Many companies and organizations are searching for cleaner, more reliable, more efficient, and affordable energy.
To meet the demand, the US Department of Energy’s Office of Distributed Energy and Electric Reliability is working with energy technology suppliers to promote advances in and adoption of distributed energy. Increasingly restrictive emissions requirements, the rising cost of energy, and the drive of the green movement have set the bar higher for producers and users of distributed energy systems. Because of the new parameters, impressively effective new methods are being designed.
In 2003, the Distributed Gas Turbine of the Future Workshop gathered experts together to discuss then-pressing issues: emissions, alternative fuels, competitive costs, reliability, and combined heat and power (CHP) efficiency. Seven years later, the issues are unchanged—only magnified. However, some progress has been made on the development of technology to address the concerns discussed at the workshop.
Around the same time, the US Department of Energy, Office of Energy Efficiency and Renewable Energy anticipated that at least half of all new power generating capacity to be added by 2010 would use gas turbines, because they can be used in a variety of applications with a range from 1 MW to 20 MW. Primary end-users include petro-chemical companies, the pulp and paper industry, pharmaceutical companies, the cement and textile industries, and oil and gas exploration, as well as universities and colleges, hospitals, and airports.
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Photo: Solar Turbines
Outside Paris, France, a Mars 100 gas turbine transforms landfill gas into electricity. |
Their predictions weren’t far off. Gas turbines are compact and simple to operate, making them popular for use at colleges, hospitals, commercial buildings, and industrial settings as a means of producing supplemental or standby power. Often located near the building benefiting from the energy they produce, these units provide a reliable power source with reduced emissions.
According to the Department of Energy, mid-sized turbines have tremendous potential as a source for baseload, CHP, peaking, and standby/backup power in commercial and industrial settings. Large-frame turbines have advanced in efficient production of high-quality heat and low emissions, particularly when heat recovery equipment, combined cycle designs, and CHP applications are added. However, transferring those accomplishments to smaller distributed systems has remained a challenge.
Other challenges include reducing environmental impact, with emissions of nitrogen oxides (NOx) of less than three parts per million (untreated); being “fuel flexible,” capable of switching between fossil, renewable, and hydrogen fuels; coming in an Internet-ready customizable prepackaged module that incorporates remote monitoring, diagnostics, and online maintenance; integrating absorption chillers for CHP and power applications; and achieving significant gains in fuel efficiency in simple cycle mode, with 90% or better fuel efficiency in packaged CHP applications. To be competitive with grid-connected energy services, they must provide at least a 20% savings in total electricity costs.
Alternate Fuel Sources for Cleaner Energy
Foreseeing increasing competition among alternative energy conversion devices, the 2003 workshop predicted the emergence of hybrid engines as the leader in turbine system technology, with reciprocating engines, fuel cells, and solar and wind energy devices playing significant roles.
While the group looked to high-pressure nuclear turbines, operated in a combined cycle mode, to capture much of the market, few are developing it. Stephen Burris, director of gas engines for Mitsubishi Power Systems Americas Inc. (MPSA), in Lake Mary, FL, says Mitsubishi Nuclear Energy Systems Inc., part of Mitsubishi Heavy Industries Ltd. (MHI) located in Japan, continues to work on nuclear power projects, including design ideas for US applications. However, it’s not widespread in the US yet.
Similarly, a photovoltaic (PV) system based on previously fielded European-compliant power panels was released in the US in 2009, but is not in extensive use in this country, although US sales of PV cells in general are growing at a fast pace, with several manufacturers competing for market segment.
Geothermal is not prevalent in North America either, but other renewable energy sources are being developed and tested. Wind turbines producing between 1 MW–2.4 MW of electricity are an established part of this continent’s power-generation landscape, despite the voltage concerns of wind fluctuations and the challenges of increased wind on the grid.
Mitsubishi Power Systems is working on large combined-cycle gas turbine designs, small and large steam turbines, boilers, and reciprocating engines, all designed to efficiently produce electrical power. In 2008, they demonstrated gasification of coal for integrated gasification combined-cycle systems at a test facility in Japan. “It’s clean coal technology without a lot of waste to the atmosphere,” states Burris.
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Photo: Solar Turbines
This Mercury 50 gas turbine generator set provides power, heating and cooling to a hospital in Texas. |
Despite many desirable characteristics, most renewable sources pose challenges chiefly related to grid fluctuation and reliability. Conventional power generators have difficulty responding quickly enough to stabilize fluctuating supplies. In order to support stabilization of the grid and address rising fuel costs, Mitsubishi expanded its lineup of pure, natural gas solutions.
Burris says they actively pursued reciprocating engines for use in distributed energy applications outside the US prior to the 2009 introduction of the KU30GSI (spark ignition) reciprocating engine, which incorporates a robust spark plug for igniting methane. The all-gas-fueled derivative engine underwent testing in 2008, successfully operating on the Yokohama, Japan, commercial grid.
The KU30GSI exceeded 47% energy efficiency when tested against the ISO 3046 standards (allowing -0 to +5% tolerance), according to Burris, which surpassed the simple cycle capabilities of the world’s most advanced gas turbines. “The engine produces only 1 pound of [Carbon Dioxide] CO2 per kilowatt generated, reducing carbon emissions. With a user interface that’s easy to understand, the engine produces 5.5 megawatts at the generator terminal. The engine advertises the ability to move from a warm standby condition to 100% load synchronized with the grid in under seven minutes—thus making 10-minute non-spinning reserve a reality for today’s power producers.”
These days, MPSA is focused on a distributed energy tie-in to distribution systems at the local level. By entering the grid closer to the end user, distributed generation systems can save consumers considerable wasted energy and can eliminate line loss by up to 10% in remote locations. “We’re looking at smaller needs,” elaborates Burris.
Viable areas include locations where the system hasn’t kept up with growth, or where additional distribution lines to the feeder are needed and there are environmental impact concerns with additional lines. The goals are to reduce transmission losses, increase the utility’s revenue by producing a product to sell to end users rather than simply transmitting it long distances, and to provide a degree of reliability to a geographical area. “In a catastrophic situation, if a key node is lost in the transmission system, this can minimize loss to several downstream customers,” claims Burris. “Our engines can’t offset entire grid loss, but we can mitigate urban loss.”
There are advantages for the utility. Plants larger than 250 MW present a prevention of significant deterioration (PSD) concern for the environment, as classified by today’s environmental laws and regulations. Multiple smaller engines can be spaced apart, reducing the point source of emissions impact on cities and communities, while still producing enough power to sustain a large community and avoiding permitting issues.
Because two engines have a combined output over 10.6 MW and three engines increase it to 15 MW or more, distributed generation plant owners have the opportunity to pull one engine offline for maintenance without a complete loss in generation. In a plant with three or more engines, 10 MW can be kept online while doing maintenance on the third unit since each engine can operate independently.
Small plants also offer insurance through their ability to restore power after disasters that disrupt major arteries of the grid, such as hurricanes Ivan and Katrina, tornados in the Midwest, and, most recently, the earthquake in Haiti. By operating in “Island Mode” until grid connectivity can be restored, small plants offer security.
Because many of their industry partners already had large heating or cooling systems in place, MHI developed several CHP configurations designed to work in a cogen application. Using the various heat release sources of the engine, it is designed for easy integration in settings such as factories, hospitals, universities, large office complexes, and industrial and agricultural applications—anywhere with high power demands and a need for hot or chilled water or steam for building heat. A single gas engine can produce over 5 MW of electricity and 1,000-plus tons of chilled water simultaneously, Burris states.
“We’re happy with the product,” says Burris, adding that the future will benefit from the combination of renewable energy sources.
Gas, in One Form or Another
Energy production has been concerned about emissions for years. The 2003 workshop insisted that “distributed gas turbines must be extremely clean by producing very little or no emissions at all without some form of after-treatment.” Because gas turbines incorporate technology to convert gaseous fuels into electricity and thermal energy, the group believed that using blends or pure feedstocks of natural gas, coal gas, oil, propane, biogas, methanol, and hydrogen could lead to an ultra-clean distributed turbine.
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Photo: Mitsubishi
Multiple KU engines |
Alturdyne Energy Systems, a San Diego, CA-based manufacturer of turbine systems for the commercial and government customers since 1971, is developing natural gas-driven chillers and cogeneration units for distributed utility applications.
“We developed turbines that are modified to burn gases,” states Frank Verbeke, president. “We bought a 200-kilowatt line in the mid-’90s that is the backbone of our company.” Units can parallel together for a combined 1,000-kW system. The capstone turbine does the same thing, he adds. They require more space, but the multiple units feature built-in redundancy.
Custom reciprocating engine power systems from 10 to 5,000 horsepower presently account for the majority of Alturdyne’s business. However, they are currently developing new systems, all based on a 200-kW turbine engine. The first burns wood chips in a fluidized bed provided by Agripower.
Another uses partially oxidized natural gas fuel, producing a hydrogen-rich stream for use in fuel cells. Verbeke notes that this design was presented in a paper at the ASME conference in 2009. The new synthesis gas production process generates mechanical power in a plant that integrates a partial oxidation reactor, a synthesis gas turbine, and an air separation unit which is run by the turbine. The gas is expanded in the turbine, which provides a very efficient method of heat recovery, compared with that of a conventional system that uses a heat recovery steam generator. The difference is as much as 10 times more power and increased efficiency of 3.7%.
The third, developed in conjunction with Flex Energy, burns landfill gas. “The time is now to burn landfill gas [that is currently] seeping into the atmosphere,” insists Verbeke. Efficiency and the “green” movement are motivators behind this new technology, but he believes emissions, CO2 limits, and the Kyoto Protocol are also driving development. The Kyoto Protocol is an international agreement linked to the United Nations Framework Convention on Climate Change that sets binding targets for 37 industrialized countries and the European community for reducing greenhouse gas emissions. These targets amount to an average of 5% against 1990 levels over the five-year period from 2008 to 2012.
The three systems are in the research and development stage, with beta site testing scheduled for this year. Perhaps the one mandate from the 2003 workshop that has been the most difficult to comply with is its requirement to lower manufacturing costs. “It takes a lot of development money,” says Verbeke, “so we look for partners. When we see a need out there, we go to them.” For the partner or customer, he believes the units will pay for themselves “in a couple years, compared with the cost of generating power with natural gas.”
Hybrid Integration
Hybrids were high on the list of priorities for the 2003 workshop. Other considerations for small gas turbines included ultra-low emissions and the capability of being fully integrated with the grid. With NOx regulations becoming stricter and CO2 emissions more heavily regulated, the role of gas turbines in relation to increasingly stringent emissions regulations is critical. Both NOx and CO2 emissions can be reduced by lean premixed, pre-vaporized combustion and by enriching the fuel stream with hydrogen fuel.
Everest Sciences Corp., in Tulsa, OK, is a young company working to address many of the issues surrounding turbines used for distributed energy. In particular, it focuses on turbine inlet cooling, a technique that improves a gas turbine’s fuel efficiency and power output. “Cooling inlet air, increases the density of the air ingested by the gas turbine, allowing the engine to operate at higher mass flow rates,” explains Marcus Bastianen, director of sales and marketing. “Besides the additional power gain that results, the increase in mass flow allows the gas turbine to operate more efficiently, thus reducing emissions per kilowatt of energy produced.”
Everest Sciences builds inlet cooling systems that provide cooler air than traditional evaporative techniques and chilled air systems that use less power than conventional refrigeration methods. “By minimizing the energy required to cool inlet air, the total NET power output of the engine increases while decreasing the total NET heat rates,” he adds.
A lower net heat rate also means that CO2 emissions decrease per incremental kilowatt produced. Traditionally, smaller gas turbines have higher heat rates relative to large industrial-sized gas turbines. However, using efficient inlet cooling can provide additional incremental net power with emissions that rival the national average emissions for natural gas-fueled power generation. That’s a hot button, because Congress is looking more closely at emissions and greenhouse gases. In fact, proposed EPA rules may require “best-available control technologies and energy efficiency measures to minimize greenhouse gas emissions” when certain industrial facilities are constructed or significantly modified.
Everest has developed technology to continue to improve on gas turbine performance, focusing on hybrid turbine inlet cooling. The heart of the hybrid design uses an indirect evaporative cooling mechanism. Although indirect evaporative methods have been used in comfort cooling for quite some time, Everest Sciences uses this technology efficiently for the high air-flows that gas turbines consume. They build hybrid systems that add supplementary mechanical chilling and/or a direct evaporative process after the indirect evaporative cooling process. The company integrates and ships complete units/systems for engines up to 25 MW and scaling larger to 50–60MW. Everest has had requests for 100-MW gas turbines.
“We’ve spent a great deal of time and effort testing and fine-tuning our heat exchange technology and configuring our products such that the pressure loss through the system is minimized while our cooling efficiency is maximized,” says Bastianen. “Our product is different.”
The three new, engineered products are ECOCool, ECO CHILL, and Hydro-Flexcool. ECOCool is a hybrid package system featuring indirect evaporation with an air wash cycle. Air washing is direct evaporation with proven filtration benefits. This combination lowers the inlet air temperature below ambient wet bulb temperature while consuming a parasitic power similar to conventional evaporative techniques. Where traditional evaporative cooling methods are limited to the wet bulb temperature, the Everest hybrid design can bring ambient air as low as 14 degrees below ambient wet bulb in certain climate conditions. Cooling the inlet air below wet bulb and only using power to turn fans and pumps makes ECOCool the most efficient water-based turbine inlet system available, Bastianen claims. This has the added bonus of improving CO2 emissions per unit of useable net power.
ECO CHILL is a hybrid, incorporating indirect evaporative cooling and chilling with an optional air washing stage. Although the engine makes more power, fuel consumption is reduced per unit of power; it is an integrated package system with lower parasitic loads than conventional refrigeration.
Mechanical chilling requires a compressor and condenser to refrigerate air. The capital cost is generally high and there is a high parasitic load: It takes a lot of power to chill air mechanically. By performing up to half the cooling with the indirect process and finishing with chilling, using similar albeit smaller compressors, less power is consumed and greater net fuel economy is achieved.
Depending on the climate conditions, an optional air wash stage can be added to ECO CHILL for use in combination. Their control processes work in conjunction with one another, so different parts of the cooling process can be turned on, minimizing the overall power consumption of the hybrid cooling process. ECO CHILL is a hybrid, incorporating indirect evaporative cooling and adding air washing and chilling. Air washing is direct evaporation with filtration benefits. Although the engine makes more power, fuel consumption is reduced; it is an integrated package system with lower parasitic loads.
The Hydro-Flexcool is Everest Sciences’ other new product and is the first integrated system safe for use with brackish or reclaimed water. Bastianen explains that the concept came from remote locations that needed cooling but didn’t want a large chilling system. The poor quality of water at these locations made water-based cooling complicated and more costly. Typical reverse osmosis systems or other water treatment techniques have an added cost. It’s not uncommon that some processes break down and need much attention in harsh remote climate locations. If the system fails, inlet cooling doesn’t work. “A lot of things can go wrong,” says Bastianen.
With the Hydroflex system, there’s no need for water treatment. The evaporated air/water stream passes in a separate channel so the air to the turbine never comes in contact with it. The system benefits plants with stringent requirements on effluent or locations that don’t have access to clean water. Plants that have effluent constraints often build retention ponds to allow the sun to evaporate the effluent. “We can run it through the Hydro-Flex system and evaporate much of it,” says Bastianen. An added benefit is that the clean, dry, and cooled air runs through the turbine for additional power and reduced heat rates.
The cooling concept is the same, he indicates, but design changes, materials, and components make the Hydro-Flex more efficient. For instance, fogging systems need demineralized water. To get that, Bastianen says, you have to “make” it. That adds cost. But the Hydro-Flex allows for use of non-treated water and eliminates the power and emissions associated with the water treatment process.
“Nobody we know of offers a product like this,” he says proudly. There hasn’t been a lot of recent innovation in this area; traditional evaporative techniques and chilling are the norm. “Our innovative hybrid is different. It provides equal chilling for less power and can perform inlet cooling with bad water.
“We are the integrator,” he continues. “We build technology around the indirect evaporative process.” The system is built to be compatible with filtration, with integrated framework and a filter house added on for easy installation. Maintenance is very minimal, with high industrial-grade components that are easily inspected and serviced, by design. “All moving parts can be worked on while the gas turbine is running, because it’s not in the air path line to the engine. All moving parts are in the secondary air stream in the heat exchange or out of the primary turbine inlet air path. It’s a well thought-through system design with redundancy so if a pump or fan goes down, all cooling is not lost.
In development for 11 years and in operation for two, the system is not widely known yet. Everest Sciences’ targets end users, gas turbine original equipment manufacturers, and engineering companies. End users include food processing facilities that use CHP; pulp and paper companies that want the flexibility of selling excess power to the grid; and other industrial plants and factories.
A CHP off the Old Block
To realize their full potential, distributed gas turbines should be integrated into a combined cooling, heating, and power system. These systems capture and use the heat produced during the combustion process for steam, hot water, or thermally activated chillers. Commercial, industrial, and institutional facilities can gain as much as 90% efficiency.
High exhaust temperatures are useful in certain industries, such as the paper and pulp, chemical, pharmaceutical, and ceramic industries, where steam is necessary for manufacturing processes. In such cases, a system has to provide more than just power requirements; it must capture exhaust heat to provide steam.
Gas turbine CHP is a proven option that can provide significant efficiency improvements and emissions reductions for existing plants using natural gas as the primary boiler fuel and for new plants planning to use natural gas.
However, the need to control volatile organic compounds (VOCs) and the cost and energy consumption of thermal oxidizers presents important challenges. VOCs are emitted mainly in the form of contaminated air exhausted from various processes. Major sources include the petrochemical industry, wood products industry, electronics, metal refining, and the automotive paint industry. Approximately 33 million tons of VOCs are discharged annually by industrialized nations around the world.
The most common way to destroy VOCs is through high temperatures to oxidize the compounds into carbon dioxide and water. Thermal oxidation consists of heating the contaminated air stream (typically to 1,400–1,800°F) and ensuring a residence time inside the reaction vessel of 1.2 seconds.
CHP systems may offer alternative VOC destruction options. According to Siemens Westinghouse Power Corp., several industry contacts have emphasized that a CHP system that also serves as a VOC oxidizer would benefit the industry and enhance the acceptance of CHP.
The SGT-300 combustion turbine offered by Siemens is a “mature product used heavily at universities,” states Bob Jones, sales manager for industrial power generators. “Its unique feature is the concept of using a standard unit for VOC destruction. It uses tried and true technology for two roles: onsite power and VOC mitigation—without modification.”
VOC-laden air is collected and fed to the turbine air intake and is combusted in a standard proprietary Dry Low Emissions combustion system. The system destroys VOCs and simultaneously produces electrical energy and thermal energy: CHP. Exhaust heat can be used for direct heating or to produce steam, hot water, or hot oil using a heat recovery steam generator.
VOCs are a byproduct of contaminated air, Jones explains, and are thus destructive to a furnace. By burning them in the turbine, they give energy back that can be sold to the grid or used steam for heating or other processes. “You get your money back, and the exhaust is clean,” summarizes Jones.
The SGT-300 provides 96% destruction efficiency, he continues, pointing out that legislation requires 90–95% efficiency. The industry standard for NOx and CO emissions for a gas turbine generator is 25 ppmvd (Parts Per Million, Volumetric Dry). Siemens’ system achieves guaranteed figures down to 10 ppmvd. “Its primary purpose is to meet VOC requirements,” says Jones. Surplus thermal energy for the facility and electricity for internal use or sale to the grid is a benefit that provides flexibility and helps with return on investment (ROI).
At a comparative initial cost, its long-term cost is more attractive because of the ROI, claims Julia Brown, product manager for SGT-300. Spatial demands are not an issue. “It needs less room than the thermal oxidizer—but you need both.” She estimates a 15 x 90’ footprint. “For an existing plant, it’s something to consider; for a new plant, it’s simple to plan for.”
Other benefits include reduction of NOx emissions, more efficient use of energy resources, increased reliability, reduced operations and maintenance costs, and built-in redundancy. “Backup is always available with the thermal oxidizer,” elaborates Jones. In addition, it can operate below 90% during maintenance periods. “This gives you coverage; it’s running all the time. If you have a money-making process, you lose money when it’s down.”
In fact, he says, the turbine is available over 97% of the time year-round, because if it goes down, the plant closes—sometimes for days to do maintenance and another two days to get it back up to temperature. The SGT-300 gets up to temperature quickly. “It keeps the plant running. If you can’t destroy the VOCs, you must close the plant.”
Routine maintenance is minimal: an annual boroscope inspection; a minor overhaul of the hot section every three years; a main turbine change at six years. Designed for a broad window of capability, it can run on multiple fuels—gas or liquid (natural gas, diesel, landfill gas) or energy content fuels (landfill gas)—without changing hardware. The standard unit will even run on future fuels. “No other unit has the same attributes,” claims Jones.
The key, Brown says, is applying known technology, not new technology. In a 2009 VOCGEN CHP market overview, Steven Sexton writes: “CHP, or cogeneration, has been around in one form or another for more than 100 years; it is proven, not speculative. Despite this proven track record, CHP remains underutilized and is one of the most compelling sources of energy efficiency that could, with even modest investments, move the Nation strongly toward greater energy security and a cleaner environment.”
Sexton estimates that if the US achieves 20% of generation capacity from CHP by 2030, it would reduce CO2 emissions more than 800 million metric tons per year, the equivalent of taking more than half of the current passenger vehicles in the US off the road, and could save an estimated 5.3 quadrillion Btu of fuel annually, the equivalent of nearly half the total energy currently consumed by US households—not to mention the technical jobs it would create.
Jones considers Siemens’ system an EPA-CHP partnership to promote CHP technology for VOC destruction. “We’re promoting CHP as a concept, but penetration is slow. It’s not always embraced. There has been no commercialization until now.”