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Distributed battery power banking


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Posted (edited)

Similar to the concept of micro-inverters for solar PV, I've been thinking about micro-battery storage to time-shift self generation in a different way. I brought it up in this thread yesterday:

 

@SteamyTea asked about the 200W target I have for a solar PV offline battery storage solution for distribution using Hi-PoE (IEEE 802.3bt). The devices I have in mind, which are powered 24/7, would be the four WiFi access points I use to give full coverage of house garage and garden - each averaging around 25W and powered by 12VDC. The other 100W is accounted for by a NAS, broadband modem and a significant number of WiFi equipped microcontrolers (ESP8266, ESP32) SBC's (Raspberry Pi cameras etc.)  and assorted Amazon Echo devices all operating on 5VDC. There is also some scope for Laptop and phone charging.

 

The idea would be to utilise four 50AH 12V LiFePo4 batteries each with their own floating 240VAC charger but wired in series to provide the 48V PoE supply. This provides 2.4kWh or up to 200W for 12 Hours for a capital outlay of £775, preferentially charged using excess solar PV. This bucks around £0.70 imported power per day with full payback after approx. 3 years. Not accounted for is the hardware required to interface non PoE Powered Devices (PD) but this is an area generally covered by inexpensive off-the-shelf solutions and a little electronic tinkering.

 

Apart from the obvious advantages of having uninterruptable power for all the key networked components, the reason for going down this route is that I can't make a case for an off-the-shelf grid-tied battery system in terms of ROI. And such a system would require additional expensive integration components to make it work offline. If every 240VAC load in the house had to be catered for, the capacity required would be more in the region or 10kWh.

 

On the other hand, all the networked devices represent a distinct group of things that are permanently sucking power and already connected to pre-wired infrastructure in the house and outbuildings. And I can just about afford the outlay!

 

If successful, I would also consider extending this principle to another targeted load - lighting. As the house lighting circuits are relatively easy to isolate and populated exclusively with LED's which will typically operate from 80V~240V (AC or DC as it happens) there is an opportunity to do some more time-shifting with relatively small batteries. Again, 2.4kWh or 20 x 10W Lamps for 12 hours seems to be in the right ball-park. The same ROI argument makes this seem like a no-brainer to me.

Edited by Radian
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38 minutes ago, Radian said:

would be the four WiFi access points I use to give full coverage of house garage and garden

The hotspot in my cheap Motorola phones has a fantastic range. Not sure how long the battery would last moving data about, may give it a go oneday.

The reason I mention this is that they are low powered, have battery storage, charge controllers, screen and networking via USB built in.

And for 20 quid a month 100GB of 4G connectivity.

So for a grand, you get 50 months of usage from an old phone.

The NAS can be a large SD card (trying a 256 GB one in my camera at the moment).

Just a thought.

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I jumped from owning a succession of Samsung Galaxy Notes when their prices got plain silly - to a Moto G Pro with a pen. Really impressed with the value for money. However, a whole house, wired ethernet backbone is non-negotiable for me.

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Do "110-230VAC" universal input inverter driven fridges exist that'll actually run ok on DC?

 

That's the other big hotel load. 

 

MVHR units with LV EC motors that should by right run on DC?

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6 minutes ago, markocosic said:

Do "110-230VAC" universal input inverter driven fridges exist that'll actually run ok on DC?

 

That's the other big hotel load. 

 

MVHR units with LV EC motors that should by right run on DC?

 

Good point. Probably. Our LG direct drive washer dryer has a SMPSU delivering a number of DC rails (I know because I fixed it a few years ago). Unfortunately it uses 240VAC to heat the water (Oh for the days of hot & cold feeds) and the dryer element. The latter is crying out for some ducted hot air (AKA @Onoff's soda can contraption).

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Even if they won't do it...is it worth inverting back up to AC for the fridges?

 

Charge at 90% invert at 90% ?

 

That's 1/(0.9*0.9) = 123% of original energy use if you're time-shifting the grid via a 90% efficient charge (and invert) and a 90% efficient discharge (and invert) 

 

If you're consuming at PV rates you don't care. Even if consuming at grid rates it's probably worth having if you had ToU with a big enough arbitrage opportunity.

 

Peak startup load on an inverter fridge?

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Posted (edited)

Fridge/freezers are certainly good candidates for solar PV as their consumption peaks on the hotter days. They can even be slightly delayed/advanced to 'bank' a certain amount of energy before the Ice cream gets too mushy/hard. The gains would probably be fairly marginal from this approach though.

 

Over a 24 hour period my fridge/freezers typically consume 3kWh (units) costing roughly £300pa at present rates:

 

image.thumb.jpeg.d5b7b89565975ab086434763b64626bb.jpeg

  • (amusing how all the three units fall into sympathetic resonance despite being in separate kitchen cabinets)

 

While I couldn't justify the expense of replacing them with inverter refrigerators, my current units are fairly close to the 2.4kWh target I've set here so yes, an "islanded" 240VAC inverter could work out.

Edited by Radian
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2 minutes ago, Radian said:

Over a 24 hour period my fridge/freezers typically consume 3kWh (units) costing roughly £300pa at present rates:

Have you thought about putting some EWI on the appropriate bits.

 

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1 hour ago, SteamyTea said:

Have you thought about putting some EWI on the appropriate bits.

 

I suspect the biggest inefficiency is with heat transfer between the external condenser loops and the inside of the fridge. No easy way of increasing the insulation there I'm afraid. It would be fantastic if you could relocate the condenser somewhere useful e.g. laid flat on the floor under the kitchen cabinets, with a fan to provide a kick-panel heater in the winter.

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Hmmm.

 

 

Who on earth designed these energy efficiency ratings?

 

 

Fridges...

 

(D) Liebherr 390 L for 98 kWh/yr @ £1079

https://ao.com/product/k4330-liebherr-comfort-fridge-white-73722-30.aspx

 

(E) AEG - 404 L for 121 kWh/yr @ £700

https://ao.com/product/rkb638e2mw-aeg-fridge-white-76030-30.aspx

 

Ok - seems legit - but then we get this...

 

(E) Beko - 101 L for 137 kWh/yr @ £199

https://ao.com/product/ur4584w-beko-fridge-white-76186-30.aspx


How is that even in the same league? Since when is 1/4 of the usable volume for the same energy consumption a sensible comparison?

 

 

Freezers...

 

(E) Liebherr 276L for 250 kWh/yr @ £1250

https://ao.com/product/agb728e2nw-aeg-upright-freezer-white-77126-34.aspx

 

(E) AEG 280 L for 248 kWh/yr @ £760

https://ao.com/product/agb728e2nw-aeg-upright-freezer-white-77126-34.aspx

 

Less difference when it comes to freezers it seems...

 

(F) Beko 86 litre for 226 kWh/r @ £245

https://ao.com/product/uff584apw-beko-under-counter-freezer-white-26887-35.aspx

 

Again LOL at the utter hopelessness of the "efficiency" ratings though at least this one scores an F rather than an E.

 

Interestingly the "fat upright" white Beko freezer nails them all on "litres per kWh per year" beating even the freestanding chest freezers on this metric:

 

(E) Beko 404 L for 290 kWh/yr @ £629 - but it's 70 cm wide and 75 cm deep

https://ao.com/product/ffep3791w-beko-upright-freezer-white-78914-34.aspx

 

 

Then fridge freezers...

 

(D) Liebherr - 268/103 L for  203 kWh/yr @ £599

https://ao.com/product/cnd5703-liebherr-fridge-freezer-white-92694-28.aspx

 

(E) AEG - 230/94 L for 248 kWh/yr @ £570

https://ao.com/product/rcb632e5mw-aeg-fridge-freezer-white-77129-28.aspx

 

Ok - there's more in it again here - and a couple of absolutely spectacular ones from Samsung:

 

(A) Samsung - 273/114 L for 108 kWh/yr @ £1350

https://ao.com/product/rl38a776asr-samsung-bespoke-fridge-freezer-stainless-steel-85149-28.aspx

 

(C) Samsung - 276/144 L for 169 kWh/yr @ £729

https://ao.com/product/rb38t602cs9-samsung-rb7300t-fridge-freezer-silver-76991-28.aspx

 

How are they managing that? Certainly the way to go if starting from scratch.

 

 

I wonder if it's worth retiring an A+++ rated fridge freezer from 9 years ago? Not at all it seems. 221/91 L for 158 kWh/yr (if the tests are comparable) and £529 each still isn't terrible. I should check their real world consumption when I get roundtuit. 

 

https://www.appliancesdirect.co.uk/p/kg39eaw40g/siemens-kg39eaw40g-iq100-freestanding-fridge-freezer

 

 

An experiment that's fun - try loading the fridge / freezer with water as they are emptied. Keeping an "I wonder what the load capacity of this shelf is" amount of bottled water in a fridge helps extend the cycle time and reduce start/stop losses I'm told.
 

 

 

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Sticking the freezer in an unheated garage and ensuring decent ventilation behind the things is the best we can probably do. MVHR extracts just above/behind the fridge/freezer perhaps?

 

beersnow-700x325.jpeg.662667b505aee413a4747e1f11c5511a.jpeg

 

 

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1 hour ago, markocosic said:

Who on earth designed these energy efficiency ratings?

 

Clearly an idiot.

 

If I were to replace my 25 year old fridge and two freezers with up-to-date units it would potentially reduce their annual electricity consumption by about 66% saving around £200 each year. Substantial, but at current like-for-like replacement costs that's only going to break even after 10 years or so. Better, I think, to see them until the end of their natural life - or mine - whichever comes first. 👻

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I have a vague recollection that the energy rating on a fridge, or freezer, is a moving target. So what was A++ 5 years ago will use more energy than an A++ today.

Think our @DamonHD knows a bit about it.

 

Small units are always going to perform worse than large units. The volume to surface area is against them.

 

My cheap fridge uses about 10W I think. So about 90 kWh a year. About £25/year on my E7 mix.

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3 minutes ago, SteamyTea said:

Small units are always going to perform worse than large units.

Great, so my big American type fridge freezer is green? 🤔

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Are the commercial solutions now economic given the increase in marginal unit rates / decrease in costs as shenzhen grade kit comes on the market?

 

£800 AC coupled inverter / £800 for 6000 cycles @ 2 kWh / 1 kW(p)

https://midsummerwholesale.co.uk/buy/sofar-storage/sofar-me3000sp

https://midsummerwholesale.co.uk/buy/sofar-storage/sofar-me3000sp-us2000-type-c-kits

 

Charge @ average 250 W for 8 hours; discharge @ average 125 W for 16 hours; bank £0.50 / day through "screw displacing gas fired generation; play the broken electricity market using batteries and PV instead" game?

 

£18/yr. Will it last 10 years? 15 years? Perhaps; if the peak charge / discharge rates are limited? Is it Depth of Discharge or the cycle count or the charge/discharge rates that kill LFP?

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You may already know this @DamonHD - do these cheapies do an acceptable job of sensing the minimum energy buckets?

 

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

 

Or might they be better connected as an "interruptible" power supply?

 

i.e. You manually pull the incoming mains once the sun goes down to force 100% of load connected to their "Emergency Power System" output over to battery; then manually flip it back on again if the "Emergency Power System" goes down (i.e. battery discharged unexpectedly early)

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1 hour ago, markocosic said:

Are the commercial solutions now economic given the increase in marginal unit rates / decrease in costs as shenzhen grade kit comes on the market?

 

£800 AC coupled inverter / £800 for 6000 cycles @ 2 kWh / 1 kW(p)

https://midsummerwholesale.co.uk/buy/sofar-storage/sofar-me3000sp

https://midsummerwholesale.co.uk/buy/sofar-storage/sofar-me3000sp-us2000-type-c-kits

 

Charge @ average 250 W for 8 hours; discharge @ average 125 W for 16 hours; bank £0.50 / day through "screw displacing gas fired generation; play the broken electricity market using batteries and PV instead" game?

 

£18/yr. Will it last 10 years? 15 years? Perhaps; if the peak charge / discharge rates are limited? Is it Depth of Discharge or the cycle count or the charge/discharge rates that kill LFP?

 

Sofar so good - what is it with these Chinese names? (often have the word fire in them somewhere)

So inverter battery charge control for £800 then add another £800 for a 2.4kWh battery. Well, the numbers match with my best-buy battery deal but the electronics still have room for improvement. However, it's real (unlike my imagined circuitry) so yes I think we're getting there!

I think I'd prefer to keep it in an outbuilding though. That's actually what I had planned for my own attempt 🧨🔥

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Definitely not indoors!

 

You almost want graded supplies for the house:

 

"Hotel load, non-discretionary/interruptible" (fire alarm, house alarm, etc?)

"Hotel load, kinda-discretionary/interruptible" (fridge, sewage system, etc?)

"Other load, don't bother trying to shift"

 

Select sockets fed from a dedicated MCB; itself fed via a widget in a discretionary outbuilding or under an awning outside etc.

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3 minutes ago, Radian said:

think I'd prefer to keep it in an outbuilding though

You are near Bovington, could be fun scaring the neighbours with a midnight pyrotechnic show.

 

My Kindle has Fire in the name as well.

Who will be the first EV maker to use they word, Fiat.

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Posted (edited)
1 hour ago, markocosic said:

might they be better connected as an "interruptible" power supply

One of my assignments when studying renewable energy was to design a system that supplied power for 3 days.

I designed a cascading system.

Basically anything could run for 4 hours, then stuff was disconnected, big loads first. So no DHW or heating, then oven, then freezers and entertainment, eventually just left with lights, I considered lighting to be a safety issue.

Costed it out at around £60k using lead acid technology and islanding inverters.

Class mate if mine found a biodiesel generator, I considered that cheating.

He got better marks.

 

He used to get better marks in the Mathematic assignments, and I did them for him.

Edited by SteamyTea
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How tightly they can control import/export.

 

Newer leccy meters have 1 Wh "energy buckets" that will increment the import or export registers. They need tight control of battery charge/discharge in order not to be constantly importing/exporting and therefore arbitraging in the wrong direction.

 

Many chargers err on the side of "always exporting a bit" in order not to accidentally import. Getting import wrong costs 30p, getting export wrong only 5p for example.

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Interestingly, while the supply meters say my Enphase is doing that job relatively well, my Eddi metering dsiagrees quite substantially, though does its own job reasonably well.

 

(This is the old banking joke about if you don't want to have to do reconcilliation, don't record more than one stream of numbers.)

 

I'm starting to assemble side-by-side streams here to enable better comparisons:

 

https://www.earth.org.uk/energy-series-dataset.html

 

So still can't help you I think, though I am prepared to add sponsored equipment under test in principle!

 

As to battery life, other than abuse (pushing to close to or beyond limits), I basically assume both calendar-life limits AND effective full-cycle limits (whether taken piecemeal or not) for both LA and Li chemistries.

 

But what do I know?

 

Rgds

 

Damon

 

 

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