Renewable energy technologies produce marketable energy by converting natural phenomena into useful forms of energy. These technologies use the sun's energy and its direct and indirect effects on the earth (solar radiation, wind, falling water, and various plants; i.e., biomass), gravitational forces (tides), and the heat of the earth's core (geothermal) as the resources from which energy is produced. These resources have massive energy potential; however, they are generally diffused and not fully accessible, and most of them are intermittent and have distinct regional variabilities. These characteristics give rise to difficult, but solvable, technical and economical challenges. Nowadays, significant progress is made by improving the collection and conversion efficiencies, lowering the initial and maintenance costs, and increasing the reliability and applicability of renewable energy systems.
Worldwide research and development in the field of renewable energy resources and systems has been carried out during the last two decades. Energy conversion systems that are based on renewable energy technologies appeared to be cost effective compared to the projected high cost of oil. Furthermore, renewable energy systems can have a beneficial impact on the environmental, economic, and political issues of the world. At the end of 2001 the total installed capacity of renewable energy systems was equivalent to 9% of the total electricity generation (Sayigh, 2001). As was seen before, by applying the renewable energy-intensive scenario, the global consumption of renewable sources by 2050 would reach 318 EJ (Johanson et al., 1993).
The benefits arising from the installation and operation of renewable energy systems can be distinguished into three categories: energy saving, generation of new working posts, and decrease in environmental pollution.
The energy-saving benefit derives from the reduction in consumption of the electricity and diesel used conventionally to provide energy. This benefit can be directly translated into monetary units according to the corresponding production or avoiding capital expenditure for the purchase of imported fossil fuels.
Another factor of considerable importance in many countries is the ability of renewable energy technologies to generate jobs. The penetration of a new technology leads to the development of new production activities, contributing to the production, market distribution, and operation of the pertinent equipment. Specifically for the case of solar energy collectors, job creation is mainly related to the construction and installation of the collectors. The latter is a decentralized process, since it requires the installation of equipment in every building or for every individual consumer.
The most important benefit of renewable energy systems is the decrease in environmental pollution. This is achieved by the reduction of air emissions due to the substitution of electricity and conventional fuels. The most important effects of air pollutants on the human and natural environment are their impact on the public health, agriculture, and on ecosystems. It is relatively simple to measure the financial impact of these effects when they relate to tradable goods, such as the agricultural crops; however, when it comes to non-tradable goods, such as human health and ecosystems, things becomes more complicated. It should be noted that the level of the environmental impact and therefore the social pollution cost largely depend on the geographical location of the emission sources. Contrary to the conventional air pollutants, the social cost of CO2 does not vary with the geographical characteristics of the source, as each unit of CO2 contributes equally to the climate change thread and the resulting cost.
All renewable energy sources combined account for only 17.6% share of electricity production in the world, with hydroelectric power providing almost 90% of this amount. However, as the renewable energy technologies mature and become even more cost competitive in the future, they will be in a position to replace a major fraction of fossil fuels for electricity generation. Therefore, substituting fossil fuels with renewable energy for electricity generation must be an important part of any strategy of reducing CO2 emissions into the atmosphere and combating global climate change.
In this book, emphasis is given to solar thermal systems. Solar thermal systems are non-polluting and offer significant protection of the environment. The reduction of greenhouse gas pollution is the main advantage of utilizing solar energy. Therefore, solar thermal systems should be employed whenever possible to achieve a sustainable future.
The benefits of renewable energy systems can be summarized as follows (Johanson et al., 1993):
■ Social and economic development. Production of renewable energy, particularly biomass, can provide economic development and employment opportunities, especially in rural areas, that otherwise have limited opportunities for economic growth. Renewable energy can thus help reduce poverty in rural areas and reduce pressure for urban migration.
■ Land restoration. Growing biomass for energy on degraded lands can provide the incentive and financing needed to restore lands rendered nearly useless by previous agricultural or forestry practices. Although lands farmed for energy would not be restored to their original condition, the recovery of these lands for biomass plantations would support rural development, prevent erosion, and provide a better habitat for wildlife than at present.
■ Reduced air pollution. Renewable energy technologies, such as methanol or hydrogen for fuel cell vehicles, produce virtually none of the emissions associated with urban air pollution and acid deposition, without the need for costly additional controls.
■ Abatement of global warming. Renewable energy use does not produce carbon dioxide or other greenhouse emissions that contribute to global warming. Even the use of biomass fuels does not contribute to global warming, since the carbon dioxide released when biomass is burned equals the amount absorbed from the atmosphere by plants as they are grown for biomass fuel.
■ Fuel supply diversity. There would be substantial interregional energy trade in a renewable energy-intensive future, involving a diversity of energy carriers and suppliers. Energy importers would be able to choose from among more producers and fuel types than they do today and thus would be less vulnerable to monopoly price manipulation or unexpected disruptions of supply. Such competition would make wide swings in energy prices less likely, leading eventually to stabilization of the world oil price. The growth in world energy trade would also provide new opportunities for energy suppliers. Especially promising are the prospects for trade in alcohol fuels, such as methanol, derived from biomass and hydrogen.
■ Reducing the risks of nuclear weapons proliferation. Competitive renewable resources could reduce incentives to build a large world infrastructure in support of nuclear energy, thus avoiding major increases in the production, transportation, and storage of plutonium and other radioactive materials that could be diverted to nuclear weapons production.
Solar systems, including solar thermal and photovoltaics, offer environmental advantages over electricity generation using conventional energy sources. The benefits arising from the installation and operation of solar energy systems fall into two main categories: environmental and socioeconomical issues.
From an environmental viewpoint, the use of solar energy technologies has several positive implications that include (Abu-Zour and Riffat, 2006):
■ Reduction of the emission of the greenhouse gases (mainly CO2, NOx) and of toxic gas emissions (SO2, particulates)
■ Reclamation of degraded land
■ Reduced requirement for transmission lines within the electricity grid
■ Improvement in the quality of water resources
The socioeconomic benefits of solar technologies include:
■ Increased regional and national energy independence
■ Creation of employment opportunities
■ Restructuring of energy markets due to penetration of a new technology and the growth of new production activities
■ Diversification and security (stability) of energy supply
■ Acceleration of electrification of rural communities in isolated areas
■ Saving foreign currency
It is worth noting that no artificial project can completely avoid some impact to the environment. The negative environmental aspects of solar energy systems include:
■ Pollution stemming from production, installation, maintenance, and demolition of the systems
■ Noise during construction
■ Land displacement
■ Visual intrusion
These adverse impacts present difficult but solvable technical challenges.
The amount of sunlight striking the earth's atmosphere continuously is 1.75 X 105 TW. Considering a 60% transmittance through the atmospheric cloud cover, 1.05 X 105 TW reaches the earth's surface continuously. If the irra-diance on only 1% of the earth's surface could be converted into electric energy with a 10% efficiency, it would provide a resource base of 105 TW, while the total global energy needs for 2050 are projected to be about 25-30 TW. The present state of solar energy technologies is such that single solar cell efficiencies have reached over 20%, with concentrating photovoltaics (PVs) at about 40% and solar thermal systems provide efficiencies of 40-60%.
Solar PV panels have come down in cost from about $30/W to about $3/W in the last three decades. At $3/W panel cost, the overall system cost is around $6/W, which is still too high for the average consumer. However, solar PV is already cost effective in many off-grid applications. With net metering and governmental incentives, such as feed-in laws and other policies, even grid-connected applications such as building integrated PV (BIPV) have become cost effective. As a result, the worldwide growth in PV production has averaged more than 30% per year during the past five years.
Solar thermal power using concentrating solar collectors was the first solar technology that demonstrated its grid power potential. A total of 354 MWe solar thermal power plants have been operating continuously in California since 1985. Progress in solar thermal power stalled after that time because of poor policy and lack of R&D. However, the last five years have seen a resurgence of interest in this area, and a number of solar thermal power plants around the world are under construction. The cost of power from these plants (which so far is in the range of $0.12 to $0.16/kWh) has the potential to go down to $0.05/kWh with scale-up and creation of a mass market. An advantage of solar thermal power is that thermal energy can be stored efficiently and fuels such as natural gas or biogas may be used as backup to ensure continuous operation.
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