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Where Does Surplus Energy Go When You Can't Backflow Onto the Grid?

Discussion in 'Tesla Energy' started by Snerruc, Feb 23, 2017.

  1. Snerruc

    Snerruc Member

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    Solar City on the Big Island can no longer tie to the grid with 2 way meters. Basically, they now install an off the grid system that can draw power from the grid in emergencies. As a result they want to put in 2 Powerwalls to store for the night time load. When they are full, apparently excess power goes to ground. My worry is if this is safe or will multiple kilowatts going to ground cause galvanic corrosion and other safety problems.
     
  2. mblakele

    mblakele radial cross member

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    I'm still learning about solar generation systems. But maybe if I post some terrible misinformation someone more informed will correct it, and I'll learn something along the way.

    My understanding is that most setups capable of working off-grid need a dummy load, aka dump load. That might be what's meant by "goes to ground". But the power won't go straight to ground: it passes through a big resistor, which will heat up. If there's enough of that heat, it could do something useful — say, heat water.

    With the right setup (only available from SolarEdge?) they could do without the dummy load. In that case the panels are cut off and float free when there's nowhere to put the power. Depending on how you look at it that's either a great feature or a bit of a waste.

    Either way, a well-designed system shouldn't damage anything.
     
  3. miimura

    miimura Active Member

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    Excess solar power is very easy to modulate. If there is no demand for the power and the batteries are full, the inverter will simply stop taking current from the panels and the operating power point will move to the open circuit voltage.
     
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  4. Snerruc

    Snerruc Member

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    Thanks. The Solar City rep was inept and couldn't answer any questions about operations. Her response was that if you saw the batteries fill up I should bake a batch of cookies. Also, their online contract they sent me had no specs on panel type or wattage of inverters,etc. No way I would sign a $36000 contract on no information. Further, the only "deal" they offered was $16000 cash upfront and $ 160000 cash on completion.
     
  5. Jack007

    Jack007 Member

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    Brutal, $176,000 for a solar system... think you're getting ripped off;)
     
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  6. Snerruc

    Snerruc Member

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    Whoops ! One too many zeros. It was $36000 total. Still, that was double what a 20 kW system cost my friend when he could put in a two way meter two years ago.
     
  7. Jack007

    Jack007 Member

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    Agreed $36K is still nuts.
     
  8. Snerruc

    Snerruc Member

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    If this is where Solar City is going, they won't be installing many systems in the future. I'm checking to see what Sunrun will do for me.
     
  9. nwdiver

    nwdiver Active Member

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    Well... sad fact is... that info would be gibberish to ~80% of their customers... not knocking Solar City customers... just people in general. kW and kWh aren't units they ever really have to deal with. I ask people how much electricity they use and ~90% of the time they use $$$ as a unit. I ask how many kWh and I get a blank stare :(

    I've had engineers at work ask what good a 10kW array is if they use 1500kWh per month.....
     
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  10. Ampster

    Ampster Member

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    Am I missing somthing? $36k for a 20kW solar installation is $1.80 per Watt.
     
  11. Snerruc

    Snerruc Member

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    Sorry, 6 kW system was quoted. I use 20 kW a day. Half house and half car. It's not in the contract, but I believe she said 22 panels and 2 Powerwalls. However, when I am out, the car may need a 50 a 60 kW charge and the system doesn't have the capacity to handle that. Then I have to buy from the grid at .34 a kWh.
     
  12. Ampster

    Ampster Member

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    Okay now I understand that is a 6kW solar system and two powerwalls. The solar system should not cost more than $24,000 ($4/Watt) so that means that the Powerwalls are coming in at about $6,000 each. I am a little confused by your mixing of kW and kWhrs. I think you meant to say your car might need 50 to 60 kWhrs and you used on average 20kWh per day.. Are there opportunities to shift loads? Is there a late night EV rate?
     
  13. Snerruc

    Snerruc Member

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    Sorry, I'm not an EE. The rate is .34 all the time. Most days I drive 10 - 20 miles, but some days I drive 100-200 miles. That is what causes that problem. I need the car in the day but not at night. With HELCO not allowing people to install
    Two way meters, there is no way you can install a system that is transparent and at 77 I'm not going to spend time messing with an electric system.
     
  14. allanfridstrom

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    The vast majority of the time, electricity supply is simply matched to demand by throttling flexible power plants up and down.

    Aside from throttling load-following plants, the grid has a large reserve of standby generators available to increase supply. Utilities can also take many types of generators offline as needed to reduce supply. Demand is actually pretty straightforward to predict -- it follows regular patterns by time of day and day of week. So utilities are quite good at supply-matching when they are allowed to pick their own generation capacity mix.

    Back when per-capita electricity consumption and population were both growing rapidly in the developed world, many people thought that the problem would always be struggling to build enough power plants to provide sufficient power at an economical price. Peak demand in mid-afternoon is particularly problematic because you don't want to build entire power plants just to run for a few hours a day in the hottest months of summer. The construction costs raise power rates across the board, just to provide capacity that's rarely needed. Electricity is cheapest when power plants can run non-stop -- it distributes the capital cost over more units of produced energy.

    But now per-capita electricity use is dropping in the developed world, and population growth is slowing, and governments are encouraging massive construction of renewables. So the bigger issue today is over-supply. Why? Because it's not always easy to take power plants offline:
    • Some types of generation capacity have economics that discourage going offline, like coal plants.
    • Some are physically difficult to take offline (and then restart) like nuclear plants.
    • Some electricity from non-utility sources is forced into the grid regardless of demand, like wind and solar power in many jurisdictions.

    Storing electricity just isn't practical. First, it's physically difficult and every method we've invented takes up a ton of space. Second, all energy storage and over-supply management requires some degree of waste. You're always doing one of these three things:
    • Losing energy in the conversion to and from a storage medium like pumped-storage hydroelectricity or battery banks, as well as all the energy and resources that went into building those storage systems
    • Losing energy by just dumping it, such as venting excess nuclear plant heat or shunting excess electricity directly to ground
    • Losing financial value by selling excess power at a loss and buying shortfall power at another loss (and even then, someone else's flexible generation capacity has to handle the variability) tranasms.nu
     

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