LARGE-SCALE SOLAR HEATING SYSTEMS IN THE SETTLEMENT AREA10 YEARS DEVELOPMENT IN GERMANY

by Prof. Dr.-Ing. M. Norbert Fisch

Institute for Building and Solar Technology (Institut für Gebäude- und Solartechnik, IGS)

Technical University Braunschweig, Mühlenpfordtstraße 23, D- 38106 Braunschweig

ABSTRACT: After the successful testing of smaller solar-supported district heating projects in the first half of the nineties inGermany (e. g. Ravensburg, Köngen, Neckarsulm, Schwäbisch-Gmünd) the first large-scale solar plants with long-term storagewere put into operation at the end of 1996 in Hamburg and Friedrichshafen. This achieved an aim which seemed utopian overmany years – to store the solar heat collected in summer for space heating in winter.

While digged hot water stores with a volume of 4,500 m³ in Hamburg (collector area 3,000Friedrichshafen (5,600 m² in the final execution) were realised, a duct storage in Neckarsulm is tested as a third demonstrationproject. The first extension step of this plant (collector area 2,700 m², storage volume approx. 20,000 m³) started operation inOctober 1997. A fourth solar supported district heating with a man-made aquifer (volume approx. 8,000 m³) is being built inChemnitz and is to be completed in the second half of 1998. Further smaller projects are under design and in construction.

In spite of the welcome cost reduction of small plants, large-scale solar plants are still more economic than the small plants bya factor of two. The solar heating plant in Friedrichshafen, for instance, was built for system cost of about 1,100 DM per m²collector area including the seasonal storage, of which 11,000 DM were spent per dwelling. This covers almost 50 % of theannual overall heating demand for space heating and hot water preparation. The investment cost at the plant in Neckarsulm isabout 900 DM per m², i. e. approximately 20 % lower than in Friedrichshafen. The gained solar heat cost of the first pilot proj-ects is between 300 and 500 DM/MWh.

1. INTRODUCTIONSolar heating plants with short-term (diurnal) storage(CSHPDS = Central Solar Heating Plants with DiurnalStorage) mainly serve for domestic hot water heating(DHW) and can cover only about 10 to 20 % of the annualoverall heating demand of a building or residential area.The "cost-benefit-ratio" of large-scale solar heating plants(>100 m²) is approximately two to three times more rea-sonable in comparison to small plants (<10 m²) for domes-tic hot water heating. The ratio between the investment costof the solar plant and the yearly solar energy gain is shownin Figure 1.

A solar contribution of 50 % up to 70 % of the overallheating demand and thereby a large covering of the spaceheating can only be achieved by the use of long-term stor-age. In this connection the ratio between the investmentcost and the yearly solar energy proceeds increases /1/.Concerning the substitution of fossil energies and the re-duction of CO2-emissions the use of solar energy is one ofthe most interesting applications. Figure 1 shows that eventhe large-scale plants with seasonal storage (f > 50 %) in-volve a more reasonable cost-benefit-ratio than small plantsfor domestic hot water heating (DHW) (f < 15 %).

2. FROM UTOPIA TO REALISATIONAt the end of 1996 the first large-scale solar heating plantswith seasonal storage started operation in Hamburg andFriedrichshafen, Germany. An aim was achieved whichseemed to be utopian over many years - to heat a buildingin the winter by solar energy from the summer. On the wayfrom utopia to realisation a lot of factors played a decidingpart:• There is a development of collector technology to

large-scale modules that can efficiently be connected

0

2

4

6

8

10

12

14

16

18

small plants (<10 m²), f<15 % large-scale plants (>100 m²), f<20 % Solar plants with long-term storage, f=50-70 %

Co

st/

be

ne

fit-

rati

o [

DM

/(k

Wh

/a)]

12 DM/(kWh/a)

8 DM/(kWh/a)

5 DM/(kWh/a)

3 DM/(kWh/a)

Flat Collectors

1,5 DM/(kWh/a)

3,5 DM/(kWh/a)

15 DM/(kWh/a)

Evacuated Tube Collectors

Figure 1: Cost/benefit-ratio for solar supported small andlarge-scale plants (f = solar fraction)

in large fields with minimum pipe expenditure. Col-lector area cost of 400 – 500 DM/m² including instal-lation and casing till the edge of the field can beachieved today for areas with more than 200 m² (seeFigure 2). The new solar roof concepts facilitate afurther improved integration and even lower cost.

• Concepts for economical long-term storage and theirsuccessful testing are developed. In comparison to thefirst feasibility studies in the middle of the eighties(project studies Mannheim and Wolfsburg) the cost ofhot water earth pit stores could be reduced about afactor 3, i. e. under 250 DM/m³ for 10,000 m³ storagevolume (see Figure 3). A development of this storagetype with almost water steam tight concrete without astainless steel liner promises further reduced buildingcost by 20 % /2/.

• The improvement of the thermal insulation of newbuildings (25 % under the limits of the WSVO ‘95)led to the result that today only 10 m² collector area

Collector field cost referred to the collector area incl. mounting and casing in the field, net, without planning

250

300

350

400

450

500

550

600

650

0 500 1000 1500 2000 2500 3000 3500

Collector area [m²]

Co

st

pe

r m

² [D

M/m

²]

Dachintegriert Montage auf Stahlkonstruktion Solarroof

Ravensburg I 1992

Ravensburg II 1992

Holzgerlingen 1996

Neckarsulm SW 1997

Neckarsulm 1997

Wiggenhausen 1996

Brenzstrasse1998

Brenzstrasse 1998

Neckarsulm I 1993 Neckarsulm Halle 1997without steel construction!

Burgholzhof 1998

Wiggenhausen 1996

Hamburg 1995

Bühlstr.1998

Rohr1997

Roof integrated Mounting on steel construction Solar roof

Figure 2: Cost of large collector fields referred to thecollector area

Figure 3: Storage cost of different projected and executedlong-term stores

for space heating and domestic hot water of onedwelling. Thus only one half of the roof of a four-storey multi-family building needs to be covered withsolar collectors.

• Through an intensive and friendly cooperation withcolleagues from Sweden and Denmark their knowl-edge could be used, and with some modifications thetechnology could be updated in a few years in Ger-many.

• Finally, the outstanding cooperation between city de-partments, house building companies, municipal deci-sion-makers and institutions, architects and experts aswell as a wise promotion policy of the BMBF withinthe programme “Solarthermie 2000” made the suc-cessful transposition of the concepts and ideas intoworking projects possible. Due to the combination ofthese persons and factors the aim could be achieved,and in consequence the door to “heating with the sun”in the next century was opened.

3. TECHNOLOGY OF COLLECTORS - FROM THEMODULE TO THE COLLECTOR ROOFWhen the first supporting projects concerning solar districtheating were built at the beginning of the nineties inRavensburg and Köngen, only module collectors with ap-propriate covering frames for roof integration existed. Forthe first project of a collector roof with about 115 m² col-lector area in Ravensburg the Wagner company, Marburg,modified their self-building system correspondingly andinstalled the collector on site.For a second plant in Ravensburg the existing large-scalecollectors were modified so that over two vertically ar-ranged collector rows a common dense level of glass-sheets

with appropriate connecting treads could be installed – bythat means interim sheets of metal could be dropped. Thecollector plant of the project in Hamburg Bramfeld wasbuilt up after a similar system by the Wagner company (seeFigure 8).

A further system for the combination of large-scale collec-tor modules was used for the first time by the companyParadigma, Karlsbad, in Neckarsulm 1993 (see Figure 4).The collectors are arranged one on top of the other withminimal distance, so that the gap can be covered by anoverlapping rubber sealing.This system was also installed on a building inFriedrichshafen; in this project it certainly represents themost successful integration in the architecture (see Fig-ure 5). It is also installed at the project “Stuttgart on two roofs with an area of 400 m². The cost forthese collector areas amount to approximately 450 DM/m²including installation and casing on the roof (compare Ta-ble 1). In /3/ to /4/ further information about the first solardistrict heating plants in Germany is arranged.

Figure 4: Roof integrated collector area on the multipledwellings in Neckarsulm-Amorbach (1993/94),

Figure 5: Collector fields on a building block inFriedrichshafen-Wiggenhausen (1996)

Very reasonable collector field costs of approx.350 DM/m² can be achieved by large-scale collectorswhich are installed on steel frameworks. If, however, oneadds the necessary substructure, this variant is, in spite ofthe favourable cost for collector and installation, withabout 500 DM/m² the most expensive one. As a conse-quence the roof integrated alternative should be chosen forcost reasons.

Table 1: Cost of large-scale collector fields(prices orientated to executed projects)Collector system /manufacturer

Project Aream²

CostDM/m²

Roof integratedmodule collectore. g. Paradigma, Karlsbad

e. g. Wagner, Marburge. g. Solar Diamant, Wettringen

Neckarsulm-Amorbach 1Friedrichshafenblock 4Brenzstraße,StuttgartNeckarsulm Amor-bach 2Hamburg BramfeldBurgholzhof,Stuttgart

700

685

400

385

3,000

1,674

450,-

Mounted flat plate

e. g. ARCON, Denmark

e. g. Sonnenkraft, Austria

Mounting

Friedrichshafenblock 1-3Neckarsulm 2,shop centreNeckarsulm 2,sports hall

2,015

444

1,200

370,-

330,-

320,-

ca. 150,-Solar roofe. g. Wagner, Marburge. g. SET, AltlußheimRoof construction (referring tothe collector area)

BrenzstraßeRohrFellbach

1601602x90

412,-

ca. 180,-

Meanwhile, three manufacturers in Germany also offercomplete collector-roofs which combine rafters, insulation,solar collector and roof sealing in one component. Thisconcept is ideal for large connected roof areas, because as acomponent from one company it avoids many cut problemsand can be integrated optimally into the architecture (seeFigure 6). Furthermore, it saves much installation time; acomplete roof with about 200 m² is installed in one day,one more day is needed for the points. The cost for a ready-made roof element amount to 500 or 600 DM/m² of whichapproximately 200 DM/m² can be credited to the replacedroof, so that the collector cost only amount to about 300 or400 DM/m², even at the so far installed relatively small ar-eas. The solar roof can also be delivered in a “light ver-sion”, i. e. without the load-bearing rafters, and then can beput onto an already existing sub-roof. In this case there donot remain any cost benefits, because the fundamental con-struction is not easier and only the rafters themselves aredropped or become smaller. This system was installed onthe elementary school in Neckarsulm-Amorbach (see Fig-ure 6).

4. LONG TERM STORES – THE DECIDINGMODULEThe seasonal store facilitates the storage of sun heat out ofthe summer into the winter. Without it a solar fraction of50 % of the overall heating demand of a house cannot beachieved by justifiable means. As each cubic metre of stor-age volume is only used once or twice a year, between 1,5(e. g. Hamburg) and 2 m³ (Friedrichshafen) water per m²collector area are needed for the storage. The stores thusoccupy a lot of space and must be manufactured economi-cally. In principle geological pre-studies (study of geologi-cal maps, test drills) at the planned store position are nec-essary. Water protectorates and large ground water flow(> several hundred m per year) have to be ruled out for thebuilding of a long-term store near to the surface.

The following criteria determine the choice of a storageconcept:

Figure 6: Mounting of the "solar roof“ onto the elementaryschool Neckarsulm – Amorbach

• The specific heat losses decrease with increasing sizeof the store; small stores can only be built with an in-sulation, very large stores without one (from approx.30,000 m³ at water stores and 100,000 m³ at an earthduct or aquifer storage).

• The building cost referred to the volume decrease withincreasing size (see Figure 3); a hot water storage canbe realised for less than 250 DM/m³ for a volume of10,000 m³ and more.

• The integration into the residential area must be pos-sible; as a rule the store should be buried for mostparts; a suitable underground then is necessary inwhich the store can be built economically. For anearth pit storage, with regard to low building cost, aground elevation should be permitted and a high earthmass compensation be striven for.

• The adaptation of the storage volume to the buildingadvance is desirable. A storage plan that is expandablestep by step as, for instance, an earth duct storage isadvantageous for a gradual extension of the residentialarea.

5. SYSTEM TECHNOLOGY – SOLAR SUPPORTEDDISTRICT HEATING WITH SEASONAL STORAGEAfter the successful realisation of the above-mentionedprojects, in the last years a number of solar supported dis-trict heating systems with long-term storage was builtwithin the BMBF-supporting programme"SOLARTHERMIE 2000" (subprogramme 3 "Solar districtheating”) (/3/ to /4/). The scientific charge and the moni-toring-programme of the plan are carried out by the ITW atthe University of Stuttgart. The first two plants are runningsince the end of 1996 in Friedrichshafen-Wiggenhausenand Hamburg-Bramfeld with hot water storage. A thirdplant in Neckarsulm-Amorbach with an earth duct storagewas put into operation in October 1997. In a fourth projectin Chemnitz a man-made aquifer was built which is to startoperation in the second half of 1998 (/16/, /17/).

5.1 Pilot projects "Friedrichshafen–Wiggenhausen” and“Hamburg–Bramfeld”Figure 7 shows the system scheme of the solar districtheating in Friedrichshafen-Wiggenhausen. The plant inHamburg-Bramfeld is technically very similar. The solarcollectors on the roofs are run by the central heating with awater/glycol mixture. The collector areas are connected in

CondensingBoilergas

Heating PlantCollectors 5600 m²

Distribution Network

Collectors NetworkSeasonalStorage

12 000 m³

sch_fn.cdr

Figure 7: System scheme of the solar plants with seasonalstorage in Friedrichshafen

such a way that the plant is run with a flow of about14 l/(m²h), i. e. "low flow".The collector circuit is coupled in the central heating over aheat exchanger into the store. The storage water forms aself-contained system and is heated by the solar collectorsup to temperatures between 40 and 90 °C. The discharge ofthe storage occurs over a further heat exchanger in thecentral heating into the district heating distribution net-work.Altogether, the heat has to pass through a heat exchangerthree times on its way from the collector over the distribu-tion network into the house, where it loses between 3 and8 K temperature every time. The reverse running tempera-ture is correspondingly raising. Especially this increase oftemperature of altogether 10 - 15 K leads on average to alower collector gain by 8 – 10 % than if the heating waterof the house was directly conducted into the collector. Lowreverse running temperatures are thus very important forthe effectiveness of the solar plant. For one more reasonthey are very important for plants with long-term storage:The store cannot be cooled down under the reverse runningtemperature, and already an increase of 5 K on averageleads to a reduction of the storage capacity of about 10 %.

Figure 8: Aerial photo of the housing scheme in Hamburg(124 terraced houses)

Figure 8 shows an aerial photo of the housing scheme inHamburg. The residential area in Hamburg consists of124 terraced houses (see Figure 9) with roof integratedcollector fields. The area in Friedrichshafen is composed of8 four-storied building blocks with approx. 570 dwellings,half of which were finished in the first phase of construc-

tion (Figures 10 + 11). The planning data of the residentialareas, the climatic conditions at the site and the interpreta-tion parameters of the solar plants are compiled in Table 2.

Figure 9: Terraced houses with solar collector fields(Hamburg-Bramfeld)

Figure 10: Multi-family houses with collector fields(Friedrichshafen-Wiggenhausen)

Figure 12 shows schematically the heat storage inFriedrichshafen: Its concrete ceiling is conically shapedand self-supported and does not need, unlike the storage inHamburg, any pillars. Figure 13 shows the store (volume12,000 m³) during the building phase. It consists of a sup-porting structure of reinforced concrete with an outsideheat insulation of mineral fibre. The insulation is only fixedat the vertical walls (ca. 30 cm) and at the ceiling (ca.40 cm). The inside of the concrete container is lined with asteam tight 1,2 mm plate of stainless steel. The buildingcost referred to the volume come to 240 DM/m³.

Figure 11: First phase of construction of the housingscheme with multiple dwellings in Friedrichshafen-Wiggenhausen

Table 2: Interpretation data of the residential areas andsolar plants in Friedrichshafen, Hamburg and Neckarsulm

Residential area HamburgBramfeld

Friedrichshafen

Neckar-sulm

Building type Terracedhouses

Multi-storey

Multi-storey

Number of buildings/Dwellings

124/124 8/586 120/210

Total living area m² 14,800 39,500 30,000Heat insulation stan-dard

Hamburg1992

20% <WSVO95

Total gas consumptionreference without solar

MWh/a 1,686 4,106 2,850

Referred to living area kWh/m²a 114 104 95ClimateHeat degree days K d 3,837 3,717Global radiation inhorizontal plane

kWh/m²a 978 1,177

Solar plantCollector area m² 3,000 5,600 700Storage volume m³ 4,500 12,000 20Fore / reverse runningtemperature

°C/°C 60/30 70/40

Gas consumption (in-cluding solar)

MWh/a 860 2,191 110

Referred to living area kWh/m²a 58 55 3,67Solar fraction % 50 47

The fuel coefficients (annual fuel consumption per m² resi-dential area) amount for the project of Hamburg to58 kWh/(m²a) and for the project of Friedrichshafen to55 kWh/(m²a). The investment cost for the complete sys-tem in Friedrichshafen are about 11,000 DM per dwelling.The solar heat prices come to ca. 30 Pf/kWh(Friedrichshafen) and to about 50 Pf/kWh (Hamburg, eachwithout support) and, as a consequence, are twice as highas large-scale systems without seasonal storage. This showsa requirement for economic storage concepts and clarifiesthat further research and development must be done. Thefirst operation experiences and measured results of the firsttwo solar district heating plants with seasonal storage aredescribed in /11/ to /13/.

19,7

5

excavetedsoil

ground

thermal insulation

concrete

stainless steel liner

O 35,2

Figure 12: Section through the hot water storage inFriedrichshafen (V=12.000m³)

5.2 Pilot project “Neckarsulm – Amorbach”The scheme of solar district heating with earth duct storage(first extension level) for the new residential area inNeckarsulm-Amorbach 2 is shown in Figure 14. In the fi-nal extension (approx. 1,200 dwellings) a collector area of15,000 m² and an earth duct store with a volume of150,000 to 175,000 m³ are planned. In October 1997 thefirst stage of construction with a collector area of 2,700 m²and about 20,000 m³ earth duct store (136 probes, each30 m deep) was officially opened. Until the end of 1999 anexpansion up to 57,000 m² collector area and 55,000 m³storage volume is planned.

Figure 13: Seasonal storage in Wiggenhausen underconstruction (volume 12,000 m³)

Planning the plant in Neckarsulm a new heat distributingand collecting system was developed that is better suited tothe specific requirements of the site. The earth duct storage(see Figure 15) requires low reverse running temperaturesto an even higher degree than a water storage; that is whythe number of heat exchangers should be reduced to aminimum. As the extended site is developed in severalsteps, the system has to be easily expandable. Apart fromthe new storage type (earth duct) a prefabricated SOLAR-ROOF was installed into a large roof area for the first time– within the framework of the new building of the elemen-tary school Amorbach (see Figures 6 and 16).

House transferstation

Collector field

District heating distribution network (3-wire)

Bufferstorage

Integrationboiler plant

Load

Unload

Earth duct storage

Solar transferstation

Heat reverse running

Solar forerunning

Heat forerunning

Heating central

Figure 14: Plant scheme for the project of Neckarsulm –three-wire distribution and collection network

Figure 15: Duct Storage under construction (Neckarsulm)

The collector area of about 2,700 m² of the first extensionstep of the project was put onto the elementary school(Figure 16), the sports hall, the shop centre, the elderlypeople´s home as well as onto two multiple dwellings. Thetotal cost of the project amount to approximately

12,000 DM per dwelling (without considering any sup-ports).

Figure 16: Façade of the elementary school inNeckarsulm-Amorbach with 604 m² collector area

The collector areas on the houses achieve, analogously tothe house transfer station for the district heating, a so-called solar transfer station, over which the solar heat is fedinto the network (see Figure 14). The plant has got thesame structure as a solar plant with short-term (diurnal)storage, with the network replacing the short-term storage.The regulation of the separate solar plants occurs analo-gously to the regulation of the plant in Friedrichshafen,switching on all collector circuit pumps by a signal fromthe heating centre. The connection with the secondarypump occurs individually when the temperature criterion isachieved. For this reason collector fields of different ori-entation can also be installed into the plant without anyfield losing a part of the heat which another one collects.The buffer storage (see Figure 14) serves as a hydraulicdecoupling between district heating network, solar fore-running and storage circuit and as a short-term storage outof which the heat needed in the network is taken after ashort time. In summer the earth duct storage is loaded outof the buffer storage; in winter the heat is taken out of theearth duct storage into the buffer storage and is then fedinto the district heating network. If the temperature in thebuffer storage is not sufficient, it is additionally heated bythe boiler. A detailed description of the project of Neckar-sulm and first measured results can be found in /7/.The three-wire network can of course be operated without along-term storage or in connection with a hot water stor-age. The two plants in Stuttgart that are being built inBurgholzhof (1,674 m² collector area) and Brenzstraße(1,000 m² collector area) are both run after this scheme.

5.3 Pilot project “SOLARIS A fourth project with a gravel/water-seasonal storage is de-veloped in Chemnitz (see Figure 17). This storage type – afirst prototype with about 1,000 m³ was tested in the mid-dle of the eighties at the ITW, University of Stuttgart – has8,000 m³ volume, and a gravel filling takes the load-bearing function so that lateral concrete walls are not nec-essary. The tightness of the storage is granted by means ofa 2,5 mm thick PE-HD film. The storage is already fin-ished, the approximately 2,000 m² solar collectors are to beinstalled and by that means the total system should startoperation in the second half of 1998 (see /8/). The buildingcost of the man-made aquifer amount to ca. 280 DM/m³(water equivalent, 8,000 m³ correspond to a storage capac-ity of 5,300 m³ water).

Figure 17: Man-made aquifer in Chemnitz underconstruction (Foto: Pfeil, ZSW Stuttgart)

6. NOTE OF THANKSThe in this article presented projects were mostly supportedby the aid programme “Solarthermie 2000” of the FederalMinistry of Education, Science, Research and Technology– the author thanks for that.

7. LITERATURE/1/ Lutz, A., Fisch, M. N., Hahne, E.: Kostenoptimale

Kombination von solarer Nahwärme, rationeller Heiz-technik und verbessertem baulichen Wärmeschutz.Proc. 9. Internationales Sonnenforum, Stuttgart 1994,volume 1, pp. 125-132

/2/ Reineck, K.H.: Erdbeckenspeicher aus Hochlei-stungsbeton. Statusbericht, Solarunterstützte Nahwär-meversorgung – Saisonale Wärmespeicherung, Neckar-sulm, 1998, in print, Steinbeis TZ/EGS, Stuttgart

/3/ Fisch, M.N., Kübler, R., Hahne, E.: Solare Nahwärme -Stand der Projekte in Deutschland. VDI-Tagung „Fort-schrittliche Energiewandlung und -anwendung“, Essen,March 29.-30.1995, VDI report 1182, p. 711-723

/4/ Schulz, M. E., Seiwald, H., Fisch, M. N.: Central solarheating plants with seasonal storage, The first pilotplants in Germany. 4th European Conference on SolarEnergy in Architecture and Urban Planning, Berlin,March 1996, H.S. Stephens & Associates, UnitedKingdom (in print)

/5/ Ebel, M.: Solarunterstützte NahwärmeversorgungHamburg-Bramfeld. Statusbericht, SolarunterstützteNahwärmeversorgung – Saisonale Wärmespeicherung,Neckarsulm, 1998, in print, Steinbeis TZ/EGS, Stutt-gart

/6/ Stanzel, B.: Betriebserfahrungen mit der solaren Nah-wärmeversorgung in Friedrichshafen/Wiggenhausen-Süd. Statusbericht, Solarunterstützte Nahwärmeversor-gung – Saisonale Wärmespeicherung, Neckarsulm,1998, in print, Steinbeis TZ/EGS, Stuttgart

/7/ Effenberger, S.: Nahwärmeversorgung Neckarsulm-Amorbach – Demonstrationsanlage Saisonaler Erd-wärmespeicher Amorbach II – Erfahrungen bei Bauund Betrieb. Statusbericht, Solarunterstützte Nahwär-meversorgung – Saisonale Wärmespeicherung, Neckar-sulm, 1998, im Druck, Steinbeis TZ/EGS, Stuttgart

/8/ Urbanek, T., Schirmer, U.: Solarunterstützte Nahwär-meversorgung – Pilotanlage SOLARIS Chemnitz. Sta-tusbericht, Solarunterstützte Nahwärmeversorgung –Saisonale Wärmespeicherung, Neckarsulm, 1998, inprint, Steinbeis TZ/EGS, Stuttgart