Sustainable Resource Utilization Parameter

One of the major factors undermining the sustainability of a production process is the depletion of the resources it uses. A quantification of process sustainability should therefore include a parameter that deals with the sustainability of resource utilization, and the construction of such a parameter begins with defining a quantitative measure for the depletion of an individual resource. One way of doing this is to classify each resource as either renewable or nonrenewable, as was done by De Wulf et al. [36]. The distinction made between renewable and nonrenewable resources is that renewable resources are created at least as fast as they are consumed (e.g., solar energy), while nonrenewable resources are consumed faster than they are created (e.g., crude oil). 

A second objection that can be made against the renewability concept is that renewability is only a part of sustainable resource utilization, since it does not involve the natural reserves of resources. Although the concept of renewability rightfully views the consumption rate of a resource in relation to its production or regeneration rate, this gap between consumption and regeneration rate should in turn be viewed in relation to the size of the natural reserves of that resource. In fact, a temporary discrepancy between the consumption and the regeneration rate of a resource does not necessarily 

instead of considering the renewable versus the nonrenewable resources in a process, it is preferred to quantitatively combine the consumption rate ( ra/ConSumption)/ the regeneration rate ((t m/production), and the extent of natural reserves (Mreserves) of a resource, as they are known at this time, in a resource depletion time (x) defined as 

Besides taking into account any gap between the consumption and regeneration rate of a resource, the depletion time relates this gap to the extent of the natural reserves. The depletion time is then a measure for the rate at which the currently known reserves of a resource are being depleted. For example, a depletion time of 100 years means that currently 1% of the known reserves is being depleted yearly likewise, a depletion time of 1000 years means that yearly 0.1% is being depleted. It should be stressed that the depletion time as defined in Equation 13.13 does not attempt to predict resource depletion in the future it merely indicates how fast a known supply of a resource is currently being depleted. 

This definition means that the depletion time of a resource is time dependent. The consumption rate of a resource may increase over time as a result of increased industrial production, or it may decrease if alternative resources are increasingly being used. Likewise, the regeneration rate may increase when more resources are recycled, and reserves may shrink after prolonged utilization or may expand when new natural deposits are found. In addition, more accurate data may become available, for example, on the extent of currently known reserves or on the natural formation rates of certain deposits. In any case, the depletion time x is variable, and it reflects the rate of resource depletion only in the present situation. 

---

With Equations 13.14 through 13.16, and with the abundance factors for oil, coal, and solar energy as determined above, it is possible to determine the parameter for sustainable resource utilization for a process that uses solely one or more of these three resources. Table 13.5 lists the relevant data for several such processes, each extracting different percentages of the total required exergy from oil, coal, and solar energy. 

The proposed method for quantitatively describing the sustainability of resource utilization in a process has several advantages. First, it considers the degree of resource renewability, which allows even subtle differences in the depletion of different resources to be accounted for. Second, it also includes the (natural) reserves of resources, making the method yet more refined. Finally, the concept of depletion times and their translation to abundance factors allow the resource sustainability parameter a of a process to be limited by its most rapidly depleting resource, which is quite realistic. 

First of all, Table 13.5 lists three processes that each extract their exergy entirely from one of the three resources (processes 1, 2, and 3). In these situations, there is only one relevant abundance factor, and its value then solely determines the values of the average and minimum abundance factor. Also, the sustainability parameter is then simply this value squared. The results show that the process using only oil (process 1) is the least sustainable in its utilization of resources, the process using only coal (process 2) is more sustainable, and the process extracting all exergy from solar energy (process 3) is practically entirely sustainable in terms of resource utilization. 

The effect of the minimum abundance factor amin on the sustainability parameter a may seem counterintuitive. However, it must be realized that the sustainability parameter a only expresses that part of process sustainability that involves the availability of the resources used. The fact that process 6 in Table 13.5 may be more sustainable than, for example, process 2 in terms of environmental impact is not relevant to this particular aspect of process sustainability. Purely in terms of resource utilization, process 2 is more sustainable than process 6, because process 2 does not depend on a rapidly depleting resource like oil. 

---