1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee EENVIRO 2015doi: 10.1016/j.egypro.2015.12.232

Energy Procedia 85 (2016) 489 – 497

ScienceDirect

Sustainable Solutions for Energy and Environment, EENVIRO - YRC 2015, 18-20 November 2015, Bucharest, Romania

PCM selection using AHP method to maintain thermal comfort of the vehicle occupants

Lavinia Socaciua*, Oana Giurgiua, Daniel Banyaia, Mihaela Simiona a Technical University of Cluj-Napoca, Faculty of Mechanical Engineering, Departament of Mechanical Engineering, Romania,

Abstract

Thermal energy storage with phase change materials (PCMs) offers a high thermal storage density with a moderate temperature variation. PCMs have attracted growing attention due to its important role in achieving energy conservation in buildings and vehicles increasing thermal comfort. Nowadays, in order to maintain the thermal comfort of vehicle occupant’s, air conditioning is used, but the problem behind the usage of air conditioning is the increasing of fuel consumption and environmental problem caused by it. PCMs can be integrated easily in the car components with low-cost in order to maintain thermal comfort of the vehicle occupants. These materials are used to store the thermal energy during the time which the car is parked or during driving in sunlight. Material selection for a given application can be a challenging task because there are lots of condition and consideration that must be taking into account. Most of researchers use PCM in particular application depending on their experience or availability of the material. Material selection is a continuous process, aiming to choose the best material for a given application and to satisfy the set of requirements imposed. To improve the quality of decision, an effective evaluation approach is essential. The problem of selecting a material for an engineering application can be treated as a multi-criteria decision-making problem. In this research paper the AHP method was used for the ranking of ten commercial PCM taking into consideration the technical specification of the materials. © 2015 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee EENVIRO 2015.

Keywords: Phase Change Materials (PCMs); Analytic Hierarchy Process (AHP); thermal comfort of the vehicle occupants.

* Corresponding author. E-mail address: lavinia.socaciu@termo.utcluj.ro.

Available online at www.sciencedirect.com

© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee EENVIRO 2015

490 Lavinia Socaciu et al. / Energy Procedia 85 (2016) 489 – 497

1. Introduction

The thermal comfort of vehicular occupants is gaining more and more importance due to the rising attention toward comfortable mobility, in addition to the growing time that people spend in vehicles (private or public transport). Furthermore, comfortable vehicular climate control in many cases not only help to reduce the driver stress but also guarantee good visibility by avoiding the fogging phenomenon, and contributing to a more secure driving. Along with today’s demand for better vehicular energy utilization and more efficient performance have led to an increased interest in investigating and analysing the system and design requirements for good indoor and vehicle environments [1].

The temperature inside the vehicle intensifies especially during daytime (12 noon-2.30PM) which lead to affect the human comfort. In order to overcome this, air conditioning is needed. The usage of air conditioning increase the fuel consumption and causes environmental problem. Due to the above mentioned problem in the usage of air conditioning, phase change material is used to maintain the temperature [2] and maintain the thermal comfort of vehicle occupants.

A lot of studies have been conducted in order to study and improve thermal comfort [3-8], one of the solutions identified to improve thermal comfort in vehicles and, at the same time, to reduce the greenhouse gases emission due to the energy conservation is the use of phase change material (PCM) for thermal energy storage inside vehicle cabin [9].

In generally, the selection of materials is done by the designer bearing in mind the availability of the material or based on their experience. The AHP method is a structured technique for organizing and analysing complex decisions (like material selection), based on mathematics and psychology. In the specialized literature, the selection of proper PCMs which can be used to maintain the thermal comfort of the vehicle occupants using multi-criteria decision-making (like: AHP, TOPSIS, VIKOR etc.) are at the beginning. Results obtained from literature review are presented below.

M. K. Rathod and all, in his study [10], deal with two methods: technique for order preference by similarity to ideal solution (TOPSIS) method and fuzzy TOPSIS method that uses linguistic variable presentation and fuzzy operation. An analytic hierarchy process (AHP) method was used to determine the local weights of the criteria. TOPSIS and fuzzy TOPSIS methods are used to obtain final ranking. To demonstrate the effectiveness and feasibility of the proposed model, was evaluated the best choice of PCM used in solar domestic hot water system. Empirical results showed that the proposed methods are viable approaches in solving PCM selection problem. TOPSIS is suitable for the use of precise performance ratings while the fuzzy TOPSIS is a preferred technique when the performance ratings are vague and inaccurate [10].

M. Rastogi and all, in his study [11], attempts to extend Multiple Criteria Decision Making approach for ranking and selecting PCMs for domestic HVAC application. Firstly, Ashby approach has been employed for determining two novel figures of merits (FOM) to grade PCMs performance. The FOMs thus obtained were subjected to Pareto Optimality test. The graded materials were ranked using TOPSIS method. The local weights for the different attributes were calculated using Shannon’s entropy method. In order to justify the rankings obtained, the top materials were subjected to a standard simulation study to evaluate their relative performance using PCM Express with the aim of maintaining human comfort temperature. It was observed that the results obtained by simulation are in good agreement with those obtained using Multiple Criteria Decision Making approach. The candidates with the best ranks showed significant improvement in ameliorating the temperature conditions. Thus it can be concluded that integration of Multiple Criteria Decision Making approach for PCMs selection would prove an economical and swift alternative technique for ranking and screening of materials [11].

C. Barreneche and all in [12] studied the available materials to be used as PCM for building application in literature were added to a database to be used with CES Selector software. More than three hundred PCM with phase change temperatures between 50°C and 150°C considering commercial and non-commercial PCM were listed and classified by their properties and the values available of the materials have been introduced in this database. The main objective of their study was to generate a PCM database and draw on it in order to facilitate the selection of the most suitable PCM depending on the building application [12].

In [13] is applied the principles and techniques of the Analytic Hierarchy Process (AHP) in order to prioritize and select the proper PCMs for comfort application in buildings. The objective was to evaluate the best choice of

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commercial PCM used for comfort application in buildings. The PCMs which can be used in above system should have phase change temperature between 22 and 28°C. The criteria used in this case study are: phase change temperature, latent heat capacity, density for solid phase, specific heat capacity and thermal conductivity of material. These criteria influences the selection of PCM used for comfort application [13].

The aim of this research paper was the selection of the proper commercial PCM used to maintain the thermal comfort of vehicle occupants with AHP method. Applying the AHP method is a new approach in this field. This research paper is limited to determine the weight of ten commercial PCMs considering only the technical specification of each PCM, but this study can be reiterated by adding other criteria.

2. Materials and Analytic Hierarchy Process (AHP) method

Phase Change Material (PCM) is a substance with a high heat of fusion which is melting and solidifying at certain temperatures [14]. PCM is capable of storing or releasing large amounts of latent heat of fusion [15]. The PCMs can have various composition, different proprieties and different application fields. Phase change proprieties do not make all these materials viable for a specific application.

In this study a set of ten commercial PCMs witch can be used to maintain the thermal comfort of vehicle occupants was analysed, using the AHP method. The criteria considered for all materials were:

Phase change temperature (PCT) which matches the application (between 0-30°C) to assure the energy storage, High value of the latent heat of fusion (LHF) to achieve high storage density compared to sensible storage, High value of specific heat capacity (SHC) at constant pressure to provide additional significant sensible storage, High thermal conductivity (TC) to assist the charging and discharging energy of the storage system, High density for solid (DS) and for liquid (DL) phase, Maximum operation temperature (MOT).

In table 1 is presented the list of selected commercial PCMs and theirs thermo-physical proprieties used to maintain the thermal comfort of vehicles occupants.

Analytic Hierarchy Process (AHP) is a method of scientific analysis of scenario and decision making using consistent valuation of hierarchies. AHP has been applied in various fields of strategic management and resource allocation, where decisions have far-reaching importance and where the decision makers need quality and reliable advice during alternatives consideration and determination of their effects in relation to the set objectives [16].

The steps to be fallowed for determining the overall weights for PCMs alternative and ranking them with AHP model are:

Step 1: Define the objectives. Step 2: Identify the criteria / attributes [17] Step 3: Select the alternatives. Step 4: Arranged in a hierarchical structure the objectives, criteria and alternatives.

A hierarchy tree has at least three levels: overall goal of the problem at the top, multiple criteria that define alternatives in the middle, and decision alternatives at the bottom [18].

In figure 1 is presented the hierarchical tree for the studied problem. The objective of this study was to select the proper PCM that can be used to maintain the thermal comfort of vehicle occupants with AHP method. The thermo-physical proprieties of PCMs represent the criteria’s used for selection the optimal material. The commercial PCMs represents the alternatives studied, taking into consideration several criteria like: phase change temperature (PCT), latent heat of fusion (LHF), specific heat capacity (SHC), thermal conductivity (TC), density for solid (DS) and for liquid (DL), respectively maximum operation temperature (MOT). The supplier of PCMs studied were: Rubitherm (with the next products: PCM 1 - Rubitherm RT0, PCM 2 - Rubitherm RT 10 HC, PCM 3 - Rubitherm RT 15, PCM 4 - Rubitherm RT 28 HC), PlusICE (with the next products: PCM 5 - Plus ICE PCM X25, PCM 6 - Plus ICE PCM X30), SavEnrg (with the next products: PCM 7 - SavEnrg PCM-Hs01P, PCM 8 - SavEnrg PCM-OM06P, PCM 9 - SavEnrg PCM-Hs22P, PCM 10 - SavEnrg PCM-Hs29P ).

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Table 1. List of selected PCMs and their thermo-physical proprieties

Criteria\Alternatives PCM 1 PCM 2 PCM 3 PCM 4 PCM 5 PCM 6 PCM 7 PCM 8 PCM 9 PCM 10

PCT (°C) 0 10 15 28 25 30 0 5.5 23 29

LHF (kJ/kg) 225 195 140 245 110 105 290 260 185 190

SHC (kJ/kgK) 2 2 2 2 1.63 1.65 4.14 n.a. 3.05 2.26

TC (W/mK) 0.2 0.2 0.2 0.2 0.36 0.36 0.55 n.a. 0.54 0.54

DS (kg/m3) 880 880 880 880 1055 1050 920 780 1840 1840

DL (kg/m3) 770 770 770 770 n.a. n.a. 1010 735 1540 1550

MOT (°C) 40 50 50 50 n.a. n.a. 50 50 80 80

References [19] [19] [19] [19] [20] [20] [21] [21] [21] [21]

Fig. 1. Hierarchical tree for the selection of the proper PCM used to maintain the thermal comfort of vehicle occupants

Step 5: Construct the pairwise comparison matrices.

Each element in an upper level is used to compare the elements in the level immediately below with respect to it. To make comparisons, we need a scale of numbers (Table 2) that indicates how many times more important or dominant one element is over another element with respect to the criterion or property with respect to which they are compared [22]. The 9-point scale was used to transform verbal judgments into numerical quantities ranging from 1 (equally important) to 9 (extremely important).

Table 2. Saaty`s 9 point scale of pair-wise comparison [17]

Scale Compare factor of i and j

1 Equally important

3 Weakly important

5 Strongly important

7 Very strongly important

9 Extremely important

2, 4, 6, 8 Intermediate value between adjacent scales

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If C={Cj׀j=1, 2, ... , n} is the set of criteria analysed, then the result of the pair-wise comparison can be summarized in an (nxn) evaluation matrix A. The element aij (i, j = 1, 2, ..., n) indicates the relative importance of criteria i with respect to criteria j:

0a,a/1a,1a,

aaa

aaa

aaa

A ijijjiii

nn2n1n

n22221

n11211

(1)

Step 6: Calculating the geometric mean of i row and normalizing the geometric means of rows in the comparison matrix [10] to obtain the relative normalized weight (Wi) of each criteria:

n1ij3i2i1ii a...aaaGM (2)

nj

1j iii GMGMW (3)

Step 7: Achieve matrix X, an n-dimensional column vector which outline the sum of the weighted values for the importance degrees of alternatives:

n

2

1

n

2

1

2n1n

n221

n112

c

c

c

w

w

w

1aa

a1a

aa1

WAX (4)

Step 8: Calculate the consistency value:

iii wcCV (5)

Step 9: Get the priority vectors (w) from the pair-wise comparison matrix A by solving an eigenvalue problem whit the relation [23]:

wAw max (6)

where λmax is the maximum eigenvalue of A [24]. Step 10: The consistency is expressed by the following equation, and the measure of consistency is called the

consistency index (CI) [25]:

1nnCI max (7)

Step 11: The random inconsistency (RI) [26]:

n2n987.1RI (8)

Step 12: The consistency ratio (CR)[25]:

RICICR (9)

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Saaty stated that a C.R. less than 0.1 are accepted otherwise, a new comparison matrix needs to be reconstructed to weight the indicators [27].

Step 13: The overall performance level of each alternative with respect to the criteria and the decision goal is obtained as [24]:

m,,1j;n,,1i,wwPmj

1jj

ni

1iik (10)

where wi (i=1,...n) are the weights of criteria, wj (j=1,...m) are the weights of alternatives with respect to criterion i. The Proper material will be the one with the biggest overall weight with respect to the decision goal.

3. Results and discussions

These research papers focus on the selection of the proper commercial PCM which can be used to keep the thermal comfort of vehicle occupants. Figure 1 explains the hierarchy tree considered to solve the material selection problem. Table 1 presents the thermo-physical proprieties of alternatives analysed (commercial PCM). The thermo-physical proprieties were considered as criteria for this study. By applying the AHP method we obtain the local weights of the criteria and the alternatives, and then we could establish the overall weights of each alternative taking into account all the criteria.

The local weights of each criterion taking into account the main objective of this study are presented in figure 2. It can be observed that LHF (latent heat of fusion) and TC (thermal conductivity) criteria are the most important criteria when we want to select the proper PCM which can be used to maintain the thermal comfort inside the vehicle when the vehicle is parked or when the vehicle runs on road in sunshine. Because the PCMs which can be used in this scope must have phase change temperature between 0 and 30°C, and because the maximum temperature between can operate this material is 50 to 80°C, the weight of PCT (phase change temperature) criteria have the next rank.

Figure 3 presents the local weights of the alternatives studied taking into consideration the each criteria considered: phase change temperature (PCT), latent heat of fusion (LHF), specific heat capacity (SHC), thermal conductivity (TC), density for solid (DS) and for liquid (DL), respectively maximum operation temperature (MOT). For the alternative PCM 8 data regarding at SHC and TC criteria are not available. For alternative 5 and 6 data regarding at DL and MOT criteria are not available. In this research, the alternative PCM 5, PCM 6 and PCM 8 was study taking into consideration only the criteria for known data.

Fig. 2. The weights of criteria for accomplish the objective

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Fig. 3. The weights of alternatives according each criterion

From figure 3 it can be observed that:

The most proper material from the point of view of PCT criteria is PCM 4 (Rubitherm RT 28 HC), PCM 6 (Plus ICE PCM X30) and PCM 10 (SavEnrg PCM-Hs29P) with a weight of 24.75%. The next materials which can be used are PCM 9 (SavEnrg PCM-Hs22P) and PCM 5 (Plus ICE PCM X25), but their weight are only the 7.78%; at three times less compared to alternatives ranking on first place.

The most proper material from the point of view of LHF criteria is PCM 8 (SavEnrg PCM-OM06P) and PCM 7 (SavEnrg PCM-Hs01P) with a weight of 30.60%. The next materials which can be used are PCM 4 (Rubitherm RT 28 HC) and PCM 1 (Rubitherm RT0), but their weights are considerable much lower.

The most proper material from the point of view of SHC criteria is PCM 7 (SavEnrg PCM-Hs01P) with a weight of 38.58%. The next materials which can be used are PCM 9 (SavEnrg PCM-Hs22P) and PCM 10 (SavEnrg PCM-Hs29P) with a weight of 22.89%, respectively 13.94%.

The most proper material from the point of view of TC criteria is PCM 7 (SavEnrg PCM-Hs01P) with a weight of 42.18%. The next materials which can be used are PCM 9 (SavEnrg PCM-Hs22P) and PCM 10 (SavEnrg PCM-Hs29P) with a weight of 18.75%.

The most proper material from the point of view of DS criteria is PCM 10 (SavEnrg PCM-Hs29P ) and PCM 9 (SavEnrg PCM-Hs22P) with a weight of 30.46%. The next materials which can be used are PCM 6 (Plus ICE PCM X30) and PCM 5 (Plus ICE PCM X25) with a weight of 11.79%.

The most proper material from the point of view of DL criteria is PCM 10 (SavEnrg PCM-Hs29P) and PCM 9 (SavEnrg PCM-Hs22P) with a weight of 35.21%. The next material which can be used is PCM 7 (SavEnrg PCM-Hs01P) with a weight of 13.20%.

The most proper material from the point of view of MOT criteria is PCM 10 (SavEnrg PCM-Hs29P) and PCM 9 (SavEnrg PCM-Hs22P) with a weight of 36.48%.

Figure 4 illustrate the overall weights of the alternatives studied taking into account each criteria considered to accomplish the goal.

496 Lavinia Socaciu et al. / Energy Procedia 85 (2016) 489 – 497

Fig. 4. The overall weights of alternatives taking into consideration each criterion to accomplish the objective

It can be observe that the proper PCM which can be used to maintain occupant’s thermal comfort inside the vehicle is PCM 7 (SavEnrg PCM-Hs01P) taking into account all the criteria analysed, with an overall weight of 29.90%. The next alternatives ranked are PCM 10 (SavEnrg PCM-Hs29P), with 14.80% overall weight and PCM 9 (SavEnrg PCM-Hs22P), with 13.17% overall weight. The overall weight for PCM 10 and PCM 9 is almost half compared with the best alternative.

4. Conclusions

Through the proposed work, the authors are ranking ten commercial Phase Change Materials using AHP method. The ranked PCMs are used to maintain the thermal comfort of vehicle occupants, when the car is parked or when the vehicle runs on road in sunshine. Suitable materials were first shortlist based on the phase change temperature (within the range 0-30°C), then taking into account the thermo-physical proprieties of PCMs.

The selection of materials is made by product designer or engineer. In generally, this decision is based on the experience of the decision maker or on the availability of the material. The AHP method is a structured technique for analysing complex decisions, based on mathematics and psychology. The local weights of criteria and of alternatives obtained with AHP method conduct us to determine the overall weights of alternatives taking into account all criteria considered.

AHP method was used to obtain the overall weight of PCMs based on the three level hierarchies which include goal (select the proper PCMs that can be used to maintain the thermal comfort of vehicle occupants), seven criteria (technical specifications) and ten alternatives (commercial PCMs).

The most important criteria (thermo-physical proprieties) for these PCM are weighted as: thermal conductivity and latent heat of fusion 36.34%; phase change temperature 13.25% and specific heat capacity 6.91%.

PCM 7 (SavEnrg PCM-Hs01P) is found to be best material using AHP method for this specific application taking into consideration all the thermo-physical proprieties known and the weights of these criteria to accomplish the goal. The next alternatives which can be chosen are PCM 10 (SavEnrg PCM-Hs29P) and PCM 9 (SavEnrg PCM-Hs22P), but their weights are half of the best material.

The systematic evaluation of AHP method can increase the efficiency of the system and decrease the hazardous selection of the material. These studies try to correlate the scientific and engineering goals for maintaining the thermal comfort of vehicle occupants. For future research, with additional criteria like: chemical criteria (i.e. chemical stability, non-corrosiveness, non-toxic, non-flammable and non-explosive) or economic criteria (i.e. availability, abundance and available to a low price), the AHP method can be reiterated.

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Acknowledgements

This work was supported by Grant of the Romanian National Authority for Scientific Research, CNCS, UEFISCDI, Projects number: PN-II-PT-PCCA-2013-4-0569 Innovative strategies of HVAC systems for high indoor environmental quality in vehicle and PN-II-PT-PCCA-2011-3.2-1212 Advanced strategies for high performance indoor Environmental Quality in Operating Rooms.

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