Demonstration of a Compact Heat Exchanger for Supercritical CO2 Heat Recovery Systems

Waste heat recovery has the potential to provide up to 20 percent of the U.S. electricity demand with no direct fuel consumption, thus reducing greenhouse gas emissions by 20 percent. This enormous potential continues to go unrealized due to various technical and financial obstacles with traditional steam and organic Rankine cycle (ORC) systems. An alternative cycle using supercritical carbon dioxide (sCO2) as the working fluid has shown considerable promise for becoming the superior heat recovery technology due to reduced footprint, lower operating costs and ability to operate over a wide range of temperatures and scales. The system offers additional advantages of a low toxicity, non-flammable working fluid, low complexity, and high energy density. Often one of the largest and most costly components of a heat recovery system is the waste heat exchanger (WHX), especially for lower temperature ( & lt;400oC) heat sources. The sCO2 cycle has the advantage of single-phase fluid in the WHX but also requires high pressures ( & gt;20 MPa). Compact, economical WHXs are obviously desired but these high pressures as well as the fouling characteristics of many heat sources have prevented faster development. Echogen Power Systems is partnering with Pacific Northwest National Laboratories (PNNL) to produce and demonstrate a highly compact waste heat exchanger. Echogen is the world leader in sCO2 heat recovery technology and is currently testing the largest sCO2 power loop in the world (7 MW output, 100 kg/s CO2 flow), one of three working power loops in their possession. PNNL has developed a micro- channel heat exchanger design with exceptionally efficient heat transfer and a potential 16x reduction in size over conventional technology. This technology has been successfully demonstrated at very small scales (~60W) for a steam system using with a relatively clean heat source (natural gas combustion). This proposal seeks to extend the technologys validation to include sCO2 cycles, larger scale and dirtier exhaust (diesel combustion). Echogen will design and construct a fully functional test rig including a multi-fuel burner and an existing sCO2 power loop. PNNL will perform detailed thermal and structural analysis, design and build a micro-channel WHX prototype for testing at the Echogen facility. Particular attention will be paid to WHX fouling and transient response. Because both the sCO2 system and the micro-channel WHX are highly scalable, the combined system has tremendous potential applicability. Echogen has begun developing solutions for markets such as power generation, marine, oil & amp; gas, industrial processes, nuclear, concentrated solar energy and geothermal energy. Results of this study will be extrapolated to assess a combined systems ability to impact such markets and increase energy output throughout.