In thermal power plants the steam is generated by burning fuels or from the heat released by nuclear fission or is extracted from underground geothermal reservoirs.
Fossil fuels such as coal, oil, natural gas
Fuels artificially prepared, such as hydrogen, alcohol and acetylene
Converted fuels, such as methane and biogas
Nuclear fuels
Geothermal steam
These fuels converting into electricity may also be grouped into solid, liquid and gaseous fuels, as follows:
Solid fuels: − Fuel wood 1 − Forest products 1 –
Coal: anthracite; bituminous coal; sub-bituminous coal; lignite (brown coal) –
Peat: peat is considered as a substance somehow between forest product and coal –
Carbon wastes Liquid fuels: These fuels result from refining crude oil: − The lighter products first to distillate are liquefied petroleum gases (LPG) − The following distillate products will give gasoline, petrol and gas-oil − The residue, which is not distillate, is fuel-oil.
There are also on the market some mixtures of gas-oil and thick fuel-oil which result in: − Diesel-oil − Burner-oil − Thin fuel-oil Other liquid fuels: − Alcohol (especially ethanol)
Gaseous fuels – Natural gas is a mixture of hydrocarbons, chiefly methane (CH4) − Liquefied petroleum gases (LPG), butane; propane − Manufactured gas: derived from the industrial petrochemical process –
Other fuel gases: hydrogen, acetylene, et al. Fossil fuels Fuel is an organic substance used for its energy content. The energy content of a fossil fuel, before any treatment or conversion, corresponds to primary energy.
A fuel is characterized, giving the common feature inherent to its heat energy generation, by the calorific value. Calorific value (GCV-Gross Calorific Value or NCV-Net Calorific Value) is the quantity of heat released by the complete combustion of a unit quantity of a fuel in a well-determined condition. Its calculation may, or not, take into account the vapour condensation of the water, determining the GCV or the NCV.
Generation Efficiency The electric power plant efficiency η is defined as the ratio between useful electricity output from the generating unit, in a specific time unit, and the energy value of the energy source supplied to the unit, within the same time.
The type of energy converted in a fuel-burning installation is variable. The output of the conversion process may either be electricity (power), heat or a mixture of both, which makes it difficult to define efficiency of the process (it is even more complex in a three-product system of electricity, heat and a high-quality syn-gas product, i.e. produced in gasification plants). Different energy conversion processes have different thermodynamic limitations. Therefore, the term “efficiency” should only be used for one process with one energy source and one energy product, specifically referring to the output, i.e. “electrical efficiency”. 6 In physics theory, η of a thermal electricity generation process is limited by the Carnot efficiency.
Carnot efficiency = (T source – T sink) / T source
Example of the Carnot efficiency: A heat engine supplied with steam at 543ºC (T source = 816K) And the choice of a sink in a river at 23ºC (T sink = 296K) In theory, the Carnot efficiency η = 64%.
In practice, the process efficiency is less than that ideal maximum, about 40%
Example: Energy value of fuel supplied in a time unit (available energy) 1 toe (tonne of oil equivalent) <> 11,628 kWh in NCV (implicit “heat equivalent of 1 kWh” = 86 grams of oil equivalent) Useful electricity output from the power station in a time unit (electricity supplied) 1 toe <> 4,505 kWh (implicit average heat consumption per 1 kWh = 222 grams of oil equivalent)
Thermoelectric power plant efficiency η = 4,505/11,628 = 38.7% (in NCV) (Or η = 86/222 = 38.7%)
Energy value of fuel – the denominator – represents its heat content, which is the product of the burned mass times the NCV or GCV. Electricity output – the numerator – represents the net power output of the power station in the same period. The difference between these two terms represents the losses.
Transformation sequence: Example At the fuel CV - Calorific value (= 100%)
The solid, liquid and gaseous fuels used in a thermal power plant are mainly hard coal, lignite, fuel-oils, gasoil and natural gas. Their values of GCV and NCV are on an average the following:
Heavy fuel-oil 42.6 MJ/kg (= 10,175 kcal/kg) 40.57 MJ/kg (= 9,690 kcal/kg)
Light fuel-oil 43.3 MJ/kg (= 10,342 kcal/kg) 41.2 MJ/kg (= 9,840 kcal/kg)
Burner-oil 44.1 MJ/kg (= 10,533 kcal/kg) 42.16 MJ/kg (= 10,070 kcal/kg)
Gas-oil 45.7 MJ/kg (= 10,915 kcal/kg) 43.75 MJ/kg (= 10,450 kcal/kg)
Natural gas 42.0 MJ/m3 (=10,032 kcal/ m3 ) 37.9 MJ/m3 (= 9,052 kcal/m3 )
Hard coal 35.4 MJ/kg (=8,448 kcal/kg) 34.1 MJ/kg (=8,145 kcal/kg)
Lignite 24.0 MJ/kg (=5,732 kcal/kg) 23.0 MJ/kg (=5,493 kcal/kg)
1 calorie = 4.1868 Joules
Processes like CHP (see page 10) have two different efficiencies. When added, these data together represent the utilization of the input energy fuel. This is called “fuel utilization” or “overall efficiency”. In electricity statistics, the share of fuels used for electricity generation is estimated based on conventional/design values of the two different efficiencies mentioned above.
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”
[28] 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.