Thermal Energy Storage is the Key to Unlocking Low Carbon Concrete Buildings

Written by: Jack Laken

The building industry has been exposed in the past several years for requiring dramatic change to status quo practices. Buildings contribute nearly 40% to annual global CO2 emissions. The pandemic has led to questions regarding ventilation safety in buildings where we spend up to 90% of our time.

These serious challenges are prompting many energy engineers to question:

If I were to start today with retrofits or new building designs, what would I do differently?

Those who are interested in successful building and business transformations are finding answers by embracing disruptive engineering designs while establishing synergistic and lucrative industry partnerships.  

Effectively scaling innovative sustainable building designs will contribute significantly to meeting global climate change goals. But these solutions should meet certain sustainability criteria.

Thermal energy storage is the key to unlocking low carbon, super sustainable buildings – and a simple and intuitive ventilation design utilizing standard hollow core concrete construction is helping to make that happen.

In an age where energy efficiency, sustainability, balancing energy supply with demand, and reducing embodied and operational carbon are important to the health of the global environment, concrete is seen as an energy and carbon intensive material.

Building in cold Canadian climate zone 7 at Brock University where this strategy is deployed.

Yet, concrete is the second most used material on earth other than water. This volume of production makes its contribution to global emissions even more pronounced.

Concrete is not going away any time soon, however. There is no substitute for its economics, strength, resilience, ease of use and global accessibility. It is used on every construction project…bar none.

Making Concrete Sustainable – It’s in the Design

So, how do we make concrete a more sustainable building material? As with any construction project aimed at sustainability and reducing carbon, there are four aspects to the strategy:

  • Reduce embodied carbon for short term carbon reductions
  • Reduce or eliminate operational carbon for long term carbon reductions
  • Extend the useful life of low operational carbon construction
  • Recycle

Embodied carbon is the calculated sum of all the Greenhouse Gases (GHG) to manufacture, ship, install, and dispose of all the materials used to construct the building – including its equipment and all the pieces and parts. Operational carbon is sum of the GHGs emitted through building operations; for example, heating and cooling the building.

Sustainable building lifecycle

Over the useful life of a building, cumulative operational carbon will eventually swamp the embedded carbon totals in a building which remain relatively stable over time. But embodied carbon may be more important to shorter-term sustainability targets as its reductions are tabulated when built, and not over time. Conversely, operational carbon is tallied over the building’s useful life. This means that savings are smaller in the beginning years but gain cumulatively as time passes.

The hot, humid climate of South Carolina helped the campus achieve Net Positive energy and has this strategy deployed.

To be effective, a low carbon concrete building strategy should employ reductions in both embodied and operational carbon.

The Concrete Kicker

The one thing that concrete has going for it that makes it an extremely sustainable material is its thermal capacity, which is currently not being leveraged to its full potential. Concrete is a great thermal energy storage mechanism. In fact, stone is the only practical construction material that beats it.

The ability of concrete to store thermal energy is well known, but successfully engaging it to accomplish a low carbon building strategy has been elusive until now. Common passive thermal strategies are not enough. They are unpredictable, inconsistent, and provide a rather small benefit.

Actively managing the thermal energy storage within a building is therefore necessary. And, believe it or not, accomplishing this can be relatively simple.

Here is how.

Integrated Thermal Ventilation Design

The concrete flooring system of a building provides an ideal mechanism to actively access a large and currently not utilized thermal storage mechanism in a building. This is especially the case if the flooring system is constructed of precast hollow core slabs (although cast in place solutions can be done as well).   Integrated Thermal Ventilation Design feeds air through ducts created in the concrete floors to take full advantage of the thermal properties of concrete. It combines four systems into one: heating, cooling, fresh air ventilation, and thermal energy storage.

Integrated thermal ventilation design feeds air through ducts created in the concrete floors to take full advantage of the thermal properties of concrete.

The objective of integrated thermal ventilation design is to maintain concrete flooring temperatures between 68°F and 72°F (20° to 22° C). This is similar to how the constant temperature of the Earth drives the effectiveness of geothermal heating/cooling systems.

By keeping the concrete flooring system at or close to the comfort set point for the building, you essentially have created a non-hydronic radiant heating and cooling system. As air passes through the labyrinth of duct in the concrete slab, thermal energy is strategically exchanged so that when the air reaches occupied spaces, it is already at the comfort set point.

The air can largely be conditioned without engaging the heating or cooling elements of the HVAC system itself. This process uses simple fan motors and outside air versus turning on chillers or heating elements of the HVAC system – saving large amounts of energy in the process.

The thermal energy storage capability of the concrete provides a strong inertia against temperature fluctuations. This helps to stabilize internal building temperatures in winter or summer and provides a largely self-regulating indoor environment.

So, how does this ventilation design perform against the sustainability measures we identified above? Let’s take a look at each step.

Reduce embodied carbon for short term carbon reductions With integrated thermal ventilation, embodied carbon Is reduced through simplification of mechanical systems in a building. Simplification is achieved through reduction in size or elimination of pieces of mechanical and other building equipment and materials that are not needed anymore. For example, the HVAC system can be reduced by 40% – 50% in size to account for the actively managed thermal capacity of the building.

Some potential impacts (reductions or elimination) on mechanical systems enabled by deploying this strategy driving cost, carbon and energy consumption reductions for the building.

Advanced efficiency equipment such as chilled beams and ice storage can be eliminated because it’s simply not needed anymore. Additionally, drop ceilings, raised floors, perimeter ducting, and other standard building pieces and parts can be eliminated. Because of operational efficiencies (to be discussed), solar systems to achieve net zero or net positive outcomes can be downsized to achieve the same result.

All of these reductions and eliminations also reduce embodied carbon for the project. 

Reduce or eliminate operational carbon for long term carbon reductions

When the concrete flooring system of a building is paired with the heating and cooling system, energy consumption reductions of 35% – 50% can be achieved with no new equipment, and no complex software applications.

Cost comparison of the Integrated Thermal Ventilation solution to other systems for school buildings.

Actively keeping the temperature of the concrete floors within a small range using outside air or internal loads at strategic times during the day and night using simple fan motors allows this system to maintain comfort levels in the building without utilizing the heating or cooling elements of the HVAC system itself. For example, cool night-time air can be circulated through the concrete floor slabs, thermally “charging” them for the next day. The cooled building serves as a heat sink during the daytime hours, reducing the need for mechanical cooling. The off-peak cooling strategy leverages discounted energy pricing from the utility, while reducing daytime HVAC demand calls. During daytime occupied hours, the thermally charged floors maintain indoor comfort with minimal mechanical cooling.

During heating season, the internal load from people, lighting and equipment is scavenged by the concrete flooring system which can then be distributed throughout the building as a free heating source. Once again, the thermally charged floors can maintain indoor comfort with minimal mechanical heating. In fact, one Canadian school building with thermal ventilation did not turn on its heating system throughout the entire winter due to the amount of heat scavenged from equipment, people, and lighting systems in the building. 

In fact, the following figure shows an actual trend log from the building management system in Hawthorne Village School located in Burlington, Ontario. You can see on a very cold winter day in climate zone 7, the heating system did not cycle on during day-time hours (bottom red oval), yet the temperature rose slowly and comfortably throughout the day (top red oval).

An actual building data log showing a modest temperature rise in the building on a very cold (sub-freezing) day in climate zone 7, yet the heating system was not required to cycle on to maintain comfort

Keep reducing operational carbon for an extended period

Designing low or net zero energy buildings that have a long or extended life cycle is the ideal situation for low carbon buildings. Long life cycles keep carbon out of the atmosphere since you don’t have to start the construction cycle again. An efficient building will continue to provide low carbon emissions throughout its lifecycle. Concrete is ideal for this scenario since it is a resilient and long-lived building material. If integrated thermal ventilation is utilized in the building, it will continue to save energy and reduce carbon emissions throughout its life.

Features inherent with the Integrated Thermal Ventilation strategy that can contribute to an overall lower cost of construction.

Recycling

Finally, concrete is fully recyclable. Recycling concrete saves a considerable amount of energy compared to mining, processing, and transporting new aggregates. It may also save quite a bit of energy in transportation of the concrete to a landfill or disposal site. Crushed concrete can be used as an aggregate in new concrete production with a few limitations.

Thermal ventilation changes the game

With integrated thermal ventilation, low carbon construction using concrete is entirely possible. In fact, when actively accessed and managed, the thermal energy storage capabilities of a building open opportunities for highly efficient low carbon buildings long into the future.

Additionally, because a thermal ventilation system can safely and strategically accommodate over-ventilation the, it may also help to prevent the spread of infectious disease. Finally, due to mechanical system and building simplification, thermal ventilation can reduce the capital cost of a construction project by $10 – $50 per square foot. 

For energy engineers, the opportunity to use this strategy in a retrofit or even new construction capacity is enormous. One concrete industry association in Canada estimates that there is over 1 billion square feet of hollow core flooring installed in the Canadian building stock, and about 5 to 7 times that area in the USA. This is massive and has the potential to dramatically change the trajectory of building efficiency projects.  

Integrated thermal ventilation is NOT a new idea or technology. There have been over 35 buildings in Canada and the USA adopt this solution over the past 15 years; two of which are the best performing K-12 schools in Canada (independently assessed). Additionally, there is a 5 building Net Positive school campus in South Carolina USA that was featured in a recent NREL report on Zero Energy K-12 schools.

These high performing buildings are examples of the benefits of meshing existing technology and building materials with an innovative yet simple design techniques to accomplish significant efficiency, sustainability, and carbon reductions – all at a reduced cost. 

Concrete is often seen as an unsustainable construction material, but maybe it should not get such a bad rap after all.


Jack Laken is President of Termobuild Canada and the only founding partner and a specialist in integrated building solutions that fills the gap with conventional designs. He brings over 30 years of North American and International experience in specialized low energy building solutions serving corporate, institutional and private clients. Recently his expertise on how to do more with less is focused on Smart City, Resilience, Zero energy buildings and how to future proof real estate value. Key expertise is focused on integrated building solutions that drive simplification and improved efficiency as a baseline. Jack has 30 years of North American and International experience in commercial, institutional and residential engineering design at his own Engineering Design Assist firm prior to founding TermoBuild.

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