A 1,400kW(p) solar PV array backed up by 6,000kWh of battery storage and a smart grid has been installed on the island of Ta’u in American Samoa. It’s reported that this system already allows Ta’u to obtain 100% of its electricity from renewable sources for 100% of the time, and this brief review suggests that it will in fact be capable of delivering 100% electricity for almost 100% of the time if and when it reaches full operation. However, the operator Solar Power (recently acquired by Tesla) foresees potential problems in integrating 100% solar energy with the grid and plans on a “phased approach” to identify and resolve them. Consequently the system will not supply 24/365 renewable electricity immediately, and its ability to deliver it in the future will be dependent on whether a grid fed by 100% solar energy can be made to work.
Ta’u, the easternmost island in American Samoa, has an area of 44.31 sq.km., a population of 790 and a 2015 per-capita GDP of $11,289. Historically it has obtained its electricity from diesel units which generate 1,300,000 kWh/year, or about 3,600kWh/day. Peak load is 229kW and average annual per-capita consumption 1,650kWh/year. (Data from various sources).
Google Earth image of the volcanic island of Ta’u, latitude 14.25 degrees south, showing the location of the solar panels. Habitation is confined to the northern coast. Note the caldera rim feature in the southern part of the island.
The Ta’u solar installation:
Project funding was provided by the American Samoa Economic Development Authority, the U.S. Environmental Protection Agency and the U.S. Department of Interior. Costs, about which more later, are reported at $8 million.
The installation consists of:
- 1,400 kW of solar pv (5,328 panels)
- 6,000 kWh of storage in 60 Tesla powerpacks
- A microgrid which “allows the island to stay fully powered for three days without sunlight and can recharge to full capacity in only seven hours.”
The image below shows the solar installation. The panels are fixed and oriented at a shallow angle to the north, presumably at or around 14 degrees to comply with fhe 14 degrees south latitude:
The Ta’u 1,400kW solar panel installation (image credit Solar City)
The image below shows the Tesla powerpacks, each of which is about eight feet high:
The Tesla powerpacks (image credit Inhabitat)
To evaluate the likely performance of the Ta’u installation I reviewed two questions: 1) what is the solar pv capacity factor going to be and 2) how large are seasonal variations in solar output? This turned out to be less problematic than I had anticipated because there are already a number of solar pv installations operating on other islands in American Samoa and the Sunny Portal website provides operating data on 20 of them. Averaging monthly output from the 18 installations that gave usable results, which range in size from 7kW to 35kW and contain 391 months of generation data between 2011 and 2014, gave the results shown in Figure 1:
Figure 1: Average monthly capacity factor for 18 solar installations in American Samoa
Capacity factors range from 14% in June up to 20% in September, October and December. Comparatively small seasonal variations of this type are typical of low latitudes. The average annual capacity factor is 18.0%, which closely matches the 17.7% capacity factor calculated from data supplied by the American Samoa Power Authority (3,800,000 kWh/year from 2,450kW of solar pv capacity). At a capacity factor of 18%, however, the 1,400kW Ta’u installation would generate 2,200,000 kWh/year, well in excess of Ta’u’s present consumption of 1,300,000 kWh/year, and as a result about 40% of Tau’s Annual solar generation would probably have to be curtailed.
The next question is whether the 6,000kWh of Tesla battery storage is enough to fill demand during extended cloudy periods. To evaluate it I took the 2012 daily generation data from the 28kW “Procurement” installation on Tafuna Island 130km to the west, scaled them up by 1,400/28 to replicate generation from the 1,400kW Ta’u installation and used them to run a “battery balance” assuming a constant daily demand of 3,600kWh. Figure 2 shows the daily generation data for the 1,400kW case. It fairly consistently exceeds the 3,600 kWh daily demand, usually by a substantial margin:
Figure 2: Ta’u daily solar generation estimated by scaling up 2012 generation from “Procurement” installation on Tafuna. The month divisions are approximate.
Figure 3 plots the battery balance, which assumes no charge/discharge limitations. The 60 Tesla powerpacks remain fully charged at 6,000kWh for over 80% of the time and become completely discharged on only four days (May 24 and 25 and June 4 and 5), i.e. for about 1% of the time. However, over 40% of the total solar generation has to be wasted or another use found for it:
Figure 3: “Battery balance” obtained from Figure 2 data.
Ta’u is the latest entrant in the growing field of “100% renewables” projects, and this brief appraisal suggests that it probably has a better chance of succeeding than some of the other projects that have been marketed under this mantra. The key, however, is whether the smart grid can be made to work with 100% solar generation and zero diesel backup. As discussed earlier Solar Power plans to adopt a “phased integration” approach under which solar energy is sent to the grid in small amounts to begin with and in increasingly larger amounts only after the grid has been shown to be capable of handling them, but this approach was adopted at King Island, Tasmania, eighteen or nineteen years ago and the goal of 100% renewables has still not been reached there. My guess is that in common with King Island and Gorona del Viento on El Hierro the island of Ta’u will never be able completely to get rid of backup diesel generation, or at least not at any time in the foreseeable future, but I would be happy to be proven wrong.
The total cost of the project is reported to have been $8 million. I don’t have enough backup cost data to check this estimate out, but here are some of the numbers I have been able to come up with:
- According to NREL installed costs for 1,400kW of pv panels in the US (Ta’u is part of the US) were around $2,000 per installed kW in 2015, which works out to $2.8 million.
- According to Electrek 54 Tesla powerpacks (5,400kW) and 10 bi-directional inverters (2,500kW) can be purchased in the US for $3.2 million excluding installation. Raising the price by 10% to include 6,000kW of powerpacks plus a few more inverters and adding, say, $1 million for installation gives $4.5 million.
- $2.8 million plus $4.5 million is $7.3 million, and we still need to allow for the costs of shipping everything to Ta’u, which has no docking or airport facilities worth speaking of, and erecting it there. How much to allow? I arbitrarily doubled the $7.3 million estimate, giving a grand total of $14.6 million.
Now $14.6 million may be too high and $8 million may be really what the project cost, although it still seems a little on the low side. But even if $8 million is the right number it still works out to $10,000 (or almost one year’s GDP) for each of the island’s 790 residents.
Further updates will follow as and when additional information becomes available.