Both types of roofs have advantages. Green roofs do not produce energy (in some cases just fruits) but they do not consume that much energy for production of panels, installation and control and do not need treatment after use. So what are the either advantages and disadvantages compared with each other?
The two are not mutually exclusive, and in fact installing PV panels over a green roof improves PV performance anywhere from 1 to 10% due to the cooling effect of the green roof.
From the presentation "Interaction between PV-systems and extensive green roofs" given at the Fifth Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show in 2007 (emphasis added):
The energy output of PV-panels depends on several factors. In this survey of over approximately 5 years of data, the effect of the green roof has been estimated at an average 6% increase in yields. But the green roof is only one factor in enhancing the efficiency of PV-panels. The total range of increased energy output varies between 1 and 10%. PV over green roofs can be mounted on fixed frames. The costs are lower in comparison to the tracking systems, which follow the sun. Under the PV panels, a variety of extensive green roof vegetation can be grown. In the case of UFA it was originally a Sedum roof but this was subsequently changed to include a wide variety of plantings. Minimum garden roof maintenance is required to reduce taller growing plants. The PV – cells must be without shade. The estimated benefit of a higher Power output from a green roof can help to offset the cost of green roof installations.
This effect is attributable to the fact that solar panels are more efficient when kept cooler, which a green roof facilitates through evaporative cooling.
Several other research studies verify this finding:
- "Quantification of the passive cooling of photovoltaics using a green roof" uses TRNSYS building energy modeling software to estimate a small (<1%) improvement in power output for a city in the Northeast US, but notes that the dual use of space increases the PV system's value regardless.
- "Green roof integrated PV canopies -- an empirical study and teaching tool for low income students in the South Bronx" uses a number of student-planned and -built test setups to measure a 2.4% increase in PV system output, and a 4.6% reduction in interior temperature over an eight-month period (covering summer, fall, and the beginning of winter).
- "Photovoltaic-green roofs: An experimental evaluation of system performance" measured increased PV output of 1.3 to 3.3% over a five-day period in the summer in Spain.
- "Integration of green roof and solar photovoltaic systems" used EnergyPlus modeling software on a hypothetical building in Hong Kong to predict a slight decrease in cooling needs, and an 8.3% increase in PV system output.
I would put these two things in separate categories of sustainable technology.
Green roofs are sustainable in the sense that the provide extra insulation and the building the cover will require less energy. They also require a great deal of maintenance and energy so the benefits are mostly superficial while the dis advantages in terms of cost and effort to maintain is high.
Photovoltaics, roof top solar panels, are going to have a much bigger impact of sustainability in the big picture.
I think green roofs are gorgeous and must be fun for architects to design but solar panels are what you want if you care about sustainability.