Tesla Powerwall

Post by **T1 Terry** » Wed, 03 Jun 2015, 16:53

mikedufty wrote: Load shifting is what they are marketing them for, isn't it? Not off grid, which means you don't need 24kwh, but also probably means you don't need more than 2kW, as that should be plenty to utilise the 8kwh of storage you have each day.

And if you did go off grid, you probably would need the 24kwh, so would get the 6kw.

How could you load shift the peak power period with only 2,000w available at any one time? Simply turning the kettle on exceeds the supply by 400w, the intended direction for future energy is all electric as gas has out priced itself with both the cost of connection and the fuel cost itself due to a twisting of the actual price facts. Using the selling price as the base cost without deducting the cost to produce or transport the product acts as an inflated price for the product used at home, this goes for LPG and natural gas, but that is some what off topic.
How many kW would be required to power the air con, TV, home entertainment centre, hot water, fridge and induction cook top? That is ignoring all the extras that would be powered up at the same time, like the kettle for a cuppa and the kids computers etc.... how many multiple of 2kW would be required? Simply removing 2kW of energy during the peak period and replacing it with stored solar energy would save roughly $1.02 in GST per hr, for the 6 hr peak period you could save $6.12 per week day, 260 week days per yr, $1591.20 per yr, how long would that take to break even?
Using the same outlay $$ you could build a system that could handle higher peaks for a short term, a 12kW battery pack with a peak continuous 6kW power output and short term 9kW (quality inverter capability) would cover the 1 hr cooking load and have energy to spare for the other 5 hrs of peak power load, a lower initial cost than 3 Powerwall type units and a greater saving potential says this is a far smarter move than a power wall or its equivalent with its pitiful output limit.

T1 Terry

Post by **offgridQLD** » Wed, 03 Jun 2015, 18:35

The way I see it though is if your just (load shifting to save money) So you have the grid to fall back on. Then why would it matter if your system could only supply 2000w. So the fast boil kettle that's on for 2min would have 400w supplied by the grid for that 2 min. Thats 13.3whrs suplyed by the grid and would be payed for at full rates (30cent kwh) (1.33% of 30 cents is 0.39 Cents...less than 1/2 a cent

I can tell you that large spike loads like kettles, toasters, microwaves, hair dryers and the like make up a very very small proportion of a homes daily KWH total.

The big players that accumulate the KWH's (remember thats what you pay for) are fridge (200w), freezer (200w), TV (80w) , Water heating (800 - 1000w..heat pump), lights (100w), computers (100w), All small loads but they add up as they are on for the longest time periods.

All together and on at the same time all the big players in the KWH total would be in the 2000w scope. With load shifting we are trying to shift the bulk of our KWH consumption to a cheaper rate not run fully off grid. Giving you a 5000w continuous inverter for the the small amount of time your making toast and boiling the kettle at the same time to save you 1/2 a cent would not pay it's in saved KWH consumption at full rates.

Kurt

Post by **Richo** » Wed, 03 Jun 2015, 20:28

I'd wait until Toshiba do their SCiB.
It looks like they have already sold MWh to some of the power plants for load leveling.

Post by **T1 Terry** » Thu, 04 Jun 2015, 01:58

Hi Kurt,
I don't think you are being realistic if you think you can load shift 2kW for 6 hrs (6kWh) from the peak period to the off peak period and save enough money to pay for the unit within an acceptable time frame. Generating that 6kWh for free via solar only saves less than $1,600 yr, what price do you expect to get one of these installed for, the saving over 10yrs is only $16,000 and that relies on the solar recharging that 6kWh every week day (no peak load on weekends so no saving)that would require 3Kw of solar to replace the 6kWh in winter as 3 peak sun hrs is being optimistic any where south of the Qld border.
If you are only reducing the cost of the peak load period by recharging during the off peak period, you are only saving 40c/kWh, or $2.40/day, $624/yr, that will take a long time to repay if you include the money costs required up front.

T1 Terry
UMMM.... a coffee later the math/reasoning needs a bit of up dating I think

There is only a max of 8kWh available from the battery pack, the 6kWh over the 6 hr period is allowing a bit in reserve so the cycle life will hopefully equal the repayment period

Post by **coulomb** » Thu, 04 Jun 2015, 02:41

offgridQLD wrote: So you have the grid to fall back on. Then why would it matter if your system could only supply 2000w. So the fast boil kettle that's on for 2min would have 400w supplied by the grid for that 2 min.

Err, if that's the way it will work. To do that, the inverter would have to sync to the mains; OK every grid interactive inverter does that. But it would have to know when not to push out power. If your load drops to 1 kW for a while, and the inverter pushes out 2 kW, then it will be exporting the precious battery energy at the rate of 1 kW (1 kWh per hour), getting nothing to 6 c per kWh for it. Indeed, from a battery, it's probably not *allowed* to export anything. So it has to somehow know what power is being used/exported. As far as I know, no normal grid interactive inverter can talk to your meter to know this. Tesla seem to be implying that you can use your ordinary inverter. But maybe I have that wrong and you have to get a new inverter from one of their partners.

Post by **offgridQLD** » Thu, 04 Jun 2015, 03:02

T1 Terry,
I would have to agree that economical it's most likely cutting it real fine for a domestic application to load shift and recoup costs. I wasn't trying to justify the economics of it (I haven't even bothered looking into /calculating it at all) I'm offgrid so it's of no concern to me.

I like systems that are fully offgrid as you don't pay the connection fee that's becoming substantial now days.Change some of your habits and learn adjust them around the weather a little.

It's the same for a electric car we didn't purchase it to save money just for the self sufficient transport in a SHTF scenario that makes a ICE car less attractive along with the fun factor and health benefits and its the same with being offgrid.

Post by **offgridQLD** » Thu, 04 Jun 2015, 03:16

Coulomb,
I guess what I was getting at it it's not going to be a lights out tripped / overload inverter if you consume more that 2000w at any one time like it would be if your offgrid with that output limitation.

I'm sure its going to require smart inverters that can manage the load sharing between mains and battery for the systems to work seamlessly for the average punter to go about consuming electricity as they please uninterrupted.

Kurt

Post by **Gabz** » Wed, 10 Jun 2015, 17:52

powerwall peak output and steady output increased http://www.teslamotors.com/2015shareholdermeeting

Post by **MDK** » Wed, 10 Jun 2015, 17:54

Tesla held their annual shareholder meeting today, and they announced that the continuous output of the powerwall will now be 5kW, with 7kW peak - not the 2kW/3.3kW as previously announced

Edit: Gabz beat me to it. I must type too slowly

Post by **Richo** » Wed, 10 Jun 2015, 20:38

So all they have to do now is change the cell chemistry, increase the cycle life to 3000+, removed the liquid cooling and sell it cheaper than locally sourced equivalents and their set.

Post by **T1 Terry** » Wed, 10 Jun 2015, 20:52

I have no issue with the cell chemistry if it is combined with liquid cooling that can hold the cell temp below their cycle life reduction point.... roughly 40*C I think the figure was, so 30*C would be a good temp for extended cycle life? Now, combine the liquid cooling with the heat pump for the household water heater and some of that heat energy can be recycled using an extremely efficient heat transfer method.... powered by solar/battery pack perhaps

5kW and 7kW peak brings it back into the viable option field, 2 of the 7kW units in parallel would give a 14kW peak load and 14kW storage capacity, enough to think seriously about going off grid. A 5kW solar system could recharge a drained pack on a sunny winters day, all those figures are within the realms of reality for every house and using mortgage redraw it would be within the reach of many households.... glad I don't have shares in any of the grid based generation mobs

T1 Terry

Post by **Richo** » Wed, 10 Jun 2015, 21:03

Even with a decently design house 10 units per day is a slim margin.

The liquid cooling wont help when one goes into thermal runaway.
Room A/C then wouldn't help it in a house bolted to a wall either.

Think of all the cell chemistries out there the powerwall cells would be near the bottom of the preference list to use in bulk attached to a house.

I just see it being chosen based on price and the convenience of a few bolts attaching it to a wall.

Post by **offgridQLD** » Wed, 10 Jun 2015, 21:15

"cell temp below their cycle life reduction point.... roughly 40*C I think the figure was, so 30*C would be a good temp for extended cycle life? "

I think it's already been mentioned a few times. There isn't a clear cut temperature point it's more of a sliding scale in the calander life of the cells due to how chemicals react to tempreture.Reducing the calendar life by 50% for every 10C increase In storage tempredure (weighted average temp) A brief excursion to 45C has way less effect that extended time at 30C.

I would think it would be pointless trying to extract the wast heat from the liquid cooling system and reuse it. The. Cells are just to eficiant so it would be a minute amount of energy. Lithium Battery's in stationary storage don't get hot from operation at sub 1c rates. Cooling would only help with keeping the thermal mass of the bank to a more stable lower average temp over time fighting the ambiant temps rather than to overcome spikes of heat gain like what is needed in EV doing 100kph up a steep hill pulling 100kw+.

I know my own stationary storage bank just follows ambiant temp (lagging a little high and low from the current ambient temp due to it's thermal mass) the loads and charge rates are just to small running a house.

Kurt

Post by **reecho** » Wed, 10 Jun 2015, 23:12

Regardless if you like or loathe the Powerwall....it is making a difference....

CleanTechnica

Plenty of speculation on what the Powerwall will sell for here. The increase of the onboard DCDC converter at no extra charge is a good move...

Existing PV inverter integration will be the tricky bit.

So what currently exists on the market today that will undercut the Powerwall?

Post by **T1 Terry** » Thu, 11 Jun 2015, 04:19

reecho wrote: Regardless if you like or loathe the Powerwall....it is making a difference....

CleanTechnica

Plenty of speculation on what the Powerwall will sell for here. The increase of the onboard DCDC converter at no extra charge is a good move...

Existing PV inverter integration will be the tricky bit.

So what currently exists on the market today that will undercut the Powerwall?

http://www.blitzability.com.au/ using Jay Whitacre's brain child the salt water battery. It has been very interesting watching the rapid development of this product and the faith the big name backers have in Jay's ability to deliver. To me, this product looks way out in front of anything on the market at the moment in both practicality and functionality... price is still an unknown though

T1 Terry
(Edited by Rob Hills to make the URL clickable)

Post by **Gabz** » Thu, 11 Jun 2015, 04:38

http://www.fusionps.com.au/ told me there where the distributors. they had "show" girls at the energy storage conf. which sorta put me of them, skirts don't make me think a product is good. there where about 3-4 guys all selling these batteries there. in one corner of the expo room it was a bit odd.

Post by **reecho** » Fri, 12 Jun 2015, 02:58

Don't forget Redflow Zinc Bromine Module.

Simon Hackett is a big investor and is also on the board. Also owns multiple Tesla's....

More expensive than Powerwall but has other advantages.

Post by **Rusdy** » Fri, 12 Jun 2015, 16:58

reecho wrote:

More expensive than Powerwall but has other advantages.

Hmmm... I find it hard to find advantage. Maybe temperature range?

Powerwall: -20 to 43
Redflow: 5 to 45

Not to mention it's full of chemical and uglier (not to mention doesn't have Tesla logo on it

68% round trip efficiency compared to 92% round trip for powerwall.

And more expensive.

Uh oh for Simon Hackett.

Post by **lesmando** » Fri, 12 Jun 2015, 21:02

Redflow batteries are most similar to the 7kW/h daily cycle powerwall model. They have about 5KW output, but hold 10kW/h ish of energy.

Redflow batteries were installed in trial homes in Newcastle - Ausgrid Trial rolled a ZBM and an inverter into a unit and called it a R510. They were very successful. They can completely discharge and have a long cycle life. They do have a caveat, when you charge/discharge you need to fully charge/discharge. I don't know the exact details. So not useful on its own, but if 100 houses have them in a suburb, then they can all share on the grid in peak times.

We have a giant Redflow at UQ (container with many ZBM inside) and it has been happily slurping up solar during the day and feeding into the grid at night. So they have a narrow range of uses, but excel in those uses. Maybe the Aquion Energy S-Line Stack would be close competitor that can fully discharge. But it has a 3000 cycle to 70%.

Toshiba have SCiB batteries that have over 90% capacity at 6000 cycles. I am not sure on their voltage or cost.

Les

Post by **T1 Terry** » Fri, 12 Jun 2015, 23:28

Gabz wrote: http://www.fusionps.com.au/ told me there where the distributors. they had "show" girls at the energy storage conf. which sorta put me of them, skirts don't make me think a product is good. there where about 3-4 guys all selling these batteries there. in one corner of the expo room it was a bit odd.

Did you check out the video on the fusionps website, you don't do a video to sell yourself as a state of the art company from the basement of a building showing serious signs of neglect, confused body language and no sign of the product or an example of one in operation. I guess the show girls went with this whole confused mash up of a marketing approach. Maybe it was an attempt to get away from the shiny flash look of many of the companies that seem to float into this "cutting edge Technology" I think he achieved his goal if that was it

Did any of them mention an actual $$ figure for either the "S" or "M" battery pack? I had planned to attend that show and tell conference but I was up to my neck in issues here trying to sorting an RV battery pack using the CTek DC to DC chargers (2) along with a Smartpass and 25 amp mains charger. I made the fatal error of assuming they used the same control system throughout all their products so once I had mastered 1 unit I had the controller sorted for all 4 units

T1 Terry

Post by **coulomb** » Sat, 13 Jun 2015, 14:31

coulomb wrote: Err, if that's the way it will work.

Duh. I think I have finally realised how they probably intend it to work, at least for the load shifting scenario.

I think they assume a common (but not universal) situation of a grid interactive inverter connected to a relatively high voltage string of PV panels, around 400 V. A small or zero feed in tariff is assumed, hence exporting to the grid is assumed to be of little benefit. The power wall would connect across the output of the PV panel, which is of course also the input to the grid interactive inverter. During the day, it would somehow know that conditions are right to draw some of the power from the PV panels to charge the battery. The presumed MPPT in the grid interactive inverter sees this as weaker PV output, so it generates less power during this time. Hopefully, this will be timed so that the battery gets charger mostly when there is excess PV power available, or when mains power is at least off-peak.

Later, either at night or possibly during the day when household demand is greater than PV supply, the power wall exports its power through its internal DC/DC converter. (Note: DC/DC, not DC/AC.) It exports its power through the same wires it imported power from, i.e. the connection to the PV panels and the inverter input. This raises the PV panel voltage. If it's at night when there is no solar input at all, the panel diodes prevent flow of power into the panels. The inverter sees this voltage, and assumes it's solar input. The DC/DC in the power wall will be designed to have a similar V-I curve to solar panels, so the inverter can't tell the difference, and will use all the power that the power wall makes available. So the grid interactive inverter generates power to support the load. Hopefully, it will not generate so much that power gets exported to the grid; this is to support the grid and reduce power imported from the grid.

So, using Kurt's example, if the power wall is set to generate a maximum of 2 kW, and ignoring losses for simplicity, then with a 2400 W load (e.g. typical kettle), then 400 W will be drawn from the grid, where it would have been 2400 W without the power wall. The fact that there is still some imported power doesn't matter much, because eventually the power wall will likely have used all its energy supporting the grid, and from then on, all power consumption will be imported anyway.

With the higher power limit from the power wall, it could cover such peaks, avoiding the importing of power till later, which may be an advantage. For example, the peak might happen before off-peak tariff comes in. Or you might be using less power that night, so it's better to use more of the energy available shaving peaks to zero, rather than leaving energy in the battery so you get less benefit from it.

Because the converter [ edit: was "inverter") in the power wall isn't directly supplying the load, it doesn't have to be big enough to supply your biggest peak load. The grid will supply any deficit. It also means that during a blackout, even though you have energy capable of running your house, you'll still have no power. That would explain why Tesla seem to consider the load shifting and the UPS (uninterruptable power supply) to be so different. I suspect most people don't realise this, even though it's similar to the situation of a daytime blackout: even though the sun is shining and you have plenty of solar power available, without the grid, it all goes to waste till the grid comes back again. (Again, assuming the typical grid interactive inverter installation.)

The power wall arrangement for load shifting sounds a little like the Enginer system for Prius cars. The extra 48 V battery, through a DC/DC converter, supports the hybrid battery, making more electrical power available to move the vehicle, but not necessarily proving all the power needed at any one point of time. Over a typical drive, all the energy from the 48 V battery ends up reducing petrol consumption. If you happen to take only a short trip, then possibly you can't take advantage of all of the electrical energy you have in the 48 V pack. The cheaper and hopefully greener electrical power reduces the use of the more expensive and less environmentally friendly petrol. With the power wall, the excess solar energy that would have been exported at a zero to low price per kWh is stored, and used to prevent importing at a higher cost per kWh.

How this gets implemented is an interesting question. Figuring out when the best time to charge or discharge the battery is a difficult decision. The optimum solution, if there even is one, depends on many factors: SOC of the battery, present power demand, the present cost of grid power, the likely availability of solar power later in the day (weather), and so on. If you have a discretionary load like a hot water heater or a pool pump, then it gets more complicated again; you don't want to run out of hot water and/or you want your pool to stay adequately cleaned. This is such a complex decision that there is a niche market for software that does this properly; it seems to be called Demand Charge Management (DCM). So Tesla are a clever company; they could do this well, or possibly allow some sort of interface to the DCM system of your choice. But that's more than a simple plug and play system; at the very least, it would need access to the current electrical demand of the house. Maybe this could be as simple as a wireless device that your electrician clips over the right cable inside your meter box. I don't know how it would access a weather forecast; perhaps it just ignores that potential source of information.

If this was obvious all along, I apologise for the unnecessary long post.

[ Edit : added a paragraph re size of DC/DC converter; added question at end re weather forecast; DC/DC inverter -> DC/DC converter ]

Post by **Gabz** » Sat, 13 Jun 2015, 17:02

implementation is a huge question because ausgrid, endevour, ergon everyone who owns the poles and wires ban this sort of system. they only allow UPS style battery connections.

ausgrid did trial hybrid systems but I don't think it's approved of outside of the trial.

Post by **weber** » Sat, 13 Jun 2015, 17:46

Hi Coulomb,

Yes, it almost certainly has a single bidirectional DC/DC converter, used in one direction to maximum-power-point track the PV array to charge the battery, and in the other direction to simulate a PV array to the MPPT input of the inverter. However I can't see it working without actually presenting two separate ports to the world, each separated from the converter's single non-battery port by a relay of some kind.

There are no panel diodes to prevent flow of power into the panels. And its hard to see how the Powerwall's MPPT could work in parallel with the one in the inverter and win. And why bother trying when a relay will solve the problem.

At one stage you mention "the inverter in the Powerwall", but there isn't one. You also mention a "DC/DC inverter". As normally understood, this is a contradiction in terms because a power inverter is a DC to AC device. This is the same misleading-marketing-nonsense language used by Elon Musk during the product launch.

The Powerwall doesn't need to do Demand Charge Management, for the simple reason that almost no households have a demand charge. So far, demand charges have been limited to large 3-phase customers, both here and in the US. A demand charge is a charge that is based on your highest power demand (in kW, not kWh, averaged over a 15 minute period) -- the highest during the billing period.

Dealing with household Time-of-use tariffs is a piece of cake, the Powerwall only needs to know the time of day and the times when the rates change. Hybrid inverters do that too. And the Powerwall won't be controlling any discretionary loads. That can be done with simple timers.

Post by **coulomb** » Sat, 13 Jun 2015, 23:42

Ah, my bad. Now I understand the term "demand charge".

I still don't think that optimising power flow to/from the battery is a piece of cake, but you've probably given it more thought than I have.

Post by **coulomb** » Sun, 14 Jun 2015, 00:20

weber wrote: However I can't see it working without actually presenting two separate ports to the world, each separated from the converter's single non-battery port by a relay of some kind.

I think you would not need the relay, which would be a somewhat expensive DC rated component. See below.

There are no panel diodes to prevent flow of power into the panels.

Duh. I should remember that, after all that time we tried to photograph the IR coming from a panel to test it. (We never did get that to work; certain old iPhones are supposed to not have IR filters, so they would see the IR radiation from panels being powered externally like a giant IR LED.) Of course there are diodes in panels, but they are across strings of cells, not in series with them.

And its hard to see how the Powerwall's MPPT could work in parallel with the one in the inverter and win. And why bother trying when a relay will solve the problem.

I think you'd want to be able to charge the battery with part of the PV output, and have the rest supply the load, and that would require the Powerwall to somehow draw power, yet make some available to the grid interactive inverter. I think that appearing as a constant impedance, or possibly as a constant current drain, would do it. The inverter's MPPT would see a distorted VI curve; it would appear that there is a PV with limited current available. It's not clear to me that the result would be optimal for the PV output; I think it might operate the panels at slightly too high or too low a voltage to optimise the overall power. I expect that the error would be slight, though. With a large diode to prevent power back-flow to the panels at night, I think the relay could be dispensed with.

At one stage you mention "the inverter in the Powerwall", but there isn't one.

I meant the DC/DC converter.

You also mention a "DC/DC inverter".

My bad again. I've corrected the post. I think I must have gotten this from Elon's announcement.