Building-integrated photovoltaics (BIPV) are solar energy materials that replace traditional materials in parts of a building, such as the roof, skylights, or walls. These systems are often used in new buildings to generate electricity as a main or additional power source. Existing buildings can also be updated with similar technology. One benefit of BIPV compared to other solar systems is that it can lower overall costs by reducing the need to buy and install separate building materials and labor. Additionally, BIPV helps more people use solar energy in buildings where appearance is important, as it avoids the need for visible solar panels that might change the building’s look.

The term building-applied photovoltaics (BAPV) is sometimes used for solar systems added to buildings after they are built. Many BIPV installations are actually BAPV. Some companies and builders distinguish between BIPV used in new buildings and BAPV added later.

History

Photovoltaic (PV) systems for buildings started being used in the 1970s. These systems, which used aluminum-framed PV modules, were attached to or placed on buildings in remote areas that did not have access to electricity grids. In the 1980s, PV modules were added to rooftops for testing. These systems were usually placed on buildings connected to the main power grid in areas with large power stations. By the 1990s, building-integrated photovoltaic (BIPV) products designed to be part of a building’s structure, such as walls and roofs, became available for purchase. A 1998 doctoral thesis by Patrina Eiffert, titled An Economic Assessment of BIPV, suggested that one day Renewable Energy Credits (RECs) might have economic value. A 2011 report by the U.S. National Renewable Energy Laboratory reviewed BIPV’s history and noted that technical challenges must be solved before BIPV costs can compete with standard PV panels. However, many experts agree that widespread use of BIPV could help achieve the European goal of zero energy buildings by 2020. Despite its potential, challenges remain, including the traditional practices of the building industry and the difficulty of fitting BIPV into densely populated urban areas. These studies suggest that long-term success of BIPV will depend on both technological progress and supportive public policies.

Forms

Building-integrated photovoltaic (BIPV) products use two main types of technology: Crystalline Solar Cells (c-SI) or Thin-Film Solar Cells. Crystalline Solar Cells use thin slices of single-cell crystalline silicon. These cells are more efficient at converting sunlight into electricity than Thin-Film cells but are more expensive to make. Thin-Film cells are less efficient but can be used in flexible materials. BIPV products are grouped into five main types:

• Standard in-roof systems: These are strips of photovoltaic cells installed on rooftops.
• Semi-transparent systems: These are used in greenhouses or cold climates where sunlight needs to enter buildings while also being captured for energy.
• Cladding systems: These are applied vertically on building walls and come in many designs.
• Solar Tiles and Shingles: These look like traditional roof materials and are the most common BIPV systems.
• Flexible Laminates: These are thin, flexible sheets that can be attached to various surfaces, especially roofs.

Except for flexible laminates, all other categories can use either c-SI or Thin-Film technology. Thin-Film technology is only suitable for flexible laminates, making it ideal for advanced designs with moving parts.

BIPV products can be used in many situations, including pitched roofs, flat roofs, curved roofs, semi-transparent walls, skylights, shading systems, and external walls. Flat and pitched roofs are best for capturing solar energy. Roofing and shading systems are most common in homes, while wall and cladding systems are more often used in businesses. Roofing BIPV systems currently dominate the market and are more efficient than systems on walls or cladding because they face the sun directly.

Building-integrated photovoltaic modules come in several forms:

• Flat Roofs: The most common type uses thin-film solar cells attached to a flexible polymer sheet on the roof. A US company produces thin-film cells with 17% efficiency, and a UK company integrates these cells into TPO single-ply membranes.
• Pitched Roofs: Solar roof tiles are ceramic tiles with built-in solar cells, developed by a Dutch company in 2013. Solar shingles look like regular shingles but use flexible thin-film cells. They protect roofs from damage by reducing condensation.
• Façades: These modules are mounted on building walls, improving the building’s appearance and value.
• Glazing: Photovoltaic windows are semi-transparent and can replace glass windows or skylights. They save energy by improving insulation and controlling sunlight.
• Photovoltaic Stained Glass: Colored solar panels use semi-transparent materials like perovskite or dye-sensitized cells. These cells can absorb specific light wavelengths and are made with layers of glass and special materials. Researchers have created red and blue solar panels with 34.7% and 24.6% light transmission, respectively. Blue panels convert 13.3% of absorbed light into electricity, making them the most efficient. Perovskite cells can be tuned to absorb red, green, or blue light by adjusting their design. Dye-sensitized cells use natural pigments like those from maqui fruit, black myrtle, and spinach to capture light. These cells have achieved up to 9.8% efficiency in some tests.

Transparent and translucent photovoltaics

Transparent solar panels use a tin oxide coating on the inside of glass panes to carry electricity from the cell. The cell contains titanium oxide covered with a special dye that helps convert light into electricity.

Most traditional solar cells use visible and infrared light to create electricity. In contrast, the new type of solar cell also uses ultraviolet light. These panels can replace regular window glass or be placed on top of it. Their large surface area allows them to be used in ways that combine power generation, lighting, and temperature control.

Another name for transparent photovoltaics is "translucent photovoltaics" (they let about half the light through). Like inorganic photovoltaics, organic photovoltaics can also be translucent.

Some non-wavelength-selective photovoltaics become semi-transparent by arranging small opaque solar cells on a transparent surface. This method uses any type of opaque solar cell but greatly reduces how well the cells convert light into electricity while increasing light transmission.

Another type of non-wavelength-selective photovoltaics uses thin, visible-light-absorbing materials with large band gaps that let some light pass through. This creates semi-transparent solar cells with a similar trade-off between efficiency and light transmission as the segmented cells.

Wavelength-selective photovoltaics achieve transparency by using materials that only absorb ultraviolet and/or near-infrared light. These were first shown in 2011. Despite allowing more light to pass through, they have lower efficiency due to challenges such as short exciton movement, scaling transparent electrodes without reducing efficiency, and material instability from organic components.

Early attempts to make semi-transparent organic photovoltaics with very thin layers that absorbed visible light achieved less than 1% efficiency. In 2011, a type of transparent organic photovoltaic using an organic chloroaluminum phthalocyanine donor and a fullerene acceptor absorbed ultraviolet and near-infrared light with about 1.3% efficiency and allowed over 65% of visible light to pass through. In 2017, researchers at MIT developed a way to apply transparent graphene electrodes to organic solar cells, achieving 61% visible light transmission and improved efficiencies of 2.8% to 4.1%.

Perovskite solar cells, known for their high efficiency (over 25%), have also shown promise as translucent photovoltaics. In 2015, a semitransparent perovskite solar cell using methylammonium lead triiodide perovskite and a silver nanowire mesh top electrode allowed 79% of light at 800 nm wavelength to pass through and achieved 12.7% efficiency.

Government subsidies

In some countries, extra support, called subsidies, is given for building-integrated photovoltaics (BIPV) in addition to the current payments for stand-alone solar systems. Since July 2006, France has provided the highest incentive for BIPV, adding an extra payment of €0.25 per kilowatt-hour (kWh) to the existing €0.30 per kWh for solar systems. These payments are given as a rate for electricity sent to the power grid.

• France: €0.25/kWh
• Germany: €0.05/kWh (façade bonus ended in 2009)
• Italy: €0.04–€0.09/kWh
• United Kingdom: 4.18 p/kWh
• Spain: For systems up to 20 kW, €0.34/kWh; for systems over 20 kW, €0.31/kWh (compared to €0.28/kWh for non-building installations, as stated in RD 1578/2008)

• United States: Incentives vary by state. For more details, refer to the Database of State Incentives for Renewables & Efficiency.

In March 2009, the Chinese government announced a subsidy program for BIPV projects, offering RMB20 per watt for BIPV systems and RMB15 per watt for rooftop systems. Later, the government introduced the "Golden Sun Demonstration Project," a photovoltaic energy subsidy program designed to support solar electricity generation and the use of photovoltaic technology. The Ministry of Finance, the Ministry of Science and Technology, and the National Energy Bureau jointly announced the program in July 2009. Qualified on-grid solar projects, including rooftop, BIPV, and ground-mounted systems, receive a subsidy equal to 50% of the total project cost, including transmission infrastructure. Qualified off-grid projects in remote areas receive subsidies covering up to 70% of the total project cost. In mid-November, China’s finance ministry selected 294 projects totaling 642 megawatts, with costs of approximately RMB20 billion ($3 billion), as part of the subsidy plan to increase the country’s solar energy production.

Other integrated photovoltaics

Vehicle-integrated photovoltaics (ViPV) work similarly in vehicles. Solar panels can be placed on parts of the car that get sunlight, like the hood, roof, and maybe the trunk, depending on how the car is designed.

Challenges

BIPV systems produce electricity on-site and are built into the structure of buildings. The main factors used to measure their performance are the amount of power they generate and their ability to manage heat. Traditional BIPV systems release less heat than PV systems mounted on racks, which causes BIPV modules to operate at higher temperatures. These higher temperatures can damage the materials inside the modules, reducing their efficiency and causing them to fail sooner. Also, BIPV systems are affected by weather conditions, and using the wrong type of BIPV system can lower the energy they produce. In terms of heat management, BIPV windows can help reduce the need for cooling in buildings compared to regular clear glass windows, but they might increase the need for heating.

The high initial cost of BIPV systems is a major challenge for using them. In addition to the cost of buying BIPV parts, the way BIPV systems are built into buildings makes the design process more complex, which raises design and construction costs. A lack of skilled workers also increases the cost of developing BIPV projects.

Although many countries support PV systems, few offer extra benefits for BIPV systems. BIPV systems must follow building and PV industry standards, which makes their implementation more difficult. Also, government policies that keep the cost of traditional energy low can reduce the advantages of BIPV systems. This is especially true in countries where traditional electricity is very cheap or supported by the government, such as in Gulf Cooperation Council countries.

Studies show that the public knows little about BIPV systems, and the cost is often seen as too high. Increasing public knowledge about BIPV through methods like policies, community activities, and buildings that demonstrate how BIPV works may help its long-term success.