The system shall be efficient. The system shall be sustainable. These two requirements are important. They are stated in nearly every specification of man-made products and services. If their missing would be criticised, their self-evidence would probably be claimed for defence. There may be some reluctance to accept these requirements as contractually binding due to the difficulties for providing acceptable verification evidence. No absolute measures exist for efficiency and sustainability. All evaluations will summarise efficiency and sustainability in ratios. It is worth to investigate further how numerators and denominators may be constructed.
System efficiency is simply the ratio of system benefits to system costs. Optimising system efficiency means maximising the benefit and minimising the costs. Utilisation is the reason to invest in a product or service. Consequently, system benefit is in the first place a measure of if and how well a system satisfies its intended purposes. System costs are all costs that are accumulated over the system’s life cycle from first conceptual ideas over development, production, deployment and utilisation to disposal. Sometimes the term total system life cycle costs is used to express the comprehensiveness of the concept. However, this wording implies an absolute measure that is not observable in reality. In practice, it would be better to talk about explicitly accumulated system life cycle costs since every system exists on-top of a socio-economic environment. As long as the hypothesis of small disturbances remains nearly valid, total system life cycle costs and explicitly accumulated system life cycle costs converge asymptotically. The basis for explicitly accumulated life cycle costs increases when its share in comparison to the size of the socio-economic environment increases. Then, additional investments in industrial infrastructure and for providing demanded numbers of artisans with specialised skills are allocatable to the system’s life cycle. The variability and uncertainty in calculating benefits and system life cycle costs make it clear that a global scale for assessing system efficiency is illusionary. However, comparing the system efficiencies of alternative solutions satisfying the same needs provides an important decision aid among other decision criteria for selecting the most appropriate solution.
Historic evidence demonstrates that open societies acting in market economies are best suited for achieving system efficiency in reality. However, the stakeholders directly involved in a system’s life cycle rarely rate their efficiency in the scope of overall system life cycle costs. Typically, consumers rate efficiency as a ratio of assumed personal benefits to purchase price. Industrial investors derive their decisions based on the total costs of ownership. Production companies are interested in profits and profit margins primarily. Regulation is required to preserve the interests of the general public. This has led to the concept of the producing enterprise. The body putting a product or service on the market has to take responsibility in terms of liability and safety over the system’s life cycle. This concept works as long as the producing enterprise has the means to bear the risks associated with the costs for compensating potential damages. Otherwise, the general public has to take care for the damages ultimately. Sound systems engineering considering the complete system life cycle provides the most advanced way for producing enterprises to eliminate or to minimise risks to a tolerable level.
Considering the complete system life cycle, system sustainability has various facets. With respect to utilisation, system sustainability is closely linked to system efficiency. Prolonging the utilisation period of the system tends to increase system benefits by keeping pre- and post-utilisation costs unaffected. However, the obvious assumption to achieve outmost system sustainability by products and services that never fail and are built for eternity is wrong. In mature civilisations at all times there have been voices claiming everything that could be invented has been invented. We know that those prophecies always failed. A system’s utilisation period is limited by physical wear and changing scenario conditions due to technical innovation, societal advance, and an improved understanding of scenarios and their further impacts.
For illustrating the evolutionary interferences, let us consider the technical evolution of the wheel. Conceptually, a wheel is a disk rotating around an axis at its centre. Most likely, first mature wooden implementations revealed the endurance of the tread and the friction between disk and axis as the issues limiting the operating life. Advances in metallurgy enabled far more endurable treads made from bronze and steel. Rethinking the scenario, roads were paved to increase ride comfort. Friction between disk and axis were reduced by lubrication and later by rolling bearings. All these innovations allowed higher rotation velocities of wheels and higher speeds of carts. The mass of the wheel and its inertial moments became a noticeable issue. The disk architecture was refined by the introduction of the system elements wheel hub, spokes and tyre. A further speed increase became feasible. The invention of pneumatic rubber tyres improved the ride comfort of motorised cars. Due to higher rotation velocities unexpected resonance effects were observed resulting in a preference for a prime number of spokes. As this is not a book about the history of wheels, please, accept the simplifications and let us return to the discussion of system sustainability.
The example of the wheel shows that the benefits of existing designs degrade when scenarios alter and may be totally lost eventually. Striving for utilisation periods beyond the duration of stable scenarios is useless. This marks the upper limit of reasonable utilisation periods. The lower limit is dominated by physical wear. Usually, physical wear does not affect all product elements evenly. For optimum system sustainability during utilisation, two solution strategies may be applied in combination. Product elements with short endurance should be replaceable by maintenance action. All other product elements should be designed and produced for similar operating lifetimes.
In public debates, system sustainability is primarily discussed in a wider scope concerned with preserving human existence in the biosphere. The mathematics of system theory provide us with a path to extent system boundaries by increasing internal complexity. Observability and controllability set theoretical limits for identifying internal states of complex systems and for driving the system into desired states. With respect to the whole biosphere, observability cannot be assumed. We know about the importance of the carbon cycle and the water cycle. Keep in mind that cycle here should not be interpreted as an option to revert to previous states. It just means that the concept of waste is foreign to the biosphere. Regarding many other phenomena of the biosphere, we know less, little or nothing. Our knowledge of all interferences and interactions is even more limited. If observability is not granted, controllability is not given, too. Of course, the high adverse human impact on living conditions supportive to human development opportunities demand considerations in wider system scopes. Furthermore, we need to be aware: When we try to manipulate more complex systems not only the opportunity impacts may be higher. Also, the risk impacts may increase dramatically. Consequently, high-integrity technical systems are demanded beyond previous conventions and practices.