Effect of the Diffuse Solar Radiation on Photovoltaic Inverter Output

Effect of the Diffuse Solar Radiation on Photovoltaic Inverter Output C.A. Balafas #1, M.D. Athanassopoulou #2, Th. Argyropoulos #3, P. Skafidas #4 and C.T. Dervos #5 #1,2,3,4,5 School of Electrical and Computer Eng., National Technical University of Athens, Zografou Campus 15780, Athens, Greece 5 cdervos@central.ntua.gr Abstract Solar global irradiance received at a horizontal level on the earth varies significantly over short intervals due to diffuse radiation changes. Experimental data on global irradiance profiles received by fast data recording systems show that the global irradiance may be enhanced for a few minute periods by as much as 40%. The diffuse radiation is intensified by dry air mass formations, airborne nanoparticles, and cloud formations at higher atmospheric levels. Evaluation factors for photovoltaic system design (i.e. sun hours, tilt angle, module direction, soiling, module reflection, and losses due to temperature, wiring and module output differences) may also have to consider possible global irradiance surges. Power monitoring of a photovoltaic park has shown that the delivered AC output power by the inverters can be increased beyond their nominal limits due to diffuse radiation effects, thus rising component reliability issues. I. THEORETICAL INVESTIGATION A. Solar radiation basics Solar radiation is the result of fusion of atoms inside the sun. Part of the energy from this fusion process heats the chromosphere, the outer layer of the sun that is much cooler than the interior of the sun, and the radiation from the chromosphere becomes the incident solar radiation on earth. The solar radiation is not much different from the radiation of any object that is heated to about 5800 Kelvin except that the 'surface' of the sun is heated by the fusion process. This radiation spans a large range of wavelengths from 200 nm to more that 50000 nm with its peak around 500 nm. Solar radiation outside the earth's atmosphere is called extraterrestrial radiation. On average the extraterrestrial irradiance is 1367 W/m 2. This value varies by ±3% as the earth orbits the sun [1-4]. Approximately 47% of the incident extraterrestrial solar radiation is in the visible wavelengths varying from 380 nm to 780 nm. The infrared portion of the spectrum has wavelengths greater than 780 nm and account for another 46% of the incident energy. Finally, the ultraviolet portion of the spectrum is with wavelengths below 380 nm and it accounts for 7% of the extraterrestrial solar radiation. As the sunlight passes through the atmosphere, a large portion of the UV radiation is absorbed and scattered. Air molecules scatter the shorter wavelengths more strongly than the longer ones. This scatters more blue light and is the reason why the sky appears blue. Water vapour and atmospheric dust further reduce the amount of direct sunlight passing through the atmosphere. On a clear day approximately 75% of the extraterrestrial direct normal irradiance passes through the atmosphere without being scattered or absorbed. Table I provides common sources of radiation absorption and scattering within the earth s atmosphere. On a day without clouds, about 25% of the solar radiation is scattered and is absorbed as it passes through the atmosphere near noon. Therefore about 1000 W/m 2 of the incident solar radiation finally reaches the earth's surface without being significantly scattered. This radiation, coming on the earth s surface from the direction of the sun, is called direct normal irradiance (or beam irradiance). Some of the scattered sunlight is scattered back into space and some of it also reaches the surface of the earth. The scattered radiation reaching the earth's surface is called diffuse radiation. Some radiation is also scattered off the earth's surface and then rescattered by the atmosphere to the observer. This is also part of the diffuse radiation the observer sees. For example, rescattered radiation may contribute significantly in those areas where ground is covered by snow. The total solar radiation on Table I. Radiation Absorption and Scattering Under Clear Sky Factor Percent absorbed Percent scattered Percent of total radiation passing through the atmosphere Ozone 2% 0% Water vapour 8% 4% Dry air 2% 7% Upper dust 2% 3% Lower dust 0% 0% Total absorbed or scattered 87% 87% 76% Fig 1. Extraterrestrial solar spectrum after [5]. 978-1-4244-5794-6/10/$26.00 2010 IEEE 58

environmental factor that can actually increase the photovoltaic module output power. B. Environmental factors affecting the performance of photovoltaics Solar module performance factors help to explain the conversion from the solar module power rating (Watts DC) Standard Test Conditions (STC) to the energy (kilowatt-hours AC) produced at the utility. Fig 2. Solar radiation components in the earth s surface: Beam, diffuse and global. Fig 3. The components of global and direct solar spectrum, for the following conditions: Air Mass AM 1.5, and 37 Tilt [6]. a horizontal surface is called global irradiance and is the sum of the incident diffuse radiation plus the direct normal irradiance projected onto the horizontal surface, Fig 2. If the surface under study is tilted with respect to the horizontal, the total irradiance is the incident diffuse radiation plus the direct normal irradiance projected onto the tilted surface plus ground reflected irradiance that is incident on the tilted surface. As shown in Fig. 3 the difference between direct and global radiation, provides the diffuse radiation received at the observation point (at any wavelength). According to the data given in Fig 3, the diffuse radiation is enhanced in the visible spectrum (i.e. 380 nm - 780 nm). Many applications are concerned with specific regions of the solar spectrum. For example, building designers are interested in lighting for the human eye, which is sensitive only to the visible part of the spectrum. On the other hand photovoltaic applications are interested on the power delivered by wavelengths corresponding to greater energy values compared to the band-gap of the semiconductor material used to produce the photovoltaic cells, in order to provide the required electron-hole pairs that will be spatially separated by the build-in potential barrier across the p-n junction contact region. So far only parameters that limit performance of photovoltaic modules are normally encountered. However, as this work points out, the component of diffused radiation may become in practice the only (i) Modules are rated in DC Watts under STC. All solar module manufacturers test the power of their solar modules under specific Standard Test Conditions in the factory, i.e. irradiance level 1000 W/m 2, AM 1.5, cell temperature 25 C, and solar spectral irradiance as per ASTM E 892 [7]. The test results are used to rate the modules according to the tested power output. For example, a module tested in the factory, which produces 100W of DC power, is rated and labelled as a 100W STC DC solar module. During operation under STC, the actual power output of a given module may vary up or down. No module power output tolerances are taken into consideration, as average tested module power output is equal to nameplate rating. Manufacturers publish separately the output power tolerance (i.e. 100W ± 5%) on the module specification sheet. (ii) Increasing Module Temperature Decreases Power. Module operating temperature increases when placed in the sun. As the operating temperature increases, the power output decreases (due to the properties of the conversion material - this is true for all solar modules). The ratings are different for each module, and can vary from approximately 87%-92% of the STC rating. A typical decrease in power output is approximately 12% for crystalline based solar modules. This decrease results in a STC rated 100 Watt DC solar module now being rated at approximately 88 Watts DC. (iii) Particulate build-up ("Soiling"). When a module is placed outdoors, airborne particulates (e.g. dust, debris) settle on the glass surface of the module [8]. Particulates block the amount of light reaching the effective surface of the module and therefore reduce the produced power. The reduction in power from particulate build-up may range from 5%-15%. A typical value can be estimated at 7%. Due to the rain water rinsing off the module's glass surface modules installed in wet weather climate have less "soiling" than module installed in dry region. The effect of particulate build-up results in power decrease from 88 Watts to approximately 82 Watts. (iv) System wiring and module output differences. Typical solar electric systems require more than one module to be connected to one another. The wires used to connect the modules create ohmic losses in the electrical flow, thus decreasing the total power output of the system. In addition, slight differences in power output from module-to-module tend to reduce the maximum power output delivered by the modules [9]. The system AC and DC wiring losses and individual module power output differences could reduce the total system rated energy output from 3%-7%. A typical value 59

for these losses is 5%. This further reduces the expected power output from 82 Watts DC to 78 Watts DC. (v) Inverter conversion losses. Power inverters need to be used in order to convert the DC power delivered from the solar modules to the standard utility AC power (used by homes and businesses). Power conversion from DC to AC results in an energy decrease by approximately 6%-10%, and it varies for each inverter (primarily due to energy lost in the form of heat) [10]. A typical value for this loss is 6%. Thus, AC conversion results in power decrease that is estimated from 78 Watts DC to 73 Watts AC. (vi) Solar module tilt angle. The module installation angle in relation to the sun affects the overall energy output [11]. The module produces more power when the light source is located perpendicular to the surface of the module. For this reason, solar module installations are often tilted towards the sun to maximize intensity of light exposure. As the sun angle changes throughout the year (higher in the sky during summer and lower in the sky during winter), the amount of light falling directly on the module changes, as does the energy output. In the Mediterranean region, a typical optimum tilt angle for average module power production over the course of a year in a fixed-tilt system is approximately 30 degrees. For flat mounted systems, the reduction in average annual energy output for a module is approximately 11% when compared to the optimal tilt of approximately 30 degrees. Typical residential roofs are tilted approximately 15 degrees. The reduction in the average annual energy output for a module, which is mounted at a South-facing roof, 15-degree tilt, is approximately 3% when compared to the optimal tilt angle of approximately 30 degrees. This results in decreasing the energy from one sun hour exposure from 73 Watts to approximately 71 Watt-hours AC. (vii) Solar Module Compass Direction. As the sun moves across the sky throughout the day, from the East in the morning to the West in the afternoon, the compass direction, "orientation", of the module affects the cumulative energy output. For this reason, it is optimal to install a South-facing module in order to obtain the maximum amount of direct light exposure throughout the day. If the module is facing East or West, it will be exposed to less direct sunlight as the sun moves across the sky. There is no loss factor for south facing modules, so the estimated energy (from one sun hour exposure) for this particular example will remain at 71 Watt hours AC. If the module is not facing South, the module energy output will be reduced. For example, in a Southwestfacing module the energy output would be reduced further by approximately as much as 3%. (viii) Sun Hours. Every location on earth has a different amount of sunlight exposure throughout the year, which is measured in kwh/m 2 or Sun Hours [11]. For example, a coastal Mediterranean city will have a lower average amount of yearly Sun Hours than a dry inland city because of coastal moisture in the air. "One Sun" is approximated as the peak noon sunlight power intensity in the middle of summer. "One Sun Hour" is energy produced by the peak noon sunlight intensity in the middle of summer, over one hour. See Fig 4. Fig 4. Daily Sun Profile and Sun Hour relation The integrated area of daily sun profile equals the product between (sun hours) x (peak sun intensity). For one particular location the amount of Sun Hours may differ from day to day. There are multiple Sun Hour data sources which slightly vary from one to another. For example, the average Sun Hours during the summer season (e.g.. approximately 7.1 hours per day) and the average Sun Hours during the winter season (e.g. approximately 3.9 hours per day) are combined to provide the seasonal average result, (7.1+3.9)/2 = 5.51 Sun Hours per day. For the aforesaid example, the daily Sun Hour average of 5.51 hours throughout the year is shown in Fig. 4. In order to estimate the energy production of a solar module per year, one simply multiplies the estimated module output energy 71 Watt hours AC (from one sun hour exposure 1000W/m 2 over one hour), by the amount of Sun Hours for the particular location, i.e. 5.51. This results in approximately 391 Watt hours AC per day or 0.391 kwh AC per day. When estimating yearly energy production, the estimated daily energy production, is multiplied by the total number of days in the year, 365. This results in approximately 142 kwh AC energy production. Thus, under the specified conditions of the aforesaid example, one 100 Watt DC module will approximately produce yearly 142 kwh of energy (AC). It should be stressed that all of the above factors that are taken into consideration to select, design and implement a photovoltaic installation tend to reduce the power delivered by the modules. However, the only environmental factor that may temporarily significantly enhance the delivered AC power, is the diffused radiation received by the panels. II. EXPERIMENTAL INVESTIGATION A. Solar radiation monitoring. The global radiation received at a horizontal surface was recorded at the National Technical University of Athens, Greece, 37 o 58'31.76"N, 23 o 47'05.51"E The sensor (Hydro 60

Lynx 4014 pyranometer) was located at an altitude of 212 m above sea level and it was placed 18m above ground level to reduce particle contamination. The employed sensor measures total sun and sky (global) radiation and provides a millivolt output signal proportional to solar radiation energy from 0 to 1400 W/m 2. The sensor responds to a 100% change in incoming radiation within one millisecond. It is calibrated for the entire solar spectrum by comparison with thermopile type radiometers in bright sunshine on a clear day. Cosine effects and Air Mass corrections have been made during the calibration by the manufacturer. The active device is sealed by a Pyrex glass dome incorporating desiccant, thus protecting it from moisture and dust. The sensor consists of a p-i-n silicon photodiode cell, having a spectral response that varies between 0.35 and 1.15 μm. The full scale response time is 1 ms. For radiation levels varying between 0 and 1400 W/m 2 it produces a voltage output 0-50 mv dc, with a linearity accuracy between incidence radiation and output voltage ±5%. Temperature compensation has been considered for photodiode temperature varying from +4 C to +60 C. The sensor output voltage is monitored by an Agilent 3458A multimeter that is capable of measuring DC voltages with an 8.5 digit resolution and a maximum sensitivity of 10 nv. The 24-hour voltage accuracy is 0.6 ppm, and the annual voltage reference stability is 8 ppm. The maximum number of readings per second is 100,000 thus, when used with a pyranometer having response time 1 ms it enables the investigation of fast global irradiance changes. The measuring unit is connected to the data bus of a PC platform by the IEEE 488 interface and the software application written in C, enables for fast sampling rates enabling up to 100 samples/s. The acquired global irradiance data are stored for further processing, and some of the daily acquired global irradiance data (sampling rate 1 sample/min) are also given in the following URL address: http://eml.ece.ntua.gr. B. Results on global irradiance profiles. Figs. 5a, 5b and 5c provide representative results obtained for a sunny clear sky day (5a), and partially clouded days (5b,c). The meteorological data for the selected global irradiance profiles are summarised in Table II. Table II. Meteorological data of selected days Nov. 20 th 2008 Nov.21 st 2008 Feb.16 th 2009 Sunrise 07:08 07:11 07:14 Sunset 17:11 17:08 18:05 Pressure 1018 mbar 1016 mbar 1019 Temperature 15-20 C 16 20 C 6 13 C Humidity 51% RH 55% RH 58% RH UV-index 2 2 2 Fig 5a provides the typical profile of global irradiance over the period of a single clear-sky day. Notice that in the first hour following sunrise the radiation level is remarkably low (below 45 W/m 2 ) and it rises abruptly after an hour or so. This phenomenon is attributed to the morphology of the greater area, and specifically the presence of a physical obstacle towards East of the monitoring point. This consists of a mountain (Hymettus) having an altitude of 1026 m located at a distance of approx. 3 km from the monitoring place. Therefore, only diffused radiation is monitored initially after the sunrise, but as soon as the direct beam reaches the sensor, global irradiance is actually measured. This response is evident in all irradiance profiles shown. However, the exact time required after sunrise till the direct beam reaches the sensor varies with the day of year. W/m 2 W/m 2 W/m 2 600 500 400 300 200 100 600 500 400 300 200 100 20 November 2008 0 06:00 07:30 09:00 10:30 12:00 13:30 15:00 16:30 18:00 Time (h) 21 November 2008 0 06:00 07:30 09:00 10:30 12:00 13:30 15:00 16:30 18:00 800 700 600 500 400 300 200 100 Time (h) 16 February 2009 0 06:00 07:30 09:00 10:30 12:00 13:30 15:00 16:30 18:00 19:30 Time (h) Fig. 5. Global irradiance profiles measured during different days (Nov. 20 th, Nov. 21 st, and Feb. 16 th ). Fig. 5b clearly points out that at 9:25 the presence of a cloud aside the direct line interconnecting the sun and the sensor (prior shadowing effect) induces momentarily a significant rise of the global irradiance by increasing the 61

diffuse radiation component. This increase of global irradiance from 300 W/m 2 to 420 W/m 2 lasts for more than 10 min in the specific example and corresponds to radiation level increase by as much as 40%. Then, as the cloud enters the area interconnecting directly the sun and the recording location the global irradiance drops to levels below 80 W/m 2. Later on at 10:45 the increase of global irradiance is observed again (from 420 W/m 2 to 500 W/m 2 without the shadowing effect following. According to Table I, the dry air masses, upperdust in the atmosphere (originating from air born particulates) or water vapour (clouds) could be some possible causes initiating diffuse radiation enhancement. Fig. 5c demonstrates the occurrence of multiple shadowing intervals without any diffuse radiation enhancement effects. Though this is the usually obtained response of global irradiance variations within partially cloudy days, it should be stated that there are not significant modifications between meteorological data corresponding to monitoring days of results given in Figs. 5b and 5c. C. Power delivered by inverter in a photovoltaic park A photovoltaic park located in Peloponnesus was used to monitor inverter output power variations with global irradiance. The park consisted of polycrystalline silicon solar modules and the DC to AC conversion took place via a 3- phase inverter system, having nominal power 35 kw each. The typical power plot received for all three inverters during a clear sunny-day, with very few (and short) shadowing intervals during the afternoon, is shown in Fig 6a. During noon all three inverters provide maximum power of the order of 30 kw, which is below inverter nominal output power levels. During the shadowing periods, the inverters output drop significantly, i.e. from 27 kw to 19 kw, as it is practically expected. The effect of diffuse radiation enhancement on the power output of the 3-phase inverter unit is clearly demonstrated by the results given in Fig. 6b. Here, the increased global irradiance levels force all three inverter units to provide enhanced output power (the first inverter produces up to 38.8 kw, the second up to 37.2 kw and the third up to 36.5 kw) all values being beyond nominal power levels. When monitoring the inverter power output levels over periods of several days (Fig. 6c) it becomes evident that during certain days with intense global irradiance variations the produced AC power by the inverters may practically exceed the theoretically predicted values, as is the case for days 10 and 11 in the power plot given in Fig. 6c. This response may practically induce aging of the electronic components, also affecting battery charging, and induce instantly unstable operating conditions, thus rising reliability issues in the photovoltaic establishments. III. CONCLUSIONS It has been shown experimentally that the global irradiance levels on flat surfaces may be increased by as much as 40% due to the diffuse radiation component. Though the intervals of increased global irradiance levels may last for only a few Fig. 6. Daily measurements of power output from a 3-phase inverter unit in a photovoltaic park. (a) Clear sunny sky and (b) partially cloudy with increased diffuse radiation levels, and (c) power plot of inverter output power for 13 day period. minutes, this phenomenon practically induces reliability issues on photovoltaic applications. For example, designers of photovoltaic plants should allow for inverters with greater nominal power capabilities compared to theoretically predicted ones, in order to compensate for unstable operating conditions relating to increased power outputs. The diffuse radiation may be enhanced significantly either by dry air formations in the atmosphere, or occurrence of dust particles in higher atmospheric levels. Nanoparticle 62

contaminants in the atmosphere may be introduced as a result of forest fires, airborne sand particles from wind gusts in the Sahara desert, and finally by industrial pollution and man made activities. The small size of atmospheric particles (few nanometers) causes their surfaces to be very active thus interacting with gaseous contaminants such as sulfur oxides, nitrogen oxides) and water vapor. The solar light beam interacts with particles and affects the diffused solar radiation component. Depending on the angle of incidence and irradiance monitoring locations the radiation may be significantly enhanced. This environmental parameter affects performance of photovoltaics. REFERENCES [1] G. W. Partridge and C. M. R. Platt, Radiative Processes in Meteorology and Climatology, Elsevier Scientific Pub. Co. (Amsterdam, New York), 1976. [2] J. A. Duffie, and W. A. Beckman,. Solar Engineering of Thermal Processes, 2nd edn., J. Wiley and Sons, New York, 1991. [3] J. J. Michalsky, The astronomical almanac's algorithm for approximate solar position (1950-2050), Solar Energy, vol. 40, pp. 227-235, 1988. [4] Watt Engineering Ltd. On the Nature and Distribution of Solar Radiation US Government Printing Office, March 1978. [5] C. Wehrli, Extraterrestrial Solar Spectrum, Physikalisch- Meteorologisches Observatorium and World Radiation Center (PMO/WRC) Davos Dorf, Publication no. 615, Switzerland, July 1985. [6] Standards (E-891) and (E-892), American Society for Testing and Materials (ASTM), 1992. [7] (Temperature) Module PTC ratings are available at the following CEC URL:http://www.consumerenergycenter.org/erprebate/eligible_pvmod ules.html. See APPENDIX, California Energy Commission List of Eligible Photovoltaic Modules PTC rating listing. [8] California Energy Commission, A Guide to Photovoltaic (PV) System Design and Installation report, Factors Affecting Output, Dirt and Dust, (Soiling) APPENDIX, section 2.3.1, page 8. [9] California Energy Commission, A Guide to Photovoltaic (PV) System Design and Installation report, Factors Affecting Output, Mismatch and wiring losses, section 2.3.1, page 8. [10] California Energy Commission, List of Eligible Inverters Inverter PTC ratings and inverter manufacturers specification sheets (SMA and Fronius). [11] California Energy Commission, A Guide to Photovoltaic (PV) System Design and Installation report, Estimating System Energy Output, Sun angle and house orientation, section 2.3.2, page 9. 63