Heat transfer fundamentals: radiant versus convection heat

Radiant Cooling Systems

Radiant cooling systems typically use chilled water running in pipes in thermal contact with the surface. The circulating water only needs to be 2-4°C below the desired indoor air temperature.Heat is removed by the water flowing in the hydronic circuit once the heat from different sources in the space is absorbed by the actively cooled surface – ceiling, floor or walls.

Majority of the cooling process results from removing sensible heat through radiant exchange with people and objects and not air, occupant thermal comfort can be achieved with warmer interior air temperatures than with air based cooling systems. Combined with higher cooling capacity of water than air, and the having a cooled surface close to the desired indoor air temperature, radiant cooling systems offer significant reductions in cooling energy consumption.

The latent loads (humidity) from occupants, infiltration and processes generally need to be managed by an independent system. Radiant cooling can also be integrated with other energy-efficient strategies such as night time flushing, indirect evaporative cooling, or ground source heat pumps.

Types of Radiant Cooling Systems:-

There are primarily two types of radiant cooling systems:

1. Chilled Slabs:

These deliver cooling through the building structure, usually slabs, and is also know as thermally activated building systems (TABS).

Laying of PEx pipes in Slab for Radiant Cooling

Radiant cooling from a slab can be delivered to a space from the floor or ceiling. Floor cooling is similar to floor heating that has been used in Europe since last few decades. However, delivering cooling from the ceiling has several advantages:

Radiant Cooling through Ceiling embedded pipes.

• It is easier to leave ceilings exposed to a room than floors, increasing the effectiveness of thermal mass. Floors have furniture, coverings and furnishings that decrease the effectiveness of the system.
• Greater convective heat exchange occurs through a chilled ceiling as warm air rises, leading to more air coming in contact with the cooled surface.

Cooling delivered through the floor makes the most sense when there is a high amount of solar gains from sun penetration, as the cool floor can more easily remove those loads than the ceiling. Chilled slabs, compared to panels, offer more significant thermal mass and therefore can take better advantage of outside diurnal temperatures swings. Chilled slabs cost less per unit of surface area, and are more integrated with structure.

2. Ceiling Panels:

These deliver cooling through specialized panels. Systems using concrete slabs are generally cheaper than panel systems and offer the advantage of thermal mass while panel systems offer faster temperature control and flexibility.

Radiant cooling panels are generally attached to ceilings, but can also be attached to walls. They are usually suspended from the ceiling, but can also be directly integrated with continuous dropped ceilings. Modular construction offers increased flexibility in terms of placement and integration with lighting or other electrical systems. Lower thermal mass compared to chilled slabs means they can’t easily take advantage of passive cooling from thermal storage, but controls in panels can more quickly adjust to changes in outdoor temperature. Chilled panels are also better suited to buildings with spaces that have a greater variance in cooling loads. Perforated panels also offer better acoustical dampening than chilled slabs. Ceiling panels are also very suitable for retrofits as they can be attached to any ceiling. Chilled ceiling panels can be more easily integrated with ventilation supplied from the ceiling. Panels tend to cost more per unit of surface area than chilled slabs.

How Does Radiant Cooling Work?

A radiant cooling system is a temperature-controlled surface that cools indoor temperatures by removing sensible heat and where more than half of heat transfer occurs through thermal radiation. Heat will flow from objects, occupants, equipment and lights in a space to a cooled surface as long as their temperatures are warmer than that of the cooled surface and they are within the line of sight of the cooled surface. The process of radiant exchange has a negligible effect on air temperature, but through the process of convection, the air temperature will be lowered when air comes in contact with the cooled surface.

Radiant cooling cools a floor or ceiling by absorbing the heat radiated from the rest of the room. When the floor is cooled, it is often referred to as radiant floor cooling; cooling the ceiling is usually done in homes with radiant panels. Although potentially suitable for arid climates, radiant cooling is problematic for homes in more humid climates.

Most radiant cooling applications have been based on aluminum panels suspended from the ceiling, through which chilled water is circulated. To be effective, the panels must be maintained at a temperature very near the dew point within the house, and the house must be kept dehumidified. In humid climates, simply opening a door could allow enough humidity into the home to allow condensation to occur.

The panels cover most of the ceiling. In all but the most arid locations, an auxiliary air-conditioning system will be required to keep the space’s humidity low.

Structures built on concrete slabs are prime candidates for radiant cooling systems, and radiant ceiling/floor cooling takes advantage of the same principle using chilled water.

In a Comfort Heating situation, Conduction (physical transfer of heat from source to target by direct contact) is not an option