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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

http://www.smartgrids.eu/

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.

http://www.dmkv.dk/

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)



Gas consumption





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)



Gas consumption





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)



Gas consumption





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)



Gas consumption





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)



Gas consumption





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.







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.







DGC-report 34

References

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Smart Grids. Expert Group 4, 2011.

[2] M. Larsen, "Regulerkraft på villakedelområdet. Overskuds-el fra

vindmøller til naturgaskedler," Dansk Gasteknisk Center, 2011.

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SGC 113, 2000.

[4] J. Lemmens, J. Darmeveil, J. Eerland, S. Hegge, J. Turkstra and J.

Westing, "Combination micro-CHP - heat pump: proof of principle," in

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[5] C. Vuillecard, "Bottom-up model for local gas and electricity

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[6] T. Afjei and R. Dott, "Heat pump modelling for annual performance,

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[7] H. Cheung and J. E. Braun, "Performance Characteristics and Mapping

for a Variable-Speed Ductless Heat pump," in International

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2010.

[8] L. van Gruijthuijsen and M. Näslund, "Description of the calculation

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