Sustainable Campus: Reducing Environmental and Financial Burdens by Using Pruned Branches for On-Campus Energy

Sustainability activities on university campuses are an important part of building a sustainable society. As a part of this effort, the present study quantitatively evaluated the reduction in greenhouse gas (GHG) emissions and disposal costs upon use of pruned branches for on-campus energy use in a Japanese university. The pruned branches are currently used off campus as fuel for biomass power generation, whereas the proposed systems include their on-campus use for heating through wood-chip heaters in a greenhouse or through wood stoves in buildings. The GHG balance was evaluated using a life-cycle assessment, and the cumulative costs over a 20 y period were estimated. Using pruned branches for on-campus heating was 70–180% more effective in reducing GHG emissions than the current off-campus power generation system. Using branches for heating eliminated the emissions produced by the transportation of pruned branches off campus and had higher thermal efficiency than power generation, which was conducive to the replacement of larger quantities of fossil fuel. In addition, the payback period of the proposed systems was estimated at 2 and 4 y for wood stove and wood-chip heater systems, respectively. For the 20 y period, the cumulative costs can be reduced by approximately 20%.


Introduction
Policies to create a sustainable society are urgently needed to mitigate climate change. In this context, universities can contribute to sustainability through research, education, social collaboration, and campus development [1]. University campuses often have abundant green spaces, as well as various buildings and facilities. Therefore, sustainable campus activities are often developed in universities to help reduce greenhouse gas (GHG) emissions and other environmental burdens, and contribute to a sustainable society [2,3]. GHGs, such as carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), and fluorinated gases, trap heat within the Earth's atmosphere, leading to climate change.
The reduction in energy consumption and use of renewable energy are especially important to improve the sustainability of campuses [4][5][6][7][8][9]. To promote the use of renewable energy, the pruned branches produced in the green areas of campuses [10][11][12] can be potentially used as renewable woody biomass. Sustainability 2021, 13

Quantification of Pruned Branches and Processing Costs
According to interviews and information collected from relevant university departments, pruning companies, and waste disposal firms, the pruning of broad-leaved trees on the Fuchu campus generated between 23,780 to 90,120 kg/y of branches from 2010 to 2018, with an average of 56,729 kg/y ( Figure 2). Currently, these pruned branches are

Quantification of Pruned Branches and Processing Costs
According to interviews and information collected from relevant university departments, pruning companies, and waste disposal firms, the pruning of broad-leaved trees on the Fuchu campus generated between 23,780 to 90,120 kg/y of branches from 2010 to 2018, with an average of 56,729 kg/y ( Figure 2). Currently, these pruned branches are transported to waste processing facilities and eventually used as fuel in biomass power plants in prefectures close to Tokyo (see Section 2.3). The average yearly cost to process these pruned branches is approximately 16,612,000 JPY, of which 70% (11,628,000 JPY) is for pruning and 30% (4,984,000 JPY) for loading, transportation, and processing. Sustainability 2021, 13, x FOR PEER REVIEW 4 of 15 transported to waste processing facilities and eventually used as fuel in biomass power plants in prefectures close to Tokyo (see Section 2.3). The average yearly cost to process these pruned branches is approximately 16,612,000 JPY, of which 70% (11,628,000 JPY) is for pruning and 30% (4,984,000 JPY) for loading, transportation, and processing.

Use for Energy
This study considers using the branches on campus for heating in wood-chip heaters in a plant research greenhouse or in wood stoves in buildings and facilities. These three energy systems (current power generation from branches, wood-chip heaters, and wood stoves) were compared with the traditional fossil fuel scenarios for the analysis.
In the current system, the trees on campus are mainly pruned by professional pruners, and the branches are temporarily stored in an on-campus collection site. The pruned branches are regularly transported from the university to a waste disposal plant in Fuchu for weighing and sorting. The pruned branches are then transported to Tokorozawa City for processing into wood chips. The processed wood chips are then distributed to eight biomass power plants in Tochigi, Ibaraki, and Kanagawa prefectures, which are close to Tokyo, to be used as fuel for power generation. Therefore, the pruned branches are used as an alternative power source in a power grid that mainly uses fossil fuels.
For the on-campus use of wood-chip heaters, the pruned branches would be processed into wood chips, dried using storage/drying sheets, and then used as fuel in the heaters in a greenhouse during the winter of the following year (November-March). Therefore, they would be an alternative to the currently used kerosene heaters. In addition, the ash generated by the burning of wood chips could be used as a soil amendment on an on-campus farm.
For use in on-campus wood stoves, the pruned branches would be dried on a vacant lot on campus using breathable flexible containers, processed into firewood, then used on campus in wood stoves of buildings and facilities during the winter of the following year (November-March). They would thus be an alternative to the current heating system that uses municipal gas as fuel. The ash generated by burning the firewood could also be used on an on-campus farm.

Use for Energy
This study considers using the branches on campus for heating in wood-chip heaters in a plant research greenhouse or in wood stoves in buildings and facilities. These three energy systems (current power generation from branches, wood-chip heaters, and wood stoves) were compared with the traditional fossil fuel scenarios for the analysis.
In the current system, the trees on campus are mainly pruned by professional pruners, and the branches are temporarily stored in an on-campus collection site. The pruned branches are regularly transported from the university to a waste disposal plant in Fuchu for weighing and sorting. The pruned branches are then transported to Tokorozawa City for processing into wood chips. The processed wood chips are then distributed to eight biomass power plants in Tochigi, Ibaraki, and Kanagawa prefectures, which are close to Tokyo, to be used as fuel for power generation. Therefore, the pruned branches are used as an alternative power source in a power grid that mainly uses fossil fuels.
For the on-campus use of wood-chip heaters, the pruned branches would be processed into wood chips, dried using storage/drying sheets, and then used as fuel in the heaters in a greenhouse during the winter of the following year (November-March). Therefore, they would be an alternative to the currently used kerosene heaters. In addition, the ash generated by the burning of wood chips could be used as a soil amendment on an on-campus farm.
For use in on-campus wood stoves, the pruned branches would be dried on a vacant lot on campus using breathable flexible containers, processed into firewood, then used on campus in wood stoves of buildings and facilities during the winter of the following year (November-March). They would thus be an alternative to the current heating system that uses municipal gas as fuel. The ash generated by burning the firewood could also be used on an on-campus farm.

GHG Balance
The GHG balance (emissions/reductions) of each system was determined using a life-cycle assessment (LCA) [24][25][26]. The functional unit was the average annual amount of pruned branches generated on campus (56,729 kg/y). Figure 3 shows the evaluation scopes (system boundaries) of the three energy systems. Figure 3b,c present on-campus wood-chip heaters and wood stoves, respectively, which were compared to the current off-campus power generation system shown in Figure 3a. As there was no available data on the manufacturing, transportation, repair, and disposal of power plants, heaters, and other equipment, these were excluded from the assessment. For each process, we examined the amounts of fossil fuel-derived GHGs (CO 2 , CH 4 , and N 2 O) that were emitted or reduced. Because the trees on campus are managed continuously, the CO 2 generated by the burning of pruned branches was not evaluated based on the carbon-neutral concept of biomass fuel, but CH 4 and N 2 O emissions were included in the assessments. The GHGs were converted to CO 2 considering their respective 100-year global warming potential [27].

GHG Balance
The GHG balance (emissions/reductions) of each system was determined using a lifecycle assessment (LCA) [24][25][26]. The functional unit was the average annual amount of pruned branches generated on campus (56,729 kg/y). Figure 3 shows the evaluation scopes (system boundaries) of the three energy systems. Figure 3b,c present on-campus wood-chip heaters and wood stoves, respectively, which were compared to the current off-campus power generation system shown in Figure 3a. As there was no available data on the manufacturing, transportation, repair, and disposal of power plants, heaters, and other equipment, these were excluded from the assessment. For each process, we examined the amounts of fossil fuel-derived GHGs (CO2, CH4, and N2O) that were emitted or reduced. Because the trees on campus are managed continuously, the CO2 generated by the burning of pruned branches was not evaluated based on the carbon-neutral concept of biomass fuel, but CH4 and N2O emissions were included in the assessments. The GHGs were converted to CO2 considering their respective 100-year global warming potential [27].   To evaluate the fossil fuel-derived GHG emissions and the reductions upon their substitution, we considered the distance the pruned branches and wood chips were transported, the amount of fuels used and replaced, and other factors obtained from the relevant university departments, pruners, waste disposal companies, and heating equipment manufacturers through interviews and collected numerical data (Table 1). GHG emission intensities of the fuels were mainly obtained from the Life Cycle Inventory (LCI) database Inventory Database for Environmental Analysis (IDEA) v2.1.3 [28] ( Table 2). The life-cycle GHG emissions/reductions were calculated by multiplying the transportation distance and the amount of fuel used and replaced in each process (Table 1) by the corresponding GHG emission intensity (Table 2).  Table 2. GHG emission intensity (kg-CO 2 eq/unit).

Life-Cycle Process Unit Emissions Note
Transport by 4-t truck (outbound, load rate 100%) (resource extraction-diesel production-diesel combustion) t·km 0.199 [28] Transport by 4-t truck (return, load rate 0%) (resource extraction-diesel production-diesel combustion) km 1.302 [28] Transport by 10-t truck (outbound, load rate 100%) (resource extraction-diesel production-diesel combustion) t·km 0.109 [28] Transport by 10-t truck (return, load rate 0%) (resource extraction-diesel production-diesel combustion) km 0.712 [28] Resource extraction-diesel production L 0.354 [28] Diesel combustion MJ-HHV 0.078 [28], 38.04 MJ-HHV/L [34] Resource extraction-gasoline production L 0.543 [28] Gasoline combustion MJ-HHV 0.083 [28], 33.36 MJ-HHV/L [34] Resource extraction-fuel production-grid power generation kWh 0.458 [35] Resource extraction-kerosene production L 0.314 [28] Kerosene combustion MJ-HHV 0.076 [28], 36.49 MJ-HHV/L [34] Resource extraction-municipal gas production-municipal gas combustion MJ-HHV 0.061 [36], 39.96 MJ-HHV/m 3 [34] Wood combustion (CH 4 and N 2 O) MJ-HHV 0.002 [28], 15.8 MJ-HHV/L [33] Ash disposal kg 0.002 [28] In Table 1, the pruned branches were assumed to have 50% moisture by weight [29]. The transportation process in the current off-campus power generation system includes the transport of pruned branches and wood chips by truck. The traveled distance (university campus-disposal plant-chip plant) was estimated based on the road distance to each location. For the wood chip transportation to the different power plants, the mean distance considering the eight power plants was used. On-campus wood-chip heaters and wood stoves did not require off-campus transport. The wood chips and firewood produced were assumed to have 20% moisture [29], and the amount of wood chips/firewood (20% moisture) was calculated by multiplying the amount of pruned branches (50% moisture) by a yield of 90% [29,30] and the ratio of the density of pruned branches with 20% moisture content (710 kg/m 3 ) to that with 50% moisture content (1100 kg/m 3 ) for broad-leaved trees [29]. The fuel consumption of the wood chipper was calculated by multiplying the amount of wood chips produced by the amount of diesel used [29]. Similarly, the fuel consumption of the log splitter was calculated by multiplying the amount of firewood produced by the amount of gasoline used [30]. We assumed that no fossil fuels would be used for air-drying the pruned branches. The power generated in the current system was calculated by multiplying the amount of wood chips by the thermal efficiency of 30% [31,32] and the wood lower heating value (LHV) [33]. The heat used by the wood-chip heaters and wood stoves was also calculated by multiplying the amount of wood chips/firewood by the equipment thermal efficiency (wood-chip heater: 85%; wood stove: 72%) and the LHV [33]. For disposal, the ash generated by the current power generation system goes through waste processing, but the ash generated by the wood-chip heater and wood stoves could be used on farms within the campus, eliminating the need for waste processing. For the comparison, the generated power and used heat from wood-chip heaters and wood stoves, respectively, were converted into their equivalent use of grid power, kerosene, and municipal gas.
In Table 2, the GHG emission intensity [28,36] associated with the combustion of each fuel is shown as a numerical value based on their respective higher heating value (HHV).
After converting the amount of fuel used and the replaced amount from the LHV (Table 1) to an HHV basis, the obtained values were multiplied by the GHG emission intensity to obtain the GHG emissions and reductions.

Costs
The costs of the three energy systems over 20 y were estimated. The annual cost of pruning the branches (11,628,000 JPY) was used as the basis to calculate the costs of the new on-campus systems, and the initial equipment, operating, maintenance, and labor costs associated with introducing the wood-chip heaters and wood stoves were also included. In addition, the initial equipment, operating, and maintenance costs associated with the current kerosene heaters and municipal gas heating system that would be saved by introducing the wood-chip heaters and wood stoves were also considered. These costs were estimated based on information provided by equipment manufacturers. Table 3 shows the unit prices (including consumption tax) of all investigated items. On-campus wood stoves  Figure 4 and Table 4 show the results of the life-cycle GHG balance of the three energy systems.  Figure 4 and Table 4 show the results of the life-cycle GHG balance of the three energy systems.    From transport 1 to disposal, annual GHG emissions of the current, wood-chip heater, and wood stove systems were 4184, 1468, and 1348 kg-CO 2 eq, respectively. Emissions in the use stage included CH 4 and N 2 O from the burning of wood chips or firewood. The current off-campus power generation system had the highest emissions, nearly three times the emissions of the two new systems. In the substitution process, the annual GHG reductions from the current, wood-chip heater, and wood stove energy systems were 17,860, 40,002, and 24,853 kg-CO 2 eq, respectively. The wood-chip heater system exhibited the largest reduction, which was approximately 2.2 times that of the power generation system. The net emissions considering the reductions in GHG emissions resulted in negative values or net reductions for all energy systems. The annual net reductions from the current, wood-chip heater, and wood stove systems were 13,676, 38,534, and 23,505 kg-CO 2 eq (0.24, 0.68, and 0.41 kg-CO 2 eq/kg-branch), respectively. Thus, the emission reduction was the smallest for the current power generation system, and the reductions from the wood-chip heater and wood stove systems were approximately 180% and 70% greater, respectively, than that of the current system. current, wood-chip heater, and wood stove systems were 13,676, 38,534, and 23,505 kg-CO2eq (0.24, 0.68, and 0.41 kg-CO2eq/kg-branch), respectively. Thus, the emission reduction was the smallest for the current power generation system, and the reductions from the wood-chip heater and wood stove systems were approximately 180% and 70% greater, respectively, than that of the current system. Figure 5 and Table 5 display the estimated cumulative costs across 20 y and those in year 20, respectively, for the three energy systems.    The estimated cost of the current off-campus power generation system over the 20 y period is approximately 332,249,000 JPY, which includes annual costs of pruning, loading, transporting, and processing. The new on-campus wood-chip heater system has initial equipment costs (purchasing and installing a chipper, heaters, storage/drying sheets, silos, etc.) and equipment renewal costs (Table 3). In addition, after the first year, there are operating, maintenance, and labor costs associated with equipment use. However, the annual costs for loading, transporting, and processing pruned branches are no longer incurred, and only the cost of pruning the branches is maintained. In addition, this eliminates the annual operating and maintenance costs associated with the current kerosene heater, as well as renewal costs to update the respective equipment. The payback period of the wood-chip heater system is only 4 y. Thus, the cumulative cost of introducing wood-chip heaters exceeds that of the current power generation system only until year 3, after which the former is economically superior. Over 20 y, the total expenditure of the wood-chip heater system was estimated as 263,940,000 JPY, which is approximately 21% lower than that of the current power generation system. For on-campus wood stoves, the initial costs (purchasing and installing a log splitter, breathable flexible container, and wood stoves) and renewal costs to update equipment (Table 3) are lower than those of the woodchip heater system. Operating, maintenance, and labor costs associated with the log splitter and wood stoves are incurred every year, but nothing is spent on loading, transporting, and processing of pruned branches. Moreover, the operating and maintenance costs associated with the current municipal gas heating are eliminated. The payback period of the wood stove system is only 2 y, after which the investments already generate economic value. Over 20 y, the cumulative expenditure was estimated as 254,095,000 JPY, which is approximately 24% lower than the costs of the current power generation system.

GHG Balance
Of the annual GHG emissions for the current power generation system, from first transport to disposal, the transport of pruned branches and wood chips represented the largest share of emissions. Transportation accounted for approximately 65% of the total emissions (22% from the transport of pruned branches, and 43% from the transport of wood chips) and was the main factor in the current system's higher GHG emissions when compared to those of the wood-chip heater and wood stove systems. The difference in fuel consumption between the wood chipper and the log splitter was the main factor in identifying the wood stove system as the producer of lower amounts of emissions. The emissions of the two on-campus heat uses differed by approximately 8%, which is trivial compared to their respective differences with the current power generation system. In the substitution process, the difference in annual GHG reduction among these three systems was mainly attributed to the differences in thermal efficiencies and substituted fuels. While the thermal efficiency of the current power generation system is only 30%, the efficiencies of the wood-chip heaters and wood stoves would be 85% and 72%, respectively. This difference created a large gap in the amount of energy used (Table 1), thereby causing a major impact on the GHG emission reduction. Furthermore, the GHG emission intensity of the substituted fuel (which includes resource extraction, fuel production, and fuel combustion) was 0.085 and 0.061 kg-CO 2 eq/MJ-HHV for kerosene and municipal gas, respectively ( Table 2). The larger emissions reduction of replacing kerosene clarifies the larger emissions reduction by the wood-chip heater system. Therefore, the net life-cycle GHG reduction is higher for the use of branches in heating on campus than for the current system of power generation off campus.
Similar results were obtained in previous studies investigating the life-cycle GHG balance of woody biomass for power generation and heating [13][14][15], in which the emission reduction of the latter was higher than that of the former. Another study reported that the life-cycle GHG emissions of pruning residues obtained from apple orchards for heating were 15 kg-CO 2 eq/GJ [19]. This value falls in the middle range of the GHG emissions (from transport 1 to disposal) that were calculated in the present study for the current power generation, wood-chip heater, and wood stove systems as 29.8, 3.7, and 4.0 kg-CO 2 eq/GJ, respectively.
A reliable supply of pruned branches is a potential issue for on-campus heating systems. Annual amounts of pruned branches varied between 23,780 and 90,120 kg/y from 2010 to 2018 ( Figure 2). Therefore, to avoid a shortage, systems and processes would be needed for maintaining stocks of pruned branches on campus or use those generated from surrounding areas.

Sensitivity Analysis
The results shown in Section 3.1 indicate that the transportation of pruned branches/ wood chips and the thermal efficiencies of the processes significantly affected the life-cycle GHG balance. However, the estimations were conducted based on interviews with pruners, waste disposal companies, and equipment manufacturers, as well as from publications by power plants, and thus include some degree of uncertainty. Therefore, we conducted a sensitivity analysis to consider the extent to which the results would be influenced by variations in the distance wood chips are transported and in thermal efficiency.
For wood-chip transportation, we used the mean distance to the eight power plants participating in the current system (132.6 km, Table 1), but the shortest distance to a plant was 73 km (Kanagawa Prefecture), and the longest was 190 km (Ibaraki Prefecture). Therefore, we performed calculations based on the shortest and longest distances. For the thermal efficiency of power generation, the 30% value was originally chosen based on the mean value of publications by power plants (Table 1). To account for possible variations, we examined efficiencies of 40%, 35%, and 25%. For the thermal efficiency of heat use, we originally used values according to equipment manufacturers (wood-chip heater: 85%, wood stove: 72%; Table 1). For the sensitivity analysis, we examined the thermal efficiency variances with five percentage points. Table 6 shows the results of the sensitivity analysis of the annual net GHG emissions. All cases yielded a negative value or a reduction in GHG emissions. Comparing the energy systems, the best case for the current power generation system (shortest transport distance, +10% thermal efficiency) led to a reduction of 20,438 kg-CO 2 eq, which was still smaller than the worst case for the wood-chip heaters and wood stoves (−5% thermal efficiency) at 36,181 and 21,779 kg-CO 2 eq, respectively. According to the calculation, power generation thermal efficiency values higher than 72% and 47% are required to surpass the net GHG reduction from the wood-chip heaters and wood stoves (the current case), respectively. Furthermore, the best case for wood stoves (+5% thermal efficiency) led to a reduction of 25,231 kg-CO 2 eq, which was smaller than the worst case for wood-chip heaters (−5% thermal efficiency) at 36,181 kg-CO 2 eq. Therefore, the net GHG reduction was still the largest for wood-chip heaters, followed by wood stoves, and the current power generation system for all cases. These results demonstrate that using pruned branches for on-campus heating is more effective in reducing GHG than using them for off-campus power generation.

Costs
A previous study reported that the investment, operating, and maintenance costs of using wood chips and pellets for energy were higher than those associated with diesel and natural gas use [20]. The results of this study also showed that the costs of wood-chip heaters and wood stoves were greater than the eliminated costs possible as a result of replacing kerosene and municipal gas ( Figure 5 and Table 5), confirming the trends observed in previous research. Although another previous study presented contradicting results, where the use of wood for heat was more expensive than that for power generation [13], the present study applied to a university campus indicates that the cost of heating from on-campus pruned branches would be lower than that for off-campus power generation. At present, pruned branches are processed as waste products that are used as fuel for power generation. Therefore, the associated costs are regarded as waste disposal costs (rather than a power generation issue), and universities have not focused on reducing such waste disposal costs. However, our results indicate that effectively shifting the off-campus processing of pruned branches to on-campus energy use could reduce these disposal costs. Because the costs described in this study are estimates, the actual costs of introducing wood-chip heaters or wood stoves could vary and should be more accurately investigated before implementation. Nevertheless, it is likely that the payback period of such systems would be only a few years. Therefore, the effective use of pruned branches in university campuses can contribute to improve both environmental and economic sustainability.

Conclusions
In this study, we quantitatively evaluated the reduction in GHG emissions and costs by using pruned branches generated on campus for energy purposes as a measure toward a more sustainable campus at Tokyo University of Agriculture and Technology in Japan.
Pruned branches are currently processed off campus by a waste disposal company and used as fuel for biomass power generation in prefectures in the Greater Tokyo Area. However, using the pruned branches on campus in wood-chip heaters in a plant research greenhouse or in wood stoves in buildings and facilities can reduce their life-cycle GHG emissions by approximately 70% compared to the current system. This is largely due to the reduction in emissions associated with the transportation of pruned branches and wood chips off campus.
The GHG reduction from replacing fossil fuels was larger for wood-chip heaters (kerosene) and wood stoves (municipal gas) than for the current power generation system (typical grid power). As a result, the net GHG reductions from the current, wood-chip heater, and wood stove systems were 0.24, 0.68, and 0.41 kg-CO 2 eq/kg-branch, respectively. Thus, the smallest reduction was obtained by the current power generation, and the reductions from wood-chip heaters and wood stoves were approximately 180% and 70% larger, respectively, than that of the current system. Particularly, the thermal efficiency was higher for heating than for power generation, which contributed to the replacement of larger amounts of fossil fuels.
The cost of using pruned branches for on-campus heating was estimated to be lower than that for off-campus power generation. The payback period of the systems was estimated at 4 and 2 y for the wood-chip heater and wood stove systems, respectively, and their cumulative costs over 20 y would be approximately 21% and 24%, respectively, lower than that of the current power generation system. This study shows that GHG emissions and disposal costs can be reduced by effectively using pruned branches, a biomass resource generated on campus, for sustainable purposes on university campuses. In addition to the sustainability of the campus itself, such efforts can have educational effects for the surrounding community. In the future, university campuses can work in partnership with local communities by accepting and using pruned branches generated in the surrounding area.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.