GiPV: Building-integrated photovoltaics with semi-transparent solar modules

Building-integrated photovoltaics (BIPV) refers to photovoltaic materials that replace conventional building materials in parts of a building envelope, such as the roof, skylights, or facade. It is increasingly being integrated into new buildings as a primary or secondary power source, and existing buildings can also be retrofitted with similar technology. The advantage of integrated photovoltaics over conventional non-integrated systems is that the initial costs can be offset by reducing the expenditure on building materials and labor that would normally be required to construct the part of the building that the BIPV modules replace. Furthermore, BIPV enables wider acceptance of solar installations when the building's aesthetics are a consideration and conventional, rack-mounted solar panels would detract from the intended appearance.

The term BAPV (building-applied photovoltaics) is sometimes used to refer to photovoltaic systems that are retrofitted into a building. Most building-integrated systems are indeed BAPV. Some manufacturers and developers differentiate between BIPV and BAPV in new construction.

Building-integrated photovoltaic (BIPV) applications emerged in the 1970s. Aluminum-framed photovoltaic modules were attached to or mounted on buildings, typically located in remote areas without access to the electricity grid. In the 1980s, rooftop PV systems began to be installed. These PV systems were generally installed on buildings connected to the electricity grid and located in areas with centralized power plants. In the 1990s, BIPV products specifically designed for integration into the building envelope became commercially available. A 1998 doctoral dissertation by Patrina Eiffert, entitled "An Economic Assessment of BIPV," hypothesized that there would one day be economic value in trading renewable energy credits (RECs). An economic assessment and brief history of BIPV by the US National Renewable Energy Laboratory in 2011 suggests that significant technical challenges remain before BIPV installation costs can compete with those of photovoltaic systems. However, there is a growing consensus that BIPV systems, through widespread commercialization, will form the backbone of the European Zero Energy Building (ZEB) target by 2020. Despite the promising technical possibilities, social barriers to widespread adoption have also been identified, such as the conservative culture of the construction industry and integration into high-density urban planning. The authors point out that long-term adoption is likely to depend as much on effective policy decisions as on technological development.

The semi-transparent solar modules offer an interesting way to integrate building-integrated photovoltaics (BIPV) into architecture and urban planning. This novel type of solar energy generation is highly likely to become an important component of global electricity production in the future.

Building-integrated photovoltaics with semi-transparent solar modules is an attractive option for constructing energy-efficient buildings. This technology can help reduce energy costs while simultaneously improving the building's exterior.

Furthermore, semi-transparent solar panels can be used to direct daylight into the interior of a building. This not only saves energy but also reduces the cost of artificial lighting.

In summary, building-integrated photovoltaics (BIPV) is a highly efficient and versatile form of renewable energy. It has the potential to sustainably improve the energy supply of buildings.

Solar modules made of crystalline silicon for ground-mounted and rooftop power plants.

Amorphous crystalline silicon thin-film solar PV modules, which can be hollow, lightweight, red, blue and yellow, used as a glass facade and transparent skylight.

CIGS-based (copper indium gallium selenide) thin-film cells on flexible modules that are laminated onto the building envelope element, or the CIGS cells are mounted directly onto the substrate of the building envelope.

Double-glazed solar modules with square cells inside.

flat roofs

The most widespread solution to date is an amorphous thin-film solar cell integrated into a flexible polymer module, which is attached with an adhesive film between the solar module's backsheet and the roof membrane. Using copper indium gallium selenide (CIGS) technology, a US company has achieved a cell efficiency of 17% for building-integrated modules in single-layer TPO membranes.

Pitched roofs

Solar roof tiles are (ceramic) roof tiles with integrated solar modules. The ceramic solar roof tile was developed and patented by a Dutch company in 2013.

Modules shaped like several roof tiles.

Solar shingles are modules that look and function like normal shingles, but contain a flexible thin-film cell.

They extend the normal lifespan of roofs by protecting the insulation and membranes from UV radiation and water damage. They also prevent condensation by keeping the dew point above the roof membrane.

Metallic pitched roofs (both structural and architectural) are now being equipped with PV functions, either by bonding a freestanding flexible module or by heat and vacuum sealing the CIGS cells directly onto the substrate.

facade

Facades can be attached to existing buildings, giving them a completely new look. These modules are mounted on the building's facade over the existing structure, which can increase the building's attractiveness and its resale value.

glazing

Photovoltaic windows are (semi-)transparent modules that can replace a number of architectural elements usually made of glass or similar materials, such as windows and skylights. They not only generate electrical energy but can also achieve further energy savings due to their excellent thermal insulation properties and ability to control solar radiation.

Photovoltaic glass windows: The integration of energy-generating technologies into residential and commercial buildings has opened up additional areas of research that place greater emphasis on the overall aesthetics of the final product. While the goal remains to achieve high efficiency, new developments in photovoltaic windows also aim to offer consumers an optimal level of glass transparency and/or the option to choose from a range of colors. Differently colored solar panels can be designed to optimally absorb specific wavelength ranges from the broader spectrum. Colored photovoltaic glass has been successfully developed using semi-transparent, perovskite, and dye-sensitized solar cells.

• Plasmonic solar cells that absorb and reflect colored light were developed using Fabry-Pérot-Etalon technology. These cells consist of two parallel, reflective metal films and a dielectric cavity film between them. The two electrodes are made of silver (Ag), and the cavity between them is made of Sb₂O₃. By changing the thickness and refractive index of the dielectric cavity, the wavelength that is best absorbed is altered. Matching the color of the absorption layer glass to the specific part of the spectrum for which the cell's thickness and refractive index are best suited improves both the cell's aesthetics by intensifying its color and minimizing photocurrent losses. Red and blue light devices achieved transmittances of 34.7% and 24.6%, respectively. Blue devices can convert 13.3% of the absorbed light into electricity, making them the most efficient of all the colored devices developed and tested.
• Perovskite solar cell technology can be tuned for red, green, and blue wavelengths by changing the thickness of the metallic nanowires to 8, 20, and 45 nm, respectively. Maximum power efficiencies of 10.12%, 8.17%, and 7.72% were achieved by adjusting the glass reflectance to the wavelength for which the respective cell is best suited.
• Dye-sensitized solar cells use liquid electrolytes to capture light and convert it into usable energy, much like how natural pigments enable photosynthesis in plants. While chlorophyll is the specific pigment responsible for the green color in leaves, other naturally occurring pigments, such as carotenoids and anthocyanins, produce variations of orange and purple hues. Researchers at the University of Concepción have demonstrated the viability of dye-sensitized colored solar cells that both appear vibrant and selectively absorb specific wavelengths of light. This low-cost solution uses natural pigments derived from maqui fruit, black myrtle, and spinach as sensitizers. These natural sensitizers are then sandwiched between two layers of transparent glass. While the efficiency of these particularly inexpensive cells remains unclear, previous research in organic dye-sensitized solar cells has achieved a “high power conversion efficiency of 9.8%.”.

Transparent solar cells use a tin oxide coating on the inside of the glass panes to conduct electricity from the cell. The cell contains titanium oxide coated with a photoelectric dye.

Most conventional solar cells use visible and infrared light to generate electricity. In contrast, this innovative new solar cell also utilizes ultraviolet radiation. If used as a replacement for conventional window glass or placed over existing glass, the installation area could be large, leading to potential applications that combine power generation, lighting, and temperature control.

Another term for transparent photovoltaics is “translucent photovoltaics” (they only allow half of the incident light to pass through). Similar to inorganic photovoltaics, organic photovoltaics can also be translucent.

Non-wavelength selective

Some non-wavelength-selective photovoltaic systems achieve semi-transparency through the spatial segmentation of opaque solar cells. This method uses any type of opaque solar cell and distributes several small cells onto a transparent substrate. This segmentation drastically reduces the energy conversion efficiency and increases transmission.

Another branch of non-wavelength-selective photovoltaics utilize visibly absorbing thin-film semi-conductors with small thicknesses or large enough band gaps that allow light to pass through. These results in semi-transparent photovoltaics with a similar direct trade off between efficiency and transmission as spatially segmented opaque solar cells.

Another branch of non-wavelength-selective photovoltaics utilizes visibly absorbing thin-film semiconductors with low thickness or sufficiently large band gaps that allow light to pass through. This results in semi-transparent photovoltaics with a similar direct trade-off between efficiency and transmission as spatially segmented opaque solar cells.

Wavelength-selective photovoltaics

Wavelength-selective photovoltaics (WSPV) achieves transparency through the use of materials that absorb only UV and/or NIR light and was first introduced in 2011. Despite the higher transmittance, energy conversion efficiencies are lower due to a number of problems. These include short exciton diffusion lengths, the scaling of transparent electrodes without compromising efficiency, and the overall lifetime due to the inherent instability of the organic materials used in WSPVs.

Innovations in transparent and translucent photovoltaics

Early attempts to develop non-wavelength-selective semi-transparent organic photovoltaics with very thin active layers absorbing in the visible spectrum achieved efficiencies of less than 1%. However, in 2011, transparent organic photovoltaics using an organic chloroaluminium phthalocyanine donor (ClAlPc) and a fullerene acceptor demonstrated absorption in the ultraviolet and near-infrared (NIR) spectrum with efficiencies around 1.3% and visible light transmittance exceeding 65%. In 2017, MIT researchers developed a method for successfully depositing transparent graphene electrodes onto organic solar cells, resulting in 61% visible light transmittance and improved efficiencies of 2.8–4.1%.

Perovskite solar cells, which are very popular as next-generation photovoltaics with efficiencies exceeding 25%, have also proven promising for transparent photovoltaics. In 2015, a semi-transparent perovskite solar cell with a methylammonium lead triiodide perovskite and a silver nanowire grid top electrode demonstrated a transmission of 79% at a wavelength of 800 nm and an efficiency of approximately 12.7%.