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How to Build Negative Carbon Buildings

Did you know buildings are responsible for approximately 40% of all carbon emissions? Real estate developers aren’t entirely to blame. But we have the power to not only reduce these emissions, but even achieve net negative carbon emissions throughout the life cycle of a building!

Before explaining how negative carbon buildings are possible, it’s important to understand that the emissions come from 2 parts of a building’s life cycle: construction, and operation.

Where Real Estate’s Carbon Emissions Come From

Building operations produce 28% of global emissions, which come from the energy requirements to run a building. I covered this in my previous post about how to build a zero energy building. So read that first because generating all of a building’s energy needs with renewables effectively eliminates all operational carbon emissions.

This article focuses on the construction emissions, or embodied carbon. Construction creates direct and indirect emissions every step of the process, producing about 11% of global emissions.

Construction Emissions

It’s not immediately apparent where all the carbon emissions come from in construction. Because if you watch a building go up, you can’t really see any pollution except from heavy construction vehicles. But the single largest source of emissions is from mixing concrete, which by itself produces 8% of world’s emissions. The other 3% is from producing steel and bricks, and manufacturing other materials and delivering them to construction sites. But going back to concrete, humans produce over 20 billion tons every year. After water, it’s the most widely used substance on earth.

The Concrete Making Process

We make concrete by mixing cement with water, which glues together aggregate (rocks). Then you set it, and let it dry. But that’s the very last step of the process that happens on the construction site. Transporting concrete to the site in a 30 ton mixing truck takes a ridiculous amount of energy. But it’s nothing compared to the energy required to make cement.

We make cement, particularly portland cement, by mixing limestone, clay, iron ore, and ash in a massive cylindrical kiln. The kiln is heated to 1450C or 2640F, which requires a ton of energy. But the actual chemical process requires the Calcination of Limestone. Limestone is made up of mostly Calcium Carbonate, or CaCO3. When we apply heat, it splits into Calcium Oxide (CaO), and Carbon Dioxide (CO2). The calcinated compound is a greyish pebbly material called Clinker. Clinker is ground and mixed with gypsum and limestone to produce the powdery cement, which will eventually be mixed with water and aggregate to make concrete. But the CO2 produced earlier? It floats away into the atmosphere.

When you factor in all the steps of making and delivering concrete to a construction site, we produce almost 1 lb of CO2 for every pound of concrete.

But there are a number of existing methods to reduce the carbon emissions from concrete.

Reducing Emissions from Concrete

One option is Limestone Calcined Clay Concrete, or LC3. It replaces most of the limestone in the Portland Cement formula with clay, which is as readily abundant as limestone all over the world. The clay requires less energy and produces fewer emissions because it only needs to be heated to 800 C. And there’s much less CO2 output from the chemical process. Overall, it leads to about 40% lower carbon emissions without sacrificing structural integrity or longevity. And the bonus… it’s cheaper to produce! The major hurdle for adoption is retrofitting existing cement plants to produce LC3 rather than Portland Cement.

Another option gaining traction is CO2 mineralization, and it can reduce emissions up to 70%. This requires first capturing carbon from the air. We can add carbon capture systems to some of our biggest carbon dioxide emitters, like coal fire power plants, or even the cement making process itself! The concrete is then cured by mixing with CO2 instead of water. And the result is even stronger than regular concrete. The captured and absorbed carbon is sequestered forever, regardless of whether the concrete is demolished.

The major hurdles here are costs. Installing carbon capture systems and producing the CO2-cured concrete need to become cheaper than regular concrete. Real estate developers don’t really care if it’s sustainable if it’s not cheaper.

Replacing Concrete

There’s one more option that might be more effective than tweaking the concrete formula- don’t use concrete at all. Use WOOD! Wood obviously comes from trees, which naturally absorb and store CO2 as they grow bigger. So when forests are harvested, the carbon is sequestered and won’t go anywhere. And forests are renewable and capture more carbon when harvested sustainably.

But you can’t replace concrete with your typical 2×4 or plywood. Developers are using Mass Timber more and more in larger buildings because it offers a ton of benefits. It’s short for Massive Timber, because it’s huge. Mass Timber includes a variety of wood products like glue-laminated (glulam) beams, laminated veneer lumber (LVL), nail-laminated timber (NLT), dowel-laminated timber (DLT), and the most popular one, cross-laminated timber, or CLT.

CLT is produced by gluing together trimmed and dried lumber boards in criss crossing layers to form really big panels. Typically 2-10 feet wide, 60 feet long, and over a foot thick. The production process doesn’t require any heat like concrete, steel, or bricks, unless you dry them in a kiln. So not only does wood naturally sequester carbon, it barely generates emissions! Furthermore, since the panels are so big, they’re lighter yet just as strong as concrete and steel. And they can be used to form the structural elements of buildings. They’re also surprisingly, more fire resistant than steel. And lastly, wood just looks better than concrete and steel.

How to Achieve Negative Emissions in Buildings

Up to this point, I’ve explained methods to reduce emissions to zero. But we need lots of creativity to push through the threshold to negative carbon buildings.

The BIQ or Bio Intelligent Quotient House in Germany implemented a really creative solution. It’s a 15-story concrete apartment building with a facade covered in algae biomass. Algae feeds off of CO2. Each one of these transparent panels is a bioreactor filled with algae and water and injected with nutrients and CO2. 

The panels act as solar thermal collectors by collecting the light that isn’t used by the algae, which is then converted into heat using heat exchangers. This heat can be used for hot water or be stored in the building’s underground geothermal system. Every 3-4 weeks, the algae is transplanted to tanks where they will undergo fermentation to produce methane gas or biofuel. This methane gas can be burned to generate electricity.

Algae covered façades can help us make slightly negative carbon buildings. But we can do better. The last piece of the puzzle doesn’t require any physical or chemical engineering.. but rather.. financial engineering!

It doesn’t make financial sense for most buildings to go carbon neutral. So the concept of a carbon credit was introduced and formalized in the Kyoto Protocol in 1997.

Carbon Credits

One carbon credit represents one metric ton, or 2,204 pounds of CO2 removed from the atmosphere. Once bought, the credit is retired because it represents a single, not a continuous removal of carbon. The Kyoto Protocol devised a quota system for participating countries to limit carbon emissions.

Countries or companies emitting beyond those quotas would need to buy carbon credits on public exchanges to offset their excess emissions. So free market mechanics and the levels of emissions from participating countries would dictate the pricing of these carbon emissions. The Paris Climate Agreement has a similar framework of national emission reduction targets with over 200 countries participating.

How Carbon Offsetting Works

So say you own a forest and you could raze the whole thing and sell the lumber. But suppose there’s a cement factory out there that’s generating a million tons more of CO2 annually than what regulations allow for. If you decide to sustainably harvest your lumber and regrow it as you go, and you hire a third party verification company to analyze the carbon offsets from your forest, you could generate cash to make up for not razing your forest by selling your carbon offsets to the cement factory. The cement factory could get back below the emissions limit, you’re compensated for it, and there’s less carbon in the air.

Carbon Credit exchanges still have some maturing to do. The value of these credits and the success of this system really depends on countries agreeing to participate in setting emissions quotas. Because it’s a lot like fiat currencies- it requires that a large enough network believes in its value for it to really be viable.

As of now it’s possible for individuals to buy carbon credits from a handful of marketplaces. So you can actually buy as many as you want to make negative carbon buildings. But whether it makes financial sense for your project really comes down to the value it creates for your building’s users. Office buildings tend to get more value out of sustainability features because a lot of companies establish environmental standards in their corporate governance.

Where We Go From Here

Although the methods to make negative carbon buildings all exist today, we have a lot of work to do to make it financially feasible. ClimateTech is entering a renaissance period with lots of funding. So you can check out which proptech companies are tackling these problems in my directory.