Hydronic Radiant Heating and Cooling System

Radiant Hydronic Cooling System

The cooling of non-residential buildings equipped with All-Air Systems contributes significantly to electrical energy consumption and peak power demand. The fans that transport cool air through the ducts consume some of the energy used to cool buildings. This energy heats the conditioned air and thus contributes to the peak internal thermal cooling load. LBNL scientists discovered that, in the case of a typical office building in Los Angeles, external loads account for only 42% of the thermal cooling peak. At the time, lighting accounted for 28% of internal gains, air transport accounted for 13%, people accounted for 12%, and equipment accounted for 5%. Better windows, as well as higher plug loads due to increased use of electronic office equipment, have most likely caused these contributions to change to some extent since then.

advertisement

HVAC systems are intended to provide thermal space conditioning while also maintaining indoor air quality. HVAC systems are typically designed as All-Air Systems, which means that air is used to perform both tasks. DOE-2 simulations for different California climates using the California Energy Commission (CEC) base case office building show that only 10% to 20% of the supply air is outside air at peak load. Only a small portion of the supply air is required to ventilate the buildings and maintain a high level of indoor air quality. Recirculated air compensates for the volume difference between supply air and outside air in conventional HVAC systems. In these systems, recirculated air is required to keep the temperature difference between supply air and room air within a comfortable range. The additional supply of air, on the other hand, frequently causes draughts as well as indoor air quality issues due to the distribution of pollutants throughout the building.

advertisement

Convection alone is used by All-Air Systems to cool a building. An alternative is to provide cooling inside the building using a combination of radiation and convection. Cool surfaces in a conditioned space are used in this strategy to cool the air and space enclosures. The systems based on this strategy are frequently referred to as Radiative Cooling Systems, despite the fact that radiation accounts for only about 60% of heat transfer. If the cooling of the surfaces is achieved through the use of water as a transport medium, the resulting systems are known as Hydronic Radiant Cooling Systems (HRC Systems).

HRC Systems enable the separation of ventilation and thermal space conditioning tasks by providing cooling to the space surfaces rather than directly to the air. While the primary air distribution system provides ventilation for a high level of indoor air quality, the secondary water distribution system provides thermal conditioning to the building. Because ventilation is provided by outside air systems without the recirculating air fraction, HRC systems reduce the amount of air transported through buildings. HRC Systems remove a given amount of thermal energy while using less than 5% of the otherwise required energy due to the physical properties of water.

The separation of tasks not only improves comfort conditions, but it also improves indoor air quality and system control and zoning. HRC Systems combine room surface temperature control with the use of central air handling systems.

Because of the large surfaces available for heat exchange in HRC Systems (typically almost the entire ceiling and sometimes entire vertical walls), the coolant temperature is only slightly lower than the room temperature. Because of the small temperature difference, heat pumps with very high coefficients of performance (COP) values or alternative cooling sources (e.g., indirect evaporative cooling) can be used to further reduce electric power requirements. HRC systems also reduce problems caused by duct leakage because the ventilation airflow is significantly reduced and the air is only circulated when necessary.

The thermal storage capacity of the coolant in HRC Systems aids in deferring the peak cooling load until later in the day. This cooling system has the potential to interact with thermal energy storage systems (TES) and looped heat pump systems due to the hydronic energy transport.

System of Hydronic Cooling

advertisement

The majority of HRC Systems fall into one of three system designs. The panel system is the most commonly used system. This system is made of aluminum panels with metal tubes attached to the back.

It is critical that the panel and tube are connected properly. Poor connections allow for only a limited amount of heat exchange between the tube and the panel, resulting in increased temperature differences between the panel surface and the cooling fluid. Water flow paths between two aluminum panels are included in panels built in a "sandwich system" (like the evaporator in a refrigerator). This configuration reduces the heat transfer issue while increasing the directly cooled panel surface. Approximately 93 percent of the cooling power is available to cool the room when panels are suspended beneath a concrete slab. The remaining 7% cools the floor of the above-mentioned room.

Cooling grids made of small plastic tubes closely spaced can be embedded in plaster, gypsum board, or mounted on ceiling panels (e.g., acoustic ceiling elements).

This second system ensures an even distribution of surface temperature. This system may be the best choice for retrofit applications due to the flexibility of the plastic tubes. It was created in Germany and has been on the market for a number of years. When the tubes are embedded in plaster, the heat transfer from above is greater than when cooling panels are used.

The heat transfer to the concrete connects the cooling grid to the slab's structural thermal storage. The thermal performance of plastic tubes mounted on suspended cooling panels is comparable to that of the panel systems described above. Tubes embedded in gypsum board can be attached directly to a wooden ceiling structure without the use of a concrete slab. To reduce the cooling of the floor above, insulation must be installed.

advertisement

A third system is based on the concept of floor heating. The tubes are embedded in the concrete core of a ceiling. The thermal storage capacity of the ceiling allows for peak load shifting, allowing this system to be used in conjunction with alternative cooling sources. This system's control is limited due to the thermal storage involved. As a result, relatively high surface temperatures are required to avoid uncomfortable conditions in the case of reduced cooling loads. As a result, the system's cooling capacity is limited. This system is ideal for alternative cooling sources, particularly heat exchange with the cold night air. By running the circulation pump for a short period of time, faster warming of rooms with a high thermal load can be avoided. short periods of the day in order to achieve a balance with rooms with a lower thermal load

Because of the placement of the cooling tubes in this system, a greater portion of the cooling is applied to it. the space above the slab's floor The circulated heat removes approximately 83 percent of the heat. The majority of the water comes from the room beneath the slab, with only 17% coming from the room above.