Power Generation Efficiency

 

Introduction

This next several pages present knowledge on power generation efficiency.  The pages provide:

  • Basic engineering definitions of electrical production efficiency,
  • Fuel energy content for a wide variety of fuels, and
  • Power production efficiencies by technology

The most current power production efficiencies are provided based on state-of-the-art equipment that is most recently available on the market or expected to be available within 18 months.  Efficiency tables can be found using the links below:

A summary of the tables appears below:

Power_Generation_Efficiency

Basic Principles of Electrical Generation

Electrical energy generation involves the conversion of primary energy into electricity.

Generation TypeConversion Processes
Hydro Power PlantThe conversion of water's "mechanical" energy or movement into electrical energy
Nuclear PowerThe conversion of the nuclear energy released by nuclear fuel into electrical energy
Thermal Power PlantThe conversion of the chemical energy of fossil fuel into electrical energy
Solar EnergyThe conversion of sunlight radiation or "protons packages" into electrical energy
Fuel CellThe conversion of the chemical energy from an oxygenation controlled reaction directly into electrical energy

In practice, the process of generating electricity relies on several transformations since most forms of primary energy are not directly convertible into electricity.  For example, the primary energy in a thermal power station is converted first to high temperature steam, an intermediate heat source, then into mechanical energy in the steam turbines, which are physically connected with power generators where the electricity is produced.  Direct energy conversion, of course, would be more efficient since electrical power could be generated without intermediate equipment and energy losses.

Carnot Efficiency

In physics theory, \footnotesize{\eta} is the efficiency of a thermal electricity generation process.  Carnot efficiency is the maximum possible efficiency for a heat engine and is defined by the temperature (T) differential of heat entering and exiting the process:

(1)   \begin{equation*} \small{ \eta_{Carnot} = \frac{T_{Hot} - T_{Cold}}{T_{Hot}} } \end{equation*}

Carnot_heat_engine

Carnot engine diagram where an amount of heat QH flows from a  high temperature source TH or furnace through the fluid of the “working body” (or engine) and the remaining heat QC flows into the cold sink TC, thus forcing the engine to do mechanical work W via cycles of contractions and expansions.

In general, the larger the difference in temperature between the hot source and the cold sink, the larger the potential thermal efficiency of the cycle. The cold side of any heat engine is limited to being close to the ambient temperature of the environment, or roughly 300 Kelvin.  Most efforts to improve the thermodynamic efficiencies of heat engines focus on increasing the temperature of the source, given material limits.

For example:

Input steam temperature
543 degrees C

Sink temperature in a river
23 degrees C

Carnot efficiency
\footnotesize{\eta} = (54323) / 543 = 64%

The actual efficiency of various heat engines used today has a large range:

  • 10 to 15 percent for a geothermal power plant
  • 25 percent for most automotive gasoline engines
  • 49 percent for a supercritical coal-fired power station
  • 60 percent for a steam-cooled combined cycle gas turbine.

All these processes gain their efficiency (or lack thereof) from the temperature drop across them.  Significant energy may be used for auxiliary equipment, such as pumps, which effectively reduces efficiency.

Practical Efficiency Defined

Electric power plant efficiency, \footnotesize{\eta}, is defined as the ratio of useful electricity output from the generating unit, in a specific time unit, and the energy value of the  fuel source supplied to the plant, within the same time.

For example:

Energy value of fuel in one time unit
1 tone of oil equivalent
11,628 kWh in net calorie value

Useful electricity output in one time unit
4,505 kWh

Thermoelectric power plant efficiency
\footnotesize{\eta} = 4,505 / 11,626 = 38.7%

Power plant energy loss
11,628 kWh – 4,505 kWh = 7,123 kWh

The basic definition is simple enough.  The denominator is the heat contact, which is the product of the burned mass over some period of time multiplied by the fuel’s net calorie value.  Electricity output, the numerator, is the net power output of the plant over the same time period.  The difference between the two terms represents power plant losses.

Sequential Conversion Efficiencies

Practical efficiency compares grid injected power to the NCV of the input fuel.  Sequential efficiencies track losses for each energy transformation.

Process StageAvg. LossDescriptionNet Efficiency
Primary Fuel0%Initial condition100%
Steam Generator11%Mass, water vapor and ash losses (if any) in combustion; Heat losses steam cooling89%
Mechanical Turbine48%Heat loss in the condenser (cold source); friction (ball bearings), and ancillary equipment (pumps)41%
Power Generator1%Minor electrical losses40%
Balance of Plant2%Primary electrical losses (transformer, substation, cabling) 38%

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