Investigating a Prototype Heat Exchanger for Steam Storage

Abstract

A coiled tube single pass counter flow heat exchanger was designed and fabricated using locally available materials and its capacity to transmit heat to water to make steam was tested. The heat exchanger was part of components used in solar thermal power production using a parabolic trough solar concentrator. The design of the heat exchanger storage system was done using Auto CAD 2010 software. Higher temperatures of steam were realized when the heat exchanger was used as a steam storage system. The heat transfer fluids used were locally available and they were water, sunflower oil, Rina oil, used engine oil, unused engine oil, 2 M sodium chloride salt solution, 4 M sodium chloride salt solution and 6 M sodium chloride solution. For each of the heat transfer fluids, heat exchanger operating points were obtained and it was operated between 1 Nm-2 to 1.0 Nm-2 of pressure. In the study the number of heat transfer units of the heat exchanger obtained was 0.61 and the thermal efficiency was found as 0.91. The average rates of heat transfer were 68.4 J for 6M sodium chloride solution, 62.8 J for 4 M sodium chloride solution, 57.9 J for 2M sodium chloride solution, 55.4 J for water, 49.1 J for sunflower oil, 46.3 J for Rina vegetable oil, 59.7 J for unused engine oil and 54.1 J for used engine oil. Thermal conductivity of the salt solutions was better than for other heat transfer fluids although they were more corrosive on the system. Thermal efficiencies of the heat transfer fluids were 6 M sodium chloride solution; 0.89, for 4 M sodium chloride solution ; 0.84, for 2 M sodium chloride solution ; 0.80, water ; 0.78, sunflower; 0.69, Rina oil ; 0.66, unused engine oil ; 0.75 and used engine oil; 0.71. Storage of solar thermal energy will address the problem of low solar density and its variability. The heat exchanger storage system achieved a thermal capacity of 3.26 kJ at a maximum temperature of 249.4 oC and at a pressure of 7.2 Nm-2. The use of heat exchangers in solar thermal storage will increase the overall efficiency of total system