# Integrated grey relational analysis and multi objective grey linear programming for sustainable electricity generation planning

- 557 Downloads
- 3 Citations

## Abstract

Sustainable energy generation is a key feature in sustainable development and among various sources of energy electricity due to some unique characteristics seems particularly important. Optimising electricity generation mix is a highly complex task and requires consideration of numerous conflicting criteria. To deal with uncertainty of experts’ opinions, inaccuracy of the available data and including more factors, some of which are difficult to quantify, in particular for environmental and social criteria, we applied grey relational analysis (GRA) with grey linguistic, and grey interval values to obtain the rank of each system. Then the obtained ranking were used as coefficients for a multi objective decision making problem, aimed to minimize the cost, import dependencies and emissions as well as maximizing the share of generation sources with better ranking. Due to existence of interval variables multi objective grey linear programming (MOGLP) method was used to solve the problem. Our results for the UK as a case study suggest increased role for all low carbon energy technologies and sharp reduction in the use of coal and oil. We argue that the integrated GRA–MOGLP approach provides an effective tool for the evaluation and optimisation of complex sustainable electricity generation planning. It is particularly promising in dealing with uncertainty and imprecisions, which reflect real-life scenarios in planning processes.

### Keywords

Electricity generation Sustainable planning MODM Interval linear programming GRA MOGLP## 1 Introduction

Uninterrupted access to energy resources is a defining factor of modern economies. Arguably energy’s role in economic systems is even greater than that of other resources and commodities simply because of the essential role of energy for the production, transformation and market delivery of all commodities and goods. Among energy resources, electricity is particularly important mainly for two reasons (Armaroli and Balzani 2011); firstly, because it can potentially substitute all other fuels in the transport, buildings and industrial sectors, and secondly because it can be generated without significant environmental emissions (Balat 2006; Kalkuhl et al. 2012). Moreover, electricity is a secondary form of energy which can be produced by a variety of fossil fuels and renewable energy sources, making use of a wide range of resource combinations depending on regional availability (Chalvatzis 2009).

Despite the aforementioned flexible characteristics, electricity is confronted with substantial challenges, such as increasingly stricter environmental constraints (Chalvatzis and Rubel 2015), social acceptability and support mechanisms of new forms of energy production facilities (Zafirakis et al. 2013), the lack of low cost storage in the electricity supply chain and the variability of renewable energy production. Within this context, power production systems have to comply with a complex set of regulations in addition to technical and societal constraints. Complexity in the power sector is arguably exacerbated with the introduction of new technologies such as distributed renewable energy generation, energy storage and electric private vehicles (Hofmann et al. 2016). This leads to an enormous growth in the number of market participants and the gradual transition from the large scale utility model and even the variability and alternation of consumer and producer roles as is possible for electric vehicles and households to both buy electricity and sell it back to the grid (Sioshansi 2012).

While complexity in decision making for power sector planning is inherently growing the literature offers a number of potential approaches. Linares and Romero (2000) used multi objective linear optimization, considering electricity derived costs and emissions minimization as their two objective functions, tried to find an optimal solution for electricity planning in Spain. Koroneos et al. (2004) generated a number of efficient solutions for the energy supply system of the island of Lesbos (Greece) by applying MODM linear optimization and concluded that renewable systems are adequate to produce optimal electricity power for this island. Unsihuay-Vila et al. (2011) proposed a Multi-Objective model for long-term expansion planning of electricity generation and transmission by considering sustainable energy development criteria. The proposed model is a well detailed mixed-integer linear model however all variables are considered as crisp valued and when it comes to sustainability, only environmental criteria have been considered. A Bi-Objective Linear Programming model was presented by Arnette and Zobel (2012) which determined the optimal mix of renewable energy sources and existing fossil fuel facilities on a regional basis, based on minimizing the cost and \(\hbox {CO}_{2}\) emissions as its objective functions. Perera et al. (2013) Combined MCDM and Multi-Objective Optimization to optimize a hybrid energy system for a standalone grid by considering levelized energy cost, unmet load fraction, wasted renewable energy and fuel consumption as objective functions. They applied TOPSIS to evaluate the solutions obtained in Pareto optimal front. Although applying TOPSIS for comparison in case of abundant solutions can be tiresome and inaccurate. Similarly, Promjiraprawat and Limmeechokchai (2013) presented a hybrid framework of multi-objective optimization and multi-criteria decision making to solve power generation expansion planning problems in Thailand. They first applied multi objective genetic algorithm to produce several solutions to the problem and then through Analytic Hierarchy Process (AHP) ranked the created optimized solutions. Purwanto et al. (2015) developed a bi-objective linear programming model which by minimizing the cost of production and \(\hbox {CO}_{2}\) emission to compute the share of renewable energy resources from generation mix in 2050 in Indonesia. In addition they have considered the technology diffusion in their model and compared the share of renewables with and without technology diffusion considerations. Sithole et al. (2016) by using an excel-based “Energy Optimisation Calculator” and utilizing assumptions about renewable share of generation and \(\hbox {CO}_{2}\) emissions restrictions required by UK government, obtained a least-cost solution for generation mix. In order to deal with uncertainty they conducted a sensitivity analysis and considering a ±30% margin for fuel costs factor. However there are various factors in a mixed generation model that should be considered and analysis the combinations of these factors through sensitivity analysis is a hard task and a comprehensive analysis is difficult to reach.

The main shortcoming in the aforementioned approaches is not to consider the uncertainty of different factors and applying fixed and crisp values for all the factors in the model. However, many system parameters such as costs associated with operation and maintenance, capital costs, fuel consumption, production efficiency, demand prediction and greenhouse gases emitted by various generation systems, as well as systems’ performance with respect to environmental and social criteria may appear uncertain and need to be presented by fuzzy or interval formats such as grey interval numbers. These uncertainties can have a significant effect over the optimization results and the decision makers’ plans based on the obtained results (Cai et al. 2007). Interval Linear programming (ILP) (also named as grey linear programming) developed by Huang et al. (1992) is a technique to deal with situations where we have interval values due to existence of inexact values, error measurements or performing sensitivity analysis (Hladı 2013). Interval linear programming since then has been developed into more complete and precise models and has proven its efficiency in solving problems in different fields.

Huang et al. (1995) further developed the interval linear programming by considering existence of interval variables and proposed a Grey Interval Linear Programming model solved through an interactive algorithm which can facilitate researchers to deal with discrete variables in allocation problems. While the most of the ILP applications have been in the field of environmental management and allocation of water supply sources (Cheng et al. 2003; Zhou et al. 2008; Han et al. 2011; Fan and Huang 2012), there has been few applications in other domains. Ren et al. (2015) developed a model for optimizing the life cycle cost of biofuel supply chain by considering uncertainties of different factors, and attempted to minimize the cost of the cycle. At the end they have developed a method to obtain an optimized solution from the solution intervals by introducing uptake coefficients and trying to minimize them so that the selected solution be more beneficial for stakeholders.

At the same time transition to a sustainable electricity sector is pressing and requires a comprehensive evaluation of each energy source in order to investigate their performance related to environmental and social attributes. These evaluations are a Multi Criteria Decision Making problem with often conflicting criteria being presented in different scales and formats. Previous research in this domain investigated different energy resources against sustainability criteria and proposed a framework for prioritizing renewable and conventional energy resources (Stein 2013; Kabak and Dağdeviren 2014; Büyüközkan and Güleryüz 2016). However despite the mentioned researches, applying fuzzy or grey MCDM techniques can overcome uncertainty within the decision makers’ judgments or imprecise information and increase the validity of evaluation significantly. Sadeghi et al. (2012) applied Fuzzy AHP to obtain the weights of each criteria and Fuzzy TOPSIS to compare 4 renewable energy resources in Iran with respect to sustainability criteria.

While application of fuzzy numbers and systems are numerous in prioritizing the electricity generation methods (Kaya and Kahraman 2009; Streimikiene et al. 2012; Tasri and Susilawati 2014; Şengül et al. 2015; Afsordegan et al. 2016), Grey based methods also have had an extensive application for evaluation of sustainable energy resources and while in the most of the cases, the imprecise information are available in the format of uniformly distributed within a lower and upper bound, defining fuzzy membership functions for them is a hard or impractical job. Thus application of grey numbers can have a significant advantageous over fuzzy numbers in these situations. Çelikbilek and Tüysüz (2016) by applying Grey-DEMATEL and Grey-ANP obtain the weights of criteria for assessment of sustainable performance of renewable energy sources and through Grey-VIKOR they compute the final ranking of the resources.

Grey relational analysis is one of the distinguished method among MCDM grey techniques and one of its advantageous features is the ability to assess quantitative and qualitative relationships between factors and variables using a relatively small amount of data (Arce et al. 2015). This method has been used by several researchers assessment of energy systems’ sustainable performance (Xu et al. 2011; Liu et al. 2013) and selecting the best technology (Sarucan et al. 2011; Manzardo et al. 2012; Ren et al. 2012).

In this paper we propose a framework for obtaining the optimal electricity generation mix, in which firstly, we have done a comprehensive GRA based evaluation for both environmental and social performance of the available sources in our case study by considering various types of data and measurements along with uncertainties associated with them. Then, by linking the MCDM evaluations and grey linear multi objective programming (GLMOP) created an MODM problem. MCDM evaluation of criteria and utilization of the results as coefficients in the optimization problem can reduce the number of required objective functions and also enable us to include more information in our model, which some of them are quantifiable and hard to include as a separated objective function. As a result the model is going to be smaller and easier to solve and at the same contain considerable amount of factors. Through considering the financial aspects, emissions and sustainability score of each source, we have covered all the sustainability aspects in our optimization and address the uncertain nature of the problem by applying grey linear programming. In this way we provide a significantly improved approach to those of the existing literature which are limited to either optimization over several objectives, by limited consideration of uncertainties, or just simply ranking the existing renewable and non-renewable sources based against various criteria.

For our case study we focus on the UK, because it combines several unique energy policy features which provide an ideally complex testbed for our proposed methodological approach. Some of the specificities of the UK power sector include its long-term commitment for energy decarbonisation (Sithole et al. 2016), energy market innovations such as capacity market and demand side management systems (Strbac 2008; Warren 2014), relatively low capacity margin (Newbery 2016) and vocal public debate about energy policy (Lilliestam and Hanger 2016).

This manuscript is structured as follows: after this introduction we continue in Sect. 2 where we present the integrated approach of GRA and IGLP and the process for solving the problem as well as the methodologies applied in this paper and then following with Sect. 3 where we present the GRA evaluation with respect to environmental and social factors and defining the related factors and criteria. In Sect. 4 we describe the optimization model and present the definition of all the coefficients. The results are shown in Sect. 5 and diagrams associated with them are depicted. Eventually we discuss our results and their policy implications in Sect. 6 and in Sect. 7 we provide concluding remarks, future research suggestions and limitations.

## 2 The proposed integrated approach

### 2.1 Grey system theory

*X*be the universal set and \(x\in X\). Then a grey set

*G*of

*X*is defined by its two mappings in Eqs. 1 and 2:

### 2.2 Grey relational analysis

*m*alternatives \(\left( {y_1 ,y_2 ,\ldots ,y_m} \right) \) and

*n*criteria \(\left( {k_1 ,k_2 ,\ldots ,k_n} \right) \) and form a decision matrix as Eq. (10).

*Step 1*Normalizing the decision matrix through one of the Eqs. 11, 12 or 13 if the criteria is belonged to benefit (higher the better), cost (lower the better) criteria or the value of the criteria has to be closer to a Desired Amount (DA) respectively:

*Step 2*Defining the reference sequence (ideal sequence) \(R_0 =\left( {R_{0,1} ,R_{0,2} ,\ldots ,R_{0,n}} \right) \) based on Eq. 14 as follows:

*Step 3*Grey Relational coefficient is being used to determine how close (more connected) is the \(r_{ij}\) to \(R_{0,j} \) and the larger grey relational coefficient is, the closer \(r_{ij} \) and \(R_{0,j} \) are together. Grey relational coefficient is shown by \(\gamma \left( {r_{ij} ,R_{0,j}} \right) \) and calculated through Eq. 15 as follows:

*Step 4*After obtaining all the grey relational coefficients, grey relational grade between alternative

*i*and reference sequence, which is shown by \({\Gamma } _i \), is computed based on Eq. 16 as follows:

### 2.3 Grey interval linear programming

*n*interval coefficients in model (1),

*k*of them be positive \((c_j^\pm \ge 0;j=1,2,\ldots ,k)\) and \(n-k\) of them be negative \((c_j^\pm \le 0;j=k+1,k+2,\ldots ,n)\), then the first sub-model for obtaining the lower bounds can be shown as follows:

### 2.4 Multi-objective grey interval programming

*l*th objective function, which has to be maximized, is \(f_l^*=\left[ {\underline{\otimes }}\,f_l^*,\overline{\otimes }\,f_l^*\right] \), so then the membership function can be as Eq. 21.

*l*th objective function, which has to be minimized, is \(f_l^{*} =\left[ \underline{\otimes }\,f_{l}^{*},\overline{\otimes }\, f_{l}^{*} \right] \), so then the membership function can be defined as Eq. 22.

## 3 Evaluation models

Environmental criteria and their descriptions

Criteria | Description | Measuring unit, type |
---|---|---|

Heavy metal emissions | Amount of heavy metals released to the environment by generating electricity from fuel combustion | Interval grey number (g/GWh) |

Water consumption | Water withdrawals mainly for use in cooling systems of thermal power stations | Crisp value \((\hbox {m}^{3}/\hbox {GWh}\)) |

Effect on global Warming | Given the amount of \(\hbox {CO}_{2}\), \(\hbox {NO}x\) and other greenhouse gases being released by each system for generating one GWh electricity, experts express their opinion on this criteria. | Linguistic term |

Land use | Land occupied by energy generation facilities against their expected energy production throughout their lifespan | Interval grey number \((\hbox {m}^{2}/\hbox {MWh}\)) |

Disturbance of ecological balance | Energy Generation systems through the land they occupy during installation and waste they produce during their operations can cause stress and disruption to ecosystems. | Linguistic term |

Particulate matter \(\hbox {PM}_{10}\) and Particulate matter \(\hbox {PM}_{25}\) | Particulate matter emissions have been considered separately for \(\hbox {PM}_{10}\) and \(\hbox {PM}_{2.5}\). Particulate matter emissions pose significant risks for human health depending on size, distribution, microstructure and chemical composition. | Interval grey number (kg/GWh) |

Social criteria and their descriptions

Criteria | Description | Measuring unit, type |
---|---|---|

Job creation | Energy production facilities are large infrastructure project that employ more people during construction phase and fewer during operation. Here we used levelised data based on expected energy output during project lifespan | Crisp value (job years/GWh) |

Social acceptability | Social acceptability expresses the overview of opinions related to the energy systems by the local population regarding the hypothesized realization of the projects under review from the consumer point of view | Linguistic term |

Mortality rate | Number of deaths occurring during the production of certain energy resources. It is a combination of actual direct deaths and epidemiological estimates | Interval grey number (Deaths/GWh) |

External costs associated with health | Energy facilities are associated with numerous externalities. Here we are only taking into consideration those that are directly linked to human health (excluding those linked to ecosystem damages etc) | Interval grey number (€/GWh) |

The power production options have been evaluated against the aforementioned criteria. For cases best described by crisp data, the precise number for the criteria has been considered. For cases with data uncertainty or data being available in an interval range, the interval value in form of a grey number has been used. Moreover, where the opinions of experts were needed for the criteria, by the means of linguistic terms their opinions have been considered and entered into the decision matrix. After that linguistic variables have been transformed into grey numbers.

## 4 Optimization model

In this section Grey Multi-Objective Linear Model is proposed for resolving the problem of optimal electricity fuel mix problem. The decision variables, objective functions, constraints and coefficients used in our model are described in detail in the following sections. The aim of our multi-objective approach is to share the electricity generation between seven available systems based on different resources i.e. Coal, Oil, Gas, Nuclear, Biomass, Hydro, Wind Power and Solar. The proposed resource mix should minimize the overall costs of generation, the amount of imported fuel, the emissions caused by power generation and simultaneously maximize the environmental and social benefits of installing and producing electricity by our systems. Basics of the model is derived from previous similar research in the field (Linares and Romero 2000; Cong 2013; Cabello et al. 2014) with few changes, some common concepts about fixed and variables costs as well as some innovative constraints and objective functions developed for this research. The importance weights of all the objective functions have been considered equally throughout the multi-criteria optimization.

### 4.1 Decision variables

The capacity that needs to be available or be installed is our first decision variable. Being denoted by \(Y_i \left( {i=1,2,\ldots ,m} \right) \) for each system *i* in Megawatts (MW) and called installation capacity, determines the nominal power output that can be generated by each system type aggregated across the country. \(X_{ki} \left( {i=1,2,\ldots ,m} \right) \) represents the hourly electricity generation of *i*th system in the *k*th period (\(k=1,2,\ldots ,T)\). In this research time periods are considered in intervals of 6 h from 00 am to 24 pm of each day continuously during a year. The number of hours in the *k*th period is denoted by \(t_k \). Moreover, the demand for period *k* is denoted by \(d_k \) and is defining the overall demand in the country during period *k* in MWh.

### 4.2 Objective functions

As previously discussed, in order to optimize the electricity generation, the objective functions consider financial, environmental, social and reduction of fuel import dependence. For financial objective annual costs, for environmental objective emissions and environmental scores and ranks, and for social objective the social scores and ranks which were obtained through GRA in Sect. 3 have been considered.

#### 4.2.1 Annual cost of electricity generation

*i*per MW, \(FOM_i \) fixed operation and maintenance cost for system

*i*per MW, \(VMC_i\) variable operation and maintenance cost for system

*i*per MW, \(FuC_i\) fuel cost for system

*i*per MW which obtained by multiplication of fuel price and fuel consumption for system

*i*to produce 1 MW of electricity.

Note that all the costs associated with Eq. 23 are in interval grey format to deal with uncertainties and imprecision exist with the price fluctuations and estimations.

#### 4.2.2 Independence from imported fuel

*i*and this coefficient is in grey interval format.

#### 4.2.3 Environmental consideration

*i*th generating system based on MWh electricity of production, \(Escore_i\) environmental scores obtained by GRA for each generation system.

#### 4.2.4 Social consideration

### 4.3 Constraint

A set of constraints for the optimization problem have been considered and are explained below.

#### 4.3.1 Electricity reliability and robustness

*i*, that is the amount of energy produced and can be turned into useful electricity to be supplied to the grid for system

*i*, \(CA_i\) capacity coefficient of system

*i*, that the fraction of the year when system

*i*is available, \(Av_i\) technical availability factor for system

*i*, that is the relation between number of hours that system

*i*is connected to the grid to the total hours in 1 year, \(d^{*}\) demand at pick time, when it is mostly probable that renewable energy systems can be unavailable,

*S*slack coefficient, which is a reliability coefficient in percentage and determining a confidence level for generating electricity more than demand in case the energy demand is higher than our predictions.

#### 4.3.2 Diversity and limitation on conventional systems

*LSC*% limitation on producing electricity by a single source or system,

*Lim*% limitation on amount of electricity that should be produced by conventional systems.

Where \(I\cup V=m,\) i.e., *I* is the number of conventional generation systems, *V* is the number of renewable generation systems and *m* is the total number of generation systems.

#### 4.3.3 Demand satisfaction

#### 4.3.4 Hydro limitation

### 4.4 Additional constraints

*i*th available system capacity is adequate to produce enough power to meet demand.

### Assumption 1

There is no import and export of electricity in and out of the grid.

### Assumption 2

75% of the demand should be satisfied by Coal, Gas, Nuclear, Oil and Biomass systems

### Assumption 3

Upper bound for electricity generation by solar system is 5% of the overall electricity.

### Assumption 4

Total available capacity to be installed by hydro system is 11,300 MWh.

Assumptions 2, 3 and 4 are based on realistic estimations of experts and The UK Renewable Energy Strategy document.

### 4.5 Data for optimization problem

Decision matrix for environmental evaluation process of different systems

Systems | Criteria | ||||||
---|---|---|---|---|---|---|---|

Heavy metal per g/GWh | Water consumption \(\hbox {m}^{3}/\hbox {GWh}\) | Global warming (tons \(\hbox {CO}_{2}/ \hbox {GWh}\)) | Land use \((\hbox {m}^{2}/\hbox {MWh}\)) | Disturbance of ecological balance | Particulate matter \(\hbox {PM}_{10}\) kg/GWh | Particulate matter \(\hbox {PM}_{2.5}\) kg/GWh | |

Coal | [666.83 806.17] | 2405 | Very high | [360 440] | Very high | [175.5 210.98] | [65.44 146.25] |

Gas | [115.11 139.31] | 1480 | Medium high | [36 44] | Medium high | [5.67 7.06] | [5.67 7.06] |

Nuclear | 0 | 2405 | Very low | [9 11] | Medium | 0 | 0 |

Oil | [4322.98 5247.98] | 2405 | High | [36 44] | Very high | [203.5 246] | [147.25 178.25] |

Biomass | [2103.66 2573.66] | 2271 | medium | [11.3 13.9] | Low | [335.6 403.41] | [291.16 350.16] |

Wind | 0 | 0 | Very low | [632 948] | Medium | 0 | 0 |

Hydro | 0 | 0 | Very low | [104 156] | Medium | 0 | 0 |

Solar | 0 | 0 | Low | [110 130] | Low | 0 | 0 |

Social decision matrix for evaluation process of different systems

Systems | Criteria | |||
---|---|---|---|---|

Job creation (Job years/GWh) | Social acceptability | Mortality rate (deaths/TWh) | External costs associated with health €/GWh | |

Coal | 0.11 | Low | 10–170 | 10200–76500 |

Gas | 0.11 | Medium | 3–5 | 2000–8000 |

Nuclear | 0.14 | Low | 0.00001–0.09 | 1640–5740 |

Wind | 0.17 | High | 0.15 | 340–1680 |

Hydro | 0.55 | High | 0.00001–1.4 | 200–6700 |

Oil | 0.11 | Medium | 36 | 2000–8000 |

Solar | 0.87 | High | 0.44 | 4380 |

Biomass | 0.21 | Medium | 24 | 1700–42500 |

Grey interval numbers for linguistic terms

Interval term | Grey value |
---|---|

Very high | [9 10] |

High | [7 9] |

Medium high | [5 7] |

Medium | [3 5] |

Low | [1 3] |

Very low | [0 1] |

Final results for grey relational degree of each of the systems based on environmental and social GRA evaluation

Coal | Gas | Nuclear | Oil | Biomass | Wind | Hydro | Solar | |
---|---|---|---|---|---|---|---|---|

GRG for environment | 3.5539 | 5.3086 | 6.0476 | 3.4192 | 4.1640 | 6.0923 | 6.5122 | 6.5809 |

GRG for social | 1.6927 | 2.6927 | 2.6751 | 2.4399 | 2.2887 | 3.3487 | 3.4735 | 3.9126 |

*T*for electricity generation is considered of a 6 h period, starting from 00:00am at January 1st of 2016. So for the 6 months under investigation we have a total of 728 time periods.

Coefficients and data (lower and upper bounds where applicable) for optimization problem

Systems | Coefficients | ||||||
---|---|---|---|---|---|---|---|

PE (%) | CA (%) | Av (%) | Capital cost (£/MWh) | FOM (£/MWh) | VMC (£/MWh) | Fuel cost (£/MWh) | |

Coal | [32 45] | 85 | 90 | [26 50] | [18.5 19.5] | 1 | [30 35] |

Gas | [45 53] | 85 | 75 | [8.77 9.22] | [3.9 4.1] | 0 | [47.77 50.22] |

Nuclear | [31 35] | 85 | 90 | [56.96 74.24] | [9.79 12.76] | 3 | [4.45 5.8] |

Oil | [28 32] | 85 | 85 | [8.1 9.9] | [3.6 4.4] | 0 | [44.1 53.1] |

Biomass | [33 37] | 70 | 85 | [8.55 9.45] | [9.5 10.5] | 1 | [81.7 90.3] |

Wind | [98 100] | 28 | 98 | [78.3 102.6] | [32.4 42.48] | 1 | 0 |

Hydro | [98 100] | 50 | 98 | [80 100] | [7 9] | 6 | 0 |

Solar | [98 100] | 20 | 99 | [123.28 144.72] | [22.08 25.92] | 0 | 0 |

As can be seen all the prices except variable cost of operation and maintenance (VMC) are considered in an interval form based on their nature, the location of implementation and other conditions and can vary between an upper and a lower bound. Although the variations of variable costs were so small that in this paper we considered them by their crisp values.

\(\hbox {CO}_{2}\) being emitted by each of the system based on MWh of electricity production

Systems | Coal | Gas | Nuclear | Oil | Biomass | Wind | Hydro | Solar |
---|---|---|---|---|---|---|---|---|

\(\hbox {CO}_{2}\) (kgs/MWh) | [807.5 892.5] | [427.5 472.5] | [17.1 18.9] | [522.5 577.5] | [17.1 18.9] | [4.94 5.46] | [12.45 13.65] | [66.5 73.5] |

## 5 Results

A direct comparison between the upper and lower values of the objective function can be considered in Fig. 3a and b. The differences are subtle and at the lower bound values benefit those resources with solar, gas, hydro, nuclear, biomass and wind.

The total electricity generation mix by each of the systems is shown in Fig. 4.

Each of the objective functions tries to optimize the electricity generation towards a different perspective. Therefore, focusing on the decomposed results of each of the solutions provided by each objective function provides a straight-forward outlook. This is particularly important for decision makers who need valuable insights about each perspective that can assist future planning.

Higher fixed cost for renewable resources, which results in lower installation capacity, and our second assumption lead to higher total generation by conventional systems. Among thermal power stations, nuclear is benefitted by low production cost and as a result has the highest share of electricity generation. Also among renewable resources, hydro by having the costliest variable operation and maintenance (£6/MWh) has approximately zero share of the electricity generation.

When evaluating power production alternatives against a broader set of environmental criteria a more diverse fuel mix occurs (Fig. 8). That is still however, based on nuclear and gas power generation as one of the system constraints requires a majority stake for thermal power stations. Moving from lower bound to upper bound of demand and increasing the feasible area for the problem wind is the most benefitted power source.

## 6 Discussion

The actual fuel mix for the UK’s power sector was in 2015 based on 29% gas, 21% nuclear, 22% coal, 24% by renewable including biomass, wind, hydro and solar as well as 3% by other fuels (UK Energy Statistics, 2015 & Q4 2015). The results obtained by this research suggest that at least 28% of the total electricity supply should be produced by renewable energy systems. This is certainly limited by problem constraints such as infrastructure cost and upper limit to variable output energy sources. However, technology development in the renewable energy field is rapid and offers declining installation and maintenance costs and increased reliability. At the same time market maturity for various energy storage technologies is growing fast and brings renewables and storage hybrid solutions off their small-scale past (Zafirakis and Chalvatzis 2014). Market ready systems provide a bundle of grid services and through variable compensation schemes are attractive for private investment (Zafirakis et al. 2016). These gradually expand to non-traditional power supply systems such as for example distributed industrial facilities (Zafirakis et al. 2014)

The UK is at the forefront of these developments. Firstly, by its commitment to offshore wind energy the UK has become a world leader in installed capacity and successfully transferred its offshore engineering know-how from oil and gas to wind turbines (IRENA 2015). Secondly energy storage has been identified as one of the key innovations for the UK’s technological strategy (UK Government 2013). Moreover, the UK actively promotes further development of the cross-border electricity market which enables electricity trade to improve grid stability and lower costs with existing interconnections to France, the Netherlands and Ireland and plans for further links to Norway and Denmark (OFGEM 2016). These connections will further increase the scope for more base-load renewables and have the potential to impact not only on the power market but also the emissions market (Zafirakis et al. 2015). At the same time the UK provides an interesting landscape because of its significantly development ICT infrastructure even at household level which supports a dual purpose; that of energy consumption and that of enabling smart energy management (Pothitou et al. 2016, 2017)

One clearer outcome of our research has been that coal as an electricity sector fuel is difficult to justify under any set of criteria. This echoes accurately with the UK Government’s schedule for shutting down all the coal-fired power stations of the country by 2025 or even earlier (Guardian 2016). At the same time throughout our analysis, withdrawal from coal, leads to large-scale reliance on nuclear energy. While there are certain benefits to nuclear energy such as its capacity to deliver large-scale reliable power and its limited environmental emissions, recent developments cast doubts about the future of nuclear energy. Since, the Fukushima disaster, nuclear safety standards have been raised, and as a result led infrastructure costs to grow substantially (Boggard 2014). Furthermore, new reactor design innovations increase project completion and cost uncertainty (Riesz et al. 2016). These issues and particularly the new increased costs have not yet been clearly incorporated in the literature and therefore are not taken into account into our modelling. This explains the disparity between the completely justified reluctance of the UK Government (Financial Times 2016) to commit to new nuclear investment because of costs (The Economist 2016) and our decision making modelling essentially recommending nuclear as the best option including cost parameters.

## 7 Conclusion

Planning for electricity generation mix is a highly complex problem which confronts decision makers with multiple conflicting priorities and potentially disproportionate objectives. Established and mostly new technologies are coming with inherent uncertainties either due to their complexity or due to them being untested at scale and can affect and mislead optimization results and generation schemes. With this manuscript we put forward a new model that can support decision making for electricity fuel mix and we demonstrated it using the UK power sector as a case study. Apart from the aforementioned uncertainties, the UK presents an even more challenging case because of its recent decision to leave the EU. While this is an ongoing process, it is expected that it will stall new large-scale investment decisions which are particularly important for the energy sector. We argue that in the face of this turbulent environment our research provides the foundation of a flexible decision making tool that will be of help to policy making and assessment.

We dealt with excessive and inaccurate factors by applying two evaluation processes based on GRA to obtain the performance score of different generation systems with respect to environmental and social criteria and used the computed scores in our optimization problem to increase the share of power generations sources with the best environmental and social performance. For parameters that do not have certain or fixed values both coefficients and left hand side constraints we applied multi-objective interval grey linear programming. We found that the application of hybrid MCDM and MODM methodology is an effective approach in addressing complex and large-scale problems that include uncertainties.

Specifically for the UK we suggest an increase in all three renewable sources considered i.e. wind, solar and hydropower and significant decrease of coal and oil due to their prohibitive environmental and social impact and reliance on imports. Our results are contextualised and contrasted with the UK Governments’ policies as we recommend significantly stronger support for renewable energy sources than is currently in place.

Two of the limitations of this research are inherent within our assumptions. Firstly in the fact that we have considered the UK power system as a closed ecosystem without any cross-border power trade which is not the case as the UK has international electricity connections which can have an impact on the country’s power mix. Secondly, we did not consider the role of new technologies, such as energy storage, which is forthcoming in the UK both in terms of stationary storage and increasingly mobile storage in electric vehicles.

Therefore, future research should expand the scope of our present work to include the interconnectors with France, Netherlands Ireland, Denmark and Norway. Moreover, we are looking forward to modelling the role of energy storage in power dispatching and in optimising the use of indigenous resources in the UK or other countries. Finally, our modelling approach can be enriched with weight determining techniques such as DEMATEL and AHP in order to provide flexible planning and create a scenario development tool that will be of direct use by policy makers and take their priorities more into account.

## Notes

### Acknowledgements

The specific study has been funded under the project TILOS (Horizon 2020 Low Carbon Energy Local/small-scale storage LCE-08-2014). This project has received funding from the European Union & Horizon 2020 research and innovation programme under Grant Agreement No. 646529. The authors would also like to thank the project ‘A cross country examination of supply chain barriers on market access for small and medium firms in India and UK’ (Ref No. PM130233) funded by British Academy, UK for supporting this research.

### References

- Afgan, N. H., & Carvalho, M. G. (2002). Multi-criteria assessment of new and renewable energy power plants.
*Energy*,*27*(8), 739–755.CrossRefGoogle Scholar - Afsordegan, A., Sánchez, M., Agell, N., Zahedi, S., & Cremades, L. V. (2016). Decision making under uncertainty using a qualitative TOPSIS method for selecting sustainable energy alternatives.
*International Journal of Environmental Science and Technology*,*13*(6), 1419–1432.CrossRefGoogle Scholar - Arce, M. E., Saavedra, Á., Míguez, J. L., & Granada, E. (2015). The use of grey-based methods in multi-criteria decision analysis for the evaluation of sustainable energy systems: A review.
*Renewable and Sustainable Energy Reviews*,*47*, 924–932.CrossRefGoogle Scholar - Armaroli, N., & Balzani, V. (2011). Towards an electricity-powered world.
*Energy and Environmental Science*,*4*(9), 3193–3222.CrossRefGoogle Scholar - Arnette, A., & Zobel, C. W. (2012). An optimization model for regional renewable energy development.
*Renewable and Sustainable Energy Reviews*,*16*(7), 4606–4615.CrossRefGoogle Scholar - Balat, M. (2006). Electricity from worldwide energy sources.
*Energy Sources, Part B: Economics, Planning and Policy*,*1*(4), 395–412.CrossRefGoogle Scholar - Boggard, N. (2014). The cost of nuclear electricity: France after Fukushima.
*Energy Policy*,*66*, 450–461.CrossRefGoogle Scholar - Büyüközkan, G., & Güleryüz, S. (2016). An integrated DEMATEL-ANP approach for renewable energy resources selection in Turkey.
*International Journal of Production Economics*,*182*, 435–448.CrossRefGoogle Scholar - Cabello, J. M., Luque, M., Miguel, F., Ruiz, A. B., & Ruiz, F. (2014). A multiobjective interactive approach to determine the optimal electricity mix in Andalucía (Spain).
*Top*,*22*(1), 109–127.CrossRefGoogle Scholar - Cai, Y., Huang, G. H., Nie, X. H., Li, Y. P., & Tan, Q. (2007). Municipal solid waste management under uncertainty: A mixed interval parameter fuzzy-stochastic robust programming approach.
*Environmental Engineering Science*,*24*(3), 338–352.CrossRefGoogle Scholar - Çelikbilek, Y., & Tüysüz, F. (2016). An integrated grey based multi-criteria decision making approach for the evaluation of renewable energy sources.
*Energy*,*115*, 1246–1258.CrossRefGoogle Scholar - Chalvatzis, K. J. (2009). Electricity generation development of Eastern Europe: A carbon technology management case study for Poland.
*Renewable and Sustainable Energy Reviews*,*13*(9), 2703–2709.CrossRefGoogle Scholar - Chalvatzis, K. J., & Rubel, K. (2015). Electricity portfolio innovation for energy security: The case of carbon constrained China.
*Technological Forecasting and Social Change*,*100*, 267–276.CrossRefGoogle Scholar - Cheng, S., Chan, C. W., & Huang, G. H. (2003). An integrated multi-criteria decision analysis and inexact mixed integer linear programming approach for solid waste management.
*Engineering Applications of Artificial Intelligence*,*16*(5), 543–554.CrossRefGoogle Scholar - Conca, J. (2012). How deadly is your kilowatt? We rank the killer energy sources.
*Forbes*. Available online at: http://www.forbes.com/sites/jamesconca/2012/06/10/energys-deathprint-a-price-always-paid/#2ffb6f6a49d2. - Cong, R. G. (2013). An optimization model for renewable energy generation and its application in China: A perspective of maximum utilization.
*Renewable and Sustainable Energy Reviews*,*17*, 94–103.CrossRefGoogle Scholar - Data Explorer \({\vert }\) National Grid. www2.nationalgrid.com. N.p., 2016. Web. 10 Jul. 2016.
- Deng, J. L. (1982).
*Grey system fundamental method*. Wuhan: Huazhong University of Science and Technology.Google Scholar - Department of Energy and Climate Change (DECC). (2013). Electricity generation costs 2013.Google Scholar
- Economist. (2016). Hinkley pointless. Available online at: http://www.economist.com/news/leaders/21703367-britain-should-cancel-its-nuclear-white-elephant-and-spend-billions-making-renewables.
- Electricity Statistics - GOV.UK.
*Gov.uk*. N.p., 2016. Web. 10 Aug. 2016.Google Scholar - European Environment Agency (EEA/EMEP). (2013). Air pollutant emission inventory guidebook. Available online at: http://www.eea.europa.eu/publications/emep-eea-guidebook-2013.
- Evans, A., Strezov, V., & Evans, T. J. (2009). Assessment of sustainability indicators for renewable energy technologies.
*Renewable and Sustainable Energy Reviews*,*13*(5), 1082–1088.CrossRefGoogle Scholar - Fan, Y. R., & Huang, G. H. (2012). A robust two-step method for solving interval linear programming problems within an environmental management context.
*Journal of Environmental Informatics*,*19*(1), 1–9.CrossRefGoogle Scholar - Financial Times. (2016). Hinkley Point: ‘I am prime minister, this is my method,’ says May. Available online at: https://www.ft.com/content/6e0b3354-5580-11e6-befd-2fc0c26b3c60.
- Grubb, M., Butler, L., & Twomey, P. (2006). Diversity and security in UK electricity generation: The influence of low-carbon objectives.
*Energy Policy*,*34*, 4050–4062.CrossRefGoogle Scholar - Guardian. (2016). Green conservatives call for earlier UK coal power phase-out. Available online at: https://www.theguardian.com/environment/2016/jun/07/uk-should-shut-down-all-coal-power-plants-two-years-before-2025-pledge.
- Han, Y., Huang, Y. F., Wang, G. Q., & Maqsood, I. (2011). A multi-objective linear programming model with interval parameters for water resources allocation in Dalian city.
*Water Resources Management*,*25*(2), 449–463.CrossRefGoogle Scholar - Hladı, M. (2013). Weak and strong solvability of interval linear systems of equations and inequalities.
*Linear Algebra and Its Applications*,*438*(11), 4156–4165.CrossRefGoogle Scholar - Hofmann, J., Guan, D., Chalvatzis, K., & Huo, H. (2016). Assessment of electrical vehicles as a successful driver for reducing CO\(_2\) emissions in China.
*Applied Energy*,*184*, 995–1003.CrossRefGoogle Scholar - Huang, G., Baetz, B. W., & Patry, G. G. (1992). A grey linear programming approach for municipal solid waste management planning under uncertainty.
*Civil Engineering Systems*,*9*(4), 319–335.CrossRefGoogle Scholar - Huang, G. H., Baetz, B. W., & Patry, G. G. (1995). Grey integer programming: an application to waste management planning under uncertainty.
*European Journal of Operational Research*,*83*, 594–620.CrossRefGoogle Scholar - International Renewable ENergy Agency (IRENA). (2015). 30 Years of Policies for Wind Energy: Lessons from United Kingdom. Available online at: https://www.irena.org/DocumentDownloads/Publications/GWEC_UK.pdf.
- Joint Research Centre (JRC). (2013). Assessment of the European potential for pumped hydropower energy storage: A GIS-based assessment of pumped hydropower storage potential by Marcos Gimeno-Gutiérrez and Roberto Lacal-Arántegui. Report EUR 25940 EN.Google Scholar
- Kabak, M., & Dağdeviren, M. (2014). Prioritization of renewable energy sources for Turkey by using a hybrid MCDM methodology.
*Energy Conversion and Management*,*79*, 25–33.CrossRefGoogle Scholar - Kalkuhl, M., Edenhofer, O., & Lessmann, K. (2012). Learning or lock-in: Optimal technology policies to support mitigation.
*Resource and Energy Economics*,*34*(1), 1–23.CrossRefGoogle Scholar - Kaldellis, J. K., Spyropoulos, G. C., Chalvatzis, K. J., & Paliatsos, A. G. (2006). Minimum \(\text{ SO }^{2}\) electricity sector production using the most environmental friendly power stations in Greece.
*Fresenius Environmental Bulletin*,*15*(11), 1394–1399.Google Scholar - Kaya, I., & Kahraman, C. (2009). Fuzzy robust process capability indices for risk assessment of air pollution.
*Stochastic Environmental Research and Risk Assessment*,*23*(4), 529–541.CrossRefGoogle Scholar - Koroneos, C., Michailidis, M., & Moussiopoulos, N. (2004). Multi-objective optimization in energy systems: the case study of Lesvos Island.
*Greece. Renewable and Sustainable Energy Reviews*,*8*(1), 91–100.CrossRefGoogle Scholar - Lilliestam, J., & Hanger, S. (2016). Shades of green: Centralisation, decentralisation and controversy among European renewable electricity visions.
*Energy Research & Social Science*,*17*, 20–29.CrossRefGoogle Scholar - Linares, P., & Romero, C. J. (2000). A multiple criteria decision making approach for electricity planning in Spain: Economic versus environmental objectives.
*Journal of the Operational Research Society*,*51*(6), 736–743.CrossRefGoogle Scholar - Liu, G., Baniyounes, A. M., Rasul, M. G., Amanullah, M. T. O., & Khan, M. M. K. (2013). General sustainability indicator of renewable energy system based on grey relational analysis.
*International Journal of Energy Research*,*37*(14), 1928–1936.CrossRefGoogle Scholar - Macknick, J., Newmark, R., Heath, G., & Hallett, K. C. (2012). Operational water consumption and withdrawal factors for electricity generating technologies: A review of existing literature.
*Environmental Research Letters*,*7*(4), 045802.CrossRefGoogle Scholar - Manzardo, A., Ren, J., Mazzi, A., & Scipioni, A. (2012). A grey-based group decision-making methodology for the selection of hydrogen technologies in life cycle sustainability perspective.
*International Journal of Hydrogen Energy*,*37*(23), 17663–17670.CrossRefGoogle Scholar - Maxim, A. (2014). Sustainability assessment of electricity generation technologies using weighted multi-criteria decision analysis.
*Energy Policy*,*65*, 284–297.CrossRefGoogle Scholar - Newbery, D. (2016). Missing money and missing markets: Reliability, capacity auctions and interconnectors.
*Energy Policy*,*94*, 401–410.CrossRefGoogle Scholar - Perera, A. T. D., Attalage, R. A., Perera, K. K. C. K., & Dassanayake, V. P. C. (2013). A hybrid tool to combine multi-objective optimization and multi-criterion decision making in designing standalone hybrid energy systems.
*Applied Energy*,*107*, 412–425.CrossRefGoogle Scholar - Pothitou, M., Hanna, R. F., & Chalvatzis, K. J. (2016). Environmental knowledge, pro-environmental behaviour and energy savings in households: An empirical study.
*Applied Energy*,*184*, 1217–1229.CrossRefGoogle Scholar - Pothitou, M., Hanna, R. F., & Chalvatzis, K. J. (2017). ICT Entertainment appliances’ impact on domestic electricity consumption.
*Renewable and Sustainable Energy Reviews*,*69*, 843–853.CrossRefGoogle Scholar - Promjiraprawat, K., & Limmeechokchai, B. (2013). Multi-objective and multi-criteria optimization for power generation expansion planning with CO\(_{2}\) mitigation in Thailand.
*Songklanakarin Journal of Science & Technology*,*35*(3).Google Scholar - Purwanto, W. W., Pratama, Y. W., Nugroho, Y. S., Hertono, G. F., Hartono, D., & Tezuka, T. (2015). Multi-objective optimization model for sustainable Indonesian electricity system: Analysis of economic, environment, and adequacy of energy sources.
*Renewable Energy*,*81*, 308–318.CrossRefGoogle Scholar - Ren, J., Dong, L., Sun, L., Goodsite, M. E., Tan, S., & Dong, L. (2015). Life cycle cost optimization of biofuel supply chains under uncertainties based on interval linear programming.
*Bioresource Technology*,*187*, 6–13.CrossRefGoogle Scholar - Riesz, J., Sotiriadis, C., Vithayasrichareon, P., & Gilmore. J. (2016). Quantifying key uncertainties in the costs of nuclear power. Working Paper. Available online at: http://nuclearrc.sa.gov.au/app/uploads/2016/02/Dr-Jenny-Riesz-20-10-2015.pdf.
- Sadeghi, A., Larimian, T., & Molabashi, A. (2012). Evaluation of renewable energy sources for generating electricity in province of Yazd: A fuzzy MCDM approach.
*Procedia-Social and Behavioral Sciences*,*62*, 1095–1099.CrossRefGoogle Scholar - Sackman, H. (1974).
*Delphi assessment: Expert opinion, forecasting, and group process*(No. RAND-R-1283-PR). RAND CORP SANTA MONICA CA.Google Scholar - Santoyo-Castelazo, E., & Azapagic, A. (2014). Sustainability assessment of energy systems: Integrating environmental, economic and social aspects.
*Journal of Cleaner Production*,*80*, 119–138.CrossRefGoogle Scholar - Sarucan, A., Baysal, M. E., Kahraman, C. & Engin, O. (2011). A hierarchy grey relational analysis for selecting the renewable electricity generation technologies. In
*Proceedings of the world congress on engineering*(Vol. 2, pp. 1149–1154).Google Scholar - Şengül, Ü., Eren, M., Shiraz, S. E., Gezder, V., & Şengül, A. B. (2015). Fuzzy TOPSIS method for ranking renewable energy supply systems in Turkey.
*Renewable Energy*,*75*, 617–625.CrossRefGoogle Scholar - Sioshansi, R. (2012). OR forum—Modeling the impacts of electricity tariffs on plug-in hybrid electric vehicle charging, costs, and emissions.
*Operations Research*,*60*(3), 506–516.CrossRefGoogle Scholar - Sithole, H., Cockerill, T. T., Hughes, K. J., Ingham, D. B., Ma, L., Porter, R. T. J., et al. (2016). Developing an optimal electricity generation mix for the UK 2050 future.
*Energy*,*100*, 363–373.CrossRefGoogle Scholar - Statistical Press Release; UK Energy Statistics, 2015 & Q4 2015. Available online at: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/513244/Press_Notice_March_2016.pdf.
- Stein, E. W. (2013). A comprehensive multi-criteria model to rank electric energy production technologies.
*Renewable and Sustainable Energy Reviews*,*22*, 640–654.CrossRefGoogle Scholar - Stirling, A. (1994). Diversity and ignorance in electricity supply investment: Addressing the solution rather than the problem.
*Energy Policy*,*22*(3), 195–216.CrossRefGoogle Scholar - Strbac, G. (2008). Demand side management: Benefits and challenges.
*Energy Policy*,*36*(12), 4419–4426.CrossRefGoogle Scholar - Streimikiene, D., Balezentis, T., Krisciukaitienė, I., & Balezentis, A. (2012). Prioritizing sustainable electricity production technologies: MCDM approach.
*Renewable and Sustainable Energy Reviews*,*16*(5), 3302–3311.CrossRefGoogle Scholar - Tasri, Adek, & Susilawati, Anita. (2014). Selection among renewable energy alternatives based on a fuzzy analytic hierarchy process in Indonesia.
*Sustainable Energy Technologies and Assessments*,*7*, 34–44.CrossRefGoogle Scholar - UK Government. (2013). Industrial strategy: Eight great technologies. Available online at: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/249255/eight_great_technologies_overall_infographic.pdf.
- Unsihuay-Vila, C., Marangon-Lima, J. W., De Souza, A. Z., & Perez-Arriaga, I. J. (2011). Multistage expansion planning of generation and interconnections with sustainable energy development criteria: A multiobjective model.
*International Journal of Electrical Power & Energy Systems*,*33*(2), 258–270.CrossRefGoogle Scholar - Wang, J. J., Jing, Y. Y., Zhang, C. F., & Zhao, J. H. (2009). Review on multi-criteria decision analysis aid in sustainable energy decision-making.
*Renewable and Sustainable Energy Reviews*,*13*(9), 2263–2278.CrossRefGoogle Scholar - Warren, P. (2014). A review of demand-side management policy in the UK.
*Renewable and Sustainable Energy Reviews*,*29*, 941–951.CrossRefGoogle Scholar - Xu, G., Yang, Y. P., Lu, S. Y., Li, L., & Song, X. (2011). Comprehensive evaluation of coal-fired power plants based on grey relational analysis and analytic hierarchy process.
*Energy Policy*,*39*(5), 2343–2351.CrossRefGoogle Scholar - Zafirakis, D., & Chalvatzis, K. J. (2014). Wind energy and natural gas-based energy storage to promote energy security and lower emissions in island regions.
*Fuel*,*115*, 203–219.CrossRefGoogle Scholar - Zafirakis, D., Elmasides, C., Sauer, D. U., Leuthold, M., Merei, G., Kaldellis, J. K., et al. (2014). The multiple role of energy storage in the industrial sector: Evidence from a Greek industrial facility.
*Energy Procedia*,*46*, 178–185.CrossRefGoogle Scholar - Zafirakis, D., Chalvatzis, K. J., Baiocchi, G., & Daskalakis, G. (2016). The value of arbitrage for energy storage: Evidence from European electricity markets.
*Applied Energy*,*184*, 971–986.CrossRefGoogle Scholar - Zafirakis, D., Chalvatzis, K. J., & Baiocchi, G. (2015). Embodied CO\(_2\) emissions and cross-border electricity trade in Europe: Rebalancing burden sharing with energy storage.
*Applied Energy*,*143*, 283–300.CrossRefGoogle Scholar - Zafirakis, D., Chalvatzis, K., & Kaldellis, J. K. (2013). Socially just support mechanisms for the promotion of renewable energy sources in Greece.
*Renewable and Sustainable Energy Reviews*,*21*, 478–493.CrossRefGoogle Scholar

## Copyright information

**Open Access**This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.