Modern urban buildings must take maximum advantage of all spaces, including rooftops. In many cities, there are regulations that require green roofs, and often architects and planners believe that it rules out solar energy. But the good news is that you can make green roof and solar a great combination!
Types of green roof
There are different types of green roofs, and many ways to build them up. Generally, a layered structure consisting of a root barrier, a drainage system, a substrate (soil) and plants build up the green roof.
Most green roofs are so-called extensive green roofs, planted with sedum plants. Extensive means that they are optimized for large area coverage, low weight and low maintenance needs. Sedum plants are succulent plants that can hold a lot of moisture and therefore resist long periods of little rain and the strong sun and heat that rooftops are exposed to. The sedum plants typically grow 5-15 cm over the rooftop, depending on the season, the substrate thickness, climate and maintenance. Extensive green roofs are designed to be light-weight, and typically weigh 50-150 kg/m2 when soaked in rainwater. Extensive green roofs are good for combination with solar panels.
The other main type of green roof is intensive green roofs, which offers thicker soil substrate and more biodiversity. The weight is high (150-800 kg/m2), and therefore put high demands on the building structure. Plants include bushes, weeds, grasses and sometimes even trees, and the typical plant height is in the range of 30-60 cm. Intensive green roofs are difficult to combine with solar panels.
Read also: Why are green roofs so important?
Checklist for combination roofs
To combine green roofs with solar energy, you need to plan well. There are many examples of installations that don’t work very well. We here want to propose a set of design principles and guidelines for planning a good combination.
Know your type of green roof
To plan a working solar installation on a green roof it is important to know the type of green roof to ensure that you select a suitable solar solution. The substrate thickness, plant height and maintenance needs are crucial to know.
Plan for the right roof load
While green roofs in themselves add substantial weight to the roof, adding a solar installation may end up with doubling the roof load. For existing buildings, this is important to know in order not to compromise the safety of the building, and new buildings may need to be reinforced with respect to original plans, adding cost and consuming more concrete and other materials in the construction process.
Ensure low conflict between plants and solar panels
It is important to plan for a solution where the green roof and the solar panels will not be “fighting against each other”. Solar panels may alter sunlight, evaporation and water flow on the roof, and might create unexpected negative consequences. It is important not to completely cover the green roof with solar panels. A conventional east-west solar installation may provide a lot of kWp in theory, but the fact is that plants will end up covering the solar panels to a smaller or larger degree. In general, a green roof solar installation will have less power (kWp) than a completely covering flat roof solar installation. We recommend planning with 50-70% of a the power of a conventional solar installation for flat roofs, otherwise it will be a suboptimal solution both for the green roof and the solar panels.
Optimize for easy maintenance
Although extensive green roofs require very little maintenance, they need more than traditional dead materials. You must make sure that the green roof is accessible through the lifetime of the building. If planned wrong, as we will see below, the combination of green roof and solar may also generate a much higher need for maintenance than what each solution separately would have.
Types of solar installations for green roofs
A solar installation consists of solar panels, a mounting system, cabling and an inverter. The type of solar panels and mounting system is extremely important to make a good combination roof.
There are three main ways used to build a solar installation on a green roof.
Conventional solar panels and conventional flat roof mounting structures
For flat roofs without green, regular solar panels that measure about 1 by 2 m in size are mounted into a mounting system in an east-west configuration. The solar panels will cover nearly the entire roof, except for some walkpaths in between the panels for most mounting systems. While this is an efficient and well-established way to build a solar installation, it is not a good idea for combination with a green roof. Why? Because the large solar panels will cover the green roof almost completely. Below the panels there will be a humid and dark environment, which favors the growth of plants that can grow fast to reach the light above the solar panels. These plants will shade for the solar installation and generates a massive maintenance need to remove them continuously through the growth season. The large solar panels will also make the maintenance very difficult, as maintenance personnel will need to lean over the large solar panels without putting much weight on them to remove the weeds in between. This combination is therefore not recommended.
Conventional solar panels with a specialized mounting structure
There are several providers of specialized structures for usage of conventional solar panels on green roofs, for example the well-established solutions from Bauder and Zinco. These solutions provide an elevated mounting of the solar panels at an angle of 15-30 degrees, with ample space between, for plants to grow and maintenance personnel to move. The elevated and sparse installation of solar panels make them highly exposed to winds and winds can generate substantial lift forces on the structure. Therefore, they need high ballast and may add up to 100 kg or more per square meter of rooftop. When planning such an installation it is crucial that the building is designed for such loads, and, for retrofitting of existing buildings with green roofs, it might not be a viable solution. The roof is relatively easily available for maintenance. However, since the tilted panels will concentrate rainwater on the lower end of the panel, there will often be accumulation of plant growth at the lower end of the panel. Good planners of such installations know this, and intentionally divide the thickness of the green roof substrate on the roof, so the lower end of the solar panel has thinner substrate, in order to avoid growth of tall plants that shade the solar panels on the lower edge. In the end, these installations are well-established in the market, but the installation may be complex, the weight might be high, and therefore it is not always a possible solution.
Specialized vertical bifacial solar panels in a specialized mounting structure
To address the challenges that arise when using conventional solar panels (weight, maintenance, etc), you might consider a third way to install solar panels on green roofs. An example of this is the system offered by Over Easy Solar. Use of vertical solar panels in combination with green roofs were first tested in Switzerland in 2015, and it offers a solution that may give the best of both worlds. The vertical solar panels do not cover the green roof, so all of the plants get access to rainwater, light and evaporation. Small vertical solar panels mounted at a suitable distance over the rooftop require little or no ballast, making it a very light-weight and easy system to mount. The VPV solution from Over Easy Solar normally only adds 11 kg/m2 to the rooftop. However, you still need to know which type of green roof is under, and for intensive green roofs this is not a recommended solution since tall plants will overgrow the solar panels.
In sum, you need to use a specialized system for green roofs. Over Easy Solar's solution with the VPV Unit is a good option, and there are also other alternatives that makes a high-quality and low-maintenance green roof solar installation possible.
