The functional units and the four levels of modelling

To address the goal of the project and provide support for decisions on the strategy for developing the Danish energy system, the modeling is performed and results are presented at four levels:

1 A unit process database including data on input and output flows of the biomass types and conversion technologies. This is provided in appendix H 2 LCA of individual biomass conversion pathways expressed per unit of

functional output (e.g. 1 kWh electricity)

3 LCA of individual biomass conversion pathways expressed per unit (MJ) of biomass input

4 LCA of whole energy systems expressed per unit of total functional output from the Danish energy and transport system

These modelling levels will be described in this section.

3.6.1 Modelling level 1

At the modeling level 1, a database is established providing the basic data on the biomass conversion technologies.

This conversion technology database at level 1 comprises data on biomass input and energy inputs to the conversion process, as well as emission data. Also data on energy conversion efficiencies and functional outputs. In case the conversion process implies several functional outputs, these are all maintained as such, and no allocation of data between outputs is done. More unit operations may be covered

by the data, and a simple outline of the involved unit operations will be included as illustrated in figure 3-2

Figure 3-2 Example of a process flow diagram illustrating the involved operations represented by the data

The illustrated process Flow diagram in Figure 3-2 and the subsequent Figure 3-3 to 4-5 are only meant to illustrate the principle of modelling. The real model for the case of Straw CHP is shown in Chapter 4.

3.6.2 Modeling level 2

Modeling level 2 comprises the Carbon Footprint assessments of the biomass conversion pathways expressed per functional output for each of the studied functional output types, i.e.:

› 1 kWh of continuous power production

› 1 kWh of flexible power production

› 1 MJ of heat - industrial process heat/steam or district heating

› 1 MJ of transport fuel

The results are normalized per one selected functional output by eliminating any other outputs by expanding the system with the avoided alternative for other outputs. See the illustration in Figure 3-3:

Figure 3-3 Principle of the process flow diagram behind the biomass conversion pathway LCAs at level 2

The model also includes the avoided marginal use of the land and/or the residual biomass when relevant. Furthermore, it includes induced and avoided marginal energy supplies.

The functional unit used to normalize results depends on the conversion pathway and the type of functional output in question, cf. the list above.

The Carbon Footprint results at this level allow comparing one pathway providing a given functional output (like e.g. continuous/non-flexible power) with another pathway providing the same type of functional output, but comparisons between pathways providing different types of functional outputs cannot be done at this level.

The Carbon Footprint assessments at this level aim at answering questions like e.g.:

How can flexible power be produced with the lowest potential impact (with respect to the Carbon Footprint) – under the energy system assumptions and other future framework conditions in question?

3.6.3 Modeling level 3

Modeling level 3 comprises the Carbon Footprint assessments of the biomass conversion pathways expressed per biomass input, i.e. per 1 MJ of biomass input, to the specific biomass conversion pathway in question. An example of a system modeled at level 3 can be seen in Figure 3-4:

Figure 3-4: Principle of the process flow diagram behind the biomass conversion pathway LCAs at level 3

The difference between the model at level 2 and 3 is that all functional outputs are modeled to replace the alternative (marginal) supply of the same functional outputs in the studied system, including the primary functional output. In this way,

conversion pathways providing different types of functional outputs can be compared, allowing to answers questions like: How is a given biomass type best

used (with respect to the Carbon Footprint) – under the energy system assumptions and other future framework conditions in question?

3.6.4 Modeling level 4

Modeling level 4 comprises the Carbon Footprint assessments of the entire Danish energy system expressed per one and the same total functional output of the whole energy and transport system. The functional output delivered by each system is based on data from the CEESA study (Lund et al., 2011). This study comprise a thorough investigation of the Danish energy and transport systems and included a variety of analyses of energy savings and structural changes (for example some personal transport shifting to rail) prior to finally defining the demanded end use services of energy and transport.

The defined annual energy consumption and transport demand identified in the CEESA study and used in the present study divided by type is found in Table 3-1.

Table 3-1 Annual demand for energy and transport used as functional unit in the whole-system designs of this study

Traditional 88.8 PJ/year 171.7 PJ/year 76 PJ/year 163,000

Mpkm²/year

164

Mtkm³/year

The total share of the transport demand covered by aviation is believed to consume 24.5 PJ jetfuel annually by 2050. The level 4 modelling has paid special attention to pathways for jetfuel production, because of the unique high quality standards defining jetfuel as a petrochemical product. The study has identified and included pathways able to produce synthetic jetfuel from biogenic resources - both with and without the addition of hydrogen. The process flows of these pathways and how they were included in the modeling of each system can be seen in appendix J.

Additionally, and considered as part of the functional output, each system design is to return 169 kT of ‘soil-stable’ carbon to the agricultural sector. In this context

‘soil-stable’ carbon is defined as the biologically slowly degradable part of the straw carbon, which stays in the soil over time. To this end, each system design must either return this after a conversion pathway, e.g. as digestate from biogas fermentation, or plough down a portion of the straw, which in turn prevents this from being used for energy purposes.

An example of the system modeled at level 4 is shown in sections 4.2 and 5.1.

Modeling level 4 is provided and applied in this project to allow comparing full system designs in which the complex system network itself defines the induced and

2 Person km

3 Tonne km

avoided marginals of heat and power. The reason for including this modeling level is that:

› the decisions and comparisons to be supported by the Carbon Footprint assessments in this project involve long term system designs which are not yet implemented. Thus the degree of freedom to choose and also compare

different approaches to and designs of a renewable energy system in e.g. 2050 is large.

› the fully renewable energy system is characterized by a very large degree of system integration, i.e. creating links and synergies between producers (e.g.

wind turbines) and consumers (e.g. battery cars and heat pumps) of electricity as well as conversions between electricity and transport fuels through

interactions between electrolysis, hydrogen and biomass conversion pathways.

› biomass is a constrained resource and potentially the main contributor to environmental impacts in the fully renewable energy system. These overall environmental properties of the whole system, therefore, lies in the elegance and synergy created in the whole system design, and not in the environmental performance of the individual conversion pathways seen in isolation.

These aspects inherently renders it quite difficult to assess a given biomass conversion pathway in isolation, as it to a wide extent is its system integration qualities that renders it attractive, and also at the end leads to its resulting Carbon Footprint implications in a systems perspective.