HUMIDITY
The atmosphere is composed of billions upon billions of charged particles. It is the molecular fabric of the climates that engender our local identities. When we reconceive of architecture as a product of our environments, we look to the elements as inspiration for form and matter.
The air that sustains life is composed of many elements, the most concentrated being Nitrogen (N2) at approximately 78%. Oxygen (O2) only makes up 21%, in addition to Argon (Ar) at 1% and Carbon Dioxide (CO2) at .038%. Other elements and molecules such as Neon (Ne), Methane (CH4), Helium (He), Krypton (Kr), Hydrogen (H2), and Xenon (Xe) roughly complete the atmosphere that we breathe. But of this composition, it is water that is most fascinating and most abundant, with concentration levels that help define the climate types and the cultures that evolve from them.
Table of the composition of dry air at sea level.
WATER
Water, or H2O is a dipole molecule, hexagonal in geometric formation with a molar mass of 18 g/mol.
A water molecule is lighter than a dioxide molecule (32 g/mol) and is less dense than air.
Its two hydrogen atoms are covalently bonded to a single oxygen atom, completing the valence electron orbitals (co-valence) of the individual atoms.
The oxygen atom has a significantly greater electronegativity than hydrogen atoms due to its greater number of protons in the nucleus, making the water molecule a net dipole structure, with the oxygen acting as a negative pole and the hydrogen atoms as the positive poles. This dipole assembly is what gives rise to water’s high boiling point – the exchange between the hydrogen atoms and the oxygen atom is in a state of extreme rapid movement creating what is called hydrogen bonds (the rapid formation and deformation of hydronium (H3O+) and hydroxide (OH-)). 1
The strength of hydrogen bonds creates the surface tension, for which give water many significant properties. One such property is high conductivity. Water is a well-known conductor, given by the ease of energy exchange between its molecules. Furthermore, its dipole-dipole connections and rapid electron exchange enable fast rates of energy transfer, giving water a high thermal diffusivity. This means that humid air has higher conductivity and diffusivity levels than dry air.
In addition to conductivity and diffusivity, water has a high heat capacity. The rapid dissociation of the hydrogen bonds establishes high degrees of freedom in electron movement that enables the absorption of high energy levels before an increase in temperature – thus a high boiling point. 2
Diagram of water molecules. 1
WATER AS ENERGY
The high heat capacity of water is used often in passive design strategies in the heating and cooling of buildings. Radiant floors and ceilings are created by placing tubes in the surfaces of spaces that transport water (or other inorganic fluids) for the absorption/emission of heat. As the energy systems in a space evolve towards thermal equilibrium, water acts as a strong stabilizer able to hold and distribute large amounts of energy per unit. Given this storage capacity in comparison to dry air without undergoing a phase change makes for an efficient heating and cooling system.
The heat capacity of water is important for not only thermodynamically-driven architectural design, but also for the sustainability of our planet. The heat capacity of water enables our atmosphere to fluctuate thermally and absorbs over 80% of the planet’s global warming. This heat absorption is called Latent Heat. There are two categories of latent heat: Latent Heat of Fusion which is the transition between a liquid and a solid; and Latent Heat of Evaporation which is the transition between a liquid and a gas. When meteorologists monitor humidity levels in the atmosphere, they do so due
Latent Heat of Fusion and Condensation. 3
to the high levels of energy in transformation between the phases of water. As water evaporates, that energy has the potential to become kinetic energy, creating and influencing barometric pressure systems. The amount of energy within an atmospheric system is called Enthaply [H] and is contingent on the pressure and volume of the system.
PSYCHOMETRIC CHART
The psychometric chart pioneered by Carrier in 1904 as a means of comparing the properties of gas-vapor mixtures graphs various enthalpy levels at a given atmospheric pressure and volume. 4 Represented by diagonal lines across the diagram, it enables the comparison between different energy systems based on their composition of air and water per a given temperature. Along the Y-axis lists Absolute Humidity levels – the total amount of water units present in a unit of air. The parabolic lines represent Relative Humidity levels – the percentage of water vapor to dry air in the system. The horizontal axis lists Dry Bulb temperatures – the average kinetic energy or temperature of air without the presence of water vapor, while step diagonal lines represent the Wet Bulb temperature which takes into consideration the effect of humidity on the temperature of the system. It is the wet bulb temperature that we register in our thermal experiences of a space, but it is the dry bulb temperature with which we set our thermostats.
The psychometric chart is useful for understanding mechanisms for changing the temperature or humidity levels of an atmospheric system. For example, a hot dry system can be transformed into a temperate system by increasing the concentration of water vapor in the air. This is achieved by introducing plants or water features into a hot dry space, also known as Evaporative Cooling. However, this strategy is limited by a maximum level of vapor that a system can hold, called the Dew Point. At this concentration, water condenses into liquid. The amount of water need to reach the dew point varies by temperature and this temperature is called the Dew Point Temperature. For hot climates, the dew point temperature when relative humidity is 100% is significantly lower, sometime by more than 20C, than dry air at that temperature, whereas in cold climates the dew point
temperature is only a few degrees cooler than dry air. This can be a problem for the construction of buildings in cold climates due to condensation occurring within walls between warm interiors and cold exteriors, leading to hazardous organic growth within cavities.
In the event that the increase of water vapor in a system is insufficient in reducing the wet bulb temperatures of a system, the system can be cooled to its dew point temperature, after which a condenser would remove the water from the system to a lower temperature and then reheat to the desired specifications. This process can be illustrated by a vapor-compressor refrigeration cycle.
PASSIVE STRATEGIES
In general, high levels of humidity can be difficult in passive design systems. In cold humid climates, thermal strategies focus on using solar gains to increase radiant temperatures (dry bulb) of a space, which is limited by the hours of sunlight and by increased cloud coverage that is typical of humid climates. In hot humid climates, thermal strategies consist of a constant internal air cycle to ventilate the spaces. Usually hot climates can benefit from heat loss by the reradiation of the building envelope materials back into the night sky (radiant transfer to the solar system), however this too is limited by cloud coverage.
One unique technique for addressing high humidity levels in hot climates is the use of water walls. A body of water of a temperature less than the dew point temperature is introduced into a space. The vapor in contact with the water will condense on the surface of the water, thus dehumidifying the adjacent space, reducing the wet bulb temperature of that space.
Vapor-Compressor Refrigerant Cycle
Additional alternative strategies to mitigate high humidity levels focus on the molecular properties of water in the creation of advanced materials. Salts are hydrophilic ionic compounds that hydrolyze to produce hydronium and hydroxides. Many desiccant materials contain salts in their composition to break apart the water molecule. Hydrophobic materials rely on the surface tension of water to ‘repel’ the water. Hydrophobic materials tend to be nonpolar in their molecular construct and therefore are unattracted to the polar H2O molecules that cluster together. The difference between hydrophilic and hydrophobic materials can be determined by the contact angle of the water droplet on the surface. A chilled hydrophobic material can cause vapor to condense on the surface to be removed from a system.
Diagram showing contact angle concept. 5
Other materials simply use the surface tension of water in a capillary action to absorb vapor for removal. Unfired clay bricks or various cloths are a timeless example in which the materials absorb water throughout the day to be evaporated during high noon. Additionally several desert plants can absorb water in high humidity climates.
It is important when addressing humidity to consider it as an energy system in a gaseous phase. The water molecules in the air alter the properties of the system, increasing conductivity and diffusivity, decreasing the density of the system through a lower volumetric mass, and increasing the internal energy of that system that, in all, affects its thermal exchange with the objects in this proximity.
WORKS CITED
1 - “Properties of Water.”Wikipedia, the Free Encyclopedia, January 2, 2015.http://en.wikipedia.org/w/index.php?title=Properties_of_water&oldid=640610171.
2 - “Heat Capacity.” Wikipedia, the Free Encyclopedia, December 26, 2014.http://en.wikipedia.org/w/index.php?title=Heat_capacity&oldid=639737190.
3 - “Latent Heat.” Wikipedia, the Free Encyclopedia, December 20, 2014.http://en.wikipedia.org/w/index.php?title=Latent_heat&oldid=636188779.
4 - “Psychrometrics.” Wikipedia, the Free Encyclopedia, December 20, 2014.http://en.wikipedia.org/w/index.php?title=Psychrometrics&oldid=638665661.
5 - “Wetting.” Wikipedia, the Free Encyclopedia, December 22, 2014.http://en.wikipedia.org/w/index.php?title=Wetting&oldid=638251371.