Simulation Modeling of Integrated Multi-Carrier Energy Systems

*Nikolai Voropai, Ekaterina Sеrdyukova, Dmitry Gerasimov and Konstantin Suslov*

## **Abstract**

Integrated multi-carrier energy systems give good possibilities to have high effectiveness of energy supply to consumers. Transformation of energy systems under the impact of internal and external factors remarkably strengthens the technological integration of those systems and supports development of integrated multi-carrier energy systems. The concept of energy hub is developed for modeling and simulation of integrated multi-carrier energy systems. Based on previous research, a simulation model of the energy hub is being developed. The basic principles of building a simulation model of an energy hub concept are discussed. Realization of simulation model using Matlab/Simulink is proposed. Simulation results for the integrated electricity and heat systems are explained to demonstrate the capabilities of the simulation energy hub model. A case study for application of the simulation model is discussed.

**Keywords:** integrated multi-carrier energy systems, simulation modeling, energy hub, energy converters, energy storage, energy consumption optimization, Matlab/Simulink software

#### **1. Introduction**

Modern energy supply systems, primarily electricity, heat and gas systems represent a developed energy infrastructure that provides consumers in the economic and social sectors with various energy types with the required reliability, the required quality and at an affordable price. The development and opposition of these energy systems is under the influence of a new paradigm of customer-oriented energy supply. Recently, the requirements for the reliability of power supply and the quality of the types of energy supplied to consumers have significantly increased due to computerization and digitalization of consumer production processes and the expansion in the use of "high" production technologies by the consumer.

The design and operation of these energy systems tend to consider them independently of each other. The systems under discussion however interact quite closely with each other, for example, when electricity and heat is generated using gas as fuel at cogeneration under normal and emergency conditions, when electric heaters are used by consumers in the case of accidents in the heating system, etc.

The new conditions for the development and computerization of the infrastructure energy systems contribute to the expansion of interaction between them as many new actors appear that can provide ancillary services. The consumers with controlled load, managing their energy load, can have self-generation sources and energy storage units, and simultaneously, depending on the current conditions, be involved in conversion, storage and generation of the required type of energy; electric vehicles can deliver stored electricity to the power supply system during peak hours, etc.

The development of information and telecommunication technologies bring about additional opportunities for joint coordinated management of the expansion and operation of the energy systems under consideration.

All the above features increase significantly the interest in the research of virtually new facilities, i.e., integrated energy systems (IESs) [1, 2]. The primary basic problem here is the technology of modeling the sophisticated IESs. This chapter focuses on the main principles of the IES simulation technology relying on the capabilities of the Matlab/Simulink system and the energy hub concept.

The further presentation is structured as follows. Section 2 presents basic information about the features of IES and the history of research in this strand. Section 3 provides an overview of the energy hub concept. Section 4 contains a description of the nature of mathematical models of IES based on the integration of traditional models of the components of the considered IES of power supply systems. Section 5 discusses the principles of energy hub modeling used in most of the studies conducted, and the advantages and disadvantages of the models. Section 6 analyzes the capabilities of the Matlab/Simulink system for IES modeling. Section 7 presents a new approach to building a simulation model of an energy hub developed by the authors. Section 8 discusses the proposed technology for constructing an IES simulation model. Section 9 contains a description of one of the problems solved using the developed simulation model. The conclusion to this Сhapter summarizes the results of the studies performed.

## **2. Integrated multi-carrier energy systems**

Objective trends in energy systems development (electric power, heat, gas, oil, oil products supply systems, etc.) lead to creation of integrated multi-carrier energy systems. These tendencies are determined by strengthening of technological integration not only during production of energy (for example, electric power and heat on the co-generation plants (CGP) by using gas as the fuel), but also under energy consumption based on implementing different kinds of energy for the same objectives. For example, it is possible to use heat from centralized heating system based on CDP or from individual electric boilers, electric or gas individual furnaces, and so on. In these cases individual energy systems (electric power, gas and heat supply systems) acquire the interdependences not only between production plants and consumption of individual systems, but also between load flows in networks of these systems. Particularly significant interrelations between individual energy systems we can meet in emergency conditions. Taking into account above mentioned peculiarities we have to consider joint operation and expansion of individual energy systems [1, 2].

In [2], the authors explain the elements of the concept of integrated energy systems as a three-layer structure in three dimensions, similar to Rubik's cube (see **Figure 1**). The groups of layers can be defined as follows:


*Simulation Modeling of Integrated Multi-Carrier Energy Systems DOI: http://dx.doi.org/10.5772/intechopen.99323*

#### **Figure 1.**

*Three-layer structure of integrated energy systems in three dimensions.*

Integrated multi-carrier energy systems, as well as their individual energy supply systems, especially electric power, heat and gas supply systems, have important infrastructural role in the enhancement of optimal operation of different economy sectors and acceptable life of citizens in any country. There are concrete requirements to necessary level of power supply reliability to consumers and high quality of supplied energy, and also to effectiveness of operation and development of above mentioned infrastructural energy systems. It is necessary to note, that the requirements to increase reliability and quality of energy supply first of all are forming under the influence of digitalization and computerization in technological processes of consumers [3, 4].

In 1999 actually the first research project started concerning energy delivery systems from production of different kinds of energy to retail markets [5]. End use energies included electricity and heat. Such kinds of energy were studied, as electric power, gas, oil, as well as conversion between different kinds of fuel (gas power plants, hydro power plants, co-generations, heating pumps, plants for production of liquid natural gas, and so on). The possibilities of alternative storages were studied, for example hydro accumulating plants and liquid natural gas storages. This project was as the stimuli for preparation of methodology of comprehensive analysis of complicate energy delivery systems with several kinds of energy including technological, economic and ecology aspects. It was planned, that such methodology will be very flexible and will allow the integrated energy companies to make comprehensive analysis their investments and general optimization of their energy supply systems.

The project "Vision of Future Energy Networks (VFEN)" was proposed by group of authors and supported by industry [6, 7]. Horizon of planning is since 30 up to 50 years. Economic, ecology and technological aspects localize the research conditions. General hybrid approach includes different kinds of energy, which consider the synergy between electrical, chemical and heat energies (it is possible, between the other kinds of energy).

An integration of different energy systems into technologically joint body gives new functional possibility, using complex innovative technologies for integrated energy system operation and creation of smart integrated multi-carrier energy systems (SIES). Such systems have many dimensional structures of functional possibilities and development properties. They consider big number of factors: intelligence, effectiveness, reliability, controllability, flexible use of technologies for energy transformation, transportation and preservation, active demand. Protection and control systems have to react to emergency and unreal behavior and to ensure SIES after such events. It is important to develop the models and software for online decision making, especially in the conditions of large disturbances [8–10].

#### **3. Energy hub concept**

Tendency towards technological integration of energy supply systems gave birth to the notion of an energy hub [1, 8], that implies an integrated facility with multiple inputs and outputs, which represent different types of energy. This facility has internal elements for the support of some functions, i.e., transformation, conversion and storage of different kinds of energy. It is necessary to note [10], that the energy hub concept can be used rather wide – from representing some individual transmission element to a building or a part of the city.

Following [7], we will consider an example of the energy hub shown in **Figure 2**. The Figure shows the inputs and outputs of the energy hub, as well as its internal components and their interconnections (electric transformer, electric battery, micro-turbine, heat exchanger, furnace, cooler and hot water storage).

In [11], an overview of the main provisions of the energy hub concept is presented. Four main functionalities of the energy hub concept are identified, including the input, conversion, storage and output of the considered types of energy. At the same time, most of the studies discussed in the overview, use electric and gas networks as the studied facilities of the energy hub. Various types of power plants especially those based on renewable energy resources, and those relying on promising innovative technologies, such as, fuel cells, for example, were studied as sources of generation.

#### **Figure 2.**

*Example of a specific energy hub containing a transformer, microturbine, heat exchanger, furnace, cooler, battery, and hot water storage.*

## **4. Conventional modeling of integrated multi-carrier energy systems**

It is necessary to take into account, that technological and market strengthening of individual energy systems requires more intensive studies of modeling integrated multi-carrier energy systems for the investigation and control of their operating conditions and expansion planning. There are two basically different approaches for modeling integrated multi-carrier energy systems: based on conventional mathematical models of individual energy systems [12, 13] and to use energy hub concept [14, 15].

Let us represent as the example conventional mathematical model of integrated multi-carrier energy system, including electric power and heat supply systems, in following form (1)–(6) [13]:

$$F\_{obj} \to \min \tag{1}$$

subject to:

$$E\_{k\text{ min}} \le E\_k^\epsilon \le E\_{k\text{ max}}, k \in N\_{par}^\epsilon, t = \overline{1, T}, \tag{2}$$

$$H\_{k\text{ min}} \le H\_k^t \le H\_{k\text{ max}}, k \in \mathcal{N}\_{par}^h, t = \overline{1, T}, \tag{3}$$

$$0 \le P\_i^\ell \le E\_{i\max}, i = \overline{1, N}, t = \overline{1, T}, \tag{4}$$

$$F\_{w\max}^{it} \ge F\_{w}^{it}, F\_{q\max}^{it} \ge F\_{q}^{it},\tag{5}$$

and balance between electricity and heat production is:

$$\sum\_{t=1}^{T} (\mathcal{W}\_t + \mathcal{Q}\_t) = \sum\_{i=1}^{N} \sum\_{t=1}^{T} \left( \mathcal{W}\_i^t + \mathcal{Q}\_i^t \right) = \sum\_{t=1}^{T} P\_i^t \Delta t,\tag{6}$$

where *Fobj* is objective function, its structure depends on the sense of solved problem for example, active power; *Fqi* is volumes of fuel used at source *i* for heat production; *Fqi* is volumes of fuel used at source *i* for electricity production; *Pi* is used (installed) capacity of source *i*; *Wi* is supply of electricity from source *i*; *Qi* is supply of heat from source *i*; *W* is is electric power output total value in the system; *Q* is heat output total value in the system; *Ek* is current state parameter of electric network; *Ek* min and *Ek* max are technically admissible current state operating parameters limits of the electric network; *Hk* is current state parameter of heat network; *Hk* min and *Hk* max are technically admissible current state operating parameters limits of the heat network; *Pi* is used (installed) capacity of source *i*; *Pi* max is maximum (installed) capacity of source *i.*

#### **5. Main current principles of the energy hub modeling**

References [14, 16, 17] present a system of algebraic equations that relate input variables of the energy hub into output variables. Both variables present different kinds of energy:

$$
\begin{pmatrix} L\_{\alpha} \\ L\_{\beta} \\ \vdots \\ L\_{\gamma} \end{pmatrix} = \begin{pmatrix} \mathbf{C}\_{\alpha a} & \mathbf{C}\_{\beta a} & \cdots & \mathbf{C}\_{\gamma a} \\ \mathbf{C}\_{a\beta} & \mathbf{C}\_{\beta\beta} & \cdots & \mathbf{C}\_{\gamma\beta} \\ \vdots & \vdots & \ddots & \vdots \\ \mathbf{C}\_{a\gamma} & \mathbf{C}\_{\beta\gamma} & \cdots & \mathbf{C}\_{\gamma\gamma} \end{pmatrix} \begin{pmatrix} E\_{a} \\ E\_{\beta} \\ \vdots \\ E\_{\gamma} \end{pmatrix} \tag{7}
$$

or, in matrix presentation,

$$L = \mathbf{C} \cdot \mathbf{E} \tag{8}$$

Energy in the input and output ports is represented by vector-columns *E* ¼ *Eα*, *E<sup>β</sup>* … *E<sup>γ</sup>* � � and *<sup>L</sup>* <sup>¼</sup> *<sup>L</sup>α*, *<sup>L</sup><sup>β</sup>* … *<sup>L</sup><sup>γ</sup>* � �, *C* is a matrix of direct relations, that describes conversion of energy forms from input to output. Each member of the matrix relates one specific input to a certain output.

In case of solving the inverse problem, a matrix of inverse conversions is introduced

$$
\begin{pmatrix} E\_a \\ \vdots \\ E\_\gamma \end{pmatrix} = \begin{pmatrix} d\_{aa} \ \cdots \ d\_{\gamma a} \\ \vdots \ \ddots \ \vdots \\ d\_{a\gamma} \ \cdots \ d\_{\gamma r} \end{pmatrix} \begin{pmatrix} L\_a \\ \vdots \\ L\_\gamma \end{pmatrix} \tag{9}
$$

Relations between coefficients of inverse and direct transformations have a unique form:

$$d\_{\beta\alpha} = \begin{cases} c\_{a\beta}^{-1} \text{ if } c\_{a\beta} \neq 0 \\ 0 \text{ else} \end{cases} \tag{10}$$

Should there be N output ports and one input port, the energy through each output channel would be distributed following the equation:

$$E\_{im} = \sum\_{n=1}^{N} d\_{im} L\_{in} \tag{11}$$

It is necessary to note, that the most part of references, which deal with the energy hub modeling, including dissertations [18, 19] for different problems investigations concerning integrated multi-carrier energy systems, are using linear energy hub models. These studied problems include calculation and optimization of power flow in integrated multi-carrier energy systems, reliability of electric power and heat supply to consumers, optimization of integrated energy system expansion, and some others [1, 10, 12, 14–17].

Above mentioned studies showed potentials of considered approach to use the linear energy hub model and at the same time the problems of its application. The matter is, that it is necessary to determine the matrix coefficients in (7), which relate inputs and outputs of the energy hub. But this determination faces some difficulties even for linear case. Really these coefficients can have complicate structure including non-linearities. Moreover, above mentioned energy hub models allow to solve only stationary problems in integrated multi-carrier energy systems. Dynamic problems consideration based on energy hub concept had not been studied yet, what had noted as the favorite direction of further investigations [18].

It is necessary to draw the attention on the first known results of dynamic problems study in [20] using conventional mathematical model of integrated energy system based on technique of the theory of singular perturbations (small parameters). This technique was used for presentation of individual energy systems in the integrated multi-carrier energy system.

The above mentioned peculiarities of energy hub modeling stimulate to search the other possibilities to solve these problems. Next Section allows such possibilities.
