It has become very fashionable to talk about more environmentally-friendly energy sources as concerns about our overuse of fossil fuels and their carbon consequences have become central issues in the 21st century. The technical feasibility of producing much of our energy requirements going forward using renewable sources remains a challenge.
Today global renewable energy capacity totals 1,274 Gigawatts. How does this compare to all the energy we consume? If U.S. consumption is an indicator renewable is just a small fraction of the total. For example, wind, solar and other non-hydro renewable energy sources generated 126 TWh of electricity in the United States in 2008, double the 64 TWh of output in 1990. The percentage share from non-hydro renewable sources amounted to 3.1% up in 2008 from 2.1% in 1990. Hydroelectric production in contrast dropped from 8.9% to 6.2%.
Compare those numbers to coal, oil and natural gas. Coal-generated electricity in the U.S. dropped as a percentage from a 51.8% in 1990 to 48.2% in 2008. In total power produced, however, coal dramatically increased. Petroleum also dropped as a percentage from 3.6% to 1.1% with little increased capacity. But natural gas on the other hand increased dramatically both in the amount of power generated and as a percentage of total generating capacity, growing from 14.7% to 21.4% over the same period.
What are renewable energy sources? They include hydroelectric from water, wind turbines, solar arrays, tidal and wave power turbines, and geothermal. Some of these energy generation types are carbon neutral but other renewable sources have a significant carbon footprint. Most of these sources can be used to generate power continuously without depleting the energy source from which the electricity is derived. Hydroelectric, however, has some significant challenges. A brief discussion of each follows.
Humanity has been converting falling and flowing water into energy for centuries. In fact water represents the single largest contributor to global renewable energy sources on the planet today. We classify the conversion of water power to electricity under the name hydroelectricity or hydro for short. Today hydro projects generate almost one quarter of the world’s electricity usually through sites where there are natural waterfalls or through the damming of rivers to create artificial drops.
The world is running out of usable new sites of this type. This is compounded by environmental concerns. Gradients are critical in establishing enough force for water to turn turbines connected to generators, but dams are extremely disruptive to the wildlife in rivers, the population displaced by flooded lands, and for people living downstream from the sites. In addition damming of rivers changes how they deposit silt. Many current dams are dealing with significant build ups of silt in their reservoirs leading to a decline in reservoir capacity. These are just some of the reasons that the 21st century will see hydroelectric power projects not at sites like Three Gorges Dam on the Yangtze River in China, but in smaller locales with less environmental consequences.
Harvesting the latent energy of the atmosphere has been one of humanities great technical achievements. From windmills to sails we have been using the energy of wind to power machinery from the milling of grain to trans-oceanic expeditions on sailing ships. Farms in rural North America were often not connected to the electrical grid and used the wind as a power source for pumping well water, milling and other purposes. Today wind power is becoming a significant source of electrical power in many countries through the development of large wind farms featuring turbines that can be several hundred meters in height with propeller vanes as large as 60 meters.
The average individual wind turbine generates enough electricity to power 600 homes when the wind is blowing. Wind farms tend to have a cluster of turbines from as few as ten to hundreds. Once installed operational costs are practically non-existent. The real challenge is the reliability of wind as a power source. When it doesn’t blow no energy can be produced making wind less reliable than hydro and other renewable sources.
Wind turbines have been blames for the killing of migratory birds and bats. Many people fear having them installed near their homes because of medical and environmental concerns. Others don’t want a forest of these giant windmills dotting their shorelines or fields on purely aesthetic grounds. Yet despite consumer resistance wind power continues to grow globally with global capacity reaching 70,000 Megawatts in 2009. At the present rate of growth wind power may contribute as much as one-third of the world’s electrical generation capacity by the mid-21st century.
All the energy of the atmosphere comes from the warming effect of our sun including wind and the water cycle that gives us the ability to harness water. Even biomass derives its energy from the sun which is then converted into plant material. But when we talk about solar energy we are usually referring to the renewable energy generated through a number of technological inventions.
What makes solar energy so attractive is its abundance. The sun is always shining somewhere on the planet and when you look at the amount of energy that reaches the surface even on cloudy days the numbers dwarf our current energy generating capacity. In fact, the upper atmosphere of our planet receives the equivalent of 174 petawatts (PW) of incoming solar radiation of which 30% is reflected back into space with the rest getting absorbed by our atmosphere, oceans and land. To put this in perspective in terms of our average requirement versus the amount of solar energy available, let’s look at Australia as an example. That continent receives 15,000 times the amount of energy it currently generates from all other sources through solar.
Although passive solar has been the traditional way humans have used the sun’s energy historically, it is the advent of photovolatics (PV) where research and development is focused today. What we call solar cells are used to convert sunlight directly into electricity. The manufacture of solar cells and photovoltaic panels is a growing industry. The industry-wide challenge is to maximize the efficiency of these devices in their ability to convert the sun’s energy intro a reliable power source.
Ultimately the evolution of 21st century photo-exchange technologies should yield artificial photosynthesis and the almost unlimited energy that we will derive from solar as a result.
Tidal and Wave
We owe the moon a lot. The gravitational forces it exerts on the world’s oceans create an enormous amount of energy in the form of tides. In addition wind blowing over open ocean has enormous energy potential. What exactly is the total potential of tides and waves?
It is estimated at 2-3 million megawatts. The Bay of Fundy off the eastern coast of Canada, the western coast of the United States and Europe, the coast of Japan and coastal New Zealand are considered to be the best sites for harnessing energy derived from waves and tides.
At the beginning of the 21st century we are only beginning to experiment with the right kind of engineering designs to to harness wave energy effectively. Today there are only small commercial wave energy projects in place but the ability to harvest the latent energy potential should yield significant breakthroughs in the near future. The immediate benefit with small wave and tidal energy plants is the effective delivery of power to local and often isolated shoreline communities.
Geothermal energy is derived from two types of sources, one natural and the other helped along by human intervention. For example, natural geothermal can be found in New Zealand and Iceland, where below ground heat sinks represent an enormous energy resource. New Zealand generates approximately 10% of its electricity from geothermal sources. Iceland derives 23% of its electrical energy this way. Other countries with existing geothermal programs include Italy and Japan. California has an active geothermal program as well. And the rest of the world is sitting up and taking notice.
Geothermal resources can be classified as high, moderate and low temperature.
High temperature geothermal is usually located on the edge of continental plates (New Zealand and Iceland for example) where magma from the Earth’s mantle is close enough to the surface that the heat from the melted rock is reachable. Typical magma heat sources provide temperatures between 200- 350 degrees Celsius at accessible depths.
Moderate temperature geothermal is usually associated with fault lines. California, Italy and Japan have these types of deep faults. Geothermal sources of this type have temperatures at accessible depths of approximately 140 degrees Celsius. This type of resource is often found close to high temperature geothermal sources.
Far more common is low temperature geothermal. A typical low temperature geo-exchange system involves drilling a number of holes in the ground to depths between 90 and 110 meters where temperatures year-round are a consistent 13 degrees Celsius. Pipes inserted into the holes are used to pump water or other heat conducting fluids. The closed system is combined with a heat exchanger, providing air conditioning in the summer and heat in the winter. In Iceland 89% of homes use heat exchangers to provide warmth in the winter.
Geothermal is a clean energy resource and low temperature geo-exchange can be implemented widely with a reasonable cost recovery whether large scale or for single residential use. The 21st century should see a rapid expansion in use of this low emission energy resource.