Marianne Kjendseth Wiik, Kristin Fjellheim, Jon Are Suul, Kamal Azrague
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Technology for the electrification of transport is currently undergoing rapid development that is necessary for reducing greenhouse gas (GHG) emissions. On a global level, the transport sector is responsible for around 12% of the world’s GHG emissions. While the introduction of battery-electric cars is leading the way in terms of commercial scale, developments are also progressing toward electrification of heavy-duty vehicles for road freight transport and coastal transport by battery-electric ships. The performance of modern Li-ion batteries is also enabling electrification of other types of machines and small vehicles that have traditionally been powered by internal combustion engines (ICEs). However, until recently, the developments toward electrification have been mainly directed toward applications with either a large market for series-produced vehicles, such as electric cars, or a high degree of individual engineering for each unit, such as battery-electric ships. Still, there are several application areas where other types of vehicles and machines contribute significantly to GHG emissions.
In many urban environments, the construction sector is a significant contributor to GHG emissions. In Oslo, Norway, around 60-100,000 tons of carbon dioxide equivalent (tCO2e) emissions arise from the construction industry. Over 90% of these GHG emissions arise from activities related to road transport to and from construction sites as well as from the operation of construction machinery on site. As a response, Oslo has set ambitious targets for cutting GHG emissions from construction site activities by 95% by 2030. These targets are currently being applied as a basis for public procurement processes intended to drive development toward zero-emission construction sites.
For the construction of buildings, the groundwork and preparation of the site is the construction phase that is traditionally the main contributor to high GHG emissions. This is mainly due to emissions from construction machinery powered by diesel engines. Among the construction machinery, recent studies on Norwegian construction sites indicate that excavators are responsible for more than 50% of these emissions. Thus, the construction industry in Norway, in collaboration with Oslo, has been responding to the challenge of reducing GHG emissions by electrifying construction machinery, with a special focus on excavators. Since the first pilot projects on fully electrifying excavators were started in Norway around 2018, the Norwegian Association for Machinery Wholesalers has reported that over 100 large electric excavators (over 8 tons) had been introduced in Norway by the end of 2021. Furthermore, around 250 electric excavators entered the Norwegian market during 2022, which corresponds to a 15% market share of all new excavators. This is an important achievement when considering that 40% of all medium and large construction machines in Norway are excavators.
This article discusses the electrification of excavators by retrofit installation of electrical drivetrains for replacing the original diesel engine. Such conversion from conventional fossil fuels to electrified operation serves to demonstrate the feasibility of reducing GHG emissions from construction machinery. Small-scale production series based on retrofit installation also enable the demonstration and gradual introduction of zero-emission technology while providing the first steps toward establishing a market that can allow for future series production of fully electrified excavators. The electrical configurations selected for retrofitting crawler excavators in three different weight classes (8.5, 17.5, and 38 tons) are discussed. Analyses of carbon footprints and costs are presented as a basis for guiding further development and future large-scale market introduction of electric excavators for reducing the emissions from the construction industry.
Internationally, there has been increasing attention toward the development of technology for reducing GHG emissions from construction equipment during the past decade. A large part of the resulting efforts in terms of scientific publications and the development of industrial products has been dedicated toward hybrid solutions for recovering regenerated energy and improving fuel efficiency by optimizing the operation of ICEs as the only energy source. However, solutions for fully electrified power supplies, either directly from the grid during operation or by battery-based operation, are also receiving increased international attention for ensuring zero-emission operation.
In the European construction machinery market, prototypes for mini, small, and middle-sized electric excavators are already becoming available. Similar developments are also expected for wheel loaders and other more specialized vehicles and machines that have traditionally been powered by ICEs. The Committee for European Construction Equipment (CECE) has published a position paper on the role of construction equipment in decarbonizing Europe and created a map of an expected path to market phases, as shown in Figure 1. The path shows three phases for market penetration, whereby the first phase involves prototype pilot projects, followed by productionization and, finally, series production. As discussed in the following, Norway has recently transitioned from phase 1 to phase 2 for the electrification of excavators.
Figure 1. The CECE path-to-market phases. Figure based on data from CECE.
To reach this point, several studies have been carried out in Norway over the past few years to investigate the required technical advancements for the electrification of larger excavators and to test electric excavator prototypes on construction sites to compile knowledge and practical experiences. An impact assessment of emission-free construction processes in Oslo has found that the demand for electric construction machinery is currently higher than the production capacity. Manufacturers resolve technological challenges in different ways; for example, one manufacturer is committed to converting its production lines to electric power trains, while another manufacturer is converting off-the-shelf diesel machinery to electric operations. For some cases, replaceable battery solutions are considered, while other applications are based on direct electrification by dynamic cables. Nearly all manufacturers have already started providing small-scale construction machinery (under 8 tons), while others have begun to electrify small (8–16 tons), medium (16–23 tons), and large (over 23 tons) machines. New actors are also emerging on the construction market in Norway to offer various mobile temporary battery container solutions, energy tracking tools, and power demand calculators. In addition, the Norwegian standardization body is working on a new technical standard (SN/TS 3770) that will give guidance for emission-free building and construction sites.
In the period since 2018, several prototypes for battery-electric excavators and excavators with direct cable-based electric power supply during operation have been developed for the Norwegian market by retrofitting conventional diesel excavators from mainly two different suppliers. During the same period, examples of electrified machinery for the zero-emission operation of a quarry have also been demonstrated by Volvo Construction Equipment in Sweden. When starting the development of the retrofit solutions in 2018, the feasibility of alternative power train technologies was assessed for excavators of various size classes available in the Norwegian market, and expectations for the introduction of prototypes were predicated. This assessment formed the basis for the development of the retrofit solutions provided in Norway by an official retailer of construction equipment from Japan. This assessment has since been updated to the market phases (unsuited, possible, prototype, production, and series production) defined by CECE; see Table 1. The following discussions refer specifically to three constructed retrofit prototypes of electrified crawler excavators, as marked by bold text in Table 1. These prototypes belong to three classes of machines, namely, 8.5-ton (small), 17.5-ton (medium), and 38-ton (large) excavators.
Table 1. An overview of the power train technologies for excavators of different size classes in the Norwegian market.
The largest excavator under study, the 38-ton unit, was developed to operate with a direct electrical power supply via a dynamic cable. An overview of the electrical system layout, indicating the main components introduced in the onboard power train and for the power supply via dynamic cables, is available in Figure 2(a). The onboard system consists of an active rectifier supplying a local dc bus, where the main load is an inverter driving the electrical motor that is used to replace the originally installed diesel engine. The system is rated for a maximum power of 197 kW. The excavator is operated from a regular 400-Vac grid, which supplies a mobile substation with a transformer for galvanic separation, a circuit breaker for protection, and any necessary instrumentation for measurements and power metering. Another mobile container solution with remote-controlled cable drums is used to manage the dynamic cables for connecting to the excavator, and this container can be moved around on the construction site by the excavator to ensure sufficient operating range. A photograph of the excavator and the container for the cable drums during operation at a construction site appears in Figure 3.
Figure 2. (a) The electrical system configuration of the 38-ton cable electric excavator. (b) The electrical system configuration of the 8.5- and 17.5-ton battery-electric excavators.
Figure 3. The 38-ton excavator and cable container on site at Biri care home, in Gjøvik, Norway. (Source: NASTA; used with permission.)
The 17.5-ton excavator was designed with the same electrical system configuration as the 38-ton prototype but extended with a small onboard battery, as indicated in Figure 2b. The battery is designed for peak shaving to limit the load on the local power system. Furthermore, the battery allows for operation without a power supply for a limited time, for instance, for operations outside the range of the supply cable and for transport between construction sites. Beyond the battery, the system components are functionally identical to what has been explained for the 38-ton prototype but with a correspondingly lower power rating. A photograph of the prototype during operation for refurbishing Olav Vs Street in Oslo is in Figure 4.
Figure 4. The 17.5-ton excavator on site at Olav Vs Street, in Oslo. (Source: SINTEF; used with permission.)
The 8.5-ton excavator also has the same electrical configuration as the 17.5-ton prototype but with a battery designed for regular operation of the machine. Thus, this machine is intended to operate without the dynamic cable arrangement explained for the 38- and 17.5-ton machines and with regular quick charging during breaks. A photograph of the prototype during charging at the same construction site as the 17.5-ton machine is presented in Figure 5.
Figure 5. The 8.5-ton excavator charging on site at Olav Vs Street. (Source: SINTEF; used with permission.)
A summary of key parameters for the three electric excavators is provided in Table 2. All three excavator prototypes are designed to have operating performances equivalent to corresponding conventional diesel units and have been tested under real conditions, as shown by the photos in the preceding figures. Thus, after the initial testing, these machines have been made commercially available, and further information on the machines can be found in the manufacturers’ product specifications.
Table 2. A summary of the key parameters of the studied excavators.
Lately, several studies have compared fossil fuel-powered vehicles with their electrified versions with respect to GHG emissions, using a lifecycle assessment methodology. In most cases, only the operation phase is considered, resulting in a comparison of the impact of the energies used (fossil fuels and electricity). The complexity of the design of the vehicles and the differences in the production processes and geographic contexts indicate that a complete lifecycle comparison should be performed, from the extraction of raw materials to the end-of-life phase, including any necessary infrastructures and potential reuse and recycling after end of life. Besides the carbon footprint, economic assessments and comparisons of electric versus fossil fuel vehicles have also been largely investigated using either lifecycle cost (LCC) or cost-benefit analysis. To our knowledge, no study has yet covered the comparison of diesel excavators with their electrified versions in terms of carbon footprints and LCCs.
The goal of the carbon footprint assessment presented is to ascertain the emission reduction potential of converting diesel excavators to the studied electrified equivalents. Lifecycle inventory data have been collected from the manufacturers and the Ecoinvent v3.1 database. The lifecycle inventory models are developed in SimaPro Analyst v9.0.0.48, and the impact assessment is carried out according to the Intergovernmental Panel on Climate Change 2013 Global Warming Potential (GWP) 100a method. A sensitivity analysis of electricity emission factors has also been carried out using the Norwegian (0.018 kgCO2e/kWh) and European (0.136 kgCO2e/kWh) electricity mixes according to the operational energy use scenarios provided in the Norwegian Standard NS 3720:2018, A Method for GHG Calculations for Buildings. The functional unit is 1 h of operation, given 1,800 h of operation per year. The reference period and service lifetime of the diesel and electric excavators are set to 10 years. The same material inventory is used for both diesel and electric excavators. However, the diesel power train components are then replaced with the components necessary for an electric excavator. An overview of the system boundary is provided in Figure 6.
Figure 6. The system boundary for the comparative carbon footprint analysis of diesel and electric excavators.
All three excavators were manufactured in Japan and transported to Larvik, Norway, by container ship. The 15-ton “hydraulic digger” process from Ecoinvent is used as a starting point and modified to 8.5-, 17.5-, and 38-ton diesel excavators. These processes are then modified further to electric excavators, using background information from the manufacturer. The European electricity mix unit processes are replaced with Japanese unit processes that use the Japanese electricity mix. When this is not available, rest-of-world unit processes are used. On arrival in Larvik, the excavators underwent a rebuilding process; this rebuilding process used Norwegian unit processes that use the Norwegian electricity mix (NO). Each excavator required different components, depending on the technology being integrated, such as batteries (8.5- and 17.5-ton excavators), electrical engines (all excavators), power trains (all excavators), inverters (all excavators), 230-m cable (17.5- and 38-ton excavators), transformer for galvanic isolation (17.5- and 38-ton excavators), and storage container (17.5- and 38-ton excavators).
For the analysis, it is assumed that the diesel-powered and electric excavators are further transported from Larvik to Oslo (128 km) and connected on site. This installation process requires charging infrastructure, such as a charging cable, storage container, and cable drum. All excavators operate for one year on the construction site before they are moved to a new construction site within the Oslo area. The distance between one construction site to the next is negligible.
Table 3 has an overview of the operational energy use of each excavator. All excavators have 1,800 h of operation per year. The emission factor for diesel is taken from Ecoinvent and is 3.32 kgCO2e/liter. According to NS 3720:2018, two GHG emission factors are supplied for electricity. The first is based on a European energy mix factor of 0.136 kgCO2e/kWh, which accounts for the exchange of electricity with the rest of Europe, and the second is based on the NO factor of 0.018 kgCO2e/kWh, which is mainly based on hydropower.
Table 3. The energy use per excavator.
When the diesel excavators reach their end of life, it is assumed that they will be dismantled and recycled. When the electric excavators reach their end of life, the batteries and cables are sent to disposal, the containers are sold on the secondhand market, the electrical engine and power train are dismantled and recycled, and the aluminum in the inverters is recycled. It is acknowledged that the lifetime of the excavators may be longer than the 10-year reference study period used in this study. In Norway, it is common practice for construction machinery to be sold on the secondhand market after a machine has accrued 10,000 h of operation (around six years). However, for the purposes of this carbon footprint assessment, the reference study period has been set to 10 years to reflect that the machines have a longer service lifetime than their first market use. It is also acknowledged that the service life of electric excavators is still an unknown factor since the electric excavators produced so far are either prototypes or small-scale production series since 2018. Therefore, the reference study period for the electric excavators has been set to the same as for the diesel excavators until better data on the lifetime of electric excavators are made available.
The carbon footprint results (Figures 7 and 8) show the total emissions for the 8.5-, 17.5-, and 38-ton diesel and electrical excavators, with both European Union (EU) and NO electricity factors. Figure 7 illustrates a significant reduction in GHG emissions by converting all types of diesel excavators to electrical excavators, regardless of whether the EU or NO electricity factor is used. The results in Figure 8 report that most of the GHG emissions from the diesel excavators are from operation energy use (96%-97%), followed by production (3%-4%). For the electrical excavators using the EU electricity factor, operational energy use also accounts for the highest emissions (45% for 8.5 tons, 49% for 17.5 tons, and 64% for 38 tons), followed by production (38% for 8.5 tons, 39% for 17.5 tons, and 31% for 38 tons). When using the NO electricity factor, the highest contribution to GHG emissions for the electric excavators is from production (62% for 8.5 tons, 69% for 17.5 tons, and 71% for 38 tons), followed by operational energy use for 17.5 tons (11%) and 38 tons (19%) and by the production stage for the 8.5-ton excavator (22%). Regardless of the size of the excavator, these results indicate that the share of GHG emissions from operation increases with the amount of fossil fuel inputs either directly as fuel or indirectly in the production of electricity.
Figure 7. The total GHG emissions per functional unit (1 h of operation) for all excavators. EU: European Union.
Figure 8. The GHG emissions for all excavators per lifecycle stage.
In terms of GHG emissions, the contribution from manufacturing the excavator presents only minor differences between the diesel and electric alternatives (Figure 7). This indicat–––es that the replacement of diesel engines with an electric motor, electronics, and a battery does not significantly impact the GHG emissions occurring during the raw material extraction and supply and during the manufacturing of the excavators. On the other hand, the electrification of excavators allows a significant reduction in GHG emissions associated with the operation phase, especially if the electricity mix includes a large proportion of renewable electricity. Compared to their diesel counterparts, electric excavators would allow, over a 10-year period, a cut in GHG emissions of 3,111 tCO2e for the 38-ton excavator, 1,309 tCO2e for the 17.5-ton excavator, and 675 tCO2e for the 8.5-ton excavator if the European electricity mix is used and 3,702 tCO2e for the 38-ton, 1,474 tCO2e for the 17.5-ton, and 751 tCO2e for the 8.5-ton excavator if the NO is used. These results correspond to a reduction of 90%–96% in GHG emissions with the conversion from diesel to electric excavators, depending on which electricity mix is used. This finding is in accordance with those reported in the literature for the electrification of transport vehicles, such as trucks, buses, and cars. The contribution of installation on the construction site is site specific, as it is associated with the transport of the excavator to the construction site, which is achieved by diesel truck. However, it can be argued that a prestigious emission-free construction project should not be transporting electric excavators with diesel transport to and from the construction site and would have lower GHG emissions from transport to the site by using electrified transport. Thus, the major contributions of the electric excavators are mostly from raw material supplies and manufacturing, while for the diesel counterparts, it is emissions from operation.
Lifecycle costing is the method used to calculate the sum of all costs associated with the excavators over their whole lifetime and to compare the cost among different alternatives. The LCC calculation is based on the net present value (NPV) of all cost elements and provides the present value of all future costs, depending on the discount rate and inflation rate. The functional unit is similar to the functional unit used in the carbon footprint, namely, Norwegian kroners (NOK) (1 NOK = 0.0998 euros) per hour of excavator operation, given a reference study period of six years and 1,800 operational hours per year. For the cost analysis, the reference period of six years is used, as this is the standard operating life span in the market. The excavators have a longer life span, but after six years, they are often either sold on the secondhand marked or undergo significant maintenance. The discount rate is set at 5% and the inflation rate at 2.2%. All prices are in NOK.
The goal of the LCC is to ascertain the economic feasibility of the 8.5-, 17.5-, and 38-ton electric excavators compared to the diesel excavators of equivalent size. The LCC has been carried out according to ISO 15686-5: 2017, Building and Construction Assets—Service Life Planning—Part 5: Life-Cycle Costing. LCC data are gathered from the manufacturer and contractor. Total costs include purchase costs (including installation costs), operating costs, maintenance costs, other costs (including insurance costs), and remnant value.
The purchase costs for the 8.5-, 17.5-, and 38-ton diesel and electric excavators are listed in Table 4. In addition to the purchase cost, there is an installation cost for the 17.5- and 38-ton electric excavators of 650,000 NOK for the electric connection, cable, container, and galvanic isolation. This does not occur for the 8.5-ton excavator, as it runs only on batteries, and it is assumed that the original charging connection is included in the purchase cost.
Table 4. The purchase, installation, operation, and maintenance costs per excavator.
Energy consumption is the main cost of operation for both diesel and electric excavators. The energy consumption for each excavator is listed in Table 3. The price of diesel and electricity is based on the price range in the Norwegian report “Klimakur 2030” for the year 2022. Diesel prices consist of the cost per liter, excluding fees but including a CO2 tax per liter. For electricity, the price includes the electricity cost per kilowatt-hour and the electricity fee. Since the energy prices are fluctuating, both the estimated prices and actual prices for 2022 are used in the analysis (collected from SSB and Circle K). Prices are listed in Table 4.
The maintenance costs include yearly service costs per hour of operation, and it is assumed that all the machines have an operating time of 1,800 h per year. Table 4 details the service cost per hour and the service cost per year given 1,800 h of operation for each excavator. The maintenance cost for electric and diesel excavators is assumed to be the same, as many of the tasks are routine tasks that must be performed on the machines regardless of the type of energy carrier. This is a conservative assumption since electric excavators should require less maintenance for the motor, while all maintenance for the hydraulic and mechanical parts will be the same.
Other costs include yearly insurance of 2.5% of the purchase cost for all excavators.
The residual value is based on an estimate of where the secondhand market for excavators will be in six years. Based on the market today, it is assumed that its minimum value should be about 25% of the purchase cost for both diesel and electrical excavators.
Figure 9 gives the accumulated cost for each excavator, calculated over a six-year planning horizon with both estimated energy prices for 2022 and actual energy prices for 2022. Figure 9 shows that the 38-ton excavator will become more economically favorable than the diesel excavator equivalent at today’s energy prices, as shown by the intersection of accumulated costs occurring in year 5. On the other hand, the electric 8.5- and 17.5-ton excavators display higher accumulated costs than their diesel version over the six-year planning horizon both with estimated and actual energy prices. This is mainly due to the initial cost of the batteries.
Figure 9. The accumulated costs (in NOK) for the six-year planning horizon. The (a) accumulated cost with estimated 2022 energy prices and (b) accumulated cost with actual 2022 energy prices.
Figure 10 describes the sum of the costs per lifecycle category after the end of the six-year planning horizon for the estimated energy cost and actual energy cost in 2022. The results show that the main cost for all electrical excavators is for acquisition, followed by operation and other costs, for the 8.5- and 17.5-ton excavators and operation costs for the 38-ton excavator. For the diesel excavators, the main cost for the 8.5- and 17.5-ton excavators is the acquisition cost, followed by operation costs, maintenance costs, and other costs, while for the 38-ton excavator, the highest cost is the operation cost, followed by acquisition, maintenance, and other costs. The higher operating costs due to higher energy prices compared to estimated energy prices are the reason the 38-ton electric excavator has a lower LCC than the diesel excavator.
Figure 10. The sum of the costs (in NOK) per lifecycle category per functional unit (operating hour) after the sox-year planning period.
Table 5 shows the NPV of the different excavators over the planning horizon of six years and the annual cost when looking at the period of six years, with estimated and actual energy prices. The table also shows the increase in total cost due to increased energy prices, showing that the operating costs for diesel excavators will be more affected by fluctuating energy prices.
Table 5. The NPV of all excavators after the six-year planning horizon and annual cost.
This article has presented the technical basis as well as the environmental and economic performance results for the electrification of 8.5-, 17.5-, and 38-ton electric excavators in Norway, compared to their diesel equivalents. The carbon footprint results show that the operational phase is the main contributor to GHG emissions for a diesel excavator, while switching to an electric engine using either the Norwegian or European electricity mix leads to much lower total GHG emissions. The cost assessment shows that electric excavators have a higher investment cost than their diesel equivalents. However, when evaluating probable market developments, electric excavators can become more cost efficient over their life span.
This article has investigated the electrification of diesel excavators in Norway with different retrofit solutions. The results from the presented study can be used to further progress the electrification of construction machinery in Norway to full-scale series production. The results may be applied to other types of construction machinery and may also be applied to other markets in Europe and internationally. The retrofit electrification of excavators may also be applied to existing diesel excavators and other types of construction machinery.
G. K. Booto, K. Aamodt Espegren, and R. Hancke, “Comparative life cycle assessment of heavy-duty drivetrains: A Norwegian study case,” Transp. Res. D, Transp. Environ., vol. 95, Jun. 2021, Art. no. 102836, doi: 10.1016/j.trd.2021.102836.
S. M. Fufa, M. Kjendseth Wiik, and I. Andressen, “Estimated and actual construction inventory data in embodied greenhouse gas emission calculations for a Norwegian zero emission building (ZEB) construction site,” in Proc. Int. Conf. Sustainability Energy Buildings, Springer, 2018, pp. 138–147, doi: 10.1007/978-3-030-04293-6_14.
T. Lin, Y. Lin, H. Ren, H. Chen, Q. Chen, and Z. Li, “Development and key technologies of pure electric construction machinery,” Renewable Sustain. Energy Rev., vol. 132, Oct. 2020, Art. no. 110080, doi: 10.1016/j.rser.2020.110080.
G. Puig-Samper Naranjo, D. Bolonio, M. F. Ortega, and M. J. García-Martínez, “Comparative life cycle assessment of conventional, electric and hybrid passenger vehicles in Spain,” J. Cleaner Prod., vol. 291, Apr. 2021, Art. no. 125883, doi: 10.1016/j.jclepro.2021.125883.
S. Verma, G. Dwivedi, and P. Verma, “Life cycle assessment of electric vehicles in comparison to combustion engine vehicles: A review,” Mater. Today Proc., vol. 49, pp. 217–222, Jan. 2022, doi: 10.1016/j.matpr.2021.01.666.
M. K. Wiik et al., “Impact assessment of zero emission building processes in Oslo,” SINTEF Academic Press, Oslo, Norway, Rep. No. 89, 2022.
The authors would like to acknowledge project partners Nasta, Skanska, Omsorgsbygg, Bellona, and Difi for their collaboration in the Zero Emission Digger project. This work was supported by the Norwegian Research Council, Innovation Norway, and Enova through the PILOT-E program 2018-2020 (grant 281804).
Marianne Kjendseth Wiik (marianne.wiik@sintef.no) is with SINTEF Community, 0373 Oslo, Norway.
Kristin Fjellheim (kristin.fjellheim@sintef.no) is with SINTEF Community, 0373 Oslo, Norway.
Jon Are Suul (jon.a.suul@sintef.no) is with SINTEF Energy Research and the Norwegian University of Science and Technology, 7013 Trondheim, Norway.
Kamal Azrague (kamal.azrague@sintef.no) is with SINTEF Community, 0373 Oslo, Norway.
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