In thermodynamics In science, thermodynamics is the study of energy conversion between heat and mechanical work, and subsequently the macroscopic variables such as temperature, volume and pressure. The first to give a concise definition of the subject was Scottish physicist William Thomson who in 1854 stated that:, the thermal efficiency () is a dimensionless In dimensional analysis, a dimensionless quantity is a quantity without a physical unit and is thus a pure number. Such a number is typically defined as a product or ratio of quantities that might have units individually, but these cancel out in the combination performance measure of a device that uses thermal energy In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of particles and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals. It includes the energy in all, such as an internal combustion engine The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable component of the engine, such as the, a boiler A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, a furnace A furnace is a device used for heating. The name derives from Latin fornax, oven. The earliest furnace was excavated at Balakot, a site of the Indus Valley Civilization, dating back to its mature phase . The furnace was most likely used for the manufacturing of ceramic objects, or a refrigerator A refrigerator is a cooling appliance comprising a thermally insulated compartment and a heat pump—chemical or mechanical means—to transfer heat from it to the external environment, cooling the contents to a temperature below ambient. Cooling is a popular food storage technique in developed countries and works by decreasing the reproduction for example. The input, , to the device is heat In physics and thermodynamics, heat is the process of energy transfer from one body or system to another due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body, or the heat-content of a fuel that is consumed. The desired output is mechanical work In thermodynamics, work performed by a system is the quantity of energy transferred by the system to another that is accounted for in a particular way; namely, by changes in the external generalized mechanical constraints on the system, , or heat, , or possibly both. Because the input heat normally has a real financial cost, a memorable, generic definition of thermal efficiency is[1]

From the first law of thermodynamics The first law of thermodynamics, an expression of the principle of conservation of energy, states that energy can be transformed , but cannot be created or destroyed, the energy output can't exceed the input, so

When expressed as a percentage, the thermal efficiency must be between 0% and 100%. Due to inefficiencies such as friction, heat loss, and other factors, thermal engines' efficiencies are typically much less than 100%. For example, a typical gasoline automobile engine operates at around 25% efficiency, and a large coal-fueled electrical generating plant peaks at about 46%. The largest diesel engine in the world The Wärtsilä RT-flex96C is a two-stroke turbocharged low-speed diesel engine manufactured by the Finnish manufacturer Wärtsilä. It is currently considered the largest reciprocating engine in the world, designed for large container ships, running on heavy fuel oil. It stands at five stories (13.5 metres ) high, is 27.3 m (90 ft) long, and peaks at 51.7%. In a combined cycle A combined cycle is characteristic of a power producing engine or plant that employs more than one thermodynamic cycle. Heat engines are only able to use a portion of the energy their fuel generates . The remaining heat (e.g., hot exhaust fumes) from combustion is generally wasted. Combining two or more thermodynamic cycles, such as the Brayton plant, thermal efficiencies are approaching 60%.[2]

There are two types of thermal efficiency- 1.Indicated thermal efficiency 2.Brake thermal efficiency

Contents

Heat engines

Heat engines transform thermal energy In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of particles and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals. It includes the energy in all, or heat, Qin into mechanical energy In physics, mechanical energy describes the sum of potential energy and kinetic energy present in the components of a mechanical system. Mechanical energy is the energy associated with the motion or position of an object, or work, Wout. They cannot do this task perfectly, so some of the input heat energy is not converted into work, but is dissipated as waste heat Machines converting energy contained in fuels to mechanical work or electric energy produce heat as a by-product Qout into the environment

The thermal efficiency of a heat engine A heat engine is a physical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle. Heat engines can be open to the atmospheric air or sealed and closed off to the outside is the percentage of heat energy that is transformed into work In thermodynamics, work performed by a system is the quantity of energy transferred by the system to another that is accounted for in a particular way; namely, by changes in the external generalized mechanical constraints on the system. Thermal efficiency is defined as

The efficiency of even the best heat engines is low; usually below 50% and often far below. So the energy lost to the environment by heat engines is a major waste of energy resources, although modern cogeneration Cogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat. It is one of the most common forms of energy recycling, combined cycle A combined cycle is characteristic of a power producing engine or plant that employs more than one thermodynamic cycle. Heat engines are only able to use a portion of the energy their fuel generates . The remaining heat (e.g., hot exhaust fumes) from combustion is generally wasted. Combining two or more thermodynamic cycles, such as the Brayton and energy recycling Energy recycling is the energy recovery process of utilizing energy that would normally be wasted, usually by converting it into electricity or thermal energy. Undertaken at manufacturing facilities, power plants, and large institutions such as hospitals and universities, it significantly increases efficiency, thereby reducing energy costs and schemes are beginning to use this heat for other purposes. Since a large fraction of the fuels produced worldwide go to powering heat engines, perhaps up to half of the useful energy produced worldwide is wasted in engine inefficiency. This inefficiency can be attributed to three causes. There is an overall theoretical limit to the efficiency of any heat engine due to temperature, called the Carnot efficiency. Second, specific types of engines have lower limits on their efficiency due to the inherent irreversibility In science, a process that is not reversible is called irreversible. This concept arises most frequently in thermodynamics, as applied to processes. Irreversibility is also used in economics to refer to investment or expenditures that involve large sunk costs of the engine cycle Every thermodynamic system exists in a particular state. When a system is taken through a series of different states and finally returned to its initial state, a thermodynamic cycle is said to have occurred. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine. The Carnot they use. Thirdly, the nonideal behavior of real engines, such as mechanical friction Friction is the force resisting the relative lateral motion of solid surfaces, fluid layers, or material elements in contact. Its colloquial opposite is slipperiness. Friction is usually subdivided into several varieties: and losses in the combustion Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame. Fuels of interest often include organic compounds in the gas, liquid or process causes further efficiency losses.

HCV and Gross CV or LCV, and Net CV

To complicate matters, there are at least two different definitions of Calorific Value in wide use, and which one is being used significantly affects any quoted efficiency. Not stating whether an efficiency is HCV or LCv renders such numbers very misleading.[3]

Carnot efficiency

The second law of thermodynamics The second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium; and that the entropy change dS of a system undergoing any infinitesimal reversible process is given by δq / puts a fundamental limit on the thermal efficiency of all heat engines. Surprisingly, even an ideal, frictionless engine can't convert anywhere near 100% of its input heat into work. The limiting factors are the temperature at which the heat enters the engine, , and the temperature of the environment into which the engine exhausts its waste heat, , measured in an absolute scale, such as the Kelvin The kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale referenced to absolute zero, the absence of all thermal energy. So by definition, the temperature of a substance at absolute zero is zero kelvin (0 K). The secondary reference point on the Kelvin or Rankine scale. From Carnot's theorem Carnot's theorem, also called Carnot's rule is a principle which sets a limit on the maximum amount of efficiency any possible engine can obtain, which thus solely depends on the difference between the hot and cold temperature reservoirs. Carnot's theorem states:, for any engine working between these two temperatures:[4]

This limiting value is called the Carnot cycle efficiency because it is the efficiency of an unattainable, ideal, reversible engine cycle called the Carnot cycle The Carnot cycle is a particular thermodynamic cycle proposed by Nicolas Léonard Sadi Carnot in 1824 and expanded by Benoit Paul Émile Clapeyron in the 1830s and 40s. It is the most efficient existing cycle capable of converting a given amount of thermal energy into work or, conversely, creating a temperature difference by doing a given amount. No device converting heat into mechanical energy, regardless of its construction, can exceed this efficiency.

Examples of are the temperature of hot steam entering the turbine of a steam power plant, or the temperature at which the fuel burns in an internal combustion engine The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable component of the engine, such as the. is usually the ambient temperature where the engine is located, or the temperature of a lake or river that waste heat is discharged into. For example, if an automobile engine burns gasoline at a temperature of and the ambient temperature is , then its maximum possible efficiency is:

It should be kept in mind that, due to the other causes detailed below, practical engines have efficiencies far below the Carnot limit; for example the average automobile engine is less than 35% efficient.

As Carnot's theorem only applies to heat engines, devices that convert the fuel's energy directly into work without burning it, such as fuel cells A fuel cell is an electrochemical cell that converts a source fuel into an electrical current. It generates electricity inside a cell through reactions between a fuel and an oxidant, triggered in the presence of an electrolyte. The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel, can exceed the Carnot efficiency.

It can be seen that since is fixed by the environment, the only way for a designer to increase the Carnot efficiency of an engine is to increase , the temperature at which the heat is added to the engine. This is a general principle that applies to all heat engines: the efficiency increases with operating temperature. For this reason the operating temperatures of engines have increased greatly over the long term, and new materials such as ceramics to enable engines to stand higher temperatures are an active area of research.

Engine cycle efficiency

The Carnot cycle is reversible and thus represents the upper limit on efficiency of an engine cycle. Practical engine cycles are irreversible and thus have inherently lower efficiency than the Carnot efficiency when operated between the same temperatures and . One of the factors determining efficiency is how heat is added to the working fluid in the cycle, and how it is removed. The Carnot cycle achieves maximum efficiency because all the heat is added to the working fluid at the maximum temperature , and removed at the minimum temperature . In contrast, in an internal combustion engine, the temperature of the fuel-air mixture in the cylinder is nowhere near its peak temperature as the fuel starts to burn, and only reaches the peak temperature as all the fuel is consumed, so the average temperature at which heat is added is lower, reducing efficiency.

The higher the compression ratio, the higher the temperature in the cylinder as the fuel burns and so the higher the efficiency. However the maximum compression ratio usable is limited by the need to prevent preignition (knocking), where the fuel ignites by compression before the spark plug fires. The specific heat ratio of the air-fuel mixture γ varies somewhat with the fuel, but is generally close to the air value of 1.4. This standard value is usually used in all the engine cycle equations below, and when this approximation is used the cycle is called an air-standard cycle.
The Diesel cycle is less efficient than the Otto cycle when using the same compression ratio. However, practical Diesel engines are 30% - 35% more efficient than gasoline engines.[5] This is because, since the fuel is not introduced to the combustion chamber until it required to ignite, the compression ratio is not limited by the need to avoid knocking, so higher ratios are used than in spark ignition engines.

Other inefficiencies

The above efficiency formulas are based on simple idealized mathematical models of engines, with no friction and working fluids that obey simple thermodynamic rules called the ideal gas law The Ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law. It can also be derived from kinetic theory, as was achieved. Real engines have many departures from ideal behavior that waste energy, reducing actual efficiencies far below the theoretical values given above. Examples are:

Another source of inefficiency is that engines must be optimized for other goals besides efficiency, such as low pollution Pollution is the introduction of contaminants into an environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat, or light. Pollutants, the elements of pollution, can be foreign substances or energies,. The requirements for vehicle engines are particularly stringent: they must be designed for low emissions, adequate acceleration, fast starting, light weight, low noise, etc. These require compromises in design (such as altered valve timing) that reduce efficiency. The average automobile engine is only about 35% efficient, and must also be kept idling at stoplights, wasting an additional 17% of the energy, resulting in an overall efficiency of 18%.[5] Large stationary electric generating plants The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday. His basic method is still used today: electricity is generated by the movement of a loop of wire, or disc of copper between the poles of a magnet have fewer of these competing requirements as well as more efficient Rankine cycles, so they are significantly more efficient than vehicle engines, around 50% Therefore, replacing internal combustion The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable component of the engine, such as the vehicles with electric vehicles An electric vehicle , also referred to as an electric drive vehicle, is a vehicle which uses one or more electric motors for propulsion. Depending on the type of vehicle, motion may be provided by wheels or propellers driven by rotary motors, or in the case of tracked vehicles, by linear motors. Electric vehicles can include electric cars,, which run on a battery An electrical battery is a combination of one or more electrochemical cells, used to convert stored chemical energy into electrical energy. Since the invention of the first Voltaic pile in 1800 by Alessandro Volta, the battery has become a common power source for many household and industrial applications. According to a 2005 estimate, the that is charged with electricity generated by burning fuel in a power plant, can greatly increase the thermal efficiency of energy use in transportation, thus decreasing the demand for fossil fuels.

Energy conversion

For an energy conversion device like a boiler or furnace, the thermal efficiency is

.

So, for a boiler that produces 210 kW (or 700,000 BTU/h) output for each 300 kW (or 1,000,000 BTU/h) heat-equivalent input, its thermal efficiency is 210/300 = 0.70, or 70%. This means that the 30% of the energy is lost to the environment.

An electric resistance heater has a thermal efficiency of at or very near 100%, so, for example, 1500W of heat are produced for 1500W of electrical input. When comparing heating units, such as a 100% efficient electric resistance heater to an 80% efficient natural gas-fueled furnace, an economic analysis Engineering economics, previously known as engineering economy, is a subset of economics for application to engineering projects. Engineers seek solutions to problems, and the economic viability of each potential solution is normally considered along with the technical aspects is needed to determine the most cost-effective choice.

Heat pumps and refrigerators

Heat pumps, refrigerators and air conditioners use work to move heat from a colder to a warmer place, so their function is the opposite of a heat engine. The work energy (Win) that is applied to them is converted into heat, and the sum of this energy and the heat energy that is moved from the cold reservoir (QC) is equal to the total heat energy added to the hot reservoir (QH)

Their efficiency is measured by a coefficient of performance (COP). Heat pumps are measured by the efficiency with which they add heat to the hot reservoir, COPheating; refrigerators and air conditioners by the efficiency with which they remove heat from the cold interior, COPcooling:

The reason for not using the term 'efficiency' is that the coefficient of performance can often be greater than 100%. Since these devices are moving heat, not creating it, the amount of heat they move can be greater than the input work. Therefore, heat pumps can be a more efficient way of heating than simply converting the input work into heat, as in an electric heater or furnace.

Since they are heat engines, these devices are also limited by Carnot's theorem. The limiting value of the Carnot 'efficiency' for these processes, with the equality theoretically achievable only with an ideal 'reversible' cycle, is:

The same device used between the same temperatures is more efficient when considered as a heat pump than when considered as a refrigerator:

This is because when heating, the work used to run the device is converted to heat and adds to the desired effect, whereas if the desired effect is cooling the heat resulting from the input work is just an unwanted byproduct.

Energy efficiency

The 'thermal efficiency' is sometimes called the energy efficiency. In the United States, in everyday usage the SEER is the more common measure of energy efficiency for cooling devices, as well as for heat pumps when in their heating mode. For energy-conversion heating devices their peak steady-state thermal efficiency is often stated, e.g., 'this furnace is 90% efficient', but a more detailed measure of seasonal energy effectiveness is the Annual Fuel Utilization Efficiency (AFUE).[6]

Energy Efficiency of heat exchangers

A counter flow heat exchanger is generally, effectively 100% efficient in transferring heat energy from one circuit to the other, albeit at a slight loss in temperature.

See also

References

  1. ^ Fundamentals of Engineering Thermodynamics, by Howell and Buckius, McGraw-Hill, New York, 1987
  2. ^ GE Power’s H Series Turbine
  3. ^ http://www.claverton-energy.com/the-difference-between-lcv-and-hcv-or-lower-and-higher-heating-value-or-net-and-gross-is-clearly-understood-by-all-energy-engineers-there-is-no-right-or-wrong-definition.html
  4. ^ a b c d e Holman, Jack P. (1980). Thermodynamics. New York: McGraw-Hill. pp. 217. ISBN 0-07-029625-1.
  5. ^ a b "Where does the energy go?". Advanced technologies and energy efficiency, Fuel Economy Guide. US Dept. of Energy. 2009. http://www.fueleconomy.gov/feg/atv.shtml. Retrieved 2009-12-02.
  6. ^ HVAC Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, USA, 2004

There are two types of thermal efficiency- 1.Indicated thermal efficiency 2.Brake thermal efficiency

Categories: Thermodynamics | Heating, ventilating, and air conditioning | Energy conversion | Mechanical engineering

 

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