Solar energy in progress and future research trends
Renewable alternatives
The sun is the primary source for new and renewable energy alternatives. Unfortunately, this source is not consumed sufficiently at its full extent everywhere in the world. This is due to the fact that all the portions of earth surface do not receive usable amounts of solar energy. For instance, places far away from the equatorial belt do not have enough irradiation intensity because annual clear and sunny days number is too small. On the contrary, zones close to the equator in the north and south have high potentials. Hence, there are unbalances in the receipt of solar energy generateable irradiation amounts.
The long-term sought energy sources are expected to have the following important points for a safer and pleasant environment in the future:
1. diversity of various alternative energy resources (conventional, non-renewable and renewable) with steadily increasing trend in the renewable source consumptions but steadily decreasing trend by time in the nonrenewable resources usage,
2. quantities must be abundant and sustainable for the long future,
3. acceptable cost limits and compatible prices with strong economic growth,
4. energy supply options must be politically reliable,
5. friendly energy resources for the environment and climate changes,
6. renewable sources are domestic resources that help to reduce the important energy alternatives, and finally,
7. they can support small to medium scale local industries.
The renewable energies are expected to play an active role in the future energy share because they satisfy the following prerequisites:
1. they are environmentally clean, friendly and do not produce greenhouse gases,
2. they should have sufficient resources for larger scale utilization. For instance, the solar energy resources are almost evenly distributed all over the world with maximum possible generateable amounts increasing towards the equator,
3. intermittent nature of solar and wind energy should be alleviated by improving the storage possibilities, and finally
4. cost effectiveness of the renewable sources is one of the most important issues that must be tackled in a reduction direction. However, new renewable energies are now, by and large, becoming cost competitive with conventional forms of energy.
Hydropower is an already established technological way of renewable energy generation. In the industrial and surface water rich countries, the full scale developments of hydroelectric energy generation by turbines at large scale dams are already exploited to the full limitations, and consequently, smaller hydro systems are of interest in order to gain access to the marginal resources. The world’s total annual rainfall is, on the average, 108.4 X 1012 l/y of which 12 X 10121 recharges the groundwater resources in the aquifers, 25.13 X 10121 appears as surface runoff, 71.27 X 1012t evaporates into atmosphere. If the above rainfall amount falls from a height of 1000 m above the earth surface, then the kinetic energy of 1.062 X 1015 kJ/y is imparted to the earth every year [92]. Some of this huge energy is stored in dams, which confine the potential energy so that one can utilize it to generate hydroelectric power.
Wind power, by now is reliable and established technology, which is able to produce electricity at cost competitive with coal and nuclear energy alternatives. Although the amount of wind energy is economically insignificant for the time being in many parts of the world, mankind took the advantage of its utilization since early years whenever he/she found the chance to provide power for various tasks. Among these early utilizations are the hauling of water from a lower to a higher elevation, grinding grains in mills by water and other mechanical power applications for many centuries. It is still possible to see at some parts of the world these type of marginal benefits from the wind speed. Perhaps, the windmills in Holland exemplify the most publicized application of wind power. All of the previous activities have been technological and the scientific investigation of wind power formulations and accordingly development of modern technology appeared after the turn of 20th century. In recent decades the significance of wind energy originates from its friendly behavior to the environment so far as the air pollution is concerned although there are to some extent noise and pollution appearance in the modern wind-farms. Due to its cleanness, wind power is sought wherever possible for conversion to the electricity with the hope that the air pollution as a result of fossil fuel burning will be reduced. In some parts of USA, up to 20% of electrical power is generated from wind energy. In fact, after the economic crises in 1973 its importance increased by forcing the economic limitations and today there are wind-farms in many western European countries [135].
Biomass is important in world energy terms since it covers almost 15% of the present world energy supply. Especially, in developing countries biomass is the major component of the national energy supply. Although biomass sources are widely available but they have low conversion efficiencies. This energy source is used especially for cooking and comfort and by burning it provides heat. The sun radiation that conveys energy is exploited by the plants through the photosynthesis, and consequently, even the remnants of plants are potential energy sources because they conserve historical sun energy until they parish either naturally after very long time spans or artificially by human beings or occasionally by forest fires. Only 0.1% of the solar incident energy is used by photosynthesis process but even this amount is 10 times greater than the present day world energy consumption. Currently, living plants or remnants from the past are reservoirs of biomass that is a major source of energy for humanity in the future. However, biomass energy returns its energy to the atmosphere partly by respiration of the living plants and by oxidation of the carbon fixed by photosynthesis is used to form fossil sediments which eventually transform to the fossil fuel types such as coal, oil and natural gas. This argument shows that the living plants are the recipient media of incident solar radiation and they give rise to various types of fossil fuels.
On the other hand, solar energy applications in buildings are of interest for heating, cooling and day lighting. Furthermore, solar electricity production from photovoltaic has already made great advancements since the beginning of 1980s. The emergence of interest in solar energy utilization took place since 1970 principally due to then rising cost of energy from conventional sources. Solar radiation is the world’s most abundant and permanent energy source. The amount of solar energy received by the surface of the earth per minute is greater than the energy utilization by the entire population in one year. For the time being, solar energy, being available everywhere, is attractive for stand-alone systems particularly in the rural parts of developing nations. Occurrences of solar energy dynamically all over the world in the forms of wind, wave and hydropower through the hydrological cycle provide abilities to ponder about their utilization, if possible instantly or in the form of reservations by various conversion facilities and technologies. It is also possible that in the very long, the human beings might search for the conversion of ocean currents and temperature differences into appreciable energy quantities so that the very end product of solar radiation in the earth will be useful for sustainable development.
The comprehensive design and assessment of solar energy systems depend, largely, on adequate information on the solar energy characteristics of the region in which the systems are to be located. The best information is obtained by measurements of radiation at any site with time. However, due to the high cost of setting up and maintaining a large number of stations for conducting such measurements, various satisfactory methods have been suggested and used in practical applications for the estimation of the radiation component. A significant group of estimation models is empirical whereby meteorological data are used in conjunction with the regression techniques [108,110]. Detailed account of solar energy prediction model alternatives will be explained in Section 10.
Design of many technical apparatus such as coolers, heaters, solar energy electric generators in form of photovoltaic requires terrestrial irradiation data at the study area. Especially, among the use of clean energy resources, the solar energy utilization gained intensive interest since 1970, principally due to the then rising cost of energy from conventional sources. Scientific and technological studies in the last three decades tried to convert the continuity of solar energy into sustainability for the human comfort. Accurate estimations of global solar irradiation need meteorological, geographical and astronomical data, and especially, many estimation models are based on the easily measurable sunshine duration at a set of meteorology stations.
