The building integration of a PV system as part of the energy supply in a building is an ongoing concept that is slowly but surely penetrating the market with the latest developments in PV modules, deeply related to the international tendency to realize energy efficient buildings.
However, besides the electrical design of a PV system, building integrated photovoltaics (BIPV) includes the overall image of the array into the building, meaning the architectural point of view must also be considered when designing a BIPV.
A well-integrated system with a good electrical design and an architecturally elegant structure will increase market acceptance and provide the building owners with a highly visible expression of their environmental commitment, which acts as the major impulse for BIPV development.
BIPV Pros and Cons
Associated with BIPV is a wide range of benefits and advantages that other types of systems cannot offer such as:
- PV installation does not consist of moving parts that would need maintenance
- No additional land is needed for the PV array
- Transmission losses are minimal due to energy consume onsite
- BIPV are more secure against theft and damage
- BIPV installation is easy to connect to the electrical system of the building
- BIPV replaces conventional building envelope materials.
Amongst all of these advantages, the main purpose of BIPV is to reduce the requirement for land and costs. After all, it is more efficient to integrate a PV system when constructing the building, rather than mounting it afterwards.
On the other hand, BIPV also presents some disadvantages that must be taken into account when considering the installation in a new building:
- Integration will influence the building physics.
- Sealed air gaps on BIPV increase the operating temperature of PV modules
- Higher levels of relative humidity that could increase the probability of moisture and damage efficiency of the cells in the long-term
- It is necessary to include adequate ventilation
- High initial cost
- Any bad design factor could affect the physical structure and integration of the building and correcting such problems is not easy once the installation is done.
BIPV can be divided into two main categories:
- Roof systems
- Façade systems.
The disposition and selection of the category will depend on technical considerations like orientation and available space, but also on the architectural integration of the PV cells to the building.
Mainly, as BIPV are intended to contribute to the aesthetics of the building, the solar cells must interact in harmony with the other structures of the building and be designed accordingly.
Roof Integrated PV
The roof-integrated option works in several ways.
Roofs can either be pitched or flat and the idea is that the PV system acts as the outdoor covering and part of an impermeable layer in the building’s construction.
Other approaches also consist of gluing the PV system onto an expanded polystyrene insulation material that is suitable for renovating large, flat roofs.
Furthermore, besides covering the entire roof with modules, other innovative ideas are the introduction of PV shingles and tiles that in small-scale produce 2 cells on a single tile, making them very convenient for housing BIPV solutions.
Also, transparent PV modules are used as roofing materials that serve as water and sun barriers while allowing the transmission of daylight as skylights.
The additional benefit in this type of roof installation is that the PV cells absorb nearly 70% of the sun’s radiation, protecting the people below the skylight from ultraviolet radiation and allowing only the diffused irradiance which offers a pleasant and harmonic lighting level.
Actually, new developments in this area have also penetrated the market environment recently. Without question, Tesla is one of the most advanced companies in this topic and recently this millionaire company released a new feature to the market: The Solar Roof.
It represents the ultimate BIPV integration, with roofs that do not look like PV modules, but instead, they actually look like a roof surface, but they produce electricity.
The idea is possible because they combine two types of glass tile, the solar tile and the non-solar tile.
Tesla’s tiles are made with tempered glass which means they are three times stronger than typical roof tiles and therefore the company can offer an infinite tile warranty.
In order to prove their durability, the company exhibits a video on its website where standard tiles and Tesla solar tiles are hit by a ball with amazing results:
The second innovation on this topic, are the Photovoltaics Thermal (PVT) modules, which consist of a typical PV module coupled with a solar thermal collector which is installed on the backside of the panel.
As the PV module generates electricity from the sun’s radiation, the excess heat is used to heat water or ventilate air for a thermal system.
This approach offers the greatest usability of solar radiation in two of its practical energy forms: heat and electricity.
Moreover, it has been widely studied that the increase in temperature reduces PV module efficiency and causes voltage drops. But if there is a heat transfer fluid like water, air or any refrigerant, then the excess heat does not reduce the efficiency or voltage values, which creates additional electricity that otherwise would be lost.
However, as efficient as this technology might be, there are still many constraints to make it a market feasible installation, as high manufacturing and installation costs make the system not cost-effective. It is only feasible in electricity markets with electricity rates beyond 30 cents/kWh with payback times of 19 years.
Façade Integrated PV
Façades are made of concrete constructions which form the structural layer which is covered with insulation and protective cladding.
In this case, the cladding is made from PV modules which is no more expensive than other materials (when considering luxurious buildings).
Sun-shading devices, canopies and louvres can be used with façade PV installations, combining shade for the building during summer while producing electricity at the same time.
For instance, terraces with a roof on the sunny side of the building are excellent examples of the application of a BIPV façade mode, as they provide shade, electricity and even protection from rain.
Technical issues arise when considering this system as the heat load, shade covering, electrical power output, shadings between panels and the design of the façades is entirely related to the orientation of the building, which cannot be changed.
Therefore, when considering this type of installation, orientation is crucial and particularly for Australia, the ideal configuration is that the façade faces North.
Now, when shading of the façade cannot be modified, then some applications develop sloped surfaces facing the sun in order to adjust the proper tilt angle of the modules, however, this configuration disables the aesthetic purpose of the BIPV, so is generally not recommended.
BIPV design must always take into account that the modules will not be added to the building, but designed as part of the building.
The BIPV system varies in colours, materials and compositions in order to contribute to the appearance and aesthetics of the building, which is the main added value of this technology, as it combines energy efficiency, renewable energies, demand reduction and architectural value all into one.
Some tools are available online to better design the PV integration to the building according to the architectural point of view.
The BIPV Tool allows you to design the PV module type according to the aesthetics of the façade or the roof, choosing between monocrystalline, polycrystalline and thin film modules. You can establish the configuration of the modules (size, space between cells, transparency, thickness, color), use a simple shading simulation to visualize the architectural view across the day with the variation of the sun into façade and roof options and finally, integrate the results in Revit.
Selection of BIPV Modules
The selection of BIPV modules takes into account all the technical criteria for the selection of typical PV panels, but in particular, on BIPV the efficiency values are extremely important and another additional parameter also influences the decision of the module type, the colour.
BIPV can integrate the typical silicon monocrystalline, polycrystalline and thin film modules and the decision of one or another type (besides electrical parameters of open circuit voltage, short circuit current and nominal power output) relies on efficiency.
The second and particular consideration for BIPV is the aesthetics.
Monocrystalline modules have a uniform appearance, with several colour options, but the most typical features either black or dark blue colours. The reasons for this is that due to the spectrum of solar radiation, the darker the colour, the less light is reflected and therefore, more efficiency is achieved.
Polycrystalline ones, on the other hand, have a blue colour making them less efficient but cheaper. Losses in reflection can be reduced by applying the anti-reflective coating.
Thin-film modules are typically dark black and are deposited onto substrates such as glass window, plastic or stainless steel, providing a wide spectrum of weight, flexibility and mechanical strength.
Additionally, a particular module is also applied for BIPV. The semi-transparent cells are made of mono silicon wafers and have deep perpendicular grooves on the front and back through which sunlight is transmitted, allowing it to have a transparent appearance.
This type of module is generally used for sunroofs as there is some orange or red tint of light passing through due to the spectrum of light absorbed by the silicon cells to produce electricity.
Now that you know some possibilities for BIPV modules, you can have a more extensive look at the different types of modules here.
Another particular consideration in the selection and design of PV modules is temperature.
Temperature coefficients are also defined for PV panels as the higher this value, the higher the loss when increasing the temperature of the environment.
The difference between typical PV installations and BIPV lies in the fact that both ground-mounted and roof-mounted installations have some airflow behind the panels, which acts as a cooling system to reduce thermal losses.
On BIPV such air flow does not exist. BIPV install the panels in close contact with building material like wall insulation, so there is a lack of airflow which increases the module’s temperature.
Therefore, when considering monocrystalline or polycrystalline panels, you can expect relative losses higher than 5% due to temperature coefficients, this is an important fact when designing the PV array.
We have examined the latest trends in BIPV, as well as design considerations for this new technology.
Development of BIPV is still new and the deployment at market level small and slow.
There are great possibilities for integration in this market, with multiple product options, efficiencies, designs and prices, therefore the main constraint for market development is not related to technology available solutions, but to the building process, codes and financing.
In spite of all this, successful BIPV solutions have been implemented in Australia; only time will tell if the solutions will be extended or not.
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Photo credit: Europa Studio, Tesla