Goodsprings Energy Recovery Station
Renewables-era combined cycle meeting expectations
(Figures in gallery below.)
When you mention the term “combined cycle” to colleagues in the electric power industry, they generally assume you’re referring to a gas turbine/generator coupled to a steam turbine/generator by means of a heat-recovery steam generator.
But that’s not necessarily the case. Another type of combined cycle, one offering renewables credits, marries a gas turbine/compressor and an expander turbine/generator. Here, a conventional heat exchanger and thermal fluid are used to transfer energy from the Brayton cycle’s gas-turbine exhaust to the low-boiling-point organic working fluid driving the Rankine cycle’s expander. Vapor exiting the expander often is returned to the liquid state in an air-cooled condenser. The Goodsprings Energy Recovery Station is a case in point (Figs 1, 2).
Ormat Technologies Inc, Kern River Gas Transmission Co, and NV Energy came together to assure the commercial success of the 7.5-MW Goodsprings project located 35 miles south of Las Vegas near the California border. It began commercial operation at the end of 2010.
Ormat, based in Reno, Nev, provided the ORC (organic Rankine cycle) and heat-recovery technology; it also manufactured key equipment and served as the EPC contractor. Kern River supplies the heat to vaporize the pentane working fluid. NV Energy is the project owner and electricity off-taker. Ormat is operating and maintaining the unmanned plant for its first three years of service.
Goodsprings is southern Nevada’s first non-solar renewable energy project and also the first renewable energy project owned by the state’s largest utility. Note that NV Energy has 46 separate geothermal, solar, biomass, small hydro, wind and waste-heat recovery projects under contract—either in commercial operation or under development.
The company’s renewable energy portfolio totals more than 1200 MW. In 2010, the utility exceeded the state-legislature-mandated Renewable Portfolio Standard of 12% kilowatt-hours from renewables as well as the solar carve-out of 5%—a first-time accomplishment.
NV Energy is no stranger to combined cycles or to dry cooling. It owns and operates eight E- and F-class 2 × 1 combined cycles at six stations. ACCs serve six of the existing combined cycles. Get details on the utility’s generating plants at www.ccj-online.com/archives, click 2Q/2009.
In terms of electric output, Goodsprings is larger than it appears. While rated only 7.5 MW, it is expected to operate at a capacity factor of 90% or greater, making the plant equivalent to a 25-MW solar or wind facility. Capacity factors of 30% are typical for wind and solar.
Performance for the first year was close to expectations. The capacity factor was lower than planned because the volume of gas transported last winter on the Kern River pipeline was less than normal over a period of several weeks. Equipment operated satisfactorily for the most part. An availability north of 95% is expected for a mature plant; a little less in Year One. O&M costs were within the 2011 budget; they are projected to decrease slightly in the coming years.
At the plant dedication in November 2010, Kern River President Gary Hoogeveen told the guests that his company is an advocate of increasing energy efficiency and of reducing greenhouse gas emissions. He added that if Goodsprings meets expectations, Kern River, owned by Warren Buffett’s Des Moines-based MidAmerican Energy Holdings Co, would likely invest in other heat-recovery projects along its 1707-mi pipeline from southwestern Wyoming to Southern California.
The Kern River system has 12 compression stations with a total installed capability of 384,220 hp. It carries 2.14 billion ft3/day, or three times the system’s capacity when it was placed in service in February 1992. More than 1300 of the line’s 1700 miles are made of 36-in.-diam stainless steel pipe. Operation is 8760 hr/yr.
The Goodsprings Compressor Station is the last booster facility on the system before gas reaches the only compression station in California near Daggett. It was part of the original network and declared operational in early January 1992 with three Mars® 90 (Solar Turbines Inc, San Diego) natural-gas-fired simple-cycle compressor drivers rated at about 11,000 hp each (Fig 3). A 525-kW emergency gas engine/generator and a 3.85-million-Btu/hr gas-fired boiler support station operations.
The Goodsprings operating permit required Kern River to install a new low-NOx combustor that was under development by Solar at the time the station was built. That modification had to be implemented before the first scheduled major overhaul after the new combustion system became available.
The original permit allowed 170 ppmvd of NOx at 15% O2 and mass emissions of 237.3 tons/yr. After the upgrade to Solar’s SoLoNOx combustion system in 1996, a new operating permit was issued. It reduced allowable NOx emissions to 42 ppm and 72.3 tons/yr. Performance test results confirmed compliance. The reduction in mass emissions dropped Goodsprings’ status from “major” source to “minor” for all regulated pollutants.
A subsequent revision to the operating permit removed all CEMS (continuous emissions monitoring system), PEMS (predictive emissions monitoring system), and quarterly reporting requirements. However, annual compliance reporting is still required; plus, turbine performance is monitored continuously by PEMS, but not reported.
In spring 2001, Kern River requested and was given permission to upgrade one of the Mars 90s to a Mars 100 with a 15,000-hp rating—this to raise system pressure and increase gas flow through the pipeline. An ATC (authority to construct) was issued in fall 2002 to upgrade the remaining two Mars 90s to 100s. At that time, a limit of 16 ppmvd CO (quarterly average of hourly values) was imposed on all three engines.
Kern River’s SCADA (supervisory control and data acquisition) system allows it to continuously monitor emissions and operating parameters at all compressor stations on a real-time basis. This is critical to company goals for maintaining emissions compliance and expected maintenance intervals. For example, power-turbine inlet temperature is monitored to assure hot-gas-path parts are not subjected to potentially damaging over-temperature conditions; bleed-valve operation is monitored to assure a relatively flat emissions profile.
Participation in the ORC project was simply Kern River’s latest effort for improving the performance and reducing the environmental impact of its pipeline infrastructure. ORC plants ranging in capability from a few hundred kilowatts to a few megawatts have been installed by many different owners over the last 25 years or so to recover energy from heat rejected by industrial processes as well as that available in diesel-engine jacket water and geothermal resources.
In the oil and gas sector, the leader in heat recovery from pipeline compression facilities is ONEOK Partners LP, Omaha, the owner/operator of a 42-in. line carrying 2.5 billion ft3/day of Canadian gas to midwestern markets. At least four of its compressor stations have been retrofitted with Ormat Recovered Energy Generation (REG) systems. TransCanada Pipeline’s Cold Creek Compressor Station has been producing 6.5 MW reliably since 1999 under demanding ambient conditions in Alberta.
Ormat perhaps is best known for its worldwide development, operation, and maintenance of geothermal plants. In that generation sector, Ormat Energy Converter (OEC) systems based on the ORC are used to extract heat from wells yielding medium-temperature brines (Fig 4). Plus, they also serve in bottoming cycles to maximize power production in plants using high-temperature geothermal resources to first drive a steam turbine.
ORC efficiency typically ranges from 10% to 20% depending on the temperature of the heat source. An economically viable, megawatt-size system probably would require a heat source with a minimum temperature in the 280F-290F range. Also, the ORC would have to be located near the heat source and a cooling medium to condense the vapor.
Here’s how REG works: Gas-turbine exhaust heat is transferred to a thermal fluid circulating through the recovery unit shown at the left in the diagram. The hot thermal oil provided to the OEC boils organic fluid (pentane at Goodsprings) in the vaporizer and then gives up some of its remaining heat to pentane in the preheater before returning to the recovery unit.
Vaporized pentane expands through the turbine and flows to the recuperator where it warms the ozone-benign, organic working fluid returning from the air-cooled condenser. Use of an ACC eliminates the expense associated with permitting, installing, operating, and maintaining a water treatment system which would have been significant—if possible—given the desert environment surrounding Goodsprings.
The attributes of pentane. The thermodynamic properties of pentane allow much higher condensing pressures than are possible for steam. This permits use of shorter turbine blades and minimizes ingress of air into the system. The latter mitigates the need for vacuum maintenance.
Another plus is that the saturation curve for hydrocarbons is such that the working fluid remains dry under all operation conditions—thereby eliminating the possibility of erosion damage to turbine buckets and nozzles often found in steam systems. CCJ
