Value chain · CLM-0941
In plain terms
In space there is no air to carry away the waste heat, so a rectenna's output is limited by how much waste heat it can radiate. The report puts that ceiling at about 2.33 kW of DC per square metre.
The claim as recorded
A rectenna cooled only by radiation, as in space, is thermally limited to about 2.33 kW/m2 of DC output at 70 per cent efficiency. SRC-336 computes that a reflecting plane at 100 degrees C radiates about 1.09 kW/m2 as a black body to a 0 K sink, that the front surface at nearly the same temperature roughly doubles it, and that after allowing for a warmer ambient and an emissivity below 1 the realistic radiated power is around 1 kW/m2 - which caps DC output at 2.33 kW/m2 for a rectenna that is 70 per cent efficient.
Evidence behind this: T3-primary-report, and the figure is a CALCULATION inside it rather than a measurement - class is ENGINEERING-MODELLED for that reason. The report's own measured results are all convectively air-cooled and do not test this case.
Scope and limits
A CALCULATION IN THE DOCUMENT AND NOT A MEASUREMENT - nothing in SRC-336 tests it, because every efficiency measurement in that report is convectively air-cooled (CLM-0939). The ceiling scales with the assumptions and is not a constant: it is linear in the radiated power, so a hotter plane raises it steeply as T^4, and it depends on the efficiency chosen - at 85 per cent the same 1 kW/m2 of waste heat would support 5.67 kW/m2 rather than 2.33, so the efficiency assumption moves the answer by more than a factor of two and the report does not justify picking 70. It also assumes both faces radiate freely, which a rectenna mounted on a structure may not. This is the space case and it is the number a reader needs, but it is a 1987 sketch and should be treated as an order of magnitude.
What it assumes
A 100 degrees C reflecting plane, radiation from both faces, and an efficiency of 70 per cent rather than the 85 per cent the same report measured in moving air.
Where it comes from
PDF p.85, verbatim at the normalised level: "If the reflecting plane is operated at 100 degC, the maximum (black body) radiation to space from the back plane at a temperature of zero degrees Kelvin would be 1.09 kW. However, the front surface would probably be at nearly the same temperature, doubling the radiation to space. After making allowances for radiation into a higher ambient than 0 degK and an emissivity of less than 1, radiated power in the range of 1 Kw/m2 is more likely." "This would seriously restrict the amount of DC power output from the rectenna. If the efficiency were 70%, the DC power output would be 2.33 Kw/m2." The degree symbols and the two instances of the digit 1 rendered as a capital I are letter-for-digit corruption in the text layer, repaired here and flagged rather than passed off as the raw string. BOTH STEPS RE-COMPUTED INDEPENDENTLY ON REGISTRATION: Stefan-Boltzmann gives 1097.6 W/m2 at 373.00 K and 1099.4 W/m2 at 373.15 K, against the stated 1.09 kW - agreeing to within 1 per cent, and the residual is the choice of what "100 degC" means to the last digit rather than an error in either; and 1 kW/m2 of waste heat at 70 per cent efficiency implies 1 x 0.70/0.30 = 2.333 kW/m2 of DC. The report's arithmetic is right.
Related records
- Answers question
- QST-002 not published in this release
- Contradicts
- CLM-0970 A 1992 computer model of the same thin-film receiving antenna, sitting in space with nothing... a 1992 finite-element model of the same hardware radiation-cooled in LEO puts the ceiling at 620 W/m2 at 85 per cent because the diode's conduction path binds before the plane's radiation does (); this record's plane figure is the looser of the two.CLM-0971 The heat from each tiny rectifier can only escape downwards into the copper it sits on, because...
Cite this
energyaperture.com/c/clm-0941
CLM-0941, Energy Aperture, as of 1987.
Connected records
Other published records linked to this one. A list, not a judgement.
- Also from SRC-336
- CLM-0938A 1988 NASA paper says a rectenna built on mylar reached 85 per cent efficiency. The 1987 final report of the contract it cites says mylar...CLM-0939The 85 per cent figure that later papers quote flat was one component on a bench, in moving air, with an error bar, at one power level. The...CLM-0940Two NASA efforts to build rectennas at much higher frequencies than the standard 2.45 GHz ended without a working device. One concluded the...CLM-0969If the tubes are only 70 per cent efficient, the transmitter's own face is not big enough to get rid of the heat they make, even spread...
- Contradicted by
- CLM-0938A 1988 NASA paper says a rectenna built on mylar reached 85 per cent efficiency. The 1987 final report of the contract it cites says mylar...CLM-0970A 1992 computer model of the same thin-film receiving antenna, sitting in space with nothing but radiation to cool it, found its diodes...