Sustainable Warehousing: Selecting The Best Warehouse for Solar Transformation

Environmental sustainability has been gaining importance not only in manufacturing but also in logistics and supply chain. Logistics sector has a significant responsibility in global energy consumption. The effect of warehouse on carbon emission demonstrates its importance of warehousing regarding sustainability. One way of developing sustainable warehouses is using renewable energy sources. In a solar warehouse; which is a new example of sustainable warehousing, roof of the warehouse is covered with solar panels so, both energy costs and carbon emission are reduced. Warehouses have been turning into solar warehouse in recent years. While there is a substantial literature regarding sustainable supply chain, literature regarding sustainable warehousing is limited. In order to contribute this gap in the literature the problem conducted here is selecting the best warehouse to be turned into a solar warehouse. In other words, the aim of the research is ranking the alternative warehouses according to solar facility location criteria in order to find best alternative. In this context first the alternative warehouses that can be turned into solar warehouse are determined then, the criteria which will be used in evaluation process is obtained from previous research. TOPSIS method is used to rank the alternative warehouses. The evaluation of alternatives according to solar facility location criteria is obtained from expert opinions and secondary data. The ranking obtained with TOPSIS method is compared with another ranking based on payback period of investment. The differences observed between two rankings is discussed and compared.


Introduction
Sustainability is one of the most crucial concepts in business world for the last decades. Sustainable Supply Chain Management (SSCM) focuses not only economic goals of the supply chainbut also environmental and social goals by coordinating processess in order to achieve long term performance (Carter and Rogers, 2008: 368). A similar concept to SSCM is Green Supply Chain Management (GSCM). GSCM can be defined as considering environmental factors in SCM. In the literature several aspects of SSCM and GSCM is analyzed from different aspects. From a supplier viewpoint sustainable monitoring and encouraging activities of the suppliers Ageron et al. (2012), selecting green suppliers Kannan et al. (2014) and sustainable environmental purchasing in GSCM (Zsidisin and Siferd;2001) may be considered as important activities. Cooperation in new green product development, green process development, using green technologies, using pollution preventive technology, reducing waste in energy usage are activities of green partnership (Vachon and Klassen, 2006). From a reverse logistics viewpoint GSCM includes recycling and remanufacturing (Zhu ve Sarkis, 2004).
Warehouses has an important role in environmental sustainability as well. Res et al. (2017) states that warehousing is responsible for huge amunt of CO2e. While warehousing is one of the most important activities in supply chain, most of the warehousing companies have little effort for environmental issues (Tan et al. 2009). Ciliberti et al. (2007) reported SSCM practices of Italian companies and found that 56% of the companies practice environmental purchasing, 20% of the companies practice sustainable transportation, 17% practice reverse logistics, 6% for sustainable packaging and only 1% practice sustainable warehousing. As well as the practical applications, academic research on sustainable warehousing is limited when compared with other supply chain activities.
This research focuses on the concept of sustainable warehousing. The problem is to determine the best warehouse to be turned in to a solar one among six alternative warehouses. Fırst the concept of sustainable warehousing is mentioned. In the next part of the study the methods of facility location selection is underlined as the problem dealed is a kind of facility location selection problem. The method used is analyzed and findigs is discussed.

Sustainable Warehousing
Warehousing can be defined as the storage of materials (packaging, finished goods and raw materials) at different stages of the supply chain (Chopra and Meindl, 2016). While in the past storage activities were defined as the activities of keeping stocks or keeping them safe, storage activities today figure far beyond that. In a way, storage might be seen as an unnecessary and compulsory activity that does not create value, but on the other hand, contemporary storage activities involve many implementations that actually create value. Disassembly, consolidation, delaying, cross-shipment, final assembly and packaging activities can be given as examples of these implementations (Bowersox, 2002). The changes in the activities carried out in warehouses can be listed as follows (Frazelle, 2001);  More transactions are performed, and the transactions have now less product range.
Alphanumeric Journal Volume 8, Issue 1, 2020  Storing and handling more products.
 Increased customer-oriented customization of products and services.
 Offering more of value-added services in warehouses.
 Management of larger volumes of product return.
 A larger amount of international orders.
 On the other hand, despite all these increases, there is less time to complete orders and less fault tolerance.
When it comes to sustainabile warehouse, it can be said that sustainability in warehouses has several aspects. Amjed et al. (2013) determined constructs of sustainable warehousing as warehouse facility design, warehouse layout, inventory management, warehouse staff, warehouse operations, onsite facilities, warehouse management system and mechanical handling equipment. Sustainability elements for facility design are reported as using renewable energy sources, daylight use, artificial lighting scheme, temperature control, noise pollution and biodiversity. One of the new activities related to sustainable warehouses is the production of electricity with solar panels in the roof of the warehouses. Therefore, carbon emissions of warehouses are reduced and warehouse revenues increase with the sale of electricity energy obtained. The literature on solar warehousing is limited as well. Knez, Bajor and Seme (2011) described the solar warehouse concept and focused on the important criteria regarding solar warehouse. Boztepe (2018) studied facility location slecetion using ANP (Analytic Network Process) with a set of criteria including economical, infrastructure. market, geographic, social environment and and solar warehouse location. Determining which warehouse will be turned into a solar warehouses is a problem of location selection and it has some differences compared to traditional location selection problems.

Facility Location for Sustainable Warehouse
Businesses want to choose the most appropriate facility location for their long-term goals. Facility location selection is closely related to other activities such as production control, material handling and organization of the site, and especially the production planning activity of the enterprise. Selection of facility or warehouse location is a complex and strategic planning problem where all activities within the logistics network between supply sources and product demand points are designed within the entire logistics system. A bad facility location decision can cause high cost, Alphanumeric Journal Volume 8, Issue 1, 2020 lack of resources or financial losses. One of the important issues such as process improvement, stock policies, routing or scheduling, which should not be ignored during the examination of a logistics network, the facility location selection decision is a business decision that directly affects balance sheet items (Meade and Sarkis, 1998). Considering the costs such as transportation cost, labor cost, taxes, raw material costs, and real estate costs, which are affected by the facility location decision, it is evaluated that the location selection decision affects 50% of the total operation costs (Heizer et al., 2017).
The facility location selection is made with a long-term planning. For this reason, the selection of location must be made by considering a number of criteria. Factors influencing the selection of the site of establishment can vary by industrial sector and by service sector (Krajewski et al., 2013).
The purpose of the facility location decision may vary depending on the type of business. The aim for the industrial sector is generally to minimize costs. In addition, increasing innovation and creativity may also be a goal. The goal for retailers and service businesses may be maximizing revenue rather than cost minimization. For example, in the decision on location selection for a warehouse, the objective may be a combination of cost and delivery speed criteria (Heizer et al., 2017). One of the criteria in faclity location may be environmental factors or sustainability factors (Uysal ve Tosun, 2014;Raut et al. 2017;Boztepe, 2018;Jha et al. 2018;Foroozesh, 2018).
According to Stevenson (1996), the main process used in the analysis of site of establishment, alternative decision points and evaluation criteria in variables are as follows:  Determining the criteria affecting location selection  Determining the weights of the criteria affecting location selection  Identifying alternative places suitable for criteria affecting location selection  Evaluation of the suitable alternatives to criteria affecting location selection.
There are many methods used in the facility location decision. In general, these methods can be listed as mathematical methods, statistical methods and multi criteria decision methods (MCDM). Kahraman et al. (2003) presented several fuzzy MCDM methods for facility location. TOPSIS method is also one of the multi criteria decision methods used in logistics for site selection. Demirel et al. (2010) provided a Choquet integral for selection problem of a real warehouse location. Ertuğrul and Karakaşoğlu (2008) used the TOPSIS method in the selection of the site of establishment of the manufacturing company, while Ulukan and Kop (2009)

TOPSIS Method
The steps of the TOPSIS method are described below (Özdemir, 2014).
Step 1: Creation of Decision Matrix (A) In the rows of the matrix, there are decision points to be ranked according to their superiorities, while in its columns there are evaluation factors to be used in decision making. Matrix A is the starting matrix created by the decision maker. The decision matrix is shown as follows: Step 2: Creating the Standard Decision Matrix (R) Obtaining the Standard Decision Matrix is a kind of normalization. Normalized Matrix (R) is calculated with the elements of matrix A, using the formula (1). The matrix R is obtained as follows: Step 3: Formation of Weighted Standard Decision Matrix (V) Firstly, weight values related to evaluation factors (WI) are determined. The sum of (Wi) will be 1.
Then, the elements in each column of the matrix R will be multiplied by the value related to (WI) to generate the matrix V. The matrix V is shown below:

 
Step 4: Creating Ideal ( The TOPSIS method assumes that each evaluation factor has either a monotonous increasing or a decreasing trend. In order to create the ideal solution set, in the matrix V, the largest value of the weighted evaluation factors, namely the largest column values, are selected. (If the relevant evaluation factor is minimization oriented, the smallest is selected) Finding the ideal solution set is shown in formula (2).
The negative ideal solution set, on the other hand, is created by selecting the smallest of the weighted evaluation factors in the matrix V, namely the smallest column values. (If the relevant evaluation factor is maximization oriented, the largest is selected) Finding the negative ideal solution set is shown in formula (3).
In both formulas, J shows benefit (maximization), and J' shows the value of loss (minimization).
In the formulas shown above; if the criteria are benefit-oriented, they show maximization J in the ideal solution set, and they show minimization J in the negative ideal solution set. Likewise, if the criterion is cost-oriented, it indicates the Alphanumeric Journal Volume 8, Issue 1, 2020 minimization J in the positive ideal solution set, and the maximization J in the negative ideal solution set.
Both the ideal and the negative ideal solution set consist of the number of evaluation factor, namely the element m.
The numbers ( * ) and ( − ), to be calculated here will naturally be same as the number of decision point.
Step 6: Calculation of Relative Proximity to Ideal Solution When calculating the relative proximity of each decision point to the ideal solution ( * ), the ideal and negative ideal discrimination measures are used. The criterion used here is the share of the negative ideal discrimination measure in the total discrimination measure. The calculation of the relative proximity value to the ideal solution is shown in the formula (6).
Here * , value is between the range of 0≤ * ≤1 and * = 1 shows the proximity of related decision point to the ideal solution, while * = 0 shows the proximity of related decision point to the negative ideal solution.

Real Life Problem
This study is conducted in a logistics company in Turkey in 2017. The Company has six warehouses alternatives that can be converted into solar warehouses. The problem is determining which warehouse to be converted to a solar one. Each alternative warehouse is located in a different city. The similar problem is studied by Boztepe (2018). Boztepe developed an ANP model for a new solar warehouse facility location selection among five alternative locations using a set of criteria including economical, infrastructure. market, geographic, social environment and solar warehouse location. Having weights of alternatives form ANP results he used TOPSIS method to rank the alternatives. He used only three criteria regarding solar warehouse location. The problem is to determine the best warehouse to be turned in to a solar one among six alternative warehouses. This research aims to make a ranking using only solar warehouse location criteria.
Alphanumeric Journal Volume 8, Issue 1, 2020 The solar criteria for a solar warehouse location selection were previously determined by Boztepe et al. (2015) with AHP method. The criteria and weights is shown in Table  1. The most important criterion is the radiation value of the region, while the least significant criterion is the self-consumption within the warehouse. The weight of self-consumption is small according to AHP results obtained by literature because Turkey government gives guarantee to buy the electricity produced by the facilities such as warehouses. So even if the warehouse cannot consume all the electricity it produces, it can sell the electricity to the government.  The criteria in Table 1. is used to rank the warehouses in this research. Ranking of alternatives is conducted by TOPSIS method. In TOPSIS method each alternative is scored according to ten criteria in Table 1.
Two different approaches are used to determine the scores of the alternatives according to the criteria. Secondary data is used for criteria that can be directly obtained. For the criteria that could not be imported directly, expert opinion is used. Two of the experts are two senior managers of the company that own the warehouses. They were managers of the company's earlier projects on solar warehouses. The third expert is the manager of the company that builds solar facilities and converts the roofs into solar roofs by solar panels. All experts are experienced in solar warehousing and have knowledge about alternative six warehouse. The experts were asked to rate the six alternatives according to ten criteria.
The criteria A1 is the annual irradiation values of the regions, and the criteria A2 is the solar annual time. These data are obtained from the past meteorological data of the Criteria C1 is Self-consumption and C2 is warehouse roof space were obtained directly from the company. Data were obtained for four out of ten criteria, while the other six criteria were asked to be scored by experts. The wind potentials of the regions, criteria A4 is also obtained from the General Directorate of State Meteorological Affairs (www.eie.gov.tr/yekrepa/repa-duyuru_01.html). However, since the winding potential varies considerably with the land, these data were not used directly but presented to the experts to use the expert opinion.
In the first step, the A1, A2, C1 and C2 criteria were directly taken as , while for the other criteria, opinions of the three experts were taken. The geometric mean of the Alphanumeric Journal Volume 8, Issue 1, 2020 scores of three experts was taken. In the second step, normalization was carried out with the data that were obtained. All criteria is about maximazing the the benefit in other words maximizing the electricity produced by solar transformation of warehouse. So criteria B1 is taken as how small is the risk of shadowiness for the alternative warehouse. The larger score of an alternative from B1 criteria shows that the shadowiness risk is smaller. For example Istanbul warehouse has the lowest score from criteria B1 as there are huge buildings around the Istanbul warehouse and new huge buildings may be constructed to the land near the warehouse. Standard Decision Matrix which is formed after normalization is seen in Table 2. In the next step, the weighted matrix was obtained. The weights were obtained from Table 1. The weighted normalized matrix is shown in Table 3.

Criteria
Alphanumeric Journal Volume 8, Issue 1, 2020 ranking is associated with ten solar criteria. Even though all criteria are about maximizing electricity production, none of them is directly about payback period.
This ranking can be compared with payback period of the investment. A ranking according to the payback period is developed as well. Considering that a facility with an installed capacity of 1 MWp can be installed at an investment cost of 975,000 USD with prices of 2017, the production capacities and annual monetary returns in the same regions are only as follows according to the data of the sunbathing potential (http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php). So investment cost and yearly production (kwh/year) is obtained for six alternatives and shown in the second and third columns of Table 6 consequently. The fourth column of Table 6. demonstrates annual income. Turkey government buys the electricity with a price of 0,133 $ per kwh. For example Istanbul warehouse produces 1.300.000 kwh and sells it to the government with a price of 0,133$ per kwh and consequently will yield (0,133 ($/kwh)*1.300.000 (kwh/year)=172.900 $/year which is illustrated n the fourth column. If we divide the investment cost (975.000 $) to yearly income (172.900 $/year) the result will be the payback period (5,64 years) which is shown on the fifth column. As investment cost is the same for all alternatives, ranking may be done by yearly income or by payback period. Instead of payback method self-consumption can be used however it is already used as a criterion in TOPSIS method. As seen on Table  6. the best alternative is Antalya warehouse. When Table 5 and Table 6 are compared, it is seen that the first row has not changed. According to both methods, Antalya is seen as the best alternative. Although the best alternative is the same according to the two rankings, the remaining of two ranking lists is different. According to TOPSIS method, the second is İzmir warehouse, while according to the payback period method it is Adana warehouse. It is expected to face some differences in two rankings because the criteria are not the same for two methods. TOPSIS is a more integrative method as it considers ten criteria.

Conclusion and Recommendations
This study focuses on the solar warehouse which is a new concept for sustainable warehousing. As a warehouse consume huge amount of electricity and transforming a warehouse to a solar one will reduce this consumption by producing own electricity, it is thought that warehouses will be transformed to solar ones in future. The aim of the study is to determining which depot should be selected to be converted to a solar one. TOPSIS method is used for the ranking of alternatives. This method considers ten solar warehouse location criteria. It can be said that this method contains both Alphanumeric Journal Volume 8, Issue 1, 2020 economic and environmental aspects. While economic aspect is about maximizing electricity production, using renewable energy by reducing environmental effects can be regarded as environmental aspect.
The results of this method is compared with the results of an another method; payback period. Although the best alternative is the same according to the two rankings, the remaining of two ranking lists is different as it is expected.
One of the main constraints of the study is that some of the evaluations are made subjectively, as all multi-criteria decision-making problems. In addition, the fact that the criteria used in this study are only solar and economic criteria, which can be considered as another constraint. Two methods may be integrated and also the social dimension of sustainability may be considered as another criterion in further research. With larger models, in future studies, an integrative criterion ranking can be made, considering not only solar criteria but also other criteria.