page 1

Photovoltaics for Buildings Market, Technology, Architecture, Energy ConceptsKarsten Voss, Hermann Laukamp, Martin UfheilFraunhofer Institute for Solar Energy Systems ISEOltmannsstr. 5, D-79100 Freiburg, GermanyTel. +49 (0)761 4588-135, Fax: +49 (0)761 4588-132email: karsten.voss@ise.fhg.deinternet: http://www.ise.fhg.de

Abstract

Grid-connected photovoltaics for buildings is a market. This is demonstrated by the numberof applications, which has increased steadily over the last three years. Technicalindependence from the other energy supply systems in a building (heating, cooling, hotwater) leads to comparatively simple systems technology for photovoltaics and clearinterfaces to the technical infrastructure of a building. Photovoltaic modules, as buildingcomponents, fulfil various functions in the building envelope. If applied correctly, they makea positive contribution both to the energy supply and the architecture of a building.The following text provides an introduction to the fundamental technology of grid-connectedphotovoltaics in buildings, illustrated with examples, and presents costs and energy yieldsin relation to other technologies for saving energy and using solar energy in buildings.

The Market for Photovoltaics

Solar generation of electricity with Photovoltaics (PV) has a broad application field. Whenthe global market is considered, the emphasis is clearly on applications where no othersource of electricity is readily available. Photovoltaic systems in the form of so-called SolarHome Systems (a set consisting of a solar module, battery, charge controller and smallappliances) supply electricity to houses without grid connections in rural areas of Asia andSouth America. Beyond this, industrial applications for PV are opening up increasingly intelecommunications and signal technology. An example for this sector is the solar-poweredvending machine for parking vouchers, which in Germany have now a market share of morethan 70 %. Small electric appliances such as clocks and watches represent anotherapplication area with increasing demand.

All of these grid-independent applications totalled about 80 % of the market for PVmodules in 1994 (fig. 1). The remaining 20 % consists of PV systems which operate incombination with the grid, feeding the generated electricity via a grid inverter into the publicelectricity grid. If the demand exceeds supply (e.g. at night), electricity from the gridreplaces or adds to the supply from the PV modules. Already in 1994, this marketcorresponded to around 13 MWp installed power and thus a module area of 130,000 m2.

page 2

remot e rura3 5%

indust rial application26 %

grid-conne ct ed2 0%

cons umer product1 9%

Fig. 1: The global market for photovoltaics and its distribution in market segments (Source:EPIA European Photovoltaic Industry Association, 1995).The market for grid-connected photovoltaic systems has developed very favourably in therecent past. In Germany and the year 1997 alone, systems were installed with a power of11 MWp, corresponding to an investment volume of 100 million ECU /1/. Numeroussubsidy programmes were a major factor in supporting this boom. At the forefront is theso-called 1000 Roofs Programme, initiated by the German Federal Ministry for Research,which resulted in the installation of 2000 photovoltaic systems, ranging from 1 to 5 kWp,between 1990 and 1995 /2/. Investment subsidies of up to 70 % made the programmeparticularly attractive. The knowledge gained in the accompanying scientific researchprogrammes is a significant resource for planning new systems today /3/.

New initiatives have now replaced the 1000 Roofs Programme. Important ones include:

investment support by government programmes or electric utilities public loans at reduced interest rates tax benefits cost-covering payment for photovoltaically generated electricity green electricity rates as a means to introduce ecological quality into the market for

electricity. share-holder models.

With a view to electricity production costs of 0.7 to 1 ECU per kWh, such initiatives remainnecessary. In connection with a future liberalisation of the European electricity market,particularly the green electricity rates are significant if electricity is to be marketed withecological arguments and not simply on the basis of the cheapest price. It has becomeevident that customers can be won, especially from private households, who are preparedto pay more for electricity with this quality.

page 3

MWp/ a

1 ,6 1,3

2 ,9 32,1

7,9

1 1

3 ,1

0

2

4

6

8

10

12

till 19 90 19 91 1 99 2 19 93 1 99 4 1 99 5 19 96 1 99 7

Fig. 2: The market development for grid-connected photovoltaic systems in Germany since1990 /2/.

Fig. 3: Example of a building with a grid-connected photovoltaic system from the 1000Roofs Programme.

page 4

Photovoltaics on Buildings - Arguments

The investment costs for small PV systems came down by some 30 % during the last 5years. Despite the rather high cost (Fig. 4), the development of photovoltaics is a successstory. Evidently, the financial value of the generated electricity is not the only significant factor.The palette of arguments for photovoltaics on buildings is wide-ranging.

They include:

Photovoltaics on buildings uses existing infrastructure. No further ground area is neededto tap the sun as a source of energy. This is particularly significant in the denselypopulated areas of Europe with regard to solar energy supply in the future.

Electricity generation directly where it is consumed avoids distribution losses. Synergy with other parts of the building envelope reduce additional investment costs (

wintergarden, balcony roofs, etc.). Photovoltaic modules are components in the buildingenvelope.

The annual energy yield from an individual photovoltaic system provides a benchmarkfor the electricity consumption of the corresponding household (average 4 personhousehold = 4000 kWh annual electricity demand = 50 m_ PV !). This often stimulateselectricity-saving measures at home and at work.

Photovoltaic modules are an architectural expression of innovation and high technology.In particular, they enhance the public image of a company when used as a component inprestigious facades of the company buildings.

The architectural quality of a PV facade gives an alternative to other well establishedcladding materials (PV instead of marble ?)

The more significant these arguments are in a specific case, the more likely it is that theinvestment in PV technology will prove to be the right decision.

electrical installation

8%

design, planning

3%labour11%

modules58%

mounting structure

6%

inverter14%

Fig. 4: The average cost structure of an 2 kWp grid-connected photovoltaic system inGermany. Total cost are about 16,000 ECU (1996). Total costs and cost structure maydiffer significantly from project to project.

page 5

Electrical Technology

Solar cells are large-area semiconductor components which convert sunlight directly to DCelectricity. Almost all of the commercially available cells are made of silicon. The propertiesof the various cell technologies are listed in Table 1. In addition to the well-known dark bluecells, cells in other colours are now on offer. The price for the other colours is a reduction inthe efficiency value of one to three percentage points, depending on the colour.

Table 1: Properties of silicon solar cells

cell type efficiency valuein %

other properties

monocrystalline 14 - 18 homogeneous cell surface, opaquepolycrystalline 11 - 15 structured cell surface, opaqueamorphous 4 - 8 homogeneous appearance, long-term stability limited, flexible

structures possible, opaque or translucent over entire area

Grid-connected photovoltaic systems consist of so-called solar moduls. A standard moduleis a glazed unit of typically 0.5 m2 area comprising of 30 to 40 solar cells connected togetherto an electric power of 45 to 60 W. Solar modules are usually installed in buildings in asimilar fashion to glass elements. The front glazing is a low iron, tempered safety glass.The back protective cover can be made of glass or less expensive laminates of plastic filmsand a vapour barrier. The DC current of the module is converted by an inverter to ACcurrent conforming to grid requirements. If the system is correctly designed, the losses arelow (app. 10 % on average over the year) that it is not worth setting up a domestic grid withDC appliances. Figure 5 shows a typical block circuit diagram for a grid-connectedphotovoltaic system.

Fig. 5: Design of a grid-connected PV system (block circuit diagram).

page 6

Building Technology

Standard modules usually have an additional frame of aluminium or stainless steel for easiermounting. The demands on module production technology are high, to ensure completefunctionality for more than 25 years (warranty by some major manufacturers: 20 to 25years!). This is a precondition for rational use of photovoltaics, but at the same time, it meansthat 40 - 50 % of the costs for the module are attributable to packaging of the actual activeelement, the solar cell /4/. The primary energy consumption to produce a completephotovoltaic system is usually amortised by the energy yield in operation over five(amorphous cells) to seven years (crystalline cells)1 /5/.Without a frame, the modules can be incorporated as panels in the usual transom-mullionsystems. Modules for this application are produced in any sizes according to the projectneeds. Thermal insulation is given by the facade or roof construction behind the module.The modules are thus passively ventilated from behind, which helps to ensure a highenergy yield, by decreasing the temperature increase at irradiation.

A word of caution: Any form of shading should be avoided. This also applies to cover stripson framing systems and other projecting building components on the wall or the roof(antennas, ventilation pipes, chimneys, outward-opening windows, etc.).

Fig. 6: Design of a frameless PV module (left: solar cells laminated between two sheets ofglass or a front glass and a plastic cover on the back) and a double glazing unit with PV(right: solar cells laminated between two sheets of glass as external galzing). Grafic: Vegla

1 These figures apply for a site in Central Europe. Higher solar radiation intensity in countriescloser to the equator leads to a higher energy yield for the same module area. In suchcases, the energy amortisation period is shortened.

page 7

trans mittance in %

0

1

2

3

4

5

0 15 30 45 6 0 75 90

angle of inclination in degre

lights olar

Fig. 7: Measured transmittance of a double glazing PV unit constructed according to fig. 6,right and applied as solar shading system (measurement: Fraunhofer ISE). The doubleglazing incorporates Argon as fill gas and a low-e coating on the inner side of the backsidepane. Both measures reduce the transmittance of solar energy trough the glazing. Theoverall transmittance properties vary significantly with the ratio of PV covered to uncoveredareas of the front glazing, namely the distance between the cells.

page 8

Photovoltaics can be incorporated into so-called functional double glazing without anydifficulties (fig. 6, right). In combination with coatings, gas fills and appropriate panes,properties such as thermal insulation, solar control, noise protection, etc. can be obtained. Inparticular, the effectivity of photovoltaic double glazing for solar control, with a total solarenergy transmittance of around 10 to 20 %, predestines it for use in inclined glazing onconservatories or the southern side of shed roofs. The remaining transparent north side ofthe shed roofs is available to illuminate the rooms below with natural lighting (fig. 8).

Fig. 8: PV in double glazing units as the roof glazing of the Centre for Art and MediaTechnology (ZKM) in Karlsruhe, Germany (Photo: Fraunhofer ISE).Another component form is also commercially available, namely roof tiles or shingles. Thismodule technology leads to a closed roof surface with all the properties of usual roofingmaterials.

All the applications share the property that the energy yield is determined decisively by theorientation of the system with respect to the sun (fig. 9). Optimal results are obtained fromunshaded systems which are inclined and orientated toward the south (northernhemisphere). Integrating PV modules into facades results in a yield which is around 25 - 30% lower than from the optimal orientation. As a consequence, this type of installation is ofinterest when there is a pronounced interest within a project that the solar generator beclearly visible (fig. 10). Such applications are also suitable for coloured solar cells (seeabove).

page 9

Fig. 9: The relative annual irradiation on inclined surfaces of various orientation towards thesun (azimut angle) for the location Freiburg, Germany

tilt angle

page 10

Fig. 10: Shading simulation for the ZKW Building, Germany (ref. Fig. 8)

Fig. 11: Photovoltaics on the facade of an administrative building in Heidelberg, Germany(Photo: Lamy)

page 11

Energy Concepts with PV - Residential Buildings

For the usual standard of residential building and Central European climatic conditions, mostof the energy demand is for space heating (fig. 12). This demand, and that for domestic hotwater, is most commonly met by local conversion of a primary source of energy (gas, oil,wood) to heat2. As a result, the percentage distribution for a building's energy demand willbe different if the final energy form is considered rather than the primary energy.

space heat ing73%

hot water10%

elec t ric it y17%

space heat ing56%

hot water8%

elec t r icit y36%

Fig. 12: Distribution of the final (left) and primary (right) energy demand of a four-personhousehold in a building corresponding to the typical current standard in Germany. Theprimary energy demand is simultaneously an indicator for climatically relevant pollution(CO2). Typical conversion factors for final energy to primary energy in Germany are: 1.15for heat supplied by a condensing boiler and 3.2 for grid electricity.

Figure 12 illustrates the fact that, with respect to buildings, concepts for energy saving andthus pollution reduction have to start with the heating demand. If appropriate measures aretaken - thermal insulation, heat recovery, thermal use of solar energy - the electricity demandfor the household and building services becomes more significant (fig. 13). Thus, once thestandard of a (thermally) low-energy house has been reached, a consistent ecological andeconomical approach demands energy concepts which take account of all the energyservices in the building.

2 At present, electric space heating does not play a significant role in Germany. The situationis fundamentally different in Scandinavian countries, with their high proportion of hydroelectricity.

page 12

0

20

40

60

80

100

120

140

160space heating domestic hot water electricity

standard low- und extra low-energy house

zero space heating house

zero heating house

"Passive"house

energy autonomous

house

zeroenergyhouse

prim

ary

ener

gy d

eman

d

Fig. 13: Qualitative distribution of the primary energy demand for different building conceptsaccording to the energy services in a residential building.

Whereas only the thermal energy budget is affected in low and zero heating-energyhouses, concepts such as the passive house (fig. 14), the zero-energy house (fig. 15) andthe self-sufficient solar house (fig. 16) treat all types of energy services, including householdelectricity. Further-reaching definitions take account of the energy for constructing, maintainingand demolishing the building, in the form of life cycle analysis.

Fig. 13: Passive houses in Darmstadt-Kranichstein (Photo: IWU). The combination ofenergy saving and passive and active use of thermal solar energy characterises a buildingconcept with minimal use of external energy. A photovoltaic system is not installed - forfinancial reasons.

page 13

Fig. 15: Zero-energy pre-fabricated house in Rheinau-Linx (Photo: Weber Haus). Thepackage of measures for an almost passive house was extended with a larger thermalsolar system (40 m2 collector area) with a seasonal storage tank (20 m2 capacity) and grid-connected photovoltaics (3 kWp, 28 m2 generator area).

Fig. 16: The Self-Sufficient Solar House in Freiburg (Photo: Fraunhofer ISE). In this rigorousapproach, the connection to the electricity grid was terminated and a stand-alone energyconcept was put into (research) practice. In order to provide sufficient electricity in winter,solar-produced hydrogen was used for seasonal storage (PV system: 4.2 kWp, 36 m2generator area).When the fossil fuel consumption is to be reduced, the costs and benefits of the selectedmeasures must be compared if the aim is to identify an economically rational order ofinvestments. Whereas the ratio of cost to benefit (energy yield) can be assumed to befixed for a grid-connected photovoltaic system, investments in building measures andsystems technology result in different amounts of saved energy, depending on the initialenergy situation. As an example, the first centimetre of thermal insulating material on a wallsaves much more energy, for the same price, than increasing the insulation thickness from 20to 21 cm. The same applies for the first square metres of collector area as compared toextending a 10 m2 system. In the case of PV systems, this situation applies analogously

page 14

only for so-called stand-alone systems, i.e. systems which are not connected to the publicelectricity grid. Like the situation for a thermal solar system, the limited storage capacity isthen the decisive aspect.

Table 2 Comparison of the primary energy coefficient of solar thermal systems and grid-connected photovoltaics. Equivalent Primary Energy Cost (PEC) = annual capital costsrelative to the annual savings in primary energy

system investmentcosts

energy yield PEC

final energy primaryenergy

ECU kWh/a kWhp/a ECU/kWhpsolar domestichot water

SDH 5.000 1.650 1.900 0,22

solar assistedspace heating,small

SSH1 7.400 2.400 2.800 0,22

solar assistedspace heating,large

SSH2 11.600 2.800 3.200 0,30

photovoltaics PV 7.400 800 2.600 0,24

Conditions:low-energy house, annual heating demand 35 kWh/(m_ a), 4-person household, locationFreiburg, Germany, calculated lifetime of solar systems 20 years, real interest rate 4 %,maintenance costs 1 %

System parameters:- SDHW - flat-plate collector 5 m2, storage tank 0.3 m3- SSH1 - flat-plate collector 10 m2, storage tank 0.7 m3- SSH2 - flat-plate collector 16 m2, storage tank 1.2 m3- PV - photovoltaic system 1 kWp with inverter to grid

Table 2makes it clear that today it is already more economic, in terms of cost per primaryenergy savings, to install a grid-connected photovoltaic system than large thermal solarsystems. The reason, apart from the system aspects already discussed, is the strongreduction in PV prices over the past few years. Also, the systems technology of a grid-connected photovoltaic system proves to be simpler than that of a large scale solar collectorsystem for heating support. This, in turn, is because there is no need for in-house storage.As a result of such considerations, fig. 18 shows a set of measures, ordered according toenergy and economic criteria, for the route from a low-energy house to a zero-energyhouse. From a certain point on, photovoltaics should be preferred to further demand-reducing measures, e.g. seasonal storage of heat. At present, most building and systemstechnology measures to reduce the heat demand (including thermal collector systems) areless expensive than an investment in photovoltaics. However, this situation could change infuture!

Figure 19 qualitatively illustrates the seasonal compensation between supply (PV yield)and demand (primary energy demand) for a balanced budget at the end of the year (zero-energy house). This approach can be extended simply to zero-energy solar settlements(fig. 19). The solar thermal alternative to this is the transition to solar district heating with large,

page 15

central, thermal reservoirs. Solar district heating systems have significantly better economics,specially compared to large scale photovoltaics.

a b c d e f g h0

5

10

15

20

25

30primary energy demand in MWh/a

0

10

20

30

40

50

60investment costs in kECU

thermalinsulation

electricitysaving

solarthermalcollector

photovoltaicventilationheat

recovery

Fig. 17: Investment costs and primary energy demand for measures along the route from alow-energy to a zero-energy house.

Fig. 18: The cumulative primary energy demand of a zero-energy house compared to theyield of a photovoltaic system dimensioned to meet the total annual demand (qualitativerepresentation).

page 16

Fig. 19: New residential complex with roof-integrated photovoltaic systems in Bremen,Germany (Photo: Osmer).

page 17

Energy Concepts with PV - Non-Residential Buildings

Most projects involving non-residential buildings are characterised by the fact that theelectricity demand gains in significance relative to the heating demand to a greater (e.g. officebuilding, shopping centre) or lesser (e.g. gymnasium) extent. Additional electricity isconsumed by computing equipment, communications technology, air-conditioning and, notto be neglected, artificial lighting. Thus, each square metre of roofing area which could beused for a photovoltaic system is confronted with such a high electricity demand that it isimpossible to achieve a significant solar fraction.

Today the prominent reason to install a PV system in non-residential buildings is non-technical: to create an image, to demonstrate environmental awareness, to display modernthinking (refer fig. 10). However, in some cases PV offers also technical advantages: In regions with a weak grid PV allows to extend operating times for UPS systems,

especially when the grid load peaks around noon. In regions with a daily peak load, e.g. Mediterranean countries, cities with a large fraction

of air conditioning equipment, tourist centres etc. the grid load shows a peak around noon.Here PV power production shows a good load matching.

The Californian Pacific Gas and Electric Company (PG&E) investigated under whichcondition grid connected PV installed at customers sites can be an option as means ofdemand side management /5/. They installed PV in an area where the transmission anddistribution capacity of the electric utility network was approaching capacity limits andwhere an upgrade was planned. In this case the investment for PV can be balanced withthe avoided cost to upgrade the network hardware. It was estimated that 5 MWp PVwould provide an effective increase in feeder capacity of 2.9 MW or 58 %.

In remote locations a PV hybrid system often is the cheapest power supply. Goodexamples are mountain huts, tourism facilities along hiking trails and the like.

In Germany recently share-holder financed PV systems gained importance. They areusually constructed on large roof areas like schools , factories, stadium etc. The electricpower is completely fed into the public grid, the large area allows efficient construction ofthe PV system. An economic advantage are the decreasing specific costs with thegenerator size. As larger the PV generator as smaller the costs per installed kWp.

Fig. 20: Advertising with PV - The Trautwein company in Emmendingen, Germany. 500 m_of PV modules on the roof of the production building are starting point for consequentenergy efficiency in the production process. (Photo: Trautwein)

page 18

Fig. 21: The Solar Fabrik, Freiburg. The buildingenvelope with integrated PV modules is part of therenewable energy supply system of the buildingdesigned to be 100% free of CO2emissions

page 19

Energy Concepts with PV Building Renovation

PV in building renovation is not the prior measure to reduce the high energy demand of theexisting building stock. But, as part of a well balanced renovation concept (thermal insulation,improved windows, improved ventilation system, solar collectors,..) it marks a further steptowards sustainable housing. As part of a facade renovation it can underline the changes inthe building appearance from old fashioned to high-tech, fig. 22. A more conventionallooking approach is the use of PV roof tiles, fig. 23.

Fig. 22: Photovoltaic as part of the renovation of the so-called Yellow-House in Aalborg,Denmark.

Fig. 23: Photovoltaic roof tiles as part of abuilding renovation in Niederurnen,Switzerland

page 20

References

/1/ Gabler, H., Heidler, K., V. U. Hoffmann: Market Introduction of Grid-ConnectedPhotovoltaic Installations in Germany, 14th European Photovoltaic Solar EnergyConference, Barcelona, 1997

/2/ 1000 Roofs Measurement and Analysis Programme, Fraunhofer ISE, Freiburg, 1995/3/ F. Sick, T. Erge (ed.): Photovoltaics in Buildings, James & James, London, 1996/4/ Hagedorn, G.: Kumulierter Energieverbrauch und Erntefaktoren von PV-Systemen,

Energiewirtschaftliche Tagesfragen, 1989/5/ Berdner, J. et al.: Installation and Performance Issues for Commercial Roof-Mounted PV-

Systems, 1st World Conference WPVSEC, Hawaii, 1994