Design and Manufacture of the Solaris 11 Solar Car Body

Technological advancement and rapid growth in human population lead to depletion of energy resources, a major requirement in today's global world. Fossil fuels, which constitute a large portion of non-renewable energy sources, are expected to be insufficient for the coming century.

Furthermore, the use of fossil fuels causes numerous adverse scenarios such as global warming and air pollution due to carbon dioxide emissions.

For this reason, the use of electrical energy obtained from solar energy, a sustainable and renewable energy source, is a strong option that could replace fossil fuel-based energy sources.

This study examined the polymer composite body manufacturing of Solaris 11, the 7th solar vehicle of the Solaris Solar Cars Team. Composite materials are materials obtained by combining two or more materials at the microscopic level and engineered to possess superior mechanical properties compared to conventional materials (iron, steel, aluminum, etc.). The purpose of producing these materials is to combine properties that each component does not possess alone or cannot possess in a single material and to obtain lighter, stronger, more rigid, and more cost-effective products. Various methods exist for polymer composite manufacturing. One of the most commonly used is the hand lay-up method. Some of the positive features of this method include low production costs, the ability to mold in place, and design flexibility. This method, which has a very broad application range, is particularly suitable for the production of large parts requiring high strength and with a smooth outer surface. [1] The application of the method involves applying liquid resin with a reinforcing material to an open mold. One of the most important advantages of composite materials is the lightness they provide in a way that increases energy efficiency of a vehicle powered by solar energy. In solar cars, electrical energy is supplied through solar panels (photovoltaic) with an approximate surface area of six square meters. [2] The electrical energy obtained from a panel of this size is not sufficient in quantity. For this reason, polymer composite materials are preferred in the production of vehicles powered by solar energy. To obtain energy more cleanly, the number of solar cars increases day by day. International competitions are organized to improve the efficiency and design of these vehicles. The largest of these competitions is the WSC (World Solar Challenge) organization held in Australia. Solaris Solar Cars Team, which participates in these competitions, has been producing vehicles powered by solar energy since 2003. In this study, the polymer composite body manufacturing of the Solaris 11 vehicle designed by the Solaris team was addressed.

Vehicle Introduction

The Solaris 11 vehicle was modeled in a computer-aided design environment (Solidworks) as shown in Figures 1 and 2. To efficiently use the limited energy obtained from solar energy, the vehicle's geometry was approximated to a water droplet form taking aerodynamic principles into consideration. Thus, the aim was to reduce aerodynamic forces that could affect the vehicle at high speeds. Additionally, the race rules established by the relevant organizations in which the team would participate were considered in the vehicle's design.

Material Selection

Composite materials used in body manufacturing are structures created by combining two distinct components using various manufacturing techniques. One of these structures is the reinforcement element, and the other is the matrix component. While the reinforcement element provides the structure with high strength, the other structure, called the matrix, tightly binds the fibers present in the reinforcement element and allows forces acting on the structure to be distributed evenly. [3] Composite materials are classified according to the matrix material and the reinforcement material used. Table 1 indicates the classification of composite materials. Generally, glass fiber, kevlar fiber (aramid), carbon fiber, etc. are used as reinforcement elements. In the Solaris 11 vehicle, carbon fiber fabric was used as the reinforcement element. The main reasons for this are that carbon fiber is an extremely strong and durable material and has a low weight-to-strength ratio. Table 2 indicates the mechanical properties of carbon fiber fabric. Polymer matrix composites are widely used in the automotive industry. Polymers are generally divided into two categories: thermoplastics and thermosets. Thermoplastics are high molecular weight materials that melt when heat is applied. Thermosets are low molecular weight polymers that harden when exposed to heat and retain this condition indefinitely. [5] Epoxy resin, a thermoset material, was chosen as the matrix material in the production process. This is because the chemical reactions occurring in the resin harden the material in a way that allows the production of high-strength and lightweight products. [1] Carbon fiber polymer matrix composites have the following basic properties: • Low density (less than aluminum), • High strength (as strong as steel), • High rigidity (as hard as titanium but with lower density), • Chemical resistance, • Vibration damping capability, • Low electrical resistance. [6] End-grain balsa was used as core material to strengthen the structural strength. Table 3 indicates the mechanical properties of end-grain balsa.

Mold Preparation for Manufacturing

Styrofoam molds were obtained to manufacture the lower body and upper shell designed in a computer-aided environment. Styrofoam molds were chosen because of their low production and material costs. In the initial stage, as the surface quality of the molds was not at a sufficient level, surface improvement work was started. Polyurethane Jelkot was selected as the surface improvement material. Additionally, sandpapers with different grit sizes were used. Sandpaper grit designation indicates the number of grits in a one-inch length. Therefore, as the sandpaper grit increases, the sand grains on the surface become finer. [7] Low-grit sandpapers are used in initial coats, medium-grit in intermediate coats, and high-grit in final coats. • A polyurethane-based putty application was made to create a base surface on the styrofoam mold. • Defects and excess material in the applied putty were sanded. • Jelkot layering was applied until the desired surface quality was achieved. Since Jelkot hardened quickly, it was applied to the entire mold surface in a short time. After each Jelkot layering operation, sanding was performed. • After the mold surface quality reached the desired level, wet sanding was applied as a finishing operation. All these operations were applied to both the lower body and upper shell molds. Mold Casting After the surface quality operations on the molds were completed, mold release film was used before casting to ensure the product could be removed from the mold without damage and easily. Additionally, composite material components were assembled using the polymer composite sandwich structure technique. Sandwich composites are structures in which high-strength thin materials are used on the outer surfaces, and low-strength, thick, and low-density materials are used in the middle section, called the core. [8] Figure 5 shows the sandwich structure. In the production of the Solaris 11 vehicle, end-grain balsa was placed as core material between carbon fiber fabrics. Additionally, hand lay-up was chosen as the casting technique. Figure 6 shows the hand lay-up method.

Upper Shell Production

• Mold release film cut to appropriate dimensions was first laid on the mold.
• Epoxy resin, the matrix material, was combined with its hardener in the appropriate ratio and stirred for approximately 10 minutes.
• Two layers of carbon fabric were placed on the lower section, end-grain balsa core material was placed on top, and one layer of carbon fabric was placed on top of that. During these operations, the prepared epoxy resin mixture was applied to each layer with the help of rollers. The purpose of using rollers is to remove air bubbles trapped between layers. [1]
• After the resin application in the final layer, peel-ply fabric was placed on the carbon fiber fabric to facilitate easy removal of the film. [9]
• Flow mesh was laid to ensure proportional distribution of epoxy resin flow. [10]
• Felt was placed to remove any excess resin.
• Spiral and T tubes were attached on top of the felt as connecting elements between the mold and vacuum.
• Vacuum bag was laid to allow vacuum creation inside the mold, and vacuum sealant was applied to the mold edges.
• The mold was placed on the vacuum system and vacuum was applied at a pressure of 600 mmHg through spiral and T tubes placed in the mold, and the mold was cured at 40 degrees Celsius for approximately 15 hours with the help of heaters. During this stage, the temperature was monitored and recorded at 5 different points on the system.
• After the curing process was completed, the peel-ply fabric was removed. The product was then easily removed from the mold.

Lower Shell Production

All operations performed on the upper shell were also applied in the lower shell production. However, foam was placed instead of balsa material in the tail section of the lower body to dampen aerodynamic forces affecting the vehicle and to shift the vehicle's center of gravity slightly forward. Additionally, due to the physical nature of the product, a decision was made regarding the placement of end-grain core material on the side walls to make the vehicle lighter, as shown in Figure 8.

Conclusion

The use of sustainable energy sources and composite materials in vehicles enables the production of vehicles that are more environmentally conscious and have higher efficiency. The Solaris Solar Cars Team, which aims to increase the use of renewable energy sources, participates in various organizations that promote the use of electrical energy obtained from solar energy. There are several important considerations for polymer composite body manufacturing for vehicles powered by solar energy. These are: • Correct mixing ratios of components in chemical reactions directly affect the properties that will result from mixing. Therefore, when mixing different components, it is necessary to do so in a measured and precise manner. Otherwise, the desired composition may not be obtained. • The use of mold release film is of great importance for the product to be removed from the mold without deformation and smoothly. Additionally, the film plays a role in reducing spots that may be present on the mold surface. • For surfaces where high surface quality of the product is desired, sanding should be done with careful work that does not damage the mold. Considering time management and labor, the sandpaper grain surface should become finer toward the final coats. • To ensure proper curing of the resin, care must be taken when applying vacuum sealant between the mold edges and the vacuum bag. Because during vacuuming, air may enter the mold from these points and may prevent vacuuming from being performed at the desired pressure value. This study described the materials used and body production processes of the Solaris 11 vehicle, the 7th solar car of the team, which was designed in a computer-aided design environment.

Acknowledgment The Solaris Solar Cars Team thanks Kordsa Teknik Tekstil A.Ş. for its carbon fabric support in the production of the Solaris 11 vehicle.

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  Mehmet Ali Pınar Dokuz Eylül University Faculty of Engineering Department of Mechanical Engineering Mustafa Harman Dokuz Eylül University Faculty of Engineering Department of Mechanical Engineering Asst. Prof. Dr. Aytaç Gören Dokuz Eylül University Faculty of Engineering Department of Mechanical Engineering Picardy Jules Verne University Innovative Technologies Laboratory