Gear pump for low power output ORC - an efficiency analysis

A pump is a necessary component of ORC units. Although it is simple in a principle and various pump types can be used, reality may easily bring many complications. When designing an ORC for very low power output applications (1-10 kW gross power production), problems such as leakages, mechanical and electrical losses are more significant. Using a tight and reliable pump, typically of a diaphragm type, results in extremely low efficiency, which negatively affects the overall cycle performance. We propose for micro ORC units a gear pump. Typical losses in a gear pump and their contributions with respect to issues that need to be overcome (hermetic seal requirement, reliability and overall efficiency) are described in general in the first part of this paper. The losses are volumetric, mechanical and also electrical of motor and variable frequency drive. In the second part of the paper we present the measured characteristics of the modified commercially available gear pump. The pump is intended for an ORC with a net power output of 1-10 kW. Characteristics are given for both a standalone pump and the pump coupled with an asynchronous motor. They show significantly higher efficiency than typically reported in the literature. Contribution of different losses is discussed. Mechanical losses influence operation of the pump at low pressures (lower hydraulic work but nearly constant power input) while volumetric losses at higher pressure have only small effect. The largest loss comes from electrical efficiencies of motor and variable frequency drive, especially for operation at low speed. Optimizing the operating conditions of the pump together with the motor is therefore the most important in terms of reducing pump power consumption. (C) 2017 The Authors. Published by Elsevier Ltd.