybrid systems. The temperature where the horizontal
electricity curves starts to decrease marks the lowest temperature where the
heat pump alone can heat the house. At a lower outdoor temperature the heat
pump will operate continuously on full load and the gas boiler will add the
extra heat needed. The heat pump operates at part load at higher outdoor
temperatures, and the interruptible power demand becomes lower. Please
note that the heat pump is assumed to operate even though the outdoor tem-
perature is very low. Some heat pumps are shut down when the outdoor
temperature is for example -7 C or lower. New heat pumps can operate
without this limitation during the entire year.
3 Dansk Mikrokraftvarme, http://www.dmkv.dk/
4 The temperatures 7/35 are the source temperature (air, soil etc) and the heating system
forward temperature.
DGC-report 7
Figure 2 Heating demand and interruptible power potential in Denmark
when a population of 100,000 externally controlled hybrid sys-
tems are used
1.1 Hybrid system definition
The definition of a hybrid heating system including a gas appliance is as
follows:
It consists of two appliances, one gas fired and one using another en-
ergy source, often electricity.
The appliances can be separate appliances or integrated in the same
package.
The two appliances can independently cover the heating demand to a
large extent.
The appliances’ operation can be integrated or totally separated. An
example of the first option is a heat pump, which can use the energy
in the flue gases from the gas boiler.
The operation of the two appliances shall be controlled either by in-
ternal or external signals.
Some manufacturers have other definitions of a hybrid system, For example
gas boiler and solar heating.
0
100
200
300
400
500
600
700
800
-15 -10 -5 0 5 10 15
He
atin
g an
d e
lec.
de
man
d (
MW
)
Ambient temperature (°C)
Overall heating demand, 20 MWH/a
Interruptible elec. demand, 20 MWh/a
Overall heating demand, 10 MWh/a
Interruptible elec. demand, 10 MWh/a
DGC-report 8
1.2 Early work and appliances
This section presents a few examples of early products and studies of the
hybrid principle.
The Swedish company IVT Elektro Standard manufactured a hybrid system
already in the 1990s. The unit Auto Term 660A consisted of a non-
condensing boiler and an electric heat pump. It was designed to integrate the
heating and ventilation system. The heat source was the ventilation exhaust
air and the flue gases when the boiler was in operation. The heat pump had
an output of 2.0 kW and a COP of 2.9. The gas burner had three burner
steps: 3.4, 7.2 and 10.6 kW. Heat pump operation was limited to return tem-
peratures below 50 °C. Measurements from installed units showed that the
energy consumption was split into 40 % gas and 60 % electricity. An evalu-
ation of the hybrid system is reported from the Swedish Gas Centre [2].
At the International Gas Union Research Conference (IGRC) in 2008 Dutch
laboratory tests of a hybrid system and micro cogeneration and a heat pump
were reported [3]. The principle of hybrid systems including heating and
cooling were tested for early evaluation.
Hybrid systems and smart grids are discussed in a paper from IGRC2011
[4]. This paper discusses the effect on the electricity grid when a larger
number of hybrid systems are in use. It thus deals with the aggregated grid
aspects and not the individual appliance performance.
DGC-report 9
2 Current hybrid system appliances
A number of hybrid systems are offered by manufacturers in Europe. In this
chapter these appliances are described based on information from websites,
journals and also a questionnaire sent out to manufacturers on the Danish
market.
2.1 Products on the market
The hybrid systems often consist of a gas boiler and an air-to-water heat
pump in separate parts. There are also more integrated solutions and also
prepared for other heat sources. The manufacturers often show system lay-
outs, which also include solar energy. Figure 3 shows the main operation of
a hybrid system. During times with high outdoor temperature the heat pump
capacity is large enough to cover the heating demand. During winter the
heating demand is covered by the gas boiler alone. The heat pump and the
gas boiler are used simultaneously in an intermediate period. The heating
costs using the condensing boiler and the heat pump are marked with red
and green curves in the figure. The costs using the condensing boiler can be
assumed not to differ significantly between various boiler models. The op-
eration of the heat pump depends on the efficiency/COP and the lowest am-
bient temperature allowed for operation. The latter can differ heavily, from
-5 C to for example -15 C for heat pumps adapted to the low winter tem-
peratures in Scandinavia. The figure shows that in this configuration the
heat pump will not be economical to operate when it is colder than -4 C.
Several control strategies are implemented in hybrid systems. Some hybrid
systems allow the customer to choose between two or more control options.
These options include:
Heat pump operation until the heating demand exceeds the heat
pump’s capacity. Ambient temperature may also restrict the heat
pump operation.
Operation to minimize the heating cost
Operation to minimize the CO2 emissions
The two last options require preset values for gas and electricity prices, al-
ternatively the price relation, and the CO2 emission factors for gas and elec-
tricity. The control system calculates the cost or CO2 emission and chooses
DGC-report 10
the appliance to be used in order to minimize the consumer energy cost or
the overall CO2 emission in an optimizing algorithm. These factors are input
given by the installer or consumer and are fixed until new factors are given
as input.
Figure 3 Hybrid system operation (Source: Bosch)
Boiler efficiency in a hybrid system will be lower than in a stand-alone in-
stallation due to the lower annual load and a higher return temperature if the
heat pump heats the return line.
The systems described in this chapter have mostly been found in internet
searches for gas heating and hybrid. Manufacturers who do not use the ex-
pression hybrid for their hybrid systems may have been omitted. The fol-
lowing hybrid systems have an outdoor heat pump part and an indoor part
and a boiler, unless otherwise stated.
2.1.1 Buderus
The Buderus hybrid system is named Logatherm WPLSH. The indoor unit
has the size 500×390×360 mm (height×width×depth) and weighs 21 kg. The
maximum supply temperature is 50 C. It operates in different modes. Hot
water is always produced using the gas boiler. At low heating loads and
moderate supply temperatures the heat pump alone operates. At low ambient
temperatures the gas boiler is the only heat source. Between these operation
modes is a range where the heating is split between the appliances, which
DGC-report 11
are operating at the same time. An existing boiler installation can be up-
graded to a hybrid system if the boilers are the Logamax plus GB145,
GB152, GB162, or GB172. Some Buderus boilers are sold in Denmark as
Milton (Nefit) Highline.
Buderus has used the word hybrid for combinations of gas boilers and solar
collectors as well.
Figure 4 Buderus Logatherm WPLSH hybrid system installation example
2.1.2 Daalderop
The Daalderop Cool is a single-unit wall-hung hybrid appliance. It uses both
ambient air and ventilation exit air as heat sources. It can also be used for
cooling purposes. The heat pump and the condensing boiler output is 3 and
24 kW, respectively. The condensing boiler assists the heat pump for heat-
ing during coldest days. The manufacturer states it is compact and easy to
install with no buffer or separate storage tanks required. The size is
896×821×552 mm (height×width×depth) and it weighs 110 kg. Figure 5
shows images of the appliance.
DGC-report 12
Figure 5 Daalderop Cube hybrid heating appliance design and installation
example
2.1.3 Vaillant
Vaillant presented a hybrid system, geoTHERM, in March 2012. The heat
pump has an output of 3 kW and can use ambient air and ventilation exit air
as heat source. The heat pump is a single-unit indoor appliance. According
to Vaillant the system is suitable both for existing and new houses. Gas
boilers from Vaillant can be upgraded to a hybrid system by adding the heat
pump. The gas boiler is used for hot water production. The image in Figure
6 shows from left to right the indoor heat pump, the hot water tank and the
gas boiler.
Figure 6 Vaillant geoTHERM hybrid heating system
DGC-report 13
2.1.4 Junkers/Bosch
Junkers presented two hybrid systems, CerapurAero and Supraeco SAS Hy-
brid, in April 2012. Junkers products are marketed under the Bosch name in
Denmark. The hybrid systems have different layouts. They are shown in
Figure 7.
The CerapurAero hybrid system has the heat pump and condensing boiler in
a common single cabinet. The size of this cabinet is 890×600×482 mm
(height×width×depth). The heat pump can use either ambient air or water as
heat source. Heat pump output is approximately 2 kW and COP = 3.5
(7/35). The gas boiler has either 14 or 24 kW nominal output. The manufac-
turer claims an overall efficiency which is 12 % higher than for the gas boil-
er alone.
The Supraeco SAS hybrid system has the indoor heat pump unit and the gas
boiler in separate cabinets. The maximum output is 5.2 kW. Ambient air is
used as heat source. The manufacturer states that it is possible to use the
installed boiler in the Cerapur product series from 2007 for an upgrade of an
existing heating installation to a hybrid system.
Three operating strategies are possible:
CO2 reducing operation,
Cost reducing operation
Gas boiler operation below a preset ambient temperature
The indoor unit size of the Supraeca is 390×500×360 mm
(height×width×depth).
DGC-report 14
CerapurAero indoor unit including heat
pump and gas boiler
Supraeco SAS indoor heat pump unit
Supraeco SAS outdoor heat pump unit
Figure 7 Junkers/Bosch hybrid heating systems CerapurAero and
Supraeco SAS
2.1.5 Viessmann
Viessmann has no dedicated heating system or product called a hybrid sys-
tem. However, the website mentions that a gas boiler and a heat pump can
be connected in a common bivalent system. Figure 8 shows a picture from
the website describing a system layout where an air-to-water heat pump is
connected.
DGC-report 15
Figure 8 A hybrid system design consisting of a boiler, a heat pump and a
storage tank suggested by Viessmann
2.1.6 Glow-worm
Glow-worm is a British hybrid system consisting of a condensing gas boiler
and an air-to-water heat pump. The appliance is also sold as AWB Genia on
the Dutch market. Figure 9 shows the system parts including control boxes
and a remote control.
DGC-report 16
Figure 9 Glow-worm hybrid system parts
The control system can set the operation in at least two modes, either as a
capacity control or a price relationship control mode.
2.1.7 MHG
MHG Thermipro is a floor-standing unit prepared also for solar energy use.
The unit consists of a heat pump indoor unit, a condensing gas boiler and a
500 l storage tank and heat exchanger for solar energy. The heat pump out-
put capacity is 7.1 kW with a COP = 4.2 (A2/W35) and the gas boiler is
modulating in the range 7.2 – 27.3 kW. The unit has a capacity significantly
higher than most of the products presented in this chapter. Figure 10 shows
a configuration. The left image shows an outside view of the hybrid unit.
The middle image shows the interior parts, condensing gas boiler, heat
pump indoor unit and storage tank for solar energy. The graph indicates that
DGC-report 17
the heat pump is not operating when the ambient temperature is below the
freezing point. The heat pump and solar energy is used above the freezing
point, while the gas boiler and solar energy are used below the freezing
point. The right-hand graph shows an example of the annual distribution of
energy as a function of the ambient temperature. The heat pump and solar
energy covers 81 % of the annual heat demand and the gas boiler and solar
energy cover 29 % of the annual heat demand. The manufacturer states that
the gas consumption is low enough for LPG operation outside the gas grid
area. The size is 1745×820×1250 mm (height×width×depth) and the weight
is 512 kg excluding water.
Figure 10 MHG Thermipro hybrid system
2.1.8 Carrier
In the North American market the expression dual-fuel is often used instead
of hybrid system. The Carrier Infinity dual-fuel hybrid system is an air-to-
air heat pump and a gas furnace (warm air) delivered either as an integrated
package for outdoor location or as separate parts for indoor and outdoor
location. The system provides heating and cooling including humidifying
the warm air for space heating. Figure 11 shows the two options with the
separate design to the left and the integrated package in the right image.
DGC-report 18
Figure 11 Carrier Infinity dual-fuel hybrid systems
2.2 Summary of hybrid systems
In Table 1 the main characteristics of hybrid systems are collected. Two
products not described above are included in the table to show that the de-
scription is not complete.
Table 1 Summary of European packaged hybrid systems
Manufacturer/Model Heat
source
Heat pump output,
COP
Possible to add
heat pump to
existing boiler?
Buderus Logatherm WPLSH Ambient air 1.6 – 5.2 kW
COP = 4.42
A7/W35
Can be com-
bined with sev-
eral Buderus
gas boilers
Daalderop Cube/Cool Cube Ambient air
Vent exit air
3 kW
COP = 2.9
No
Brötje/Bosch/Junkers
Cerapur Aero
Supraeco SAS
Ambient air/
water
2 kW, COP = 3.5
5.2 kW, COP =
4.11
Yes
(Supraeco)
Viessmann Ambient air - Yes
Vaillant Ambient air
Vent exit air
3 kW Yes
Glow-worm Clearly Hybrid
(AWB GeniaHybrid)
Ambient air 4.41 kW
COP = 3.73
A7/W35
-
MHG ThermiPro Ambient air
(solar ener-
gy)
7.2 kW
COP = 4.2
A2/W35
No, delivered as
a complete
package
Carrier Infinity Dual Fuel
System
Ambient air US energy efficien-
cy labels.
SEER = 15.0 (heat
pump)
AFUE >81 % (gas
Delivered either
as package or
heat pump and
gas furnace
separately
DGC-report 19
boiler, HCV)
HSPF = 8.0
Techneco Elga Ambient air 5 kW Can be com-
bined with boil-
ers from a num-
ber of manufac-
turers
Liechti EcoStar Hybrid
ThermiAir Hybrid
Ambient air
3.5-1.2 kW Oil boiler
Gas and electricity meters are an essential part of the smart-grid concept. To
use the full potential of external monitoring and control in order to fit into
the smart-grid concept the gas and electricity meters need to be equipped
with two-way communication. These options are not necessary for the oper-
ation of a hybrid heating system. In the future, smart meters with communi-
cation between the heating system and external connections will facilitate
operation based on dynamic energy prices and correct billing. Today, only
preset fixed values for gas and electricity can be used for price control of the
hybrid system operation. The gas and electricity utilities will also get an
overview of the electricity demand in real time and the possible interruptible
electricity power.
DGC-report 20
3 Hybrid model and calculations
The annual performance of a hybrid system is modelled and simulated in
this chapter. The system consists of an electric air-to-water heat pump and a
gas boiler.
The calculations can only include internal control systems. The two control
systems used are:
The heat pump is the primary heat source. The gas boiler is used
when the heat pump output is not large enough for the heating de-
mand.
The appliance chosen for heating is based on the price difference be-
tween natural gas and electricity.
The calculations are made using the model (Boilsim) for energy labelling of
gas boilers in Denmark. A simplified heat pump model is incorporated in
the Boilsim calculations. The Boilsim model is described in [6].
The Coefficient of Performance (COP) for the heat pump is modelled as
plyout BTATCOP sup
where Tsource is the source temperature at the evaporator, and Tsupply is the
supply temperature to the heating system. The compressor input is also a
calculation input and used together with the COP to calculate the heat pump
output in each climate step. The coefficients A and B are fixed and describe
the heat pump performance map. An example of a heat pump performance
map is shown in Figure 12. The source temperature is equal to the outdoor
air temperature, and is input in the climate steps. Tsupply is calculated in each
climate step.
The shape of the performance map in Figure 12, and the fact that heat
pumps are often characterized by the COP in only 2-3 operating points,
show that modelling the COP as a plane is a reasonable simplification.
DGC-report 21
Figure 12 Performance map for an air-to-water heat pump [5]
3.1 Calculation parameters
Input to the calculation of annual performance and heating costs for hybrid
systems are summarized in Table 2.
Table 2 Parameters in hybrid system performance calculations
Technical parameters
Heat pump COP (A7/W35) 3.85, 4.25 and 4.65
Heat pump compressor input 1.0 and 1.5 kW
Heat pump output (7/35) 3.85 kW, 4.25 kW and 4.65 kW (1.0 kW)
4.81 kW, 5.31 kW and 5.81 kW (1.5 kW)
Operation control 1. The gas boiler is used for hot water production and peak load
2. The gas boiler is used for hot water production and when the heating cost is lower for the boil-er.
Gas boilers Two condensing gas boilers with modu-lating burners, 3–15 kW (boiler 1) and 6–25 kW (boiler 2). Annual efficiency in the Danish energy labelling system: 101-102 % at 20 MWh annual heating demand and 2,000 kWh hot water de-mand.
Economic parameter
Price relation electricity/gas 2.7 (only used for capacity controlled operation) in base case. 2.2 and 3.2 in calculations for price relation sensitivity
DGC-report 22
The calculation procedure is as follows.
The calculations are made in the same manner as the calculations of annual
efficiency for the Danish energy labelling system. The Boilsim model is
used. The heating season is divided into 13 climate steps, representative of
the Danish climate. For each of these climate steps the operation conditions
are calculated, i.e. the heat demand and the supply and return temperatures.
Calculations are made for an annual heating demand of 10, 20 and 30 MWh.
The annual hot water demand is 2,000 kWh. The gas boiler is supposed to
cover the entire hot water demand. The heat pump is assumed to be able to
operate down to -15 °C ambient air temperature.
Two operational cases are calculated as:
Capacity controlled operation. Heat pump COP is calculated using
the ambient air temperature and the heating system supply tempera-
ture. The calculated COP and heat pump output is compared to the
heat demand. If the heat pump capacity is not enough, the gas boiler
adds the remaining heating part. Gas boiler efficiency is calculated
for this reduced heat demand.
Energy price controlled operation. The heat pump COP and the gas
boiler efficiency are calculated for the heating demand. If the rela-
tion between the calculated heat pump COP and gas boiler efficiency
is less than the price relation between electricity and gas, the gas
boiler covers the entire heat demand. The heat pump covers the heat
demand if the result is the opposite. If the heat pump output is not
sufficient, the gas boiler operates as in the capacity controlled opera-
tion. The price relation between electricity and gas is given as a
fixed input. The effect of dynamic prices set by the electricity utility
cannot be calculated with the method.
The 12 heating systems in the base case calculations are described in Table
3.
DGC-report 23
Table 3 System layout for hybrid system evaluation
System Boiler Heat pump elec-
tricity input (kW)
(A7/35)
Sys 1 1 (3-15 kW) 1.0 3.85
Sys 2 1 1.0 4.25
Sys 3 1 1.0 4.65
Sys 4 1 1.5 3.85
Sys 5 1 1.5 4.25
Sys 6 1 1.5 4.65
Sys 7 2 (6–25 kW) 1.0 3.85
Sys 8 2 1.0 4.25
Sys 9 2 1.0 4,65
Sys 10 2 1.5 3.85
Sys 11 2 1.5 4.25
Sys 12 2 1.5 4.65
The air-to-water heat pump performance map is shown in Figure 13. Actual
COP in the calculations are shown in the graphs showing the annual system
performance.
Figure 13 Performance map of air-to-water heat pump used in calcula-
tions of hybrid system annual performance
The results are presented as graphs with the annual gas consumption as
function of price relation, heat pump performance (COP) and annual heating
demand. The energy demand on the x-axis is the space heating demand. In
the calculations an annual hot water demand of 2,000 kWh is added. The
DGC-report 24
heating system efficiency is expressed as a weighted efficiency with an up-
per and lower limit. The lower limit is calculated with electricity energy
multiplied by a factor 2.5 to get the primary energy consumption. The elec-
tricity consumption is treated in the same manner in the current Danish en-
ergy labelling system. The upper limit is calculated as if the electricity gen-
eration is carbon-free, for example wind power, solar power or hydro power.
The box at the upper right corner contains information about the heat pump
performance and the electricity input to the compressor. The number at each
point of the weighted annual efficiency (green dotted line) is the heat pump
seasonal COP calculated as the heat pump output divided by the electricity
consumption. An electricity consumption of 100 W is assumed for fans,
pumps and heat pump electronics. The heat pump COP is assumed to be
constant regardless of the output, i.e. the part-load COP is equal to the full-
load COP.
It should be noted that the electricity consumption is the sum of electricity
to the heat pump compressor and electronics, fans and circulation pumps.
The latter part is approximately 500 kWh. Thus the electricity consumption
in the graphs cannot be multiplied by the adjacent COP to get the output
energy from the heat pump. The gas consumption is composed of gas con-
sumption for hot water production and heating when the heat pump is not
covering the entire heating demand. The hot water demand is 2,000 kWh
annually in all calculations. For the boilers chosen the hot water efficiency
is approximately 75-80 %. This means that 2,700 kWh gas is consumed for
hot water production and is not affected by the heat pump operation.
The operating conditions for the gas boiler are different from the situation
when a boiler alone covers the heating demand. Firstly, the return tempera-
ture increases as the pump operates and, secondly, the boiler load is re-
duced. As a result the boiler efficiency decreases. For the 20 MWh case the
annual efficiency is decreasing to approximately 95 %, which leads to an
increase in gas consumption. This is equivalent to 500-700 kWh annual in-
crease in gas consumption.
3.2 Gas boiler operates as peak load – capacity control
The graphs in Figure 14 and Figure 15 show from top to bottom systems
with the same gas boiler but with heat pumps with increasing COP. The
DGC-report 25
result is an increased heat pump output and decreased gas consumption. The
graphs in Figure 15 where the compressor input is 50 % higher (1.5 kW)
show that this tendency is further enhanced.
Figure 14 and Figure 15 show the result for boiler 1 (3-15 kW) and air-to-
water heat pumps with 1.0 and 1.5 kW compressor input, respectively. The
three graphs in each figure show an increasingly higher heat pump COP and
follow a vertical line between the performance maps in Figure 13.
The graphs show performances for hybrid systems including boiler 1 with a
modulating range of 3-15 kW. Boiler 2 with 6-25 kW modulating range
shows no significant differences compared to the results shown in Figure 14
and Figure 15.
3.3 Price controlled operation
The results for calculations where the hybrid system operation is controlled
by the price relation between electricity and gas are shown in Figure 16 and
Figure 17. The current price relation for gas and electricity has been used. In
this control strategy the control system will actively move heat production
from the heat pump to the gas boiler in order to minimize the customer’s
heating bill. In a real installation where this option is possible the boiler ef-
ficiency and the heat pump COP are in some way compared. No applicable
algorithms have been found.
If the capacity control results are compared to the corresponding price con-
trolled results we observe slightly lower electricity consumption when a
price controlled operation is used. This is more visible for the lower annual
heating demands where the heat loads are lower.
DGC-report 26
Figure 14 Annual performance for hybrid system - Boiler 1 and 1 kW com-
pressor input. Capacity controlled operation.
DGC-report 27
Figure 15 Annual performance for hybrid system - Boiler 1 and 1.5 kW
compressor input. Capacity controlled operation.
DGC-report 28
Figure 16 Annual performance for hybrid systems - Boiler 1 and 1 kW
compressor input. Price controlled operation.
3,40
3,44 3,49
90
110
130
150
170
190
210
0
5000
10000
15000
20000
25000
0 5 10 15 20 25 30 35
An
nu
al e
ffic
ien
cy (
%)
Gas
an
d e
lect
rici
ty c
on
sum
pti
on
(kW
h)
Annual space heating demand (MWh) + 2 MWh hot water
Price control - Energy consumption and annual efficiency. El/gas price = 2,7
Gas consumption
Electricity consumption
Weighted annual efficiency
Annual efficiency, green elec
COPA7/W35A0/W55Pcomp
3,852,551000
3,55
3,63 3,69
90
110
130
150
170
190
210
0
5000
10000
15000
20000
25000
0 5 10 15 20 25 30 35
An
nu
al e
ffic
ien
cy (
%)
Gas
an
d e
lect
rici
ty c
on
sum
pti
on
(kW
h)
Annual space heating demand (MWh) + 2 MWh hot water
Price control - Energy consumption and annual efficiency. El/gas price = 2,7
Gas consumption
Electricity consumption
Weighted annual efficiency
Annual efficiency, green elec
COPA7/W35A0/W55Pcomp
4,252,951000
3,84
3,94 4,01
90
110
130
150
170
190
210
0
5000
10000
15000
20000
25000
0 5 10 15 20 25 30 35
An
nu
al e
ffic
ien
cy (
%)
Gas
an
d e
lect
rici
ty c
on
sum
pti
on
(kW
h)
Annual space heating demand (MWh) + 2 MWh hot water
Price control - Energy consumption and annual efficiency. El/gas price = 2,7
Gas consumption
Electricity consumption
Weighted annual efficiency
Annual efficiency, green elec
COPA7/W35A0/W55Pcomp
4,653,351000
DGC-report 29
Figure 17 Annual performance for hybrid systems - Boiler 1 and 1.5 kW
compressor input. Price controlled operation.
3,27
3,40 3,44
90
110
130
150
170
190
210
0
5000
10000
15000
20000
25000
0 5 10 15 20 25 30 35
An
nu
al e
ffic
ien
cy (
%)
Gas
an
d e
lect
rici
ty c
on
sum
pti
on
(kW
h)
Annual space heating demand (MWh) + 2 MWh hot water
Price control - Energy consumption and annual efficiency. El/gas price = 2,7
Gas consumption
Electricity consumption
Weighted annual efficiency
Annual efficiency, green elec
COPA7/W35A0/W55Pcomp
3,852,551500
3,55
3,57 3,63
90
110
130
150
170
190
210
0
5000
10000
15000
20000
25000
0 5 10 15 20 25 30 35
An
nu
al e
ffic
ien
cy (
%)
Gas
an
d e
lect
rici
ty c
on
sum
pti
on
(kW
h)
Annual space heating demand (MWh) + 2 MWh hot water
Price control - Energy consumption and annual efficiency. El/gas price = 2,7
Gas consumption
Electricity consumption
Weighted annual efficiency
Annual efficiency, green elec
COPA7/W35A0/W55Pcomp
4,252,951500
3,83
3,86 3,94
90
110
130
150
170
190
210
0
5000
10000
15000
20000
25000
0 5 10 15 20 25 30 35
An
nu
al e
ffic
ien
cy (
%)
Gas
an
d e
lect
rici
ty c
on
sum
pti
on
(kW
h)
Annual space heating demand (MWh) + 2 MWh hot water
Price control - Energy consumption and annual efficiency. El/gas price = 2,7
Gas consumption
Electricity consumption
Weighted annual efficiency
Annual efficiency, green elec
COPA7/W35A0/W55Pcomp
4,653,351500
DGC-report 30
3.4 Energy costs for the hybrid system
The annual heating costs for hybrid systems are illustrated in two graphs in
Figure 18. The upper graph shows the costs for capacity controlled systems
and the lower graph shows the results for price controlled systems. The solid
blue graph at the far right shows the heating cost for a gas boiler alone. Dan-
ish energy prices have been used. The gas price is 8.12 DKK/m3 and the
electricity price used is 2.04 DKK/kWh.
The graphs clearly show that the heating cost can both lower and higher
than the heating cost for a condensing boiler alone. Price controlled opera-
tion shows slightly lower heating cost where the gas cost has a larger share
of the overall cost than in capacity controlled operation mode.
Figure 18 Heating cost for hybrid systems installed in a house with 20,000
kWh annual heat demand and 2,000 kWh hot water demand
DGC-report 31
The influence of the price relation (e/g) between electricity and natural gas
is illustrated in Figure 19. Heating with a gas boiler only has the heating
cost equal to 1.0. Systems 4–6 are selected for this calculation, i.e. boiler 1,
1.5 kW compressor input and heat pump COP = 3.85, 4.25 and 4.65.
Figure 19 Influence of the price relation (e/g) between electricity and nat-
ural gas on the relative heating cost using hybrid systems com-
pared to a condensing gas boiler alone
The calculations show that the lowest heating cost for the consumer is ob-
tained when the electricity price is favourable and thus maximizing the op-
eration with the most efficient heat generator, the heat pump. A possible
reduction of up to 30 % of the heating cost is possible with the input data
used. However, current Danish gas and electricity prices rather indicate a
possible reduction of 15-20 % for the consumer.
Only the cost for gas and electricity is reasonably known today. Due to the
early market stage the investment and maintenance costs are not known well
enough to make an overall economic evaluation for the consumer. This
ought to be assessed in further studies and field tests.
The advantages for the gas and electricity utilities lie mainly in the possibil-
ity of using the hybrid systems as part of a smart grid where gas and elec-
tricity interact. Renewable electricity can be used in the best possible way.
The hybrid systems can be switched over to gas operation when for example
the wind power output is low and the option is expensive power generation
with high CO2 emissions.
DGC-report 32
4 Conclusions
Hybrid systems have recently been introduced on the market. An electric
heat pump covers the base load while a gas boiler covers peak heating load
and the hot water production. These heating systems are offered either as
integrated units, separate heat pump and boiler or as add-on heat pump to
existing gas boilers.
The hybrid system operation may be controlled according to several prin-
ciples. Most common is a capacity controlled operation where the gas boiler
is used for heating only when the heat pump output is not large enough for
the heating demand. In a price controlled operation the lowest heating cost
determines heat pump or boiler operation. The heat pump COP is checked
against the boiler efficiency and the relationship between electricity and gas
price. In a similar way the lowest CO2 emission can control the operation. In
the future, an external control by gas or electricity utilities as part of a
smart-grid concept will be possible in order to adapt the operation to the
availability of renewable electricity.
In this report hybrid systems are described and the operation is simulated.
These simulations show the annual gas and electricity consumption for ca-
pacity and price controlled operation as a function of heat pump COP, size
and the price relationship between electricity and gas.
The heating cost is not automatically reduced when hybrid heating systems
are used compared to a condensing boiler only. The heat pump COP and
energy prices play an important part in the economic bottom line for the
consumer. If the heat pump COP is not high enough and the electricity price
is not favourable the heating cost may even increase.
These calculations show that the lowest heating cost is obtained when a
price control mode is used and the electricity price is favourable, thus max-
imizing the operation with the most efficient heat generator, the heat pump.
The calculations in this study show a possible reduction of up to 30 % of the
heating cost. However, current Danish gas and electricity prices rather indi-
cate a possible reduction of 15-20 % for the consumer. This ought to be ver-
ified by controlled field tests. The investment and maintenance cost for the
DGC-report 33
consumer are today not known due to the fact that hybrid systems are only
at a market introduction stage.
The advantages for the gas and electricity utilities lie mainly in the possibil-
ity of using the hybrid systems as part of a smart grid where gas and elec-
tricity interact. Renewable electricity can be used in the best possible way.
The hybrid systems can be switched over to gas operation when for example
the wind power output is low and the option is expensive power generation
with high CO2 emissions.
DGC-report 34
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