Synthetic evaluation of integrated energy system based on AHP-fuzzy comprehensive assessment method

. Integrated energy system (IES) is an essential ingredient of energy reform. The construction of IES can promote the coupling utilization of multiple energy sources, and improve the reliability and flexibility of energy system. Based on amounts of literature study and experts’ recommendations, 9 first-class indexes, 13 second-class indexes and 11 third-class indexes are confirmed to assess the integrated energy system from three dimensions of technology, economy, and environmental protection. Analytic Hierarchy Process (AHP) and fuzzy comprehensive assessment method are adopted to assess the overall quality of chosen integrated energy system. An industrial park is chosen to be an example and examine the evaluation condition of IES.


Introduction
Integrated energy system (IES) is an important part of energy internet, and it's not only the research hotspot in the field of energy, but also the development direction of energy restricting. The core of IES is carrying out energy conversion, utilization, and multi-energy complement [1][2]. U.S. Department of Energy had proposed a smart grid evaluation index system, which aimed to evaluate smart grid from six aspects [3]. The evaluation system can measure the reliability, security, and the interactive service for the user side of smart grid [4]. Europe also published smart grid earning evaluation index system to achieve the aim of improving grid earnings from nine fields and 21 key indexes [5].
In current literatures, methods of evaluation index always focus on power grid, and ignore the hot grid and cool gird [6][7][8]. Li et al. [9] built universal flow system models from the aspects of energy distribution, conversation, and storage. On this basis, the assessment system of IES from four dimensions of energy, devices, information, and system was come up. Luo et al. [10] studied the equivalent features of microgrid, and put forward the characteristics of intermittent power source, the islanding state of microgrid, and the benefits of microgrid. Zhang et al. [11] established an index evaluation model of park-level integrated energy system for microgrid from the aspects of economy, reliability, energy consumption and environmental protection.
Currently, the main methods of comprehensive evaluation method conclude principal component analysis (PCA) [12], AHP [13,14], entropy weight method (EWM) [7,15,16], Data Envelope Approach (DEA) [17] and the method for order performance by similarity to ideal solution (TOPSIS) [15]. However, PCA and EWM always have errors when the evaluation indexes with small range of changes, and DEA is easily affected by extremes. Cavallaro et al. [15] used fuzzy Shannon entropy and fuzzy TOPSIS to assess five CHP technologies in terms of economy and efficiency. Ren et al. [13] chose various of evaluation indexes such as primary energy consumptions, investment costs, carbon emissions, and applied AHP to analyse the Japanese power supply system.
The AHP-Delphi method can combine the data and practical experience to a certain degree, and obtain more objective and reasonable evaluation indexes of IES. This method is used to determine the indexes and weight, and a park will be tested by fuzzy comprehensive assessment method.

Establishment of synthetic evaluation
Selection of evaluation index is both the foundation and key point of evaluation system of IES. Because the pros and cons of index always determine the quality of evaluation. Based on the construction concept of IES, this paper will expand the synthetic evaluation from three dimensions: technology, economy, and environment protection. Generally, IES is a multi-flow system, and it contains power flow, heating flow, cooling flow and so on. Third-class index aims at these questions to launch the discussion, and the synthetic assessment system of IES is illustrated in Figure 1.  Figure 1. The synthetic assessment system of integrated energy system.

Establishment of weightiness and fuzzy discriminant vector
For IES, synthetic evaluation always used to indicate the advantages and disadvantages of the single index. AHP combines both qualitative analysis and quantitative analysis. Fuzzy comprehensive assessment method uses exact mathematics model to deal with indefinable and fuzzy things. Therefore, AHP -fuzzy comprehensive assessment method is used in this study. The flow diagram of synthetic evaluation is shown in Figure 2.
Initial index weight of first-class index W, eigenvalue of maximum and coincidence indicator CI can be calculated by AHP.

Second-class index
In a similar way, judgement matrix, normalization matrix and initial index weight of second-class indexes can be calculated and the final weightiness of evaluation indexes are shown in Table 1.

Partition of factors set
Combined with the synthetic assessment system shown in Figure 1, index set F of first-class indexes is exhibited in equation (4)

Calculated membership
Evaluation sets is shown in equation (5) and the score range is illustrated in Table 2.

Calculation example
An industrial park is chosen to be a calculation example. Based on expert evaluation, 5 pieces of data were collected to evaluate this example. After normalization, the memberships of each indicator are as shown in Table  3.   where, ° is fuzzy matrix composition operator, the calculation method is as follows: In a similar calculation way, the vectors of others first-class indexes can be calculated, and fuzzy comprehensive evaluation matrix is shown in equation (11).
Thus, according to the combined weightiness in It means that the score of this industrial park is 85.78, and the evaluation level of this integrated energy system is "good".

Conclusion
The synthetic evaluation of IES is a very important work to monitor the operation status, and provide guidance for the future improvement and optimization. In this paper, a complete evaluation index system is established by AHP and Delphi method from three dimensions of technology, economy, and environmental protection. Also, an industrial park is selected to be a calculation example. After calculation based on the fuzzy comprehensive assessment method, evaluation score is obtained and corresponding evaluation level is good. The effectiveness and broad applicability of the synthetic assessment system are verified by the example.