Energies 2019 , 12 , 247
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is available for use [40]. However, exergy is rarely, if at all, mentioned in the literature addressing energy management and using SEC. Instead, the same variable expressed in thermodynamic units per physical units is most often referred to as specific energy consumption. The wording of the term can be misinterpreted, because it is the exergy that is used during the process—the part of the energy that is available to be used for work. Some studies refer to the same term as “typical energy demand”, e.g., power demand and heat demand per paper grade [38] or SEC—specific energy demand [41]. In conclusion, it would be better to use the thermodynamically correct names, i.e., specific exergy consumption or specific energy use. Nevertheless, although specific exergy consumption and specific energy use are thermodynamically correct, it could be possible in theory that the change of names could cause some unforeseen/unintended challenges. 4.2. Calculations of SEC Generally (e.g., [8]), SEC is calculated as a ratio of energy used for producing a product:
Energy used Product s amount
SEC =
(1)
Units are seemingly chosen arbitrarily depending on the intended usage of SEC, e.g., GJ/t for heat (e.g., [10]), and GWh/kt for electricity (e.g., [6]). When calculating the total energy, including electricity and heat, units are also chosen arbitrarily depending on the purpose of the study. For example, GJ/tonne as in e.g., [8,10], while Lawrence et al. [6] used GWh/kt because some of the large companies also used GWh/kt for their SEC in their annual reports (e.g., [42,43]). Calculation of SEC as presented in the Equation (1) is generic, meaning that generally SEC is calculated as a ratio between the energy used and a unit of product. However, Equation (1) can be modified if needed for specifying the type of energy carriers and/or product. For example, Farla et al. [8] calculated SEC for one single individual product for one single individual process as:
Ex Px
(2)
SECx =
In Equation (2), Ex—the energy use of the single individual manufacturing process x and Px—physical unit of a single individual product x. Meanwhile aggregated SEC (SECagg) for a group of products, e.g., for a sector, can be calculated as a ratio of the sum of all the energy used with the sum of all the products produced, providing the production output has the same composition during the time that is to be compared [8]:
∑ E ∑ P
SECagg =
(3)
In Equation (3), ∑ E—sum of all the energy used and ∑ P—sum of physical units of all products. Following the principles of Equations (1)–(3), in order to evaluate the usage of individual energy carriers for individual products or a sum of products the individual energy carriers can be used instead. For instance De Beer et al. [16] calculated SEC for electricity and SEC for heat. Furthermore, Laurijssen et al. [10], calculated specific primary energy use by a process unit:
Epr P
(4)
SECpr =
In Equation (4), Epr—the total annual primary energy used by a process unit and P—the annual amount of products processed by the specific process unit. Generally, energy use is related to weight units of an air-dried product, which is assumed to consist of 90% oven-dry pulp and 10% water content, even if the actual conditions vary in different environments [44]. The majority of the studies that have used SEC neither mentioned anything about the moisture content nor correction for the moisture content. The exception was a study at process
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