a critical analysis of factors affecting photovoltaic-green roof performance

A critical analysis of factors affecting photovoltaic-greenroof performance

Chr. Lamnatou n, D. ChemisanaApplied Physics Section of the Environmental Science Department, University of Lleida, c/Pere Cabrera s/n, 25001 Lleida, Spain

a r t i c l e i n f o

Article history:Received 5 March 2014Received in revised form10 September 2014Accepted 4 November 2014

Keywords:Photovoltaic (PV)-Green roofsPlant/PV and plant/building interactionPV output increaseAlbedo and other critical factors

a b s t r a c t

Photovoltaic (PV)-green roofs combine PVs with green roofs, are a new tendency in the building sectorand they provide additional benefits (in comparison with the simple green roofs) such as in situproduction of electricity. The present study is a critical review about multiple factors which are relatedwith PV-green roofing systems. Representative investigations from the literature are presented alongwith critical comments. The studies reveal that plant/PV interaction results in PV output increasedepending on parameters such as plant species, climatic conditions, evapotranspiration, albedo, etc.Furthermore, by comparing a PV-green roof with a PV-gravel one from environmental point of view, itcan be seen that the PV-green system, on a long-term basis, compensates its additional impact due to itshigher production of electricity. Moreover, in the frame of the present study, a systematic classification ofMediterranean plant species in terms of their appropriateness for PV-green roofs is also conducted. Theresults reveal that PV output increase which is provided by PV-green roofs depends on several factorsand among the studied plant species, Sedum clavatum shows the best interaction with the PVs and thebuilding. Experimental results and findings about the environmental profile of PV-green roofs are alsopresented and critically discussed. Conclusively, PV-green roofing systems are promising, especially forwarm climates.

& 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2642. Critical issues related With PV-green roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

2.1. Increase of PV output due to plant/PV interaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2652.2. Albedo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2672.3. Benefits of the PV-green roof for the building during its operational phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

2.3.1. Benefits due to the soil/plant layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2692.3.2. Benefits due to plant/PV interaction: PV-green vs. simple, green configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

2.4. Environmental impact: PV-green roof vs. PV-gravel roof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2702.5. Additional benefits of the PV-green roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2702.6. Improvement of PV-green roof cost-effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2712.7. Selection of appropriate plant species for PV-green roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

2.7.1. Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2712.7.2. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273

2.8. A comparison between a PV-gazania and a PV-sedum roof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2783. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

Contents lists available at ScienceDirect

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Renewable and Sustainable Energy Reviews

http://dx.doi.org/10.1016/j.rser.2014.11.0481364-0321/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author.E-mail address: [email protected] (Chr. Lamnatou).

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

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1. Introduction

Green roofs are roofs covered with a soil/plant layer and theyare of great interest since they have multiple benefits such asmoderation of heat island effect, temperature regulation, soundinsulation, envelope protection for the building, etc [1]. On theother hand, photovoltaic (PV) modules are another option forutilizing building roof since they provide environmentally-friendlyelectricity production. These two different technologies can becombined together for the utilization of building roof and theresult is the PV output increase. This is attributed to evapotran-spiration (ET) cooling effect and in general to plant/PV interaction.

PV-green roofs are a recent tendency in the building sector;thereby, in the literature there are only a few studies. Thesestudies are experimental as well as modeling, regarding severalclimatic conditions (Mediterranean, etc.) and several plant species(Sedum, Gazania, etc.) and they are analytically presented inSection 2.1 and Table 1. Based on these works it is provedthat there is an increase in PV output due to plant/PV synergyand this increase varies from 0.08% [7] to 8.3% [3] depending onfactors such as climatic conditions, plant species, reflected radia-tion from plant canopy (albedo), etc. In general, the PV-green roofstudies reveal that there are several crucial factors which influenceplant/PV interaction and thus, PV-green benefits e.g. for thebuilding.

In the field of green roofs, there are several investigationswhich examine the radiation that it is reflected from plant canopy.Albedo of plant canopy is of great importance because it is relatedwith plant/PV synergy and therefore also with PV output increase.Certain plant species are beneficial because they reflect incidentirradiance 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 studyof Coutts et al. [8] which regards the comparison of insulatingproperties, radiation budget and surface energy balance of fourexperimental rooftops, including an extensive green roof (Sedum)and a cool roof (uninsulated rooftop coated with white elastomericpaint), in Melbourne. The high albedo of the cool roof substantiallyreduced net radiation, leaving less energy available at the surface(for sensible heating during day). Under warm/sunny conditions,when soil moisture was limited, ET from the green roof was low,resulting in high sensible heat fluxes during day. Irrigationimproved green roof performance by increasing ET.

It has also been concluded from the literature that there areonly few studies which examine roof reflectivity for the specificcase of PV-green roofing systems [5,9]. The above mentionedinvestigations along with other studies about albedo of simplegreen roofs are analytically presented in Section 2.2. In general, forPV-green roofing systems, plant canopies which have light-colorleaves and high percentage of soil cover are desirable because oftheir higher albedo.

Furthermore, there are studies which examine green-roofbenefits for the building by focusing on building energy savingsdue to the soil/plant layer. There is an investigation for the case ofSpain [10]: the energy performance of a building in Madrid wassimulated and the results verified green-roof benefits for buildingenergy consumption. That study along with other studies aboutgreen-roof advantages for the building, are analytically presentedin Section 2.3.1. The benefits in terms of energy savings for asimple green-roof building could also regard a PV-green roofbuilding but it should be noted that for the specific PV-green casethese values are expected to be slightly different due to theshading effect of the PV modules.

Another aspect is the environmental impact of a PV-green roof.In the literature the only Life Cycle Analysis (LCA) study about PV-green roofs is that of Lamnatou and Chemisana [11] with emphasis

on the comparison of a PV-green roof with a PV-gravel one.Different scenarios in terms of PV output increase were adoptedfor the PV-green system. The results revealed that although thePV-green roof it has an additional impact (in comparison with thePV-gravel roof) due to its green (soil/plant) part, this additionalimpact on a long-term basis can be compensated.

On the other hand, there are studies which focus on the plantsand examine their specific characteristics. For example there is astudy which evaluated the effect of substrate depth on initialestablishment and survival of 25 succulent plant taxa for greenroof applications in the midwestern United States. Several Sedumplants were tested [12]. Sedum has extensively been studied bymany authors [2–5,8,10,13] given the fact that it is a common plantfor extensive green roof applications [1]. However, there are nostudies which examine plant species in terms of their appropri-ateness1 for PV-green roofs.

From the above mentioned references it can be seen that plant/PV synergy is complicated and depends on several parameters.Nevertheless, in the literature there are only few PV-green roofstudies while there are no review studies which focus on thecrucial parameters which are related with this specific type ofroofing system. Thus, in the frame of the present study a criticalreview about important factors such as PV output increase, albedo,benefits of a PV-green roof building is presented. Moreover, asystematic classification of Mediterranean plants appropriate forPV-green roofs based on certain criteria/weighting factors andwith emphasis on plant/PV and plant/building interaction, isconducted. In this way, the present study offers information whichcan be useful for academic/research purposes and in general, forfuture studies/developments in the field of PV-green roofs.

2. Critical issues related With PV-green roofs

2.1. Increase of PV output due to plant/PV interaction

The increase of PV output is related with factors such as ETcooling effect. Certainly, the improvement of PV efficiency isconsiderably beneficial, from environmental as well as fromeconomic point of view, on a long-term basis during buildingoperational phase. Following representative PV-green roof studiesare presented proving PV output increase because of plant/PVsynergy.

Regarding theoretical/modeling investigations about PV-greenroofing systems:

� Scherba et al. [9] examined the role of roof reflectivity. A modelwas developed and validated by using data from a fieldexperiment (Portland, Oregon) while several roof configura-tions were studied: a control dark membrane roof, a highlyreflective (cool) roof, a vegetated green roof and PV panelselevated above various base roofs. The energy balance modelswhich were developed and validated were used to estimatesensible fluxes in cities located in six climate zones across US(New York, NY; Los Angeles, CA; Chicago, IL; Houston, TX;Minneapolis, St. Paul, Mn; Portland, OR). The results showedthat by replacing a black roof with a white or green roofresulted in a substantial decrease in the total sensible flux.A black membrane roof replaced by a PV-covered white or aPV-covered green roof showed reduction in total sensible fluxof the order of 50%.

1 The authors refer to the appropriateness of the plants for PV-green roofapplications e.g. in terms of plant density (the density of the foliage), albedo (colorof the leaves of the plants), etc. The word appropriateness it is specific for theinteraction of the plants with the PV panels.

Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280 265

� Hui and Chan [3] conducted a study (in Hong Kong) whichincluded a theoretical as well as an experimental part. Based onthe theoretical part, the findings of a year-round buildingenergy simulation (by using EnergyPlus), for a low-rise com-mercial building, showed that the PV-green roof produced 8.3%more electricity than the PV roof. It should be noted that a roof-mounted PV roof with a few inches gap was considered andthus, the fact that there is no air circulation behind the PVsmeans that the difference in yield between that system and thePV-green one is expected to be higher. Sedum was adopted asthe green roof plant.

� Sui and Munemoto [14] proposed a simulation methodologyfor the evaluation of the performance of CO2 Emission (CE) andInvestment Value (IV) of the shape of a Green Roof IntegratedPhotovoltaic System (GRIPVS) of a wooden detached house, byusing genetic algorithm. Three typical locations in Japan wereexamined: Sapporo, Tokyo and Naha. Feasible solutions showedthat GRIPVSs with a larger southern roof area and fully coveredwith PVS performed the best concerning IV than others whilethe pitches of GRIPVSs in the given locations should be lessthan their local optimal solar absorption pitches. In order tominimize CE, GRIPVSs with larger roof pitches for enlarging thePVS installation area to generate more electric power andlarger greening area to absorb more CO2 are required. For thatstudy, also Sedum was considered.

� Witmer and Brownson [15] developed a model about a PV-green roof. That model focused on energy balance and itincluded the microclimate effects. Moreover, Witmer [7] devel-oped an energy balance model of a green-roof integrated PVsystem. The model was analyzed in a transient system simula-tion by means of a FORTRAN code base in TRNSYS energysystem simulation tool. Simulations for several locations in USshowed a small efficiency gain (ranging from 0.08 to 0.55%) inpower output. It should be mentioned that the author of thatstudy ([7]) noted that further development of that model (interms of experimentation and benchmarking) is necessary inorder to refine the model for regional comparisons.

� An experimental study about a large field project in Pittburg,PA [6] also included some theoretical calculations. Regressionequations were derived from Pittsburgh data and they wereapplied in other climates. The results for San Diego andHuntsville showed that the considered PV-green roof produced

slightly more power than the PV-black one (0.04 kW and0.03 kW, respectively) when averaged across the entire yearof daylight hours. The results for Phoenix revealed that the PV-green roof out produced the PV-black one by 0.08 kW (1.3%).In terms of experimental investigations about PV-green roofs:

� The study of Hui and Chan [3] which was previously cited alsoincluded an experimental part. Measurements were taken in arooftop garden in the University of Hong Kong during a sunnysummer day, from 11 am to 2 pm. Two PV modules were placedon a bare and a green roof in order to be compared. The PV-green configuration produced around 4.3% more electricitythan the PV on the bare roof (for the experiment the PVs werenot stacked on the roof) during the time period of themeasurements. Regarding plants, Sedum was considered.

� Köhler et al. [2] investigated several PV-green roof systemsbased on several configurations, dominated by Sedum species.The PV-green systems were compared with PV-bitumen ones,in Berlin. For some cases, the PV-green configurations showedincreased efficiency while for other cases the PV-bitumen roofsshowed higher output, depending on the specific characteris-tics of each system. It should be mentioned that the authors of[2] noted that since there were many overlapping effects (suchas reflection from other PVs, etc.) it would be desirable tocontinue their research and get results from other sites also inorder to verify their findings.

� Furthermore, there is a study about CIGS (Cadmium–Indium–

Gallium di-Selenide) PV cylinders combined with a Sedumgreen roof [16]. That study was based on the analysis of PennState’s 2009 “Natural Fusion” home [17]. Gains in performancewere outlined [16]; nevertheless, in the literature there are nospecific results about the increase of PV output due to plant/PVsynergy for the above mentioned system.

� Another PV-green experimental study was conducted in Pitts-burg (Pennsylvania) [6]: measurements over one year (1-7-2011to 30-6-2012), from a large field project in Pittsburgh, were usedto examine the differences in power output from green andblack roofs. The results revealed that the PV-green roof, underthose climatic conditions (73% of ambient temperatures o25 1Cand 90% of solar irradiance values o800W/m2), can provideonly a small positive impact of 0.5% in power generation in July,whilst for all the year the PV-black roof outperformed the PV-

Table 1Studies about PV-green roofs.

Reference Type ofstudy

System Region Time periodconsidered

Plant species Findings about PVoutput increase

Systems which werecompared

Köhler et al. [2] Experimental Large-scale

Berlin, Germany 5-year data Mainly Sedum species Depending on theconfigurationa

PV-green vs. severalconfigurations

Hui and Chan [3] Experimental Large-scale

Hong Kong, China Sunny summer day,11–2 pm

Sedum 4.3% PV-green vs. PV-bareroof

Perez et al. [4] Experimental Small-scale

New York, USA June Varietal Sedum 2.56%b PV-green vs. PV-gravel

Chemisana andLamnatou [5]

Experimental Small-scale

Lleida, Catalonia,Spain

June–July Gazania rigens, Sedumclavatum

1.29% (G. rigens), 3.33% (S.clavatum)

PV-green vs. PV-gravel

Nagengast et al. [6] Experimental Large-scale

Pittsburg, PA, USA Julyc Mosses 0.5% PV-green vs. PV-black

Witmer [7] Modeling Different locationsin USA

0.08–0.55%

Hui and Chan [3] Modeling Hong Kong, China 1 year 8.3% PV-green vs. PV roofmounted

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 ofbitumen 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%.

Chr. Lamnatou, D. Chemisana / Renewable and Sustainable Energy Reviews 43 (2015) 264–280266

green one by 0.5%. For days with temperatures higher than 25 1Cand/or irradiances higher than 800 W/m2, the PV-green roofstarted to outperform the PV-black one. Finally, it should bementioned that moss was utilized for that PV-green system.

� In addition, Perez et al. [4] investigated multiple, small-scaleroofing systems: gravel, green, PV-gravel and PV-green, oversmall model houses, in New York. Sedum species were adoptedfor 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 onthe gravel house were 10.69% higher on the gravel-roof housethan the PV-green roof house during the same month. Meaninternal and surface temperatures were found to be 5.1% and1.73% 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 (Gazaniarigens) (Fig. 1a: [5]), PV-sedum (Sedum clavatum) (Fig. 1b: [5])and PV-gravel (reference case) (Fig. 1c: [5]) were developed andtested at the University of Lleida, in Spain (June–July, 2013).Five-day average percentages of maximum power outputincrease for PV-gazania and PV-sedum were found to be1.29% 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 Sedumdaily difference was 26.1% (25.9% over five days) lower. Sedumkept soil temperature 5.95% cooler (at 3 cm depth). Sedum leafcharacteristics improved the effective incident irradiance onthe module 1.43% more than Gazania. PV-green roofs alsoaffected the incident irradiance on the PV panel, obtaininghigher relative incident irradiances in comparison with thegravel 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 contentleaves). Under those climatic conditions, PV-green roofs wereproved to be more beneficial than the conventional gravel rooffor both, PV module and temperature of the roof surface. Theauthors noted that further research should be addressed forlong-term characterization (e.g. over winter) and large-scaleinstallations.

In Table 1, the above mentioned theoretical and experimentalstudies are presented. From Table 1, it can be observed that themost ‘pessimistic scenario’ in terms of PV output increase is that ofWitmer [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 incidentsunlight 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 thesereferences regard simple (without PVs) green roofs since there areonly a few PV-green roof investigations which examine reflectedradiation issues.

Among the PV-green studies is that of Chemisana and Lamna-tou [5] which was previously cited in Section 2.1. Both, Sedum andGazania, reflected higher quantity of light to the PV module thangravel, from the sunrise to approximately 3:00 pm (Fig. 2: [5]).From that moment, gravel layer implied progressively higherincident irradiance until the time when a building (which was infront of the experimental roofs) shaded the experimental set-upand hided the symmetric effect for the sunset. In order to obtain

the real effect of that difference, the values were referred to theirradiance level at that moment obtaining the relative valuesdepicted in dark colors. In Fig. 2, the relative irradiance differencefor the PV-green roofs and the PV-gravel system, are illustrated. Aninteresting behavior is denoted on the left part of Fig. 2. It can be

Fig. 1. PV roofs developed by the authors at the University of Lleida, in Spain:(a) PV-Gazania rigens roof; (b) PV-Sedum clavatum roof; (c) PV-gravel roof(reference system) .Source: [5].


seen that PV green roofs increased the incident irradiance on thePV module by up to 32% (maximum) for the Sedum/gravelcomparison. This could be related with albedo which is higher atthe plants as well as with sun position (when the sun loci wasbehind the PV plane, the rays that in a normal situation would notimpact on the module fell on it because the plant canopy acted as a´diffused reflector´). Sedum was proved to be better in terms ofincident irradiance increase (on the PV module) throughout thewhole day, achieving an average improvement with respect toGazania of 1.41%.

In addition, the study of Scherba et al. [9] which was also citedin Section 2.1 was a modeling study about the impacts of roofreflectivity. As it was previously mentioned, models for multipleroofing systems were developed and validated with data fromPortland (Oregon) while annual models were run for six cities in5 different climate zones across US. Across all six cities which werestudied, the black roof and PV-black roof had the highest totaldaily sensible flux levels (the average value ranged from 331 to405 W/m2). For the case that the black roof was replaced by thewhite or the green roof, the peak flux was reduced by around 70%,while the total daily flux was reduced by around 80% with a whiteroof and 52% with a green roof. When PVs were added to the blackroof, there was negligible impact on peak flux; nevertheless, thetotal flux was reduced from the unshaded black roof levels byaround 11%. Compared to the flux for the unshaded black roof, thePV-white roof showed a peak flux reduction of around 40% and atotal flux reduction of 55%. The PV-green roof had a peak reductionof around 45% and a total flux reduction of approximately 42%. Thegreen roof showed higher total daily flux than the white roof. Theauthors noted that this is attributed to green roof thermal massthat prevents roof from cooling below ambient temperatures atnight. Thus, green roof flux was usually positive while black andwhite roofs had negative fluxes at night.

Regarding investigations about simple (without PVs) greenroofs, Coutts et al. [8] compared insulating properties, radiationbudget and surface energy balance of four experimental rooftops,including an extensive green roof (Sedum) and a cool roof (unin-sulated rooftop coated with white elastomeric paint), during the

summer of 2011–2012 in Melbourne, Australia. The four experi-mental roofs (2.4 m�2.4 m) which were examined: a conven-tional steel sheet roof (STEEL), a steel sheet roof covered withwhite, high albedo paint (WHITE), a vegetated roof (VEG), a roofwith just the soil layer (no vegetation) (SOIL). WHITE albedo wasvery high (0.71 at solar noon, January 2012). SOIL albedo was low(0.10) due to its dark color. The presence of the lighter coloredvegetation increased the albedo slightly for the case of VEG (0.15).STEEL albedo was 0.21, resulting in a value of reflected shortwaveradiation higher than both VEG and SOIL. In terms of the outgoinglongwave radiation, WHITE showed the lowest levels, largely dueto the high albedo of the surface. Despite the higher albedo ofSTEEL relative to VEG and SOIL, outgoing longwave radiation washigher as the STEEL surface heated more easily than VEG and SOIL.The influence of the vegetation layer also reduced outgoing long-wave radiation, since the vegetation itself and the shading of thesoil surface served to reduce surface temperature and thus,emitted longwave radiation. During night, outgoing longwaveradiation of STEEL and WHITE were similar and lower than VEGand SOIL.

D’Orazio et al. [18] experimentally investigated the yearlythermal performance of a green roof in comparison with otherpassive cooling technologies. All of the roofs were installed on areal-scale, experimental building in the vicinity of Ancona, in Italy(the building was 8.20�10.50 m). The green roof was extensivewith low/evergreen vegetation (officinalis type). The resultsshowed 13% albedo for the green configuration, 31% for the clay-tile roof and 9% for the copper roofing system. For the greensystem, the experimental findings revealed that the plant canopyreflected 13% of the incident global solar radiation and absorbed56%, so that the solar radiation entered the system could be thenestimate as 31% of the incident global solar radiation.

MacIvor et al. [19] studied the performance of dryland andwetland plants on green roofs. The experimental study wasconducted over two growing seasons (2007–2008) on top of thePatrick Power Library (Saint Mary’s University) in Halifax, NovaScotia, Canada. Albedo values ranged from 15.81% (Empetrumnigrum) to 19.12% (Sibbaldiopsis tridentata; Kalmia polifolia)

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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)

Irrad

ianc

e di

ffere

nce

(W/m

2 ), R

elat

ive

irrad

ianc

e di

ffere

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Gazania vs Gravel. Irradiance relative difference referred to gravelSedum vs Gravel. Irradiance relative difference referred to gravelSedum vs Gazania. Irradiance realtive difference referred to GazaniaSedum vs Gazania. Irradiance difference referred to GazaniaSedum vs Gravel. Irradiance difference referred to gravelGazania vs Gravel. Irradiance difference referred to gravel

Fig. 2. Relative irradiance difference: PV-green roofs (Gazania rigens and Sedum clavatum) and PV-gravel roof (reference system). Incident irradiance .Source: [5].


depending on the plant species. In general, albedo values tendedto increase with increasing diversity of dryland plants, butdecreased with increasing the number of wetland species.

Yujiro and Toshiaki [20] investigated the evaporative coolingeffect of rooftop vegetation. The surface heat budget and severalrelated parameters regarding a rooftop of a three-story officebuilding planted with Sedum were measured (two clear-sky daysin September, Japan). The results showed that albedo and emis-sivity of Sedum were 0.153 and 0.995, respectively.

Gaffin et al. [21] developed a green-roof environmental mon-itoring and meteorological network in New York. Mix of Sedumspecies was adopted for the experimental systems. The reportedalbedo values (July 2008; Bronx, NY green roof network site) wereanalyzed. A pronounced diurnal U-shaped cycle for albedo wasobserved. Albedo was minimum at noon when incidence anglewas minimized. The authors noted that the U-shape was largelydue to the change in solar incidence angle during the day, affectingsurface reflectivity; nevertheless, other factors including probablyleaf responses may be playing a role. The time averaged albedo forthe July data was found to be 19.6%.

From the above mentioned studies it can be seen that albedo isa crucial factor and it is associated with parameters such as leafcolor and soil cover percentage. Leaves with lighter color showhigher albedo. On the other hand, green roofs with high percen-tage of bare soil exposed are not desirable because are expected toshow low reflectivity. Thereby, for the PV-green roofs it isimportant to maintain almost total plant coverage in order toachieve high reflectivity and thus, greater benefit for the PVmodules. High infrared reflectivity of the plants is very promisingfor keeping PV cells cool while still supplying a large part of theuseful spectrum to the PV solar cells. In general, green roofs havehigh albedo values and they provide several advantages incomparison with the conventional, cool-roof technologies [1].

2.3. Benefits of the PV-green roof for the building during itsoperational phase

2.3.1. Benefits due to the soil/plant layerIn the literature there are no PV-green roof studies which

examine the PV-green benefits for building energy consumption.Thus, in the present paragraph investigations about simple (with-out PVs) green roofs which verify the benefits of the soil/plantlayer for building energy needs are presented.

There is a study about green-roof benefits for the climaticconditions of Spain [10]. In the frame of that study, the energyperformance of an eight-storey, residential building in Madrid,with flat roof accounting for 17% (677 m2) of the external envel-ope, was simulated. Three different roofs: a common flat roof, agreen roof and a green roof with water storage capacity werecompared. In terms of the plants, Sedum sempervivum, Opuntiaaciculata (cactus), Larrea divaricata (desert shrub) were adopted.Green roof without water storage capacity and Aljibe green roofwith water storage capacity were considered. The results showed aslight reduction in energy consumption for heating for both greenroofs. The reduction in annual energy consumption for heatingwas 0.12% for the simple green roof and 0.2% for the Aljibe roof.However, the studied roofs offered a considerable reduction in theannual energy consumption for cooling and peak energy con-sumption on the hottest day of the summer. More analytically, theannual energy consumption for the simple green roof and theAljibe roof dropped by 6.2% and 6.4%, respectively, while the totalenergy consumption for the peak day dropped by 12% for both roofconfigurations. In terms of the adopted method, the building wasdefined in ESP-r (according to actual project plans and specifica-tions). Simulation of annual and peak energy consumption wasconducted for the base case or reference building with a

conventional roof. The green-roof subsystems were modeled bymeans of ESP-r by taking into account the thermal properties ofthe components provided by the manufacturers. Simulation ofannual and peak energy consumption of the building with thegreen roofs was conducted and compared to that of the referencebuilding.

In addition, Wong et al. [22] used DOE-2 energy simulationprogram in order to determine the effects of rooftop garden (turf,shrubs and trees) on the annual energy consumption, cooling loadand roof thermal transfer value of a five-storey, hypotheticalcommercial building in Singapore. The installation of rooftopgarden on that building resulted in savings of 0.6–14.5% in theannual energy consumption while shrubs were found to be themost effective plants in reducing building energy consumption.Moreover, the results showed that the increase of soil thicknesswould further reduce building energy consumption. Furthermore,the presence of the plants on the typical roof also reduced the heatgain into the building; however, the reduction was less significantthan that caused by the installation of a rooftop garden on theexposed roof. The annual energy consumption for the assumedbuilding was calculated to be 200 MW h (for the exposed roof).This value showed: (1) 19 MW h (10%) reduction for 100%-turfcovering, (2) 29 MW h (15%) reduction for 100%-shrub covering.

The study of Ascione et al. [23] also verifies green-roof benefitsfor the building. More specifically, that study was about greenroofs in Europe and their potential for energy savings in buildingair conditioning. Several green roofs were studied: Sedum andgramineous (short and tall height), grass lawn. The resultsrevealed that in warm climates green roofs are suitable forreducing the energy demand for space cooling (without penalizingthe scarce heating demand): the annual reduction of the primaryenergy ranged between 1% and 11% for Tenerife, 0% and 11% forSevilla, 2% and 8% for Rome. In cold climates green roofs are usefulfor reducing energy demand for space cooling but also in order todecrease winter heating needs (for example for Amsterdam andLondon, the annual savings ranged between 4% and 7%).

In addition, in the review article of Castleton et al. [24] thebenefits and important issues about green roofs were presented.Among these the following aspects were highlighted: green roofscan significantly reduce energy use in buildings with poor insula-tion values (in terms of summer cooling as well as winter heating);for modern buildings with high U-values associated with betterroof insulation, green roofs could save small (or zero) amount ofenergy; thicker soil substrate on the roof means higher reductionof the heat gain/loss into/out of the building; a less dense soil hasmore air pockets and thus, it is a better insulator for the building.

The main findings of the above mentioned studies are pre-sented in Table 2. It can be seen that the energy savings because ofthe soil/plant layer, depend on several factors (climatic conditions,etc.). More specifically, the studies reveal that considerable savingsfor cooling of a building during summer under Mediterraneanclimatic conditions such as in Spain could be achieved. On theother hand, at this point it should be noted that the energy savingswhich are reported in the above mentioned studies are for simple(without PVs) green roofs. For the specific case of the PV-greenroofs, these values are expected to be slightly different given thefact that the PV panels shade the roof of the building.

Finally, it should be mentioned that thermal inertia which isprovided to the building by the soil/plant layer (because of leaf andsoil water content) is another important issue. Thermal inertia is a“climate moderator” and it is related with the attenuation oftemperature fluctuations. This benefit could be valuable forexample when phase change materials are used for thermalstorage in a building. Roof is one of the most sensitive buildingelements during warm months of the Mediterranean climate dueto the fact that it is exposed to solar radiation for the greatest part


of the day. The soil/plant layer on the roof can reduce the rate atwhich indoor temperature rises and drops. During summermonths, by means of a green roof, a delay of cooling load peak(and thus, a reduction of air-conditioning energy consumption)can be achieved. On the other hand, during the winter season theenergy which is available from the sun (solar gain) is stored andthen, it is slowly released in the interior space of the building. Atthis point it should be noted that there are some plants withconsiderable thermal inertia. For example, a thick, succulentcanopy can positively contribute to roof thermal inertia.

2.3.2. Benefits due to plant/PV interaction: PV-green vs. simple, greenconfiguration

Based on experimental studies about PV-green roofs [2–6], thebenefits of a PV-green roof in comparison with a simple (withoutPVs) green roof are multiple. These benefits are followingpresented:

� A basic advantage of a PV-green roof is related with the in situproduction of electricity [2–6] by means of an environmentallyfriendly technology. In this way, all or part of building energyneeds can be covered. Moreover, the electricity generated bythe PVs could be utilized to power the water pumps for plantirrigation [3].

� PVs shade roof; thus, soil temperature is reduced [3,5].� The benefit due to the higher electricity production from a PV-

green roof could offset the cost for the “green part” (relatedwith soil/plant layer) of the roof installation [2].

� By means of a PV-green system there is better utilization of theavailable space of a roof [3].

� With a PV-green configuration, during winter the plants (dueto their thermal capacity) protect the PV panels from the cold.On the other hand, PVs protect plants from direct exposure tosunlight and in this way plant growth and plant species varietyare enhanced [4,5].

2.4. Environmental impact: PV-green roof vs. PV-gravel roof

For the evaluation of the environmental impact of a product,LCA is a useful tool. However, as it was mentioned in theintroduction, in the literature the only LCA study about PV-greenroofing systems is that of Lamnatou and Chemisana [11]: severalroofs were examined (PV-green (extensive), PV-gravel, green

(extensive and intensive), gravel) while emphasis was given onthe PV-green roof and its comparison with the PV-gravel one. Forthe case of the PV-green configuration, different scenarios (basedon certain literature references: Table 1) in terms of PV outputincrease (because of plant/PV interaction) were adopted. Theresults (based on three different Life Cycle Impact Assessment(LCIA) methodologies: Ecoinvent 99 (EI99), IMPACT 2002þ andCumulative Energy Demand (CED)) revealed that although the PV-green roof it had an additional impact (because of its components/materials related with soil and plants) in comparison with the PV-gravel roof, this additional impact on a long-term basis (duringbuilding operational phase) can be compensated.

Based on the above mentioned study [11], in Fig. 3(a) the totalimpact points (for all the considered phases) by the LCIA metho-dology IMPACT 2002þ (based on the four damage categories:human health, ecosystem quality, climate change, resources) forthe PV-green and the PV-gravel roof are illustrated. It can be seenthat the PV-green case has an additional impact, as it wasexpected, due to its additional components of the soil/plant layer.However, in Fig. 3(b), where the total impact points per kWh ofproduced electricity (based on IMPACT 2002þ , damage cate-gories) are shown, it can be observed that the PV-green roof (ona long-term basis) compensates its additional impact. Morespecifically, Fig. 3(b) regards several scenarios (based on certainliterature references of Table 1) in terms of the PV output increaseof the PV-green configuration (the PV-gravel impact points/kWhare used as reference and they are represented as a straight linesince the adopted scenarios regard only the PV-green case). FromFig. 3(b) it can be seen that after around 6.2% increase of PVoutput, PV-green system compensates its additional environmen-tal impact and it becomes more environmentally friendly than thePV-gravel one.

2.5. Additional benefits of the PV-green roofs

Following two additional benefits of the PV-green roofs incomparison with the simple (without soil/plant layer) PV roofs arepresented:

(1) Carbon sequestrationAn additional advantage of PV-green roofs, from CO2-reduc-tion point of view, is their potential for carbon sequestrationby the soil/plant layer. The green roofs can fix carbon in plantsand soil. The CO2 captured by the plants is the result of

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

Reference Type ofstudy

Plant species Building Region Findings about energy savings Additional comments

AlcazarandBass[10]

Modeling Sedum, cactus,shrubs

8-storey, residential withflat roof 677 m2

Madrid,Spain

Reduction of annual energy consumption:6.2% for the simple green roof; 6.4% for thegreen roof with water storage

Common flat roof vs. green roofs (with/without water storage)

Ascioneet al.[23]

Modeling Sedum,gramineous(short/tallheight), grasslawn

Office building: maximalfloor dimensions:70.7 m�17.7 m; overallheight: 4.3 m

Europe(Spain andothercountries)

Annual reduction of primary energy: 1–11%for Tenerife; 0–11% for Sevilla; 2–8% forRome

For warm climates, green roofs aresuitable for reducing energy demand forspace coolinga

Wonget al.[22]

Modeling 5-story hypotheticalcommercial building

Singapore Rooftop garden led to 17–79% reduction inspace cooling, 17–70% in peak space load;cost savings were also found

Thermal resistances (R-values) of turf/shrubs/trees were estimated by datafrom measurements

Castletonet al.[24]

Reviewarticle

Sedum, etc. Several configurations Severalregions

Green roofs-significant reduction ofenergy use in buildings with poorinsulation (for summer cooling as well asfor winter heating)

Thicker soil substrate-higher reductionof heat gain/loss into/out of building; lessdense soil-more air pockets-betterinsulator

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%).


differences between atmospheric CO2 absorbed during photo-synthesis and CO2 emitted to the atmosphere during respira-tion. This difference is converted into biomass [25].

(2) Urban agricultureFor the case that food crops are selected for the green space ofthe building, an additional advantage has to do with theavoided CO2 emissions due to the transportation of the foodfrom the food production area to the consumer. Food produc-tion in cities has not only environmental but also economicand social benefits [25]. Certainly, the selected food cropsshould fulfill certain criteria. These criteria are analyticallypresented in Section 2.7.

2.6. Improvement of PV-green roof cost-effectiveness

In terms of PV-green roof cost-effectiveness, subsidies from thegovernment for the promotion of green roof construction couldprovide benefits and considerable reduction of the initial invest-ment cost. Other options for further cost-effectiveness improve-ment could include:

� Reduction of the construction cost through further cost reduc-tions among the industrial companies

� Inclusion of the social costs/benefits.� Reusing some of the waste materials.� Innovative policies.� Inclusion of air pollution mitigation technologies.� Partially transfer of the social benefits to the investors.� Use of eco-friendly fertilizers.� Adoption of environmentally-friendly and cost-effective dispo-

sal systems.� Standardization/certification of green roof products.

� Specialization/training for the installation.� Selection of low-cost plant species.

Another aspect, very important for the case of the PV-greenroofs is the increase of PV efficiency (Table 1). PV efficiencyimprovement should be considered as an additional advantagewhich can increase the profitability of a PV-green roof investmentalong with all other advantages that PV-green roofs offer. Theimprovement of PV efficiency and the benefits due to the green(soil/plant) layer which protects the building could be consider-ably beneficial on a long-term basis, from environmental as well asfrom economic point of view. The increase of PV output due toplant/PV synergy has been proved to be considerable e.g. duringwarm months of the Mediterranean climate [5].

2.7. Selection of appropriate plant species for PV-green roofs

In this section, a systematic classification of several Mediterra-nean plants appropriate for PV-green roof applications has beendone. The criteria and the scores for each plant are presented intables. Most of the data of the tables are based on certainreferences while some of them are derived based on the specificcharacteristics of each plant. For each plant, critical comments interms of its appropriateness for PV-green (or simple green) roofapplications, are made.

2.7.1. Materials and methods2.7.1.1. Scores and weighting factors. In the tables, the criteria arepresented aggregated into 6 categories: (1) Suitability forextensive green roofs; (2) resistance to weather conditions;(3) interaction with the PVs; (4) interaction with the building;(5) interaction with the external environment; (6) other factors.

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


A score for each criterion, ranging from 1 to 3 (1¼ low (worstoption); 2¼medium; 3¼high (best option)) is given. In this way, atotal score for each plant is derived. Higher total score meansgreater appropriateness for PV-green roof applications. Inaddition, the data are further elaborated by adding weightingfactors and by adopting the following assumptions: for the plant/PV and the plant/building interaction, 30% weighting factor isassumed, for each of these two criteria. For the other four criteria(suitability for extensive green roofs; resistance to weatherconditions; interaction with the external environment; otherfactors) this weighting factor is considered to be 10% for eachone of these four criteria. In this way, the importance of plant/PVand plant/building interaction is taken into account. Following theselected criteria are analytically presented.

These criteria regard the extensive (shallow-substrate systems)and not the intensive (deep-substrate systems) green roofs becausethe extensive configurations are more appropriate for PV-green roofapplications [5]. The fact that the intensive green roofs are lessappropriate is related with factors such as their high height of the

soil/plant layer, their high weight (weight of the whole system) [1] aswell as with aesthetic/building integration factors [5].

2.7.1.2. Selected criteria

1) Suitability for extensive green roofs (12 points maximum)In this category, criteria which are related with: (a) root system:shallow roots are needed since the growing medium for exten-sive green roof applications should be r20 cm depending onthe selected plant [26]; (b) nutrient/irrigation requirementswhichshould be low (low-maintenance plants), the plants should beselected for their ability to thrive with minimal to no inputs(water, fertilizers, etc.) after establishment [27]; (c) disease/pestresistance: plants with no severe pest susceptibility are needed[27]. Certainly, it is important this category to have very hightotal score because first of all the selected plants should beappropriate for extensive green roof applications.

2) Resistance to weather conditions (12 points maximum)In the frame of this category, the resistance to: (a) direct solar

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

Gazania rigens Asteriscus maritimus Coreopsis pubescensCriteria Comments/Refs. Comments/Refs. Comments/Refs.

1. Suitability for extensive green roofsShallow roots G. rigens “Sun Gold”: shallow rooting plant [33] 3 It grows in rocky, coastal

Mediterranean regions [40]3 Coreopsis: deep-rooted [44] 1

Nutrient needs Low [34] 3 Maintenance: low [41] 3 Coreopsis spp.: low food needs [45] 3Irrigation needs Low [34] 3 Low (it is drought tolerant [34]) 3 Coreopsis spp.: low water needs [45] 3Disease/pestresistance

Long-term health usually not affected by pests [35] 3 No known serious insect or diseaseproblems [41]

3 Coreopsis spp.: no serious pests arenormally seen [46]

3

Score 12 12 102. Resistance to weather conditionsResistance to directradiation

Heat tolerant; full sun [34] 3 Full sun; heat tolerant [34] 3 Coreopsis spp.: plant grows in full sun[46]

3

Resistance to drought Drought/heat tolerant [34] 3 Drought/heat tolerant [34] 3 Coreopsis spp.: high drought tolerance[46]

3

Resistance to frost Medium [36] 2 Medium [42] 2 Medium (it can take some cold weather[45])

2

Resistance to wind H–DFa 3 H–DF 3 H–DF 3Score 11 11 113. Interaction with the PVsAlbedo (is relatedwith leaf color)

Greyish/whitish leaves [37] 3 Grey-leaved [40] 3 C. pubescens ‘Sunshine Superman’:medium green leaves [47]

3

ET Hairy leaves [37] 1 A. maritimus cv. Gold Coin has hairyleaves [42]

1 Stems with hairs [48] 1

Height 15–30 cm [34] 3 15–30 cm [41] 3 Coreopsis pubescens Ell. var. debilis: 25–50 cm [49]

2

Compactness Compact series [36]; compact [5] 3 Compact [41] 3 ‘Sunshine Superman’: compact, 25–30 cm tall [47]

3

Score 10 10 94. Interaction with the buildingLeaf water content Non-succulent 1 Non-succulent 1 Non-succulent 1Dense foliage(insulation)

Leaves densely clustered along the stems [37] 3 The leaves appear in dense clusters[42]

3 C. pubescens Ell.var. debilis: plant body:dense [49]

3

Score 4 4 45. Interaction with the external environmentRed. urb. temp.; Abs.s-w; C seq.b

H–DF 3 H–DF 3 H–DF 3

Attr. i/p, incr. urb.biod c

Gazania: visited by honey bees/solitary bees [38]; itattracts butterflies [36]

3 Attracts butterflies [41] 3 Coreopsis: visited by honeybees/solitarybees [38]

3

Score 6 6 66. Other factorsAcoustic benefits H–DF 3 H–DF 3 H–DF 3Interest in medicine,etc.

Honey, medicinal, food (edible flowers) [39] 3 Anti-insect activity [43] 3 Coreopsis spp.: dyes [50] 3

Aesthetics High [32,36,37] 3 High [41,42] 3 High [45,47] 3Score 9 9 9Total score 52 52 49

a H–DF¼ It 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


radiation, (b) drought, (c) frost, (d) wind, are included [1]. Theseparameters are crucial for the survival of the plants on a roofwhere extreme weather conditions can occur. As it waspreviously mentioned (Section 2.3.2), the plants protect thePVs from winter frost while during summer the PVs protectplants from direct exposure at solar radiation [5]. Finally, itshould be noted that for the case of the Mediterranean climate,e.g. for Spain, since it is characterized by dry summers withhigh temperatures, it is important the selection of drought-tolerant plant species [5].

3) Interaction with the PVs (12 points maximum)A relevant criterion is albedo and it is associated with factorssuch as leaf color. The importance of albedo is related with thefact that the part of solar radiation which is reflected and itreaches PV module surface, it leads to PVs output increase. Forthe PV-green roofs it is important to maintain almost totalplant coverage with light-colored plant canopy in order toachieve higher reflectivity and thus, greater benefit for the PVsand the building [5,9].On the other hand, another criterion is ET because it increasescooling effect and thus, PV electrical efficiency. Weather andmicroclimate ET is the combination of evaporation of waterfrom the soil and transpiration from the plants while apotential ET can be adjusted to a particular type of plant, plantpopulation, plant growth stage, vigor and stress. In terms of thedrought-tolerant plants, they go dormant or near dormantwhen soil water is unavailable and then, become active whenwater is available [28]. From the above mentioned it can beseen that the relationship drought-tolerant plant/ET/water useis complicated. During the PV-green roof experiments of theauthors Chemisana and Lamnatou [5], a wet irrigation regime(wet irrigation regime means prevention of moisture deficit:[29]) was adopted given the high summer temperatures andthe high irradiation of Lleida. In general, for green roofs duringwarm months of Mediterranean climate, it is desirable theadoption of a wet irrigation regime in order to increase thecooling effect of the soil/plant layer. In the frame of the presentstudy, in order to facilitate the classification of the selectedplants from ET point of view, a rating of 1 is given to the plantswith “hairy” leaves since this type of leaves have reducedtranspiration [30] and a rating of 3 is given to the plants withglossy leaves because the glossiness is associated withincreased cuticular transpiration [31].Finally, other criteria regard the selection of plants with low-height/compact canopies. These criteria are also critical sincethe plants should fit perfectly at the space below the PV panels.A canopy with greater height can create practical problemswhich are associated with: (1) the placement of the PVs, (2) thepartially shading of the PV panels because of the presence ofthe plants. Thus, only low-height/compact canopies are appro-priate for PV-green roofs [5]. For facilitating the classification ofthe selected plants, a rating of 3 is given to the plants withaverage height less than 25 cm, a rating of 2 is given to thespecies which have an average height from 25 to 45 cm and arating of 1 is given to the plants with average height greaterthan 45 cm. The selection of the height of 45 cm as the upperacceptable limit is based on the assumption that the PV panelscould be placed at around 50 cm above roof surface.

4) Interaction with the building (6 points maximum)The first issue regards the water content of the leaves because itinfluences roof thermal inertia. Succulent plants with thick,high-water content leaves and dense foliage can positivelycontribute to roof thermal inertia. Thus, a rating of 1 is givento the plants with non-succulent leaves while a rating of 3 isgiven to the succulent plant species. The second factor has to dowith the density of the foliage. A dense foliage can also reduce

convective heat losses given the fact that it has the ability to“enclose” air volumes and thus, to act as insulation layer. In thisway, plants with a good interaction with the building can leadto energy savings. Studies which verify green roof benefits forthe building were previously presented (Section 2.3.1 andTable 2).

5) Interaction with the external environment (6 points maximum)In this category, the first group of parameters regards theability of the green roof to: reduce urban temperature andthus, mitigate the heat island effect, absorb storm water andmitigate air pollution by means of carbon sequestration [1]. Thesecond group of factors has to do with the attraction of insects/pollinators and the increase of urban biodiversity [1].

6) Other factors (9 points maximum)Plant canopy has the potential to reduce noise pollution [1] andthis is an additional advantage. A rating of 3 is given to theplants which have dense foliage (which means that theseplants are expected to provide high acoustic protection forthe building). On the other hand, some plants have greatinterest in medicine and in other sectors (food industry, etc.)which means that for these cases the green roof can also offeragricultural and other products. Finally, the aesthetic is anotheraspect [1] and it is also taken into account.

2.7.2. Results and discussion

1) Compositae (Asteraceae)Gazania rigensGazania rigens is a plant appropriate for decorations [32].In Table 3, the scores for this plant are shown and it can be seenthat G. rigens is a very appropriate plant for PV-green roofs since itshows very good interaction with the building and the PV panels.The appropriateness of this plant for PV-green roof applicationshas been also confirmed by the experimental study of Chemisanaand Lamnatou [5]: G. rigens plants were transplanted in a box0.9�1.3 m2 which contained soil substrate (10 cm height) andthis small-scale, extensive green roof was combined with apolycrystalline silicon (p-Si), PV panel (Fig. 1a). The plant showedaway of growth ideal for the placement of the PV panels above itsfoliage because G. rigens spread over the ground and it formed avery dense canopy with low height. In this way, the plant fittedperfectly under the PVs and at the same time it created a“protective” layer over the building. The results revealed thatthere is a good interaction between the plants and the PVs(details about the results of that study were previously presentedin section 2.1).Conclusively, G. rigens is a very promising plant for the develop-ment of PV-green roofs and it provides considerable benefitsduring warm months of the Mediterranean climate [5]. Finally, itshould be mentioned that this plant has high aesthetic value:flowers of multiple colors depending on the variety. Moreover,there are some varieties with silvery/white leaves [32] (goodinteraction with the PVs from albedo point of view). In generalterms, Gazania is strongly suggested as plant for decorations ofindoor and outdoor environment (ground covers, rock gardens,etc.) providing a wide variety of beautiful colored flowers [32].

Asteriscus maritimusAsteriscus maritimus is a native plant of the lands surrounding theMediterranean Sea and it is common in Spain [43]. In Table 3, thecriteria/scores for this plant are presented. The results reveal thatthis plant shows very good interaction with the building becauseof its dense foliage. From PV-green point of view, the interactionplant/PV is also good and this is mainly related with the fact thatthis plant has a relatively low height and it is compact. In


addition, A. maritimus is a low-maintenance plant with highresistance to severe weather conditions and thus, it could be avery good choice for the development of PV-green roofs (even incoastal areas with sandy soil; it is also called “Sea aster” or “Seadaisy”: [34]) adding a high aesthetic value to the building with itsimpressive flowering.

Coreopsis pubescensCoreopsis pubescens is an herbaceous perennial plant withdaisy-like flowers [46]. Based on the results of Table 3, thisplant is more appropriate for semi-intensive green roofapplications because its root is not shallow. This plant hasgood interaction with the building (because of its densefoliage) while it also has other advantages (low-mainte-nance/resistant plant, attraction of bees, etc.). For the case of

the PV-green roofs, in general, C. pubescens is not an appro-priate plant. However, a compact variety such as ‘SunshineSuperman’ (25–30 cm height [47]) could be a possible solu-tion, keeping in mind that there are other plants which aremore appropriate for PV-green roofing systems.

2) LamiaceaeRosmarinus officinalisRosmarinus officinalis is an evergreen shrub with intenselyfragrant foliage [51]. It is a native plant of the Mediterraneanregions, especially of Spain and Portugal [52]. As it can be seenfrom Table 4, R. officinalis is more appropriate for simple(without PVs) semi-intensive green roof applications (becauseits root is not shallow), providing that appropriate growingconditions are fulfilled. It is a plant with dense foliage and

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

Rosmarinus officinalis Origanum vulgare Lamium maculatumCriteria Comments/Refs. Comments/Refs. Comments/Refs.

1. Suitability for extensive green roofsShallow roots Minimum root depth: 35 cm [53] 1 Origanum: root depth:

30 cm [56]1 L. maculatum: shallow root system [58] 3

Nutrient needs Responds well to additional applications ofnitrogen [54]

2 Low [57] 3 Low [59] 3

Irrigation needs The plants should not dry out completely[54]

2 Water: dry to medium[57]

3 Medium [59] 2

Disease/pestresistance

Rosemary is vulnerable to spider mites,mealybugs, whiteflies and thrips [54]

2 No serious insect ordisease problems [57]

3 Mid family with no disease problems [59] 3

Score 7 10 112. Resistance to weather conditionsResistance todirectradiation

It needs full sun [54] 3 Full sun [57] 3 Medium [59] 2

Resistance todrought

Mature plants can cope with drylandconditions if rainfall 4500 mm/year [54]

2 It tolerates drought[57]

3 Low [59] 1

Resistance tofrost

It can tolerate frost [54] 3 It is grown inmountainous areas[56]

3 Thrives in many climates including the cold mountain [60] 3

Resistance towind


Score 11 12 93. Interaction with the PVsAlbedo (isrelated withleaf color)

The leaves are dark green above and downywhite below [54]

3 Leaves: dark green[57]

2 Silver-variegated leaves [59] 3

ET Hairy leaves [55] 1 Hairy leaves [56] 1 Hairy leaves [61] 1Height 1–2 m [54] 1 30–60 cm [56,57] 2 15–20 cm [59] 3Compactness Medium 2 High 3 High 3Score 7 8 104. Interaction with the buildingLeaf watercontent

Non-succulent 1 Non-succulent 1 Non-succulent 1

Dense foliage(insulation)

The foliage is dense [52] 3 It shoots spriggy [56] 3 Texture: thick density [59] 3

Score 4 4 45. Interaction with the external environmentRed. urb. temp.;Abs. s-w; Cseq.


Attr. i/p, incr.urb. biod

It is visited by bees/bumblebees [38] 3 It is visited by bees/small Apoidea [38]

3 L. maculatum and other Lamium species: important for bees andbumblebees; great importance as nectariferous plants [38]

3

Score 6 6 66. Other factorsAcoustic benefits H–DF 3 H–DF 3 H–DF 3Interest inmedicine, etc.

Food and flavoring, industrial uses,pharmaceutical/therapeutic, cosmetics, etc.[54]

3 Food, pharmaceuticaluses, etc. [56]

3 Lavender, groundcover [59] 3

Aesthetics High [52,54] 3 High [56,57] 3 High [59,60] 3Score 9 9 9Total score 44 49 49


great interest in medicine, cosmetics and food industry [54].Based on Table 4, R. officinalis has a very good interaction withthe building and the external environment (increase of biodi-versity and other benefits). It is a very melliferous plant,frequently visited by bees and it provides a clear unifloralhoney, with delicate odor, flavor and fine crystallization in theMediterranean area [38].

Origanum vulgareOriganum vulgare is an herbaceous perennial with specificblooming period [57]. From Table 4 it can be observed that O.vulgare is a plant more appropriate for the development ofsemi-intensive green roofs (due to its deep root). It has densefoliage and it shows good interaction with the building and theexternal environment. Other positive aspects are related to itsapplications in food, pharmaceuticals, etc. For the case of PV-green roofs this plant is not suitable; however, O. vulgare can beconsidered for the development of simple (without PVs) greenroofs in the Mediterranean region since it is a resistant plantand it can provide multiple advantages.

Lamium maculatumLamium maculatum is a flowering plant and Table 4 shows thattheoretically it could be used for the development of PV-greenroofs because it is compact, low-height and some varieties havesilvery/white leaves (good interaction with the PVs from albedopoint of view). In addition, plant foliage is dense [59] and thus,the interaction plant/building is also very good. However, L.maculatum has low tolerance to direct radiation and drought.

3) Fabaceae (Leguminosae)From Table 5 it can be observed that the studied plants fromFabaceae are not appropriate for PV-green roof (and in generalfor green roof) applications for several reasons (for examplethey have low/medium resistance to drought).

4) Other plant familiesa) Crassulaceae

Sedum clavatumSedum clavatum is a leaf succulent plant. The droughttolerance of the succulent plants makes them useful for

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

Trifolium dubium Trifolium repens Trifolium rubensCriteria Comments/Refs. Comments/Refs. Comments/Refs.

1. Suitability for extensive green roofsShallow roots It has a very well developed root system on a

relatively small area of soil [62]3 T. repens: shallow-rooted [67] 3 Red clover (T. rubens): thick tap root

grows 60–90 cm/year [72]1

Nutrient needs Low [63] 3 Ladino (T. repens) has high requirementof nutrients [68]

2 It grows on a wide variety of soilconditions [72]

3

Irrigation needs T. dubium Sibth.: low to medium tolerance toprolonged drought [64]

2 T. repens needs moist to heavy-moistureconditions [68]

2 Water requirements: average [73] 2


Infected by scorch caused by Kabatiella caulivora[65]

2 Medium [69] 2 There are pest-resistant varieties [74] 3

Score 10 9 92. Resistance to weather conditionsResistance to directradiation

Full sun [66] 3 Medium [70] 2 Full sun [73] 3


T. dubium Sibth.: low to medium [64] 2 It tolerates moderate drought [70] 2 Water requirements: average [73] 2

Resistance to frost High [64] 3 High [70] 3 High [74] 3Resistance to wind It is expected to be low because the foliage is

sparse1 H–DF 3 H–DF 3

Score 9 10 113. Interaction with the PVsAlbedo (is relatedwith leaf color)

Albedo is expected to be low because the foliageis sparse

1 Light green stems [71] 3 The leaves have silver hairs [73] 3

ET Transpiration is expected to be low because thecanopy has low density

1 Hairless leaves/stems [71] 3 Hairy leaves [73] 1

Height 30 cm [62] 2 15 cm [71] 3 45–60 cm [73] 1Compactness Low 1 High 3 Medium 2Score 5 12 74. Interaction with the buildingLeaf water content Non-succulent 1 Non-succulent 1 Non-succulent 1Dense foliage(insulation)

The foliage is not dense 1 Dense foliage [70] 3 Dense foliage [74] 3

Score 2 4 45. Interaction with the external environmentRed. urb. temp.;Abs. s-w; C seq.

It is expected to be low because the foliage issparse

1 H–DF 3 H–DF 3

Attr. i/p, incr. urb.biod

Trifolium: European unifloral honeys-nectanirefous species [38]

3 Trifolium: European unifloral honeys-nectanirefous species [38]

3 Trifolium: European unifloral honeys-nectanirefous species [38]

3

Score 4 6 66. Other factorsAcoustic benefits It is expected to be low because the foliage is

sparse1 H–DF 3 H–DF 3

Interest inmedicine, etc.

Lawn [66] 3 Hay; silage [70] 3 Forage [74] 3

Aesthetics Medium 2 Medium 2 High [73,74] 3Score 6 8 9Total score 36 49 46


the development of green roofs [1,5]. In the literatureseveral studies about Sedum have been reported [2–5,8,10,13]. Historically, Sedum species have been the mostcommonly used plants for green roofs because are tolerantto extreme temperatures and high winds while they needlimited water supply [13]. In terms of the reflectivity ofSedum green roofs, in Section 2.2 a related study waspresented [20]. In Table 6, the selected data for S. clavatumare given and Sedum considerable advantages can be seen.

This plant has high scores and thus, it is a very good choicefor the development of PV-green (as well as for simpleextensive green) roofs providing valuable benefits for thePVs (compact/low-height plant with good interaction withthe PVs, etc.) as well as for the building (energy savings dueto the succulent/protective layer over building roof). At thispoint it should be mentioned that the benefits which areprovided by S. clavatum (and in general by a succulent plantwith dense foliage) to the building are also associated with

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

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

1. Suitability for extensive green roofsShallow roots Sedum/green roof:

6 cm media depth [13]3 Shallow root

system [77]3 Sufficient growth: 7.5-cm

soil [81]3 Shallow root [86] 3 Sallow roots [88] 3

Nutrient needs Low [13] 3 Light fertilization[77]

2 Dianthus: medium [82] 2 It tolerates a wide rangeof soil types [86]

3 It needs fertilizing to keephigh turf quality [89]

2

Irrigationneeds

Low [13] 3 Medium [77,78] 2 Dianthus: water average [82] 2 It requires dry soils [86] 3 Water requirementsdepending on the use [89]

2


Sedum spp: powderymildew- resistant[75]

3 No disease or pestproblems [77]

3 Dianthus: insect/fungalattacks [83]

1 Resistant [87] 3 Pest problems [89] 2

Score 12 10 8 12 92. Resistance to weather conditionsResistance todirectradiation

High [1,13] 3 Full sun to partialshade [77,78]

2 Dianthus: sun to part shade[82]

2 It requires plenty ofsunlight [86]

3 High [89] 3


High [13] 3 Medium [77,78] 2 Medium [82] 2 It requires dry soils [86] 3 Medium [89] 2

Resistance tofrost

It is tolerant of extremetemperatures [13]

3 Medium [77,78] 2 High [82] 3 It is found in NorthAmerica, BritishColumbia, etc. [86]

3 Medium [89] 2

Resistance towind

High [13] 3 H–DF 3 High because of the strongfibrous root system [81]

3 H–DF 3 It is expected to be mediumbecause the foliage is notdense

2

Score 12 9 10 12 93. Interaction with the PVsAlbedo (isrelated withleaf color)

High [5,20] 3 Grey-green leaves[78]

3 Glaucous leaves [84] 3 Dull green [86] 3 Dark green [89] 2

ET Sedums limit waterloss due totranspiration [13]

2 Stems with hairs[79]

1 Glabrous leaves [84] 3 Hairy leaves/stems [86] 1 Green roof study: grassET4Sedum [90]

3

Height Low (r10 cm) [5] 3 10 cm [77] 3 50 cm [81] 1 Branched stems: 10–40 cm [86]

3 Upright shoots: 15–25 cm[88]

3

Compactness High [5] 3 High [77,78] 3 Medium 2 Medium 2 High 3Score 11 10 9 9 114. Interaction with the buildingLeaf watercontent

Succulent [1,13] 3 Non-succulent 1 Non-succulent 1 Non-succulent 1 Non-succulent 1

Dense foliage(insulation)

Dense foliage [5] 3 Dense foliage[77,78]

3 Bushy [81] 3 Dense foliage 3 Dense [89] 3

Score 6 4 4 4 45. Interaction with the external environmentRed. urb.temp.; Abs.s-w; C seq.

H–DF 3 H–DF 3 H–DF 3 H–DF 3 H–DF 3

Attr. i/p, incr.urb. biod

Sedum: visited byhoney bees, solitarybees [38]

3 Light yellowpollen [38]; beecrop [80]

3 Dianthus: important forbees-good quantities ofpollen [38]

3 It is visited by bees/bumblebees [38]

3 Attraction of butterflies [91] 3

Score 6 6 6 6 66. Other factorsAcousticbenefits

H–DF 3 H–DF 3 H–DF 3 H–DF 3 H–DF 3

Interest inmedicine,etc.

Sedums: few are usefulas food or medicinal[76]

2 Seeds formedicinalpurposes [80]

3 Dianthus spp.: edible flowers[85]

3 Medicinal [87] 3 Numerous uses: lawns,parks, playgrounds, etc. [89]

3

Aesthetics Medium 2 High [77,78] 3 High [82,84] 3 High [86] 3 Medium 2Score 7 9 9 9 8Total score 54 48 46 52 47


its advantages from thermal inertia point of view, as it wasmentioned in Section 2.3.1.The advantages of a PV-sedum (S. clavatum) roof have beenexperimentally verified by the study of Chemisana andLamnatou [5], for the climatic conditions of Spain. Moredetails about the results of that study were previouslypresented in Section 2.1 while the developed system isillustrated in Fig. 1(b).

b) BrassicaceaeLobularia maritimaLobularia maritima is also known as “Sweet Alison” and it isa plant native to the Mediterranean region [77]. In Table 6,the results for L. maritima are given. It can be seen thattheoretically this plant could be used for PV-green as well asfor simple green roofs (without PVs) (advantages: goodplant/PV and plant/building interaction; high aestheticvalue), taking into account that there are other plants moreresistant to extreme weather conditions and with lowermaintenance needs.

c) CaryophyllaceaeDianthus fruticosusDianthus fruticosus has busy appearance and it is a Medi-terranean plant. In the literature there is a study about theuse of this plant for green roof applications [81]. The resultsof that study revealed that D. fruticosus sub. fruticosus is apromising native plant for extensive green roofs in theMediterranean region. Nevertheless, from Table 6 it can beseen that D. fruticosus has some disadvantages (nutrient/irrigation requirements: medium; low pest/disease resis-tance, etc.). For the specific case of the PV-green roofsystems, this plant has one crucial disadvantage: its rela-tively high height. Therefore, even if albedo and ET coolingeffect have high scores (because of leaf grey color and leafglossy texture, respectively) other plants more resistant andmore compact should be considered. Conclusively, for sim-ple green roofs (without PVs) and under certain conditions,D. fruticosus could be a possible option with high aesthetics,taking into account that there are other plants moreresistant and with lower maintenance needs.

d) GeraniaceaeGeranium molleGeranium molle is a low-growing plant with small pinkflowers and it is also known as dove’s-foot crane’s-bill [86].In Table 6 the results for this plant are presented. It can beseen that G. molle has good interaction with the building aswell as with the PVs (low-height canopy; dense foliage,etc.); thus, it can be used for PV-green (as well as for simpleextensive green) roofs, taking into consideration that thereare other plants more compact.

e) EragrostoideaeCynodon dactylonCynodon dactylon is also known as bermuda grass and it is acreeping perennial grass [88]. From Table 6 it can be seenthat this grass is compact/dense and theoretically it could beadopted for PV-green (or simple green) roofs. However,there are other plant species (more resistant and with lowermaintenance needs) which are more appropriate for PV-green (or simple green) roofing systems.

Elaboration of the data for the considered plantsIn this section the data (scores) of Tables 3–6, for all theplants, are presented as graphs: without and with using

weighting factors (the symbols NWF (no weighting factor)and WF (with weighting factor) are used in the text). As itcan be seen from Fig. 4(a) and (b), the family Compositaeincludes plants with high scores: G. rigens and A. maritimus.They both have very good interaction with the PVs and thebuilding. On the other hand, C. pubescens could not beconsidered for PV-green roofs unless a compact variety suchas ‘Sunshine Superman’ is adopted. In terms of the familyLamiaceae, R. officinalis and O. vulgare are of interest butonly for simple (without PVs) green roofs. L. maculatumcould be a possible choice for PV-green configurations (highalbedo; low-height canopy) with main disadvantage its lowtolerance to direct radiation/drought. Regarding the selectedplants from the family Fabaceae, in general, are not promis-ing for PV-green applications.From Fig. 4(a) and (b), in the category “other families”, S.clavatum shows the highest NWF (54) and WF total score(8.8). This is mainly associated with the fact that this planthas a very good interaction not only with the PVs (highcanopy albedo, low-height/compact plant) but also with thebuilding (high-water content leaves; dense foliage). Anotherquite suitable plant for PV-green roof applications is G. mollewith basic disadvantage its low compactness. In additionfrom Fig. 4(a) and (b) it can be seen that L. maritima showshigh scores in terms of its interaction with the PVs and thebuilding but it is not resistant to extreme weather condi-tions.If only the criteria plant/PV and plant/building interactionare taken into account, from Fig. 4(b) it can be observed thatmost of the studied plants have good interaction with thebuilding (WF scores Z1.2). S. clavatum has the highest“plant/building interaction” WF score (1.8) and it showsthe best interaction with the building in comparison with allthe other studied plants; certainly, because of its ability to

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Fig. 4. The total scores for all the studied plant species: (a) without weightingfactors (NWF) and (b) with weighting factors (WF).


form a succulent “protective layer” over the roof. In terms ofthe plant/PV interaction, even if several plants show highWF scores (Z3), in practice only G. rigens and S. clavatumcould be considered as very good choices for PV-green roofsfor the reasons that were previously explained and alsobased on the experimental study of the authors (for thewarm months of Mediterranean climate [5]). Other alter-native solutions for PV-green roofs could be A. maritimusand G. molle. These plants, based on the results ofTables 3 and 6, are also expected to show very goodinteraction with the PVs and the building; however, in theliterature there are no studies which verify their suitabilityfor PV-green roofs under experimental conditions.

2.8. A comparison between a PV-gazania and a PV-sedum roof

In this section, results from the experimental study of Chemi-sana and Lamnatou [5] (Lleida, Spain; June–July 2013) are pre-sented and commented. In the frame of that study, a comparisonbetween the developed PV-gazania (G. rigens) and the PV-sedum(S. clavatum) roof was also conducted. The results revealed that themain relative power difference, for the five-day period considered,resulted in a percentage of 2.24%. In Fig. 5(a) (Ref. [5]), the poweroutput for PV-gazania and PV-sedum roof are illustrated. It can beobserved that PV-sedum system has 2.21% higher efficiency thanPV-gazania. The irradiance profiles are illustrated in Fig. 5(b) (Ref.[5]). The authors noted that the differences in the behaviorbetween PV-gazania and PV-sedum roof can be attributed to thehigher irradiance received on the module over Sedum because of

its higher albedo/reflection characteristics. In terms of plantcharacteristics, S. clavatum is a succulent plant with fleshy leaveswhich have the ability to store water and form a thick/high-watercontent layer over the soil. This succulent layer positively con-tributes to roof thermal characteristics and it keeps soil colderthan G. rigens which is an ornamental plant with narrow leaves.

Further research should be addressed regarding long termcharacterization, large-scale installations and behavior of the sys-tems during winter in order to extract more general results. Inaddition, for the experiments, sunny days with no wind and noclouds were selected (high temperature days) since these are veryrepresentative of the Mediterranean conditions. In Lleida the annualamount of sun hours over 2012 was 3064, it rained only 52 days(282.7 mm) while the mean monthly maximum temperature fromMarch to October was 26.6 1C (Source: [92]). In terms of thecomparison of the results of the experimental study of Ref. [5] withthe few PV-green experimental studies which are available in theliterature (Table 1), the authors noted that good agreement wasobserved in the power output differences, taking into account thedifferences because of the location and its associate climate [5].

3. Conclusions

PV-green roofs combine PV panels with simple green roofs, area new tendency in the building sector and they provide severalbenefits such as PV output increase because of plant/PV synergy.The present study is a critical review about multiple/crucial factorswhich are related with PV-green roofing systems. Representativestudies from the literature are presented and critical commentsare made for each case. Additional information about plant speciesappropriate for PV-green applications is also provided. The resultsreveal that:

� PV output increase depends on several factors such as plantspecies, climatic conditions, evapotranspiration (ET), albedo,etc. High soil cover percentage is important in order to increasealbedo. The reflected radiation depends also on leaf color andplants with light-colored leaves have higher albedo. For thespecific case of hot, dry climates, it is desirable the adoption ofa wet irrigation regime during summer in order to increase ETcooling effect.

� PV-green roofs provide additional benefits in comparison withsimple (without PVs) green roofs, such as: in situ production ofelectricity; PVs protect plants from direct exposure to sunlightand thus, plants growth better; plants protect PVs duringwinter; there is better utilization of roof available space.

� During the operational phase of the building, the benefits dueto the soil/plant layer and due to plant/PV interaction areremarkable.

� A crucial parameter for the development of a PV-green roof isthe selection of appropriate plant species. Certain factors suchas plant resistant to extreme weather conditions, albedo, ET,maintenance/irrigation needs, plant compactness/foliage den-sity should be taken into account. In general, succulent plantswith compact canopies are desirable because they fit at thespace below the PVs while they create a protective layer overthe building and they contribute positively to roof thermalinertia.

� The systematic classification of certain Mediterranean plants interms of their potential for PV-green roof applications andbased on certain criteria (with emphasis on plant/PV and plant/building interaction) reveals that S. clavatum shows the bestinteraction with the PVs and the building.

� Based on a recent experimental study [5], PV-gazania and PV-sedum are promising roofing systems under warm months of

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Mediterranean climate, e.g. for the case of Spain: they increasePV output while they reduce roof temperature in comparisonwith a PV-gravel roofing system. For cold climates, with lowtemperatures and low solar irradiances, PV-black roofs couldshow higher performance than PV-green roofs.

� Future experimental research about large-scale PV-green roofs(e.g. during winter in Mediterranean climate) it is necessary toprovide a more complete picture of the PV-green benefits. Ingeneral, further experimental investigations are needed toexamine PV-green roof performance under several conditions.

� In terms of PV-green environmental impact, the Life CycleAnalysis (LCA) study of Lamnatou and Chemisana [11] revealedthat PV-green roofs, depending on factors such as climaticconditions, plant species, ET, albedo, etc. which affect plant/PVsynergy, on a long-term basis, can compensate their initialimpact. Certainly, future improvements in terms of PV cellmanufacture and green layer materials can lead to furtherreduction of PV-green roof environmental impact.

� There are some crucial factors which are related with PV-greenroof cost-effectiveness (use of eco-friendly fertilizers, reusingsome of the waste materials, etc.).

Conclusively, PV-green roofing systems are a new technologyand should be promoted since they can provide considerableadvantages towards the development of environmentally friendlybuildings, especially for warm climates. Certainly, further researchis needed for the evaluation of large-scale PV-green roofs (e.g.during winter months, under Mediterranean climate). The presentstudy, by highlighting crucial factors, provides useful informationfor the development of future PV-green roofs.

Acknowledgements

The authors would like to thank “Ministerio de Economía yCompetitividad” of Spain for the funding (grant referenceENE2013-48325-R).

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