In Ahmedabad, the Bus Rapid Transit System (BRTS) that is popular by the name ‘Janmarg’ began operation in October 2009 over a route that spanned 12 kms, while the total number of private vehicles in the city was 18 lakhs. Today, the BRTS route is about 160 kms and with a daily ridership of 3.5 lakhs. However, the number of additional private vehicles registered over the past 5 years was 20 lakhs, and only 18% of the city population is using public transport.
One of the design features of the BRTS system was a dedicated bicycle track that could offer the last mile connectivity – a vital component to popularise public transportation. The BRTS won several prestigious awards on account of this. A smart city project to ease last mile connectivity through dockless bikes was also planned a few years back. However, it is a pity that the Ahmedabad Municipal Corporation has decided last month to dismantle all the bicycle tracks following encroachment complaints.
It is clear that BRTS has failed to increase the number of public transport users. India is still dreaming big on the obsolete US (car) model. The more recent Netherlands’ (bicycle) model is superior and sustainable. This article is part of a series of initiatives, under Project Savitré, that aims to disrupt / correct this distortion.
What is Project Savitré?
Project Savitré aims to bootstrap ‘solar’ over the Netherlands’ (bicycle) model. One of the initiatives – namely, replacing ‘Net Metering’ tariff with a fair ‘Net Billing’ tariff to encourage EV prosumers to maximise consumption from their own solar generation was well received by the readers of Electrical India – Jan & Feb 2023 issues.
Project Savitré is a term that refers to the ‘Worldwide First Solar Bicycle Rollout’ program – a disruptive innovation in electric mobility. This project assumes significance particularly because 2023 is the year when India has taken up the presidency of G20 (i.e. 19 countries and EU) with the theme – ‘One Earth, One Family, One Future”. A month earlier, namely November 2022, is also when the United Nations Climate Change Conference – COP27 brought together more than 45,000 participants. Governments were urged to not just listen to solutions put forward by young people, but to incorporate those solutions in decision and policy making.
While solar bicycles have been developed earlier by cycling enthusiasts and hobbyists, they were neither engineered for efficiency nor for mass production. Project Savitré is the first commercial venture with a production line that is geared towards meeting the G20 and COP27 objectives. In March 2023, BEM’s Project Savitré received the prestigious India Smart Grid Forum (ISGF) Innovation Award under the category – ‘Emerging Innovation in Electric Mobility Domain – Electric Vehicle (EV) (2/3/4 Wheelers). Clearly, the world is waking up.
Project Savitré is based on a well-engineered solar bicycle design. One of the challenges that had to be overcome was to reduce the size and weight of the solar panel to make it appropriate to fit onto a bicycle. A bicycle is light (typically weighs 20 kg), and is sleek (a bicycle steering handle measures about 60 cm across). Hence, we need to limit the weight of the solar panel, to say 2 kg, and width. to say 35 cm. To achieve this, without compromising on (or increasing) the battery charging time, it is necessary to develop a solar charger that is efficient and capable of extracting maximum power from the small solar panel. In this article, I describe such a solar charger – also called the MPPT Charge Controller (see Figure2).
Disadvantage of Conventional AC to DC Chargers
The conventional EV chargers are designed to charge the Direct Current (DC) EV battery from the utility mains supply that is typically a single phase 240V Alternating Current (AC) supply. These chargers are designed to first convert the AC voltage to DC voltage and then to control the DC output by either limiting the DC current or regulating the output DC voltage. Nowadays, a fast switching action is used to control the AC to DC conversion, and hence these chargers are called ‘Switched Mode Power Supplies’ or SMPSs. Most of the SMPSs available in the market do not respect the sinusoidal nature of the input voltage and are seen to inject large amounts of harmonic currents back into the power system. Harmonics are not only a sign of inefficiency but also responsible for lowering down the power factor, and damaging other sensitive electrical load and capacitors connected in its vicinity.
In case you are having a solar roof top and a grid-tie inverter, then charging your EV during the day from your solar generation, represents two levels of conversion inefficiencies. The inverter takes in DC power from the solar panels and converts it to 230V AC; an EV battery charger then converts this AC power back to DC. From an environmental angle, charging your EV at night instead, is worse, since you would then be charging from fossil fuels.
Solar Charge Controller
A solar EV charger has a numerous advantages over an AC mains EV charger. First and foremost, it draws power from an environmentally friendly and sustainable source – the sun. Energy is harvested from the sun and not from fossil fuels and hence does not contribute to global warming. Secondly, since the solar charger is a DC to DC controller, it is not only more efficient but it escapes the sensitive issue of power factor.
However, a challenge that a solar charge controller needs to overcome is the odd current-voltage characteristic curve of a solar panel. Let us compare a solar panel with another DC source, say a battery. The current-voltage characteristic curve of a battery (see Figure 3) is a vertical line. A battery allows you to draw less or more current at a fairly constant voltage (a minor drop in voltage is due to a small internal resistance). Hence, the power vs voltage of a battery is also a vertical line (see Figure 3).
On the other hand, the current-voltage characteristic curve of a solar panel (see Figure 4) shows that a solar panel’s voltage is sensitive to the current drawn. More the current drawn from a solar panel, the lesser the voltage. However, if one attempts to draw more current beyond the knee bend, the voltage falls sharply while the current no longer increases significantly. The maximum current is the short circuit current, Isc.
While the power output from a solar panel is zero at both open circuit and short circuit conditions, it is maximum somewhere in between. The vertical line marked as Pm in Figure 4 gives the location on the V-I and P characteristic curves that gives the maximum power output from the panel. The area of the blue rectangle also denotes the power.
Figures 5 and 6 show the solar power when the operating voltages are less and more, respectively, than the voltage corresponding to the Maximum Power point (Figure 4). The area of the blue shaded rectangles in Figures 5 and 6 is observed to be considerably lower indicating that the solar power delivered is considerably lower.
Thus, to get the maximum power out of a solar panel, a charge controller should be able to choose the optimum current-voltage point on the curve (Figure 4). Not all the charge controllers have the capability to choose the optimum point. Essentially, there are two types of solar charge controllers available in the market: (a) The PWM charge controller and (b) The MPPT charge controller.
The PWM Charge Controller
The conventional, cheaper and more popular charge controller is the PWM charge controller. Actually the name is a misnomer. There is very little control during the charging process. The PWM controller is merely a switch that connects the solar panel to the battery – and hence the panel voltage drops down to match the battery voltage while the battery gets charged. As the state of charge of the battery increases, its voltage increases and so does the panel voltage to equal the battery voltage. Only when the absorption voltage is reached, will the PWM controller start to disconnect and reconnect the panel to prevent overcharge – hence the name Pulse Width Modulated controller.
From Figure 7, it is clear that the battery voltage decides the operating point – and hence the power delivered by the panel depends on the battery voltage and not on the controller when a PWM controller is used.
The MPPT Charge Controller
The MPPT charge controller has a microcontroller and sophisticated software to detect the Maximum Power Pm point. The software also has the ability to control and limit the charging current to the load (battery), so that the solar panel operation is controlled to a voltage Vm corresponding to the maximum power point Pm, to deliver maximum power.
This controller is a DC to DC transformer that can be designed as a step-down transformer (buck controller) or a step-up transformer (boost controller). Both the step-down and the step-up transformers are highly efficient – the output power delivered to the battery is only marginally lower than the power extracted from the solar panel. The loss in the transformation process is small, typically less than 8%.
Figure 8 shows the characteristics of a buck MPPT charge controller where Pbat = Pm while Vbat < Vm .
The maximum power that a solar panel can deliver depends on various factors including the temperature of the panel. As the temperature increases, the maximum power that the panel can deliver reduces considerably. Moreover, the voltage (MPPT point) at which the maximum power gets delivered also shifts (decreases). Hence, the MPPT point needs to be repeatedly computed using a sophisticated algorithm.
…to be continued
Vithal Kamat has a Doctorate in Artificial Intelligence from the University of New Brunswick, Canada as a Commonwealth Scholar in 1996. He completed Masters in Control and Instrumentation from IIT Bombay. His current role – reviving a sick industry as a Managing Director of Baroda Electric Meters Ltd. Current interest lies in exploring ways to replace the Human centric Judiciary with an AI Judiciary, to replace the 24-hour clock with Ghati clock, and to replace ICE vehicles with solar vehicles.