An integrated radiant cooling system is a radiant system that is embedded into the structure of the building, rather than installed post-construction. Integrated radiant systems are called thermo-active building systems (TABS), as they activate and use the thermal mass of the building structure to heat or cool the space.
Integrated radiant cooling can be used in a variety of places, including residences, industrial locations, and commercial spaces. I have covered the range of benefits of radiant systems in previous blogs, but I have not yet discussed exactly how integrated radiant systems actually work. In this blog I will give some information on how these systems work, specifically in commercial spaces.
Construction and installation
Before I get to how integrated radiant systems work, I will just reiterate that integrated radiant cooling systems are installed during construction, and are therefore practically unnoticeable within finished commercial spaces. They are also silent. Therefore, integrated radiant systems work out of sight and out of mind in commercial spaces – at least until occupants think about what makes their working environment so comfortable.
Broad scale functioning of high mass integrated radiant systems
Now, how radiant cooling systems actually work, TABS are high mass systems, which means that they are capable of storing a large amount of energy and releasing it slowly over time. This is the key feature of TABS, the ability to shift the peak energy requirement outside of peak occupancy hours. This allows utilization of the lower night time electricity rates, as well as passive cooling afforded by cool night breezes, or ground heat exchangers. Radiant cooling is provided throughout occupied hours with minimal energy input on the water side.
Figure 1 – energy input profile over a 24h period, showing the ‘peak shaving effect’. Peak energy input on the water side is overnight, thus providing cooling during occupied hours with minimal energy expenditure.
The system is cooled down to a temperature that will be able to heat up slightly through the occupied hours as it absorbs heat energy to cool the space, and still provide adequate cooling at the end of the day. The predicted mean vote (PMV) is an internationally used index of thermal comfort, and ASHRAE guidelines recommend values within the range of 0.5 to -0.5 for interior spaces. At the beginning of occupied hours the PMV is slightly below 0 (occupants may feel slightly cool), and as the day progresses the radiant slabs heat up slightly, increasing the operative temperature, such that by the end of the day the PMV is slightly above 0 (occupants may feel slightly warm). However, the PMV always remains within the recommended range for optimal thermal comfort.
Figure 2 – temperature profiles of the various aspects of operative temperature within the space as a function of time. The predicted mean vote (PMV) is an index of thermal comfort, on a scale of 3 to -3 (3 = Hot, -3 = Cold, 0 = Neutral; right axis).
Specifics on how high mass integrated radiant systems provide cooling
With the concept of how the whole system functions over time understood, I can now get to how the system actually cools the occupants within commercial spaces.
TABS circulate water through pipes embedded in the structural concrete of a building (either in the walls, ceiling, or more commonly, the floors) to control the temperature of the concrete itself. This turns the building’s actual structure into a temperature regulator for the internal spaces. As the water is circulated within the concrete, energy is transferred between the water and the concrete in the direction of the temperature difference; if the water is colder than the concrete, energy will travel into the water, cooling the concrete, and vice versa. For radiative cooling, the water is pumped through the pipes and then passes through a mechanism to cool the water (there are various options for cooling, including chillers, cooling towers, and ground water). Once the water is cooled it is recirculated, creating a cycle of energy transfer out of the internal spaces. Notably, the water is usually only cooled to 2 – 4 °C below the room air temperature. This means that the risk of condensation is easily removed by keeping the surface of the concrete above the dew point temperature.
The principle of how the cooled concrete then regulates thermal comfort relies on the fact that heat energy travels from high to low energy between surfaces that are in direct sight of each other. Thus, any part of an occupant in direct line of the cooled surface will transfer radiative heat to the concrete, thus cooling the occupant. Any object separating the radiant panel and the surface that you wish to cool will prevent cooling. Therefore, radiant cooling systems in commercial spaces have to be designed carefully such that the people that need to be cooled will be within the direct line of the cooled surface.
A key point to make here is on thermal comfort itself. Thermal comfort is a subjective measure of how comfortable someone is with their thermal environment. People judge their thermal comfort based on the operative temperature in the space. This is a temperature measure that accounts for all avenues of heat gain and loss, including metabolic heat, air temperature, and solar heat gain among others. Upwards of 50% of the operative temperature humans experience in the mean radiative temperature (MRT). This means that the radiative environment is a vital consideration for the thermal comfort of a person in any given environment. For this reason, radiant systems should be installed with not only components that not only measure air temperature, but also the MRT. By monitoring the MRT, TABS can regulate it, and thus provide superior thermal comfort compared to all-air heating, ventilation, and air condition (HVAC) systems which only monitor and regulate air temperature.
A critical part of how radiant cooling systems work in commercial spaces is how they are controlled. This determines when they will be actively cooled, and how much cooling they provide during occupied hours. The control strategy also governs how different zones function. Generally different rooms, or zones within a building will have different heating / cooling requirements. As a result, each zone may need its own set of sensors and cooling strategy.
The control strategy for a whole building in general depends on the design characteristics, such as building envelope, thermal inertia, the system response times and others. In modern systems, the control will be automatically operated, using complex algorithms to ensure proper regulation. Even so, these strategies will need to be modified for each new application. Generally, control can either be constant flow with variable temperature, or variable flow with constant temperature. Regardless of which of these is used, commercial buildings can be designed such that each zone is individually managed to provide optimal thermal comfort.
Overall, integrated radiant cooling systems are ideal for regulating thermal comfort in most commercial spaces. They work efficiently and quietly to provide effective, low maintenance cooling.