PV-cladding as an energy resource for the UK

Renewable Energy, Vol.5, Part I, pp. 348-355, 1994 Elsevier Science Lid Printed in Great Britain

Pergamon

0960-1481/94 $7.00+0.00

PV-CLADDING AS A N ENERGY RESOURCE FOR THE UK

N. M. PEARSALL, R. HILL and P. CLAIDEN* Newcastle Photovoltaics Applications Centre, University of Northumbria at Newcastle, Newcastle u p o n Tyne, NE1 8ST, UK * D e p a r t m e n t of Built Environment, University of N o r t h u m b r i a at Newcastle

ABSTRACT Photovoltaic cladding of building roofs and facades is receiving considerable attention in many European countries, due to the potential for significant contributions to the electricity supply, even in densely populated countries with little spare land area. The cladding of facades is particularly attractive for high latitude countries, such as the UK, due to the low sun angles during much of the year. This paper will discuss the methodology and results of a study to assess the potential electricity resource from the PV-cladd_~ng of buildings in the UK. It will also describe briefly the first UK demonstration project on PV-cladding, which is presently in the detailed design phase and scheduled to be operational in the autumn of 1994.

KEYWORDS Photovoltaics, solar energy, PV-cladding, UK resource assessment, PV demonstration project.

INTRODUCTION It is widely recognised that photovoltaic (PV) technology can provide an effective electricity supply with low environmental impact and the use of PV in electricity supply systems is growing rapidly. However, the potential contribution of PV to the overall electricity demand of a country is highly dependent on both the prevailing climatic conditions and the nature of the electricity supply and demand within that country. In northern climates, the large variation between winter and summer insolation and the variability of insolation present problems in the implementation of solar technologies. These are compounded if the country already has an extensive electricity supply infrastructure. However, the use of PV cladding, on the roofs or facades of buildings, can be attractive under these conditions and many European countries are now investigating such applications. PV-cladding has advantages over conventional PV systems for countries such as the UK. Firstly, it can utilise existing unused areas of building facades and roofs rather than taking up valuable land area in densely populated regions. The cost of the PV module can also be partially offset against the cost of the facade or roof element which would have been used. Secondly, the electricity is generated at point of use, which is assumed to be within the building on which the system is mounted or in the near vicinity of that building. These means that the cost of the PV generated electricity should be compared with the cost of conventionally generated electricity inclusive of distribution costs. In addition, the environmental effects of electricity distribution are reduced. Following a study of PV systems for the UK by the Energy Technology Support Unit (Taylor, 1991), PV cladding was identified as the most promising option. However, in order to determine the potential contribution 348

349 of PV-clad buildings to the UK electricity supply, it was necessary to carry out a more detailed resource assessment (Hill et al, 1992). This paper reviews the procedure adopted and discusses the results and implications of the study. Although the study considered the UK specifically, the methodology and many of the conclusions can be applied to many other countries. Following this work and other related studies (I-lalcrow Gilbert Associates, 1993, Lord et al, 1993), a major demonstration project of PV-cladding has been commenced at the University of Northumbria and the final section of the paper will discuss this project, together with other examples within the European Community.

PV RESOURCE ASSESSMENT For traditional energy sources, such as oil and coal, the resource is usually taken to be the amount identified as existing or which can be inferred to exist, regardless of the cost-effectiveness of extraction. That portion of the resource which is cost-effective to extract under present economic and technical conditions is known as the reserve. Strictly speaking, therefore, the resource assessment was not required to consider either the technical or economic barriers to PV cladding implementation, but merely to assess the amount of electricity which could be generated using this technology. However, some account was taken of the likely usage of the generated electricity in arriving at the resource value and an initial consideration of the cost-effectiveness was also included. At present prices, PV-cladding is not cost-effective in the UK in most cases, although it is expected to become increasingly attractive during the next 10-20 years. This aspect is discussed further in the next section. The methodology for the resource assessment was as follows: 1) Determination of typical insolation values on facades and roofs of buildings, including considerations of shading, seasonal and orientational variation and suitability of building surfaces. 2) Assessment of likely PV conversion efficiency. 3) Determination of typical generating capacity for buildings, using values from 1) and 2). 4) Extrapolation of typical values to UK building stock.

Buildin2 Tvoes The first task was to divide the building stock into categories and assign characteristics of the PV clad_d_~ngwithin each category. For simplicity, only three categories of building were used, these being termed domestic, commercial and industrial. The domestic category included all residential buildings and, for the assessment, an area including a range of house designs, sizes and orientations was considered. PV modules were taken to be installed in roof areas only for domestic dwellings. The only exception to this was for high-rise blocks of fiats, for which facade mounted systems were also allowed, but these form a very small proportion of the domestic building stock of the UK and are therefore insignificant in terms of the resource assessment. The commercial category represented the office buildings generally located in urban centres and considered to be the most promising building type for PV-cladding in the UK within the next decade or two. This is because there are large areas of facade available and the demand for electricity is more closely matched to the availability of sunlight (i.e. a daytime load) than for domestic dwellings. City centre areas containing commercial buildings were analysed in detail, taking particular account of losses due to shading from surrounding buildings, and the data for these areas were then used as the basis for the calculations for the other building types. Both roof and facade cladding were considered for commercial buildings. The final category was termed industrial buildings and these included factories, large retail stores (particularly out-of-town), leisure complexes etc. In this case, only roof mounted systems were considered. Clearly, there are a number of buildings which cannot be included in any of the categories listed, most notably large structures such as town halls, cathedrals, churches and other such buildings. Since most of these would be considered unsuitable for PV-cladding, their omission from the resource assessment was not significant.

350 Assessment of Insolation on Commercial Buildin~ Surfaces The first requirement was to determine insolation levels on the surfaces of typical commercial buildings. These are usually multi-storey structures and generally located in urban centres together with other office blocks and retail and service facilities. It was important to assess the effect of surrounding buildings in terms of shading of facades, together with the influence of building type, building mix and orientation. It is reasonably straightforward to determine the potential electricity generation from the PV-cladding of a single building, but requires considerable computation for a group of closely-packed buildings of different shape and orientation. Rather than attempt to create a "typical" urban centre, data from actual city centre sites were used. Eight UK city centre sites, each of around 1 km 2in area and ranging in latitude from Plymouth (50.40N) on the south coast of England to Glasgow (55.9°N) in Scotland, were chosen. The sites also covered a wide range of UK climatic conditions. Table 1 shows the areas and latitudes of the eight sites. All sites were medium to high rise development areas in the town or city centres. Where possible, the site boundaries consisted of obvious features such as ring roads, rivers or significant demarkations between commercial and residential areas. This had the effect ofrandomising the types of commercial building within the site boundary and avoided the unrepresentative selection of a site consisting solely of modern office blocks. Photogrammetric analysis of aerial stereophotographs of each site was used to provide the coordinates of all buildings on the site. This allowed a wire-frame simulation of each building to be generated by the computer. Sites typically contained between 200 and 400 buildings and, for the largest site, in excess of 600 buildings were included in the analysis. Firstly, the surface areaofthe buildings in the site was determined, allowing for all surfaces which were common to two structures or were so close as to be constantly shaded (the tolerance was taken as 0.5 m for this study). Each surface was then divided into grid elements. In this case, elements of lm x lm in size were used. These are equivalent to approximately three standard PV modules and are thus neither too large to give a reasonable approximation of the unshaded portion of the surface nor too small to be practically capable of being excluded from or included in the PV system. Each grid element was tested for shading by any other surface element in the site, for a particular solar position. If any part of the grid element was found to be shaded, the whole of that element was removed from the calculation of irradiated area. Clearly, the shading of surfaces varies throughout the day and throughout the year. This calculation was by far the most time consuming process of the analysis, particularly for the larger sites. As a compromise between Table 1. Details of city centre sites studied,

Site

Latitude

Area (sq. km.)

Plymouth

50.4

0.60

Barbican

51.5

1.06

Victoria

51.5

0.55

Birmingham

52.5

1.30

Sheffield

53.3

0.67

Manchester

53.4

0.83

Newcastle

55.0

0.40

Glasgow

55.9

1.53

accuracy and computing time, it was chosen to calculate shading coefficients for every hour of the day for the middle day of each month (i.e. twelve sets of shading coefficients for each site) and to use these values for all days within that calendar month. Additional computations for selected sites indicated that this introduced a significant error only during December, when the solar elevation is at its lowest. Since the central day of the month was used, this would tend to lead to a slight overestimate of the shaded area at the beginning and end of the month. Most UK cities have developed on sites where there have been settlements for tens and, in many cases, hundreds of years and have grown around the original settlements. The resultant orientation of the buildings is therefore rather random, depending on natural features such as ground topography and other issues which have determined where buildings are constructed. The ordered north-south, eastwest grid system, such as commonly found in US cities, rarely occurs in the UK, except for small areas of old urban centres or new towns. Thus, the sites studied presented a mixture of surface orientations. To allow assessment of the

351 results, each wall surface was placed in one of four categories, north, south, east or west-facing. If the normal to the surface fell within the quadrant defined by the directions south-east and south-west, the surface was deemed to 'be south-facing. Similar definitions were used for the other three quadrants. Shading coefficients were determined for each of the four orientation categories. In any random mix of buildings, there will be some which are unsuitable for PV-cladding due to their construction or their nature (e.g. buildings of historical interes0. The determination of these buildings is very time consuming, since it cannot be performed automatically by the computer. Each such building must be identified visually from photographs, site records or a site visit by a trained operator. As a compromise, the identification was carried out fully for the two London sites andpartially for three further sites. For the remaining sites, an average of the figures for Barbican and Manchester were used, resulting in 38% of the buildings being assumed to be unsuitable. The only exception was Victoria, which had an actual assessment of 60% of unsuitable buildings, due to the high proportion of old and historic buildings in the site. In addition, a proportion of the facade of all buildings is unavailable for cladding, due to fenestration, external service features (e.g. drainpipes) and, possibly, decorative and architectural features. A value of 33% of the total wall area was assumed to be unavailable for the fixing of PV-cladding. Both these corrections were applied retrospectively to the output of the computer analyses and, thus, all buildings were included in the shading calculations. Once the unshaded surface area had been determined, the insolation failing on all unshaded surfaces was calculated hourly throughout the year. This could then be summed over an appropriate period or used to investigate the variability of insolation. Solar data for the appropriate site (or the closest site available) were obtained from the FACET weather files. Direct irradiation was corrected for surface orientation, whilst the diffuse irradiation was assumed to be isotropic. This latter assumption is clearly a simplification of the actual case and it is hoped to refine the treatment of diffuse radiation in subsequent studies. Since solar energy is available only during daylight hours, unless some form of storage is employed, the solar insolation was calculated for a nine hour period from 08.30 to 17.30 each day (i.e. a typical office day). This was in accordance with the assumption that the electricity generated would be used within the building on which the cladding was mounted, replacing electricity normally supplied by the distribution company. Clearly, if storage technology is developed to allow economic use of electricity generated outside these hours, then the calculated resource would rise accordingly.

Re~ll~s of Commerf,ial Buildin~ Analvsis It is clearly not possible to provide a detailed discussion of the extensive results of the study within this paper. The major points will be summarised. Interested readers are referred to the full report for further information. The computation provided information on the insolation on the building surfaces within the sites studied. By assumption of an appropriate PV module conversion efficiency, the insolation data could be used to determine the potential electricity generation from PV-cladding of those surfaces. The insolation levels depend on the climate at the site and the nature of the buildings, including density, structure and surface orientation.

Seasonal Variations. The seasonal variations observed were generally as expected, with minimum values obtained in December. However, maximum values were usually offset from June, either to the spring months (April/May) or later into the summer (July/August). This is attributed to the detail of the solar data, with high diffuse content occurring in June. The ratio between the summer peak and the winter minimum was between 5 and 9 for all surfaces. These values are lower than would be measured for a horizontal surface, since winter direct insolation values are enhanced on a vertical surface, due to the low solar elevation, at the expense of the summer values. In addition, the constraint of using only the 08.30-17.30 period further penalises the summer values when the day length is considerably longer than nine hours.

352

Table 2. Percentage of solar insolation falling on building surfaces for Plymouth site for selected months MONTH

NORTH

EAST

SOUTH

WEST

ROOF

March

11.7

14.1

23.0

19.0

32.2

June

14.7

15.9

17.7

18.1

33.6

September

13.0

14.5

21.6

19.2

31.6

December

13.0

15.1

27.2

17.0

27.7

ANNUAL

13.$

15.2

20.4

18.4

32.6

Note: All values are pe~ unit area of surface.

~lY,agg.l~f, a g l i ¢ ~ For PV-cladding of facades, the module orientation is constrained within stringent limits and the optimum angle for power generation cannot generally be chosen. Table 2 shows the percentage of the insolation falling on surfaces of different orientations for the Plymouth site for selected months. It is clear that the high diffuse content of the insolation in the UK leads to significant insolation levels (at least in percentage terms) on walls other than those which are designated as south-facing. Inspection of the solar data shows, for example, that, for Glasgow, for one third of Jtme days, the diffuse content was in excess of 85% for the entire day (i.e. overcast cloud conditions). Whilst the use of an isotropic model for diffuse radiation may lead to this quantity being slightly overestimated for north facing walls, the analysis stiU implies that these surfaces may provide a significant proportion of the power available. Thus, whilst south-facing wails will clearly provide cost-effective electricity generation from PV-cladding in advance of all other walls, non-south facing walls cannot be discounted and all available surfaces have been included in the resource calculation. In order to extrapolate the results of the study of specific city centre sites to the whole of the UK, it was decided that the average solar input per unit area of land (rather than per unit area of building surface) should be used. This parameter was calculated for all sites and Table 3 shows the land area corrected values expressed as an equivalent installed power capacity. This has been derived by dividing the total energy available (in kWh) by the number of hours in the period under consideration, using the nine hour daily period for which the energy data were calculated. Perhaps the most interesting feature is the consistency of the annual mean values, despite the differences in latitude, solar data and building mix. This provides a strong justification for the use of the mean value for all sites of 107 MW/km zfor the calculation of the resource. Calculations of the solar input power capacity for the two other buildings types were obtained from direct comparison with the appropriate calculations for commercial buildings and using theoretical domestic and industrial sites containing a representative mix of buildings within each site. Values of 76.1 MW/km 2 and 38.4 MW/km z were determined for the industrial and domestic sectors respectively.

To convert the solar inputs discussed previously to electrical power output from the PV cladding, an appropriate module efficiency must be assumed. This value will depend on the time at which the resource is calculated, since module efficiencies have been steadily increasing due to technical advances since the mid- 1970's. Thus, it may be argued that the increasing module efficiency, coupled with an increase in urban areas and hence available building surfaces, will lead to an increasing resource for PV electricity rather than a decrease as occurs for traditional fossil fuel energy sources. In this study, a module conversion efficiency of 13%, equivalent to the present standard commercial modules, was assumed for the 1995 resource estimate and an increased efficiency of 20% was used for the 2020 resource calculation.

Land Area. For the method of calculation chosen, it was necessary to determine the total UK land area attributable to the three building types defined. The urban land area was calculated from 1991 statistical tables for a population density over 500 persons/km 2. This gave a total urban land area of 23,267 km 2, which is

353 Table 3. Average electrical generating capacity ( M W / k m 2)

AVEILAGING PERIOD

SITE June

December

April.September

Annual

Plymouth

160

30

150

103

Barbican

191

29

168

115

Victoria

187

31

165

115

Birmingham

168

27

129

91

Sheffield

142

28

175

98

Manchester

231

26

179

123

Newcastle

171

23

167

115

Glasgow

156

18

147

97

AVERAGE

176

26

160

107

equivalent to approximately 10% of the total UK land area (242,520 kin2). City areas were defined as those with population densities exceeding 2,000 persons/kin 2, leading to a value of 6,908 km 2(about 30% of the total urban area). The distribution of industrial, commercial and residential buildings within typical city and town areas was determined from detailed inspection of Ordnance Survey maps. Table 4 shows the percentages of each building type derived. As expected, the city shows a higher proportion of commercial buildings. However, in both cases, over 50% of the area is categorised as "open space". These percentages were used to determine the total land area attributable to each of the three building types and, by using the solar input per unit land area and the module conversion efficiency, to arrive at a total UK resource from PV-cladding. Table 4. Distribution of land within UK cities and towns

Category

City

Town

Commercial

4.0

1.8

Industrial

5.6

6.2

Residential

39.3

32.9

Open Space

51.2

59.1

For the projection to 2020, it was assumed that there was no change in the total urban land area, but that there would be a redistribution between categories. Consideration of recent changes to commercial areas in city centres implies two scenarios for redevelopment of obsolete building stock. Firstly, the building may be replaced by a modern version of a more suitable design for PV-cladding. Secondly, it may be replaced by an alternative development, commonly low-rise or "town square" designs usually for retail activities. It was assumed that the balance of these activities leads to a situation where the total building stock available for cladding remains unchanged. However, industrial and housing areas are assumed to increase by 20% and 10% respectively, at the expense of land area allocated to open space. Table 5 shows the resulting resource calculations for 1995 and 2020.

ECONOMIC ANALYSIS Whilst the primary purpose of this study did not include economic analysis of PV-cladding, it was felt to be important to provide some consideration of this aspect. The cost calculations assumed a module efficiency of 13% in 1995 rising to 20% in 2020, with a module cost decreasing from £3/Wp to £0.7/Wp over the same period, a lifetime of 30 years and a real discount rate of 8%. Since the PV-cladding is a replacement for conventional cladding, it is also appropriate to consider offsetting some or all of the cost of the conventional cladding which would have been used. Table 6 shows the estimated costs of the electricity from PV-cladding for an insolation value of 700 kWh/m2/year, which is typical of the average for all walls in the south of England. The avoided

354 Table 5. Electrical resource from PV-cladding of UK building stock 1995

2020

Building

Power

Category

Capacity per sq. km (MW)

Commercial

107.1

8.0

26.4

131.3

15.2

49.9

Industrial

76.1

14.1

46.3

76.1

26.0

85.5

Residential

38.4

41.1

135.1

38.4

69.5

228.3

63.2

207.8

110.7

363.7

TOTAL

Total Power Electrical Capacity Energy p.a. (GW) (TWh)

Power Capacity per sq. km (MW)

Total Power Electrical Capacity Energy p.a. (GW) (TWh)

cost of £-50/m 2 represents the case where there is no allowance for replacement of the conventional cladding and the system costs of £50/m 2 must be met fully. The zero avoided cost category represents replacement of the cheapest present wall cladding and those of £100 and £200/m 2 represent single and double glazed curtain walls respectively. It can be seen that, for the most expensive cladding, costs fall below the current electricity price of 8p/kWh before 2005 and, even for the cheapest cladding, costs fall to about 10p/kWh by 2020. This would represent current electricity prices with a small addition for the environmental costs of conventional energy sources. Of course, for selected areas of south facing walls and roofs, where the insolation exceeds 850 kWh/m2/year, the costs of electricity from the PV cladding would be reduced still further.

PV-CLADDING DEMONSTRATIONS The study of the UK building stock has shown that there is a large electricity resource available from PVcladding, even in a high latitude country. However, there are still technical problems, especially with regard to the variability of the output, the matching to building loads and the interaction with the grid supply system, which must be overcome. In addition, issues such as design, reliability and appearance must be addressed, together with planning regulations, both for the initial installation,and continued use of the PV system. Finally, for widespread use of this technology, public acceptance must be gained. It is important, therefore, to begin demonstration projects, where specialists such as building engineers, architects and planners can gain information and which can be used to increase public awareness. Table 6. Projected cost of electricity from PV-cladding Many European countries are now engagedin a programme of demonstration projects on PV in buildings, some using purpose built structures such as at the Fraunhofer Institut in Freiburg AVOIDED COST (£/sq. m) YEAR (Goetzberger et al. 1993) and others employing standard -50 0 100 250 houses or office blocks, such as the 1000-roof programme in 17 Germany (Decker et al, 1992) or the Scheidegger building in 1995 43 39 30 Switzerland (Posnansky et al, 1992) The first UK demonstration 2000 31 28 20 8.4 project is currently in the detailed design phase and is due for 3.8 completion in the autumn of 1994. The project is funded by the 2005 24 21 14 -2.2 Commission of the European Communities (DG XVII) under 2010 17 14 7.4 the Thermie programme, the UK Department of Trade and 2015 14 ll 4.9 -4.2 Industry, the project partners (University of Northumbria, Ore Arup & Partners, BP Solar and IT Power Ltd.) and by 2020 12 9.2 3.4 -5.2 private sponsors including the local electricity company, Northern Electric.

355 The south facing wall of one of the buildings on the city campus of the University of Northumbria will be fitted with PV-cladding in the course of a major refurbishment of the external fabric of the building. PV modules will be incorporated into aluminium rainscreen cladding and it is estimated that approximately 40kWp of modules will be installed. The electricity produced will be used within the building, which houses staff offices, lecture rooms and the main University computer system. The system will be extensively monitored in accordance with European guidelines. Unlike many projects where modules are retrofitted to existing walls or roofs, the modules will be fully architecturally integrated, as they would be when PV-cladding becomes widespread. The construction of the building is typical of a 1960's office block, many hundreds of which require recladding in the near future. The building location, close to the city centre of Newcastle upon Tyne, will also give the maximum opportunity for viewing by students, members of interested professions and the general public. Information on the design and performance of the building will be published in the open literature during the course of the three year project.

CONCLUSIONS Even in high latitude countries such as the UK, there is the potential for the generation of significant amounts of electricity from the PV-cladding of buildings. Indeed, the cladding of facades can be particularly attractive in such areas since it maximises the winter insolation levels when the sun is at its lowest elevation. Furthermore, for climates with a high diffuse content, it is possible to consider cladding of facades other than south facing. Cost estimates, using reasonable projections of module efficiency and cost, show that electricity from PV-clad facades could be competitive with conventionally generated electricity well before 2020 for replacement of all but the cheapest conventional cladding. The inclusion of environmental costs in conventional energy pricing would bring forward the date at which PV-cladding became cost-effective. Studies of PV-cladding in the UK agree that the first application will be on commercial buildings where the daytime load is in reasonable agreement with the supply. The study discussed in this paper paid special attention to city centre sites and, in particular, the issue of shadowing by surrounding structures. The UK is now commencing demonstration of the technology, which should allow development of an appropriate infrastructure to further the use of PV-cladding as it becomes cost-effective. ACKNOWLEDGEMENTS The authors would like to acknowledge the financial support of the UK Department of Trade and Industry (Energy Division) for the resource assessment study, together with the support of staff from the Energy Technology Support Unit at Harwell.

REFERENCES Decker, B. et al, 1992, Proceedings of the 1 lth European Photovoltaic Solar Energy Conference, Montreux, Switzerland, p. 1497-1500 Goetzberger, A. et al, 1993, Record of the 23rd IEEE Photovoltaic Specialists Conference, Louisville, KT, USA, p. 1152-1158 Halcrow Gilbert Associates Ltd., 1993, Grid Connection of Photovoltaic Systems, ETSU S 1394-P1 Hill, R. et al, 1992, The Potential Generating Capacity of PV-Clad Buildings in the UK, ETSU S 1365-Pl Lord, B. et al, 1993, A Study of the Feasibility of Photovoltaic Modules as a Commercial Building Cladding Component, ETSU S/P2/00131/REP Posnansky, M. et al, 1992, Proceedings of the 1 lth European Photovoltaic Solar Energy Conference, Montreux, Switzerland, p. 1676-1678 Taylor, E.H., 1991, Review of Photovoltaic Power Technology, Report No. ETSU-R-50