I don't mean to make most of you jealous; in fact it is I who am jealous of most of you. If it were practical I'd gladly take a good chunk of snow and blizzards off your hands, though truth be told it's cooled down nicely here over the past week or so. It's another of my harebrained schemes, and this one comes up in my thinking every summer, when my inability to deal with heat conflicts with my distaste for air conditioning in cars. Because I really do hate air-con in cars. It's illustrative of all that has gone wrong in the dominant automotive technological trajectory. I hate the sense of co****-like isolation, the unreality; I hate the white noise of multiple electric fans. There are altogether too many electric fans in the world today — even if I struggle to sleep in summer without one. I don't want to be locked up in a sealed bubble surrounded by electric fans when I drive; I want to be physically there in the street, with the windows open. Radiant cooling is a thing. It works really well if you're after comfort rather than refrigeration. The idea is to cool certain surfaces in an environment, so that they do not radiate heat onto you, and also receive radiant heat from you. In buildings it takes the form of chilled-water pipes embedded in hard floors or in suspended overhead panels. Apparently, compared to air conditioning, you need to cool air a lot more than you need to cool radiant surfaces to achieve the same gain in comfort. In a closed car, especially a HAMB-era car with smaller, more vertical windows, the main source of interior radiant heat is the roof. If we could chill the roof even slightly, this would increase comfort significantly. The obvious first thought would be to have the evaporator coil of a conventional vapour-compression cooler, as developed by Willis Carrier in the early 20th century, in the form of a mat of small-diameter tubing between the headlining and some stout insulation: but that would require an engine-driven A/C compressor and a radiator-mounted condenser, which is fine if it's that sort of build. And it's likely to cool a whole lot more than we'd need or want. The obvious second thought, then, is the ammonia-absorption process used in RV refrigerators. Its history is also situated well within our era. Indeed an early and very simple application of the principle was manufactured by none other than Powel Crosley in the '30s. It has the advantage of requiring only a heat source for its operation: no electricity, no engine-driven ancillaries — provided constant-pressure plumbing containing hydrogen is added, and attention is paid to the heights of components relative to each other. This p***ive thermo-siphon principle appeals immensely to me, as I have an abiding desire to have as little as possible in a car running on electricity. The system can take heat from a liquid-to-liquid heat exchanger plumbed into the engine's cooling system. The only problem is, where to put the condenser coil. Compared to that of a vapour-compression system, the condenser is tiny: a single length of tubing about 2' long, with a heat sink attached to it; but it needs to be at least as high as the evaporator we're putting in the headlining, or we're back to needing a pump. On some cars it can be hidden in a visor; in others it can live under a small scoop, something like a WWII aircraft's wing radiator, near the back of the roof. It could even be exposed, if it were designed and detailed neatly, perhaps chrome-plated or made in stainless, and fitted as a bit of jewellery over the rear window. Reinventing the wheel: probably a textbook case but, if it's done for fun, why not?
Sounds very practical for coupes and sedans. Could you also use it ICW one of the old swamp coolers that are window mounted? Tell me more!
Ammonia absorption is a life limited technology. The heat crystallizes the ammonia causing it to block the cooling p***ages. The average life span on a RV Fridge is about 10 years. Way less of it’s not leveled when parked. It also requires an open flame and a 12v power source to control the gas valve. I am trying to retrofit my 1963 Dometic to a more modern gas valve. It has a thermal disc to shut the flame off if the lack of flame is detected. It’s a mess. Most aircraft use air cycle machines which are basically a stand alone compressor for operating on the ground when boarding. Using ram air works good at 300 knots but not so much when stagnant. Aircraft also use bleed air for cooling off of the turbine engines. I know you hate the whole idea of ac in a car, but the systems used today have been refined for the intended purpose. I love to see folks challenge the norms. You seem to be an intelligent guy, I am sure you can come up with something.
I had a thought. To achieve any given reduction in sensible heat, radiant cooling requires a significantly smaller drop in refrigerant temperature than air conditioning does. Doing this with a vapour-compression process would therefore require a significantly smaller installation than an air conditioning setup of comparable effectiveness: moreso still if the cabin is small. If a conventional A/C compressor is used, it need not turn as fast as it would normally have to, or if a different compressor solution is used, if could be much smaller. Then: the pressure difference capacity of a conventional A/C compressor is often surplus to requirements. Using a York or Mopar RV2 for engine-driven OBA (an idea I really love) represents ΔP of up to 120psi or so. For actual air conditioning — or radiant cooling — ΔP of only about 25psi is needed, and in a much lower range. It should be quite possible to run the cooling off the OBA via a low-ratio gas booster i.l.o. a compressor, and have ample CFM to spare: no need to plumb the cooling to the engine or bring rotational power to the compressor. Everything sits under the floor or behind interior panels or wherever it's convenient.
A flat roof panel as described would absorb heat from the cabin, but wouldn’t it also be working to absorb heat from the outside? I never considered what you are talking about. My quick thoughts would be, is my head going to get cold as my but is sweating? It seems like radiant heat in reverse... I wouldn’t want my head 4” from a radiator while the rest of my body is cold. Would this not be the same thing? Also, are you going to start a rain shower in your car with headliner condensation?
Ya lost me at "I got an idea".... How about a bucket of ice from the cooler, place it on the floorboard and open the cowl vent. Instant air conditoning...AND a place to keep your beer cold when "drinkin' & driving'"
For radiant cooling to be effective in terms of what it feels like, you only need to cool the headlining about 5°F cooler than the desired interior temperature. By contrast, the evaporator coil in a car air conditioning unit is likely to be around 20°F cooler than the desired interior temperature. You won't have like a slab of ice above your head! That's why you probably wouldn't have to get that close to the dew point temperature in any but the most humid of climates, and why this would work best with the ample ventilation provided by the open-window conditions I like anyway. It would be sensible to insulate the roof as far as possible, above the cooling coil. The coil handles heat which makes it through to the underside of the insulation. We're taking away the heat you feel radiating from the roof first, and improving the prospects for convection currents off your body second. Rising warm air no longer ends up congregating around your ears, but gets cooled and sinks again.
My experience with this kind of cooling is with propane powered refrigerators. They are painfully weak in the cooling department, taking many hours to cool down when initially fired up. How about spraying the roof with water via oil burner nozzles?
I followed your post on air conditioning in old cars to your post here. As the owner an ammonia absorption refrigerator in my motorhome, it is exactly as Roothawg said and is entirely impractical, like who wants a flame or electrical heat source to boil the ammonia in their car? Then the rate of heat absorption is so low you would need to hermetically seal your cabin completely and just one door opening would lose much of your cooling effect. Since the heat absorption rate is so low you would need to leave it on for 24 hours before you used the car to cool the m*** of the interior of the car to get a head start on cooling it and once you put it in a hot and sunny environment it would be quickly overwhelmed by the radiant heat. In addition to all that you would have to maintain airflow over the condenser coil to get rid of the heat that is absorbed. That would take a fan, especially when stationary. I have already installed two fans in my RV refrigerator, one thermostatically controlled to move air over the condenser coils and out of the roof vent and one to circulate the air inside the refrigerator. I hate the damn things because they really struggle to cool a well sealed and insulated 13 cu. ft. refrigerator in the hot summer time. The only way they can keep up is to cool the m*** in the refrigerator and then barely maintain it. When mine dies I'll be replacing it with a 12 volt compressor type refrigeration unit that will primarily be powered by a solar array and my batteries, like this: https://jc-refrigeration.com/product/norcold-2118-hvac-12-24v-dc-conversion/ I would suggest you might find an easier tried and true solution, like this.
Alcohol evaporates much faster than water which is why it feels so cold when applied to your skin (a complete waste of resources). How about using those nozzles to spray vodka instead? And rather than outside the car I am thinking inside the vehicle and perhaps pour rather than spray would be more beneficial. Much cooler
@Ned Ludd I enjoyed the write up, got me thinking about cooling systems, then I started thinking the same thing @gnichols was. Get this man a swamp coolers. Effective, traditional.
Ammonia refrigerator ****. Just as stated, the recovery is very slow. Total heat capacity is very low compared to a conventional frig. Also ammonia fig are very sensitive to being level, which is not practical for a car. So while I understand the concept of radiant cooling being proposed, using ammonia is not going to work. Also consider that a big function of conventional A/C is the dehumidification of the air. In very humid environments this can be as important as the cooler air temperatures.
One thing I don't see mentioned is that ammonia is extremely toxic in case of a leak. I worked in a milk bottling plant just out of school, the raw milk was cooled by ammonia. It was hunky dory until one of the lines got a leak, then they had to evacuate the entire building and the service guys had to go in in oxygen masks and tanks and fix it, then they had to air the building out to get the ammonia out. It only takes a small leak to overcome the oxygen in the air. That's why RV fridges are sealed to the outside, so not much of the ammonia can get inside if a leak happens.
But I want to reinvent the wheel! Seriously, I'm not wedded to ammonia. Post 4 was where it struck me that a vapour-compression process could be suitable, given the reduced demands due to: Small cabin volume — moot, really, given radiant cooling; Small window area and more vertical gl*** angle — I'm thinking '20s/'30s cars — compared to typical modern cars i.e. much reduced solar heat gain through the windows; and Small temperature reduction requirement compared to air conditioning: we need to keep the rooflining at around 65-70°F, rather than an evaporator coil buried in the dash at 45-50°F. If it's 95°F out, that's around half the temperature reduction. That adds up to enough that we're talking about a different kind of refrigeration plant, i.e. something of the same order, maybe, as the icemaker @Ebbsspeed mentioned in the other thread? I've got two problems with the swamp cooler. First, you have to keep refilling it, because it operates on constant water loss to evaporation; and second, it imposes an aesthetic of its own, which might be cool for some builds but completely wrong for others. I could add visibility and aerodynamics, but I'm not likely to daydream about a 200mph Pacer anyway ...
Tell me about it! When I started working in drawing offices the standard print technology was the ammonia-based Diazo whiteprint process. Every office would have a print room with a guy manning it: not a long-term career prospect. But why is everyone on about the ammonia-absorption part? This thread is about radiant cooling, not about absorption processes as such. Perhaps it's because people have a lot of experience of the latter and very little of the former? The ammonia bit really doesn't matter.
If you're slopping alcohol on your body, it's time to switch to coffee until you can hit your mouth again!
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