Solar Power Generation
Sustainable Technology – Solar Power Generation
Power Generation from Fossil Fuels and Global Warming
Global warming, as defined by Lineman, Do, Kim and Joo (2015), is the long-term trend of increasing average temperature of the lower atmosphere. It is widely known that this phenomenon is related to the concentration of greenhouse gases that trap heat in the atmosphere, such as carbon dioxide, methane, nitrous oxide, among others.
Activities related to human development such as industrial processes, agriculture, cattle raising for meat consumption, transportation, deforestation and power generation are the major causes of greenhouse gases emissions. Specifically, power generation based on the burning of fossil fuels (coal, oil, natural gas, etc.) is responsible for 25% of all global greenhouse gases emissions, as reported by the United States Environmental Protection Agency (2014). In consideration, other forms of power generation that are proven to be more environmentally-friendly, as they utilize renewable primary energy sources, have been developed as alternate energy sources to help in the fight against global warming. Among these are hydroelectric power generation, wind power generation and solar power generation. This paper will focus on the latter, and will analyze if it can be considered as a sustainable method for the generation of electric power.
Fundamentals of Solar Power Generation
Solar Power Generation is the conversion of the energy contained in sunlight into electricity. There are two different methods that are used to perform this conversion: Photovoltaic Cells (PV) and Concentrating Solar Power Systems (CSP).
Solar panels, which are arrangements of photovoltaic cells, are composed of semiconductor material with p-n junctions (union of opposite-doped semiconductor regions). The concentration of sunrays on the cell’s surfaces causes the excitation of the contained electrons by raising their energy level and freeing them from their atomic shells (Penick and Louk, 3). The physical characteristics of the semiconductor material forces the electrons to move in a single direction, thus establishing a flow of electric current. Figure 1 is a diagram that illustrates the basic function of a photovoltaic cell. There are several types of PVs, with varying efficiencies and costs. The most common are crystalline silicon (c-Si) cells, which have a rated efficiency of approximately 15%, followed by thin-film cells with a lower efficiency of 8%.
Figure 1. - How solar cells work from Penick and Louk; “Photovoltaic Power Generation”; December 1998
The electricity obtained through the use of PV cells is direct current (DC), which is not the standard for power systems. Therefore, inverters are needed to convert the output into alternating current for subsequent distribution. Moreover, given the dependence on weather conditions of power output in solar generation, battery systems are typically installed to store energy produced during times of intense sunlight and radiation that can later be supplied as per system requirements (during nighttime, for example, when electricity generations ceases), preventing voltage and frequency issues on the power grid and improving system stability.
Photovoltaic power generation is implemented through solar farms, which are large-scale systems composed of many panels distributed among extensive fields, used to supply power to grids shared by multiple users or, conversely, through individual systems used to power specific homes or businesses, mostly through the installation of solar panels on rooftops.
As previously explained, solar cells allow for the direct conversion of sunlight into electricity. Conversely, Concentrating Solar Power Systems use mirrors or lenses to concentrate sunrays to produce thermal energy that is used to raise the temperature of Heat Transfer Fluids, which are later turned into super-heated steam to complete a typical cycle of thermal-to-electric power conversion. This steam is used to drive a turbine which effectively converts the thermal energy of the steam into mechanical energy; the turbine’s rotating shaft is used to drive a generator that converts the mechanical energy into electricity, completing the process.
There are several types of CSP systems: line-focusing systems such as solar dishes, point-focusing systems that used dish-shaped mirrors similar to a satellite, and solar tower systems. According to the NREL (2016), solar tower systems “use a large field of flat, sun-tracking mirrors known as heliostats to focus and concentrate sunlight onto a receiver on the top of a tower”. IRENA (2012) indicates the efficiency levels of CSP systems, which are: 11 to 16% for parabolic trough systems, 7 to 20% for solar power systems, 22 to 24% for linear Fresnel systems and 25 to 28% for dish-stirling systems.
Solar Power Generation and Sustainability – Environment
The Brundtland Commission defines sustainability as the “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. It is clear that environmental concerns play a role in sustainability, as the indiscriminate exploitation of natural resources for human activities leads to potentially irreversible environmental degradation, endangerment of flora and fauns species and contributes to global warming.
Greenhouse gas emission is the main environmental concern in the analysis of power generation; however, for solar power generation systems, CO2 emissions are significantly lower than those resulting from fossil fuel burning. According to Nelson, Gambhir and Ekins-Daukes (2014), the carbon emission intensity (CO2 emissions per unit of energy generated) of solar power generation during operation is so low that may be disregarded, and the actual gas emission impact is solely derived from the manufacture process of the required materials. Photovoltatic cell systems analyzed in Europe showed a range of carbon emission intensity of 15 – 38 gCO2/kWh, while the carbon intensity associated to CSP systems ranges from 20 to 50 gCO2/kWh. In contrast, the average carbon emission intensity of fossil fuel generation in the United Kingdom is 500 gCO2/kWh. Therefore, solar power generation technology makes a valuable contribution in the reduction of carbon dioxide emissions.
Though solar power generation appears to be solely beneficial in this aspect, gas emission is not the only factor that may cause adverse effects on the environment. Solar power generation systems raise concerns related to land use, land degradation and habitat loss. However, this argument is not necessarily valid, as according to Tsoutsos, Frantzeskaki and Gekas (2015), sites considered for solar power generation systems are usually desert areas with fragile soil and plant communities. Therefore, if site selection avoids ecologically sensitive areas, land use is not a major factor affecting the environment.
Moreover, a study conducted by Turney and Fthenakis (2011) analyzed 32 environmental impacts of solar power systems compared to traditional fossil fuel generation, and found that 22 of them were beneficial, 4 are neutral and 6 require further impact.
Solar Power Generation and Sustainability – Social
Murphy explains that the four pre-eminent concepts of the social pillar of sustainable development are equity, awareness for sustainability, participation and social cohesion. Regarding this aspect, the development of solar power technology can mostly be related to the concept of participation.
Creation of solar power plants positively affects local communities and promotes their participation through the creation of jobs. According to Schinke and Schetter (2015), the Noor power station project in Morocco created over 1800 jobs, 700 of which were assigned to local workers, 850 to workers from other parts of the country and only 250 were awarded to international engineers. Job creation leads to social development, which in turn results in improved welfare, education and healthcare services.
Regarding this project, the Wuppertal Institute for Climate, Environment and Technology (2015) explored the social dimensions of the establishment of the CSP project, and surveys concluded that the plant had only limited influence on the social and cultural environment, and did not negatively affect the social cohesion and stability of the region.
However, social cohesion may be disrupted in communities where negative perception of solar power plant aesthetics is predominant. In this case, development of projects of this type may result in protests and conflicts.
Additionally, establishment and operation of solar power stations results in little air pollution, therefore citizens of nearby areas enjoy cleaner air compared to those living near fossil fuel burning plants and may show overall better health, as the replacement of fossil fuels with renewable energy has been linked to reduction in premature mortality, breathing problems and cancer.
Finally, communities in remote areas (mountains, forests, islands, etc.) that do not have access to power grids and therefore do not receive electricity, can take advantage of solar power generation systems to generate power locally, thus fostering their social and economic development and improve their educational and medical facilities.
Solar Power Generation Sustainability – Economics
According to Nunez, prices for residential solar systems have drastically fallen an approximate 70% since 2008, even as technologies have become smarter. However, electric billing for power consumption derived from fossil fuels has remained static. In consequence, the competitiveness of solar power is rising; evidence of this is the quick growth of global solar power generation, which according to the Fondation Energies Pour Le Monde (2013) accounted for 0.5% of the electricity produced worldwide in 2012 and rose to an approximate 1% in 2015.
As solar power costs have become a better option than purchasing electricity from utility companies in some countries, many homes and businesses are taking the initiative to completely or partly cover their energy demand through renewable sources. For instance, according to Frankel, Ostrowski and Pinner, Wal-Mart stored have vowed to completely shift to renewable energy by 2020.
Though for the user it seems advantageous to turn to solar power, this creates a negative economic impact for power industries, given that as their client bases become smaller due to the growing reliance on solar power, their income to undertake the fixed costs of maintaining the power grid declines.
Solar Power Generation Sustainability – Governance
The governance aspect of solar power generation is related to the general governance issue of renewable energy. Stakeholders’ interest to plan, develop and fund solar power generation projects is highly dependent on location, economic benefits and other factors. For example, an oil-exporting country’s government may not promote solar power generation given the economic implications of reducing oil or other fossil fuel exportation. Conversely, oil-importers would certainly benefit from reducing their oil dependence through the increase of renewable energy reliance. In consequence, governance on solar power generation comprises complex social relations, practices and visions that influence how societies perceive and utilize natural resources.
There is a globally-accepted need to reduce greenhouse gases emissions by shifting towards renewable energy. To meet the set targets, robust governance systems must be designed; these shall be based on strong legislation, economic commitments, close monitoring and enforcement, regional cooperation and unified policies.
Governance schemes must involve both public (governments) and private sectors, through the realization of actions to achieve long-term milestones. For instance, governments should provide incentives for the development of solar power generation such as tax reductions while, in parallel, reduce subsidization of fossil fuels.
Moreover, as stated by the milestones set by the IEA (2010), utility companies should facilitate grid access to solar power plant developers and take an active participation role in these projects.
Future Developments
Constant research and technological advances are made aimed at improving solar power system efficiency and reducing manufacturing and implementation costs. Regarding efficiency, as previously mentioned photovoltaic cells have a sunlight-to-electricity average rated efficiency of approximately 15%. To improve this rating, several experiments have been conducted modifying solar cell structure.
Nayfeh and Stupca discovered that coating the cell’s surface with silica nanoparticles significantly increases UV light absorption and turns than energy into electricity, improving the cell’s efficiency. Moreover, Connor (2014), explains that instead of utilizing silicon semiconductors, which are do not perform well on sunlight absorption, new PV cells will use a thin layer of cadmium telluride or magnesium chloride, materials that only need a thickness of 1% relative to the needed silicon to absorb the same amount of sunlight.
Moreover, companies are working on the development of concentrating photovoltaic cells that rely on lenses to concentrate sunlight on their surface, similar to the effect of a magnifying glass. The most advanced cells currently in the market have proven to reach an efficiency of over 40%.
Another important advance for the promotion of solar power is the development of micro-inverters. Current generated by PV cells is direct current, which cannot be used to power typical residential loads. In consideration, several companies such as Smart Spark and Enphase Energy are working on the development of micro-inverters that will be incorporated into the PV cell, reducing system complexity.
Regarding Concentrating Solar Power Systems, the engineers of the National Renewable Energy Laboratory are conducting researching aimed at the replacement of molten salt as a heat transfer fluids, given the complications related to its transportation and conditioning. Research is focused on the utilization of solid particles that can be stable at temperatures above 1000 °C, and can thus increase system efficiency.
The International Energy Agency (2010) set a series of milestones for technology improvements in the field of solar power generation, to be accomplished by 2020 and 2030. These include the improvement of parabolic troughs, through the replacement of costly curved mirrors by troughs based on more economic technologies, such as acrylic substrates with silver or aluminum coating, or aluminum sheets glued to glass-fiber substrates. For solar towers and dishes, supercritical steam cycles are being tested to improve process efficiency, and potentially reach the levels of thermal power generation ranging from 40 to 46%. Among other proposed milestones are improvements in storage technologies, and cost reduction by augmenting overall temperature of the solar plants.
References
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Lineman, M., Do, Y., Kim, J., & Joo, G. (2015). Talking about Climate Change and Global Warming. PLOS one.
National Renewable Energy Laboratory. (2016, February 17F). Improved Concentrating Solar Power Systems. Retrieved from U.S. Department of Energy Website: http://techportal.eere.energy.gov/technology.do/techID=1316
National Renewable Energy Laboratory. (n.d.). Concentrating Solar Power Basics. Retrieved April 13, 2016, from NREL Website.
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Nunez, C. (2015, October 2). Solar Energy Sees Eye-Popping Price Drops. Retrieved April 12, 2016, from National Geographic.
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