Renewable and Sustainable Energy Reviews 43 (2015) 264–280

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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 /PV interaction results in PV output increase Available online 25 November 2014 depending on parameters such as plant , 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, 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- 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 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 taxa for green result is the PV output increase. This is attributed to evapotran- roof applications in the midwestern . 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 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- 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. in Section 2.3.1. The benefits in terms of energy savings for a A black membrane roof replaced by a PV-covered white or a simple green-roof building could also regard a PV-green roof PV-covered green roof showed reduction in total sensible flux building but it should be noted that for the specific PV-green case of the order of 50%. these values are expected to be slightly different due to the shading effect of the PV modules. 1 The authors refer to the appropriateness of the plants for PV-green roof Another aspect is the environmental impact of a PV-green roof. applications e.g. in terms of plant density (the density of the foliage), albedo (color In the literature the only Life Cycle Analysis (LCA) study about PV- of the leaves of the plants), etc. The word appropriateness it is specific for the green roofs is that of Lamnatou and Chemisana [11] with emphasis interaction of the plants with the PV panels. 266 Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280

Table 1 Studies about PV-green roofs.

Reference Type of System Region Time period Plant species Findings about PV Systems which were study considered output increase compared

Köhler et al. [2] Experimental Large- Berlin, Germany 5-year data Mainly Sedum species Depending on the PV-green vs. several scale configurationa configurations Hui and Chan [3] Experimental Large- Hong Kong, China Sunny summer day, Sedum 4.3% PV-green vs. PV-bare scale 11–2pm roof Perez et al. [4] Experimental Small- New York, USA June Varietal Sedum 2.56%b PV-green vs. PV-gravel scale Chemisana and Experimental Small- Lleida, Catalonia, June–July , Sedum 1.29% (G. rigens), 3.33% (S. PV-green vs. PV-gravel Lamnatou [5] scale Spain clavatum clavatum) Nagengast et al. [6] Experimental Large- Pittsburg, PA, USA Julyc Mosses 0.5% PV-green vs. PV-black scale Witmer [7] Modeling Different locations 0.08–0.55% in USA Hui and Chan [3] Modeling Hong Kong, China 1 year 8.3% PV-green vs. PV roof mounted

a For some cases of Köhler et al. [2] PV-bitumen roof had higher electrical output than PV-green one because of: cold weather, reflection properties of the specific type of bitumen that was adopted for some systems, overlapping effects (e.g. reflection, tracking), etc. b The collection of the data started May; this percentage (2.56%) regards June [4]. c Reference [6] regards measurements over one-year. Under that cold climate, the considered PV-green roof provided only 0.5% increase in power generation in July, whilst for all the year the PV-black roof outperformed the PV-green one by 0.5%.

Hui and Chan [3] conducted a study (in Hong Kong) which slightly more power than the PV-black one (0.04 kW and included a theoretical as well as an experimental part. Based on 0.03 kW, respectively) when averaged across the entire year the theoretical part, the findings of a year-round building of daylight hours. The results for Phoenix revealed that the PV- energy simulation (by using EnergyPlus), for a low-rise com- green roof out produced the PV-black one by 0.08 kW (1.3%). mercial building, showed that the PV-green roof produced 8.3% In terms of experimental investigations about PV-green roofs: more electricity than the PV roof. It should be noted that a roof- mounted PV roof with a few inches gap was considered and The study of Hui and Chan [3] which was previously cited also thus, the fact that there is no air circulation behind the PVs included an experimental part. Measurements were taken in a means that the difference in yield between that system and the rooftop garden in the University of Hong Kong during a sunny PV-green one is expected to be higher. Sedum was adopted as summer day, from 11 am to 2 pm. Two PV modules were placed the green roof plant. on a bare and a green roof in order to be compared. The PV- Sui and Munemoto [14] proposed a simulation methodology green configuration produced around 4.3% more electricity

for the evaluation of the performance of CO2 Emission (CE) and than the PV on the bare roof (for the experiment the PVs were Investment Value (IV) of the shape of a Green Roof Integrated not stacked on the roof) during the time period of the Photovoltaic System (GRIPVS) of a wooden detached house, by measurements. Regarding plants, Sedum was considered. using genetic algorithm. Three typical locations in Japan were Köhler et al. [2] investigated several PV-green roof systems examined: Sapporo, Tokyo and Naha. Feasible solutions showed based on several configurations, dominated by Sedum species. that GRIPVSs with a larger southern roof area and fully covered The PV-green systems were compared with PV-bitumen ones, with PVS performed the best concerning IV than others while in Berlin. For some cases, the PV-green configurations showed the pitches of GRIPVSs in the given locations should be less increased efficiency while for other cases the PV-bitumen roofs than their local optimal solar absorption pitches. In order to showed higher output, depending on the specific characteris- minimize CE, GRIPVSs with larger roof pitches for enlarging the tics of each system. It should be mentioned that the authors of PVS installation area to generate more electric power and [2] noted that since there were many overlapping effects (such

larger greening area to absorb more CO2 are required. For that as reflection from other PVs, etc.) it would be desirable to study, also Sedum was considered. continue their research and get results from other sites also in Witmer and Brownson [15] developed a model about a PV- order to verify their findings. green roof. That model focused on energy balance and it Furthermore, there is a study about CIGS (Cadmium–Indium– included the microclimate effects. Moreover, Witmer [7] devel- Gallium di-Selenide) PV cylinders combined with a Sedum oped an energy balance model of a green-roof integrated PV green roof [16]. That study was based on the analysis of Penn system. The model was analyzed in a transient system simula- State’s2009“Natural Fusion” home [17]. Gains in performance tion by means of a FORTRAN code base in TRNSYS energy were outlined [16]; nevertheless, in the literature there are no system simulation tool. Simulations for several locations in US specific results about the increase of PV output due to plant/PV showed a small efficiency gain (ranging from 0.08 to 0.55%) in synergy for the above mentioned system. power output. It should be mentioned that the author of that Another PV-green experimental study was conducted in Pitts- study ([7]) noted that further development of that model (in burg (Pennsylvania) [6]: measurements over one year (1-7-2011 terms of experimentation and benchmarking) is necessary in to 30-6-2012), from a large field project in Pittsburgh, were used order to refine the model for regional comparisons. to examine the differences in power output from green and An experimental study about a large field project in Pittburg, black roofs. The results revealed that the PV-green roof, under PA [6] also included some theoretical calculations. Regression those climatic conditions (73% of ambient temperatures o25 1C equations were derived from Pittsburgh data and they were and 90% of solar irradiance values o800 W/m2), can provide applied in other climates. The results for San Diego and only a small positive impact of 0.5% in power generation in July, Huntsville showed that the considered PV-green roof produced whilst for all the year the PV-black roof outperformed the PV- Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280 267

green one by 0.5%. For days with temperatures higher than 25 1C and/or irradiances higher than 800 W/m2, the PV-green roof started to outperform the PV-black one. Finally, it should be mentioned that moss was utilized for that PV-green system. In addition, Perez et al. [4] investigated multiple, small-scale roofing systems: gravel, green, PV-gravel and PV-green, over small model houses, in New York. Sedum species were adopted for the systems. Variability of temperatures inside the gravel- roofed house was found to be 16.5% higher than in the PV- green roof house (June). Variability of surface temperatures on the gravel house were 10.69% higher on the gravel-roof house than the PV-green roof house during the same month. Mean internal and surface temperatures were found to be 5.1% and 1.73% higher on the gravel roof than the PV-green roof, respec- tively and the PV performance had a 2.56% increase (June). Regarding the experimental study of Chemisana and Lamnatou [5], three small-scale, roof configurations: PV-gazania (Gazania rigens)(Fig. 1a: [5]), PV-sedum (Sedum clavatum)(Fig. 1b: [5]) and PV-gravel (reference case) (Fig. 1c: [5]) were developed and tested at the University of Lleida, in Spain (June–July, 2013). Five-day average percentages of maximum power output increase for PV-gazania and PV-sedum were found to be 1.29% and 3.33%, respectively, in comparison with the PV- gravel roofing system. In terms of temperature at 3 cm depth, Gazania green roof showed an average daily value 17.8% (17.5% for five-day period) lower than the gravel roof while Sedum daily difference was 26.1% (25.9% over five days) lower. Sedum kept soil temperature 5.95% cooler (at 3 cm depth). Sedum characteristics improved the effective incident irradiance on the module 1.43% more than Gazania. PV-green roofs also affected the incident irradiance on the PV panel, obtaining higher relative incident irradiances in comparison with the gravel configuration. The differences between the two PV- green roofs are related with factors such as plant type (Gazania: flowers and narrow leaves vs. Sedum: thick, high-water content leaves). Under those climatic conditions, PV-green roofs were proved to be more beneficial than the conventional gravel roof for both, PV module and temperature of the roof surface. The authors noted that further research should be addressed for long-term characterization (e.g. over winter) and large-scale installations.

In Table 1, the above mentioned theoretical and experimental studies are presented. From Table 1, it can be observed that the most ‘pessimistic scenario’ in terms of PV output increase is that of Witmer [7] with an increase of 0.08% (different locations in US) while the most ‘optimistic scenario’ is that of Hui and Chan [3] with 8.3% increase in PV output (Hong Kong).

2.2. Albedo

The plants may positively change the quantity of incident sunlight on the PV modules due to the reflective effect; thus, albedo of the selected plant species is a crucial factor. Following, studies about albedo of several plants are presented. Most of these references regard simple (without PVs) green roofs since there are only a few PV-green roof investigations which examine reflected Fig. 1. PV roofs developed by the authors at the University of Lleida, in Spain: radiation issues. (a) PV-Gazania rigens roof; (b) PV-Sedum clavatum roof; (c) PV-gravel roof Among the PV-green studies is that of Chemisana and Lamna- (reference system) . tou [5] which was previously cited in Section 2.1. Both, Sedum and Source: [5]. Gazania,reflected higher quantity of light to the PV module than gravel, from the sunrise to approximately 3:00 pm (Fig. 2: [5]). the real effect of that difference, the values were referred to the From that moment, gravel layer implied progressively higher irradiance level at that moment obtaining the relative values incident irradiance until the time when a building (which was in depicted in dark colors. In Fig. 2, the relative irradiance difference front of the experimental roofs) shaded the experimental set-up for the PV-green roofs and the PV-gravel system, are illustrated. An and hided the symmetric effect for the sunset. In order to obtain interesting behavior is denoted on the left part of Fig. 2. It can be 268 Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280

35 Gazania vs Gravel. Irradiance relative difference referred to gravel Sedum vs Gravel. Irradiance relative difference referred to gravel 30 Sedum vs Gazania. Irradiance realtive difference referred to Gazania Sedum vs Gazania. Irradiance difference referred to Gazania Sedum vs Gravel. Irradiance difference referred to gravel 25 Gazania vs Gravel. Irradiance difference referred to gravel

20

15

), Relative irradiance difference (%) 10 2

5

0

-5 Irradiance difference (W/m

-10 6:40 7:20 8:00 8:40 9:20 10:00 10:40 11:20 12:00 12:40 13:20 14:00 14:40 15:20 16:00 16:40 17:20 18:00 18:40 19:20 Time (hh:mm)

Fig. 2. Relative irradiance difference: PV-green roofs (Gazania rigens and Sedum clavatum) and PV-gravel roof (reference system). Incident irradiance . Source: [5]. seen that PV green roofs increased the incident irradiance on the summer of 2011–2012 in Melbourne, Australia. The four experi- PV module by up to 32% (maximum) for the Sedum/gravel mental roofs (2.4 m 2.4 m) which were examined: a conven- comparison. This could be related with albedo which is higher at tional steel sheet roof (STEEL), a steel sheet roof covered with the plants as well as with sun position (when the sun loci was white, high albedo paint (WHITE), a vegetated roof (VEG), a roof behind the PV plane, the rays that in a normal situation would not with just the soil layer (no vegetation) (SOIL). WHITE albedo was impact on the module fell on it because the plant canopy acted as a very high (0.71 at solar noon, January 2012). SOIL albedo was low ´diffused reflector´). Sedum was proved to be better in terms of (0.10) due to its dark color. The presence of the lighter colored incident irradiance increase (on the PV module) throughout the vegetation increased the albedo slightly for the case of VEG (0.15). whole day, achieving an average improvement with respect to STEEL albedo was 0.21, resulting in a value of reflected shortwave Gazania of 1.41%. radiation higher than both VEG and SOIL. In terms of the outgoing In addition, the study of Scherba et al. [9] which was also cited longwave radiation, WHITE showed the lowest levels, largely due in Section 2.1 was a modeling study about the impacts of roof to the high albedo of the surface. Despite the higher albedo of reflectivity. As it was previously mentioned, models for multiple STEEL relative to VEG and SOIL, outgoing longwave radiation was roofing systems were developed and validated with data from higher as the STEEL surface heated more easily than VEG and SOIL. Portland (Oregon) while annual models were run for six cities in The influence of the vegetation layer also reduced outgoing long- 5 different climate zones across US. Across all six cities which were wave radiation, since the vegetation itself and the shading of the studied, the black roof and PV-black roof had the highest total soil surface served to reduce surface temperature and thus, daily sensible flux levels (the average value ranged from 331 to emitted longwave radiation. During night, outgoing longwave 405 W/m2). For the case that the black roof was replaced by the radiation of STEEL and WHITE were similar and lower than VEG white or the green roof, the peak flux was reduced by around 70%, and SOIL. while the total daily flux was reduced by around 80% with a white D’Orazio et al. [18] experimentally investigated the yearly roof and 52% with a green roof. When PVs were added to the black thermal performance of a green roof in comparison with other roof, there was negligible impact on peak flux; nevertheless, the passive cooling technologies. All of the roofs were installed on a total flux was reduced from the unshaded black roof levels by real-scale, experimental building in the vicinity of Ancona, in Italy around 11%. Compared to the flux for the unshaded black roof, the (the building was 8.20 10.50 m). The green roof was extensive PV-white roof showed a peak flux reduction of around 40% and a with low/evergreen vegetation (officinalis type). The results total flux reduction of 55%. The PV-green roof had a peak reduction showed 13% albedo for the green configuration, 31% for the clay- of around 45% and a total flux reduction of approximately 42%. The tile roof and 9% for the copper roofing system. For the green green roof showed higher total daily flux than the white roof. The system, the experimental findings revealed that the plant canopy authors noted that this is attributed to green roof thermal mass reflected 13% of the incident global solar radiation and absorbed that prevents roof from cooling below ambient temperatures at 56%, so that the solar radiation entered the system could be then night. Thus, green roof flux was usually positive while black and estimate as 31% of the incident global solar radiation. white roofs had negative fluxes at night. MacIvor et al. [19] studied the performance of dryland and Regarding investigations about simple (without PVs) green wetland plants on green roofs. The experimental study was roofs, Coutts et al. [8] compared insulating properties, radiation conducted over two growing seasons (2007–2008) on top of the budget and surface energy balance of four experimental rooftops, Patrick Power Library (Saint Mary’s University) in Halifax, Nova including an extensive green roof (Sedum) and a cool roof (unin- Scotia, Canada. Albedo values ranged from 15.81% (Empetrum sulated rooftop coated with white elastomeric paint), during the nigrum)to19.12%(Sibbaldiopsis tridentata; Kalmia polifolia) Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280 269 depending on the plant species. In general, albedo values tended conventional roof. The green-roof subsystems were modeled by to increase with increasing diversity of dryland plants, but means of ESP-r by taking into account the thermal properties of decreased with increasing the number of wetland species. the components provided by the manufacturers. Simulation of Yujiro and Toshiaki [20] investigated the evaporative cooling annual and peak energy consumption of the building with the effect of rooftop vegetation. The surface heat budget and several green roofs was conducted and compared to that of the reference related parameters regarding a rooftop of a three-story office building. building planted with Sedum were measured (two clear-sky days In addition, Wong et al. [22] used DOE-2 energy simulation in September, Japan). The results showed that albedo and emis- program in order to determine the effects of rooftop garden (turf, sivity of Sedum were 0.153 and 0.995, respectively. shrubs and trees) on the annual energy consumption, cooling load Gaffin et al. [21] developed a green-roof environmental mon- and roof thermal transfer value of a five-storey, hypothetical itoring and meteorological network in New York. Mix of Sedum commercial building in Singapore. The installation of rooftop species was adopted for the experimental systems. The reported garden on that building resulted in savings of 0.6–14.5% in the albedo values (July 2008; Bronx, NY green roof network site) were annual energy consumption while shrubs were found to be the analyzed. A pronounced diurnal U-shaped cycle for albedo was most effective plants in reducing building energy consumption. observed. Albedo was minimum at noon when incidence angle Moreover, the results showed that the increase of soil thickness was minimized. The authors noted that the U-shape was largely would further reduce building energy consumption. Furthermore, due to the change in solar incidence angle during the day, affecting the presence of the plants on the typical roof also reduced the heat surface reflectivity; nevertheless, other factors including probably gain into the building; however, the reduction was less significant leaf responses may be playing a role. The time averaged albedo for than that caused by the installation of a rooftop garden on the the July data was found to be 19.6%. exposed roof. The annual energy consumption for the assumed From the above mentioned studies it can be seen that albedo is building was calculated to be 200 MW h (for the exposed roof). a crucial factor and it is associated with parameters such as leaf This value showed: (1) 19 MW h (10%) reduction for 100%-turf color and soil cover percentage. Leaves with lighter color show covering, (2) 29 MW h (15%) reduction for 100%-shrub covering. higher albedo. On the other hand, green roofs with high percen- The study of Ascione et al. [23] also verifies green-roof benefits tage of bare soil exposed are not desirable because are expected to for the building. More specifically, that study was about green show low reflectivity. Thereby, for the PV-green roofs it is roofs in Europe and their potential for energy savings in building important to maintain almost total plant coverage in order to air conditioning. Several green roofs were studied: Sedum and achieve high reflectivity and thus, greater benefit for the PV gramineous (short and tall height), grass lawn. The results modules. High infrared reflectivity of the plants is very promising revealed that in warm climates green roofs are suitable for for keeping PV cells cool while still supplying a large part of the reducing the energy demand for space cooling (without penalizing useful spectrum to the PV solar cells. In general, green roofs have the scarce heating demand): the annual reduction of the primary high albedo values and they provide several advantages in energy ranged between 1% and 11% for Tenerife, 0% and 11% for comparison with the conventional, cool-roof technologies [1]. Sevilla, 2% and 8% for Rome. In cold climates green roofs are useful for reducing energy demand for space cooling but also in order to 2.3. Benefits of the PV-green roof for the building during its decrease winter heating needs (for example for Amsterdam and operational phase London, the annual savings ranged between 4% and 7%). In addition, in the review article of Castleton et al. [24] the 2.3.1. Benefits due to the soil/plant layer benefits and important issues about green roofs were presented. In the literature there are no PV-green roof studies which Among these the following aspects were highlighted: green roofs examine the PV-green benefits for building energy consumption. can significantly reduce energy use in buildings with poor insula- Thus, in the present paragraph investigations about simple (with- tion values (in terms of summer cooling as well as winter heating); out PVs) green roofs which verify the benefits of the soil/plant for modern buildings with high U-values associated with better layer for building energy needs are presented. roof insulation, green roofs could save small (or zero) amount of There is a study about green-roof benefits for the climatic energy; thicker soil substrate on the roof means higher reduction conditions of Spain [10]. In the frame of that study, the energy of the heat gain/loss into/out of the building; a less dense soil has performance of an eight-storey, residential building in Madrid, more air pockets and thus, it is a better insulator for the building. with flat roof accounting for 17% (677 m2) of the external envel- The main findings of the above mentioned studies are pre- ope, was simulated. Three different roofs: a common flat roof, a sented in Table 2. It can be seen that the energy savings because of green roof and a green roof with water storage capacity were the soil/plant layer, depend on several factors (climatic conditions, compared. In terms of the plants, Sedum sempervivum, Opuntia etc.). More specifically, the studies reveal that considerable savings aciculata (cactus), Larrea divaricata (desert shrub) were adopted. for cooling of a building during summer under Mediterranean Green roof without water storage capacity and Aljibe green roof climatic conditions such as in Spain could be achieved. On the with water storage capacity were considered. The results showed a other hand, at this point it should be noted that the energy savings slight reduction in energy consumption for heating for both green which are reported in the above mentioned studies are for simple roofs. The reduction in annual energy consumption for heating (without PVs) green roofs. For the specific case of the PV-green was 0.12% for the simple green roof and 0.2% for the Aljibe roof. roofs, these values are expected to be slightly different given the However, the studied roofs offered a considerable reduction in the fact that the PV panels shade the roof of the building. annual energy consumption for cooling and peak energy con- Finally, it should be mentioned that thermal inertia which is sumption on the hottest day of the summer. More analytically, the provided to the building by the soil/plant layer (because of leaf and annual energy consumption for the simple green roof and the soil water content) is another important issue. Thermal inertia is a Aljibe roof dropped by 6.2% and 6.4%, respectively, while the total “climate moderator” and it is related with the attenuation of energy consumption for the peak day dropped by 12% for both roof temperature fluctuations. This benefit could be valuable for configurations. In terms of the adopted method, the building was example when phase change materials are used for thermal defined in ESP-r (according to actual project plans and specifica- storage in a building. Roof is one of the most sensitive building tions). Simulation of annual and peak energy consumption was elements during warm months of the Mediterranean climate due conducted for the base case or reference building with a to the fact that it is exposed to solar radiation for the greatest part 270 Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280

Table 2 Studies about green-roof benefits during building operational phase.

Reference Type of Plant species Building Region Findings about energy savings Additional comments study

Alcazar Modeling Sedum, cactus, 8-storey, residential with Madrid, Reduction of annual energy consumption: Common flat roof vs. green roofs (with/ and shrubs flat roof 677 m2 Spain 6.2% for the simple green roof; 6.4% for the without water storage) Bass green roof with water storage [10] Ascione Modeling Sedum, Office building: maximal Europe Annual reduction of primary energy: 1–11% For warm climates, green roofs are et al. gramineous floor dimensions: (Spain and for Tenerife; 0–11% for Sevilla; 2–8% for suitable for reducing energy demand for [23] (short/tall 70.7 m 17.7 m; overall other Rome space coolinga height), grass height: 4.3 m countries) lawn Wong Modeling 5-story hypothetical Singapore Rooftop garden led to 17–79% reduction in Thermal resistances (R-values) of turf/ et al. commercial building space cooling, 17–70% in peak space load; shrubs/trees were estimated by data [22] cost savings were also found from measurements Castleton Review Sedum, etc. Several configurations Several Green roofs-significant reduction of Thicker soil substrate-higher reduction et al. article regions energy use in buildings with poor of heat gain/loss into/out of building; less [24] insulation (for summer cooling as well as dense soil-more air pockets-better for winter heating) insulator

a Study [23] showed that for the cold climates green roofs are useful for reducing the energy demand for the space cooling but also for decreasing winter heating needs (e.g. for Amsterdam and London, the annual savings ranged between 4 and 7%). of the day. The soil/plant layer on the roof can reduce the rate at (extensive and intensive), gravel) while emphasis was given on which indoor temperature rises and drops. During summer the PV-green roof and its comparison with the PV-gravel one. For months, by means of a green roof, a delay of cooling load peak the case of the PV-green configuration, different scenarios (based (and thus, a reduction of air-conditioning energy consumption) on certain literature references: Table 1) in terms of PV output can be achieved. On the other hand, during the winter season the increase (because of plant/PV interaction) were adopted. The energy which is available from the sun (solar gain) is stored and results (based on three different Life Cycle Impact Assessment then, it is slowly released in the interior space of the building. At (LCIA) methodologies: Ecoinvent 99 (EI99), IMPACT 2002þ and this point it should be noted that there are some plants with Cumulative Energy Demand (CED)) revealed that although the PV- considerable thermal inertia. For example, a thick, succulent green roof it had an additional impact (because of its components/ canopy can positively contribute to roof thermal inertia. materials related with soil and plants) in comparison with the PV- gravel roof, this additional impact on a long-term basis (during building operational phase) can be compensated. 2.3.2. Benefits due to plant/PV interaction: PV-green vs. simple, green Based on the above mentioned study [11],inFig. 3(a) the total configuration impact points (for all the considered phases) by the LCIA metho- Based on experimental studies about PV-green roofs [2–6], the dology IMPACT 2002þ (based on the four damage categories: benefits of a PV-green roof in comparison with a simple (without human health, ecosystem quality, climate change, resources) for PVs) green roof are multiple. These benefits are following the PV-green and the PV-gravel roof are illustrated. It can be seen presented: that the PV-green case has an additional impact, as it was expected, due to its additional components of the soil/plant layer. A basic advantage of a PV-green roof is related with the in situ However, in Fig. 3(b), where the total impact points per kWh of production of electricity [2–6] by means of an environmentally produced electricity (based on IMPACT 2002þ, damage cate- friendly technology. In this way, all or part of building energy gories) are shown, it can be observed that the PV-green roof (on needs can be covered. Moreover, the electricity generated by a long-term basis) compensates its additional impact. More the PVs could be utilized to power the water pumps for plant specifically, Fig. 3(b) regards several scenarios (based on certain irrigation [3]. literature references of Table 1) in terms of the PV output increase PVs shade roof; thus, soil temperature is reduced [3,5]. of the PV-green configuration (the PV-gravel impact points/kWh The benefit due to the higher electricity production from a PV- are used as reference and they are represented as a straight line green roof could offset the cost for the “green part” (related since the adopted scenarios regard only the PV-green case). From with soil/plant layer) of the roof installation [2]. Fig. 3(b) it can be seen that after around 6.2% increase of PV By means of a PV-green system there is better utilization of the output, PV-green system compensates its additional environmen- available space of a roof [3]. tal impact and it becomes more environmentally friendly than the With a PV-green configuration, during winter the plants (due PV-gravel one. to their thermal capacity) protect the PV panels from the cold. On the other hand, PVs protect plants from direct exposure to 2.5. Additional benefits of the PV-green roofs sunlight and in this way plant growth and plant species variety are enhanced [4,5]. Following two additional benefits of the PV-green roofs in comparison with the simple (without soil/plant layer) PV roofs are 2.4. Environmental impact: PV-green roof vs. PV-gravel roof presented:

For the evaluation of the environmental impact of a product, (1) Carbon sequestration

LCA is a useful tool. However, as it was mentioned in the An additional advantage of PV-green roofs, from CO2-reduc- introduction, in the literature the only LCA study about PV-green tion point of view, is their potential for carbon sequestration roofing systems is that of Lamnatou and Chemisana [11]: several by the soil/plant layer. The green roofs can fix carbon in plants roofs were examined (PV-green (extensive), PV-gravel, green and soil. The CO2 captured by the plants is the result of Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280 271

Fig. 3. PV-green (extensive) vs. PV-gravel roof (LCIA methodology: IMPACT 2002þ, damage categories): (a) total impact points for stages of the phases material manufacturing, transportation, use phase and disposal, (b) impact points per kWh of produced electricity . Source: [11].

differences between atmospheric CO absorbed during photo- Specialization/training for the installation. 2 synthesis and CO2 emitted to the atmosphere during respira- Selection of low-cost plant species. tion. This difference is converted into biomass [25]. (2) Urban agriculture Another aspect, very important for the case of the PV-green For the case that food crops are selected for the green space of roofs is the increase of PV efficiency (Table 1). PV efficiency the building, an additional advantage has to do with the improvement should be considered as an additional advantage avoided CO2 emissions due to the transportation of the food which can increase the profitability of a PV-green roof investment from the food production area to the consumer. Food produc- along with all other advantages that PV-green roofs offer. The tion in cities has not only environmental but also economic improvement of PV efficiency and the benefits due to the green and social benefits [25]. Certainly, the selected food crops (soil/plant) layer which protects the building could be consider- should fulfill certain criteria. These criteria are analytically ably beneficial on a long-term basis, from environmental as well as presented in Section 2.7. from economic point of view. The increase of PV output due to plant/PV synergy has been proved to be considerable e.g. during warm months of the Mediterranean climate [5]. 2.6. Improvement of PV-green roof cost-effectiveness

2.7. Selection of appropriate plant species for PV-green roofs In terms of PV-green roof cost-effectiveness, subsidies from the government for the promotion of green roof construction could In this section, a systematic classification of several Mediterra- provide benefits and considerable reduction of the initial invest- nean plants appropriate for PV-green roof applications has been ment cost. Other options for further cost-effectiveness improve- done. The criteria and the scores for each plant are presented in ment could include: tables. Most of the data of the tables are based on certain references while some of them are derived based on the specific Reduction of the construction cost through further cost reduc- characteristics of each plant. For each plant, critical comments in tions among the industrial companies terms of its appropriateness for PV-green (or simple green) roof Inclusion of the social costs/benefits. applications, are made. Reusing some of the waste materials. Innovative policies. Inclusion of air pollution mitigation technologies. 2.7.1. Materials and methods Partially transfer of the social benefits to the investors. 2.7.1.1. Scores and weighting factors. In the tables, the criteria are Use of eco-friendly fertilizers. presented aggregated into 6 categories: (1) Suitability for Adoption of environmentally-friendly and cost-effective dispo- extensive green roofs; (2) resistance to weather conditions; sal systems. (3) interaction with the PVs; (4) interaction with the building; Standardization/certification of green roof products. (5) interaction with the external environment; (6) other factors. 272 Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280

A score for each criterion, ranging from 1 to 3 (1¼low (worst soil/plant layer, their high weight (weight of the whole system) [1] as option); 2¼medium; 3¼high (best option)) is given. In this way, a well as with aesthetic/building integration factors [5]. total score for each plant is derived. Higher total score means greater appropriateness for PV-green roof applications. In 2.7.1.2. Selected criteria addition, the data are further elaborated by adding weighting factors and by adopting the following assumptions: for the plant/ 1) Suitability for extensive green roofs (12 points maximum) PV and the plant/building interaction, 30% weighting factor is In this category, criteria which are related with: (a) root system: assumed, for each of these two criteria. For the other four criteria shallow roots are needed since the growing medium for exten- (suitability for extensive green roofs; resistance to weather sive green roof applications should be r20 cm depending on conditions; interaction with the external environment; other the selected plant [26];(b)nutrient/irrigation requirements which factors) this weighting factor is considered to be 10% for each should be low (low-maintenance plants), the plants should be one of these four criteria. In this way, the importance of plant/PV selected for their ability to thrive with minimal to no inputs and plant/building interaction is taken into account. Following the (water, fertilizers, etc.) after establishment [27];(c)disease/pest selected criteria are analytically presented. resistance: plants with no severe pest susceptibility are needed These criteria regard the extensive (shallow-substrate systems) [27]. Certainly, it is important this category to have very high and not the intensive (deep-substrate systems) green roofs because total score because first of all the selected plants should be the extensive configurations are more appropriate for PV-green roof appropriate for extensive green roof applications. applications [5]. The fact that the intensive green roofs are less 2) Resistance to weather conditions (12 points maximum) appropriate is related with factors such as their high height of the In the frame of this category, the resistance to: (a) direct solar

Table 3 Criteria for the plants: Gazania rigens, Asteriscus maritimus and pubescens.

Gazania rigens Asteriscus maritimus Coreopsis pubescens Criteria Comments/Refs. Comments/Refs. Comments/Refs.

1. Suitability for extensive green roofs Shallow roots G. rigens “Sun Gold”: shallow rooting plant [33] 3 It grows in rocky, coastal 3 Coreopsis: deep-rooted [44] 1 Mediterranean regions [40] Nutrient needs Low [34] 3 Maintenance: low [41] 3 Coreopsis spp.: low food needs [45] 3 Irrigation needs Low [34] 3 Low (it is drought tolerant [34])3Coreopsis spp.: low water needs [45] 3 Disease/pest Long-term health usually not affected by pests [35] 3 No known serious insect or disease 3 Coreopsis spp.: no serious pests are 3 resistance problems [41] normally seen [46] Score 12 12 10 2. Resistance to weather conditions Resistance to direct Heat tolerant; full sun [34] 3 Full sun; heat tolerant [34] 3 Coreopsis spp.: plant grows in full sun 3 radiation [46] Resistance to drought Drought/heat tolerant [34] 3 Drought/heat tolerant [34] 3 Coreopsis spp.: high drought tolerance 3 [46] Resistance to frost Medium [36] 2 Medium [42] 2 Medium (it can take some cold weather 2 [45]) Resistance to wind H–DFa 3H–DF 3 H–DF 3 Score 11 11 11 3. Interaction with the PVs Albedo (is related Greyish/whitish leaves [37] 3 Grey-leaved [40] 3 C. pubescens ‘Sunshine Superman’: 3 with leaf color) medium green leaves [47] ET Hairy leaves [37] 1 A. maritimus cv. Gold Coin has hairy 1 Stems with hairs [48] 1 leaves [42] Height 15–30 cm [34] 315–30 cm [41] 3 Coreopsis pubescens Ell. var. debilis: 25– 2 50 cm [49] Compactness Compact series [36]; compact [5] 3 Compact [41] 3 ‘Sunshine Superman’: compact, 25– 3 30 cm tall [47] Score 10 10 9 4. Interaction with the building Leaf water content Non-succulent 1 Non-succulent 1 Non-succulent 1 Dense foliage Leaves densely clustered along the stems [37] 3 The leaves appear in dense clusters 3 C. pubescens Ell.var. debilis: plant body: 3 (insulation) [42] dense [49] Score 44 4 5. Interaction with the external environment Red. urb. temp.; Abs. H–DF 3 H–DF 3 H–DF 3 s-w; C seq.b Attr. i/p, incr. urb. Gazania: visited by honey bees/solitary bees [38];it 3 Attracts butterflies [41] 3 Coreopsis: visited by honeybees/solitary 3 biod c attracts butterflies [36] bees [38] Score 66 6 6. Other factors Acoustic benefits H–DF 3 H–DF 3 H–DF 3 Interest in medicine, Honey, medicinal, food (edible flowers) [39] 3 Anti-insect activity [43] 3 Coreopsis spp.: dyes [50] 3 etc. Aesthetics High [32,36,37] 3 High [41,42] 3 High [45,47] 3 Score 99 9 Total score 52 52 49

a H–DF¼It is expected to be high because the foliage is dense. b Red. urb. temp.; Abs. s-w; C seq.¼Reduction of urban temperature; ability to absorb storm water; carbon sequestration. c Attr. i/p, incr. urb. biod.¼Attraction of insects/pollinators, increase of urban biodiversity Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280 273

radiation, (b) drought, (c) frost, (d) wind, are included [1]. These convective heat losses given the fact that it has the ability to parameters are crucial for the survival of the plants on a roof “enclose” air volumes and thus, to act as insulation layer. In this where extreme weather conditions can occur. As it was way, plants with a good interaction with the building can lead previously mentioned (Section 2.3.2), the plants protect the to energy savings. Studies which verify green roof benefits for PVs from winter frost while during summer the PVs protect the building were previously presented (Section 2.3.1 and plants from direct exposure at solar radiation [5]. Finally, it Table 2). should be noted that for the case of the Mediterranean climate, 5) Interaction with the external environment (6 points maximum) e.g. for Spain, since it is characterized by dry summers with In this category, the first group of parameters regards the high temperatures, it is important the selection of drought- ability of the green roof to: reduce urban temperature and tolerant plant species [5]. thus, mitigate the heat island effect, absorb storm water and 3) Interaction with the PVs (12 points maximum) mitigate air pollution by means of carbon sequestration [1]. The A relevant criterion is albedo and it is associated with factors second group of factors has to do with the attraction of insects/ such as leaf color. The importance of albedo is related with the pollinators and the increase of urban biodiversity [1]. fact that the part of solar radiation which is reflected and it 6) Other factors (9 points maximum) reaches PV module surface, it leads to PVs output increase. For Plant canopy has the potential to reduce noise pollution [1] and the PV-green roofs it is important to maintain almost total this is an additional advantage. A rating of 3 is given to the plant coverage with light-colored plant canopy in order to plants which have dense foliage (which means that these achieve higher reflectivity and thus, greater benefit for the PVs plants are expected to provide high acoustic protection for and the building [5,9]. the building). On the other hand, some plants have great On the other hand, another criterion is ET because it increases interest in medicine and in other sectors (food industry, etc.) cooling effect and thus, PV electrical efficiency. Weather and which means that for these cases the green roof can also offer microclimate ET is the combination of evaporation of water agricultural and other products. Finally, the aesthetic is another from the soil and transpiration from the plants while a aspect [1] and it is also taken into account. potential ET can be adjusted to a particular type of plant, plant population, plant growth stage, vigor and stress. In terms of the drought-tolerant plants, they go dormant or near dormant 2.7.2. Results and discussion when soil water is unavailable and then, become active when water is available [28]. From the above mentioned it can be 1) Compositae () seen that the relationship drought-tolerant plant/ET/water use Gazania rigens is complicated. During the PV-green roof experiments of the Gazania rigens is a plant appropriate for decorations [32]. authors Chemisana and Lamnatou [5], a wet irrigation regime In Table 3, the scores for this plant are shown and it can be seen (wet irrigation regime means prevention of moisture deficit: that G. rigens is a very appropriate plant for PV-green roofs since it [29]) was adopted given the high summer temperatures and shows very good interaction with the building and the PV panels. the high irradiation of Lleida. In general, for green roofs during The appropriateness of this plant for PV-green roof applications warm months of Mediterranean climate, it is desirable the has been also confirmed by the experimental study of Chemisana adoption of a wet irrigation regime in order to increase the and Lamnatou [5]: G. rigens plants were transplanted in a box cooling effect of the soil/plant layer. In the frame of the present 0.9 1.3 m2 which contained soil substrate (10 cm height) and study, in order to facilitate the classification of the selected this small-scale, extensive green roof was combined with a plants from ET point of view, a rating of 1 is given to the plants polycrystalline silicon (p-Si), PV panel (Fig. 1a). The plant showed with “hairy” leaves since this type of leaves have reduced a way of growth ideal for the placement of the PV panels above its transpiration [30] and a rating of 3 is given to the plants with foliage because G. rigens spread over the ground and it formed a glossy leaves because the glossiness is associated with very dense canopy with low height. In this way, the plant fitted increased cuticular transpiration [31]. perfectly under the PVs and at the same time it created a Finally, other criteria regard the selection of plants with low- “protective” layer over the building. The results revealed that height/compact canopies. These criteria are also critical since there is a good interaction between the plants and the PVs the plants should fit perfectly at the space below the PV panels. (details about the results of that study were previously presented A canopy with greater height can create practical problems in section 2.1). which are associated with: (1) the placement of the PVs, (2) the Conclusively, G. rigens is a very promising plant for the develop- partially shading of the PV panels because of the presence of ment of PV-green roofs and it provides considerable benefits the plants. Thus, only low-height/compact canopies are appro- during warm months of the Mediterranean climate [5].Finally,it priate for PV-green roofs [5]. For facilitating the classification of should be mentioned that this plant has high aesthetic value: the selected plants, a rating of 3 is given to the plants with flowers of multiple colors depending on the variety. Moreover, average height less than 25 cm, a rating of 2 is given to the there are some varieties with silvery/white leaves [32] (good species which have an average height from 25 to 45 cm and a interaction with the PVs from albedo point of view). In general rating of 1 is given to the plants with average height greater terms, Gazania is strongly suggested as plant for decorations of than 45 cm. The selection of the height of 45 cm as the upper indoor and outdoor environment (ground covers, rock gardens, acceptable limit is based on the assumption that the PV panels etc.) providing a wide variety of beautiful colored flowers [32]. could be placed at around 50 cm above roof surface. 4) Interaction with the building (6 points maximum) Asteriscus maritimus The first issue regards the water content of the leaves because it Asteriscus maritimus is a native plant of the lands surrounding the influences roof thermal inertia. Succulent plants with thick, Mediterranean Sea and it is common in Spain [43].InTable 3,the high-water content leaves and dense foliage can positively criteria/scores for this plant are presented. The results reveal that contribute to roof thermal inertia. Thus, a rating of 1 is given this plant shows very good interaction with the building because to the plants with non-succulent leaves while a rating of 3 is of its dense foliage. From PV-green point of view, the interaction given to the succulent plant species. The second factor has to do plant/PV is also good and this is mainly related with the fact that with the density of the foliage. A dense foliage can also reduce this plant has a relatively low height and it is compact. In 274 Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280

addition, A. maritimus is a low-maintenance plant with high the PV-green roofs, in general, C. pubescens is not an appro- resistance to severe weather conditions and thus, it could be a priate plant. However, a compact variety such as ‘Sunshine very good choice for the development of PV-green roofs (even in Superman’ (25–30 cm height [47]) could be a possible solu- coastal areas with sandy soil; it is also called “Sea aster” or “Sea tion, keeping in mind that there are other plants which are daisy”: [34]) adding a high aesthetic value to the building with its more appropriate for PV-green roofing systems. impressive flowering. 2) Coreopsis pubescens Rosmarinus officinalis Coreopsis pubescens is an herbaceous with Rosmarinus officinalis is an evergreen shrub with intensely daisy-like flowers [46]. Based on the results of Table 3,this fragrant foliage [51]. It is a native plant of the Mediterranean plant is more appropriate for semi-intensive green roof regions, especially of Spain and Portugal [52]. As it can be seen applications because its root is not shallow. This plant has from Table 4, R. officinalis is more appropriate for simple good interaction with the building (because of its dense (without PVs) semi-intensive green roof applications (because foliage) while it also has other advantages (low-mainte- its root is not shallow), providing that appropriate growing nance/resistant plant, attraction of bees, etc.). For the case of conditions are fulfilled. It is a plant with dense foliage and

Table 4 Criteria for the plants: Rosmarinus officinalis, Origanum vulgare and maculatum.

Rosmarinus officinalis Origanum vulgare Lamium maculatum Criteria Comments/Refs. Comments/Refs. Comments/Refs.

1. Suitability for extensive green roofs Shallow roots Minimum root depth: 35 cm [53] 1 Origanum: root depth: 1 L. maculatum: shallow root system [58] 3 30 cm [56] Nutrient needs Responds well to additional applications of 2 Low [57] 3 Low [59] 3 nitrogen [54] Irrigation needs The plants should not dry out completely 2 Water: dry to medium 3 Medium [59] 2 [54] [57] Disease/pest Rosemary is vulnerable to spider mites, 2 No serious insect or 3 Mid with no disease problems [59] 3 resistance mealybugs, whiteflies and thrips [54] disease problems [57] Score 7 10 11 2. Resistance to weather conditions Resistance to It needs full sun [54] 3 Full sun [57] 3 Medium [59] 2 direct radiation Resistance to Mature plants can cope with dryland 2 It tolerates drought 3 Low [59] 1 drought conditions if rainfall 4500 mm/year [54] [57] Resistance to It can tolerate frost [54] 3 It is grown in 3 Thrives in many climates including the cold mountain [60] 3 frost mountainous areas [56] Resistance to H–DF 3 H–DF 3 H–DF 3 wind Score 11 12 9 3. Interaction with the PVs Albedo (is The leaves are dark green above and downy 3 Leaves: dark green 2 Silver-variegated leaves [59] 3 related with white below [54] [57] leaf color) ET Hairy leaves [55] 1 Hairy leaves [56] 1 Hairy leaves [61] 1 Height 1–2m[54] 130–60 cm [56,57] 215–20 cm [59] 3 Compactness Medium 2 High 3 High 3 Score 7 8 10 4. Interaction with the building Leaf water Non-succulent 1 Non-succulent 1 Non-succulent 1 content Dense foliage The foliage is dense [52] 3 It shoots spriggy [56] 3 Texture: thick density [59] 3 (insulation) Score 4 4 4 5. Interaction with the external environment Red. urb. temp.; H–DF 3 H–DF 3 H–DF 3 Abs. s-w; C seq. Attr. i/p, incr. It is visited by bees/bumblebees [38] 3 It is visited by bees/ 3 L. maculatum and other Lamium species: important for bees and 3 urb. biod small Apoidea [38] bumblebees; great importance as nectariferous plants [38] Score 6 6 6 6. Other factors Acoustic benefits H–DF 3 H–DF 3 H–DF 3 Interest in Food and flavoring, industrial uses, 3 Food, pharmaceutical 3 Lavender, groundcover [59] 3 medicine, etc. pharmaceutical/therapeutic, cosmetics, etc. uses, etc. [56] [54] Aesthetics High [52,54] 3 High [56,57] 3 High [59,60] 3 Score 9 9 9 Total score 44 49 49 Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280 275

great interest in medicine, cosmetics and food industry [54]. Lamium maculatum Based on Table 4, R. officinalis has a very good interaction with Lamium maculatum is a flowering plant and Table 4 shows that the building and the external environment (increase of biodi- theoretically it could be used for the development of PV-green versity and other benefits). It is a very melliferous plant, roofs because it is compact, low-height and some varieties have frequently visited by bees and it provides a clear unifloral silvery/white leaves (good interaction with the PVs from albedo honey, with delicate odor, flavor and fine crystallization in the point of view). In addition, plant foliage is dense [59] and thus, Mediterranean area [38]. the interaction plant/building is also very good. However, L. maculatum has low tolerance to direct radiation and drought. Origanum vulgare Origanum vulgare is an herbaceous perennial with specific 3) (Leguminosae) blooming period [57]. From Table 4 it can be observed that O. From Table 5 it can be observed that the studied plants from vulgare is a plant more appropriate for the development of Fabaceae are not appropriate for PV-green roof (and in general semi-intensive green roofs (due to its deep root). It has dense for green roof) applications for several reasons (for example foliage and it shows good interaction with the building and the they have low/medium resistance to drought). external environment. Other positive aspects are related to its applications in food, pharmaceuticals, etc. For the case of PV- 4) Other plant families green roofs this plant is not suitable; however, O. vulgare can be a) considered for the development of simple (without PVs) green Sedum clavatum roofs in the Mediterranean region since it is a resistant plant Sedum clavatum is a leaf succulent plant. The drought and it can provide multiple advantages. tolerance of the succulent plants makes them useful for

Table 5 Criteria for the plants: Trifolium dubium, and Trifolium rubens.

Trifolium dubium Trifolium repens Trifolium rubens Criteria Comments/Refs. Comments/Refs. Comments/Refs.

1. Suitability for extensive green roofs Shallow roots It has a very well developed root system on a 3 T. repens: shallow-rooted [67] 3 Red (T. rubens): thick tap root 1 relatively small area of soil [62] grows 60–90 cm/year [72] Nutrient needs Low [63] 3 Ladino (T. repens) has high requirement 2 It grows on a wide variety of soil 3 of nutrients [68] conditions [72] Irrigation needs T. dubium Sibth.: low to medium tolerance to 2 T. repens needs moist to heavy-moisture 2 Water requirements: average [73] 2 prolonged drought [64] conditions [68] Disease/pest Infected by scorch caused by Kabatiella caulivora 2 Medium [69] 2 There are pest-resistant varieties [74] 3 resistance [65] Score 10 9 9 2. Resistance to weather conditions Resistance to direct Full sun [66] 3 Medium [70] 2 Full sun [73] 3 radiation Resistance to T. dubium Sibth.: low to medium [64] 2 It tolerates moderate drought [70] 2 Water requirements: average [73] 2 drought Resistance to frost High [64] 3 High [70] 3 High [74] 3 Resistance to wind It is expected to be low because the foliage is 1H–DF 3 H–DF 3 sparse Score 9 10 11 3. Interaction with the PVs Albedo (is related Albedo is expected to be low because the foliage 1 Light green stems [71] 3 The leaves have silver hairs [73] 3 with leaf color) is sparse ET Transpiration is expected to be low because the 1 Hairless leaves/stems [71] 3 Hairy leaves [73] 1 canopy has low density Height 30 cm [62] 215cm[71] 345–60 cm [73] 1 Compactness Low 1 High 3 Medium 2 Score 5 12 7 4. Interaction with the building Leaf water content Non-succulent 1 Non-succulent 1 Non-succulent 1 Dense foliage The foliage is not dense 1 Dense foliage [70] 3 Dense foliage [74] 3 (insulation) Score 2 4 4 5. Interaction with the external environment Red. urb. temp.; It is expected to be low because the foliage is 1H–DF 3 H–DF 3 Abs. s-w; C seq. sparse Attr. i/p, incr. urb. Trifolium: European unifloral honeys- 3 Trifolium: European unifloral honeys- 3 Trifolium: European unifloral honeys- 3 biod nectanirefous species [38] nectanirefous species [38] nectanirefous species [38] Score 4 6 6 6. Other factors Acoustic benefits It is expected to be low because the foliage is 1H–DF 3 H–DF 3 sparse Interest in Lawn [66] 3 Hay; silage [70] 3 Forage [74] 3 medicine, etc. Aesthetics Medium 2 Medium 2 High [73,74] 3 Score 6 8 9 Total score 36 49 46 276 Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280

Table 6 Criteria for the plants: Sedum clavatum, maritima, Dianthus fruticosus, Geranium molle and Cynodon dactylon.

S. clavatum L. maritima D. fruticosus G. molle C. dactylon Criteria Comments/Refs. Comments/Refs. Comments/Refs. Comments/Refs. Comments/Refs.

1. Suitability for extensive green roofs Shallow roots Sedum/green roof: 3 Shallow root 3 Sufficient growth: 7.5-cm 3 Shallow root [86] 3 Sallow roots [88] 3 6 cm media depth [13] system [77] soil [81] Nutrient needs Low [13] 3 Light fertilization 2 Dianthus: medium [82] 2 It tolerates a wide range 3 It needs fertilizing to keep 2 [77] of soil types [86] high turf quality [89] Irrigation Low [13] 3 Medium [77,78] 2 Dianthus: water average [82] 2 It requires dry soils [86] 3 Water requirements 2 needs depending on the use [89] Disease/pest Sedum spp: powdery 3 No disease or pest 3 Dianthus: insect/fungal 1 Resistant [87] 3 Pest problems [89] 2 resistance mildew- resistant problems [77] attacks [83] [75] Score 12 10 8 12 9 2. Resistance to weather conditions Resistance to High [1,13] 3 Full sun to partial 2 Dianthus: sun to part shade 2 It requires plenty of 3 High [89] 3 direct shade [77,78] [82] sunlight [86] radiation Resistance to High [13] 3 Medium [77,78] 2 Medium [82] 2 It requires dry soils [86] 3 Medium [89] 2 drought Resistance to It is tolerant of extreme 3 Medium [77,78] 2 High [82] 3 It is found in North 3 Medium [89] 2 frost temperatures [13] America, British Columbia, etc. [86] Resistance to High [13] 3H–DF 3 High because of the strong 3H–DF 3 It is expected to be medium 2 wind fibrous root system [81] because the foliage is not dense Score 12 9 10 12 9 3. Interaction with the PVs Albedo (is High [5,20] 3 Grey-green leaves 3 Glaucous leaves [84] 3 Dull green [86] 3 Dark green [89] 2 related with [78] leaf color) ET limit water 2 Stems with hairs 1 Glabrous leaves [84] 3 Hairy leaves/stems [86] 1 Green roof study: grass 3 loss due to [79] ET4Sedum [90] transpiration [13] Height Low (r10 cm) [5] 310cm[77] 350cm[81] 1 Branched stems: 10– 3 Upright shoots: 15–25 cm 3 40 cm [86] [88] Compactness High [5] 3 High [77,78] 3 Medium 2 Medium 2 High 3 Score 11 10 9 9 11 4. Interaction with the building Leaf water Succulent [1,13] 3 Non-succulent 1 Non-succulent 1 Non-succulent 1 Non-succulent 1 content Dense foliage Dense foliage [5] 3 Dense foliage 3 Bushy [81] 3 Dense foliage 3 Dense [89] 3 (insulation) [77,78] Score 6 4 4 4 4 5. Interaction with the external environment Red. urb. H–DF 3 H–DF 3 H–DF 3 H–DF 3 H–DF 3 temp.; Abs. s-w; C seq. Attr. i/p, incr. Sedum: visited by 3 Light yellow 3 Dianthus: important for 3 It is visited by bees/ 3 Attraction of butterflies [91] 3 urb. biod honey bees, solitary pollen [38]; bee bees-good quantities of bumblebees [38] bees [38] crop [80] pollen [38] Score 6 6 6 6 6 6. Other factors Acoustic H–DF 3 H–DF 3 H–DF 3 H–DF 3 H–DF 3 benefits Interest in Sedums: few are useful 2 Seeds for 3 Dianthus spp.: edible flowers 3 Medicinal [87] 3 Numerous uses: lawns, 3 medicine, as food or medicinal medicinal [85] parks, playgrounds, etc. [89] etc. [76] purposes [80] Aesthetics Medium 2 High [77,78] 3 High [82,84] 3 High [86] 3 Medium 2 Score 7 9 9 9 8 Total score 54 48 46 52 47

the development of green roofs [1,5]. In the literature This plant has high scores and thus, it is a very good choice several studies about Sedum have been reported [2– for the development of PV-green (as well as for simple 5,8,10,13]. Historically, Sedum species have been the most extensive green) roofs providing valuable benefits for the commonly used plants for green roofs because are tolerant PVs (compact/low-height plant with good interaction with to extreme temperatures and high winds while they need the PVs, etc.) as well as for the building (energy savings due limited water supply [13]. In terms of the reflectivity of to the succulent/protective layer over building roof). At this Sedum green roofs, in Section 2.2 a related study was point it should be mentioned that the benefits which are presented [20].InTable 6, the selected data for S. clavatum provided by S. clavatum (and in general by a succulent plant are given and Sedum considerable advantages can be seen. with dense foliage) to the building are also associated with Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280 277

its advantages from thermal inertia point of view, as it was mentioned in Section 2.3.1. 60 The advantages of a PV-sedum (S. clavatum) roof have been 50 experimentally verified by the study of Chemisana and Lamnatou [5], for the climatic conditions of Spain. More 40 details about the results of that study were previously 30 presented in Section 2.1 while the developed system is 20 illustrated in Fig. 1(b). scores without weighting factors 10 b) 0 Lobularia maritima Lobularia maritima is also known as “Sweet Alison” and it is Sed. Cl. Tr. Rep. Tr. Rub. Dian. Fr. Or. Vulg. Ast. Mar. Gaz. Rig. Trif. Dub. Lob. Mar. a plant native to the Mediterranean region [77].InTable 6, Cor. Pub. Ger. Moll. Lam. Mac. Cyn. Dact. the results for L. maritima are given. It can be seen that Rosm. Off. theoretically this plant could be used for PV-green as well as

for simple green roofs (without PVs) (advantages: good total score PV score Building score plant/PV and plant/building interaction; high aesthetic 9 value), taking into account that there are other plants more 8 resistant to extreme weather conditions and with lower 7 maintenance needs. 6 5 c) Caryophyllaceae 4 Dianthus fruticosus scores with 3 weighting factors 2 Dianthus fruticosus has busy appearance and it is a Medi- 1 terranean plant. In the literature there is a study about the 0 use of this plant for green roof applications [81]. The results of that study revealed that D. fruticosus sub. fruticosus is a Sed. Cl. Tr. Rep. Tr. Rub. Dian. Fr. Or. Vulg. Ast. Mar. Gaz. Rig.

promising native plant for extensive green roofs in the Trif. Dub. Cor. Pub. Lob. Mar. Ger. Moll. Lam. Mac. Cyn. Dact. Mediterranean region. Nevertheless, from Table 6 it can be Rosm. Off. seen that D. fruticosus has some disadvantages (nutrient/ Fig. 4. The total scores for all the studied plant species: (a) without weighting factors (NWF) and (b) with weighting factors (WF). irrigation requirements: medium; low pest/disease resis- tance, etc.). For the specific case of the PV-green roof systems, this plant has one crucial disadvantage: its rela- weighting factors (the symbols NWF (no weighting factor) tively high height. Therefore, even if albedo and ET cooling and WF (with weighting factor) are used in the text). As it effect have high scores (because of leaf grey color and leaf can be seen from Fig. 4(a) and (b), the family Compositae glossy texture, respectively) other plants more resistant and includes plants with high scores: G. rigens and A. maritimus. more compact should be considered. Conclusively, for sim- They both have very good interaction with the PVs and the ple green roofs (without PVs) and under certain conditions, building. On the other hand, C. pubescens could not be D. fruticosus could be a possible option with high aesthetics, considered for PV-green roofs unless a compact variety such taking into account that there are other plants more as ‘Sunshine Superman’ is adopted. In terms of the family resistant and with lower maintenance needs. Lamiaceae, R. officinalis and O. vulgare are of interest but only for simple (without PVs) green roofs. L. maculatum d) Geraniaceae could be a possible choice for PV-green configurations (high Geranium molle albedo; low-height canopy) with main disadvantage its low Geranium molle is a low-growing plant with small pink tolerance to direct radiation/drought. Regarding the selected flowers and it is also known as dove’s-foot crane’s-bill [86]. plants from the family Fabaceae, in general, are not promis- In Table 6 the results for this plant are presented. It can be ing for PV-green applications. seen that G. molle has good interaction with the building as From Fig. 4(a) and (b), in the category “other families”, S. well as with the PVs (low-height canopy; dense foliage, clavatum shows the highest NWF (54) and WF total score etc.); thus, it can be used for PV-green (as well as for simple (8.8). This is mainly associated with the fact that this plant extensive green) roofs, taking into consideration that there has a very good interaction not only with the PVs (high are other plants more compact. canopy albedo, low-height/compact plant) but also with the building (high-water content leaves; dense foliage). Another e) Eragrostoideae quite suitable plant for PV-green roof applications is G. molle Cynodon dactylon with basic disadvantage its low compactness. In addition Cynodon dactylon is also known as bermuda grass and it is a from Fig. 4(a) and (b) it can be seen that L. maritima shows creeping perennial grass [88]. From Table 6 it can be seen high scores in terms of its interaction with the PVs and the that this grass is compact/dense and theoretically it could be building but it is not resistant to extreme weather condi- adopted for PV-green (or simple green) roofs. However, tions. there are other plant species (more resistant and with lower If only the criteria plant/PV and plant/building interaction maintenance needs) which are more appropriate for PV- are taken into account, from Fig. 4(b) it can be observed that green (or simple green) roofing systems. most of the studied plants have good interaction with the building (WF scores Z1.2). S. clavatum has the highest Elaboration of the data for the considered plants “plant/building interaction” WF score (1.8) and it shows In this section the data (scores) of Tables 3–6, for all the the best interaction with the building in comparison with all plants, are presented as graphs: without and with using the other studied plants; certainly, because of its ability to 278 Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280

form a succulent “protective layer” over the roof. In terms of its higher albedo/reflection characteristics. In terms of plant the plant/PV interaction, even if several plants show high characteristics, S. clavatum is a succulent plant with fleshy leaves WF scores (Z3), in practice only G. rigens and S. clavatum which have the ability to store water and form a thick/high-water could be considered as very good choices for PV-green roofs content layer over the soil. This succulent layer positively con- for the reasons that were previously explained and also tributes to roof thermal characteristics and it keeps soil colder based on the experimental study of the authors (for the than G. rigens which is an ornamental plant with narrow leaves. warm months of Mediterranean climate [5]). Other alter- Further research should be addressed regarding long term native solutions for PV-green roofs could be A. maritimus characterization, large-scale installations and behavior of the sys- and G. molle. These plants, based on the results of tems during winter in order to extract more general results. In Tables 3 and 6, are also expected to show very good addition, for the experiments, sunny days with no wind and no interaction with the PVs and the building; however, in the clouds were selected (high temperature days) since these are very literature there are no studies which verify their suitability representative of the Mediterranean conditions. In Lleida the annual for PV-green roofs under experimental conditions. amount of sun hours over 2012 was 3064, it rained only 52 days (282.7 mm) while the mean monthly maximum temperature from March to October was 26.6 1C(Source: [92]). In terms of the 2.8. A comparison between a PV-gazania and a PV-sedum roof comparison of the results of the experimental study of Ref. [5] with the few PV-green experimental studies which are available in the In this section, results from the experimental study of Chemi- literature (Table 1), the authors noted that good agreement was – sana and Lamnatou [5] (Lleida, Spain; June July 2013) are pre- observed in the power output differences, taking into account the sented and commented. In the frame of that study, a comparison differences because of the location and its associate climate [5]. between the developed PV-gazania (G. rigens) and the PV-sedum (S. clavatum) roof was also conducted. The results revealed that the fi main relative power difference, for the ve-day period considered, 3. Conclusions resulted in a percentage of 2.24%. In Fig. 5(a) (Ref. [5]), the power output for PV-gazania and PV-sedum roof are illustrated. It can be PV-green roofs combine PV panels with simple green roofs, are fi observed that PV-sedum system has 2.21% higher ef ciency than a new tendency in the building sector and they provide several fi PV-gazania. The irradiance pro les are illustrated in Fig. 5(b) (Ref. benefits such as PV output increase because of plant/PV synergy. [5]). The authors noted that the differences in the behavior The present study is a critical review about multiple/crucial factors between PV-gazania and PV-sedum roof can be attributed to the which are related with PV-green roofing systems. Representative higher irradiance received on the module over Sedum because of studies from the literature are presented and critical comments are made for each case. Additional information about plant species appropriate for PV-green applications is also provided. The results 6.0 15 reveal that:

5.0 10 PV output increase depends on several factors such as plant

4.0 species, climatic conditions, evapotranspiration (ET), albedo, 5 etc. High soil cover percentage is important in order to increase 3.0 albedo. The reflected radiation depends also on leaf color and plants with light-colored leaves have higher albedo. For the

Power (V) 0 2.0 specific case of hot, dry climates, it is desirable the adoption of

Relative difference (%) a wet irrigation regime during summer in order to increase ET -5 1.0 Sedum. Maximum power cooling effect. Gazania. Maximum power Difference relative to gazania PV-green roofs provide additional benefits in comparison with 0.0 -10 simple (without PVs) green roofs, such as: in situ production of

9:00 electricity; PVs protect plants from direct exposure to sunlight 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 Time (hh:mm) and thus, plants growth better; plants protect PVs during winter; there is better utilization of roof available space. During the operational phase of the building, the benefits due 1200 to the soil/plant layer and due to plant/PV interaction are Sedum remarkable. Gazania 1000 A crucial parameter for the development of a PV-green roof is the selection of appropriate plant species. Certain factors such ) 2 800 as plant resistant to extreme weather conditions, albedo, ET, maintenance/irrigation needs, plant compactness/foliage den- 600 sity should be taken into account. In general, succulent plants with compact canopies are desirable because they fit at the

Irradiance (W/m 400 space below the PVs while they create a protective layer over the building and they contribute positively to roof thermal 200 inertia. The systematic classification of certain Mediterranean plants in 0 terms of their potential for PV-green roof applications and 9:00 12:00 13:00 15:00 19:00 10:00 11:00 14:00 16:00 17:00 18:00 based on certain criteria (with emphasis on plant/PV and plant/ Time (hh:mm) building interaction) reveals that S. clavatum shows the best interaction with the PVs and the building. Fig. 5. PV-sedum vs. PV-gazania roof: (a) power output and (b) irradiance profiles. Results for the day 6/7/2013 . Based on a recent experimental study [5], PV-gazania and PV- Source: [5]. sedum are promising roofing systems under warm months of Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280 279

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