**5. Conclusions**

Over the coming years, it is anticipated that the Irish governmen<sup>t</sup> will come under increased pressure to enact measures to mitigate the negative environmental impact of the agricultural sector. This will be further heightened by the targeted growth of the dairy sector, increasing to 1.7 million dairy cows by 2025, with the average herd size growing to over 100 cows [106]. Of the renewable energy technologies available, SSAD is particularly promising for both the reduction of GHG emissions and the economic value in the form of on-site energy generation. This study uses a non-linear model to determine the technical, environmental, and economic viability of SSAD on Irish dairy farms ranging from 50 to 250 dairy cows. The study found the technology to be profitable within the lifespan of the plant on farms with dairy herds exceeding 100 cows (payback periods of 12.87 to 7.75 years). In addition, all scenarios with dairy herds sizes >100 cows showed a net CO2 reduction ranging between 2059 and 173,237 kg CO2-eq. yr<sup>−</sup>1.

Although SSAD plants were shown to be viable, significant governmen<sup>t</sup> supports are still needed to achieve financial returns that are attractive to investors. One support explored in this study was the inclusion of a capital subvention gran<sup>t</sup> at rates similar to schemes in other EU countries. Incorporating the result had a significant economic impact, reducing payback periods by 3.88 years to 14.62 years. Furthermore, there is a need for the reintroduction of an electricity feed-in tariff applicable to SSAD plants. Without such a mechanism, the size of plants is limited to the electrical demand of local applications, significantly limiting expansion and financial returns. Both measures provide potential pathways for the governmen<sup>t</sup> to support and accelerate a domestic biogas industry.

For future research, we sugges<sup>t</sup> the analysis of the seasonal feedstock supply, parasitic energy consumption and net energy production variabilities experienced by farm-scale AD plants. Such seasonal variabilities can negatively affect the sustained operability and economic viability of plants as they often have contractual obligations to provide a consistent energy output year-round with minimum variations in the quantities and quality of energy produced. In addition, a greater understanding of Irish farmer's perception of AD is needed. Key information essential to the long-term success of AD in Ireland is still lacking in the literature, such as characteristics of potential adaptors, uptake rates, and perceived barriers.

**Author Contributions:** Conceptualization, S.O., E.E., and J.B.; validation, S.O., E.E., and J.B.; writing—original draft preparation, S.O.; writing—review and editing, S.O., E.E., S.C.P., G.L., and J.B.; supervision, E.E., S.C.P., and J.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the European Union's INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB), with match funding provided by the Department for the Economy, and Department of Jobs, Enterprise and Innovation in Ireland, gran<sup>t</sup> number IVA5033.

**Acknowledgments:** In this section you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials used for experiments).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
