Looking back at 2020, one thing will naturally come up in most people’s mind. The pandemic’s reach is global and very much ongoing, with only a handful of countries now flirting with a“back to normal”. Beyond the ubiquitous health crisis, we are all familiar with the economic implications that it brought about as well (Portugal’s GDP dropped a historical -7.6%, for instance). However, every so often a news outlet somewhere would communicate the rare good piece of news which would circle the world and quickly fade. Most might remember the before-and-after image of air pollution in China’s cities during the country’s lockdown. Or the slump in carbon emissions that took place virtually everywhere. But why is this? Remember the word “carbon” is a concept that standardises and groups all greenhouse gases.
Well, economic activity and energy use are proven to be tightly coupled. And energy use is in turn coupled to carbon emissions . Rarely has a country grown without worsening their emissions due to higher consumption, transport, construction, and more. With economic activity returning after a drastic drop, emissions are expected to surge at historical values. We are all aware of the urgent need to deepen the change from a strictly economic to a broader sustainable development. And the question of decoupling GDP and emissions is more relevant than ever. The answer to this is either to reduce the amount of energy used per unit of activity, or to reduce the amount of carbon emitted per unit of energy used (or even be “energy positive”). Put simpler: use less or use cleaner. Of course, both can be attacked at the same time.
When this problem is brought to building energy use, using less can be understood as having more efficient buildings. And using cleaner, switching to renewable sources of energy such as onsite solar PV systems. But there’s a catch: renovating a façade or installing a PV system (with wiring, batteries, an inverter, and more) are economic activities that bring about energy use and emissions themselves. But this doesn’t mean that they are useless — what we need to look at is their overall effect by quantifying how much carbon their existence brings and avoids in the different phases of their lifetime. In other words, understanding their embodied carbon and comparing them to business as usual, with our environmental goals in mind.
Let’s concentrate on the use cleaner part. Research on carbon that is embodied by different electricity generation technologies is widely available. These studies typically offer a range of values, as each installation carries with it specific mining, manufacturing, transport, and installation/construction needs which use up different amounts of energy (in short, Life Cycle Assessment studies). However, a mid-point estimation can be taken for reasonably accurate comparisons against other technologies.
Take the Portuguese case. The graph below compares 3 scenarios for a typical Portuguese dwelling’s electricity consumption profile of 3293 kWh/year . The first scenario, in yellow, shows what emissions look like with the current portuguese energy mix. That is, considering the grid’s carbon intensity based on the existing power plants and onsite generation (a combination of low-carbon and high-carbon sources ). The second, in green, shows what would happen if that dwelling installed PV panels. A typical residential PV installation is considered, replacing 1720 kWh/year of what is taken from the grid  (a bit over half of the total of 3293 kWh/year). The third scenario, in red, shows an extreme hypothetical situation in which all electricity is produced in a coal plant in the baseline scenario. This is not true for Portugal, but it is somewhat representative of other countries, such as Poland (with a whopping 74% of electricity coming from coal ).
What we see is that PV mitigates emissions, no matter the case. When replacing coal, less than 2 years of using 1720 kWh/year of PV-generated electricity already offsets its embodied carbon (see the red line crossing the green line). When comparing to the current Portuguese scenario, carbon flow for the PV implementation proves to be better somewhere around the 7th year. Keep in mind we are being practical and considering a real decision case, so we are not accounting for the embodied carbon of the existing grid infrastructure (as the damage has already been done). So we are being very demanding with PV in this study — even unfair. The embodied carbon of the PV installation is the reason why the green line starts above the red and yellow ones in year 0. Once this is offset, every year is pure mitigation (remember: renewables don’t make carbon “disappear” nor do they absorb it, they just avoid emissions from other dirty sources). And why stop here? Buildings can be low-carbon, but they can even be “energy-positive”. This means they generate more (cleaner) electricity than they consume, which can be injected into the grid and be used up by another building that needs it.
What all this shows is that even by conservative estimations, PV offsets its own negative effects. And this needs to be common knowledge. Almost all technology has negative effects. In fact, virtually all activity implies direct or indirect carbon emissions. This in itself isn’t a valid reason to dismiss technological changes or energy uses, as the whole picture must be analysed. A combination of use less and use cleaner is what engineers and designers should strive for. And why stop there? Let’s start thinking energy positive and start generating more than we consume.
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 Haberl, Helmut et al. (2020). A systematic review of the evidence on decoupling of GDP, resource use and GHG emissions, part II: Synthesizing the insights. Environmental Research Letters.
 PV carbon data: Stephen Finnegan, Craig Jones, Steve Sharples, The embodied CO2e of sustainable energy technologies used in buildings: A review article, Energy and Buildings, Volume 181, 2018, Pages 50–61, ISSN 0378–7788