Torben Schmidt Ommen

Two Thermoeconomic Diagnosis Methods Applied to Representative Operating Data of a Commercial Transcritical Refrigeration Plant

In order to investigate options for improving the maintenance protocol of commercial refrigeration plants, two thermoeconomic diagnosis methods were evaluated on a state-of-the-art refrigeration plant. A common relative indicator was proposed for the two methods in order to directly compare the quality of malfunction identification. Both methods were applicable to locate and categorise the malfunctions when using steady state data without measurement uncertainties. By introduction of measurement uncertainty, the categorisation of malfunctions became increasingly difficult, though depending on the magnitude of the uncertainties. Two different uncertainty scenarios were evaluated, as the use of repeated measurements yields a lower magnitude of uncertainty. The two methods show similar performance in the presented study for both of the considered measurement uncertainty scenarios. However, only in the low measurement uncertainty scenario, both methods are applicable to locate the causes of the malfunctions. For both the scenarios an outlier limit was found, which determines if it was possible to reject a high relative indicator based on measurement uncertainty. For high uncertainties, the threshold value of the relative indicator was 35, whereas for low uncertainties one of the methods resulted in a threshold at 8. Additionally, the contribution of different measuring instruments to the relative indicator in two central components was analysed. It shows that the contribution was component dependent.

Design and optimization of the heat exchanger network for district heating ammonia heat pumps connected in series

Denmark presents ambitious climate policies, and in order to fulfil these visions electrically driven large-scale heat pumps (HP) are often mentioned as an important technology for future district heating (DH) systems. To reach the high temperatures needed in current DH systems, the suggested HP installations become complex systems, where heat transfer between the HP cycle and the heat sink takes place at several temperature levels. In this study the heat exchanger network (HEN) between a HP installation consisting of two serially connected two-stage ammonia HP units and a heat sink being heated from 50 °C to 80 °C was investigated. The study applied pinch analysis to estimate the highest attainable Coefficient of Performance (COP) with the given HP configuration. Based on the result of the pinch analysis, a HEN reaching the highest COP was suggested and compared with COPs obtained with three other solutions for a HEN. The result revealed an estimated highest COP of 3.46. The three other design suggestions yielded reductions in the COP of -2.3%, -2.0%, and -1.8% compared to the highest. From this it was concluded that the HEN has an influence on the COP, and that the pinch analysis can be used to estimate the highest COP for a given HP installation. Furthermore, the COP obtained by practical installations was accordingly shown to come close to the target.