Development of a Test Bench for Cryogenic Thermoelectric Generators

Abstract

In 2016, the EU's energy imports amounted to around half (53%) of its own energy consumption [1]. Consequently, the EU will also have to import a large proportion of the energy it consumes in the future. However, this energy will be direved from renewable sources in order to achieve the climate targets. Established import paths that rely on chemically bound energy sources, such as hydrogen or natural gas, should also be utilised. In order to achieve a high density for transport and storage, energy carrier such as hydrogen (LH2) or natural gas (LNG) will be liquefied at low temperatures. The energy required for liquefaction is up to 18 % of the calorific value of hydrogen [2]. It therefore seems essential to recover some of this energy. Thermoelectric generators (TEG), which a heat flow to electrical energy, form the technical basis for energy recovery. The possible applications are extremely diverse, as they can be introduced in all technical configurations which provide a useful temperature difference. The utilisation of thermoelectric generators at low temperatures has hardly been investigated until now. Accordingly, there are virtually no suitable concepts and measurement data in this area. At the same time, thermoelectric generators can be used to provide the heat required for vaporisation and thus ensure that the consumer, for example a fuel cell or a hydrogen combustion engine, is supplied with a gaseous energy source. The challenges are the development of heat exchangers and the selection of suitable/high-performance thermoelectric modules. A high-performance test bench was designed and built-up for the development of a so-called cryo-TEG. The boundary conditions for cryogenic energy recovery is realised with liquid nitrogen. Through advanced process control, the phase transistion of the cryogenic fluid inside the thermoelectric generators can be avoided to ensure high heat transfer and thus high efficiency. The control valves can be used to set different cryogenic fluid mass flow rates to satisfy the requirements of the various applicaions and different operating points for commercial vehicles. To demonstrate the usability of the test bench a generic TEG was used and measured to show an initial performance indication.