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Note for Renewable Energy System - RES By JNTU Heroes

  • Renewable Energy System - RES
  • Note
  • Jawaharlal Nehru Technological University Anantapur (JNTU) College of Engineering (CEP), Pulivendula, Pulivendula, Andhra Pradesh, India - JNTUACEP
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NOMENCLATURE c cp cv F g h H I m p P Q t T u v V w W x y z speed of light specific heat at constant pressure specific heat at constant volume force acceleration of gravity specific enthalpy heating value electric current mass pressure power thermal energy, heat time temperature specific internal energy specific volume = 1/ρ electric tension velocity energy, general distance parameter defining physical state vertical distance η ρ energy efficiency density Subscripts el fuel heat loss marg th electricity fuel useful heat loss marginal thermal LIST OF ABBREVIATIONS ARC cap GNP IPCC PAH PV RME SEK USD VOC Anti-reflection coating capita Gross national product Intergovernmental Panel on Climate Change polyaromatic hydrocarbons Photo-voltaic rapsmetylester Swedish crown US Dollar volatile organic compounds

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1. INTRODUCTION 1.1 Scope of these notes The objective of the lecture notes is to give an introduction to the current issues of energy engineering with a focus on the role of energy supply for development of the human societies. Technical issues are treated only to the extent that this is necessary for understanding of the limitations and potential of the current technologies. These notes discuss the potential and technologies of renewable energy sources for generation of energy carriers in industrial and developing countries. The approach is source-oriented rather than user-oriented, but this does not mean that it is implied that a switch from a “conventional” energy source to a renewable energy source requires that the same energy carrier or the same amount of energy shall serve the needs of the user after the switch. Since use of renewable energy is often more expensive than use of the conventional energy sources, introduction of energy conservation measures is often economically justified when the switch to renewable energy is made. 1.2 Definition of renewable energy In the strict thermodynamic sense energy cannot be destroyed or produced, only converted from one form to another. These conversions are associated with an increase of the total entropy, which means a loss in the “quality” of the energy i.e. a loss of exergy. Apparently, renewable energy is a contradiction from a strict thermodynamic point of view. The term is nevertheless used frequently, not only by politicians and laymen, but also by those who understand their thermodynamics. Renewable energy is then understood as energy that is supplied directly or indirectly from the Sun or the Moon and thermal energy stored or generated below the crust of our planet, the Earth. In other words, renewable energy represents energy that can be expected to be available for as long as the Earth is habitable for the human race. Certainly these energy flows will deteriorate at some stage, but in the human perspective they may be considered as perpetual. This is in contrast with the large but much more limited deposits of fossil fuels, coal, petroleum and natural gas and the likewise limited deposits of fissile uranium. Even though renewable energy is based on energy sources that are expected to be available very far into the future, there is no guarantee that an energy system is sustainable just because it relies on renewable energy sources. A discussion of different requirements for sustainability is the subject of a forthcoming lecture. It may be sufficient at this stage to mention two requirements for sustainability that must be fulfilled in addition to a sustainable energy source. These are economic sustainability and environmental sustainability. Economic sustainability requires that the resources generated by utilisation of the energy source exceed those needed for the utilisation. Environmental sustainability requires that first of all the resource base is not deteriorated by excessive utilisation. This can easily be the case when biomass energy is over-utilised. Environmental sustainability also requires that the impacts and risks caused by the utilisation are 1

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acceptable and do not jeopardise the possibilities for future generations to survive on the Earth. 1.3 Overview As mentioned, renewable energy is either energy that is supplied directly or indirectly from the Sun or the Moon or thermal energy stored or generated below the crust of our planet, the Earth. The annual energy flow from the sun is very large compared to the present use of technical energy, about 0,112 EWh, as illustrated by figure 1. The problem with utilising this energy flow is that the energy density is much lower than needed in most technical applications, see some examples in table 1. Figure 1. Comparison between natural and technical energy flows Table 1. Energy densities for solar energy and technical energy Energy source or user application Solar radiation to Earth Single family house in Sweden Car Electric light bulb Average power kW 121.1012 3 100 0,6 Projected area for supply or user application, m2 510.1012 120 10 0,003 Average power density, W/m2 2371 25 10000 200000 The power density in most applications for technical energy is much higher than the power density of solar energy on the projected surface of the device used for final energy conversion. Also, there is generally a mis-match between the daily and 1 Average to whole Earth. In Stockholm 114 W/m2, year 2

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seasonal variations of solar radiation and energy demand. The consequence is that use of solar energy for technical purposes often requires methods for concentration of the energy flux and storage of the energy. Actually, heating of a single family house is one of the few applications where the average in-coming solar heat exceeds the average annual heat demand but also in that application is seasonal storage necessary of the heating shall rely on solar radiation only. Concentration and storage of solar radiation energy by means of technical devices is expensive. It is therefore attractive to use natural processes for concentration and storage of solar energy. Biomass is solar energy concentrated and stored by the photosynthesis reaction. Hydropower is solar energy concentrated by evaporation of water that collects and is stored, after condensation as rain, in lakes located above the ocean level. Wind, waves and ocean currents are also concentrated and temporarily stored natural energy. It can be noticed that the annually available amount of these forms of concentrated solar energy is larger than the present use of technical energy, but the difference is not dramatic, in particular if the fact that about 73% of the Earth is covered with water is taken into account. Table 2 shows a comparison between the concentrated natural energy flows and the present use of technical energy. Table 2. Technical energy use and concentrated natural energy flows Concentrated natural energy Energy PWh/year Hydropower2 Photosyntesis Winds, waves ocean currents Tidal energy World total 91 400 3200 26 3717 flow Natural energy divided by present technical energy 112 PWh/year 0,8 3,6 28,6 0,2 33,2 When the numbers for the energy flows are compared, it must be remembered that the numbers for natural energy flows refer to the total global potential and that the practical and economical constraints have not been taken into account. The per capita use of primary technical energy in Sweden 2003 was about 69 MWh. A world population of 10 billion 2050 with the same per capita use of technical energy as in Sweden today would need a supply of at least 690 PWh. The present use of renewable energy is about 14,5 PWh. Creating a sustainable energy system with equal energy standard on a global level, supplied by the natural energy flows, appears like a very big challenge. 2 It is difficult to find estimates for the global potential. The estimate shown here is based on data in table 6 from lecture notes on energy sources and energy carriers and the assumption that the potentials in Asia and Latin America can be estimated from that in North America with the same potential per land area unit. 3

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