*3.3. Economic Results*

A comprehensive economic analysis was carried out to investigate the revenues, expenditures, and financial indicators of each of the scenarios under investigation over a 20-year life span, as illustrated in Table 7. The results of this analysis showed SSAD plants to be economically feasible and profitable for commercial dairy farms with >100 dairy cows. However, the payback periods of farm sizes between 100 and 200 dairy cows were relatively long, which may dissuade potential investors.


**Table 7.** Economic results of small-scale anaerobic digestion plants over a 20-year lifespan.

The largest revenue generators were electricity sold to the national grid and thermal energy sold to a nearby district heating system (where available). These two applications should be key considerations in the planning process for any such development considered.

The capital expenditure required decreased significantly as the capacity of the plant increases, primarily due to the economies of scale that occur. In addition to the economic analysis of the scenarios under study, this work also explored the adoption of a capital gran<sup>t</sup> subvention in an attempt to provide a possible political pathway to increase the adoption of SSAD in Ireland. Such subvention has proven successful in countries such as Sweden, France, Wales and England, where capital grants of up to 50% have been applied [5]. As shown in Figures 5 and 6, the addition of a 50% capital subvention gran<sup>t</sup> had a significant impact on the scenarios payback periods, resulting in all scenarios having a discounted payback period of under 17 years, with herd sizes above 100 cows particularly attractive with a payback period of under eight years.

**Figure 5.** Comparison of discounted payback periods of scenarios.

**Figure 6.** Comparison of discounted payback periods of scenarios (including 50% capital subvention grant).
