Green Cooling Factor

How much heat do Plants absorb?
0.68 kWh/lt

Research studies into the cooling potential of plants used to create Green Roofs and Walls are ongoing and always updated. The known effects are extraordinary.
Today it is possible to calculate how much green surface is needed to compensate the heat emitted by a building and substantially contribute to the city’s surroundings atmosphere cooling.
Plants convert water into aqueous vapour through Evapotranspiration.
During this event, thermal energy (heat) is absorbed by the surrounding environment; this process is known as Evapotranspirative Cooling (adiabatic cooling).
As water transitions from a liquid to a gaseous state, it absorbs energy, causing the temperature of the atmosphere around foliage to drop.

How much water does a plant contain?

The water content in its tissues can exceed 90% in organs in full vegetative activity and fall below 10% in seeds or dormant buds.
The water that enters in the plant’s constitution is called constitutional water. 99% of this, is eliminated in the atmosphere through transpiration process.

0.68 kWh/lt

The Energy needed to convert water from the liquid state to aqueous vapour mainly depends on temperature and other, less relevant factors such as relative humidity and atmospheric pressure of the surrounding environment. One litre of water at a temperature of 20 °C evaporates into 1673lt of vapour; this process requires 0.68kWh per litre of evaporated water.
The value may vary with the air temperature (Betzler 2012b; Kravcik et al. 2008).

How big is the green building cooling efect?

If we multiply the value of 0.68 kWh by the quantity of litres of water evaporated from the green surfaces (Green Roofs and Green Walls), we get the building’s evapotranspiration energy; released as cooling energy (2450KJ/lt) into the surrounding atmosphere..

Is it possible to measure the cooling capacity of Plants?

THE GREEN COOLING FACTOR
Green roofs and walls provide an option for reducing the thermal load on buildings, reducing the requirement for mechanised air conditioning and helping to mitigate urban heat islands.

The Green Density Factor

The Green Density Factor relates the amount of vegetation installed on the building to the surface area of the building’s footprint.
Therefore, the greater the surface
installed with Green Roofs or Green Walls, the greater the GDF.
The following parameters and subsequent calculations make it possible to measure the Green Density Factor of a building:

  • Length and width of the base of the building
  • Building base area
  • Height of a single floor and number of floors in the building
  • Total height of building
  • Green Roof area
  • Green Wall area

Higher buildings with Green Walls have a higher GDF due to the greater overall amount of vegetation installed. A GDF factor of more than 1 indicates a building with a green area greater than its base area.

The Green Cooling Factor

Based on the GDF obtained, it is possible to calculate the total energy and heat absorbed by green roofs and walls installed on the building, through Evapotranspirative cooling (EVE: Evapotranspiration Cooling Energy).
The building releases energy as sensible heat in the surrounding microclimate, due to the operation of HVAC (heating, ventilation, air conditioning) systems and electricity usage.
The Green Cooling Factor is the ratio between the energy used by plants, absorbed from the environment to complete Evapotranspiration, and the energy/heat emitted
in the same environment by a building’s HVAC systems.

Measuring parameters of the two Factors on the building:

GDF
“Green Density Factor”

  • L, W (mt) Length and width of the base of the building.
  • FP (mq) Building base area.
  • H (mt) Height of a single floor.
  • N number of floors in the building
  • H (mt) Total height of building.
  • Fr Fraction of Rooftop Garden (e.g.: Fr =0.8 means 80% green roof).
  • GRA (mq) Total green roof area (Fr x FP).
  • Fff Fraction of Green Wall (e.g.: Ff =0.25 means 25% green wall).
  • GFA (mq) Total Green Wall area (Ff x 2 x (l+w) x H.

GCF
“Green Cooling Factor”

  • R (lt7mq/gg.) Water percentage. Evapotranspiration value / square metre of vegetated surface. (es.: 3lt /mq /Day).
  • WC (lt) Total water consumption. (Evapotranspiration) (R x (GRA +GFA) x 365 days).
  • EEF (KWh/lt) Energy needed per litre of evaporated water (0.68kWh/lt).
  • EVE (kWh) Evapotranspiration Energy (WC x EEF).
  • NFA (mq) Total net floor area of building (0.8 x FB x N); 80% of the building base area ( effective iveability ) that requires HVAC systems. All floors are considered.
  • HVAC (kWh) total energy consumed for air conditioning, ventilation, heating (HVAC x NFA).

The data and arguments addressed in this section can be explored in further detail by consulting the text:
GDF –Green Densiy Factor and GCF – Green Cooling Factor
“A specific calculation method to integrate green roofs, green facades and their evapotranspiration cooling rate into the general planning procedure of architects and planners”
Written by Arch. Florian Betzler, ed. Tectum Verlag Marburg, 2016

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