• Energy Research

Abstract The environmental concerns due to global warming are pushing A/C industries to new eco-friendly refrigerants in several fields. In this paper a model to simulate the dynamic evolution of the temperature inside an air-conditioned high-speed train compartment is presented. The dynamic modeling of both the reversible heat pump unit and the thermal loads of the cabin are presented, including the possibility of adapting the frequency of the compressor and the return air fraction for maintaining the internal comfort conditions. Under different dynamic load conditions (in terms of ambient temperature, solar radiation, train speed, number of passengers) the energy consumptions and the TEWI related to the use of new refrigerants, (like R1234yf and R1234ze), are calculated being the R134a a baseline for comparison.

The growth of Internet of Things (IoT) technologies is leading to new opportunities in the heat pump sector for residential air conditioning, in which fault detection and diagnosis (FDD) techniques are being developed from measured data to better manage maintenance operations. This paper presents a physics-based digital model simulating the effects of soft faults (refrigerant leakages and condenser/evaporator fouling) concurring with different intensities on a residential heat pump system. Firstly, their effect on cooling capacity and COP has been analysed. Then, each thermodynamic cycle (as a sequence of measured pressures and temperatures) uniquely related to a peculiar combination of fault intensities is determined. In this way, the field of faults which can be uniquely diagnosed and evaluated has been calculated, and the interrelationship between this field and the uncertainty of the measurements has been demonstrated. The results are encouraging, since common instruments available on the market may detect penalizations of the energetic performance as small as 5%.

Abstract CO2 is one of the most interesting non toxic, nor flammable, low GWP natural fluid to be used in applications with a large direct contribution to global warming, such as vending machines. In this paper the results of an experimental investigation on a small capacity, air-forced refrigerating plant working with CO2 are presented. The air inlet temperatures at gas cooler and evaporator were varied between 16 °C and 31 °C and between −25 °C and 25 °C, respectively, to cover the range of temperatures under conditions typical of commercial refrigeration, during quasi-steady operation. The thermodynamic analysis of the system performance was carried on varying independently the pressure at the gas cooler, by overfeeding the liquid receiver to point out the effect of refrigerant charge. The results allow to describe the performance of the system in a map varying the boundary conditions; at the same time the corresponding variations of the thermodynamic cycle, the COP and the mass flow rate are reported.

This paper presents a critical analysis of possible data reduction procedures for the evaluation of local heat transfer coefficient during flow boiling experiments. The benchmark method using one-dimensional (1-D) heat transfer in a heated tube was compared to a new data reduction method in which both radial and circumferential contributions to the conductive heat transfer inside a metal tube are considered. Using published experimental flow boiling data, the circumferential profiles of the wall superheat, inner wall heat flux, and heat transfer coefficients were independently calculated with the two data reduction procedures. The differences between the two methods were then examined according to the different heat transfer behavior observed (symmetric or asymmetric), which in turn was related to the two-phase flow regimes occurring in a channel during evaporation. A statistical analysis using the mean absolute percentage error (MAPE) index was then performed for a database of 417 collected flow boiling data taken under different operating conditions in terms of working fluid, saturation temperature, mass velocity, vapor quality, and imposed heat flux. Results showed that the maximum deviations between the two methods could reach up to 130% in the case of asymmetric heat transfer. Finally, the possible uses of the two data reduction methods are discussed, pointing out that the two-dimensional (2-D) model is the most reliable method to be employed in the case of high-level modeling of two-phase flow or advanced design of heat exchangers and heat spreader systems.

Abstract Electric heat pumps are recognized as a key technology for decarbonization and have received increasing policy support in several countries over the last few years. These devices offer a highly efficient form of electric heating and could make an important contribution to the transition to a low carbon future, especially in case of district heating systems. Within this context, this paper presents a thermo-economic optimization analysis of a 7.35 MW electric heat pump system employed for district heating purposes and using ammonia as working fluid. The implemented code gathers multiple sub-models calibrated ad-hoc from real data or manufacturers’ datasheets. The heat exchangers are simulated by considering phenomenological equations and well-known heat transfer coefficient and pressure drop prediction methods. The optimization analysis is performed at constant condensation temperature that guarantees in/out hot water temperatures of 30/62 °C and is focused on the matching between compressor and evaporator heat exchanger, by considering either a direct expansion system or a chilled water heat pump. Both the coefficient of performance (COP) and the set-up costs are considered as optimization performance indicators for the construction of the Pareto front.

Abstract Long-period food storage vertical freezers are small cooling capacity appliances usually working with HC. The particular system architecture and the small thermal load allow the use of natural draft wires and tube heat exchangers with large heat transfer areas. Despite of the good performance achieved, high refrigerant charges with respect to the cooling capacity of the system are required, due to the high volumes of the heat exchangers. In this work an experimental analysis of an apparatus with a typical arrangement of the heat exchangers and capillary tube is presented. The system was first designed according to the current standards to reach high performances. Then the working conditions of the system were completely characterized experimentally in the whole range of refrigerant charges compatible with steady operation. Some general conclusions about the systems with this kind of architecture are furnished about the determination of the thermodynamic optimum operating condition and, hence, the determination of the optimum charge.

To decarbonize Europe achieving almost zero emissions in 2050, more stringent regulations are going to be applied. Particularly, Europe is investing in the emissions’ re-duction of buildings (existing and new ones), and strong improvements in energy performance of building are expected according to novel energy performance of building directive (at last phase of negotiation). At the same time, the production and use of fluorinated gases will be further reduced with the novel F-Gas regulation (under Parliament approval). New F-Gas will affect remarkably the small size, air-to-air split systems for air-conditioning, since no fluorinated gases will be used after 2035, forcing manufacturers to the use of natural refrigerants. Being propane the most efficient among the non-toxic natural refrigerants, less refrigerant would be charged into systems according to current safety standards: this would potentially reduce the heat transfer surfaces and, consequently, for the same capacity, the energy efficiency or, for the same efficiency, the capacity would decrease. In this paper, some scenario analyses, complying with actual and future plausible dispositions, are presented, in order to showing the margins for de-sign and commenting criticalities. In particular, the optimal design options are proposed for different fluids, in terms of costs vs energy performance, under representative cases, in terms of weather conditions and building types in Italy (existing ones and new ones respecting high-efficiency standards, trying to meet the requirements of hypothesized national law following the draft of the novel EPBD).

The main target of this paper is to numerically study a multi-source (air/sun/ground) heat pump with the implementation of a thermal energy storage, using either water or PCM, for residential space heating. The system was modelled considering several sub-models for each of the components (compressor, solar panels, storage tank, heat exchangers etc.). A control strategy has been established to decide which operating mode of the system provides the highest coefficient of performance (COP). A multi-objective optimization through genetic algorithm of several decisional variables of the system was carried out, in different configurations and climate conditions, by considering different scenarios in terms of total investment and energy consumption costs, to optimize seasonal performances and investment cost of the entire system. Results show that solar thermal and solar photovoltaic collectors coupled with water storage tank give higher seasonal energy performance, especially in warmer climates, whereas the exploitation of the ground source can be more advantageous for colder climates. From the optimization analysis, it results that optimal non-dominated solutions characterized by a SCOP increase between 50% and 250% are characterized by higher investment costs between 215% and 730%, depending on the climate conditions. None of the solutions employing a PCM storage tank results economically feasible, due to a slight effect on system performance, and a much higher effect on investment costs. Finally, several cost scenarios in terms of incentives on investment costs and increased energy prices were analysed, for which the employment of scenarios with higher capital investment can be more advantageous in terms of lower total costs. This work was partially funded by Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) (PID2021-123511OB-C31 - MCIN/AEI/10.13039/501100011033/FEDER, UE and RED2018-102431-T - MCIU/AEI). This work is partially supported by ICREA under the ICREA Academia programme. The authors form University of Lleida would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREiA is certified agent TECNIO in the category of technology developers from the Government of Catalonia.