Energy efficiency measures intended to reduce a building’s carbon footprint may also have negative impacts; for example, both insulative foam and heat pumps increase F-gas (fluorinated greenhouse gas) emissions. On the other hand, mechanical ventilation with heat recovery (MVHR) can greatly reduce heating demand. These complexities mean it is important to take a lifecycle approach to reducing a building’s carbon footprint.
There are a number of ways that a building’s carbon footprint can be assessed, listed below in increasing complexity:
GLOBAL WARMING POTENTIALS
Different gases cause varying amounts of warming while they are present in the atmosphere. The instantaneous warming effect is known as ‘radiative forcing’. The lifespan of a gas in the atmosphere is also of significance. A gas which causes a lot of radiative forcing but only lasts for a short time may not be as significant as carbon dioxide that lasts for several hundred years. GHGs are therefore compared to carbon dioxide in terms of how much global warming they will cause over a particular time period. This is technically known as the global warming potential (GWP) at a given time horizon. It is stated as a multiple of carbon dioxide’s global warming potential.
Fluorinated gases are particularly powerful and in some cases long-lived GHGs. The main types of F-gas are hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (SF6) and nitrogen trifluoride (NF3).HFCs and PFCs are both commonly used for refrigeration, including in air conditioners.
F-gases currently account for just a few percent of global warming, but rapid growth in their use means they could overtake CO2. F-gases are particularly concerning as they can remain in the atmosphere for very long periods and are inert, meaning that chemical removal from the atmosphere is virtually impossible. While carbon capture could in the future be scaled to remove CO2, there is unlikely to be any way to remove long-lived F-gases such as PFCs.
Using global warming potentials, the combination of different GHGs can be converted into a single figure, equivalent to that amount of carbon dioxide over the given time horizon. The mass of each gas is multiplied by its GWP to give a carbon dioxide equivalent (CO2e). For example, releasing 1 kg of methane, with a 100-year GWP of 28, would add 28kg CO2e to a carbon budget.
While F-gases are very powerful, they are generally circulated within the air conditioning unit, with only very small quantities leaking. This is very different to the CO2 released when fossil fuels are burned to produce power or heat. Because this CO2 is being continuously released, it typically contributes much more to a building’s carbon footprint.
Chlorofluorocarbons (CFCs) were once commonly used as refrigerants. They are powerful GHGs as well as damaging to the ozone layer. CFCs have now been phased out, as are their immediate replacement, hydrochlorofluorocarbons. HFCs are now the most common refrigerants in air conditioning, despite their high GWP. The 2016 Kigali Accord commits most of the world to phase down HFCs, with the most developed countries reducing use by 45% by 2024 and 85% by 2036.
R32, difluoromethane, is currently the alternative of choice. Although an HFC, its short life of just five years results in a relatively low 100-year GWP of 675, putting it below the F-gas regulation limit. However, the slight flammability of R32 requires compatible equipment.
Alternative refrigerants with much lower GWP and very high performance include ammonia, propane, and isobutane. Although toxicity and flammability may be a concern, they are generally considered safe for HVAC applications, although significantly different equipment designs may be required.
Carl Dickinson, consultant sales manager at Mitsubishi Electric, says that the key elements of the F-gas regulations are two-fold: contractors need to label, recover, report and destroy refrigerants, and clients with F-gas in that have a leak are obliged to repair that leak as soon as physically possible.
THE FUTURE: CARBON NEGATIVE HVAC
Direct air capture (DAC) takes CO2 from the air to be stored underground or used to produce carbon-neutral electro-fuels. Although currently very expensive and only operating on a very small scale, it is anticipated that DAC will eventually play a significant role in achieving net zero. In a 2019 paper in Nature, led by Prof. Roland Dittmeyer, it was suggested that air conditioning units could play a key role in making DAC economic. The paper considered the three largest supermarket chains in Germany and found their air conditioners currently process enough air to capture 8.75 Mt CO2, which could be converted to 3 Mt of e-fuel – 8% of Germany’s diesel consumption, or 30% of its kerosene. “This analysis impressively demonstrates that air conditioning systems in place, if equipped with the appropriate technology, could indeed capture a very significant amount of carbon dioxide,” says Dittmeyer.
When attempting to reduce a building’s carbon footprint, the HVAC system plays a big role. It’s also important to remember that this isn’t all about energy efficiency. For new systems, air conditioners and heat pumps should utilise low-GWP refrigerants. For older systems, it is critical that refrigerant leaks are prevented. Having said this, energy efficiency remains the most important way to reduce the overall carbon footprint.Carbon dioxide emissions during operation: This is the simplest way to measure a carbon footprint, considering only the actual CO2 released during operations, for example due to burning natural gas in a heating boiler or burning coal in a power station used to generate the electricity for an air conditioning unit. It ignores other GHG emissions that might be released during operation, as well as all of the emissions caused during manufacture and end-of-life. This is the incomplete thinking that assumes a battery electric vehicle is ‘zero emissions’.GHG emissions during operation: This is an extension of the first method, with other GHGs also taken into account, such as nitrous oxides that may be produced when a fuel is burned. Although an improvement, considering only operational emissions can dramatically underestimate the carbon footprint. As energy efficiency improves, this becomes increasingly true.Lifecycle emissions: This is the only method which at least attempts to consider all of the emissions associated with an activity. It considers emissions due to energy, materials and land use, during manufacture, transport, operation and end-of-life. Doing this comprehensively is highly complex and certainly not an exact science, being closely related to the politics of carbon accounting.