Performance Indicators

Scenarios for biomass and geothermal technologies are evaluated based on economic and environ- mental performance indicators (PIs).

Total annual cost

The total annual cost of the energy system (Ctot, Eq. C.1) is chosen as objective of the optimization

model under the assumption that for a given pathway the sizing and operation of the energy system is determined by economic criteria.

The total annual cost results from the sum of the total annualized investment and maintenance cost of technologies, and the yearly operating cost of resources. For technologies and resources within the city boundaries (in terms of ownership) investment and O&M costs are accounted for, while a purchasing price is attributed to all imported resources. As an example, if natural gas is imported by a public service provider, only the cost paid at the import is accounted for. The profit made by the public service provider when selling the resource to private consumers is not accounted for in the total cost as it constitutes just a transfer of money within the system. This global approach to urban energy systems cost calculation has two main advantages: i) it avoids the need of assuming prices for produced fuels and exchanges within the urban system boundaries and ii) it allows the definition of only one indicator representing the global cost for the public.

Technologies investment costs are annualized based on their economic lifetime. In this framework, annualized investment cost of existing technologies is also accounted for. This is coherent with the fact that at the end of their lifetime these technologies need to be replaced. In this way, the cost of technologies is spread over their whole lifetime, whereas financial depreciation would only attribute this cost to their early years of operation, leaving an upfront investment cost to future generations. As detailed in Appendix A, a real discount rate is adopted and cost values are expressed in real 2015 currency in order to provide a common basis for comparison.

LCA environmental impact indicators

As shown in Figure C.1, environmental impact indicators are calculated for each scenario after the optimization phase. Environmental impact is calculated following a LCA approach, i.e. taking into account emissions of technologies and resources from cradle to grave. The reference database for impact assessment is ecoinvent [168]. Data used in the model are reported in Appendix D. The impact categories of interest in this work are the GWP and the impact on human health, the latter included in order to account for the impact of biomass combustion. A different calculation approach is followed for these two categories.

For GWP calculation the “IPCC 2013 - GWP 100a” impact assessment method [263] is selected. The

global annual emissions GWPtot, expressed in ktCO2-eq./year, are calculated with an approach

symmetrical to the one used for the cost calculation (Eq. C.17). They are defined as the sum of the emissions related to the construction (C) and end-of-life (E) of the energy conversion technologies (TECH), allocated to one year based on the technology lifetime n, and the emissions related to resources (RES). The latter are the emissions associated to fuels (from cradle to combustion) and im- ports of electricity. For resources, the construction phase corresponds to the extraction, processing and transportation whereas operation (O) corresponds to fuel combustion. Operating emissions of technologies, mainly corresponding to auxiliary materials and maintenance, are accounted for only if they are non-negligible.

The conceptual separation between technologies and resources for GWP calculation allows the integration of biofuels without increasing the model complexity. As an example, Figure C.2 shows that when SNG is produced it can be input in the natural gas layer, thus replacing its fossil equivalent. As a consequence, the total GWP emissions are reduced as the utilization of the fossil natural gas resource is lower. If emissions related to combustion were allocated to technologies, instead, unit models would need to be duplicated in order to account for the different emissions of fossil resources and their biogenic alternatives.

GWPtot=  j∈TECH GWPC,E( j ) n( j ) + GWPO( j )  + i∈RES GWPC,O(i ) (C.17)

On the other hand, human health emissions are technology-dependent. In this case, combustion emissions can not be allocated to the resources as the combustion processes vary based on the

technology. Thus, for this category the operating emissions of technologies (human health (hh)O)

include the resource combustion as well. Extraction, processing and transportation remain allocated to the resources (Eq. C.18).

hhtot=  j∈TECH hhC,E( j ) n( j ) + hhO(i)  + i∈RES hhC(i ) (C.18)

Since there is no consensus on an impact assessment method for the human health indicator, two methods are chosen to address different aspects. The “impact 2002+” method [264], which includes an endpoint indicator for the human health, is widely used by the scientific community. It integrates a wide range of pollutants and health effects, such as respiratory effects, ionizing radiation or human toxicity. The Swiss Eco-factors 2013 [265], based on the method of the ecological scarcity (“ecoscarcity 2013”), provide a wide range of midpoint indicators related to specific environmental issues, and are based on the scientifically supported goals of the Swiss environmental policy. For this method, the category “main air pollutants and PM” has been chosen as another indicator representative of the human health. This indicator is more focused on air emissions, which are of particular concern in the case of wood combustion.