Overview of renewable energy sources
Renewable energies are characterised by the fact that they are, well, renewable. I.e., we can use as much as we want of them without ever depleting them. This is a major difference to fossil fuels, such as crude oil and natural gas, which exist in large quantities in the earth but will eventually be depleted. Estimates for the point at which we reach total depletion vary from around 40 to up to 100 years.
There are basically four main types of renewable energy: solar, wind, water and biomass. Very recently, geothermal energy has joined the group, but it is not yet widely employed on a large scale (although it is an interesting option for home owners).
Out of these, biomass occupies by far the leading position with an 80% share on a global level according to the International Energy Agency. This is mostly due to the burning of wood (and to a minor extent dried animal manure, e.g. cow pads) in 3rd world countries, where it often serves as the main source of heat and energy. Hence, we have to distinguish between this "traditional biomass", and "new biomass", which is treated in this section.
Apart from being inexhaustible, renewable energies have another major advantage: they produce no emissions of any kind, as opposed to fossil fuels when burned. The only exception is biomass, which however produces only the amount of (CO2) emissions it took out of the atmosphere while growing.
Solar Energy
| Solar energy comes in two "flavours": Photovoltaic (PV) and solar thermal. In the case of photovoltaic, sun rays are converted into electric energy in solar cells. Solar thermal modules on the other hand contain a fluid (e.g. water) that is heated up thanks to the impact of the radiation on the module. |  |
Photovoltaic solar energy
PV modules are based on silicon, a semiconductor which under certain conditions enables the flow of an electric current. Two main types of solar cells exist today: those based on silicon crystals and so-called amorphic or thin-film based cells. The first type yields higher efficiency levels of around 15% in practice, but is also more expensive to produce due to the high amount of silicon required. Most rigid solar cells in use today are based on this technology. If you think that 15% efficiency is low, then you should keep in mind that traditional coal fired plants have an efficiency of only 30-35% (the remainder being transformed into heat and wasted). Thus, while a solar energy plant may be half as efficient as a coal plant, the source it derives its energy from is readily available and virtually infinite, emissions are zero and operating costs are low.
Thin-film technology, on the other hand, is cheaper to produce since only a thin film of silicon is applied to an inexpensive base material. The efficiency level is below 10%, but a major advantage of thin-film technology is - as the name indicates - that it is quite flexible in its physical properties with respect to rigid crystal based modules. It can be easily applied to curved building exteriors, for instance. In addition, this kind of technology seems to work better when the sun exposure is not direct or not very intense, as often occurs in the cloudy conditions in Northern Europe.
The efficiency of solar cells is continuously being increased. Most recent technologies exceed 40% efficiency in the laboratory. At the same time, increase in demand and production volumes creates economies of scale, bringing prices down. For instance, between 1990 and 2004, prices of PV modules have decreased by around 60% (from Euro 15,000 to Euro 6,000 per kWp, i.e. per kilo Watt of energy produced under ideal = peak conditions).
Nevertheless, at the current stage PV is not yet competitive with fossil fuels. Today, one kWh of energy produced with PV costs around Euro 40 - 50 cts (depending on the cost of acquisition of the modules, as well as the level of sun exposure and the lifetime of the modules), which is around 3-5 times the cost of a kWh produced from fossil energy. For this reason, solar energy requires incentives to become viable.
Such incentives are being offered in several countries, e.g., Germany, Japan, Italy, Spain and to some extent the US. For instance in Germany, home owners purchase a PV system at their own expense (the total cost of which can reach up to Euros 25 K for a 4-person family house). The system is then connected to the energy grid, and all the energy produced by the PV system being passed on to the grid. The grid operator is obliged by a Government legislation to pay the house owner Euro 45 cent for every kWh produced by the PV system. Considering that a solar panel has a lifetime of about 20-25 years, the initial investment is amortized after around 10-15 years of operation, making for a financially interesting opportunity for home owners. For this reason, Germany is today the major market for solar modules after Japan. Italy has recently adopted a similar legislation.
Thermal solar energy
Thermal solar energy is usually employed for heating water consumed in households. It is best suited to warm geographies, with high levels of sunshine. In those contexts, it can substitute most or all of the water heating requirements, thus allowing to save significant amounts of e.g. natural gas. Another advantage is the relative inexpensiveness of these systems, since they basically consist of tubes and dark paint. Several countries, Spain being a leading example, are incentivizing or even obliging developers of new buildings to include solar thermal systems in their structures.
Wind Energy
| Wind has been used to drive mills for hundreds of years. In the 60s, the first windmills were conceived that turned wind power into electric energy by means of a generator. Like solar, wind energy has experienced strong growth over the last decade or so across the world, fueled by environmental and economic considerations. |  |
Over this period scientific research as well as economies of scale have made energy produced by windmills fully competitive to that gained from fossil fuels. In countries with strong winds like Denmark and in Northern Germany, windmills are an almost ubiquitous sign. This has caused complaints from individuals and protection organisations, who don't want to see the landscape "cluttered" with windmills.
Discussions about the beauty or lack of if of windmills are of rather personal nature. In any case, in recent years new wind farms have increasingly been built offshore, e.g. several miles off the coast of Germany and the UK. The advantages such as stronger and more consistent winds and less controversy offset the higher construction and operating costs. A modern windmill can reach an output of up to 5 MW. A wind farm consisting of, say, 50 turbines, has thus a combined power generation capacity of 250 MW. This is a very sizeable output (for comparison, coal fired plants have power outputs of 1 GW, i.e. 1000 MW, and above), and explains the attractiveness of this type of renewable energy, to environmentalists and investors alike.
Most recently, entrepreneurs have been looking into ways to enable the generation of wind energy in our backyards. In the UK, for instance, micro wind turbines are available at Do-it-Yourself stores for about £ 2-3K. The manufacturer claims that the acquisition cost can be offset within 4-5 years through savings in energy consumption.
Water Power
| Water power is yet another type of renewable energy that has been used by humans for many centuries. Today, many countries with sufficiently large rivers have some form of hydro power plant installed along them. In countries like Brazil, power generated from water accounts for more than 50% of total energy production. In most cases, energy produced from water power is fully cost competitive with fossil fuels. |  |
Another form of water power is tidal power. The tides of the sea can provide virtually endless power, but harnessing energy out of the sea movements is not as straight forward as putting a turbine into a river stream. In addition, the potential impact on the immediate environment and ecosystems is considerable. For this reason, only a limited number of tidal power plants are in operation today (the first one was built in France in the 60s, and has a capacity of 240 MW).
Biomass
| As stated above, biomass is by far the most important source of renewable energy on a global level. That is, if the burning of wood for heat and energy production in 3rd world countries is taken into account. This however can have devastating effects for the forests, if they are not maintained in a sustainable way. |  |
In recent years, biomass has become an increasingly popular source of renewable energy also in industrialized countries. Here, it consists not of wood but mostly of harvested plants such as rapeseed, corn and sugar cane. These can be processed and turned either directly into fuel for vehicles (biodiesel) or into ethanol, which can then be used as vehicle fuel or to power gas turbines of power plants.
Scientists and entrepreneurs are currently working on ways to increase the fuel yield from plants. The objective is two-fold: Use the entire plant to produce fuel, i.e. in most cases ethanol (rather than only the seeds or fruits of the plant), and extend the process to organic material that is more readily available or exists in high quantity and is not used otherwise, such as special types of grass and wood chips, respectively.