Photovoltaic Systems on Buildings
PV- modules can be mounted on tilted roofs, flat roofs but also integrated into building walls and windows. Transparent modules can be mounted on solar greenhouses.
In many cases, pv- systems can be retro-fitted to buildings, either on tilted or flat roofs. A number of mounting systems have been developed specifically for PV- modules on tilted roofs such that no major building work has to be undertaken. The modules tend to be made from aluminium and can grip onto the roof tiles.
The crucial characteristics of these systems are:
- Snow Load: Weight that the structure is able to carry.
- Wind Speed: Wind between the roof and the panels can create a significant force. Some mounting systems may therefore be limited to low-wind zones only.
- Roof Slope: Most mounting systems will not allow roof tilt angles higher than 60°.
- Lightning Protection: It is essential that the mounting system comes with lightning protection.
Naturally, orientation of the roof is important. Ideally, facing due south at an angle of latitude - 10% - 25%.
Flat roofs provide an ideal space for solar systems, as these are often unused spaces. Moreover, flat roofs don't have the limitations of the pre-existing orientation of tilted roofs. In fact, in the U.S. alone, in excess of 100million square meters of reflective roofs are installed annually. Commercial rooftops in the U.S. alone could provide an average of 5GW!
On flat roofs, flat panels are installed at a fixed tilt angle, similar to stand-alone pv- installations. However, this option may not be feasible if the panels would be too heavy or if there is too much wind, which may affect the stability of the tilted up panels.
Solyndra, an American company, provided an alternative which was a thin film module in cylindrical shape. Although of lower efficiency, they were lighter and not susceptible to wind. Thin film is wrapped around cylinders. As a result, light can be collected from anyn angles, rendering tilting unnecessary. In addition, the thin film may also catch light that has been reflected off the roof itself. Due to its unique geometry, there are also no issues with wind activity. However, Solyndra filed for bankruptcy in 2011.
Building Integrated PV
There are many shapes and forms where pv panels, especially roll-on thin films can be used in buildings.
Some of the major characteristics are:
- Colour of the surface. Unlike crystalline photovoltaics, thin film pv does not necessarily have to be blue.
- Transparency: Some thin films are transparent to the visible spectrum. This way they can be put on big window fronts of office blocks, and still generating some electricity while not blocking the light for the offices inside.
- Flexibility: Some panels come protected with glass while others are more like sheets.
Using transparent solar modules, greenhouses can combine agricultural food production with power production.
Depending on the location and the needs of the plants to be grown underneath, the light transparency of the modules must be chosen.
The greenhouses will typically allow for cucumber, lettuce, tomatoes, water melons, cabbage and other crops.
In hydroponic plants, the roofs above the plants are not transparent, as inside an artifical climate is created while plants are in water. In this instance, standard solar modules can be used on top of the roof.