Timbo - things are looking up for solar. There are now triple-junction cells that are over 35% efficient, and bigger gains in the works. These cells work in conjunction with a concentrator, so maybe not a good solution for the average homeowner's roof, but a step up for big solar facilities. More info at - http://www.spectrolab.com/prd/terres/cell-main.htm There is indeed a shortage of silicon need to make solar cells, but some manufacturers have committed to increasing capacity. This is an indication of real demand.

Mary - just google "tidal power" and you'll find more than you ever wanted to know about it. There are tidal power plants in operation, and more under consideration. It's all about choosing the right locations.