Glass

To reduce the embodied carbon of glass, a set of actions is required. They include shifting energy-intensive glass production to best available technologies and low-carbon energy sources; establishing a policy framework that incentivises flat glass recycling from buildings through local solutions that avoid contamination of recycling streams, and designing glass that minimises unwanted heat absorption into the interior and instead captures solar energy for heating, cooling and lighting.

Demand for Glass in Construction and Renovation Is Rising

The buildings sector is the second largest end user of glass after packaging.

The glass sector is divided into flat glass (51 per cent; for buildings, automotive and electronics), container glass (45 per cent; for food and beverages) and other glass (4 per cent; e.g., domestic glass and tableware) (International Year of Glass [IYOG] 2020). The buildings sector accounts for around two-thirds of flat glass production, with glass used in most building façades as well as in many interior applications. Around 60 per cent of the world’s flat glass manufacturing capacity is in China (IYOG 2020).

Glass is Energy Intensive to Produce and Involves Emissions Trade-offs

Multi-paned windows save energy during operations but are more energy-intensive to produce.

Glass production is a high-temperature (between 1,400 and 1,600 degrees Celsius), energy-intensive process that is responsible for 0.3 per cent of global carbon emissions (86 million metric tons) (Westbroek et al. 2021). Glass production reached 209 million metric tons in 2019 and is growing rapidly at 5.2 per cent annually (IYOG 2020). The raw materials for virgin glass production are sand, lime or calcium carbonate, and soda ash. Mining these materials poses a high risk of forced labour (Grace Farms Foundation 2022). Melting the raw materials for glass leads to two main sources of CO2 emissions: 1) energy emissions from melting and 2) process emissions from adding limestone and soda ash to the melt (Westbroek et al. 2021). The energy intensity of production depends greatly on the technology and fuel source used.
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Glass is among the most controversial building materials, with trade-offs between embodied and operational emissions. High-efficiency windows with double or triple panes provide substantial energy savings during the operation of buildings but are more energy intensive to produce. Glass coatings reduce operational emissions by providing shading and reducing the need for artificial lighting (Arup and Saint-Gobain Glass 2022), but they complicate recycling with their complex material composition. Variations in design and expected building lifetimes greatly influence emissions over the glass life-cycle.

At the manufacturing and use phases, the most promising measures for improving the material efficiency of glass (and thus reducing emissions) are the re-use of container glass (68 per cent re-use brings emission savings of 38 per cent), using less material in the design of containers (10 per cent reduction in mass brings 6 per cent emission savings) and extending the lifespans of buildings and vehicles (Westbroek et al. 2021).

Hardly Any Glass in the Built Environment Is Recycled

Recycling is a powerful but underused tool for decarbonizing glass, particularly in the buildings sector.

Glass is a material that in theory could be produced in a low-carbon manner and be infinitely recyclable. In practice, only a third of container glass and hardly any flat glass is recycled. Container glass, used mostly for beverages, typically faces short lifetimes (less than one year) and has well-established recycling technologies; its average recycling rate is 32 per cent globally, although in some countries it reaches 70 per cent (Westbroek et al. 2021). In contrast, flat glass is used mainly for buildings with long lifetimes, estimated at 75 years, delaying recycling opportunities (Westbroek et al. 2021).
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The little glass from the built environment that is recycled is rarely recycled as flat glass; instead, after removal it is downcycled for use in insulation, containers, construction aggregates and road paint, among others (Westbroek et al. 2021). In Europe, the recycled content of flat glass is 26 per cent, but most of it comes from pre-consumer scrap (Glass for Europe 2020), as post-consumer material currently cannot reliably meet the strict quality requirements in flat glass manufacturing. Also, the high weight-to-volume ratio of glass makes its transport costly, with high environmental impacts; for this reason, it is important to set up local and regional recycling infrastructures (Bristogianni and Oikonomopoulu 2022).

Figure 5.14 Advanced glass façades

Glass façades of the future need to capture solar energy for use in heating, cooling, lighting and electrical loads.

Source: Novelli et al. 2022.

KEY STEPS TOWARDS

Decarbonizing Glass

Shift glass production to low-carbon energy sources and best available technologies

Solutions include switching to low-carbon fuels, melting using renewable electricity, improving energy efficiency in processing and operations, and waste heat recovery (Zier et al. 2021).

Analysis of 16 emerging glass production technologies showed potential energy savings of 20-70 per cent (Springer and Hasanbeigi 2017).

Large regional differences in emissions indicate that a shift to best available (and emerging) technologies is a key decarbonisation tool (Scalet et al. 2013).

Provide incentives for local production and recycling

Using recycled glass (“cullet”) in glass manufacturing can reduce energy use in furnaces by 2.5 to 3 per cent for every 10 per cent of cullet input, on top of the savings from avoided soda use (IEA 2007) (or 30 per cent if all glass were manufactured from cullet).

Through the proper handling and recycling of building glass, the European Union could avoid the landfilling of 925,000 metric tons of glass waste annually and save around 1.23 million metric tons of primary non-renewable raw materials (Hestin, de Veron and Burgos 2016).

These measures require education and close collaboration of contractors and recyclers, standards and legislation that encourage such practices (e.g., landfill tax, incentives for locally based production and recycling), rewards for recycling and re-use in certification systems, and focusing sustainability assessment credit systems (e.g., BREEAM, LEED) on the re-use and recycling of glass.

Improve glass renovation and demolition practices to maintain quality and enable recycling

Increasing the use of flat glass cullet requires ensuring that the recycled glass is clean, with only minimal contamination (e.g., no mixing in of special heat and fire-resistant glass types like Ceran).

Discarded windows should be disassembled off-site in clean environments that allow for efficient separation and for closed-loop recycling processes that maintain the high quality of flat glass and the re-use of coated glass where possible (Glass for Europe 2013).

Glass recycling needs to be optimised for the local context, balancing the needs for high collection efficiencies and material quality (Hestin, de Veron and Burgos 2016).

An alternative glass recycling path proposes the local, low-tech and contamination-tolerant casting of cullet into voluminous cast glass components for structural applications in architecture and interior design (Bristogianni and Oikonomopoulu 2022).

Improve glass design and related components by adopting best available technologies

Typically, glass is not used on its own in buildings but is associated with a range of other materials and components. The supply chains for glass curtain walls in particular can be complex.

Decisions made during the design stage can have impacts on the embodied carbon of glass systems. Incentives in education and enforcement by building codes would greatly increase the availability of circular glass.

In commercial buildings, use bio-based framing materials, such as engineered timber or bamboo, rather than high-carbon materials such as aluminium.

Improve glass design for windows and curtain walls to optimally absorb, store and redistribute solar energy for building functions

Glass façades often drive up the energy demand for cooling because they either let in too much heat and glare (increasing the size and emissions of cooling equipment) or they reflect the excess solar energy onto urban pavements, worsening the heat island effect and driving up cooling loads.

By using building information modelling in the design phase, a building’s shape and façade can be designed to let in more solar energy during cold periods and remain self-shaded during hot periods. However, these strategies are limited in hot climates.

Far more research and development is needed to adapt glass façades to capture solar energy for use in heating, cooling, lighting and electrical loads in the building interior (see Figure 5.14). Glass is key to the future on-site solar collection technologies that can enable net zero buildings (Novelli et al. 2022).