The PV performance in terms of standard reporting conditions (SRC) or standard test conditions (STC) is commonly expressed in terms of a peak watt rating or an efficiency. At the research level, an internationally accepted set of standard reporting conditions is essential to prevent the researcher from adjusting the reporting conditions to maximize the efficiency. The procedures for measuring the performance with respect to SRC must be quick, easy, reproducible, and accurate for the research cell fresh out of the deposition system or for the module on a factory floor with production goals. The PV conversion efficiency (n) is calculated from the measured maximum or peak PV power (Pmax), device area (A), and total incident irradiance (Etot):
_Рта^шо Etot A
Parameters which directly influence the measurement or calculation of the efficiency, and therefore must be well controlled and well-defined, are the area of the device, the spectrum and intensity of incident radiation which determine Etot, and the temperature of the device. The term “reporting,” rather than “reference” or “test,” is used because in practice a test is performed at other than SRC and then corrected to be equivalent to being measured at SRC. ASTM standards use SRC while the international IEC standards use STC. The currently accepted SRC for rating the performance of cells and modules are summarized in Table 18.1 [15-17]. The direct and global air
|
Table 18.1 Standard reference conditions (SRC) for rating photovoltaic cells, modules and systems. The irradiance listed is the reference irradiance and the reference spectrum may not integrate to this value
^linear regression of power to project test conditions, 850 W m-2 with a 5° field of view for concentrator systems. *At present, no consensus standards exist, de facto conditions are listed. |
mass 1.5 (AM1.5), and air-mass 0 (AM0) reference spectra are shown graphically in Figure 18.1 and in tabular form on the Web site http://rredc. nrel. gov/solar/spectra/am1.5/.
It should be noted that neither the direct reference spectrum nor the global reference spectrum actually integrates to the 1-sun reference total irradiance of 1000Wm-2 [10, 14, 15, 18]. In 2008, the international terrestrial standards community revised the reference spectrum from a slightly modified version of ASTM G159 to a slightly modified version of ASTM G173 [10, 14, 15]. The differences are in the digits of precision versus significant digits and how the data past 4000nm are treated [10, 14, 15]. The ASTM standards committee attempted to have the global spectrum ASTM G173 integrate to 1000Wm-2 using the open-source spectral irradiance model SMARTS 2 developed by Gueymard [19, 20]. The global reference spectrum integrates to about 1000.4Wm-2, and the direct reference spectrum integrates to about 900.1 Wm-2. The structure in the spectral irradiance is due to atmospheric absorption and scattering. The full width at half maximum (FWHM) bandwidth in the spectral irradiance at any given wavelength is approximately the difference in wavelength between adjacent points and is a function of the bandwidth of the measurement system.
The PV community has arbitrarily taken the term “one sun” to mean a total irradiance of 1000Wm-2 [18]. In fact, the spectral irradiance of the ASTM G173 direct reference spectrum normalized to a “1-sun” value of 1000 Wm-2 exceeds the AM0 spectral irradiance in the infrared (IR), which is not physically possible without concentration. The term “global” refers to the spectral irradiance distribution on a 37°-tilted south-facing surface with a solar zenith angle of 48.19° (AM1.5). The term “direct” refers to the direct-normal component (5° field of view) of the global spectral irradiance distribution [14, 19]. The term AM1 or AM1.5 is often used to refer to standard spectra, but the relative optical air mass is a geometrical quantity and can be obtained by taking the secant of the zenith angle or the sine of the solar elevation (see Chapter 22). For AM1, the zenith angle is 0°. The relative optical air mass can be pressure corrected to an absolute air mass by multiplying by the barometric pressure and dividing by the sea-level pressure. In outer space, the pressure is zero so the absolute air mass is always zero. The fact that the reference spectrum
only approximates the “real-world” spectra at solar noon is unimportant as long as the differences between the photocurrents are the same for various PV technologies.
The technical basis for the direct spectra has recently been reexamined and found to have a diffuse component that is substantially greater than concentrators would normally encounter [21, 22]. Examination of the US solar radiation database has found that when the global-normal irradiance is near 1000 Wm-2, the direct-normal component is near 850 Wm-2 and not the 767 Wm-2 to which the direct standard spectrum integrates [21]. This difference has been attributed to a high turbidity [22]. This has not been a problem for concentrators in the past because of their relative insensitivity to the specific direct spectra [23, 24]. Recent high-efficiency structures such as the GaInP/GaAs/Ge triple-junction solar cell exhibit a significant difference in the efficiency between the global and direct reference spectra (>10%). It has been suggested that the global reference spectrum may be a better spectrum than the direct reference spectrum to optimize concentrator cells for use in sunny climates [22, 25]. The previous ASTM G159 direct-beam reference spectrum may be more appropriate for regions with high aerosol content that have direct-beam resources above 5kW hm-2d-1 such as Saudi Arabia [26]. The IEC TC82 Working Group 7 Standards Committee is considering what additional reference spectra should be considered for evaluating concentrator cells and modules. At present there is no consensus among the calibration labs around the world on alternatives to ASTM G173 direct and there is no IEC standard being drafted to address applications where concentrators are deployed in high aerosol climate. It is the author’s recommendation that G159 direct be used as the reference spectrum for concentrator applications in sunny but high aerosol regions of the world.
The extraterrestrial spectral irradiance distribution at one astronomical unit distance from the sun is commonly referred to as the AM0 spectrum. International consensus standards for AM0 measurements have been developed [17]. Measurements of the total AM0 irradiance used by the aerospace community have varied from 1353 to 1372Wm-2 [8, 16, 27-30]. Many groups still rely on the less accurate value of 1353 Wm-2 total AM0 irradiance [27, 28]. Recently, a new ASTM AM0 standard has been adopted that uses more accurate spectral irradiance measurements given in Figure 18.1 [8]. The best estimate for the solar “constant” is 1367 Wm-2 recommended by the World Radiation Center [16], or 1366.1 recommended by ASTM [8]. Both of these values were obtained from long-term monitoring of the solar irradiance with an active-cavity radiometer on the Solar Max and Nimbus 7 and other satellites [30]. Fortunately, the 1353 Wm-2 total AM0 irradiance, used by many groups for efficiency measurements and reporting purposes, does not enter into the spacecraft PV power measurements. This is because primary balloon or space-based AM0 reference cells are calibrated at whatever irradiance exists at the time of calibration, corrected for 1 astronomical unit distance from the sun. However ISO standard 15387 allows terrestrial based calibrations with respect to this synthetic AM0 spectral irradiance, as discussed in Section 18.3.3 and Chapter 22 [17].
A variety of definitions for cells and modules have been proposed [1, 5, 32, 33]. A module consists of several encapsulated, environmentally protected, electrically interconnected cells. The area of a cell is taken to be the total area of the space-charge region including grids and contacts. The standard definitions of cell area replace the term “space-charge region” with “frontal area,” but this term does not adequately account for devices with multiple cells on a single substrate or superstrate. The area of a concentrator cell is based on the area that is designed to be illuminated
[5] . This area is taken to be the area of the space-charge region minus the area of any peripheral bus bars or contacts. A submodule or minimodule is an unencapsulated module.
The PV efficiency (n) is inversely proportional to the area definition used (Equation 18.1). In fact, differences in the area definition often account for the greatest differences in reported efficiencies between various groups and values published in the literature [33, 34]. The largest differences occur when the so-called active area (total device area minus all area that is shaded or not active)
is used. The use of an active area in the efficiency neglects the trade-off between lower resistance losses and increased shading. Several thin film PV device structures do not have any shading losses, so the active and total areas are the same. To prevent an artificial increase in the efficiency, care must be taken to ensure that light outside the defined area cannot be collected by multiple internal reflections or incomplete electrical isolation. Incomplete electrical isolation is always a possibility when the device area is defined by the contact area and the junction area is larger than the device area. This effect increases as the cell size decreases. Larger perimeter-to-area ratios increase the potential effect of current being collected outside the defined area. This phenomenon is why a 1-cm2 minimum area is required for inclusion in the Progress in Photovoltaics efficiency tables [35]. To be sure the region enclosed by the total area is the only active region, an aperture should be used [35].
At the module level, the total area including the frame is used. For prototype modules, where the frame design is less important than the encapsulation and cell interconnections, an aperture-area definition is often used. The aperture-area definition is the total area minus the frame area. This aperture area may be defined by opaque tape if there is no frame to eliminate the possibility of the module collecting current outside the defined aperture area by multiple internal reflections or light piping. Plastic tape may or may not be opaque enough in the IR depending on the tape and on the PV materials.
The most common method of performance rating for modules is the PV power conversion efficiency under SRC (Table 18.1). The power or peak watt rating on the module’s nameplate is usually given with respect to SRC, as shown in Table 18.1 using a 25 °C module temperature. Unfortunately, prevailing conditions under natural sunlight do not commonly match nameplate conditions. The nameplate rating that the manufacturer assigns to a given module model number is often higher but rarely lower than the measured power output in the field [36-38]. While the nameplate rating is determined with the module temperature controlled at 25 ° C, the actual power produced is often less than this because the module will typically run around 35 °C above air temperature on a sunny day. The temperature coefficient for the peak power is negative. The nameplate rating also does not include long-term degradation or system losses. System losses include the power-conditioning unit’s efficiency, ability of the power conditioner to operate at the maximum PV power point, orientation, shading, resistance losses in the wiring, and mismatch in the power of different modules.
The nominal operating cell temperature (NOCT) is a rating designed to give information about the thermal qualities of a module and a more realistic estimate of the power in the field on a sunny day at solar noon. The NOCT of a module is a fixed temperature that the module would operate at if it is exposed to the nominal thermal environment (20 °C air temperature, 800 Wm-2 total irradiance, and a wind speed of 1ms-1) [7, 39]. Typical NOCT values found on the module nameplate range from 35 to 45 °C. The term “standard operating conditions” (SOC) is sometimes used for flat-plate or concentrator terrestrial modules operating at NOCT. The actual determination of the NOCT of a module with an uncertainty of less than ±2 °C has proved difficult because of difficulties in measuring the temperature of cells in an encapsulated module, uncertainties in the total irradiance, and secondary environmental effects such as wind direction, ground reflections, mounting, and electrical loading [39, 40]. The installed NOCT is up to 15 °C warmer for roof – mounted applications than a free-standing module depending on the stand-off distance between the module and roof [39, 40]. The module temperature can be calculated from the NOCT or installed NOCT and air temperature using
T = Tair + (NOCT – 20 °C)Etot/800 Wm-2. (18.2)
A wind-speed correction can also be applied to Equation (18.2) [7, 39].
For a fair and meaningful comparison of efficiencies between technologies, the measurements should be performed after any initial degradation or transient behavior has stabilized. Commercial silicon modules have shown small changes in performance after the first hours of
operation [41, 42]. At the present time, all amorphous silicon PV technologies degrade when exposed to sunlight. Fortunately, this degradation stabilizes at a level of 80-90% of the initial value. Partial recovery occurs in the field during the summer when the higher module temperature leads to partial annealing or when amorphous silicon modules are annealed in the laboratory at 60-70 °C [43, 44]. The efficiency continues to decrease after 500 hours of light exposure at lower temperatures even if the light level is reduced [43-45]. For a fair and meaningful comparison of improvements in amorphous silicon module development, the performance at SRC is now reported after illumination of about 1000 Wm-2, at a module back-surface temperature of nominally 50 °C, for at least 1000 hours, with a resistive load near Pmax, and low humidity [33, 34]. These conditions were chosen to approximate one year of outdoor exposure without the humidity or temperature cycling. Thin-film module stabilization procedures allow for shorter times based upon the power changing less than 2% over two consecutive periods of 43kWhm-2. Other thin-film module technologies may undergo reversible and irreversible changes during the first few hours of light exposure [34, 46, 47].
