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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorgravelld
    • CommentTimeJul 25th 2018
     
    Posted By: djh
    Presuming the relay/contactor was suitably rated that should be possible. But most diverters will divert arbitrary fractions of the power and connect and disconnect frequently and arbitrarily. I can't think of many inductive devices that are designed for an intermittent supply like that.
    We use a oil filled electric radiator like this to contribute to heating in the shoulder months.
    •  
      CommentAuthorDamonHD
    • CommentTimeJul 25th 2018 edited
     
    Interesting.

    Looking at our usage, the only time we have excess electricity generation over usage and have heating demand is maybe one month, eg March last year:

    http://www.earth.org.uk/saving-electricity-2017.html

    Maybe a bit of April and October in 2016 also...

    Rgds

    Damon
    •  
      CommentAuthordjh
    • CommentTimeJul 25th 2018
     
    Posted By: gravelld
    Posted By: djh
    Presuming the relay/contactor was suitably rated that should be possible. But most diverters will divert arbitrary fractions of the power and connect and disconnect frequently and arbitrarily. I can't think of many inductive devices that are designed for an intermittent supply like that.
    We use a oil filled electric radiator like this to contribute to heating in the shoulder months.

    An oil-filled radiator is resistive, not inductive, isn't it?
    •  
      CommentAuthorDamonHD
    • CommentTimeJul 25th 2018
     
    I'd expect it to be, though there will be some parasitic inductance unless there is special care taken in winding heating elements, etc.

    Rgds

    Damon
    • CommentAuthorgravelld
    • CommentTimeJul 25th 2018
     
    Posted By: djh
    An oil-filled radiator is resistive, not inductive, isn't it?
    I might have misunderstood the discussion, but here I thought resistive/inductive was being used as a synonym for variable/fixed demand.

    While the heating element in an oil filled rad is resistive, the electronics that drive it aren't and demand a particular draw.

    I think I realise that I'm not talking about the fundamentals of what makes a resistive load resistive, rather the practicalities of these situations - I'm ready to be educated.
  1.  
    Posted By: gravelldWhile the heating element in an oil filled rad is resistive, the electronics that drive it aren't and demand a particular draw.

    I'm not sure what sort of oil filled rad you have but any that I have used are about as far from electronics as my teapot. For mine you plug them in, the heating element heats up and when the set temp is reached a mechanical switch (driven by either a bi-metal strip or a capillary tube) switches of the heating element. 100% resistive plus a bit of electromechanics.
    •  
      CommentAuthorDamonHD
    • CommentTimeJul 29th 2018
     
    I've been having fun, creating a simulation of a single Emphase AC battery and my night loads, now with some pretty graphs!

    http://www.earth.org.uk/electricity-storage-whole-household-2018.html#SMAXCDW

    Rgds

    Damon
    • CommentAuthorJamster
    • CommentTimeAug 1st 2018
     
    Damon, and other interested by-standers - stumbled across this after my supplier went under:

    https://octopus.energy/agile/

    Their text:

    "Plunge Pricing pays you to take excess energy off the grid

    "Agile Octopus introduces Plunge Pricing – a world first that lets you take advantage of negative price events, and get paid for the electricity you use. Receive SMS alerts whenever prices drop below zero, or use our API to program your smart devices."

    Don't know if its worth thinking about since you appear to have a fair idea what you are doing with these things!
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 1st 2018
     
    Hi,

    That is very interesting, thanks! (And Octopus seems very interesting to keep an eye on in general.)

    But (a) I'm doing this much more for my education than for money and (b) intend to stay with Ecotricity.

    Time of Use tariffs are how life is going to be. The suggestion that I heard from an eminent prof is that those people who stay on all-day single-price tariff will be paying several times more than those who show some flexibility to support solar and wind which are already cheaper than new fossil generators...

    Rgds

    Damon
    •  
      CommentAuthordjh
    • CommentTimeAug 2nd 2018
     
    Posted By: DamonHDI've been having fun, creating a simulation of a single Emphase AC battery

    Don't want to put a dampener on your fun, but have you priced the Enphase batteries?
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 2nd 2018
     
    I'm part way through paying for them!

    Very very much more expensive by the usable kWh than my lead-acid off-grid storage, but wrapped in an interestingly elaborate control and monitoring system including BMS. Yes, I wish I was seeing that utopian US$100/kWh wholesale price of the near future!

    Part of this experiment is (potentially) to see if systems from different manufacturers can be paralleled post hoc as things become cheaper. I have always detested having to junk older stuff prematurely because new stuff won't play nicely.

    Rgds

    Damon
    •  
      CommentAuthordjh
    • CommentTimeAug 2nd 2018 edited
     
    DamonHD wrote: "Very very much more expensive by the usable kWh than my lead-acid off-grid storage"

    Overnight, it occurred to me that there's a simple way to price them.

    Take the price of the battery and its capacity. For the Enphase that's £1,699 for 1.2 kWh according to https://www.which.co.uk/reviews/solar-panels/article/solar-panels/solar-panels-and-energy-storage#batterytable

    Take the warranty, in this case 10 years. Then if we make the simplistic assumption that we want to recover the cost of the unit within it's warranty period, and also that we can fully charge and discharge it every day, we can calculate how many units we can save in the waranty period: 10*365*1.2 = 4,380 kWh and if we divide the cost by that number, we'll see how expensive the electricity has to be to make the unit cost effective: £1,699 / 4380 kWh = 39p /kWH

    So in this case, there's no real possibility of the unit being cost effective. Now we can change the assumptions of course. Firstly, we'd probably want to earn twice as much to account for days when we didn't charge or discharge the battery completely and to account for the cost of money. But then we might make money from grid regulation payments, for example, or we might be able to charge more than once per day from the mains. But unless and until the tariffs change in nature, I'd say the whole thing is pretty uneconomic.
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 2nd 2018 edited
     
    At the moment, in the UK general retail market, it is uneconomic.

    In some countries/markets the grid just won't take your exported power or may not let you connect if you have PV, or may charge a large fee to handle PV-related flows.

    ToU tariff arrival here is going to make a huge difference.

    But, for the record, I could nominally switch my gas heating to heat-pump and completely disconnect from the gas and electricity grids right now: I'm exergy-negative. Then I *just* have to store the energy seasonally. About 250kWh for the electricity alone, ignoring heat/heatpump. A sizeable chunk of the value of my house.

    I know at least one person that does live in the Styx (in the US) without any grid connection and a huge battery. If the grid-connection costs are huge, storage at current prices looks like a bargain, and will go on getting better.

    Rgds

    Damon
    •  
      CommentAuthorfostertom
    • CommentTimeAug 2nd 2018
     
    "The Styx is a river in Greek mythology that formed the boundary between Earth and the Underworld"

    "... one person that does live in the Styx (in the US)" = an American hedging his bets?
    • CommentAuthorowlman
    • CommentTimeAug 2nd 2018
     
    Fiddle-styx Tom; It's North of the Wash,( Danelaw Hel ), according to our Government.:bigsmile::wink:
    • CommentAuthorgyrogear
    • CommentTimeAug 2nd 2018
     
    Styx and Stone's in Staffs...

    gg
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 2nd 2018
     
    Commonly called "in the sticks", but I was being fancy. I believe that's where the expression comes from. Being marooned on the shores of the Styx without money to pay the ferryman was a BadThing(TM)... But there are other claimed etymologies involving woodland... %-P
    •  
      CommentAuthorfostertom
    • CommentTimeAug 2nd 2018
     
    ah - credible
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 2nd 2018 edited
     
    .
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 7th 2018
     
    The new battery is in, and behaving nicely:

    http://www.earth.org.uk/Enphase-AC-Battery-REVIEW.html

    Yesterday evening the house basically ran off battery after sunset until we went to bed.

    It'll be a couple of days before I start to get access to stats.

    Rgds

    Damon
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 13th 2018
     
    FWIW the stats are coming in and it's working as designed:

    http://www.earth.org.uk/electricity-storage-whole-household-2018.html#Data

    Very good to import just 0.2kWh out of 2.2kWh demand even when roughly half the demand happens when the PV wasn't generating (second graph).

    However, the effects are generally more subtle since the Enphase is not speced to prevent the big loads hitting the grid, just to take the edge off them.

    Rgds

    Damon
    • CommentAuthorgravelld
    • CommentTimeAug 13th 2018 edited
     
    Thanks! Any chance you could use colours other than red and green for the graph, I'm colour blind!

    I'm a little puzzled why the export W doesn't remain flat for a short while during the charging of the battery, e.g. from 6am to 8am in your 2018/08/11 graph. Shouldn't it be charging the battery rather than exporting, or is there a throttle on the charging?
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 13th 2018 edited
     
    That dip before 8am is us using more electricity (for a short while) to make breakfast than the PV+battery can cover.

    Sorry about the colour scheme: that's gnuplot's default for the first two traces and I hadn't thought to override it. I'll have a think.

    Rgds

    Damon
    • CommentAuthorgravelld
    • CommentTimeAug 14th 2018 edited
     
    Posted By: DamonHDThat dip before 8am is us using more electricity (for a short while) to make breakfast than the PV+battery can cover.
    No, I'm asking why are you exporting while the battery is charging - shouldn't all excess power be going into the battery and charging it faster?
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 14th 2018
     
    Each battery's charge (and discharge) limit is about 260W in the Enphase system. So a full charge or discharge takes about 4 hours.

    Rgds

    Damon
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 31st 2018
     
    The Enphase increased self-consumption while we were away from about 50% (as you'd expect with ~12h of PV generation at this time of year) to about 90%.

    So we imported 2.4kWh for the whole 15 days that we were away, even though daily consumption was ~1.6kWh/d over that time.

    So far, so good.

    Rgds

    Damon
  2.  
    Can you correlate the time of day of consumption with your grid intensity report?

    IE work out:
    A) the CO2 that could have been displaced if you had exported at time of generation during the day
    B) the CO2 that was avoided when you drew battery instead of importing at night

    Is the difference +ve or -ve, did you actually save some CO2 overall?
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 31st 2018
     
    In principle yes I can do all that, and it's on my to-do list. Indeed I may compute a much more accurate nominal electricity CO2 for the house for the year in 5-minute chunks, looking at import and export at prevailing marginal intensity.

    The argument about whether it's really a CCGT somewhere that spins up or down in response to marginal changes in grid demand is still there!

    Note also, I'm nominally able to claim some possible savings by simply reducing flows across the grid (~5% distribution network, ~2% transmission network) if that's more than the round-trip through local storage.

    Rgds

    Damon

    PS. Don't tell the rest of my family, but I still harbour an intention to do this "properly" with ~250kWh interseasonal battery storage to kill grid imports more or less entirely!
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 31st 2018 edited
     
    Posted By: gravelldThanks! Any chance you could use colours other than red and green for the graph, I'm colour blind!


    Would it help if I thickened up one of the lines so that you're not solely reliant on colour?

    Like here: http://www.earth.org.uk/_off-grid-stats.html

    Rgds

    Damon
    •  
      CommentAuthorDamonHD
    • CommentTimeSep 30th 2018
     
    See http://www.earth.org.uk/Enphase-AC-Battery-REVIEW.html#Self

    Self-consumption, ie (Net - Import) / Net, where Net consumption = Import - Export + Generation, rose from 48% in August 2017 to 72% in August 2018 (~0.5kWh/d reduced imports). The Enphase was only installed on the 6th, and August is a special low-consumption case.

    September 2018 roughly halved daily imports compared to September 2017 (1.6kWh/d vs 3.2kWh/h), with self-consumption up to 69% from 37%. Some behaviour change and better PV generation may have helped.
   
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