5 ms·

To compare to an air conditioner. This device has about 40W of cooling per m2. Apparently in Australia you should size between 80 and 120 W/m2 of air condition

To compare to an air conditioner. This device has about 40W of cooling per m2. Apparently in Australia you should size between 80 and 120 W/m2 of air conditioning (I think this is cooling watts rather than power usage watts) - https://www.google.com/amp/s/www.crownpower.com.au/blog/choosing-the-right-sized-air-conditioner/ https://www.google.com/amp/s/www.crownpower.com.au/blog/choo... So that means every square meter of living space needs 2 square meters of radiative cooling (assuming no other passive cooling infrastructure). I suspect you'd see further inefficiencies getting the heat to the passive cooler. So it's within the same order of magnitude of an ac, but not powerful enough that it would be straightforward to retrofit.

varjag 5y ago
There's a lot of potential applications outside of habitat AC in Australia. For example cooling down stand-alone hardware in the field.
kumarvvr 5y ago
A typical AC cycles its power input. Its not constantly On. One benifit of using this system could be to continuously remove heat from the house and then use an AC on top of it. There is always a consumption value to free beer.
- sandworm101 5y ago
  >> A typical AC cycles its power input. If it has reached the requested temperature. Like basically all consumer thermostats, it is a bang-bang controller. There is no set on-off cycle. If the AC unit is running at capacity, ie it is properly sized for requirements, it will just be on all the time.
  - PaulHoule 5y ago
    But the temperature goes up and down all the time so there is no "fixed requirement". If the AC is on all the time it is most likely undersized for the requirement at that time and can't maintain the desired temperature.
    - silon42 5y ago
      Check out interter ACs. Good ones should scale from 20-100% in power or better, so should be able to stay on for most of the time.
      - tempestn 5y ago
        Yeah, most central heat pumps installed now are inverter models (also referred to as variable speed). More efficient and more comfortable since you've got a continuous flow of cool air, rather than blasts of cold interspersed with nothing.
    - sandworm101 5y ago
      In a better-than-consumer setup you will have multiple chillers. Most will just stay on, with one going on-off to handle the variable bit of the load. Starting and stopping electric motors is less efficient tha just keeping them running as much as possible.
the8472 5y ago
The technology has advanced since then, now white paints with high emissivity in the infrared window are being researched. So if you cover the entire building with that you would get some free, always-on cooling that way.
- PaulHoule 5y ago
  Opening the windows at night and running fans to equalize the temperature, then putting space blankets over the windows for the day works wonders on hot days in upstate NY.
  - dsr_ 5y ago
    It's very good as long as two conditions apply: - the humidity has to be comfortably low - the outside temperature has to be low enough Historically, this is usually the case. In the last few weeks, I've had one or both of those fail to apply on the majority of nights. Dropping to 65F doesn't help when the outside air is also at 99% humidity. If the overnight low is 75F, we're not getting much cooling out of it.
    - PaulHoule 5y ago
      Any system has to deal with time-variable conditions. I dream of getting a geothermal heat pump for my 1850s farm house which is normally heated with two wood stoves but has a propane backup. (e.g. the kind of compact heater that you see all the time in people's apartments in anime) At points south the capacity of that kind of system is set by cooling demand but where I live it is set by heating demand. The woodstove could pick up the slack on the coldest days, but that defeats the main selling point of the heat pump which is extreme comfort (e.g. it switches seamlessly from heating to cooling)
    - hinkley 5y ago
      - the outside air quality has to be okay Between pollen count and pollution this is not always the case, and keeping the windows closed and running both the AC and an air filter unfortunately has health benefits for some individuals.
PaulHoule 5y ago
The weakness it has it that it needs a clear sky to work. For overcast muggy days or even a high coverage of cumulus clouds, performance will be absent or degraded. It works at night, however, and for best results you could maximize insulation and "thermal mass" inside the building and minimize radiative transport through the windows. The best thing about air conditioning, however, is de-humidification and that is a matter of cooling the air more than you have to and then re-heating it. I live in an 1850 farmhouse and the reason I want a ground source heat pump is that the humidity destroys books and other printed matter. I have inkjet prints curling off the walls and detailed logs of how 3M's best products only work 90% of the time in my applications.
- eloff 5y ago
  Adhesives are not built for high-humidity environments. My parents live in the tropical rainforest of Panama. One of the challenges is that anything stuck together with adhesive usually comes apart over time. The glass panel on the door of my mom's oven fell off.
  - PaulHoule 5y ago
    As an engineer I don't accept that things have to suck. If other people think failure is OK I can't do anything about it, but if I have the problem that "Adhesive X does not work in Environment Y" I am going to change the adhesive, change the environment, or not use an adhesive.
- etskinner 5y ago
  I'm having trouble understanding why a clear sky is important. Surfaces radiate based on their temperature and emissivity only, right? So why would it matter what the surface is emitting toward? Perhaps what I'm missing is that clouds emit some radiative heat back to the surface, whereas a clear sky emits very little, so the net heat loss from the surface under a cloudy sky would be lower.
  - Sanzig 5y ago
    > Perhaps what I'm missing is that clouds emit some radiative heat back to the surface, whereas a clear sky emits very little, so the net heat loss from the surface under a cloudy sky would be lower. This is exactly it, yes.
fy20 5y ago
Might as well just get solar PV then, that's around 150W per m2.
- hatsunearu 5y ago
  150W electricity, so you need to run a refrigerator cycle to pump that heat out of your house, which comes at a massive efficiency penalty.
  - zplizzi 5y ago
    Refrigerators actually have greater than 100% efficiency, often like 300% or so. Because you're just moving the heat, not creating it. Sounds like magic but it's not.
    - hatsunearu 5y ago
      ah true. though that number is coefficient of performance, not efficiency
- thescriptkiddie 5y ago
  And an efficient heat pump can move four times as much energy as it consumes. But if some special paint/tile/insulation can reduce your needed cooling capacity by 30% you'll save a lot of money on equipment.
mannykannot 5y ago
I am trying to figure out whether one should take into account the cooling power of the surfaces being replaced. The figure of 80-120 W/m2 for air conditioning is presumably based on conventional building materials, which have negative cooling power. In the paper, the figure of 40 W/m2 seems to be the net cooling power, which is defined in equation 1 as being the power radiated away minus various inflows of heat: radiatively, from the atmosphere; radiatively, from the sun; and by conduction and convection. As far as I can see, these corrections are all for this particular surface, not the surface it might be replacing. These will not, in general, be the same, and, given that this new surface is both highly reflective and vacuum-insulated, I would guess that its values for these properties are lower than the conventional building materials on which the a/c rule-of-thumb is based. Nevertheless, I doubt that replacing the entire roof with this material would be sufficient cooling, on its own, in the Australian case, and I agree that this would not likely be a straightforward retrofit, to say the least!
- parineum 5y ago
  This was my thought as well. I have an attic fan and monitor the temperature inside to control it. It regularly gets 120+ F in there on an 80 F day. I have to think the majority of the heating of my house is coming from the attic.
hatsunearu 5y ago
Keep in mind there is nothing preventing you from just angling the device (in its extreme, vertically) and just get an arbitrary amount of radiative surface with a given flat footprint. (other than of course, it looking unsightly and construction costs) edit: it would probably help a lot of you angle it such that it is normal to the sun rays, like where i live the sun sweeps from the east to the west, so if you angle the device north or south it would probably work even better.
jryb 5y ago
That's true, but I don't see why the two couldn't be used in tandem, thus reducing AC power consumption substantially.

varjag 5y ago

There's a lot of potential applications outside of habitat AC in Australia. For example cooling down stand-alone hardware in the field.

kumarvvr 5y ago

A typical AC cycles its power input. Its not constantly On. One benifit of using this system could be to continuously remove heat from the house and then use an AC on top of it. There is always a consumption value to free beer.

sandworm101 5y ago
>> A typical AC cycles its power input. If it has reached the requested temperature. Like basically all consumer thermostats, it is a bang-bang controller. There is no set on-off cycle. If the AC unit is running at capacity, ie it is properly sized for requirements, it will just be on all the time.
- PaulHoule 5y ago
  But the temperature goes up and down all the time so there is no "fixed requirement". If the AC is on all the time it is most likely undersized for the requirement at that time and can't maintain the desired temperature.
  - silon42 5y ago
    Check out interter ACs. Good ones should scale from 20-100% in power or better, so should be able to stay on for most of the time.
    - tempestn 5y ago
      Yeah, most central heat pumps installed now are inverter models (also referred to as variable speed). More efficient and more comfortable since you've got a continuous flow of cool air, rather than blasts of cold interspersed with nothing.
  - sandworm101 5y ago
    In a better-than-consumer setup you will have multiple chillers. Most will just stay on, with one going on-off to handle the variable bit of the load. Starting and stopping electric motors is less efficient tha just keeping them running as much as possible.

the8472 5y ago

The technology has advanced since then, now white paints with high emissivity in the infrared window are being researched. So if you cover the entire building with that you would get some free, always-on cooling that way.

PaulHoule 5y ago
Opening the windows at night and running fans to equalize the temperature, then putting space blankets over the windows for the day works wonders on hot days in upstate NY.
- dsr_ 5y ago
  It's very good as long as two conditions apply: - the humidity has to be comfortably low - the outside temperature has to be low enough Historically, this is usually the case. In the last few weeks, I've had one or both of those fail to apply on the majority of nights. Dropping to 65F doesn't help when the outside air is also at 99% humidity. If the overnight low is 75F, we're not getting much cooling out of it.
  - PaulHoule 5y ago
    Any system has to deal with time-variable conditions. I dream of getting a geothermal heat pump for my 1850s farm house which is normally heated with two wood stoves but has a propane backup. (e.g. the kind of compact heater that you see all the time in people's apartments in anime) At points south the capacity of that kind of system is set by cooling demand but where I live it is set by heating demand. The woodstove could pick up the slack on the coldest days, but that defeats the main selling point of the heat pump which is extreme comfort (e.g. it switches seamlessly from heating to cooling)
  - hinkley 5y ago
    - the outside air quality has to be okay Between pollen count and pollution this is not always the case, and keeping the windows closed and running both the AC and an air filter unfortunately has health benefits for some individuals.

PaulHoule 5y ago

The weakness it has it that it needs a clear sky to work. For overcast muggy days or even a high coverage of cumulus clouds, performance will be absent or degraded. It works at night, however, and for best results you could maximize insulation and "thermal mass" inside the building and minimize radiative transport through the windows. The best thing about air conditioning, however, is de-humidification and that is a matter of cooling the air more than you have to and then re-heating it. I live in an 1850 farmhouse and the reason I want a ground source heat pump is that the humidity destroys books and other printed matter. I have inkjet prints curling off the walls and detailed logs of how 3M's best products only work 90% of the time in my applications.

eloff 5y ago
Adhesives are not built for high-humidity environments. My parents live in the tropical rainforest of Panama. One of the challenges is that anything stuck together with adhesive usually comes apart over time. The glass panel on the door of my mom's oven fell off.
- PaulHoule 5y ago
  As an engineer I don't accept that things have to suck. If other people think failure is OK I can't do anything about it, but if I have the problem that "Adhesive X does not work in Environment Y" I am going to change the adhesive, change the environment, or not use an adhesive.
etskinner 5y ago
I'm having trouble understanding why a clear sky is important. Surfaces radiate based on their temperature and emissivity only, right? So why would it matter what the surface is emitting toward? Perhaps what I'm missing is that clouds emit some radiative heat back to the surface, whereas a clear sky emits very little, so the net heat loss from the surface under a cloudy sky would be lower.
- Sanzig 5y ago
  > Perhaps what I'm missing is that clouds emit some radiative heat back to the surface, whereas a clear sky emits very little, so the net heat loss from the surface under a cloudy sky would be lower. This is exactly it, yes.

fy20 5y ago

Might as well just get solar PV then, that's around 150W per m2.

hatsunearu 5y ago
150W electricity, so you need to run a refrigerator cycle to pump that heat out of your house, which comes at a massive efficiency penalty.
- zplizzi 5y ago
  Refrigerators actually have greater than 100% efficiency, often like 300% or so. Because you're just moving the heat, not creating it. Sounds like magic but it's not.
  - hatsunearu 5y ago
    ah true. though that number is coefficient of performance, not efficiency
thescriptkiddie 5y ago
And an efficient heat pump can move four times as much energy as it consumes. But if some special paint/tile/insulation can reduce your needed cooling capacity by 30% you'll save a lot of money on equipment.

mannykannot 5y ago

I am trying to figure out whether one should take into account the cooling power of the surfaces being replaced. The figure of 80-120 W/m2 for air conditioning is presumably based on conventional building materials, which have negative cooling power. In the paper, the figure of 40 W/m2 seems to be the net cooling power, which is defined in equation 1 as being the power radiated away minus various inflows of heat: radiatively, from the atmosphere; radiatively, from the sun; and by conduction and convection. As far as I can see, these corrections are all for this particular surface, not the surface it might be replacing. These will not, in general, be the same, and, given that this new surface is both highly reflective and vacuum-insulated, I would guess that its values for these properties are lower than the conventional building materials on which the a/c rule-of-thumb is based. Nevertheless, I doubt that replacing the entire roof with this material would be sufficient cooling, on its own, in the Australian case, and I agree that this would not likely be a straightforward retrofit, to say the least!

parineum 5y ago
This was my thought as well. I have an attic fan and monitor the temperature inside to control it. It regularly gets 120+ F in there on an 80 F day. I have to think the majority of the heating of my house is coming from the attic.

hatsunearu 5y ago

Keep in mind there is nothing preventing you from just angling the device (in its extreme, vertically) and just get an arbitrary amount of radiative surface with a given flat footprint. (other than of course, it looking unsightly and construction costs) edit: it would probably help a lot of you angle it such that it is normal to the sun rays, like where i live the sun sweeps from the east to the west, so if you angle the device north or south it would probably work even better.

jryb 5y ago

That's true, but I don't see why the two couldn't be used in tandem, thus reducing AC power consumption substantially.