Use of Fossil Fuels for Power Generation and the Need for Renewable Energy
According to Nelson (2011), more than 85% of the world’s energy consumption, which is estimated at 520 exajoules, is generated through the burning of fossil fuels such as petroleum, natural gas and coal (p.19). Electric power generation from non-renewable sources will, unequivocally, result in their exhaustion and creation of a major energy crisis. For this reason, it appears primordial to shift in a greater scale to alternate power generation, such as solar, wind, hydroelectric, etc. Amongst these, Sorensen (2011) explains that though hydroelectric power does represent one of the major forms of power generation, its growth is stagnant due to the limitation of locations with potential hydro sources (p. 3).
Nelson (2011), illustrates this concern by calculating the cumulative consumption of non-renewable resources, and by estimating with a constant rate of consumption increase, computes in how much time the resource will be depleted. In regards to conventional oil, a yearly consumption growth rate of 3% results in the disappearance of conventional oil in only 25 years. Thus, resource depletion should be a major factor leading towards renewable energy sources which can be replenished within human lifespan, as well as the more efficient use of energy. In consequence, he states: “we do not have an energy crisis, since you will learn energy cannot be created or destroyed. We have an energy dilemma because of the finite amount of readily available fossil fuels, which are our main energy source today” (p. 9).
Besides the concerns related to resource depletion, one of the major factors that ignite interest in renewable energy is the worsening phenomena of global warming which, according to Sorensen (2011), causes by the increased injection of greenhouse gases into the atmosphere. Activities inherent to human development, which include but are not limited to power generation, industrial activity, deforestation, agriculture and transportation are the principal sources of greenhouse gases (p. 31).
According to Boyle (2012), burning fossil fuels is the principal contributor to greenhouse gases emissions, which have significantly increased since 1950 resulting in a temperature increase of approximately 0.8 °C. In consideration, he summarizes the benefits of renewable energy in three major points: it is not substantially depleted by continued use, it does not entail significant major problems, and does not perpetuate health hazards or social injustices.
This shift towards renewable energy aims to prevent the temperature increase from reaching 2 °C before 2050, through the reduction of carbon dioxide emissions by approximately 80%.
Solar Energy and Solar Power Generation Methods
Solar energy refers to the power obtained through the transformation of the sun’s radiation and heat intro electricity. According to Nelson (2011), the sun radiates energy into space from its surface with a power of over 3.8 x 1023 kW. This inexhaustible source of energy can be used to generate electric power through the use of certain technologies. Boyle (2012) explains that solar energy can be absorbed in solar collectors to provide hot water or space heaters, it can be concentrated by mirrors also to obtain high temperatures for solar thermal-electric plants, and, finally, it can be directly converted into electricity using photovoltaic cells (p. 15).
Solar Thermal Energy Conversion or Concentrating Solar Power (CSP)
Sorensen (2011) indicates that this method of power generation is performed through collectors, which according to him “are able to distribute the absorbed radiant energy over internal degrees of freedom associated with the kinetic energy of motion at the molecular level” (p. 450). In other words, collectors concentrate the radiation emitted from the sun and subsequently use it to raise their temperature or transfer the energy to a reservoir, known as heat transfer fluids. The most efficient collectors maximize sunlight absorption and minimize reflection.
These systems can be divided into passive and active. The former is characterized by natural energy flows that do not depend on man-provided energy inputs. A mainstream example of passive systems are windows and walls, which can absorb radiation when facing the sun. On the contrary, active systems are characterized by added energy (through pumps, for example) in order to establish an energy flow from the collector to load areas. Sorensen describes several types of collectors, such as flat-plate collector (absorbers with flat appearance), stalled and operating collectors and flat-plate collectors with heat storage. Moreover, other devices known as focusing are a different type of solar collectors but perform a similar purpose, for very high-temperature applications (p. 466). The types of collectors and CSP systems will be further explained.
Nelson (2011) explains the fundamentals of CSP systems as follows: “the collector is in the pond, and the salt gradient allows for stratification of the thermal energy, which can be used to drive the turbine to generate electricity” (p. 125). The thermal energy of the heat transfer fluids is used to generate steam to drive a turbine that converts it’s into mechanical energy, subsequently converted into electric power by a generator that is driven by the turbine’s rotating shaft. Sorensen (2011) explains this as a two-step process: a sun-tracking solar collector performs a certain measure of concentration and then a thermodynamic engine cycle, such as Rankine cycle, is used to generate electricity. The Rankine cycle is characterized by a steam expansion (obtained from the heating of the transfer fluids, in this case) through a turbine.
Nelson (2011) classifies CSP systems in four categories:
Power tower: They are composed of large fields of heliostats, flat mirrors that track the sun, which concentrate sunlight onto a receiver placed atop a tower, where the concentrated sunlight is used to heat a fluid which is used for steam generation. These systems provide the advantage of steam-storing, which signifies that electricity can be generated even in the absence of sunlight for a few hours. Nelson (2011) points out that the concentrated beam has sufficient energy to vaporize a bird flying into it. Considering thermal storage, the annual capacity factor of power towers can reach up to 80% during the seasons of highest sun radiation, as during nighttime the turbines can be powered with the stored thermal energy (p. 127). Among the first solar power towers were one constructed at Sandia National Laboratories, followed by the Solar-One and Solar-Two projects in California.
Line-focusing systems: Ensembles of mirrors such as parabolic troughs, reflecting mirrors or linear Fresnel reflectors, adequately tilted in the sun’s direction (generally oriented in the north-south direction) to collect its energy. The earliest of these systems were the Solar Energy Generating Systems in California. By 2009, line-focusing systems accounted for approximately 620 MW of generation through parabolic troughs, mostly by the United States and Spain.
The difference between parabolic troughs and linear Fresnel reflector systems is that in the former, the receiver tube follows the trough while in the latter, the mirrors rotate individually focusing on a fixed receiver tube.
Dish/engine systems: These are composed of dish-shaped mirrors which resemble satellites. According to Nelson (2011) the parabolic shape is generally made from individual mirrors or reflecting membranes (p. 133). The Stirling engine is the most common heat engine, which utilizes the heated fluid to move its pistons, generating mechanical energy which is then used to drive a generator.
Point-focusing systems: This is a form of a hybrid-systems, as it combines collector technology with photovoltaic cells, which will be explained in further detail later in the paper. They use individual Fresnel lenses to concentrate the sun’s radiation towards the surface of photovoltaic cells, thus reducing the amount of PV material to generate the same amount of electricity compared to regular PV systems.
Photovoltaic Cells (PV)
Contrary to the process of CSP systems, photovoltaic devices directly convert sunlight radiation into electricity through the use of semiconductor materials, on which “the electron excitation caused by impinging light quanta has a strongly enhancing effect on the conductivity” (Sorensen, 2011, p. 418). This means that when exposed to sunlight, electrons become excited and can move more freely as they are released from their atomic shells. Semiconductor materials use p-n junctions to force the electrons to move in a single direction, establishing a flow of electric current. A p-n junction is a junction of p-type doped and n-type doped semiconductors, forming a gradient of electrical potential as the electron density is higher in the n-type material, while the hole density in the valence band is higher in the p-type material.
Solar cells are fabricated by having an n and p layer, forming one of these junctions, forming the basis of PV systems. A solar panel is a group of solar cells used for large-scale power generation applications.
Razykov, Ferekides, Morel, Stefanakos, Ullal and Upadhyaya (2011) describe the different types of solar cells (p.1583 – 1587), which are:
Crystaline Silicon (c-Si) solar cells: These typically use boron-doped single-crystal wafers of approximately 400µ. The efficiency of c-Si solar cells has evolved along with technological developments, increasing from 17% in 1948, to 17% in the 1970s, to 25% in the first decade of the XXI Century. As of today, the improvements in contact and surface passivation and light-trapping technology as allowed for a maximum recorded efficiency of 28.8%. Those commercially available, however, typically range between 15 and 22%.
Polycrystalline wafer Si solar cells: Their commercial efficiency ranges from 12 to 15% and do not have a significant cost difference compared to c-Si cells. They have a transmissive surface and increased light (of approximately 40%) due to a process called reactive-ion etching. According to Nelson (2011) the efficiency of pc-Si is lower due to boundaries between crystals (p. 104).
Thin-film c-Si and p-Si solar cells: These are created by deposition or thin ribbon. The have some superior aspects over regular cells, including significantly lower thickness (can be reduced to almost 1µ) and can be fabricated on module-sized substrates. The efficiency of these cells depends on their thickness, but in average the efficiency of a c-Si thin film cell is 8.2%, and 13.7% for –Si thin film cells (Razykov, Ferekides, Morel, Stefanakos, Ullal and Upadhyaya, 2011, p.1584).
Thin-film amorphous and nano-Si solar cells: Amorphous silicon is an alloy of silicon with hydrogen. This material is vaporized and deposited on glass, and its absorption coefficient is almost double than in c-Si cells. Therefore, to affectively absorb sunlight they require a thickness of only a couple of microns. However, the lifespan of these cells is limited due to a large number of defect state in the conventional p-n junction configurations of these types of cells. Nelson (2011) estimates their efficiencies within 7 and 8%.
Research conducted by Razykov, Ferekides, Morel, Stefanakos, Ullal and Upadhyaya (2011) indicates that presently over 80% of all electricity generated by the PV industry is done based on c-Si and pc-Si technologies, but also note that single-junction c-Si cells are reaching their maximum theoretic efficiency (p. 1604). This is the reason for which so much research is directed towards improving the efficiency of other types of solar cells, as well as developing new ones.
Among other types of cells are cadmium-telluride (Cd-Te) cells, hybrid solar cells and flexible solar cells. An important point regarding photovoltaic systems is that the end product is electricity in the form of Direct Current (DC), therefore for most applications the use DC/AC inverters to perform the conversion into alternating current.
Solar Power Generation and Sustainability
Environmental Perspective
As previously explained, energy consumption needs to balance CO2 concentration in the atmosphere. According to Razykov, Ferekides, Morel, Stefanakos, Ullal and Upadhyaya (2011), the use of photovoltaics (PV) and other renewables for electricity generation is the simplest scenario to stabilize CO2.
Though much less significant in greenhouse gas emissions compared to fossil fuel generation, large renewable energy projects including those for solar energy, create environmental issues that deserve analysis. Nelson (2011) indicates that some of these concerns regard biological impact on wildlife, plants habitat, as well as detrimental visual impact, among others (p.297). However, as stated by Boyle (2012), even though there is not an ideal method for power generation, renewable sources appear much more sustainable than fossil fuels as they are essentially inexhaustible and entail fewer health hazards and gas emissions (p. 2).
Political Perspective
In order to successfully achieve the transition towards renewable energy, certain policies must be established to provide incentives, inflict penalties and educate the industry. According to Nelson (2011), incentives are primarily in the form of tax breaks, subsidies, mandates an regulations to promote renewable project developments, while penalties are increased taxes and stricter regulations (p. 300). Similarly, education refers to the creation of public awareness regarding the long term cost-benefits of renewable energy.
Specifically referring to solar power systems, a great example of policies promoting this technology is Japan. Nelson (2011) explains that in 1994, Japan began to subsidize the installment of photovoltaic systems. By 2004, it was the country with the highest installed PV capacity (p. 308). Similarly, Germany and Spain began large scale feed-in tariffs in the years 2000 and 2007, respectively. Moreover, Israel has regulations that require water heating by solar energy in all new buildings, for which over 80% of its annual water heating needs are met by solar systems.
Economic Perspective
The first commercially available technologies for solar power generation were too expensive for the average person, and therefore their widespread adoption was unpractical. However, these costs have been reduced significantly, especially in the latest years. According to Razykov, Ferekides, Morel, Stefanakos, Ullal and Upadhyaya (2011), the market of photovoltaic systems has an annual growth rate of 35 to 45%, and this most likely derived from the cost reductions achieved through the development of new technologies (p. 1604).
Similarly, Nelson (2011) indicates that the cost of energy (cost per watts generated) of Concentrating Solar Power Systems was approximately $125-225/MWh, but predicted future costs are estimated to be $43-62/MWh for trough plants and $35-55/MWh for tower plants (p. 332). These costs are competitive with those from energy generated through fossil fuels.
However, the increase in residential solar power generation poses a disadvantage for utility companies. Homes with solar panels installed are still commonly connected to power grids, as a backup source of electricity in case of solar system failure, or for power consumption during cloudy days or nighttime, but their billing is considerably reduced. Therefore, utility companies face the issue of assuming the fixed costs of maintain the grid when their income has decreased.
Social Perspective
Renewable energy power generation has inherent social benefits derived from its implementation. Regarding solar power, communities that did not previously have access to large-scale power grids due to limitations (geographical, infrastructural or otherwise) can take advantage of solar power stations to generate electricity, promoting their economic, social and educational development.
Moreover, Nelson (2011) indicates that one of these benefits is “the benefit everyone gets from cleaner air from installation of renewable energy systems” (p. 308). For instance, coal-fueled Chinese cities have a serious smog problem, which has in the present and will continue to detriment the overall health of citizens in the future, including children.
In addition, the social benefits obtained from the installment of solar power plants can be related to job creation. Solar plants are generally located in areas with fragile soil, not suitable for crops or cattle raising. Therefore, the increased activity resulting from solar plants can benefit the overall life quality of communities by promoting economic, social and educational development.
Works Cited
Boyle, G. (Renewable Energy: Power for a Sustainable Future). 2012. Oxford: Oxford University Press.
Nelson, V. (2011). Introduction to Renewable Energy (Energy and the Environment). Boca Raton: CRC Press Taylor & Francis Group.
Razykov, T., Ferekides, C., Morel, D., Stefanakos, E., Ullald, H., & Upadhyaya, H. (2010). Solar photovoltaic electricity: Current status and future prospects. Solar Energy, 1580-1608.
Sorensen, B. (2011). Renewable energy: physics, engineering, environmental impacts, economics and planning (4th ed.). Burlington, Massachusetts: Elsevier Academic Press.
