Comparison of multistage thermoacoustic engines in serial versus parallel configurations

Abstract

In industrial processes about 80% of the energy demand is in the form of heat. One of the problems of the industrial waste heat is the low temperature of this heat. The department of energy efficiency and infrastructure, 'E&I' of the Energy research Center of the Netherlands 'ECN' runs projects in order to use industrial waste heat with a thermoacoustic heat pump. Thermoacoustic engines can deliver the power to drive a thermoacoustic heat pump and can be driven by waste heat. The desired hot temperatures exceed the temperatures of the waste heat. It is believed that more than one thermoacoustic engine is necessary in order to gain sufficient temperature lift. In this study a parallel configuration of thermoacoustic engines is compared to a serial configuration of thermoacoustic engines. Additionally the losses in a coaxial thermoacoustic engine are analyzed. In two sets of experiments two regenerator units were placed in parallel and serial configuration inside a symmetric resonator. The regenerators were heated with an electrical heater and cooled with cooling water. Power was subtracted with a load. The power that can be subtracted by the load is the same as the power that could be used in a heat pump. The main objective is to gain maximum load power with a hot regenerator temperature as low as possible. The experiments show that the serial configuration produced more acoustic power in the load than the parallel configuration. In contrast the efficiency of the power produced in the resonator is found to be about the same for both configurations. This is because of the assumption that the power dissipated in the resonator is equal to twice the power dissipated in an empty half of the resonator, while in reality the power dissipated in the resonator depends also on the internal geometries in the resonator, and the internal velocity. In the present experimental setup the pressure and velocity profile are not the same for serial and parallel configurations. The acoustic power measured in the load is used to compute the external efficiency. The internal efficiency is calculated from the resonator losses and the power measured in the load. The resonator losses were originally determined from two microphone measurements of an empty resonator half, without the consideration of internal geometries. Hence internal efficiencies of the two configurations can not be compared, whereas the external efficiencies can be compared to draw conclusions. In the parallel configuration at a pressure of 19.9 bar, a drive ratio of 3.03 and a heater input of 324 W, 5.96 W of acoustic power can be subtracted while the hot regenerator temperature rises till 274 oC. In the serial configuration at a pressure of 19.5 bar, a drive ratio of 2.95 and a heater input of 367 W, 11.63 W of acoustic power can be subtracted while the hot regenerator temperature rises till 290 oC. This means the 2nd law external efficiency of the serial configuration is 47% better than for the parallel configuration. A calculation predicts that the serial configuration could perform as much as 80% better if the load was placed at the engine side. In cooling, heating, and other processes where the external power is the useful power the serial configuration is the better configuration.