Policy makers often debate the constraints and opportunities of renewable energy.
Renewable electricity production, from sources such as wind power and solar power, is sometimes criticized for being variable or intermittent. The International Energy Agency has stated that its significance depends on a range of factors, such as the penetration of the renewables concerned.International Energy Agency (2007). Contribution of Renewables to Energy Security IEA Information Paper, p. 5.
There have been concerns relating to the visual and other impacts of some wind farms, with local residents sometimes fighting or blocking construction. In the US, the Massachusetts Cape Wind project was delayed for years partly because of such concerns. Residents in other areas have been more positive, and there are community wind farm developments. According to a town councillor, the overwhelming majority of locals believe the Ardrossan Wind Farm in Scotland has enhanced the area.
The market for renewable energy technologies has continued to grow. Climate change concerns, coupled with high oil prices, peak oil, and increasing government support, are driving increasing renewable energy legislation, incentives and commercialization.United Nations Environment Programme Global Trends in Sustainable Energy Investment 2007: Analysis of Trends and Issues in the Financing of Renewable Energy and Energy Efficiency in OECD and Developing Countries (PDF), p. 3. New government spending, regulation and policies helped the industry weather the 2009 economic crisis better than many other sectors.Clean Edge (2009). Clean Energy Trends 2009 pp. 1–4.
The concerns about environmental impacts of renewable energy are presented by the proponents of theories like degrowth and steady-state economy as one of the proofs that for achieving sustainability technological methods are not enough and there is a need to limit consumption.
Definition of renewable energy The International Energy Agency defines renewable energy saying Renewable energy resources exist over wide geographical areas, in contrast to other energy sources, which are concentrated in a limited number of countries.
Variable renewable energy
Variability inherently affects solar energy, as the production of electricity from solar sources depends on the amount of light energy in a given location. Solar output varies throughout the day, the seasons, with cloud cover and by latitude on the globe. Windblown sand erodes glass in dry climates, protective layers add expenses. These factors are fairly predictable, and some solar thermal systems make use of molten salt heat storage to produce power when the sun is not shining.
Wind-generated power is a variable resource, and the amount of electricity produced at any given point in time by a given plant will depend on wind speeds, air density, and turbine characteristics (among other factors). If wind speed is too low (less than about 2.5 m/s) then the wind turbines will not be able to make electricity, and if it is too high (more than about 25 m/s) the turbines will have to be shut down to avoid damage. While the output from a single turbine can vary greatly and rapidly as local wind speeds vary, as more turbines are connected over larger and larger areas the average power output becomes less variable.
Capacity factors for PV solar are rather poor varying between 10 and 20% of the rated nameplate capacity. Onshore wind is better at 20-35% and offshore wind is best at 45%. This means that more total capacity needs to be installed in order to achieve an average output for the year. The capacity factor relates to statements about capacity increases, generation may have increased by a much smaller figure.
The International Energy Agency says that there has been too much attention on issue of the variability of renewable electricity production. This issue only applies to certain renewable technologies, mainly wind power and solar photovoltaics, and to a lesser extent run-of-the-river hydroelectricity. The significance of this "predictable variability depends on a range of factors which include the market penetration of the renewables concerned, the nature of the energy sources used to balance the intermittentcy, as well as demand side flexibility. Variability will rarely be a barrier to increased renewable energy deployment. But at high levels of market penetration it requires careful analysis and management, and additional costs may be required for dispatchable back-up or system modification.Contribution of Renewables to Energy Security Renewable electricity supply in the 20-50+% penetration range has already been implemented in several European systems, albeit in the context of an integrated European grid system:
In 2011, the Intergovernmental Panel on Climate Change, the world's leading climate researchers selected by the United Nations, said "as infrastructure and energy systems develop, in spite of the complexities, there are few, if any, fundamental technological limits to integrating a portfolio of renewable energy technologies to meet a majority share of total energy demand in locations where suitable renewable resources exist or can be supplied". IPCC scenarios "generally indicate that growth in renewable energy will be widespread around the world". The IPCC said that if governments were supportive, and the full complement of renewable energy technologies were deployed, renewable energy supply could account for almost 80% of the world's energy use within forty years. Rajendra Pachauri, chairman of the IPCC, said the necessary investment in renewables would cost only about 1% of global GDP annually. This approach could contain greenhouse gas levels to less than 450 parts per million, the safe level beyond which climate change becomes catastrophic and irreversible.
Mark Z. Jacobson says that there is no shortage of renewable energy and a "smart mix" of renewable energy sources can be used to reliably meet electricity demand: Because the wind blows during stormy conditions when the sun does not shine and the sun often shines on calm days with little wind, combining wind and solar can go a long way toward meeting demand, especially when geothermal provides a steady base and hydroelectric can be called on to fill in the gaps.
As physicist Amory Lovins has said: The variability of sun, wind and so on, turns out to be a non-problem if you do several sensible things. One is to diversify your renewables by technology, so that weather conditions bad for one kind are good for another. Second, you diversify by site so they're not all subject to the same weather pattern at the same time because they're in the same place. Third, you use standard weather forecasting techniques to forecast wind, sun and rain, and of course hydro operators do this right now. Fourth, you integrate all your resources — supply side and demand side..." The combination of diversifying variable renewables by type and location, forecasting their variation, integration with dispatchable renewables, flexible fueled generators, and demand response has the potential to create a power system that could reliably meet energy needs. Integrating ever-higher levels of renewables has had some successful demonstrations in the real world, in previous decades in the real world:Amory Lovins (2011). Reinventing Fire, Chelsea Green Publishing, p. 199. In 2009, eight American and three European authorities, writing in the leading electrical engineers' professional journal, didn't find "a credible and firm technical limit to the amount of wind energy that can be accommodated by electricity grids". In Fact, not one of more than 200 international studies, nor official studies for the eastern and western U.S. regions, nor the International Energy Agency, has found major costs or technical barriers to reliably integrating up to 30% variable renewable supplies into the grid, and in some studies much more. Renewable electricity supply in the 20-50+% range has already been implemented in several European systems, albeit in the context of an integrated European grid system: In 2010, four German states, totalling 10 million people, relied on wind power for 43–52% of their annual electricity needs. Denmark isn't far behind, supplying 22% of its power from wind in 2010 (26% in an average wind year). The Extremadura region of Spain is getting up to 25% of its electricity from solar, while the whole country meets 16% of its demand from wind. Just during 2005–2010, Portugal vaulted from 17% to 45% renewable electricity.
Integration of renewable energy has caused some grid stability problems in Germany. Voltage fluctuations have caused problems with sensitive equipment. In one case, Hydro Aluminium plant in Hamburg was forced to shut down when the rolling mill's highly sensitive monitor stopped production so abruptly that the aluminum belts snagged. They hit the machines and destroyed a piece of the mill. The malfunction was caused when voltage off the electricity grid weakened for a millisecond. A survey of members of the Association of German Industrial Energy Companies (VIK) revealed that the number of short interruptions to the German electricity grid has grown by 29 percent in the years 2009–2012. Over the same time period, the number of service failures has grown 31 percent, and almost half of those failures have led to production stoppages. Damages have ranged between €10,000 and hundreds of thousands of euros, according to company information.
Minnkota Power Cooperative, the leading U.S. wind utility in 2009, supplied 38% of its retail sales from the wind.
Mark A. Delucchi and Mark Z. Jacobson report that there are at least seven ways to design and operate variable renewable energy systems so that they will reliably satisfy electricity demand: (A) interconnect geographically dispersed, naturally variable energy sources (e.g., wind, solar, wave, tidal), which smooths out electricity supply (and demand) significantly. (B) use complementary and non-variable energy sources (such as hydroelectric power) to fill temporary gaps between demand and wind or solar generation. (C) use "smart" demand-response management to shift flexible loads to a time when more renewable energy is available. (D) store electric power, at the site of generation, (in batteries, hydrogen gas, molten salts, compressed air, pumped hydroelectric power, and flywheels), for later use. (E) over-size renewable peak generation capacity to minimize the times when available renewable power is less than demand and to provide spare power to produce hydrogen for flexible transportation and heat uses. (F) store electric power in electric-vehicle batteries, known as "vehicle to grid" or V2G. (G) forecast the weather (winds, sunlight, waves, tides and precipitation) to better plan for energy supply needs. Jacobson and Delucchi argue that wind, water and solar power can be scaled up in cost-effective ways to meet our energy demands, freeing us from dependence on both fossil fuels and nuclear power. In 2009 they published "A Plan to Power 100 Percent of the Planet With Renewables" in Scientific American. The article addressed a number of issues, such as the worldwide spatial footprint of wind turbines, the availability of scarce materials needed for manufacture of new systems, the ability to produce reliable energy on demand and the average cost per kilowatt hour. A more detailed and updated technical analysis has been published as a two-part article in the journal Energy Policy.