Decision point for an electric vehicle and our home's photovoltaic array expansion

Discussion in 'EV (Electric Vehicle) Discussion' started by Nords, Feb 1, 2010.

  1. Nords

    Nords Member

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    I've searched the old threads here, but I still have two questions for a design decision:
    1. Roughly how many kilowatt-hours does it take to recharge an EV? I understand this depends on the battery type/size/efficiency and the miles driven, but I'm looking for either a calculator or an estimate.
    2. What are the EV options for a 50-mile range that includes some 55 MPH highway driving? I see that GreenCar sells the occasional Hymotion Prius (Green Car Company), which is tempting, but there's no hurry.


    Here's the background:
    Spouse and I could see ourselves buying a used EV (or another hybrid) in the next five years. We're happy with our Prius for highways & hauling and we don't need a second car, but it'd be convenient. An EV that could hold a 9'0" longboard inside the passenger compartment would be an unexpected bonus.


    Oahu is small enough that we could cope with a 50-mile range. Most highways are limited to 55 MPH or have secondary routes, so highway speeds would be desirable but not essential. For the last five years we've driven less than 100 miles per week and most weeks are closer to 75 miles. Gas currently costs about $3.10/gallon but in 2008 it was closer to $4/gal.


    Most of Oahu's power is generated by burning oil, and that fuel cost is passed right along to the customer. Electricity costs 23 cents/kilowatt-hour and has been as high as 30. However over 80% of our current consumption is supplied by our grid-tied photovoltaic array. It's almost paid for itself but we've already maxed it out. Adding an EV to our consumption would require a second inverter & more panels. Oahu's grid-tied net-metering rules allow us to roll over our excess production credit for up to 12 months, so we could recharge an EV during the day or at night. All we'd have to figure out is how many kilowatt-hours an EV would add to our consumption.


    We've just about maxed out our roof, too. We could barely squeeze another 500 watts of panels on the remaining space. I'm not interested in high-efficiency cells or high-power-density panels or any of the integrated-PV roof tiles-- they're great concepts but way too expensive right now.


    However in a separate home-improvement project we're considering building a pergola along our south wall. A pergola roof would offer plenty of space for more panels in a variety of designs. One local firm actually sells a pergola whose roof consists of PV panels.


    So today's pergola design might be complicated by the discussion over how much future generating capacity we'd need, which depends on a vehicle that we might be five years from buying. I could just wing it with another 1000-2000 watts of panels, but I'd rather come up with an estimate that would also help us decide what type of EV (or plug-in) we want.


    Any "easy" answers to our questions? Are we missing any other issues?
     
  2. rpatterman

    rpatterman Thinking Progressive

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    Not an expert here, but the commonly used number for EVs is 4 miles/kWh.
    So worst case: 100 miles per week is 25 kWh/week, 100 kWh/month, 1200 kWh/year.

    Great job on getting to 75 to 100 miles per week and 80% EV!

    What size system do you now have that provides 80%?

    At 100 miles per week in a Prius, even at $5/gal, you are only spending $10/week on gas!!!

    Might be hard to justify from a purely economic point of view either EV or more PV.
    Nice situation to be in!
     
  3. vegasjetskier

    vegasjetskier New Member

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    I used to drive my Jet Electrica back and forth to work, 22 miles roundtrip. It would recharge overnight (12 hours) with a 15 amp, 120 volt charger. 15 X 120 X 12 = 21.6 Kwh. So about 1 kwh/mile. This was about half stop-and-go and about half freeway at 70 mph. So at 55mph or less you should do better. 4 miles/kwh might be hard to get to, especially considering charger and conversion inefficiencies.

    If I'm doing the math right (someone correct me if I'm wrong), with 4 miles/kwh, you would need 50 miles / 4 = 12.5 kwh/day. If you get 12.5 hours useful sun, divide by 12.5 = 1 kw system (net). Add in conversion inefficiencies: 1 kw / 80% (inverter efficiency, my guess, could be wrong) / 95% charger efficency = 1.31 kw system size (net). If you have a setup where you can connect the PV directly to the EV you'll need a smaller system, but will have to size and design it just right to work. If you don't get the 4 miles/kwh from your EV you will need a commensurately larger PV system, of course.
     
  4. Nords

    Nords Member

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    4 miles/KWh? 0.25 KWh/mile? Sounds great-- we can work with that. Thanks!

    12.5 hours of sun during the summer is no problem, but quality varies dramatically. Our Xantrex inverter is rated at around 95% efficiency, but I don't have to do any more calculations to come up with an estimate. We regularly produce 12-13 KWh/day in the summer months and 8-10 KWh/day during winter. Of course that varies with cloud cover, vog, and rain.

    Our array is 3300 watts of used panels-- 1990s-vintage Siemens & Solarex models of 50-75 watts. We also have Evergreen factory seconds (blemishes, some bad cells) rated at 115 watts. I doubt that every panel is producing rated output, and the Xantrex 3 KW inverter manual says 10% over rating is fine. We're not approaching any voltage or current limits.

    We bought the panels off eBay & Craigslist for $3-$4/watt. Made our own racks out of scrap aluminum left over from a lanai-screening project. Paid full retail for the inverter, conduit, & wiring. Had a very patient solar installer teach us the mechanical & electrical code requirements, mounted everything on the roof ourselves, strung all our wiring, and paid him $750 to do the grid connection. State & federal tax credits paid for 65% of our $16K cost.

    It's always easier to reduce consumption than to raise production, and we're in a good location. No heating or air conditioning. Tradewind cooling with ceiling fans and solar-powered attic exhaust fans. Radiant foil insulation in the attic and most walls. Mostly CFL lighting. EnergyStar appliances. The dishwasher gets all the hot water it needs from the solar water heater. Two old CRT TVs, no big screens or LCDs, Energystar desktop PC/monitor. Power strips for vampire loads. We still use an electric clothes dryer but spouse drew the line on that. We only do 1-2 loads of laundry a week anyway.

    We could downsize our 25 cu ft fridge to about 20 cu ft-- even an EnergyStar 25 cu ft fridge is an electricity hog. We use an old MagicChef countertop convection/microwave oven that we're eventually going to upgrade to a more efficient GE Profile of the same technology. Our main oven is a Kenmore convection model with a glass cooktop but that doesn't get much use anymore. We're adding one more solar exhaust fan to an attic void that gets a lot of sun, and we might put some decorative holes in an interior wall to improve airflow. ("Hole" is straightforward engineering, but "decorative" is a challenge.) Our next roof (probably in 20 years) will use whatever's current in reflective solar insulation.

    The biggest energy sink in our house is our 17-year-old, but she's leaving for college in just 187 days...

    These days the highest component cost of a PV system is the inverter. I should easily be able to find used/blemished PV panels at $3/watt. To recharge an EV under full-use conditions, it looks like we'd want an entire new 3 KW system to have the reserve capacity. I'll have to figure out what dimensions spouse wants for the pergola (that's a décor decision, not engineering design) and then see how many kilowatts the pergola contractor can cram onto it.

    You're right about fuel consumption. 55 mpg and $4/gallon gas is about the same as 4 miles/KWh and 30 cents/KWh electricity. At an energy use of ~$7.50/week, even a $25K PV array and 65% tax credits would take over 20 years to pay for itself. And for a trip of over 50 miles, or for hauling a load, we'd still use the Prius. But I guess I'd have to worry about fuel stabilizer in its gas tank...

    It's not compellingly cost-effective to install a second PV array, even if energy inflation costs will cut down on the payback time. Of course I enjoy the design & "cool" aspects of this type of project, and a second car would be a convenience.
     
  5. drees

    drees Senior Member

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    Per watt, inverters are typically $1/watt or less. Wholesale prices on new panels are $2-3, so I think panels are still the most expensive per watt. You can find new UL rated panels online for $3.50/watt or so, so you might be able to find used panels even cheaper now.
     
  6. daniel

    daniel Cat Lovers Against the Bomb

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    My Zap Xebra gets 3 miles per kWh measured at the wall outlet. The electric Porsche was getting between 350 and 400 Wh per mile measured at the battery (add in whatever is dissipated by the charger, as I have no way to measure amperage at the 220 volt outlet). Neither of these cars is likely to be as efficient as a purpose-designed EV would be. Note, of course, that the Xebra is a low-speed car, while the Porsche was driven harder and on the freeway.

    Darell (EVnut.com) may be able to give you numbers regarding his Rav4EV, which he also charges from his PV array.
     
  7. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    go with daniel's number since the only thing that counts is what is coming out of the wall. an average EV does do about 4 miles per KWH, but conversion losses, battery bleeds, etc. actually puts the real # to just under 3 miles per KWH.

    my Zenn figures cannot be considered because i plug in at work (amongst other free plug ins around town) and since i dont pay for it, i dont track that electricity usage. (plus its a hassle to haul around my Kill-a-watt meter.
     
  8. Nords

    Nords Member

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    Good point. We were buying panels a few at a time.

    Thanks. The Porsche's a bit out of our price range but the RAV looks like it would've more than fit our needs.

    Even at 3 miles/KWH we'd probably still go with a 3-4 KW array. I'm going to have to take another look at inverter/panel prices and see how much space the pergola affords. I'd hate to spend top dollar for high-power-density panels and an oversized inverter.

    I can see that we'd get to know the location of every plug-in station around town...
     
  9. daniel

    daniel Cat Lovers Against the Bomb

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    I suspect that the Xebra is not terribly efficient. On the other hand, it's slow, which uses less energy. A purpose-designed EV from a good company like Nissan should be more efficient, but you'd want to drive at highway speed and accelerate faster than the Xebra does, which uses more. So maybe it would come out about the same.

    Still, it shows some numbers for a car that goes on the highway and accelerates more like a normal car than the Xebra does. But again, a purpose-designed EV should be able to do better.

    It's really too bad that the Aptera seems to have gotten bogged down. They were really headed for great efficiency.

    I don't remember ever hearing what the RAV4EV gets. But even that may not be a good final number because a small sedan would have better aerodynamics and might be lighter.
     
  10. drees

    drees Senior Member

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    When you've got a variety of panels in your array, I'd suggest looking at micro inverters like the ones from Enphase.

    The micro inverters should make the most of your panels as they enable each panel to produce maximum output regardless of what the other panels on the string are doing. The drawback is that they are a bit more expensive than traditional inverters and they are not compatible with all panels.

    But with traditional inverters you have to worry about panel matching and string sizing.

    Something to look at, anyway.
     
  11. Nords

    Nords Member

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    I've been seeing those at the home shows, and they probably raise reliability while solving some problems of bigger inverters. But I'm not sure that their power optimization is a significant improvement over blocking diodes and a conventional inverter's MPPT algorithms. I'm pretty sure that we're macro-charged for the micro-privilege.


    Although we have a variety of panels, their operating voltages are pretty similar. We've arranged the two strings to keep their total voltages within 5% across the day. Performance is "good enough" for our purposes, and I don't know how much better we'd get by paying up for a matched set. I'd love to see an objective study by someone like Consumer Reports.



    Anyway if we went with a PV pergola then we'd match the panels because that's how the contractor wants to build it-- with a standard set of dimensions instead of whatever I dredge up from eBay. They also put some effort into hiding the connectors & wires among the pergola's roof framing while maximizing the panel exposure to cast a nice uniform shade effect through the translucent panels, so I don't know how they'd deal with microinverters. But those are décor decisions, not engineering, and we've learned to defer all décor decisions to my spouse...


    A local solar contractor does EV conversions as a hobby. His last one was a Geo Metro and he's always casting about for the next project. It'd be tempting to see what he could put together from a typical hatchback but I suspect we'd despair over the lead-acid lifecycle.
     
  12. daniel

    daniel Cat Lovers Against the Bomb

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    Lead batteries can last if they are very closely monitored and maintained. And floodies do better than sealed. A bigger problem is range: They are so heavy that you're not likely to get more than 20 or 30 miles, and for optimum life, you need to avoid more than 50% discharge, effectively cutting the range in half. Lithium solves many of those problems but is expensive, though proponents claim a much longer life.
     
  13. thefortunes

    thefortunes New Member

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    I know Dave has been disappointed in his Zenn, and maybe we will be over the long run, but so far (about 1300 miles over 2 months here in frigid Wisconsin) it has performed better than we expecteed.

    We can go 40mph (well, 38 per gps), and even in our 20deg F average temperature over the last month we have averaged 4.65 miles/kWh. That equates to a range of almost 30 miles (and remember, we are COLD right now).

    We have about a 5 mile trip anywhere, so we typically drive about 10 miles roundtrip, then recharge (even if it is only for a short time between trips). The batteries have only seen a DOD > 50% once or twice, so I hope they last longer than those who have had problems.

    All this for $4500 new (after the $3500 tax credit). IMHO it is a great second car (actually fourth for us, but who is counting :)).
     
  14. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    cant say i am disappointed really, just wish i knew then what i know now. my only real mistake was thinking that the battery upgrade was better than the standard batteries they offered. that turned to be very very wrong
     
  15. ljbad4life

    ljbad4life New Member

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    Has anyone had any success with a home primarily powered with wind power? A property I am looking at is near a river and has very constant and strong (at times) wind. I found this one that would really blend in to the buildings. The site I believe is in dutch so I can't read it.
     
  16. miscrms

    miscrms Plug Envious Member

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    From what I've seen 50 miles is a pretty tough range spec for a converted EV without spending pretty big money on a Li-ion battery and BMS. If you can, I'd wait a few months and see what the prices are going to be like for the Nissan LEAFs. The RAV4EVs are awesome, but $$$ due to so few around and so much interest.

    3 miles / AC kWh seems like a pretty safe estimate for a smaller vehicle. The PbA EV1 for example was tested by EVA at 164 Wh/mi on the driving cycle, with an overall charging efficiency of 248 AC Wh/mi. The NimH was a bit less efficient in this regard, but a lot of this probably had to do with poor pack design requiring active battery cooling. Note the RAV4EV efficiency is very similar between the PbA and NimH versions.

    EV1 PbA: http://avt.inel.gov/pdf/fsev/eva/genmot.pdf
    EV1 NimH: http://avt.inel.gov/pdf/fsev/eva/ev1_eva.pdf
    RAV4EV PbA: http://avt.inel.gov/pdf/fsev/eva/toyrav96.pdf
    RAV4EV NimH: http://avt.inel.gov/pdf/fsev/eva/toyrav98.pdf
     
  17. Nords

    Nords Member

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    I've looked into it, but the numbers aren't there for us.

    First you have to decide that you want to live in a place with a constant breeze-- it might get very annoying after a while. We have tradewinds blowing across the islands pretty consistently but our yard doesn't see a breeze more than a few hours a day and some neighborhoods hardly see anything at ground level.

    Second, you want wind data. A good wind-turbine contractor will set up a data-gathering box at the site and leave it for a month or two. I don't know how common those contractors are. We talked with our local university and eventually located a high-school science project that used a similar system on a home near ours. The conclusion was that our breezes were gusty at times but overall too light & intermittent.

    No rush-- after considerable décor discussion (thanks to the feedback in this thread!) I don't think we're ready to do the pergola this year, although we'll re-evaluate when the summer weather heats up. The design is straightforward but the construction will need a lot of yard work.

    Anyway we'd wait a few years until new EVs started showing up on Craigslist or Autotrader or similar used-car markets.
     
  18. miscrms

    miscrms Plug Envious Member

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    If you have the space, you might want to look at thin film panels. Their area efficiency is poor, but they are getting down near $1/W. Here's an example, built in Japan, UL listed $1.20/W for orders of 25 panels or more.

    Kaneka GSA60 : Solar Panels Direct, Powered by Nature!
     
  19. hill

    hill High Fiber Member

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    Isn't the cost of a good wind generator somewhere on par with similarly high efficient PV panels? . . . in the neighborhood of $4 per watt (after 30% fed. credit) installed?

    .
     
  20. ljbad4life

    ljbad4life New Member

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    The specific wind turbine was about $3 per watt installed (with a friend discount :) ) I am still looking at this property and I will have my friend come down and evaluate it. My personal experience was that no matter when I went to the property (i know the owner) it was always consistently windy

    so i will report back my experience
     
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