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P.W. Gri ffi n et al.
based on available statistical data, and uses this data to determine en- ergy use, output, energy intensity and other measures for which data is available. This approach has the advantage of covering a large pro- portion of energy demand, but it is limited by the level of disaggrega- tion available from industry-wide statistical sources. Thus, the conclu- sions that can be drawn from such top-down studies are often only indicative in nature. In contrast, a bottom-up approach would typically focus on a single industrial sub-sector. Energy use can then be separated into lower order sub-sectors, processes or manufacturing plants. The data used for this type of bottom-up study typically comes from more speci fi c information sources, such as trade associations, company re- ports, and case studies. Such a bottom-up study can therefore be useful in terms of presenting more accurate fi ndings [22,23], although it will be limited in the breadth of its application. An innovative hybrid approach was employed to develop the in- dustrial Usable Energy Database (UED) [9,10], produced by the present authors for the whole of the UK industrial sector as part of the research programme of the UK Energy Research Centre (UKERC). Aspects of both top-down and bottom-up models were adopted, with detailed bottom- up studies set within a top-down framework. Using this novel approach would normally entail focusing on a number of sub-sectors for the bottom-up study [7], with the remainder of the sector being treated in a generic manner. Sub-sectors that use a large amount of energy are obviously prioritised for bottom-up studies. In additional, sub-sectors that use energy in a relatively homogeneous manner are easier to analyse, and this may also be considered when selecting appropriate sub-sectors. Sub-sectors that are not the subject of detailed bottom-up modelling require a focus on the potential reduction in emissions through widely used, ‘ cross-cutting ’ technologies can be useful [7,9,10].
fuel consumption. Paper is formed and dried from pulp, and fi nished into paper products. Just two mills were fully integrated pulp and paper operations. Final energy demand at typical mills is dominated by the dryer section in which steam-heated cylinders heat the paper fi bres to around 100 °C [21]. The physical unit of production for the sector is tonnes of paper and board (tpb). UK sector energy demand in 2010 was 60 PJ; of which fuel demand was 53 PJ. Imported electricity was 8.5 PJ, whilst the corresponding power supplies exported was 1.5 PJ. The UK paper-making industry reduced its total energy consumption by 34% per tonne of paper made between 1990 and 2010 [19]. Production was 4.3 Mtpb in 2010; resulting in a direct speci fi c energy consumption (SEC) of 12.2 GJ/tpb and primary SEC of about 19 GJ/tpb. Energy costs amount to about 30% of the total cost of paper-making [19]. Direct GHG emissions were some 2.3 MtCO 2 e; a reduction of 42% over the period 1990 – 2010, due to investment in lower carbon energy sources [19]. The corresponding total emissions, including those attributable to net electricity, were 3.3 MtCO 2 e. Large and complex paper mills typi- cally take control of their energy supplies by building CHP plants that are more e ffi cient than separate supply of electricity and heat, and re- duce GHG emissions and generating costs [19]. A number of such CHP plants use biogenic (wood) waste, which is a renewable resource and gives rise to further reductions in GHG emissions. The UK paper sector is the largest user and producer of bioenergy in Europe [19].
3. Methods and materials
3.1. A Hybrid top-down/bottom-up approach
There are two broad ways to modelling the industrial sector [7]: top-down and bottom-up approaches, as illustrated in Fig. 5 (adapted and elaborated from those presented by Dyer et al. [21] and Gri ffi n et al. [7]). A top-down approach splits industry into sub-sectors, usually
Fig. 5. Schematic representation of an integrated top-down and bottom-up modelling approach for the UK industrial sector. Source: elaborated from the diagrams presented in Dyer et al. [21] and Gri ffi n et al. [7].
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