Organic Rankine Cycle Heat Recovery System - An Emerging Energy Technology Grant Project
This project sought the demonstration of a 50 kW Organic Rankine Cycle (ORC) system designed to generate electricity through the utilization of “waste heat” from diesel electric generators (DEGs). Given the prevalence of DEGs and the high cost of fuel in many Alaska communities, a diverse range of groups expressed interest in the applications of this technology for Alaska. While waste heat is currently recovered for space heating applications in many of these communities, there is need to formally assess the comparative opportunities of recovered heat utilized for electricity production.
The isolated rural villages in Alaska annually consume about 370,000 MWh of electrical energy, which comes from individual diesel-fired generator sets. Because the generators must be sized for the maximum usage a village will need, the generators are running at less than full capacity during off-peak times; the ratio of electrical power produced to fuel energy consumed is generally less than 40%. The rest of the fuel energy is lost as heat. While some power plants utilize a portion of this for other heating needs, such as space and water heating, the majority of this energy in Alaska is wasted.
The goal of adding Organic Rankine Cycle (ORC) products to an existing power cycle is to reclaim some of this heat to generate a bit more power, increasing the overall fuel efficiency of the power plant. While this technology is mature for larger-scale power generation, the products for smaller-capacity generator sets, appropriate for the typical size of Alaska village power plants, are still new to the market or in prototype phase. ElectraTherm’s “Green Machine” was identified as one of the devices with the highest potential for success. The Green Machine is designed to generate up to 50 kW of power using the ORC, which is a process used to obtain energy from lower-value (lower temperature) heat sources than are commonly used for power generation.
The goal of this project was to investigate the applicability of a commercial-scale ORC system designed to generate electricity by utilizing waste heat from DEGs in rural Alaska communities. The technology used in the investigation was a 50 kW ORC system, manufactured by ElectraTherm and known as the Green Machine (GM). Project tasks included modeling, technology demonstration, and performance and economic assessment.
Before project testing activities commenced, a thermodynamic and heat transfer model of the GM system was created in order to predict performance characteristics as a function of adjustable system parameters such as heat source temperature and flow rate.
The GM was installed in the UAF power plant building, which had access to the necessary heating supply and cooling water required for operation. The heat source was taken from the power plant steam supply and transferred to a hot-water loop via a heat exchanger. This hot water loop then functioned as the heat source for the GM. The heat sink was municipal water made available from a fire hydrant and connected directly to the GM using a bypass valve to control the temperature of the heat sink.
Two tests were conducted on the GM to obtain data related to its operation and performance. The first, a reliability test, was designed to test the GM under full load and operating capacity to verify its steady-state operation including net power production. The second was a performance test designed to test the GM under different input (flow rates and temperatures of heat source and heat sink) conditions to determine the performance (e.g., power output and efficiency) of the GM at each of the given input conditions.
The project team found the fuel efficiency of a diesel generator may be improved by about 4% with an ORC system on experimental results, the maximum net efficiency experienced at the heating fluid temperature of 195°F is about 7.4%. If more than 50% of fuel energy is wasted as heat (in jacket water and exhaust), the potential of fuel efficiency improvement is about 3.7%.
Overall, the project team found that “An example to evaluate the present ORC system using the field diesel engine data is presented for jacket water heat recovery, and combined jacket water and exhaust heat recovery systems using the developed performance curves. The example shows that the performance data obtained from this experiment can be used to simulate and evaluate the application of this ORC system to Alaska village genset for power output, efficiency, payback period, emissions reductions etc.”
Laboratory testing for the GM has been completed and the project team has commenced field-testing with funding of $472,787 provided through the Alaska Energy Authority Renewable Energy Fund (link to Tok project page). Field-testing is further investigating performance, operations, and the economics of the GM such as the installation and maintenance costs, fuel savings, and emissions. Tok, a TCC community, is the location of the field test; the powerhouse is owned and operated by Alaska Power and Telephone (AP&T), the utility for Tok. Installation of the GM took place during the summer of 2013. The system was commissioned by McKinley Services and is undergoing demonstration with potential for long-term deployment and data monitoring contingent on initial performance.
The ongoing field demonstration of this project is critical to validate the reliability and performance findings of this project and to further inform the widespread applicability of this technology in rural Alaska communities. In particular, system performance information within the context of a rural community-scale and quality heat source and sink is needed to refine the preliminary model developed through these project activities. Information regarding the installation, operation, and maintenance of the system is needed to refine preliminary economic assessments.
Photo 1: ACEP Researchers Review System Instrumentation. Courtesy of Todd Paris, UAF.
Photo 2: Green Machine Laboratory Placement. Courtesy of Todd Paris, UAF.
Photo 3: Green Machine Utility Connections. Courtesy of Todd Paris, UAF.