Fossil fuels such as coal, oil, natural gas
Fuels artificially prepared, such as hydrogen, alcohol and acetylene
Converted fuels, such as methane and biogas
Coal and other non-gaseous fossil fuels can also be converted into electricity (and heat in CHP power plants) in combined gas-steam-cycle if the fuel is gasified in advance. Such IGCC (Integrated Gasification Combined Cycle) power plants offer large potential for higher efficiencies. On the other hand, these plants are very complex and difficult to operate, which reduces flexibility and availability. Other advanced techniques concentrate on special firing systems like fluidized bed combustion (FBC; attractive for medium scale and low-quality coal) and increased steam parameters (600°C, 270 bar and more; affords new materials). Currently, there are four main available “Clean Coal Combustion Technologies”
 in various sizes:
CONVERTING NATURAL GAS INTO ELECTRICITY IN COMBINED CYCLE
In the case of CCGT (Combined Cycle Gas Turbine processes) power is generated more efficiently than in a simple gas turbine cycle: the hot exhaust gases of the gas turbine are used to produce steam that generates electricity in a steam turbine cycle.
CONVERTING BIOMASS AND BIOGAS INTO ELECTRICITY
Biomass results from the joint combustion of organic materials of vegetal or animal origin, and also including materials resulting from their transformation. Biogas is a mixture resulting from the anaerobic fermentation of organic materials.
We may consider as main sources of biomass the following: forest; waste materials from forestry and sewage; skin and residues from agro-industrial activities; residues from agricultural plantation; sewage from animal wastes; urbane waste; energy farm. Waste-to-energy Utilization of waste for power generation should be treated as “Renewable” because it prevents the use of exhaustible fuels. Moreover, it reduces the need for waste landfills and related methane emissions. The incineration of biomass and organic waste is CO2 neutral, because the carbon dioxide that is released into the atmosphere practically offsets the CO2 absorbed by biomass during its growth.
As far as nuclear energy is concerned, the fact that the fission of one gram of U235 releases approximately 24 MWh or 1 MWday (MWd) of thermal energy makes it convenient to use the concept of combustion rate, also known as “burn-up”, which is expressed in MWdays per tonne of heavy metal (MWd/tHM). Over the past 30 years, burn-up has steadily increased: for light water reactors, the most common type in the western world, it has moved from 33,000 to around 65,000 MWd/tHM and is expected to increase further. The total thermal energy released by nuclear fuel is proportional to the burn-up it reaches at the end of its reactor life5. One fuel assembly containing typically 460 kg of uranium and reaching a burn-up of 65,000 MWd/tU would therefore release 65,000 MWd/tU x 0.46 tU x 24h/d = 717,600 MWh of thermal energy over its reactor life. The thermal efficiency of a nuclear power station is defined in exactly the same way as for any other thermal plant: it is the efficiency of the thermodynamic cycle by which the heat generated by the fuel is converted into steam through steam generators. The thermal efficiency of a conventional nuclear power station is around 33%.
Geothermal energy comes from the thermal earth inner activity, mainly where there is volcano activity. The deposits of heat may be exploited with almost constant power supply. Once steam reaches earth surface through wells, it is used to produce electricity, in some cases used for non-electric purposes (e.g. building heating), or saving energy otherwise produced through conventional methods. Inside geothermal plants steam supplies power to move the turbines producing electricity. Waste water derived from steam is then injected in deep wells in order to keep a constant pressure level and to avoid steam pollution. In some areas of the world, including Europe, geothermal energy plays a leading role. The type of use – heating or power generation – depends on the quantity and quality (level of temperature) of the geothermal source. In some regions, it has been produced commercially in the range of hundreds of MW for many decades [EU Blue Book on Geothermal Resources].
The efficiency of existing organic Rankine Cycle plants generally range from 10% to 15.5% for resources at 100°C to 160°C and is slightly higher (17%) for temperatures up to 190°C with a two-phase geothermal fluid [quote from EGEC / Geothernet]. Advanced cycles like the Kalina Cycle offer large potential but are not commercially available. Regarding the high density and the constant availability of the energy source – that is, for a renewable technology, only comparable with hydro – the focus is not on increasing efficiencies but at reducing costs. Just for heating purposes the use of heat pumps is very attractive, especially if the temperature of the geothermal source is not very high (low quality). Heat pumps require external energy input like electricity but are able to generate much more heat (at medium quality) than the quantity included in the fuel for generating this electricity. For domestic heating, even the upper ground or ambient air suffices as geothermal source.
In the case of Combined Heat and Power (CHP), or co-generation, part of the converted thermal energy is used for generating useful heat: either by utilising the low-temperature steam at the steam-turbine exit for district heating or branching off a certain amount of steam directly from the steam turbine i.e. for process heat. This reduces the electrical efficiency slightly (~14 % of extracted heat for district heating), but the input fuel energy is better used in total. The loss of electrical output results from the pressure difference by condensing steam at back pressure instead of vacuum conditions. For high temperature steam extraction, the loss is higher. For example, a 112 MW (electric) plant operating in a mode without heat extraction has an electrical efficiency of 36.3%. By producing 152 MW additional heat the overall efficiency increases to 84.9%.
The “overall efficiency” is higher than the electrical efficiency and results from adding the efficiency of the generated heat (= useful heat / energy of fuel supplied). The overall efficiency is therefore defined as: 11 Overall efficiency = (Electrical Power Output + Useful Heat Output) / Total Fuel Input. Comparing separated heat and power supply to CHP or two different CHP solutions on the basis of overall efficiencies is possible with the same amount of electricity and heat at uniform temperature levels6 . CHP applications provide potential for better fuel utilization especially if the volume of heat demand is high and relatively constant (in the summer period too), as in industry or in some northern regions of Europe. Examples for CHP power stations in Finland show highest figures for heat output and overall efficiency compared to others and in contrast to other countries, without any subsidies being provided.
Renewable energies are sources of energy that renew themselves constantly through natural processes and, seen on a human-time scale, will never run out. Renewable energies come from three primary sources: solar radiation; heat from inner earth; tidal power. These three sources can be used either directly or indirectly, in particular the form of biomass, wind, wave energy and ambient heat. Renewable energy sources (RES) can be converted into electricity, heat and also fuel.
Solar systems for electricity generation purposes are based on the concentration of sunlight. There are three different concentration solar power systems: parabolic trough systems; solar power tower; parabolic dish technology using a stirling motor Their efficiency values are the following: Parabolic trough 14 – 18% Power tower 14 – 19% Dish sterling 18 – 23% Ref: Figures agreed through peer review between EURELECTRIC and VGB experts Solar energy may also be used directly to produce electricity (photovoltaic effect) that involves photovoltaic cells and, sometimes, grouped on photovoltaic panels. Although it is difficult to generate a high output solar energy compared with fossil fuel or nuclear energy, solar energy is of major importance because it is a nonpolluting and renewable energy source. The efficiency value of photovoltaic cells is the ratio of the electrical energy produced by the cells to the incident solar radiant energy.
The most effective power conversion method is found in hydro turbines, the oldest and most widely used renewable energy source.
To calculate a generator's or power plant's efficiency as a percentage, divide the equivalent Btu content of a kWh of electricity (3,412 Btu) by the heat rate. If the heat rate is 10,000 BTU, the efficiency is 33%. The efficiency is 45% if the heat rate is 7,500 Btu.
For coal and oil-fired facilities, typical thermal efficiency is approximately 37%, while for combined-cycle gas-fired plants, it is 56 - 60% (LEV). Plants intended to attain maximum efficiency while working at capacity will be less efficient while operating outside of their design parameters (i.e. temperatures too low.)
The changes of efficiency of different power plant attributes mostly to different temperature , pressure, superheat steam and Reheat steam temperatures.
Nuclear Power Plant: 0.27%
Super Critcial Thermal Plant: 42%
Hydro Power Plant: 85-90%
Sub Critical Thermal plant: 35-38%
It is worthwhile to conduct an audit of your company's energy use to determine where and how much energy is being wasted. You may put measures in place to increase your company's energy efficiency and save costs once you've determined where energy is being wasted. Check your company energy tariff one last time. You might not be on the best rate if you've been using the same supplier for a time. It can be wise to compare commercial energy providers right away and switch to a more affordable offer.