Radiant Cooling

What Is The Difference Between Latent and Sensible Loads in Radiant Cooling?

In previous blogs, I have spoken about the advantages that a dedicated outdoor air system (DOAS) combined with radiant cooling/heating system has over conventional systems. One of the key advantages I repeatedly highlight is the ability of a “DOAS + Radiant” system to decouple the handling of sensible and latent energy loads.

However, you may not be completely familiar with what latent and sensible energy are, and what they mean in the context of a radiant cooling system. Therefore, in this short blog, I will cover what the differences between latent and sensible loads in radiant cooling are.

Latent load

 

Latent energy is the energy absorbed by a system, causing a change in phase, without a change in temperature. For example, the energy added to change water from liquid to gas or vice versa will not change the actual temperature of the water.

 

Sensible load

 

Sensible energy is the energy in the system that alters the temperature without a change in phase. For example, adding energy to a pot of water at 10 °C using a stove will cause the water to increase in temperature, without changing to water vapor until it reaches 100 °C.

 

Latent and sensible loads in radiant cooling

Latent and sensible loads in radiant cooling

Regarding HVAC, latent energy essentially refers to the relative humidity or moisture in the air. A high latent load would mean that the relative humidity was high.

Sensible energy is the energy that alters the mean radiant temperature (MRT) and air temperature that you can feel. In HVAC it is important to remember that latent and sensible loads interact to affect the operative temperature (overall thermal environment) that occupants experience. For instance, an air temperature of 28 °C at a relative humidity of 20 % (high sensible load, low latent load) would probably feel comfortable.

However, an air temperature of 28 °C at a relative humidity of 90 % would feel hot and clammy. Moreover, the air temperature determines how much moisture the air can hold. The warmer air is, the more moisture it can hold. This is why the moisture content of the air is referred to as the relative humidity. The same absolute amount of moisture in the air at 30 °C will correspond to a much lower relative humidity than that amount of moisture at 15 °C.

Radiant cooling systems

For radiant cooling systems, both latent and sensible energy are critical concerns.

Firstly, sensible load hugely affects comfort. Both radiant and convective energy transfer that affect operative temperature needs to be accounted for to help regulate thermal comfort.

Secondly, regulating the latent load (i.e. keeping the relative humidity within the desired range of 30 – 65 %) is vital to both occupant health and comfort, but also to the functionality of the radiant cooling system. A dew point higher than the temperature of a radiant cooling surface could risk issues with condensation.

Radiant cooling systems monitor and regulate the MRT as well as the air temperature (often in conjunction with other air temperature control systems). However, radiant cooling systems do not independently control the latent energy load. Radiant cooling systems have to be coupled with systems such as DOAS that control the latent energy load, as well as energy recovery.

 

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