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Technology Spotlight: Organic Rankine Cycle harnesses moderate waste heat for combined heat and power
Many industrial processes result in waste heat at temperatures less than 1,000°F, Combined heat and power systems (CHP) can cost-effectively recover this heat to serve onsite thermal needs directly. But if the facility does not have a direct application for the heat, a CHP based on traditional steam turbine cycle systems cannot use such low-grade heat to produce electricity. On the other hand, Organic Rankine Cycle (ORC) turbine systems may feasibly generate electricity from waste heat at moderate temperatures (between about 200°F and 750°F). Established technologyORC turbines are an established technology with a long history, primarily at geothermal installations. Ormat—just one of several manufacturers—has installed more than 800 MW of total capacity over the last 40 years, and demonstrated equipment life spans of 20 to 30 years without major overhaul. As equipment costs decline and energy costs increase, ORC systems are being used in industrial applications to recover heat from process heating exhausts, reciprocating engines, gas turbines, thermal oxidizers and kilns. Technology descriptionORC systems and steam systems both have four primary components: a boiler or evaporator to evaporate the working fluid, a turbine fed with vapor from the boiler to drive the generator, a condenser or other means of condensing the exhaust vapors from the turbine, and a unit (such as a pump) for recycling the condensed fluid to the boiler. In a steam cycle, water circulates through these components as the working fluid. In an ORC system, the working fluid is a liquid that has a lower boiling point than water, typically a refrigerant such as R134a or R245fa, silicon oil, ammonia or a hydrocarbon such as iso-pentane. ORC systems can also be compared to air conditioning systems operating in reverse. In fact, some designs of ORCs make use of standard heating, ventilation and air-conditioning (HVAC) equipment, reducing cost by taking advantage of off-the-shelf technology. For example, in early 2007, Carrier Corporation and United Technologies Corporation (UTC) began marketing an ORC system derived from a centrifugal compressor design. Much like large HVAC equipment, ORC systems are available as packaged, modular units, and so are relatively easy to transport, install and interface with the hot and cold sources on site. Most ORC systems range in size from 50 kW to about 2 MW. Smaller units, down to 5 kW, are either under development or have recently entered the market. Temperature requirementsThe capacity and cost effectiveness of the system generally increase with source temperature. The minimum economic source temperature depends on factors such as the manufacturer's equipment design, flow rate of the waste stream, the temperatures of both the heat source and the heat sink (i.e., the cooling source for the ORC), and the cost of electricity. At least three manufacturers—Infinity, UTC and ElectraTherm—build ORC systems that can operate with waste heat temperatures less than 200°F. Maximum temperatures also vary by manufacturer but can be as high as about 750°F. In addition to minimum and maximum source temperature, a minimum temperature difference between the source and sink is required. Ormat’s "OEC," for example, requires a temperature difference of at least 100°F. CostInstalled costs vary widely, depending on size and on the available temperature difference between the heat source and sink. Typically, installed cost will range from $2,000 to $4,000 per kilowatt and can be as low as about $1,300 per kilowatt. This is pricier than a reciprocating engine, but the greater installed cost may be offset by its low maintenance costs and zero fuel use. UTC reports that their geothermal demonstration project at Chena Hot Springs, which used an HVAC-derived ORC, demonstrated that the cost of power production using ORCs can be reduced to below 5¢ per kWh. Maintenance costs per kilowatt-hour are typically a fraction of comparably sized fossil-fuel generators because of their lower speeds, closed loop and few moving parts. Maintenance and repair activities include replacing filters, checking oil, lubricating engine parts and recharging the working fluid. ORCs operate at a lower pressure than steam turbines and so generally, no operator attendance is required. Cost effectiveness can be improved by recovering heat from the ORC’s condenser to produce hot water or to meet air conditioning or refrigeration needs using an absorption chiller. Also, incentives may be available for installation of ORC systems. Thirteen states now include combined heat and power or waste heat recovery as an eligible resource to meet renewable portfolio standards. ManufacturersORC systems available in the U.S. include Infinity Turbine, Ormat, UTC Power, ElectraTherm, Cryostar and Barber-Nichols. References
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ResourcesUnited Technologies Corporation |
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