Introduction
In addition to conventional electrical power sources for electric vehicles such as batteries and fuel cells, there is a range of alternative options including solar photovoltaics, winddriven generators, flywheels and supercapacitors. There are also older systems which may be important in the development of electric vehicles, particularly electric supply rails either with mechanical pick-ups or modern ones with an inductive supply. In this lecture, considering stores of electrical energy, energy conversion devices, and energy transfer systems.
The following topics are covered in this lecture:
• Solar Photovoltaics
• Flywheels
• Supercapacitor
Solar Photovoltaics
Photovoltaic cells are devices that convert sunlight or solar energy into direct current electricity. They are usually found as flat panels, and such panels are now a fairly common sight, on buildings and powering roadside equipment, to say nothing of being on calculators and similar electronic equipment. They can also come as thin films, which can be curved around a car body.
Solar radiation strikes the upper atmosphere with a value of 1300Wm-−2 but some of the radiation is lost in the atmosphere and by the time it reaches the Earth's surface it is less than 1000Wm-−2 , normally called a ‘standard sun'. Even in hot sunny climates solar radiation is normally less than this. Typical solar radiation on a flat plate constantly turned towards the sun will average around 750Wm-−2 on a clear day in the tropics and around 500Wm-−2 in more hazy climates. For a flat plate such as a solar panel placed on a car roof, the sun will strike the plate at differing angles as the sun moves around the sky, which halves the amount of energy falling on the plate.
The exact average will depend on the latitude, being larger on the equator and less at higher latitudes. Solar radiation is split into direct radiation which comes from the direction of the sun which is normally prominent on cloudless days, and indirect radiation which is solar radiation broken up by cloud and dust, comes from all directions and is prominent on cloudy days. Photovoltaic cells convert both types of radiation into electricity with an efficiency of conversion of around 14%. So the power which could be obtained from a photovoltaic panel will be less than 100Wm-−2 when tracking the sun, and around half of this for a fixed panel on a horizontal car roof.
There are two methods of using solar panels,
• Onboard
• Off-board the vehicle.
Clearly even if the whole of a car plan area were covered with cells only a very limited amount of power would be obtained. For example, a car of plan area 5m−2 would produce a maximum of around 375W at the panel output, and an average of around 188 W, giving 1.88 kWh of energy over a 10 hour day, equivalent to the energy stored in around 50 kg of lead acid batteries. This energy could be stored in a battery and used to power the vehicle for short commuter and shopping trips; but basically this amount of energy is insubstantial and would normally only give an impracticably limited range.
Solar panels mounted off-board could give as much power as needed. The electricity could either charge the vehicle battery from a suitable charging point or could be supplied to the vehicle via supply rails. The idea of a solar roof could be wasteful, in the sense that it is expensive and when the car is not being used the power will go to waste, unless of course it is used for some other purpose such as charging domestic batteries at a remote residence. The surplus power could be sold back to the grid in some cases.
Apart from the disadvantage of low power per square meter, solar panels are not cheap, costing around £4000 per peak kW, when bought in bulk. Bearing in mind that a peak kW is rarely achieved even in very sunny places, the actual cost per kW achieved is considerably more than £4000. Despite this, the idea of solar photovoltaic cells fitted to vehicles should not be written off entirely. The efficiency will improve and may one day in the future be as high as 50%. The cost of photovoltaic cells has already fallen dramatically and the long-term cost of solar photovoltaic panels is predicted to fall still further. Apart from supplying power to drive the vehicle, solar photovoltaic cells may be used for other useful purposes, such as compensating for natural battery self-discharge, and also for cooling or heating the car whilst at rest. A small fan powered by a photovoltaic roof panel could be used to draw air through a vehicle and keep it cool when parked in the sun.
Flywheels
Introduction
Flywheels are devices that are used for storing energy. A plane disc spinning about its axis would be an example of a simple flywheel. The kinetic energy of the spinning disc is released when the flywheel slows down. The energy can be captured by connecting an electrical generator directly to the disc as shown in Figure 1 , power electronics being required to match the generator output to a form where it can drive the vehicle motors. The flywheel can be re-accelerated, acting as a regenerative brake. Alternatively the flywheel can be connected to the vehicle wheels via a gearbox and a clutch.
Whether mechanical or electrical, the system can also be used to recover kinetic energy when braking. The flywheel can be accelerated, turning the kinetic energy of the vehicle into stored kinetic energy in the flywheel, and acting as a highly efficient regenerative brake.