TESLA POWERWALL 2.0 FOR STABILIZING DISTRIBUTION GRIDS
Tesla markets a wall-hung, 14 kWh Powerwall 2.0 battery unit, with lithium-ion cells made by Panasonic, either in Japan or Nevada. The unit is designed for daily charging and discharging.
The turnkey cost of the unit = $6,200, factory FOB + S & H + Contractor markup of about 10 percent + Misc. hardware + Installation by 2 electricians, say 16 hours @ $60/h = $8,200, or $586/kWh of battery capacity. This is a Tesla estimate, which likely would be higher, depending on customer site conditions. Here is a schematic drawing of the miscellaneous equipment required for a turnkey system.
Tesla offers a 10-y warrantee for manufacturing defects, but does NOT cover performance. Tesla estimates a 10% degradation in performance by year 10.
The unit likely contains enough batteries to store about 20 kWh, so by marketing a unit with a rating of 14 kWh (usable 13.5 kWh), it likely never gets charged above 95% (charging to 100% shortens life) or discharged below 25% (discharging below 25% shortens life), thereby achieving the warranty 10-year life, 3600 cycles, likely with less than 10% degradation. See fig 6 of URL.
PV Solar Systems on Distribution Grids: A distribution grid with many small or a few large PV solar systems would be destabilized during variable cloudy conditions, as in southern California, southern Germany and New England. The output of the PV systems would rapidly increase and decrease as clouds pass over the systems.
Damping PV Power Variations: The PV system output variations, especially between 10 am and 2 pm, could be reduced to acceptable levels by Powerwall 2.0 units distributed throughout a distribution grid among ratepayers. The battery units act as dampers. Their average state-of-charge would be about 60% so they could be charged and discharged, up to 95% and down to 25%, as needed.
System Operation and Energy Losses: A Powerwall 2.0 unit has it own built-in inverter. The unit takes AC energy from the grid, converts it to DC for charging, and converts it from DC to AC for discharging. Batteries have fast charge and discharge rates, which makes them suitable for this damping service. The unit AC-to-AC efficiency is 89% at rated conditions, per Tesla. The unit efficiency would be less at low charge and discharge rates. The losses are dissipated as heat.
Energy Storage and Demand Reduction: A utility, such as GMP in Vermont, could have a few thousand Powerwall 2.0 units on its distribution grids to reduce its peak demand on the New England grid, which would reduce its ISO-NE-imposed forward demand charges and forward transmission charges. Each unit delivers continuous power at 5 kW and peak power at 7 kW. The units would be charged and discharged once per day.
Two thousand units would reduce GMP peak demand by at least 10 MW, which would significantly reduce GMP forward charges. See URL for explanation of forward charges.
However, if the units had been used for balancing, as above described, they would need to be topped off to about 95% by the PV energy, if available, or from the grid, in afternoon hours, i.e., before the daily peak demand hours occur, to be useful for demand reduction during those peak demand hours.
The battery units could be used for balancing services, as required by ISO-NE, assuming GMP is the owner of the batteries, and they are not needed for other services. Much would depend on the season and the weather.