Hydronic Radiant Cooling (Commercial Buildings)
Cooling of non-residential buildings equipped with All-Air Systems significantly contributes to the electrical energy consumption and to the peak power demand. Part of the energy used to cool buildings is consumed by the fans that transport cool air through the ducts. This energy heats the conditioned air, and therefore adds to the internal thermal cooling peak load. Scientists at LBNL found that, in the case of the typical office building in Los Angeles, the external loads account for only 42% of the thermal cooling peak. At that time, 28% of the internal gains were produced by lighting, 13% by air transport, 12% by people, and 5% by equipment. The implementation of better windows, together with higher plug loads due to increased use of electronic office equipment, have probably caused these contributions to change to some extent since then.
HVAC systems are designed to maintain indoor air quality and provide thermal space conditioning. Traditionally, HVAC systems are 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, at peak load, only 10% to 20% of the supply air is outside air. Only this small fraction of the supply air is in fact necessary to ventilate the buildings in order to maintain a high level of indoor air quality. For conventional HVAC systems the difference in volume between supply air and outside air is made up by recirculated air. The recirculated air is necessary in these systems to keep the temperature difference between supply air and room air in the comfort range. The additional amount of supply air, however, often causes draft as well as indoor air quality problems due to the distribution of pollutants throughout the building.
All-Air Systems achieve the task of cooling a building by convection only. An alternative is to provide the cooling through a combination of radiation and convection inside the building. This strategy uses cool surfaces in a conditioned space to cool the air and the space enclosures. The systems based on this strategy are often called Radiative Cooling Systems, although only approximately 60% of the heat transfer is due to radiation. If the cooling of the surfaces is produced using water as transport medium, the resulting systems are called Hydronic Radiant Cooling Systems (HRC Systems). By providing cooling to the space surfaces rather than directly to the air, HRC Systems allow the separation of the tasks of ventilation and thermal space conditioning. While the primary air distribution is used to fulfill the ventilation requirements for a high level of indoor air quality, the secondary water distribution system provides thermal conditioning to the building. HRC Systems significantly reduce the amount of air transported through buildings, as the ventilation is provided by outside air systems without the recirculating air fraction. Due to the physical properties of water, HRC Systems remove a given amount of thermal energy and use less than 5% of the otherwise necessary fan energy. The separation of tasks not only improves comfort conditions, but increases indoor air quality and improves the control and zoning of the system as well. HRC Systems combine temperature control of the room surfaces with the use of central air handling systems.
Due to the large surfaces available for heat exchange in HRC Systems (usually almost a whole ceiling, and sometimes whole vertical walls), the temperature of the coolant is only slightly lower than the room temperature. This small temperature difference allows the use of either heat pumps with very high coefficient of performance (COP) values, or of alternative cooling sources (e.g., indirect evaporative cooling), to further reduce the electric power requirements. HRC Systems also reduce problems caused by duct leakage, as the ventilation air flow is significantly reduced, and the air is only conditioned to meet room temperature conditions, rather than cooled to meet the necessary supply air temperature conditions. Furthermore, space needs for ventilation systems and their duct work are reduced to about 20% of the original space requirements. Beside the reduction of space requirements for the shafts that house the vertical air distribution system, floor-to-floor height can be reduced, which offsets the initial cost of the additional system.
The thermal storage capacity of the coolant in HRC Systems helps to shift the peak cooling load to later hours. Because of the hydronic energy transport, this cooling system has the potential to interact with thermal energy storage systems (TES) and looped heat pump systems.
The connection between the panel and the tube is critical. Poor connections provide only limited heat exchange between the tube and the panel, which results in increased temperature differences between the panel surface and the cooling fluid. Panels built in a "sandwich system" include the water flow paths between two aluminum panels (like the evaporator in a refrigerator). This arrangement reduces the heat transfer problem and increases the directly cooled panel surface. In the case of panels suspended below a concrete slab, approximately 93% of the cooling power is available to cool the room. The remaining 7% cools the floor of the room above.
Cooling grids made of small plastic tubes placed close
to each other can be imbedded in plaster, gypsum board, or mounted on ceiling
panels (e.g., acoustic ceiling elements).
This second system provides an even surface temperature
distribution. Due to the flexibility of the plastic tubes this system might
be the best choice for retrofit applications. It was developed in Germany
and has been on the market for several years. When the tubes are imbedded
in plaster, the heat transfer from above is higher than in the case of cooling
The heat transfer to the concrete couples the cooling grid to the structural thermal storage of the slab. Plastic tubes mounted on suspended cooling panels show thermal performance comparable to the panel systems described above. Tubes imbedded in a gypsum board can be directly attached to a wooden ceiling structure without a concrete slab. Insulation must be applied to reduce cooling of the floor above.
A third system is based on the idea of a floor heating system. The tubes are imbedded in the core of a concrete ceiling. The thermal storage capacity of the ceiling allows for peak load shifting, which provides the opportunity to use this system in association with alternative cooling sources. Due to the thermal storage involved, the control of this system is limited. This leads to the requirement of relatively high surface temperatures to avoid uncomfortable conditions in the case of reduced cooling loads. The cooling power of the system is therefore limited. This system is particularly suited for alternative cooling sources, especially the heat exchange with cold night air. The faster warming of rooms with a particular high thermal load can be avoided by running the circulation pump for short times during the day to achieve a balance with rooms with a lower thermal load.
Due to the location of the cooling tubes in this system, a higher portion of the cooling is applied to the floor of the space above the slab. Approximately 83% of the heat removed by the circulated water are from the room below the slab, while 17% are from the room above.
Obviously, these three system types can also be used to heat a building.
Heating and Cooling (Residential)
While there are many examples of hydronic radiant heating and cooling installations in non-residential buildings available, very little has been reported about residential applications. For some time floor heating systems were installed in residences because of their superior thermal comfort. Besides thermal comfort issues, hydronic floor heating installations avoid recirculation air and the bothersome cycling of the air handling system. A positive side-effect is the elimination of duct leakage (which wastes up to 30% of conditioning energy) for naturally ventilated buildings or at least a significant reduction of the duct leakage effect for mechanically ventilated houses.
The installation of a floor heating system can also be used to cool the residence. There are several issues related to floor cooling:
* cooling power is limited due to small temperature difference
between supply water and room air
* floor surface temperature should not be less than 19 degree C (66 degree F)
* the dew point of the indoor air has to be kept below the supply water temperature (condensation)
In humid climates, condensation can be avoided by dehumidifying the ventilation air either by mechanical cooling or by desiccant systems.
Floor heating and cooling are often designed as concrete core conditioning systems, which provide good heat transfer form the embedded tube to floor surface. Systems with the tubing installed between floor joists usually have a reduced heat transfer, which requires larger temperature differences between the supply water and the room air. While this might not be a problem when the system is in heating mode, large temperature differences in the cooling mode might create a danger for condensation or at least diminish energy savings because of the need to dehumidify the ventilation air to levels beyond thermal comfort criteria.
When designed well, hydronic radiant heating and cooling systems operate with temperatures close to design room air temperatures. When mated with a ground-source heat pump, these systems provide excellent energy efficiency. High heating supply water temperature and low cooling supply water temperature reduce the energy efficiency.
Stetiu, C. and H.E. Feustel
Development of a Model to Simulate the Performance of Hydronic/Radiant Cooling Ceilings
LBL-Report, LBL-36636 (1994)
Feustel, H.E. and C. Stetiu
Hydronic Radiant Cooling - Preliminary Assessment
Energy and Buildings 22 (1995) 193-205
Stetiu. C., H.E. Feustel, and Y. Nakano
Ventilation Control Strategies for Buildings with Hydronic Radiant Conditioning in Hot Humid Climates
Lawrence Berkeley National Laboratory, Internal Report (1996)
Radiant Cooling in US Office Buildings: Towards Eliminating the Perception of Climate-Imposed Barriers
Lawrence Berkeley National Laboratory, LBNL-41275, 1998
Ph.D. Thesis, Energy and Resources Group, University of California, Berkeley, 1997
Please see also the International Energy Agency's Future Building Forum on "Low-Temperature Heating and High-Temperature Cooling"
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