**4. Discussion**

Few approaches to renewable integration acknowledge the resource and capital constraints common within isolated communities, ye<sup>t</sup> both represent significant barriers to uptake of renewable generation. In acknowledging the urgen<sup>t</sup> need for power system decarbonization, low-load diesel application has a role to play in providing greater flexibility to isolated power systems, resulting in improved renewable hosting and acceptance. In this regard, low-load diesel is identified as a low complexity technology solution, o ffering both hosting capacity relief and battery storage rationalisation. Future research e ffort should be directed to improved flexibility of dual fuel generation technologies, which remain less compatible to renewable integration than diesel due to increased ignition delay.

## **5. Conclusions**

The presented King Island and Moloka'i case studies represent isolated power systems at di ffering states of technology progression and refinement. King Island is one of the world's first high-penetration isolated power systems, able to run 100% renewable for extended periods. Unfortunately, the King Island experience has limited commercial relevance given the high cost and complexity of the approach. In this regard low-load diesel has shown to reduce the systems battery requirements by 50%, while reducing annual fuel consumption by 6.3%. The result highlights the gains possible under improved generator flexibility, o ffering reduced barriers to renewable integration.

For Moloka'i the challenges in progressing past low annual solar PV penetrations are very di fferent. In this environment, low-load diesel is shown to provide near term relief from the hosting capacity constraint currently preventing connection of additional solar PV generation. On Moloka'i, the adoption of low-load applications allows for the interconnection of another 800 kW of approved, but stalled solar PV connections. For a system dependent on residential deployment of renewable generation, the hosting relief o ffers both commercial and social benefit, reducing community frustration regarding interconnection delay. In addition to the 27% improvement in hosting capacity low-load applications also provided for a 2.7% reduction in operational expenditure. In improving the flexibility of diesel generation on Moloka'i low-load applications provide for improved system efficiency, a recommencement of solar PV connection and a rationalisation of any future battery storage requirement (low-load diesel provided for a 43% reduction in optimal BESS sizing for a hypothetical doubling of renewable capacity). The results identify low-load diesel as a valuable near-term enabling technology, able to deliver significant value with or without BESS integration. Of the available, commercial technologies low-load diesel is unique for its ability to benefit both low and high-penetration isolated power systems. Its accessibility makes it a natural precursor to storage. The results challenge the conventional practice of prioritising high e fficiency, low-speed diesel generation as the base load within diesel-based power systems, instead advocating for flexible thermal applications as systems transition towards renewable economies.

**Author Contributions:** Conceptualization, J.H. and E.S.; methodology, J.H., M.N. and X.W.; software, J.H.; validation, E.S.; formal analysis, M.N. and X.W.; investigation, J.H.; writing—original draft preparation, J.H.; writing—review and editing, X.W. and M.N.; visualization, J.H.; supervision, M.N.; project administration, X.W.; funding acquisition, J.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by The Australian Research Council, award LP170100879, and the O ffice of Naval Research Global, award N00014-19-1-2161.

**Acknowledgments:** The authors would like to acknowledge Richard Rocheleau, from the Hawai'i Natural Energy Institute and Ray Massie from Hydro Tasmania for their assistance during the case study preparation.

**Conflicts of Interest:** The authors declare no conflict of interest.
