Solar Energy Engineering

Processes and Systems

Soteris A Kalogirou

• Publisher: Academic Press
• Year: 2009
• ISBN:
  9780080922874
• Topic: Tecnología e ingeniería
• Subtopic: Recursos de energía renovable

As perhaps the most promising of all the renewable energy sources available today, solar energy is becoming increasingly important in the drive to achieve energy independence and climate balance. This new book is the masterwork from world-renowned expert Dr. Soteris Kalogirou, who has championed solar energy for decades. The book includes all areas of solar energy engineering, from the fundamentals to the highest level of current research. The author includes pivotal subjects such as solar collectors, solar water heating, solar space heating and cooling, industrial process heat, solar desalination, photovoltaics, solar thermal power systems, and modeling of solar systems, including the use of artificial intelligence systems in solar energy systems, modeling and performance prediction.*Written by one of the world's most renowned experts in solar energy*Covers the hottest new developments in solar technology, such as solar cooling and desalination*Packed with quick look up tables and schematic diagrams for the most commonly used systems today'

Chapter one

Introduction

1.1 General Introduction to Renewable Energy Technologies

The sun is the only star of our solar system located at its center. The earth and other planets orbit the sun. Energy from the sun in the form of solar radiation supports almost all life on earth via photosynthesis and drives the earth’s climate and weather.

About 74% of the sun’s mass is hydrogen, 25% is helium, and the rest is made up of trace quantities of heavier elements. The sun has a surface temperature of approximately 5500 K, giving it a white color, which, because of atmospheric scattering, appears yellow. The sun generates its energy by nuclear fusion of hydrogen nuclei to helium. Sunlight is the main source of energy to the surface of the earth that can be harnessed via a variety of natural and synthetic processes. The most important is photosynthesis, used by plants to capture the energy of solar radiation and convert it to chemical form. Generally, photosynthesis is the synthesis of glucose from sunlight, carbon dioxide, and water, with oxygen as a waste product. It is arguably the most important known biochemical pathway, and nearly all life on earth depends on it.

Basically all the forms of energy in the world as we know it are solar in origin. Oil, coal, natural gas, and wood were originally produced by photosynthetic processes, followed by complex chemical reactions in which decaying vegetation was subjected to very high temperatures and pressures over a long period of time. Even the energy of the wind and tide has a solar origin, since they are caused by differences in temperature in various regions of the earth.

Since prehistory, the sun has dried and preserved humankind’s food. It has also evaporated sea water to yield salt. Since humans began to reason, they have recognized the sun as a motive power behind every natural phenomenon. This is why many of the prehistoric tribes considered the sun a god. Many scripts of ancient Egypt say that the Great Pyramid, one of humankind’s greatest engineering achievements, was built as a stairway to the sun (Anderson, 1977).

From prehistoric times, people realized that a good use of solar energy is beneficial. The Greek historian Xenophon in his “memorabilia” records some of the teachings of the Greek philosopher Socrates (470–399 BC) regarding the correct orientation of dwellings to have houses that were cool in summer and warm in winter.

The greatest advantage of solar energy as compared with other forms of energy is that it is clean and can be supplied without environmental pollution. Over the past century, fossil fuels provided most of our energy, because these were much cheaper and more convenient than energy from alternative energy sources, and until recently, environmental pollution has been of little concern.

Twelve autumn days of 1973, after the Egyptian army stormed across the Suez Canal on October 12, changed the economic relation of fuel and energy as, for the first time, an international crisis was created over the threat of the “oil weapon” being used as part of Arab strategy. Both the price and the political weapon issues quickly materialized when the six Gulf members of the Organization of Petroleum Exporting Countries (OPEC) met in Kuwait and abandoned the idea of holding any more price consultations with the oil companies, announcing at the same time that they were raising the price of their crude oil by 70%.

The rapid increase in oil demand occurred mainly because increasing quantities of oil, produced at very low cost, became available during the 1950s and 1960s from the Middle East and North Africa. For the consuming countries, imported oil was cheap compared with indigenously produced energy from solid fuels.

The proven world oil reserves are equal to 1200 billion barrels (2005) and the world natural gas reserves are 180 trillion m³ (2004). The current production rate is equal to 80 million barrels per day for oil and 7.36 billion m³ per day for natural gas. Therefore, the main problem is that proven reserves of oil and gas, at current rates of consumption, would be adequate to meet demand for only another 41 and 67 years, respectively (Goswami, 2007). The reserves for coal are in a better situation; they would be adequate for at least the next 230 years.

If we try to see the implications of these limited reserves, we are faced with a situation in which the price of fuels will accelerate as the reserves are decreased. Considering that the price of oil has become firmly established as the price leader for all fuel prices, the conclusion is that energy prices will increase continuously over the next decades. In addition, there is growing concern about the environmental pollution caused by burning fossil fuels. This issue is examined in Section 1.3.

The sun’s energy has been used by both nature and humankind throughout time in thousands of ways, from growing food to drying clothes; it has also been deliberately harnessed to perform a number of other jobs. Solar energy is used to heat and cool buildings (both actively and passively), heat water for domestic and industrial uses, heat swimming pools, power refrigerators, operate engines and pumps, desalinate water for drinking purposes, generate electricity, for chemistry applications, and many more operations. The objective of this book is to present various types of systems used to harness solar energy, their engineering details, and ways to design them, together with some examples and case studies.

1.2 Energy Demand and Renewable Energy

Many alternative energy sources can be used instead of fossil fuels. The decision as to what type of energy source should be utilized in each case must be made on the basis of economic, environmental, and safety considerations. Because of the desirable environmental and safety aspects it is widely believed that solar energy should be utilized instead of other alternative energy forms because it can be provided sustainably without harming the environment.

If the world economy expands to meet the expectations of countries around the globe, energy demand is likely to increase, even if laborious efforts are made to increase the energy use efficiency. It is now generally believed that renewable energy technologies can meet much of the growing demand at prices that are equal to or lower than those usually forecast for conventional energy. By the middle of the 21^st century, renewable sources of energy could account for three fifths of the world’s electricity market and two fifths of the market for fuels used directly.¹ Moreover, making a transition to a renewable energy-intensive economy would provide environmental and other benefits not measured in standard economic terms. It is envisaged that by 2050 global carbon dioxide (CO₂) emissions would be reduced to 75% of their 1985 levels, provided that energy efficiency and renewables are widely adopted. In addition, such benefits could be achieved at no additional cost, because renewable energy is expected to be competitive with conventional energy (Johanson et al., 1993).

This promising outlook for renewables reflects impressive technical gains made during the past two decades as renewable energy systems benefited from developments in electronics, biotechnology, material sciences, and in other areas. For example, fuel cells developed originally for the space program opened the door to the use of hydrogen as a non-polluting fuel for transportation.

Moreover, because the size of most renewable energy equipment is small, renewable energy technologies can advance at a faster pace than conventional technologies. While large energy facilities require extensive construction in the field, most renewable energy equipment can be constructed in factories, where it is easier to apply modern manufacturing techniques that facilitate cost reduction. This is a decisive parameter that the renewable energy industry must consider in an attempt to reduce cost and increase the reliability of manufactured goods. The small scale of the equipment also makes the time required from initial design to operation short; therefore, any improvements can be easily identified and incorporated quickly into modified designs or processes.

According to the renewable energy-intensive scenario, the contribution of intermittent renewables by the middle of this century could be as high as 30% (Johanson et al., 1993). A high rate of penetration by intermittent renewables without energy storage would be facilitated by emphasis on advanced natural gas-fired turbine power-generating systems. Such power-generating systems—characterized by low capital cost, high thermodynamic efficiency, and the flexibility to vary electrical output quickly in response to changes in the output of intermittent power-generating systems—would make it possible to back up the intermittent renewables at low cost, with little, if any, need for energy storage.

The key elements of a renewable energy-intensive future are likely to have the following key characteristics (Johanson et al., 1993):

1. There would be a diversity of energy sources, the relative abundance of which would vary from region to region. For example, electricity could be provided by various combinations of hydroelectric power, intermittent renewable power sources (wind, solar-thermal electric, and photovoltaic), biomass,² and geothermal sources. Fuels could be provided by methanol, ethanol, hydrogen, and methane (biogas) derived from biomass, supplemented with hydrogen derived electrolytically from intermittent renewables.

2. Emphasis would be given to the efficient mixing of renewable and conventional energy supplies. This can be achieved with the introduction of energy carriers such as methanol and hydrogen. It is also possible to extract more useful energy from such renewable resources as hydropower and biomass, which are limited by environmental or land-use constraints. Most methanol exports could originate in sub-Saharan Africa and Latin America, where vast degraded areas are suitable for re-vegetation that will not be needed for cropland. Growing biomass on such lands for methanol or hydrogen production could provide a powerful economic driver for restoring these lands. Solar-electric hydrogen exports could come from regions in North Africa and the Middle East that have good insolation.

3. Biomass would be widely used. Biomass would be grown sustainably and converted efficiently to electricity and liquid and gaseous fuels using modern technology without contributing to deforestation.

4. Intermittent renewables would provide a large quantity of the total electricity requirements cost-effectively, without the need for new electrical storage technologies.

5. Natural gas would play a major role in supporting the growth of a renewable energy industry. Natural gas-fired turbines, which have low capital costs and can quickly adjust their electrical output, can provide excellent backup for intermittent renewables on electric power grids. Natural gas would also help launch a biomass-based methanol industry.

6. A renewables-intensive energy future would introduce new choices and competition in energy markets. Growing trade in renewable fuels and natural gas would diversify the mix of suppliers and the products traded, which would increase competition and reduce the possibility of rapid price fluctuations and supply disruptions. This could also lead eventually to a stabilization of world energy prices with the creation of new opportunities for energy suppliers.

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