Difference between revisions of "Cooling"
(Add brief description of the solutions and challenges for cooling on Mars.) |
|||
| Line 1: | Line 1: | ||
| − | Because industrial, residential, or agricultural activities use energy, they will produce heat. This heat eventually needs to be dissipated into the environment, or the [[temperature]] will continue to rise. Heat can be transported via convection, conduction, and radiation. | + | Because industrial, residential, or agricultural activities use energy, they will produce heat. This heat eventually needs to be dissipated into the environment, or the [[temperature]] will continue to rise. Heat can be transported via convection, conduction, and radiation, or by mass transfer and phase change. Agriculture in particular requires large amounts of light, typically between 400 and 600 W/m2 to be productive. This light turns into heat, that must be removed from the greenhouse or grow room. A large part of the heat is turned into water vapor by the plants by evapo-transpiration. This water must be removed from the air as condensation. This is a form of mass transfer heat transportation. |
| − | == Convection == | + | ==Convection== |
<b>Convection</b> on Mars is minimal due to its very thin atmosphere, with even fan-assisted convection appearing to be less mass efficient than radiative cooling<ref>von Arx and Delgado, "Convective heat transfer on Mars", AIP Conference 1991 https://aip.scitation.org/doi/abs/10.1063/1.40133?journalCode=apc</ref>. | <b>Convection</b> on Mars is minimal due to its very thin atmosphere, with even fan-assisted convection appearing to be less mass efficient than radiative cooling<ref>von Arx and Delgado, "Convective heat transfer on Mars", AIP Conference 1991 https://aip.scitation.org/doi/abs/10.1063/1.40133?journalCode=apc</ref>. | ||
| − | == Conduction == | + | the convection equation is Q=h*A*dT, where Q is the power (Watts), A is the area(m2), h is the experimental coefficient for convective heat transfer (J/m2*°K) and dT is the temperature difference (°K) between the surface and the convecting fluid. |
| + | |||
| + | ==Conduction== | ||
<b>Conduction</b> into Mars regolith or megaregolith (soil or bedrock) may be feasible, since the ground's average temperature is around -60C. On Earth ground-source heat pumps are feasible for cooling. On Mars, depending on the ground conditions, sufficient cooling may be available via the building's foundation alone, or this could be augmented with cooling channels, which could be combined with existing utility trenches used for power or materials. | <b>Conduction</b> into Mars regolith or megaregolith (soil or bedrock) may be feasible, since the ground's average temperature is around -60C. On Earth ground-source heat pumps are feasible for cooling. On Mars, depending on the ground conditions, sufficient cooling may be available via the building's foundation alone, or this could be augmented with cooling channels, which could be combined with existing utility trenches used for power or materials. | ||
| − | Challenges include the low temperature of the ground requiring a careful choice of working fluid, and interior humidity may deposit frost on cooling panels. | + | Challenges include the low temperature of the ground requiring a careful choice of working fluid, and interior humidity may deposit frost, and will certainly condensate, on cooling panels. |
Some regolith, such as dry dust or loose rock, may have poor thermal conductivity, requiring either additional conduction area such as drilled cooling pipes or channels, or a soil treatment such as water injection to increase thermal conductivity by filling the soil pore voids with ice. | Some regolith, such as dry dust or loose rock, may have poor thermal conductivity, requiring either additional conduction area such as drilled cooling pipes or channels, or a soil treatment such as water injection to increase thermal conductivity by filling the soil pore voids with ice. | ||
| − | == Radiation == | + | ==Radiation== |
<b>Radiative</b> cooling is a standard solution for spacecraft, since the large temperature difference between outer space (around 3K) and human habitable areas (around 300K) gives substantial radiative cooling from high emissivity surfaces. | <b>Radiative</b> cooling is a standard solution for spacecraft, since the large temperature difference between outer space (around 3K) and human habitable areas (around 300K) gives substantial radiative cooling from high emissivity surfaces. | ||
| Line 20: | Line 22: | ||
One challenge with radiative cooling is keeping sunlight from warming the radiator panels. A possible mitigation is a careful arrangement of mirrors<ref>Lunarpedia Lunar Radiator https://lunarpedia.org/w/Lunar_Radiator</ref> to reflect sunlight away, or a paint with high visible reflectance but high thermal emittance. | One challenge with radiative cooling is keeping sunlight from warming the radiator panels. A possible mitigation is a careful arrangement of mirrors<ref>Lunarpedia Lunar Radiator https://lunarpedia.org/w/Lunar_Radiator</ref> to reflect sunlight away, or a paint with high visible reflectance but high thermal emittance. | ||
| + | == Mass transfer and phase change == | ||
| + | The mass transfer equation is Q=m*Cp*dT where Q is the power in Watts, m is the mass flow in kg/s, Cp is the specific heat in Joules/kg/°K and dT is the temperature difference of the moving flow. | ||
| + | The phase change equation is Q=m*Phe where m is the mass flow in kg/s and Phe is the phase change energy in J*/kg. | ||
[[Category: HVAC]] | [[Category: HVAC]] | ||
| + | <references /> | ||
Revision as of 12:08, 13 September 2020
Because industrial, residential, or agricultural activities use energy, they will produce heat. This heat eventually needs to be dissipated into the environment, or the temperature will continue to rise. Heat can be transported via convection, conduction, and radiation, or by mass transfer and phase change. Agriculture in particular requires large amounts of light, typically between 400 and 600 W/m2 to be productive. This light turns into heat, that must be removed from the greenhouse or grow room. A large part of the heat is turned into water vapor by the plants by evapo-transpiration. This water must be removed from the air as condensation. This is a form of mass transfer heat transportation.
Convection
Convection on Mars is minimal due to its very thin atmosphere, with even fan-assisted convection appearing to be less mass efficient than radiative cooling[1].
the convection equation is Q=h*A*dT, where Q is the power (Watts), A is the area(m2), h is the experimental coefficient for convective heat transfer (J/m2*°K) and dT is the temperature difference (°K) between the surface and the convecting fluid.
Conduction
Conduction into Mars regolith or megaregolith (soil or bedrock) may be feasible, since the ground's average temperature is around -60C. On Earth ground-source heat pumps are feasible for cooling. On Mars, depending on the ground conditions, sufficient cooling may be available via the building's foundation alone, or this could be augmented with cooling channels, which could be combined with existing utility trenches used for power or materials.
Challenges include the low temperature of the ground requiring a careful choice of working fluid, and interior humidity may deposit frost, and will certainly condensate, on cooling panels.
Some regolith, such as dry dust or loose rock, may have poor thermal conductivity, requiring either additional conduction area such as drilled cooling pipes or channels, or a soil treatment such as water injection to increase thermal conductivity by filling the soil pore voids with ice.
Radiation
Radiative cooling is a standard solution for spacecraft, since the large temperature difference between outer space (around 3K) and human habitable areas (around 300K) gives substantial radiative cooling from high emissivity surfaces.
The Stefan-Boltzmann law describes the thermal emission of a black body radiator as j=e σ T4, where the radiated power in watts j is equal to the surface emissivity e (between 0 and 1), a constant σ, and the fourth power of thermodynamic temperature T. For a surface at 293K (about 20C) with emissivity 0.8, the black body radiative cooling is 334 W/m2 when facing the cold dark of space.
On Mars, during the nighttime a structure's roof could be used for radiative cooling, which could be as simple as a high-emissivity coating applied to the existing roof.
One challenge with radiative cooling is keeping sunlight from warming the radiator panels. A possible mitigation is a careful arrangement of mirrors[2] to reflect sunlight away, or a paint with high visible reflectance but high thermal emittance.
Mass transfer and phase change
The mass transfer equation is Q=m*Cp*dT where Q is the power in Watts, m is the mass flow in kg/s, Cp is the specific heat in Joules/kg/°K and dT is the temperature difference of the moving flow.
The phase change equation is Q=m*Phe where m is the mass flow in kg/s and Phe is the phase change energy in J*/kg.
- ↑ von Arx and Delgado, "Convective heat transfer on Mars", AIP Conference 1991 https://aip.scitation.org/doi/abs/10.1063/1.40133?journalCode=apc
- ↑ Lunarpedia Lunar Radiator https://lunarpedia.org/w/Lunar_Radiator
