Prepared for Energy Design Resources by Financial Times Energy, Inc. (PG&E, SDG&E, SCE / CPUC).
Unlike most cooling systems in California, which circulate cold air to maintain comfort, most radiant cooling systems circulate cool water through ceiling, wall, or floor panels. "Coolth" from that water is absorbed by occupants and interior spaces via thermal radiation. Radiant cooling systems are more efficient, more comfortable, more attractive, and more healthful than systems that circulate air, and over their lifetimes are less expensive to own and operate.
Two barriers to wider adoption: lack of familiarity with the technology, and memories of moisture control problems in early systems. The latter is mitigated by recent sensor and control advances; the former should diminish as information on successful applications in Europe, North America, and California spreads. Healthcare facilities, which benefit from separating comfort cooling from ventilation, may lead the way.
Designers should consider radiant cooling in new buildings in any of California's climate zones. Commercial buildings primarily cooled by radiant means are more comfortable than those cooled by traditional HVAC. First costs are comparable with variable-air-volume (VAV) systems, but lifetime energy savings over VAV are routinely 25 percent or more.
With radiant systems, people are cooled by radiant heat transfer to adjacent surfaces (ceilings, walls, or floors) held a few degrees cooler than ambient. Water has roughly 3,500 times the energy transport capacity of air; a hydronic system can transport a given amount of cooling with less than 5 percent of the energy required to deliver cool air with fans. With radiant space conditioning, the ventilating function is separate—air volume and components can be roughly five times smaller. Fan power is saved and ducts can be smaller.
Advantages over VAV:
In the example in the brief, energy savings exceed 42 percent vs. conventional (less in high-humidity areas). Corina Stetiu (LBNL) simulated a prototypical office in nine U.S. cities: on average, radiant cooling saves 30% on cooling energy and 27% on demand (range 17% in cold, moist areas to 42% in warmer, dry areas).
First and lifecycle costs: Buildings with radiant cooling routinely show slightly lower first costs but substantially lower lifecycle costs than four-pipe fan coil systems (Sean Timmons, Arup). Using German prices, Franc Sodec (Krantz-TKT) reported up to 20% first-cost savings (ceiling panels vs. standard VAV at ~14–18 Btu/ft² cooling) and 40–55% savings in space due to less ducting.
Radiant energy can be delivered at temperatures only a few degrees from the conditioned space. For cooling, surface temperature must stay above dewpoint to avoid condensation → buildings must be at least moderately well sealed; in highly humid areas, ventilation air may need dehumidification. Cooling surfaces must be large enough to deliver adequate cooling at small temperature differences; large radiant areas also give a uniformly pleasant environment.
Helmut Feustel (LBNL): Hydronic radiant cooling has become the new standard in Germany. Strategies include suspended metal panels with tubes (e.g. mixed-use Düsseldorf building), concrete core activation, and others.
The brief discusses familiarity, moisture/condensation concerns, and the role of documentation and standards.
Davis Energy Group has been involved in three projects using "natural cooling" (evaporative coolers, cooling towers, or roof spray at night; water stored and/or circulated through slab tubing).
3,620 ft² mini-mart west of Sacramento: 15-ton RTU downsized to 10 tons; evaporative precooler for condenser and ventilation air; at night, unit cools water in sub-slab tubing. 51% cooling energy savings, 47% demand savings vs. conventional RTU. Payback under 1 year overall; incremental cost ~$1/ft², 2.7-year payback with PG&E downsizing credit.
Five-story, 30,000 ft², downtown San Francisco. Retrofit: tubing in light concrete above existing floors; cooling tower (spray / low fan / high fan, no compressor). Installed for $140,000 vs. $500,000 quoted for chiller + fan-coils. Projected 84% demand savings, 61% electric energy savings over conventional.
70,000 ft² facility, Vacaville (1998). Night spray array cools water in 10,000-gal tank; radiant slab + five fan-coils. 10-ton chiller used off-peak as needed. NightSky supplies 67% of cooling. 70% energy savings, 87% demand savings; payback 2.5 years.
Corina Stetiu: An adequately designed and operated radiant cooling system can function in a state-of-the-art office building at any U.S. location with low risk of condensation. Retrofits in existing buildings are possible, but cost-effectiveness drops when buildings are leaky, especially in humid areas—still leaving a large potential market.
Strong evidence from simulations, European experience, and California projects shows radiant cooling works and is cost-effective. Full embrace in North America will likely follow once more buildings across climate zones are built and documented. Healthcare facilities (single-pass ventilation + radiant conditioning) are prime candidates; Oak Park Hospital, Cook County Hospital (Chicago), UCLA, and University of Michigan are cited.
Source: Energy Design Resources design brief (PG&E, SDG&E, SCE). Text from PDF.
PDF: 2000-01-01-design-brief-radiant-cooling.pdf