Solar energy in progress and future research trends

Solar thermal collectors and applications

Extensive fossil fuel consumption in almost all human activities led to some undesirable phenomena such as atmospheric and environmental pollutions, which have not been experienced before in known human history. Consequently, global warming, greenhouse affect, climate understood scientifically by experiments and researches that these phenomena are closely related to fossil fuel uses because they emit greenhouse gases such as carbon dioxide (CO2) and methane (CH4) which hinder the long wave terrestrial radiation to escape into space, and consequently, the earth troposphere becomes warmer. In order to avoid further impacts of these phenomena, the two concentrative alternatives are either to improve the fossil fuel quality with reductions in their harmful emissions into the atmosphere or more significantly to replace fossil fuel usage as much as possible with environmentally friendly, clean and renewable energy sources. Among these sources, solar energy comes at the top of the list due to its abundance, and more evenly distribution in nature than any other renewable energy types such as wind, geothermal, hydro, wave and tidal energies. It must be the main and common purpose of humanity to sustain environment for the betterment of future generations with sustainable energy developments. On the other hand, the known limits of fossil fuels compel the societies of the world in the long run to work jointly for their gradual replacement by renewable energy alternatives rather than the quality improvement of fossil sources.

Solar radiation is an integral part of different renewable energy resources. It is the main and continuous input variable from practically inexhaustible sun. Solar energy is expected to play a very significant role in the future especially in developing countries, but it has also potential prospects for developed countries. The material presented in this paper is chosen to provide a comprehensive account of solar energy sources and conversion methods. For this purpose, explanatory background material has been introduced with the intention that engineers and scientists can have introductory preliminaries on the subject both from application and research points of view. Applications of solar energy in terms of low and high temperature collectors are given with future research directions. Furthermore, photovoltaic devices are discussed for future electric energy generations based on solar power site-exploitation and transmission by different means over long distances such as fiber-optic cables. Another future perspective use of solar energy is its combination with water and as a consequent electrolysis analysis generation of hydrogen gas, which is expected to be another form of clean energy sources. Combination of solar energy and water for hydrogen gas production is called solar-hydrogen energy. Necessary research potentials and application possibilities are presented with sufficient background. Possible future new methodologies are mentioned and finally recommendations and suggestions for future research and application directions are presented with relevant literature review.

a continuous driving power for the social and technological prospective developments. Energy sources are vital and essential ingredients for all human transactions and without them human activities of all kinds will not be progressive at all. On one hand, the energy sources are limited and on the other hand, the population growth at present average rate of 2% inserts extra pressure on additional energy demands.

The oil crises of the 1970s led to a surge in research and development efforts that are dedicated to the development of solar energy alternatives. These efforts were strongly correlated with the fluctuating market price of energy, and suffered a serious setback as this price later plunged.

Nomenclature

a linear model coefficient

b linear model coefficient

a' restrictive model parameter

b' restrictive model parameter

c light velocity

cj linear model coefficient

c2 linear model coefficient

c3 linear model coefficient

c4 linear model coefficient

c5 linear model coefficient

C constant

dij distances

Dj half-squared differences

E electric field

E0 eccentricity correction factor

fdear sunshine fraction

f(R) astronomical factor

hg geographical elevation

h Plank constant (6.626 X 10_34 J s) or geo­

graphical elevation

H daily global radiation on a horizontal surface

Hf heat flow per unit area of cross-section

Hb daily direct (beam) radiation

Hbc monthly-averaged potential daily clear-sky

beam irradiation on a horizontal surface Hcg cloudless global irradiation Hcg

Hd daily diffuse radiation on a horizontal surface

Hm magnetic field

Ho maximum daily radiation

Ho extraterrestrial irradiation on a horizontal plane

Hb. dean monthly average of daily clear sky horizontal

surface beam radiation

Hd the monthly mean daily horizontal surface

diffuse radiation I global radiation

Ib direct (beam) radiation

Id diffuse radiation

Ih horizontal surface radiation

In normal radiation

Isc solar constant (1360 W/m[1])

k thermal conductivity

K monthly mean daily clearness index

-Kciear monthly average clear sky clearness index

n the number of hours of bright sunshine per month

N total number of daylight hours in the month

Nd day number of the year

P radiation power

Q(R) radiation flux per unit area

r radius of the earth

rhs cross-correlation coefficient between global

solar irradiation

R distance from the sun

Rf photosphere radius

RM maximum distance

S sunshine duration

Si measured solar irradiation value

Ss total solar radiation

SE solar irradiance estimation at a site SO

Sc correction for the irradiance threshold of sun­

shine recorders

Sc monthly-averaged day length

So maximum possible sunshine duration

T temperature

Ts surface temperature

Tf fluid temperature

VE estimation variance

Vm cumulative semivariogram value corresponding

to RM

Var() variance of the argument

wi weighting factors

x distance along x direction

Z the angle between the normal to the surface and

the direction of the beam S solar declination angle

Г day angle

g(di;j) cumulative semivariogram

l wavelength

Amax maximum wavelength

n frequency

ф latitude angle

s Stefan’s constant (5.67 X 10_8 W/m2 K4)

dz zenith angle

vs sunset hour angle

The missing ingredient in such a process was a long-term perspective that hindered the research and development policies within the wider context of fossil and solar energy tradeoffs rather than reactions to temporary price fluctu­ations. The same events also gave rise to a rich literature on the optimal exploitation of natural resources, desirable rate of research and development efforts to promote competitive backstop technologies [139]. There is also vast literature on energy management in the light of atmospheric pollution and global warming processes [19,36,37,59,88,89,90,91,138,139,143].

 

Since the energy crisis in 1973, air pollution from combustion processes has caused serious damage and danger on the forests, monuments, human health as has been documented by official studies and yearly statistics. Many environmental damages including acid rains, and their forest damaging consequences, incurred economical losses in short, and especially, long-term durations. Hence, seemingly cheap energy inflicted comparatively very high expenses on the society. Fig. 1 shows three partners in such a social problem including material beneficiary, heat beneficiary and in between the third

 

Fig. 1. Energy usage partners.

party who has nothing to do with these two major actors.

The climate change due to use of chloroflorocarbons (CFCs) is a major cause of imbalance and natural absorption of CO2 is another example as possible social costs from the energy use which are handed over to the future generations by today’s energy consumers. Again the major source of climate change is the combustion of bad quality fossil fuels.

Today, the development scale of any country is measured by few parameters among which per capita energy consumption holds the most significant rank. In fact, most industrialized countries require a reliable, efficient and readily available energy for effective transportation, indus­trial, domestic and military systems. This is particularly true for developing countries especially those that do not possess reliable and sufficient energy sources. Importation of energy from other countries is expected to be one of the main items in the national budgets of many countries in an increasing manner in the future. However, recently many countries have launched projects to optimize, update and search for their internal energy resources, whatever are the availabilities.

Non-renewable energy resources are limited to cover all the foreseeable future energy consumptions of the world. As a whole electricity production based on fossil or nuclear fuels induce substantial social and environmental costs, whereas it would appear that the use of renewable energy sources involves far less and lower costs. There are a number of different energy cost categories born by third parties, which ought to be taken into consideration in the comparison of different energy resources and technologies. Hohmeyer [60] has given the following seven effective categories for consideration. [2] 2

3. Long-term cost of resource depletion:

3.1 Structural macro-economic impacts like employ­ment effects.

4. Subsidies for:

4.1 research and development,

4.2 operation costs,

4.3 infrastructure, and

4.4 evacuation in cases of accidents.

5. Cost of an increased probability of wars due to:

5.1 securing energy resources (like gulf war), and

5.2 proliferation of nuclear weapons.

6. Cost of radioactive contamination of production equip­ment and dwellings after major nuclear accidents.

7. Psycho-social cost of:

7.1 serious illness and death, and

7.2 relocation of population due to construction or accidents.

Economic growth and prosperity rely heavily on adequate energy supply at reasonably low costs [20]. Unfortunately, energy is the main source of pollution in any country on its way of development. It is well known, by now, that the SO2 emission from the fossil fuels is the main cause of acid rain as a result of which more than half the forests in the Northern Europe have already been damaged. On the global scale, increase in the emission rates of greenhouse gases and in particular CO2 represents colossal treat to the world’s climate. Various theories and calculations in the atmospheric research circulations have already indicated that over the last half century, there appeared a continuously increasing trend in the average temperature value up to 0.5 °C. If this trend continues in the future, it is expected that in some areas of the world, there will appear extreme events such as excessive rainfall and consequent floods, droughts and also local imbalances in the natural climatic behaviors giving rise to unusual local heat and cold. Such events will also affect the world food production rates. In order to decrease degradation effects on the environment and atmosphere, technological developments are sought since 1973 oil crisis. It has been recently realized that renewable energy sources and systems can have a beneficial impact on the following essential technical, environmental, and political issues of the world. These are:

1. major environmental problems such as acid rain, strato­spheric ozone depletion, greenhouse effect and smog,

2. environmental degradation,

3. depletion of the world’s non-renewable conventional sources such as coal, oil, natural gas,

4. increasing energy use in the developing countries, and

5. world population increase.

The use of conventional energy resources will not be able to offset the energy demand in the next decades but steady increase will continue with undesirable environmental consequences. However, newly emerging renewable alterna­tive energy resources are expected to take increasing role in the energy scenarios of the future energy consumptions.

Oil rich countries do not have energy shortages as long as the fossil fuel resources are economic and available within the country. The net return from industrial material produced in a country is the reflection of energy consump­tion in an efficient way. Otherwise, burning fossil fuels without economic industrial return may damage any country in the long run, especially, with the appearance of renewable energy resources that are expected to be more economical, and exploitable in the long run. Fossil fuel reservoir availability steadily decreases at an unprecedented rate and hence, there are future non-sustainability alarms on the energy source. It is, therefore, necessary to reduce consumption rates even starting from today by partial replacements through the sustainable alternatives, such as solar energy. Solar energy is practically unlimited, envir­onmentally clean and friendly. Unfortunately, for the time being large-scale fossil energy production is cheaper than the available solar alternatives as stated by Chakravorty et al. [18]. Parallel to the fossil energy exploitation and consumption technological advancements, solar energy consumption has also developed but to the level that for today such developments are marginal. Abundance and cheap exploitation of fossil fuels leave room only for technological developments, in order to reduce the environ­mental pollution. Researches and technological develop­ments are concentrated on clean coal and oil technology rather than improving fossil fuel conversion efficiencies. There is hardly any study towards the reduction of fossil energy cost, but studies are directed rather to energy demand mitigation. Solar energy, on the other hand, has many prospects of future developments and technological renew­als that are appropriate for research and development.

Especially, the oil crises in 1973s have led to a surge in solar energy research and development efforts. The rate of these developments is dependent on the fluctuating oil prices. Perhaps, the most significant side view was the individual and rivalry developments in fossil and solar alternatives rather than their joint exploitations.

Although an adequate supply of energy is a prerequisite of any modern society for economic growth, on the other hand, energy is also the main source of environmental pollution, particularly in industrialized countries. In an indirect way, it is also known that acid rains as a result of sulfur dioxide emission from fossil fuel plants have already damaged plant and forest life, which are observable especially in the developed countries. Additionally, on the global scale, increasing emissions of air pollutions are main causes of greenhouse gases. If the increasing trend of carbon dioxide continues at the present rate, then major climatic disruptions and local imbalances in the hydrological as well as atmospheric cycles will be the consequences leading to excessive rainfall or drought, excessive heat and cold. Such changes are already experienced by those who are at their 1950s and will also affect the world’s potential for food production which is the major survival supply for the human life. The continual use of conventional energy resources is expected to affect adversely the natural environmental conditions, and consequently, social energy related prob­lems are expected to increase in the future. A new factor, however, which may alleviate the environmental and social problems of future energy policies, or even solve them, is the emerging new forms of renewable energies such as solar, wind, biomass, small hydro, ocean and geothermal energies as well as the solar hydrogen energy possibilities. Up to now, the renewable sources have been completely dis­criminated from the conventional alternatives due to economic reasons. However, the trend in recent years steadily favors the renewable energies in many cases over the conventional sources.

Today consumption of fossil fuel quantities is so high that even minor imbalances between supply and demand may cause considerable societal disruptions. In order to get rid of such disruptions at least for the time being, each country imports coal, and especially, oil to cover the energy imbalances. Oil embargo, by Organization of Petroleum Exporting Countries (OPEC) in 1973, gave the first serious warning and alarm to even industrialized countries that energy self-sufficiency is an essential part of the country concerned for her economic, social and even cultural survival. In fact, the technological and industrial develop­ments in the last 150 years rendered many countries to energy dependable status.

Worldwide use of energy for several decades appeared to be increasing dramatically, but in the last decade, it has leveled off, and even dropped to a certain extent as shown in Fig. 2. In this graph all forms of energy uses are represented in terms of the amount of coal that would provide the equivalent energy. Around 1970s most of the predictions foresaw that energy demand would continue to accelerate causing expected severe energy shortages. However, just an opposite situation developed, and today, there is energy surplus on the worldwide market that has resulted from economic downturn coupled with many-fold increase in the oil prices during the last 20 years.

Fossil fuel reserves in the form of oil and natural gas are still considerable at present consumptive rates for the next 50 years. However, with increasing amounts of renewable sources and discoveries of new reservoirs, this span of time is expected to extend for almost a century from now onwards.

With the unprecedented increase in the population, the industrial products and the technologic developments, the human beings started to search for new and alternative ways of using more energy without harming or perhaps even destroying the natural environment. This is one of the greatest unsolved problem facing mankind in the near future. There is an unending debate that key atmospheric energy source, solar radiation should be harnessed more

Подпись: Fig. 2. Changes in annual energy consumption in the world [35].

effectively and turned directly into heat energy to meet the growing demand for cheaper power supplies.

The main purpose of this paper is to discuss the potential of solar energy for future uses as the major alternative among the renewable sources in addition to its environmen­tally friendly clean characteristics. An extensive literature review on recent and future directions are also presented for solar energy prediction models. The practical uses of solar energy in the forms of low and high temperature collectors and photovoltaic possibilities are also given with future potential research directions.

Solar energy in progress and future research trends

Spatial solar radiation estimation

In the previous sections, the modeling of solar radiation is discussed on a given site. However, in practical solar energy assessment studies, it is also necessary to have spatial (multiple …

Heat transfer and losses

As explained earlier the easiest way of solar radiation collection is for low temperature heating purposes. It is well known that black surfaces absorb solar radiation more than any other …

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