Testing the Potential Synergyof Green Roof-Integrated Photovoltaicsat the University of Toronto Green RoofInnovation Testing (GRIT) Laboratory

Dalia El Helow;

Jennifer Drake, PhD;

and

Liat MargolisUniversity of Toronto

35 st. George street, Toronto, on M5s 1a4

Phone: 647-716-3226

e-mails: dalia.elhelow@mail.utoronto.ca and jenn.drake@utoronto.ca and margolis@daniels.utoronto.ca

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Abstract

The 2009 City of Toronto Green Roof Bylaw requires green roofs on all new construction, yet solar and green roof technologies are rarely combined. Combining green (vegetative) roofs with photovoltaic (PV) technology may improve energy efficiency through evaporative cooling, while also providing other benefits such as stormwater capture and improved urban ecol-ogy. The speaker’s research investigates the effect of green roofs on ambient temperature, PV surface temperature, and power output. This is the first research of this kind in Canada and will benefit the building industry by producing Canadian-based performance data and generating design guidelines and tools.

Speaker

Dalia El Helow – University of Toronto, Toronto, ON

Dalia El Helow is in the second term of her master’s program in the Department of Civil Engineering at the University of Toronto. El Helow’s undergraduate degree is in mechanical engineering with concentrations in both energy and the environment. She brings a unique skill set to the GRIT Lab group and is an asset for bridging between environmental civil engi-neering and PV energy. Her focus is to investigate the effects of vegetated roofs on PV panel performance (specifically energy production and longevity).

Nonpresenting Coauthors

Jennifer Drake and Liat Margolis, University of Toronto, Toronto, ON

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Testing the Potential Synergyof Green Roof-Integrated Photovoltaicsat the University of Toronto Green RoofInnovation Testing (GRIT) Laboratory

ABSTR AC T This study examined the summer

performance of vegetative roof-integrated PVs at the Green Roof Innovation Testing Laboratory (GRIT Lab), located on the roof-top of the John H. Daniels Faculty of Architecture, Landscape, and Design at the University of Toronto. The study, conducted between June and August 2016, compared the temperature of PV on two high-perfor-mance roof surfaces (a vegetative “green roof” system and a reflective “white” roof membrane) at two PV module heights (2 and 4 ft.) for optimal PV performance.

The 2009 City of Toronto Green Roof Bylaw requires the construction of green roofs on all new construction above 2000 square meters (City of Toronto, 2013). That same year, the Province of Ontario launched the Feed-In Tariff program to encourage the development of renewable energy technolo-gy, attract investment, and create new clean energy jobs in Ontario (Ontario Ministry of Energy, 2016).

However, solar and green roof technolo-gies are rarely combined. Their integration simultaneously addresses climate change mitigation and adaptation goals and would contribute to the advancement of the energy and sustainable building sectors in Canada and beyond.

As a PV module heats up from solar radiation, its efficiency and durability are negatively affected by the increase in ambi-ent temperature (Hoffmann & Koehl, 2014; Singh, Singh, & Husain, 2008; Wyscoki & Rappaport, 1960). Combining green roofs (GR) with PV arrays can improve energy efficiency through evaporative cooling, while also providing other benefits such as storm-water capture and improved urban ecology. The combined GR and PV system provided noticeable improvement of PV performance by cooling PV surfaces above a green roof. Data analysis showed significant differenc-es among PV surface temperature, average

ambient temperature above GR and GR-PV surfaces, and ambient air temperature from the weather station. PV surface tempera-tures were up to 11ºC (4 ft.) and 9.9ºC (2 ft.) cooler than the local ambient temperature.

As expected, minimum and maximum average surface temperatures for PV mod-ules placed 2 ft. above the GR were 0.4°C and 0.2°C cooler than PV modules placed 4 ft. above the GR. Ambient air temperatures directly above the GRs were noticeably dif-ferent from a nearby weather station; the GR was cooler during the morning but warmer during the afternoon. Air temperatures above the GR and GR-PV systems were essentially identical—an average of 30°C (86ºF) during the summer study period.

In addition to current findings, power output data and PV surface temperature data over a white roof are being collected and will be used to verify and add to find-ings in this study.

INTRODUC TION An urban heat island (UHI) effect is cre-

ated by changes in the land surface and waste heat generated by building develop-ments, industrial activity, transportation, and their energy uses (Santamouris, 2014). Green roofs are one potential technology that can serve to counterbalance this phe-nomenon. Green roofs (also known as veg-etative roofs, eco-roofs, or living roofs) are a vegetative layer grown on a rooftop and typi-cally consist of a series of layers, including a root barrier, drainage board, filter fabric, geotextile, soil-less growing media, plants, and irrigation system, installed on top of a waterproof roofing membrane. GR sys-tems are a sustainable building technology that provides numerous environmental ben-efits, including cooling of local surroundings via evaporative cooling during hot summer months (Natural Resources Defense Council [NRDC], 2012). Localized cooling near a green roof is caused by the evaporation of

water from the growing media, as well as transpiration from plants as a byproduct of photosynthesis.

In addition to cooling benefits, which reduce the UHI effect, a GR can provide a number of benefits to individual build-ing owners, neighborhoods, and cities as a whole. At the building level, a GR can increase the lifespan of a building’s roof, reduce the energy used and associated costs for cooling the building, and improve aesthetics. At the neighborhood or city level, a GR can reduce greenhouse gas emissions by reducing cooling needs, reduce storm-water runoff volume and pollutant loading, improve air quality by reducing tempera-ture and capturing air pollutants, and pro-vide habitat space (NRDC, 2012).

According to researchers at the GRIT Lab in Toronto, Canada, cooling effects depend on GR design characteristics (Maclvor, Margolis, Perotto, & Drake, 2016). Maclvor et al. observed that roofs with sedum and compost-based growing media provide more cooling effects than roofs with grasses, wildflowers, and mineral-based media. Irrigation of a GR provides water to support plant health and increases transpiration rates. In Japan, Onmura et al. (2001) found that a well-irrigated GR provided higher evaporative cooling effects and cooled the slab below the green roof by about 30°C (86°F) compared to concrete slabs without a GR. The irrigated GR also provided a 50% reduction in the heat flux into the room below the roof slab.

Toronto is the first city in North America to adopt a bylaw that requires the con-struction of GR on new developments with a gross floor area of 2,000m2 (21,527 sq. ft.) or greater. The bylaw was adopted by Toronto City Council in May 2009, under the authority of Section 108 of the City of Toronto Act (City of Toronto, 2013). The Toronto Green Roof Construction Standard within the Toronto Green Roof Municipal

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Code sets out minimum requirements that must be conformed to, which meet the city’s objectives and the Ontario Building Code requirements.

Key objectives of the bylaw include reduc-ing UHI effect, stormwater impact, building energy consumption, and improving air qual-ity. Within the United States, programs such as the City of Chicago Green Permits, Green Roofs for Healthy Cities North America, and the Washington DC Green Roof Rebate Program all encourage the development of GR, with the aim of creating healthier urban environments and reducing building energy consumption (Green Roofs for Healthy Cities, 2016; City of Chicago, 2016; Department of Energy and Environment, 2016). The top five North American cities that have supportive policies and programs that encourage GR implementation (listed in order of highest area of GR installed in 2014) are Washington DC, Toronto, Philadelphia, Chicago, and New York City (Green Roofs for Healthy Cities, 2014).

In addition to GRs, PV technology is one of the fastest-growing sources of elec-tricity in Canada (Government of Canada, 2016) and the United States (Solar Energy Industries Association [SEIA], 2016). In 2013, the Province of Ontario launched the Feed-In Tariff program to encourage the development of renewable energy technolo-gy, attract investment, and create new clean energy jobs in Ontario (Ontario Ministry of Energy, 2016). In the first quarter of 2016, the U.S. installed 1.7MW of PV, adding more new capacity to the U.S. during this period than coal, natural gas, and nuclear energy

combined (SEIA, 2016). Currently, the U.S. has a total installed capacity of 29.3GW in solar energy (SEIA, 2016).

As a PV module heats up from solar radiation, the power output decreases and the degradation rate of PV cells increases (Hoffmann & Koehl, 2014; Singh, Singh, & Husain, 2008; Wyscoki & Rappaport, 1960). Singh, et al. (2008) observed that the performance of solar cells is negatively influenced by temperature as its perfor-mance parameters, open-circuit voltage, and efficiency decrease linearly with tem-peratures. In another study, a decrease in maximum voltage of approximately 2mV/°C was observed (Wyscoki & Rappaport, 1960). It has been observed that PV thermal degra-dation follows the Arrhenius behavior. This means that the rate of thermal degrada-tion increases exponentially as temperature increases (Hoffmann & Koehl, 2014; Kurtz, et al., 2011).

PVs are made of semiconductors; the parameter most affected by an increase in temperature is the open-circuit voltage. This is due to a decrease in the semiconductor electron mobility, which is limited by dif-ferent forms of scattering (David Wolpert, 2011). Phonon scattering, one of the most important sources of scattering in semicon-ductor material, refers to the potential for an electron to be scattered by a lattice vibration. As temperature increases, lattice vibrations increase, and the probability of an electron being scattered also increases, thus decreas-ing mobility (David Wolpert, 2011). Some research has suggested that combining GR with PV can improve PV energy efficiency

and reduce the rate of thermal degradation through evaporative cooling by reducing the operating temperature of the PV panel surface (Chemisana & Lamnatou, 2014; Hoffmann & Koehl, 2014; Meral & Dinçer, 2011; Singh, Singh, & Husain, 2008), while providing their associated renewable energy and urban vegetation benefits.

The goal of this study is to investigate the effects of GR on ambient temperature and PV back-surface temperature. In preparation for this study, two hypotheses were devel-oped. The first was that the GR will provide cooler ambient conditions, and thus, a cooler back-surface temperature of the PV modules compared to ambient conditions. The second hypothesis was that a vegetated surface that is positioned closer to a PV panel would have a more enhanced cooling effect than the higher height. This research is ongoing and is carried out at the GRIT Lab. This is the first research of this kind in Canada and will benefit the building industry by pro-ducing Canadian-based performance data, generating design guidelines and tools, and building technical experience that can be applied to the next generation of sustainable building design. This paper will present pre-liminary temperature data collected in July and August 2016.

METHODOLOGY Site

The GRIT Lab (Figure 1) is located on the roof of 230 College Street at the Daniels Faculty of Architecture, Landscape, and Design building at the University of Toronto’s St. George Campus.

Figure 1 – Green roof integrated PV system on the roof of the GRIT Lab at the Daniels Faculty of Architecture, Landscape and Design building, 230 College Street, University of Toronto, St. George Campus.

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Figure 2 – Thermocouple distribution at the GRIT Lab (Duke, 2015).

The integrated installation includes 40 solar modules (MEMC-P280AMA) arranged in four rows above extensive GR beds at two heights (2 and 4 ft.), planted with a blend of grasses and wildflower species in a bio-module tray system 5.7 in. deep (Bioroof Systems). Each PV module has a maximum power rating of 280 W, and together, the array is capable of generating up to 11.2 kW. The approximate annual power output that can be expected from the GRIT Lab PV system is 14,062 kWh, approximately 7% of the 180,600 kWh annual consumption of electricity by the Daniels Faculty build-ing. This study builds on previous research projects completed at the GRIT Lab that focused on air temperature cool-ing by extensive GR (Maclvor, Margolis, Perotto, & Drake, 2016), shading effects of PV panels on the evapotranspiration process of extensive GR (Jahanfar, Drake, Sleep, & Margolis, 2016), energy performance simulation for build-ing envelopes integrated with veg-etation (Li, Byrne, & Kesik, 2014), as well as an ongoing research into the effects of PV systems on GR hydrolog y.

Temperature Sensor Layout and Data Collection

The GR-PV test setup is equipped with three different ther-mopile arrangements, including differential, averaging, and dis-

crete. Thermocouples function based on the Seebeck Effect; with a voltage created by the junction of two dissimilar metals, as the temperature increases, the volt-age increases (Brenig, 1989). Twenty-four thermopiles (Type T thermocouple wiring – 24 AWG copper/constantan, Multi/Cable Corporation) were installed and are collect-ing data at five-minute intervals (Campbell Scientific, CR1000 Data Logger & AM25T Multiplexer) including: eight differential PV surface temperature readings, eight aver-age PV surface temperature readings, and eight discrete air temperature readings. All thermopile arrangements were calibrated at an external lab with measurements

Figure 3 – Averaging and differential thermopile locations over a white roof (WR) and a GR for one row.

representing a 95% con-fidence level (ISO 17025 Accredited calibration). T RCA’s S ust a inable Technologies Evaluation Program (STEP) solar technologies technicians provided technical and design guidance for the layout and construction of the thermopiles.

The PV surface dif-ferential and averaging thermopile distribution is shown in Figure 2. Each differential ther-mopile measures the difference of the aver-age temperature of three points on the back sur-face of a PV module over a green and white roof

as shown in Figure 3. Averaging thermopiles measure the average surface temperature of three points located on the top corner and middle and bottom of the center column cells on the back of 16 PV modules over green and white roof surfaces (Figure 2). These locations were chosen to provide the closest measurement representing the entire panel’s environment and diverse conditions. There are two averaging thermopiles and two dif-ferential thermopiles installed on each of the four rows of panels and over green and white surfaces. An additional, averaging thermopile will be installed in the fall of 2016 on the second module of the second row for differential thermopile data validation.

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Figure 4 – Average hourly temperature data from July 27, 2016, to August 10, 2016.

Figure 5 – Hourly temperature data from July 27, 2016, to August 10, 2016.

Discrete thermopiles measure the ambient air temperature via the average of four thermocouples installed at 29.5 and 59 in. above green and white roof surfaces, respectively. All discrete thermopiles are installed via solar radiation shields (Davis Instruments Solar Radiation Shield), serv-ing to keep the thermocouples at or near ambient air temperature, while the shield’s white color reflects solar radiation.

The ambient air temperature recorded by the onsite weather station at the GRIT Lab (HMP45C relative humidity and air temperature probe with radiation shield) is used for comparison with measured air temperatures.

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Temperature data collection began in June 2016, and solar array operation began in July 2016. Data collection is ongoing and will be continuing into 2017. As of August 31, the GRIT lab PVs have generated over 1800 kWh.

Data Analysis Descriptive statistics, including range,

mean median, and standard deviation were calculated for all thermopile locations. Time-series plots were created to examine differ-ences in surface and ambient temperature compared to ambient air temperature mea-sured by the weather station. A box and whisker plot was created to examine differ-

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ences in temperatures for different PV-GR heights, and a t-test was used to determine if the datasets were statistically different. All statistical analysis was performed for a 95% confidence level using a normal distribution.

RESULTS AND DISCUSSION PV Surface Temperature Above a GR Surface Compared to Ambient Air Temperatures

Figure 4 and Figure 5 illustrate a sample of diurnal PV surface temperatures over a green surface at two heights (2 and 4 ft.), ambient temperatures above a GR surface, ambient temperatures above GR and below PV (GR-PV), and ambient air temperature from

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the GRIT Lab weath-er station. Figure 4 illustrates the average hourly temperatures daily over 24 hours of the two GR beds at 4- and 2-ft. heights from the integrated PVs from July 27 to August 10, 2016. Table 1 – Descriptive statistical results.

Measurement Description Mean Median Range σ

Average PV surface temperature above GR (4 ft.) 23.5 21.6 5.2 – 45.4 8.2

Average PV surface temperature above GR (2 ft.) 23.6 21.7 4.8 – 45.2 8.2

average ambient GR air temperature 29.8 31.8 11.6 – 47.6 8.0

Average ambient GR-PV air temperature (4 ft.) 29.9 31.9 11.6 – 47.6 8.0

Average ambient GR-PV air temperature (2 ft.) 29.9 31.9 11.7 – 47.4 7.9

Ambient air temperature (weather station) 25.6 26.8 14.3 – 34.7 3.2

Figure 5 illustrates hourly temperatures over the same 14-day period as a time series.

As shown in Figure 4, on average, at 12 p.m., the PV surface temperatures were 11ºC and 9.9ºC cooler (at 4 and 2 ft. GR-PV, respectively) than the ambient air tempera-tures recorded by the weather station. The largest temperature difference occurs at 2 p.m., when the PV surface temperatures are 15.4ºC and 13.8ºC cooler (at 4 and 2 ft. GR-PV, respectively) than the ambient air temperature 29.5 in. below the panel. Surprisingly, and contrary to the study’s initial hypothesis, the PV surfaces of the 4-ft. GR-PV system are consistently cooler than PV surface temperatures of the 2-ft. GR-PV system. If vegetative cooling were a dominant factor in determining the PV surface temperature, it would have been expected that the roof with a smaller dis-tance (i.e., 2-ft. GR-PV set-up) between the PV panels and the GR vegetation would be cooler. These early observations suggest that the thermal effect of plant evapotrans-piration is too small to have an observable effect on PV panel surface temperature. Instead, other factors, such as shading from nearby buildings and/or obstructions, or localized wind effects created by the PV array, may be governing the PV surface temperatures.

Ambient air temperatures above the 4- and 2-ft. GR are essentially identical, and temperature differences are always less than 0.3ºC. Lastly, average ambient GR-PV air temperatures were approximately 2°C cooler than the weather station ambient air temperatures during the morning, but were significantly warmer (up to 10°C), than the weather station by late afternoon. At this time, it is unclear why air temperatures recorded at the GR-PV test setup are so much warmer during the afternoon than air temperatures recorded at the lab weather station.

Relatively high ambient temperatures, up to 47°C, are recorded from the average

any observable effect on PV power output. Figure 6 illustrates that overall, the average surface temperatures for PV arrays placed 4 and 2 ft. above the GRs between 6 a.m. and 9 p.m. from July 27, 2016, to August 10, 2016, are essentially identical. Descriptive statistics in Table 1, using a larger data set with measurements taken at five-minute intervals, display that the minimum and maximum average surface temperatures for PV modules placed 2 ft. above the GR are 0.4°C and 0.2°C cooler than PV modules placed 4 ft. above the GR. Furthermore, a statistically significant (p < 0.05) difference of 2.1°C and 2°C (Table 1) was observed between the mean ambient temperature at the weather station and the mean PV surface temperature for PV arrays placed 4 and 2 ft. above the GR, respectively.

CONCLUSION Data analysis showed significant tem-

perature differences between PV surface temperature, average ambient GR tempera-ture, average ambient GR-PV temperature, and ambient air temperature from the weather station. However, many of these differences are too small to have observ-able effects on PV performance. The aver-age ambient GR and GR-PV temperatures maintained the highest during the daylight hours and could have been due to solar radiation being reflected from the white roof surface and the white back surface of the PV modules, causing a rise in the surround-ing ambient temperatures of the integrated system and below the PV modules (U.S. Department of Energy, 2010). Additional analysis of the installed differential thermo-piles will allow for comparison of PV surface temperature over a GR and PV surface temperature over a white roof, as well as ambient temperature below PV arrays over a white roof and PV surface temperature over a white roof.

While other researchers (Chemisana & Lamnatou, 2014) have observed improvements

ambient GR and GR-PV air temperature sensors. This may be a result of solar radia-tion being reflected from the surrounding white roof surface and the white back-surface of the PV modules, causing a rise in the ambient temperature around the inte-grated system and below the PV modules (U.S. Department of Energy, 2010). Unlike trapping radiation at the roof layer via a GR, which absorbs solar energy for vegetation photosynthesis and solar panels for ener-gy generation, white roofing reflects solar radiation and emits solar heat back into the atmosphere to reduce the amount of heat absorbed by the roof and transferred to the building below. This can be supported by the significantly cooler ambient tempera-tures recorded by the GRIT Lab weather station, which is more than two meters above the roof surface. Going forward, the solar reflectance and thermal emittance values, using aged (weathered reflectance) values as opposed to initial values, will be calculated to predict the solar reflectance of the white roofing at the GRIT Lab. It is important to note that roofs that are not washed frequently or made of materials that absorb solar reflectance well can contribute negatively to the benefits of white roofs (U.S. Department of Energy, 2010).

Daytime PV Surface Temperatures Above the GR Surface

Figure 4 and Figure 5 illustrate that PV arrays placed 4 ft. above the GR surface are generally cooler (up to 1.6°C cooler) than those placed 2 ft. above the GR sur-face (Figure 2). Table 1 includes descriptive statistics, including mean, median, range, and standard deviation (σ) for the collected temperature data (July 27, 2016, to August 10, 2016).

Although paired t-tests confirm that the difference in mean temperature, 0.1°C (Table 1), between the two PV surfaces is statisti-cally significant (p < 0.05), a temperature difference of this size is too small to have

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Figure 6 – Boxplots for daytime average PV surface temperatures for different PV to GR heights, from July 27, 2016, to August 10, 2016. Measurements taken in five-minute intervals.

in PV power output as a result of integration with vegetation, these studies have relied upon lab-scale installations (i.e., small) with a PV panel sitting directly above the vegetation. This study examines a full-scale GR-PV installation, and the physical sepa-ration between the GR and PV surfaces is quite large. One of the most important out-comes of this work will be the successful demonstration that GR and PV technologies can be successfully constructed and oper-ated within the same roof space. To date, the presence of the GR has had no negative impact on PV operation or power produc-tion.

The results presented in this paper are only for two weeks in July and August. In general, the weather during this time was extremely hot and dry. The green roof plants, while irrigated, were likely experi-encing water stress conditions and likely reduced their daily rate of transpiration in order to conserve water. In order to deter-mine whether or not the GR surfaces have an impact on PV surface temperature, and thus PV power output, a multiyear data set is needed. Data collection at the GRIT lab is ongoing and will be continuing through to fall 2017.

The integration of GRs and solar PVs may be the answer to enhancing PV per-formance and durability by the observed cooling of PV surface temperatures and the studied shading effects (Jahanfar, Drake,

Sleep, and Margolis, 2016). While the data presented show no substantial benefit from integrating GRs with the PV system, this can only be confirmed through long-term monitoring. Further temperature analysis using differential thermopiles, thermal pat-tern analysis using thermal imaging, and energy analysis using data collected from the currently installed TIGO energy invert-ers is planned as a continuation to this study to further understand the degree of effectiveness GRs and PV integration bring to the Toronto climate and beyond.

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