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PV-INTEGRATION IN SOLAR SHADING: A RETROFIT CASESTUDY (NL)

THERMIE SE 100/97 NL/DK

Dr. Ir. Henk F. Kaan (ECN) and Ir. Tjerk Reijenga (BEAR Architecten)

Netherlands Energy Research Foundation ECN

P.O. Box 1, 1755 ZG Petten, the Netherlands

tel. +31 224 56 4689, fax. +31 224 56 3214

e-mail: kaan@ecn.nl

BEAR Architecten

Thorbeckelaan 2, NL 2805 CA Gouda, The Netherlands

tel. +31 182 529899, fax. +31 182 582599

email: office@bear.nl, internet: http://www.bear.nl

ABSTRACT

Photovoltaics (PV), though not yet competitive in the economic sense, is a promising technology for future energy supply. In order to make PV more economical, PV integration in buildings is an option that may save money by easier mounting systems, savings in roofing materials, and savings in floor space ­ an argument which is quite important in densely populated areas like in Western Europe.In order to stimulate application of renewable energy technologies like PV in the built environment, the European Union has defined the Thermie programme for demonstration projects. This paper deals with such a demonstration project, located in the Netherlands. It concerns a laboratory building, built in 1963 and going to be renovated. Apart from improving the energy performance of the building, which is beyond the scope of this paper, the building will be provided with an innovative PV-integrated sunshading and roofing system. The total output of the system is approx. 70,000Wp, producing 56 440 kWh per year. From the Thermie programme, the project is subsidized up to 40% of the specific system costs. The Dutch government will subsidize the project with an additional 9% .The process and results of developing and designing the PV-integrated sunshading system will be discussed in this paper.

1. Introduction

Since the energy crises of the seventies, and, with more emphasize, since CO2 reduction has become a worldwide political issue, it is generally understood that in the shorter or longer term fossil fuel generated energy has to be replaced by forms of renewable energy. The EC, realizing this, has emphasized the need for developing and demonstrating renewable energy technologies among other by the research& development and demonstration programmes Joule and Thermie. Within the Thermie programme, a lot of attention has been paid to building integrated PV systems. One of the conditions for obtaining subsidy is an innovative approach of PV integration in buildings. A project that was honoured with subsidy from the Thermie programme is a laboratory building that is located at the site of the Netherlands Energy Research Foundation ECN in Petten, The Netherlands. For this building a PV-integrated sunshading system, a PV cladding system and PV roofing system have been developed.

2. The Building

The building concerned is the so-called General Laboratory of the Netherlands Energy Research Foundation ECN; a building built in 1963, total floor surface 3530m2. A survey showed that the building has several major technical and thermal shortcomings, which will be taken away in the renovation process. Regarding energy performance after renovation, several targets were set that exceed the Dutch Energy Performance Standard. Elements that influence the energy performance of the building in a positive way are among other things: balanced ventilation with heat recovery, passive solar building, passive and natural cooling, heat and cool storage, heat pumps and photovoltaics.

The total yearly electricity consumption of the building is 286,000 kWh, being 80 kWh/m2. This is high, compared to other buildings of the same period. The large number of computers causes this, as do the inefficient lighting system and the inefficient heating and ventilation system. At present, the total yearly heat demand is 1750 GJ, which is about 140 kWh/m2, a normal energy consumption for a Dutch building of the sixties. However, the heat gains from the computers are not included in this number, so in fact heat demand is higher.

The comfort in the building is far from ideal. Partially this is caused by failings in the facade (cold in winter), partially by overheating in summer.

3. Targets for energy savings

When defining the energy targets for the laboratory building after renovation, it was decided not to exceed the level of heat demand of 50kWh/m2/year. Electricity demand should not exceed 40 kWh/m2/year. These targets are quite ambitious. However, they do not include the process related energy demand. Simulations with the TRNSYS program showed that the goal of reducing primary energy demand to 80 kWh/m2 could be obtained. In this case, co-generation and application of photovoltaics can substantially contribute to this goal.
As said before, overheating is quite an important problem. In order to avoid airconditioners ­the building should be cooled by natural ventilation and summer night cooling- heat load should be as low as possible. Apart from efficient office machines and well-used automatic switch-off appliances, outside sunshading devices should be applied as much as possible. This requirement is one of the points of departure for the PV system design. Thus, exceed of temperature during summer months is far below the standard. In addition PV generates a considerable amount of electricity. The amount of PV that can be applied is about 700 m2: 300m2 integrated in the facade shading device, 300 m2 as a sunshading roof construction and 100 m2 mounted as a cladding system on the staircase wall. All together the PV system produces 56,440 kWh per year.

From the Thermie programme, the EC supports 40% of the costs of the PV system, whereas the Dutch government pays an additional 9 %.

4. The design of the PV-system

What requirements were defined for the PV system?
As said before, the system consists of three parts: the sunshading integrated system, the roofing system and the PV cladding.
The latter has no special requirements; it is just for electricity generation and for architectural expression. The south facade has a problem of overheating in summer. It was clear that the PV modules should be integrated in the outside sunshading system. Such a solution may:

  • optimize solar gain;
  • give good shading of the building in summer;
  • make daylight diffuse;
  • ease maintenance of the building and cleaning the windows (maintenance walkway).

But there is more than one reason that justifies the use of integrated PV modules in the shading device: construction costs will be optimized by the elimination of costs of a conventional PV module support system; interior light and temperature will be improved and energy is produced directly where needed.
The choice should be made whether the shading device should be mounted close to the facade or at a certain distance. Furthermore, the size of the lamellas had to be discussed: should a few, wide lamellas be chosen or a larger number of slim ones? What should be the length of the lamellas?
From the point of view of maintenance, accessibility and window cleaning it was decided to have the shading/PV device constructed as a separate facade, about 80 cm from the building, connected to the main structure of the building. The length of the lamellas followed from the width of the rooms behind.

In order to make a choice for the width of the lamellas various solutions for an integrated system were examined:
  • two large lamellas with modules, at a vertical distance of 1.5 m in a fixed position;
  • idem with moveable tracking system;
  • seven small lamellas with modules, at a vertical distance of 0.5 m in a fixed position;
  • idem with moveable tracking system.

The study, which was carried out with a model of a laboratory room scale 1:10 in a daylight chamber and on a solar table, focused on the solar gain, the heat load of the building, shading of the building, shading of the modules, outside view from the interior and daylighting conditions. This study showed, that the best results for solar gain, shading and daylighting were obtained with a model using 4 fixed lamellas per floor. Considering the solar ratio between a fixed system and a moveable system, the solar gain is only approximately 10% higher with a moveable one.
Considering the high costs of a moveable system compared to a fixed structure and the small difference of solar gain it was decided to select a system that is fixed in the optimal position (in the Netherlands 37o with the horizon). However, the occupant of the room behind can move one lamella, at eye level, in a horizontal position, in order to have a good outside view. After 20 minutes or so, the lamella will automatically take its position of 37o again. Thus, a continuously varying architectural view of the facade is created.
Each lamella will be about 840 mm wide, 3000 mm long and will be covered by three standard multi-crystalline PV modules on the front part. Because of the dimensions of the lamellas the building is shaded during the summer period. The efficiency of the shading system is about 85 %. For fine-tuning the glare, especially in winter, a second, very simple interior shading system is provided.

Because of the new exterior PV/shading system overheating of the south facade will be avoided and an expensive, much energy consuming air-conditioning system will not be necessary.

As far as can be predicted from the study, the specific position of the lamellas might improve the distribution of the daylight in the rooms compared to the existing situation. The daylighting situation through the rooms might be more equal. However, to be sure about the effect of the integrated PV/sunshading system on the daylighting of the rooms behind, a separate study will be carried out. This study, which had just started when this paper was written, consists of advanced daylighting computer simulations. After evaluation of the results a mock-up will be built, followed by measurements. But not only daylighting aspects are examined by means of the mock-up. Also constructive implications, deterioration of moveable construction parts, questions of manufacturing, color and acceptance by the users of the building will be examined in the prototype stage. When some time and money is spent for the prototype, mistakes can be avoided in the construction phase.

The PV roofing system was originally meant as a kind of a parasol, a passive-cooling device for the roof. The roof construction underneath should provide water tightness. As the design of the interior of the building got more and more shape, it became clear that the space between the parasol and the existing roof should be used for technical devices such as ventilators and air ducts. So it was decided to construct the parasol as a watertight part of the building. As the construction still had to be worked out when this paper was written, details cannot yet be discussed here.

5. How will it go on?

Apart from having a good demonstration project within the aims of the Thermie programme, one of the purposes of the demonstration project is to have a case study for the International Energy Agency Task VII, Photovoltaic Power Systems in the Built Environment. The Dutch and Italian partners in the above project are members of the Task VII Expert Group. Within Task VII, the experts discuss questions regarding design, production problems, wiring, interconnection of PV and lamellas, and non-technical aspects like involvement and role of utility, local government, and owner/user. The above project certainly contributes to that goal.

The main conclusions and results until now are the following:
The interest in the project, both from national and international sides, is large. Thus the project may encourage the application of PV systems as an energy-conscious retrofit.
The innovative constructions for PV integration in the roof and in the facade contributes highly to the reduction of heat load of the building. Thus, energy consuming aircondition equipment can be avoided.
The project is regarded as innovative because of its contribution to new developments in the sun shading industry, the architectural solution, the integration of PV, shading, passive cooling and daylighting, and the good outside view by moveable lamellas.
Integration of PV systems in shading devices can result in considerable cost reduction.

The design of the PV facade and PV roof will be finalized in September 1998. For organizational reasons construction of the PV facade and roof will start in September 1999 and the project will be completed in June 2000.

6. Participants

The project is a cooperation between the Dutch partners ECN, BEAR Architects, the local utility ENW and Shell Solar Energy, and the Italian architect Cinzia Abbate of Rome. A Danish manufacturer, Dasolas, in cooperation with ALCO, is involved in manufacturing the combined PV support / sunshading system.