The Ergosphere
Wednesday, November 19, 2014
 

The mild PHEV: the next little thing?

Leveraging existing assets is one of our best prospects for cleaning up the air and cutting petroleum consumption.  An under-appreciated possibility is using conventional hybrids, even retrofitting existing ones, to substitute a bit of grid power for petroleum fuel.

In an article on advanced lead-carbon batteries for mild hybrids at GCC, there are these notable phrases:
The state-of-charge (SoC) of current lead-carbon batteries is typically maintained at between 30 and 50%.... Advanced lead-carbon batteries for vehicles currently under development will be capable of operating in the 30 to 70% SoC range at 12.5kW.
That's operation.  What about off-line?  Lead-acid likes to be held at 100% SOC, and I doubt that lead-carbon suffers at all from it.  NiMH batteries also appear to prefer to be held at 100% SOC and cycled only shallowly.  Neither could accept regenerative braking power when full, but when the vehicle is starting and driving shortly after start there is no braking energy to recover.

The GCC article continues:
Future battery developments will most likely combine advanced lead-carbon electrochemistry with ... substantially reducing the size of a 1 kWh battery required for mild electrification of the powertrain.
The hybrids and mild hybrids of tomorrow will have on the order of 1 kWh (above) to 1.3 kWh (base Prius) of battery, and operate it in a SOC range centered between 40% and 50%.  But if the battery was charged to 100% SOC off-line, there would be between 500 and 700 Wh of extra energy to move the car (or for other functions, like instant high-power defrost).  This is enough for perhaps 2 miles of petroleum-free driving, perhaps more if the first stretch after starting is creeping in traffic for an extended distance.

How much fuel could this save?  Approximately 930,000 hybrids (not plug-ins or BEVs) were sold in the USA in 2012-3.  If we assume 10 battery top-offs per week 52 weeks a year, 2 miles range per top-off and 40 MPG consumption avoided, the 2012-3 fleet would avoid about 1 gallon consumption per vehicle per week, 52 gallons/vehicle/year, 48 million gallons/year for the fleet.  The total for the hybrid fleet going back to the oldest Priuses on the road would be multiples of this, perhaps a good fraction of a percent of total US gasoline consumption.

What could this do for the grid?  1.2 kWh/day 5 days a week isn't much, but if you can draw it on demand it might be worth something.  The J1772 Level 1 spec is 120 VAC @ 12 A, or 1440 watts.  930,000 vehicles @ 1.44 kW/vehicle is 1.34 GW of potential demand.  It would take only about 25 minutes to put 600 Wh into a battery at that pace, but 25 minutes of demand equal to a large nuclear plant available twice a day (perhaps 3x, if vehicles are plugged in during after-work errands) might be very useful to ISOs for down-regulation of the grid.

This is a small possibility, not a big possibility.  But there is a big push for mild hybrids (perhaps multiples of the number of conventional hybrids), and a lot of littles make a lot. 
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