Here’s a really neat energy project:

In this instructable, we describe how to build a photo-bioreactor that uses algae to convert carbon dioxide and sunlight into energy. The energy that is produced is in the form of algae biomass. The photo-bioreactor is built from plastic recycled water bottles. By using algae as a biofuel, we can increase the world’s supply of oil while at the same time we decrease the amount of atmospheric carbon dioxide used during its production. The resulting product is a sustainable biofuel whose carbon footprint is neutral inasmuch as the CO2 produced on consumption is essentially balanced by the CO2 used in its production. In this instructable, we first make the carbon dioxide delivery system, then mount the water bottles on a rack, and then inoculate the bottles with algae. After letting the algae grow for a week, we extract the biomass.