The question of the performance achievable with solar pumped dye lasers will soon be addressed by experiments to be performed by Dr.
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Winston's group at the University of Chicago. A third topic of debate was the application of solar pumped dye lasers to laser isotope separation. However, the implementation of laser isotope separation in its current form requires laser pulses with a fairly low duty cycle. This is a mode of operation in which solar pumped lasers are very inefficient, because the low duty cycle wastes much of the collected solar power. This question must be considered and, if necessary, its impact on the cost estimates should be included.
In the paper by Christiansen the novel idea of pumping lasers with the solar radiation trapped in a blackbody cavity is discussed and demonstrated. In principle, the recycling of the radiation trapped in the blackbody cavity might provide much more efficient utilization of the collected solar flux than direct solar pumping. This is illustrated by the relatively high efficiency predicted by Christiansen for a neodymium activated solid state laser. It remains to be demonstrated, however, that this advantage can be achieved in practical blackbody cavities.
The loss of thermal energy from the blackbody must be not much larger than, and preferably smaller than, the laser output in order to achieve the predicted efficiencies. Such a demonstration should have a high priority in future research plans. A potential advantage of blackbody pumping is that the collected solar energy is stored in the blackbody during periods of interruption of the solar flux, during which laser operation might continue, perhaps with the infusion of substitute forms of heating of the blackbody during long interruptions.
The solar pumped gas laser discussed by Lee uses an imaging concentrator, which is adequate for pumping gases of molecules containing iodine to above their laser threshold. The utility of such a laser for space-based applications, such as communications and power transfer to space vehicles is being explored.
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This laser concept has the advantage of readily scaling up to large sizes for high output powers. The anticipated efficiency of this type of laser about a tenth of a percent or so is rather low for prospective terrestrial applications in competition with other technologies. At this stage of their development, it might be said that solar pumped lasers are a possible solution in search of possible problems. While judging the merits of solar pumping of lasers for any application, it is, of course, important to keep in mind the competing technologies that employ either solar or nonsolar energy sources.
For example, semiconductor diode laser pumped solid state lasers powered by solar pumped photoelectric cells may provide stiff competition in the future for direct solar pumping in prospective laser applications. At this time too little is known about the performance and cost of solar pumped lasers to make meaningful assessments of their suitability for applications or comparisons with other technologies.
It is therefore important to shape research and development programs on the various solar pumping schemes with a view toward clearly establishing the limits to their performance. At the same time, efforts should be made to identify the promising applications of solar pumped lasers and to make credible assessments of their suitability for those applications.
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If progress is not made in parallel in these two areas, it may be difficult to come up with either the 'solution' or the 'problems' in this period of intense competition for funding. Of the variety of solar energy conversion schemes that have been explored, the conversion of solar flux to coherent laser radiation is a relative newcomer. Actually, demonstration of solar pumped lasing goes back to the s[ 1 ]. However, the number of groups engaged in solar laser research is extremely small [ 2 , 3 ]. Part of the reason for this is technological.
Solar flux at sufficiently high concentrations to overcome threshold for the really important laser materials has simply not been available. This technological inhibition has recently been overcome through the application of nonimaging optics [ 4 ] through the demonstration of concentration levels of 84, ''suns'' at the University of Chicago in a refractive medium sapphire and of over 20, "suns" in air at the Solar Energy Research Institute SERI High Flux Facility [ 6 ].
One can therefore expect renewed interest and increased activity in solar lasers, this latest and least developed frontier of solar energy utilization. It is useful to view solar lasers in the context of other more familiar and more highly developed solar utilization technologies. The Second Law of Thermodynamics provides a useful unified framework for this purpose. The difference q 1 -q 2 is available for supplying work W.
A similar discussion could be constructed for the photovoltaic cell cycle.
Recalling that the brightness of blackbody radiation Planck spectrum can be written as. This then is what the solar laser does, increase brightness by a huge amount. Let us analyze how well the solar laser performs. While one might be left with an impression that the input numbers have been chosen to make the answer "come out right," the legitimate conclusion that can be drawn is that the solar laser operates near the Second Law limit of brightness gain.
The solar laboratory at our university is situated on the roof of the high-energy physics building. A 1 m 2 heliostat redirects solar radiation to a small primary reflector. The heliostat is ordinary float glass covered with 3Z silver film. The light from the primary is then further concentrated by a nonimaging concentrator. In our early work the primary reflector was a cm diameter telescope mirror.
Initially the concentrator was made of acrylic, which has an index of refraction of about 1. To keep the concentrator from melting, a spinning wheel with a hole in it was placed in front of the concentrator to chop the sunlight. The concentrator "compressed" the 1 cm spot from the mirror to a 1. One way of thinking about the experiment, then, is that all the light from the 40 cm diameter mirror was funneled' into a 1.
The light exiting the tip of the concentrator was then directed into one end of a laser crystal; the tip of the concentrator was optically coupled to the crystal. Both crystals were 30 mm long and 1. Since the light exiting the tip of the concentrator forms a hemispherical front, the effective concentration area is increased from 1. The coupled end of the laser rod was coated with a high-reflection HR film at 1. The HR coated end served as the end mirror of the laser cavity.
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The other end of the crystal was antireflected AR coated at 1. An external partially transmitting mirror completed the laser cavity. We successfully lased both types of crystals. In the next stage of our research we sought to increase the efficiency of the Nd:Cr:GSGG system and to lase alexandrite, which has the advantage of being tunable.
Experiments and Lab Ideas
Two tactics were crucial to this work. First we switched from acrylic to sapphire, which has an index of refraction of 1. Since concentration is proportional to the square of the index of refraction, this boosts our concentration by a factor of 1.
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Second, we decided to grind the outer diameter of the laser crystals to match the diameter of the concentrator tip and thereby avoid the loss in concentration due to oversizing of the laser rod. Developing the required sapphire concentrators proved to be a significant challenge in itself.
Sapphire is almost as hard as diamond, and is therefore difficult to work with. Nevertheless, in the past few years our learning curve for machining and polishing this material has rocketed almost exponentially. We owe our success in mastering this task to the excellent university machine shop and its extraordinarily skilled craftsmen. Unfortunately, we discovered that grinding the laser rods to 1. The grinding process apparently introduces stress into the crystals, which prevents lasing from occurring. We also had to make the sapphire concentrators correspondingly larger.
We are exploring the possibility of applying an HR coating to the alexandrite crystal as well. Order Code. ExWorks Price. User manual. Description Solar water heaters are becoming a popular alternative to conventional water heaters because they save the homeowner money, and they reduce carbon emissions. Renewable Energy with Vernier 2 Includes student instructions, teacher tips, safety information, and sample data. Efficiency is reduced at higher temperatures. In sunny, warm locations, where freeze protection is not necessary, an ICS batch type solar water heater can be cost effective.
This increases initial costs, but not life-cycle costs. The biggest single consideration is therefore the large initial financial outlay of solar water heating systems. Payback times can vary greatly due to regional sun, extra cost due to frost protection needs of collectors, household hot water use etc. For instance in central and southern Florida the payback period could easily be 7 years or less rather than the The payback period is shorter given greater insolation.