ENERGY CONVERSION ES 832a Eric Savory www.eng.uwo.ca/people/esavory/es832.htm Lecture 5 – Main energy conversion principles Department of Mechanical and.
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ENERGY CONVERSION ES 832a Eric Savory www.eng.uwo.ca/people/esavory/es832.htm Lecture 5 – Main energy conversion principles Department of Mechanical and Material Engineering University of Western Ontario
Main energy conversion principles Before the detailed analysis of an energy conversion system and its components, the main primary sources of energy and the principal energy conversion mechanisms are reviewed.
Chemical energy: Combustion In the context of this course, combustion is the oxidation of a carbon-based substance (e.g. octane, propane, coal) for the production of heat. The heat is then: –Used directly for heating purposes –Used to cause a large volumetric expansion of gases: o Directly in an internal combustion engine or turbine (small-scale applications (< 300 MW), especially where weight is an issue) o Indirectly to generate steam (large-scale applications (>500 MW) where efficiency and cleanliness is critical). Advantages: cheap, very compact (high energy yield per weight), known technology. Disadvantages: non-renewable, environmentally costly.
Fuel cells The basic principle is that a chemical solution, either a liquid or an active membrane is placed between two electrodes. The active elements strip electrons from the fuel molecule. The electrons are then drawn at the anode and return to the cathode where these unite again with the cations (ions with +ve charge). Unlike a battery, the electrodes do not chemically react. Advantages: Renewable energy, clean production (water, no greenhouse gases), portable. Disadvantages: Technology still not well developed, relatively low power production, expensive materials (e.g. platinum catalyst).
Gravitational energy The potential energy stored through the change of elevation (hydrostatic pressure) of a liquid (water) is used to cause a flow rate. This kinetic energy is then converted using turbines to mechanical energy (and usually into electrical energy). The most common type is by damming a large body of water. However, the increase in elevation due to tidal motion can also be used. Advantages: If weather patterns hold it is renewable. No gas emissions. Disadvantages: Large areas modified (environmental, areas flooded can be the size of SW Ontario), large energy transmission losses, seasonal variations, only economical at very large scale.
Solar energy Is the extraction of energy from the sun by either: Direct Extraction: Through solar panels, where the sun radiation is captured by a substrate and then used to heat a liquid, or by solar-cells, where the photovoltaic effect is used to convert solar radiation into electricity directly. Advantages: Renewable, well suited for individual residential needs. Disadvantages: Low yields (10 W/m 2 is a typical average yearly flux for industrialized latitudes), expensive installation, dirty production energy for cells, weather and climate dependent. Indirect Extraction: Wind energy is used to drive a turbine. This will be covered in detail in the last part of the course. Advantages: Renewable, good for residential and small- scale applications. Disadvantages: Large surface area needed for wind farms, noise generation and weather dependent.
Conversion systems (1) Primary conversion system: Heat or fluid power is converted to mechanical energy. Internal combustion engines: The expansion of a gas is a closed system is used to drive work (these are not covered in this course). Turbines: The kinetic energy of a fluid (and for a gas, the internal energy) are converted to mechanical energy by imparting rotation in a rotor.
Secondary system: Electrical Generators. Typically, the rotating shaft, mounted with magnets, is placed in a casing. The casing contains field coils. The change in electrical field due to the rotation induces a current inside the coils. The power developed by the current in the coils is equal to the mechanical power needed to drive the shaft less the irreversible losses. Conversion systems (2)