As oil and natural gas reserves are being depleted, the world’s attention is increasingly turning to plant-based energy sources. These include food crops, forest industry byproducts, sugar industry byproducts, plantations of fast-growing trees, crop residues, and urban tree and yard wastes — all of which can be used for electrical generation, heating, or the production of automotive fuels.

The potential use of plant-based sources of energy is limited because even corn — the most efficient of the grain crops — can convert just 0.5 percent of solar energy into a usable form. In contrast, solar PV or solar thermal power plants convert roughly 15 percent of sunlight into a usable form, namely electricity. In a land-scarce world, energy crops cannot compete with solar electricity, much less with the far more land-efficient wind power.

In the forest products industry, including both sawmills and paper mills, waste has long been used to generate electricity. U.S. companies burn forest wastes both to produce process heat for their own use and to generate electricity for sale to local utilities. The 11,000 megawatts in U.S. plant-based electrical generation comes primarily from burning forest waste.

Wood waste is also widely used in urban areas for combined heat and power production, with the heat typically used in district heating systems. In Sweden, nearly half of all residential and commercial buildings are served with district heating systems. As recently as 1980, imported oil supplied over 90 percent of the heat for these systems, but by 2007 oil had been largely replaced by wood chips and urban waste.

In the United States, St. Paul, Minn. — a city of 275,000 people — began to develop district heating more than 20 years ago. It built a combined heat and power plant to use tree waste from the city’s parks, industrial wood waste, and wood from other sources. The plant, using 250,000 tons or more of waste wood per year, now supplies district heating to some 80 percent of the downtown area. This shift to wood waste largely replaced coal, thus simultaneously cutting carbon emissions by 76,000 tons per year and providing a sustainable source of heat and electricity.

The sugar industry recently has begun to burn cane waste to cogenerate heat and power. This received a big boost in Brazil, when companies with cane-based ethanol distilleries realized that burning bagasse, the fibrous material left after the sugar syrup is extracted, could simultaneously produce heat for their fermentation process and generate electricity that they could sell to the local utility. This system, now well established, is spreading to sugar mills in other countries.