Feasibility of using power steering pumps in small-scale solar thermal electric power systems

Abstract

The goal of this study was to determine performance curves for a variety of positive displacement pumps in order to select an efficient and low cost option for use as a boiler feed pump in a 1-kWe organic Rankine cycle (ORC) system built by the Solar Turbine Group in Lesotho. The pumps tested included OEM plunger and piston pumps, and rotary vane-type power steering pumps purchased from a junk yard. Motor speed and torque were measured at different flow rates to determine the power consumed to move fluid in the prescribed pressure regime. The test station was designed to pump deionized water; it was intended that measurements and calculations would then be non-dimensionalized and used to predict the ORC working fluid's properties. Unfortunately, deionized water caused the power steering pump shafts to seize; the efficiencies were below anticipated and the pumps were unable to operate under the specified pressures. It was discovered, after WD-40 was added to the water, that power steering pumps performed best when moving fluids with more lubricity. The optimal pump was selected based on how the pump efficiency affected the overall ORC system efficiency, defined as the electrical work output divided by the heat input, and the net electric power output. Power steering pumps achieved efficiencies between 34%-54% under the desired ORC operating conditions with water-oil emulsion as the working fluid. For that pump efficiency range, the overall solar thermal electric ORC system efficiency would be 7.4%-8.5% and the overall system cost would be USD 4.59-5.27 per installed Watt. Made specifically for pumping hydroflurorcarbons, the working fluid used in STG's ORC, the OEM Dynex pump exhibited poorer performance than predicted. The pump efficiency of 31% gave a system efficiency of 7.1% and a cost of USD 6.40 per installed Watt. The OEM water piston and plunger pumps made by Hypro achieved efficiencies of 70% and 81%, respectively, under the same ORC operating conditions described above.

(cont) For those pump efficiencies, the overall system efficiencies would be 9.0% and 9.2% and the costs would be USD 4.58 and 4.63 per installed Watt, respectively. The most optimal pump is the HyproPiston pump; although it costs nearly six times that of a power steering pump, the overall system cost is lower when normalized over the power output.