Beyond the general rules for constructing variable names described before, for energy-related variables the naming convention follows the general structure of energy balances which groups all energy use along the conversion chain into the following categories (note that not all of these categories are included in all energy balances):
Primary Energy[|Source][|Secondary Energy Carrier] , where "Source" might include several hierarchy levels, for example specifying a technology or process that the source was used with (e.g., Wind|Onshore, Solar|PV, Coal|w/ CCS). The optional �Secondary Energy Carrier� can be appended to count the use of the primary energy source in a certain conversion technology or sector that generates this secondary energy carrier (e.g., Electricity). In case the �Secondary Energy Carrier� is added, the technology or process addition to the source would be moved to the end of the variable name as this would further specify the conversion technology (i.e. Primary Energy|Coal|Electricity|w/ CCS instead of Primary Energe|Coal|w/ CCS|Electricity). It should be noted that the different �Secondary Energy Carrier� categories belonging to one �Source� in general do not add up to the total, because at the regional level the total may be composed of a combination of unprocessed primary energy (e.g., crude oil) and processed primary energy (=secondary energy, e.g., petroleum products). The purpose of these variables is primarily to be able to calculate conversion efficiencies from primary to secondary energy and therefore this non-additivity may not be a problem.
There are a number of different subcategories which are typically reported for primary energy. A standard primary energy variable tree is shown in the figure below with folders representing totals that the subcategories within these folders should add up to. It includes the breakdown of combustible fuels (fossil and biomass) into application with and without Carbon Capture and Storage (CCS). For example, the sum of Primary Energy|Coal|w/ CCS and Primary Energy|Coal|w/o CCS should add up to Primary Energy|Coal.
In addition to the breakdown in the standard tree above, various other subcategories exist in the set of primary energy variables which also follow an hierarchical structure in which subcatgories should add up to subtotals. In the figure below, the variables in the various folders illustrate which subcategories should add up to subtotals. For example, Primary Energy|Fossil should be the sum of Primary Energy|Coal, Primary Energy|Oil and Primary Energy|Gas. Note that variables can occur multiple times in the list below, because aggregation along different dimensions is possible (e.g., Primary Energy|Biomass can be broken down by application in traditional vs. modern biomass or by type of feedstock.
The accounting of combustible sources, including all fossil energy forms and biomass, includes some ambiguities related to the definition of the heating value of combustible fuels. The higher heating value (HHV), also known as gross calorific value (GCV) or higher calorific value (HCV), includes the latent heat of vaporisation of the water produced during combustion of the fuel. In contrast, the lower heating value (LHV) (also: net calorific value (NCV) or lower calorific value (LCV)) excludes this latent heat of vaporization. For coal and oil, the LHV is about 5% less than the HHV, for most forms of natural and manufactured gas the difference is 9-10%, while for electricity and heat there is no difference as the concept has no meaning in this case (IEA, 2011). The reporting of energy quantities, including primary energy, should use the LHV which is consistent with the IEA energy balances.
The accounting of primary energy from non-combustible energy sources, i.e. nuclear energy and all renewable energy sources except biomass, is more ambiguous and different conventions exist. Primary energy reporting should follow the direct-quivalent accounting method which counts one unit of secondary energy provided from non-combustible sources as one unit of primary energy, i.e. 1 kWh of electricity or heat is accounted for as 1 kWh = 3.6 MJ of primary energy.
Traditional biomass which is used predominantly in the residential sector of many developing countries should also be reported as part of primary energy use. If it is not represented, it should be documented making use of the comments tab in the reporting template. To be able to compare indentical quantities across models, it is important to report traditional biomass use separately under Primary Energy|Biomass|Traditional if it is represented in a specific model (Figure 2).