A Methodology for the Assessment of Efficiency in Systems Under Transient Conditions: Case Study for Hybrid Storage Systems in Elevators

Vertical transportation in buildings (elevators, vertical conveyors, escalators, etc.) moves more than one billion people worldwide every day, accounting for a significant portion of the energy consumption of an average building. This work discusses the operation of an energy management system in a hybrid energy storage system for an elevator in a commercial building. Besides energy support in case of mains failure, the storage units are also coordinated to attain an efficient management of the power flows in the full system, allowing for the implementation of a peak-shaving grid power strategy. The design procedure of such a storage system needs to take into account the effects in key performance parameters of the intended varying operating conditions. Among these key performance parameters, the system efficiency plays a vital role, namely, the power losses, the thermal performance, the reliability of the system, or even exploitation costs. Keeping this in mind, the main contribution of the work is the proposal of a systematic methodology for the selection of the optimal configuration of the power electronic conversion systems, in terms of energy efficiency performance, for the case of study of the storage system in an elevator previously defined. But in any case, the proposal is formulated as a general approach, valid for any system in which the general functionality is defined as a sequence of transient intervals, rather than based on a fixed steady-state operating point. The algorithm, intended for evaluating the performance of a given power conversion stage, includes a procedure for the power electronic topology selection and for the dynamic control parameters’ adjustment. The methodology is introduced and described in detail, and then it is applied to two different topologies for the power converter configuration. Additionally, it forms the basis of an optimal controller design. One of the major benefits of the methodology is that it provides the same information obtained from the thorough computation of mission profiles of the power demanded by the system, defined for full operating periods, but using a simplified characteristic power profile. Therefore, by applying such simple profile-based strategy, an optimal configuration of the control parameters can be derived in cases where the complete power profiles are unknown. An additional advantage of this contribution is that this approach provides very accurate results with a reduced number of calculations. This last aspect opens the possibility to implement the resulting low computational burden algorithm in real-time control schemes.