Can We Power Our Car With the Sun?
We oversized our PV system so that can use solar energy to power around-town driving with a plug-in hybrid car
I’ve written about a lot of the features we included in our new house in Dummerston, Vermont, to reduce its energy use and environmental footprint, but there’s another one — a big one — that doesn’t really relate to the house.
We are hoping to power a plug-in hybrid car using the electricity generated on our barn. We have 12 kilowatts (kW) of photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) modules installed on the barn roof (there is also a 6-kW group-net-metered PV system that belongs to a neighbor), and we’re hoping that the 12 kW will be enough to not only power our all-electric house on a net-zero-energy basis, but also to power our car for around-town use.
Trading up to a plug-in hybrid
My wife and I have two cars: a nine-year-old Subaru Forester with 128,000 miles on it and a ten-year-old Honda Civic Hybrid (manufactured the first year that the Civic was offered in a hybrid version) with 180,000 miles. Aside from being embarrassed by how many miles we drive — less now that our daughters are out of college — I’m aware that end-of-life decisions might be coming up soon, at least with the Honda.
Our hope is to trade it in on a Chevy Volt, a Toyota Plug-in Prius, or a plug-in hybrid made by some other manufacturer. I first began thinking about a plug-in hybrid before they were commercially available, and I’m glad we waited and invested in new batteries on the Honda when the original hybrid battery system was failing a year ago at about 170,000 miles.
My brother-in-law loves his Chevy Volt. Through conversations with a number of car experts I’ve generally gotten the sense that General Motors leapfrogged Toyota with its own plug-in technology — but I’m still doing research on this. I’m hoping that by the time we really need to replace the Honda there will be even more choices.
I’d really love it if VW introduced an affordable plug-in diesel hybrid. Prior to our two current cars, I drove VW diesels for years and loved them — first a 1983 Diesel Rabbit (which was a little too noisy and smoky) and then a 1996 TDI Passat wagon that we loved. We drove the Rabbit well past 200,000 miles without ever having to make any major repairs to the engine, transmission, or clutch. And we sold the Passat with 140,000 miles after running it for a couple of years on biodiesel.
How much solar electricity would I need for around-town driving?
Back when I first started thinking about powering a car with the sun, I asked my friend Steven Strong, of Solar Design Associates in Harvard, Massachusetts, how many extra kilowatts of capacity I would need on my PV system to provide for driving. Steven had converted his standard Prius to a plug-in version by adding additional battery capacity and the necessary controls (this was before the Chevy Volt or Prius plug-in models were available).
Back in mid-2011, Steven told me that his plug-in conversion Prius required 265-275 Watt-hours (Wh) per mile in Eastern Massachusetts where it’s reasonably flat and winter temperatures are more moderate than in Vermont. He thought 300 Wh/mile would be a more realistic estimate here. He also said that the Prius conversion isn’t an optimal electric vehicle and that the next-generation, factory-engineered EVs and plug-in hybrids should provide better performance.
I just looked online and saw some claims as low as 200 Wh/mile for a Volt, but most are in the 250 to 300 Wh/mile range. If I go with Steven’s estimate of 300 Wh/mile (0.30 kWh/mile) and estimate our commuting and around-town driving to be ten trips per week at 18 miles round-trip, or 9,360 miles/year, then our annual electricity usage for that commuting and around-town driving would be 2,800 kWh.
If I assume 1,200 kWh of output per kW of rated capacity for a PV system (typical for Vermont), that works out to 2.3 kW of additional PV to generate enough electricity for that amount of driving.
Despite the fact that our HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. score (Home Energy Rating System) showed that my house will need the full output of a 12 kW PV system plus a little bit of heat from our wood stove, I’m hoping that we’ll be using less energy than the HERS model predicts (once we have our low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. storm windows up), and we’ll have enough electricity left over for powering our around-town driving.
The proof will be in the pudding.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
- Alex Wilson
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