THERMO-FLUIDIC AND MECHANICAL LOSSES IN A SCROLL EXPANDER FOR AN R134a ORGANIC RANKINE CYCLE

asme-orc2015 Tracking Number 156

Unmatched demand and supply of energy keeps motivating the engineering community to efficiently realize even the marginal potential of low temperature heat (~150 °C) which is available in plenty in form of renewable resources such as geothermal or via a low concentration solar field. Further, these sources are distributed in space ranging from kWth to MWth making the scalability a key feature of the technology to be chosen to convert this heat into electricity. Organic Rankine cycle (ORC) is a promising technology which is scalable and can efficiently generate electricity in the above mentioned range. However, the choice of expander becomes crucial at power scales below 100 kWe as the conventional turbine expanders tend to have high rotational speeds (>104 rpm) and suffer from low isentropic efficiencies. Positive displacement device such as a scroll expander is a possibility in the range of 1 to 100 kWe. Existing literature on scroll covers thermal-fluidic losses by using a lumped model to represent the mechanical losses (such as friction between solid components due to relative motion). However, these models do not represent the true mechanical losses of a scroll machine which are dependent on a number of parameters ranging from the basic geometry to thermodynamic interactions, which need to be represented in the model.. The present paper tries to establish the need for optimizing the scroll geometry by simultaneously minimizing both thermo-fluidic and mechanical losses. The methodology is based on a three step approach. First the scroll geometry is generated based on the prescribed operating conditions, next the scroll analyzed using a thermodynamic model. Herein, thermo-fluidic losses due to supply pressure drop, flank and radial leakage are calculated. Finally, the pressure variations are fed to the mechanical losses model wherein the losses at journal bearing, thrust bearing and Oldham coupling are calculated using force and moment analysis. A case study using above approach is performed for an ORC with R134a as a working fluid for various expander inlet temperatures ranging from 100 to 175 °C and a condenser temperature of 45 °C. Condenser pressure of 12 bar results in high densities at the expander exhaust which in turn makes the volumetric flow rates low even at a power scale of 100 kWe. Thus, a 100 kWe R134a scroll is justified despite being traditionally limited to power levels below 10 kWe. Further, for the given operating conditions, scroll involute base circle radius and scroll height are selected as independent variables are optimized for maximum isentropic efficiency. Thermo-fluidic and mechanical losses for the optimized geometric configuration are found to be ~10 and ~15 kW respectively in a 100 kWe scroll, thus establishing the importance of the latter. The procedure described in this paper is a universal design tool applicable to any working fluid or operating conditions for arriving at an optimum scroll geometry for ORC applications.