Journal of Engineering Technology Volume 6, Special Issue on Technology Innovations and Applications Oct. 2017, PP

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1 Performance analysis of a solar powered wheel chair S. Algarni, S. Mellouli, E. Abhilash College of Engineering, Mechanical Engineering Department, King Khalid University, Abha, Saudi Arabia. Abstract: Devices for enhancing personal mobility of physically challenged people are of great concern to the modern society. There are various wheel chair designs to suit the individual needs, enabling the users to lead a more active life and to participate in as many activities as possible. The main aim of this paper is to discuss on a new design of Solar Powered Wheelchair (SPW) considering various design aspects and functionalities. This includes the modification of a motorized wheel chair to use solar energy with the help of retractable solar panels, use of solar panels as a desk (an additional functionality), and an optimal compact design to ensure the transportation and the storage of the SPW. Performance analysis of this new design was also carried out to understand the effective mobility of wheelchair in Abha city (KSA) considering a barrier free environment. Energy characteristics study of the photovoltaic (PV) system added to this design for the weather conditions of Abha city reveals that the average power generation is 490 Wh/day. Hence, it can be inferred that PV system added to the motorized wheelchair (with a rated power 336 W) can provide an additional uninterrupted journey of 9 km at a maximum speed of 6 km/hr. Keywords: Solar wheelchair, Photovoltaic solar cell, Energy performance. 1. Introduction The wheelchair for physically challenged people as well as for elderly people is a growing necessity. Currently, the wheelchairs are used by a significant portion of the population. With the recent technological advancements, the wheelchairs have been modified to use electric power providing much comfort to the disabled person allowing him to propel the wheelchair for long distances or over terrain which would be fatiguing in a manual wheelchair. The increase in the usage of wheelchairs has attracted many researchers to optimize the design and the concept of the wheelchairs in order to achieve most reliable operations. A major problem in using the electric wheelchair is that this system has only limited mobility depending on the battery life. Further, recharging of batteries is very time consuming. One of the alternatives to overcome these inconveniences is the use of renewable energy systems. Various modifications of the SPW have attracted much attention to the researchers and engineers in the last decade. Many of these researches show that the addition of solar-power to the motorized wheelchair can extend the mobility range of the wheelchair. In most of the SPW designs, the panels were placed on roof-like attachment to the wheelchair. Even though, the roof is a heavy attachment, there may be a few mechanisms to detach and fold it [1]. The folding certainly requires an aid of another person; however, it allows the wheelchair to be compacted and easily transported. There have been many innovations for improving the solar power system of the motorized wheelchair [1-9]. A literature review has been made to understand various SPW designs and is summarized in Table 1. A few of these designs have obtained patents [7-9]. In most of these designs, a flexible solar cell panel is arranged on the back side of the chair and is used for supplying electricity to the wheelchair. The upper portion of the solar cell panel is hinged to the chair-back and a movable support is arranged on the lower portion of the solar cell panel; therefore, the included angle between the solar cell panel and chair-back can be adjusted to improve efficiency in power generation. The energy production depends on the number of panels and its inclination to 212

2 solar irradiation. This study largely aims to modify and improve the basic design of SPW by analyzing the energy characteristics of the PV system added to it. Table 1: Details on various solar powered wheelchair system Design Comments Reference Design Comments Reference Capable of travelling 120 km in 14-hours e-energy- news.info/solar- powered-wheelchai Two 50W solar panels Battery 12v -80Ah 39 km Shahidul 2012 Four 18W solar panels Battery 48V 8 km Manohr 2012 Ref: Patent Number: US 2014/ A1. Kurt Sheneider 2014 Two 75 W solar panels Battery 24V 100Ah Kandasmy 2013 Ref: Patent Number: US 2014/ A1. Bradford Three 160W solar panels Battery 24V 100Ah g.com/solarpoweredwheelchair-winsfirst-place-incompetition Ref: Patent Number: US 8,448,728 B2. Kurt Sheneider / 2. Solar Powered Wheelchair design and analysis 2.1. Solar panel box In the present design, a main frame to hold the panels was built around the base of the wheelchair. Four solar panels were attached to a convertible-like structure which can be moved forward and can be used as a desk. The solar panel box was made retractable in order to reduce its area when used as a desk and to increase effective area for solar irradiation. The retractable panel box includes a stationary solar panel on the top and a sliding panel at the bottom. To reduce the weight of the panels, the panel box was fabricated using aluminum. Fig. 1 shows a combination of serial and parallel connection of four solar panels arranged to obtain adequate voltage and current to charge the battery system of SPW. This combination allows increased absorption of solar power and thus provides more output current and voltage. This type of PV array has to be carefully designed to generate sufficient power to charge the battery. 213

3 Figure 1: The electrical circuit and arrangement of solar panels in SPW. In the electrical circuit composed of four solar panels, each panel has 12 cells, 12 cm x 13 cm, with a nominal open circuit voltage of Vcc = 0.5 V. The cells are attached in series, in order to achieve 17.6 V per panel. Working voltage is between 12 and 24 volts. The total area covered by the solar panels is 0.82 m 2. Each cell is specified for 19.5% efficiency. Assuming ideal conditions, the power generated can be theoretically obtained as 120 W, adding 35% increase in power to the motorized wheelchair. The top surface of the solar panels is fixed with light weight plexiglass instead of the standard glass sheets typically used in the construction of solar panel. This not only reduces the weight of the panels but also increases the safety and the impact resistance. Since the 17.6 V panels were being used to charge a 24 V battery pack, a DC-DC boost converter was included in the design. The boost converter is capable of charging battery packs at any voltage beyond 5 V and also acts as a maximum power point tracker, with a nominal electrical efficiency of 96-98% Main frame The design of the structure holding the solar power hood (solar panel box) consists of two supporting frames, a fixed one and another movable. The fixed supporting frame locks and support the solar panel box while acting as a roof and the movable supporting frame locks and support the solar panel box while acting as a desk. The placement and the length of these frames were chosen by considering the arrangement of control system fitted on the motorized wheelchair as well as the ergonomics. The specialty of this novel design is that there are only a few moving parts. The supporting frames are allowed to turn at the pivot and the main assembly pivots in a bushing assembly on both side of the chair. This simple design limits the number of parts which might deform or break while in usage. Fig.2 illustrates the movement of the retractable panel box to demonstrate its usage as a desk and as well as a roof. 214

4 Figure 2: Drawings illustrating the movement of solar panel box to show its usage as a desk Materials and construction Prototyping involved choosing appropriate materials for the main frame and the panel box, and appropriate assembly of various parts. The choice of the materials was based on an optimization of the both need and the cost. The wheelchair used for this prototype was a Foshan FS110A. The selection of this commercial wheelchair was made by considering the cost and their high reliability in operation due to the use of lead acid battery system. Table 2 provides main specification of the wheelchair system. Protection for overcharging was included in the solar panel circuit to prevent damage to the batteries. A booster fit to charge controllers, maintains a steady output voltage from the solar panels and ensures consistent battery charging even at lowlight (solar irradiance) conditions. Table 2: Main specification of the wheelchair system Specifications Details Power storage 2 x 24V, 28 Ah batteries Motors 2x 168 w motors Size: width, height, length 63x50x91cm Weight 110 kg Range (without solar power) 20 km Solar charge controller PWM-10 A-24V The solar power system of the motorized wheelchair as shown in Fig. 3 consists of solar panels (120 W), solar charge controller (Maximum Power Point Tracking), and Lead-acid battery (12 V-80 Ah). In the SPW system, 215

5 four photovoltaic cells (solar panels) were installed in the box which is used as roof and desk, the batteries were installed under the wheelchair and the regulator was installed on the backside of the wheelchair. Figure 3: Circuit diagram of solar power system. The SPW system with the solar panel box in charging position as roof and as a desk is shown in Fig.4. When sufficient sun light is available, the control system gives priority to solar power allowing SPW to run long distances and when sun light is insufficient, it takes power from battery storages. The wheelchair test run was made on a plain ground to understand the required current. During the test, the SPW was made to run at maximum and minimum speed. From these tests, assuming 24 Ah full power storage in battery and neglecting the power consumption during the acceleration of SPW, it was concluded that the wheelchair could run for an additional period of 1.5 hours. Figure 4: The SPW system with solar panel box in charging position as roof and as a desk. 3. Energy performance analysis of the SPW The solar energy generation and utilizing it as an energy source for motorized wheelchair should start with an assessment of energy potential at the site or region of interest. The energy characteristics study of the SPW is 216

6 made using an online software PVGIS (Photovoltaic Geographical Information System) [10]. PVGIS is a solar PV performance assessment tool developed by the JRC (Joint Research Center), European Commission's inhouse science services. It is a map-based inventory of solar energy resources and can be used to assess the electricity generation from PV systems in Europe and Africa as well as large part of Asia. The PV performance can be estimated for any geographical location in the area under investigation. This virtual tool allows the investigator to retrieve and share the data online [11]. The data for the PV system in this design was computed in three steps: i. Computation of clear-sky global irradiation on a horizontal surface ii. Calculation and spatial interpolation of the clear-sky index and computation of maps of global irradiation on a horizontal surface iii. Deriving the diffuse and beam components of the clear-sky index and computation of maps of global irradiation on inclined surfaces. This virtual tool calculates the monthly and the annual solar electricity production, E (kwh) of the PV system with a few defined parameters for a particular panel inclination and orientation using formula [12]: (1) Where, P k, r p and H h,i are respectively the peak power of PV system, the system performance ratio and the monthly or yearly average of daily global irradiation on the horizontal or inclined surface. Moreover, the calculator estimates the optimum inclination of the PV modules to harvest maximum electricity within a year. The PVGIS is a powerful tool that can be used to estimate the solar electricity production of any SPW system. The energy characteristics study of the SPW was made to estimate the power generation of the PV system for a span of one year. The test location for this study was Abha city in Saudi Arabia with geographical coordinates 18 13'0" North, and 42 30'19" East. Fig. 5 shows hourly mean irradiance values (W/m 2 ) at Abha for a particular day in the month of May As observed from the plot, the maximum irradiation is at noon. As mentioned earlier, the SPW is installed with PV system of 120 W (crystalline silicon) and battery storage system rated 24 V, 24 Ah. Hence, the energy loss due to temperature and low irradiance is estimated as 11.2% (using local ambient temperature) and energy loss due to angular reflectance effects is estimated as 2.8%. Figure 5: Hourly solar irradiance at Abha on a particular day in the month of May

7 The performance of SPW system is a strong function of surface azimuth angle and angle of inclination of the solar PV panel with the horizon. Fig. 6 shows the annual variation of optimal panel inclination angles. An optimal inclination is at which PV panel surface must have the beam incidence angle (at solar noon) equal to zero ensuring a maximum solar irradiation. It can be observed that during the first six months the optimal inclination has a decreasing trend form 45 to -15, but in the second half of the year it is increasing up to 46. Hence, to harvest the maximum possible power from PV panel, it can be concluded that the average optimal panel inclination angle is 19. Also, it indicates the need of adjusting the inclination of PV panels every month. Thus, SPW system may be incorporated with an automatic tracking system. Figure 6: The annual variation of optimal panel inclination angle. The average monthly profile of solar insolation received by the PV system for a span of one year is shown in Fig. 7. The tool also predicts the profiles of horizontal irradiation, irradiation at optimal angle, irradiation at 90 degree and direct nominal irradiation. Fig. 7 also indicates the solar irradiation for power generation at the optimal inclination angle showing a maximum value of 7000 Wh/m 2 /day in the month of October and a minimum of 5100 Wh/m 2 /day in January while the irradiation on horizontal plane showing a maximum value of 7000 Wh/m 2 /day in May and a minimum of 4200 Wh/m 2 /day in January. Figure 7: Annual global irradiation for Abha city in KSA 218

8 In the energy performance analysis of this SPW system, the PV panel orientation angle is assumed zero to ensure the maximum solar irradiation irrespective of the direction of the travel. Fig. 8 shows that the average power generation is 490 Wh/day. Hence, it can be inferred that the PV system added to the motorized wheelchair with a rated power 336 W can provide an additional uninterrupted journey of 9 km at a maximum speed of 6 km/hr. Figure 8: The monthly energy output from SPW system. 4. Conclusion This paper discussed the design and performance analysis of a SPW system and summarized its energy performance characteristics. This newly developed SPW system is suitable for both outdoor and indoor use in all weather conditions. Light weight and robust materials were used in the system in addition to high efficiency solar cells. Design was optimized to effectively utilize available space for solar power generation and ensuring the transportation and storage of SPW. The solar panels were made retractable to obtain maximum solar power of approximately 120 W and to use as a desk. This new SPW system with enhanced mobility and functionality is expected to bring new quality standards to wheelchair market. Monthly optimal inclination angles for the PV panels were identified in energy performance analysis and the maximum optimal angle is found 45 o for the month of December and -15 o for June. The average monthly optimal panel inclination angle is found to be 19 o. Also, for Abha city in Saudi Arabia, a monthly estimation of power production was executed for this SPW system when the panels are at horizontal position and the average power production is found 490 Wh/day. Hence, it can be inferred that with an addition of solar PV system in a wheel chair system of a rated power 336 W can provide an additional uninterrupted journey of 9 km for a maximum speed of 6 km/hr. Acknowledgments The authors wish to thank the College of Engineering, King Khalid University, Abha, (KSA) for financially supporting this project. References [1] T. Zhao, Light power-saving foldable electric wheelchair having a solar energy charging unit. Patent 219

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