Chemical Energy Storage with Piston Engines? Theory and Experiment
R. Hegner(1), M. Werler(2), R. Schießl(2), U. Maas(2), B. Atakan(1)
(1)
University of Duisburg-Essen, Institute of Combustion and Gas Dynamics, Thermodynamics
(2)
Karlsruhe Institute of Technology, Institute of Technical Thermodynamics
The present study investigates the question, whether motored piston engines can be
used to convert work into storable chemical exergy in a power-to-gas-like process. The
usual purpose of piston engines is the conversion of fuel to mechanical energy by
means of combustion. This approach, however, limits the applicability of engines
compared to other means of energy conversion, like batteries or fuel cells. Like
combustion engines, these devices can convert chemical energy to work (or electrical
energy), but they are also capable of the reverse conversion. Providing this flexibility
with piston engines as well, could change the way engines are perceived and used in
our society. Methane pyrolysis where methane is converted to ethylene and hydrogen
at elevated temperatures, is a suitable process for this task. It can be realized in
conventional engines, which run in a compressor-like mode. In this case, the
compression work would be partially used to increase the exergy (here, mainly the
chemical energy) of the mixture.
In order to investigate this unusual approach, a crank-angle-dependent engine model
is used together with chemical reaction mechanisms to predict the kinetics of product
formation. Available energy (or exergy) and its losses were used to evaluate the
efficiency of the engine pyrolysis. Since the product gas is usually highly diluted and
consists of different hydrocarbons and hydrogen, a process concept that enables the
separation of valuable species is designed as well. Key parameters of this process,
like fuel conversion, storage power and product gas composition were analysed,
depending on various degrees of freedom. In order to prove the predictions, exemplary
experiments in a rapid compression machine (RCM) were also carried out and
compared to single stroke simulations.
The investigations indicate that the mere compression of pure methane leads to too
low temperatures and conversion rates, due to the high heat capacity and low reactivity
of the fuel. In order to reduce the heat capacity of the compressed gas, dilution with
gases of low heat capacity was considered; argon proved to be most suitable. With an
argon content of 95% an inlet temperature of 500 K is already sufficient to achieve 80%
methane conversion. These conditions would lead to 1.8 kW storage power at 600 rpm
or 8 kW at 3000 rpm engine speed. This is accomplished while maintaining high
exergetic efficiencies above 90%. Therefore performing methane pyrolysis inside of
piston engines seems to be a promising and flexible contribution to today's energy
issues.
Acknowledgement
Funding by the DFG as part of the research unit FOR1993 “Multifunctional conversion
of chemical species and energy” is gratefully acknowledged.