1/4/2014
Bachelorthesis Earth Sciences
|
Els Aarts, 10002850
U
NIVERSITY
OF
A
MSTERDAM
C
OMBINATION OF
G
REEN ROOFS
&
PV
2
T
ABLE OF CONTENTS
Abstract ... 2
1. Introduction ... 3
2. Theoretical framework: Green roofs, solar panels and their relation. ... 5
2.2 What are solar panels and their temperature coefficient? ... 7
2.3 Why combine solar panels and green roofs? ... 9
3. Methods ... 9 4. Results ... 12 5. Discussion ... 22 6. Conclusions ... 25 7. Acknowledgements ... 25 8. Literature list... 26 9. Appendix ... 28 9.1 Materials ... 28 9.2 Thermal pictures... 29
A
BSTRACTIn this research, the effect of the presence of a green roof (a roof covered with sedum plants), on the efficiency of solar panels is examined. According to different companies, the efficiency of solar panels would increase with 6% when they are placed on a green roof, due to the cooling effect that a green roof has on its surroundings. This is because high temperatures have a negative influence on solar panel efficiency. Two roof plots are made on a green and black roof, with two solar panels on each plot. Measurements are taken on temperature, radiation, wind and power output. A comparison is made between these measurements, to examine the eventual differences between the roof plots. The results did not provide a significant conclusion on the hypothesis that a green roof influences the efficiency of solar panels. The results are not consistently indicating either a positive or a negative correlation. The solar panels on the black roof seem to be slightly warmer, but this does not seem to influence the power output of the solar panels negatively. Some reasons for the lack of significant results could be that the plots were very small, the measuring time short and a shortage of warm, sunny days. Therefore, this research could be seen as a pilot study for further research.
3
1. I
NTRODUCTION
In the rapidly changing world of today, humankind is trying to keep in pace with the ever-growing energy demand of civilisation. Technological choices largely determine the long-term characteristics of society, including impacts on the environment. (Grübler et al., 1999) Because of global warming and the depletion of conventional energy sources, a change in both the production and use of energy is needed. This global warming is caused by the emission of anthropogenic greenhouse gases, of which the most important one is CO2. (Summary for
policymakers, IPCC 2007). A large part of the CO2 emitted is caused by the use of fossil fuels
(Bernstein et al., 2007). Therefore, technologies are developed to produce energy in a more sustainable way, such as the generation of renewable energy from renewable sources, as sunlight, wind and water. Also, technologies to increase energy use efficiency are developed, on both the larger industrial scale as in smaller, domestic appliances. These two concepts, energy efficiency and renewable energy, are said to be the twin pillars of sustainable energy policy. (Prindle, 2007). Innovative renewable energy technologies have a positive effect on energy security, employment and air quality. (Sims et al., 2007). These include hydropower, wind, geothermal, bio and solar energy. (Metz et al., 2007). The appliance of these technologies is often included in environmental policies; for example the use of solar photovoltaic (PV) panels has been part of the policy in both Japan and California. (Levine et al., 2007). The other part of sustainable energy policy is increasing energy use efficiency. It is often more cost-effective to invest in end-use energy efficiency improvement than in increasing energy supply to satisfy energy demand for energy services. (Metz et al., 2007). A potential environment to combine these two goals of renewable and efficient energy, is integrating technologies in the building sectors. The potential of reducing the emission of greenhouse gases in the building sector is between 21% and 54% of the national baseline for buildings in developed countries. (Barker et al., 2007). Two kinds of technologies that have potential of increasing energy efficiency and producing renewable energy are the use of PV solar panels for generation of electricity and the implementation of green roofs, to reduce energy use in buildings. However, the potential of the combination of these two is not clear; how they can be integrated in the existing energy flow in buildings and especially how they influence each other and may improve each other’s performance. This influence is specified to the effect that a green roof might have on the performance of the solar panels. According to different companies, such as ZinCo (Weng, 2011) and SolarSedum, green roofs enhance the performance of solar panels with their cooling effect. This relation will be explained further in section 2.3, “Why combine green roofs and solar panels?”
Although a lot of research is done on the effect of green roofs on the heat flux on a roof, which mostly concludes that a green roof has a mitigating effect on the roof temperature. (Kolokotsa et al., 2013), not much research is done on the relation with solar panels yet. In the 2007 IPCC report, no research on the combination of green roofs with solar panels is mentioned. Also, when searching through databases such as Science Direct, no peer reviewed articles are found. However, research that is done by Dominguez et al. (2011), states that PV solar panels have a cooling effect on the bare roof underneath. The research that is done by the company Zinco, which concluded that a green roof increases the efficiency of solar panels by 6% (Weng, 2011), is not freely available. Currently, research is done by both the Hogeschool van Rotterdam and the NIOO on this subject; however, this research is also not available and not finished yet.
4
Research question
The aim of this current research, for the University of Amsterdam, is to answer the following research question,
“Does a positive relation exist between the presence of a green roof under a solar system, and the efficiency of the solar system? “
From this, a null hypothesis and alternative hypothesis can be derived as follows;
H0: No difference in efficiency exists between a PV solar system on a bare roof and a green roof plot.
H1: The solar panels on the green roof plot have a higher efficiency (>6%), than the solar panels on the bare roof plot.
The questions below can be used to help answering the research question.
- Is a difference in efficiency caused by the mitigation of temperature fluctuations by the green roof, and not by other factors such as wind speed and direction, precipitation and solar radiation?
- Is this mitigation caused by the different circumstances in the environment above green roofs and bare bitumen roofs, in either evapotranspiration or net radiation, or both? - Is the investment in a combination of a green roof with a solar system economically
viable in a time period of 20 years?
In this paper, at first solar panels and green roofs are discussed and the combination of the two. Then, the research methods and the results are discussed. Finally, an answer to the research question is formulated and the hypotheses will be accepted or rejected.
5
2. T
HEORETICAL FRAMEWORK
:
G
REEN ROOFS
,
SOLAR PANELS
AND THEIR RELATION
.
2.1
G
REEN ROOFSA green roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane (Kolokotsa et al., 2013). It may also include additional layers such as a root barrier and drainage and irrigation systems. Two kinds of green roofs exist; intensive and extensive green roofs. Intensive roofs are thicker and can support a greater variety of plants, but are also heavier (which is the main limiting factor in the green-roof business) and require more maintenance. (Kolokotsa et al., 2013). Having an intensive roof is comparable to having a garden on the roof. Another kind of a green roof is the extensive roof, which are covered in a light (not heavy) layer of vegetation, for example sedum, which is a succulent plant. These extensive roofs need barely any maintenance, and are more suitable to combine with a solar system.
Green roofs serve several purposes for both the building it is structured on as the environment.
Effects on the building
Green roofs have a stabilizing effect on temperature fluctuations on the roof. This is because green roofs have a higher albedo than bare, often black roofs and therefore absorb less heat and reflect more radiation. The mitigation of temperature fluctuations is also caused by the evapotranspiration of the plants on the roof (Oberndorfer et al., 2007).
One of the advantages of having a green roof is that a green roof has a much longer durability than a normal, ‘bare’ roof. (Teemusk and Mander, 2009). These conventional roofs are often made of (black), bitumen material or from plastics. On such a roof, temperatures can vary between -20 °C in a cold winter night and 80 °C at a hot summer day. Every year, the roof is exposed to these extreme temperature fluctuations, and in summer the difference between night-time and daytime temperatures on the roof can be as much as 70 °C. This causes a rapid wear and tear and a fast aging process for the roof. Expected is that green roofs can extend the lifetime of the roof with decades. (Teemusk and Mander, 2009).
The mitigation of temperature fluctuations also affects the energy exchange with the space under the roof. In summer, the roof has a cooling effect on the upper rooms in the building. In winter, with dry weather the roof also has an isolating effect. (Kolokotsa et al., 2013). However, with rainy winter weather, which occurs quite often in the Netherlands, neither an isolating nor a cooling effect has been scientifically proven. (van Praag, 2011).
Effects on urban climate
The effect on the urban climate is that green roofs have a stabilising effect on temperature- and humidity fluctuations in the city. Therefore, they also have a mitigating effect on the Urban Heat Island (UHI) effect, which is a phenomenon that the temperature in urban areas is higher than in the surrounding rural areas. (Kolokotsa et al., 2013). This effect is one of the causes for higher mortality rates in vulnerable groups during heat waves in summer, and also increases energy
6 use for air conditioning during summer (van Praag, 2011). The UHI is caused by the use of ‘dark’ surfaces in the city, such as asphalt or bitumen roofs, because these surfaces absorb solar radiation and thereby increase temperature, and by heat of anthropogenic origin, such as heating, cars, and industry. (Rizwan et al., 2007).
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2.2 W
HAT ARE SOLAR PANELS AND THEIR TEMPERATURE COEFFICIENT?
A photovoltaic (PV) solar panel is a package of connected photovoltaic cells, which convert the energy of light directly into electricity by the photovoltaic effect. The panels are connected to an SMA enphase microconverter, which hangs under each solar panel. This converter changes the AC to DC, and also reports the output of the solar panels every 5 minutes. For the efficiency of the solar panels, a few factors are important, for example the infrared radiation. According to the manufacturer of the used solar panels, the efficiency of the solar panels is shown in Table 1.
Table 1: Varied Radiation Efficiencies
Irradiance 200W/m2 400 W/m2 600 W/m2 800 W/m2 1000 W/m2
Efficiency 15.8 % 16.2 % 16.2 % 16.1 % 16.0 %
The efficiency of solar panels is not only dependent on the amount of irradiance, but also on temperature (Al-Baali, 1989); how this works is explained briefly in the following section.
The maximum power of one solar panel is 250 W under optimal conditions; however, in reality this amount is never reached. The power output under NOCT (Normal Operating Cell Temperature) is 185 W, according to the manufacturer of the solar panels. (Renesola).
The electrical performance of photovoltaic modules is tested under “Standard Test Conditions” (STC) with different parameters. The Module Operating Temperature is the optimal temperature at which the photovoltaic modules operate. This is usually between 25 – 28 °C. (Weng, 2011). The temperature coefficient (pTemp) determines how much the efficiency of the solar panels changes when ambient temperature increases. (King, 1997). For the panels used in this research, the pTemp is -0.40%/°C. This works as follows ;
With the evaluation of temperature coefficients in correlation to the performance of installed systems, temperature behaviour of PV systems can be characterized (Makrides et al., 2009). In the paper of Makrides et al., 13 different PV systems which are placed in different climates are tested for performance in relation to the ambient temperature. It concludes that with a onstant irradiance, a decrease in voltage is evident when the temperature rises. (Makrides et al, 2009).
Because the power output P is defined by multiplying the voltage U and the current I, a change in voltage also influences the power output.
Also, the temperature coefficients where calculated through analysis of the obtained data by sensors that were installed outdoors. The PMPP (mpp means maximal power point) was plot
against the module temperatures. The phrase ‘normalized DC power’ means that P is normalized, which is a method to be able to compare power output of a PV panel that is measured over short time intervals. (Haeberlin & Beutler, 1995). A linear fit provides the temperature coefficient γPMPP (see Figure 1)
8
Figure 1: Data points of the SolarWorld multicrystalline-Si system in Stuttgart. A linear fit gives the temperature coefficiency PMPP=−0.424 %/K (Makrides et al., 2009)
When the measured temperature coefficients of Makrides’ research were compared to the coefficients provided by the manufacturer, the results demonstrated good agreement with an average relative error of <10%. (Makrides et al., 2009).
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2.3 W
HY COMBINE SOLAR PANELS AND GREEN ROOFS?
As said before and according to literature, a big difference in surface temperature between bare roofs and green roofs exists. Also, the ambient temperature above the roofs is influenced by the surface temperature. According to Weng (2011), temperature above bare roofs is about 50 – 60 °C, according to other studies temperatures can even rise to 85 °C. On the contrary, temperature above green roofs reaches (in the Netherlands) a maximum temperature of 35 °C, according to the website of Solar Sedum.
The combination of these two statements leads to the hypothesis that the placement of a PV (PhotoVoltaic) system on a green roof enhances the efficiency of the solar panels. According to research done by (Weng, 2011), green roofs increase the efficiency of solar panels with an average of 6 %.
3. M
ETHODS
To be able to falsifythe above hypothesis and to be able to provide an answer to the research question, a measurement setup is installed at the roof of the Solar Sedum office, at Joan Muyskensweg 19, Amsterdam. A green roof of 16,3 m2 is constructed, and right next to it a plot
of approximately 17,3 m2 is made clear of gravel, resulting in a ‘black roof’ plot. Two solar panels
are placed upon each of these two plots, so the system consists of four solar panels in total. A scheme of the two plots is added in the Appendix. For the construction of this green roof and the solar panels, the help and expertise of Solar Sedum is used, which is a company that is specialised in the combination of sedum roofs and solar systems.
Then, the measurement gear was installed at the two different plots; on the bare bitumen roof (Blackplot), and at a green roof, covered with sedum containers(GreenPlot).
- Temperature at 3 places per plot, namely (SurfaceTemp) at the surface of the (green) roof, SolarTemp, which is the temperature at the solar panels, measured with a temperature sensor right at the underside of the solar panels, in the middle of the surface. (AirTemp) is the temperature measured at approximately 80 cm above the surface. It is important to not measure these variables too much near the edges of the green- or bitumen roofs, because then they will be likely to influence each other.
- Humidity is measured at the two plots, at approximately 80 cm above the surface. This is done to measure a difference in evapotranspiration between the roofs; if the absolute humidity above the green roof is higher, that indicates a higher evapotranspiration. - Net short radiation and Albedo are measured at the two plots.
- Wind speed at both sites, at approximately 80 cm above the surface - Soil moisture
- Short wave radiation received by solar panels, at 1 site, with the same inclination as the solar panels
The instruments are fixed to two poles, one for every site, and two dataloggers are used to measure all instruments every second and store the averages every 5 minutes.
10 Also, the efficiency, or performance of the solar panels is measured. This is done with the micro converters which are connected to the solar panels, each panel has its own micro converter. The power output, along with other variables such as temperature, are sent to the SMA Envoy, which registers the power output of the solar panels only, and makes it accessible over the internet. A picture of the roof plots is provided in Figure 2.
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Figure 3: Schematic visualisation of roof plots
In Figure 3, the surfaces of the roofs and the solar panels are visualised at the right scale. Also, the measurement gear is placed in the map, but these are not at scale and not placed exactly as the map indicates. Below a legend for the different types of measurement gear. G = Pyranometer,
DL = Datalogger, A = Albedometer, P = Tipping bucket (precipitation), W = Wind Speed, T = Temperature, H = Relative Humidity, M = Soil Moisture
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4. R
ESULTS
In this section, the results of the research are visualised and discussed. The time period over which these measurements are taken, spans from June 5th (day 156), 2013 at 03.55 hours until
June 12th (day 163), 13.20 hours. Then, problems with the equipment on the green roof
occurred. When the equipment was replaced, a second time series is measured which spans from June 18th (day 169) until June 24th (day 175). Unfortunately, for day 169, the
measurements start at 15.30, due to these technical problems. This second time series is added wherever it either clarifies or disturbs the results of the first time series, and not added for the measurements where they do not add extra information.
The variables are measured every second, and an average is saved over every 5 minutes. This has resulted in 2131 values per variable. The results are plotted and discussed per variable. Thereafter, some infrared pictures of the whole plot will be displayed and discussed.
Figure 4: Roof Surface Temperature: Temperature on the green roof and the black roof: A: day 156-163 (5-12 June) 0 5 10 15 20 25 30 35 156 156 157 157 157 158 158 158 158 159 159 159 160 160 160 161 161 161 162 162 162 162 163 Tem p e ratu re ° C Date
Surface Temperature
SurfTempGreen SurfTempBlack13
Figure 5: Roof Surface Temperature on the green roof and the black roof: day 169-175 (18-24 June) Table 2, Roof Surface Temperature
green Black mean 16,5 17,5 std 5,24 5,52
Figure 4: Roof Surface TemperatureFigure 5 temperatures measured above the black roof and the green roof, in different time series. A clear difference between daytime and night-time is shown; the peaks are often measured at midday, when solar radiation is strong, especially when the weather is not cloudy, and the lowest points express temperature drops at night-time.
An important note that has to be made is that until day 161, which is June 10th, both the sensors
where placed on the surface right below, and at the low front of, the solar panels. At June 10th
around 4.30 pm, the sensors where placed further from the solar panels, so that the solar panels probably do not influence them. Before this displacement green roof seems to be slightly cooler than the black roof. After the displacement the black roof seems to be cooler in the peaks. At night-time, no significant difference is found. However, in the second time series, the surface at the black roof is higher during daytime.
0 5 10 15 20 25 30 35 40 169 169 170 170 170 170 171 171 171 171 172 172 172 172 173 173 173 174 174 174 174 175 175 Tem p e ratu re o C Date
Surface temperature
SurfTempBlack Surftempgreen14
Figure 6: Temperature beneath Solar Panels on the Green roof and the Black roof, day 156-163 (7-12 June)
Figure 7: Temperature beneath Solar Panels on the Green roof and Black roof, day 169-175 (18 – 24 June)
0 10 20 30 40 50 60 169 169 170 170 170 170 171 171 171 171 172 172 172 172 173 173 173 174 174 174 174 175 175 Tem p e ratu re o C Date
Temperature under Solar Panels
TempSolarBlack TempSolarGreen 0 5 10 15 20 25 30 35 156 156 157 157 157 158 158 158 158 159 159 159 160 160 160 161 161 161 162 162 162 162 163 Tem p e ratu re ° C Daynumber
Temperature under Solar Panels
TempSolarGreen TempSolarBlack
18 – June 19- June 20-June 21- June 22- June 23- June 24- June 5 – June 6- June 7-June 8- June 9- June 10- June 11- June 12- June
15
Tabel 3: Temperature under Solar Panels
green Black Mean 16,8 16,2 Std 5,2 4,5
The temperature of the solar panel on the green roof is higher in the peaks (Figures 6 and 7)
Table 4: Relative Humidity
green black Mean 63,1% 63,8% Std 11,5% 11,6%
The relative humidity above the plots is almost the same for the two plots (Figure 8). Relative humidity is also dependent on temperature, so the absolute humidity can be different when the air temperature fluctuates. However, the average air temperatures above the roof plots were 15,30 °C (black) and 15,26 °C (green) respectively, so the absolute humidity above the plots is comparably the same.
Figure 8: Relative Humidity above the Green roof and the Black roof, day 156-163 (5-12 June)
0 10 20 30 40 50 60 70 80 90 100 156 156 157 157 157 158 158 158 158 159 159 159 160 160 160 161 161 161 162 162 162 162 163 R e lativ e Hu m id ity % Date
Relative Humidity
AirMoistBlack AirMoistGreen16
Figure 9: Insolation on the PV panels
The pyranometer measures incoming solar radiation on the PV panels. The results clearly show which days were sunny and which days were more cloudy, resulting in a peakier graph. (Figure 9) The pyranometer peaks are higher, of almost 1000 W/m2, whereas the global radiation peak
values are around 850 W/m2. This difference can be explained by the angle the measurers are
placed in; the pyranometer is placed in the same angle as the solar panels, which are more directed to the south and are more directed to the sun. Therefore, more radiation is collected with the pyranometer, compared to the global radiation.
Figure 10: Shortwave Reflection
Figure 10: Shortwave Reflection displays the reflection of both the roofs. As can be seen, the green roof reflects more radiation than the black roof, especially at the peaks of the more sunny days. -200 0 200 400 600 800 1.000 1.200 1 127 253 379 505 631 757 883 1009 1135 1261 1387 1513 1639 1765 1891 2017 R ad ian ce (W/m 2) Date
Insolation on PV panels
Insolation 0 20 40 60 80 100 120 140 160 156 156 157 157 158 158 158 159 159 160 160 160 161 161 162 162 162 163 R e fl e ction (W/m 2) DateReflection
ReflectionBlack ReflectionGreen5 – June 6- June 7-June 8- June 9- June 10- June 11- June 12- June
17
Figure 11: Absorption
Absorption is calculated by subtracting the radiation reflected by the roof from the global radiation. (Figure 11) The results show that the Black roof absorbs a little bit more radiation, especially during peak radiation.
Figure 12: Wind Speed
-100 0 100 200 300 400 500 600 700 800 900 1 103 205 307 409 511 613 715 817 919 1021 1123 1225 1327 1429 1531 1633 1735 1837 1939 2041 A b sor p tion (W/m 2) Date
Absorption
AbZW AbG 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 156 156 157 157 157 157 158 158 158 159 159 159 159 160 160 160 160 161 161 161 162 162 162 162 163 Wi n d S p e e d ( m /s) DateWind Speed
WindSpeedBlack WindSpeedGreen5 – June 6- June 7-June 8- June 9- June 10- June 11- June 12- June
18
Table 1: Wind Speed
green black mean 0,30 0,30 stdev 0,049 0,052
Figure 13: Wind Speed
Figure 12 & Figure 13 show the wind speed for both plots. They seem to be almost the same at both roofs, according to their means.
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 169 169 169 170 170 170 170 171 171 171 171 172 172 172 172 173 173 173 173 174 174 174 174 175 175 Wi n d S p e e d ( m /s) Date
Wind Speed
WindSpeedBlack WindSpeedGreen19
Figure 14: Energy Production per Panel Table 5: Energy production per Panel
green black mean 0,93 0,97 stdev 0,36 0,37
In Figure 14: Energy Production per Panel, the power production per module is displayed per day. No clear difference between the solar panels that are placed on either the black and the green roof are seen. The variance between the panels reciprocally seems to be higher than the difference between the green and the black roof.
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 8 -ju n 9 -ju n 10 -ju n 11 -ju n 12 -ju n 13 -ju n 14 -ju n 15 -ju n 16 -ju n 17 -ju n 18 -ju n 19 -ju n 20 -ju n 21 -ju n 22 -ju n 23 -ju n 24 -ju n 25 -ju n 26 -ju n 27 -ju n En e rg y Pr o d u ction (k Wh)
Energy Production Per Panel
Green1 Green2 Black1 Black2
20
Figure 15 Difference in Energy Production
To make the difference in power production more clear, the relative difference with the best performing module, which is Black1, is shown in Figure 15. This is done by subtracting Black1 from the other values, which returns the negative difference in power production.
Below is a picture that is made with an infrared camera, which shows the temperature
differences that exist on both plot sites. Pictures at made at June 5th, more pictures are made and
discussed in the Appendix.
-0,16 -0,14 -0,12 -0,1 -0,08 -0,06 -0,04 -0,02 0 8 -ju n 9 -ju n 10 -ju n 11 -ju n 12 -ju n 13 -ju n 14 -ju n 15 -ju n 16 -ju n 17 -ju n 18 -ju n 19 -ju n 20 -ju n 21 -ju n 22 -ju n 23 -ju n 24 -ju n 25 -ju n 26 -ju n 27 -ju n D iff e re n ce w ith B lac k1 (k Wh)
Difference in Energy Production Compared with
Best Performing Panel
Green1 Green2 Black2 Black1
21
Figure 16: Thermal photo of roof plots
Figure 16 shows an indication of the temperature differences in the different solar panels and the roofs. The black roof is warmer than the green roof, and also the solar panels on the black roof seem to be warmer, though the temperature index of this picture is not correct; a
temperature of -3 degrees is not realistic in this case. This is probably caused by the very low infrared emission coefficient of the metal caps. This makes the infrared camera measure a low ‘temperature’, because it does not emit much infrared radiation.
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5. D
ISCUSSION
The measured results do not seem to confirm the hypotheses. The most important data that are retrieved are the data that show the power yield of the solar panels. To confirm the hypotheses, the power yield of the panels on the green roofs should be higher than the power yield of the panels on the black roof. This is not the case; the panels on the black roof perform slightly better, and the variance between the solar panels is higher than between the two plots. The claim that is tested is a difference of 6%, but the differences between the solar panels themselves are also in that range. Therefore, no hard conclusions can be made.
For the temperatures, some differences are shown in the graphs. The air above the surface of the black roof is clearly warmer than the air above the green surface. However, this difference does not seem to be translated to the temperature of the solar panels. In fact, especially in the graph that was made at the 18th of June, the temperature under the green solar panels is a lot higher
than the temperature under the black panel. A possible cause for this could be that the wind speed is higher on the black roof, though no significant prove for this theory is found.
Also, a point of discussion is that the temperature is only measured under one of the two solar panels. The green solar panel under which the temperature meter is placed, becomes very hot but is also the panel that performs the worst. It could be that for some unknown reason, this panel becomes very hot, in spite of the green roof that is under it, and performs worse because of this high temperature. Though, this enhances the assumption that solar panels perform worse under warm conditions.
The surface temperature differences that are measured are lower than described in the literature. The literature (Weng, 2011) describes differences of about 15 ° C - 25 ° C, while the differences that are found in this research are more in the order of 5 ° C – 10 ° C. It would have been better to stick the temperature meters into the soil and roof, but that was not possible on the black plot, because leakages should be prevented. Also, the temperature meters where not very suitable for measuring surfaces. The difference in surface temperature may have been caused by two factors. The first is the cooling effect of evapotranspiration on the green roof. This is not measured by the air moisture meters; the relative air moisture was the same for both roofs. The second factor is that more radiation is absorbed by the black roof. This is confirmed by the measurements of the albedometers.
Another point of discussion is the shadow that was casted over the roof by a nearby building. This shadow reached the roof plots in the late afternoon, but reached the green roof earlier than the black roof. Therefore, the green roof received less radiation every day in the time that the shadow was already on the green roof, but not the black. Figure 17 shows the received radiation for both plots at the same time, of the measurements of the first four sunny days. The red dots indicate the measuring points in which the shadow was casted on the green roof plot. The difference in received radiation is about 7%. This may be a cause for the lower performances of the solar panels that were placed on the green roof plot. The difference in performance is indeed about 6%.
23
Figuur 17: Comparison betweer insolation on black roof and green roof.
Furthermore, the measured wind speeds do not seem to be correct. The average measured wind speed is about 0.3 m/s, but the average for the same time period at Schiphol is 5,6. (KNMI) Although the wind speed is probably lower on the roof plots due to surrounding trees and buildings, the difference between the two measurements is unrealistic. However, no errors in the equipment or the used programme could be found.
Also, the roof plots probably ought to have been bigger to show a significant difference. The roof plots were both very small and close to eachother. Eventual differences in temperature and wind speed could be influenced easily by the surroundings of the roof plots, and so they could
influence eachother. For example, in the research on heat fluxes on green roofs which is done by Liu (2005), green roofs with an area of 460 m2 where constructed, which is a whole other order
of magnitude than the 16,3 m2 that was created for this research. On the other hand, the purpose
of this research is to resemble the ‘real’ situation of a green roof on a private house, which often does not have a roof of 460 m2.
The weather caused two very important restrictions on this research. At first, warm and sunny days where needed, because on those days, the highest temperature differences between the roofs where expected. However, in the past few weeks not many days like that day have passed.
y = 0,9255x
R² = 0,9989
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24 The 18th of June was the most suitable day to do the measurements, with a measured maximum
air temperature of 45 ° C (above the roof). Unfortunately, the datalogger on the green roof was broken on that day, and the replacement of the datalogger took until 14.30, so only the afternoon and evening of that day are measured. The best way to get significant results in this research is to measure on a longer period of warmer, sunnier days. Therefore, this study may be seen best as a pilot study for a study that would last a whole year, or at least a whole summer.
The hypothesis about the economical viability of the combination of a green roof with solar panels is not relevant anymore; according to these results, the green roof does not increase the efficiency of the solar panels placed upon them. Therefore, the time that is needed to earn back the investment that is made in solar panels is only lengthened by the placement of a green roof. However, the real economical advantage of a green roof seems to be the lengthening of the roof durability.
However, a note should be made on the research as a whole. The fact that this research did not confirm the hypotheses does not mean that they should be rejected. Mostly, further research is needed in which the researcher has learned from the flaws in this research and improves them where possible. Also, an insight that could be drawn from this research is that even though the green roof does not seem to improve the performance of the solar panels, it does not decrease it, either. In some cities where roof space is limited, green roofs and solar systems could compete each other for roof space. Maybe comparable research to this can conclude that they do not affect each other negatively, and thus do not need to compete.
Also, the other advantages of green roofs and solar panels that are not taken into account in this research should be used in making a cost benefit analysis.
Further research
Further research is done with these measuring gear; the measurements will continue until September. Also, this study is used as an example for a study from the Hogeschool van Rotterdam, which will be done over a longer time period and with more and more suitable measuring gear.
- Note (01-11-2013) This research is not finished yet but seems to have approximately the
25
6. C
ONCLUSIONS
The aim of the research was to research the relation between the presence of a green roof under a solar system and the efficiency of a solar system. This relation is not proven, even no indication for a positive relation is found. This could be caused by an absence of measuring days with suitable, cloudless and sunny weather, or by the plot size, which was very small. On average, the solar panels on the black plot performed better than those on the green plot, but this effect is probably caused by the fact that the solar panels on the black roof received more solar radiation. This means that the hypothesis of an increase or difference of 6 % is also not accepted nor rejected. The variance in power output between the solar panels is greater than the difference between the plots.
On most days, the green roof has a lower peak surface temperature, though this difference in temperature is not as high as found in the literature. This difference in surface temperature is not translated to the temperature that is measured under the solar panels.
The wind speed on the black roof is slightly higher, but the wind speed results are rather unrealistic and therefore not reliable.
According to the literature, two factors contribute to the surface temperature difference; evapotranspiration and a difference in radiation absorbance. The eventual difference in evapotranspiration is not found; therefore the difference is likely caused by the higher absorption of radiation by the black roof.
The hypothesis on the economic viability is not relevant anymore, because there is no indication of a higher efficiency of the solar cells on the green roof.
7. A
CKNOWLEDGEMENTS
I have received a lot of help from different people, especially in constructing the roof plots. Therefore, my thanks go to:
• Matthijs Bourdrez and other employees from Solar Sedum • Dhr. Dr. John van Boxel as my supervisor
• Mathilde van Otterloo, Melissa Vleugel, Joris Solleveld and Elsa Buijs for their physical and mental contributions while constructing the roof
26
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9. A
PPENDIX
9.1 M
ATERIALS
Provided by UvA
- 4x P107 Campbell sensors for measuring soil temperature.
- 2x HMP45AC Vaisala sensor for measuring relative humidity and air temperature. - 2x A100R Vector Instrument (cupanemometer) for measuring wind speed. - 2x CR10x datalogger.
- Dell D600 Laptop.
- 2x T351-RS Radiation Shield with 405-1 for HMP45AC Vaisala humidity and
temperature sensor
- Radiation measurement, either the 3x Kipp CM7 Albedo or the 2x Campbell Science Q 7.1 - 2x 12V battery for CR10x datalogger.
- 2x Waterproof box for CR10x datalogger and battery. - 2x Pole
Provided by Solarsedum
4 SMA enphase micro converters
4 Renesola 250 Wp polycrystalline solar panels 16,5 m2 sedum tiling
29
9.2 T
HERMAL PICTURESFiguur 18: Temperature under solar panel (13:25, 05-06-2013)
This picture shows the temperature range that surrounds one of the solar panels on the black roof. The roof itself is very hot, the green on the left is cooler. Also, the aluminium frame is hot, whereas the aluminium rim of the solar panel is fairly cool. The shadow that the solar panel casts on the roof is cooler as well. The red spot right under the solar panel is not very clear, it could be either the air under the solar panel, or the surface of the roof. The latter option is more realistic, because air temperature differences would be more gradual.
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Figuur 19: Solar panels on green roof (13:18, 05-06-2013)
Above picture represents the two solar panels that are placed on the green roof. The sedum plants have over all a lower temperature than the solar panels. Also, the solar panel on the right is somewhat cooler at the uttermost right part.
Figuur 20: Close up solar panels on green roof (13:22, 05-06-2013)
This picture shows that the temperature of the solar panels are in the middle approximately 45 ° C. In the shadow of the panels, the temperature is cooler, around 20 degrees. Also, the aluminum frame has a very high temperature. Apparently, heat conduction through the aluminum frame takes place. Again, the uttermost right part of the solar panel seems cooler than the rest of the panel.
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Figuur 21: Solar panels with purple/yellow color scheme (13:45, 05-06-2013)
This picture is made with another colour range. This picture also shows the differences between the solar panels. The solar panels on the black roof seem to be slightly warmer, as well as the black roof itself.
