Sensible Heat vs. Latent Heat
An air conditioning system’s ‘heat load’ comprises of two components: Sensible Heat and Latent Heat. Understanding the difference between these two types of heat (energy transfer) can make a huge difference when it comes to properly caring for and growing plants.
Sensible Heat: Sensible heat causes a change in overall temperature, which ismeasurable by a thermometer.
Latent Heat: Latent heat is the amount of heat (absorbed or released) that’s requiredto change the physical state, or phase, of a substance with no change in the temperature of the substance.
|This graph shows the principle differences between sensible heat and latent heat. As you can see, the energy required to change the actual temperature of a solid is defined as sensible heat, whereas latent heat refers to the period where a phase change occurs (ice melting, for example).|
Back to the Plants
In an air-conditioned building, where plants are growing, sensible and latent heat loads are remarkably similar. Lights, and other electronic or motorized equipment, produce most of the sensible heat in the area—plants generate nearly all of the latent heat load.
Through a process known as transpiration, plants release water vapor into the air through specialized cells, called stomata or stomatal apertures. Hydrostatic pressure, a chemical property of water, allows plants to naturally absorb water into the roots, then into the xylem. After making its way into the xylem, water moves into mesophyll cells located in the leaves. Then, water evaporates from this area through the stomata in the form of water vapor. This evaporation of water forces the roots to absorb more water from the soil through a very complex hydrostatic vacuum created within a plants’ natural structures and membranes.
|Stomata are very small pores within the outer membrane, or epidermis, of plant leaves. Plants have the ability to open or close their stomata in instances of drought or heavy rains.|
Why it Matters
Anywhere from 97% to 99.5% of the water taken up by the roots of a plant ends up being released back into the air. During this process, a physical change in the state, or phase, of water occurs, liquid water to water vapor. Implementing a watering system that uses less water on average effectively reduces the amount of water getting into the plants, which causes a reduction in the overall amount of latent heat within the system. By reducing the amount of latent heat by 50% reduces the total heat of an air conditioning system by as much as 25%.
A liquid-gallon of water weighs 8.33 pounds and it takes 970 BTUs (British Thermal Units) of latent heat to convert just 1 pound of water vapor back into liquid water. Reducing the amount of water available to the plants by just 1 gallon reduces the total heat load of an air conditioning system by a whopping 8080.1 BTUs. To put this into perspective, reducing the total amount of water in your system by 10 gallons ends up reducing the total heat load by roughly 23 kW hours.
The rate of transpiration in plants has been observed to be affecting by the following factors:
- Wind and/or air movement
- Soil Moisture
The four factors above have been shown to have a significant effect on the amount of water vapor that gets released into the system by plants, more specifically, their stomata. This causes a direct effect on the amount of latent heat. As I stated before, this has a drastic effect on the total heat load of a system.
To wrap it all up, I suggest implementing a watering system that uses significantly less water than others. Blumat Systems, a company out of Boulder, CO, has developed a system that reduces overall water usage by as much as 90%. As weird as it sounds, this system promotes stronger and more prolific plant growth due to the science behind the biological make-up of plants. Over-watering can be just as bad, or worse, than under-watering for plants.