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Organic Rankine Cycle Waste Heat Solutions and Opportunities in Natural Gas Compression > the Renewable Energy Source

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Organic Rankine Cycle Waste Heat Solutions and Opportunities in Natural Gas Compression > the Renewable Energy Source Organic Rankine Cycle Waste Heat Solutions And Opportunities In Natural Gas Compression > The renewable energy source BY JOHN FOX t takes a significant amount of en- commonly found on reciprocating en- engine or in electricity purchased from ergy to transport the ever-growing gines, where an ORC generator can the grid. By adding an ORC to an en- supply of natural gas in the United increase engine performance and de- gine installation, the parasitic load of States from where it is produced to crease fuel consumption by convert- cooling the engine can be greatly re- where it is consumed. At each natu- ing the engine’s jacket water and/or duced or even eliminated. The ORC ral gas compression station there exhaust into additional electricity for turns the cost of cooling into the pay- are multiple opportunities to con- the site. The typical engine runs at ap- back of cooling. vert the existing waste heat streams proximately 35% efficiency, resulting to additional electricity for the site in considerable waste heat from the Excess heat abounds and potentially more horsepower for jacket water and the exhaust. Most industrial processes, though increased compression and plant Engine applications include prime designed with efficiency in mind, shed throughput. The existing waste heat power production in remote areas, excess/unused heat in some form streams can be converted to more island and developing nations, bio- and in significant amounts. Heat may power with no added fuel or emis- gas gen-sets such as landfill and originate from boilers, engines, fur- sions, using technology that is prov- wastewater treatment plants, and naces, incinerators, etc., or it may en and established worldwide. renewable biofuels. The thousands originate from other processes, in- Organic Rankine Cycle (ORC) tech- of natural gas compression engines cluding gas compression, chemical nology has been around for decades, across the globe provide a great op- reactions and more. but only recently established itself portunity for waste-heat-to-power. The majority of the heat resulting as a proven source of power gen- The abundance of waste heat in its from the combustion of fuel in an in- eration from low-temperature waste many forms provides for the renew- ternal combustion engine is lost in the heat streams. Such heat streams are able you already have. coolant and exhaust, each of which Because ORC generators work as represents an opportunity for heat re- John Fox is the chief executive officer at cooling devices, they can act as the covery (Figure 1). This level of heat ElectraTherm. He holds a BS in mechani- engine’s radiator, essentially working rejection is common to diesel, gas, or cal engineering and a master’s degree in as a “radiator with a payback.” For all biomass-powered reciprocating en- engineering mechanics from Penn State engine applications, there is an as- gines. Exhaust stack gases from virtu- University. He also has an MBA from the sociated cost of cooling the engine, ally all combustion processes (ovens, University of Pittsburgh. either in horsepower provided by the continued on page 34 32 flow across a difference in tempera- ture generates power. See the basic cycle in Figure 2. Steps in the process include: • Surplus heat is used to boil a work- ing fluid in an evaporator. • Under pressure, the vapor is forced through a twin-screw expander (the power block), turning it to spin an electric generator. • The vapor is cooled and condensed back into a liquid in the condenser. • The working fluid liquid refriger- ant is pumped to higher pressure and returned to the evaporator to repeat the process. Replacing water with alternative low boiling-point fluids allows a modified version of the traditional Rankine cycle to successfully use kilns, furnaces, incinerators, thermal nize a power generation opportu- heat — which is typically at tempera- oxidizers and boilers) contain a large nity, be able to design and install tures too low to drive a steam engine fraction of the original energy of the heat capture hardware plus ORC, — to produce electricity. Such fluids fuel consumed. and operate and maintain power include organic molecules, e.g., hy- In addition to industrial processes, station-type equipment. drocarbons like pentane, or hydro- waste heat is available in alternative • Finally, low relative energy costs, fluorocarbon refrigerants, hence the energy applications such as as well as the historically intangible moniker ORC. geothermal, biogas, solar, biomass, nature of the environmental benefits ElectraTherm’s ORC waste-heat-to- and oil field geothermally heated co- associated with reduced emissions, power generators use a hydrofluoro- produced fluids. have made the payback period of carbon called R-245fa (1,1,1,3,3-pen- Historically low-grade waste heat small waste-heat-to-power equip- tafluoropropane), a nonflammable, has been ignored for several reasons: ment too long. However, histori- nontoxic liquid with a boiling point • Low-temperature waste heat can- cally rising costs of energy — in- slightly below room temperature, about not drive conventional heat engines cluding the environmental costs of 58°F (15°C). such as water-based Rankine cy- fossil fuel combustion — coupled cle systems (steam-turbine power with enabling new technology, Return on investment plants). A lower-temperature meth- have reduced the return on invest- To take low-temperature heat ener- od of converting a fraction of this ment for waste-heat-to-electricity gy from the jacket water of a 1341 hp heat into electricity is needed. conversion equipment. (1 MW) engine-powered generator, • Although the total amount of heat for example, and convert it to a valu- can be very large, it is geographi- How it works able form of energy such as electric- cally diverse. There is insufficient The ORC process follows that of ity, ORC technology is bound by the heat available at individual process the steam engine, the principle differ- same laws of thermodynamics that sites to utilize the most commonly ence being the replacement of water apply to the engine itself. Heat must available heat engine solutions, with a working fluid with a much lower be rejected. typically rated in megawatts. There- boiling point. Consider the ORC a re- Figure 3 shows heat flowing from fore, the distribution and variation frigerator running in reverse, i.e., heat a high-temperature TH through the of available waste heat demands smaller, modular and distributed generation technology. • Industries produce heat as a by- product of their primary enterprise, so they generally do not consider waste-heat-to-power as an inte- is converted to gral part of their operations. Often sites do not have the personnel with the skills or desire to recog- 34 working fluid of the ORC and into the cold sink TC, forcing the working sub- stance to do mechanical work, in this case on a generator. The fraction of heat that is theoreti- cally recoverable is limited by the Car- not efficiency equation, given as: 1 - (TH / TC) provided that temperatures are mea- sured in Kelvin or Rankine degrees. Thermal efficiencies range between 6 and 12%. Although the numbers seems small, since the heat is already going to waste, the fuel source is es- sentially free. The heat from the condenser can be reused, though not for generating more electricity because of its low temperature, but in applications for low-temperature heat, such as district heating, greenhouses, aquaculture, do- mestic hot water preheating, radiant heating, swimming pools, de-icing, etc. ORC efficiency R&D ElectraTherm, a manufacturer of low-temperature ORC generators, in working with the U.S. Department of Defense (DoD), researched ORC ef- ficiencies and output, and discovered the possibility of replacing the engine radiator completely. The company began its first proj- ect with the DoD to simulate various U.S. Navy-owned engine models and ORC integration schemes, and fully test those configurations on Elec- traTherm’s test cell. A matrix was developed consisting of five engine models and two-engine configura- tions over different ambient condi- tions for waste heat capture: jacket water only and jacket water boosted continued on page 36 35 freed up for compression. In effect, the engine’s waste heat becomes a source of cost savings by displacing the radiator’s capital cost, reducing or eliminating the associated parasitic load on the engine and producing electricity for the site. Figure 5 shows the configuration that will deploy later this year, comprised of two 40 ft. (12.2 m) ISO (International Standards Organization) shipping con- tainers. The Cummins gen-set, engine controls, switch gear and exhaust-gas heat exchanger are housed in a com- bined heat and power (CHP) module packaged under ElectraTherm’s direc- tion by Cummins Rocky Mountain, lo- cated in Denver, Colorado. The ORC module will contain the ORC and associated controls, liquid loop radiator (the combined radiator for the engine and ORC), and the cor- with exhaust energy. Navy personnel vanced engine cooling with a pay- responding balance of plant, including visited ElectraTherm several times back was born, accomplished with an piping, pumps and expansion tank, during the first project for training and intermediate heat exchanger to op- etc. The system will be tested first inspection of the test cell and facili- timize the return temperature to the at ElectraTherm’s facility, and then ties. At the conclusion of the project engine and a bypass to ensure the shipped to the field for a full year of a favorable report was issued by the engine cooling remained operating if performance monitoring and fuel sav- Navy and is available for U.S. Gov- the ORC is not running. The impact to ings validation. ernment employees upon request. the overall installed cost for an ORC The next project, which is underway, can now be reduced by 20 to 30%.
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