Journal of Engineering Technology Vol. 1: 60-67, 2011 ISSN 2231-8798 © 2011 UniKLBMI

60

Maximum Power Transferred For Solar Car Application

S. Yub1, M.I. Abu Bakar 2, R. Hussain2 & N. Mohamad2 1Department of Energy Competency Unit

2Department of Electrical Technology Universiti Kuala Lumpur – British Malaysian Institute,

Batu 8 Jalan Sg Pusu, 53100 Gombak, Selangor, Malaysia.

Corresponding email: shaharudin@bmi.unikl.edu.my

Abstract: Throughout the history of the technological advancements in our day and age, there have been numerous breakthroughs that have proven to test the limits of electrical and mechanical inventions as well as help us advance in every way possible. Solar power would be the most interesting and efficient type of energy to have power a vehicle for technological advantages such as having a powerful source of energy, and also not needing any type of fuel. Another vital reason for our choice to use solar energy is the high price of gas and fuel today. The feeling is that solar energy is the next vital advancement in the everlasting search for a powerful yet more cost efficient source of power. Thus, this project is compromised of designing and building a transportation vehicle that will use solar power in order to get maximum power transferred for solar car application. This vehicle will be composed of a previously purchased and stripped golf cart chassis, solar panels, a solar power inverter or peak power tracker, rechargeable batteries, and a high output 3.2 horsepower DC motor. Keywords: DC motor, solar panels, power inverter, rechargeable batteries

1.0 INTRODUCTION

A golf cart or similar small cart type model will be used as the basis for this design. The suspension, brakes and steering components will remain “stock” on this model barring any modifications that need to be performed in order to mount the DC motor. Typical frameworks for these carts weigh between 400-800 lbs [1].

The idea to cut down on weight is essential to allowing optimal battery efficiency and motor longevity. The built cart will not employ doors or windows and only have a plastic cover for the windshields and side panels. The option to add door and window panels will be explored in further designs. The suspension of a typical golf cart includes a solid axle and small gas tube shocks mounted straight onto the chassis [2].

No modifications need to be made to this setup unless the shock mounting interferes with the drive axle assembly in which the DC motor will mount to. The DC motor mounting will be done straight on to the chassis and the drive axle. Motor mounts will be used to allow the power plant to “hover” over the drive axle.

Output shafts will run from the motor to each wheel. At this point in the design phase the motor will mount on the rear axle. This was done to avoid major modification to the steering column and steering components on the front of the cart. In Fig. 1, the motor assembly can be seen.

Fig. 1 View of DC motor

Journal of Engineering Technology Vol. 1: 60-67, 2011

61

In order for the DC motor to function efficiently, a proper motor controller must be used. The motor controller in this case is capable of regulating up to 1000 watts of one-shot power. This means that it can send 1 KW of energy to the motor within 1 second of being activated. The power can then increase depending on the depressing of the accelerator pedal [3].

In this relative case the accelerator pedal is connected directly to the motor controller like a variable resistor knob and will act as a stop and go feature. When the accelerator pedal is not depressed the engine is not running. Conserving battery power will be an optimal part of this design project. In order to transfer the correct amount of current to the motor controller a power tracker must be installed into the system.

The peak power trackers condition the electricity coming from the solar array to maximize the power and deliver it either to the batteries for storage or to the motor controller for propulsion. When the solar array is charging the batteries, the peak power trackers help to protect the batteries from being damaged by overcharging [4].

Peak power trackers can be very lightweight and commonly reach efficiencies above 95%. A schematic of a peak power tracker can be seen in Fig. 2 below.

Fig. 2 Power tracker schematic

Solar panels used in 12-volt applications have very distinct power curves (volts x amps = watts). A graph of voltage vs. current will range from maximum current at zero volts (no power) to maximum volts at zero current (also no power). Somewhere in the middle there will be the Maximum or Peak Power Point (MPP) where the

maximum output wattage can be extracted from the solar panel. In solar panels designed for 12 volts systems this MPP is usually between 16 and 17 volts. This process is illustrated in the graph as in Fig. 3[4].

Fig. 3 Peak power tracker output

The power tracker is essential to the correct functioning of the solar car and great care will be taken when the assembly of this part is started and completed. The power tracker will be mounted near the DC motor, yet considering its small size it can mount anywhere on the chassis away from weather and road hazard [5].

The NiMH batteries (or equivalent) will be placed in the rear compartment below the trunk area. There will be a total of 6 of these batteries. The output wires will be sent to the motor controller and the input will be coming from the power tracker. An indicator on the dashboard will be created in order to monitor the charge on these batteries.

A low charge light will illuminate when the motor will not have enough capacity to continue more than 10 miles and must be shut off so the batteries can recharge via solar power. In order to Fig. out the mileage range of the batteries, various tests will be run to determine this value [6].

The main component of this project is the solar panel array. This array will consist of 2 solar panels each rated at 250 watts. This rating is an estimate of the power each can output when the sun is at its highest point with no obstructions to the panel.

To help with the insight on this concept, Fig. 4 shows the power transfer through a solar panel.

Journal of Engineering Technology Vol. 1: 60-67, 2011

62

Fig. 4 How a solar panel works

The sunlight is directed to the base (a compound of silicon and its attributes) beyond the anti-reflective coating, where it creates movement of the photons causing the electrons to discharge and move apart. These electrons move away from the photons and are captured by the external load (in this case the power tracker).

The solar array will be mounted on the vehicle at its 2 highest points, the roof and the trunk lid. The electronics will be wired down to the power tracker. The expected charge time after dissipation from the power tracker is 5 hours for the NiMH batteries. In Fig. 5 the configuration of the solar array can be seen.

Fig. 5 Solar Array Configuration

2.0 OPERATION

In essence this solar car will have a variety of systems working in harmony to promote its effortless movement. This system includes the following components (each labeled with its purpose):

2.1 Golf cart framework

A golf cart as in Fig. 6 is an electric or gas-powered vehicle used to transport golfers and their equipment around the course during play. Designed to meet golfers' needs, the carts offer a number of specialized safety and comfort features. For example, the fact that they are built low to the ground gives them a low center of gravity, preventing spills when they are driven over uneven terrain. Many electric carts also come with portable battery chargers. Often, the center of the steering wheel (where the horn would be in a normal automobile) features a metal clipboard to which players can attach their score cards. The vehicles can be ordered with ball and cup holders, plastic enclosures to zip up in case of rain, sun canopies, and racks to hold bags, sweaters, and sand trap rakes. The frames of golf carts are usually made out of steel plates, rods, and tubing. The bodies may be made of sheet aluminum, fiber glass, or sheet steel. Other components, usually plastic or metal, are generally purchased from outside suppliers and assembled to the vehicle. These include components such as tires, which are made out of rubber; seat cushions, which typically consist of foam cushion covered by vinyl; steering mechanisms, made of metal; and motors, brakes, batteries, transaxles, suspensions, drive trains, and electrical cables.

Fig. 6 Golf cart of Club Car

Journal of Engineering Technology Vol. 1: 60-67, 2011

63

2.2 3.2 Horsepower DC motor

Motors are devices that convert electrical energy into

mechanical energy. The D.C. motors as in Fig. 7 that we have been dealing with here convert electrical energy into rotational energy. That rotational energy is then used to lift things, propel things, turn things, etc.

Fig. 7 D.C motor torque/speed curve for maximum power

When we supply the specified voltage to a motor, it rotates the output shaft at some speed. This rotational speed or angular velocity, is typically measured in radians/second {rad/s}, revolutions/second {rps}, or revolutions/minute {rpm}. From the angular velocity,

, we can find the tangential velocity of a point anywhere on the rotating body through the equation tangential velocity, v = r* , where r is the distance from the axis of rotation.

This relation can be used to compute the steady state (constant speed - no acceleration) speed of a vehicle if the radius and angular velocity of a wheel is known, or the linear speed of a rope as it is wound up by a winch. In order to effectively design with D.C. motors, it is necessary to understand their characteristic curves. For every motor, there is a specific Torque/Speed curve as shown in Fig. 8 below.

Fig. 8 D.C motor torque/speed curve[7]

The graph above shows a torque/speed curve of a typical D.C. motor. Note that torque is inversely proportional to the speed of the output shaft. In other words, there is a tradeoff between how much torque a motor delivers, and how fast the output shaft spins. Recall that earlier we defined power as the product of torque and angular velocity. This corresponds to the area of a rectangle under the torque/speed curve with one corner at the origin and another corner at a point on the curve as in Fig. 8. Due to the linear inverse relationship between torque and speed, the maximum power occurs at the point where = ½

, and = ½ .

Fig. 8 D.C motor torque/speed curve for maximum power[7]

Journal of Engineering Technology Vol. 1: 60-67, 2011

64

2.3 Power trackers

BETA PWM 20A as shown in Fig. 9, solar charge controllers (Voltage Controller, PV Charge Controller) are of pocket size and light weight, which have been widely applied in solar photovoltaic system, coordinating the operation of solar panels, batteries and loads, ensuring rapid and stable charging for batteries.

Fig. 9 BETA 20A Auto 12V/24V solar charge controller

It is design collects battery charge current, discharge current, battery voltage, battery temperature, charge time, discharging time and other battery working datum. Make use of discharge characteristic of storage battery and patented system of numbering by us, it can detect all the movements of remnant capability when battery is in charge and discharge states. The precise controlling of charge process can extend the battery lifetime, also exert the performance farthest.

Solar charge controller units have what is known as a 3 stage charge cycle that goes as below: 1) During the Bulk phase of the charge cycle, the voltage gradually rises to the Bulk level (usually 14.4 to 14.6 volts) while the batteries draw maximum current. When Bulk level voltage is reached the absorption stage begins. 2) During this phase the voltage is maintained at Bulk voltage level for a specified time (usually an hour) whiles the current gradually tapers off as the batteries charge up.

3) After the absorption time passes the voltage is lowered to float level (usually 13.4 to 13.7 volts) and the batteries draw a small maintenance current until the next cycle.

The relationship between the current and the voltage during the 3 phases of the charge cycle can be shown visually by the Fig. 10.

Fig. 10 Relationship between the current and voltage

during the 3 phases of the charge cycle

2.4 Used of Maximum Power Point Tracking (MPPT)

Maximum Power Point Tracking, frequently referred to as MPPT, is an electronic system. That operates the Photovoltaic (PV) modules in a manner that allows the modules to produce all the power they are capable of. MPPT is not a mechanical tracking system that “physically moves” the modules to make them point more directly at the sun. MPPT is a fully electronic system that varies the electrical operating point of the modules so that the modules are able to deliver maximum available power.

Additional power harvested from the modules is then

made available as increased battery charge current. MPPT can be used in conjunction with a mechanical tracking system, but the two systems are completely different. The MPPT system as in the Fig. 1-11, using a Solar Boost, charge controller calculates the voltage at which the module is able to produce maximum power.

In this example the maximum power voltage of the

module (VMP) is 17V. The MPPT system then operates the modules at 17V to extract the full 75W, regardless of present battery voltage. A high efficiency DC-to-DC

Journal of Engineering Technology Vol. 1: 60-67, 2011

65

power converter converts the 17V module voltage at the controller input to battery voltage at the output. If the whole system wiring and all was 100% efficient, battery charge current in this example would be VMODULE¸ VBATTERY x IMODULE, or 17V¸ 12V x 4.45A = 6.30A.

A charge current increase of 1.85A or 42% would be

achieved by harvesting module power that would have been left behind by a conventional controller and turning it into useable charge current. But, nothing is 100% efficient and actual charge current increase will be somewhat lower as some power is lost in wiring, fuses, circuit breakers, and as well as in the Solar Boost charge controller[8].

Fig. 11 Typical 75W PV module power/voltage/current at

standard test conditions

2.5 Motor controller

A motor controller works like a dimmer switch for a light. It varies “power” by modulating voltage. By using a controller, it allows the user to vary the current to the motor thus adjusting the speed of the vehicle. A larger controller is capable of providing more current thus allowing the motor to produce more torque.

The motor controller as shown in Fig. 1-12 below, controls the power to the motor by a transistor, a type of high-speed electronic switch. The transistor does not send a constant current to the motor. Instead, the transistor sends tens of thousands of pulses of electricity every second. If the transistor wants to lower the current by a certain amount, it omits pulses regularly. For example, if the transistor wanted to lower the current by one-fifth, it would turn off for one of every five pulses. This lowers the average current, making the motor run at lower power when lower power is needed.

Fig. 12 Motor controller

2.6 Solar panel array

Solar Panels; An eco-friendly electric generator. Solar panels generate free power from the sun by converting sunlight to electricity with no moving parts, zero emissions, and no maintenance. The solar panel, the first component of an electric solar energy system, is a collection of individual silicon cells that generate electricity from sunlight. The photons (light particles) produce an electrical current as they strike the surface of the thin silicon wafers.

A single solar cell produces only about 1/2 (.5) of a volt. However, a typical 12 volt panel about 25 inches by 54 inches will contain 36 cells wired in series to produce about 17 volts peak output. If the solar panel can be conFig.d for 24 volt output, there will be 72 cells so the two 12 volt groups of 36 each can be wired in series, usually with a jumper, allowing the solar panel to output 24 volts. When under load (charging batteries for example), this voltage drops to 12 to 14 volts (for a 12 volt configuration) resulting in 75 to 100 watts for a panel of this size.

Multiple solar panels can be wired in parallel to increase current capacity (more power) and wired in series to increase voltage for 24, 48, or even higher voltage systems. The advantage of using a higher voltage output at the solar panels is that smaller wire sizes can be used to transfer the electric power from the solar panel array to the charge controller & batteries. Since copper has gone up considerably in the last few years, purchasing large copper wiring and cables is quite expensive.

The 3 basic types of Solar Panels;

Monocrystalline solar panels as shown in Fig. 13 below: The most efficient and expensive solar panels are made with Monocrystalline cells. These solar cells use very pure silicon and involve a complicated crystal growth process. Long silicon rods are produced which are cut into slices of .2 to .4 mm thick discs or wafers which

Journal of Engineering Technology Vol. 1: 60-67, 2011

66

are then processed into individual cells that are wired together in the solar panel.

Polycrystalline solar panels: Often called Multi-crystalline, solar panels made with Polycrystalline cells are a little less expensive & slightly less efficient than Monocrystalline cells because the cells are not grown in single crystals but in a large block of many crystals. This is what gives them that striking shattered glass appearance. Like Monocrystalline cells, they are also then sliced into wafers to produce the individual cells that make up the solar panel.

Amorphous solar panels: These are not really crystals, but a thin layer of silicon deposited on a base material such as metal or glass to create the solar panel. These Amorphous solar panels are much cheaper, but their energy efficiency is also much less so more square footage is required to produce the same amount of power as the Monocrystalline or Polycrystalline type of solar panel. Amorphous solar panels can even be made into long sheets of roofing material to cover large areas of a south facing roof surface.

Fig. 13 Monocrystalline solar panels

2.6 NiMH rechargeable

Storage Batteries; The fuel tank of your solar power system. Without batteries to store energy you would only have power when the sun was shining or the generator was running. RV or Marine type deep cycle batteries are basically for boats & campers and are suitable for only very small systems. Regular or Car type batteries should not be used at all because they cannot be discharged very much without internal damage. A very popular battery for small systems is the Golf Cart battery as shown in the Fig. 14. They are somewhat more expensive than deep cycle recreational batteries but are probably the least expensive choice for a small system.

Fig. 14 Golf cart battery

3.0 DISCUSSION

The project has successful completed and it has been used by the UniKL-BMI as the transporting vehicle for maintenance purposes. With the limited time frame of one year, we cannot cover all scope described in the objectives and need to extend the project if it were to cover the pedagogical approach to learn in term of designing the circuits especially the motor controller in which to generate maximum power.

Thus, the project is focusing on the existing performed of the components used and plus with the used of solar energy as the alternating power sources to charge the batteries. Furthermore, there are one presentation have been achieved through this project by one of the project member at 4th Indonesia Japan Joint Scientific Symposium 2010 (IJJSS 2010 – International Conference) with the paper title ‘’Modeling for High Power of 5KW Single Phase Inverter by using PWM Switching Technique’’ on October 2010. 4.0 CONCLUSION

The project, Maximum Power Transferred for Solar Car Application, has been developed in UniKL BMI under the Renewable Energy Research Group members for the purpose of investigating the maximum power transferred from solar energy. It can now be used as the test bed in the development of the solar practice for vehicle in the future either for students or staffs.

Further development on project needs further funding and extension of at least one year or so. The project needs more attention because so far is not fully accomplish due

Journal of Engineering Technology Vol. 1: 60-67, 2011

67

to financial constraint in constructing the aerodynamic body frame for vehicle.

As for recommendations, there are some features to be upgraded in this project, namely;

1. For aerodynamics body frame, it is useful to have this structure since that we can estimate the weight used against the acceleration of the vehicle.

2. For solar energy efficiency, the used of different solar cells will give different performance of the power consumption.

3. For motor efficiency; the used of hub motor will reduce the transmission losses for the higher efficiency of the power to propel the vehicle.

REFERENCES [1] Patterson, D. J., “Electrical System for a Solar Powered

Vehicle,” Recordings of 21st Annual IEEE Power Electronics Specialists Conference PESC ‘90, pp. 618-22, 1990.

[2] Holtz, J., “Pulsewidth Modulation for Electronic

Power Conversion,” Proceedings of the IEEE, vol. 82, no. 8, pp. 1194-213, August 1994.

[3] Gasser, S. C., Cochrane, I. C., “High Efficiency PWM-VSI

for a Solar-Electric Racing Car,”, pp. 353-8, 1995. [4] Caricchi, F., Crescimbini, F., Honorati, O., Napoli, A. D.,

Santini, E., “Compact Wheel Direct Drive for EVs,” IEEE Industry Applications Magazine, pp. 25-32, November/December 1996.

[5] Kogure, S., Kawabata, Y., Yamada, E., Kanamori, A.,

“Development of Motor Control Technology for Electric Vehicles,” EVS 14 Conference, Toyota Motor Corporation, pp. 1-14.

[6] http://www.howstuffworks.com/solar-cell.htm

[7] http://www.siliconsolar.com/solar-powered-

cars.php?OVRAW=Solar%20cars&OVKEY=solar%20car&OVMTC=standard&OVADID=6398143521&OVKWID=73991058521

[8] http://www.flashkit.com [9] http://www.solarcharger.com/site/sunsei.html