Accounting land as natural resource for energetic and exergetic LCA: a new method

Life Cycle Assessment (LCA) is an environmental assessment methodology that considers the life cycle perspective of products (goods and services). It is divided in four steps, and in the Life Cycle Impact Assessment (LCIA) stage is where the potential environmental impacts are raised [1, 2]. Some other assessment methodologies can be considered as LCIA methods when the life cycle approach is considered. For energetic and exergetic analysis, three resource-based LCIA methods are found in literature: Cumulative Energy Demand (CED) [3], Cumulative Exergy Demand (CExD) [4], and Cumulative Exergy Extraction from the Natural Environment (CEENE) [5]. These methods are well structured for evaluation of fossil fuels, but for biofuels there is still room for some advances regarding the biotic resources, which in the one way are accounted by the energy or exergy content in biomass (in CED and CExD, respectively), while in other way the land area where the biomass is grown is accounted (CEENE). There is no consensus in which is the best approach, but the methods are flexible, and the researcher can decide what to account (the energy/exergy of the biomass or the land for growing). When this happens, special attention has to be given because only one way should be chosen; otherwise double-counting may occur. The way of accounting for land in the CEENE method (by solar radiation) can also be found in other energetic and exergetic analysis [6], but might be considered misleading, since the fraction of solar radiation used for photosynthesis is influenced by several factors, including water availability, soil quality, temperature, etc. The objective of this paper is to introduce a way of accounting for land through its potential Net Primary Production (NPP). NPP is the amount of biomass production of a certain area and it represents how much energy is available to transfer from plants to other trophic levels in the ecosystem [7, 8]. It is an output indicator influenced by several factors [9] and it is in accordance with ecosystem thermodynamics theory [10, 11]. The potential NPP is an estimation of the possible natural biotic production that would occur in a certain area if there was no land use or land use change. A regionalized database over the World is available [8]. Therefore, regarding the challenge of considering regional aspects in LCA [12], to account for land occupation through potential NPP might give better results since other site-dependent factors would be considered. In NPP data, the unit usually considered is mass of carbon (gC/ma) or dry matter (kgDM/ma), but since the aforementioned resource-based LCIA methods produce single score results (in energy or exergy), the units of NPP have to be transformed. This obstacle may be overlapped through the creation of generic energy/exergy conversion factors for biomass, which may be regionalized in biome level. This new approach was applied in a sugarcane case study from Brazil and the results were confronted with the three original LCIA methods. The analysis was considered until the farm gate (“cradle-to-gate”), and we used data from ecoinvent database. CED and CExD presented a slight increase on the total value (around 1%), while CEENE had a decrease of 42%. The Potential NPP appears to be a good indicator for accounting land as natural resource, having advantage of availability of a regionalized database. It is important to point that the potential NPP mentioned here is the “natural” NPP, considering that no land use change would have occurred; not a potential NPP “produced” by men through forestry or agricultural practices.