EV Engineering News

It all adds up: extending EV range through efficiency increases

Want to increase the range of your EV without adding capacity to the costly battery pack? Optimize. In parallel with global research efforts to improve the power density of batteries, many are looking for new ways to use less energy per mile.


Mitsubishi’s New Motor

Mitsubishi Electric is hard at work redefining what a motor and inverter look like. It has recently announced a prototype motor-inverter-in-one combo pack. The company says the new system is half the size and significantly lighter than their existing motor and external inverter combo of equal power.

Mitsubishi Electric’s new cylinder-shaped inverter and motor combo.
Photo courtesy of Mitsubishi Electric.

“Mitsubishi Electric’s existing system consists of separate motors and inverters driving the motors, which requires more space for these components and their wiring. The newly developed cylinder-shaped inverter matches the diameter of the motor, enabling them to be connected coaxially within a chassis, resulting in a substantial downsizing of the motor system.”

Furthermore the new inverter is silicon carbide (SiC) based, as opposed to the regular old silicon chips, which are widely used in power electronic switching applications. SiC is recognized as a more suitable material for chips due to its electrical characteristics. Mitsubishi says the loss reduction is as much as 50% compared to its silicon-based inverter system.


Engineering Notes: Silicon Carbide

SiC advantages include a breakdown electric field that is 10 times greater compared to silicon chips. This greater breakdown electric field enables thinner chips, which reduces electrical resistance and lowers loss.

So if SiC is so efficient, why isn’t it used everywhere? There’s the rub. SiC, along with GaN (gallium nitride), have been touted as the Next Big Thing in semiconductors for a few years. While they are better than standard silicon/silicon dioxide semiconductors, they’re still relatively expensive and of limited availability.

SiC does look promising, but – like many technologically superior solutions – to date it has lost out to more easily produced (i.e. – cheaper) products. It’s likely SiC diodes will be embraced first, because they are the easiest semiconductor devices to manufacture, and SiC MOSFETs and IGBTs (used in inverters) will be a long time in coming to market.


Stay Cool

Heating and cooling a vehicle’s passenger compartment can be a real drag (on an EVs battery pack). 

DESNO, a global automotive systems supplier, recently launched a new and improved electric air conditioning compressor. The company has been working on optimizing their e-compressor design since 2003, when they first introduced it for use in hybrids.

DENSO’s previous (left) and new (right) electric air conditioning compressor designs.
Photos courtesy of DENSO.

When moving from an internal combustion engine to an electric motor, not only is the powertrain electrified but the accessories are as well. Conventional compressors are powered by an engine’s belt drive, but EVs don’t have belt-driven engines that energize the peripheral systems (hence, the move to an electrically-driven compressor). This electrified compressor is a little more sophisticated in that it has its own motor, inverter and circuit board.

DENSO was the first to integrate the inverter with the compressor into one component in 2005, at the time reducing the compressor size by approximately 60% compared with a conventional electric compressor and inverter. They had to develop a new cooling method using air conditioning refrigerant in the compressor, because the conventional inverter is cooled by the engine’s coolant system.

To further reduce the weight and size, DENSO improved the packaging by repositioning the e-compressor’s inverter in-line with the motor as opposed to the current integrated inverter that is stacked above the motor.

The new and improved design will make its global debut on Ford Motor Company’s Focus Electric, which began production this past December with retail production ramping up the first half of the year. It will also appear on the Fusion and C-MAX Hybrids and Energi Plug-in Hybrids, due to hit North American roads in 2012.


A Lot of Hot Air

Meanwhile, some researchers are looking past the standard kilowatt-sucking heating and cooling cycles. 

Internal combustion engines generate a lot of heat, which is great for heating the passenger cabin in winter. EVs, however, produce very little waste heat, so providing electricity for the same amount of heat can reduce their driving range by as much as 40%. 

Keeping a Nissan LEAF nice and toasty in the dead of winter comes with a drop in range of as much as 30 miles.

Pacific Northwest National Laboratory (PNNL) scientists Pete McGrail and Praveen Thallapally, and University of South Florida chemists Mike Zaworotko and Ma Shengqian received $800,000 from an ARPA-E grant to develop a material called an electrical metal-organic framework, or EMOF for short, for vehicle heating and cooling systems. 

The EMOF would work as a molecular heat pump, which efficiently circulates heat or cold as needed. By directly controlling the EMOF’s properties with electricity, their design is expected to use much less energy than traditional heating and cooling systems. 

For example, a five-pound EMOF-based heat pump the size of a two-liter bottle could theoretically handle the heating and cooling needs of an EV with far less impact on driving distance.


Engineering Notes: Absorption Heat Pump
Excerpt courtesy of Energy Efficiency & Technology 

To understand PNNL’s work it helps to first review the principle of operation for a conventional adsorption heat pump.

A standard adsorption chiller uses heat to a boil refrigerant, usually water. Commercial adsorption units might use propane for a heat source, but waste heat and even solar power can serves as sources as well. The adsorptive material is usually silica gel. 

When the silica gel is dry, it has a great affinity for water. When a source heats up the adsorber, the high temperature induces a rise in pressure, from the evaporation pressure up to the condensation pressure. 

The cycle starts with the opening of a valve between the adsorptive silica material and the condenser. The adsorbent temperature continues to rise, which induces desorption of water vapor. The condenser liquifies the desorbed vapor. Meanwhile, the condensation heat is released at an intermediate temperature.

During cooling and depressurization, the adsorber releases heat while the valve is closed. The adsorbent temperature falls, which reduces the pressure from that of the condensation pressure down to the evaporation pressure.

During a cooling, adsorption and evaporation period, the adsorber continues releasing heat while being connected to the evaporator. The adsorber continues to cool, which lets it suck up vapor. 

So, essentially the process is one of a cooling mode where silica is absorbing refrigerant off the evaporator, then once the silica is saturated, begin applying heat to drive off as much of the water as possible, condense it, reject the heat outside, and repeat.

PNNL is trying to miniaturize the system by switching from silica gel adsorbant material to the new EMOF nanomaterial. EMOF material, says McGrail, can have triple and quadruple the mass uptake of refrigerant compared to silica gel. Its kinetics are also about a hundred times faster than that possible with silica gel, with about half of the adsorption penalty. Computer models show that the chiller can be about a factor of three smaller than when using a silica gel adsorbing material, with a coefficient of performance (COP) that’s better by a factor of two. And that is without optimizing the heat transfer aspects of the system, McGrail says.


This article originally appeared in Charged APR/MAY 2012


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