Energy systems

In the simulation model technical equipment units are connected to each other to form a network of components across which the use of energy is balanced. The specific way the components are connected is named energy system. In the following any change in how the components are connected is considered to result in a similar but different energy system. An example of a multi-sector energy system for an urban strict of two buildings is shown in the figure below.

A multi-sector example energy system

There, arrows indicate the flow of energy between the components, with the colors of the arrows representing the respective medium. A main heat bus supplies two secondary heat busses, which each connect with a buffer tank and the demand side of a building. The main heat bus draws heat from the waste heat of a hydrogen electrolyser (HEL), elevated to the required temperature by a heat pump, as well as a combined heat-and-power plant (CHPP). A gas boiler supplies one of the buildings for additional peak load capacity. In order to shift available energy between seasons, a seasonal thermal energy storage (STES) is connected to the main heat bus.

Such an energy system requires several operational strategies and control mechanisms to operate as expected. How these can be modeled for this example is described in more detail in the chapter on operation and control. For the following we focus on the graph structure that is formed from the components and the connections between them.

Characteristics of an energy system

The example in the introduction above shows some of the important characteristics that an energy system requires for a correct simulation. Using one of the available graphical user interfaces to construct an energy system should automatically fulfill these as invalid configurations are not allowed, but it is good to keep these in mind when writing a project file from scratch.

No unmatched inputs / outputs

Each component has defined inputs and outputs. A necessary condition for the simulation to work is that every input and output connects to something. If an input or output is not relevant for the analysis / question that underlies the simulation, an undesired input or output can be connected to a grid connection that does not appear in the simulation output.

Bidirectional flow only for storage

Energy flow is always in one direction, namely from the output of one component to the input of another. While it is possible to construct a cycle by connecting the output of a component to the input of another component and in turn the output of that back to the the input of the first component, this will lead to problems in calculating the correct order of execution.

The exception to this rule are storage component, which typically are connected to both the input and output of a bus component. In the example energy system this is displayed as a bidirectional arrow between the bus and the storage.

No mixing of media

Along with the number of inputs and outputs a component has, each also has a defined medium. This is used to ensure that the components in an energy system are connected in such a way that the output of a component works on the same medium as the input of the other component. It is not possible to connect mismatching inputs / outputs.

Exactly one component per input / output

Each output of a component must connect to exactly one input of another component and vice versa. If there is a need to connect multiple components to the same receiver, there must be a bus used as an intermediary. This was chosen for several reasons:

  • It simplifies calculations as there is always exactly one other component to consider.
  • It improves readability of the energy system as, while it requires additional busses, it reduces the overall connectedness of the graph.
  • It enables an important layer of control behavior as input priorities can be handled by the intermediate bus.
  • Storage components work better if they are connected to a bus because large demand fluctuations might overload the storage capacity leading to flickering values of the storage as it is filled or depleted within a single timestep. Being connected to a bus enables the energy sources on the same bus to side-step the storage and feed into the demand side directly.

Energy media

All energy handled by the simulation model exists in the form of some medium that carries that energy. This includes but is not limited to the two major forms of energy used in buildings, namely electricity and heat. For a complete physical simulation each medium would carry its own set of restrictions and modeling challenges, however for ReSiE this was simplified. Different media are reduced to categories that differ only in name.

For example, alternating current of a certain voltage can be converted to a different voltage or to direct current. The actual energy carried by this current is not simply a scalar value but depends on how the current is used to perform work. For the simulation model this exact simplification has been done, which results in different energy media for different currents, each of which carries a scalar amount of energy. For other physical media (especially water) similar simplifications are used.

The following lists which media are currently implemented as default and what they represent.

Note: Temperatures of fluids are crucial for a correct simulation even in this simplified model due to how they are utilized to carry energy and how they affect the efficiency and performance of energy system components. Currently, this is implemented in a simplified way where temperatures are handed over between components together with the supplied or requested energies. This does not allow to simulate the temperature change in a district heating grid due to energy mismatch of supply and demand! This may change with later versions of ReSiE. Currently, if different temperatures are present at one bus, always the highest temperature will be set as bus temperature which can lead to inefficient operation of connected components like a HP.

Electricity

  • m_e_ac_230v: Household electricity at 230 V AC and 50/60 Hz.

Chemicals - Gasses

  • m_c_g_natgas: A natural gas mix available through the public gas grid.
  • m_c_g_h2: Pure hydrogen gas.
  • m_c_g_o2: Pure oxygen gas.

Heat - Low temperature water

Different low temperature regimes:

  • m_h_w_lt1
  • m_h_w_lt2
  • m_h_w_lt3
  • m_h_w_lt4
  • m_h_w_lt5

Heat - High temperature water

Different high temperature regimes:

  • m_h_w_ht1
  • m_h_w_ht2
  • m_h_w_ht3
  • m_h_w_ht4
  • m_h_w_ht5

User definable media names

The names of all media can also be user-defined. Therefore, the name of each medium of each in- and output of all components can be declared in the input file. Alternatively, only a few default media names can be overwritten by user-defined media names. They have to match exactly the medium name of the interconnected component.

For busses, grids, demands, storages (except seasonal thermal energy storage), the medium name of each component can be given with the specifier medium (String) in the input file. For transformers and seasonal thermal energy storages, user-definable media names of each in- and output can be given using the specifier m_heat_in, m_heat_out, m_gas_in, m_h2_out, m_o2_out, m_el_in or m_el_out depending on the inputs and outputs of a transformer.

Interfaces

When writing the implementation of components a problem has emerged in the functionality handling the processing1 of energy. There must be a way to track the energy balances between components which is the same for all types of components, so that the processing code does not need to know which types of component it can connect to and how to transfer energy. In particular this has been shown to be a problem with control and processing calculations for components that are supposed to feed into a demand and fill a storage at the same time.

Illustration how interfaces connect components

An energy system component A connected via an interfaces to an other component B.

To solve this problem interfaces have been introduced, which act as an intermediary between components. The output of a component connects to the "left" of an interface and the input of the receiving component on the "right". That way energy always flows from left to right.

When a component outputs energy, it writes a negative amount of energy to the right side of the interfaces of all its inputs and writes a positive amount of energy to the left side of all its outputs. The connected components can then maintain the energy balance by writing matching positive / negative energy values to their inputs / outputs. In addition, this mechanism is also used to differentiate between energy demands and the loading potential for storage components.

This mechanism has proven useful as otherwise the implementation of every component would have to check if it is connected to a bus or a single other component as well as if it is a storage component or not. The interfaces simplify this behavior and decouple the implementations of components, which is important to maintain the flexibility of the overall simulation software in regards to new components.

Units

Currently, ReSiE is based on the following units:

  • time in seconds [s]
  • energy in watt-hour [Wh]
  • power in watt [W]
  • temperatures in degree celsius [°C]

Values provided in the project input file or in profiles should have these units and the plots created directly from ReSiE has to be labeled accordingly (while offering the change of scale by a scale factor - but then the unit displayed in the plots has to specified respectively in the input file).

Theoretically, all provided values can also be scaled by any order of magnitude, e.g. into [kW] and [kWh]. But keep in mind that this has to be done uniformly in every input value and profile and that the naming of the outputs has to be adjusted accordingly!

Output units:

Note that all energy-related output values requested in the input file to plot or to write to the output.csv file are returned as energies and not as power! For example, if all inputs are given in [W] and [Wh], the output value of the heat delivered by a heat pump is given as energy in [Wh] delivered within the current time step. If the simulation time step is set to 15min, an output of 100 kWh equals a thermal power of 400 kW.


  1. Here "processing" is a stand-in for the transport, transfer or transformation of energy. The term is used to differentiate the "action" from the control of a component.