Macroscopic Energy at the system (large)level Potential Energy (PE): -elevation in a gravitational mg field compared to some reference Z level F Reference plane PE =mgz pe g3 V=0 V>0 Kinetic Energy (KE): -motion relative to some m m reference frame v2 V2 KE ke reference m 2 2 Are they properties of a system? 上降文通大学 March 5,2019 6 SHANGHAI JLAO TONG UNIVERSITY
March 5, 2019 6 Macroscopic Energy Potential Energy (PE): –elevation in a gravitational field compared to some reference level Kinetic Energy (KE): –motion relative to some reference frame at the system (large) level Are they properties of a system?
Example:Wind Energy 8.5m/s A site evaluated for a wind farm is observed to have steady winds at a speed of 8.5 m/s.Determine the wind energy (a)per unit mass (b)for a mass of 10 kg,and (c)for a flow rate of 1154 kg/s for air. Solution A site with a specified wind speed is considered.Wind energy per Discussion unit mass,for a specified mass,and for a given mass flow rate of air are to be determined. Assumptions Wind flows steadily at the specified speed. =8.5m/s,12m section, Analysis The only harvestable form of energy of atmospheric air is the Pair=1.2 kg/m3 kinetic energy,which is captured by a wind turbine. (a)Wind energy per unit mass of air is A wind turbine with a V2(8.5m/s)2 1 J/kg e =ke= 2 1m2/s =36.1J/kg wind spanΦl2 m has a 2 power generation (b)Wind energy for an air mass of 10 kg is potential of 41.7 kW. E=me=(10kg)(36.1J/kg)=361J Real wind turbines (c)Wind energy for a mass flow rate of 1154 kg/s is convert about 1/3 of this E=me=(1154kg/s)(36.1J/kg) 1kW potential to electric 1000J/s 41.7kW power. 上海究通大学 March 5,2019 SHANGHAI JIAO TONG UNIVERSITY
March 5, 2019 7 Example: Wind Energy A site evaluated for a wind farm is observed to have steady winds at a speed of 8.5 m/s . Determine the wind energy (a) per unit mass, (b) for a mass of 10 kg, and (c) for a flow rate of 1154 kg/s for air. A wind turbine with a wind span Φ12m has a power generation potential of 41.7 kW. Real wind turbines convert about 1/3 of this potential to electric power. = 8.5m/s, Φ12m section, ρair=1.2 kg/m3 Discussion
What about Microscopic Energy? 上游充通大学 March 5,2019 8 SHANGHAI JLAO TONG UNIVERSITY
March 5, 2019 8 What about Microscopic Energy?
Microscopic Energy at the molecular (small)levels Translational energy:molecules move around Rotational energy:individual atoms rotate about the axis forming the molecular bond Vibrational energy:individual atoms vibrate about the center of mass of the molecular bond Electronic energy:electrons in an atom or molecule are bound in particular configurations combine these to get sensible energy that depends on temperature for ideal gases 上游充通大 March 5,2019 9 SHANGHAI JIAO TONG UNIVERSITY
March 5, 2019 9 – Translational energy: molecules move around – Rotational energy: individual atoms rotate about the axis forming the molecular bond – Vibrational energy: individual atoms vibrate about the center of mass of the molecular bond – Electronic energy: electrons in an atom or molecule are bound in particular configurations Microscopic Energy at the molecular (small) levels combine these to get sensible energy that depends on temperature for ideal gases
Microscopic Energy at the molecular (small)levels Chemical energy:binding energy between electrons and nuclei of atoms - Nuclear energy:binding energy between protons and neutrons (within the nucleus) - Electric/magnetic/surface tension effects usually negligible in most engineering device 上游充通大学 March 5,2019 10 SHANGHAI JLAO TONG UNIVERSITY
March 5, 2019 10 – Chemical energy: binding energy between electrons and nuclei of atoms – Nuclear energy: binding energy between protons and neutrons (within the nucleus) – Electric/magnetic/surface tension effects usually negligible in most engineering device Microscopic Energy at the molecular (small) levels