High-temperature heat pumps represent a valuable technology to decrease fossil fuel consumption in industry, as they can use renewable electricity to cover a given heat demand. Conventional heat pumps provide heat at temperatures around a maximum of 80 °C. Nowadays, heat production up to 150 °C can be achieved with high-temperature heat pumps. For higher temperatures between 150 °C and 250 °C, specialized designs, such as Brayton heat pumps, are required. This paper aims to investigate the transient response of the DLR's CoBra prototype, an innovative Brayton-cycle heat pump intended to provide heat above 250 °C and currently under commissioning at the DLR facility in Cottbus, Germany. First, a comprehensive transient thermodynamic model of the system is developed, accounting for heat exchangers and piping thermal inertia. Furthermore, a control logic is presented that ensures safe operation throughout off-design conditions and start-up manoeuvres. In particular, several control parameters are considered to avoid potential operational issues, such as critical temperature gradients, compressor surge, and critical mechanical vibration phenomena due to resonance. The performed simulations aim to reduce start-up time and energy consumed during start-up. Results show that with the help of the described controller, the system can reach design operation via a transient trajectory safely and quickly. Therefore, the capability of the CoBra prototype to flexibly supply high-temperature heat is demonstrated.

Rotary tables are often used to speed up the acquisition time during the 3D scanning of complex geometries. In order to avoid manual registration of the point clouds acquired with different orientations, automatic algorithms to compensate the rotation were developed. Alternatively, a proper calibration of the rotary axis with respect to the camera system is needed. Several methods are available in the literature, but they only consider a single-axis calibration. In this paper, a method for the simultaneous calibration of both axes of the table is proposed. A checkerboard is attached to the table, and several images with different poses are acquired. An optimization algorithm is then setup to determine the orientation and the locations of the two axes. A metric to assess the calibration quality was also defined by computing the average mean reprojection error. This metric is used to investigate the optimal number and distribution of the calibration poses, demonstrating that the optimum calibration results are achieved when a wider dispersion of the calibration poses is adopted.

Acoustic pyrometry is a non-intrusive measurement technique that may have several applications in turbomachinery. This methodology estimates the gas temperature by measuring the time of flight of an acoustic wave moving through a medium. It can be accomplished by placing a sound source (emitter) and a set of microphones (receivers) on opposite sides of a section. The emitter generates a sound pulse, and the receivers detect it. Since the emitter-receiver distances are known and fixed, the average temperatures of the paths traversed by the acoustic pulse can be computed by estimating the time-of-flight through deconvolution techniques. However, despite the straightforward principle, an acoustic wave suffers a variation of amplitude when propagating within a medium because of energy losses and ambient noise. Hence, time-of-flight estimation becomes a critical task, especially when considering high-frequency waves or short distances between sensors. It is then fundamental to select proper acoustic waves to maximise the cross-correlation between the signals of the emitter-receiver couples, thus improving the accuracy of the time-of-flight measurements and, consequently, the estimation of the spatial temperature distribution within a specific area. This study is a preliminary investigation, based on a modelling approach, to estimate the impact of different acoustic waves on the accuracy of the time-of-flight measurement. The results of this analysis will be useful to design and setup an acoustic pyrometry application.

The need for storage capacity is continuously increasing due to the progressive penetration of renewable energy sources in the power grids. New storage technologies for large-scale applications are emerging in this context, among which Pumped Thermal Electricity Storage (PTES) is a promising alternative. PTES is based on closed Brayton cycles: a Brayton heat pump is operated in the charging phase, using the off-peak electricity to transfer heat between two reservoirs. A Brayton heat engine is operated between the same reservoirs in the discharging phase, producing electric energy. The purpose of grid-scale storage facilities is to provide flexibility to the grid. Therefore, the proposed technologies must effectively operate in part-load conditions and quickly react when the load must be adjusted. In this study, the transient response of the Brayton PTES technology is studied, with particular emphasis on its off-design operation and control strategy. To this end, a detailed thermodynamic model of the system is developed. Particular attention is given to the sizing of the components relevant to the control. Furthermore, a control strategy suited for the power regulation of closed Brayton cycles, the Inventory Control, is investigated, and implementation is proposed. As it resulted, the system can operate in part-load with negligible performance degradation and is characterized by a rapid transient response. Therefore, Brayton PTES resulted as a promising storage technology for grid-scale applications.