Using the System Complexity Metric (SCM) to Compare CO2 Removal Systems

A fundamental cause of difficulty in large engineering projects is their inherent
complexity. An impression of complexity occurs if a system is simply difficult to understand,
where there is no obvious mental model that correctly predicts its behavior. Higher system
complexity is usually associated with higher cost and higher failure rate. Complexity is
perceived if a system has many diverse components, multiple interactions and feedback
loops, transients and dynamic behavior, and unanticipated failure modes. Identifying and
removing these signs of complexity should improve performance and reduce the cost and
failure rate. Complexity can be directly measured by the number of components and their
interactions. The System Complexity Metric (SCM) is defined as the sum of the number of
parts in a system, N, plus the number of the one-way interconnections between them, I. SCM
= N + I. The SCM is easily determined by direct inspection of the system block diagram.
SCM can be used to compare systems and to guide their redesign to reduce cost and failure
rate. Carbon dioxide removal systems are analyzed using SCM, cost, and failure rate. As in
previous work, cost is directly proportional to SCM and that failure rate increases as a
power of SCM for large differences in SCM. The SCM ranking of carbon dioxide removal
systems is the same as their ranking in detailed analysis and practice.