looking at the all matt black prototype and remembering that the heat from batteries and posibly even the A/C system are supposed to dissapate all excess energy from micro colant channels threwout the vehicles outer skin.
is anyone else concerned that this will be impossible to achieve on a hot day in direct sunlight. ive reserved the Sol white color just for that reason and was considering painting/wrapping the upper side of the car between the solor cells to help whith heat dissipation.
as far as the increas in drage from a radiator, i would prefer the simplicity and reliablilty especially in hotter climates where it may prove better for batery degradation and charging speed.
I hear you Riley. My reservation is silver in color for the same reason.
How about we lobby for flaps, one in the roof and another in the "hood" of the car that only open when the batteries, interior, and/or solar panels get too hot.
The temperature-activated flaps would allow natural convective heat transfer to remove the heat when parked and dynamic convective heat transfer when moving.
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automatic flaps that open when needed is a great idea, i think the best location would probably be under the vehicle behing the rear most battery pack. it doesnt need to be a large radiator and the idea of having micro tubes dissapating heat is an awesome idea. if these tubes can actively cool the solar cells to increase efficiency would make more sense to me.
Riley:
Thanks for the shout out.
I don't want to dampen your creative process, but placing temperature-activated vents under the vehicle, especially if the vent led to a micro tube air-to-air heat exchanger, might tend to get clogged with road dirt.
Thoughts?
@Tom Kruer many motercycles place their radiator directly behind the front tire and regular cleening is needed to maintain thermal effiency. that could be a problem if the apteras radiator is tucked behind a flap under the vehicle. getting crammed full of mud while overlanding would be devistating. unfortunatly the apteras super sliperry desighn has backed it into a thermal corner. I cant think of any reaonable options to this dilemma.
@Riley:
Obviously, you see my point about road debris.
Thus the reasoning behind suggesting placement of the proposed temperature-activated vents for the micro tube heat exchangers on the roof and hood.
Repeating, we are trying to mitigate the problem of efficiency loss when the batteries, solar panels (below), and motors get too hot. (Assuming the experts at Aptera have not already come up with a solution).
The Thermo classes we had in school taught that natural convention can be very effective at removing heat... if air is allowed to flow. Therefore, when our cars are sitting in the hot sun recharging, some air flow over an air-to-surface micro fin heat exchanger mounted to the back side of the solar cells might help.
When we are cruising down the highway on a hot day, just a little dynamic air convection flow over a heat exchanger could help cool the motor, motor controller, and passengers.
Using a heat pipe to transfer the heat from the motor, etc. up to the same air stream of the roof vents could make for a system with no moving parts, other than the vents.
And I am thinking the vents would only have to open a millimeter or two with the exhaust out the tip of the "tail" (or whatever the aft of a wingless animal is called).
Aptera's low drag means lower cooling requirements when driving*, since there's less motor power draw, which means less power flow heating the battery.
Power flow is higher than that while grid-charging, but at that time the vehicle is stopped so if more cooling is required, a retractable radiator that extends out into ambient air would be much less problematic. However a simple A/C-to-battery-coolant heat exchanger would be smaller, lighter, completely enclosed (zero added air drag), & has far fewer moving parts.
*Ball-parking heat-creating power flow: I'm pretty sure I saw that Aptera range specs are based on the EPA test cycle which has an average speed of about 35mph (about 20 city & 50 highway). Divided by the 10mi/kWh spec, that's only 3.5kW, which is only 53% of the 6.6kW from charging at most public L2 chargers.
@Kiteboarder, I understand your point but must ask for support of your claim that a retractable radiator, and A/C to battery coolant heat exchanger has less parts than a fixed grill in the tail and a small, vent (perhaps of shape memory alloy) at the top edge of the windshield.
When moving, the air and heat flows from front to back. When stationary and charging, the air and heat flows from back to front (or from the lower to higher points).
Thanks for reading my post, but if you take another look you'll see that I didn't "claim that a retractable radiator, and A/C to battery coolant heat exchanger has less parts than" anything.
I claimed that "a simple A/C-to-battery-coolant heat exchanger ... has far fewer moving parts." compared to a retractable radiator.
My 500e has a small one of those battery coolant heat exchangers, circled in the picture below, with I believe 1 moving part (coolant flow valve). I've paid very close attention to it, & even in 113F, driving at highway & city speeds & immediately L2 charging, it has never activated.
The radiator/fan have always been sufficient. The most battery cooling I've ever seen is high coolant flow, & high fan speed about 3 times in over 5 years.
All that is on a car with about 250% as much battery-heating current flow while driving as an Aptera. & the exact same charging current flow on any public L2 I've ever seen, since those have all matched the Fiat's maximum 6.6kW.
I think most of the heat is being dissipated through the belly of the Aptera, and less on the top and sides. The poor thermal conductivity of the composite and foam materials should keep the bottom much cooler than the top and sides. With that said, it is a legitimate concern for a black car, but one they should be able to explore with the prototype. All this movable parts, flaps, radiators, etc. talk will add weight, complexity, cost and lower reliability. It violates the KISS principle.
I've never heard of a car having micro channels on its outer skin to dissipate heat and to me is the opposite of keeping it simple. An easy to repair flap and standard radiator may be another point of failure but at least can be repaired by anyone.
I honestly don't know what their micro channels look like. They have talked about replacing the belly pan when something goes wrong, so it could be that the belly pan is made out of aluminum or composite, and could be thought of as a radiator without fins. There are a lot of options I can think of: graphene film, aluminum foil, pitch fiber - all of these would provide thermal conductivity just under the skin, in plane with the skin. The microchannels are probably a compliant mat of very thin plastic, glued or ultrasonically welded to create the channels.
As for the thread topic, I recently read of a test, I think by GM, where body color only made a few degrees difference in baking sun, but black INTERIOR had much higher temp than light colors, inside (where the battery is?).
I've heard mention of that article, I think the Aptera will have a fan to maintain the interrior temp similar to ambient temp. The next time it's a very hot day place your hand onto both a white and black car. Hopefully having the majority of the cooling system partially shaded is enough to offset the additional heatload thats added.
@Riley Yes, I've felt the difference on the OUTSIDE of the car, but haven't done the real test of checking INSIDE temp when parked side-by-side when my friend had the same car in a different color.
There are paints and coatings called selective surfaces that reflect visible light, but emit IR (think white radiators on the space station). In this way, they absorb very little visible light that can heat up the surface, but they do a great job of emitting the IR (heat) to the environment. The most radical version of this are films being made that emit at a specific wavelength range where the atmosphere is transparent to space. They are currently being commercialized as a way to reduce the outside temperature of buildings and AC equipment to reduce their AC energy requirements by about 15%. What is mind blowing is that these surfaces can be lower than the ambient temperature because they can see through the atmosphere to the cold of space, and be very effective radiators.
The cooling will be on the under belly of the Aptera. The top is only for gathering heat for the batteries in winter.
@david.maddon i remember hearing on one of the interviews that they are debating whether to use convection or pumped cooling for the batteries. If they are aiming for that level of simplicity than I don't think they will go the extra mile of running liquid threw the upper parts of the vehicle.
@Riley he stated I believe in the Monro interview that the top was to draw heating down to the batteries, not to cool the motors. That’s why the top is black in addition to the solar cells which are also a heat magnet. The bottom is for cooling both the A/C and motor. Though the said the motor during testing was hooked up and it didn’t over heat due to the car being so efficient at speed.
@Riley Liquid-cooling is specifically required for the motors, & almost certainly also for the power electronics, unless they make an efficiency breakthrough (air-cooled Leaf & eGolf still have liquid-cooled power electronics).
How is the heat gathered? Are they running fluidic channels all the way up there? It seems like using the belly pan as the main heat exchanger between the inside and the outside of the car makes a lot of sense since it sounds like it can be replaced in the event of damage in an accident. It has always sounded as though they were embedding microfluidic channels in the composite shell, but that doesn't seem like a good idea to me for many reasons. My guess is if they need more cooling/heating than the belly pan, they are using the hood/roof panels to provide the surface area required.
He said the underbelly could be metal and composite or just metal. He said they are working with suppliers for to decide. They were clear about gathering heat from the top and dumping heat the underbelly.