“Energy can neither be created nor destroyed” First Law of Thermodynamics…. Mayer.
Coal Fired Power Plants
Coal based power accounts for almost 41 % of the world’s electricity generation. Coal fired power plants operate on the modified Rankine thermodynamic cycle.The efficiency is dictated by the parameters of this thermodynamic cycle. The overall coal plant efficiency ranges from 32 % to 42 %. This is mainly dictated by the Superheat and Reheat steam temperatures and Superheat pressures. Most of the large power plants operate at steam pressures of 170 bar and 570 °C Superheat, and 570 ° C reheat temperatures. The efficiencies of these plants range from 35 % to 38 %. Super critical power plants operating at 220 bar and 600/600 °C can achieve efficiencies of 42 %. Ultra super critical pressure power plants at 300 bar and 600/600 °C can achieve efficiencies in the range of 45% to 48 % efficiency.
Natural Gas Fired Power Plants
Natural Gas fired (including LNG fired) power plants account for almost 20 % of the world’s electricity generation. These power plants use Gas Turbines or Gas Turbine based combined cycles. Gas turbines in the simple cycle mode, only Gas turbines running, have an efficiency of 32 % to 38 %. The most important parameter that dictates the efficiency is the maximum gas temperature possible. The latest Gas Turbines with technological advances in materials and aerodynamics has efficiencies upto 38 %. In the combined cycle mode, the new “H class” Gas turbines with a triple pressure HRSG and steam turbine can run at 60 % efficiency at ISO conditions. This is by far the highest efficiency in the thermal power field.
Hydro turbines, the oldest and the most commonly used renewable energy source, have the highest efficient of all power conversion process. The potential head of water is available right next to the turbine, so there are no energy conversion losses, only the mechanical and copper losses in the turbine and generator and the tail end loss. The efficiency is in the range of 85 to 90 %.
Wind turbines have an overall conversion efficiency of 30 % to 45 %.
These two renewable sources, though efficient, are dependent on availability of the energy source.
Solar thermal systems can achieve efficiency up to 20 %. The moving path of the sun and the weather conditions drastically alter the incident solar radiation. The efficiency on an annual basis, around 12 %, is considerably less than on a daily basis.
Geo thermal systems, on the other hand, also use the Rankine cycle with steam temperatures at saturation point. Since there is no other conversion loss, this plant can achieve efficiencies in the range of 35 %.
The efficiency of nuclear plants is little different. On the steam turbine side they use the Rankine thermodynamic cycle with steam temperatures at saturated conditions. This gives a lower thermal cycle efficiency than the high temperature coal fired power plants. Thermal cycle efficiencies are in the range of 38 %. Since the energy release rate in nuclear fission is extremely high, the energy transferred to steam is a very small percentage – only around 0.7 %. This makes the overall plant efficiency only around 0.27 %. But one does not consider the fuel efficiency in nuclear power plants; fuel avaliabity and radiation losses take center stage
Diesel engines, large capacity industrial engines, deliver efficiencies in the range of 35 – 42 %.
The power industry is trying to increase this conversion efficiency of power plants to maximise elctricity generation and reduce environmental impact.