Carnot's theorem (thermodynamics)

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https://en.wikipedia.org/wiki/Carnot%27s_theorem_(thermodynamics) 

 

Carnot's theorem, developed in 1824 by Nicolas Léonard Sadi Carnot, also called Carnot's rule, is a principle that specifies limits on the maximum efficiency any heat engine can obtain. The efficiency of a Carnot engine depends solely on the temperatures of the hot and cold reservoirs.

Carnot's theorem states that all heat engines between two heat reservoirs are less efficient than a Carnot heat engine operating between the same reservoirs. Every Carnot heat engine between a pair of heat reservoirs is equally efficient, regardless of the working substance employed or the operation details.

The maximum efficiency is the ratio of the temperature difference between the reservoirs and the temperature of the hot reservoir, expressed in the equation ${\displaystyle \eta _{\text{max}}=1-{\frac {T_{\mathrm {C} }}{T_{\mathrm {H} }}}}$, where T_C and T_H are the absolute temperatures of the cold and hot reservoirs, respectively, and the efficiency ${\displaystyle \eta }$ is the ratio of the work done by the engine to the heat drawn out of the hot reservoir.

Carnot's theorem is a consequence of the second law of thermodynamics. Historically, it was based on contemporary caloric theory, and preceded the establishment of the second law.

Proof

An impossible situation: A heat engine cannot drive a less efficient (reversible) heat engine without violating the second law of thermodynamics.

The proof of the Carnot theorem is a proof by contradiction, or reductio ad absurdum, as illustrated by the figure showing two heat engines operating between two reservoirs of different temperature. The heat engine with more efficiency (${\displaystyle \eta _{M}}$) is driving a heat engine with less efficiency (${\displaystyle \eta _{L}}$), causing the latter to act as a heat pump. This pair of engines receives no outside energy, and operates solely on the energy released when heat is transferred from the hot and into the cold reservoir. However, if ${\displaystyle \eta _{M}>\eta _{L}}$, then the net heat flow would be backwards, i.e., into the hot reservoir:

${\displaystyle Q_{\text{hot}}^{\text{out}}=Q<{\frac {\eta _{M}}{\eta _{L}}}Q=Q_{\text{hot}}^{\text{in}}.}$

It is generally agreed that this is impossible because it violates the second law of thermodynamics.

We begin by verifying the values of work and heat flow depicted in the figure. First, we must point out an important caveat: the engine with less efficiency (${\displaystyle \eta _{L}}$) is being driven as a heat pump, and therefore must be a reversible engine. If the less efficient engine (${\displaystyle \eta _{L}}$) is not reversible, then the device could be built, but the expressions for work and heat flow shown in the figure would not be valid.

By restricting our discussion to cases where engine (${\displaystyle \eta _{L}}$) has less efficiency than engine (${\displaystyle \eta _{M}}$), we are able to simplify notation by adopting the convention that all symbols, ${\displaystyle Q}$ and ${\displaystyle W}$ represent non-negative quantities (since the direction of energy flow never changes sign in all cases where ${\displaystyle \eta _{L}\leqslant \eta _{M}}$). Conservation of energy demands that for each engine, the energy which enters, ${\displaystyle E_{in}}$, must equal the energy which exits, ${\displaystyle E_{out}}$:

${\displaystyle E_{\text{in}}^{M}=Q=(1-\eta _{M})Q+\eta _{M}Q=E_{\text{out}}^{M},}$

${\displaystyle E_{\text{in}}^{L}=\eta _{M}Q+\eta _{M}Q\left({\frac {1}{\eta _{L}}}-1\right)={\frac {\eta _{M}}{\eta _{L}}}Q=E_{\text{out}}^{L},}$

The figure is also consistent with the definition of efficiency as ${\displaystyle \eta =W/Q_{h}}$ for both engines:

${\displaystyle \eta _{M}={\frac {W_{M}}{Q_{h}^{M}}}={\frac {\eta _{M}Q}{Q}}=\eta _{M},}$

${\displaystyle \eta _{L}={\frac {W_{L}}{Q_{h}^{L}}}={\frac {\eta _{M}Q}{{\frac {\eta _{M}}{\eta _{L}}}Q}}=\eta _{L}.}$

It may seem odd that a hypothetical heat pump with low efficiency is being used to violate the second law of thermodynamics, but the figure of merit for refrigerator units is not efficiency, ${\displaystyle W/Q_{h}}$, but the coefficient of performance (COP), which is ${\displaystyle Q_{c}/W}$. A reversible heat engine with low thermodynamic efficiency, ${\displaystyle W/Q_{h}}$ delivers more heat to the hot reservoir for a given amount of work when it is being driven as a heat pump.

Having established that the heat flow values shown in the figure are correct, Carnot's theorem may be proven for irreversible and the reversible heat engines.

Reversible engines

To see that every reversible engine operating between reservoirs ${\displaystyle T_{1}}$ and ${\displaystyle T_{2}}$ must have the same efficiency, assume that two reversible heat engines have different values of ${\displaystyle \eta }$, and let the more efficient engine (M) drive the less efficient engine (L) as a heat pump. As the figure shows, this will cause heat to flow from the cold to the hot reservoir without any external work or energy, which violates the second law of thermodynamics. Therefore both (reversible) heat engines have the same efficiency, and we conclude that:

All reversible engines that operate between the same two heat reservoirs have the same efficiency.

This is an important result because it helps establish the Clausius theorem, which implies that the change in entropy is unique for all reversible processes.,

${\displaystyle \Delta S=\int _{a}^{b}{\frac {dQ_{\text{rev}}}{T}},}$

over all paths (from a to b in V-T space). If this integral were not path independent, then entropy, S, would lose its status as a state variable.

Irreversible engines

If one of the engines is irreversible, it must be the (M) engine, placed so that it reverse drives the less efficient but reversible (L) engine. But if this irreversible engine is more efficient than the reversible engine, (i.e., if ${\displaystyle \eta _{M}>\eta _{L}}$), then the second law of thermodynamics is violated. And, since the Carnot cycle represents a reversible engine, we have the first part of Carnot's theorem:

No irreversible engine is more efficient than the Carnot engine operating between the same two reservoirs.

Definition of thermodynamic temperature

The efficiency of the engine is the work divided by the heat introduced to the system or

${\displaystyle \eta ={\frac {w_{cy}}{q_{H}}}={\frac {q_{H}-q_{C}}{q_{H}}}=1-{\frac {q_{C}}{q_{H}}}}$

(1)

where w_cy is the work done per cycle. Thus, the efficiency depends only on q_C/q_H. 

Because all reversible engines operating between the same heat reservoirs are equally efficient, all reversible heat engines operating between temperatures T₁ and T₂ must have the same efficiency, meaning the efficiency is a function only of the two temperatures:

${\displaystyle {\frac {q_{C}}{q_{H}}}=f(T_{H},T_{C})}$

(2)

In addition, a reversible heat engine operating between temperatures T₁ and T₃ must have the same efficiency as one consisting of two cycles, one between T₁ and another (intermediate) temperature T₂, and the second between T₂ and T₃. This can only be the case if

${\displaystyle f(T_{1},T_{3})={\frac {q_{3}}{q_{1}}}={\frac {q_{2}q_{3}}{q_{1}q_{2}}}=f(T_{1},T_{2})f(T_{2},T_{3}).}$

Specializing to the case that ${\displaystyle T_{1}}$ is a fixed reference temperature: the temperature of the triple point of water. Then for any T₂ and T₃,

${\displaystyle f(T_{2},T_{3})={\frac {f(T_{1},T_{3})}{f(T_{1},T_{2})}}={\frac {273.16\cdot f(T_{1},T_{3})}{273.16\cdot f(T_{1},T_{2})}}.}$

Therefore, if thermodynamic temperature is defined by

${\displaystyle T=273.16\cdot f(T_{1},T)\,}$

then the function viewed as a function of thermodynamic temperature, is

${\displaystyle f(T_{2},T_{3})={\frac {T_{3}}{T_{2}}},}$

and the reference temperature T₁ has the value 273.16. (Of course any reference temperature and any positive numerical value could be used—the choice here corresponds to the Kelvin scale.) 

It follows immediately that

${\displaystyle {\frac {q_{C}}{q_{H}}}=f(T_{H},T_{C})={\frac {T_{C}}{T_{H}}}}$

(3)

Substituting Equation 3 back into Equation 1 gives a relationship for the efficiency in terms of temperature:

${\displaystyle \eta =1-{\frac {q_{C}}{q_{H}}}=1-{\frac {T_{C}}{T_{H}}}}$

(4)

Applicability to fuel cells and batteries

Since fuel cells and batteries can generate useful power when all components of the system are at the same temperature (${\displaystyle T=T_{H}=T_{C}}$), they are clearly not limited by Carnot's theorem, which states that no power can be generated when ${\displaystyle T_{H}=T_{C}}$. This is because Carnot's theorem applies to engines converting thermal energy to work, whereas fuel cells and batteries instead convert chemical energy to work. Nevertheless, the second law of thermodynamics still provides restrictions on fuel cell and battery energy conversion.

Cogeneration

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Cogeneration 

 

Diagram comparing losses from conventional generation vs. cogeneration

Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Trigeneration or combined cooling, heat and power (CCHP) refers to the simultaneous generation of electricity and useful heating and cooling from the combustion of a fuel or a solar heat collector. The terms cogeneration and trigeneration can be also applied to the power systems generating simultaneously electricity, heat, and industrial chemicals – e.g., syngas or pure hydrogen (article: combined cycles, chapter: natural gas integrated power & syngas (hydrogen) generation cycle). 

Cogeneration is a more efficient use of fuel because otherwise-wasted heat from electricity generation is put to some productive use. Combined heat and power (CHP) plants recover otherwise wasted thermal energy for heating. This is also called combined heat and power district heating. Small CHP plants are an example of decentralized energy. By-product heat at moderate temperatures (100–180 °C, 212–356 °F) can also be used in absorption refrigerators for cooling.

The supply of high-temperature heat first drives a gas or steam turbine-powered generator. The resulting low-temperature waste heat is then used for water or space heating. At smaller scales (typically below 1 MW) a gas engine or diesel engine may be used. Trigeneration differs from cogeneration in that the waste heat is used for both heating and cooling, typically in an absorption refrigerator. Combined cooling, heat and power systems can attain higher overall efficiencies than cogeneration or traditional power plants. In the United States, the application of trigeneration in buildings is called building cooling, heating and power. Heating and cooling output may operate concurrently or alternately depending on need and system construction.

Cogeneration was practiced in some of the earliest installations of electrical generation. Before central stations distributed power, industries generating their own power used exhaust steam for process heating. Large office and apartment buildings, hotels and stores commonly generated their own power and used waste steam for building heat. Due to the high cost of early purchased power, these CHP operations continued for many years after utility electricity became available.

Overview

Masnedø CHP power station in Denmark. This station burns straw as fuel. The adjacent greenhouses are heated by district heating from the plant.

 

Many process industries, such as chemical plants, oil refineries and pulp and paper mills, require large amounts of process heat for such operations as chemical reactors, distillation columns, steam driers and other uses. This heat, which is usually used in the form of steam, can be generated at the typically low pressures used in heating, or can be generated at much higher pressure and passed through a turbine first to generate electricity. In the turbine the steam pressure and temperature is lowered as the internal energy of the steam is converted to work. The lower pressure steam leaving the turbine can then be used for process heat.

Steam turbines at thermal power stations are normally designed to be fed high pressure steam, which exits the turbine at a condenser operating a few degrees above ambient temperature and at a few millimeters of mercury absolute pressure. (This is called a condensing turbine.) For all practical purposes this steam has negligible useful energy before it is condensed. Steam turbines for cogeneration are designed either for extraction of some steam at lower pressures after it has passed through a number of turbine stages, with the un-extracted steam going on through the turbine to a condenser. In this case, the extracted steam causes a mechanical power loss in the downstream stages of the turbine. Or they are designed, with or without extraction, for final exhaust at back pressure (non-condensing). The extracted or exhaust steam is used for process heating. Steam at ordinary process heating conditions still has a considerable amount of enthalpy that could be used for power generation, so cogeneration has an opportunity cost.

A typical power generation turbine in a paper mill may have extraction pressures of 160 psig (1.103 MPa) and 60 psig (0.41 MPa). A typical back pressure may be 60 psig (0.41 MPa). In practice these pressures are custom designed for each facility. Conversely, simply generating process steam for industrial purposes instead of high enough pressure to generate power at the top end also has an opportunity cost. The capital and operating cost of high pressure boilers, turbines and generators are substantial. This equipment is normally operated continuously, which usually limits self-generated power to large-scale operations.

A cogeneration plant in Metz, France. The 45MW boiler uses waste wood biomass as energy source, and provides electricity and heat for 30,000 dwellings.

A combined cycle (in which several thermodynamic cycles produce electricity), may also be used to extract heat using a heating system as condenser of the power plant's bottoming cycle. For example, the RU-25 MHD generator in Moscow heated a boiler for a conventional steam powerplant, whose condensate was then used for space heat. A more modern system might use a gas turbine powered by natural gas, whose exhaust powers a steam plant, whose condensate provides heat. Cogeneration plants based on a combined cycle power unit can have thermal efficiencies above 80%. 

The viability of CHP (sometimes termed utilisation factor), especially in smaller CHP installations, depends on a good baseload of operation, both in terms of an on-site (or near site) electrical demand and heat demand. In practice, an exact match between the heat and electricity needs rarely exists. A CHP plant can either meet the need for heat (heat driven operation) or be run as a power plant with some use of its waste heat, the latter being less advantageous in terms of its utilisation factor and thus its overall efficiency. The viability can be greatly increased where opportunities for trigeneration exist. In such cases, the heat from the CHP plant is also used as a primary energy source to deliver cooling by means of an absorption chiller.

CHP is most efficient when heat can be used on-site or very close to it. Overall efficiency is reduced when the heat must be transported over longer distances. This requires heavily insulated pipes, which are expensive and inefficient; whereas electricity can be transmitted along a comparatively simple wire, and over much longer distances for the same energy loss.

A car engine becomes a CHP plant in winter when the reject heat is useful for warming the interior of the vehicle. The example illustrates the point that deployment of CHP depends on heat uses in the vicinity of the heat engine.

Thermally enhanced oil recovery (TEOR) plants often produce a substantial amount of excess electricity. After generating electricity, these plants pump leftover steam into heavy oil wells so that the oil will flow more easily, increasing production. TEOR cogeneration plants in Kern County, California produce so much electricity that it cannot all be used locally and is transmitted to Los Angeles.

CHP is one of the most cost-efficient methods of reducing carbon emissions from heating systems in cold climates and is recognized to be the most energy efficient method of transforming energy from fossil fuels or biomass into electric power. Cogeneration plants are commonly found in district heating systems of cities, central heating systems of larger buildings (e.g. hospitals, hotels, prisons) and are commonly used in the industry in thermal production processes for process water, cooling, steam production or CO₂ fertilization.

Types of plants

Topping cycle plants primarily produce electricity from a steam turbine. Partly expanded steam is then condensed in a heating condensor at a temperature level that is suitable e.g. district heating or water desalination. 

Bottoming cycle plants produce high temperature heat for industrial processes, then a waste heat recovery boiler feeds an electrical plant. Bottoming cycle plants are only used in industrial processes that require very high temperatures such as furnaces for glass and metal manufacturing, so they are less common.

Large cogeneration systems provide heating water and power for an industrial site or an entire town. Common CHP plant types are:

• Gas turbine CHP plants using the waste heat in the flue gas of gas turbines. The fuel used is typically natural gas.
• Gas engine CHP plants use a reciprocating gas engine which is generally more competitive than a gas turbine up to about 5 MW. The gaseous fuel used is normally natural gas. These plants are generally manufactured as fully packaged units that can be installed within a plantroom or external plant compound with simple connections to the site's gas supply, electrical distribution network and heating systems.
• Biofuel engine CHP plants use an adapted reciprocating gas engine or diesel engine, depending upon which biofuel is being used, and are otherwise very similar in design to a Gas engine CHP plant. The advantage of using a biofuel is one of reduced hydrocarbon fuel consumption and thus reduced carbon emissions. These plants are generally manufactured as fully packaged units that can be installed within a plantroom or external plant compound with simple connections to the site's electrical distribution and heating systems. Another variant is the wood gasifier CHP plant whereby a wood pellet or wood chip biofuel is gasified in a zero oxygen high temperature environment; the resulting gas is then used to power the gas engine. Typical smaller size biogas plant see 
• Combined cycle power plants adapted for CHP
• Molten-carbonate fuel cells and solid oxide fuel cells have a hot exhaust, very suitable for heating.
• Steam turbine CHP plants that use the heating system as the steam condenser for the steam turbine.
• Nuclear power plants, similar to other steam turbine power plants, can be fitted with extractions in the turbines to bleed partially expanded steam to a heating system. With a heating system temperature of 95 °C it is possible to extract about 10 MW heat for every MW electricity lost. With a temperature of 130 °C the gain is slightly smaller, about 7 MW for every MWe lost. A review of cogeneration options is in 

Smaller cogeneration units may use a reciprocating engine or Stirling engine. The heat is removed from the exhaust and radiator. The systems are popular in small sizes because small gas and diesel engines are less expensive than small gas- or oil-fired steam-electric plants.

Some cogeneration plants are fired by biomass, or industrial and municipal solid waste (see incineration). Some CHP plants utilize waste gas as the fuel for electricity and heat generation. Waste gases can be gas from animal waste, landfill gas, gas from coal mines, sewage gas, and combustible industrial waste gas.

Some cogeneration plants combine gas and solar photovoltaic generation to further improve technical and environmental performance. Such hybrid systems can be scaled down to the building level and even individual homes.

MicroCHP

Micro combined heat and power or 'Micro cogeneration" is a so-called distributed energy resource (DER). The installation is usually less than 5 kW_e in a house or small business. Instead of burning fuel to merely heat space or water, some of the energy is converted to electricity in addition to heat. This electricity can be used within the home or business or, if permitted by the grid management, sold back into the electric power grid.

Delta-ee consultants stated in 2013 that with 64% of global sales the fuel cell micro-combined heat and power passed the conventional systems in sales in 2012. 20.000 units were sold in Japan in 2012 overall within the Ene Farm project. With a Lifetime of around 60,000 hours. For PEM fuel cell units, which shut down at night, this equates to an estimated lifetime of between ten and fifteen years. For a price of $22,600 before installation. For 2013 a state subsidy for 50,000 units is in place.

MicroCHP installations use five different technologies: microturbines, internal combustion engines, stirling engines, closed cycle steam engines and fuel cells. One author indicated in 2008 that MicroCHP based on Stirling engines is the most cost-effective of the so-called microgeneration technologies in abating carbon emissions; A 2013 UK report from Ecuity Consulting stated that MCHP is the most cost-effective method of utilising gas to generate energy at the domestic level. However, advances in reciprocation engine technology are adding efficiency to CHP plant, particularly in the biogas field. As both MiniCHP and CHP have been shown to reduce emissions  they could play a large role in the field of CO₂ reduction from buildings, where more than 14% of emissions can be saved using CHP in buildings. The University of Cambridge reported a cost-effective steam engine MicroCHP prototype in 2017 which has the potential to be commercially competitive in the following decades. Quite recently, in some private homes, fuel cell micro-CHP plants can now be found, which can operate on hydrogen, or other fuels as natural gas or LPG. When running on natural gas, it relies on steam reforming of natural gas to convert the natural gas to hydrogen prior to use in the fuel cell. This hence still emits CO
2 (see reaction) but (temporarily) running on this can be a good solution until the point where the hydrogen is starting to be become distributed through the (natural gas) piping system.

Trigeneration

Trigeneration cycle

A plant producing electricity, heat and cold is called a trigeneration or polygeneration plant. Cogeneration systems linked to absorption chillers or adsorption chillers use waste heat for refrigeration.

Combined heat and power district heating

In the United States, Consolidated Edison distributes 66 billion kilograms of 350 °F (180 °C) steam each year through its seven cogeneration plants to 100,000 buildings in Manhattan—the biggest steam district in the United States. The peak delivery is 10 million pounds per hour (or approximately 2.5 GW).

Industrial CHP

Cogeneration is still common in pulp and paper mills, refineries and chemical plants. In this "industrial cogeneration/CHP", the heat is typically recovered at higher temperatures (above 100 deg C) and used for process steam or drying duties. This is more valuable and flexible than low-grade waste heat, but there is a slight loss of power generation. The increased focus on sustainability has made industrial CHP more attractive, as it substantially reduces carbon footprint compared to generating steam or burning fuel on-site and importing electric power from the grid.

Utility pressures versus self generated industrial

Industrial cogeneration plants normally operate at much lower boiler pressures than utilities. Among the reasons are: 1) Cogeneration plants face possible contamination of returned condensate. Because boiler feed water from cogeneration plants has much lower return rates than 100% condensing power plants, industries usually have to treat proportionately more boiler make up water. Boiler feed water must be completely oxygen free and de-mineralized, and the higher the pressure the more critical the level of purity of the feed water. 2) Utilities are typically larger scale power than industry, which helps offset the higher capital costs of high pressure. 3) Utilities are less likely to have sharp load swings than industrial operations, which deal with shutting down or starting up units that may represent a significant percent of either steam or power demand.

Heat recovery steam generators

A heat recovery steam generator (HRSG) is a steam boiler that uses hot exhaust gases from the gas turbines or reciprocating engines in a CHP plant to heat up water and generate steam. The steam, in turn, drives a steam turbine or is used in industrial processes that require heat.

HRSGs used in the CHP industry are distinguished from conventional steam generators by the following main features:

• The HRSG is designed based upon the specific features of the gas turbine or reciprocating engine that it will be coupled to.
• Since the exhaust gas temperature is relatively low, heat transmission is accomplished mainly through convection.
• The exhaust gas velocity is limited by the need to keep head losses down. Thus, the transmission coefficient is low, which calls for a large heating surface area.
• Since the temperature difference between the hot gases and the fluid to be heated (steam or water) is low, and with the heat transmission coefficient being low as well, the evaporator and economizer are designed with plate fin heat exchangers.

Cogeneration using biomass

Biomass is emerging as one of the most important sources of renewable energy. Biomass refers to any plant or animal matter in which it is possible to be reused as a source of heat or electricity, such as sugarcane, vegetable oils, wood, organic waste and residues from the food or agricultural industries. Brazil is now considered a world reference in terms of energy generation from biomass.

A growing sector in the use of biomass for power generation is the sugar and alcohol sector, which mainly uses sugarcane bagasse as fuel for thermal and electric power generation. 

Power cogeneration in the sugar and alcohol sector

In the sugarcane industry, cogeneration is fuelled by the bagasse residue of sugar refining, which is burned to produce steam. Some steam can be sent through a turbine that turns a generator, producing electric power.

Energy cogeneration in sugarcane industries located in Brazil is a practice that has been growing in last years. With the adoption of energy cogeneration in the sugar and alcohol sector, the sugarcane industries are able to supply the electric energy demand needed to operate, and generate a surplus that can be commercialized.

Advantages of the cogeneration using sugarcane bagasse

In comparison with the electric power generation by means of fossil fuel-based thermoelectric plants, such as natural gas, the energy generation using sugarcane bagasse has environmental advantages due to the reduction of CO2 emissions.

In addition to the environmental advantages, cogeneration using sugarcane bagasse presents advantages in terms of efficiency comparing to thermoelectric generation, through the final destination of the energy produced. While in thermoelectric generation, part of the heat produced is lost, in cogeneration this heat has the possibility of being used in the production processes, increasing the overall efficiency of the process.

Disadvantages of the cogeneration using sugarcane bagasse

In sugarcane cultivation, is usually used potassium source's containing high concentration of chlorine, such as potassium chloride (KCl). Considering that KCl is applied in huge quantities, sugarcane ends up absorbing high concentrations of chlorine.

Due to this absorption, when the sugarcane bagasse is burned in the power cogeneration, dioxins and methyl chloride ends up being emitted. In the case of dioxins, these substances are considered very toxic and cancerous.

In the case of methyl chloride, when this substance is emitted and reaches the stratosphere, it ends up being very harmful for the ozone layer, since chlorine when combined with the ozone molecule generates a catalytic reaction leading to the breakdown of ozone links.

After each reaction, chlorine starts a destructive cycle with another ozone molecule. In this way, a single chlorine atom can destroy thousands of ozone molecules. As these molecules are being broken, they are unable to absorb the ultraviolet rays. As a result, the UV radiation is more intense on Earth and there is a worsening of global warming.

Comparison with a heat pump

A heat pump may be compared with a CHP unit as follows. If, to supply thermal energy, the exhaust steam from the turbo-generator must be taken at a higher temperature than the system would produce most electricity at, the lost electrical generation is as if a heat pump were used to provide the same heat by taking electrical power from the generator running at lower output temperature and higher efficiency. Typically for every unit of electrical power lost, then about 6 units of heat are made available at about 90 °C. Thus CHP has an effective Coefficient of Performance (COP) compared to a heat pump of 6. However, for a remotely operated heat pump, losses in the electrical distribution network would need to be considered, of the order of 6%. Because the losses are proportional to the square of the current, during peak periods losses are much higher than this and it is likely that widespread (i.e. citywide application of heat pumps) would cause overloading of the distribution and transmission grids unless they were substantially reinforced.

It is also possible to run a heat driven operation combined with a heat pump, where the excess electricity (as heat demand is the defining factor on utilization) is used to drive a heat pump. As heat demand increases, more electricity is generated to drive the heat pump, with the waste heat also heating the heating fluid.

Distributed generation

Most industrial countries generate the majority of their electrical power needs in large centralized facilities with capacity for large electrical power output. These plants benefit from economy of scale, but may need to transmit electricity across long distances causing transmission losses. Cogeneration or trigeneration production is subject to limitations in the local demand, and thus may sometimes need to reduce e.g. heat or cooling production to match the demand. An example of cogeneration with trigeneration applications in a major city is the New York City steam system.

Thermal efficiency

Every heat engine is subject to the theoretical efficiency limits of the Carnot cycle or subset Rankine cycle in the case of steam turbine power plants or Brayton cycle in gas turbine with steam turbine plants. Most of the efficiency loss with steam power generation is associated with the latent heat of vaporization of steam that is not recovered when a turbine exhausts its low temperature and pressure steam to a condenser. (Typical steam to condenser would be at a few millimeters absolute pressure and on the order of 5 °C/11 °F hotter than the cooling water temperature, depending on the condenser capacity.) In cogeneration this steam exits the turbine at a higher temperature where it may be used for process heat, building heat or cooling with an absorption chiller. The majority of this heat is from the latent heat of vaporization when the steam condenses.

Thermal efficiency in a cogeneration system is defined as:

${\displaystyle \eta _{th}\equiv {\frac {W_{out}}{Q_{in}}}\equiv {\frac {\text{Electrical power output + Heat output}}{\text{Total heat input}}}}$

Where:

${\displaystyle \eta _{th}}$ = Thermal efficiency

${\displaystyle W_{out}}$ = Total work output by all systems

${\displaystyle Q_{in}}$ = Total heat input into the system

Heat output may be used also for cooling (for example in Summer), thanks to an absorption chiller. If cooling is achieved in the same time, Thermal efficiency in a trigeneration system is defined as: 

${\displaystyle \eta _{th}\equiv {\frac {W_{out}}{Q_{in}}}\equiv {\frac {\text{Electrical power output + Heat output + Cooling output}}{\text{Total heat input}}}}$

Where:

${\displaystyle \eta _{th}}$ = Thermal efficiency

${\displaystyle W_{out}}$ = Total work output by all systems

${\displaystyle Q_{in}}$ = Total heat input into the system

Typical cogeneration models have losses as in any system. The energy distribution below is represented as a percent of total input energy:

Electricity = 45%

Heat + Cooling = 40%

Heat losses = 13%

Electrical line losses = 2%

Conventional central coal- or nuclear-powered power stations convert about 33-45% of their input heat to electricity. Brayton cycle power plants operate at up to 60% efficiency. In the case of conventional power plants approximately 10-15% of this heat is lost up the stack of the boiler, most of the remaining heat emerges from the turbines as low-grade waste heat with no significant local uses so it is usually rejected to the environment, typically to cooling water passing through a condenser. Because turbine exhaust is normally just above ambient temperature, some potential power generation is sacrificed in rejecting higher temperature steam from the turbine for cogeneration purposes.

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Costs

Typically, for a gas-fired plant the fully installed cost per kW electrical is around £400/kW (US$577), which is comparable with large central power stations.

History

Cogeneration in Europe

A cogeneration thermal power plant in Ferrera Erbognone (PV), Italy

 

The EU has actively incorporated cogeneration into its energy policy via the CHP Directive. In September 2008 at a hearing of the European Parliament's Urban Lodgment Intergroup, Energy Commissioner Andris Piebalgs is quoted as saying, “security of supply really starts with energy efficiency.” Energy efficiency and cogeneration are recognized in the opening paragraphs of the European Union's Cogeneration Directive 2004/08/EC. This directive intends to support cogeneration and establish a method for calculating cogeneration abilities per country. The development of cogeneration has been very uneven over the years and has been dominated throughout the last decades by national circumstances.

The European Union generates 11% of its electricity using cogeneration. However, there is large difference between Member States with variations of the energy savings between 2% and 60%. Europe has the three countries with the world's most intensive cogeneration economies: Denmark, the Netherlands and Finland. Of the 28.46 TWh of electrical power generated by conventional thermal power plants in Finland in 2012, 81.80% was cogeneration.

Other European countries are also making great efforts to increase efficiency. Germany reported that at present, over 50% of the country's total electricity demand could be provided through cogeneration. So far, Germany has set the target to double its electricity cogeneration from 12.5% of the country's electricity to 25% of the country's electricity by 2020 and has passed supporting legislation accordingly. The UK is also actively supporting combined heat and power. In light of UK's goal to achieve a 60% reduction in carbon dioxide emissions by 2050, the government has set the target to source at least 15% of its government electricity use from CHP by 2010. Other UK measures to encourage CHP growth are financial incentives, grant support, a greater regulatory framework, and government leadership and partnership.

According to the IEA 2008 modeling of cogeneration expansion for the G8 countries, the expansion of cogeneration in France, Germany, Italy and the UK alone would effectively double the existing primary fuel savings by 2030. This would increase Europe's savings from today's 155.69 Twh to 465 Twh in 2030. It would also result in a 16% to 29% increase in each country's total cogenerated electricity by 2030.

Governments are being assisted in their CHP endeavors by organizations like COGEN Europe who serve as an information hub for the most recent updates within Europe's energy policy. COGEN is Europe's umbrella organization representing the interests of the cogeneration industry.

The European public–private partnership Fuel Cells and Hydrogen Joint Undertaking Seventh Framework Programme project ene.field deploys in 2017 up 1,000 residential fuel cell Combined Heat and Power (micro-CHP) installations in 12 states. Per 2012 the first 2 installations have taken place.

Cogeneration in the United Kingdom

In the United Kingdom, the Combined Heat and Power Quality Assurance scheme regulates the combined production of heat and power. It was introduced in 1996. It defines, through calculation of inputs and outputs, "Good Quality CHP" in terms of the achievement of primary energy savings against conventional separate generation of heat and electricity. Compliance with Combined Heat and Power Quality Assurance is required for cogeneration installations to be eligible for government subsidies and tax incentives.

Cogeneration in the United States

The 250 MW Kendall Cogeneration Station plant in Cambridge, Massachusetts

Perhaps the first modern use of energy recycling was done by Thomas Edison. His 1882 Pearl Street Station, the world's first commercial power plant, was a combined heat and power plant, producing both electricity and thermal energy while using waste heat to warm neighboring buildings. Recycling allowed Edison's plant to achieve approximately 50 percent efficiency.

By the early 1900s, regulations emerged to promote rural electrification through the construction of centralized plants managed by regional utilities. These regulations not only promoted electrification throughout the countryside, but they also discouraged decentralized power generation, such as cogeneration.

By 1978, Congress recognized that efficiency at central power plants had stagnated and sought to encourage improved efficiency with the Public Utility Regulatory Policies Act (PURPA), which encouraged utilities to buy power from other energy producers.

Cogeneration plants proliferated, soon producing about 8% of all energy in the United States. However, the bill left implementation and enforcement up to individual states, resulting in little or nothing being done in many parts of the country.

The United States Department of Energy has an aggressive goal of having CHP constitute 20% of generation capacity by the year 2030. Eight Clean Energy Application Centers have been established across the nation whose mission is to develop the required technology application knowledge and educational infrastructure necessary to lead "clean energy" (combined heat and power, waste heat recovery and district energy) technologies as viable energy options and reduce any perceived risks associated with their implementation. The focus of the Application Centers is to provide an outreach and technology deployment program for end users, policy makers, utilities, and industry stakeholders.

High electric rates in New England and the Middle Atlantic make these areas of the United States the most beneficial for cogeneration.

Energy industry

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Energy_industry 

 

The energy industry is the totality of all of the industries involved in the production and sale of energy, including fuel extraction, manufacturing, refining and distribution. Modern society consumes large amounts of fuel, and the energy industry is a crucial part of the infrastructure and maintenance of society in almost all countries.

In particular, the energy industry comprises:

• the fossil fuel industries, which include petroleum industries ( oil companies, petroleum refiners, fuel transport and end-user sales at gas stations) coal industries (extraction and processing) and the natural gas industries (natural gas extraction, and coal gas manufacture, as well as distribution and sales);
• the electrical power industry, including electricity generation, electric power distribution and sales;
• the nuclear power industry;
• the renewable energy industry, comprising alternative energy and sustainable energy companies, including those involved in hydroelectric power, wind power, and solar power generation, and the manufacture, distribution and sale of alternative fuels; and,
• traditional energy industry based on the collection and distribution of firewood, the use of which, for cooking and heating, is particularly common in poorer countries.

The increased dependence during the 20th century on non-renewable fossil fuel and nuclear power, means that the energy industry has frequently been an important contributor to pollution and environmental impacts of the economy. Until recently, fossil fuels were the main source of energy generation in most parts of the world, and are a major contributor to global warming and pollution. As part of human adaptation to global warming, many economies are investing in renewable and sustainable energy.

Energy consumption in kilograms of oil equivalent (kgoe) per person per year per country (2001 data). Darker tones indicate larger consumption, while dark grey areas are missing from the dataset. Red hue indicates increasing consumption, green hue indicates decreasing consumption, in the time between 1990 and 2001.

History

The use of energy has been a key in the development of the human society by helping it to control and adapt to the environment. Managing the use of energy is inevitable in any functional society. In the industrialized world the development of energy resources has become essential for agriculture, transportation, waste collection, information technology, communications that have become prerequisites of a developed society. The increasing use of energy since the Industrial Revolution has also brought with it a number of serious problems, some of which, such as global warming, present potentially grave risks to the world.

In some industries, the word energy is used as a synonym of energy resources, which refer to substances like fuels, petroleum products and electricity in general, because a significant portion of the energy contained in these resources can easily be extracted to serve a useful purpose. After a useful process has taken place, the total energy is conserved, but the resource itself is not conserved, since a process usually transforms the energy into unusable forms (such as unnecessary or excess heat).

Ever since humanity discovered various energy resources available in nature, it has been inventing devices, known as machines, that make life more comfortable by using energy resources. Thus, although the primitive man knew the utility of fire to cook food, the invention of devices like gas burners and microwave ovens has increased the usage of energy for this purpose alone manyfold. The trend is the same in any other field of social activity, be it construction of social infrastructure, manufacturing of fabrics for covering; porting; printing; decorating, for example textiles, air conditioning; communication of information or for moving people and goods (automobiles).

Economics

Production and consumption of energy resources is very important to the global economy. All economic activity requires energy resources, whether to manufacture goods, provide transportation, run computers and other machines.

Widespread demand for energy may encourage competing energy utilities and the formation of retail energy markets. Note the presence of the "Energy Marketing and Customer Service" (EMACS) sub-sector.

The energy sector accounts for 4.6% of outstanding leveraged loans, compared with 3.1% a decade ago, while energy bonds make up 15.7% of the $1.3 trillion junk bond market, up from 4.3% over the same period.

Management

Since the cost of energy has become a significant factor in the performance of economy of societies, management of energy resources has become very crucial. Energy management involves utilizing the available energy resources more effectively; that is, with minimum incremental costs. Many times it is possible to save expenditure on energy without incorporating fresh technology by simple management techniques. Most often energy management is the practice of using energy more efficiently by eliminating energy wastage or to balance justifiable energy demand with appropriate energy supply. The process couples energy awareness with energy conservation.

Classifications

Government

The United Nations developed the International Standard Industrial Classification, which is a list of economic and social classifications. There is no distinct classification for an energy industry, because the classification system is based on activities, products, and expenditures according to purpose.

Countries in North America use the North American Industry Classification System (NAICS). The NAICS sectors #21 and #22 (mining and utilities) might roughly define the energy industry in North America. This classification is used by the U.S. Securities and Exchange Commission.

Financial market

The Global Industry Classification Standard used by Morgan Stanley define the energy industry as comprising companies primarily working with oil, gas, coal and consumable fuels, excluding companies working with certain industrial gases.

Environmental impact

Government encouragement in the form of subsidies and tax incentives for energy-conservation efforts has increasingly fostered the view of conservation as a major function of the energy industry: saving an amount of energy provides economic benefits almost identical to generating that same amount of energy. This is compounded by the fact that the economics of delivering energy tend to be priced for capacity as opposed to average usage. One of the purposes of a smart grid infrastructure is to smooth out demand so that capacity and demand curves align more closely. Some parts of the energy industry generate considerable pollution, including toxic and greenhouse gases from fuel combustion, nuclear waste from the generation of nuclear power, and oil spillages as a result of petroleum extraction. Government regulations to internalize these externalities form an increasing part of doing business, and the trading of carbon credits and pollution credits on the free market may also result in energy-saving and pollution-control measures becoming even more important to energy providers.

Consumption of energy resources, (e.g. turning on a light) requires resources and has an effect on the environment. Many electric power plants burn coal, oil or natural gas in order to generate electricity for energy needs. While burning these fossil fuels produces a readily available and instantaneous supply of electricity, it also generates air pollutants including carbon dioxide (CO₂), sulfur dioxide and trioxide (SOx) and nitrogen oxides (NOx). Carbon dioxide is an important greenhouse gas which is thought to be responsible for some fraction of the rapid increase in climate change seen especially in the temperature records in the 20th century, as compared with tens of thousands of years worth of temperature records which can be read from ice cores taken in Arctic regions. Burning fossil fuels for electricity generation also releases trace metals such as beryllium, cadmium, chromium, copper, manganese, mercury, nickel, and silver into the environment, which also act as pollutants.

The large-scale use of renewable energy technologies would "greatly mitigate or eliminate a wide range of environmental and human health impacts of energy use". Renewable energy technologies include biofuels, solar heating and cooling, hydroelectric power, solar power, and wind power. Energy conservation and the efficient use of energy would also help.

In addition, it is argued that there is also the potential to develop a more efficient energy sector. This can be done by:

• Fuel switching in the power sector from coal to natural gas;
• Power plant optimisation and other measures to improve the efficiency of existing CCGT power plants;
• Combined heat and power (CHP), from micro-scale residential to large-scale industrial;
• Waste heat recovery

Best available technology (BAT) offers supply-side efficiency levels far higher than global averages. The relative benefits of gas compared to coal are influenced by the development of increasingly efficient energy production methods. According to an impact assessment carried out for the European Commission, the levels of energy efficiency of coal-fired plants built have now increased to 46-49% efficiency rates, as compared to coals plants built before the 1990s (32-40%). However, at the same time gas can reach 58-59% efficiency levels with the best available technology. Meanwhile, combined heat and power can offer efficiency rates of 80-90%.

Politics

Since now energy plays an essential role in industrial societies, the ownership and control of energy resources plays an increasing role in politics. At the national level, governments seek to influence the sharing (distribution) of energy resources among various sections of the society through pricing mechanisms; or even who owns resources within their borders. They may also seek to influence the use of energy by individuals and business in an attempt to tackle environmental issues.

The most recent international political controversy regarding energy resources is in the context of the Iraq Wars. Some political analysts maintain that the hidden reason for both 1991 and 2003 wars can be traced to strategic control of international energy resources. Others counter this analysis with the numbers related to its economics. According to the latter group of analysts, U.S. has spent about $336 billion in Iraq as compared with a background current value of $25 billion per year budget for the entire U.S. oil import dependence.

Policy

Energy policy is the manner in which a given entity (often governmental) has decided to address issues of energy development including energy production, distribution and consumption. The attributes of energy policy may include legislation, international treaties, incentives to investment, guidelines for energy conservation, taxation and other public policy techniques.

Security

Energy security is the intersection of national security and the availability of natural resources for energy consumption. Access to cheap energy has become essential to the functioning of modern economies. However, the uneven distribution of energy supplies among countries has led to significant vulnerabilities. Threats to energy security include the political instability of several energy producing countries, the manipulation of energy supplies, the competition over energy sources, attacks on supply infrastructure, as well as accidents, natural disasters, the funding to foreign dictators, rising terrorism, and dominant countries reliance to the foreign oil supply. The limited supplies, uneven distribution, and rising costs of fossil fuels, such as oil and gas, create a need to change to more sustainable energy sources in the foreseeable future. With as much dependence that the U.S. currently has for oil and with the peaking limits of oil production; economies and societies will begin to feel the decline in the resource that we have become dependent upon. Energy security has become one of the leading issues in the world today as oil and other resources have become as vital to the world's people. However, with oil production rates decreasing and oil production peak nearing the world has come to protect what resources we have left in the world. With new advancements in renewable resources less pressure has been put on companies that produce the world's oil, these resources are, geothermal, solar power, wind power and hydro-electric. Although these are not all the current and possible future options for the world to turn to as the oil depletes the most important issue is protecting these vital resources from future threats. These new resources will become more useful as the price of exporting and importing oil will increase due to increase of demand.

Development

Producing energy to sustain human needs is an essential social activity, and a great deal of effort goes into the activity. While most of such effort is limited towards increasing the production of electricity and oil, newer ways of producing usable energy resources from the available energy resources are being explored. One such effort is to explore means of producing hydrogen fuel from water. Though hydrogen use is environmentally friendly, its production requires energy and existing technologies to make it, are not very efficient. Research is underway to explore enzymatic decomposition of biomass.

Other forms of conventional energy resources are also being used in new ways. Coal gasification and liquefaction are recent technologies that are becoming attractive after the realization that oil reserves, at present consumption rates, may be rather short lived.

Energy is the subject of significant research activities globally. For example, the UK Energy Research Centre is the focal point for UK energy research while the European Union has many technology programmes as well as a platform for engaging social science and humanities within energy research.

Transportation

All societies require materials and food to be transported over distances, generally against some force of friction. Since application of force over distance requires the presence of a source of usable energy, such sources are of great worth in society.

While energy resources are an essential ingredient for all modes of transportation in society, the transportation of energy resources is becoming equally important. Energy resources are frequently located far from the place where they are consumed. Therefore, their transportation is always in question. Some energy resources like liquid or gaseous fuels are transported using tankers or pipelines, while electricity transportation invariably requires a network of grid cables. The transportation of energy, whether by tanker, pipeline, or transmission line, poses challenges for scientists and engineers, policy makers, and economists to make it more risk-free and efficient.

Crisis

Oil prices from 1861 to 2007

Economic and political instability can lead to an energy crisis. Notable oil crises are the 1973 oil crisis and the 1979 oil crisis. The advent of peak oil, the point in time when the maximum rate of global petroleum extraction is reached, will likely precipitate another energy crisis.