The potential for these alternative-energy sources is vast, but they face very different challenges. June 1, 2009 SOLARSize of industry:Worldwide: $29.6 billionU.S.: $3 billion, accounting for 0.4% of the nation's electricityMajor players: First Solar, SunPower, China Sunergy (China), Suntech Power (China)Potential: Could provide 10% of the nation's electricity by 2025. Worldwide sales could grow to $81 billion by 2018.Problems: Solar energy wonÕt be cost-competitive with other forms of energy until 2015Ñtoday it relies heavily on subsidies. Also, growth depends heavily on the development of nationwide power grids. WIND Size of industry:Worldwide: $51 billionU.S.: $17 billion, accounting for 1.3% of the nation's electricityMajor players: General Electric, Vestas (Denmark), Gamesa (Spain), Siemens (Germany), Mitsubishi Heavy Industries (Japan)Potential: The industry seeks to produce 20% of U.S. electricity by 2030. Sales could hit $35 billion a year in a few years. Worldwide: $139 billion by 2018. Problems: Getting the electricity to major cities from remote wind farms will require billions for expanding the electrical grid. GEOTHERMAL Size of industry:U.S.: $3 billion, accounting for 0.4% of the nation's electricityMajor players: Ormat Technologies, Calpine, Siemens (Germany), Sierra Geothermal Power (Canada)Potential: The industry says it will double capacity in the U.S. in the next few years, perhaps aided by $440 million set aside for geothermal projects in the Obama administration's stimulus plan.Problems: Discovering heat sources is expensive, and industry infrastructure is still in development. Advertisement SOLID WASTE Size of industry:U.S.: 0.3% of power generationMajor players: VeoliaEnvironnement (France), Waste Management, CovantaPotential: The industry, with 89 facilities in the U.S., is mature. Growth will be slow.Problems: Businesses -- which burn trash and generate methane gas from landfills -- must contend with "not in my backyard" resistance to new plants. BIOFUELS Size of industry:Worldwide: $40 billionU.S.: $14 billion Major players: Poet,* Archer Daniels Midland, Cargill,* Valero Energy, Cosan (Brazil)Potential: Some analysts estimate that the industry will grow to more than $80 billion worldwide by 2017. The industry is on the verge of breakthroughs that will allow the use of many other biofuel sources beyond corn and sugar.Problems: Critics say biofuels put too much demand on food sources and do little or nothing to reduce emissions of greenhouse gases. Note: Figures are for 2008. *Privately held. Sources: American Wind Energy Association, Clean Edge, Kiplinger's Biofuels Market Alert, Geothermal Energy Association, Integrated Waste Services Association.