Direct Generation of Compressed Air from Waste Heat by Reciprocating Thermocompressors

• Applicant: Privatdozent Dr.-Ing. Hans-Detlev Kühl
• Subject Area: Technical Thermodynamics
• Term: from 2017 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 388388693

Compressed air is one of the most important, but also one of the most expensive technical energy sources, as the efficiency from generation to consumption is typically only about 5% due to various conversion and transmission losses. Therefore, considerable savings from both an economic and an environmental point of view can be achieved by substituting conventional compressed air generation by electrically driven compressors at least partially by other techniques, which feature a reduced or possibly zero primary energy consumption.For this purpose, the direct generation of compressed air from heat flows - preferably from waste heat - by a cascade of so-called oscillating thermocompressors is the objective of this project. The underlying principle of thermal compression has in principle been known from the Stirling cycle for a long time and was patented as "Bush Thermocompressor” in the USA as early as 1935, but has so far only been explored for a few low-pressure niche applications. It is advantageously implemented by double-acting displacer pistons that cause pressure fluctuations by periodically moving the process gas - in this case the air to be compressed - back and forth between a "hot" and a "cold" cylinder volume via a regenerator (and thus reversibly in the ideal case) at a constant overall cycle volume. By means of check valves, compressed air can thus be generated directly and very efficiently without any detours via electrical or mechanical energy, utilizing an available heat flow. In order to achieve technically usable pressure levels with such a system, a series connection of several compressor stages in a cascade is necessary. This can be achieved cost-effectively by combining units of identical design and size, despite the fact that the state variables of the air vary significantly between the inlet and the outlet pressure. As a further simplification, so-called overdriven free pistons are used, which are self-actuated without any external supply of auxiliary energy.In a first project phase, a method was developed to simulate and design one single stage of such a cascade using both analytical and numerical models. For this purpose, the theoretical principles and simulation tools mainly developed for closed regenerative gas cycles so far were transferred to this so far scarcely investigated field of open, valve-controlled cycles. Initial preliminary tests with a single, experimentally variable thermal compressor stage constructed for this purpose constitute the end of the first project phase.The aim of the proposed second project phase is to realize a cascade of several stages based on the findings of these preliminary tests. This cascade is planned to be heated and cooled with temperature-controlled heat transfer medium circuits in order to investigate and optimize the expected self-regulating properties during operation of such a cascaded machine and to compare them with the results of analytical considerations and simulations.

• DFG Programme: Research Grants