Energy Transition Scenarios in Off-grid Communities using SOFC-CHP/Battery Hybrid Systems

Abstract

Today, most communities are connected to large energy infrastructures that constitute the electricity and gas grids. But these infrastructures may not be present in extremely low populated regions or regions that are difficult to access, thus considered as off-grid islands or remote regions. Currently, the electricity in these communities is still mostly supplied by centralized or decentralized diesel generators, even though the supplemental use of renewable energy from water, wind and solar is increasingly part of their energy mix. Additionally, centralized or decentralized furnaces, also based on fossil fuels or regenerative fuels (as wood), may be used to supply heat needed for room heating and domestic hot water.The first step for energy transitions towards low emissions in this specific application scenario has the goal to implement prime mover that have higher electrical efficiencies. Consequently, the amount of primary energy (fuel) and the associated CO2 emissions are reduced. Secondly, the efficiency of fuel utilization shall be increased by using the waste heat for room heating, thus representing a typical combined heat and power (CHP) system.While diesel generators are very load flexible, available in different power ranges at reasonable costs their electrical efficiency is limited to 40% or less due to the thermodynamic cycle with several energy conversion steps. Due to the low number of conversions steps in fuel cells, higher electrical efficiencies up to 65% are possible for Solid Oxide Fuel Cell systems specifically. Furthermore, Solid Oxide Fuel Cell systems have a high fuel variety due to operation at high temperatures and enables the CHP operational mode. The overall efficiency of CHP SOFC systems ranges to more than 90%. The main drawback for this technology is the limited flexibility to adapt to load variations, especially regarding start/stop cycles and associated power degradation. To overcome this drawback, batteries in conjunction with SOFC systems may be used for the off-grid scenario.In this regard, SOFC-/Battery Hybrid systems are investigated in an annual simulation analysis for diffent scenarios and 3 different sized example communities. In each scenario waste heat from the SOFC combined with decentralized furnaces is used to fulfill the heat demand.Figure 1 shows the shares of the electrical and the thermal power supplied with a 2.8 MWel SOFC system, the operational states of the system as well as the battery State of Charge (SOC) over one year. It is visible how the battery is charged and discharged and when back-up diesel gensets are necessary to supply sufficient electricity.Based on the annual simulations, the performance and the benefit of using such hybrid systems can be shown in terms of primary energy saving and reduction of CO2 emissions, as well as in terms of system configurations and operation patterns. The analysis also gives information about the necessary sizes of the SOFC systems and the battery capacity to be installed.Figure 1: Exemplary simulation of the SOFC-CHP / Battery Hybrid system with a battery capacity of 2.5 MWh Figure 1