Status (updated 2/13/2013): Ongoing
Funding sources: CBE Industry Consortium, California Energy Commission PIER, and Industry In-kind Support
Project Objective
The overarching goal of this project is to contribute to improved understanding of applications, design, and optimization of radiant systems, and to develop guidelines, tools and resources for system designers and operators. This research area is currently underway as multi-project research topic.
Significance to Industry
Radiant cooling is a common building conditioning system in Europe, and is now gaining popularity in North America for its potential to reduce cooling energy consumption and peak loads. Radiant cooling refers to any system where interior surface temperatures are lowered to remove sensible heat gain. Some systems circulate water in specialized panels, others cool the building structure (typically floor or ceiling slab). Because radiant surfaces are often cooled only a few degrees below the desired air temperature, there are many opportunities for innovative cooling sources such as nighttimecooling, ground-source heat exchangers, and indirect evaporative cooling.
High performance buildings with lower cooling loads may offer the best opportunities for radiant cooling applications. In addition, the limited cooling capacities of some low-energy cooling strategies such as displacement ventilation and natural ventilation can be extended when combined with radiant cooling.
Research Approach
During the initial stages of this research, we conducted a literature search on radiant cooling systems, focusing on current design issues, case studies and open research questions. We also interviewed industry professionals with expertise with these systems, and produced a technical paper summarizing our assessment of radiant cooling applications in North America. In addition, we evaluated design and simulation tools to assess which are most effective and where further development is most needed.
The current research is being conducted via three related research approaches: (1) simulation studies; (2) laboratory studies; and (3) field studies of advanced buildings with radiant cooling systems.
Simulation studies.
Ongoing studies
In one recent simulation study, we used EnergyPlus to investigate the zone level cooling load difference between radiant and air systems. Cooling load calculations are a critical step in radiant system design and the current design standards do not provide much guidance on this topic. Results show that radiant systems respond to radiative heat gain faster than air systems so that the majority of the cooling load removed from the zone occurs during occupied daytime hours. This results in higher zone level peak cooling loads when compared to air systems. In general, even if the zone level cooling load may be higher for the radiant systems, there are verified advantages of using hydronic-based radiant systems such as improved plant side equipment efficiency with warmer cold water temperature, possibility of night pre-cooling and utilization of natural cooling resource, and energy efficiency in transporting heat with water compared to air. All of these factors combine to produce better overall energy performance in radiant cooling systems. Detailed findings are published here.
In support of a new CBE project initiated in April 2012, EnergyPlus simulations have been conducted to develop guidelines for addressing how to downsize an air-side cooling system when combined with radiant floor cooling in potentially high solar loaded spaces (e.g., perimeter zones and atriums). Simulations have shown that waterside peak cooling capacity for the simulated embedded radiant floor system when exposed to direct solar radiation can be 92% higher than its air system counterpart, and this is consistent with rules of thumb used by some system practitioners. This research has been accepted for presentation at Clima 2013.
Another on-going simulation study aims to evaluate the cooling performance of thermally activated building system (TABS) with evaporative cooling source for typical United States climates. An EnergyPlus model of a prototype medium office building is used to study the significance of different design parameters. Various design options for air system sizing and comfort zone expansion (for example, using personal comfort systems) are investigated to extend the range of application.
Previous studies
For our first simulation study we conducted whole-building simulations of slab-integrated hydronic radiant cooling with mechanical ventilation, compared to a more conventional all-air cooling system as a baseline, for Denver, Sacramento, Los Angeles, and San Francisco climates. Results suggest strong energy-saving potential for radiant cooling systems in both Colorado and California climates. This research was published in 2008 (PDF).
We then conducted simulations for a hybrid HVAC system that combines a UFAD system with a cooled radiant ceiling slab. In the simulations, cooling tower water pre-cools the structural slabs during the night and early morning period. For the Sacramento, California climate this radiant/UFAD hybrid system shows an energy reduction of 21-25% during the peak cooling months, and peak electricity demand reduction of 27%, and also improved occupant thermal comfort. These findings were published in 2010 (PDF).
We also conducted simulations using CBE’s Thermal Comfort Model to determine the combinations of air temperatures and floor or ceiling temperatures that would create comfortable conditions. These findings provide valuable guidance for building designers and professionals, and were published in 2009.
We are currently working in collaboration with Lawrence Berkeley National Lab on the development of powerful tools to enable accurate design and optimization of radiant systems. This work includes improving the “auto sizing” functions for EnergyPlus for hydronic tubing spacing and depth in slab, and water flow and temperature.
Laboratory studies. Radiant chilled ceilings with displacement ventilation (DV) represent a promising integrated approach that combines the energy efficiency of both sub-systems with the opportunity for strong ventilation performance. We collaborated with CBE partner Price Industries on laboratory experiments for a typical interior zone office to investigate how room air stratification is affected by the ratio of cooling load removed by a chilled ceiling to the total cooling load for two radiant ceiling configurations. This work has led to improved design resources and was published in 2011. We are continuing our collaboration with Price, and among other topics, will be conducting additional lab studies to understand the risk of condensation with radiant cooling systems.
Field studies. We are also conducting field studies in buildings using radiant systems to meet ultra-low energy goals. Our current focus is the David Brower Center in Berkeley, California, which includes a hybrid radiant/UFAD system in combination with natural ventilation and other energy-conserving measures. We are conducting advanced monitoring of the energy performance, evaluating occupant satisfaction with the occupant IEQ survey, and investigating methods to optimize the control of the system. We also conducted a pilot field study of the IDeAs net zero-energy building in San Jose, California.
In the fall of 2012, we obtained new funding from the California Energy Commission to support detailed field studies in two near zero-net-energy (ZNE) buildings using radiant slab cooling and heating systems. These studies will be conducted over the next 2 ½ years.
Publications and Reports
Feng, D., S. Schiavon, and F. Bauman, 2013. Impact of Solar Heat Gain on Radiant Floor Cooling System Design. Proceedings of the 11th REHVA World Congress-CLIMA 2013.
Feng D., S. Schiavon and F. Bauman, 2012. Comparison of Zone Cooling Load for Radiant and All-Air Conditioning Systems. Proceedings of the International Conference on Building Energy and Environment.
Schiavon, S., F. Bauman, B. Tully, and J. Rimmer, 2011. Room Air Stratification in Combined Chilled Ceiling and Displacement Ventilation Systems. HVAC&R Research Journal.
Raftery, P., K.H. Lee, T. Webster, and F. Bauman, 2010. Analysis of Hybrid UFAD and Radiant Hydronic Slab HVAC System. Proceedings of ICAE 2010, Singapore, April.
Wang, Z., H. Zhang, D. Lehrer, E. Arens, C. Huizenga, T. Yu, and S. Hoffmann, 2009. Modeling Thermal Comfort with Radiant Floors and Ceilings. 4th International Building Physics Conference, Istanbul, June.
Moore, T., 2008. Simulation of Radiant Cooling Performance with Evaporative Cooling Sources. CBE Summary Report, October. Executive Summary.
Moore, T., F. Bauman and C. Huizenga, 2006. Radiant Cooling Research Scoping Study. CBE Internal Report, April.
Presentations
Bauman, F., T. Webster, H. Zhang, and E. Nahman, 2008. Radiant Program Overview, Research Overview, Berkeley, CA. October.
Zhang, H., Z. Wang, C. Huizenga, T. Yu, E. Arens, and T. Moore, 2008. Evaluating Thermal Comfort of Radiant Floors, Research Overview, Berkeley, CA. April.
