Introduction
For thousands of years before the invention of electricity, civilizations like the Persians in the Arabian desert or Celtic people in the British Isles designed their homes and cities around ancient practices to help regulate the temperature in their homes.
Passive heating and cooling have been developed over decades, each generation refining and developing the knowledge of those who came before. Unfortunately, electricity has become so widely used that we have nearly forgotten those passive strategies for controlling our environments.
Due to the need to go carbon neutral, these ancient strategies are having a resurgence in the mainstream construction and design industry to help curb anthropogenic climate change
So let us look at the basic principles that must be considered when designing and implementing these passive heating and cooling systems.
Know the basics
In the simplest of terms, the principle of heat transfer is concerned with only two things: temperature and heat flow. Temperature relates to the amount of thermal energy available, whereas heat flow relates to the movement of thermal energy from place to place.
On an atomic scale, thermal energy is related to the kinetic energy of molecules. The greater a material’s temperature, the greater the thermal agitation of its neighbouring molecules. It is natural for regions containing greater molecular kinetic energy to pass this energy to areas with less kinetic energy.
Several material properties can modulate the heat transferred between two regions at various temperatures. These include thermal conductivities, specific heats, material densities, surface emissivities, fluid velocities, viscosities, and more. Together, these properties make up the solution to many heat transfer problems an involved processes.
Regions with greater molecular kinetic energy will pass their thermal energy to areas with less molecular energy through direct molecular collisions, a process known as conduction. In metals, a significant portion of the transported thermal energy is also carried by conduction-band electrons.
Heat conducting through a fluid or gass leads to a local volumetric expansion. It displaces the less dens fluid, thereby transporting heat by fluid motion (i.e. convection) in addition to conduction. As a result of gravity-induced pressure gradients, the expanded fluid parcel becomes buoyant.
All materials radiate thermal energy in amounts determined by their temperature. Photons of light carry energy in the infrared and visible portions of the electromagnetic spectrum. When temperatures are uniform, the radiative flux between objects is in equilibrium, and no net thermal energy is exchanged. When temperatures are not uniform, thermal energy is transported from higher to lower temperatures.
Passive Cooling Strategies
A yakhchal is an ancient form of refrigeration invented by the Persians to create ice and store food in the heart of the desert. Covered with a beehive-shaped dome with a large ventilation hole at the top, it encourages convection in the yakhchal. It creates a temperature difference between 10 and 20 degrees Celsius by submerging the cavern into the soil and using the more stable thermal mass of the subsoil. The cavern was sealed with a watertight lime plaster called Sarooj, creating an airtight seal.
By using a slow trickle of water from the shadow side into the cavern, the bricks are saturated and retain the moisture that helps stabilize the internal temperature of the yakhchal.
Densely packed desert cities employed the principle of convection to rid street-level temperatures. Occasional towers were created to catch the predominant wind, and due to rising hot aid are sucked out and replaced with fresh cool air.
Also called natural ventilation, passive ventilation uses natural systems like the buoyancy of hot air and wind to create convection in a building, pulling in fresh cool air and expelling stale hot air.
Passive cooling can be achieved in several ways. Passive cooling is primarily created by opening and closing windows and vents. Wind-driven ventilation creates pressure differences around a building and Buoyancy-driven ventilation created by temperature differences. Night cooling can be achieved by opening windows or louvres for pre-set periods at night to allow the structure’s thermal mass to purge heat that is then replaced the following day.
When water changes from a liquid to a gas through evaporation, it absorbs heat from the atmosphere. This form of cooling has been used for thousands of years to control indoor temperatures, and even appears on Egyptian murals dating back to 2500BCE when porous clay pots filled with water were stacked in front of windows or doors and fanned to induce evaporation.
More modern technology uses two-stage evaporative cooling that doesn’t add additional; moisture to the air.
Passive heating strategies
A Trombe wall is a heating method that uses thermal mass and convection to trap and redistribute heat—consisting of an outer glass wall and a thick wall (generally painted black) with ample space for air to circulate. Sunlight heat the air in the void space that is absorbed by the brick wall’s body mass and dissipates into the interior at night when the temperature plummets outside.
One of the main drawbacks of a Tromb wall is for them to be most effective, they need to face towards the main direction of the sun (North in the Southern Hemisphere and South in the Northern). Unfortunately, this is also where light most effectively penetrates to illuminate and heat our homes, so a combination of strategies must be employed to retain efficacy and comfort.
Solar Heat Gain (SHG) describes how solar radiation is converted into heat.
If not adequately controlled, this can lead to overheating and uncomfortable internal temperatures, but if properly utilized can create comfortable ambient temperatures indoors without much additional heating or cooling.
It is advised that proper calculations described by the Solar Heat Gain Coefficient to calculate how much heat is gained and how much needs to be controlled for a comfortable environment. If you gain too much heat, louvres or shading elements might be needed to control light levels or even require double glazing or smaller openings. These strategies are all site and environmental-dependent, and design specialists are needed.
The thermal mass of an object is dependent on density, conductivity, size and temperature. Objects with significant thermal mass can store energy in the form of heat for long periods. Soil is an excellent source of thermal mass and can be bermed on the outside of a building to aid the structure regulate temperatures.
But walls and other parts of the structure can also store thermal heat if it has sufficient mass. Rammed Earth walls are great, while timber and glass have a low thermal mass.
Conclusion
With rising energy costs and the effects of climate change, it is becoming increasingly important to cut energy consumption. Heating and cooling account for almost 60% of the consumption of an average household.
By harnessing the natural processes and strategies above during the design phase, the embedded energy and lifelong consumption can be kept to a minimum, leading to better, energy efficient and more comfortable living spaces.
