Gas Technology: Capturing the heat
Making CHP pay with efficient heat recovery.
Combined Heat and Power—CHP—is no longer a secret. Energy users have found the benefits of generating their own electricity on-site, and using the byproduct heat for a wide range of applications. CHP has been shown to provide significant reductions in operating costs, while increasing energy supply security. In some cases, owners even successfully sell surplus electric energy back to the grid.
CHP Increasing in Importance
According to a report by the Combined Heat and Power Partnership of the U.S. EPA, “The existing 82 GWe of CHP capacity at almost 3,600 industrial and commercial facilities represents approximately 8% of current U.S. generating capacity and over 12% of total electricity generated. The report indicates that a reasonable goal would be to meet 20% of the country’s electric needs at CHP facilities.
Benefit the Community and the Planet
CHP systems fired on natural gas help the country reduce dependence on overseas oil, and lower the environmental impact of central-station electric generation. Further, because of the high overall efficiency of CHP systems, they reduce total levels of greenhouse gases emitted. On a local level, CHP reduces the need for new power plants, large transmission lines and substations for electric grid utilities. To the extent that CHP systems sell excess capacity into the grid, they can also improve local power supply reliability.
Equipment Selection is Critical
Key to a successful CHP installation is selecting equipment for optimum heat recovery. Natural gas is widely used as a primary fuel for CHP. The most common gas technologies are reciprocating engines and gas turbines or microturbines. Other potential applications for on-site power include steam turbines and fuel cell systems. All these options have potential for byproduct heat recovery.
Multiple Heat Sources from Engines
Reciprocating engine generators are often chosen for their rapid startup times, high electrical efficiencies, ability to respond to changing load patterns, and proven reliability, especially with multiple units. Byproduct heat from an engine-generator set comes from several sources. These include engine jacket cooling water and heat recovered from exhaust gases. In some cases there are additional streams of heated water from the engine oil cooler or from other minor engine cooling systems. These minor streams are typically combined with the jacket cooling water stream.
The jacket stream is typically 200°F to 250°F. In a typical CHP engine installation, this stream goes to a heat exchanger to heat water to between 180°F to 220°F. This level of hot water can be used for a wide variety of process purposes, can provide building heat, or with the use of a single-stage absorption chiller, can provide chilled water for building cooling or process purposes.
Capturing the Exhaust Heat
The other major source of heat from engine generation is the exhaust stream. Although the exhaust gases may be as hot as 500°F, not all of this heat can be captured. Some exhaust heat should remain to prevent stack condensation or other exhaust discharge problems. Typically the exhaust stream passes over a heat exchanger to produces low-pressure hot water in the range of 230 to 260°F. Alternatively it can be used to produce steam from 120 to 150 psig. This steam can be used for a variety of plant purposes, or again it could go to an absorption chiller.
General Electric offers both its Waukesha line of engines, and the Jenbacher series, for global markets. According to GE spokesperson Gina DeRossi, with new technologies, the efficiencies of engines are increasing. As an example, she points to the introduction of two-stage turbocharger technology for engines. The Jenbacher J624 engine, when fitted with the new turbocharger, goes from 4.0 to 4.4 MWe capacity, with an efficiency increase of 1% to reach 46.5%. She notes, “This engine is particularly well-suited for operation in hot environments and CHP applications.”
DeRossi states that engine CHP is particularly attractive for district heating and cooling, commercial buildings, critical care facilities, airports, industrial buildings and parks, and greenhouses for the horticultural industry. The greenhouse applications can also use exhaust CO2 to support plant growth.
Gas Turbine Systems
An alternative to CHP engine generation is the gas turbine. These can range from systems as small as a single 30 kW microturbine to large combustion turbines sized in the tens of megawatts. Larger gas turbines are a good choice for CHP installations where a large volume of higher-temperature water or steam can be used, including in combination with an absorption cooling plant.
Microturbines such as the Capstone Microturbine are increasingly popular, especially in multiple unit configurations. The Capstone machines are available in sizes of 30, 60 and 200 kW, with control arrangements available for multiple units. Microturbine byproduct heat is widely used for comfort and process applications.
Solar Turbines is a Caterpillar Company, and manufactures a broad line of gas turbine-generator sets ranging in size from 1.1 MW to 22.4 MW. Solar spokesman Chris Lyons points out that the electric generation efficiency increases as one goes up in size, typically from 25% for the smaller machines to 39% for the larger units. He adds, “In combined heart and power operations, these same units range in total efficiency from 70% to 90%.
Various Factors in Sizing Units
He notes, “Most applications in the larger size ranges are for hospitals, pharmaceuticals, breweries, and university and college campuses. They also serve a variety of other industries that have thermal or chilling loads for process needs.” He indicates that in selecting equipment, most size for the necessary thermal load. But, he adds, “It is also very dependent upon overall electric rates and factors such as plant reliability.”
Lyons cites a large paper company that uses a 5.7 MWe MW Solar turbine for plant electric service, and uses the byproduct heat for a dryer for tissue paper. Another example is a ceramic plant that uses the heat from a 7.5 MWe unit to dry the slurries used to make ceramics. In yet another application at Cal State Fullerton, a 4.5 MWe unit supplies campus power, and the exhaust heat supplies a Thermax absorption chiller for cooling a data center.
Lyons notes that the food and dairy industries also are using gas turbines for plant power, and using the byproduct heat to generate steam for sterilization and other plant purposes. He says, “With the introduction of Boiler MACT (maximum available control technology), we have seen many applications of replacing older coal boilers with steam from exhaust heat from gas turbines.” He explains, “The most common industrial steam use is in the range of 120 to 250 psig saturated steam, with some applications of 50° to 100° F of superheat.”
The technology for gas turbines has become very reliable, and service intervals have been extended for today’s gas turbines. Lyons explains, “Most of our turbines are designed for 30,000 hours between overhauls. However some of the smaller units go well beyond 40,000 hours before being overhauled.”
Selecting a System
In making the decision to go with CHP, and in choosing the right sizes of units, it is essential that owners accurately characterize their current energy usage patterns, both for heat and electricity. Remember to consider potential reductions in electrical demand charges as well as energy charges. Sometimes operating a unit for relatively few hours per day can have a major impact on electric demand charges.
Include in your considerations potential future changes in operations that might necessitate more capacity. Get help from an engineer with experience in onsite power systems who is also familiar with local energy suppliers and their tariffs. Then evaluate potential thermal contributions from a CHP system, keeping in mind that not effectively using the thermal energy can dramatically affect the overall system efficiency.
Your electric supplier may also offer attractive buyback payments for unused electric production. This might support installation of a system with higher electrical output than for site use alone. CHP will continue to be an important energy option for the future. Making the right decisions now could have a major effect on your bottom line in the future.
Energy Solutions Center CHP Site
EPA Combined Heat and Power Partnership
General Electric Distributed Power
This article originally appeared on Gas Technology Spring 2015 issue.
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