A study on lubricant oil supply for positive-displacement expanders in small-scale organic Rankine cycles

Positive-displacement expanders, which are widely used in small-scale ORCs (Organic Rankine Cycles), need reliable LOS (Lubricant Oil Supply) to get well lubrication and sealing. In the present paper, the characteristics of two traditional LOS schemes are examined. Moreover, a modified one is proposed. Analyses of those elements that lead to work loss of lubricant oil supply have been carried out for all the three LOS schemes. The work loss of lubricant oil supply, which is caused by the employment of lubricant oil pumps, pressure drop in lubricant oil separator and other components contributing to work loss, is evaluated by a definition of WLLS (Work Loss Factor of Lubrication Oil Supply). Based on the thermodynamic model of ORC established, the calculation methods of WLLS are presented. Through analyses of LOS schemes and calculation of WLLS in two typical ORCs, it was found that the traditional LOS schemes either can not work reliably, or might cause up to 11.5% and 9.5% power decrease. The values can be reduced by half in the proposed LOS scheme, which can also work reliably. Accompanied with the advantages, the defects of the new scheme were also investigated.