Tesla deployed over 300 Powerwalls in Hawaiian schools to cool down classrooms | Hacker News Reader

Tesla deployed over 300 Powerwalls in Hawaiian schools to cool down classrooms

electrek.co

189 pointsdoener102 comments

This perhaps says a lot more about the state of the Hawaii power grid than the benefits of the Powerwall.

Due to economies of scale, unless your grid infrastructure is really poor, grid attached energy storage is going to be cheaper than distributed energy storage. Tesla just wants to sell batteries, and consumers are easier to target than utilities.

See my other comment with links down thread, but HELCO (and other Hawaiian Electric Utilities) no longer support net metering, because it is not profitable (enough) for them. So they sell retail at $0.30-0.50/kWh and buy at $0.10/kWh making a lot of people upset and crashing the solar market in the islands. That makes battery storage cost effective where standard installs are much less so.

That's really expensive electricity! I assume there are obstacles to developing more power plants in Hawaii?

Mostly because each island is its own small grid. Everything is fossil fuel, no hydro, little wind, no nuclear. Cost of transporting fuel. Environmental concerns about air pollution (eg: no China style cheap coal power plants).

Kauai (home of one of the rainiest places on Earth) has had hydro power for decades. It's currently only a small percentage of the island's total generation capacity, but it absolutely exists: http://website.kiuc.coop/content/hydroelectric

It's always been strange to me that an island with abundant solar and geothermal resources relies on imported oil for most of their power generation.

https://www.eia.gov/state/?sid=HI

"In 2015, Hawaii imported 91% of the energy it consumed, mostly as petroleum."

They seem to be working hard on it at least, although I'm surprised that only 20% of the renewable energy production is geothermal. I guess it's just more expensive than solar nowadays.

Only one island has geothermal and it's not the main island.

Iceland is exploiting the hell out of it's geothermal energy, attracting energy intensive industries like aluminium smelters and bitcoin mining.

Doesn't the Big Island have similar levels of available geothermal resources?

Is the difference that Iceland is an independent nation while Hawaii's decisions are made in Washington DC?

A long time ago they started using geothermal energy but some poisonous gas was released. So they shut it down permanently.

Source?

Most of the population lives on Oahu, which doesn't have any volcanic activity anymore as far as I know. I would be a little curious as the why the Big Island doesn't do geothermal, though. If Hawaii could get cheap geothermal power I don't see why Washington DC would block it, I don't think the power generation companies there are controlled federally but I don't know.

As others have noted, outside of Hawai'i and maybe Maui, the islands aren't active (Haleakala is thought to have a few more eruptions left in it). Most of the people live on Oahu, although pretty much everyone else in Hawaii lives on Hawai'i and Maui. I don't know what the residual heat is on Oahu and the other islands; there might be enough to drive geothermal energy, but it's certainly not a renewable resource in those circumstances.

One thing that people haven't mentioned: geothermal energy relies on groundwater or injected water to extract electricity. Hawaii doesn't have particularly plentiful groundwater reserves; and the rock is generally porous enough that there is some concern for the leaching of toxics into the water supply.

Don't forget H-power, Hawaii incinerates trash on Oahu and has been pretty successful with it.

In theory, HVDC cables could be installed to transfer power between the islands (see the recent submission https://news.ycombinator.com/item?id=16452926).

>Due to economies of scale, unless your grid infrastructure is really poor, grid attached energy storage is going to be cheaper than distributed energy storage.

What about transmission loss? Distributed energy storage would presumably be more efficient in that regard.

Why does this project need battery storage? The power demand for air-conditioning during the day follows the supply of solar. Is it to cater for hot but cloudy days?

You can get very hot and humid days when it's overcast.

Yes, with complete overcast even a very large PV array such as quantity 80, 370W high-efficiency 72-cell panels, will not produce more than 3 to 5kW. In bright sun it'll be well above 23kW. Under STC (standard test conditions) that's a 29.6kWh array...

what you should care about is your kWh/month produced.

I know electricity is very expensive in Hawaii since it's almost all natural gas. Maybe they charge the batteries off school hours to save on the electricity bill?

For large loads, at $0.32 to $0.50/kWh in Hawaii it is now much more economical to disconnect from the grid entirely, unless you have some limiting factor that would prohibit large scale low cost mounting of a huge number of 1.65 x 1.0 (60-cell) or 1.99 x 1.0 meter (72-cell) size high efficiency PV panels. The ROI can be well under 6 years.

Sure, but if you can save up extra electricity from your pv when you don't need to cool the building, it doesn't hurt.

Does Hawaii allow you to sell electricity back to the grid, or it that prevented by the local electricity monopoly?

What do we mean by "high efficiency" PV panels in the context of commercial roof tops ?

AFAIK thin film are around 10%, standard PVs around 20% and the multi junction are 30% but cost x10 for special uses only..

In this context I am not referring to lower-cost, but lower efficiency thin films, nor triple junction or concentrator type cells...

High efficiency = 5W STC rating per 156mm cell or better. More W per square meter. For example you can get cheap 72-cell panels that are rated at 320W, the best ones which are exactly the same dimension will be rated at 370, 375 or 380W. That is for typical modules which measure exactly 1.99 x 0.99 meters.

In large-scale solar, but not large enough to use a huge land area and low-cost thin films, there are two standard sizes of panels. 60-cell 1.65 x 0.99m and 72 cell 1.99 x 0.99 meter. These are standardized to work with a wide variety of different mounting systems.

There are many different grades and quality levels of polycrystalline and mono crystalline silicon solar panels.

The very best monocrystalline Si panels are made by Sunpower, but they have a significant price premium as compared to a pallet load that I could buy right now with my Visa card of qty 20, 370W panels. The commodity 370W are 65% of the price but only a few % less efficient.

Your numbers are outdated. Thin film = 23%, multi-junction are 46%.

"Standard PV" is probably what's available in bulk and cheap, is around 22-25%.

https://upload.wikimedia.org/wikipedia/commons/0/01/PVeff%28...

Those are best in class research cell efficiency for each technology type. The silicon cell based panels you can buy now are 17.5-22% unless you are spending a lot of money.

Special thin films cells in a lab may be 23%, what you can buy economically for a grid scale utility power plant is more like 14-15%.

This is at the very top end of the thin film market for what is now commercially available, and is 17%. See datasheet.

http://www.firstsolar.com/en/Modules/Series-6

I thought oil was used in Hawaii?

https://www.washingtonpost.com/graphics/national/power-plant...

Sorry, you are correct.

So where is the cost break even point?

You can't run air conditioners directly off solar. PV panels natively produce DC. There are units with microinverters built into the back of the panel that produce AC. Even in a sunny climate clouds will pass in front of the sun. Unless you have a massively oversized PV array, you can't just run the DC array's output into a bank of sinewave inverters and then straight into the input of multiple 2 to 5kW load air conditioners.

I'm assuming here that a typical AC unit will look something like a 12000, 18000 or 24000 BTU/H LG, Daikin or Samsung split type, ductless, cooling-only unit.

The cumulative kWh/day and kWh/month produced by your PV is what you should care about. As compared to what your load will consume in the same time period.

For off grid, which now economically makes sense in much of Hawaii because fossil-fuel powered grid power can costs $0.32 to $0.50/kWh.

You need a battery buffer so that your load (the air conditioners) doesn't brown out and fail when the a few clouds pass in front of your PV panels. At a given time when it is a little bit cloudy out you might have 12kW of load but even a very big PV array might only be producing 4kW.

The load is constantly running off the battery and inverter system, while the PV panels and charge controller are constantly doing their best to keep the battery full.

You used the word PV a lot but never said what it means. For anyone wondering, PV means photovoltaic.

https://en.wikipedia.org/wiki/Photovoltaics

Yes, I forget that not everyone know the term. Work with solar on a regular basis and the whole industry is just as full of acronyms as x86-64 hardware.

Any developed industry has it's own terms and language - healthcare, military, government, programming, finance...

Always fun learning new languages inside languages.

I’d also assume that you get weekends to fully charge to give you a reserve buffer.

Coming from Europe, I've never seen an AC unit expressed in BTU/hour. To me it seems much more logical to express it in kW.

The only "strange" (but correct) notation was on a french train, the AC was expressed in kcal/hour

Then you've probably never had to plan cooling for a server room. BTU's are often the only metric you get from the servers datasheet.

I find that these are occasionally very inaccurate. If possible put a prototypical server on a watt meter and artificially load cpu, disk and ram to determine its real world max wattage and thermal values. Then compare to what its wattage is while idle.

At the scale of a server room, you're estimating over an aggregate and probably want some overhead/room to expand.

At the scale of a small to medium colocation facility you define W per cabinet... Such as a typical 44U height cabinet in a row designed for front to back airflow. 5kW per cabinet would be a normal low range, up to 10-12kW. Then figure out your floor spacing and sq ft usage to determine how many cabinets.

"Then you've probably never had to plan cooling for a server room."

For an American one you mean

i'm german, so no.

but i admit that its incredibly rare to find localized data sheets. they're generally en_us, not that it really matters.

1 watt = 3.412 BTU/hr.

Even British people don't use BTU

Every AC I've bought in the UK (5 so far) have been rated only in BTU.

IANAEE but would it be possible to use a DC compressor and rectify the mains, instead of using an AC compressor and inverting the PV?

Yes, but 3-phase AC motors require less maintenance and are more efficient than brushed DC motors. Most traction applications are now using AC motors with inverters instead of DC.

Powerwalls only output single phase AC, not 3-phase.

Brushless DC motors actually have an awful lot in common with 3 phase motors, though, in terms of operating characteristics and performance.

You theoretically could, but then you would have a special weird setup that doesn't match 99.9% of the similar installation base of air conditioners on the same island. Making repairs, maintenance and such very costly. You generally want to generate usable AC power from any significant sort of solar system, so that you can use it for more than just air conditioners.

I think we need smarter ACs, not more batteries. A smarter AC would just continue to run with reduced output when there is less available power.

Is that not exactly what a brownout would cause, as GP alluded to? (I don't know the effect of low voltage on A/C so I'm guessing here...)

I don't think a brownout would cause the desired effect (GP said "brownout and fail"). But what you could do is automatically switch off a selected set of A/C units when power output is low. That way you can prevent a brownout (drop in voltage), while still having some cooling. Of course, the A/C units should be designed such that frequent shut-offs are not a problem.

Anyway, I'm no expert either but I think the approach should consider the whole system (PV + A/C) rather than view them as different domains.

Modern mini-split heat pumps are inverter driven and can step up/down their power consumption and cooling capacity pretty much in 1% steps — at least that’s what the marketing material claims.

Normally the cooling output is a function of the set point temperature vs current ambient and the fan speed.

So I’m not sure what you would need to add to allow the units to operate/respond to variation in available power from the panels but certainly they are able to operate at variable output levels.

> You can't run air conditioners directly off solar. PV panels natively produce DC. There are units with microinverters built into the back of the panel that produce AC.

Why can't an air conditioner run off of DC? Just lack of availability?

Can you run an electric motor on DC power?

Yes?

https://en.wikipedia.org/wiki/DC_motor

I think he meant more ‘can you run an AC motor on DC power’ (benefit of the doubt), since almost all AC units sized for residential/business operate on AC power (to my knowledge?).

Modern ACs are powered by AC, but they run IGBT inverters to power a brushless DC motor. So you could build ACs to run on straight DC, but they would be a special application (so expensive initially).

There are 48v DC air conditioners, again though you typically need a buffer with solar, either batteries or grid unless your way over paneled and even then you might get a cutout if a cloud goes over.

It's implicitly in your post, but basically a battery is a lot cheaper than a ton of extra solar panels since you can use your full day of solar during the peak period.

Tesla's latest thing with powerwalls is to build "virtual power plants". This means the batteries are all under central control and can absorb extra power if wind/solar/nuclear are providing more than needed and later reduce load by running from battery when loads are higher.

In this case the schools probably just provide a convenient set of distributed locations, owned by a single authority. The money they earn from providing this service can be partially kicked back to the schools to offset air con costs.

a) PR b) From what I understand, Hawaii's power utility either doesn't allow or strongly discourages new net metering customers (e.g. where excess generated power goes back to the grid and you get credited). Accordingly, on-site battery systems make sense for most PV solar installs to help deal with the difference in generation and usage over time.

Net metering is no longer supported in Hawaii, Several different programs have been implemented that are more profitable for the utility. As a result, without batteries and self-supply it is no longer nearly as cost-effective to install solar even at $0.35-0.50/kWh. The utility will only buy at ~$0.10 and sell at 3-5x that rate.

https://www.greentechmedia.com/articles/read/rooftop-solar-i...

https://www.hawaiianelectric.com/clean-energy-hawaii/produci...

They're not running the air conditioners directly from the solar. They're using solar power to offset the energy cost of the AC.

Nowhere did they say what Powerwall actually is, and I’d never heard of them. Powerwall is a battery made by Tesla to store excess solar energy.

https://www.tesla.com/powerwall

This "article" reads like an ad.

There is probably a fund raising event coming up.

Tesla is a public company, every day is a fund raising event.

It certainly does with the Tesla video ad.

As a Tesla fan, I dislike the writer for this site. He often makes any Tesla failure sound like a success, and everything reads as heavily biased

Worth noting that the author is /u/FredTesla on reddit, moderator of /r/TeslaMotors as well as the founder of Electrek. His career is essentially built on being a Tesla fanboy.

There are dozens of battery companies creating storage units, but Tesla are always superb with their marketing efforts to brand...in this case, batteries as 'powerwalls'.

This is the 'Hoover' logic of making a generic device synonymous with a brand name.

Whatever else you might say about Musk and company and their various ventures, they are actually solving real problems, with real innovation.

NOT a bunch of talk about "too soon" or "focusing on core competencies".

I wonder if the power budget in Hawaii would permit the other kind of air conditioning, where you soak down a large water thermal mass, and then use it to do the chiller side. I know nothing is free, but its a model which quite a lot of places are moving to, in search of a different pattern of energy usage. Hawaii might be one of the hard cases: I believe it has really high, and until recently diesel backed power and this kind of off-peak power may simply not exist.

It's a complementary technology to this use of powerwall. it's not an either-or thing.

Isn’t that form of air conditioning a non-starter in humid climates like Hawaii?

Mostly, yeah. Common term for them on a small scale is a swamp cooler.

I interpreted the parent post as talking about chilled water thermal energy storage as opposed to evaporative cooling.

Yes. its not evaporative cooling: you have a watermass you chill down with offpeak power, and then use as the coolth source to chill dry air in the aircon, rather than using a normal fridge chiller circuit. Its power-shifting. The total powercost is probably comparable to on-demand chilling but you shift the power budget to off-peak, or other sources.

Does anyone know why Hawaii doesn't utilize more geothermal energy? There are a few hits for geothermal projects, but I would have expected these to be much more popular.

Oddly, only 20% of just the renewable generation is geothermal. They might be including biofuels though, and since their generation is dominated by oil right now that might be attractive just for compatibility with their existing infrastructure.

But yeah, this confuses me too.

It upsets pele. I'm totally serious. Google for why do native hawaiians oppose geothermal.

This sounds odd. When I spent some time in Hawaii, houses had a gap between the top of the walls and the roof, to let the breeze blow through the house. While being impossible to air condition such a structure, it was not needed. The ambient temperature with the breeze was nearly always comfortable.

Does it seem unusual that it's costing $100mm for 2300 classrooms, which is about $43k each?

How many powerwalls per classroom? Solar? Electrical upgrades?

https://www.tesla.com/powerwall

"Your final design and pricing will be based on your electrical panel, home energy usage, number of Powerwalls, and where you’d like your Powerwall installed. Typical installation cost ranges from $800 to $2,000. This does not include solar installation, electrical upgrades (if necessary), taxes, permit fees, or any retailer / connection charges that may apply."

I assume shipping would add a HEAFTY cost in Hawaii as well.

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