In Indonesia the Bahasa Indonesia word for diesel fuel is "Solar", which is cute and a bit confusing when first entering the country. I have no idea what the origin of this word is, but it could be any number of things considering the diverse linguistic origins of a language that is made up largely of Dutch and several traditional local languages with Portuguese, English and a huge number of Asian languages thrown in in varying amounts. Whatever the linguistic origins of the word "Solar" being used for diesel fuel it seems strangely prophetic to an English speaker.
Solar Thermal Electrical Generation
Use of Electricity
Photovoltaics have been around for half a century, and significant improvements in the technology have been made in that time. Ironically though the major advances which have made photovoltaics more functional and more practical are mostly the result of better and cheaper electronic controls. The basic panels made up of sliced silicone wafer cells have also improved, but only slowly. The major advances have come in packing the cells more tightly to increase panel efficiency and reducing manufacturing costs. Efficiency has remained stubbornly around 13% for most of the history of the technology, although there are some notable exceptions to this as well. Thin film panels (previously known as amorphous silicone) became very popular for a short time, but when consumers learned that they lasted only a few years in direct sunlight interest dwindled. The thin film panels do however hold great promise simply because they could be so much cheaper to produce. With efficiency normally around 5% thin film panels really do have to be dirt cheap to make sense. For a few years not long ago there seemed to be great promise for increased efficiency panels that could operate at up to 25% efficiency. By the time these increased efficiency panels reached the market they were operating at only about 17% efficiency though. The big stickler for photovoltaics has always been the life of the panels. Since they are solid state silicone devices it would seem that they would last forever, but this has not been the reality. The early panels were not very robust and they lost capacity over a period of decades. Even after many decades in service though these early panels usually would still put out half of what they did when they were new. This was disappointing performance to be sure, but they did continue to function. Then for several decades from the 1990's through the first decade of the 21st century panels were being guaranteed to put out at least 80% of their rating for a 20 or 25 year period. This seemed pretty good considering that everyone assumed that the panels would continue to function at gradually diminishing output for a long period of time like the old models had done. What happened though was that panels came on the market that just died all of a sudden after a decade or so depending on where they were installed. Obviously the climate and exposure have a lot to do with how long panels last, with more sun leading to shorter panel life. In areas with a short summer and long stormy winters the panels do not work as well, but they would also tend to last much longer simply because they don't wrack up as many hours. Heat has a lot to do with panel life as well. Panels that are not well ventilated from behind will get a lot hotter, and open circuit operation also has something to do with panel life. The more efficient a solar cell is the more difference there will be between an operational state and an open circuit state. Said another way the more efficient a cell is the more energy will be removed from the cell electrically, and this removed energy is energy that does not go into heating the cell.
With efficiency seeming to be stuck at 13% the land area of solar installations tends to seem very significant. If solar is used for providing a small amount of electricity for an individual household just a few square meters of panel area can do a tremendous job. The key to using a small photovoltaic system for a house is keeping power hungry appliances chained. Fluorescent and/or LED lighting can provide illumination for several rooms on a surprisingly small amount of electrical energy each day. Likewise LED and LCD displays for televisions and computers tend to use about a tenth as much power as the old CRC displays. When the current system of fossil fuel burning power plants and conspicuous consumption is considered though meeting electrical demand with photovoltaics appears to be a daunting task that would require huge areas of land. One estimate from the early 21st century was that the entire area of the Sahara Desert, the Mohave Desert, Australia's outback and other smaller areas of ideal insolation would have to be covered with solar panels to meet the worlds current electrical demand. Electricity can be moved over long distances, and the technology for efficient conversion to and from high voltage AC signals is much better than it was in the past. Still though more localized and more efficient power generation seems to make much more sense. When solar electric is produced closer to where it is used the land area is a huge concern.
The appeal of using the heat of the sun to create electricity is that overall efficiency in the neighborhood of 40% is not only possible but quite practical. Photovoltaics have been made to attain this same 40% efficiency as well, but practicality and longevity of these high efficiency solar cells is entirely unknown. If 17% efficient cells can not even be made to last two decades then the prospects for 40% efficient cells does not look good. Solar thermal on the other hand uses simple well understood technology that appears to hold great promise. The basic idea is that the power of the sun is concentrated with parabolic trough or parabolic dish mirrors and used to heat a coolant to high temperatures. This high temperature coolant (pressurized water or liquid sodium) is used to power a heat engine of some kind. Steam engines can be made to work quite well with efficiencies of up to 44% having been attained and there are other heat engines that hold promise as well.
How electricity is used is very significant for how well any large scale solar electric generation system would work. The first additions of solar power to an electrical grid are pure profit. That is there are no problems with the time of day availability of solar power when the solar is only a tiny fraction of the total electrical production. In face diversifying production with some solar, some wind and some hydroelectric tends to make the grid more flexible and more efficient. When any one of these time dependant sources becomes more than a small fraction of total production though problems can arise. For much of the early 21st century Germany had been investing heavily in photovoltaics with substantial federal government subsidies for new installations. What was interesting was how quickly this heavy investment apparently lead to a situation where their electrical grid was saturated with photovoltaic resources. Within just a few years a situation occurred where enough photovoltaics had been brought online that bright sunny summer days saw all of the easily shut down natural gas fired power plants off line and a glut of electricity on the market. This excess of electricity sounds good for uses like charging plug-in hybrid electric cars, but the realities are much darker. Occasional excesses of electricity might be good for a free afternoon trip to the zoo, but people who rely on cars every day to do their job or get to work have little use for a free trip now and again. If a plug-in hybrid is to make good overall sense for regular use it has to be plugged in each time it is used.
One of the reasons that a glut of electricity can become a problem so easily when photovoltaics are heavily used is that owners of any type of fossil fuel fired power plant tend to lose money if they shut down. Instead of shutting down natural gas fired power plants to keep the price of electricity constant there is a tendency for the price of electricity to drop by a large amount before plants are shut down. This of course is an economic problem not a technological problem, but it does point to the necessity of more flexibility if solar power is to be used effectively. The big challenge of course is how to provide solar power 24/7. There are really only two ways this could be accomplished, one is long distance transport and the other is storage. Long distance transport involves electricity being bought and sold over long distances so that "the sun never sets" on the electricity infrastructure. The Pacific and the Atlantic are the major obstacles to this concept, but just within Eurasia a system of long distance distribution could make solar power much more practical. Storing solar electricity might be done either with batteries or with thermal storage if solar thermal electrical generation was used. Neither of these options seem much good for large scale electrical storage, but there are only limited options.
Growing crops to produce biofuel has been proposed, and although this does work well on a small scale meeting current energy demand with biofuels is extremely far from practical. Few would object to covering the Sahara Desert with solar panels, but finding new agricultural land to produce huge biofuel crops is wrong in so many ways. For a very long term sort of idea it might be considered that there would be a time when petroleum was not burned for energy at all, and in this situation there would of course be huge demand for small quantities of biofuels for use in combustion engines in applications where rechargeable batteries would not be practical. Commercial fishing is one example of this, and other types of maritime expeditions as well might never give up diesel engines.
The "hydrogen economy" has long been mused over, and there are some possibilities here as well. The basic idea of the hydrogen economy is the electolization of water into hydrogen gas and oxygen gas (hydrolization). The hydrogen gas (and possibly some of the oxygen gas as well) is then compressed for storage. Using the hydrogen is as easy as feeding it into a fuel cell or burning it in a gasoline or diesel engine. If the hydrogen were burned in a diesel engine it would simply be run through a pressure regulator and then fed into a common rail injection system. The major obstacles are the loss of efficiency both in the hydrolization and at the fuel cell or combustion engine but the energy required to compress the hydrogen has to be considered as well. Since such a large amount of energy is stored in the very high pressure hydrogen it almost seems mandatory to run a compressed air engine alongside the fuel cell or gasoline engine to recover the energy stored in the high pressure gas. In a diesel engine a portion of this stored energy in the compressed gas would be recovered as the hydrogen was injected into the combustion chamber, but the single stage expansion into a diesel engine would recover only a small amount of the stored energy in the compressed gas. The reason that the recovery of the energy stored in the compressed hydrogen is so significant is that it takes a whole lot more energy to compress a gas than it does to compress a liquid. In other words it takes many times more energy to compress one pound of hydrogen to 10,000psi than it does to pressurize one pound of liquid diesel fuel to 10,000psi. The energy required to pump the fuel in a diesel engine usually remains rather low even when very high pressures are used, but it is a whole different situation when it is the compression of a gas that is considered. The low energy density of compressed hydrogen is also of concern, but it is not beyond the realm of possibilities that the hydrogen could also be liquefied for storage and transport. The liquefaction of the hydrogen would still represent a huge input of energy, but a compressed air engine could still be used to recoup a portion of this energy input. How much of the energy required to compress or liquefy the hydrogen that could be recouped would depend both on the efficiency of the compressor and the efficiency of the compressed air engine. An unfortunate reality also would be that the faster the hydrogen was compressed the less efficient the compressor would be. Large slow turning compressors with a large number of stages would attain the highest efficiency, but this would be a larger investment in materials than smaller faster spinning compressors. The key to efficient liquefaction would be to use sufficiently large heat exchangers so that the energy released from the actual change of state could be fully dissipated. For powering a gasoline engine extremely cold fuel delivery would not necessarily be a problem so long as the engine was designed to take advantage of the intake air cooling that would be provided.
As with solar thermal electrical generation hydrogen storage tends to be appealing because it uses mechanical systems that wear out in well understood ways as opposed to batteries that degrade in a huge number of ways.
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