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                                                Wolfgang Streicher
  Institute of Thermal Engineering , Graz University of Technology, Inffeldgasse 25b, A-8010 Graz, Austria, Phone:
                    +43-316-873-7306, Fax: +43-316-873-7305, E-Mail:

                                                   Markus Peter
                  p² - Energienutzung mit Verstand, Markus PETER, D-34431 Marsberg, Germany,
                           Phone: +49 172 72 85 316, E-mail:

                                                Data delivered by:
Bales, Ch., SERC, Borlänge, Sweden, Bony, J, EIVD, Yverdon-les-Bains, Switzerland, Brender, E., BATEC, Denmark,
 Ellehauge, K., Arhus, Denmark, Heimrath, R., TU-Graz, Austria, Jaehnig, D., AEE-Intec, Gleisdorf, Austria, Krause,
  Th, SOLVIS, Braunschweig, Germany, Papillon, Ph, Clipsol, Aix-les-Bains, France, Peter, M., Uni. Oslo, Norway,
                    Shah, L., J., DTU, Lyngby, Denmark, Riva, R., AEE-Intec, Gleisdorf, Austria

      Abstract – Solar thermal systems delivering domestic hot water as well as space heating are called solar
      combisystems. Task 26 of the Implementing Agreement of Solar Heating and Cooling of the International
      Energy Agency was dealing with such systems. It was finished in summer 2003. The following paper
      describes the work that has been performed in Task 26 concerning the material demand and the energy
      payback time of solar combisystems. For ten systems the material demand for the main components, solar
      collector, water stores, hydraulic station, controller and the heat carrier were collected. The materials
      were grouped according to their structure and specific energy content (e.g. metals, polymers), which was
      taken from literature. By summing up all parts used, the energy demand to produce a particular system
      was calculated. Some systems include the burner, others not, while some systems do not need space
      heating water stores because they use the concrete of a floor heating system as heat storage. To overcome
      parts of this problem a typical gas burner was added to the systems that do not include a burner. It can be
      seen that there is a wide range of specific energy demand per collector area for solar combisystems. The
      energy payback time i.e. the time that is needed before the energy savings meet the energy demand for
      building the solar combisystems, vary between 1.3 to 3.5 years, depending on system and location for
      common layouts of collector area and store volume.

1. INTRODUCTION                                              2. METHODOLOGY

  Solar thermal systems for domestic hot water (DHW)         2.1 Investigated Materials and their Production Energy
and space heat preparation are called solar combisystems.    Demand
Their market share has increased in the last years up to       The materials used in solar combisystems were grouped
50 % of all new plants in Austria, Switzerland and           according the characteristics and the Accumulated
Germany. Therefore the International Energy Agency           Energy Expense (AEE) for their production to define a
(IEA) started in 1998 the Implementing Agreement on          base enabling a brief evaluation of the analysed
Solar Heating and Cooling, Task 26 on solar                  combisystems. Table 1 provides the main figures of these
combisystems. It includes a system survey of 21 different    materials and material groups
generic combisystems, research on how to test                  It should be noted that packing material, all kind of
combisystems and the comparison and optimization of 10       transportations, assembling, maintenance and repair work
systems under the same reference conditions using the        as well as disposal and recycling was not taken into
simulation program TRNSYS as well as a new developed         account. The space heating loop as well as the domestic
comparison method (Weiss et al. 2003). One other item        hot water distribution system is defined not to be part of
was the documentation and analysis of the material           the combisystem.
demand of eight combisystems, which is presented in the        Due to the difficulties in checking up the different
following paper.                                             recycling rates for the materials (e.g. aluminium), fixed
                                                             ratios of recycling rates based on available European data
                                                             are assumed.
                                                               Because of the different building traditions in the
                                                             participating countries the replacement of conventional
                                                             building materials by the installation of a combisystem
(e.g. roof material like roof tiles spared by the roof           both, the investigated solar combisystem boiler and the
integration of the collectors etc.) is not taken into            reference system boiler,ref, have a strong impact on the
account.                                                         values of fsav,therm.
  To calculate the overall Accumulated Energy Expense
(AEE) for a particular combisystem out of the material                               Qboiler Qel.,heater
demand, the amount of different materials or material                                        
groups has to be multiplied by the specific energy                                   ηboiler     ηel.,heater      E
                                                                 f sav,therm     1                          1  aux      (1)
included per unit of the respective material. The result is                                Qboiler,ref            E ref
a value for AEE, which represents the amount of energy
embodied in the components of the complete system and                                      ηboiler,ref
necessary to produce it.
                                                                  Qboiler             Yearly heat output of auxiliary boiler
Table 1     Primary energy demand of different groups of          Qel,heater          Yearly heat output of electrical heater
            materials represented in solar combisystems                               (Norwegian case)
            (data from Büro für Umweltchemie 1995,                Qboiler,ref         Yearly heat output of boiler of reference
            Dornier, Gibo-Solar 1996, Ffe, 1999,                                      system
            Ökoinventare, 1996, Bruck, 2001, Tinox,
            Wagner, 1997)                                         This target function is also used in chapter 3.3 to
                                                                 compare the energy pay back time of solar combisystems
                                          AEE       AEE
           Material              Unit                            2.3 Data Collection
                                        [MJ/unit] [kWh/unit]
                                                                  The material demand was collected for the following
aluminium, 27.6% recycling                                       components of the solar combisystem
                                 kg      151.7       42.1              solar collector
steel high alloyed               kg       96.6       26.8              tubing between collector and storage including
steel un- and low alloyed        kg       33.9        9.4                  pumps, fittings etc.
sheet steal, galvanized          kg       58.0       16.1              heat storage
copper                           kg       96.6       26.8              fluid of collector loop
glass (flat shape) without                                             controller
                                 kg       13.3        3.7              auxiliary burner (if integrated part of the
glass and mineral wool           kg       18.6        5.2                  combisystem)
EPDM rubber, PUR hard
foam polypropylene, diff.                                          The comparison of the systems is not easy, because
polystyrol types polyethy-       kg       93.6       26.0        some systems include the burner, others not, while some
lene diff. densities and other                                   systems do not need space heating water stores because
polymers                                                         they use the concrete of a floor heating system as heat
PVC, hard quality                kg       81.8       22.7        storage. To overcome parts of this problem a typical gas
                                                                 burner was added to the systems that do not include a
propylen glycol                  kg       48.9       13.6
bitumen, PVC sealing strip       kg       49.8       13.8
                                                                   It must be carefully noted and paid attention to the fact
wood (raw)                       kg        1.4        0.4        that different combisystems may differ a lot in
wood (fibreboards, …)            kg       7.4         2.1        comparison to those from other manufacturers depending
                                                                 on the components, which are included in or defined to be
                                                                 part of the respective combisystem. This might be
2.2 Fractional Energy Savings – Definitions in Task 26           especial the case in respect to the auxiliary heating. For
                                                                 that reason a comparison with the objective of a ranking
  One of the target functions for the solar combisystems
                                                                 between the systems is not possible.
defined in Task 26 was the fractional thermal energy
savings (fsav,therm). It is defined as the saved combined
auxiliary energy consumption of the solar combisystem
(Eaux) compared to the final energy consumption of a             3. RESULTS
reference system (Eref) when no electrical devices other
                                                                 3.1 Material demand of solar combisystems
than a heating element are taken into account (eq. 1). The
                                                                   Some of the combisystems analysed include a burner as
boiler efficiency of the reference system boiler,ref as well
                                                                 integrated part of the system, the others can be coupled to
as the efficiency of an electrical heater el,heater (only the   any burner with the required power. For the latter systems
case for a Norwegian system) was assumed to be 90 %.
                                                                 the burner material demand is not included in the data of
With this definition of fsav,therm the boiler efficiencies of
the system. To compare systems on the same level the                                        lighter. The relative weight of the total system per
material demand of conventional gas burners was                                             collector area is therefore very low. System #3 uses only
evaluated from literature. Figure 1 shows the results from                                  a small DHW storage tank and no space heating tank.
two literature sources.                                                                     Instead, the concrete floor of the building is used as space
  It can be seen, there is a wide range of total weight for                                 heating store. Again a low relative weight per collector
the burner. Whereas the first burner is a heavy                                             area occurs. On the opposite, system #8 uses steel
construction standing on the floor the second burner is a                                   collectors, steel tubes and a steel tank. This results in a
light wall mounted device as it is more and more used in                                    relative heavy total construction.
houses without basement.
  In the following the lighter burner was used for all

                                                                                                Specific weight [kg/m²collector]
systems except system #19, the only system designed for                                                                            120
multi family buildings, where a heavy gas burner was                                                                                                                                          Systems as is
                                                                                                                                   100                                                        Including burner
                               EPDM rubber, PUR hard foam polypropylene, diff. polystyrol
                               types polyethylene diff. densities and other polymers                                               60
                               Glass and mineral wool
                               steel un- and low alloyed

                                                                                                                                                        # 8-8m²

                                                                                                                                                                  # 8-12m²

                                                                                                                                                                             # 8-16m²





                                                                                                                                                                                               #11 cost opt.

                                                                                                                                                                                                               #12 opt.
                               Steel high alloyed

                               Aluminium, 27.6% recycling material


                                                                                            Figure 2 Specific material demand of 10 different solar
    Weight of materials [kg]

                               120                                                                   combisystems of Task 26
                               100                                                            In Figure 3 the average material distribution of the ana-
                                80                                                          lysed systems is shown. The main material used is steel
                                                                                            (low- and high alloyed) for the storages, fittings, tubing
                                                                                            and parts of the collector, followed by the glass covers of
                                40                                                          the collector, copper for absorber and tubing, mineral
                                                                                            (glass)wool for the insulation of collector and storages,
                                                                                            and aluminium and wood for the collector frame.
                                      10 kW Gas burner      12-25 kW Gas
                                                                                                                                                 Wood                        Burner + Pumps
                                        (Ökoinvenare,        burner, wall                                                                  (fibreboards, …);                   + controller
                                            1996)          mounted (Stiwa,                                                                        0.02                         (average of
                                                                                                                                         Wood (raw); 0.06                     85 MJ/kg with
                                                                2003)                                                                                                           steel, alu.,                              Electronics; 0.00
                                                                                                                                          Bitumen, PVC                           copper,
                                                                                                                                          sealing strip;                                                             Aluminium,
                                                                                                                                                                             plastic…); 0.02
Figure 1 Material demand of gas burners from literature                                                                                        0.00                                                                27.6% recycling
                                                                                                                                         Propylen glycol;                                                           material; 0.07
                                                                                              EPDM rubber,
  Figure 2 shows the specific weight (in dependency of                                       PUR hard foam                                      PVC, hard                                                                   Steel high
                                                                                             polypropylene,                                    quality; 0.00                                                              alloyed ; 0.12
the collector area) of the ten combisystems analysed. The                                     diff. polystyrol
left hand bars show the systems as they are defined by the                                          types
                                                                                            polyethylene diff.
manufacturer and the right hand bars for the same                                             densities and                                    Glass and
                                                                                                                                              mineral wool;
                                                                                             other polymers;
systems but additionally calculated with a gas burner for                                            0.05                                         0.09

those systems, which do not include a burner as part of
the system. The impact of the burner to the whole system                                                                                    Glass (flat shape)                                                            Steel un- and low
                                                                                                                                            without coating;
is rather small.                                                                                                                                   0.16
                                                                                                                                                                                                                            alloyed; 0.30

  There is nearly a factor of 1 : 5 between the specific                                                                                                          Copper; 0.08
                                                                                                                                                                                          Sheet steal,
weight of the lightest and the heaviest systems. This can                                                                                                                               galvanized; 0.00
be explained by the different concepts used. System #9b,
which has the lowest relative weight, has a high collector                                  Figure 3 Average material demand distribution of 10
area and uses plastic collectors, that are not that efficient                                        different solar combisystems of Task 26
as collectors with copper and selective surface, but much                                            (values in % of total weight)
  In Figure 4 the specific weight is plotted against the                                                                                                                         Figure 6 shows the average distribution of the AEE of
total collector area. The differences of the systems clearly                                                                                                                   the materials used similar to Figure 3. Due to the high
have higher impact on the specific weight than the scale                                                                                                                       material production energy demand the impact of
of the plant. Nevertheless the largest system (100 m²                                                                                                                          aluminium, copper and high alloyed steel is increasing
collector area for a multi family house, system #19) has a                                                                                                                     whereas the impact of glass and unalloyed steel is
relatively low specific weight.                                                                                                                                                decreasing in comparison to the pure weight analysis. All
                                                                                                                                                                               other materials have a minor impact to the AEE.
                                                                                                                                                                                                    (fibreboards, …);    Burner + Pumps
                                      Specific weight [kg/m²collector]

                                                                         100                                                                                                                               0.00            + controller
                                                                                                                                                                                                                           (average of
                                                                                                                                                                                                        Bitumen, PVC      85 MJ/kg with
                                                                         80                                                                                                                             sealing strip;      steel, alu.,
                                                                                                                                                                                  EPDM rubber,               0.00            copper,              Selective
                                                                                                                                                                                 PUR hard foam        Wood (raw); 0.00   plastic…); 0.03        coating, 0.02
                                                                         60                                                                                                      polypropylene,
                                                                                                                                                                                  diff. polystyrol   Propylen glycol;                         Electronics; 0.01
                                                                                                                                                                                        types               0.03
                                                                         40                                                                                                     polyethylene diff.
                                                                                                                                                                                                                                                27.6% recycling
                                                                                                                                                                                  densities and
                                                                                                                                                                                                                                                 material; 0.21
                                                                                                                                                                                 other polymers;
                                                                         20                                                                                                              0.07
                                                                                                                                                                                                Glass and
                                                                                                                                                                                              mineral wool;
                                                                          0                                                                                                                        0.04
                                                                               0        20          40              60          80            100                120
                                                                                                                                                                                        Glass (flat shape)
                                                                                             Total collector area [m²]                                                                  without coating;
                                                                                                                                                                                                                                                   Steel high
                                                                                                                                                                                                                                                 alloyed ; 0.20
                                                                                                                                                                                                   Copper; 0.15
Figure 4 Specific weight versus collector area of 10                                                                                                                                                    Sheet steal,
                                                                                                                                                                                                      galvanized; 0.00
         different solar combisystems of Task 26                                                                                                                                                                          Steel un- and low
                                                                                                                                                                                                                            alloyed; 0.20

3.2 Accumulated Energy Expense of Solar Combisystems
  Using the primary energy demand of different groups of                                                                                                                       Figure 6 Average material energy demand distribution
materials of Table 1 the Accumulated Energy Expense                                                                                                                                     of 10 different solar combisystems of Task 26
(AEE) of the analysed combisystems is calculated. Figure                                                                                                                                (values in % of total accumulated energy
5 shows the specific AEE per m² collector area for the                                                                                                                                  expense, AEE)
systems as they were given by Task 26 participants and
for the same systems but additionally calculated with a
                                                                                                                                                                               3.3 Energy Payback Time of Solar Combisystems
gas burner for those systems which do not include a
                                                                                                                                                                                 The energy payback time was calculated by the AEE for
burner as part of the system. Similar to the specific
                                                                                                                                                                               the systems divided by the annual end use energy saved
material demand shown in Figure 2 a wide range of
                                                                                                                                                                               by the solar combisystems in comparison to a reference
values from 1600 to 5800 MJ/m²collector (a ratio of 1 to
                                                                                                                                                                               system with the same building (characterized by a space
3,5) area can be seen and the impact of the burner is
                                                                                                                                                                               heating demand of 60 kWh/m²a for Zurich climate) and
relatively small.
                                                                                                                                                                               for several climates (Stockholm, Zurich, Carpentras) but
                                                                                                                                                                               only pure conventional heating system with gas burner. A
                                                                                                                                                                               more detailed description of the reference system and the
    Specific material energy demand

                                                 6000                                                                                 system as is                             combisystems data can be found in Weiss et al. (2003),
                                                                                                                                      including burner                         Streicher, W. (2003) and the various system reports given

                                                 4000                                                                                                                          in the references.
                                                                                                                                                                                 Figure 7 shows the energy payback time of the analysed
                                                                                                                                                                               systems and the three climates mentioned above in
                                                                                                                                                                               dependency of the fractional energy savings Fsav,therm as
                                                                                                                                                                               defined in Chapter 2.2. Again a wide scatter of the points
                                                                                                                                                                               can be found. If system #8, which is responsible for the 9
                                                                                              # 8-8m²

                                                                                                         # 8-12m²

                                                                                                                     # 8-16m²




                                                                                                                                                      #12 opt.
                                                                                                                                      #11 cost opt.

                                                                                                                                                                               points above and around the energy payback time of 3
                                                                                                                                                                               years is taken out of the analysis, the other systems have
                                                                                                                                                                               energy payback times between 1.3 and 2.4 years.
                                                                                                                    Combiystem                                                 Compared to an estimated life time of about 20 years this
                                                                                                                                                                               seems to be a fairly short time. Amazingly the energy
Figure 5 Specific accumulated energy expense (AEE) of                                                                                                                          payback time seems to be not dependent on the fractional
         10 different solar combisystems of Task 26                                                                                                                            savings of the system. This can be due to the fact the
                                                                                                                                                                               fractional thermal energy savings Fsav,therm for all the
studied systems lie in a relatively small range, from 23 to                  REFERENCES
46 % for the 60 kWh/(m²a) building and Zurich climate.
  High fractional savings up to 75 % occur in southern
                                                                             Bales, Ch. (2002) Generic System #11, Space Heating
climates, where the space heating demand is low. Low
                                                                             Store with DHW Load Side Heat Exchanger(s) and
fractional savings for Stockholm can result in similar
                                                                             External Auxiliary Boiler, Report of IEA Task 26, Solar
energy savings to those for high fractional savings for
  Therefore the energy Payback time, which is only                           Bales, Ch. (2002) Generic System #12, Space Heating
dependent on the total conventional energy saved, is                         Store with DHW Load Side Heat Exchanger(s) and
similar for all locations. This would differ, when high and                  External Auxiliary Boiler (Advanced Version), Report of
low fractional savings would be compared for the same                        IEA Task 26, Solar Combisystems
climate. In this case the high fractional savings would
                                                                             Bony, J., Pittet, Th. (2002) Generic System #8, Space
have less specific energy yield per collector area and the
energy payback time would increase.                                          Heating Store with Double Load-Side Heat Exchanger for
                                                                             DHW, Report of IEA Task 26, Solar Combisystems
                                                                             Büro für Umweltchemie (1995) Graue Energie von
                                                                             Baustoffen, Zürich
  Energy Payback Time [a]

                                                                             Dornier, Angabe Fa. Dornier
                                                           System #8
                            2,50                                             Ellehauge, K. (2002) Generic System #2, A Solar
                                                                             Combisystem Based on a Heat Exchanger between the
                                                                             Collector loop and the Space Heating Loop, Report of
                            1,50                                             IEA Task 26, Solar Combisystems
                                                                             Ffe (1999) Forschungsstelle für Energiewirtschaft Ffe,
                                                                             München: Ganzheitliche energetische Bilanzierung von
                            0,00                                             Grundstoffen, Teil IV, Kunststoffe, Juli 1999
                                   0   20   40        60      80       100
                                                                             GIBO-SOLAR (1996), Brief von Herrn Pusch an Herrn
                                                                             Styri-Hipp zur Diskussion über umweltfreundliche
                                                                             Beschichtungsverfahren zur Vergabe des Umweltzei-
Figure 7 Energy payback time versus fractional energy                        chens vom 24.09.96
         savings Fsav,therm of 10 different solar combisys-
                                                                             Jaehnig, D. (2002) Generic System #15, Two Stratifiers
         tems of Task 26 for three climates (Stockholm,
                                                                             in a Space Heating Storage Tank with an External Load-
         Zurich and Carpentras)
                                                                             Side Heat Exchanger for DHW, Report of IEA Task 26,
                                                                             Solar Combisystems
                                                                             Heimrath, R. (2003) Generic System 19, Centralized Heat
                                                                             Production, Decentralized Heat Load, Report of IEA
  Regarding the energy payback time a comparison of
                                                                             Task 26, Solar Combisystems
solar combisystems is not easy, because some systems
include the burner, others not, while some systems do not                    Ökoinventare (1996) Ökoinventare von Energiesystemen
need space heating water stores because they use the                         Bundesamt für Energiewirtschaft, Switzerland
concrete of a floor heating system as heat storage. To
                                                                             Bruck, M. (2001) ECO-Building Optimierung von
overcome parts of this problem a typical gas burner was
                                                                             Gebäuden durch Total Quality Assessment (TQ-
added to the systems that do not include a burner.
                                                                             Bewertung), Österreichisches Ökologie-Institut, Vienna
Nevertheless it can be seen that there is a wide range of
specific weight and specific energy demand per collector                     Papillon, P., Chéze, D. (2002) Generic System #3a,
area for solar combisystems. The energy payback time                         Advanced Direct Solar Floor, Report of IEA Task 26,
i.e. the time that is needed before the energy savings meet                  Solar Combisystems
the energy demand for building the solar combisystems,
                                                                             Peter, M. (2003) Generic System #9b, Space Heating
varies between 1.3 to 3.5 years, depending mainly on the
                                                                             Store with Immersed DHW Tank and External DHW
system for common layouts of collector area and store
                                                                             Store with Auxiliary, Report of IEA Task 26, Solar
volume. This result should encourage manufacturers to
think not only in terms of price and performance but also
in terms of kind and amount of material demand when                          Stiwa, (2003) 'test April 2003' Sonne tanken - 11
designing new or optimizing old systems.                                     Solaranlagen für Warmwasser und Heizung, Zeitschrift
                                                                             der Stiftung Warentest, Seite 69 - 73.
Streicher, W., Heimrath, R. (2003) Structure of the
Reference Buildings of Task 26, Technical Report of
Subtask C of IEA Task 26, Solar Combisystems
Tinox, Angabe Fa. TiNOX
Wagner, H.-J. (1997) Emissionen von Luftschadstoffen
und CO2 bei Herstellung und Betrieb von ausgewählten
thermischen Solaranlagen; VDI Fortschrittsberichte
Reihe 6 Energietechnik Nr. 366, Düsseldorf
Weiss, W. , ed. (2003) Solar Heated Houses – A Design
Handbook for Solar Combisystems, James&James

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