Multi-criteria selection of wind farms locations using AHP method

Igor Provči, HEP Croatian transmission system operator company, igor.provci@hops.hr

Prof. Marinko Stojkov, Faculty of Mechanical Engineering, Josip Juraj Strossmayer University of Osijek, Croatia mstojkov@sfsb.hr

Prof. Damir Šljivac, Faculty of Electrical Engineering, Josip Juraj Strossmayer University of Osijek, Croatia damir.sljivac@etfos.hr

SUMMARY

Production of electricity from wind energy is a technology with the highest annual growth rate of nearly 20 percent compared to other „classic“ technologies used in producing of electricity. The paper presented the possibility of applying multi-criteria analysis in selecting the location of wind farms. Multi-criteria analysis has been made using AHP method (Analytical Hierarchy Process). AHP is one of the methods of multicriteria analysis, and in general is one of the most useful mathematical methods for decision support. The complexity of decision making is reduced to a set of comparison criteria and alternatives, and the synthesis of the final result. Criteria such as wind potential, access to the power system, the impact on the environment, significantly influence on the priorities of the alternatives, and ultimately to the goal, and that is the best location for wind farm. AHP method is implemented in program tool Expert Choice, in versions for individual and group decision making. The Expert Choice (EC) software package wich apply AHP method for decision making is one of the most commonly used program tool. Expert Choice allows structuring of problem in different ways and comparing the alternatives and criteria in pairs in many ways. Additionally, it has ability to conduct sensitivity analysis and visualization based on a simple interactive mode that change the weight of criteria and alternatives. After determining the criteria and definition of alternatives, setting up all the necessary weight sensitivity analysis will be presented in several graphs.

Key words - AHP (Analytical Hierarchy Process), multi-criteria analysis, site selection, wind farms

1. INTRODUCTION

Galloping climate change, dwindling fossil fuel reserves as well as negative effects of

fossil fuels on the environment is reflected in higher prices of fossil fuels. Energy crisis is raising the

question of operation of power systems and security of supply , further more energy growing

needs suggest the necessity of energy conservation measures and the implementation of energy

efficiency. About this a set of EC Directive and policy objectives for renewable energy sources and

energy efficiency are clearly indicates and formulated as "20-20-20 by 2020" (20% share

of renewable energy, 20% reduction in consumption and 20% reduction in CO2 emissions by 2020).

All above has forced many countries, particularly developing countries, to begin

using renewable energy sources. From many renewable energy technology wind energy is the fastest

growing technology . The real expansion of wind power worldwide has resulted, that only in Europe

in last two years has been installed more wind power than any other source of electricity (11).

According to this, production of electricity from wind energy is increasing its capacity by 19 to

20 percent annually, which can be compared with areas that are characterized by strong growth,

such as Internet or mobile communications.

Benefits of electricity production from wind energy are numerous: wind is free source which

ensures energy independence, diversify the production of electricity and reduces dependence on

fossil fuels, wind is not expendable, and there is no harmful gaseous emission and environmental

pollution, opening of new workplaces.

From shortfalls it is necessary to mention the stochastic nature of wind as an energy source,

necessary energy for the secondary control of power system and possible impacts on human and

animal health that careful planning can minimize (quiver, noise, visual effect, threat to birds and

bats). All these issues are solved by selecting wind farm with good location.

Ideal wind farm for commercial exploitation of wind energy would be built on a site with

favorable wind regime, with good road access, close to the electricity networks, with good

capabilities of power evacuation, without conflict with allocation of land and under conditions of

environmental protection. But, ideal locations are rare, and selection of locations is always looking

for a comprehensive multidisciplinary approach with consideration of larger number of relevant

parameters, which includes various kinds of activities and a variety of professions.

The first step in the development of wind farm project is to find appropriate location taking

into consideration many criteria with respect to conditions. Finding a favorable location to build a

wind farm is a long and iterative process that involves many different factors and criteria. Precisely

for this reason we use the AHP method as a multi-criterion method for making a decision.

2. Criteria for selection of wind farm location

In selecting locations for construction of wind farm undoubtedly the most important criteria

is the wind potential of location. Another important criterion is the possibility for connecting wind

farm to the electrical infrastructure which is, as well as wind potential, on most potential locations

unknown, and without systematic analysis is not possible to evaluate their impact on the project.

Therefore, in this paper will be, based on network elements as well as the actual load on the network

but without a detailed calculation, perform evaluation of connection opportunities and power

acceptance from wind farms. In addition to these two criteria there is whole range of environmental

and other criteria for the construction of wind farm. These criteria, we will be reduced to a one

common criteria - the impact on the environment.

Wind farms are usually built in areas for which we found that possess a appropriate wind

potential. These are usually rounded hilltops that are open to air flow from all directions, ridges of

the hill, plateau and open spaces which are usually away from urban areas thinly populated with

poor infrastructure resources. Wind farms usually occupy about 1% of the land on which are found

and their work does not represent an obstacle to perform other activities which is up to construction

of wind farm in a region dominated, such as livestock.

Construction of wind farm is usually precede the measurement of wind parameters which are

indicative indicator of available wind potential and a key factor in deciding the viability of

investments in wind farm. Furthermore, on the basis of the available wind potential at the selected

location is calculate potential production of electrical energy.

2.1 Wind-potential

Winds farms are usually built in areas for which determine that possess an appropriate wind

potential. Wind potential is initially tested in the wind atlas or data from weather stations around the

locations. This method of testing can be used only for orientation while for the proper assessment of

wind potential it is necessary at least one year of wind measurements at the site. One year

measurement covers all four seasons and avoids the effect of seasonality. But this measure is not

enough for the study of wind. It is possible that the year in which it measure is above or below

average year in terms of wind potential. This means that is necessary to put one year data in the

context of several years measurement. It is necessary to make long-term correlation of measurement

data measured for many years (ten or more years) with data from measurement station (usually

weather station) near the location. Such long term correlated data is used for the study of wind and

mapping of the energy potential of sites. Also, this data is used to calculate expected production of

electricity from wind power.

Measurement of wind potential is mostly performed on measuring towers with the

anemometer and signposts of minimum two different heights. According to international

recommendations, the height of measurement tower should be at least two thirds expected hub

height of wind turbine. For example, for wind turbines with a height of 90 feet, measuring tower

should be at least 60 feet tall. Although measurements using measurement tower is still most

common methods for measuring wind potential but there are also newer measurement techniques

that use sound (SODAR) and light (LIDAR) to measure wind potential(4). In general, rule for

measuring wind potential is „more measurement data is better" because it gets a better description

of the actual location. Measurements using different techniques are also recommended, in order to

avoid potential problems as a result of using only one technique.

2.2 Connection to electric power system

For construction wind farms it is necessary to build access roads, adequate transportation

infrastructure, and often new transmission lines to the appropriate connection points to transmission

and distribution system. Communication with the transmission or distribution system operator in the

early stages of the project is very important, so that operator can be inform in advance about the

project, but also to obtain initial feedback on the possible points and the method of connection.

Location of potential wind power should be close to the transmission or distribution network, and

substations. It is necessary to take in account the regulations, concerning the method of connection

(on transmission line or SS), the voltage level of connection (depending on the desired power of the

wind farm), and consult with the transmission or distribution system operator on the possibilities and

conditions for connecting wind farms. Wind farm connection to the grid is a significant problem

considering that wind farms can have a significant impact on system stability and power quality in

the grid. Connection criteria are defined in the form of Wind grid codes.

2.3 Impacts on the environment

While the most of the protected areas are specified in physical plans, it is necessary to additionally

verify the potential negative impacts on the environment at the location. This particularly refers to

protected species of birds (raptors, migratory species) and other animals, plants, protected species,

water protected areas, etc. Also, it is necessary to take into account the nearby villages (noise and

shadowing), roads and infrastructure, protected cultural property and other impacts that

preparation, construction or operation of wind turbines could have on the environment. Most of

these effects are investigated within the formal procedures assessment of the impact on the

environment. But with preliminary examination of locations and consultation with experts, it is

possible prior to making the environmental impact study to determine whether the location in the

area where it is likely that construction will have a significant impact on the environment. The

European Union has a directive related to the current assessment of the impact on the environment

(3), and special rules relating for a Bird Protection (4) and habitat (5).

In Republic of Croatia for a wind power above 20 MW it is obliged to make environmental impact

assessments (EIA) while for the wind power installed capacity greater than 10 MW Ministry of

Environmental Protection, Physical Planning and Construction (MEPPPC) perform evaluation of the

need for environmental impact assessment (Official Gazette No. 67/2009). Within the procedure of

environmental impact assessment, in most cases it is necessary to make the impact assessment for

the National Ecological Network (Official Gazette No. 118/2009).

3. AHP for the selection of Wind farm location

AHP is a popular method used in finding a solution to the problem of multi-criteria decision choice

(MCDM). One of the reasons for the popularity of AHP as an applicable method is the fact that it

takes into consideration not just tangible but also intangible criteria. For instance, determining the

best location for a Wind farm is a problem that involves both many numerical and non-numerical

criteria. Therefore, AHP method seems to be an easily applicable method in finding a solution to the

problem of where exactly to build a Wind farm. AHP approach categorizes a decision problem into

several levels and thus uses a hierarchic structure in order to define this problem. The first and the

most important step to be taken as far as a decision-making process through AHP approach is

concerned is to determine goal, alternatives a criteria with a view to forming a hierarchic structure

regarding the decision-making problem. In this approach, each element existing in the hierarchy is

assumed to be independent of one another (10). The goal that forms the first level of the hierarchic

structure is the goal to solve the problem. The ultimate goal in our case is to determine the most

suitable location for a possible Wind farm based on the other levels of hierarchy. In the second level

of hierarchy are present main criteria. In setting up the, three alternative locations are considered as

probable choices. The layout of these locations is shown in Fig. 1

Figure 1. Layout of the three alternatives locations.

AHP is a process that starts with the establishment of the hierarchy shown in Fig. 2 and ends with the

determination of importance values as to the alternatives by making pairwise comparisons. The

hierarchy model of the decision problem is developed in such a way that the goal is positioned at the

top, with criteria and subcriteria on lower levels and finally alternatives at the bottom of the model.

Figure 2. AHP hierarchy model of the decision structure

Lokacija 3 Lokacija 2

Lokacija 1

Judgement scales

The pair-wise comparisons on each hierarchy structure level should be done by comparing all

possible pairs of the elements of this level, starting with the top of the hierarchy and working this

way to the lowest level. A pair-wise comparison in Expert Choice is the process of comparing the

relative importance, preference or likelihood of two elements with respect to another element (the

goal) in the level above.

Pair-wise comparisons are based on the Saaty scale of relative importance that assumes values

between 1 and 9. Saaty scale is ratio scale which has nine intensity of importance and each of one

corresponds to the value judgments about how many times one criterion is more important than

another. The same scale is also used when comparing two alternatives, but in this case, the scale

values are interpreted as an evaluation of how many times one alternative is more preferred than

another(9).This scale can be applied with ease to criteria that can be defined numerically as well as to

those cannot be defined numerically.

The fundamental Saaty scale (1).

Intensity of

importance

Definition Explanation

1 Equal importance Two activities contribute equally to the objective

2 Weak Between equal and moderate

3 Moderate importance Experience and judgment slightly favor one activity over

another

4 Moderate plus Between moderate and strong

5 Strong importance Experience and judgment strongly favor one activity over

another

6 Strong plus Between strong and very strong

7 Very strong or demonstrated

importance

An activity is favored very strongly over another; its

dominance demonstrated in practice

8 Very, very strong Between very strong and extreme

9 Extreme importance The evidence favoring one activity over another is of the

highest possible order of affirmation

One of AHP’s strengths is the possibility to evaluate quantitative as well as qualitative criteria and

alternatives on the same preference scale of nine levels. These can be numerical (figure 3), verbal

(figure 4) or graphical (figure 5).

Figure 3. Numerical scale

Figure 4. Verbal scale

Figure 5. Graphical scale

Analyses were made through Expert Choice packet programme following the completion of pair-wise

comparisons. The first of analyses is to check the consistency of judgments. In particular, the decision

maker may inadvertently misevaluate comparisons of large-scale decision-making problems.

Therefore, the consistency of matrixes in a pair-wise comparison should be ensured. If the matrix is

inconsistent, reassessment should be made until a consistency is achieved between the judgments

given by the decision maker. Expert Choice programme provides the ratio of inconsistency for each

pair-wise comparison matrix. ‘‘Inconsistency Ratio’’ can also be used in determining whether

pairwise comparisons are consistent with one another. For a pairwise comparison matrix, to be

accepted as consistent, inconsistency ratio should be smaller than 0.10.

Further more, on the basis of the pair-wise comparisons, relative significance (weights) of elements

of the hierarchy structure is calculated. The calculation of relative priorities for each decision making

element through a number of numerical calculations are made. Finally, these results are eventually

synthesised into an overall priority list of alternatives. Decision maker is allowed to change

preferences and to test the results if the inconsistency level is considered high. Results are priorities

of the alternatives in the form of priority list of alternatives and hierarchy tree with objectives’

relative significance. As a result of the analysis conducted using AHP method, „Lokacija 1“ seems to

be the best location for a wind farm as it shown in Figure 6.

Alternative PriorityLokacija 1Lokacija 2Lokacija 3

,521,246,233

Figure 6. Synthesis of the final result

Sensitivity analysis

A decision maker might be very interested to know the consequences of varied weights after he gets

the first results from his model. Some influence could have been underestimated or a slight variation

in one criteria weight could lead to a completely different decision. For that reason, almost all

decision analysis tools come with a sensitivity analysis. Sensitivity analysis is used to determine the

sensitivity of the alternatives to changes in the objectives’ priorities. If the ranking does not change,

the results are said to be robust. The sensitivity analysis is best performed with an interactive

graphical interface. Expert Choice allows different sensitivity analyses, where the main difference is

the various graphical representations (figure 7).

Figure 7. An example of four possible graphical sensitivity analyses in Expert Choice

DISCUSSION:

The European Commission adopted a directive 2009/28/EC which sets of ambitious targets for all EU

member states. According this directive till 2020, 20% of EU energy needs should be met from

renewable energy sources. It is assumed that in 2020. 35% of EU electricity needs will be settle from

renewable energy sources. It is expected that wind energy most contribute to this goal , about 35%

of the total energy produced from renewable sources. In Croatia according to the “Decision on the

minimum share of renewable energy (without large hydro plants) in the structure of the electricity

that Energy Service Company shall supply to the customers” it's suggested the amount of 300 MW of

total installed capacity from renewable sources by year 2010. from which the electricity will be

purchased at fixed rates. We can assume that most will be wind farms with respect to their economic

profitability. If they remain at the proposed amount, this would mean a 6-7% share of wind power in

the total installed capacity of Croatian power plants.

It is expected that the installed capacity of wind power plants in Croatia in 2020. will reach amount of

1200 MW. Since Croatia is on the way to EU, these objectives should be a guideline for future use

planning of wind farms in Croatia.

CONCLUSION:

Wind power is certainly represents an interesting energy source, which should be used in the most

efficient possible way. It is evident that wind farms are profitable technology that could soon become

competitive with other electricity generation technologies.

Making the possible models for decision of the future location of wind power plants based on AHP

method certainly should be taken into consideration.

Expert Choice, the user-friendly supporting software, has certainly largely contributed to the success

of the AHP method. It incorporates intuitive graphical user interfaces, automatic calculation of

priorities and inconsistencies and several ways to process a sensitivity analysis.

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