Solar PV Knowledge Bank

Solar Glass

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Introduction

Solar glass is used to replace conventional construction materials such as glazing or cladding, whilst also generating electricity on site.

In multi-storey buildings where roof space is often limited, façades and windows usually offer the most potential for electricity generation, thus the development of solar glazing is set to play an important role in the growth of ‘urban micro-generation’.

As well as being aesthetically pleasing and visually innovative, solar glass can improve the return on investment from the building, the more so if the product used is MCS certified and thus able to qualify for the Feed-in Tariff. 

Transparency varies from 0% (fully opaque) to 50%, with a choice of colours / aesthetics on offer.

Applications

Solar glazing can be used in many ‘BIPV’ (Building Integrated PhotoVoltaic) applications:

  • translucent or semi-transparent solar windows;
  • rain screens, curtain walling, rear-ventilated facades;
  • solar protection fins and louvres;
  • atriums, skylights;
  • privacy protection panels;
  • balustrades and fencing;
  • greenhouses;
  • bus shelters;
  • barns with transparent solar roofs.

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Polysolar PS-A opaque series panels (4.6 kWp), Future Business Centre, Cambridge

Solar glass needs to be multi-functional

The design of any building integrated (BIPV) solar system needs to optimise solar energy generation whilst complying with Building Regulations, meeting the desired aesthetic, meeting economic constraints and allowing for future maintenance. 

Solar glazing can achieve the following functionalities:

        • Solar electricity generation The obvious functionality is the ability to generate electricity, even from vertical and sloped facades, and other sub-optimal environments. The profile of electricity generation needs to be modelled for the given situation. It will depend primarily on latitude, orientation, temperature/ventilation and shading. 
        • Achieving target light transmission levels, shading and glare control Solar glazing can achieve light transmission levels of between 0% and 50%.  The product provides shading and glare control. Thin film modules may absorb UV wavelength, reducing bleaching and other degradation. 
        • Weather proofing Solar modules can provide rain-proofing, wind-proofing, wind load resistance and ageing resistance. 
        • Structural strength As most BIPV glazing is laminated it tends to meet the structural and safety requirements of overhead glazing.  Where additional strength is required, strengthened back glass or triple lamination can be used.
        • Sound protection Solar facades and modules possess sound insulating properties thanks to their multi-layer structure. Sound insulation can be adjusted by increasing the thickness of the glazing and by using asynchronous cover layers or specific intermediate layers, such as polycarbonate.
        • Thermal control: thermal insulation, reduction in thermal gain The temperature of a photovoltaic module can increase significantly when the module is exposed to radiation. The heat that the modules then radiate into the environment can be harnessed to provide heating or can be utilized to enhance passive ventilation systems.
        • Depending on their thickness, the multilayer glass structures of PV modules can be used to provide thermal insulation. In addition, most solar modules can also be integrated into insulation double or triple glazing structures. U-values can be as low as 1.2W/m2K or 0.9W m2K (argon-filled).
        • Solar naturally reduces heat gain by a combination of shading and absorption, converting some photons to electricity. In addition, the conductive coatings on the glass in transparent thin film modules may reflect the infra-red light spectrum in a similar manner to the low emissivity glass used in conventional commercial office windows. This reduces thermal gain and reflects heat back within the building in winter.
        • Aesthetic Solar glazing is invariably visually exciting and innovative.  Different colours and effects are available depending on the product chosen.

           

          Types of solar glass

          As with standard roof-mounted solar panels, there are two types of solar glass available, performing in line with their non-building integrated counterparts:

        • crystalline cells (monocrystalline or polycrystalline);
        • thin film (e.g. amorphous silicon, cadmium telluride).

           

          Crystalline technologies generally provide the highest ‘STC’ (standard test conditions) efficiency values, between 12% and 17% for multicrystalline modules and up to 20% for monocrystalline modules.

          The STC efficiency of thin film technologies typically ranges from 6% - 8% for amorphous silicon and up to 13% for CIS modules.  However ‘standard test conditions’ – radiation of 1000W per square metre at a temperature of 25°C – rarely apply in reality, and the STC efficiency does not necessarily reflect the yield, or energy output that can be derived from these modules over a year in a given location or position.

          Thin film technologies tend to operate optimally at 700 to 800 W/m2 radiance and continue working down to very low radiance levels: ~10% sunlight.  Unlike crystalline modules which require direct sunlight, thin film works in ambient and reflected light, on the dullest days. Thin film exhibits less degradation than crystalline cells do at high temperatures.

          The performance differentiation in different light levels should be considered with reference to the geographic location of the project, the local climate, the orientation of the panels on the building and the application. PV panels on a vertical façade will receive lower light levels than those optimally angled on a roof.  Equally those facing North will receive less direct light than those facing South.

          As well as different aesthetics (see below) thin film solar glazing has various advantages which need to be considered with reference to each specific project.  These are set out on our dedicated 'thin film' page (under 'Why thin film?').

          Achieving transparency

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Left: Polysolar PS-CT-64/48 units (2.8 kWp), Solar Bus Shelter, Canary Wharf

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Right: Polysolar PS-MC-SE monocrystalline glass panels 

The effect of transparency is commonly achieved in the PV module by the combination of transparent unoccupied areas and a pattern of opaque solar cells. The arrangement and distribution of the solar cells within the module thus controls the degree of transparency. This makes it possible to create interesting and innovative light effects. If the module is required to have less transparency, the intermediate areas not filled with cells can also be coloured.

Solar windows have an opacity varying between 50% transparent and fully opaque.

Achievement of ‘semi-transparency’ depends on the underlying technology. With crystalline cells, light transmission is adjusted by altering the spacing between the cells. With thin film, the active layer is partially removed to allow the light to pass through, or an ultra thin film deposition of the active solar materials is combined with two layers of transparent conductive coatings.

Colour

Conventional solar cells are generally black or blue in the case of crystalline silicon and brown or black with thin film.

To achieve colour effects that differ from the cell colour, coloured laminates, coatings or films can be used. This makes it possible to create interesting effects such as logos on PV modules or colours that match the existing building.  A compromise between design and output must always be found when designing coloured solar modules.

Product specification: Romag

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The Romag PowerGlaz product is manufactured in the UK. Each pane is custom made, ensuring a high quality finish.

The glass can be ordered with a tinted finish to meet specific project requirements. It can be single-glazed or double-glazed as required.

Dimensions are specified on a case-by-case basis but the maximum sizes are 4m x 2.2m for single-glazed units and 3.6m x 2.1m for double-glazed units.

The PowerGlaz solar glass comes with the usual solar PV warranty package:

Performance is guaranteed to be at 90% of original output after 10 years and at 80% after 25 years.

The product is guaranteed against visual defects for 5 years.

Product specifications: Polysolar

Colourless / Black Opaque Thin-Film PV Glazing (cadmium telluride)

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Polysolar PS-CT-64 20% transparent panels (7.68 kWp), Donnington Park Farmhouse Hotel

  • Thin film, cadmium telluride (CdTe) cells.
  • Colour: grey tinted, through to black opaque.
  • Variable transparency of 0-50%.
  • Efficiency: 5% (50% transparent) to 12% (black).
  • Peak output from 50W per m2 to up to 118W per m2, depending on transparency.
  • Glass/glass frameless design.
  • Single or double glazed available.
  • MCS Approved. 

Partially Transparent / Opaque Amber Thin-Film PV Glazing (amorphous silicon)

 

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Polysolar PS-C901 transparent panels (15.7 kWp), Sainsbury's Petrol Station, Bishop's Waltham


This glazing is made using amorphous silicon technology, utilising the equivalent of powdered silicon in very small quantities.  This is vacuum deposited along with transparent, conductive oxides on both glass surfaces with the active PV material between as a semiconductor. The glass is then laminated together as a sandwich to create a uniquely translucent module.

  • Thin film, amorphous silicon (a-Si) cells.
  • Colour: amber tinted.
  • 20% transparent and opaque versions available.
  • Efficiency: 6% (20% transparent) to 10% (opaque).
  • Peak output 66W per m2 (20% transparent),  101W per m2 (opaque).
  • Glass/glass laminate design.
  • Laminated for compliance with overhead and safety glass
  • Thermal properties similar to K glass.
  • Single or double glazed available.
  • MCS Approved.

Partially Transparent / Opaque Black Thin-Film PV Glazing (amorphous silicon tandem)

Incorporates micromorph technology to achieve high efficiency and sleek aesthetics. Ideal for BIPV curtain walling and facades.

  • Thin film, amorphous silicon tandem (a-Si / uc-Si)cells.
  • Colour: black.
  • 0-50% transparency available.
  • Efficiency: 6%-16%.
  • Peak output 65W per m2 (50% transparent),  155W per m2 (opaque).
  • Glass/glass laminate design.
  • Laminated for compliance with overhead and safety glass.
  • Thermal properties similar to K glass.
  • Sustained performance in high ambient temperatures
  • Single or double glazed available.
  • MCS Approved, product warranty 5 years, power warranty 20 years.

Glass/glass monocrystalline and polycrystalline (PS-PC-SE) PV panels

Similar in appearance to standard solar panels, glass / glass monocrystalline and polycrystalline panels achieve the highest power densities available from solar glass. The panels are available in a range of colours and transparencies.

Key features are as follows:

  • Polycrystalline silicon.
  • Colour: dark blue as standard, range available.
  • Efficiency: up to 15.5%.
  • Peak output up to 160 W per m2.
  • Laminated for compliance with overhead and safety glass.
  • Thermal properties similar to K glass.
  • Single or double glazed available.
  • MCS Approved, product warranty 5 years, power warranty minimum 80% after 30 years.