My PV project: frequently asked questions
The first step is to find out the customer’s wishes. This has to be matched with the existing space of shadow-free roof
The roof must be analyzed in order to check it there is no need to reinforce it or to renovate it before installing PV panels. It is also important to find out how much area is not shadowed and could be used for a PV system. Some solutions can be found for shadowing during winter time or during early morning or late afternoon, but shadowing always reduces the system’s performance.
Once the area is clear, the next step is to choose the typo of modules to be installed. The dimensions of the modules and the electrical characteristics need to be taken into account. The module layout has to match the working point of an inverter.
The best indicator for sizing a PV system is the historical electrical usage or the number of kWh (kilowatthours) consumed each year. It is especially important to determine an annual average for the kWh usage, because many families experience seasonal peaks in usage. This average gives a starting point for comparing the energy output of various systems.
A PV system of between 7m²-15m² (1KWp installed), depending on the technology, can produce about 1000kWh every year in Munich and 1400KWh in Malaga. If the yearly electricity consumption is known, the calculation for a system size can easily be made.
Sizing a BIPV systems depends as well on the available surface area of the building shell you would like to cover with PV-Modules. To keep a homogenious appearance it might be sometimes better you select a BIPV system which might be larger or smaller than the required power demand of the building might ask for.
The energy payback time (EPBT) of photovoltaic (PV) systems is an important criterion in understanding the sustainability of PV. The EPBT is the amount of time a PV system has to operate in order to compensate for the energy required to fabricate the system itself. It takes the following actors into consideration: the impact of a product throughout the entire life cycle – from material sourcing, through manufacturing, construction, operation, dismantling and product collection and recycling
The major contributors to the EPBT are energy inputs that are primarily driven by the requirements during module manufacturing and the energy output, which is specific to the characteristics of the PV technology used in addition to the location of the system. For areas with high solar irradiance, such as Southern Europe, the EPBT is shorter as compared to areas with lower solar irradiance.
Recent EPBT calculations have been made in the major European Integrated PV R&D Projects, Crystal Clear, Performance and within the EPIA Sustainability Working Group. Depending on the type of PV system and the location of the installation, the EPBT at present is between 0.7 and 1.4 years
The technical lifetime of PV systems is 30+ years; hence they produce net clean electricity for more than 95% of their lifetime.
Note: For rooftop installations in Southern Europe (1700 kWh/m² yr). Irradiation on optimally-inclined modules.
Is it possible to install PV modules using different directions and angles than tilted and facing south without major reductions in energy yield?
There are no mandatory installation requirements. Certain shifts with respect to the optimum installation requirements for PV are possible without incurring substantial power loss.
For instance, considering the mean latitude value for Central Europe, a +/- 15° tilt shift can involve a slight 2% loss, while the same shift from the southern direction is merely capable of reducing a system‘s performance by 3%.
The PV cells in a standard mono- or polycrystalline module are connected in series, so even if only one of the cells is shadowed, the module production will decrease drastically. Usually the cells are connected in two independent series, so a partial shadowing of one of them would reduce production by roughly 50%, leaving the other series producing. In the same way, the least producing module in a string of modules connected in series determines the production of the whole string. Therefore, even partial shadowing should be avoided whenever possible. In situations where snow or falling leaves may cover the lower half of the panels temporarily, the panels should be installed in such a way that one of the series of PV cells in the module stays is above the other, so that shadowing the lower half of the module doesn’t disable the whole module.
Thin film solar modules are usually less affected by shadows, only decreasing module production by the percentage of shadowed surface of a module.
The yearly production of a system depends on the system size (power in kWp), structural and electrotechnical design, the irradiation, weather conditions and the system components.
Many tools already exist in the market to calculate the estimated energy production of the system during its whole life cycle. These programs normally calculate the electricity (kWh) produced based on the nominal power of the system (kWp) and takes into consideration irradiance, weather conditions and type of system (technology, direction towards the sun, angle, possible shadows, losses, etc.).
Photovoltaic Geographical Information System (PVGIS) is a free and online tool from the Joint Research Centre, EC. It is available at:
Mono- and polycrystalline solar modules are made of a series of solar cells, cut from a block of semiconductor material (ingot). The monocrystalline cells have slightly higher efficiency than polycrystalline ones, and the visible difference is that monocrystalline cells have a uniform dark blue colour while polycrystalline cells look a bit fragmented. Monocrystalline modules are slightly more efficient and slightly more expensive than polycrystalline modules.
For thin film modules, thin layers of semiconducting material are deposited on a substrate, usually glass but flexible substrates can also be used. Thin film modules are cheaper than crystalline modules but have lower efficiency, requiring more space for the same amount of installed power. Due to the flexibility of the production process, thin film modules can be made in almost any size and shape and with variable transparency, making them very suitable for integration in buildings (BIPV).
Apart from these three types of solar modules, new types like for example organic solar technology are currently being developed, but are not commercially available yet.
PV systems constitute a reasonable alternative for construction materials to be considered by architects. Besides replacing an existing building component, PV systems also produce energy making it a multi functional building component.
The possibility of using solar panels as new building components such as for roofs, façades or blinds offers a wide new spectrum and very important new design lines and challenges for both architects and module manufacturers.
PV modules are multifunctional building components that, besides generating electricity, can fulfil many other functions such as shading systems, weather protection, heat insulation and sunlight modification, creating excellent lighting effects.
Furthermore, the integration of renewable energy sources in a building will provide the building designer/owner a ‘green image’, developing positive relations with ‘green investors’ and achieving better ranking in ‘green investment funds’.
Today‘s PV systems can easily be integrated into the existing electrical system of the home. They produce clean energy - no air pollution, no greenhouse gas emissions. Furthermore, they can greatly reduce or even eliminate one’s electricity bills. Photovoltaic power systems can now be integrated into the design of a building, combining energy production with other functionalities of a building‘s external structure.
Roof tiles, windows, facades, canopies and skylights can all be incorporated with PV technology, and the combination of functionalities can lead to substantial cost savings.
A well designed and installed PV system is often considered a beneficial feature on a house and can even increase the value of the property.
It is considered that a Building Integrated Photovoltaic (BIPV) System is integrated successfully, if it can be incorporated into a building, with judicious design and structure and with a sensible energy concept. In this case PV modules form an integral part of the buildings into which they are integrated. Therefore, PV modules are part of the whole building design and may also replace traditional construction materials.
BIPV or Building Integrated Photovoltaics is an industry buzz phrase, which indicates PV modules integrated in various kinds of ways into conventional construction materials Solar roof tiles are a good example.
PV systems constitute a reasonable alternative for construction materials to be considered by architects. Besides replacing an existing building component, PV systems also produce energy making it a multi functional building component. Architects should consider the use of renewable energy sources in the design phase.
The possibility of using solar panels as new building components such as for roofs, façades or blinds offers a wide new spectrum and very important new design lines and challenges for both architects and panel manufacturers.
Active involvement by building developers, engineers, architects, module manufacturers and installers is therefore of the utmost importance from the project design phase on to resolve the total integration of the panel in the building in order to avoid potential problems with leaks, dampness and insulation that may occur.
PV systems have to be seen not only as energy generators, but as multi-functional elements enhancing the look of the building and the environment where they are placed.
The versatility of the PV component, obtained by means of design tools, ensures the possibility to enlarge its utilization to many contexts, like urban and natural environments, archaeological areas, etc. in a creative way.
There are no limits for installing PV systems when a good exposure is achievable.
Beyond roofs, some of the most common ways to implement PV systems are for example on façades which offer huge possibilities for architectural integration and visibility, or the following:
- Noise mitigation barriers
- Of course, directly on the ground or using a solar tracker, especially for large-scale applications
Standard mono- or polycrystalline modules are relatively light, around 10 to 15 kg per square meter. This load must be carried by the roof structure to which the system is attached but in most circumstances there is no need to reinforce existing structures for this extra weight.
However, wind load must also be taken into account. Depending on wind direction, panels can be blown off the roof; this force can be considerable during a storm. Most existing buildings were not designed to support photovoltaic systems, so problems could appear due to wind load, water tightness, snow and ice etc. The roof structure should always be checked by an architect or structural engineer so as to ensure a successful and safely installed photovoltaic system. This is of vital importance for installers of photovoltaic systems.
For new buildings, it is recommended to adapt the new roof to the pitch and orientation requirements for PV systems.
It is essential that PV installations comply with local building regulations and safety codes.
Transparent PV modules are generally one of two principal types:
- Normal cells in a double glass frame, the gaps between the cells are transparent
- Thin films deposed on a glass surface;
the PV layer is thin enough to let a certain amount of light through. Modern production methods allow for almost unlimited possibilities in the design of thin film modules: any transparency or even graphic design (letters or logos that are more or less transparent than the rest of the module).
The gaps between normal PV cells in a double-glass module can be increased or decreased to change the transparency level of the module. Generally, the gaps between cells are such that the transparency is between 5% and 30%. A classic double-glass module will have a transparency of roughly 5%.
The transparency of thin-film modules depends on the transparency of the support and the thickness and type of cell used.
Nearly any degree of transparency desired can be achieved (subject to potential extra costs) made to order, but it is common to balance the natural light gains against potential overheating due to increased thermal gains.