TECHNIQUE
du SOLAIRE THERMIQUE
Actualité internationale du moteur Stirling
2005
TECHNIQUE
du SOLAIRE THERMIQUE
| Le Golden State veut produire 20% de
son électricité à partir d'énergies renouvelables
en 2010 en s'appuyant sur la technologie du moteur Stirling.
Début novembre, la California Public Utilities Commission, qui régule l'approvisionnement en énergie en Californie, approuvait le plus important contrat solaire conclu aux Etats-Unis. Grâce à lui, la Southern California Edison (SCE) va pouvoir fournir de l'électricité à une partie de ses 13 millions de clients en utilisant la technologie originale de Stirling Energy Systems. «Ce sera le plus grand projet solaire du monde», promet Gil Alexander, porte-parole d'une entreprise qui se considère comme un «leader industriel en matière de production d'énergie renouvelable». Grâce à son système de concentration d'énergie solaire, développé à l'origine par McDonnell Douglas, dont la start-up a patiemment amélioré la technologie en s'appuyant sur l'expertise de laboratoires, dont le Sandia Labs, en Californie et contrairement à la plupart des systèmes concurrents qui s'appuient sur des technologies photovoltaïques (conversion directe de l'énergie solaire en électricité grâce à des cellules semi-conductrices), celui de Stirling Energy repose sur une vieille connaissance des motoristes: le moteur à combustion externe fonctionnant sur le cycle de Rankine. L'installation comprend des antennes recouvertes de miroirs incurvés qui captent le rayonnement thermique du soleil. Ces paraboles sont programmées pour faire face en permanence à la plus grande exposition possible. Leurs miroirs concentrent et transmettent l'énergie à un collecteur qui chauffe un réservoir d'hydrogène liquide. Le gaz chaud agit sur les pistons d'un moteur installé au coeur du système. Ce dernier entraîne à son tour un alternateur produisant de l'électricité sans consommation de carburant fossile ni rejet polluant dans l'atmosphère. Cette technique a bénéficié de l'incorporation des technologies les plus récentes ces dernières années via une collaboration entre Stirling Energy et le Sandia National Labs, qui dépend du ministère américain de l'Energie. Une volonté politique
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Portant sur une durée de vingt ans, le contrat
entre SCE et Stirling Energy prévoit même un quasi-doublement
de cette puissance lors de la prochaine décennie pour alimenter
en énergie les villes californiennes alentour. «Ce sera
le plus important projet de production d'électricité d'origine
solaire des Etats-Unis, plus important que toutes les installations actuelles
combinées», assure Gil Alexander.
Ce projet s'inscrit dans une volonté politique exprimée par le gouverneur Arnold Schwazenegger en matière d'énergie propre, depuis qu'il est arrivé au pouvoir. Déjà très engagé dans un projet pour attirer les premières voitures à hydrogène, il veut que 20 % de l'électricité domestique produite dans le Golden State en 2010 le soit à partir d'énergie renouvelable. Et ce pourcentage devrait passer à un tiers au milieu de la décennie suivante. Un chemin en faveur du développement durable que la classe politique américaine souhaite aussi favoriser au niveau national. L'été dernier, le Congrès a voté la loi Energy Policy Act qui accorde de nombreux avantages fiscaux destinés aux foyers qui s'équipent de systèmes de production d'électricité d'origine solaire. Des aides importantes ont aussi été votées pour des projets industriels de grande ampleur permettant d'améliorer les technologies existantes. Améliorer les coûts
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| For years now, electricity shortages
have encouraged power companies to look for alternative sources of energy.
And state governments are getting onboard as well. So far, 20 states, including
Colorado, Massachusetts, New Jersey, and New Mexico, have established renewable
energy production standards.
Add in the current sky-rocketing oil prices, and energy providers will be pushed even more to develop alternative energy sources. Nowhere is this trend more apparent than in California, where rolling blackouts still affect a power-hungry population. It's not surprisingly, then, that California may host the largest solar-energy project in history. Southern California Edison (SCE), with 13 million customers, has just announced a deal with Phoenix-based Stirling Energy Systems that could result in a huge solar farm. The California utility is already the nation's largest purchaser of renewable energy, providing its customer with more than 2,500 megawatts of wind, geothermal, solar, biomass, and small hydroelectric-derived energy, or around 18 percent of its total power load. Now SCE has agreed to purchase upwards of 500 MW of electricity from Stirling Energy Systems -- enough to provide all the energy needs to 278,000 homes -- or more than all other U.S. solar projects combined. While neither company has disclosed the financial details, SCE said the system will not require state subsidies. The effort will begin with a pilot project: a proof-of-concept facility with 40 solar dishes producing one megawatt of energy. The test will take place over the next 18 months, and, if successful, Stirling Energy Systems will construct a 20,000-dish array over four years, covering 4,500 acres -- more than four times the size of the National Mall in DC -- in the desert northwest of Los Angeles. "From our perspective, Stirling has established the viability of this at a laboratory level," says SCE spokesperson Gil Alexander. "This could be a turnaround point for solar." Stirling's dish technology, which was first developed by McDonnell-Douglas in the mid-1980s, makes use of a heat-driven engine, rather than photovoltaic panels. The company's deal with SCE marks its first utility-scaled energy application. In the Stirling solar system, each dish is a round, mirrored surface measuring 37 feet in diameter that reflects and focuses light into the receiving end of a Stirling engine. The engine itself, which was actually invented in 1816 by a Scotsman, Robert Stirling, is driven by the heating and cooling of a closed gas (see Notebook). To date, Stirling engines -- with their minimal emissions, long life spans, and quiet operation -- have produced refrigeration and even powered submarines. In the solar version, the dishes concentrate heat, which can rise to more than 720 degrees Celsius, causing hydrogen gas to expand, which in turn drives pistons and an electricity generator. Stirling Energy Systems has been operating a six-dish system since January at the Sandia National Laboratories test facility in New Mexico. There, the company converted its centuries-old technology into an efficient means of energy generation by using modern materials and programming that tracks solar progression and accounts for cloudiness and winds. The six dishes generate enough power for six homes, with their peak energy flow coming at the hottest parts of the day -- when utility needs are greatest. "Our systems have peak efficiency of 29.4 percent -- that's the record for converting solar to grid-quality energy," says Stirling CEO Bruce Osborn. A neutral observer has also given the Stirling solar design a good review. "This is a very high efficiency system," says Frank Wilkins, solar thermal team leader in the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy. "It's modular and has low water consumption, which is critical in desert areas. Of all the solar energies of the moment, this is at the top...You have to figure, this exercise is going to get [Stirling Energy Systems] more competitive in the energy market," Wilkins says. |
Despite his optimism, though, Wilkins also wonders
how easily the system would translate into a utility-sized operation.
"Even 40-dish systems haven't been built before, so there's a lot that hasn't been scaled to large systems," says Wilkins. In particular, he points to unknown operation and maintenance demands, as well as cost limitations. "[Cost] been a sticking point with the other thermal technologies I work on that use solar heat to produce electricity [like heliostats]," Wilkins says. "The cost has to come down, whether through research breakthroughs or industries deploying the system." Producing enough energy to offset the cost is what Stirling Energy Systems hopes to accomplish with its SCE deal, since high-volume fabrication should drop costs. The Department of Energy has stated that the prototype dishes at Sandia cost $150,000 to build; Stirling has estimated that large-scale production could bring down the cost to under $50,000 per dish. Although it will provide environmentally friendly energy, the Stirling project will still make a mark on the Mojave landscape, covering as much as 4,500 acres when completed. Daniel Patterson, a desert ecologist with the Center for Biological Diversity, wants to be sure concerns for nativen wildlife are also addressed as planning proceeds. Parts of the desert are home to endangered animals like the desert tortoise, whose habitat has become encroached upon by mining, development and livestock grazing. "We want to be very supportive of curbing fossil fuels," Patterson says, "but citing the actual location of the projects is important." Stirling CEO Bruce Osborn says that their concerns will be addressed. "We're looking at a combination of Bureau of Land Management and private land, and we will certainly have to go through environmental studies to be sure it's good with the flora and fauna," Osborn says. Osborn also reiterates that the system should bear less impact on the environment than other existing energy production methods: "the Stirling system takes less land than other solar systems, and requires minimal land grading. Plus, there are no toxic chemicals, and we use minimal water -- only a little to wash the mirrors every month...From our standpoint, we're very enviro-friendly." In April at a DOE workshop (see links in Notebook), top solar scientists from academia and industry assessed the state of solar research. According to their findings, while solar power is improving, significant technological breakthroughs are still needed before it makes a dent in carbon-based fuel consumption. "We need to double worldwide energy production by 2050," said Dr. George Crabtree, the Director of Materials Science at the Argonne National Laboratory in Illinois, who co-chaired the meeting, adding that in the long run, though, our society will "need something other than fossil." To Crabtree, Solar is the most promising energy source because of its sheer volume: the sun provides more energy to the Earth in one hour than all the energy consumed by the planet in a year. Nevertheless, solar remains largely untapped, making up around one millionth of the world's total electrical supply, according to the report. "If you want to have solar 50 years from now, you have to invest in doing it dramatically better," says Crabtree, "because the learning curve [for scientists and engineers] is steep." Crabtree hesitates to put a figure on how much he thinks federal funding should increase, but asserts that current levels are not nearly enough. Current estimates put federal funding for solar research at approximately $10 million, while industry experts estimate the need for at least $30 million annually to support research. Although Crabtree doesn't see Stirling Energy Systems' dish technology as necessarily a technological revolution, he does think it's encouraging to see industry players adopting solar energy. "Making it work, putting it out in the field -- you might call that a cultural advance," says Crabtree. |
