# Basics of the model structure

In SpineOpt.jl, the model structure is generated based on the input data, allowing it to be used for a multitude of different problems. Here, we aim to provide you with a basic understanding of the SpineOpt.jl model and data structure, while the Object Classes, Relationship Classes, Parameters, and Parameter Value Lists sections provide more in-depth explanations of each concept.

## Introduction to object classes

Essentially, Object Classes represents different types of objects or entities that make up the model. For example, every power plant in the model is represented as an object of the object class unit, every power line as an object of the object class connection, and so forth. In order to add any new entity to a model, a new object has to be added to desired object class in the input data.

Each object class has a very specific purpose in SpineOpt.jl, so understanding their differences is key. The Object Classes can be roughly divided into three distinctive groups, namely Systemic object classes, Structural object classes, and Meta object classes.

### Systemic object classes

As the name implies, system Object Classes are used to describe the system to be modelled. Essentially, they define what you want to model. These include:

• commodity represents different goods to be generated, consumed, transported, etc.
• connection handles the transfer of commodities between nodes.
• node ensures the balance of the commodity flows, and can be used to store commodities as well.
• unit handles the generation and consumption of commodities.

### Structural object classes

Structural Object Classes are used to define the temporal and stochastic structure of the modelled problem, as well as custom Unit Constraints. Unlike the above system Object Classes, the structural Object Classes are more about how you want to model, instead of strictly what you want to model. These include:

### Meta object classes

Meta Object Classes are used for defining things on the level of models or above, like model output and even multiple models for problem decompositions. These include:

• model represents an individual model, grouping together all the things relevant for itself.
• output defines which Variables are output from the model.
• report groups together multiple output objects.

## Introduction to relationship classes

While Object Classes define all the objects or entities that make up a model, Relationship Classes define how those entities are related to each other. Thus, Relationship Classes hold no meaning on their own, and always include at least one object class.

Similar to Object Classes, each relationship class has a very specific purpose in SpineOpt.jl, and understanding the purpose of each relationship class is paramount. The Relationship Classes can be roughly divided into Systemic relationship classes, Structural relationship classes, and Meta relationship classes, again similar to Object Classes.

### Systemic relationship classes

Systemic Relationship Classes define how Systemic object classes are related to each other, thus helping define the system to be modelled. Most of these relationships deal with which units and connections interact with which nodes, and how those interactions work. This essentially defines the possible commodity flows to be modelled. Systemic Relationship Classes include:

### Structural relationship classes

Structural Relationship Classes primarily relate Structural object classes to Systemic object classes, defining what structures the individual parts of the system use. These are mostly used to determine the temporal and stochastic structures to be used in different parts of the modelled system, or custom Unit Constraints.

SpineOpt.jl has a very flexible temporal and stochastic structure, explained in detail in the Temporal Framework and Stochastic Framework sections of the documentation. Unfortunately, this flexibility requires quite a few different structural Relationship Classes, the most important of which are the following basic structural Relationship Classes:

Furthermore, there are also a number of advanced structural Relationship Classes, which are only necessary when using some of the optional features of SpineOpt.jl. For Investment Optimization, the following relationships control the stochastic and temporal structures of the investment variables:

For Unit Constraints, which are essentially generic data-driven custom constraints, the following relationships are used to control which variables are included and with what coefficients:

### Meta relationship classes

Meta Relationship Classes are used for defining model-level settings, like which temporal blocks or stochastic structures are active, and what the model output is. These include:

## Introduction to parameters

While the primary function of Object Classes and Relationship Classes is to define the system to be modelled and it's structure, Parameters exist to constrain them. Every parameter is attributed to at least one object class or relationship class, but some appear in many classes whenever they serve a similar purpose.

Parameters accept different types of values depending on their purpose, e.g. whether they act as a flag for some specific functionality or appear as a coefficient in Constraints, so understanding each parameter is key. Most coefficient-type Parameters accept constant, time series, and even stochastic time series form input, but there are some exceptions. Most flag-type Parameters, on the other hand, have a restricted list of acceptable values defined by their Parameter Value Lists.

The existence of some Constraints is controlled based on if the relevant Parameters are defined. As a rule-of-thumb, a constraint only gets generated if at least one of the Parameters appearing in it is defined, but one should refer to the appropriate Constraints and Parameters sections when in doubt.

## Introduction to groups of objects

Groups of objects are used within SpineOpt for different purposes. To create a group of objects, simply right-click the corresponding Object Class in the Spine Toolbox database editor and select Add object group. Groups are essentially special objects, that act as a single handle for all of its members.

On the one hand, groups can be used in order to impose constraints on the aggregation of a variable, e.g. on the sum of multiple unit_flow variables. Constraints based on parameters associated with the unit__node__node, unit__to_node, unit__from_node, connection__node__node, connection__to_node, connection__from_node can generally be used for this kind of flow aggregation by defining the parameters on groups of objects, typically node groups. (with the exception of variable fixing parameters, e.g. fix_unit_flow, fix_connection_flow etc.). See for instance constraint_unit_flow_capacity.

On the other hand, a node group can be used to for PTDF based powerflows. Here a node group is used to enforce a nodal balance on system level, while suppressing the node balances at individual nodes. See also balance_type and the node balance constraint.