Value chain · CLM-1486
In plain terms
A 1977 NASA study of how receiving stations would join the US power grid looked at two kinds of shadow. For about six weeks around each equinox, the Earth would shade each satellite once a night near midnight; the study judged this easy to cover, because demand then is about half its peak. If more than a hundred satellites were built, they would also shade each other each morning and evening, close to peak demand. By the study's table, that would cut the number of five-gigawatt receiving stations the US grids could take in 2025 from about 360 to about 60. The study added that direct-current links between the regional grids would largely remove the problem, but gave no figures for that.
The claim as recorded
NASA Johnson Space Center's 1977 grid-interface study lists, among the "primary technical factors for operating 5.0 GW rectenna systems with the grids", "Eclipses of the solar collectors by the earth" and "Eclipses of the solar collectors by other SPS's", both at the equinoxes. It states that each rectenna "would be shutdown daily for short periods of time near 12:00 a.m., for 45-days around each equinox", when "the grid demand is only about 50 percent of its peak demand", so that for an outage it puts at fifteen minutes at most "the demand could be handled by other capacity on the grid". With the 112 satellites of the programme's Scenario B in 2025, the satellites "would eclipse each other on a daily basis at 6:00 p.m. and 6:00 a.m. for a 45-day period around each equinox", near the daily demand peak, which reduces the number of rectennas a grid can take. Its Table IV-D-2, with no storage, a 10 per cent reserve margin and a throttle range of 64.2 per cent of full load to full load, allows a printed total of 364 rectennas of 5 GW across the nine US regional grids in 2025 when only the Earth's eclipse is counted, and 60 when eclipse by other satellites is counted.
Evidence behind this: T3: a section of NASA Johnson Space Center's 1977 in-house progress report on the reference satellite, read first-hand; a planning estimate on the authors' own groundrules and on projections of the 2025 US grid, not a study by a utility.
Scope and limits
THE "FIFTEEN MINUTES MAXIMUM" IS THE PAGE'S OWN FIGURE AND IS NOT DERIVED THERE. By geometry a geostationary satellite spends up to about 70 minutes in the Earth's shadow at equinox (CLM-0895), and the utilities' 1980 reading of the reference system adds about 5 minutes to shut down and 16 to 60 minutes to restart (CLM-1081). The study's judgement that other capacity could carry the midnight outage rests on the shorter figure; this record does not test it against the longer one. TABLE IV-D-2 DOES NOT FULLY FOLLOW ITS OWN EQUATION (our arithmetic). Seven of the nine Earth-eclipse entries match it. The printed 16 for NPCC is the equation's figure for MARCA's capacity (16.1), and the printed 30 for MARCA is one below the equation's figure for NPCC's capacity (31.1), so the two entries look transposed, one of them also one short; the equation's rows give 365 against the printed total of 364. The other-satellite entries sum to 61 against a printed total of 60, read at a crop where the scan's zero is clear in other cells; the page gives that column's method only as "the same technique". The counts are a 1977 planning estimate for US grids in 2025 with no storage, and a limit set by throttling and reserve, not by transmission or stability; they are not a statement about any current design or about Great Britain. The page does not show the geometry of satellites shading each other. The section ends by saying that DC links between regional grids "would virtually eliminate the effects of SPS outages" on its Scenario B assumptions; Table IV-D-2 does not count that, and the page gives no figures for it.
What it assumes
The study's groundrule that the rectenna must not change the reliability of supply; its projection of 2025 US generating capacity by region (Table IV-D-1); a 10 per cent reserve margin and a 64.2 per cent minimum throttle setting for the projected 2025 mix; no energy storage; 5 GW per rectenna; for the mutual eclipse, the programme's Scenario B of 112 satellites in 2025.
Where it comes from
PDF p.148 (printed IV-D-8), read at the render: the groundrule "The rectenna/ utility interface does not change the reliability of electrical service to its customers"; then "the following parameters are considered to be the primary technical factors for operating 5.0 GW rectenna systems with the grids", listing the grid's daily and seasonal demand profiles, the throttling range of its machines, "Eclipses of the solar collectors by the earth at the vernal and autumnal equinoxes" and "Eclipses of the solar collectors by other SPS's at the vernal and the autumnal equinoxes". PDF p.151 (printed IV-D-11), read at the render: "Due to its size (5.0 GW) the rectenna would serve as a base load generator"; the rectenna "would be shutdown daily for short periods of time near 12:00 a.m., for 45-days around each equinox", brought about by the Earth's eclipse of the satellite in geosynchronous orbit; with Scenario B's 112 satellites in 2025, the solar collectors of each satellite "would eclipse each other on a daily basis at 6:00 p.m. and 6:00 a.m. for a 45-day period around each equinox"; and, in paraphrase, outages of these kinds are unknown in the existing grid operating structure and need new methods of power management. PDF p.153 (printed IV-D-13), read at the render: "An outage at this time of day has a minimum impact on grid power management. The reason being, when this outage occurs the grid demand is only about 50 percent of its peak demand. For the period of the outage (fifteen minutes maximum), the demand could be handled by other capacity on the grid."; the number of rectennas = IC (1 - RM) (1 - TR min) GW / 5 GW per rectenna, with IC the installed capacity, RM the reserve margin and TR the throttle position; for Table IV-D-2 the reserve margin is 10 per cent and the minimum throttle range 64.2 per cent, from the projected 2025 mix of nuclear, fossil-fired and hydro generation. PDF p.154 (printed IV-D-14), Table IV-D-2, titled in paraphrase the number of 5 GW rectennas allowable on each regional grid due to eclipses at the equinoxes for 2025, with no storage system, read at a crop: nine councils, installed capacities from 249.6 to 1225.2 GW; rectennas due to the Earth's eclipse 53, 29, 30, 30, 16, 79, 36, 65 and 26, total 364; due to eclipse by other satellites 10, 5, 5, 5, 2, 13, 6, 11 and 4, total 60. PDF p.155 (printed IV-D-15), in paraphrase: in Scenario B the satellites eclipse each other at 6 a.m. and 6 p.m., and by the same technique the number of possible rectennas is reduced because the outage occurs near the daily demand peak, which reduces the throttling range of the grid; spreading rectenna power over several grid networks reduces the effect of losing power at any one site, and, if DC links between regional grids came into wide use, "Such a power management scheme would virtually eliminate the effects of SPS outages based on the assumptions given in scenario B".
Basis: 5 GW rectennas; number of rectennas per US regional grid in 2025; per cent reserve margin and throttle range; days and minutes of outage
Related records
- Answers question
- QST-012 not published in this releaseQST-021 not published in this release
Cite this
energyaperture.com/c/clm-1486
CLM-1486, Energy Aperture, as of 1977.
Connected records
Other published records linked to this one. A list, not a judgement.
- Also from SRC-549
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