Transpiration, Evaporation and Condensation
Plants at the Heart of the Blue-Green System
The key mechanism in both the cooling and solar energy collection modes start with the utilisation of living plants as an integral component of the working blue/green system. Living plants have an amazing capacity to convert heat energy into what is called the ‘latent heat of evaporation’. Before this process is examined more closely, however, a quick glance at plant biology might prove beneficial.
Transpiration is the process of water evaporation from plant foliage, mostly from pores called stomata on the underside of leaves. This process affects the ascent of moisture and nutrients from the roots to the leaves supplying the food-manufacturing cells with water needed for photosynthesis. In addition the process prevents the plant tissue from overheating and provides the moisture necessary to diffuse carbon dioxide into plant cells and enable the emission of oxygen. The amount of water transpired by a plant is staggeringly large. For every kilogram of plant tissue gained in growth, a plant will transpire 200-1000 kilograms of water. Transpiration is a passive process largely controlled by the humidity of the surrounding atmosphere and the moisture content of the soil.
The Latent Heat of Evaporation
On a molecular level this is the energy (in the form of heat) required to break the bonds between molecules in order to turn a liquid into a gas. In the case of transpiration, the ‘phase change’ is from water to water-vapour. Note that the temperature of the water does not change as it becomes vapour, but rather some of the heat energy from the plant is absorbed by the process of molecular change as the water is transpired.
Release of Latent Heat Energy to Cooling Film During Condensation
Cooling the inner skin with the surfactant film provides a cool surface within the interior environment for water-vapour, with its latent heat of evaporation, to condensate. During condensation the same heat energy that was taken from the plant to evaporate (transpire) its water is released to the interior skin as liquid water molecules are remade. Since water is thermally-conductive and the skin is thin, the transfer of heat energy from condensate to surfactant film is almost instantaneous. Thus negligible heat energy is radiated back into the environment during the condensation phase change.