Energy prosumerism using photovoltaic power plant and hydrogen combustion engine: Energy and thermo-ecological assessment

Renewable Energy Sources (RES) represent an attractive way to save natural resources and improve the overall impact of power systems on the environment. A continuous increase of share of RES in national energy mixes is observed, and due to the energy policy of the European Union and many other countries, further increase is expected. A disadvantage of RES is their random, weather-dependent availability, which requires energy storage. A promising method of integrating RES with the energy system is the use of hydrogen as an energy carrier (e.g. coupling RES with electrolyzers in order to directly use the renewable electricity for production of hydrogen). In the present work, a simulation of cooperation of a photovoltaic power plant with a gas piston engine fueled by hydrogen was performed, with and without the presence of energy storage. The aim of the analysis is twofold. First, the “compensation losses” due to forced part-load operation of the engine coupled with RES are evaluated and compared with “storage losses” resulting from the thermodynamic imperfectness of the storage; this allows to calculate the minimum round-trip efficiency of storage required for positive energy effect. The “compensation losses” have been determined to be of the order of magnitude of 2%, and the minimum round-trip efficiency of storage to be at the level of 85%. Second, a thermo-ecological analysis was carried out to determine the impact of the source of hydrogen on the overall ecological effectiveness of the system. Contrary to the commonly used measure of “energy efficiency” describing a local balance boundary, thermo-ecological cost (TEC) evaluates the consumption of non-renewable exergy within a global balance boundary. The analysis confirmed that comparing various hydrogen production methods (especially renewable and non-renewable) in terms of local energy efficiency is inadequate, because it does not tell much about their sustainability. For a hydrogen energy system basing on the water electrolysis – hydrogen transport/storage – combustion in a gas piston engine pathway to be considered sustainable, the input electricity to the electrolysis process should be characterized by TEC lower than ∼0.15 J∗/J, a value which even some renewable energy sources fail to achieve.