E.ON Energy Research Center Series

Hardware-in-the-Loop Test Bed for Home Energy Systems

Kan Chen, Rita Streblow, Dirk Müller Christoph Molitor, Andrea Benigni, Antonello Monti Marco Stieneker, Rik W. De Doncker

Volume 4, Issue 2

Contents

1 Executive summary 1

2 Introduction 22.1 Goals of the project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2 State of the art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.3 Positioning of project within E.ON ERC strategy . . . . . . . . . . . . . . . . . . . . . . . 4

3 Results 53.1 The data basis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2 Performance data of heat pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.1 Analysis of a simple AWHP system . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2.2 Analysis of a AWHP system with domestic hot water generation . . . . . . . . . 16

3.2.3 Analysis of a BWHP system with ground source heat exchanger . . . . . . . . . . 17

3.3 Major performance factors and cross correlations . . . . . . . . . . . . . . . . . . . . . . 20

3.4 Enhancement for heat pump systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.5 Simulation models for heat pump systems . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.5.1 Heat pumpmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.5.2 Ground source heat exchanger models . . . . . . . . . . . . . . . . . . . . . . . . 29

3.5.3 Stratified Storage Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.5.4 Validation of models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.5.5 Verification of combined models . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.6 Analysis of the refrigerant circuit performance under realistic boundary conditions . . 35

3.6.1 Test building for parameter studies . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.6.2 Study on building insulation standard . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.6.3 Study on buffer storage volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.6.4 Study on heat sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.6.5 Study on bivalent heat pump systems . . . . . . . . . . . . . . . . . . . . . . . . . 43

4 Conclusion 48

5 Further steps, future developments and proposed actions 50

6 Literature 51

7 Attachments 53

i

Contents

7.1 List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

7.2 List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

7.3 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

7.4 Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

7.5 Short CV of scientists involved in the project . . . . . . . . . . . . . . . . . . . . . . . . . 56

7.6 Project timeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

7.7 Activities within the Scope of the project . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

ii

1 Executive summary

The efficiency of electrically driven compression heat pumps for existing dwellings has been eval-

uated with the help of data from a field test of the E.ON Energie AG, conducted through the Fraun-

hofer Institute for Solar Energy Systems in Freiburg. The data has been verified and grouped to

allow a detailed statistical analysis. A numerical calculation procedure has been developed that

models the total system consisting of heat pump, heat source and heat sink modeling the most

important influencing parameters.

A detailed analysis of the heat pump field test shows that for 43 field test objects evaluable data ex-

ists. Thereof 21 are air-to-water heat pumps, 17 brine-to-water heat pumpswith horizontal ground

source heat exchangers and 5 are brine-to-water heat pumps with vertical ground source heat ex-

changers.

A mean seasonal performance factor of 2.3 for air coupled devices and 2.9 for ground coupled

devices shows that the application of heat pumps of differentmanufacturers in existing buildings is

questionable on a level of primary energy savings and is economically inefficient. The best systems

achieved seasonal performance factors of 3.0 respectively 4.0. The great differences to the mean

values on one hand show the high potential of the technology and on the other side the necessity

of an optimization of the total system.

On the basis of field study data, a new dynamic calculation procedure has been developed using

the modeling language Modelica. It allows a detailed simulation of the total heat pump system.

The heat pumpmodel works with lookup table data, all other components are modeled according

to their physical properties. Numerical studies have shown that notably speed controlled heat

pumps and a combination of air-to-water heat pumps and boilers (the bivalent or hybrid heat

pump) have potential for primary energy savings in existing and in new buildings.

1

2 Introduction

2.1 Goals of the project

Goal of this research project is to evaluate the data from a field study experiment on domestic heat

pumps. The data will be used for a second level analysis as well as for a detailed numerical study

based on a calibrated simulation model. The following objectives are to be accomplished:

1. Performance data of different kinds of heat pumps.

2. Major performance factors for heat pump systems.

3. Cross correlations between different performance factors.

4. Enhancement for heat pump systems.

5. Simulation models for heat pump systems.

6. Analysis of the refrigerant circuit performance under realistic boundary conditions.

The data will be forwarded to original equipmentmanufacturers in order to encourage heat pump

development activities.

2.2 State of the art

Theheat pumpsusedwithin the field test are heat pumpswith standard refrigerants such as R407C,

R404A and R410A. They are generally built with a scroll compressor and controlled by an on/off-

controller. The systems usually need buffer storages with volumes of more than 500 l to ensure a

high inertia of the heating system that is needed for such a control.

Today still many systems are built this way. Nevertheless, a lot of research has been done in the

field of heat pumps. Inverter controlled heat pumps are available on the European market that

allow a compressor speed control. In the future alternatives to classic refrigerants with lower global

warming potential will be needed.

For the evaluation of heat pump systems today static calculations are used such as the VDI 4650

in Germany or in the near future the European guideline "Energy using Products" [2009-125-EC].

These guidelines use outcomes of steady state test procedures, i.e. operating points according to

the European standard DIN EN 14511. The guidelines provide simple calculation procedures that

2

Introduction

are based on standard load profiles of buildings. Different kinds of control such as a compressor

speed regulation or an innovative storage loading concept are not fully accounted for.

The European standardDIN EN 15316 uses a balance room that contains storages and load pumps

and thus focuses more on the system analysis. With this standard it is also possible to do a more

detailed hourly calculation. Nevertheless, this procedure is also based on steady state operating

mode. Being easy to calculate, in Germany, for the determination of governmental financial sup-

port, heat pump systems are generally evaluated by VDI 4650.

Figure 2.1:Overview of important standards in the field of heat pumps.

In the calculation procedure of the VDI 4650 ("Simplified method for the calculation of the sea-

sonal performance factor of heat pumps – Electric heat pumps for space heating and domestic hot

water") the SPF is determined through a simple calculationmethod (see figure 2.2). It is calculated

for heatingmode and for hot water production separately and the overall SPF is calculated weight-

ing the SPF for heating and the SPF for hot water demand on a pro-rata basis. Heat produced

through secondary heat generators is accounted for, too. The assumptions for the calculation are

a ground water temperature that is constant over the heating season (water-coupled heat pumps),

an average ground temperature is used (BWHPs) and climate data is used (AWHPs). Influences

that are not accounted for are buffer storages including their loading cycles, user influences (room

temperature, airing habits, controller adjustments etc.), controller settings and complex system

setups. A comparison of the SPF calculated by this guideline and the SPFmeasured in the field test

is done in section 3.2.

3

Introduction

Figure 2.2: Calculation procedure of the VDI 4650 guideline (simplified overview).

2.3 Positioning of project within E.ON ERC strategy

The project fits into the total strategy of E.ON ERC regarding the research in the field of complex

home energy systems. Therefore the detailed understanding of heat pump systems is important.

The models developed within this project are used within different projects at the E.ON ERC such

as the Phase Change Material (PCM) storage project. The Hardware-in-the-Loop (HiL) test bed

which is used by the institutes ACS and EBC will use the Modelica models developed within this

project. New cooperations with heat pumpmanufacturers could be acquired using the knowledge

gained within this work.

4

3 Results

3.1 The data basis

The field test analysed within this project was conducted through the Fraunhofer Institute for So-

lar Energy Systems in Freiburg. It contains 77 heat pump systems all over Germany and started in

2007. Air-to-water heat pump (AWHP) systems as well as brine-to-water heat pump (BWHP) sys-

tems are analysed. All test objects were formerly heated with oil boilers. Heat pumps of different

manufacturers were installed by local companies. Thus the test does not show the "state of the

art" but "as built in" results for heat pump systems in existing one family houses. All analyses in

sections 3.2 to 3.4 show findings that derive from the field test and should be seen in that context.

The field test objects are very inhomogeneous with respect to system layout, which complicates

the analysis and makes it difficult to extract single effects from the data.

The field test data exist as time series of measurements of temperatures, volume flows and electric

currents (1 minute time steps). Not all data is available for every time and every object due to

breakdown of measurement or data transmission. In addition not every value can be calculated

with the available time series. Generally, in thiswork all field test objects are regarded that allow the

determination of the desired value in the years 2008 and 2009. To allow a comparability of objects

and an economic analysis, a small amount of missing data (15 %) is tolerated and filled up with

data calculated by a ambient temperature dependent regression function fitted to the available

data (function according to BGW [2006]). Missing data of the ambient temperature is taken from

the Deutscher Wetterdienst (GermanMetrological Service) weather station nearest to the field test

object. Figure 3.1 shows a sample regression for the daily heating energy of a field test object.

−10 −5 0 5 10 15 200

50

100

150

200

250

300

Temperature in °C

Hea

ting

ener

gy in

kW

h measured daily valueregression

Figure 3.1: Regression of the daily heating energy by the daily mean ambient temperature.

5

Results

Finally 43 objects had a sufficient data basis whereof 21 are AWHPs, 17 are BWHPs with vertical

ground source heat exchangers (GSHX) and 5 are BWHP with horizontal GSHXs. Figure 3.2 shows

the construction years of the buildings within the field test and the according specific heat de-

mands. Insulation standards according to the building construction year can’t be identified. Even

some of the newest buildings achieve high values probably generated through a corresponding

user behavior. A comparable observation can be done related to design flow temperatures of heat-

ing systems: Some of the newest buildings are equipped with heating systems that have high flow

temperatures.

1900 1920 1940 1960 1980 2000

80

100

120

140

160

180

200

220

240

Year of building construction

Ann

ual h

eat d

eman

d pe

r hea

ted

area

in k

Wh/

m2

field test object

Figure 3.2: Construction years of the field test objects analysed and the according specific heatdemands.

6

Results

3.2 Performance data of heat pumps

The performance factor PF of a heat pump system is defined as the quotient of the heating energy

used to the electrical energy supplied to the heat pump:

PF= QuseWel

(3.1)

As simple as this definition is, it leaves the freedom to set according system boundaries. In this

work, the system boundary is chosen in a way that makes the heat pump system comparable to

other heating systems. Thatmeans that for example the energy used for loading pumps (which are

not necessary in boiler systems) is included. This system boundary is given the index ’1’ whereas

another systemboundary according to a German guideline VDI 4650 is given the index ’2’. The sys-

tem boundary is shown in Figures 3.3 and 3.4 by dashed lines. The green arrows represent the elec-

tric energy input and the red arrows stand for the heat output. Two important influences are the

electric energy consumption for the loading pumps and the losses of a buffer storage (included in

system boundary 1). System boundary 1 assumes the storage being located outside of the thermal

building shell whichmeans that all storage losses are heat losses for the system. The PF calculated

in the system boundaries 2 will be higher than the one calculated within system boundary 1 (see

figure 3.5). The Seasonal Performance Factor (SPF) stands for the PF that is calculated for the time

of one year. In the field test the mean difference between SPF2 and SPF1 is 11.3 %.

All calculations concerning primary energy demand and costs are done using the systemboundary

1, allowing a comparison to a boiler system. Unless indicated otherwise, all calculations are based

on the data of the years 2008 and 2009.

Table 3.1: Comparison of system boundary 1 and 2.

system boundary 1 system boundary 2heat energy - delivered to heating system - delivered to the buffer storage

(+heating energy delivered by heat-ing rod, if in buffer storage)

- for domestic hot water generation - for domestic hot water generationelectric work of - compressor - compressor

- source pump or ventilator - source pump or ventilator- heating rod - heating rod- loading pumps

Using the system boundary 1 the performance factor directly indicates a certain heat pump sys-

tem’s savings in primary energy compared to a boiler: Assuming a fixed efficiency of a condensing

gas boiler of 96 % and a primary energy factor of fP,ng = 1.1 for natural gas and of fP,el = 2.6 forelectricity in Germany [DIN SPEC 4701], a SPF1 of 2.3 is the boundary to savings in primary energy.

7

Results

Figure 3.3: System boundary 1 used for analysis.

Figure 3.4: System boundary 2 according to VDI 4650.

1.5 2 2.5 3 3.5 4 4.51.5

2

2.5

3

3.5

4

4.5

SPF2

SP

F 1

AWHPBWHP

Figure 3.5: Comparison of SPFs within different system boundaries measured in the field test.

8

Results

2 3 4 51.5

2

2.5

3

3.5

4

4.5

5

SPFvdi

SP

F 2

AWHPBWHP

Figure 3.6: Comparison of SPFs calculated by VDI 4650 andmeasured in the field test within samesystem boundaries.

In section 2.2 the guideline VDI 4650 is described. In figure 3.6 the SPFs calculated by this guideline

(SPFvdi) are compared to those measured within the field test (SPF2). The SPFvdi is calculated with

the design data available for the field test objects. It is usually the input to the calculation with

VDI 4650 because there is nomeasurement data availablewhen a heat pump systemgets evaluated

by the guideline to get classified e.g. for a federal support. This is usually done before a system is

installed.

There are differences between the calculated andmeasured SPFs. The guideline itself accentuates

its assumptions. It unlikely gets the same results as the measured data. The mean difference is

23 % and the SPFvdi in most cases is higher than the SPF2 for the evaluated field test objects. It

is questionable if the VDI 4650 is an adequate manner to evaluate the efficiency of heat pump

systems. In addition it is questionable if the system boundaries chosen are adequate to describe

the efficiencywith respect to primary energy demand. It cannot foresee the primary energy savings

compared to a boiler system as the loading pumps and storages are not accounted for. As the field

test results show, a lot of parameters do influence the heat pump efficiency that are not considered

within the guideline.

A German federal program (cf. BAFA 2010) supports heat pump systems that achieve a SPFvdi of

above 3.7 (AWHPs) respectively 4.3 (BWHPs). These limit values are high enough to ensure the

heat pump system to have a high efficiency. But it also is quite inexact: Within the field test only 10

systems achieve these values (2 AWHP systems, 8 BWHP systems). These 10 objects indeed achieve

high SPF2-values and they also do save primary energy compared to boilers. But there are other

systems that have a higher SPF2 and -more important concerning the energy demand of the whole

9

Results

system - a higher SPF1 than these objects.

In figure 3.7 relative savings in primary energy are shown compared to a hypothetical new con-

densing boiler that produces the same amount of heat as the heat pumps with a fixed efficiency

of 0.96. Each field test object is indicated through its number on the abscissa. It shows, that only

BWHPs achieve savings in primary energy in the field test. The majority of AWHP systems uses

more primary energy than condensing boilers would (2.4%more). Whereas BWHP systemsmerely

achieve savings of primary energy, on average 18.8 %. Only three of 21 ground coupled systems are

less efficient than a new boiler. It should be considered that primary energy factors can change e.g.

with increasing percentage of renewable energy.

The investment for heat pumps follow a clear principle of lower specific costs with higher heat

pumpnominal power rates (see figure 3.8). The nominal power is set to the output heat flowat con-

ditions of an ambient air temperature of 2 ◦C and a heating flow temperature of 35 ◦C (’A2W35’) forAWHPs. For BWHPs the nominal heat flow is defined accordingly butwith a source temperature set

to a inlet brine temperature of 0 ◦C (’B0W35’). The mean investment costs for heat pumps withinthe evaluated objects are 16,640 EUR for AWHPs and 20,940 EUR for BWHPs. These investment

costs include costs for the heat source exploitation and for auxiliary components such as storages.

The specific investment costs are 1,257 EUR/kW for AWHPs and 1,659 EUR/kW for BWHPs.

The investment costs of BWHPswith horizontal GSHXs are comparable to those of AWHPs (see fig-

ure 3.8). Figure 3.9 shows that these systems achieve high performance factors. Unfortunately the

installation of horizontal GSHXs is challenging especially in existing buildings because it requires

a large open area near the building to be equipped with it.

Figure 3.9 also shows again that generally BWHPs have higher SPFs than AWHPs but in most cases

have higher specific investment costs, too (vertical GSHX). Within the groups of heat pumps there

is no relation between the investment costs and the achieved SPF1.

For an economical evaluation, analogous to the calculation of primary energy demands, the heat

pump systems are compared to a new boiler system, but also to further operation of the old boiler.

The assumptions to this calculation are summarized within tables 3.2 and 3.3. Figure 3.10 shows

the net present value of the field test heat pump systems compared to a hypothetical new condens-

ing boiler system. It is assumed that a new gas condensing boiler is installed substituting the old

oil boiler. All heat pump systems but one have negative values, which means that economically

it is more efficient to install a modern boiler system. Nevertheless, it should be considered that

the inputs to the calculation in reality aren’t fix. For example changing energy prices do distinctly

influence the results. Even with BWHP systems being more efficient on a level of primary energy

their mean net present value is lower than that of AWHP systems. It shows that the higher invest-

ment costs of ground coupled systems carry the risk of a high economic losses. Figure 3.10 also

shows, that within the group of BWHPs, systems with vertical GSHX tend to achieve the highest

net present values because of low investment costs.

10

Results

Table 3.2: Investment costs for new condensing boilers according to Ene [2009]

condensing gas boiler

up to 27 kW 3590 EURup to 35 kW 3,897 EURup to 49 kW 4,433 EURup to 66 kW 5,213 EURup to 80 kW 7,979 EURup to 105 kW 8,792 EURup to 130 kW 9,792 EUR

installation 1,500 EUR

demounting oil tank 1,700 EUR

installation of gas grid connection 2,000 EUR

additional equipment 500 EUR

Table 3.3: Assumptions for the economical evaluation. Investment and annual costs according toEne [2009]

gas price 6 ct/kWhth

electricity price 17 ct/kWhel

imputed interest rate 4 %life-span 20 a

investment new boiler see table 3.2efficiency of new boiler 0.96

annual fixed costs of heat pump 20 EUR/aannual fixed costs of new boiler 122 EUR/aannual fixed costs of old boiler 240 EUR/a

If investment costs are not taken into account, the specific costs of heat supply of BWHPs (mean

6.18 ct/kWh) are lower than for the new boiler (mean 6.84 ct/kWh). Systems with air as heat source

have higher costs of heat supply than the new boiler (mean 7.77 ct/kWh). Compared to the old

boiler (mean 8.35 ct/kWh) only nine systems have slightly higher costs. The fixed costs for boiler

systems are higher which can be explained through higher maintenance costs. The cost savings

compared to the new boiler are summarized in figure 3.11. AWHPs in average have 13 % higher

costs of heat supply than the according new boiler systems, BWHPs in average save 9.4 % in costs

of heat supply compared to new condensing boilers. Some objects have higher absolute values

in heat supply costs, but save more compared to the boiler system than other objects. As for all

objects the same costs for supplied energy and the same boiler efficiency is assumed, the reason

for that are different heat demands. They lead to different ratios of variable costs to fixed costs.

11

Results

−40

−30

−20

−10

0

10

20

30

40

Prim

ary

ener

gy s

avin

gs in

%

15 28 32 1 16 37 8 14 6 22 38 25 35 27 4 33 3 13 20

AWHP radiator and floor heatingAWHP floor heatingAWHP radiator heating

mean: −2.4%

−40

−30

−20

−10

0

10

20

30

40

(a) AWHP

−60

−40

−20

0

20

40

Prim

ary

ener

gy s

avin

gs in

%

11 29 12 5 36 21 43 19 7 23 30 34 24 18 26 41 10 17 31 2

BWHP radiator and floor heatingBWHP floor heatingBWHP radiator heating

mean: 18.8%

−60

−40

−20

0

20

40

(b) BWHP

Figure 3.7: Relative savings in primary energy compared to a new gas boiler.

12

Results

5 10 15 20 25 30 35500

1000

1500

2000

2500

Heat pump nominal power in kW

Spe

cific

inve

stm

ent c

osts

in E

UR

/kW

AWHPBWHP, vertical GSHXBWHP, horizontal GSHX

Figure 3.8: Relative investment costs of heat pumps related to heat pump nominal power (atA2W35 (AWHPs) respectively B0W35 (BWHPs)).

500 1000 1500 2000 25001.5

2

2.5

3

3.5

4

4.5

Investment costs per heat pump nominal power in EUR/kW

SP

F 1

AWHPBWHP

Figure 3.9: SPF1 and relative investment costs of heat pumps related to heat pump nominal power(at A2W35 (AWHPs) respectively B0W35 (BWHPs).

13

Results

−25

−20

−15

−10

−5

0

5N

et p

rese

nt v

alue

in 1

000

EU

R

28 8 15 16 35 32 22 38 20 6 14 25 37 1 3 4 33

AWHP radiator and floor heatingAWHP floor heating

mean: −9.21

−25

−20

−15

−10

−5

0

5

(a) AWHP

−25

−20

−15

−10

−5

0

5

Net

pre

sent

val

ue in

100

0 E

UR

29 5 12 24 30 19 26 23 10 9 21 31 7 41 2 36 43 34 17 18

BWHP with vertical GSHXBWHP with horizontal GSHX

mean: −9.37

−25

−20

−15

−10

−5

0

5

(b) BWHP

Figure 3.10:Net present value of the field test heat pump systems compared to a new condensingboiler system (investment costs included).

14

Results

0

0.02

0.04

0.06

0.08

0.1

15 28 16 32 1 8 14 37 6 38 25 22 35 27 33 4 3 13 20

Hea

t sup

ply

cost

s in

ct/k

Wh

−40

−30

−20

−10

0

10

20

30

−38 %

−9 %

+2.4 %

+13 %

Sav

ings

in %

AWHP radiator and floor heatingAWHP floor heatingAWHP radiator heating

savings compared to new condensing boiler

(a) AWHP

0

0.02

0.04

0.06

0.08

0.1

11 29 12 5 43 19 21 7 36 23 30 18 24 34 41 26 10 17 31 2

Hea

t sup

ply

cost

s in

ct/k

Wh

−40

−30

−20

−10

0

10

20

30

−38 %

−0.5 %

+4.3 %

+27 %

Sav

ings

in %

BWHP (vertical GSHX)BWHP (horizontal GSHX)

savings compared to new condensing boiler

(b) BWHP

Figure 3.11: Savings in specific costs of heat supply for the evaluated field test objects comparedto a new condensing boiler system (investment costs not included).

15

Results

3.2.1 Analysis of a simple AWHP system

A simple system is analysed. It is an AWHP system that has a buffer storage and is only used for

heating, not for domestic hot water generation. A scheme of the system is shown in figure 3.12.

The heat delivery system is built with radiators in a building of 1975. The buffer storage volume

is 500 l. The heat pump installed has a power output of 19.8 kW at A2W35. The heat load of the

building is 8.5 kW and the maximum flow temperature of the heating system is 55 ◦C.

Figure 3.12: Scheme of field test object 25.

Still with the buffer storage the heat pump has 4000 operating intervals within one year. The heat

pump system has a SPF1 of 2.17 in the year of 2009. The storage losses are 6.3 % throughout the

year. Figure 3.13 shows how the buffer storage works. The heat flow in front of the buffer storage

is intermittent because of the on/off-control of the heat pump. Behind the storage, the heat flow

is alternating but does not get to zero. Still the flow temperature behind the storage is alternat-

ing strongly (see figure 3.14) within an interval of approximately 5 K. Simulations including the

building show that even the room temperature is influenced through that (see section 3.6.2).

3.2.2 Analysis of a AWHP system with domestic hot water generation

Object 4 is an AWHP system as shown in figure 3.3. It has two loading circuits: One supplying a

domestic hot water (DHW) storage and one for the buffer storage that supplies the heating sys-

tem. The building year of construction is 1991 and the buffer storage volume is 500 l. Figure 3.15

shows monthly mean values for the year 2009. In the upper part of the chart the temperature lift

is shown between the blue and red line. The red line represents a weighted mean value of the flow

temperature in the heating circuit (red circles) and the temperature in the loading circuit in the

DHW loading circuit according to the energy supplied to the corresponding circuits. The monthly

performance factor (see lower part of figure) is reciprocally proportional to the temperature lift.

The SPF1 of this object is 2.48 (2009). This means a small amount of savings in primary energy

compared to gas boilers. This is achieved through relatively low flow temperatures in the heating

system.

16

Results

0 8 16 240

5

10

15

Time in h

Hea

t flo

w in

kW

Qflow,1Qflow,2

Figure 3.13:Heat flow in front of and behind the buffer storage of object 25, January day.

3.2.3 Analysis of a BWHP system with ground source heat exchanger

In different ways field test object 31 is favorable: As figure 3.16 shows, it is a BWHP system with

a vertical ground source heat exchanger (GSHX). This leads to a high source temperature. The

heating system operates with a floor heating allowing a low flow temperature. In addition floor

heating systems have a high inertia allowing a operation without buffer storage. Thus no storage

losses occur. The construction year of the building is 1982.

The source temperature is above 2 ◦C throughout the whole year 2009. There is a recreation phasein summer time, when little heat is extracted from the ground. Heat can flow from the surrounding

ground to the ground source heat exchanger. The increase of source temperature can also be seen

for field test object 31 in figure 3.17. The relatively low temperature lift leads to a high SPF1 of

3.45 (2009). Due to little operation of the heat pump during the summer months, these months

do only have a small impact on the SPF. The figure shows, that even during the summer months a

small amount of heating energy ismeasured. The reason for this can be a controlling that heats the

building within cold nights or a three-way-valve not fully shutting the heating circuit (cf. scheme

in figure 3.16).

17

Results

0 8 16 24

45

50

55

60

Time in h

Tem

pera

ture

in °

C

Tflow,1Tflow,2

Figure 3.14: Flow temperature in front of (1) and behind (2) the buffer storage of object 25, Januaryday.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0

1

2

3

4

5

Per

form

ance

fact

or

SPF1=2.48

Frac. QDHW

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec−10

0

10

20

30

40

50

60

Tem

pera

ture

in °

C

source sink Load. circ. DHW flow heating

Figure 3.15: Temperature lift and SPF1 for field test object 4 in 2009.

18

Results

Figure 3.16: Scheme of field test object 31.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0

1

2

3

4

5

Per

form

ance

fact

or

SPF1=3.45

Frac. QDHW

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0

10

20

30

40

50

60

70

Tem

pera

ture

in °

C

source sink load. circ DHW flow heating

Figure 3.17: Temperature lift and SPF1 for field test object 31 in 2009.

19

Results

3.3 Major performance factors and cross correlations

The major parameters influencing the SPF1 of heat pump systems are determinated. The main

features of the field test objects are analysed by a statistical feature selection. The parameters

most influencing the performance factor of heat pumps are the heat source type (air/brine), the

heat sink type (radiators/floor heating), the heating flow temperature and the existence of a buffer

storage. Tables 3.4 and 3.5 show the mean SPF1 for groups of field test objects classified according

to heat source and heat sink type and storages.

Table 3.4: Cross correlations between heat source and sink types

SPF1 (number of systems) Radiators Floor heating All heating systemsAWHP 2.16 (13) 2.77 (5) 2.33 (18)BWHP 2.76 (9) 3.05 (10) 2.91 (19)All Sources 2.40 (22) 2.96 (15) 2.63 (37)

Table 3.5: Cross correlations between heat source and storage

SPF1 (number of systems) with storage without storage All systemsAWHP 2.33 (18) n/a (0) 2.33 (18)BWHP 2.79 (9) 3.17 (6) 2.91 (19)All Sources 2.52 (22) 3.17 (6) 2.63 (37)

Themean SPF1 for AWHPs is 2.33, for BWHPs 2.91. Figure 3.18 shows that for both heat pump types

achieve a wide range of performance factors (1.77. . .3.05 for AWHPs, 1.93. . .3.95 for BWHPs). For

AWHPs the systems with floor heating systems are the most efficient. For ground coupled systems

this effect cannot be seen that clearly.

There is a clear effect of higher heat pump efficiencies with lower design flow temperatures of the

heating system (cf. figure 3.19). The real flow temperaturemeasured for the field test objects at very

low ambient temperatures often are below these design values. For some objects a clear heating

curve could be extracted and a measured maximum flow temperature could be determinated. In

figure 3.20 the performance factor is given depending on these temperatures. With these values

the effect described above is even clearer.

The buffer storage volumes in the field test differ from 0 l (no storage) to 950 l. The two largest

buffer storages are installed in the objects with the highest heat pump nominal power. Except that,

a principle of larger volumes with higher nominal power cannot be identified. There is a tendency

of decreasing heat pump efficiencies with larger buffer storage volumes (cf. figure 3.21).

As detected in section 3.1, some of the newest buildings within the field test have the highest spe-

cific heat demands. And newer buildings also have a high variety of flow temperatures, some of

20

Results

them being higher than the flow temperatures in the oldest buildings within the field test. It is not

reasonable to classify the heat pump efficiency according to the buidling’s year of construction (cf.

figure 3.22).

21

Results

0

1

2

3

4

5S

PF 1

15 1 28 32 16 8 14 37 6 22 25 35 27 4 33 3 13 20

AWHP radiator and floor heatingAWHP floor heatingAWHP radiator heating

mean: 2.33

0

1

2

3

4

5

(a) AWHP

0

1

2

3

4

5

SP

F 1

11 29 12 5 36 21 19 7 23 30 34 24 18 26 41 10 31 17 2BWHP radiator and floor heatingBWHP floor heatingBWHP radiator heating

mean: 2.91

0

1

2

3

4

5

(b) BWHP

Figure 3.18: SPF1 according to type of heat source and heat sink.

22

Results

25 30 35 40 45 50 55 60 65 701.5

2

2.5

3

3.5

4

4.5

Design flow temperature in °C

SP

F 1AWHPBWHP, vertical GSHXBWHP, horizontal GSHX

Figure 3.19: SPF1 and design flow temperature.

25 30 35 40 45 50 55 60 65 701.5

2

2.5

3

3.5

4

4.5

Measured maximum flow temperature in °C

SP

F 1

AWHPBWHP, vertical GSHXBWHP, horizontal GSHX

Figure 3.20: SPF1 andmeasured flow temperature.

23

Results

0 200 400 600 800 10001.5

2

2.5

3

3.5

4

4.5

Buffer storage volume in l

SP

F 1AWHPBWHP

Figure 3.21: SPF1 and buffer storage volume.

1900 1920 1940 1960 1980 20001.5

2

2.5

3

3.5

4

4.5

Year of building construction

SP

F 1

AWHPBWHP

Figure 3.22: SPF1 and year of building construction of field test objects.

24

Results

3.4 Enhancement for heat pump systems

Several enhancements for heat pump systems derive from the study of the field test data. The

major enhancements are listed in table 3.6.

Table 3.6:Major enhancements for heat pump systems.

Enhancement Limiting factors

1 Heat source with high temperature Investments, local availability2 Heat sink with low temperature Heat delivery system, building insulation

standard, design of basic control (e.g. heatcurve)

3 Small storages Refrigerant circuit control

These enhancements can be subdivided into practical advices which are explained as follows:

1. The heat source should provide a high temperature. Thatmeans that BWHP or groundwater

coupled heat pumps aremore efficient than AWHP. Generally, vertical GSHXs provide higher

source temperatures than horizontal ones. Nevertheless, this effect could not be observed

analysing the performance factors, as objects with horizontal GSHXs have lower heating flow

temperatures in the field test.

2. The heat sink should need a low temperature. Heating delivery systems with large surfaces

are advantageous (e.g. floor heating). But also radiator systems often can be operated at

lower temperatures than it is done. The flow temperature of the heating system should be

controlled through a heating curve that is optimally fitted to the heating system and the

building insulation standard.

3. Storages increase the inertia of the heating system but they also increase the heat emitting

surface, if it is installed outside the heated building envelope. For certain heating systems

a buffer storage is required to avoid a strong chopping of the heat pump compressor and to

ensure its lifetime. Nevertheless buffer storages are also important as hydraulic switches to

assure a high mass flow rate in the heat pump condenser.

Standard heat pump systems as analysed in the field test are designed as shown in figure 3.23 a).

The characteristic of the heat pump power output depending on the ambient temperature is con-

trarious to the heat load of the building. That means that only for one ambient temperature the

heat pumppower fits the heat load. Below that temperature (part ’1’ in the figure) a certain amount

of the building’s heat load has to be provided through an additional heat generator. In most cases

an electric heating is installed therefore. Above that temperature (called ’bivalence point’) in part

’2’ of figure 3.23 a) the heat pump has a higher heat output than the building’s heat load. That

means that the heat pump has to be controlled though clocking. Clocking leads to a workout of

25

Results

components such as the heat pump compressor. This is why clocking has to be minimized. That

can be done by increasing the thermal inertia of the heating system. A heating system with a floor

heating has a high inertia than one with radiators. Heat pump systems with radiators as heat sup-

ply system in most cases are equipped with buffer storages. The inertia of the heating system is

increased by that and operating intervals are reduced. On the other hand buffer storages do have

heat losses and therefore should be as small as possible (see also section 3.6.3).

(a) Standard (b) Controlled

(c) Bivalent

Figure 3.23:Dimensioning of heat pump systems.

The power control of heat pumps through clocking also leads to a constant rise and fall of the

heating flow temperature. That can lead to a higher variability of the room temperature (see sec-

tion 3.6.2). All the masses in the heating system are constantly heated up and do cool down again

which leads to needless heating demand.

Regarding these facts, an enhancement for heat pumps is a heat pumpwith a controlled compres-

sor. Within a certain range of the ambient temperature the heat output of the heat pumpmatches

the heat load of the building (see ’5’ in figure 3.23 b)).

26

Results

As seen in section 3.2 heat pump systems are often less efficient regarding primary energy demand

then boiler systems. This counts especially for air-to-water heat pumps and especially for low

ambient temperatures. A bivalent system is a system that combines the advantages of boiler and

heat pump systems. Contrary to the dimensioning of a standard heat pump system in figure 3.23 a)

a bivalent system is designed as it is shown in figure 3.23 c) (partly-parallel operation). At ambient

temperatures below the bivalence point only a boiler operates (’4’). Above the bivalence point

the heat pump operates supported by the boiler (’3’) or it operates alone (’2’). In section 3.6.5 a

bivalent system is analysed numerically and compared to reference cases.

The field test also showed the dependence of the SPF on storages (see section 3.3). This could

be verified numerically (see section 3.6.3). Heat pump systems without storages are more effi-

cient than systems with. That again leads to a recommendation of heat pumps with controlled

compressors and to heating systems with high inertia such as floor heating systems. Both allow

systems without storages. Nevertheless the function of buffer storages as hydraulic switches are to

be considered.

27

Results

3.5 Simulation models for heat pump systems

The Modelica [Modelica Association, 2011] model libraries developed at the Institute for Energy

Efficient Buildings and Indoor Climate allow a detailed modelling of the whole thermo-hydraulic

system of a building. New models have been developed for heat pump systems. The Modelica

language allows an acausal modelling and thus it allows to consider the interaction of all system

components. Simulations are done using the software Dymola [Dassault Systemes, 2011] for the

graphical connection and for the compilation and simulation.

The building models represent structural effects and user influences [Hoh et al., 2005], [Müller

and Badakhshani, 2010]. They are compatible with the building services installation models de-

scribed below. The library contains multilayer walls, windows and doors including the phenom-

ena involved such as heat conduction, convection and radiation. The air volume of the room is

calculated by the mediummodels of the Modelica.Media library. User influences are described by

internal heat loads and variable air exchange. The library includes an extensive weather model

based on test reference year data of the German Meteorological Service, offering boundary con-

ditions for the simulation. The model calculates the ambient temperature, air pressure, humidity

and solar exposure rates on sighted surfaces.

The building services installation library contains basic components of building services instal-

lations, such as pumps, pipes, boilers, heaters and valves. It uses medium models of the Mod-

elica.Media and components of the Modelica_Fluid libraries. Simple components calculate state

changes of fluid by look-up tables, more complex models use finite volume methods and empiric

correlations [Hoh et al., 2005], [Müller and Badakhshani, 2010]. Pipes, heat exchangers and ther-

mally activated building parts are implemented in the latter way.

3.5.1 Heat pump model

The heat pump model is implemented as a black box model consisting of two heat exchangers

that are connected to a module that calculates the heat flows and compressor power by look-up

tables using manufacturer data. Generally this data is given at working points standardized by

DIN EN 255 or DIN EN 14511. The more working points are given the better the real heat pump’s

dynamic behaviour is reproduced. The basic working scheme of this black-box model is shown in

figure 3.24.

Working points are defined through the temperature of the sourcemedium (ambient air, brine) en-

tering the heat pump and the temperature of the sink medium (heating water) temperature leav-

ing the heat pump. An additional information is the temperature difference inside the heat pump

condenser. It is usually 5 K or 10 K at test conditions. Temperature differences different to test con-

ditions are considered in the model by linear functions according to VDI 4650. The electric power

28

Results

Figure 3.24: Scheme of the heat pumpmodel.

consumption of auxiliary components in measurement data is often not separable from the elec-

tric power consumption of the compressor. Therefore an additional electric power consumption

can be set in the heat pumpmodel.

3.5.2 Ground source heat exchanger models

Ground source heat exchangers serve as heat source for the heat pump system. They are using

either the upper ground’s or the relatively deep ground’s (up to 200 m depth) heat capacity. The

models describe coaxial pipes as well as u-formed pipes discretized in axial direction connected

to a ground model. The ground model is an axially and radially discretized volume enclosing the

borehole. At the borders of the simulated ground volume a boundary condition is assumed. In

figure 3.25 a coaxial pipe is modelled and a constant temperature is assumed as boundary condi-

tion in a certain radius to the borehole. The pipe model can also be initialized with an increasing

temperature representing the geothermal coefficient. The boundary condition is then adapted

accordingly.

Figure 3.25:Model of a ground source heat exchanger and ground.

The simulation of a model with a given heat extraction time series taken from field test data leads

to a funnel-shaped distribution of temperature in the surrounding ground (see figure 3.26). The

groundmodel assumes time-invariant conditions. Ground-water flow can strongly influence such

distributions of temperature in reality.

Most ground source heat exchangers aren’t built with a coaxial pipe but with a u-pipe. Therefore

models for u-pipes have been developed, that model the complex heat transfer between the two

or four pipes in the cross-section of the single- or double-u-pipe borehole with a simplifiedmodel

29

Results

Figure 3.26: Temperature field in the profile of the groundmodel.

of form-factors (cf. [Glück, 2008]). Figure 3.27 shows the model set-up. The heat exchange in each

pipe discretisation element is described according to Glück [2008].

Figure 3.27: Scheme of u-pipe ground source heat exchanger.

Horizontal ground source heat exchangers are pipes that are installed in the upper ground in a

depth of up to 5 m [Baden-Württemberg, 2005] where one finds a seasonal change of tempera-

ture. A modelling is done using the discretized active wall model described in Hoh et al. [2005].

Figure 3.28 shows the model design with different layers that describe the surrounding ground.

3.5.3 Stratified Storage Model

Storages are important components of the heat pump system. They are used to handle variable

heating demand with on-off controlled heat pumps or serve as drinking water storages. A wide

range of different storage types exist, such as storages with heat exchangers, combined buffer and

drinking water storages as well as stratified water storages. Basically, every storage has to be mod-

30

Results

Figure 3.28: Scheme of horizontal ground source heat exchanger model.

elled separately. However, a unifiedmodelling can be done for simple buffer storages that generally

consist of a fluid volume with several fluid inlets and outlets. To implement stratification inside

this volume, the buffer storage model consists of several fluid volumes representing fluid layers

(see figure 3.29). The layers are connected to each other allowing heat and fluid flow. Buoyancy

effects are taken into account by an effective heat conductance depending on the temperature dif-

ferences between the layers. The turbulent heat conductance is a function of the layer thickness, its

temperature and the temperature difference to the above layer . It is based on the work of Viskanta

et al. [1977]. The buffer storagemodel has fluid inlets and outlets in the top and bottom layers. The

quality of stratification in the specific storage can be calibrated by choosing the number of layers

in the model.

Figure 3.29:Model of the stratified buffer storage.

3.5.4 Validation of models

A validation of library components is done using the data from the field test (cf. section 3.1). This

data is taken as input to the components or combined components. This way the model’s reac-

31

Results

tion can be compared to the behavior of the real component. This is done for the table-based

heat pump model in the set-up shown in figure 3.30. Generally the heat pump behavior can be

simulated well by the manufacturers’ data. Other components are validated analogously. For the

validation of ground source heat exchangermodels the field test data is complemented by thermal

response test data (see Gehlin and Hellström [2003]).

Figure 3.30: Validation of models and comparison of simulated values to field test data.

3.5.5 Verification of combined models

The combined simulation of the thermal building behavior, the hydraulic components, weather-

and user influences and, in case of geothermal heat pumps, geothermal heat exchangers and

groundmodels is required to describe the main influences on the performance of the heating sys-

tem. Figure 3.32 shows a model according to the sample heat pump system in figure 3.31. The

simulation results of different temperatures occurring in the system are displayed in figure 3.33.

The ambient temperature TA is a given time series from field test data. At ambient temperatures

above 15◦C the heating is turned off by the controller, so that the buffer storage is not discharged.During that time the top and bottom buffer storage temperatures (TBS,t and TBS,b) decline because

of storage heat losses. The reaction of the evaporator and condenser temperature on the water

side (TE and TC) is shown, too. The highest peaks of the condenser temperature occur when the

drinking water storage is charged. The brine temperatures (TFlow and TReturn) show an effect of

short term rebound of ground temperature during the turn-off interval of the heat pump. A room

temperature TR of 20◦C can be ensured throughout the whole day.

32

Results

Figure 3.31: Sample scheme of a heat pump system.

Figure 3.32:Model of the total system.

33

Results

Figure 3.33: Simulation results of different temperatures of the total system.

34

Results

3.6 Analysis of the refrigerant circuit performance under realistic boundaryconditions

3.6.1 Test building for parameter studies

The heat sink for the parameter studies in section 3.6 is a family home of 192 m2. The building is

modelled having two floors with four rooms each. The rooms have two windows sighting to two

directions. The air exchange ratio is 0.5 1/h. Table 3.7 shows the two window types used within

the studies. Table 3.8 describes the wall layers. Two wall configurations are considered: A simple

wall consisting of layer 1 to 3 that represents a wall with a 1970s insulation standard in Germany

(cf. IWU [2003]). The insulated wall has an additional insulation layer (between the brick and the

exterior plaster). Within all simulations the test reference year number 4 (Potsdam) is used (cf.

Christoffer et al. [2004]).

Figure 3.34: Scheme of a floor of the buildingmodel and the outer impacts on the buildingmantle.

Table 3.7:Data of the two window types.

Window Type Thermal transmittance Energy transmittance rateW/(mK) %

Type 1 2.875 80Type 2 0.750 60

3.6.2 Study on building insulation standard

In a numeric study of a complete system consisting of the components described above, different

building insulation standards are analysed. The system’s heat source is the ambient air and the

35

Results

Table 3.8:Material data of the wall layers (* insulation layer is only considered within the insulatedwall).

Thickness Density Spec. heat capacity heat conductancem kg/m3 J/(kgK) W/(mK)

1 Interior plaster 0.01 1400 1000 0.7002 Solid brick 0.35 1400 1000 0.5803 Exterior plaster 0.01 1400 1000 0.7004 Insulation* 0.10 25 1250 0.035

Table 3.9: Building variants.

Building Type Wall Window Transmission coefficientin W/K

"not insulated" no insulation Type 1 147.3"not insulated, new windows" no insulation Type 2 121.8

"insulated" insulation Type 2 37.2

heat sink is the building described in section 3.6.1. Three variants of the system are modelled (cf.

table 3.9), each with a different building insulation standard representing typical configurations

in dwellings. The first variant represents a not retrofitted home with a three-layer wall and dou-

ble glazing. The second variant is the same building equipped with new windows and the third

has new windows and an additional insulation layer. The data describing the buildings coating is

summarized in section 3.6.1, too.

As a reference system a gas condensing boiler system is analysed. Besides the heat pump and

the heat source the gas boiler systems include the same components as the other systems (see

figure 3.35). Table 3.11 summarizes the examined variants within this study.

Table 3.10: Examined variants within the study on insulation standards.

Variant Heat generator Building

1 AWHP not insulated2 AWHP not insulated, new windows3 AWHP insulated1R Gas condensing boiler not insulated2R Gas condensing boiler not insulated, new windows3R Gas condensing boiler insulated

The heat pump has a heat output of 14.8 kW and a COP of 3.60 (A2W35, EN255). The storage has a

volume of 780 l. The supply temperature is controlled through a heating curve in each system. The

heat pump is turned off depending on the temperatures in the buffer storage. If the temperature

36

Results

(a) AWHP system

(b) Reference gas boiler system

Figure 3.35: Scheme of the heat pump system and the reference system.

falls below a certain value at the upper sensor the heat pump turns on and stays on until a certain

temperature at the bottom sensor is reached. The control of the boiler allows a continuous control

of the heating load. Table 3.11 shows the analysed variants. All systems are working with a control

switching off the heating at ambient air temperatures above 15 ◦C.

The outcomes of the annual simulation of the variants are given in figure 3.36 as the primary en-

ergy demand. Primary energy factors are fP,el = 2.6 for electricity and fP,ng = 1.1 for natural gas.Different insulation standards of the modelled building do show a clear effect on the simulated

annual energy demand. Figure 3.37 pictures the performance factors of the heat pumps for one

year.

37

Results

1R 2R 3R 1 2 30

20

40

60

80

100

120P

rimar

y en

ergy

dem

and

in %

+11%*

−3.4%*

−9.7%*

* compared to 1R/2R/3R

Figure 3.36: Annual primary energy demand for the variants. For variants with heat pump (1, 2, 3)the savings compared to the boiler variants (1R, 2R, 3R) with same insulation standardare indicated.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year0

0.5

1

1.5

2

2.5

3

3.5

Per

form

ance

fact

or

123

Figure 3.37: Performance factors for the heat pump systems (variants 1, 2, 3).

38

Results

An evaluation of the heating quality is done in figure 3.38. It shows histograms of the room tem-

perature in the heat pump and the gas boiler reference system for one month. The intermittent

working behaviour of the heat pump affects the room temperatures in the way of a bigger band-

width of temperatures compared to the constantly working boiler.

There are operating times of the heating system in summermonthswhen the ambient temperature

falls below 15 ◦C. In real systems such heating times would be avoided. They are included in themodel due to the simple on/off-control (see above).

18 20 22 240

100

200

300

400

5003

Temperature in °C

Tim

e in

h

18 20 22 240

100

200

300

400

5003R

Temperature in °C

Tim

e in

h

Figure 3.38: Comparison of the room temperatures for month of February (variant 3 - AWHP, vari-ant 3R - boiler).

3.6.3 Study on buffer storage volume

A sensitivity analysis is done, varying the volume of the buffer storage in an AWHP system (Sys-

tem 1). Small volumesmean smaller heat losses but also a high number of operating intervals. The

latter has to be avoided to ensure the heat pump’s lifetime.

Figure 3.39 shows the heat pump electric power for the time of one day each. For a January day

with a high and constant heating demand a small buffer storage cannot ensure the temperatures

required by the heating system. The temperature in the buffer storage gets low faster than the

heat pump is allowed to turn on again by the controlling system. Minimum turn-off intervals are

implemented in the controls. With a large buffer storage the required supply temperatures can

be ensured and a reduced amount of operating intervals can be achieved. These results are sum-

marized in figures 3.40 and 3.41. The reference volume (764 l) is set to 100% in each chart and

represents the volume of a reference field test object’s buffer storage. The results show a lower heat

pumpwork using smaller buffer storages, because of storage heat losses being smaller. But a strong

39

Results

(a) January, small Buffer storage (b) January, large Buffer storage

(c) April, small Buffer storage (d) April, large Buffer storage

Figure 3.39:Heat Pump electric power on one January or April day each for a small (85 l) and alarge (2123 l) buffer storage.

increase of operating intervals can be observed, too. Figure 3.41 shows the number of operating

intervals and the heat pump work for one month for different tested volumes.

3.6.4 Study on heat sources

The following study looks at heat pump systems as shown in the section above. The systems are

modelled using different heat source models. The building and the building services installations

are the same as before. The tables of the heat pump model are changed if a brine-to-water heat

pump is used. The heat pump then has a heating power of 14.0 kW and a CoP of 4.59 (B0W35,

EN255). As heat sources the ambient air is chosen as well as a single and a double GSHX or a

Ground Source Collector. All other components are staying the same. The variants of this study

are shown in table 3.11. The first three variants are matching the according variants from before.

40

Results

85 340 764 1359 212360

70

80

90

100

110

120

*

* standard volume set to 100%

Storage volume in l

Prim

ary

ener

gy d

eman

d in

%

January

85 340 764 1359 212360

70

80

90

100

110

120

*

* standard volume set to 100%

Storage volume in lP

rimar

y en

ergy

dem

and

in %

April

Figure 3.40: Primary energy demand for heat pump systems with different buffer storage volumes.

Table 3.11: Examined variants within the study on heat sources.

Variants Heat source Building type

1R Gas condensing boiler not insulated3R Gas condensing boiler insulated1 Ambient Air not insulated3 Ambient air insulated4 GSHX not insulated5 GSHX insulated6 Ground Source Collector not insulated7 Ground Source Collector insulated

Figure 3.43 shows the electric power consumption of the heat pump for one day. One can see the

differences between the air-to-water heat pump and the ground coupled systems. The air heat

pump’s performance is strongly influenced by the ambient air temperature at a certain point of

time. Differences between the ground coupled systems can hardly be recognised. Only observing

a longer period of time differences are noticeable. Ground coupled heat pumps achieve the highest

efficiencies. Here the temperature of the heat source is constant independent of the ambient air

temperature.

41

Results

85 340 764 1359 21230

500

1000

1500

Storage volume in l

Num

ber o

f ope

ratin

g in

terv

als

January

85 340 764 1359 21230

500

1000

1500

Storage volume in lN

umbe

r of o

pera

ting

inte

rval

s

April

Figure 3.41:Number of operating intervals for the systems with different buffer storage volumes.

Boiler Air Borehole Collector60

80

100

120

Prim

ary

ener

gy d

eman

d in

%

February, non−insulated building

Boiler Air Borehole Collector60

80

100

120

Prim

ary

ener

gy d

eman

d in

%

February, insulated building

Figure 3.42: Primary Energy demand of heat pump systems with different heat sources comparedto a gas boiler system. Results for month of February.

42

Results

(a) AWHP (b) BWHP with vertical GSHX

(c) BWHP with horizontal GSHX

Figure 3.43:Heat pump electric power for a February day, different heat sources.

3.6.5 Study on bivalent heat pump systems

The AWHP system presented here is a possible solution to the problems of standard systems in

existing buildings described above. Its idea is based on the following facts:

• AWHPs are less efficient at low ambient temperatures.

• Most heating systems in Germany are over-dimensioned due to design conditions at very

low ambient temperatures.

• Existing boiler systems are often quite efficient. But future policies require the use of a cer-

tain amount of renewable energy in buildings. A solution to these challenges could be a

bivalent heat pump system that meets the following requirements:

• It should be easily integrated in existing boiler systemswithout changing the hydraulic setup.

The latter makes the systems error-prone.

43

Results

• The system should be cheap. The heat pump should be of a lownominal power. That reduces

material costs such as for the heat exchangers, the ventilators and the compressor.

• The heat pump shall only work at ambient temperatures where the Coefficient of Perfor-

mance (CoP) of the heat pump is above 2.4 .

Bivalent heat pump systems aren’t a new idea. In the beginning of the 1980s when heat pumps be-

came popular the first time, studies where done evaluating the optimal controlling of such systems

(e.g. Eberhard [1986]). And also today AWHPs are often equipped with a second heat generation

system (mostly an electrical heating rod) that works at very low temperatures. Central questions

concerning the layout of bivalent AWHP systems are:

• The ratio of heat pump power and the second heat generator’s power.

• The operating modes: Parallel, partly parallel or alternative operation.

• The bivalence point (partly parallel and alternative operation modes).

Thinking of future energy grids, bivalent systems will be a solution to changing energy offers. An

energy system that uses the electric and the gas grid can have a balancing effect on both.

Figure 3.44: Scheme of the bivalent heat pump system.

Figure 3.45 shows the simulated primary energy demand for the insulated (a) and non-insulated

(b) building. For the insulated building we see that the AWHP system has low savings in primary

energy compared to the boiler system. The bivalent system achieves savings of 19%. For the non-

insulated building the AWHP has a higher demand of primary energy than the boiler system. The

savings of the bivalent system are 9 %.

According to the static calculations, the simple AWHP system saves primary energy compared to

the boiler system. Dynamic simulations considering heat losses and energy demands of auxil-

iary components show, that in the non-insulated building the AWHP system has a higher demand

of primary energy than the boiler system. Even for the insulated building only small savings are

achieved.

In Figure 3.46 the annual number of operating intervals for the six simulated systems is shown.

They are relatively low for the boiler and AWHP systems. The components in the bivalent system

44

Results

are stressedmore than in the other systems. The number of operating cycles is much higher which

means a faster wear out of the components. The AWHP of the bivalent system in the insulated

building is overdimensioned. This is why a high number of operating intervals is achieved. It is a

clear disadvantage and should be improved.

Boiler AWHP Bivalent0

20

40

60

80

100

120

Prim

ary

ener

gy d

eman

d in

%

47% gas

53% electr.

Insulated Building

electricitygas

(a) insulated building

Boiler AWHP Bivalent0

20

40

60

80

100

120

Prim

ary

ener

gy d

eman

d in

%

70% gas

30% electr.

Non−Insulated Building

electricitygas

(b) non-insulated building

Figure 3.45: Primary energy demand for the variants within the study of bivalent systems.

In figure 3.47 the behaviour of the bivalent system is shown for two days of the year. In (a) a winter

day is shown. The ambient temperature decreases from 3 ◦C to −2 ◦C during the day. At 13 h itfalls below 0 ◦C and the heat pump is turned of (AWHP heat flow gets 0 kW ). That means, that theboiler heat output has to rise. This can be seen in the lower part of figure 3.47 (a).

In figure 3.47 (b) a warmer day is shown. Ambient temperatures are below 10 ◦C at night and riseto 14 ◦C during daytime. At night the heat pump operates constantly and additionally the boilerworks at the lower boundary of its operating range. As soon as the ambient temperature rises (i.e.

the building’s heat load decreases) at about 8 h the boiler begins to clock. One hour later ambient

temperatures are high enough for the heat pump to meet the heat load alone. At 11 h the heat

pump begins to clock. When the boiler again begins to operate at around 15 h, suboptimal system

behaviour occurs: The boiler works exactly when the heat pump is turned off. In view of the high

ambient temperatures it would be preferable if the heat pump would work constantly and the

boiler would stay off. The reason for this behaviour is that the safety control deactivates the heat

pump because of a too high temperature at the condenser. Here, too, future investigations have to

be undertaken to fit the system layout. E.g. a larger storage could optimize the system.

Bivalent heat pump systems can save primary energy compared to simple gas boiler systems and

AWHP systems. The results of dynamic simulations show that savings depend on the system ar-

rangement and the ratios of heat load, boiler and heat pump. A simple add-on heat pump system

45

Results

Boiler AWHP Bivalent0

2000

4000

6000

8000

10000

12000O

pera

ting

inte

rval

sInsulated Building

AWHPBoiler

(a) insulated building

Boiler AWHP Bivalent0

1000

2000

3000

4000

5000

Ope

ratin

g in

terv

als

Non−Insulated Building

AWHPBoiler

(b) non-insulated building

Figure 3.46: Annual number of operating intervals for the variants within the study on bivalentsystems.

for existing buildings is proposed and possible energy savings are demonstrated by simulations.

The behaviour of the system regarding operating intervals still has to be improved. Every build-

ing and heating system type needs its own adjustment. Optimal sizes of each component (among

other the storage size) and controller settings have to be found for different insulation standards

and heating systems in further research.

46

Results

0 4 8 12 16 20 24−10

0102030

Tem

pera

ture

in °

C

Room AirOutside Air

0 4 8 12 16 20 240

2

4

Hea

t flo

w in

kW

AWHP

0 4 8 12 16 20 240

5

10

Time in h

Hea

t flo

w in

kW

AWHP

(a) winter day

0 4 8 12 16 20 24−10

0102030

Tem

pera

ture

in °

CRoom AirOutside Air

0 4 8 12 16 20 240

2

4

Hea

t flo

w in

kW

AWHP

0 4 8 12 16 20 240

5

10

Time in h

Hea

t flo

w in

kW

AWHP

(b) transition season day

Figure 3.47: Ambient and room temperature, AWHP and boiler power for a winter day and a tran-sition season for one day (non-insulated building)

47

4 Conclusion

Heat pump systems in buildings can save primary energy compared to standard boiler systems.

The performance of heat pump systems depends onmany systemparameters. Most important are

the type of heat source and heat sink, or more explicitly their supply temperatures. Both directly

influence the temperature lift that the heat pump has to bear. But also other components do have

a distinct influence on the efficiency, e.g. the existence of a storage and the storage volume.

A detailed data analysis of the heat pump field test showed that 43 field test objects can be eval-

uated within the chosen balance room that represents the substitution of a boiler. Thereof 21 are

air-to-water heat pumps, 17 brine-to-water heat pumps with horizontal ground source heat ex-

changers and 5 are brine-to-water heat pumps with vertical ground source heat exchangers. An

evaluation comparing the heat pump systems to new condensing boiler systems on a level of pri-

mary energy and on an economic level has been done. Considering actual primary energy factors,

performance factors of heat pumps have to be above 2.3 to be more efficient than condensing

boilers.

Heat pumps in existing buildings can be economically and energetically advantageous compared

to boilers, nevertheless in the field test a lot of them that are not. A mean seasonal performance

factor of 2.3 for air coupled devices and 2.9 for ground coupled devices shows this circumstance for

the application of heat pumps of differentmanufacturers in existing buildings. The best systems in

the field test achieved seasonal performance factors of 3.0 (air) respectively 4.0 (brine). The great

differences to the mean values show the high potential of the technology and the necessity of an

optimization of the total heating system.

These results have to be seenwithin changing boundary conditions. Energy prices and investment

costs for heat pump and boiler devices are variable. The same applies to borehole drilling, which

involves a high economic risk for ground coupled systems. Primary energy factors will change due

to rising dues of renewable energy, especially in producing electricity. Thus, in the coming years,

the basic conditions can be beneficial for the heat pump technology.

Standard calculation procedures of existing guidelines and standards do not describe the real be-

havior of heat pumps. The dynamic calculation procedure developed within this work, circum-

stantially models the total heat pump system. It allows parameter studies within fixed boundary

conditions as they are not possible within the field test. Numerical studies have shown that no-

tably speed controlled heat pumps and a combination of air-to-water heat pumps and boilers (the

bivalent or hybrid heat pump) have potential for savings in primary energy in existing and in new

buildings.

48

Conclusion

In all intents and purposes heat pumps require the consideration of the whole system including

the heat source, the hydraulic system and the building with its thermal behaviour. The installation

of heat pumps should result from a detailed calculation of all influences occurring in the respective

object.

49

5 Further steps, future developments and proposed actions

For the detailed evaluation of a field test the measurements should be expanded to the recording

of user behaviour and additional weather data. Details about the building and the heating system

should be known, e.g. a comprehensive heat load calculation should be made. This particularly

applies to heat pumps.

To achieve an expedient information about its efficiency, a heat pump should be tested under dy-

namic boundary conditions instead of static test procedures. Different building and heating sys-

tem designs should be considered. These testing methods are to be developed.

Further investigations should be done in the field of hybrid heat pump systems. A reasonable

dimensioning of the heat pump power and the bivalent temperature point is to be found. Speed

controlled heat pumps are able to achieve savings in primary energy, too. Further research has to

be done to maximize the operating range of the controlled compressor and to fit the heat pump to

the building’s thermal behavior. It has to be considered that within modern heating systems there

will not be "one fits all"-solutions of heat generators.

50

6 Literature

Praxisinformation P 2006 / 8 Gastransport / Betriebswirtschaft - Anwendung von Standardlastpro-

filen zur Belieferung nichtleistungsgemessener Kunden. Technical report, Hrsg.: Bundesver-

band der deutschen Gas- undWasserwirtschaft (BGW), Berlin und Brüssel, 2006.

Energieberater 7. Hottgenroth Software, CD, 2009.

2009-125-EC. Establishing a framework for the setting of ecodesign requirements for energy-

related products, Oktober 2009.

Umweltministerium Baden-Württemberg. Leitfaden zur Nutzung von Erdwärme mit Erd-

wärmesonden. Stuttgart, 2005.

BAFA 2010. Antrag auf Basisförderung einer effizienten Wärmepumpe., 2010.

J. Christoffer, T. Deutschländer, and M. Webs. Testreferenzjahre von Deutschland für mittlere und

extremeWitterungsverhältnisse. Deutscher Wetterdienst, Offenbach amMain, 2004.

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www.3ds.com/products/catia/portfolio/dymola.

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angetriebenen Verdichtern für die Raumbeheizung und Kühlung, Juni 2007.

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forderungen und Nutzungsgrade der Anlagen, September 2008.

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angetriebenen Verdichtern - Heizen, Juli 1997.

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trinkwassererwärmung, lüftung; änderung a1, Juli 2010.

V. Eberhard. Untersuchungen zum Energieverbrauch bivalenter Wärmepumpenheizungen, bei ver-

schiedenen Schaltungen von Kessel, Wärmepumpe und Speicher, volume 19/10. VDI-Verlag Düs-

seldorf, 1986.

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Wärmequellen, Wärmesenken und Wärme-/Kältespeichern. Rud. Otto MEyer-Umwelt-Stiftung,

Goethestraße 18, D-08547 Jößnitz (Plauen), 2008.

A. Hoh, T. Haase, T. Tschirner, and D. Müller. A combined thermo-hydraulic approach to simula-

tion of active building components. In Proceedings of the 4th International Modelica Conference

2005, Hamburg, 2005. URL http://www.modelica.org/events/Conference2005/online_

proceedings/Session6/Session6b2.pdf. The Modelica Association and the Department of

Thermodynamics, Hamburg University of Technology.

IWU. Deutsche gebäudetypologie - systematik und datensätze. Technical report, IWU Institut

Wohnen und Umwelt, Annastraße 15 64285 Darmstadt, 12 2003.

Modelica Association. Modelica and the modelica association, 2011. URL https://www.

modelica.org/.

D. Müller and A. Badakhshani. Gekoppelte Gebäude- und Anlagensimulation mit Modelica. In

Proc. of BauSim Conference, Wien, September 2010.

VDI 4650. Berechnung vonWärmepumpen - Kurzverfahren zur Berechnung der Jahresarbeitszahl

vonWärmepumpenanlagen - Elektro-Wärmepumpen zur Raumheizung undWarmwasserbere-

itung, März 2009.

R. Viskanta, M. Behnia, and A. Karalds. Interferometric observations of the temperature structure

inwater cooled or heated from above. Advances inWater Resources, 1(2):57–69, 1977. URL http:

//www.sciencedirect.com/science?_ob=MImg&_imagekey=B6VCF-4888G51-2B-1&_cdi=

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c&wchp=dGLbVlW-zSkzk&md5=e4ad0ab7abba6e03b5aab9e67005731a&ie=/sdarticle.pdf.

52

7 Attachments

7.1 List of figures

2.1 Overview of important standards in the field of heat pumps. . . . . . . . . . . . . . . . 3

2.2 Calculation procedure of the VDI 4650 guideline (simplified overview). . . . . . . . . . 4

3.1 Regression of the daily heating energy by the daily mean ambient temperature. . . . . 5

3.2 Construction years and specific heat demands. . . . . . . . . . . . . . . . . . . . . . . . 6

3.3 System boundary 1 used for analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.4 System boundary 2 according to VDI 4650. . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.5 Comparison of SPFs within different system boundaries measured in the field test. . . 8

3.6 Comparison of SPFs calculated by VDI 4650 andmeasured in the field test. . . . . . . . 9

3.7 Relative savings in primary energy compared to a new gas boiler. . . . . . . . . . . . . . 12

3.8 Relative investment costs of heat pumps related to nominal power. . . . . . . . . . . . 13

3.9 SPF1 and relative investment costs of heat pumps related to nominal power. . . . . . . 13

3.10 Net present value of the field test heat pump systems. . . . . . . . . . . . . . . . . . . . 14

3.11 Savings in specific costs of heat supply for the evaluated field test objects. . . . . . . . 15

3.12 Scheme of field test object 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.13 Heat flow in front of and behind the buffer storage . . . . . . . . . . . . . . . . . . . . . 17

3.14 Flow temperature in front of and behind the buffer storage . . . . . . . . . . . . . . . . 18

3.15 Temperature lift and SPF1 for field test object 4 in 2009. . . . . . . . . . . . . . . . . . . 18

3.16 Scheme of field test object 31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.17 Temperature lift and SPF1 for field test object 31 in 2009. . . . . . . . . . . . . . . . . . . 19

3.18 SPF1 according to type of heat source and heat sink. . . . . . . . . . . . . . . . . . . . . 22

3.19 SPF1 and design flow temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.20 SPF1 andmeasured flow temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

53

Attachments

3.21 SPF1 and buffer storage volume. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.22 SPF1 and year of building construction of field test objects. . . . . . . . . . . . . . . . . 24

3.23 Dimensioning of heat pump systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.24 Scheme of the heat pumpmodel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.25 Model of a ground source heat exchanger and ground. . . . . . . . . . . . . . . . . . . . 29

3.26 Temperature field in the profile of the groundmodel. . . . . . . . . . . . . . . . . . . . . 30

3.27 Scheme of u-pipe ground source heat exchanger. . . . . . . . . . . . . . . . . . . . . . . 30

3.28 Scheme of horizontal ground source heat exchanger model. . . . . . . . . . . . . . . . 31

3.29 Model of the stratified buffer storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.30 Validation of models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.31 Sample scheme of a heat pump system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.32 Model of the total system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.33 Simulation results of different temperatures of the total system. . . . . . . . . . . . . . 34

3.34 Scheme of the test building model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.35 Study of insulation standard: Heat pump and reference system. . . . . . . . . . . . . . 37

3.36 Primary energy demand for the variants . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.37 Performance factors for the heat pump systems . . . . . . . . . . . . . . . . . . . . . . . 38

3.38 Comparison of the room temperatures for month of February . . . . . . . . . . . . . . 39

3.39 Heat Pump electric power for one day (different storage volumes). . . . . . . . . . . . . 40

3.40 Primary energy demand for heat pump systems with different buffer storage volumes. 41

3.41 Number of operating intervals for the systems with different buffer storage volumes. . 42

3.42 Primary energy demand of heat pump systems with different heat sources. . . . . . . . 42

3.43 Heat pump electric power for a February day, different heat sources. . . . . . . . . . . . 43

3.44 Scheme of the bivalent heat pump system. . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.45 Primary energy demand (study on bivalent systems). . . . . . . . . . . . . . . . . . . . . 45

3.46 Annual number of operating intervals (study on bivalent systems). . . . . . . . . . . . . 46

3.47 Temperatures, AWHP and boiler powers for one day operation of the bivalent system. 47

54

Attachments

7.2 List of tables

3.1 Comparison of system boundary 1 and 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2 Investment costs for new condensing boilers. . . . . . . . . . . . . . . . . . . . . . . . . 11

3.3 Assumptions for the economical evaluation . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.4 Cross correlations between heat source and sink types . . . . . . . . . . . . . . . . . . . 20

3.5 Cross correlations between heat source and storage . . . . . . . . . . . . . . . . . . . . 20

3.6 Major enhancements for heat pump systems. . . . . . . . . . . . . . . . . . . . . . . . . 25

3.7 Data of the two window types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.8 Material data of the wall layers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.9 Building variants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.10 Examined variants within the study on insulation standards. . . . . . . . . . . . . . . . 36

3.11 Examined variants within the study on heat sources. . . . . . . . . . . . . . . . . . . . . 41

7.3 Nomenclature

Quse Used heat energy

Wel Electric work

AWHP Air-to-water heat pump

BWHP Brine-to-water heat pump

CoP Coefficient of Performance

DHW Domestic hot water

GSHX Ground source heat exchanger

HiL Hardware-in-the-Loop

PCM Phase Change Material

PF Performance Factor

SPF1,2 Seasonal Performance Factor according to system boundary 1,2

SPFvdi Seasonal Performance Factor calculated according to guideline VDI 4650

55

Attachments

7.4 Publications

• K.Huchtemann,D.Müller. Thermohydraulische Simulationsve