Small drones could be better for climate than delivery trucks, says study
Automated, unmanned drones are poised to revolutionise the package delivery industry, with a number of companies already testing drone-based delivery methods.
A new study in Nature Communications looks at the climate impact of a shift from truck-based to drone-based package delivery. It finds that while small drones carrying packages weighing less than 0.5 kg would reduce greenhouse gas emissions compared to diesel or electric trucks anywhere in the US, the same is not true for larger drones carrying heavier packages.
§ Game of drones
The cost of drones has fallen dramatically over the past decade. At the same time, improvements in battery energy storage and a reduction in battery costs have led to drones that can fly much larger distances. Advances in automation and computer vision has made it possible to conceive of drones that could deliver packages to individual homes without requiring a human operator.
The combination of these factors has led companies – including Amazon, Google, UPS, DHL and others – to start developing programmes for drone-based delivery of packages. Due to their fast delivery speed, drones are a natural fit for same-day delivery of orders, something that a number of different companies have been pursuing in recent years.
While the commercial use of drones is strictly regulated today, both the US Federal Aviation Administration and the European Aviation Safety Agency are developing regulations to allow the use of drones for delivery purposes.
Current in-city delivery of packages is primarily via diesel trucks in both the US and Europe. Some companies are piloting electric delivery trucks. While their deployment to date has been fairly small, they are expected to take a larger market share in the future.
In most prototype programmes developed by companies so far, the drones used for delivery have been relatively small battery-powered quadcopters or octocopters. These drones can only fly for around 10-15 minutes before their batteries are exhausted, limiting the range in which they can deliver packages.
The new study looks at the climate impacts of two specific types of drones: small quadcopters capable of transporting 0.5kg and large octocopters that can carry up to 8.1kg. Both types of drones are capable of carrying a single package up to 4km and returning to refuel before their battery runs out.
§ More drones, more warehouses
The limited range of drones compared to traditional diesel delivery trucks would require companies to build many drone-enabled warehouses in locations close to customers. The authors of the new study suggest that this would increase direct energy use by these additional warehouses, as well as require additional deliveries to these distributed warehouses.
The authors give an example of how warehouses for drone delivery in the San Francisco Bay Area might work, as shown in the figure below. As drones only have a range of around 4km, a warehouse would be needed every few kilometres to cover an entire region. They suggest that most of San Francisco itself could be supplied from only four warehouses given its density (left map), but that 112 warehouses would be needed to supply the entire San Francisco Bay Area (right map).
Example of optimal warehouse locations to supply most of the city of San Francisco (left map) and greater San Francisco Bay Area (right map) assuming a small quadcopter with a range of 3.5km. Taken from Figure 3 of Stolaroff et al 2018.
§ Small drones can reduce emissions
CO2 equivalent: Greenhouse gases can be expressed in terms of carbon dioxide equivalent, or CO2eq. For a given amount, different greenhouse gases trap different amounts of heat in the atmosphere, a quantity known as the global warming potential. Carbon dioxide equivalent is a way of comparing emissions from all greenhouse gases, not just carbon dioxide.
All types of delivery drones are much more efficient per mile travelled than diesel trucks. However, delivery trucks generally do not carry only one parcel. Thus, the energy used per package is considerably smaller. The researchers point out that “delivery trucks carry many packages, delivering on average 0.94 packages per km (1.51 per mile), and their impacts need to be allocated per package”.
To account for this, the authors calculate the total greenhouse gas (GHG) emissions in grams of CO2-equivalent (CO2eq) per package from small drones, large drones, diesel trucks, natural gas trucks, electric trucks and petrol vans. They further examine how these findings vary across the different electricity generation mixes found in the US. You can see their main results in the figure below.
Stolaroff et al 2018HighchartsThe authors find that small drones that carry packages lighter than 0.5kg have the lowest overall GHG emissions of any delivery system they examined. This is true both in places such as California (where electricity is relatively low-carbon), for the US average grid mix and for areas of the US with the most carbon-intensive electricity.
Small drones result in a 50% reduction in GHG emissions per package delivered compared to diesel trucks in California and 37% less carbon than diesel trucks on average across the whole US. In Missouri, which has among the most carbon-intensive electricity in the country, small drones still result in a 23% reduction in emissions.
If it is assumed that the alternative to drone delivery is electric delivery trucks rather than diesel trucks, small drones reduce GHG emissions by 26% in California and 33% for the whole US. Interestingly, for the current US electricity grid mix, electric trucks result in about the same emissions as diesel trucks.
The picture is quite different for large drones, which carry packages up to 8.1kg. While they still reduce emissions in “low-carbon” California, they only result in a 9% reduction compared to diesel trucks. In the US as a whole, their emissions are actually 24%. This will change in the future as the US grid mix becomes cleaner.
In general, drones have a large potential for reduced transport emissions, while potential carbon reductions for diesel vehicles are more limited.
§ Breaking down the sources
The paper further breaks down emissions from each delivery option based on the portion that comes from transporting the packages, energy used for storing packages in warehouses, and the producing batteries used in drones and electric trucks.
The figure below shows the breakdown for various delivery options for the average US grid mix.
Stolaroff et al 2018HighchartsSmall drones have a remarkably low transportation-related electricity use, with almost all their emissions coming from the additional warehouses that need to be built. By contrast, large drones require much more electricity. Their emissions per package delivered is higher, with their combined transport electricity and battery production emissions roughly equal to the diesel fuel emissions from conventional diesel delivery trucks for the US average grid mix. Electric trucks also show similarly high transportation-related emissions.
While reducing the carbon intensity of electricity generation would reduce both large drone and electric truck emissions compared to diesel trucks, the authors say there is little reason to prefer large drones over electric trucks for package delivery due to the extra warehouse-related emissions that large drones require.
§ ‘Significant promise’
Overall, the researchers find “significant promise in the use of drones to reduce energy use and greenhouse gas emissions”. However, they also warn that “there are plausible scenarios where drones lead to overall higher energy use and greenhouse gas emissions compared to ground vehicles”.
As Dr Costa Samaras, the director the Center for Engineering and Resilience for Climate Adaptation at Carnegie Mellon University and one of the authors of the paper, tells Carbon Brief:
“Our research finds that small package delivery by drones likely has lower GHG emissions than diesel trucks, even when accounting for the potential additional warehouse space needed. For larger packages, we found that it’s probably better to use electric vans or electric trucks rather than large drones.
However, if warehouses to support urban drone delivery are constructed, they are likely to be used by both small and large drones – presenting a challenge for reducing GHGs from package delivery.
To maximise the climate benefits, firms should focus on smaller drones and an electrified ground fleet, while policy makers should focus on continued decarbonisation of electricity, as well as improved energy efficiency in buildings.”