What is renewable energy?

How is energy renewable?

Thank heavens for the sun!

The sun is a formidable star.  The amount of energy produced by this massive stellar performer is almost unimaginable. If the Earth were any closer? … Game over.

The sun radiates more energy than we could possible use– but that energy is spread across the surface of the planet. Capturing that solar energy directly is simple for devices that heat up water (like a solar water heater), or that focus the solar energy to produce  a higher temperature–like a solar coooker.

As the Earth spins around the sun, the regional differences in atmospheric temperatures produce pressure gradients and cause winds. The winds never stop.  In one place or another they are always blowing.

Since the beginning of recorded history, man has figured out how to use the wind to propel a simple boat. And it wasn’t long before the first wind driven mills were constructed on the Mediterranean islands.

The sun not only drives the wind; it drives the hydrological cycle–evaporating water from the land and the oceans, that is moved by the wind until it is eventually precipitated as rain. Without rain, nothing grows.

The sun’s radiation, the wind, and flowing water are the most important sources of renewable energy–because they can be directly harnessed to produce the one form of energy that is indispensable for modern economies–electricity.

But first there was fire.

Wood was the first fuel to be used by early man.  And it is still an important fuel.  As long as the sun shines and rain falls, trees will grow–and men and women will cut them down, and collect the wood for fuel. Over a billion people in the less developed countries have no access to electricity.  They have no other fuel except wood–or the charcoal which is made from wood.  So trees and other forms of biomass are also a source of renewable energy.

But there’s a catch.

Trees grow slowly, and if you cut them all down at once, you quickly run out of fuel. So biomass is only really renewable, meaning always available as a fuel, if it is managed so that the supply of wood is extracted and consumed at the same rate at which the trees grow.

There are other sources of energy that are also classified as renewable energy.  Geothermal energy draws heat from the high temperatures that exist underground in many regions. Iceland is renowned for the way that geothermal energy is used for generating electricity and for district heating.

New technologies that capture the energy of the tides and waves are also renewable energy technologies. OTEC is another. Ocean Thermal Energy Conversion uses the temperture difference between warm water on the surface of a  lake or coastal zone, and cooler water at depth to drive a thermodynamic engine that can produce electricity. But temperatures differences are small and the conversion is inefficient and therefore expensive.

Is renewable energy clean?

Solar energy, wind, and hydropower do not produce any air or water pollution.  Some people object to the noise of large wind turbines–and call it noise pollution. But most people don’t have a problem with wind turbines and understand the huge advantages of clean renewable electricity.

Renewable energy is generally referred to as clean energy because it is contrasted with the fossil fuels: coal, oil, and natural gas.  These sources of energy–which have powered the economies of countries since the industrial revolution– are far from clean.  They are the cause of widespread of air and water pollution, and one component of that air pollution–carbon dioxide–is the principal cause of global warming.   Compared to fossil fuels, renewable sources of energy are totally clean.

But once again there’s a catch with biomass.  Wood contains carbon–just like the fossil fuels. When it is burned, wood produces carbon dioxide–a greenhouse gas. It is also releases other toxic substances that are naturally in the wood, such as sulfur and mercury.

So biomass is not a clean fuel.  It may be renewable but it is not clean–although compared to coal and oil it is definitely a better alternative.

Biogas is a special case.  A combustable fuel gas generated from the anaerobic digestion of biomass waste, it is a mixture of methane and air.  So when burned it produces carbon dioxide.  But little else.  So it is a relatively clean fuel compared to the fossil fuels. But not as clean as the other sources of renewable energy.

For more on solar energy technologies and wind power look here.

For more on photovoltaic energy check out this page.