31-01-2012, 10:39 AM
Organic Rankine Cycle
1.0 INTRODUCTION
There is an urgent need for renewable energy sources. The renewable energy industry has experienced dramatic changes over the past few years. Deregulation of the electricity market failed to solve the industry’s problems. Also unanticipated increases in localized electricity demands, and slower than expected growth in generating capacity, have resulted in an urgent need for alternative energy sources; particularly those that are environmentally sound. Consequently, the renewable energy industry is in a far different situation compared to the period prior to the electricity market deregulation. Instead of struggling to compete in a competitive deregulated electricity market, renewable energy operators suddenly faced requests to accelerate deployment of new renewable energy capacities and restore facilities that had been closed due to poor economics. Review of a renewable portfolio may provide some assurance to long-term funding of renewable energy facilities and lead to a resurgence in new renewable energy facilities. However, a number of factors and issues will require development of these renewable energy facilities both in the short and long-term. In the short term, there will be increasing pressure to deploy renewable energy facilities to help add generating capacity, improve system reliability, and stabilize electricity prices. Organic Rankine Cycle is a well-known and widely spread form of energy production, mostly in biomass and geothermal applications, but great rises in solar and heat recovery applications are also expected. Environmental concern over climate change and rising oil prices are powerful reasons supporting the explosive growth of this efficient, clean and reliable way of producing electricity. However, the strategic installation of these renewable energy facilities will be hindered by a lack of understanding of how the renewable energy facilities integrate into the existing fossil-based generation systems
The Organic Rankine Cycle (ORC) is a non-superheating thermodynamic cycle that uses an organic working fluid to generate electricity. The working fluid is heated to boiling, and the expanding vapor is used to drive a turbine. This turbine can be used to drive a generator to convert the work into electricity. The working-fluid vapor is condensed back into a liquid and feed back through the system to do work again. Today, ORC systems are being evaluated to improve the working efficiency of distributed generation systems, to generate electricity from geothermal or solar natural heat sources, or to recover waste heat from industrial processes and convert that heat into usable power.
2.0 RANKINE CYCLE
The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 80% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is named after William John Macquorn Rankine, a Scottish polymath. The Rankine cycle is the fundamental thermodynamic underpinning of the steam engine.
The Rankine cycle most closely describes the process by which steam-operated heat engines most commonly found in power generation plants generate power. The two most common heating processes used in these power plants are nuclear fission and the combustion of fossil fuels such as coal, natural gas, and oil.
The Rankine cycle is sometimes referred to as a practical Carnot cycle because, when an efficient turbine is used, the TS diagram begins to resemble the Carnot cycle. The main difference is that heat addition (in the boiler) and rejection (in the condenser) are isobaric in the Rankine cycle and isothermal in the theoretical Carnot cycle. A pump is used to pressurize the working fluid received from the condenser as a liquid instead of as a gas. All of the energy in pumping the working fluid through the complete cycle is lost, as is most of the energy of vaporization of the working fluid in the boiler. This energy is lost to the cycle because the condensation that can take place in the turbine is limited to about 10% in order to minimize blade erosion; the vaporization energy is rejected from the cycle through the condenser. But pumping the working fluid through the cycle as a liquid requires a very small fraction of the energy needed to transport it as compared to compressing the working fluid as a gas in a compressor (as in the Carnot cycle).
The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure reaching super critical levels for the working fluid, the temperature range the cycle can operate over is quite small: turbine entry temperatures are typically 565°C (the creep limit of stainless steel) and condenser temperatures are around 30°C. This gives a theoretical Carnot efficiency of about 63% compared with an actual efficiency of 42% for a modern coal-fired power station. This low turbine entry temperature (compared with a gas turbine) is why the Rankine cycle is often used as a bottoming cycle in combined-cycle gas turbine power stations.
The working fluid in a Rankine cycle follows a closed loop and is reused constantly. The water vapor with entrained droplets often seen billowing from power stations is generated by the cooling systems (not from the closed-loop Rankine power cycle) and represents the waste energy heat (pumping and vaporization) that could not be converted to useful work in the turbine. Note that cooling towers operate using the latent heat of vaporization of the cooling fluid. While many substances could be used in the Rankine cycle, water is usually the fluid of choice due to its favorable properties, such as nontoxic and unreactive chemistry, abundance, and low cost, as well as its thermodynamic properties.
One of the principal advantages the Rankine cycle holds over others is that during the compression stage relatively little work is required to drive the pump, the working fluid being in its liquid phase at this point. By condensing the fluid, the work required by the pump consumes only 1% to 3% of the turbine power and contributes to a much higher efficiency for a real cycle. The benefit of this is lost somewhat due to the lower heat addition temperature. Gas turbines, for instance, have turbine entry temperatures approaching 1500°C. Nonetheless, the efficiencies of actual large steam cycles and large modern gas turbines are fairly well matched.
2.1 THE FOUR PROCESSES IN THE RANKINE CYCLE
There are four processes in the Rankine cycle. These states are identified by numbers (in brown) in the diagram to the left.
Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.
Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor. The input energy required can be easily calculated using mollier diagram or h-s chart or enthalpy-entropy chart also known as steam tables.
Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur. The output in this process can be easily calculated using the Enthalpy-entropy chart or the steam tables.
Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant temperature to become a saturated liquid.
In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the T-S diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheat region after the expansion in the turbine which reduces the energy removed by the condensers.
2.2 REAL RANKINE CYCLE (NON-IDEAL)
In a real Rankine cycle, the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible and entropy is increased during the two processes. This somewhat increases the power required by the pump and decreases the power generated by the turbine.In particular the efficiency of the steam turbine will be limited by water droplet formation. As the water condenses, water droplets hit the turbine blades at high speed causing pitting and erosion, gradually decreasing the life of turbine blades and efficiency of the turbine.
The easiest way to overcome this problem is by superheating the steam. On the Ts diagram above, state 3 is above a two phase region of steam and water so after expansion the steam will be very wet. By superheating, state 3 will move to the right of the diagram and hence produce a drier steam after expansion.
3.0 ORGANIC RANKINE CYCLE (ORC)
The Organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. The fluid allows Rankine cycle heat recovery from lower temperature sources such as biomass combustion, industrial waste heat, geothermal heat, solar ponds etc. The Organic Rankine Cycle's principle is based on a turbogenerator working as a normal steam turbine to transform thermal energy into mechanical energy and finally into electric energy through an electric generator. Instead of the water steam, the ORC system vaporizes an organic fluid, characterized by a molecular mass higher than water, which leads to a slower rotation of the turbine and lower pressure and erosion of the metallic parts and blades.The low-temperature heat is converted into useful work that can itself be converted into electricity. A prototype was first developed and exhibited in 1961 by solar engineers Harry Zvi Tabor and Lucien Bronicki. ORC plants are relatively silent. The highest noise emissions occur at the encapsulated generator and amount to about 80 dB(A) at a distance of 1 m. The turbine forms the most determining factor in the reliability of ORC units. The silicone oil used as working medium has the same lifetime as the ORC, since it does not undergo relevant ageing. The ORC The usual lifetime of ORC units is greater than 20 years, as has been proven by geothermal applications. Unit at Admont (Austria) has been running for more than 20.000 operating hours in almost three years. This amounts to an availability of 76%. Regarding maintenance, periodic weekly checks by the operator excepted, a routine one to two day inspection is recommended once a year by the manufacturer.
An Organic Rankine Cycle, (ORC), engine is a standard steam engine that utilises heated vapour to drive a turbine. Figure 2 illustrates the basic components of an Organic Rankine Cycle. However, this vapour is a heated organic chemical instead of a superheated water steam. The organic chemicals used by an ORC include Freon and most of the other traditional refrigerants, isopentane, CFCs, HFCs, butane, propane, and ammonia. The traditional refrigerants require a high temperature heat source. What differentiates the quaternary refrigerant mixture from the traditional refrigerants, is that the quaternary refrigerant mixture boils at extremely low temperatures and is capable of capturing heat at temperatures less than 150ºF (65ºC); thus generating power from low and medium waste heat. Figure 2 presents a typical P-H diagram of the mixture (R125/R123/R124/R134a), where the saturation temperature varies at constant pressure. The degree of variation or gliding temperature depends upon the mixture components and their boiling points as well as thermodynamic and physical properties. The composition of refrigerant mixture can be adjusted to boil the mixture and generate power at a wide range of temperatures from aslow as 150ºF (65ºC) to 1100ºF (593ºC). Typical refrigerants require a minimum of 500ºF (260ºC) to generate power.
Using the quaternary refrigerant mixture the system can produce power from captured low and medium heat in applications such as process industries, solar energy and geothermal energy. Using this quaternary refrigerant mixture, the ORC reduces emissions. Compared with using a typical fossil fuel, using the ORC described reduces NOx by over 4 tons per year and significantly reduces CO2. Further, the quaternary refrigerant mixture has a long life-cycle and requires reduced maintenance and repair costs. These factors result in a relatively short payback period for the initial investment compared to using existing ORC systems. Therefore, we are able to use ORC technology to recover what is typically waste heat. Apart from utilizing for environmentally sound power regeneration what is typically an unrecoverable waste heat source from, for example, hot flue gases wasted at smoke stacks at various temperatures, solar energy using different collector geometries, and geothermal energy as well as grey water, a by-product at process industries, the author is able to produce cheaper, more ecologically-friendly power, due to the lower boiling temperature of his patented quaternary refrigerant mixture and its higher latent heat of evaporation.
1.0 INTRODUCTION
There is an urgent need for renewable energy sources. The renewable energy industry has experienced dramatic changes over the past few years. Deregulation of the electricity market failed to solve the industry’s problems. Also unanticipated increases in localized electricity demands, and slower than expected growth in generating capacity, have resulted in an urgent need for alternative energy sources; particularly those that are environmentally sound. Consequently, the renewable energy industry is in a far different situation compared to the period prior to the electricity market deregulation. Instead of struggling to compete in a competitive deregulated electricity market, renewable energy operators suddenly faced requests to accelerate deployment of new renewable energy capacities and restore facilities that had been closed due to poor economics. Review of a renewable portfolio may provide some assurance to long-term funding of renewable energy facilities and lead to a resurgence in new renewable energy facilities. However, a number of factors and issues will require development of these renewable energy facilities both in the short and long-term. In the short term, there will be increasing pressure to deploy renewable energy facilities to help add generating capacity, improve system reliability, and stabilize electricity prices. Organic Rankine Cycle is a well-known and widely spread form of energy production, mostly in biomass and geothermal applications, but great rises in solar and heat recovery applications are also expected. Environmental concern over climate change and rising oil prices are powerful reasons supporting the explosive growth of this efficient, clean and reliable way of producing electricity. However, the strategic installation of these renewable energy facilities will be hindered by a lack of understanding of how the renewable energy facilities integrate into the existing fossil-based generation systems
The Organic Rankine Cycle (ORC) is a non-superheating thermodynamic cycle that uses an organic working fluid to generate electricity. The working fluid is heated to boiling, and the expanding vapor is used to drive a turbine. This turbine can be used to drive a generator to convert the work into electricity. The working-fluid vapor is condensed back into a liquid and feed back through the system to do work again. Today, ORC systems are being evaluated to improve the working efficiency of distributed generation systems, to generate electricity from geothermal or solar natural heat sources, or to recover waste heat from industrial processes and convert that heat into usable power.
2.0 RANKINE CYCLE
The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 80% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is named after William John Macquorn Rankine, a Scottish polymath. The Rankine cycle is the fundamental thermodynamic underpinning of the steam engine.
The Rankine cycle most closely describes the process by which steam-operated heat engines most commonly found in power generation plants generate power. The two most common heating processes used in these power plants are nuclear fission and the combustion of fossil fuels such as coal, natural gas, and oil.
The Rankine cycle is sometimes referred to as a practical Carnot cycle because, when an efficient turbine is used, the TS diagram begins to resemble the Carnot cycle. The main difference is that heat addition (in the boiler) and rejection (in the condenser) are isobaric in the Rankine cycle and isothermal in the theoretical Carnot cycle. A pump is used to pressurize the working fluid received from the condenser as a liquid instead of as a gas. All of the energy in pumping the working fluid through the complete cycle is lost, as is most of the energy of vaporization of the working fluid in the boiler. This energy is lost to the cycle because the condensation that can take place in the turbine is limited to about 10% in order to minimize blade erosion; the vaporization energy is rejected from the cycle through the condenser. But pumping the working fluid through the cycle as a liquid requires a very small fraction of the energy needed to transport it as compared to compressing the working fluid as a gas in a compressor (as in the Carnot cycle).
The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure reaching super critical levels for the working fluid, the temperature range the cycle can operate over is quite small: turbine entry temperatures are typically 565°C (the creep limit of stainless steel) and condenser temperatures are around 30°C. This gives a theoretical Carnot efficiency of about 63% compared with an actual efficiency of 42% for a modern coal-fired power station. This low turbine entry temperature (compared with a gas turbine) is why the Rankine cycle is often used as a bottoming cycle in combined-cycle gas turbine power stations.
The working fluid in a Rankine cycle follows a closed loop and is reused constantly. The water vapor with entrained droplets often seen billowing from power stations is generated by the cooling systems (not from the closed-loop Rankine power cycle) and represents the waste energy heat (pumping and vaporization) that could not be converted to useful work in the turbine. Note that cooling towers operate using the latent heat of vaporization of the cooling fluid. While many substances could be used in the Rankine cycle, water is usually the fluid of choice due to its favorable properties, such as nontoxic and unreactive chemistry, abundance, and low cost, as well as its thermodynamic properties.
One of the principal advantages the Rankine cycle holds over others is that during the compression stage relatively little work is required to drive the pump, the working fluid being in its liquid phase at this point. By condensing the fluid, the work required by the pump consumes only 1% to 3% of the turbine power and contributes to a much higher efficiency for a real cycle. The benefit of this is lost somewhat due to the lower heat addition temperature. Gas turbines, for instance, have turbine entry temperatures approaching 1500°C. Nonetheless, the efficiencies of actual large steam cycles and large modern gas turbines are fairly well matched.
2.1 THE FOUR PROCESSES IN THE RANKINE CYCLE
There are four processes in the Rankine cycle. These states are identified by numbers (in brown) in the diagram to the left.
Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.
Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor. The input energy required can be easily calculated using mollier diagram or h-s chart or enthalpy-entropy chart also known as steam tables.
Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur. The output in this process can be easily calculated using the Enthalpy-entropy chart or the steam tables.
Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant temperature to become a saturated liquid.
In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the T-S diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheat region after the expansion in the turbine which reduces the energy removed by the condensers.
2.2 REAL RANKINE CYCLE (NON-IDEAL)
In a real Rankine cycle, the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible and entropy is increased during the two processes. This somewhat increases the power required by the pump and decreases the power generated by the turbine.In particular the efficiency of the steam turbine will be limited by water droplet formation. As the water condenses, water droplets hit the turbine blades at high speed causing pitting and erosion, gradually decreasing the life of turbine blades and efficiency of the turbine.
The easiest way to overcome this problem is by superheating the steam. On the Ts diagram above, state 3 is above a two phase region of steam and water so after expansion the steam will be very wet. By superheating, state 3 will move to the right of the diagram and hence produce a drier steam after expansion.
3.0 ORGANIC RANKINE CYCLE (ORC)
The Organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. The fluid allows Rankine cycle heat recovery from lower temperature sources such as biomass combustion, industrial waste heat, geothermal heat, solar ponds etc. The Organic Rankine Cycle's principle is based on a turbogenerator working as a normal steam turbine to transform thermal energy into mechanical energy and finally into electric energy through an electric generator. Instead of the water steam, the ORC system vaporizes an organic fluid, characterized by a molecular mass higher than water, which leads to a slower rotation of the turbine and lower pressure and erosion of the metallic parts and blades.The low-temperature heat is converted into useful work that can itself be converted into electricity. A prototype was first developed and exhibited in 1961 by solar engineers Harry Zvi Tabor and Lucien Bronicki. ORC plants are relatively silent. The highest noise emissions occur at the encapsulated generator and amount to about 80 dB(A) at a distance of 1 m. The turbine forms the most determining factor in the reliability of ORC units. The silicone oil used as working medium has the same lifetime as the ORC, since it does not undergo relevant ageing. The ORC The usual lifetime of ORC units is greater than 20 years, as has been proven by geothermal applications. Unit at Admont (Austria) has been running for more than 20.000 operating hours in almost three years. This amounts to an availability of 76%. Regarding maintenance, periodic weekly checks by the operator excepted, a routine one to two day inspection is recommended once a year by the manufacturer.
An Organic Rankine Cycle, (ORC), engine is a standard steam engine that utilises heated vapour to drive a turbine. Figure 2 illustrates the basic components of an Organic Rankine Cycle. However, this vapour is a heated organic chemical instead of a superheated water steam. The organic chemicals used by an ORC include Freon and most of the other traditional refrigerants, isopentane, CFCs, HFCs, butane, propane, and ammonia. The traditional refrigerants require a high temperature heat source. What differentiates the quaternary refrigerant mixture from the traditional refrigerants, is that the quaternary refrigerant mixture boils at extremely low temperatures and is capable of capturing heat at temperatures less than 150ºF (65ºC); thus generating power from low and medium waste heat. Figure 2 presents a typical P-H diagram of the mixture (R125/R123/R124/R134a), where the saturation temperature varies at constant pressure. The degree of variation or gliding temperature depends upon the mixture components and their boiling points as well as thermodynamic and physical properties. The composition of refrigerant mixture can be adjusted to boil the mixture and generate power at a wide range of temperatures from aslow as 150ºF (65ºC) to 1100ºF (593ºC). Typical refrigerants require a minimum of 500ºF (260ºC) to generate power.
Using the quaternary refrigerant mixture the system can produce power from captured low and medium heat in applications such as process industries, solar energy and geothermal energy. Using this quaternary refrigerant mixture, the ORC reduces emissions. Compared with using a typical fossil fuel, using the ORC described reduces NOx by over 4 tons per year and significantly reduces CO2. Further, the quaternary refrigerant mixture has a long life-cycle and requires reduced maintenance and repair costs. These factors result in a relatively short payback period for the initial investment compared to using existing ORC systems. Therefore, we are able to use ORC technology to recover what is typically waste heat. Apart from utilizing for environmentally sound power regeneration what is typically an unrecoverable waste heat source from, for example, hot flue gases wasted at smoke stacks at various temperatures, solar energy using different collector geometries, and geothermal energy as well as grey water, a by-product at process industries, the author is able to produce cheaper, more ecologically-friendly power, due to the lower boiling temperature of his patented quaternary refrigerant mixture and its higher latent heat of evaporation.