Field of the invention

[0001] The present invention is related to discharge lamps, in particular discharge lamps that are used to simulate solar light, and to the use of such lamps as sources in test characterisation of photovoltaic systems. Description of related art

[0002] High intensity discharge lamps (HID lamps) form one of the most widely used forms of lighting. An electrodeless lamp is a form of discharge lamp in which the discharge is obtained at the interior of a sealed

transparent bulb by use of a RF or microwave energy. The bulbs in electrodeless lamps include a chemically inert gas and one or more active components, like for example mercury, sulphur, tellurium, or metal halides.

[0003] Electrodeless lamps tend to have a longer lifetime and to maintain uniform spectral characteristics along their life than electrode discharge lamps. While requiring a radiofrequency power supply, they use bulbs of very simple structure, without costly glass-metal interfaces.

Moreover, they can use filling compositions that would be chemically incompatible with metals electrodes.

[0004] Many HID lamps are filled with a composition containing mercury. This is advantageous for what the light emission is concerned, mercury, however, is a toxic and environmentally hazardous substance, and it is expected that its use will be limited or phased out in the future. Other variants are known for the composition used to fill the bulb of an

electrodeless lamp. A fill containing selenium or sulphur is known from US5606220, and US66331 1 1 describes a fill comprising Snl ₂ . WO08120171 A and US6469444B disclose a fill with sulphur in association with antimony halides. US5866981 discloses a composition comprising rare earth and metal halides such as antimony iodide (Sb ) or indium iodide, while

WO10044020, US20101 17533 describe a fill including to monoxide compounds and metal halides. These documents are generally concerned with lamps for general illumination applications, and strive to produce a fill that delivers high luminous efficiency and colour rendition.

[0005] Test and characterisation of photovoltaic systems are carried out, with solar simulators that include light sources designed to simulate the characteristics of natural solar illumination. It is desirable, to ensure exact and repeatable test results, that the simulated solar light should match the intensity and spectrum of solar light, as it is received at the surface of earth. There exist several international standards aiming to regulate and

standardise the spectral characteristics of solar simulators, for example IEC60904, ASTMG173 and IS09845-1 , as well as the testing protocols for photovoltaic elements, like IEC601215, IEC61646. These standards prescribe, for example, that photovoltaic systems used for terrestrial applications at fixed orientation should be tested with an illumination following, within prescribed tolerances, the AM1 .5G spectrum given in table 1 .

[0006] In the art, it is known to use Xenon discharge lamps, or different combinations of discharge lamps and halogen lamps to provide an emission spectrum that closely matches the solar illumination. In some cases, the match can be improved by the use of appropriate filters. US320281 1 , US2010007301 1 and US7431466 describe examples of solar simulators of this kind.

[0007] These solar simulators provide a light with a spectrum that matches the solar emission, but at the cost of combining several sources and filters. It is desirable, therefore, a lamp that could directly generate a light that matches closely the sun spectrum in a form that is more compact, economical, and energy efficient than the solutions of the state of the art.

Brief summary of the invention

[0008] According to the invention, these aims are achieved by means of the lamp that is the object of the independent claim, while dependent claims relate to preferred embodiments and useful variants. Brief Description of the Drawings

[0009] The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which: Figure 1 is a conceptual simplified representation of a discharge lamp according to an embodiment of the invention.

Figures 2 to 9 show emission spectra of discharge lamps according to various examples and embodiments of the invention. The relative light intensity, in ordinates, is plotted against the wavelength in nm. The emission spectra are superposed to a standard AM1 .5G solar spectrum (dashed line).

Detailed Description of possible embodiments of the Invention

[0010] Plasma lamps are per se known in the art, and their structure and manufacture will be discussed here summarily. Figure 1 illustrates a possible structure of a discharge lamp suitable to embody the invention. The lamp includes a transparent sealed bulb 20, enclosing a volume 24 that is filled with a suitable fill composition, as it will be seen in the following. The bulb 20 is placed in an electromagnetic enclosure 32 to which radiofrequency energy is supplied, in order to bring the fill to a light-and infrared-radiating plasma state.

[0011] In a typical realization a magnetron 40 generates a

radiofrequency signal of appropriate intensity, and is coupled to the cavity 32 by waveguide 35 and opening 36. This variant is advantageous because magnetrons emitting in the open 2.45 GHz band with powers of the order of 1 kW are readily available at attractive prices, but the invention could be realized with any suitable means for coupling excitation power into the bulb to generate a light- and infrared-radiating plasma within the bulb. The invention could use, for example, a solid-state RF source in the UHF band or at other frequencies, for example in the LF or HF bands. It would also be conceivable to insert electrodes into the bulb, and transfer energy to the fill by an electric discharge.

[0012] The present invention is not limited to a specific coupling arrangement either. The waveguide 35 and opening 36 could in fact take any suitable form. In a possible variant the waveguide 35 could be suppressed entirely, and the magnetron or the RF source coupled directly to the enclosure 32. According the frequency of the excitation radiation, the coupling could include magnetic elements, ferrite cores or the like.

[0013] The purpose of electromagnetic enclosure 32 is to confine the radiofrequency field and concentrate it on the bulb 20. In embodiments of the invention, however, the enclosure 32 could be suppressed: for example if the lamp is fully enclosed in a larger system. In other cases the enclosure could include light reflecting and light transmitting surfaces, in order to project a light beam. In typical instances, the enclosure 32 may be an electromagnetic cavity tuned to the magnetron's frequency, whose walls are made of conductive mesh or perforated metal, in order to concentrate RF energy on the bulb 20 while letting the light out.

[0014] Optionally, the electric motor 60 is used to drive the bulb in rotation by the insulating stem 26. This is useful to prevent the formation of hot spots on the surface of the bulb itself.

[0015] The bulb itself is preferably made of quartz, or of any suitable transparent material capable to stand high operating temperatures, for example of 600-900 °C, and chemically compatible with the fill. According to the desired power, the size of the bulb may vary between 0.5 cm ³ and 100 cm ³ , typically around 10-30 cm ³ . As to the filling pressure, the bulb is typically filled at a pressure of 10-100 hPa at standard temperature, the pressure at operation being for example comprised between 0.1 MPa and 2 MPa (1 and 20 bar absolute).

[0016] The present invention aims to provide a discharge lamp suitable for the use in solar simulators, with an emission spectrum following, as much as possible, the AM1 .5G standard. With respect to conventional illumination applications, the spectrum of the lamp of the invention follows more closely the sun in the red and infrared, for example in the region between 700 and 1000 nm. These wavelengths do not add much to the perceived illumination level and colours, but contribute significantly to the thermal and electrical behaviour of photovoltaic cells and panels. The source of the present invention is also suitable to simulate other spectrum standard, like for example AMG1 .0.

[0017] According to a preferred embodiment of the invention, the bulb is filled with a composition comprising an inert gas, for example N ₂ , He, Ne, Ar, Kr, Xe or a mixture thereof, and a first and a second active components, the first active component being an antimony or bismuth halide or a mixture of antimony halides; while the second component is preferably Snl ₂ , but also other halides or a mixture of halides of: In, Sn, Ag, Bi, Cu have proven valid alternatives. Preferably, the halides are bromides or iodides or chlorides due to their favourable volatilities.

[0018] Experimentation has shown that this composition provide an emission matching closely the standard solar spectrum, and good overall efficiency. Antimony fills have proved somewhat superior in these respects than bismuth fills.

[0019] It has also been found that the spectral match can be improved by adding an additional active component like metallic indium, or, in alternative, copper or silver.

[0020] The concentration of active components in the bulb can vary between 0.1 and 5 and mg/cm3. Best results are obtained at concentrations between 0.5 and 2 mg/cm3. As to the gaseous part, good ignition of the discharge has been obtained with filling pressures of about 30 mbar at atmospheric pressure. The tests have used, with equivalent results : pure argon, Ar/Xe mixtures, or other inert gases. [ Example I ]

[0021] According to a first example, the bulb 20 is a quartz spherical vessel of 1 5.6 cm ³ internal volume, and it is filled as follows:

SbBr ₃ 10 mg

Snl ₂ 7 mg

In(metallic) 7 mg

Ar 30 mbar at 25

[0022] The bulb is inserted in a lamp having the structure of figure 1 , spun at 3000 rpm and excited by a microwave source at 2.45 GHz and 720 W. The emission spectrum obtained is shown in figure 2. The temperature of the bulb, measured by a FLIR camera, was 678 °C. This combination provides an excellent spectrum and good efficiency.

[ Example II ] [0023] According to another example, an identical quartz bulb of 1 5.6 cm ³ internal volume, it is filled as follows:

BiBr ₃ 10 mg

Snl2 5 mg

In(metallic) 5 mg

Ar 30 mbar at 25

[0024] The bulb is inserted in a lamp having identical to that of exampl I and excited by a microwave source at 2.45 GHz and 828 W. The emission spectrum obtained is shown in figure 3. The temperature of the bulb, not spinning in this test, was 810 °C. The spectrum shows higher peaks above the continuous component, and matches the solar distribution somewhat worse than the one in example I. [ Example III ]

[0025] According to another example, an identical quartz bulb of 1 5.6 cm ³ internal volume, it is filled as follows:

• BiBr ₃ 10 mg

· In(metallic) 10 mg

• Ar 30 mbar at 25 °C

[0026] The bulb is inserted in a lamp having identical to that of example I, spun at 3000 rpm and excited by a microwave source at 2.45 GHz and 795 W. The emission spectrum obtained is shown in figure 4. The temperature of the bulb was not measured. In term of spectral quality, this fill is clearly less satisfactory than the antimony fill of example I.

[ Example IV ]

[0027] According to another example, an identical quartz bulb of 1 5.6 cm ³ internal volume, it is filled as follows:

• SbBr ₃ 1 5 mg

• In(metallic) 10 mg

• Ar 30 mbar at 25 °C [0028] The bulb is inserted in a lamp having identical to that of example I, spun at 3000 rpm and excited by a microwave source at 2.45 GHz and 700 W. The emission spectrum obtained is shown in figure 5. The temperature of the bulb was 663 °C. The match with the solar spectrum is fair, but inferior to that of example I. [ Example V ]

[0029] According to another example, an identical quartz bulb of 1 5.6 cm ³ internal volume, it is filled as follows: • SbBr ₃ 14 mg

• Snl ₂ 5 mg

• In(metallic) 9 mg

• Ar 30 mbar at 25 °C

[0030] The bulb is inserted in a lamp having identical to that of example I, spun at 3000 rpm and excited by a microwave source at 2.45 GHz and 720 W. The emission spectrum obtained is shown in figure 6. The temperature of the bulb was 652 °C. This fill is qualitatively the same to that of example I, with different proportions, and also yielded an excellent spectrum.

[ Example VI ]

[0031] According to another example, an identical quartz bulb of 1 5.6 cm ³ internal volume, it is filled as follows:

• SbBr ₃ 10 mg

· InCla 10 mg

• In(metallic) 7 mg

• Ar 30 mbar at 25 °C

[0032] The bulb is inserted in a lamp having identical to that of example I, spun at 3000 rpm and excited by a microwave source at 2.45 GHz and 735 W. The emission spectrum obtained is shown in figure 7. The temperature of the bulb was 791 °C. In this case the substitution of InCU for Snl ₂ still gives a good spectrum, but a lower intensity.

Table 1 : AM1.5G spectrum λ [nm] intensity λ [nm] intensity

305 0.005833231 757.5 0.721908388

310 0.025973229 762.5 0.39494044

315 0.066191821 767.5 0.632997667

320 0.111138401 780 0.694645708

325 0.151602603 800 0.664251504

330 0.242785214 816 0.521552253

335 0.239592288 823.7 0.48207049

340 0.267346187 831.5 0.562814687

345 0.269556674 840 0.589524745

350 0.297064964 860 0.601191207

360 0.319538254 880 0.573130296

370 0.409185804 905 0.459720005

380 0.43761513 915 0.409922633

390 0.442650129 925 0.42398379

400 0.622190839 930 0.247881616

410 0.711285767 937 0.158602481

420 0.727188997 948 0.192558025

430 0.658295469 965 0.323529412

440 0.799643866 980 0.397028122

450 0.937185313 993.5 0.458614761

460 0.982377502 1040 0.424106595

470 0.97095665 1070 0.391501903

480 1 1100 0.253346433

490 0.945290434 1120 0.06692865

500 0.951123664 1130 0.116111998

510 0.974333784 1137 0.081174014

520 0.911948913 1161 0.208215645

530 0.965676041 1180 0.282512587

540 0.952351713 1200 0.2601007

550 0.958983176 1235 0.295100086

570 0.922141717 1290 0.253714847

590 0.857055139 1320 0.153628884

610 0.912194523 1350 0.01995579

630 0.880756478 1395 0.000982439

650 0.87197593 1442.5 0.034201154

670 0.855028859 1462.5 0.064533956

690 0.693970281 1477 0.064779565

710 0.808670023 1497 0.111813828

718 0.620471571 1520 0.161304188

724.4 0.640672971 1539 0.16842687

740 0.743829056 1558 0.168856687

752.5 0.733206435 1578 0.150190348