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Plea2004 - The 21th Conference on Passive and Low Energy Architecture. Eindhoven, The Netherlands, 19 - 22 September 2004 Page 1 of 6

Comparison of the solar energy utilisation potential of different urban environments M Montavon1, J-L Scartezzini1, R Compagnon2 1

2

Swiss Federal Institute of Technology (EPFL), Solar Energy and Building Physics Laboratory (LESO-PB) CH-1015 Lausanne, Switzerland E-mail: [email protected], [email protected]

University of Applied Sciences of Western Switzerland (HES-SO), Ecole d’Ingénieurs et d’Architectes (EIAF) CH-1705 Fribourg, Switzerland E-mail: [email protected] ABSTRACT: Besides a more efficient energy use, large scale application of solar energy technologies in the urban context will be necessary in the upcoming decades to achieve a drastic reduction of greenhouse gas emissions in the biosphere. Two different urban sites, representative of two of the largest Swiss cities (Basel and Geneva), were examined under this aspect in order to assess their respective solar potential. Spatial distributions of solar irradiation and daylight fluxes over the overall building facades and roofs were calculated using ray-tracing simulation techniques to determine the appropriate placement of different solar technologies (passive and active solar, photovoltaic and daylighting). Several performance indicators were used to assess the solar utilisation potential of these urban sites (e.g. statistics of sky view factors and daylight factors). A comparison of the results observed for the two urban sites will be presented in this paper. It has been observed that in both cases the values and useful surface areas appropriate to solar technologies are significant. Conference Topic: 4 Energy and urban planning Keywords: Solar energy, utilisation potential, performance indicators, Swiss cities, urban morphology

1. INTRODUCTION

2. THE STUDY CONURBATIONS

In the mid-nineties, after publication of a new policy for sustainable development by the Swiss federal government and almost twenty years of permanent debate on energy policy, the Council of the Swiss Federal Institutes of Technology (CEPF) launched the “2000 Watt Society” Programme [1-2], aiming to lower the Swiss energy consumption per capita expressed as an average power from 5000 to 2000 Watts. The innovating energy policy followed by the canton of Basel-Stadt earned it the selection as a pilot area for the implementation of a “2000 Watt Society”. The program was launched in 2001 by the Basel authorities in the framework of “Strategy for Sustainable Development”. The present research [3], supported by the Swiss Federal Office of Energy (SFOE), aims to promote sustainable urban architecture by allowing a better direct use of solar energy in the urban environment. The pilot site of Matthaeus in Basel, as well as the mixed use area of Meyrin in Geneva were considered and evaluated regarding their solar energy utilisation potential.

2.1 Geographical locations. The city of Basel is located in the northeastern part of Switzerland and Meyrin in the south-western, in the suburbs of Geneva (Fig. 1).

Figure 1: Map of Switzerland. Location of Basel and Geneva.


2.2 Matthaeus/Basel Switzerland, Germany and France converge at one point on the course of the river Rhine at Basel, forming a regional, cultural and economic entity. Of the overall cantonal population of 402 000 inhabitants, 190 800 (48%) live in this town whose surface area equals 37 km2. The northern part is a predominantly industrial area, where major chemical and pharmaceutical industries are based as well as an important share of the country’s watch industry: the district of Matthaeus is located in this part of the city of Basel, close to the town center. Owing to its restricted area, the Matthaeus district’s potential development is very limited. As a consequence, the potential of every refurbishing project is exploited in an optimal way and subject to careful reflection. With an area of approximately 550 000m2 for an overall population of 15 300 inhabitants, this town site is characterised by one of highest population densities of Swiss cities.

it has benefited from the evolution of the economic attractiveness of Geneva, which has led to a rarefaction of constructible grounds. Faced this shortage, the cantonal political authorities are aiming to increase existing urban density. Settlements and infrastructure represent currently 54% of the Meyrin area.

2.3 Meyrin/Geneva Meyrin, located at 9 km of the center of Geneva was conceived as a satellite town for Geneva : one of the rare witnesses of the architecture and functionalist town planning of the sixties in Switzerland. Built at the edge of the city, between the airport and the large CERN (European Organization for Nuclear Research) infrastructures, Meyrin accommodates various research centers of international renown, which represent 26 000 employment positions for a total population of 20 500 inhabitants and a surface of 998 hectares. This city fragment has experienced intensive growth for more than a decade. Simultaneously

2.4 Morphology of Matthaeus/Basel The first considered urban site, named “Klein Basel – Matthaeus”, is located near the city centre of Basel alongside the Rhine river. The river divides the city into two parts, which are connected by six bridges. The district of Matthaeus is one of the districts built for workmen during the industrialization of the city at the end of the 19th century. The great urban density of this working-class district (Fig. 2) is perceived by its inhabitants as a principal problem. The old medieval city, bought in 1392 by Grossbasel, presents a checkerboard plan orientated along a north-south/east-west axis; its streets are parallel to the Rhine. Unlike the regular models of their Romanesque ancestors, the medieval cities generally presented an irregular structure which made a general description of the urban form impossible [4]. The population tripled in the first half of the 19th century. After 1959, with the dismantling of the walls and the filling of the ditches, construction became extensive. Public green areas were created on the site of the old fortifications, the main roads were corrected and new streets were built. The rules of construction were formulated in a law on urban expansion on June 28, 1859.

Figure 2: Matthaeus district (Basel city) [5].

Figure 3: Zoom of Meyrin [6].


With the turning of the 19th century and the development of trade and industry, the city gradually started to lose its medieval aspect. From 1960, new constructions frequently came to replace the original buildings. Most are characterized by a commercial ground floor and often backyards made up of workshops. 2.5 Morphology of Meyrin/Geneva Before World War One, Meyrin was an agricultural village populated by one thousand inhabitants and its development articulated around Geneva and its perpendicular. It kept its primarily agricultural vocation until the end of the sixties (Fig. 3). In 1948, a study by the Commission for Geneva’s Development decided that the territorial rebalancing required the relief of congestion in existing agglomerations and transfer to peripheral zones. It meant making operational the Athens Charter of town planning concepts (1933), which recommended, among others, the rational organisation of the various activities [7]. The urban concepts of the modern city [4] thus dictated the principles of the Meyrin block plan (1957). The urban settlement of this city shows clearly defined zones with regard to both morphology as well as functionality.

Features Altitude District area Population Population density Inhabitants/Household Habitable area/Inhabitants Years buildings Topography (Fig. 4)

254 meters a.s.l. 59,1 hectares 15 300 inhab. 259,1 inhab./hect. 1,96 32 m2 66% before 1945 light slope

The Vaudagne Avenue was integrated into the composition and partially determines the direction and the geometry of the Meyrin commune. The determining elements of the industrial park were Mandement Road as well as Nant-d’Avril Road traced parallel to Meyrin Road (Fig. 3). The internal roads were built parallel to Mandement Road, in order to rationalise the parcels [8]. The two city morphologies differ by their access to solar irradiation and daylight, which is limited both by physical and socio-economical factors Solar irradiation and daylight exposure was examined through computer simulation of the two urban sites. 3. COMPARISON OF URBAN SITES The following tables and figures give a comparison of the principal statistical data of the Matthaeus and the Meyrin district, as well as a typical view.

Features Altitude District area Population Population density Inhabitants/Household Habitable area/Inhabitants Years buildings Topography (Fig. 5)

464 meters a.s.l. 998 hectares 20 500 inhab. 20,6 inhab./hect. 2,3 35 m2 70% after 1960 slightly undulating

Table 1: General statistics of Matthaeus (Basel).

Table 2: General statistics of Meyrin (Geneva).

Figure 4: Typical street view of “Matthaeus” district.

Figure 5: Typical view of “Meyrin” district.


4. SIMULATION METHODOLOGY

5. SOLAR PERFORMANCE INDICATORS OF URBAN SITES.

4.1 General approach The computer methodology used to assess the solar energy utilisation potential of “Matthaeus” [9] and “Meyrin”, is based on an approach developed by R. Compagnon within the framework of PRECis project [10]. The overall procedure, based on the Radiance ray tracing program [11], was modified, however, to take into account the larger area of the considered urban sites (59 hectares for Matthaeus and 998 hectares for Meyrin), as well as the higher model complexity of the districts (detailed building shapes and typology, higher urban density). The potential for the use of different solar technologies on this urban site (passive and active solar use, photovoltaic and daylighting) was assessed in several steps: - Climate data of both sites were obtained from the Meteonorm database. They were processed statistically in order to build up representative average radiometric and photometric sky models [12]. - A site ground relief, as well as additional building features (surface reflectance), were incorporated in the 3D digital model of the urban site. - These models were processed by computer simulation (ray tracing techniques) to determine the solar irradiation and daylight illuminance distributions on the building envelopes (facades and roofs). - Finally, several performance indicators were calculated according to these distributions in order to determine the optimal solar energy utilisation strategy for an urban site (relative fraction of building surfaces appropriate to a solar technology, appropriate surface locations)

The density of urban areas largely affect the Sky View Factor (SVF). In a first analysis, the façade and roof SVF values have been calculated for both sites. The urban character of Matthaeus is responsible for a significant reduction of the sky fraction viewed by the buildings facades: most of the building facades (and roofs) are subject to rather unfavourable conditions: Sky View Factor values lie between 20-50% for the facades and 80-100% for the roofs of Matthaeus. Sky View Factor values lie between 45-55% for the facades and 95-100% for the roofs of Meyrin: they are definitely more favourable than Matthaeus. The orientation rose [12] appearance is mostly due to the rectangular shape of buildings and their regular arrangement on both sites. This type of graph shows the facades oriented in each direction. The areas were aggregated into several azimuth sectors (15° wide each). The diagram takes into account that some facades may be highly obstructed and therefore fail to perform as potential collectors (SVF corrected). The Matthaeus rose indicates that the site is slightly more influenced by the NorthSouth/East-West pair of azimuths. For Meyrin, which is mainly occupied by long building rows, the orientation rose is clearly dominated by a single pair of directions North-West South-East / North-East South-West.

N

90

90 120

120

60

150

SVF corrected

30

60

150

SVF corrected

30

Raw area

Raw area

180

0 -0.00

0.05

0.10

210

330

240

180

0 -0.00

0.15

300 270

Figure 6: Orientation rose of the overall building facades of Matthaeus (Raw area and Sky View Factor corrected).

0.05

0.10

210

0.15

0.20

330

240

300 270

Figure 7: Orientation rose of the overall building facades of Meyrin (Raw area and Sky View Factor corrected).


Figure 8: Statistical distribution of yearly irradiation on the building facades and roofs of Matthaeus.

Figure 9: Statistical distribution of yearly irradiation on the building facades and roofs of Meyrin.

This is confirmed by the statistical distributions of the solar irradiation of the building facades and roofs (Figures 8 and 9). The threshold for the PV facade systems is 800 kWh/m2. Similar figures were drawn for the daylight illuminance distribution. Solar technology

Passive*

Active

PV

Daylighting**

Relative fraction: Facade area 31.9%*** Roof area -

46.7% 92.2%

1.3% 49.4%

51.1% -

Required minimal threshold: 167 400 800 Facade [kWh/m2] [kWh/m2] [kWh/m2] 600 1000 Roof [kWh/m2] [kWh/m2]

10 [kLux] -

Solar technology

Passive*

Relative fraction: Facade area 46.5%*** Roof area -

Active

76.7% 99.2%

PV

Daylighting**

21.9% 94.9%

78.5% -

Required minimal threshold: 187 400 800 Facade [kWh/m2] [kWh/m2] [kWh/m2] 600 1000 Roof [kWh/m2] [kWh/m2]

10 [kLux]

Table 3: Relative fraction of building facade/roof areas appropriate for a given solar technology in Matthaeus. * Calculation over the heating season ** Yearly calculation for a 8h00 to 18h00 daylight period *** η=0,7 Utilisation Factor of Solar Energy.

Table 4: Relative fraction of building facade/roof areas appropriate for a given solar technology in Meyrin. * Calculation over the heating season ** Yearly calculation for a 8h00 to 18h00 daylight period *** η=0,7 Utilisation Factor of Solar Energy.

These statistics show that on both sites, very significant fractions of building facades and roofs benefit from yearly irradiation and illuminance values that are perfectly adequate for operating different solar technologies. The thresholds themselves were determined according to procedures presented in detail in [12], based on the following assumptions: - The minimal vertical irradiation required for passive solar energy leads to a positive energy balance for state-of-the-art glazings in the Basel climate (167 [kWh/m2] for a heating season) and in the Geneva climate (187 [kWh/m2] for a heating season). - The values required for active solar energy and photovoltaics, which differ for facades and roofs, correspond to the edge of normal operating conditions (e.g. 800 [kWh/m2year] for PV in vertical facades and 400 [kWh/m2 year] for active solar energy use).

- The mean illuminance required for daylighting techniques used on vertical openings (utilisation coefficient of 0.05) leads to a 500 Lux illuminance on a workplane (10 [kLux] average illuminance required over the whole year). Tables 3 and 4 show that there is not much difference between Matthaeus and Meyrin as regards the use of passive solar techniques. The surprising values relate to PV: the potential is relatively weak for Matthaeus facades (1,3%) while it rises to 21,9% for Meyrin. The façade values remain relatively low compared with PV roofs which reach 49,4% for Matthaeus and 94,9% for Meyrin. This last number is not really surprising since the daylighting fraction is 78,5% for Meyrin while it amounts to 51,1% for Matthaeus: Meyrin’s roofs are practically not shadowed by surrounding buildings.

-


The daylighting values confirm also what was already mentioned on Meyrin’s construction bases (modern city theory) : this settlement is definitely more favorable to daylight utilization than the Matthaeus medieval fabric. Considering however the very large façade and roof areas of the sites: Surfaces Total vertical area Irradiation facades

Matthaeus Meyrin 858’030 m2 754’944 m2 334'647 MWh 423'734 MWh

Horizontal roof area Irradiation roofs

373’365 m2 406’765 m2 345'057 MWh 466'792 MWh

Table 5: Façade and roof areas of both sites.

a very significant part of the surfaces of both sites remains appropriate for solar technology, and this in spite of Matthaeus' urban character, which is less favourable than Meyrin in all the stated points. 6. CONCLUSION Preliminary assessment of the overall utilisation potential of solar energy in the urban context is necessary to achieve a sound and reasonable implementation of these technologies in large cities. Such an evaluation has been undertaken in Switzerland for two different urban sites: - Matthaeus/Basel-Stadt - Meyrin/Geneva The solar potential of the two sites is different, though not diametrically opposed: the smallest difference appears for passive solar techniques level (+=14,6%), the largest concerns the PV facades (+=45%). The differences are explained partly by the morphological fabric of these two districts. That of Matthaeus consists of imperfectly formed island sites with gardens of often large surfaces in the center. This checkerboard plan is organized with variable dimension streets of which some are rather narrow (urban canyons), while in Meyrin we find urban bars surrounded by green parks controlled by long broad avenues. It was shown moreover that a very significant part of the building facades and roofs is appropriate for the different considered solar technologies. A drastic reduction of non renewable energy consumption of these urban sites, as targeted within the framework of the “2000 Watt Society” project, can reasonably be envisaged for the future.

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