Radiant cooling

Last updated May 04, 2019

Radiant cooling is the use of cooled surfaces to remove sensible heat by radiation and convection. It is related to radiant heating. Radiant systems that use water to cool the radiant surfaces are called hydronic systems. Unlike “all-air” air conditioning systems that circulate cooled air only, hydronic radiant systems circulate cooled water in pipes through specially-mounted panels on a building’s floor or ceiling to provide comfortable temperatures. There is a separate system to provide air for ventilation, dehumidification, and potentially additionally cooling.^[1] Radiant systems are less common than all-air systems for cooling, but can have advantages compared to all-air systems in some applications.^[2]^[3]^[4]

Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic variables of the body or system, such as volume or pressure.

In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:

Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids, including molten rock (rheid). Convection includes sub-mechanisms of advection, and diffusion.

Some well-known buildings using radiant cooling include Bangkok’s Suvarnabhumi Airport,^[5] the Infosys Software Development Building 1 in Hyderabad, IIT Hyderabad,^[6] and the San Francisco Exploratorium.^[7] Radiant cooling is also used as a design strategy in some net zero energy buildings.^[8]^[9]

Suvarnabhumi Airport, also known unofficially as Bangkok Airport, is one of two international airports serving Bangkok, Thailand. The other older one is Don Mueang International Airport. Suvarnabhumi covers an area of 3,240 ha, making it one of the biggest international airports in Southeast Asia and a regional hub for aviation.

The Exploratorium is a museum in San Francisco that allows visitors to explore the world through science, art, and human perception. Its mission is to create inquiry-based experiences that transform learning worldwide. It has been described by the New York Times as the most important science museum to have opened since the mid-20th century, an achievement attributed to "the nature of its exhibits, its wide-ranging influence and its sophisticated teacher training program". Characterized as "a mad scientist's penny arcade, a scientific funhouse, and an experimental laboratory all rolled into one", the participatory nature of its exhibits and its self-identification as a center for informal learning has led to it being cited as the prototype for participatory museums around the world.

A zero-energy building, also known as a zero net energy (ZNE) building, net-zero energy building (NZEB), net zero building or zero-carbon building is a building with zero net energy consumption, meaning the total amount of energy used by the building on an annual basis is roughly equal to the amount of renewable energy created on the site, or in other definitions by renewable energy sources elsewhere. These buildings consequently contribute less overall greenhouse gas to the atmosphere than similar non-ZNE buildings. They do at times consume non-renewable energy and produce greenhouse gases, but at other times reduce energy consumption and greenhouse gas production elsewhere by the same amount. A similar concept approved and implemented by the European Union and other agreeing countries is nearly Zero Energy Building (nZEB), with the goal of having all buildings in the region under nZEB standards by 2020.

Background

Definition

By definition, radiant cooling systems primarily remove sensible heat through thermal radiation. ASHRAE defines radiant systems as temperature-controlled surfaces where 50% or more of the design heat transfer takes place by thermal radiation.^[1]

Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. Particle motion results in charge-acceleration or dipole oscillation which produces electromagnetic radiation.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers is a global professional association seeking to advance heating, ventilation, air conditioning and refrigeration (HVAC&R) systems design and construction. Founded in 1894 it now has more than 50,000 members worldwide, composed of building services engineers, architects, mechanical contractors, building owners, equipment manufacturers' employees, and others concerned with the design and construction of HVAC&R systems in buildings. The society funds research projects, offers continuing education programs, and develops and publishes technical standards to improve building services engineering, energy efficiency, indoor air quality, and sustainable development.

Physics

Thermal (longwave) radiation travels at the speed of light, in straight lines.^[1] It can be reflected. People, equipment, and surfaces in buildings will warm up if they absorb thermal radiation, but the radiation does not noticeably heat up the air it is traveling through.^[1] This means 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 direct or indirect line of sight of the cooled surface. Some heat is also removed by convection because the air temperature will be lowered when air comes in contact with the cooled surface.

History

Radiant heating systems have been used for thousands of years, notably in ancient Korea, China, and Rome.^[10] Hydronic radiant cooling systems are relatively more recent. Early radiant cooling systems were installed in the late 1930s and 40's in Europe^[11] and by the 1950s in the US.^[12] They became more common in Europe in the 1990s and continue to be used today.^[13]

System design

Radiant cooling systems are usually hydronic, cooling using circulating water running in pipes in thermal contact with the surface. Typically the circulating water only needs to be 2–4 °C below the desired indoor air temperature.^[14] Once having been absorbed by the actively cooled surface, heat is removed by water flowing through a hydronic circuit, replacing the warmed water with cooler water.

Since the 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. As a result of the high cooling capacity of water, and the delivery of a cooled surface close to the desired indoor air temperature, radiant cooling systems potentially offer reductions in cooling energy consumption.^[2] The latent loads (humidity) from occupants, infiltration and processes generally need to be managed by an independent system. Radiant cooling may also be integrated with other energy-efficient strategies such as night time flushing, indirect evaporative cooling, or ground source heat pumps as it requires a small difference in temperature between desired indoor air temperature and the cooled surface.^[14]

System types

While there are a broad range of system technologies, there are two primary types of radiant cooling systems. The first type is systems that deliver cooling through the building structure, usually slabs. These systems are also named thermally activated building systems (TABS).^[15] The second type is systems that 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.

Chilled slabs

Radiant cooling from a slab can be delivered to a space from the floor or ceiling. Since radiant heating systems tend to be in the floor, the obvious choice would be to use the same circulation system for cooled water. While this makes sense in some cases, delivering cooling from the ceiling has several advantages.

First, it is easier to leave ceilings exposed to a room than floors, increasing the effectiveness of thermal mass. Floors offer the downside of coverings and furnishings that decrease the effectiveness of the system.

Second, 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 gain from sun penetration, because the cool floor can more easily remove those loads than the ceiling.^[14]

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.

Ceiling panels

Radiant cooling panels are generally attached to ceilings, but can be attached to walls also. 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.^[1] Perforated panels also offer better acoustical dampening than chilled slabs. Ceiling panels are also very suitable for retrofits because 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.

Advantages

Radiant cooling systems offer lower energy consumption than conventional cooling systems based on research conducted by the Lawrence Berkeley National Laboratory. Radiant cooling energy savings depend on the climate, but on average across the US savings are in the range of 30% compared to conventional systems. Cool, humid regions might have savings of 17% while hot, arid regions have savings of 42%.^[2] Hot, dry climates offer the greatest advantage for radiant cooling as they have the largest proportion of cooling by way of removing sensible heat. While this research is informative, more research needs to be done to account for the limitations of simulation tools and integrated system approaches. Much of the energy savings is also attributed to the lower amount of energy required to pump water as opposed to distribute air with fans. By coupling the system with building mass, radiant cooling can shift some cooling to off-peak night time hours. Radiant cooling appears to have lower first costs ^[16] and lifecycle costs compared to conventional systems. Lower first costs are largely attributed to integration with structure and design elements, while lower life cycle costs result from decreased maintenance. However, a recent study on comparison of VAV reheat versus active chilled beams & DOAS challenged the claims of lower first cost due to added cost of piping ^[17]

Limiting factors

Because of the potential for condensate formation on the cold radiant surface (resulting in water damage, mold and the like), radiant cooling systems have not been widely applied. Condensation caused by humidity is a limiting factor for the cooling capacity of a radiant cooling system. The surface temperature should not be equal or below the dew point temperature in the space. Some standards suggest a limit for the relative humidity in a space to 60% or 70%. An air temperature of 26 °C (79 °F) would mean a dew point between 17 °C and 20 °C (63 °F and 68 °F).^[14] There is, however, evidence that suggests decreasing the surface temperature to below the dew point temperature for a short period of time may not cause condensation.^[16] Also, the use of an additional system, such as a dehumidifier or DOAS, can limit humidity and allow for increased cooling capacity.

Related Research Articles

Heating, ventilation, and air conditioning (HVAC) is the technology of indoor and vehicular environmental comfort. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics and heat transfer. "Refrigeration" is sometimes added to the field's abbreviation, as HVAC&R or HVACR or "ventilation" is dropped, as in HACR.

In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.

Heat transfer transport of thermal energy in physical systems

Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.

Roughly speaking in the context of building and construction, the R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive flow of heat. R-value is the temperature difference per unit of heat flux needed to sustain one unit of heat flux between the warmer surface and colder surface of a barrier under steady-state conditions.

The mean radiant temperature (MRT) is defined as the uniform temperature of an imaginary enclosure in which the radiant heat transfer from the human body is equal to the radiant heat transfer in the actual non-uniform enclosure.

Hydronics is the use of a liquid heat-transfer medium in heating and cooling systems. The working fluid is typically water, glycol, or mineral oil. Some of the oldest and most common examples are steam and hot-water radiators. Historically, in large-scale commercial buildings such as high-rise and campus facilities, a hydronic system may include both a chilled and a heated water loop, to provide for both heating and air conditioning. Chillers and cooling towers are used either separately or together as means to provide water cooling, while boilers heat water. A recent innovation is the chiller boiler system, which provides an efficient form of HVAC for homes and smaller commercial spaces.

Electric heating is a process in which electrical energy is converted to heat energy. Common applications include space heating, cooking, water heating and industrial processes. An electric heater is an electrical device that converts an electric current into heat. The heating element inside every electric heater is an electrical resistor, and works on the principle of Joule heating: an electric current passing through a resistor will convert that electrical energy into heat energy. Most modern electric heating devices use nichrome wire as the active element; the heating element, depicted on the right, uses nichrome wire supported by ceramic insulators. A warning that these can go to very high temperatures and create excruciating burns

Displacement ventilation (DV) It is a room air distribution strategy where conditioned outdoor air is supplied at a low velocity from air supply diffusers located near floor level and extracted above the occupied zone, usually at ceiling height.

Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control for thermal comfort using conduction, radiation and convection. The terms radiant heating and radiant cooling are commonly used to describe this approach because radiation is responsible for a significant portion of the resulting thermal comfort but this usage is technically correct only when radiation composes more than 50% of the heat exchange between the floor and the rest of the space.

Solar air conditioning refers to any air conditioning (cooling) system that uses solar power.

Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption. This approach works either by preventing heat from entering the interior or by removing heat from the building. Natural cooling utilizes on-site energy, available from the natural environment, combined with the architectural design of building components, rather than mechanical systems to dissipate heat. Therefore, natural cooling depends not only on the architectural design of the building but on how the site's natural resources are used as heat sinks. Examples of on-site heat sinks are the upper atmosphere, the outdoor air (wind), and the earth/soil.

Building insulation is any object in a building used as insulation for any purpose. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation. Often an insulation material will be chosen for its ability to perform several of these functions at once.

A chilled beam is a type of convection HVAC system designed to heat or cool large buildings. Pipes of water are passed through a "beam" either integrated into standard suspended ceiling systems or suspended a short distance from the ceiling of a room. As the beam chills the air around it, the air becomes denser and falls to the floor. It is replaced by warmer air moving up from below, causing a constant flow of convection and cooling the room. Heating works in much the same fashion, similar to a steam radiator. There are two types of chilled beams. Some passive types rely solely on convection, while there is a "radiant"/convective passive type that cools through a combination of radiant exchange (40%) and convection (60%). The passive approach can provide higher thermal comfort levels, while the active type uses the momentum of ventilation air entering at relatively high velocity to induce the circulation of room air through the unit.

A fan coil unit (FCU) is a simple device consisting of a heating and/or cooling heat exchanger or 'coil' and fan. It is part of an HVAC system found in residential, commercial, and industrial buildings. A fan coil unit is a diverse device sometimes using ductwork, and is used to control the temperature in the space where it is installed, or serve multiple spaces. It is controlled either by a manual on/off switch or by a thermostat, which controls the throughput of water to the heat exchanger using a control valve and/or the fan speed.

A snowmelt system prevents the build-up of snow and ice on cycleways, walkways, patios and roadways, or more economically, only a portion of the area such as a pair of 2-foot (0.61 m)-wide tire tracks on a driveway or a 3-foot (0.91 m) center portion of a sidewalk, etc. They function even during a storm thus improve safety and eliminate winter maintenance labor including shoveling or plowing snow and spreading de-icing salt or traction grit (sand). A snowmelt system may extend the life of the concrete, asphalt or under pavers by eliminating the use salts or other de-icing chemicals, and physical damage from winter service vehicles.

Radiators and convectors are heat exchangers designed to transfer thermal energy from one medium to another for the purpose of space heating.

HVAC is a major subdiscipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy efficiency. The discipline of HVAC includes a large number of specialized terms and acronyms, many of which are summarized in this glossary.

Underfloor air distribution air distribution strategy for providing ventilation and space conditioning

Underfloor air distribution (UFAD) is an air distribution strategy for providing ventilation and space conditioning in buildings as part of the design of a HVAC system. UFAD systems use an underfloor supply plenum located between the structural concrete slab and a raised floor system to supply conditioned air through floor diffusers directly into the occupied zone of the building. UFAD systems are similar to conventional overhead systems (OH) in terms of the types of equipment used at the cooling and heating plants and primary air-handling units (AHU). Key differences include the use of an underfloor air supply plenum, warmer supply air temperatures, localized air distribution and thermal stratification.Thermal stratification is one of the featured characteristics of UFAD systems, which allows higher thermostat setpoints compared to the traditional overhead systems (OH). UFAD cooling load profile is different from a traditional OH system due to the impact of raised floor, particularly UFAD may have a higher peak cooling load than that of OH systems. This is because heat is gained from building penetrations and gaps within the structure itself.

A dedicated outdoor air system (DOAS) is a type of heating, ventilation and air-conditioning (HVAC) system that consists of two parallel systems: a dedicated system for delivering outdoor air ventilation that handles both the latent and sensible loads of conditioning the ventilation air, and a parallel system to handle the loads generated by indoor/process sources and those that pass through the building enclosure.

Cooling load is the rate at which sensible and latent heat must be removed from the space to maintain a constant space dry-bulb air temperature and humidity. Sensible heat into the space causes its air temperature to rise while latent heat is associated with the rise of the moisture content in the space. The building design, internal equipment, occupants, and outdoor weather conditions may affect the cooling load in a building using different heat transfer mechanisms. The SI units are watts.

References

1 2 3 4 5 ASHRAE Handbook. HVAC Systems and Equipment. Chapter 6. Panel Heating and Cooling Design. ASHRAE. 2016.
1 2 3 Stetiu, Corina (June 1999). "Energy and peak power savings potential of radiant cooling systems in US commercial buildings". Energy and Buildings. 30 (2): 127–138. doi:10.1016/S0378-7788(98)00080-2.
↑ Higgins C, Carbonnier K (June 2017). Energy Performance of Commercial Buildings with Radiant Heating and Cooling (Report). pp. 9–12. Retrieved November 8, 2017.
↑ Karmann, Caroline; Schiavon, Stefano; Bauman, Fred (January 2017). "Thermal comfort in buildings using radiant vs. all-air systems: A critical literature review". Building and Environment. 111: 123–131. doi:10.1016/j.buildenv.2016.10.020.
↑ Simmonds, P.; Holst, S.; Reuss, S.; Gaw, W. (1 June 2000). "Using Radiant Cooled Floors to Condition Large Spaces and Maintain Comfort Conditions". ASHRAE Transactions: Symposia. ASHRAE Winter Meeting. Dallas, TX (US): American Society of Heating, Refrigerating and Air-Conditioning Engineers. pp. 695–701. OSTI 20104826.
↑ Sastry, Guruprakash; Rumsey, Peter (May 2014). "VAV vs. Radiant - side by side comparison". ASHRAE Journal. Atlanta, GA (USA): ASHRAE. Archived from the original on 9 November 2017. Retrieved 8 November 2017.
↑ Wenisch, Joseph; Gaunt, Lindsey (Spring 2015). "Inspiring explorers - Case study: The Exploratorium" (PDF). High Performing Buildings. Atlanta, GA (USA): ASHRAE. eISSN 1940-3054 . Retrieved 8 November 2017.
↑ 2016 List of Zero Net Energy Buildings (Report). New Buildings Institute. 13 October 2016. p. 8. Retrieved 8 November 2017.
↑ Maor, Itzhak; Snyder, Steven C. (Fall 2016). "Evaluation of Factors Impacting EUI from High Performing Building Case Studies". High Performing Buildings. Atlanta, GA (USA): ASHRAE. eISSN 1940-3054 . Retrieved 8 November 2017.
↑ Bean, Robert; Olesen, Bjarne; Kim, Kwang Woo (February 2010). "History of Radiant Heating and Cooling Systems – Part 2" (PDF). ASHRAE Journal. Atlanta, GA (USA): ASHRAE. Retrieved 8 November 2017.
↑ Giesecke, Frederick E. (1947). "Chapter 24 - Radiant cooling". Hot-water heating and radiant heating and radiant cooling. Austin, Texas: Technical Book Company. 24-6. The first large building in Zurich equipped with a combination radiant heating and cooling system is the department store Jelmoli (Fig 24-1). The first sections of this store were erected during the period from 1899 to 1932 and equipped with a standard radiator-heating system using low-pressure steam; the latest section was erected in 1933-37 and equipped with a combination radiant heating and cooling system...The Administration Building of Saurer Co. in Arbon and the Municipal Hospital in Basel are among the more important buildings recently equipped with radiant cooling systems.
↑ Manley, John K., ed. (1954). "Radiant cooling and air conditioning". Radiant Heating, Radiant Cooling. Bulletin No. 1. Pratt Institute School of Architecture. pp. 24–25. OCLC 11520430. This type of system has proved successful in several installations. It was first attempted in a few sample rooms in Radio City about five years ago. Since that time, it has appeared in the 30-story Alcoa Building as well as in another multi-story building in Canada. Both of the latter structures are heated in winter and cooled in summer by the same coils of pipe in metal ceilings.
↑ Olesen, Bjarne W. (February 2012). "Thermo Active Building Systems Using Building Mass to Heat and Cool" (PDF). ASHRAE Journal. Vol. 54 no. 2. Atlanta, GA (USA): ASHRAE. Retrieved 20 November 2017.
1 2 3 4 Olesen, Bjarne W. (September 2008). "Hydronic Floor Cooling Systems". ASHRAE Journal.
↑ Gwerder, M.; B. Lehmann; J. Tödtli; V. Dorer; F. Renggli (July 2008). "Control of thermally-activated building systems (TABS)". Applied Energy. 85 (7): 565–581. doi:10.1016/j.apenergy.2007.08.001.
1 2 Mumma, S.A. (2002). "Chilled ceilings in parallel with dedicated outdoor air systems: Addressing the concerns of condensation, capacity, and cost". ASHRAE Transactions. 108 (2): 220–231.
↑ Stein, Jeff; Steven T. Taylor (2013). "VAV Reheat Versus Active Chilled Beams & DOAS". ASHRAE Journal. 55 (5): 18–32.