Next, we investigated the cooling potential of a 22,500 m2 area of trees in terms of its effect in reducing the greenhouse gas CO2. In the 2006 Forest Sink Measures to Prevent Global Warming (Forestry Agency of the Japan Ministry of the Environment), 50-year-old cedars are given as a guideline for assessing the amount of CO2 absorbed by trees. The same publication states that the average amount of CO2 absorbed by one 50-year-old cedar is approximately 14 kg/year (Ministry of the Environment and Forestry Agency 2006), and the average area occupied by one tree in a cedar forest is 12 m2. As approximately 12 m2 is required to grow
one Japanese cedar, 1,875 cedars can be planted in an area of 22,500 m2. Looking at the amount of CO2 absorbed by these trees, because one cedar absorbs 14 kg CO2 annually, 1,875 cedars would absorb and sequester 26,250 kg CO2 in 1 year.
In a typical office building equipped with air conditioning, setting aside the waste heat in sewage, anthropogenic heat emissions for the most part would equate to the heat discharged outdoors from external air conditioning units, which are run on electric power to lower the room temperature in an office environment that is warmed by thermal radiation from the sun and is further heated by the use of heat sources such as lights, computers, and kitchen equipment (Tokyo Metropolitan Government 2005). Based on this premise, if we subtract sewage waste heat from the 681.8 GJ of heat released per day from approximately 70 commercial buildings, each with a floor area of about 3,000 m2, and then calculate the heat emissions from those buildings, we get 681.8 GJ/day minus the sewage heat waste (sewage heat intensity, 23.76 kJ/m2/day x 3,000 m2 x 70 buildings = 5 GJ/day), which comes to 676.8 GJ/day.
In contrast to air conditioners, which run on supplied power whose generation produces CO2, trees and woodlands cool the atmosphere through shade and evapo – transpiration and can exhibit cooling potential without producing any CO2. It would seem essential to take this point into account in assessing the functions of green space.
With this idea in mind, we tried converting the 676.8 GJ/day heat emissions into the power consumption of air conditioning equipment.
The basic approach used to calculate the heat discharged from external air conditioning units is taken from the Ministry of Land, Infrastructure, Transport and Tourism and Ministry of the Environment (2004). Using Eq. (2.3) given in that report, we calculated the amount of fuel (calorific value) used in running an air conditioner as follows:
Air conditioner heat discharge = fuel used (calorific value) to run the air conditioner
x energy efficiency x air conditioner COP
where COP (coefficient of performance) is the value of the cooling capacity (W) divided by the power consumption (W) while the air conditioner is in cooling operation.
In regard to the energy efficiency in Eq. (2.3), we referred to data from the Energy Conservation Center, Japan (ECCJ), which states the average all-day efficiency as 36.9 %, assuming the installation site is some distance from the power station.
For air conditioner COP, we used the value (COP = 4.159) given in the ECCJ (2006). It lists the cooling capacity (kW), power consumption (kW), cooling COP, and other characteristics of ten medium-size air conditioner models (cooling capacity, 10.0 kW class; four-way cassette) for offices and stores being marketed by
leading manufacturers as of March 2006. In this study, we took the mean value (4.159) of the cooling COPs of these ten models (2.87-4.90). Assigning these values to the variables in Eq. (2.3) above, and calculating the fuel used (calorific value) during air conditioner operation, we obtain the value 441 GJ/day. Because the conversion factor is given as 1 kW h = 3.6 MJ17, converting this 441 GJ/day calorific value into power consumption gives 122,600 kW h/day. From this result, if 676.8 GJ of anthropogenic heat is discharged per day from a commercial building, the power station needs to generate 122,600 kW h of power per day.
According to the ordinance issued by the Ministry of the Environment on March 23, 2006, which amends part of the Act on Promotion of Global Warming Countermeasures, the emission factor for calculating how much CO2 is produced in generating 1 kW h of electricity is given as 0.555 kg CO2. Therefore, the amount of CO2 produced in generating 122,600 kW h/day of electricity will be 68,043 kg CO2/ day. Because this value equates to the daily average for the month of August, if we take the cumulative effect during the period July to September (90 days) when coolers are in frequent use, the presence of woodlands would reduce the amount of CO2 in the atmosphere by approximately 6,124 t CO2.
As already discussed, the amount of carbon dioxide absorbed by planting a 22,500 m2 area of cedars would be of the order of 26 t CO2 (26,250 kg CO2) per year. However, because tree species are not necessarily planted at a uniform density, if we estimate the cooling potential of a 22,500 m2 plantation using conservatively estimated GIS data, we find that the expected reduction in CO2 from the trees’ cooling potential would be as much as 236 times the amount of CO2 that the trees absorb during the months of July to September (6,1241CO2/26 t CO2).
From these findings, any future assessment of woodlands would require a more multifaceted approach: their role needs to be examined not only from the well – canvassed perspective of CO2 absorption and sequestration, but must also take into account the potential of woodlands to moderate the amount of CO2 in the atmosphere, their capacity in summertime in particular to reduce power consumption by air conditioning equipment, effectively helping to reduce the amount of CO2 released from power stations and related facilities.