Transient modelling of heat pipe heat exchangers with vertical thermosyphons for the purpose of waste heat recovery from industrial exhaust gases

Abstract

Heat Pipe Heat Exchangers (HPHEs) can be effectively installed within challenging waste heat streams where traditional heat exchangers fail or the heat from the stream is deemed ‘unrecoverable’. Waste heat recovery using HPHEs in energy intensive industries can enhance energy efficiency, decrease fuel consumption and reduce emissions. Traditional methods of predicting HPHE performance rely on steady state or averaged input values, although the majority of process streams have fluctuating characteristics and, as such, it is important to determine the transient performance of a heat exchanger. The software, TRNSYS, has been used to achieve this capability for multiple configurations by creating a dedicated simulation component with personally developed internal coding. This component has been validated against two full-scale industrial units and a laboratory-scale unit, all of which have been installed and tested over the lifetime of this PhD programme. Alongside this work, a steady state model developed in MATLAB has been produced. The models can predict heat source and sink outlet temperatures, heat transfer rate, the conductance value and the pressure drop of the fluids across the heat exchanger. The end use for the recovered waste heat is a site-specific decision and there may not be the capacity in one process to reintroduce all of the recovered heat. Consequently, HPHEs have been developed with various heat sink fluids or even to function with multiple streams in one system to fully utilise the waste heat across a site. The TRNSYS component was not created just for the validation of the experimental installations tested, but to be as useful and versatile as possible to the wider scientific community; to scale to any size of architecture design required for the quantity of energy recoverable, with the potential to simulate exhaust inlet gases across a wide range of temperatures and flow rates and with the ability to perform calculations on heat pipes that are finned or unfinned (smooth). The development of this simulation methodology aids visualising energy recovery transiently and can use real collected data as inputs, allowing specific examples to be tested and evaluated. This aids the design process and provides increased confidence in the expected performance prior to installation, as well as assessing the applications for the energy content recovered. When comparing the temperature outlets between simulation predictions and experimental results, for the ceramic kiln exhaust-to-air full-scale unit, the component achieved an average accuracy of +3.76% for the exhaust and +1.30% for the air. In other words, the outlet temperatures were higher for the simulation than for the experiment. For the aluminium furnace exhaust-to-air finned full-scale demonstration, an average simulated outlet temperature accuracy of +10.17% for the exhaust and +3.55% for the air was achieved. In this case, thermal losses had a larger influence due to higher temperature inlets, which reduced the experimental outlet temperatures. The laboratory-scale exhaust-to-water unit achieved an average accuracy of +0.67% for the exhaust with a spread of ±6%, and +2.97% average accuracy with a spread of -8 to +9% for the water condenser section, more than sufficient for engineering applications. This experimental validation has ensured that the TRNSYS component can now be used confidently in overall system simulations.