*4.1. Financial Significance*

The scenarios explored in this work showed SSAD plants to be economically feasible and profitable for dairy farms with >100 dairy cows. However, due to the study's boundaries, some costs were not considered, such as grid connection, civil works, etc. Such considerations are deemed important for the overall viability and future implementation of SSADs in practice and should be further investigated.

The need for further governmen<sup>t</sup> supports and financial incentives is still apparent, where the relatively long payback periods projected may dissuade investors. Incentives available in Ireland, such as the REFIT scheme, have had a significant economic influence in reducing payback periods. Although the scheme provided only two tariffs, at a rate of 15.8 c€ kWh−<sup>1</sup> for plants with a CHP capacity up to 500 kW and 13.7 c€ kWh−<sup>1</sup> for plants exceeding this capacity [72]. Consequently, this puts smaller capacity plants at a disadvantage, as they have higher costs due to economies of scale. Comparing Irish rates to other EU countries, Germany provides a rate of 23.73 c€ kWh−<sup>1</sup> to plants with a total installed capacity of less than 75 kWe. Likewise, the United Kingdom provides a tari ff of 4.50 p£ kWh−<sup>1</sup> to plants with a capacity of less than 250 kW. The issue reappears with the recently introduced *Support Scheme for Renewable Heat,* which provides a single tari ff of 2.95 c€ kWh−<sup>1</sup> to all plants generating less than 1000 MWh yr<sup>−</sup><sup>1</sup> [75]. To maximise the potential deployment of SSAD plants, governmen<sup>t</sup> support schemes need to recognise the additional costs associated with smaller capacity plants and, therefore, implement policy that counteracts such expenditures.

Based on the literature and the findings of this study, the cost of finance has been the overriding barrier in the deployment of SSAD plants across Europe [11,63,81]. Issues cited include investors being uneasy with the technology due to limited case studies, the relative newness of the technology, and a lack of expertise within financial institutions to assess such plants. A potential governmen<sup>t</sup> support explored in this study was the adoption of a capital gran<sup>t</sup> subvention. Such legislation has proven successful in countries such as Sweden, France, Wales and England, where capital grants of up to 50% have been applied [68]. As shown in Table 7, the addition of a 50% capital gran<sup>t</sup> subvention reduced the payback period by 3.88 years to 14.62 years, providing a possible pathway for the Irish governmen<sup>t</sup> to support the deployment of SSAD.

Over the next few years, it is anticipated that the capital and operational costs of such plants will reduce dramatically. This is based on the most recent technological advancements, where a growing emphasis on smaller capacity plants has led to cost reductions, primarily through the development of modular systems and plug and play design. Several companies are in the testing phase or have fully commercialised such systems in the European market place, with a wide variety of technologies at various sizes now in development [82–87].
