6       Integration of photovoltaic panels

In cooperation with ITW

6.1       General principles of a photovoltaic system

A photovoltaic (PV) cell is a device that converts sunlight into electrical energy. The material the cells are made of is primarily silicon, except for some of thin-film cells, which are made e.g. of copper-indium-diselenid (CIS) or cadmium-tellurid (CdTe). When light shines on the cell it creates an electric field across the layers causing electrons or electricity respectively to flow. The greater the intensity of the light, the greater the flow of electricity. PV cells only produce energy whilst there is daylight. The produced power can either be transferred to the conventional electricity grid and/or be consumed directly or stored in an accumulator.

There are several types of PV cells: monocrystalline, polycrystalline and amorphous silicon (thin film). Monocrystalline silicon is made using thin wafers of a single crystal of silicon all arranged in orderly patterns. Polycrystalline or multicrystalline are similar to monocrystalline but are made from less refined silicon. Amorphous silicon cells or thin film are made up of silicon atoms in a thin layer deposited on a wide range of substrates, such as glass or metals. Monocrystallines are the most efficient, have a longer life span and are more expensive than the other types [ENG08].

PV cells are always arranged on a panel to form a solar module. Modules are then linked in series to form a solar array. The capacity of an array is given in terms of its peak power production (kWp). The cell efficiency depends of the material and the set-up used and is roughly between 7.5 % (amorphous-silicon) and 17 % (mono-crystalline).

The major components of photovoltaic systems are: Module field (group of cells), which absorbs the solar radiation in order to produce electricity, an inverter which converts the direct current (DC) generated by the photovoltaic cells to alternating current (AC) so that it can be used for common electrical appliances or fed back to the power grid.

In general there are many options for installing a solar PV system: fixing the array over the roof finish, tiles integrated into the roof finish, building integrated photovoltaics (BIPV) or installation away from the building. Photovoltaics can also be fitted as an array to a conservatory or glass and provide shading.

In applications of building integrated photovoltaics (BIPV) different types of modules can be used: classic (framed) modules, flexible crystalline or thin-film on metal substrate, roof-tiles with solar cells, transparent monocrystalline modules, modules with coloured solar cells, semitransparent micro perforated amorphous etc. Some of the most important ways of integration are described in the following chapters.

Upon request almost all module (mechanical and electrical) parameters can be customized. Customization includes module shapes, cell type and colour, cell transparency, laminate construction, laminate/module size, heat/noise isolation properties, module voltage and peak power etc.

Points of attention

By installing the array an appropriate orientation should be chosen to generate a significant yield of energy and it should be assured that no trees or other structures cast shadows on the array. Besides it should be looked out if bats or birds use the roof for nesting.

It is recommended to fix collectors to thatch roofs, where the material is organic, as the thickness of the thatch decreases over time [ENG08].

When planning the installation it should be considered that a PV array and its associated equipment can have a life of typically 25 years, so a building could have several installations over the years. By carefully planning the installation and how it can be removed at the end of its useful life, damage to the building fabric can be limited.

Equipment should be located to permit easy access for maintenance and repair. Where equipment is to be fixed to building walls, the number of fixing points can be minimised by the use of a wooden frame system.

Maintenance carries the potential for damage to the fabric of the building. It is important to talk to the installer about the methods and frequency of routine maintenance. Typical maintenance periods are in the range of one to two years.

Rules of thumb

A PV system with 20 m² will cover in average approx. half of the electric power demand of a family with 4 persons, if the modules are south orientated, not shaded and have a inclination of 30 to 45°. In contrast, a vertical integration in the façade will have losses of approx. 30 to 40 % [ALT08].

For the generation of 1 kWp approx. 8 - 10 m² of silicium PV modules and approx. 15 to 20 m² of thin film modules are necessary [ENE07].

Costs

The investment costs of standard modules are around 6000 to 8000 €/kWp (incl. components like power inverter, installation and tax). For tailor-made solutions costs can come up to 18000 €/kWp [ENE07].

Besides the investment costs there are also annual operating costs which have to be considered with approx. 1-2 % of the investment costs per year [ENE07].

 

6.2       Roof integration of photovoltaic panels

The PV array can be fixed over the roof covering (Figure 49), so it sits above the tiles or slates, or it can be integrated into the roof covering so it sits flush (Figure 50). It is also possible to use tiles, which integrate solar cells.

When the PV arrays are fitted over the roof covering, the array is fixed to the roof structure by drilling through the roof covering (tile, slate) directly into the rafters. Careful planning is required. The holes must also be made into the loft space for cabling to and from the PV array to the inverters. These holes should be weather-sealed with roofing sealant.

To integrate an array into the roof finish, PV tiles are used that replace individual ordinary roofing tiles or slates. Either a part of the roof can be replaced with PV tiles or the whole covering. The PV tiles are anchored onto the roofing battens and are screwed in place. The tiles overlay like single-lap roof tiles and are connected electrically together with the cabling taken back to the electrical inverter.

On a flat roof a frame can be used to hold the collector at the optimum 30° angle. The frame has to be held in place with ballast and has to be permanently fixed with the roof structure. Where the roof covering must be penetrated it is important to ensure that it is sealed against the weather.

Where a tilted PV array may have an undesirable visual impact it may be mounted horizontally, allowing the array to be hidden from ground-level view behind a parapet wall like it was realized at Dunster Castle, UK (Figure 51).

Figure 51: Flat roof installation of solar panels at Dunster Castle in West Somerset [NAT08]

Figure 52: Church in Mecklenburg-Vorpommern, Germany [PFL08]

Points of attention

For roof-integrated PV modules it is important to make sure that the modules are sufficiently rear-ventilated, as increasing temperature of the modules reduces the efficiency.

Many of these systems will only be compatible with certain roof tiles and slates. It is necessary to check this in advance with the manufacturer.

Before the installation a structural survey should be carried out, as the PV array and fixing framework will be attached to the roof rafters, which will need to be capable of supporting them [ENG08].

During the installation it is normal for tiles and slates to get broken. Replacements for roofs with stone or old handmade tiles can be expensive and difficult to find. Drilling through them will render them unusable. Therefore it is advisable it should be investigated what type of roof covering is existing and how to get replacements before undertaking any work.

Flat roof coverings have a life of 10 to 15 years before replacement. PV arrays usually have lifetimes of around 25 years. It is advisable to plan the installation at the same time as re-covering of the roof.

Best practice examples

§  Flat roof installation of solar panels at Dunster Castle in West Somerset, Source: The National Trust, UK / http://www.nationaltrust.org.uk/main/w-global/w-localtoyou/w-wessex/w-wessex-news/w-wessex-news-dunster_solar_panels.htm;

§  First Austrian church with a roof-integrated photovoltaic system: Reformierte Stadtkirche, Wien, Austria (oldest protestant church of Vienna, late 18^th century, location approx. 300 m away from the Stephansdom). / http://homepage.univie.ac.at/viktor.schlosser/vie_1.htm

§  Integration of 88 monocrystalline modules Terra-Piatta-Solar with peak power 5 kWp into church's roof in Mecklenburg-Vorpommern, Northern Germany (

§  Figure 52.

 

6.3       Façade integration of photovoltaic panels

Façade-integrated photovoltaic systems could consist of different transparent module types, such as crystalline and micro-perforated amorphous transparent modules. In such case a part of natural light is transferred into the building through the modules.

The BIPV (building integrated photovoltaic systems) façade system is designed to act as an outer skin and weather barrier as part of the building envelope. An example is a BIPV system used for rainscreen overcladding. Glass BIPV products are typically used as façade systems. BIPV façade systems include laminated and patterned glass, spandrel glass panels, and curtain wall glazing systems. These BIPV products can displace traditional construction materials. Laminated glass is a standardized BIPV product. It is composed of two pieces of glass with PV solar cells sandwiched between them, an encapsulant of ethylene-vinyl acetate (EVA) or another encapsulant material, and a translucent or colored tedlar-coated polyester backsheet. It can also be made with only one piece of glass and a tedlar backsheet. Architects can specify the colour of the tedlar backsheet.

Using semi-transparent thin-film modules, or crystalline modules with custom-spaced cells between two layers of glass, designers may use PV to create unique daylighting features in façade, roofing, or skylight PV systems. The BIPV elements can also help to reduce unwanted cooling load and glare associated with large expanses of architectural glazing.

In comparison with roof integration, the integration of PV panels in façades shows a considerable lower energy gain. On the roof solar power gains up to 90 % of maximum irradiation can be reached. Due to unfavourable inclination, urbanistic shading etc. only a gain of 50 to 60 % of the maximum irradiation can be reached with façade integrated PV panels [ENE07]. However, the profitability of façade-integrated modules can be increased when the modules take over additional building functions like sun shading elements. By simultaneously serving as building envelope material and power generator, BIPV systems can provide savings in materials and electricity costs.

Therefore from an economic point of view PV façades are more appropriate for new buildings and often not suitable as refurbishment solution.

Practical example

Points of attention

PV conversion efficiencies are reduced by elevated operating temperatures. This is more true with crystalline silicon PV cells than amorphous silicon thin-films. To improve conversion efficiency, appropriate ventilation should be allowed behind the modules to dissipate heat. Furthermore partial shading of PV modules has to be avoided.

 

6.4       Window integration of photovoltaic cells

PV cells can also be integrated into glazing (Figure 54). Integrating PV modules into a glass roof prevents overheating in summer as well as providing shade and diffuse light.

Transparent solar modules offer probably the most attractive BIPV (building integrated photovoltaic) solutions. Modules with different transparency rates and different technologies are available on the market. Most common they consist of transparent crystalline cells, very often are also modules with transparent backside and with standard crystalline cells. Another interesting solution are thin film transparent amorphous modules. Transparent modules can be used as window glazing in usual windows. Transparent modules can also be part of energy efficient glazing, where they are used instead of usual glass. With coloured backside interesting architectural visual effects can be obtained. Such solutions are often used in old architectural protected heritage buildings.

Very attractive are thin film modules because they offer some tailor made solutions - different shapes or patterns within modules are possible.

	Figure 54: Schüco E² façade, integrated solar thermal, sunscreen and integrated photovoltaic cells [SCH08]

Points of attention

Exact shadowing analysis should be made before the system is constructed, high temperature conditions should be avoided by crystalline modules (decreased efficiency).

External links

§  www.brita-in-pubs.com (BIT = Brita in pubs information tool) BRITA = Bringing Retrofit Innovation to Application, demonstration of retrofit projects, computer tool for the first planning phase

§  http://www.renewables-made-in-germany.com/en/photovoltaik/

§  http://www.pvresources.com Photovoltaic Systems - Technologies and Applications

§  http://www1.eere.energy.gov/femp/pdfs/25272.pdf Building-Integrated Photovoltaic Designs for Commercial and Institutional Structures - A Sourcebook for Architects - Patrina Eiffert, Ph.D., Gregory J. Kiss

§  http://www.english-heritage.org.uk/upload/pdf/49357-SolarElectric.pdf

§  http://www.hausderzukunft.at/nw_pdf/fofo/fofo3_07_en.pdf

§  http://www.wbdg.org/resources/bipv.php

 

6.5       References

[ENG08]          English Heritage. Small scale solar electric (photovoltaics) energy and traditional buildings. 2008. http://www.english-heritage.org.uk/upload/pdf/49357-SolarElectric.pdf

[ALT08]           Altbauten sanieren. Energie sparen, Solarpraxis, 2008, ISBN 978-3-934595-78-1

[ENE07]          Energieeffizient sanieren. 2007, ISBN 978-3-934595-72-9

[GRE08]          http://www.german-renewable-energy.com/Renewables/Redaktion/PDF/en/en-Solar-thermal-and-Photovoltaics-8211-Potential-for-the-use-of,property=pdf,bereich=renewables,sprache=en,rwb=true.pdf

[NAT08]          Flat roof installation of solar panels at Dunster Castle in West Somerset, Source: The National Trust, UK / http://www.nationaltrust.org.uk/main/w-global/w-localtoyou/w-wessex/w-wessex-news/w-wessex-news-dunster_solar_panels.htm

[PFL08]           Pfleiderer Dachziegel GmbH, http://www.pfleiderer-dach.de/

[PVR08]          Building integrated photovoltaic, CLER, photo Solarte, http://www.pvresources.com/en/transparent.php

[SCH08]          Schüco International KG, Bielefeld, Germany