Optimizing the Design of Off-Grid Micro Grids Facing Interconnection with an Unreliable Central Grid Utilizing an Open-Source Simulation Tool (Hoffmann 2019)

While national grids are traditionally the predominant electrification projects of governments, recent multi-track plans also include the utilization of decentralized supply, such as off-grid micro grids. In their Energy for All Case aiming at full electrification by 2030, the IEA expects that 44 % of the households will gain electricity access by connecting to a micro grid. Uncertainty regarding national grid extension plans poses the main implementation barrier: Investors are especially cautious when the possibility of a future national grid extension to their project site implies the danger of rendering their whole investment to sunk costs. This can leave whole communities, otherwise feasible project sites, without electricity. However, there are successful examples of micro grid interconnection. But research into postinterconnection options, especially connected to often unreliable national grids, is rare.

This study contributes to this gap. In order to evaluate possible post-interconnection options of previously off-grid micro grids with the arrival of both a reliable and an unreliable national grid, an open-source techno-economic optimization and simulation tool based on the Open Energy Modelling Framework (oemof) is developed. The energy system simulated can include grid connection, generator, wind power, PV panels and storage as well as inverters and rectifiers, supplying both DC and AC demand. The capacity and dispatch optimization can allow annual shortage or require a stability or minimal renewable share criterion to be fulfilled. From the optimization results, the simulation tool calculates the systems performance indicators: supply reliability, renewable share, autonomy, Net Present Value, Levelized Costs of Electricity and others.

The tool is applied to and validated by a case study based on an electrification plan for the Nigeria’s Plateau State, analysing 544 potential project sites. The paradigm of micro grids becoming stranded assets after the arrival of a reliable grid is confirmed, if no appropriate policies are in place. If, however, the arriving grid has a low supply reliability, 17 % of the project locations could supply their communities at lower cost off-grid. The post-interconnection options of a micro grid, if the national grid was to arrive after five years of off-grid micro grid operation, are evaluated, considering: on-grid operation, capacity adaptation (refitting), transformation into a Small Power Producer (SPP) or Small Power Distributor (SPD) as well as reimbursement and abandonment. This reveals, that the micro grid operator would favour on-grid operation, if subsidies were provided, or reimbursement. In case of an arriving unreliable national grid however, the operator would choose reimbursement over continued operation. The micro grid operation would be terminated and communities left without reliable electricity supply. From an electrification planners perspective, continued on-grid operation of the micro grid would be preferable, as it is the least-cost option ensuring reliable supply.

Therefore, governments should consider the value of reliable supply and, next to reimbursements, develop subsidy schemes that encourage operators to continue operation, especially if the national grid experiences blackouts.

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