The objective was to demonstrate the economic conversion of low temperature heat into electricity.  For this project, the researchers repurposed heat from a boiler stack from a helicopter hangar on United States Army Ft. Drum base and used data from a related installation at a biomass facility at the same location. Extracting value from stack heat in this manner was consistent with the Army’s Net Zero installation strategy of “repurposing” waste energy as well as compliant with existing Federal mandates to reduce the energy intensity of installations. Current practice allows a large percentage of the fuel burned to climate control facilities to escape into the atmosphere. Using data from this implementation and another implementation on the Ft. Drum base at a biomass facility, researchers were able to create a model to determine the economic implementation of a given potential installation. 

Technology Description

This demonstration project evaluated the performance, economic model and efficiency implications of using Ener-G-Rotors’ flagship product, the ORCA™ and ancillary systems needed to support the heating and cooling needs of the ORCA system. The ORCA basically contains an Organic Rankine Cycle (ORC) with the necessary controls to interface with the grid. A condensing economizer was needed to convert the boiler stack gas into a usable heat source and an air cooler was necessary to provide a cold sink in the hangar. Different installations of an ORCA system will require different needs for ancillary systems, for example, a biomass facility where researchers installed a similar system did not need either an economizer or air cooler.

Demonstration Results

Due to the unusual arrangement of the boiler systems at the hangar, reliable, consistent performance data could not be generated. However, installation costs were acquired for both installations and performance data was generated at the biomass site. The original plan was to have a 235°F heat source which would provide 6% efficiency. Researchers only had about 215°F on the heat source, so the efficiency was lower than expected, at ~5%, although they were able to achieve the output goal of ~20 kilowatt (kW). On the cost side, the long term goal of <$2/W capital cost for the ORCA system was achievable. Installation costs for the two sites varied dramatically for a variety of reasons, but were within the $0.60/W goal. Clearly, the installation costs and needs for ancillary systems (economizer and air cooler) will have a large impact on the economics of any installation. Researchers did not have a long enough test to determine maintenance costs. The carbon savings are based on kW generated and a diminished testing schedule made achieving a carbon saving goal difficult and affected their ability to assess reliability. 

Using all the data collected researchers were able to build a model that shows the Adjusted Internal Rate of Return (AIRR) for different configurations of systems at different sizes. Assuming a 7.5% AIRR is required, then a 50kW system using an economizer and air cooler or a 40kW system using one or the other ancillary systems, or a 30kW ORCA only system are all economical. As waste heat sources are evaluated, this simple model, and the lessons learned from the effort, will aid in deciding upon the economics of any future installation.

Implementation Issues

The most important lessons learned from the implementation were to properly characterize stack temperature and flow early to be able to accurately know the conditions that will be experienced after installation. This was the first experience Ener-G-Rotors had with an economizer and air cooler and a number of lessons were learned from installation and operation of those systems including: installing a drain-back system in case the economizer gets too hot when the ORCA system is not running, using the proper temperature probe for the air cooler, and dealing with the weight of the economizer itself. All these lessons are easily internalized and subsequent installations by Ener-G-Rotors have gone relatively smoothly.

  • Waste heat recovery