Thermal Generation

Thermal Generation Efficiencies

Thermal generation relies on fossil fuels and renewable fuel sources like biomass, biogass, waste=to-energy and geothermal.  The following section indicates thermal efficiency ranges for converting primary energy into electricity based on standard market and state-of-the-art equipment.  The efficiency values reported do not include losses attributed to plant availability such as planned maintenance, unforced outages, and grid curtailments.

Oil to Power Efficiency

PowerPlant_CrudeOilThe conversion of crude oil and refined fuel oil into electricity is most prevalent in the Middle East, Japan and Russia with secondary capacity in the UK, US, and Canada.   Crude oil was previously shown to have a higher NCV when compared to fuel oil, but crude oil power plants are only found in the Middle East where the ambient temperature is significantly higher than normal, undermining realized efficiency.  The low end of the ranges for the other fuels below also correspond to Middle East power plant performance.

Steam turbine open cycle crude oil plant (Saudi Arabia)10 to 30%
Steam turbine open cycle crude oil plant35 to 41%
Steam turbine open cycle fuel oil plant38 to 44%

Coal to Power Efficiency

PowerGeneration_CoalThe appropriate coal to use in a coal-fired thermal power station is “steam coal.” Steam coal is a grade between bituminous coal and anthracite.  Steam coal was once widely used as the fuel for steam locomotives.  In practice, fuel sourcing will often blend lower cost bituminous coal with steam coal to save on fuel costs if environmental compliance allows.

There are four basic types of coal plants:

  • Conventional coal plants with pulverized coal firing are basic steam turbines.
  • Fluidised bed combustion have special firing systems, which are attractive for medium scale and low-quality coal.
  • High temp clean coal technologies which rely on increased steam and pressure parameters (600o C, 270 bar and more) as well as alternative plant materials.
  • Combined cycle plants employ newer combustion technologies that gasify the fuel in advanced.  Such Integrated Gasification Combined Cycyle (IGCC) power plants offer larger potential for higher efficiencies and can also accommodate a broader range fuel inputs (from natural gas to renewable fuels).  However, these plants are very complex to operate, which reduces availability and increases cost.

Steam turbine coal-fired power plant39 to 45%
Pulverized coal boilers with ultra-critical steam parameters41 to 47%
Atmospheric circulating fluidized bed combustion (CFBC)40 to 46%
Pressurized fluidised bed combustion (PFBC)40 to 46%
Coal fired IGCC43 to 48%

Natural Gas to Power Efficiency

PowerGeneration_GasA Combined Cycle Gas Turbine (CCGT) generates electricity more efficiently than in a simple Open Cycle Gas Turbine (OCGT).  CCGT technology includes the extra step of capturing the hot exhaust of the gas turbine to produce additional steam.  The captured steam may be fed into the original steam turbine or a separate turbine designed for unique temperature and pressure values.

Open cycle gas turbine (OCGT)34 to 40%
Combined cycle gas turbine (CCGT)54 to 60%

Biomass to Power Efficiency

PowerGeneration_BiomassThermal generation of biomass involves the combustion of organic materials, typically forest, crop, animal, and sewage waste.  Some biomass fuels, like wood, are also transformed through drying and reshaping to maximize NCV and to accommodate improved handling by coal transport equipment.  Biogas is a methane mixture resulting from anaerobic fermentation of organic materials, typically in landfills.  Finally, waste-to-energy fuel streams can include household, hospital and commercial waste products.  Proponents of biomass burning will contend, right or wrong, that the activity is CO2 neutral since any emissions during combustion are offset by CO2 absorption during the biomass growth stage.

Biomass and biogass steam plant32 to 38%
Biomass gassification combined cycle power plant34 to 40%
Waste-ro-electricity steam plant22 to 28%

Nuclear Power Efficiencies

PowerPlant_NuclearThe efficiency of a nuclear power plant relies on the same basic input-output ratio used for conventional thermal plants.  However, the nature of the fuel is materially different and relies on fission, instead of combustion.  By way of reference, one gram of U235 releases approximately 24 MWh or 1 MWday (MWd) of thermal energy.

The standard unit of energy output is MW days per tone of heavy metal (MWd/tHM).  Over the past three decades, the standard energy extraction per tone of heavy metal has increased: for light water reactors the increase has gone from 33,000 MWd/tHM to 65,000 MWd/tHM. Hence, if the nuclear fuel assembly had 430 kg of uranium, the total energy released and input into the power generation process in one day would be 65,000 MWd/tHM * 0.430 tU235 * 24 hours/day = 670,800 MWh.

Geothermal Power Efficiencies

PowerGeneration_GeothermalGeothermal energy is thermal energy generated and stored in the Earth. The geothermal energy used in modern power plants typically originates with volcanic activity.  However, the heat of the Earth’s crust is also driven by conductive heat from the Earth’s core (20%) and from the radioactive decay of minerals (80%). These different heat process support different types of geothermal heat processing and power generation.

The typical power plant exploits steam generated by volcanic activity.  Once the steam reaches the Earth’s surface, it is used to drive turbines and produce power.  Waste water derived from the steam is then injected in deep wells in order to keep a constant pressure level and to avoid steam pollution.  In some areas, geothermal energy plays a leading role.

New geothermal generation processes rely on much less heat and are viable in areas with no volcanic activity.  The use of heat pumps is typical when the the temperature of the geothermal source is not very high.  Heat pumps require external energy input, like electricity, to operate, but they are able to generate much more heat than the quantity in the primary fuel source.

Resource temp: 100 to 160 degrees C10-15%
Resource temp: 160 to 190 degrees C16%

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