During photosynthesis, plants absorb radiant energy from the sun (kinetic) and convert and store it as chemical energy (potential).

What do plants have to do with energy? Believe it or not, in one way or another they’re related to almost every energy-oriented topic in the news today!

Another good question is, "What is energy?" As defined by the National Energy Education Development Project (NEED), energy is "the ability to produce change or do work." In concrete terms, "energy produces light, heat, motion, sound, and growth, and powers technology." Scientists divide energy into two categories, potential and kinetic (see diagram at right). Potential energy is stored energy, and includes chemical, nuclear, stored mechanical, and gravitational energy. Kinetic energy is produced by motion, including that of waves, electrons, atoms, molecules, and substances. Kinetic energy includes radiant, thermal, motion, sound, and electrical energy. Check out the NEED curriculum's Introduction to Energy for more extensive teacher background information.

The energy sources in the table below are either renewable or nonrenewable. Renewable resources can be replenished in a short amount of time. Biomass is an example because new plant material can be grown and harvested within one growing season. Nonrenewable resources cannot be replenished in a short period of time. Though fossil fuels started out as plant and animal matter, it took millions of years to transform it into usable fuel.

The top three energy sources in this table are known as fossil fuels. They were created from plants and animals that lived millions of years ago. Huge amounts of decaying organic matter was transformed into concentrated substances under conditions of extreme heat and pressure within the earth’s crust. When we burn these substances, we’re taking advantage of energy that ancient plants captured from the sun.

As you look over the list, you’ll see that a majority of the energy we currently use comes from nonrenewable sources (petroleum, natural gas, coal, uranium, propane) instead of renewable sources (biomass, hydropower, geothermal, wind, solar). As human populations grow and fuel supplies wane, it becomes ever more apparent that this rate of consumption isn’t sustainable. We must work to reduce our energy consumption and shift to renewable sources. Changes are happening slowly, but some progress is being made. For instance, just a fraction of our overall energy consumption now comes from wind power, but the U.S. Department of Energy forecasts that 20 percent of our electricity will be wind-generated by 2030.

Plant-derived Biomass: A Renewable Energy Source

Although statistically biomass makes up a small portion of our current energy consumption, it’s actually the oldest source of energy after the sun. Humans have been burning plant material for heat and cooking for thousands of years, so our species would not have evolved without it.

Though many people still depend on biomass (in the forms of wood and dried peat) as their main source of home heating and cooking fuel, biomass is also at center stage as a source for fueling transportation. Biomass-based fuel sources -- often referred to as biofuel – are renewable and potentially more environmentally friendly because they produce less carbon monoxide.

Click for larger image Source: U.S. Department of Energy

The most common biofuel in production today is ethanol, made by fermenting plant sugars into alcohol. Although it's often presented as innovative technology, ethanol has been used as an energy source for centuries. In fact, in the 1850's approximately 90 million gallons were produced each year! When the government increased alcohol taxes during the Civil War, ethanol production dropped. As prices rose, kerosene (a petroleum product) became the cheaper fuel choice. The liquor tax was repealed in the 1900's, and ethanol's popularity rebounded until its production was limited by Prohibition just decades later. When Prohibition ended in 1933, supplies increased again, but ethanol was eventually replaced by cheap foreign oil. As we move towards a market where foreign oil is no longer affordable and abundant, the interest in ethanol is returning -- proof that history repeats itself!

Corn is the most common source of ethanol, but using corn for fuel is controversial. First, the crop requires a fair amount inputs, including water, fertilizer, and pesticides, and contributes little to overall environmental health. Second, using it for fuel reduces the amount of food available for humans and livestock worldwide. Overall, the amount of energy it takes to produce, process, and distribute corn-based ethanol makes it an inefficient energy source. But scientists are developing techniques to make ethanol from plants such as perennial grasses and fast growing trees. These plants provide environmental benefits such as preventing soil erosion and using fewer resources. Check out our lesson on Plant-based Biofuels to learn more about efforts to make fuel from switchgrass.

Learn about the United States Department of Energy’s biomass energy program

A green roof in New York City Photo by Alyson Hurt Creative Commons Licence

Although exploring renewable energy sources such as biofuel will decrease dependence on nonrenewable sources, ultimately the only way to balance the Earth's resources with our growing population is to emphasize conservation. Plants have roles to play in this realm as well:

• Shade trees around buildings reduce the energy needed for air conditioners. Air temperatures directly under trees can be as much as 25°F (14°C) cooler than surrounding open areas.
• Trees planted as windbreaks can slow winter winds around buildings and reduce energy needed to heat them.
• Green roofs (roofs partially or completely covered with a planting medium and vegetation over a waterproof membrane) provide insulation, thus reducing energy needed to heat and cool buildings.
• Fruits and vegetables grown and consumed locally reduce the amount of fuel used for transportation.
• Lawns are resource intensive. One hour of mowing produces the same emissions as driving a car 20 miles. It’s estimated that mowers use 580 million gallons of gas a year! Thus, a yard landscaped with native perennials, shrubs, and trees can reduce fuel usage.

• Install solar-powered accent lights, or use solar powered pumps to circulate water in ponds or water features.
• Use hand tools rather than gas-powered tools whenever possible.
• Decrease fuel usage by using local materials in the garden. Compost waste to reduce the amount of energy used to haul it away.
• Install a sun clock to inspire discussions about the sun's path and relate it to the impact of sunlight on your garden and surrounding buildings.
• Install a weather station that measures wind direction and speed, wind chimes, and/or a wind catcher to begin discussions about wind energy.
• Plant ferns. They were among the first plants to grow on Earth (350 million years ago), and decomposed ferns are a major component of coal deposits, an important energy source.
• Plant a common biofuel feedstock, such as corn or switchgrass, to provide a point of reference for a discussion of energy/resource inputs versus energy/resource output when evaluating biomass as an energy source.
• Recycle plastic. Plastic is made from petroleum, so recycling and reusing all forms of plastic conserves energy. For instance, reuse plastic planting pots and flats, use plastic cutlery as plant labels, and make plastic milk jugs into watering cans.