Batteries

Thursday, May 08, 2014

Tags :  alternative-energy

The development of electrical technology seems to have been exponential. Had the power failed a century ago, for example, some reasonably wealthy people would have been inconvenienced. If it happened today though, it is likely that civilisation would collapse into anarchy for a time, such is our dependence on electricity, the technology it allows and all its attendant benefits. It has been embedded in our cars and motorcycles for a long time: mechanical spark timing and fuel metering is something of the past, and in future electrical technology will also be responsible for an increasing amount of vehicular propulsion.

At the moment, we are on a cusp. There is growing acceptance of hybrid powertrains, from hybrid city/commuter cars to hypercars such as the McLaren P1. Motorsport has long been aware of these powertrains’ benefits to vehicles with a need for heavy braking and rapid acceleration. From touring cars, through Prototypes to Formula One, hybrid propulsion has been embraced wherever it has been allowed. The chance to recycle some energy for propulsion that would otherwise have been lost to the atmosphere in the form of heat is an attractive proposition. At the moment, the electric hybrid, using a motor-generator unit combined with a battery is the most common choice, but there is at least one prominent alternative to this.

Beyond hybrids there is the fully electric vehicle, where all the motive power is supplied from an energy storage device. Fully electric racing is already with us and is on the rise, while fully electric roadcars are becoming a more common sight each year, at least in Europe.

What is also with us, if we are users or designers of electric hybrid technology or fully electric drivetrains, is the battery. Wherever energy is converted, it is never 100% efficient, and the process of charging and discharging batteries creates heat. Where the rate of energy conversion is high, there may be a need for cooling, and this can be as simple as ducting cooling air from the front of the vehicle where pressure is highest, through the battery to a low-pressure area to create a through-flow of air while the vehicle is in motion.

This method tends to work well only at high speed. Forcing the air to flow by using a fan allows more control of cooling, including being able to cool at low vehicle speeds. However, in order to remove serious quantities of heat, we probably need to look towards liquid cooling, simply in order to increase the mass flow of the coolant. Air is also lacking in specific heat capacity compared to liquids. The result is that liquid coolants are often more than 1000 times more effective in terms of removing heat per unit volume than air. Water is particularly effective as a coolant, but is generally not what you might want to have in contact with high-voltage batteries, so there are oils specifically designed for cooling electrical equipment such as transformers.

Although we still need to reject heat from the liquid coolant to the air via a cooler, in terms of packaging, liquid cooling of batteries can be a far more attractive proposition than air cooling. However, the practicalities in terms of battery design will be more complex than for an air-cooled installation. Leaks of any liquids are not generally welcomed, so the battery container needs to be sealed against them. There is also the issue of needing multiple electrical connections to a battery for both energy transmission and monitoring the ‘health’ of the battery. The designer will also need to consider the flow of the coolant through the battery to ensure that no cells are so poorly cooled that they are likely to overheat. In the case of lithium-ion batteries, the consequences of this can be spectacularly destructive.

Written by Wayne Ward

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