The definition of boundaries in an energy analysis is as crucial as the one adopted for the entire LCA; usually, after an initial definition of the boundaries in terms of LCA, there is a subsequent redefinition in order to discard insignificant energy contributions: regarding the classification of energies in an energy analysis, energy inputs are accounted for based on the type of contribution and not on their nature.
1. Investment Energy: The first contribution to the overall energy load comes from the implementation of machinery and infrastructure necessary for the system itself to achieve the desired function. This energy component corresponds economically to investment costs (capital cost) and for this reason is commonly referred to as "investment energy" or "capital energy." Investment energy and the associated emissions can also be significant due to the large amounts of materials used. The period of use of machinery and infrastructure is usually quite long, which is why the contribution in terms of an eco-profile is generally considered negligible when compared with that of any other product. However, there are cases where such energy cannot be neglected; examples include transport systems, oil extraction operations, limited and/or experimental productions.
2. Direct and Indirect Energy: The first represents the energy consumed by the process operation while the second includes the energy needed to produce and transport energy and the materials used in the production process. With regard to indirect energy, it is important to emphasize the fundamental contribution due to the production and transportation of the fuels directly used in the process: this portion is defined as "production and delivery energy." From an operational point of view, to determine the shares of direct and indirect energy of a production system, the basic criterion of processes is used, which involves dividing production into two phases: the first one including the obtaining of raw materials (materials and energy) and their transportation, while the second one involves the transformation of these into the desired product. It is thus correct to consider as direct consumption of materials and energy that related to the activity in question (process energy) while indirect consumption is related to everything upstream or downstream of the activity considered that enables its realization (plant energy). Optimization is achieved when the sum of the two energy components is minimal (Silvestri, 1988). When designing new plants, it is therefore necessary to properly assess the plant exergy share: if this share is not recovered within a period of time equal to the attributable life of the plant, the overall operation would correspond to a loss-making investment.
3. Feedstock Energy: Represents the energy contained within materials, potentially combustible, which are used as such: an example is organic products used in the petrochemical industry. This portion is defined as the energy content of the input materials which in principle can potentially be recovered from the outgoing products (e.g., combustion of plastic or paper). Keeping separate the energy spent as fuel from that contained in potentially combustible materials is important because, while the former is irreversibly lost, the latter is still potentially available at the end of the product's useful life. It is also important to remember that the energy content of the inputs may differ from that of the outputs due to changes in chemical structure or material loss during the production process; consequently, evaluating feedstock energy based on the energy content of the outputs can be misleading. Feedstock thus refers to the calorific potential of the input materials of the system, conventionally referring to their higher heating value.
4. Energy Provided by Workers: Energy supplied by workers participating in a production process essentially comes from the food they are fed. Considering a typical situation (daily intake of about 40 kcal per kg of weight, equivalent to 0.1672 MJ/kg of weight), minus the energy used for vital activities maintenance (around 80%), the energy available for useful work is about 2-3 MJ per day, which is less than 1 kWh. This is essentially a negligible contribution.
In summary, it can be said that the overall energy consumption of a production system or services is given by the sum of all relevant energy shares of each individual operation, namely direct energy, indirect energy, and feedstock energy. The total energy consumption corresponds to the total energy that needs to be "extracted" from the Earth in order to have that specific unit of goods available. This share is also known as "gross energy requirement" (GER), defined as the energy that must be made available under normal conditions from energy resources in their natural state and consumed by the system in order to keep the same system in production.
