There are numerous renewable sources of energy available to power home and industry. These include, wind power, hydro-electric, biomass, and solar to name a few. Understanding which ones are best suited comes down to a question of geography and economics. Home and business owners are often presented with colorful options on the wide variety of alternative energy sources available, but can find themselves faced with a large up-front cost and only a vague idea of what the savings and payback period would be. Now, I want to point out that the savings and payback period should not be the only criteria for making a decision. I firmly believe that environmental responsibility should be a high priority, but am also sympathetic with anyone who refuses to pay tens of thousands of dollars to install a system in order to completely live off the grid. There should be serious effort on the part of industry and government to invest in the advancement of the technology, and to drive down the cost, in order to make it easier for homeowners and businesses to make a shift in their energy use.
Shifting global energy use towards cleaner sources is very possible, but it will require a utilization of all the renewable sources available worldwide. This will mean that the areas of the world that are rich in geothermal energy, would contribute to the energy "pot" along with those areas that are rich in say, wind energy or solar energy. There are few places in the world that will have all their energy needs satisfied by a single local and renewable source. This will mean that "sharing" of renewable energy must occur across all borders. It will take a keen understanding of what best applies to different areas and then proceeding forward. For the most part this must happen on the industry scale, where the economics of scale will make it most feasible. Individual home and business owners can certainly do their part as well, either in using less energy and/or reducing dependence on the grid by installing some solar panels, or maybe a wind turbine, which they can afford. Certainly those people with larger properties, such as farmers, can make the most of this. But most don't have this opportunity. So for the most part, renewable energy must come from large scale applications designed to power hundreds, if not thousands of homes, per installation.
Let's look at the key areas of renewable energy potential.
Solar Energy
Photovoltaics
Photovoltaics (solar panels) can generate electricity year round but their output can vary depending on if it is sunny or cloudy. Their output efficiency is 10-15%, meaning that 10-15% of the solar energy that reaches the solar panel is converted into electricity, which is somewhat low. Furthermore, their cost is high, around $10/Watt if you're looking at a home installation. The cost per Watt tends to be lower for large utility-scale installations.
Solar panels utilize the "photoelectric effect" to produce electricity. The sun's photons excites the electrons in the photosensitive panel material to produce electricity. The technological hurdle in constructing solar panels is that the photosensitive material made out of high-purity silicon, has to be "grown", which is a slow process. Because of this the economics of scale don't benefit large-scale production of solar panels as much as say, wind turbines which can be produced at a faster rate. Nonetheless, in certain applications solar panels are valuable since they are a consistent source of power as long as there is regular daylight intervals, which is true almost everywhere. If budget is not an issue and you want steady reliable power from one single source that is almost entirely independent of geographic location, then solar panels are for you.
A further advantage of solar panels is that they require little maintenance once they are installed. There are no moving parts. All that is required is to keep the surface clean so that sunlight can get through.
Solar Thermal Energy
This source of power is most suitable in areas where there is little cloud cover year round. It relies on the unimpeded intensity of the sun, just like a magnifying glass will only focus sunlight when there are no clouds blocking the sun. The energy efficiency achieved by solar thermal can be twice that of photovoltaics.
There are two main methods to capture the sun's thermal energy. The first is with flat-plate solar collectors. These are collectors with a clear glass plate on top to help "trap" the sun's heat, much like a "greenhouse" effect. This causes the interior of the collector to heat up. On the interior of the collector there is tubing which is painted black in order to better absorb the sun's heat energy. This, combined with the "greenhouse" effect, will result in the tube temperature reaching 80-90 degrees Celsius.
Inside the tubes there flows a coolant that heats up and then flows out of the collector and exchanges heat with another device, such as a water heater. This helps reduce water heating costs. This heat can also be used for space heating, reducing costs there as well.
Flat-plate solar collectors can also be "evacuated". A vacuum is created between the collector tubes and an outer glass tube. This allows the collector temperature to become even hotter, over 100 degrees Celsius. The reason it gets hotter is because, in the absence of air, there is no heat loss due to convection, which is basically the transfer of heat to the glass tube, with air as the transfer path. With this heat loss medium removed, the collector heats up more.
Solar collectors are among the least expensive of all solar installations, costing only a few thousand dollars for a typical home installation.
The other way to collect the sun's thermal energy is by focusing the sun's energy on to a small area, much like a magnifying glass, resulting in localized very high heat. This high heat can be utilized for electricity generation.
One application for concentrated solar heat is in "power towers". Using many heliostats (mirrors), the sun is focused onto a central tower using tracking mechanisms. The power tower receives this concentrated energy to heat up water and run a steam turbine to generate electricity. Heat from the power tower is initially stored in a salt compound which becomes liquid at high temperature, and its heat is then used to turn water to steam. Because of the ability of salt to act as an efficient thermal storage at high temperatures and low pressure, it can be easily transported through piping and into a storage tank for subsequent use in the steam generator. It can store sufficient heat energy to provide electricity generation well into the night, well after the sun goes down.
The efficiency of a power tower can be around 20%, which means that 20% of the sun's energy falling on the mirrors can be converted into electricity. Efficiency as high as 25% can be achieved with larger installations. Efficiency is somewhat proportional to the size of the installation.
Another means to utilize the sun's thermal energy is with a solar concentrator, such as a SunCatcher system. A Stirling engine is placed near the focal point of a large parabolic dish, around 11 m in diameter. The dish is pointed directly at the sun by using a tracking mechanism. This allows the sun's energy to be focused onto a receiver placed at the focal point. This receiver channels the heat energy into the Stirling engine which then generates electricity.
The Stirling engine is unique among engines in that all it needs to run is a hot and cold source. In this case the hot source is the sun's heat and the cold source is the ambient air. The concentrated heat energy of the sun hitting the receiver can be as much as 800 degrees Celsius! The higher the temperature, the more efficient a Stirling engine will be, which is why a parabolic dish is much more effective than a solar collector as a source of heat. It generates more power per unit area of sun exposure. In the SunCatcher, the efficiency is around 30%, which means that 30% of the energy that reaches the parabolic dish is converted into electricity. This is the highest efficiency of any device that converts solar energy to electricity. It is comparable to coal generating stations which can operate with an efficiency of 35-40%. Each SunCatcher can generate an average of 25 kW of power. That is enough power for a dozen homes.
The SunCatcher is ideal for scalability in that it is modular, meaning that to get more power you simply add more units. This type of scalability is not as easy to implement for a power tower.
Areas such as the southwestern United States are ideal for solar thermal energy. Any areas that have ample sun and little cloud cover year round, are suitable. The outdoor temperature does not have to be hot year round. As long as the sun is shining the receiver can reach a high temperature. In fact, the efficiency record for a SunCatcher was set on a cold and sunny winter day. This is because the cold "source" temperature, which is the ambient air, is colder than in summer months. And since the efficiency of a Stirling engine increases with increasing temperature difference between the hot and cold source, it resulted in a higher than normal power output.
The disadvantage of a SunCatcher power plant is that it can only output power during the day. There is no overnight storage capacity as is the case with a power tower. However, it is still valuable as it can help reduce grid load by offsetting some of the peak power use which is generally during the day.
Currently, these SunCatcher units are very expensive, around $250,000 per unit. However, this is the cost of making prototypes. Economics of scale can greatly reduce the cost, since these are essentially mechanical devices which can be manufactured at high volume, the same way as automobiles. And indeed, they are not as technologically complex as many of the cars produced today.
Wind Turbines
Wind turbines have little environmental impact and in high volume can greatly contribute to grid quality electricity. For large installations (on the order of MW), the rated power output is 1 Watt per 1-3 dollars of installation cost. Compare this to large installations of photovoltaics that have a rated power output of 1 Watt per 4-7 dollars of installation cost. Wind is (currently) a better return.
Wind turbines are one of the largest sources of renewable energy production in the world. In Germany they supply nearly ten times as much energy as photovoltaics. The same can be said of the European union, which leads the way in renewable energy development.
As said before, it's easier to produce wind turbines in large volume than photovoltaics, since they are basically mechanical devices. This no doubt contributes to their much larger numbers.
Wind turbines work best on a large scale in large open areas, where one can install towers tall enough to capture the full brunt of the wind energy available. Coastal areas and areas near large bodies of water are often best suited for wind power.
Wind turbines can be very large, up to 200 meters tall, with 125 meter blade diameter, providing up to 6 MW capacity. One such turbine can power thousands of homes.
To have the best chance of producing steady output power, it is necessary to install wind turbines in numerous locations, spread out across large regions. This way, even if one area has insufficient wind speed, statistically speaking there is enough wind blowing in different areas such that the wind turbines will produce steady output on average. Wind speed varies much more with respect to local regions than does other sources of energy such as solar, so multiple installations covering a large area is essential.
Geothermal Power
This is heat energy that is captured from underground, which is escaping from the earth's core and venting close enough to the earth's surface to be accessible. Certain areas of the world are rich in geothermal energy, such as Iceland. This energy can be used to heat water and create steam to power turbines that generate electricity. It can also be utilized to heat homes.
The technology is progressing so that areas that were previously considered "too deep" to access geothermal energy can now be drilled and tapped. This will contribute to the number of areas that can be harvested for geothermal power.
Wave and Tidal Power
Wave power is generated by using the "up and down" motion of waves to drive a generator using an actuator such as a hydraulic ram.
Tidal power is generated by placing turbines underwater that rotate due to the bulk motion of the water. This is similar to how wind turbines rotate in the presence of wind.
Biomass
This entails the burning of plant matter for energy. It is carbon-neutral because during the life of a plant, it captures carbon dioxide from the air, using photosynthesis. This carbon is stored in the plant tissue. When the plant is burned it releases its stored energy as heat and the carbon dioxide returns to the atmosphere. So there is no net gain or loss of atmospheric carbon dioxide. Therefore it is carbon-neutral. Sources of biomass can be wood or plant husks. Ethanol derived from sugarcane is another example.
Hydro Electric Power
Many are familiar with this mode of power generation. It is the kind of power that uses the gravitational potential of falling water to rotate large turbines. Niagara Falls is an example, along with the Three Gorges Dam in China, which will produce 22 GW of power when completed. That is enough power to supply almost the entire province of Ontario.
Hydro-power is an excellent source of power when used from natural sources. It can become an environmental problem when large stretches of land are deliberately flooded to achieve it. Notwithstanding, it remains one of the largest sources of renewable energy.
Looking at all these options, it becomes clear that the solution is not a single one, but all of them put together. Different areas of the planet would lend themselves to different renewables. Some areas would be best suited to solar energy. Others may be better suited to wind power. While others may be best suited for geothermal or even tidal power. Combinations are certainly possible if the natural conditions favor it.
It's also clear that mechanical means of extracting energy from nature will form the bulk of clean, non-polluting, energy sources. It is a fact that the motion of wind and water lends itself more easily to power generation in more areas of the world than does solar. And it's fairly easy to scale up these sources. You just "add" more units.
The job of government and industry is to push for large scale installations where they are feasible, as they will supply the bulk of the electricity needed by society. Economics is not the only hurdle here. There has to be a willingness to invest in something which cannot have a price tag on it. Maintaining a habitable planet should be a priority no matter what.
The role of the home and business owner is to participate in the effort to use less energy, and conserve more. This could be by driving less, using public transit more, installing more energy efficient appliances even if it costs more, and being more cognizant of where energy use in general can be reduced. Doing this and making this part of a way of life need not be perceived as a sacrifice, but a responsible way of living harmoniously with our surroundings.
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