Concrete

Thermal Mass and Energy Performance

This article is condensed from the sustainable design module of the Concrete Thinking seminar that formed part of the RAIC’s 2009 continuing education program.

By Brian Hall

How Thermal Mass Behaves

Concrete has high thermal mass and hence thermal storage capacity relative to other materials [Figure 1]: which means it is able to absorb heat, store it for a period of time and then gradually release it. For example, during the winter a strategically placed concrete floor may absorb solar radiation. The floor will warm up during the day, and then in the evening, when the space begins to cool, the heat stored in the floor will be released into the space.
During the summer, this floor can be shaded from the sun. Overnight, the windows can be opened to allow cool air to enter the space and cool the floor slab. The floor will then be cool in the morning and once again able to absorb heat gains during the day.
Thermal mass interacts with the environment, both internal and external, to delay the effect of changes in the internal or external thermal loads.

How energy is used in buildings

Buildings are designed to meet occupants’ needs for thermally comfortable, well-lit, well ventilated spaces. Buildings use energy for lighting, appliances, equipment, service hot water and to condition interior spaces with ventilation and temperature control.
To maintain temperatures in occupied spaces so that occupants will be comfortable, building systems remove or add heat to the space and building envelope elements resist the flow of heat into or out of the space. Energy is input to balance the envelope gains and losses as well as internal heat gains from heat added by occupants, lighting, equipment and solar radiation entering through the glazed envelope.
Thermal mass helps to reduce the amount of heating and cooling energy used to meet the desired balance point. For example, in a cooling-dominated building, such as a commercial office, the savings in cooling energy will be more conspicuous because the cooling energy is large compared to heating energy.
A thermally massive building has the same external and internal loads as a low mass building, but it responds differently because it has the capacity to store heat. Thermal mass moderates indoor temperature fluctuations. When it is incorporated in massive wall and roof elements, it helps slow the transfer of heat through the building envelope. Acting somewhat like a battery, mass can also store energy thus shifting demand to off-peak periods, potentially reducing peak loads and avoiding peak utility rate periods where time-of-use rate structures are in effect. [Figure 2]
The thermal behaviour of thermally massive envelope components is complex. Thermal dynamics within massive walls and roofs smooth heat flow rates, and as a result, skin losses and gains are reduced in buildings with heavy mass envelopes.

Passive and Active Thermal Mass Strategies

Thermal mass can be used passively or actively in a building. For example, a medieval cathedral has massive construction and shows a thermal lag due to the storage effect of the building mass. Rather than responding rapidly to changing outdoor temperatures and internal occupant loads, the heavy walls permit a heat flow which varies only slightly over the course of the day. Such a structure shows use of passive thermal mass.
Buildings can now be designed to actively promote and control the exchange of energy between building occupants and the mass of the structure. For example, the temperature of the building mass can be controlled using floor slabs made of hollow-core concrete, raised floor systems that pass ventilation air over a concrete floor slab, or a radiant system in which heating/cooling coils are cast into concrete floor or wall elements.
Precast concrete hollow core slabs are strategically placed between the floor and ceiling finishes, thereby facilitating the principle of thermal mass in which naturally-occurring heating and cooling is captured, stored and released on demand. Forced air ventilation systems are used to draw in cool night air through the voids in the warm concrete slabs, cooling the slabs’ mass in the process. The following day, the now cooled concrete slabs are used to reduce the higher daytime indoor air temperature. [Figure 3]
A similar mechanism can be used to cool solid concrete slabs with raised access floors, using outdoor air drawn through the floor plenum. [See Greening the High Rise Office – this Issue] In cast-in-place concrete buildings, mass surface temperatures can also be controlled using concrete floor slabs which incorporate tubing arrays through which water can be passed. Cool water can be circulated through the slabs to pre-cool them, delaying the requirement for refrigeration-based cooling.
In this way, passive floors and ceilings become “smart floors and ceilings” that can be primed with night pre-cooling to better absorb daytime heat derived from the sun and internal building elements including occupants’ body heat and devices such as computers, lighting, etc.
By storing cool energy at night, daytime demand peak cooling loads are reduced and the mechanical system capacity can also be reduced. By linking the thermal storage capabilities of buildings to smaller, conventional mechanical systems, potentially reducing both capital and operating costs, the building’s value increases.
Concrete slabs using embedded pipe radiant heating and cooling systems are becoming more common in Canada. There are also a growing number of buildings employing active thermal mass using hollow-core slabs. One example is the Brock University Plaza 2006 [Photo 1 – see also SABMag January/February 2008] – another is Humber College. [Photo 1]

Comfort Considerations

Reducing energy consumption can reduce the operating costs of a building. However, energy-saving strategies should not compromise occupant comfort. The importance of occupant comfort is recognized by standards like ASHRAE 55-1992 [Thermal Environmental Conditions for Human Occupancy] and ASHRAE 62-1999 [Ventilation for Acceptable Indoor Air Quality].
Human thermal comfort is defined mainly by air properties such as temperature, relative humidity and air movement. A person’s perception of thermal comfort depends in part on the temperature of the surrounding air and surfaces enclosing the space occupied by that person. Humans rely on this temperature difference bec-ause our bodies are constantly generating heat, transferred to cooler air and objects, by convection, radiation, evaporation and conduction.
If the ambient temperature is too high, it becomes stressful for our bodies to lose heat because increased sweating, for evaporative cooling, is required. If ambient temperature is too low, we lose heat faster than we can replace it. Either of these situations gives us a feeling of thermal discomfort, which can also be caused by the surrounding surface temperature and the flow of air over our bodies.
Designing for thermal mass requires consideration of radiant heat transfer and the temperatures of thermal mass elements with the rise and fall of temperatures during the daytime and nighttime respectively. As radiant heat travels in straight lines, the temperature of surfaces within the line of sight of building occupants will affect their thermal comfort. ASHRAE 55-1992 offers guidance on acceptable parameters in situations where occupants are exposed to warm or cold walls and slabs.
Sustainable building design, therefore, requires strategies that can provide occupant comfort while reducing the energy-related environmental impacts.

Thermal Mass Effects: A Simulation

As we have seen, thermal mass influences heat flows through the building envelope and also between interior components of the building. The characteristics of these aspects of thermal dynamics influenced the choice of approach for a building energy simulation study carried out by the Cement Association of Canada in 2004.

Methodology

To illustrate the thermal mass effect, a typical four-storey building was modelled using alternate methods of construction. Three primary structural systems were modelled: light mass [steel], medium mass [precast concrete] and heavy mass [cast-in-place concrete]. The simulation was run using locations in Vancouver, Regina, Toronto, Montreal and Halifax.
The thermal mass construction options were based on current, typical building construction practices. Minor adjustments were made to the widths of building components to ensure that the overall R-values of each option were equal to the MNECB requirement for each location.
In lightweight buildings, an increase in temperature on the outside quickly causes a corresponding increase in heat flow to the inside. In a more massive building, the corresponding heat flow increase is delayed and is small compared to that in the lightweight building. Thermal mass reduces heat flow spikes and delays them relative to the change of temperature that causes them.
To illustrate the effect of variations in internal thermal mass, the building energy simulation included adjustments to the room weighting factor for buildings of different mass. This input affected the manner in which internal heat gains are translated into cooling loads. For rooms with heavier weighting factors, heat gains are more significantly affected, as the additional mass delayed heating and cooling requirements for longer periods.

Findings

This analysis confirmed that the use of thermal mass in energy design contributes to operating energy savings in buildings and can be used as a sustainable design strategy. Under appropriate conditions, and when compared with light-weight buildings, thermally massive buildings are expected to show energy savings benefits for three reasons:
1. There are fewer spikes in heating and cooling requirements, since mass slows the building thermal response time.
2. Thermal mass can shift some loads so that instead of superimposing, they are more spread out over a 24-hour cycle, with a resulting dec-rease in peak loads.
3. Energy for heating and cooling is reduced because heat flow in either direction through massive envelope elements is reduced.
Incorporating thermal mass into a building’s design has multiple benefits. The heat sink/heat source effects of thermal mass will help reduce peak and non-peak heating and cooling demands on the mechanical systems. Reducing peak demand allows mechanical systems such as chillers and air handling equipment to be downsized. Smaller equipment and reduced peak loads saves capital costs of equipment, energy usage [kWh] and energy demand [kW]. Operator and maintenance costs are also reduced. The buildings studied showed a trend toward reduced peak cooling demands as thermal mass increased [Figures 5 and 6].
Further studies have confirmed that energy performance benefits can be maximized by optimizing solar orientation, glazing size and specification, solar shading and the location of thermal mass within the subject building. In some instances, particularly cooling dominated buildings, the addition of active systems [eg: mechanically ventilated hollow core slabs or night flushing to reduce the temperature of structural elements] may be needed to regulate the diurnal thermal cycle and ensure that performance in practice replicates that predicted by the energy simulation.
In the form of precast units, cast-in-place slabs and columns, or concrete masonry unit walls, concrete in its many forms remains the most versatile and economical material for increasing performance in today’s buildings.

Progress on the Life Cycle front

The Canadian Precast/Prestressed Concrete Institute, the Prestressed Concrete Institute and the National Precast Concrete Association recently completed the first phase of a life-cycle assessment [LCA] of a five-storey precast concrete commercial office building from the “cradle to project completion” stage, meaning the analysis of the environmental effect of raw material extraction and precast product fabrication, through to delivery and erection.
Three primary structural systems [precast concrete, cast-in-place concrete, and steel] were modelled and five interchangeable wall envelope systems [curtain wall, brick and steel stud, precast concrete, insulated precast concrete, insulated precast concrete with brick veneer] making a total of 15 scenarios modelled. The study and model also include six regional climate zone locations, which together with the 15 scenarios made for a total of 90 scenarios. Phase Two, to begin later in 2010, will look at the environmental effects of building operation, maintenance, and end-of-life issues. Info: www.cpci.ca

Brian J. Hall, B.B.A., MBA is National Marketing Director for the Canadian Precast/Prestressed Concrete Institute.

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