Renewable and Sustainable Energy Reviews 43 (2015) 264–280 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser A critical analysis of factors affecting photovoltaic-green roof performance Chr. Lamnatou n, D. Chemisana Applied Physics Section of the Environmental Science Department, University of Lleida, c/Pere Cabrera s/n, 25001 Lleida, Spain article info abstract Article history: Photovoltaic (PV)-green roofs combine PVs with green roofs, are a new tendency in the building sector Received 5 March 2014 and they provide additional benefits (in comparison with the simple green roofs) such as in situ Received in revised form production of electricity. The present study is a critical review about multiple factors which are related 10 September 2014 with PV-green roofing systems. Representative investigations from the literature are presented along Accepted 4 November 2014 with critical comments. The studies reveal that plant/PV interaction results in PV output increase Available online 25 November 2014 depending on parameters such as plant species, climatic conditions, evapotranspiration, albedo, etc. Keywords: Furthermore, by comparing a PV-green roof with a PV-gravel one from environmental point of view, it Photovoltaic (PV)-Green roofs can be seen that the PV-green system, on a long-term basis, compensates its additional impact due to its Plant/PV and plant/building interaction higher production of electricity. Moreover, in the frame of the present study, a systematic classification of PV output increase Mediterranean plant species in terms of their appropriateness for PV-green roofs is also conducted. The Albedo and other critical factors results reveal that PV output increase which is provided by PV-green roofs depends on several factors and among the studied plant species, Sedum clavatum shows the best interaction with the PVs and the building. Experimental results and findings about the environmental profile of PV-green roofs are also presented and critically discussed. Conclusively, PV-green roofing systems are promising, especially for warm climates. & 2014 Elsevier Ltd. All rights reserved. Contents 1. Introduction . 265 2. Critical issues related With PV-green roofs . 265 2.1. Increase of PV output due to plant/PV interaction. 265 2.2. Albedo...................................................................................................... 267 2.3. Benefits of the PV-green roof for the building during its operational phase . 269 2.3.1. Benefits due to the soil/plant layer . 269 2.3.2. Benefits due to plant/PV interaction: PV-green vs. simple, green configuration . 270 2.4. Environmental impact: PV-green roof vs. PV-gravel roof . 270 2.5. Additional benefits of the PV-green roofs . 270 2.6. Improvement of PV-green roof cost-effectiveness . 271 2.7. Selection of appropriate plant species for PV-green roofs. 271 2.7.1. Materials and methods . 271 2.7.2. Results and discussion. 273 2.8. A comparison between a PV-gazania and a PV-sedum roof . 278 3. Conclusions . 278 Acknowledgements . 279 References.............................................................................................................279 n Corresponding author. E-mail address: [email protected] (Chr. Lamnatou). http://dx.doi.org/10.1016/j.rser.2014.11.048 1364-0321/& 2014 Elsevier Ltd. All rights reserved. Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280 265 1. Introduction on the comparison of a PV-green roof with a PV-gravel one. Different scenarios in terms of PV output increase were adopted Green roofs are roofs covered with a soil/plant layer and they for the PV-green system. The results revealed that although the are of great interest since they have multiple benefits such as PV-green roof it has an additional impact (in comparison with the moderation of heat island effect, temperature regulation, sound PV-gravel roof) due to its green (soil/plant) part, this additional insulation, envelope protection for the building, etc [1]. On the impact on a long-term basis can be compensated. other hand, photovoltaic (PV) modules are another option for On the other hand, there are studies which focus on the plants utilizing building roof since they provide environmentally-friendly and examine their specific characteristics. For example there is a electricity production. These two different technologies can be study which evaluated the effect of substrate depth on initial combined together for the utilization of building roof and the establishment and survival of 25 succulent plant taxa for green result is the PV output increase. This is attributed to evapotran- roof applications in the midwestern United States. Several Sedum spiration (ET) cooling effect and in general to plant/PV interaction. plants were tested [12]. Sedum has extensively been studied by PV-green roofs are a recent tendency in the building sector; many authors [2–5,8,10,13] given the fact that it is a common plant thereby, in the literature there are only a few studies. These for extensive green roof applications [1]. However, there are no studies are experimental as well as modeling, regarding several studies which examine plant species in terms of their appropri- climatic conditions (Mediterranean, etc.) and several plant species ateness1 for PV-green roofs. (Sedum, Gazania, etc.) and they are analytically presented in From the above mentioned references it can be seen that plant/ Section 2.1 and Table 1. Based on these works it is proved PV synergy is complicated and depends on several parameters. that there is an increase in PV output due to plant/PV synergy Nevertheless, in the literature there are only few PV-green roof and this increase varies from 0.08% [7] to 8.3% [3] depending on studies while there are no review studies which focus on the factors such as climatic conditions, plant species, reflected radia- crucial parameters which are related with this specific type of tion from plant canopy (albedo), etc. In general, the PV-green roof roofing system. Thus, in the frame of the present study a critical studies reveal that there are several crucial factors which influence review about important factors such as PV output increase, albedo, plant/PV interaction and thus, PV-green benefits e.g. for the benefits of a PV-green roof building is presented. Moreover, a building. systematic classification of Mediterranean plants appropriate for In the field of green roofs, there are several investigations PV-green roofs based on certain criteria/weighting factors and which examine the radiation that it is reflected from plant canopy. with emphasis on plant/PV and plant/building interaction, is Albedo of plant canopy is of great importance because it is related conducted. In this way, the present study offers information which with plant/PV synergy and therefore also with PV output increase. can be useful for academic/research purposes and in general, for Certain plant species are beneficial because they reflect incident future studies/developments in the field of PV-green roofs. irradiance increasing the amount of radiation over the PV module. In the literature most of the ‘albedo’ studies are for simple (without PVs) green roofs. Among these investigations is the study 2. Critical issues related With PV-green roofs of Coutts et al. [8] which regards the comparison of insulating properties, radiation budget and surface energy balance of four 2.1. Increase of PV output due to plant/PV interaction experimental rooftops, including an extensive green roof (Sedum) and a cool roof (uninsulated rooftop coated with white elastomeric The increase of PV output is related with factors such as ET paint), in Melbourne. The high albedo of the cool roof substantially cooling effect. Certainly, the improvement of PV efficiency is reduced net radiation, leaving less energy available at the surface considerably beneficial, from environmental as well as from (for sensible heating during day). Under warm/sunny conditions, economic point of view, on a long-term basis during building when soil moisture was limited, ET from the green roof was low, operational phase. Following representative PV-green roof studies resulting in high sensible heat fluxes during day. Irrigation are presented proving PV output increase because of plant/PV improved green roof performance by increasing ET. synergy. It has also been concluded from the literature that there are Regarding theoretical/modeling investigations about PV-green only few studies which examine roof reflectivity for the specific roofing systems: case of PV-green roofing systems [5,9]. The above mentioned investigations along with other studies about albedo of simple À Scherba et al. [9] examined the role of roof reflectivity. A model green roofs are analytically presented in Section 2.2. In general, for was developed and validated by using data from a field PV-green roofing systems, plant canopies which have light-color experiment (Portland, Oregon) while several roof configura- leaves and high percentage of soil cover are desirable because of tions were studied: a control dark membrane roof, a highly their higher albedo. reflective (cool) roof, a vegetated green roof and PV panels Furthermore, there are studies which examine green-roof elevated above various base roofs. The energy balance models benefits for the building by focusing on building energy savings which were developed and validated were used to estimate due to the soil/plant layer. There is an investigation for the case of sensible fluxes in cities located in six climate zones across US Spain [10]: the energy performance of a building in Madrid was (New York, NY; Los Angeles, CA; Chicago, IL; Houston, TX; simulated and the results verified green-roof benefits for building Minneapolis, St. Paul, Mn; Portland, OR). The results showed energy consumption. That study along with other studies about that by replacing a black roof with a white or green roof green-roof advantages for the building, are analytically presented resulted in a substantial decrease in the total sensible flux.