Analytical estimation of the presence of non-condensable gases in the condensate tank of an Organic Rankine Cycle

Abstract

The presence of non-condensable gases (NCGs) can have a negative impact on the ORC performance due to the reduced expansion ratio over the expander. The presence of NCGs can reduce the turbine expansion ratio significantly, especially in such systems in where siloxanes or high molecular weight hydrocarbons are used, requiring condensing pressures well below the atmospheric pressure. The ORC facility that uses siloxane MDM as the working fluid at LUT University has been considered in the present study. Experimental and simulation data of the facility from the previously published studies suggest that defining the working fluid state in the condenser and condensate tank suffers from significant uncertainty due to the existence of NCGs. Therefore, an attempt has been made to analytically estimate the quantities of NCGs present in the condensate tank of the ORC system using 1-D calculations based on ideal gas assumptions. It was assumed, that air is the NCG that is present inside the condensate tank for simplifications. Since the compressibility factor of MDM vapor is close to 1 at such low condensing temperatures, its behavior is close to ideal gas and hence, the assumption of ideal conditions seems viable. Data from three different experimental measurements with varying temperatures have been tested using the proposed analytical method and the corresponding quantities of the MDMair mixture during the system operation have been estimated. The results showed that the presence of the NCG was dominant, especially in the case of measurement data with lower temperatures, and was reduced for higher condensate tank temperatures.