Better Cogeneration through Chemistry: The Organic Rankine Cycle
No process that creates energy or does useful work is 100% efficient. The laws of thermodynamics see to that. For example, the typical efficiency of a coal burning electrical power plant is only about 40%. That is, only 40% of the heat energy produced by burning the coal actually becomes an equivalent amount of electrical energy. The other 60% is lost as waste heat. The trick is to capture this waste heat and make it perform useful work. The potential of waste heat as an energy source has remained largely out of reach for lack of a cost-effective technology that can recover this heat and use it to generate electrical power. While waste heat is normally insufficient to boil large quantities of water, often it is sufficient to boil a family of organic chemicals that can be used in place of water.
The Rankine cycle is the heat-engine operating cycle used by all steam engines since the start of the industrial age. As with most engine cycles, the Rankine cycle is a four-stage process. Simply put, the working fluid (usually water) is pumped into a boiler. While the fluid is in the boiler, an external heat source superheats the fluid. The hot water vapor then expands to drive a turbine. Once past the turbine, the steam is condensed back into liquid and recycled back to the pump to start the cycle all over again. Pump, boiler, turbine, and condenser are the four parts of a standard steam engine and represent each phase of the Rankine cycle (see Figure 1).
However, a more detailed examination of the pressure, volume, and temperature relationships of each stage of the Rankine cycle is necessary to understand the necessity for using organic chemicals instead of water for extracting energy from waste heat. A typical pressure and volume diagram for an ideal Rankine Cycle is shown in Figure 2.
Phase 1 to 2 occurs in a pump and involves an isentropic (as a result of being adiabatic) increase in pressure of the working fluid by the pump. In an isentropic process, the entropy of the working fluid remains constant. During adiabatic processes, no heat is lost or gained to or from the surrounding environment (nor is there any self-generated heat due to internal chemical reactions. In other words, the working fluid is pumped to a higher pressure without heat transfer into or out of the working fluid, and the temperature of the working fluid remains constant. Since water in its liquid form is essentially incompressible, there is no significant reduction in the working fluid’s volume. Pumps are usually powered by energy taken from the electrical power generated by the engine itself.
Phase 2 to 3 occurs in the boiler and involves isobaric heating of the working fluid. A fluid experiencing an isobaric process has no increase or decrease in its pressure. Per the ideal gas law, as pressure remains constant the volume expands and the temperature increases. The heat is applied from an external source such as the burning of fossil fuels
Phase 3 to 4 occurs in the turbines and involves isentropic expansion of the working fluid as the expanding steam drives the blades of the turbine, spinning the turbine and generating electrical power. As with the pressure increase by the pump, this expansion occurs without the entropy of the working fluid changing since no heat is lost or gained. As a result, the temperature of the working fluid remains the same. However, in accordance with the Ideal Gas Law, a fluid at constant temperature experiencing an increase in volume also experiences a decrease in pressure.
Phase 4 to 1 occurs in the condenser and involves isobaric cooling. As in the boiler, a working fluid experiencing an isobaric process has no change in its pressure. Condensers convert the working fluid from a gas back to its initial liquid state by acting as a heat exchanger. The fluid’s heat that it gained from the boiler is radiated off and, as the temperature drops, the volume of the working fluid decreases in accordance with the Ideal Gas Law.
However, the above describes an ideal Rankine cycle; the real thing isn’t quite so neat and tidy. In the real world, the compression by the pump and expansion in the turbine are not isentropic. Heat is gained during pumping and lost in the turbine. The exchange of heat during both processes increases the power required by the pump and decreases the power generated by the turbine.
The Rankine cycle can be further modified by reheating and regeneration. In reheating, two turbines work in series. The first turbine receives steam directly from its initial heating in the boiler. Then, instead of proceeding to the condenser, the steam is redirected back to the boiler for a second heating. It then proceeds to a second turbine, which operates at a lower pressure. A regenerative Rankine cycle utilizes steam from the hot portion of the cycle (phase 2 to 3 and phase 3 to 4) to reheat the liquid cooled by the condenser, to greatly increase the efficiency of the work performed by the system. Both the reheating and regeneration modifications to the Rankine cycle are examples of utilizing internally generated waste heat. The organic versions of the Rankine cycle do something similar with external sources of waste heat.
The amount of work (W) produced by the steam engine per cycle is equal to the area enclosed by the four pressure-volume lines (pounds x feet), or roughly the difference in pressure (pounds per square foot) multiplied by the difference in volume (cubic feet). The efficiency (e) of the engine is equivalent to the work divided by heat (Qin) that enters the engine during the “2 to 3” phase of the cycle (W/Qin).
To increase the work performed by a Rankine cycle heat engine and its efficiency, the engines resulting cycle must either produce much higher pressures or increase the volume occupied by the working fluid as it goes from expansion to condensation. Since a potential waste-heat source of a Rankine cycle heat engine is less intense and more diffuse than the direct heat of a fuel-burning boiler, working fluid other than water is needed to efficiently capture and use this energy, which brings us to organic working fluids.
Organic Rankine Cycle Engines
So what makes an Organic Rankine Cycle (ORC) engine so special? Like a standard steam engine, the ORC utilizes heated gas to drive a turbine. However, this gas is a heated organic chemical instead of superheated water steam. The organic chemicals used by an ORC include freon and most of the other traditional refrigerants—iso-pentane, CFCs, HFCs, butane, propane, and ammonia.
Refrigerants seem like an odd choice for a heat engine’s working fluid. However, these gases boil at extremely low temperatures. For example, a typical refrigerant will boil at a mere 150°F, generating significant pressures. The cycle of pressurization with a pump—expansion resulting from applied heat, using the heat to turn a turbine to create energy, and condensation of the fluid back to its liquid state—is identical to the steam engine. It just occurs at much lower temperatures. There are a few differences. Heating and expansion occurs with the application of heat to an evaporator, not a boiler. The condenser can utilize ambient air temperatures to cool the fluid back into a liquid. There is no need for direct contact between the heating source at the evaporator or the cooling source at the condenser.
For those applications where higher temperatures are available to heat the organic working fluid, a regenerator is often added to increase the efficiency of the system. Regenerators are typically constructed of a wire metal mesh or a series of closely spaced thin metal plates. The void spaces between the metal wires and plates allow for easy flow of the working fluid through the regenerator. The relatively large surface area of the metal permits conduction of heat. As the heated organic fluid leaves the boiler it passes through the regenerator, and some of its heat remains. When the cooled organic fluid leaves the condenser it passes through the regenerator in the opposite direction, acquiring some of the previously deposited heat, and preheating the fluid before it enters the boiler. Less heat is needed to boil the liquid, which increase the efficiency of the engine, since it is doing the same amount of work.
Though the amount of work performed by a typical ORC cannot compare with its steam-engine big brother, the ORC has many advantages. First, it has a very high cycle efficiency. For the relatively small amount of waste heat used to drive the engine a comparatively high amount of work can be performed. This results in high turbine efficiency—as high as 85%. That is, the amount of electricity generated by the turbine can be equal to 85% of the equivalent energy generated by the engine. This is the result of the relatively low peripheral speed of the turbine. Again, though the amount of electricity generated is small compared to the behemoth steam-driven turbines, the turbines driven by ORC engines operate at much higher efficiencies.
The low peripheral speed has several other advantages. First, it results in less mechanical stress on the turbine and no erosion of the turbine blades (though this is also a result of the lack of moisture corrosion). Low speeds allow for direct drive of the turbine, as there is no need for a reduction gear. All of the above result in a long operating life, less maintenance, and fewer repairs. Most ORC systems are essentially self running and do not need the constant supervision of a human operator.
Organic Rankine Cycle Working Fluids
But which organic working fluid should be used for each application? The “ideal” working fluid should have the following general characteristics: the heat content/capacity should be small (low enthalpy); the fluid’s critical point (the combination of pressure and temperature where the fluid transitions from a liquid state to a gaseous state) should be above the engine’s operating temperature in order to allow it to absorb all the heat available up to that temperature. The required operating pressure should not pose a danger of explosion or rupture. The fluid’s pressure inside the condenser should be above ambient air pressure in order to prevent air inflow into the system. The required volume of fluid in its gaseous state should be small enough to avoid the need for costly, over-sized turbines, boilers, and condensers.
Specifically for ORC applications, organic working fluids have additional requirements.
First and foremost, the cost of the working fluid has to be economical as exotic and expensive fluids defeat the purpose of providing energy from waste heat at marketable prices.
The gas should not require superheating. The major disadvantage of using steam for small Rankine cycles (<1,000 kW output) is its low molecular weight. Therefore, ORC working fluids should have a high molecular weight to avoid the need for high turbine rotational speed.
Like water, the ORC working fluid needs to be in a liquid state at ambient air pressure and temperature—with a freezing point lower than the lowest temperature in the condenser—while maintaining its stability at the highest temperatures in the boiler.
The fluid needs to be able to absorb and reject heat easily (low heat latency). Finally, the fluid needs to be nonflammable, non-corrosive and nontoxic.
The table summarizes the key characteristics of each type of organic working fluid compared with water used in standard Rankine steam engines
Applications and Field Trials
Where can an ORC heat engine be used? One application is in geothermal plants with low heat content (or enthalpy as measured by the volume of the geothermal steam times its pressure; either a small amount of steam or a low pressure head will result in low enthalpy). Though such sources of geothermal energy may be remote, such a heat source can be efficiently tapped for energy production for smaller communities.
Similarly, solar applications are a potential source of energy for the ORC engine. Unlike photovoltaic solar cells that produce direct current, which then has to be converted (with significant energy losses) into alternating current used by households, the turbine driven by the heated organic fluids produces directly useable alternating current. Solar collectors reflect and focus sunlight onto a central tube containing the organic working fluid. This flash-boils the fluid and allows it to drive a small turbine.
Where the ORC is most useful is in the recovery and use of waste heat. Two primary applications include Combined Heat and Power (CHP) plants (especially those utilizing biomass as fuel), and general heat recovery applications from many potential sources. In most cases, the best use of the ORC engine in waste heat recovery applications is in the 400-kW to 1,500-kW power range.
The major competitor to the ORC engine isn’t the standard steam engine. At the lower temperatures generated by waste heat it would be prohibitively expensive to try and recover this heat with steam because of the enormous volumes of steam required compared to the amount of energy available. Furthermore, steam has to be superheated to avoid erosion of the turbine blades. Organic working fluids operate at temperatures below 400°C (752 °F) and do not need to be superheated. ORC engines can recover waste heat effectively at temperatures as low as 70°C (158°F). At low operating temperatures, the ORC engine must compete with heat pumps, but the maximum operating temperatures of heat pumps limits their usefulness. Furthermore, unlike heat pumps, ORC engines do not require an additional energy source, such as an electric motor or combustion engine, to operate. ORC engines are the best technology for waste heat recovery with temperatures between 150°C to 200°C.
Existing Applications, Systems, and Fluids
Honeywell manufactures an ORC working fluid called Genetron 245fa (1,1,1,3,3-pentafluoropropane), a nonflammable liquid with a boiling point slightly below room temperature at standard one atmosphere air pressure. It is not considered a volatile organic compound, has zero ozone depletion and global warming potential, and is environmentally safe. It has better heat transfer characteristics than standard HFCs. Genetron 245fa is a good choice for waste heat recovery from low-pressure steam systems. It operates at a boiler temperature of 300°F (149°C) and a condenser temperature of 100°F (38°C).
UTC Power, a United Technologies Co., has developed the Pure Cycle TM 200 power system utilizing ORC technology. The Pure Cycle system can utilize waste heat, at temperatures greater than 500°F, from a 1-MW electrical power plant to generate 200 kW of electricity. This reduces the demand from the grid by an impressive 20%. Since the system uses waste heat, its fuel is essentially “free.” It generates no additional emissions such as NOx, CO2, or particulates.
Using the Pure Cycle system reduces emissions of NOx by over 4 tons per year and a 100% reduction in CO2 compared with a typical fossil fuel–burning electrical plant generating the same kilowatts. It has low life-cycle (maintenance and repair) costs, and a relatively short payback period. Maintenance and repair activities include replacing filters, checking oil, lubing engine parts, and recharging the working fluid; all of which are preformed at a cost of only $0.01 per kilowatt-hour. The system consists of a power module, evaporator (to heat the organic working fluid), condenser, pumps, and controls.
Its power module includes a 19xR turbine within-line generator. Its working fluid consists of 1,800 pounds of HFC. It can be mated with a wide variety of waste-heat sources including reciprocating engines, gas turbines, thermal oxidizers, landfill flares, kilns, and incinerators.
Turbonen, an Italian company, is a European leader in the application of ORC technology to waste heat recovery. Their ORC turbo-generator is a factory pre--assembled modular unit with a capacity of 800 kW. The modules are easy to transport and ready to install. It is built on a single skid-mounted assembly, and contains all the necessary equipment for electrical production (evaporators, condensers, piping, working-fluid reservoirs, feed pumps, turbine, electric generator, control, and switch-gear). Larger systems can be constructed from multiple modules. An optional regenerator is added for higher temperature applications, such as biomass-powered CHP facilities.
Ormat Technologies Inc. of Nevada specializes in recovered energy generation for a variety of industries and applications. Its Ormat Energy Converter (OEC) utilizes a hermetically sealed organic Rankine cycle generating system, which contains only one smoothly rotating part—the shaft driving the turbine’s alternator rotor. Defined as a closed-cycle vapor turbogenerator, it is a self-contained power package suitable for tapping into waste heat from remote locations. It will provide 0.2 kW to 6 kW of continuous electrical power with minimal maintenance or repairs. The system has been used worldwide for the recovery of waste heat from power and industrial applications. The Heidelberg Cement AG plant in Germany operates a turnkey Ormat system generating 1.5 MW from a heat recovery system. Operation of the power plant results in a reduction of 7,000 tons of CO2 emissions each year. The Minakami Tsukiyono-Niiharu Sanitary Facility in Japan uses an Ormat system to generate 550 kW of electricity from the burning of refuse-derived fuel. A 1.3-MW Ormat generator is used by the Shijiazhuang Heating and Power Plant in China to create electricity from waste heat recovered from flue gases.
Barber Nichols Inc., (BNI), a Colorado manufacturer of high-performance specialty turbo-machinery, has been designing and building ORC systems since 1970. BNI has built and operated numerous geothermal and solar energy systems utilizing ORC engines. Two of BNI’s geothermal plants are located in California. The plants utilize relatively low-temperature geothermal water (240°F) to produce electricity (700 kW and 1.5 MW) that is sold to the local utility. As with most ORC systems, these units operate continuously without the need for a human operator.
Author's Bio: Daniel P. Duffy, PE, writes frequently on the topics of landfills and the environment.