Structural Slab Radiant Cooling

Due to open in September this year at the University of Calgary is an information and communications technology (ICT) teaching and research facility. The 16,000-m2, $32.5 million, seven-storey building will house the departments of electrical and computer engineering, and computer science. It is comprised of academic office space and computer and electronic laboratories.

In high-tech facilities such as the ICT Building, one of the most significant mechanical engineering challenges is finding a means to offset the high internal heat gains that result from the sheer density of electronic equipment. A traditional all-air cooling system will often require high air change rates and this can make the occupants feel thermal discomfort. The large size of the distribution ductwork and the transport energy used to move the large air quantities are further disadvantages of the traditional solution.

In an effort to overcome these problems in the ICT building we have taken an innovative approach. To the best of our knowledge, this is the first use of structural slab radiant cooling in Canada.

Looking to Europe

Earth Tech Canada’s sustainable building design initiative has taken us to Europe on several occasions to explore technologies that have gained wide acceptance there but have not yet reached North America in a meaningful way. Radiant cooling was one of several technologies we have identified as being transferable and of potential benefit in high tech buildings.

The second initiative that led to the use of the structural slab radiant cooling (SSRC) system, came from our client, the University of Calgary. With utility deregulation and increasing fuel costs in Alberta, the province has never had a greater need to construct more energy-efficient buildings. The University of Calgary made a commitment to address this issue in both their existing and new buildings.

In the fall of 1999 the university issued a Request for Proposals to engineering consultants for the design of the ICT Building. The request challenged the proponents to demonstrate design creativity, excellence and innovation. In response, Earth Tech Canada presented the concepts of displacement ventilation, structural slab radiant cooling and natural ventilation. The building under construction incorporates the latter two of the three concepts proposed. This article describes only the SSRC system.

Radiant heating v. cooling

The use of radiant heating and cooling is not new. The Romans, 2000 years ago, used underfloor radiant heating and thermal mass heat storage in their hypocausts. In Turkey, stream water was run through channels in walls and floors to cool palaces in the warm summers.

Today, radiant heating is a well accepted method of providing heating in a building, whether by a radiant floor, metal panels or high intensity overhead units. Radiant cooling, however, is far less understood and misapplications in the past have hindered its acceptance.

While the installation of a structural slab radiant cooling system is very similar to the installation of conventional in-floor radiant heating, that is where the similarity ends. SSRC is quite the opposite of in-floor heating in every other respect, as Table 1 indicates.

Perhaps the most common example of the effectiveness of radiant cooling is in an open air, multi-level concrete parkade on a hot summer day. The air temperature may be 27-30C, but as you walk into the parkade you feel cooler. You have removed yourself from the radiant heat source of the sun and find yourself in an area surrounded by large areas of cool concrete. The air temperature has not changed significantly but the concrete acts as a radiant cooling source.

How the system works

HVAC engineers have long known the value, from an energy perspective, of decoupling the heating and ventilation requirements of buildings. Substantial savings result if fans do not need to run during unoccupied hours to maintain a building’s space temperature. Similarly, decoupling the cooling and ventilation offers substantial energy conservation opportunities, primarily because one reduces the need for energy to transport the air. In the ICT Building, we achieved a partial decoupling of the cooling and ventilation by the SSRC system.

In such systems, the concrete structural slab of a building is cooled by means of a “mat” of chilled water flowing through plastic piping embedded in the lower portion of the slab. The slab acts as a source of radiant cooling, primarily affecting the spaces below the exposed slab. Clearly, for such a system to be possible the building must be designed with exposed concrete slabs over the spaces to be cooled, i.e. there can be no suspended ceiling system.

In the case of the ICT building, the decision to adopt an open ceiling concept preceded the decision to employ a structural slab radiant cooling system. The designers preferred an open ceiling because it gives easier access to overhead services. Much effort was then given to the logical and aesthetic organization of the overhead services.

As buildings change over their life it is often necessary to drill into or core the floor slabs to accommodate new services or equipment. Because of the piping mat in the slabs, we had to avoid or at least minimize this kind of future intrusion. Two measures were taken. Drilling is avoided by the use of a 1.5 m x 1.5 m grid of inserts on the underside of the slab. A 1.5 m x 3.0 m Unistrut grid is suspended from the inserts. The Unistrut supports all building systems and any suspended laboratory equipment.

Coring is avoided by the installation of a series of 150 mm dia. sleeves through the slabs. The sleeves are located along building grid-column lines, between in-slab piping circuits. Beyond these measures there were only minor implications for the other building systems.

The SSRC system uses 16 mm plastic pipe at 150 mm centres embedded in the structural slab immediately above the lower reinforcing steel. The 150 mm pipe spacing is achieved by overlaying and offsetting two separate pipe circuits each installed at 300 mm centres. We selected 16 mm piping because it can be bent 180 degrees at 300 mm spacing and it provides the required output at an acceptable chilled water flow rate and pressure drop.

The building floor plate is based on a 9.0-m x 9.0-m structural grid. Each structural bay is sub-divided into three 3.0-m x 9.0-m areas for the purposes of the in-floor cooling piping. Within each 3.0-m x 9.0-m area of the floor slab two separate chilled water loops are provided from the header located below the slab. The two-loop approach reduces the pressure drop through the system and, by allowing for the isolation of each loop, it provides flexibility with respect to both capacity and for dealing with a possible single-point failure.

Energy economies

The structural slab radiant cooling system does not require the 7C water traditionally used for air-conditioning. The large thermal mass of the structural slab allows effective cooling at a slab temperature only nominally below room temperature. Chilled water at 13-18C is quite sufficient.

Using this higher temperature chilled water can result directly in energy savings, or, as is the case in the ICT Building, “used” chilled water can be re-used. The chilled water supplied to the SSRC system can be drawn either directly from the campus primary chilled water supply system or from the ICT Building secondary return system. The return chilled water from the air handling unit coils can be directed to the SSRC system, thus extending the chilled water system temperature difference within the building.

As in any building, the perimeter and interior areas of the ICT Building are thermally dissimilar at any one time, so to accommodate these variations the in-slab piping system is divided into east, west and interior zones. The west zone, for example, covers the perimeter three metre strip on the west side of the building, on all floors. The chilled water temperature and flow rate to each of the three zones can be varied independently. The SSRC system is largely passive
aside from this central flow and temperature control. There is no active local control of the slab cooling capacity.

The SSRC will provide the base cooling for a space while the variable volume air supply system will provide fine “topping” control of the space temperature. Sensors at key locations in the slab monitor the slab temperature. This data, along with room dew point temperatures and the chilled water supply temperature reset schedule, allows the building operators to avoid potential condensation on the slab surface.

The hybrid SSRC/VAV system was installed at no premium over a traditional all-air design. The in-slab piping “additional” cost was offset by savings in the size of the air handling systems, which was reduced by 50% as a result of the base load cooling provided by the SSRC. The reduction in duct sizes allowed each above-grade floor height to be reduced 375 mm. And without the SSRC the fan systems for the ICT Building would have required 300 BHP, whereas the installed fans require only 135 BHP.

Installation and benefits

The in-slab piping was installed by the mechanical contractor’s existing plumbing crews; no specialized labour was required. The consultant and the piping supplier closely monitored the laying of the first few sections. Several details of the installation were refined at this time and work proceeded well through the remainder of the project. In total 68 kilometres of piping, requiring 2,400 man-hours to install, are provided over a 9,700-m2 floor area.

The in-slab piping is expected to last for the life of the building. Should a pipe be punctured by drilling at some time in the future, two options exist. The “half” circuit with the punctured pipe can be isolated, drained and abandoned, leaving 50% of the area radiant cooling capacity in place. Alternatively, the punctured pipe can be exposed and a union installed.

The benefits of the SSRC are as follows;

Comfort levels are improved as radiant loads are treated directly and air motion in the space is at normal ventilation levels.

Peak loads are reduced as a result of thermal energy storage in the structure, resulting in smaller mechanical plant.

Energy consumption is reduced.

Flexibility for future change is provided by:

The modular nature of the SSRC and the air supply system; and,

The provision for the possible future addition of a second air handling unit in the penthouse. This would allow a future total cooling capacity of 150% of that possible with a traditional all-air system

Parallel with the design of the building, an application was submitted for a Natural Resources Canada Commercial Building Incentive Program (CBIP) grant. The energy use simulations performed indicated that the proposed design was greater than 25% more energy-efficient than the National Model Energy Code reference building, and the building has therefore qualified for the full $80,000 grant.

In addition the University of Calgary Faculty of Environmental Design has successfully applied to Alberta Innovation and Science for funding of a three-year project to monitor the performance of the ICT Building and determine the effectiveness of several innovations, including the SSRC.

Structural slab radiant cooling is a new and innovative technology that the ICT Building introduces to the Canadian HVAC industry. The building commissioning and subsequent monitoring will lend much to our understanding of the technology and will be the subject of future technical papers.CCE

Client: University of Calgary

Mechanical & electrical consultant: Earth Tech Canada, Calgary (Jim Sawers, P.Eng., Jim Dixon, P.Eng., Michel Vachon, P.Eng., Matthew Peachman, Daniel Sverdlov, Herb Haekel, Terry Griffith)

Table 1: Aspects of radiant heating v. radiant cooling

Prime consultant: Stantec Architecture

Structural consultant: Stantec Consulting

Project manager: DMC Resources

Construction manager: EllisDon Construction Services