Availability Modelling and Cost-Optimal Preventive Maintenance Scheduling of a Residential Off-Grid Photovoltaic System: A Case Study of University of Ilorin Staff Quarters, Nigeria
DOI:
https://doi.org/10.63746/njtd.v23i3.5402Keywords:
Off-grid photovoltaic system, operational availability, imperfect preventive maintenance, virtual age, value of lost loadAbstract
Off-grid photovoltaic systems supply a growing share of Nigerian households, yet their maintenance follows generic manufacturer schedules rather than any quantitative assessment of local cost and reliability. This paper develops a framework coupling component-level failure-time modelling, series reliability block diagram aggregation and imperfect preventive maintenance optimisation under a Kijima Type I virtual-age model, applied to a 4.2 kVA system at residential apartment in the University of Ilorin staff quarters. Wear-out subsystems are represented by Weibull distributions and externally-driven subsystems by exponential distributions, with unavailability valued at ?500 per kWh against measured annual demand. Under the default schedule the system attains an operational availability of 0.99405 at an annual ownership cost of ?224,634. Optimisation raises availability to 0.99549 and reduces annual cost to ?147,578, a saving of 34.3% requiring no additional up-front investment, though at the cost of a higher exposure to unplanned replacement expenditure which is quantified in the paper. The cost-optimal service interval is 16.9 months for the battery bank and 31.9 months for the inverter. The schedule is invariant across a 7.5-fold range in the value of lost load. Storage rather than the inverter governs availability in off-grid systems, reversing the pattern reported for grid-connected systems. Reliability parameters are declared as literature-derived priors and the evidence required to displace them is quantified.
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