2. Tutorial - Structure of energy system models

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.