The holonic system structure presented in this work is a formalization and extension of the work presented by the authors of [
18]. Our main contributions in this section are the improvement, extension, and formalization of the underlying model of the HOLEG simulation environment. In particular, we extend the model with the concepts of
flexibilities (see
Section 4.1.3) to enable leveraging local resources and their prioritization via priority classes. Furthermore, we improve their holonic system model by replacing their concepts of holon objects and subnetworks with one unified concept of holons. This simplifies the recursive application of the holon concept. Our system model is structured bottom-up and comprises two core-concepts, namely
holon elements and
holons.
4.1.1. Holon Elements
Holon elements represent atomic building blocks of the holon model proposed in this work. From the perspective of a holonic energy grid, these entities cannot be further separated into smaller controllable parts. Therefore, holon elements are considered as the smallest entities that can be controlled using an information and communication technology (ICT)-infrastructure and actuators within SGs. Such parts include—but are not limited to—individual appliances (e.g., TV, air conditioning), parts of systems whose parts cannot be controlled individually (e.g., a floor within a house), and entire sub-systems (e.g., legacy consumers in the grid that do not have the required technology, like houses without ICT or actuators to facilitate fine-grained control).
It is assumed that three types of holon elements exist, namely
consumers,
producers, and
prosumers. Holon elements are considered as being consumers if they consume more electricity than they produce. In contrast, holon elements are considered as being producers if their electricity production exceeds their consumption. Finally, holon elements are prosumers if they are capable of operating as both consumers and producers. Consequently, holon elements of this type can change between the two other types. Holon elements (
he) in the proposed model are formally represented as four-tuples of
power magnitude, role, state and
priority class:
A holon element denotes the holon element at layer l within a holarchy. Time is measured at discrete points , where T depends on the granularity which is assigned for measurements within the energy grid (e.g., T = 17,280 for a measurement interval of 5 s during a day). At each point in time t, holon elements are in one of three mutually exclusive states , which specify their current behavior within the energy grid. In an active state, holon elements produce or consume electricity according to their electrical profile (e.g., charging rate of a battery). Inactive holon elements do not produce or consume electricity. In a standby state, holon elements consume a small but constant amount of electricity.
More formally, the consumption and production behavior of a holon element
is specified as the
power magnitude :
Function represents the power consumption (negative values) or production (positive values) of the holon element at time t. The constant represents the power consumption required for maintaining the standby state and is the null-function and the corresponding integration constant is zero. The function takes a holon element as input and returns its current state. Inactive holon elements do not produce or consume electricity.
Depending on the orientation of the power magnitude (positive for production, negative for consumption) a holon element takes on a specific role . We emphasize that the role can change at any point in time t. As the number of prosumers in energy grids increases (e.g., battery storage units), the possibility for such changes needs to be taken into account.
Extending holon elements with priority classes allows expressing their relative importance to a responsible entity at each point in time. For instance, appliances in private households can differ in the relative importance for a user depending on the time of the day. Relative importance is also a measure of user satisfaction: a water heater may be more important to the user in the morning than at midday. Such information can be used to derive a measure of dissatisfaction if the holon element is not in the expected state when a responsible entity observes its state. A higher priority of a holon element yields a stronger decrease in user satisfaction if expectations are violated. Priority classes can be adjusted at any point in time to address changes in the perceived importance of holon elements.
4.1.2. Holons
Holons are controllable non-atomic entities in our proposed holonic model. Holons can represent aggregated entities within the grid, like individual prosumers (e.g., houses, enterprises), streets, communities, or larger parts of the grid. In comparison to holon elements, holons are assumed to have the following properties:
Communication. Communication is conducted between different holons in the proposed model. Three categories of communication can be distinguished depending on the direction of the communication. Holons can negotiate with other holons on the same holarchy layer. The communication from holons of a higher layer with holons of a lower layer is specified as delegation. The communication from lower layers towards higher ones is denoted as propagation.
Change of affiliation. A holon can be affiliated with a higher-layer holon. In this work, it is assumed that holons can change their affiliation dynamically, based on delegated information or the pursuit of individual goals of the holon.
Strive for unification. Holons possess the inherent desire to become parts of larger systems. As part of a larger system, holons are willing to give up (parts of) their autonomy and corresponding control.
Janus-faced. Derived from the two-faced image of the roman deity “Janus”, holons are considered to possess two views. An internal view that aims to pursue inherent goals of the entities encompassed within the holon (e.g., optimize local use of electricity). An external view that is responsible for considering aspects that are important for higher layers in the grid.
Split. Splitting operations allow holons to separate one or more previously encompassed holons of lower layers from their internal structure. This operation forms new holons at the same layer of the holon conducting the splitting operation.
Merge. Holons can merge to form a new larger holon represented at a higher layer in the holarchy.
Holons encompass lower-layer holons and corresponding holon elements. More formally, they can be defined as a seven-tuple:
The individual parameters of holons are explained next.
Holrachy Set
Here, the
holon
of holarchy layer
encompasses a holarchy
of lower layer holons, which is specified as follows:
where
denotes the number of holons that comprise layer
l from the perspective of the
holon of layer
and are affiliated to it. A holon can be affiliated to only a single holon of the next higher holarchy layer. Therefore, the following property must hold at each point in time t:
Consequently, holons that are located on the same layer within a holarchy do not share holons of the next lower layer. The recursive application of this definition yields a unique affiliation of each holon to a holon of the next higher layer.
Holon Element Set
Similar to lower layer holons, holon elements can be present at each layer within a holarchy. The set
comprises the holon elements of layer
l from the perspective of the corresponding higher layer holon. The set is defined as follows:
Parameter
denotes the number of holon elements that are encompassed within layer
l from the perspective of the
and are affiliated to a holon of the next higher layer. Similar to the disjointness requirement of holon affiliations presented above, the following property holds for all sets of holon elements:
Therefore, holons at the same layer do not share holon elements.
Power Magnitude
The power magnitude
of a holon represents its electrical behavior—which is the aggregated amount of consumed or produced electricity—at time t. The power magnitude depends on the encompassed holarchy and the holon elements located at the current layer. The power magnitude for a holon is calculated as follows:
Here, denotes the holarchy encompassed in the holon at layer and represents the set of holon elements located at layer l from the perspective of the holon. At each point in time, the power magnitude of a holon is either positive, which indicates that it consumes more electricity than it produces, or the power magnitude is negative and the holon provides excess electricity.
Forecasts
The set of available forecasts for the consumption or production behavior of a holon is denoted as the set for and . In this work, it is assumed that the majority of areas within an energy grid that are capable of operating as a holon can provide a forecast that indicates their future electricity demand and production. Towards this end, each holon calculates an aggregated forecast based on the electricity demand and production of its encompassed entities for m future time intervals. If a holon is not capable to calculate a forcast, the set is empty. Parameter m defines the number of time intervals that are part of the forecast and is the expected, aggregated power magnitude anticipated for a certain time interval.
Role and State
The current role of a holon is defined similarly to the role of holon elements, such that holons can be either producers or consumers at each time t. This role depends on the current electrical behavior of the holon. Consequently, holons can change their roles at any point in time, when their local production exceeds their demand and vice versa.
The state
of a holon is more complex compared to the state of holon elements as it (partially) depends on the state of its encompassed entities. The proposed model differentiates between six mutually exclusive states. At each point in time, the following definition must hold:
Consequently, a holon remains in a dedicated state at least for the time interval . The possible states for holons are the following:
Producing. The holon produces excess electricity for the grid. The aggregated demand of the holon is fully covered.
Inactive. All encompassed holons are inactive and do not produce or consume electricity.
Unsupplied. Not a single encompassed holon can be supplied with the available electricity.
Partially supplied. The available electricity is sufficient for at least partially supplying one of the encompassed holons.
Fully supplied. All encompassed holons are fully supplied.
Oversupplied. The ongoing electricity production exceeds the demand of the holon. This state is similar to the producing state, but the excess electricity is currently undesired.
Flexibility
Finally, is the set of available flexibilities within the holon. Note that this set comprises all flexibilities that are available within the encompassed holarchy of the holon. Consequently, a holon at layer has access to r flexibilities, which is the number of flexibilities on all lower layers within its scope. This allows higher-layer holons to have access to larger number of resources for the mitigation of hazardous situations. However, this also increases the complexity of managing and using these resources accordingly. The flexibility concept is explained next in detail.
4.1.3. Flexibility Concept
Holon elements located within holons can be offered as so-called
flexibilities —virtual energy resources—for supporting the mitigation of demand and supply deviations. The set
represents the set of all flexibilities—throughout the entire encompassed holarchy—that is currently available within a holon. Consequently, if problems are to be addressed on lower layers, fewer resources might be available compared to addressing problems on higher layers. The general flexibility concept is presented in
Figure 3.
For a holon element to be offered and safely operated as a flexibility, a lot of information (
element properties) needs to be accessible by control authorities. Such information mainly depends on the properties of the underlying holon element but may also be configurable by the entity that offers the flexibility. Additionally,
constraints for the safe use of flexibilities and the underlying holon element are required. More formally, a flexibility
is represented as a ten-tuple:
The parameter denotes the power capacity (Watt) provided by a flexibility. The corresponding type of flexibility is defined by . Numerous performance parameters are required to assess a flexibilities applicability to resolve a problem situation. Parameters , , and denote the delay, duration, and cool-down period of a flexibility. Parameter specifies the reimbursement required to use the flexibility and represents the priority class. The sets and comprise technical and personal constraints specified for the flexibility and represents the state of a flexibility. All parameters are explained next in more detail.
Power
Parameter represents the power (Watt) that is provided by a flexibility for a specified amount of time. This power can be provided as production or consumption capacity (e.g., 3 kW (dis-) charging rate for a battery).
Type
Flexibilities can be classified as positive or negative. A flexibility is considered to be positive if using it increases the grid frequency (by providing electricity or by reducing consumption). A flexibility is considered to be negative if it lowers the grid frequency (by increasing the consumption or reducing the production) in the grid.
Delay
Holon elements may require a certain amount of time until they can operate according to the specification of the flexibility, for example, the time to power up or shut down safely. The delay specifies the amount of time (in seconds) that is required until a flexibility provides its specified power and affects the power grid frequency. This information is important to plan the use of flexibilities depending on the time criticality of the problem to solve.
Duration
The duration defines the expected amount of time (in seconds) for which a flexibility provides its service (e.g., charging a battery for 3600 s). The duration depends on numerous aspects, like the current state of the holon element and its requirements for safe operation, but the duration can also be affected by environmental conditions. For instance, activating an air conditioning lowers the temperature in a room but increases the internal temperature of the corresponding holon element. The duration of a flexibility based on this air conditioning may be limited according to the current room temperature or the internal temperature of the holon element.
Cool-Down
Device operation can be restricted due to user preferences or operational constraints. The Cool-down parameter specifies the amount of time (in seconds) for which a flexibility is not usable after successful prior activation. Note that a time-based specification for reusing a flexibility may not be sufficient to decide at which point a flexibility may be usable again but can depend on various other factors. For instance, prosumers can decide that certain holon elements can only be used with a certain frequency. Other limitations may depend on the state of a holon element, like the state-of-charge of batteries, which can only be charged again after their state-of-charge is below a certain threshold.
Costs
The parameter represents a form of reimbursement, which has to be paid for using a flexibility. This payment should act as both a form of incentive for prosumers to offer flexibilities in the first place but also to compensate for (some of) the emerging expense resulting from offering flexibilities. Activating flexibilities depletes local resources (e.g., stored electricity) and leads to a wear of corresponding holon elements (e.g., by turning devices on/off). Therefore, a compensation is required. Costs are currently represented as monetary compensation , but other means for compensation may be applicable in the future.
Priority
Each holon element is assigned to a priority class
to indicate its relative importance to the entity that is responsible for it (see
Section 4.1.1). For the proposed model it is assumed that flexibilities
inherit the priority class
from the underlying holon element. However, for flexibilities the meaning of the class is diametrical to the meaning of priorities for holon elements. More precisely, a higher priority class of a flexibility reduces its priority for being used for mitigating demand and supply problems. The assumption for this is that if the underlying holon element is assigned to a high priority class, it is currently considered
important to the entity to which it belongs. Therefore, it is likely that the entity needs the holon element at this specific point in time and a change in its state (e.g., by using it as a flexibility) may likely cause a decrease in user satisfaction.
Constraints
Constraints represent both requirements and limitations for the use of flexibilities. Towards this end, constraints specify rules that need to be fulfilled for a flexibility to be usable in the first place and to ensure the safe operation of the underlying holon element, its environment, and to take into account the preferences of the entities who offer the flexibilities. For each flexibility, two sets of constraints that need to be fulfilled can be defined. If the constraints are not met, the respective flexibility cannot be offered. The set denotes the set of technical constraints and the set for comprises personal constraints.
Technical constraints. These constraints specify technical requirements that need to be met; otherwise, the safe operation of the underlying holon element cannot be guaranteed. For instance, the internal temperature for the safe operation of holon elements can be represented as a technical constraint for limiting the use of the holon element as a flexibility. Other constraints can be interdependencies with other devices.
Personal constraints. The constraints limit the use of flexibilities based on personal preferences and requirements defined by the entity that is responsible for managing the flexibility. For instance, the use of flexibilities can be limited according to the specified priority class of the underlying holon element, which allows flexibilities to be used only if the priority class is sufficiently low.
Constraints limit the use of flexibilities mainly in two ways: firstly, if any constraint is violated during the configuration phase of a flexibility, the flexibility cannot be offered. This aims to prevent entities from offering flexibilities that cannot be operated under safe conditions. Secondly, constraints must be repetitively checked—within reasonable time intervals—during the lifetime of a flexibility and need to remain fulfilled during this process. If constraints are violated during use, the flexibility may become unavailable. To represent the various operational conditions of flexibilities, they can be in different Flex-States .
State
A flexibility has four mutually exclusive Flex-States , namely Not-offered, Offered, In-use, and Cool-down:
Not-offered. A flexibility remains in the Not-offered state after its configuration process until it is offered by the responsible entity. Whenever an offering command is issued, the constraints are checked, and if all constraints are met, the flexibility transitions into the Offered state.
Offered. A flexibility is in the Offered state if it fulfills the specified constraints and, successively, was offered to support the energy grid operation. In this state, it can be used to support the mitigation of unbalanced demand and supply. If a control entity decides to use a flexibility, the flexibility transitions into the In-use state.
In-use. A flexibility in this state is currently used in a problem mitigation process and cannot simultaneously be used for other purposes. During the use of a flexibility, the constraints are checked repeatedly, and, if any violations occur, the flexibility transitions into the Not-offered state.
Cool-down. After a flexibility was used for its intended duration, it transitions to the Cool-down state for its specified amount of time. In this state, the constraints are checked and it automatically transitions into the Offered state if no violations occur and the specified amount of time has passed; otherwise, the flexibility transitions into the Not-offered state. If no cool-down is specified, the flexibility immediately transitions into the Offered state and becomes available again.
A flexibility can transition between the different Flex-states based on events.
Figure 4 shows the four Flex-states and corresponding transitions between them. The events that trigger a change of state are explained next:
configured. The configured event is the entry event after the successful configuration of a flexibility.
offer. The offer event occurs if an entity starts the process of offering a flexibility to support the mitigation of problems in the energy grid. This event leads to a state change to the Offered state if no constraint violation occurs.
violated. The violated event indicates that at least one constraint of the flexibility is not met. Therefore, the flexibility cannot be operated according to its specification and transitions into the Not-offered state.
withdraw. The withdraw event occurs if an entity manually withdraws a flexibility from being offered to support the grid. Flexibilities can only be withdrawn from states where they are not actively used to support the grid.
inquire. The inquire event occurs if a control entity decides to use a specific flexibility to support the mitigation of a problem situation. If an offered flexibility is inquired, and no constraint violations occur, this event leads to a state change towards the In-use state.
duration. The duration event occurs when a flexibility has been used for its specified duration. This event leads to a transition of the flexibility into the Cool-down state.
cool-down. This event indicates that a flexibility currently remains in a Cool-down state. After the event, the flexibility transitions into the Offered state or, if it is withdrawn, changes to the Not-offered state.
4.1.4. Grid Control
Our holonic system model allows for the addressing of issues on appropriate, yet varying layers of the holarchy by utilizing the resources of individual holons. The emerging problem size varies significantly and can be adjusted according to the problem severity.
We consider two problem cases: firstly, small-scale problems, where a reduced number of flexibilities is sufficient to mitigate the problem situation. For instance, a production shortage in a lower-layer holon can be compensated by a smaller number of geographically close flexibilities. Let there be a holon that encompasses 25 offered flexibilities. Resolving an issue using the locally available resources yields a resource allocation problem of the size (the decision of using a flexibility is binary). The complexity of this problem can be addressed by various methods, like optimal solvers.
Secondly, large-scale problems are considered, which may have—if not mitigated properly—more severe consequences for the energy grid operation (e.g., damage to larger producers in the network may destabilize the entire grid). Locally available resources on lower layers in the holarchy are insufficient for mitigating problems of this scale. However, higher layers in the holarchy possess an extended scope and can therefore utilize more resources for the mitigation of the problem situation. Simultaneously, the availability of an increasing number of resources on higher layers in the holarchy increases the problem size significantly. For instance, a higher layer holon may contain 100 houses that offer two flexibilities each. Furthermore, potential flexibility configurations need to be explored in order to find high-quality solutions. Such a problem size exceeds the solving capabilities of numerous solvers and efficiently addressing such problem sizes requires heuristic approaches.