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7. Terms in RAINS-OPT

This chapter describes important terms used the RAINS Optimization module.

Feasibility

An absolute prerequisite for the existence of an optimal solution is the feasibility of the problem, i.e., that at least one solution exists which satisfies the specified constraints. In formulating an optimization problem, two types of infeasibilities can occur:

• 'Policy constraints' on emissions could be outside the range of technically feasibility, as determined by the cost curves. For instance, one could impose for a country an upper limit on emissions, which is below the level of the maximum technically feasible reductions (i.e., the end of the cost curves).
• Specified deposition targets could be so low that they cannot be achieved even with the maximum possible emission reductions.

Since there exists no optimal solution for an infeasible problem, there is no use in starting the optimization solver. Consequently, the RAINS software checks already during the problem specification phase the feasibility of the specified constraints and informs the user if an infeasibility occurs. Infeasible policy constraints are identified immediately during the data entry. Selected deposition targets can be displayed in the deposition targets subwindow. To filter the infeasible targets, select the 'All infeasibilities' option in the deposition targets subwindow.

Target Deposition

The target deposition is the maximum allowed deposition for an EMEP-grid, used as constraint for the optimization. Deposition targets have to be specified by the user, and could be derived, e.g., from critical loads.

Deposition targets refer to the critical loads of a specific ecosystem and take account of all compounds contributing to the environmental effect under consideration (i.e., acidification and/or eutrophication). In line with the theory of critical loads, the relative importance of sulfur and nitrogen compounds is described for a selected ecosystem with a critical load function (see Figure below). For acidification, this function consists of two linear pieces: One horizontal line, limiting sulfur deposition, and one line giving the maximum sulfur/nitrogen deposition. The slope of this line is determined by the denitrification rate of deposited nitrogen compounds.

Figure 7.1: Effect of sulfur and nitrogen deposition on ecosystem:

In RAINS, deposition targets are expressed in acid equivalents per hectare per year, allowing for the substitution of sulfur and nitrogen compounds.

Policy Constraints

In RAINS, the optimization can vary emissions for each country between given limits. Basically, these limits are determined by technical aspects. The upper bound results from the unabated emission levels, i.e., if no emission control is applied to the projected levels of energy consumption. Conversly, the lower bound is determined by the maximum technically feasible emission reductions, i.e., if all available emission control measures are applied.

In practice, however, it proved useful to have the possibility of further constraining the allowed range of emissions, e.g., for taking account of already implemented measures, policy plans or for sensitivity analyses. Such additional limits are termed 'policy constraints'.

For technical reasons (e.g., for estimating costs of policies), policy constraints must lie within the constraints of the national cost curves. This means for instance that a policy target must not be lower than the emissions achievable by the application of the maximum technically feasible reductions.

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