Solar Modules and Solar Generators

4.1 Solar Modules

Commercial crystalline silicon solar cells have open-circuit voltage VOC ranging from around 0.55 to 0.72 V at a cell temperature of 25 °C. With a cell area of AZ ~ 100 cm2 (4 inch wafer) such cells have short-circuit current ISC ranging from 3 to 3.8 A; with AZ ~ 155 cm2 (5 inch wafer) the figure ranges from around 4.6 to 6 A; with AZ ~ 225 cm2 (6 inch wafer) the figure ranges from around 6.8 to 8.5 A; and with 400 cm2 (8 inch wafer) the figure ranges from around 13 to 15 A. For optimal power yield (MPP; see Section 3.3.3), the voltage is around VMpp ~ 0.45 to 0.58 V. Hardly any appliance can be operated at such low voltages. Hence voltages that are usable for PV system operation can only be generated using multiple solar cells wired in series.

If higher voltages are needed for a specific application, a series of solar cells must be wired in parallel. Series and parallel connection of solar cells allow for interconnection of an unlimited number of such cells to create massive solar generator fields with many megawatts of power.

In some cases it is useful to wire in series anywhere from around 32 to 72 solar cells and to house them in a single enclosure to protect them against the environment. The entities thus created are referred to as solar cell modules, solar modules or simply modules.

Solar modules with 36 cells, operating voltages from 15 to 20 V and output ranging from 50 to 200 Wp are very widely used, since a viable 12 V power supply is unobtainable with only one module and battery. On the other hand, such modules comprising 72 cells for operating voltages ranging from 30 to 40 V are suitable for stand-alone 24 V system voltage installations. Modules with output up to around 200 Wp that measure 1.5 m2 and weigh around 18 kg can be readily handled by one person.

The output of the largest mass-produced polycrystalline module currently on the market (sold by Schott) is 300 Wp (80 cells wired in series). The largest monocrystalline module with an output of around 315 Wp (96 cells wired in series) is made by Sunpower (see Table 4.1 for details concerning these modules). Larger modules for integration into buildings can be custom ordered from specialized vendors.

The life span of such modules is largely determined by how well they are protected against the ambient environment. Some vendors indicate a 30-year life span and grant 2- to 5-year full warranties and in some cases limited performance guarantees for 10 to 26 years. The fronts of most such products are well protected against hail via specially tempered low-iron glass that is highly transparent. In addition, solar cells are hermetically packed in a transparent plastic material such as ethyl vinyl acetate (EVA). The rear protection elements are made of plastic or glass, depending on the manufacturer. A classic solar module integrates relatively thin (e. g. 3-4 mm) glass and has a robust metal frame (usually made of aluminium) that provides the requisite mechanical stability and good edge protection. Modules with thin-film silicon solar cells or the like also have plastic frames.

Photovoltaics: System Design and Practice. Heinrich Haberlin.

© 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

Подпись:
Examples partially from 2007, possibly some modules no longer available now. *No longer available on the market.

Подпись:** Ground + pole of PV array by a high resistance for full power.

electrical test of solar ce

 

sorting

 

image341

Подпись: Л5Лstring layout visual inspection

ana soldenng

Подпись:cell soldeiin

connection contact

documentation

 

Подпись: string layout forПодпись: matrix configuration

assembly of sandwich of

glass, foils and cell matrix
lamination of sandwich

(vacuum, temperature)

cell

connection

 

glass glass compound

 

ЇІГІУ-.’

 

L foil

 

– I Г!!’

 

МсКіПМІ

 

framing

 

connections to module

 

Подпись: connection boxvisual inspection cleaning

moduli мігм

 

image347
image348
image349

shipment

 

documentation

 

image350

Figure 4.1 The manufacturing process for crystalline silicon solar cells (Partially based on documents from AEG)

The module manufacturing process for crystalline solar cells is relatively labour intensive. Figure 4.1 displays the workflow for the production of such solar cells. Figures 4.2 and 4.3 display some of the intermediate stages in the production process.

The above packing method is extremely cost intensive in that the requisite materials – particularly for modules with aluminium frames – consume considerable amounts of grey energy, i. e. energy that is used to manufacture these elements. That said, anyone who has seen a solar module that has been exposed to the relatively damp Central European climate for a lengthy period will instantly understand why such modules need extremely robust protection against the elements. By the same token, it is no easy matter to manufacture building windows that will remain perfectly watertight and transparent for two to three decades without maintenance.

image351

Figure 4.2 Assembling individual solar cells into strings. For purposes of illustration, a somewhat older but more readily understandable apparatus is shown here. © Solar-Fabrik AG, Freiburg

Laminates with thicker (e. g. 6-10 mm) glass that can be set into facades or roofs as is done with plate glass are often integrated into buildings. However, owing to the absence of a frame, such laminates must be handled with extreme care in order to avoid breakage. For a number of years now, some manufacturers have also been selling special plastic or glass solar roof tiles for rooftop PV systems. These tiles are somewhat larger than normal and are used for roofing in lieu of standard roofing tiles.

Figure 4.4 displays a finished SPR220 monocrystalline solar module with a new kind of rear that obviates shading by the front electrodes. Figure 4.5 displays a polycrystalline and multicrystalline solar module with conventional BP3160 front electrodes. Figure 4.6 displays an ST40 CIS module with a highly homogeneous appearance. All of these modules are framed.

image352

Figure 4.3 In the laminator, the sandwich comprising the cover glass, films and the intermediate, pre-wired strings are baked together in a vacuum. For purposes of illustration, a somewhat older but more readily understandable apparatus is shown here. © Solar-Fabrik AG, Freiburg

image353

Figure 4.4 Sunpower SPR220 solar module with 72 monocrystalline silicon solar cells and a new type of rear contact (220 Wp, Zm — 17.7%) (Photo: Sunpower Corporation)

Figure 4.7 displays a laminate with polycrystalline cells. Figure 4.8 displays a Uni-Solar US64 amorphous silicon triple-cell solar module (solar cell structure as in Figure 3.51). Figure 4.9 displays the Newtec SDZ36, which was one of the first solar roof tiles to come on the market, integrates 24 monocrystalline solar cells, and can be mounted on a roof in lieu of standard tiles; such tiles can also be walked on.

Framed modules are superior to their laminate counterparts in terms of handling, mechanical stability and lightning protection. However, in framed modules – particularly those mounted flush with the roof – over time a permanent layer of grit forms between the outermost cells and the frames that reduces energy yield. Hence the frames on the outside of such modules (i. e. the side facing the Sun) should be as low as possible and approximately 5-15 mm clearance should be left on all sides between the cells and module frames.

The solar cell module wiring diagrams in this chapter use the symbol shown in Figure 4.10. The triangle at the positive end is somewhat similar to the diode connection symbol in Figure 3.8.

In cases where higher voltages are needed, a series of modules must be wired in series into a string. For stronger current, a series of modules or strings is wired in parallel.

In the past, in order to display characteristic waveforms for a 36-cell solar module in a product and vendor neutral manner, the then widely used (but since discontinued) Siemens M55 module with 36 cells of 103 mm x 103 mm wired in series and 55 Wp of rated STC power output was employed.

But as in recent years it has become increasingly difficult to obtain detailed characteristic curves from vendors for their modules under various operating conditions, in the interest of obtaining data that are as consistent as possible with the scant data disclosed by vendors and with my own measurements for selected modules, the characteristic curves in Figures 4.11-4.15 were computer generated using the full equivalent

image354

Figure 4.5 BP Solar BP3160 solar module with 72 polycrystalline silicon solar cells and a conventional contact system (175Wp, zm = 12.7%) (Photo: BP Solar)

circuit as in Figure 3.12. Figure 4.11 provides an overview of the I-Vcharacteristics of the M55 module at cell temperatures of 25 and 55 °C, and with three different insolation levels (100, 400 and 1 kW/m2).

Figure 4.12 displays the I-V characteristic curves for an M55 module at a constant cell temperature of 25 °C and various levels of insolation. The short-circuit current is exactly proportional to insolation, whereas at low insolation the open-circuit voltage is almost as high as for 1 kW/m2. Maximum output PMpp at the MPP exhibits a somewhat larger increase relative to insolation, i. e. at lower insolation efficiency is also somewhat poorer than with 1 kW/m2.

Figure 4.13 displays the I-V characteristic curves for an M55 module at various cell temperatures TZ and constant 1 kW/m2 insolation. Short-circuit current increases very slightly as temperature rises, but this induces a reduction in the open-circuit voltage, filling factor and maximum output at the MPP; as a result efficiency declines sharply as temperature rises.

The I-V characteristic curves in Figure 4.12 for various insolation levels and at a constant cell temperature display module characteristics under laboratory conditions, but provide insufficient infor­mation concerning practical applications, since insolation will of course make a solar module quite hot. Depending on mounting modality, module design and wind conditions, cell temperature TZ at 1 kW/m2 insolation normally ranges from around 20 to 40 ° C above ambient temperature Tv If the module’s available electrical power is drawn off by the outer circuit, TZ will be somewhat lower than at open or short circuit, where insolation is converted into heat. This phenomenon, which is attributable to the law of conservation of energy, can be used for purposes such as thermographic searches for inactive modules in large solar generator fields.

The cell temperature increase relative to ambient temperature Tv attributable to module design can be determined using nominal operating cell temperature (NOCT), which is defined as the fixed temperature

image355

Figure 4.6 Shell Solar ST40 CIS solar module (40 Wp). Photo: Shell Solar/SolarWorld

the module would exhibit in the AM1.5 spectrum at open circuit at an ambient temperature of 20 °C, 1 m/s wind speed and Gnoct — 800 W/m2 irradiance. Assuming that the temperature increase relative to Tu is proportional to module irradiance Gm and solar generator irradiance GG, the following is obtained for Tz:

Gm

Gnoct

 

Cell temperature Tz

 

Tu + (NOCT – 20 °C)

 

(4.1)

 

image356

Figure 4.7 Solarfabrik SF125 laminate with a 36-string series array (Pmax — 125 Wp). © Solar-Fabrik AG, Freiburg

 

image357

image358

Figure 4.8 Uni-Solar US64 amorphous solar module, Pmax — 64 Wp, with triple cells as in Figure 3.51.As the cells are relatively large and, according to the vendor, also bypassed via a bypass diode, these modules are far more shading tolerant than standard crystalline cell modules (Photo: Uni-Solar)

 

image359

Figure 4.9 Newtec walkable plastic solar roof tile with 24 monocrystalline solar cells, Pmax — 36 Wp. This product, which was one of the first reasonably sized solar roof tiles to come on the market, could be series interconnected to form strings without screws using a simple insertion system and took the place of four standard roof tiles

 

Figure 4.10 Solar module connection symbol

 

Voltage V in Volts

Подпись: I-V-Characteristics of a Solar Module M55 Figure 4.11 Overview of the I-V characteristics of the Siemens M55 module (55 Wp, 36 cells connected in series) for three different insolation levels (100,400,1 kW/m2) and cell temperatures of 25 and 55 °C. The maximum power points (MPPs) are also indicated on each curve
where: Tv is the ambient temperature in °C;NOCT is the nominal (normal) operating cell temperature in °C according to the datasheet; GM — Gg, i. e. irradiance at the solar module and solar generator; GNOct — 800 W/m2 is the irradiance at which NOCT is defined; and NOCT in conventional modules ranges from around 44 to 50 °C.

Hence it is more in keeping with operational PV systems to plot a solar module’s I-V characteristic curves for various insolation levels, but at constant ambient temperatures. The characteristic curves for an M55 module are illustrated in Figure 4.14 for 25 °C ambient temperature (e. g. for an average summer day) and in Figure 4.15 for 5 °C ambient temperature (e. g. for an average winter day).

Characteristics I = f(V) of a Solar Module M55 at 25°C

image361

Figure 4.12 Characteristic I — f( V) curves for the M55 monocrystalline solar module, at various insolation levels and a cell temperature of 25 ° C

image362

Figure 4.13 Characteristic I = fiV) curves for the M55 monocrystalline solar module, at various cell temperatures and 1 kW/m2 insolation

These characteristic curves are predicated on a 30 °C temperature increase relative to ambient temperature at 1 kW/m2 insolation. These figures would be reached, for example, with still air and average mounting conditions, i. e. attached to a building but with good rear ventilation. For ground-based PV installation modules, which are continuously wind cooled, the temperature increase is appreciably lower and the voltage somewhat higher than shown in Figures 4.14 and 4.15. For example, the temperature increase for ground-based PV installation modules mounted in a pasture or the like is likely to be around 22 °C at 1 kW/m2, whereas the temperature increase can be upwards of 40 °C and the voltage somewhat lower for poorly rear-ventilated modules in the presence of still air.

As Figure 4.14 shows, the MPP voltage at an ambient temperature of 25 °C for all indicated insolation levels is well over 12 V. Hence in moderate climates a 12 V battery bank can be fully charged using only 32 or 33 series-connected cells, although modules with only 33 cells and hence somewhat lower voltage are available from only a handful of manufacturers (e. g. the Isofoton I-47 and in the past the Siemens Solar M50). Sunpower makes a highly efficient 32-cell module (SPR95) with about the same operating voltage as the company’s earlier 36-cell module.

Mainstream solar module manufacturers make solar cells that they then assemble into modules that are subject to a limited warranty. In recent years, various vendors have sprung up that do not make their own solar cells and that specialize in the manufacture of modules or laminates, some of which are customized and are mainly integrated into buildings. These companies use solar cells that are purchased from various vendors. It is of course convenient for architects to be able to obtain (as with windows) standard solar modules and laminates in virtually any size. However, if problems such as unduly low power, gradual power loss, delamination, discoloration and the like arise that may be attributable to the solar cells and solar cell packing, it will probably be far more difficult to assert warranty claims as the lines of responsibility are blurred in such cases.

Today hundreds of different types of solar modules are available from many vendors. Table 4.1 lists key technical data concerning selected solar modules. This information was provided by the manufacturers themselves and makes no claim to completeness. Each year in its February issue, Photon magazine publishes more complete technical data concerning several hundred modules.

Characteristics I = f(V) of a Solar Module M55
at an
Ambient Temperature of 25°C

image363

Figure 4.14 Characteristic I = f(V) curves for the M55 monocrystalline solar module, at various insolation levels and 25 °C ambient temperature

In addition to solar modules whose cells are all wired in series, hybrid solutions that combine series and parallel wiring are also available, particularly for larger modules.

Various problems can arise when solar cells and solar modules are wired to solar generators, and it is necessary to take account of these problems to avoid damage under unusual operating conditions (see Section 4.2).

Characteristics I = f(V) of a Solar Module M55
at an
Ambient Temperature of 5°C

image364

Figure 4.15 Characteristic I = f(V) curves for the M55 monocrystalline solar module, at various insolation levels and 5 ° C ambient temperature

Updated: August 4, 2015 — 11:29 am