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Published on September 28th, 2019 | by Danny Parker

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Electricity Resilience In Florida: Hurricane Dorian vs. Tesla Powerwall

September 28th, 2019 by  


As a researcher at the Florida Solar Energy Center (FSEC) in Cocoa, Florida, I’ve lived with hurricanes my entire professional career. Each year is like a bad lottery since Florida is a hurricane magnet. Given the paths of past hurricanes, there’s plenty of reason for concern. Living in the state in September suggests a shooting gallery experience — with these hits by major storms, but a lot of near misses.

While the annual numbers of Atlantic hurricanes is highly variable (more in La Niña years) and has not increased statistically in the satellite era, the intensity of the storms each year is growing. Based on NOAA data from the National Hurricane Center, the number of major hurricanes (Category 3–5) has grown significantly. The number of major hurricanes in the Atlantic basis has grown by an average of 0.45 (+0.35) storms per year each decade — a statistic that is significant beyond a 95% confidence level. And I’m far from the only one to make this observation, as the scientific experts in tropical cyclones, such as MIT’s Kerry Emanuel, find undeniable evidence that the strength of these storms is increasing as global warming extends to the world’s oceans.

Atlantic Hurricanes and their intensity in the satellite era

Atlantic Hurricanes and their intensity in the satellite era K. Fenaughty, FSEC)

Indeed, as I write this, there are no fewer than four tropical systems boiling up in the Atlantic.

The year 2019 has not been kind so far. Hurricane Dorian was a massive killer storm. It is tied with the 1935 Labor Day hurricane as the most powerful Atlantic cyclone to make landfall in history. With sustained winds of 185 mph that gusted to over 220 mph, it was by far the strongest hurricane to strike the Bahamas. Although the grim job of combing through the wreckage left behind in Abacos is still ongoing, it has evidently killed hundreds and perhaps thousands there (many are missing and presumably swept out to sea). The humanitarian disaster there is nearly unfathomable, and I encourage readers to help.

Dorian has deeply scarred the islands for decades to come. And each year we seem to have another such disaster. In 2018, it was Category 5 Hurricane Michael that slammed Mexico Beach near Panama City. In September 2017 it was the devastation to Puerto Rico from Hurricane Maria and and a similar beast, Hurricane Irma, that blasted southwestern Florida and left 7 million without power. One year after another, Florida is under threat from hurricanes, and not only Florida, but Louisiana and the Gulf region as well as the Carolinas in the South.

During my career, I have specialized in studying how to greatly reduce overall residential energy use and especially space cooling needs in Florida’s tough hot-humid climate. I have a white metal roof, low-conduction solar control windows and good insulation, efficient lighting and appliances, and solar hot water. I’ve also written extensively about our energy situation in Florida and some of the things homeowners can do who wish to trim back their consumption.

And beyond the personal level, Florida will constantly face hurricanes each year, as well as the long-term future threats posed by sea level rise from global warming. But there are things we could do personally as well as collectively. The CleanTechnica series I wrote two years ago about the unique energy challenges in Florida presents several solutions.

For the last 10 years, I’ve had a 6 kW solar electric photovoltaic or PV system on my roof. Each year, it reliably pumps out about 23 kWh per day and sends it back into the grid. Generally, I produce about as much power as I use over the year, which mostly zeros things out with net metering. The problem has been that when a hurricane comes through and knocks down the grid, I have no electric power, just like everyone else, even though the solar electric system is operating fine. In spite of five hurricanes interrupting power over the years, it has never skipped a beat — completely reliable operation. That’s because the inverter properly shuts down when there is a power interruption to protect linemen working to restore power.

Solar home in Cocoa Beach: Solar hot water, PV pumped pool, and 6 kW PV system.

Our home in Cocoa Beach: Solar hot water, PV pumped pool and 6 kW PV system.

Not surprisingly, a large complaint in the wake of Hurricane Irma was that individuals who had installed solar electric PV systems were unable to use that solar power when the grid was down. As I’ve covered before, the PV systems themselves almost universally survived the storm. Generally, homeowners have spent thousands of dollars for a PV system, but even when the storm clouds parted and the sun came out on the rain soaked streets, they were unable to use the electricity being generated by the PV modules. This problem is seen with all grid-connected PV systems without battery backup. While off-grid systems with battery backup work well when the grid is down after power interruption, the vast majority of the roughly half a million PV systems in the U.S. are net-metered. However, this problem does not extend to all grid-tied systems.

Some of the newer SMA “Sunny Boy” inverters feature its Secure Power Supply (SPS). This is an add-on component that allows two 120 volt plugs from the inverter during a power outage that provides up to 16 amps of loads (2000 watts). Post Irma, many of those in Florida with these systems were able to plug in refrigerators, phones, and fans when the sun was up. A key recommendation to those installing net-metered PV systems in Florida is that these systems should be preferred unless battery backup systems are installed. Currently, the SMA inverters with this feature are the TL-22, TLS-US, or US-40 inverters. Also recommended: “How to explain Secure Power Supply to homeowners.”

danny parker houseAfter Hurricane Irma we were with without power for 6 miserable days without refrigeration, cooling, or lights. Being without electricity for most of a week after Hurricane Irma last year, I finally decided to do something about this ridiculous situation. This year, I installed two Tesla Powerwall 2 battery systems in the car port so that in the event of power interruption, I can still use the batteries and the PV system to potentially operate the house and main power systems, assuming that the roof and PV system survives. In winter 2019, I obtained bids for two Powerwalls to be installed at my home. A very professional crew from Tesla Orlando came on 22 May to install the system. This went without a hitch and it was all installed in a single day. The total cost turnkey was $18,100, which included two Powerwall 2s, all wiring, the gateway with the inverter, control software, and the full installation. (Because everyone will want to know, the cost of installing a single Powerwall 2 with all the rest would have been $11,600.)

Yes, you can install a generator for $1,000 or less, but as seen in Puerto Rico in October 2017, what happens when you no longer have gasoline easily available? There will be long lines at best. Welcome to the brave new world of 21st century climate change in the Southeastern U.S.

Installation of two Tesla Powerwalls

Installation of two Tesla Powerwalls on May 22, 2019.

The system includes 27 kWh of electrical storage that essentially islands the power within the house when the grid is interrupted. This means that both the PV on the roof as well as the full storage can be used in the case of a major power interruption. Because my home is a very efficient one (using two low-wattage mini-split heat pumps for cooling), I was able to tie in the entire home electrical service to the Powerwall so that theoretically it would be possible to operate the entire home in the event of a power outage. But would it work?

Solar home in Cocoa Beach: Solar hot water, PV pumped pool, and 6 kW PV system.

Our home in Cocoa Beach: Solar hot water, PV pumped pool, and 6 kW PV system.

My idea was that this hurricane season I would use the close approach of a tropical depression during August or September (virtually assured in Florida) as a way of testing the system. My plan was to set the Tesla Powerwall to self-powered (an available mode) at the height of the tropical depression and then see if we could go for 5 days without grid power. Needless to say, I had no idea that Hurricane Dorian would alter my idea of a simulated test to an active threat from a monster hurricane. Be careful of what you ask for. What took place was not what I would have wanted — or even close. Instead, we had to contemplate leaving Cocoa Beach altogether, leaving the Tesla Powerwall to provide power in the event of an outage and keep the house systems operating. That was assuming the house survived.

On Wednesday, August 28, while the ferocious storm pummeled the Bahamas, I disconnected the second refrigerator in the garage (which uses about 3 kWh per day in the hot summer). We transferred contents to the main refrigerator. I then left the central mini-split set to 78°F inside and we ran the house normally, to see how we might do when powered down for a storm. The Tesla Powerwall app is very helpful to understand exactly how much power one is drawing at any given time, as well as how things go over a daily period. Every second, it displays the power being used by the house, coming from solar and to and from the Powerwall and the electric grid. Indeed, so helpful is the app that you can use its display to gauge the power usage of air conditioners, fans, computers, car chargers, and the like so that you can see what household items are gulping power. Knowing more about what you are using means much greater chance for success using the energy storage system. For instance, the power draw of your laptop or most fans will not even show, but the toaster oven draws a full kilowatt. A level 1 electric vehicle (EV) charger draws about the same power.

Powerwall app showing 2kW being produced by solar; 1.3 kW going to grid and the house electric load at 0.7 kW.

Powerwall app showing 2kW being produced by solar; 1.3 kW going to grid and the house electric load at 0.7 kW.

The following day, August 29, showed that I was able to get my home power use down to about 11.7 kWh per day — less than half what the two Powerwalls have available and half of what solar produced on that sunny day (23.5 kWh). That gave me faith that with some reasonably sunny days following the storm, the Powerwall and solar would be able to easily carry the household load. And even with cloudy days following, we should be able to last several days without any loss of household functionality.

Tesla Powerwall app

Measured house total loads on Thursday, August 29. Blue is house loads in kW, white to grid, green to Powerwall, yellow is solar output. The recurrent square waves in house power represent the cycling main home refrigerator.

My wife Lisa and I buttoned down: shuttered the house, tied down everything, and elevated valued possessions above the tile floor. We bunkerized the entire home with aluminum panels, closing in the exposed porch. Alas, we are at risk on a barrier island and only about 600 meters from the beach. Hurricane Dorian was a monster and a killer. At the end of August, the monster Category 5 storm stalled over the Abacos in the Bahamas with 180 mph winds — destroying the entire island and doing unimaginable destruction.

Tesla Powerwalls.

Lisa ties down her boats in the carport with the Tesla Powerwalls. She drives a Ford C-Max Energi with 20 miles of battery range level 2 charger on the wall).

We prepared to evacuate. I charged up the Model 3 to its full range of 310 miles with home power while many gas stations around Cocoa Beach had long lines or no gas at all. As with Hurricane Matthew and Irma in years before, we prepared to escape to Gainesville, Florida, to spend a weekend as storm refugees with Lisa’s best friend. Fearfully watching the catastrophe unfold in the Bahamas, we prepared to leave.

Our home all shuttered.

However, Monday afternoon, September 3, before we departed, two rather marvelous things happened: all of the forecast tracks for the storm began showing it passing well off the coast of Florida, similar to Hurricane Matthew three years before. Also, the wind-field intensity suddenly fell to a Category 3 — still a major storm, but not the killer variety of a Category 5 hurricane. Of course, Cocoa Beach was still in the forbidden cone. What to do?


Although there was very reasonable concern for flooding, we recalled that with Matthew we did not have such trouble in our area. Indeed, when we evacuated to Gainesville ahead of Matthew, we drove north only to have that storm hit our place of refuge more completely.

So, this time, at the last minute, we elected to stay — as many of our neighbors did as well. I’ll be honest here and tell readers that I don’t recommend the decision we made, as real storm surge associated with hurricane-related flooding can be a killer. We made a calculated bet and don’t necessarily recommend the same decision for others in similar circumstances. In other words, do what the National Hurricane Center says, and not as we did.

Determined to ride it out, on the afternoon of September 3 as the storm approached, we cooled the house down by running the second multi-split system using grid power. The weather was cloudy and windy with periodic rain squalls, but the PV panels still pumped out 13.4 kWh against the 21.7 kWh consumed, and even sent power back to the grid. The Powerwall system remained topped off at 100% capacity with 27 kWh in waiting. In the late afternoon, we carefully walked down to the beach and found ourselves nearly blown about by the winds. The surf was actually low, but the white-capped waves roared frothy and angry. However, walking back it was impressive how much lower the wind speeds were just a few hundred yards away at our home. Such were the shielding effects of landscape and other buildings.

Performance of solar, grid, storage, and home energy systems on September 3 as hurricane approached.

On September 4, as the storm made its closest approach to Florida, winds picked up around our home all night. We have a small weather station one meter above our rooftop and we saw the highest wind speeds at approximately 5:00 am. However, at rooftop, the peak gust was only about 30 mph (47 km/h). At the beach, or at a 10m weather tower height, the peak wind speed would be much greater. At the nearby Cocoa Beach Pier, a peak gust of 53 mph was measured, and a gust of 81 mph was clocked at Playlinda Beach near Titusville.

While the Space Coast dodged a bullet with Hurricane Dorian, we still experienced plenty of tropical force winds and a good soaking rain.

Measured rooftop wind speed on Sept 3-4 as Hurricane Dorian approaches

September 4, 2019: Performance of solar & Powerwall system during Hurricane Dorian. Closest approach at 4:00 am. Power transferred from grid to Powerwall at 9:00 am.

As rooftop wind speeds fell from their maximum at 3–5:00 am, it became apparent that we were not likely to lose power — a godsend to everyone in the neighborhood. We were also safe and there was no flooding at all. Feeling grateful and in keeping with my original idea for the experiment, I set the Powerwall to “self powered” at 9:00 am, which minimizes any energy from the grid and uses the storage as you would with power interruption. One can see the green bump in power from the Powerwall at 9:00 am in the image above. Note that for the remainder of the day, there was no further energy brought in from the grid. The sun and Powerwall took care of all energy loads for the remainder of the day.

After the storm passed, on September 5, we kept the Powerwall on “self-powered” and found we had plenty of electricity to do all normal activities at home. As is often the case, after a hurricane passes, it draws in much of the regional atmospheric moisture leaving one with sunny, incredibly muggy days in its wake. The big difference with our experiment versus a real post-storm power interruption was sonic. After an actual storm with grid down, the monotonous drone of gas-powered generators can go on for days. After previous storms, we experienced the misery of interior temperatures in the mid-80s with a similar relative humidity.

However, we remained completely comfortable inside. We ran the central cooling mini-split 24 hours a day (maintaining 78°F and >50% RH inside), ran the refrigerator, had all lights, charged cars, and had amenities running. And later during the evening we powered up the second multi-split system to cool the master bedroom for complete nighttime comfort.

Interior temperature (orange) and relative humidity (blue) inside home from September 3–9, 2019.

It was still partly cloudy after the storm, but there was enough solar to bring the Powerwall back from 62% capacity to 100% by 1:00 pm on September 5. For the rest of the afternoon, when the PV system output was high, we had power to burn, so we used a level 1 charger to top off both of the EVs at home in the afternoon, we ran the dishwasher, and we turned down the the multi-split air conditioner. As the sun went down, we throttled back on the extras, but kept up air conditioning, computers, and lighting.

Performance of solar system, home, and Powerwall after storm.

And it went on similar to September 5th for four more days until September 9th when we suspended the experiment and returned to grid operation. The minimum Powerwall state of charge in the morning was 41% to 62% of its 27 kWh capacity when the sun came up over the days. We showed that even moderately cloudy days allowed the Powerwall to recover its capacity for the next night. Further, we found that even with a lot of mini-split cooling, we had power to burn during daytime hours and used that power to charge the two EVs at home, do laundry, and run the dishwasher.

And yes, with solar charging of the two cars at home, after the storm we were literally driving on the sun.

Table 1, below, summarizes the day-to-day performance, showing that after 3 September the combination of solar plus the Powerwall was able to meet all household electrical needs.

Summary

To summarize, we found that a homeowner with a typical-size solar system and backup batteries can come through a storm that would knock out the grid and get along for 5 days or more afterwards. Such a system is designed to help guard against the miseries of a week-long power interruption:

  • Refrigeration: Without refrigeration, food is lost and the ability to run and feed a household is severely compromised.
  • Lighting, fans and charging: One needs lighting to see and modest amounts of power are needed for radios and phones. Without air conditioning, fans are a must, as it is otherwise very difficult to sleep in muggy, hot Florida. Charging is important, both for cell phones and battery-powered power tools that are needed to make inevitable repairs around the house.
  • Cooling: This one is at the top of every post-hurricane survivor’s list. Central AC systems are power hungry, drawing 3000–4000 watts of 240 volt power, which is out of the question for most inexpensive generators. However, this is not true of mini-split heat pumps, which provide cooling for only about 500 watts, with some 120 volt models available. Indeed, with efficiency and potentially using a central supplemental mini-split heat pump, the household would have more efficient air conditioning and be able to operate normally after the hurricane has passed. You’ll also find that with a supplemental mini-split heat pump, you save year round as well as having a redundant cooling system.
  • Peace & quiet: The other big benefit from being able to air condition the home with electrical storage post storm: the ability to keep the house closed so that the constant noise of neighborhood generators doesn’t make sleep even harder.

Lessons Learned

Relative to our experience, there were several lessons learned:

  1.  Minimize electricity loads for your home, particularly during nighttime hours. Turn off unneeded secondary refrigerators and the main central air conditioning system. The Powerwall app is ideal for this, as it provides second-by-second feedback on household electricity use.
  2.  Install a central high-efficiency mini-split heat pump. These often draw only 300–500 watts and can provide 24 hour cooling in the event of an outage. (They can also save a typical homeowner a lot of kWh for cooling to begin with and provide a redundant system to boot.)
  3. Install a high efficiency heat pump water heater with the idea to run the system hard during daytime hours when solar availability is high but otherwise leave it off.
  4. The larger the PV system installed, the more rapid will be the battery storage during the day and the more power users will have to burn during daytime. Run dishwashers, do laundry, and charge cars during the day if you have excess.

Homes that have a modest PV system with battery backup can provide all of these essentials with batteries to make it through the evening with daytime charging of electrical storage. I do have a few suggestions for Tesla and the utilities in turn:

Tesla

Using a level 1 charger with solar the day after Hurricane Dorian to add range to a Tesla Model 3.

Using a level 1 charger with solar the day after Hurricane Dorian to add range to a Tesla Model 3. Power draw was 1.4 kW, adding about 5 miles range each hour of charging.

  • On the Powerwall app, it would be nice to make it easier to download data across time periods using a calendar. At the moment, it’s beautifully presented, but clunky to save.
  • As the measurements are obviously at a greater resolution than 0.1 kW (seen in the graphic displays), it would be useful for users to have that resolution available regardless of what it is (e.g, 0.105 kW). This makes it easier to judge the power use of many household items that are less than 100 watts, such as ceiling fans, stereos, and so forth.
  • It would be useful to add the current state of charge of the Powerwall to the data that can be charted or downloaded.
  • And a major suggestion for Tesla: As many Powerwall system owners such as myself also have a Tesla car, you should consider something like the planned Nissan Leaf Vehicle-to-Home capability for Tesla cars.

While a small 13.5 kWh single Powerwall would seem completely adequate backup for most power interruptions, imagine if Tesla could “unlock” 10–20 kWh of added storage from the vehicles in the case of a localized emergency. Dispatching this over the web would prevent the expensive car batteries from being routinely used, but would also allow them to play an important role in augmenting the Powerwall in an emergency. Thus, a single Powerwall could provide all the wiring and gateway components at the same time that a larger virtual power bank from the vehicle is potentially available if there is an outage. This would seem to reduce costs for such systems while providing double utility for the consumer of the expensive car battery.

Utilities

Florida’s utilities should be incentivized to encourage PV systems with storage. Currently, some utilities are adversarial towards home PV systems, largely because of lost revenue from self-generated power. Jacksonville Electric Authority (JEA) is a future-looking exception. Florida’s utilities should be encouraging PV systems with distributed electrical storage, which the utilities could then manipulate to smooth out loads (e.g., the so-called “duck curve”) under normal operation. This would be distributed solar with distributed storage which could help smooth out demand for utilities and homeowners alike. With future time-of-day pricing almost inevitable, this will allow for maximum flexibility.

Meanwhile, post-storm with no grid power, such home battery systems would be a godsend to those who own them.

Also keep in mind that while Florida utilities are madly installing solar photovoltaics to produce clean renewable power to sell to you, that power does the consumer no good in the event of a power grid interruption from a storm.

So, let’s not forget: when the grid is down, unless you use gas generators, it is only your rooftop solar electricity and your home electrical storage that can provide you with assured power. And that is true whether you are at risk of losing power from hurricanes, earthquakes, or other natural disasters. 
 
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About the Author

Danny is principal research scientist at the Florida Solar Energy Center where he has worked for the last thirty years. His research for the U.S. Department of Energy has concentrated on advanced residential efficiency technologies and establishing the feasibility of Zero Energy homes (ZEH) — reducing the energy use in homes to the point where solar electric power can meet most annual needs. The opinions expressed in this article are his own and do not necessarily reflect those of the Florida Solar Energy Center, the University of Central Florida or the U.S. Department of Energy.



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