Introduction
With TOP-Energy you can determine the optimal plant size for a new plant according to various criteria. Two different methods are available, depending on which energy system is involved and for how many plants the optimum size is to be determined.
Determining the Plant Size Using Sensitivity Analysis
The simplest way to optimize the plant size is to use the 
eSensitivity Sensitivity Analysis module. Sensitivity Analysis can be used to vary the nominal power of a plant or the capacity of a storage tank. A simulation with an Evaluation of Economical Efficiency is then carried out for each value of the Variable Parameter. As Sensitive Variables, you can select the target variables (primitives, e.g., Dynamic Payback Period, Overall Primary Energy, Internal Rate of Return).
The result is the size of the plant depending on the Variable Parameter. If this function has a minimum or a maximum, the optimum plant size is at this point.
The tutorial Structural Optimization of Combined Heat, Power, and Cooling shows the process. For example, the size of the CHP is optimized there. For this purpose, the Nominal Electrical Capacity is selected as a variable parameter. A simulation, a variant evaluation, and a variant comparison are carried out for each size. The Net Present Value and the Dynamic Payback Period of the investment in the CHP are then evaluated. The input data are shown in the following figures.
As a result, a diagram shows the Net Present Value as a function of performance. The Net Present Value reaches its maximum at 59 kW. Accordingly, a plant size of 59 kW is the best choice in relation to the Net Present Value.
Discrete Plant Sizes in Sensitivity Analysis
If the plant size in the analysis is to assume discrete values, you can specify discrete values in the Sensitivity Analysis. Instead of scanning the solution space linearly, the values from the table are then used and evaluated. The selection is shown in the following screenshot.
Varying Nominal Power and Investment Costs
In the course of the variation of the leading Variable Parameter (e.g., electrical Nominal Power), other parameters can also be varied in parallel. This is useful, for example, if the efficiency depends on the nominal power of the system. If, for example, the electrical efficiency increases with the size of the CHP, the Nominal Electrical Efficiency and the Fuel Demand at Nominal Operation (see the input data of the component) can be selected as second and third parameters in eSensitivity. Then the First value and the Last value belonging to the respective nominal power can be entered. The selection is shown in the following screenshot.
Determination of Plant Size by Structural Optimization
The optimal size of the plant can be found with the help of Structural Optimization as an alternative to Sensitivity Analysis. Structural Optimization is particularly useful when several systems need to be optimized simultaneously. This is not possible with a sensitivity analysis because it can only vary one parameter (and, as described above, directly proportional parameters).
For Structural Optimization, there is a separate set of components for which the nominal power is not specified, but rather optimized. Structural Optimization is described in detail in the article How Structural Optimization Works.
The process is also described in the tutorial Structural Optimization of Combined Heat, Power, and Cooling. In this tutorial, the optimum plant sizes for an Absorption Refrigerating System and a CHP unit are sought. The component templates Absorption_Refrigerating_System_Opt and CHP_Opt from the subfolders Structural Optimization are used for this purpose. In these cases, the nominal power is not specified but determined by optimization.
A disadvantage of Structural Optimization is the somewhat more difficult parameterization of the partial load behavior because this can only be specified via relative and not via absolute partial loads. Furthermore, the Electricity Tariff CHP Surcharge is not available for Structural Optimization, as it depends on the size of the plant.
