This Small Business Innovation Research Phase I project will demonstrate a highly efficient supersonic ejector vapor compression technology that converts low-to-medium grade thermal energy (200-400F) into useful refrigeration (20F-50F) at high condenser temperatures (100-120F). The proposed multi-fluid jet cooler maximizes heat transfer efficiency by using a propellant with relatively low latent to continuously entrain and compress an immiscible low temperature refrigerant of relatively high latent heat. Initial prototypes have 400% higher efficiencies than conventional single-fluid ejectors. Phase I research will extend these gains while operating at elevated condenser temperatures. Specific research foci are: 1) Highly efficient jet nozzles to supersonically expand a high molar mass, low specific heat ratio propellant without the expansion/compression losses observed when using conventional nozzle designs; 2) Mixing of subsonic refrigerant into the supersonic propellant with minimal kinetic energy loss by avoiding sonic choking of the refrigerant; and 3) Maximum pressure recovery diffusers utilizing weak, oblique compression waves instead of strong, normal shock waves to transition the mixed supersonic flow to subsonic velocity. Potential applications for the two-fluid ejector compression technology include natural gas powered air conditioning, concentrated solar thermal chiller plants, lower cost combined heating power and cooling plants, and thermally-driven water desalination.
The broader impact/commercial potential of this project is reduced economic and climate burden associated with the world?s growing demand for space cooling. Air conditioning is the leading usage for peak-time electricity in the U.S. and largest energy expense for commercial buildings. Globally, the $65billion air conditioning equipment market is growing at 5% p.a.; and because it is dominated by the electrically-driven mechanical vapor compression cycle the strain on electrical grids - and by extension the environment and economy - is rising likewise. A quiet, clean, reliable and cost effective heat-driven solution would greatly reduce these risks. Unfortunately, status quo technologies suffer low efficiencies, large form factors, and require expensive water-cooled condensers. Initial R&D efforts have proven an ejector vapor compressor using optimized fluid pairs built with low cost components can operate at efficiencies competitive with electric compressors when operating in moderate ambient conditions. Massmarket adoption, however, requires efficient operation at extreme outside temperatures. Phase I research will maximize cooling power and discharge pressure (thus operability at high condenser temperatures) by minimizing irreversible losses incurred during supersonic expansion and compression of the immiscible fluid pairs. Project findings will benefit adjacent fields of hypersonic avionics and low atmosphere jet propulsion
The aim of this NSF-sponsored Phase I SBIR project was to theoretically and empirically demonstrate a heat-driven refrigeration system with adequately high energy efficiencies when operating under real-world simulated conditions to be an economically viable alternative to grid-tied vapor compression air conditioners. The stated goals of the effort were to achieve a Coefficient Of Performance (COP, or cooling power delivered divided by thermal power input) of up to 1.0 using a low-to-medium grade heat source (<350F) while providing useful coolth (<50F) in hot climates (>100F condenser). The long term benefit of this project is to enable low cost air conditioning and refrigeration in locations where electrical power is either expensive or unreliable but where low cost thermal energy is abundant, low-cost and environmentally benign. Phase I research commenced on July 1, 2012 and concluded on December 31, 2012. The research team was led by the PI, Joe Boswell of ThermAvant Technologies, LLC, and utilized two, immiscible fluids in an ejector vapor compression cycle. The cooling process was as follows: a low latent heat propellant fluid evaporated in a <350F boiler powered by an external heat source (e.g. gas burner, exhaust gas from internal combustion engine, concentrated solar collector, etc.). The relatively high pressure propellant gas was supersonically expanded through a jet nozzle to generate low pressure in the ejector’s mixing chamber. A separate, high latent heat refrigerant fluid in a low temperature (e.g. 50F) evaporator was fluidly-connected to the mixing chamber. When the pressure in the mixing chamber was below the refrigerant’s saturation pressure, the refrigerant gas was entrained into the mixing chamber. As with conventional air conditioners, the cooling is produced as low temperature heat from the controlled environment continually evaporates the refrigerant and utilizes the high latent heat of phase change. In the mixing chamber, the two gases exchange momentum and eventually slow down and recover total pressure as they exit the ejector’s diffuser. The goal of the ejector is to simultaneously maximize refrigerant flow per unit of propellant flow and to recover enough pressure at the diffuser’s exit to allow the gases to condense into immiscible liquids at ambient temperatures; so the fluids can return to the boiler (propellant) and evaporator (refrigerant) and repeat the cycle. The selected high molar mass, low latent heat propellant required about 1/17th of the thermal energy to go from liquid at ambient temperatures to gas at 350F in the boiler as did the refrigerant to go from liquid at ambient to gas at 50F. By using less energy to flow propellant through the jet nozzle, the prototype had much higher efficiencies than prior single-fluid ejectors. Further, fhe fluids proved chemically stable and were easily separable in the condenser using only gravity. The research team designed, built and tested tested dozens of nozzle, mixing chamber and diffuser combinations designed specifically for the thermal and fluid dynamic properties of the working fluids. The COPs of the ejector prototypes proved highly sensitive to condenser (i.e. ambient) temperatures. At higher condenser temperatures, the gases require higher ejector discharge pressures to liquefy. Balancing tradeoffs between cooling power and recoverable pressure were a key focus during the Phase I efforts. The best results achieved during the Phase I testing are summarized below where <50F cooling was held constant under varying condenser/ambient temperatures: At 70F condenser = COP 2.8 At 85F condenser = COP 1.4 At 100F condenser = COP 0.2 (however, COP = 0.8 at 98F condenser temperatures) The team is continuing to develop ejector designs with COPs of 1.0 at >100F condenser temperatures. While not able to reach its efficiency goals when operating at high ambient temperatures, the team did significantly raise COPs of ejectors at moderate condenser temperatures to levels above those found in existing heat-driven cooling systems. Near-term market opportunities for ejector-based, heat-driven air conditioning where either water- or ground-sourced condensers were made possible by this research effort and are being pursued.
