The simplest way of getting energy from biomass is to burn it, but many believe that a more sensible strategy would be to convert it to gaseous and liquid fuels and chemicals needed by the chemical industry.
Hydroelectric power, another indirect form of solar energy, is renewable, creates no air pollution, and is relatively inexpensive. However, sediments fills in reservoirs, giving them an average lifespan of 50 to 100 years.
The earth harbors a great deal of energy from the decay of naturally occurring radioactive materials in the crust and from magma, molten rock. The most useful geothermal resource is the hydrothermal convection zone, where magma penetrates into the crust and heats rock formations containing large amounts of groundwater. The heat pressurizes the groundwater and drives it to the surface through fissures. Currently these zones are exploited for space heating and electricity. Such systems produce steam and hot water laden with toxic minerals, salts, metals, and hydrogen sulfide. Noise pollution is also a problem.
Hydrogen fuel is produced by heating or passing electricity through water in the presence of a catalyst. Water breaks down into hydrogen and oxygen. Hydrogen is a clean-burning fuel that could replace gaseous and liquid fuels. It is easy to transport but is explosive, has a low energy yield.
Conservation is one of the key untapped energy resources for tomorrow. By reducing energy waste in homes, factories, and transportation, we could inexpensively unleash an enormous supply of energy. The largest gain can be had when new homes and offices are built with passive and active solar systems and heavy insulation. Mass transit by bus and train have to replace much of the automobile traffic, both in cities and between them.
A smooth transition from nonrenewables such as oil, natural gas, coal, and nuclear power to renewable energy resources can be made into a sustainable future, but it will require an immediate investment in renewable energy resources by governments and individuals.