Some people have serious difficulty with some naturally occurring levels of heat. The CEO of Singapore, Lee Kuan Yew, has credited air conditioning with allowing his (tropical) country to develop economically; and in recent years, as I age, I find myself suffering increasing levels of impairment and ill-being from the Buenos Aires summer. But air conditioning has some disadvantages. It requires large, capital-intensive machinery; it consumes a lot of energy to cool large spaces, and a lot of power to cool spaces with a lot of surface area to a hot outdoors; and it isn’t portable to many places people would like to go.
Years ago I read a 2007 Mother Jones article by Dennis Gaffney about hypermilers in which the protagonist, Wayne Gerdes, sometimes wears an “ice vest”, “which he uses at the nuclear plant when he has to work in really hot rooms,” because, says Wayne, “You can drive at 95 degrees [Fahrenheit; 35°] with an ice vest, and it doesn’t feel like 95 [35°].... No electricity, no air, no fans.”
Also in 2007, my then wife, Beatrice, was suffering from the Buenos Aires summer heat in our house. I don’t remember if we didn’t yet have an air conditioner in the bedroom, or if it wasn’t powerful enough, or if the power was going out, but she took to freezing plastic bottles in the freezer and wrapping them in a towel to snuggle up to at night, a trick which I’ve used to great effect ever since.
What if you had an optimized, portable version of this? Could you eliminate the need for air conditioning? What would such a design look like?
However, in situations demanding one is exposed to a hot environment for a prolonged period or must wear protective equipment, a personal cooling system is required as a matter of health and safety. There is a variety of active or passive personal cooling systems;[14] these can be categorized by their power sources and whether they are person- or vehicle-mounted.
Because of the broad variety of operating conditions, these devices must meet specific requirements concerning their rate and duration of cooling, their power source, and their adherence to health and safety regulations. Among other criteria are the user’s need for physical mobility and autonomy. For example, active-liquid systems operate by chilling water and circulating it through a garment; the skin surface area is thereby cooled through conduction. This type of system has proven successful in certain military, law enforcement, and industrial applications. Bomb-disposal technicians wearing special suits to protect against improvised explosive devices (IEDs) use a small, ice-based chiller unit that is strapped to one leg; a liquid-circulating garment, usually a vest, is worn over the torso to maintain a safe core body temperature. By contrast, soldiers traveling in combat vehicles can face microclimate temperatures in excess of 65 °C and require a multiple-user, vehicle-powered cooling system with rapid connection capabilities.
I think I want one of these for daily use.
An adult human normally must consume about 2000 to 2500 kcal per day (8.5–10.5 MJ) to remain normally active without losing weight. Only athletes or people with very extremely demanding jobs eat significantly more than this. Essentially all of this energy is converted into heat inside our bodies — either by bacteria in our intestines, by the various metabolic activities of our brain, liver, muscles, and other tissues, or by damping work done on our bodies. And there are no other significant thermal influxes under normal circumstances.
These last two points deserve some elucidation. The other day, I walked up 12 flights of stairs because the power was out in the elevator, about 50 meters. I weigh about 110 kg, so this amounted to about 54 kJ of mechanical work. Roughly speaking, as I walked up the stairs, my leg and butt muscles converted about 220 kJ of glucose and other metabolic fuels to exhaust — carbon dioxide and water — and converted about three-quarters of this directly into heat. The other one-quarter, however, was converted into the gravitational potential energy of my body.
So, you could argue, if I kept walking up stairs all day long, only about three-fourths of the calories I had consumed would be converted to heat. I could probably walk up about 900 flights of stairs in a 12-hour day of walking up stairs, at which point I would have done about 4 MJ of mechanical work and gained about 4 MJ of potential energy, and I would have expended about another 12 MJ of heat. I would be very hungry and my knees would hurt a lot.
I would also have reached an altitude of 3750 m above where I started, a bit less than climbing Mauna Kea or Mount Ranier. At some point, I would probably have to come back down, which would convert my gravitational potential energy back into mechanical energy, in my leg and butt muscles. Since my leg and butt muscles are not equipped to convert mechanical energy back into glucose, they would convert it back into heat.
(There are cases where the mechanical energy is eventually dissipated in something that isn’t your body — when you go swimming or kayaking, for example, or if you’re climbing a stair climber instead of actual stairs. Or if the power comes back on and you take the elevator down instead of the stairs. But these are unusual cases.)
I say there are no other significant thermal influxes under normal circumstances because when you put people in an environment where they cannot reject body heat to the environment, they die, usually in minutes to hours. So the net heat flux in a survivable situation is always from your body into the environment.
Porticool is a brand name for a liquid-CO₂-cooled vest marketed to HAZMAT specialists and other emergency responders, developed on a DHS SBIR Phase I grant, using an open-loop system that releases 500psi gaseous CO₂ from some of the tubes running through the vest, after running it through non-porous tubes as a liquid.
While other cooling solutions (ice vests, cooled air vests and liquid circulated garments) tested have shown favorable responses, none provide the flexibility and mobility afforded by the Porticool PCS. Nor could they compete with the light weight and thin garment size that the Porticool PCS achieved.
This doesn’t sound like something you could safely wear all day, if at all ever, because you’d have to be replacing the liquid CO₂ and you’d be constantly at risk of being poisoned by it. (I guess I should calculate how much CO₂ is emitted, but I think the answer is in the range of a few kilograms or cubic meters per day, therefore liters per minute.)
The DHS published a superficial TechNote on personal cooling systems in 2013. It divides them into “passive systems” (like ice vests) which contain no moving parts and “active systems” which do, for example because they involve a circulating fluid, and therefore need a power source. (I suppose directly applying Peltier coolers would also be “active” and without moving parts, but that wouldn’t be practical, so probably nobody does it.)
It mentions evaporative cooling systems with “water absorption crystals”; vests with phase-change material pockets, usually with paraffin, which last up to 2 hours; “gel or ice pack vests”, erroneously implied to not be phase-change materials; “ambient air systems”, which blow ambient air under your clothes; and “liquid circulating products” with vapor-compression or thermoelectric cooling, or ice, to chill the liquid. It shows a Veskimo ice-backpack-powered cooling vest as its example of this last category. Apparently ASTM F2300 is the testing standard.
In 2014, Adam Savage of Mythbusters and some other guys talked about building a cooling suit. The background is that, when he went to ComiCon in a hot costume, he was trying a gel-vest system made of “Polysorb... like probably diaper crystals” and although “it’s a brilliant design” he nearly passed out from heat exhaustion because he didn’t have a freezer to freeze it in previously. He said it should have been able to keep him cool for an hour if he had frozen it. He also mentions a liquid-circulating cooling shirt called “CoolShirt” which is sold for (auto?) racing, with “recirculating pumps that are too big”; he didn’t think it was up to the job.
The video is a waste of time; it’s just three guys talking on microphones for half an hour, and they haven’t even built the suit.
The Veskimo cooling vest mentioned in the DHS report runs microtubing through a thin vest. It explains:
NASA pioneered the use of garments employing circulating chilled liquid in the 1960's to keep astronauts cool during space walks. The design and construction of these systems are well documented. Systems of similar construction are currently in use by military personnel in aircraft and armored land vehicles. These systems are very expensive because they use compressor-based refrigeration units to chill the circulating liquid coolant. By substituting ordinary ice to chill the water, the cost and complexity of the system is greatly reduced, making Veskimo Personal Cooling Systems affordable, yet still truly effective.
...The Veskimo Personal Microclimate Body Cooling Vest is made from lightweight breathable mesh fabric that will not inhibit the evaporation of perspiration, and has a thickness of less than one-quarter inch, so it fits easily under any close fitting garment or protective gear. Its zippered front and adjustable-length elastic side straps make it easy to take on and off and adjust for best fit, comfort and the desired degree of tube-to-skin contact.
Their page also mentions “evaporative garments” usually use sodium polyacrylate crystals, like diapers, flowerpots, and maxi pads. Because apparently the Veskimo people aren’t scientifically illiterate like DHS employees, they class ice vests with other “phase-change garments”. They say their ice “backpack or cooler” is “typically enough for 4 hours or more”.
More details on the Veskimo system:
Approximately 8 pounds of ice can fit in our 4.4 Quart Hydration Backpack, and as much as 16 pounds in the 9 Quart Cooler. The useful heat capacity of 8 pounds of ice is approximately 400 Watt-hours, and 800 Watt-hours for 16 pounds of ice. Studies performed by NASA and the US Military conclude that 100 Watts of cooling power is effective in maintaining body core temperature in all but the most extreme heat conditions. The Veskimo Personal Microclimate Body Cooling Vest is capable of providing over 100 Watts of body cooling. If the system and the user are well insulated from the external environment, the useful cooling duration is approximately 4 hours (400 Watt-hours / 100 Watts) for the Backpack and 8 hours for the 9 Quart Cooler System at 100 Watts cooling output. If the user wears a lightweight windbreaker-type jacket over the vest to minimize loss of cold to the atmosphere (which we recommend), these are reasonable estimates for the system's cooling performance. Some users have reported even longer cooling duration because they had their system adjusted to provide less than 100 Watts of cooling.
They run their pump on 4 watts on a 12-volt Li-ion battery. Their list price is US$1116 for the Veskimo vest and backpack, and on top of that they’re out of stock, so a homemade substitute would be worth considerable work.
A possible alternative to an ice vest would be ice pants, which would be less of a weight burden (since only your legs would carry the weight, not also your back) but perhaps more cumbersome. Your legs are similar in surface area to your torso and have very substantial blood flow to them.
A 100-W Peltier device is typically 10 A at 12 V and costs about US$15 retail, but you need to run Peltier elements at substantially less than their maximum rating to get reasonable efficiency; at ΔT = 0 and I/Imax ≈ 0.3, Q/Qmax ≈ 0.5 and CoP ≈ 3.0 (that is, 3 joules of heat are removed from the cold side of the reservoir for every joule of electrical energy dissipated). So maybe the 100-W device can give you 50 W of heat rejection at 4 A (and, I suppose, very close to 12 V, since thermocouples are closely approximated as constant-voltage devices.) As ΔT rises, the CoP falls, eventually past 0 as the resistive heating effect of the current outweighs the Peltier effect.
In the case of a human in an environment that is only mildly hostile — say, 30°–40° — ΔT ≈ 0, so this might be a reasonable region to work in. (XXX is it actually going to be adequate to keep your skin at 30° or 35°, or is that going to result in dangerously low heat transfer rates from the body core? What skin temperature is adequate? Presumably it’s about the same as the bearable air dewpoint.) CoP ≈ 3.0 means that you only need ≈30 W of battery power to reject 100 W of heat to the environment, which means that each hour of cooling consumes about 110 kJ of battery. (The energy cost of pumps and fans was 4 W in the Veskimo case, which is small compared to the Peltier devices, though not insignificant.)
An 18650 cell typically weighs 47 g and costs about US$10, and is commonly 2000 mAh and 3.7 V, for an energy capacity of 27 kJ, so you’d be emptying on the order of 4 18650s per hour, weighing 190 g. This compares very favorably to the 1080 g of ice (100 W / (333 kJ/kg) = 300 mg/s = 1081 g/hour) that you would need to carry to absorb the same amount of heat by melting. Ice is, of course, substantially cheaper; equaling the four-hour 100-W capacity of the 4-kg-of-ice Veskimo unit described above would require 16 18650s at a price of about US$160, plus another US$100 or more for the Peltier modules and other materials. But weighing only 750 g rather than 4 kg could be a decisive advantage for the battery-driven unit under normal weather conditions.
However, the humans require for comfort not just a low temperature, but a low dewpoint; cooling the air next to the skin reduces the temperature but not the dewpoint, at least until condensation begins. A dewpoint of 15–20° is required, which I think means that the cooling vest or pants actually need to reach a temperature of 15–20°. With an external temperature of 35°, that’s a ΔT as high as 20°, so according to Meerstetter’s guide we can’t expect a CoP of better than about 1.2, and that at (again) I/Imax ≈ 0.30, at which point Qh/Qmax ≈ 0.35 and Qh/Qc ≈ 1.75, so Qc/Qmax ≈ 0.20. So we’d need 5 “100W” Peltier elements to reject 100W of heat, and we need 83 W of battery power, emptying 11.1 18650s per hour, for a weight of 520 g of batteries emptied per hour. This is still almost twice as dense as the ice, but much pricier; your 4-hour unit now needs US$440 of batteries in it. Also, you have the potential safety issue of carrying 1.2 megajoules of highly volatile lithium batteries strapped to your body.
Under extreme conditions like those described in the Wikipedia article (as high as 65°) the ice-based system would continue to function exactly as well, while the Peltier-based system’s efficiency would degrade enormously, and it might cease to work entirely. But perhaps under normal weather conditions the Peltier approach might work better.