Final Report: Low Temperature Stirling Engine for Waste Heat Recovery from Distributed Power Sources

EPA Contract Number: EPD11045
Title: Low Temperature Stirling Engine for Waste Heat Recovery from Distributed Power Sources
Investigators: Weaver, Samuel P
Small Business: Cool Energy Inc.
EPA Contact: Manager, SBIR Program
Phase: I
Project Period: March 21, 2011 through September 19, 2011
Project Amount: $79,096
RFA: Small Business Innovation Research (SBIR) - Phase I (2011) RFA Text |  Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Green Buildings


Cool Energy, Inc. (CEI) has identified several waste heat recovery (WHR) candidates whose waste heat could be recovered and converted to electric power using one or more of CEI’s 20 kWe Stirling engines. The primary applications are those in which the waste heat is present the highest percentage of time, in order to maximize the benefit generated by the economic investment. Operational information has been collected for coffee roasting machines, glass firing kilns, and diesel generators, among other waste heat sources. Although the 20 kWe design electric power output objective was chosen to be an aggressive but achievable jump beyond CEI’s current experience with 2 and 3 kWe Stirling engines, some of these applications very clearly could use even larger engines, or an array of 20 kWe engines. The initial proposal for this SBIR project had a 25 kWe design size, but subsequent to the proposal and based on customer feedback, CEI has decided to offer models of 10, 20, and 40 kWe in scale, and focused work under this project on the 20 kWe design. The design process includes looking at multiple candidate engine configurations, selecting the most optimal layout, and then determining the best performing operating speed, component sizing, and component materials. Methods used include Stirling engine numerical simulations, heat recovery system simulation, and computer-aided design.

Summary/Accomplishments (Outputs/Outcomes):

CEI evaluated three initial Stirling engine designs, each producing 20 kWe electric power output, using CEI’s Stirling engine analysis program. The first had a 55 cm heat exchanger and regenerator diameter (all equal to each other and hereinafter called the canister diameter), with non-metallic film structure for the regenerator. The other two had 35 cm canister diameters, one with non-metallic film and the other with stainless steel foil for the regenerators. The 55 cm canister diameter engine was considered impractically large. Further analysis was conducted on the two 35 cm canister diameter engines for performance, speed and cost, resulting in down-selecting the polyimide film regenerator version. This engine layout was configured to fit into a pressure vessel having a diameter comparatively smaller than the length, resulting in a four-Stirling cycle, V-8 cylinder arrangement. Optimal designs of this engine layout were computed for each of three standard pressure vessel diameters, for each of 25 design operating speeds ranging from 600 to 1200 rpm. A 36-inch outside diameter pressure vessel was ultimately chosen, having the best balance between cost/size and thermal efficiency. An optimum design operating speed of the engine then was determined, based on improved models for the thermodynamic and transport properties of the nitrogen working fluid and CEI’s piston ring design and dynamic simulation models. Because of reliability concerns about the piston rings operating at high velocities within the cylinders, the speed was chosen to be 600 rpm, to keep this velocity low. Specifications for the waste heat recovery heat exchanger were developed in complete thermal detail. Specifications for a representative hot gas waste heat stream temperature of 400°C and the mass flow rate necessary to power one 20 kWe Stirling engine at 300°C were chosen to submit to heat exchanger manufacturers for quotation. Operation protocols and ancillary hardware for plant balance were made similar to CEI’s existing designs.

Figure 1. Thermal-to-electrical efficiency map at 5.168 MPa charge pressure and 600 rpm, as a function of the heat supply and heat rejection heat transfer fluid temperatures.


A compact, multi-cycle Stirling engine producing 20 kWe can be designed to fit in a reasonably sized pressure vessel of 36 inches outside diameter and 76 inches length. Mass of the pressure vessel is less than 1,000 kg, exclusive of closure and mounting hardware. Favorable quotes for the waste heat recovery heat exchanger were obtained. Because other attributes of the 20 kWe Stirling engine are the same as CEI’s current SolarHeart engine, it was determined that much of the instrumentation, electric power handling, and operating and emergency procedures can be applied to the 20 kWe engine by merely scaling up those attributes concerned with only the increase in power output. Success with a related engine of 3 kWe capacity during the period of performance of the Phase I SBIR project lends significant credibility to the prospects of successfully implementing a 20 kWe version of the engine.
Commercialization (Potential applications of the research):  During the project performance period, CEI devoted significant amounts of resources to the following commercialization activities:
1.    Fundraising for continued business operations (successfully raised $1M in private investment).
2.    Sales of 3 kWe P3 engines (three under contract currently).
3.    Initial assembly and operation of 3 kWe Stirling engines (see photo in Figure 2).
4.    Business development discussions and visits held with potential contract manufacturers in India 
       and China.
5.    Business development discussions held with potential partners for use of 20 kWe engines in
       solar thermal power applications.
6.    Worked with Foresight on a commercialization plan to identify important contacts in the
       industrial waste heat recovery market for ceramic and brick kilns.
7.    Ongoing discussions with potential WHR customers such as manufacturers of pizza ovens,
       coffee roasters, nut roasters, and LNG re-gasification operations.
8.    Establishment of supplier quality controls, and process improvements for component manufacturing.
9.    Establishment of potential testing partnership with Colorado State University Engines and Energy
       Conversion Laboratory for 3 kWe and 20 kWe WHR demonstration program.
Figure 2. Photo of Cool Energy P3 3 kWe Stirling engine prepared for testing (left); CAD model of 20 kWe Stirling engine designed during SBIR Phase I (right).