2. Tutorial - Structure of energy system models

Grids, nodes and units

To optimize an energy sytem, the real system must be mapped to a structured, simplified model, which can be understood by the computer. The model structure we use consists of grids, nodes and units. They will be explained in the following chapters.

Grids

Grids are essentially groups of nodes with a common form of energy. The primary purpose of the grid dimension is to organize the nodes into grids so that the results are easier to decipher. Furthermore, the diffusion and transfer of energy between nodes located in different grids is not permitted directly. Instead, controlled transfer of energy between grids is referred to as “conversion” and handled by certain types of units, which will be explained later. However, it would make no difference for the functioning of said conversion unit even if all the nodes were included in the same grid. Diffusion between grids is not currently possible, even though it could possibly have some niche applications.

Source: VTT. Energy Network Structure: Introduction to Grids, Nodes and Units. Gitlab.vtt.fi/backbone, 2019.

Nodes

Nodes are what constitute the “network” part of Backbone, and they are arguably the most important part of the model framework. The nature of the nodes depends heavily on their properties, which makes them a little difficult to explain in any concise way. However, the one common thing with all the nodes is that energy balance is enforced at each defined node.

Nodes have properties in addition to their (unique) name. The most important properties of nodes are the following:

• State: E.g. the energy content or the temperature of the node. Nodes are not required to have a state, which means that the node cannot store energy. The quality of the state is defined by various parameters, and one has to be careful to assign the values of said parameters correctly. This is one of the reasons why grids prevent direct energy transfer between each other, as direct transfer of e.g. temperature into electricity would not make sense. This is mostly just a safeguard, because linear conversion could be considered using transfer efficiency parameters between grids. Various boundary conditions can be imposed on the state of a node, ranging from the simple absolute upper and lower bounds to “softer” bounds that can be exceeded at the cost of a separately defined penalty. These bounds can be set to be constant, or follow some pre-determined time series. It is also possible to constrain a state of a node relative to the state of some other node.

• Transfer: Nodes can be connected to other nodes in the same grid via controlled transfer, which can be defined as both one- or two-directional. Naturally, boundaries on the transfer capabilities of nodes can be imposed using various parameters.

• Contain units: Even though units are a separate entity altogether, each unit must be connected to at least one node.

In backbone, fuels/commodity are also modelled as nodes, which lie within their own fuel grid. Units convert energy from the fuel node in the fuel grid to the electricity node in the electric grid.

Units

While nodes handle the flow of energy in the different grids, they lack the capability to create, consume, and convert energy between grids. Similar to nodes, there is a lot of different ways that units can be made to function, depending on the parameters given in the input data. The most important properties of units are briefly explained below.

• Generation and consumption of energy: The quintessential property of units is the capability to generate energy to a node, or consume it.

• While units can be defined to generate limitless free energy out of thin air, more often the generated energy is defined to increase the consumption of defined fuels (which usually have a cost attributed to them), or have limited generation capabilities based on data (solar, wind, hydro).

• Consumption of energy is treated as “negative generation” when it comes to units, and unless some form of energy conversion is defined for the unit in question, the consumed energy is transferred out of the model boundaries.

• While nodes could be used to emulate some of the functionality of the units, units provide parameters capable of defining the way energy generation and consumption work in much more detail.

• Conversion of energy between grids: While energy can diffuse and be transferred between nodes within each grid, units are currently the only way of transferring (referred to as conversion for units) energy between nodes in different grids.

• This functionality essentially means that a unit can be connected to multiple nodes, and the energy generation and consumption variables in each node are linked to each other according to desired conversion rules and constraints.

Task: Structure of energy system

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