Energies 2019 , 12 , 247
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level presented by Laurijssen et al. [10] that the SEC values for sections pre- and after drying were showed in units of absolute dry end product not in units of absolute dry product that was processed for comparing the two dryers. Calculations of both of the variables “energy” and “product” are based on assumptions. Roughly speaking, the variable “product” relies on several assumptions. One of the assumptions regards the definition of “product”—is it product sold, i.e., all the produced products minus the products of inadequate quality that could not be sold, or is it all the products produced including the products of inadequate quality to be sold? For example, during the production of paper, e.g., when changing from the production of one paper grade to another or when a paper web breaks, some paper that was produced can be classified as internal waste due to inadequate quality [10]. The internal waste can be re-pulped, which could eliminate or minimize material waste. Nevertheless, the energy that was used for the production of paper of inadequate quality has been wasted [10]. Another example is assumptions originating from the challenges involved in dealing with partitioning of the products, i.e., joint production could cause difficulties in allocating how much energy was required for one specific product [12]. It is possible that this problem can be overcome by individual factories having access to e.g., continuous data, however such data is rarely—if ever—all available to external parties [12]. Furthermore, uncertainties in energy use because of variations in the analysed system boundaries can also be a challenge. For example, when benchmarking at process level, uncertainty can originate from a lack of clarity about whether only the energy used by the main equipment is included in the analyses. According to Laurijssen et al. [10], it is often the case that only the energy used by the main equipment is included and that used by auxiliary equipment is excluded, even though the auxiliary equipment (e.g., pumps, peripheral systems for water) also uses energy and is needed in production. For example, >50% of the electricity used for drying pulp can be used for pumping [20]. Furthermore, according to Thollander et al. [45], the primary energy use is a function of the sum of the production processes and the product-dependent support processes’ energy use per product multiplied by the number of products, plus the support processes’ energy use. This ax + b function also tells us that the primary energy use is a function of the number of products, but also consists of a base load. This function differs tremendously between different types of companies, where for example a non-energy intensive mechanical engineering company may have support process-related base load energy use of 70–80% on an annual basis, while the same figure for an energy-intensive pulp mill might be few percent in comparison to the energy used, depending on production. Consequently, differences in how to set system boundaries can make the benchmarking of similar processes between different studies challenging. The studied system boundaries for calculating SEC varied substantially in the studies presented in Tables. Generally, the precision of the boundary description tended to increase the more detailed the intended benchmarking by using SEC such as in benchmarking at process level. Namely, detailed descriptions were common while studying SEC at process level, as in e.g., refs. [10] or [7], whereas for national and international comparison general description such as e.g., PPI in the country when studying SEC at national level without mentioning whether energy was used for not directly production-related processes such as support processes e.g., [6,46]. Regarding the total amount of energy, uncertainty comes from using energy from non-primary energy carriers, such as electricity. Since electricity is not a primary energy source, in order to calculate the total energy used, the amount of electricity needs to be multiplied by the primary energy factor (PEF). For example, the fixed PEF in Europe is 2.5 and emanates from the European Energy Efficiency Directive [47]. Using fixed PEF is convenient, but actual PEFs are directly related to country- and year-specific conditions which can deviate from the fixed PEF. This is due to the fact that PEFs tend to vary over time depending on the mix of energy carriers that were used to produce electricity specifically for individual countries. Using the fixed PEF has been criticized by e.g., [47]. PEF converts the usage of non-primary energy carriers such as electricity to the usage of primary energy carriers. PEF is the ratio of primary energy use with the final energy use [48], and can be shown as in [45]. In Sweden, for example, the annual gross electricity, which is a non-primary energy carrier, production
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