The primary goal of the Tanana Chiefs Conference (TCC) project was to improve the overall efficiency of the diesel power plant generators in villages by 10%. This was to be accomplished through the use of a 50kW Organic Rankine Cycle (ORC) system, designed to recover waste heat from the diesel generators through their exhaust and/or jacket water.
This project differs from past Alaska heat recovery projects in several ways. Primarily, the selected system was retrofitted with robust instrumentation so that there would be usable, documented data. In addition, the project was commissioned to see if ORC heat recovery being used for power generation was economically viable in rural Alaska. There have not been any studies comparing the economics of different heat recovery options such as space heating versus power generation. Finally, heat recovery is being utilized to generate electricity, as opposed to other more conventional applications such as space heating, utility heating, freeze protection, etc.
- The evaluation of the feasibility, operation and maintenance requirements of such a system. In addition, the payback period after applying such a system will be analyzed to see if this would be an economical endeavor for villages in the future.
- The developing of guidelines for potential ORC system selection, operation, and maintenance. The evaluation of the potential all round impact of applying waste-heat recovery in rural Alaskan communities, specifically the reduction in fuel consumption and greenhouse gas emissions.
- A performance and economic comparison of two ORC systems. A 50kW system, which has been upgraded to a 65kW system, developed by Electratherm and a 250 kW system developed by Pratt & Witney.
An ORC system employs the same principles as a normal steam Rankine Cycle system; the difference is the working fluid that is used in each. An ORC system typically is designed to operate with lower temperature sources. Generally the organic working fluid in an ORC system has an extremely low boiling point compared to something universal such as water as well as a high stability temperature, high latent heat and density.
The Rankine cycle is a closed thermodynamic loop of four phases where a fluid, the working fluid, absorbs or transfers energy to accomplish work through continual cycles. A working fluid can be used when it is in a gas phase or in a liquid phase. The various stages include the evaporation of the working fluid by absorbing heat from the steam in a heat exchanger, expansion to lower the pressure and produce work, condensation to release heat and lower the temperature of the working fluid, and then compression by accepting pump work to increase the pressure of the fluid.
Due to its long development history, the traditional Rankine cycle is generally considered a mature technology, while Organic Rankine Cycles are often considered an emerging technology. ORC systems have, however, been installed all over the world and been applied to a variety of heat sources, including geothermal, solar, biomass, exhaust, and stripper wells. Existing commercial ORC systems are generally large scale units, designed to be used in industrial settings, with capacities from a few hundred kilowatts to a few Megawatts such as the 2 MW ORC system in Biessenhofen, Germany which is fueled by biomass and used for district heating. In Alaska, ORC technology has been applied to a geothermal plant in Chena Hot Springs just outside of Fairbanks. The owner, Chena Power, has two 225 kW PureCycle ORC units developed by United Technologies Corporation (UTC) and one 280 kW UTC unit. This system provides the Chena Hot Springs Resort its own self-sustaining energy source.
Heat recovery is the process of reusing the waste exhaust heat that a diesel generator produces. This exhaust heat is conventionally used for either power generation or space heating. The exhaust heat is commonly recovered using stack heat recovery and jacket water recovery.
Stack Heat Recovery
Stack heat recovery, captures heat directly from the exhaust port. The exhaust is piped to a heat exchanger where the heat is transferred to a different fluid or medium; generally water, which is then used in a power generation process or space heating application. In the past, stack heat has been somewhat dangerous due to the potential of fires breaking out due to the build up of soot in the exhaust stacks of the diesel engines. However, technology has been developed that allows for diesel engines to be much more efficient . Higher efficiency means that less heat is lost through the burn cycle, therefore creating less exhaust and less soot build up. Additionally, ultra low sulfur diesel is now being used creating a “cleaner” burn compared to conventional diesel fuel.
Jacket heat Recovery
Jacket water heat recovery is utilized when water that is being used as a coolant, grabs heat from inside the engine, that heat is then carried to a heat exchanger. From there the heat is transferred to a separate fluid loop which can then be used for other applications such as space heating or utility heating. Some power plants use the heat from their jacket water to maintain a warm temperature around their standby generators.
The most economical use of recovered heat is through heating applications. Recovered heat can be used for space heating, domestic water heating, or freeze protection. Using recovered heat for space heating helps to reduce the heating costs for other buildings by decreasing the amount of fuel needed to maintain the desired temperature and by raising the amount of energy utilized per gallon of diesel fuel. Recovered proves very useful in high energy water utility applications, where the heat lost from treatment plants can bed used to heat water supplies, cutting heating costs for water plants while at the same time providing freeze protection for the water supply.
Taking into account the high cost of petroleum and environmental considerations, one relatively recent research and development activity is the recovery of waste heat from mid-sized engines. Groups involved in this activity include national laboratories, power and engine industries, and private government, funding agencies. Utilized technologies include Rankine cycle, absorption cycle, Brayton cycle, and thermoelectrics. ORC and absorption cycles are commonly used and consided the most promising methods. In theory, absorption systems have been considered more efficient in recent decades; however, this hasn't been verified in practical applications. The most recent development in ORC technology is the Trilatekal Flash Cycle (TFC), in which expansion starts from saturated liquid to minimize the irreversibility. TFC is claimed to have better efficiency than conventional ORC, but no real-life TFC applications have been found by the applicant.
The Green Machine
The Green Machine (GM) is a self-contained ORC waste-heat recovery generator designed to capture heat from sources such as stationary engine jacket water, biomass boilers, solar thermal or geothermal fluid and generate electricity. ElectraTherm patented a twin-screw expander in its designs for the GM, which it claims, increases the efficiency of the unit. A normal expander can only operate with a single-phase superheat vapor or steam. The twin-screw expander can operate with a working fluid in either of two phases; liquid, vapor or a combination of both. In other words, the working fluid can pass through as a fluid, as steam, or in a combination of both. Expanders used in other ORC machines only operate well with a working fluid in only one of the phases.
Important to note: The Green Machine is a 50 kW unit with the capacity to be upgraded to work as a 65 kW unit.
The TCC project required the verification of the performance and reliability of the selected ORC system (the GM) through laboratory testing. The testing included a 600 hour reliability test of the GM under full load and a 50 hour test of the GM's performance under controlled environmental conditions. The performance test was designed to help evaluate the readiness of ORC technology for rural applications and how to optimize its performance under varying load and weather conditions. All testing was done at the UAF power plant in Fairbanks, Alaska.
The work plan included the following tasks:
- The procurement of a reliable ORC system and supporting test equipment, including a cooling system (radiator and temperature control loop components), a heating system (flow rate control), thermal flow measurement devices (flow meters, digital pressure gauges, thermocouples, etc), ORC electrical power consumption and measurement devices (resistors, heat dissipation unit, and electrical power measurement devices), and flow control systems (pump, valves, thermal couple, etc). This work was a continuation of previous UAF research on available industrial ORC products, the reliability, and cost of the systems. After consideration the Electra Therm Green Machine was selected. This stage lasted from January 1st, 2010, to December 31, 2011.
- Stage two included the Installation and instrumentation of the GM, electrical harness, heating and cooling loops and other modifications. The Green Machine was installed at the UAF power plant, utilizing plant waste heat. This stage lasted from March 16, 2011, to November 11m 2011.
- Stage three consisted of controlled testing of the selected ORC system (the ElectraTherm Green Machine) for performance verification and optimization of the ORC system under varying engine loading an ambient conditions. The two components of this test included:
- Conduction of the 600 hour running test to verify performance and reliability of the GM
- Conduction of 50 hour performance test under controlled conditions to optimize the performance of the GM.
The laboratory testing stage ran from December 12, 2011 through January 31, 2012.
Data and Analysis
The reliability test was conducted for 600 hours with the GM under full load. It produced 50.1 kW of power with hot water (225 °F) flowing at 160 gpm (gallons per minute) and cold water (50 flowing at 50 °F flowing at 160 gpm. Net output when parasitic loads were taken into account was close to 46 kW.
On January 3rd, 2012 an automatic shutdown occurred 7 times. After talks with ElectraTherm it was determined that the expander high-pressure switch was malfunctioning. The project team bypassed the switch and the rest of the test was performed without incident. The switch is due to be replaced at some point in the future; it is an important safety feature that prevents the GM from over pressurizing itself during operation.
The performance test was designed to determine optimal performance for the GM using different combinations of cooling and heating conditions. Each case was operated for 30 minutes to obtain data. The highest efficiency obtained during the test was 7.6%.
Currently, all lab testing for the GM has been completed and it has been disassembled and removed from the UAF power plant. The Original project proposal included two phases, the currently completed laboratory test, followed by a demonstration of the operational and economic feasibility of the ORC unit. Due to limited EETG funding, the second stage was removed. However, field testing is now planned, funded by the Renewable Energy fund and will examine the economic feasibility of the GM, focusing specifically on the installation and maintenance costs, fuel savings and emissions.
The community of Tok is being considered as a possible location to field test the Green Machine since it is in the Tanana Chiefs Conference region and has the power generation capacity to provide adequate waste heat for the GM to utilize.
The comparative analysis of the 50 kW GM and 250 kW P&W systems has been delayed due equipment malfunctions and shipping problems of the P&W system. As soon as there is sufficient data the comparative analysis of the systems will proceed, outside the reporting of this project.
Funding and Partnerships
This project is an Denali Commission EETG Program project. The funding goal of the EETG program is to develop emerging energy technology that has the potential of widespread deployment in Alaska and has the long-term goal of reducing energy costs for Alaskans.
Tanana Chiefs Conference (TCC) is the traditional tribal consortium of the 42 villages of Interior Alaska, based on a belief in tribal self-determination and the need for regional Native unity. TCC incorporated in 1971, representing the not only the villages but the approximately 10,000 Alaska Natives of the region. Finding opportunities for smart, clean and affordable energy solutions in rural Alaska has become a priority in order to ensure the sustainability of their cultural identity and way of life. For these villages, nestled in Interior Alaska, off the road system and far removed from the state’s electric grid, the cost of energy is typically the highest in the nation. However, this region is rich in untapped renewable resources, motivated people, and opportunities to create efficiencies in their current fossil fuel systems to decrease their dependence on expensive and unpredictably priced energy.
The Alaska Center for Energy and Power (ACEP), an energy research group housed under the Institute of Northern Engineering at the University of Alaska, Fairbanks, is serving as the program manager of the EETG solicitation. As the projects deal with emerging energy technology and by nature are high risk, high reward, ACEP’s technical knowledge and objective academic management of the projects, specifically for data collection, analysis, and reporting, is a vital component to the intent of the solicitation, i.e., providing lessons learned and recommendations.
TCC-ACEP Joint Position
TCC recognized the need for their communities to implement smart, successful and integrated projects in a brief timeframe. They enlisted the assistance of the Alaska Center for Energy and Power through the creation of a jointly managed and funded position which is held by Ross Coen. The immediate goal is to create a region-wide plan to access available funding for projects and to link the region to existing and emerging technologies with the highest impact for their current and future energy portfolio.
The diverse energy needs and available resources for each village precludes a cookie cutter approach and requires solutions that originate within each community but are integrated and shared with the entire region. This partnership matches the community and tribal connections of TCC with the technical expertise of the ACEP to seek practical solutions that decrease high energy prices utilizing the same commitment to sustainable living that has served them well for centuries in this beautiful and often extreme climate.
Ross Coen is acting as project manager for this project, on behalf of TCC.
AEA Reimbursable Services Agreement
Due to limited funding availability, TCC had to reduce the scope and budget of their original EETG proposal to the Denali Commission. The Alaska Energy Authority awarded TCC a Reimbursable Services Agreement for the amount of $54,306 to bridge this budget shortfall, and supplement the funding needed for project equipment.
GVEA Laboratory Space
Golden Valley Electric Association (GVEA) is providing laboratory space in its building in Fairbanks, Alaska for the ORC testing.
ORC Unit Supplier
Through work plan activities, ElectraTherm has been selected by the project team to supply the ORC unit. Since selection, the project team has worked closely with ElectraTherm engineers to help design and engineer the ORC test bed.
Links, Resources, and Documents
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