2.3. Energy
To investigate sustainability in a sub-system of planet Earth any energy transfer into the sub-system will be disallowed except for sustainable energy such as direct solar radiation (for example for plant growth), indirect solar energy (wind power, hydro power), or geothermal energy across the system boundary. Otherwise, the sub-system must be enlarged to include the energy source, for example fossil-driven power plants and their fuel reservoirs for electricity, fossil fuel fired boilers (with their fuel reservoir) for steam production, etc. It is extremely important to include all non-sustainable energy sources within the system boundaries. Otherwise, one can certainly chemically convert, for example, virtually any carbon source into virtually any desired liquid carbon-based fuel, given a sufficient quantity and quality of energy. Quantifying sustainability would be meaningless with vast non-renewable energy resources available at will since the “behind the stage” energy production may or may not be sustainable.
To investigate sustainability in a sub-system of planet Earth any energy transfer into the sub-system will be disallowed except for sustainable energy such as direct solar radiation
A familiar example may be instructive. To evaluate sustainability for example of a biofuel such as corn-based ethanol one can conceive a first sub-system that comprises the land to grow corn, atmosphere and water needed, transportation and cultivation systems, the biomass-to-ethanol conversion process, and the end use of the bio-ethanol, all enclosed by a virtual system boundary (Fig. 1 right, dashed line, arrows indicate major carbon mass flows, not all flows are shown for simplicity). Individual items shown for the sub-system in Fig. 1 are unit operations in chemical engineering terminology. Steady-state is defined as, on average, no accumulation or depletion of mass over time within a unit operation. The mass flows (here for carbon, similarly for any other chemical element, or total mass) into and out of each individual unit operation must be balanced since otherwise the unit operation is not sustainable due to mass depletion or accumulation with time. If a unit operation “soil” for example contains a certain volume of agricultural land including the soil to some depth then the carbon flows into and out of this unit operation must balance since a net outflow will alter and perhaps degrade the land and a sustained net inflow of carbon will raise carbon concentrations steadily until agriculture will be impacted. This is qualitatively and mathematically shown in Fig. 3. Usually mass flows from different information sources have to be used for complex unit operations such as “soil” which always introduces issues of consistency. However, there is a built-in check with a mass balance based analysis since the mass flows must add up to zero. This rigorous check on data consistency is absent in LCA, which also does not allow for elemental balances.
2.3. Energy
To investigate sustainability in a sub-system of planet Earth any energy transfer into the sub-system will be disallowed except for sustainable energy such as direct solar radiation (for example for plant growth), indirect solar energy (wind power, hydro power), or geothermal energy across the system boundary. Otherwise, the sub-system must be enlarged to include the energy source, for example fossil-driven power plants and their fuel reservoirs for electricity, fossil fuel fired boilers (with their fuel reservoir) for steam production, etc. It is extremely important to include all non-sustainable energy sources within the system boundaries. Otherwise, one can certainly chemically convert, for example, virtually any carbon source into virtually any desired liquid carbon-based fuel, given a sufficient quantity and quality of energy. Quantifying sustainability would be meaningless with vast non-renewable energy resources available at will since the “behind the stage” energy production may or may not be sustainable.
To investigate sustainability in a sub-system of planet Earth any energy transfer into the sub-system will be disallowed except for sustainable energy such as direct solar radiation
A familiar example may be instructive. To evaluate sustainability for example of a biofuel such as corn-based ethanol one can conceive a first sub-system that comprises the land to grow corn, atmosphere and water needed, transportation and cultivation systems, the biomass-to-ethanol conversion process, and the end use of the bio-ethanol, all enclosed by a virtual system boundary (Fig. 1 right, dashed line, arrows indicate major carbon mass flows, not all flows are shown for simplicity). Individual items shown for the sub-system in Fig. 1 are unit operations in chemical engineering terminology. Steady-state is defined as, on average, no accumulation or depletion of mass over time within a unit operation. The mass flows (here for carbon, similarly for any other chemical element, or total mass) into and out of each individual unit operation must be balanced since otherwise the unit operation is not sustainable due to mass depletion or accumulation with time. If a unit operation “soil” for example contains a certain volume of agricultural land including the soil to some depth then the carbon flows into and out of this unit operation must balance since a net outflow will alter and perhaps degrade the land and a sustained net inflow of carbon will raise carbon concentrations steadily until agriculture will be impacted. This is qualitatively and mathematically shown in Fig. 3. Usually mass flows from different information sources have to be used for complex unit operations such as “soil” which always introduces issues of consistency. However, there is a built-in check with a mass balance based analysis since the mass flows must add up to zero. This rigorous check on data consistency is absent in LCA, which also does not allow for elemental balances.
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