WUWT is hosting a guest post by Reed Coray. If his thought experiment is as I read it then it is an epic fail.
I got as far as this:
Take for example an internal combustion engine whose metal surface is exposed to a vacuum. In addition to doing useful work, the engine produces thermal energy (heat). That thermal energy will produce a rise in the temperature of the engine’s surface such that in energy-rate equilibrium the rate energy is radiated to space from the engine’s surface is equal to the rate thermal energy is generated within the engine. By attaching radiating plates to the engine’s surface, some of the energy radiated to space from the engine’s original surface will be absorbed/blocked by the plates; but because thermal energy can be transferred from the engine to the plates via both radiation and conduction, the temperature of the engine’s original surface will be lowered.
Emphasis mine.
In one respect it is correct – increasing the mass of the engine means more work must be done to raise its temperature but air-cooled engines rely on increased conduction to air via the fins to cool the engine. In a vacuum the fins as used on air-cooled engines will not help because the opposing faces, so valued in directing air flow, will absorb as much energy from neighboring fins as they radiate. Only radiation directed away from the engine’s surfaces actually cool this in-vacuum engine. The only change in his scenario comes from the increased mass.
Attempting to relate this to earth’s greenhouse effect is wrong.
Earth, as seen from space (imagine extraterrestrials being so interested) has an effective emission temperature of 255 K. This is controlled by the energy arriving from the sun, less albedo (solar energy reflected without affecting earth’s temperature). If earth had no atmosphere its effective emission altitude would be zero but greenhouse effect raises the altitude at which incoming radiation from the sun is balanced by outgoing radiation from the earth (whether by the non-gaseous surface or the atmosphere or a mixture of both is irrelevant). Many people misunderstand this basic fact.
We denizens of the surface are extremely ethnocentric – we don’t really care about earth’s effective temperature so much as we do about the temperature deep in the atmosphere, where we live.
Here, cosily tucked in our humidicrib, we enjoy temperatures higher than earth’s effective temperature due to internal feedbacks within the climate system. This is due entirely to greenhouse effect which is really a feedback from the atmosphere. Most of the energy involved in warming the atmosphere is obtained by conduction and evapotranspiration from the earth, some also comes from absorbing outbound radiation from the earth and a trivial amount comes from absorbing some of the inbound energy from the sun. Because the atmosphere is composed of molecules of non-zero temperature it radiates energy as a spontaneous function of entropy, the frequencies of which radiation can be determined by Planck’s Law. Earth’s non-gaseous surface is not transparent to these emissions and so absorbs energy from the atmosphere, raising the non-gaseous surface temperature and increasing the altitude at which incoming solar radiation is balanced by outgoing earth radiation.
Earth’s effective temperature is unaffected but its non-gaseous surface temperature is.
Our voyeuristic extraterrestrials will note no difference in earth temperature unless albedo changes, affecting solar input or the emission temperature of the sun changes, forcing earth to a new equilibrium state.
I strongly hope the article on WUWT is not as dumb as its opening appears but, as I said, that was plenty for me to abandon it.
All automotive engines are air-cooled.
Some use a liquid heat transfer medium with an external radiator.
With a hat tip to our resident pedant (‘morning, Gc
) the distinction here is the common-use one of air-cooled engines lacking a separate coolant circulation and radiator system.
All internal combustion engines also require air movement through the radiator/fins to keep the heat from rising, thereby causing internal damage usually in the form of piston seizure, gasket failure, or blown hoses. In the case of liquid cooling, a thermostat is used to regulate the temperature, and a cooling fan for the radiator to keep the engine at optimum efficiency without boiling the liquid coolant which is aided by a pressure retaining radiator cap.
Back to the original point, it depends on a balance of factors
1: The conductivity of the radiating plates.
2: The emissivity of the engine surface versus the emissivity of the radiating plates.
You definitely do need a different design than air-cooling fins. However, four plates 90 degrees apart from each other will not interfere with each other at all, and you will have reflectivity to consider, even if the emitted radiation does strike another fin, it will partially bounce off.
I have to agree that it is a narrow thought-experiment that really does not address the point. Furthermore, it’s implications are wrong since CO2 has a LOWER conductivity than oxygen or nitrogen. It’s a dissapointing post for WUWT
I agree on the basic points Ben but not on the 4 plates at 90 degrees – that assumes all emissions are perpendicular to the plane of the plate in order to guarantee non-interception by another plate and that isn’t true. Plates would need to be at 180 degrees to avoid the event horizon of emissions.
Valid, but that just beomes a fin effectiveness factor, nothing you don’t cover in an undergraduate heat transfer course.
Ouch! How telling is that:
“Take for example an internal combustion engine whose metal surface is exposed to a vacuum.”
This sounds like another “spherical cow” argument:
http://en.wikipedia.org/wiki/Spherical_cow
Although I was initially schooled in theoretical chemistry and physics, my ‘life experience’ in the working world centered around Analytical Chemistry, where hypotheses are recklessly butchered by empirical measurements. I can understand and appreciate the ‘elegance’ of a beautiful hypothesis, but if it does not account for the actual observations (i.e. the lack of a tropospheric hot spot, the ambiguous behaviour of clouds), then it is a *fail*.
One of the serious problems with using Stephan Boltzman to compute the energy radiating from the surface and the air is involved in this issue. For SB to be correct the material radiating must NOT be irradiating itself. The surface of the earth is quite rough even on a sandy beach so the numbers obtained will be distorted, The numbers from the atmosphere will be ridiculous as the atmosphere either is nothing but self irradiating or you would need to compute each particle separately. Then there is the actual emissivity which the Climatologists seem to find quite high…