Friday, October 12, 2007

Chevrolet Volt, an environmental makeover

If energy for an electric vehicle is supplied by renewable sources, greenhouse gas emissions are low. What would that cost? Prices for photovoltaic (PV) solar panels have been steadily dropping for many years, although recently prices have firmed because of heavy demand in Europe. A practical approach could use fixed-mount PV panels to supply daily energy to the electrical grid equal to the energy needed on workdays to power the vehicle.

The following analysis focuses on installing PV panels to supply the energy for a Chevrolet Volt, as used for workday commuting, providing enough low-pollution, renewable energy to replace high-pollution energy from the electrical grid. See "Chevrolet Volt, a plug-in polluter," September 27, 2007. The grid is fed during the day by solar power and tapped at night to charge batteries, a beneficial offset to the typical patterns of grid activity.

Chevrolet Volt electricity use, at 10,000 mile/year --
-- 14 kWh/workday * 240 workdays/year = 3360 kWh/year

Practical utilization factor for fixed-mount PV installation --
-- Applied Materials, Sunnyvale, CA, March 14, 2007, via WebWire
-- 2300 MWh / (1.9 MW * 8760 hr) = 0.138 utilization factor

Nameplate PV system power rating, fixed-mount installation --
-- 3360 kWh/year / (8760 hr/year * 0.138 utilization factor) = 2.8 kW AC

Configured cost, 3.15 kW DC nameplate PV and inverter connector, $17,494 --
-- October, 2007, California, Solatron,
-- 5% inverter loss and 5% lower tolerance leaves minimum 2.8 kW AC

Mortgage rates, 30-year fixed, national average, 6.1 percent --
-- October, 2007,
-- monthly mortgage payment for $17,494 initial principal, $106.01

Annual capital cost of PV energy for Chevrolet Volt --
-- $106 per month * 12 months = $1,272

Annual cost of gasoline for Chevrolet Volt --
-- $2.45 per workday * 240 workdays = $588

Annual cost of electrical grid energy for Chevrolet Volt --
-- $1.55 per workday * 240 workdays = $372

This simplified analysis assumes no system maintenance costs, no charges for occupancy of land, no installation or mortgage closing costs, and no credit for nighttime electricity used from the grid versus daytime energy supplied to the grid. On such a basis, photovoltaic energy cost will be about 2.2 times the cost of gasoline. The installed cost of an adequate photovoltaic system is a substantial fraction of the cost of the vehicle. That is a social view of costs, while a consumer's view is influenced by opportunities for government subsidies and tax avoidance.

In California, a combination of state and federal subsidies would reduce consumer cost of the photovoltaic system to about $8,441, bringing the estimated cost of photovoltaic energy down to $614 per year. With government subsidies paying for near half of the photovoltaic system and with consumers paying no gasoline taxes to support highway maintenance, a Chevrolet Volt's cost for low-pollution photovoltaic energy becomes little different from its cost for gasoline and only modestly higher than its cost for high-pollution energy from the electrical grid.

Other practical issues with photovoltaics should be considered. The system used as an example involves an array of panels about five feet high and fifty feet long. For rated performance, the panels need to be mounted at an angle within 10 degrees of perpendicular to the average sun direction over the year. Many houses lack enough roof space, or they have unfavorable orientations. Few if any multifamily apartments have enough space. Widespread substitution of photovoltaic energy for electrical grid energy will require community or regional facilities.

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