Auxiliary photovoltaic power supply system for lightweight electric

PROCEEDINGS OF THE INSTITUTE OF VEHICLES 2(106)/2016
Wojciech Skarka1, Michał Szymon2
AUXILIARY PHOTOVOLTAIC POWER SUPPLY SYSTEM FOR
LIGHTWEIGHT ELECTRIC VEHICLE
1. Introduction
In European edition of Shell Eco-marathon (SEM) race [4] in Rotterdam about 200
best teams take part in energy efficiency car race. Many various categories and energy
sources are allowed according to the official rules [3]. Electric cars category is the fastest
growing category in the competition. Every year changes are made in racing regulations,
and these changes relate mainly to the electric cars group. In the recent years, solar car
category has been discarded and instead of this category it has become possible to use
photovoltaic panels power in electric battery cars. These changes necessitate the need for
rapid adaptation of new technology changes and increasing investment in the
construction of racing cars. For subsequent years further changes in solar power are
announced. In 2014 the two vehicles [6] of Smart Power team [5] of Silesian University
of Technology were equipped with auxiliary power system powered by solar energy.
The article describes the issues related to the design and integration of auxiliary power
systems for photovoltaic panels in a more complex design of our team i.e. a vehicle
competing in the category Urban Concept – Bytel (Fig. 1).
Fig. 1. Bytel vehicle designed for Shell Eco-marathon race at Urban Concept category
1
Wojciech Skarka, PhD, DSc, SUT prof., Head of the Team of Mobile Systems, Institute of Fundamentals of
Machinery Design, Silesian University of Technology
2
Michał Szymon, student of Automatic Control and Robotics at Faculty of Mechanical Engineering, Silesian
University of Technology, member of Smart Power team at the Institute of Fundamentals of Machinery
Design, Silesian University of Technology
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The subject of the study described in the paper was design and integration of highly
effective photovoltaic cells, their numbers, method of connection and locations on the
body of a vehicle which is being built to participate in Shell Eco-marathon event. The
research has been conducted as a part of work of Smart Power Urban Team of Silesian
University of Technology.
2. Design of auxiliary photovoltaic power system
The limits regarding total combined surface of photovoltaic cells were regulated by
the Shell Eco-marathon 2014 Official Rules [3] and set to less than 0.65 m2 (Urban
Concept category) and less than 0,17 m2 (Prototype category). The solar panels were
supposed to charge a 48 V lithium-based battery through a dedicated charging controller.
The calculations of possible energy gain were based on Average Daily Solar Irradiance
data from CM SAF database [2].
The assortment of photovoltaic panels to be placed on highly efficient vehicle Bytel
taking part in Shell Eco-Marathon event was carried out in five main stages:
2.1. Photovoltaic technology studies
The search of optimal photovoltaic cells began with studying the photovoltaic
technology.
Standard monocrystalline silicone cells can have efficiency ratio of up to 25% [1]
and provide around 0.5 V each. The current increases with the amount of solar energy
delivered to the panel. Cells are rigid and brittle, what makes them hard to bent without
breaking, but they can be cut and connected in parallel or in series, allowing the increase
of output current and voltage. If connected in series, the voltage adds up; when
connected in parallel, the currents add up.
Produced current is not relevant only to how much sunlight in total falls on the
series of cells – the panel will only produce as much current as the least illuminated cell.
As long as all cells are clean, not covered and evenly illuminated, the produced current
depends on the amount of energy delivered by the Sun to their surface. The cell
generates the highest amount of energy if its surface is perpendicular to the sunlight rays.
As a result of this stage, it was decided to use as small cells as possible, in order to
adjust the shape of the solar panels to the shape of cars body. Since it was not at first
planned to include the solar panels in the project, minor differences of the angle between
the vector normal to cell surface and sunlight were inevitable.
2.2. Market research
During this stage of work there were two major objectives: verifying the availability
of retail high efficiency solar cells in Poland and learning about possible sizes of single
cells to be used in the panels.
The results of this stage were less satisfying than expected – available cells were
only 14% efficient, but their sizes varied from 156x156 mm to 78x39 mm (whole cell –
1/8 of cell), what allowed to create panels flexible enough to use them on the vehicle.
2.3. Panels placement
There were only 3 surfaces suitable for photovoltaic panels placement – roof, trunk
hatch and hood, as shown in Figure 2. Other surfaces were too curvy and their angles
against the sun would change too much during the race (as they are nearly vertical),
resulting in reduction of possible energy gain.
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During this stage the selected photovoltaic cells (78x39 mm and 78x78 mm) had to
be divided into three panels, allowing their assembly on the vehicle body. According to
the rules [3], it was allowed to use up to 0.65 m2 of combined cell surface area.
However, the surfaces of the vehicle body made the team use 80 cells of 78x78 mm size
and 40 cells of size 78x39 mm, giving 0.6084 m2 total.
Two panels made of 40 78x78 mm cells each were planned to be placed on the roof
and hood, while the third panel consisting of 40 70x39 mm cells was to be placed on the
trunk hatch.
According to this data, the CAD model of vehicle was equipped with a visualization
of solar cells on its body, as shown in Figure 3.
Fig. 2. Possible solar panels locations on the vehicle body
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Fig. 3. Visualization of solar panels placed on the vehicle body
2.4. Cells and panels connection
Each panel was supposed to be placed on a surface with a different angle to the
ground, while being nearly flat by itself. Due to the mentioned fact that a set of cells
connected in series produces only as much current as the least illuminated cell, the best
solution was to connect cells on the panels in series, while connecting the panels in
parallel. This allowed to reduce the losses of energy caused by the car changing its
position against the sun to the minimum.
The output voltage of designed panels was around 22 V at the moment, so in order
to charge a 48 V battery (power source of the vehicle) a proper DC-DC boost converter
was required.
2.5. Calculations of possible energy gain
Based on the Average Daily Solar Irradiance data downloaded from CM SAF
database [2], the average irradiance on a fixed plane and clear sky irradiance on a fixed
plane were calculated for selected time range (10:07 AM- 4:07 PM), resulting
respectively 491.48 W/m2 and 757.36 W/m2.
The calculated energy gains during the race (39 min) were 25 Wh for average
weather conditions and 42 Wh for clear sky .
3. Conclusion
Both the Prototype category vehicle - MuSHELLka and Urban Concept category
vehicle - Bytel are equipped with additional electrical energy supply system in the form
of photovoltaic panels in SEM 2014. During the race Shell Eco-marathon in 2014 the
auxiliary power supply system was not used because it was not approved by technical
commission during scrutineering. The vehicles took part in the races without these
systems. According to estimates by the team on the sunny days of the race it was
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possible to improve the result of power consumption for the vehicle category Urban
Concept by approx. 15% and in the Prototype category, even more than 30%.
Unfortunately regulations set limits on energy profit form auxiliary sources to 20%. It
should be noted that almost all the teams (except one team), which have been
outperforming Smart Power team had auxiliary power source systems in the form of
additional photovoltaic panels. The Smart Power team continues the integration of
photovoltaic panel for this year Shell Eco-marathon race.
References:
[1]
Best research cells effectiveness
{Available – date: 01.02.2015}
http://www.nrel.gov/ncpv/images/efficiency_chart.jpg.
[2]
CM
SAF
database
{Available
–
date:
01.02.2015}
http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php?lang=en&map=europe}
[3]
Shell Eco–marathon Rules. {Available – date: 01.02.2015} http://s02.staticshell.com/content/dam/shellnew/local/corporate/ecomarathon/downloads/pdf/global/sem2015-global-ruleschapter1-updated-020914.pdf
[4]
Shell Eco-marathon web page {Available – date: 01.02.2015}
http://http://www.shell.com/global/environmentsociety/ecomarathon/events/europe/location-and-track.html
[5]
Students’ Scientific Association of Machinery Design at Institute of
Fundamentals of Machinery Design {Available – date: 01.02.2015}
http://mkm.polsl.pl
[6]
Skarka W., Sternal K., Cholewa A., Targosz M.: Electric vehicle for the
students’ Shell Eco-marathon competition. Design of the car and telemetry
system.W: Telematics in the transport environment. 12th International
Conference on Transport Systems Telematics. TST 2012, Katowice-Ustroń,
Poland, October 10-13, 2012. Selected papers. Ed. Jerzy Mikulski. Berlin :
Springer, 2012, s. 26-33, bibliogr. 9 poz. (Communications in Computer and
Information Science ; vol. 329 1865-0929)
Abstract
The article presents the project along with the specifications of the auxiliary power
supply system using photovoltaic panels. Additionally article presents the technical
specifications of designed power supply system. The system is designed for an electric
vehicle designed to compete Shell Eco-marathon (SEM) in the Urban Concept category
of vehicles with the electric power. Power system design was prepared for the release of
the vehicle taking part in the 2014 competition in Rotterdam in the SEM. The next
version of the vehicle was prepared by a team of students from the University of Silesia
for the competition in Rotterdam in 2015. Described in the article, the project includes
an overview of photovoltaic technology with an evaluation of applicability to power the
vehicle, the choice of installing photovoltaic panels and electrical system parameters and
the necessary power performance estimates. The characteristics allow the justification
for the use of this type of system power solely in electric vehicles with extremely low
energy consumption they are the vehicles taking part in races Shell Eco-marathon.
Keywords: urban concept, race car, solar panels, photovoltaic cells, solar energy.
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SYSTEM WSPOMAGANIA ZASILANIA POJAZDU ELEKTRYCZNEGO Z
ZASTOSOWANIEM PANELI FOTOWOLTAICZNYCH
Streszczenie
W artykule przedstawiono projekt wraz ze specyfikacją systemu wspomagania
zasilania z zastosowaniem paneli fotowoltaicznych. Przedstawiono specyfikację
techniczną zaprojektowanego systemu zasilania. System zaprojektowano dla pojazdu
elektrycznego przeznaczonego do startu w zawodach Shell Eco-marathon (SEM) w
kategorii pojazdów Urban Concept z zasilaniem elektrycznym. Projekt systemu zasilania
został przygotowany dla wersji pojazdu startującego w roku 2014 w Rotterdamie w
zawodach SEM. Kolejna wersja pojazdu została przygotowana przez zespół studentów
Politechniki Śląskiej do zawodów w Rotterdamie w 2015 roku. Opisany w artykule
projekt obejmuje przegląd technologii fotowoltaicznej wraz z oceną możliwości
zastosowania do zasilania pojazdu, wybór miejsca instalacji paneli fotowoltaicznych
oraz parametrami instalacji elektrycznej oraz niezbędnymi szacunkami osiągów
zasilania. Przedstawione charakterystyki pozwalają na uzasadnienie zastosowania
zasilania tego typu układem wyłącznie w pojazdach elektrycznych o niezwykle niskim
zużyciu energii jakimi są pojazdy startujące w wyścigach Shell Eco-marathon.
Słowa kluczowe: urban concept, samochód wyścigowy, panel słoneczny, panel
fotowoltaiczny, energia słoneczna.
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