Evaluación y optimización de bombas de calor energéticamente eficientes integradas en diferentes aplicaciones para refrigeración y calefacción simultáneas con refrigerantes alternativos

Resumen

Global warming represents one of the most critical challenges that humankind has faced in modern times owing to increased global mean surface temperatures and the risk of water shortage, fire threats, drought, and weed and pest invasions. Although heating, ventilation, and air conditioning provide comfort to the occupants of a building, these devices probably further degrade the Earth¿s atmosphere. The performance of air-source heat pumps degrades at low ambient temperatures in the heating and cooling modes when they operate at high ambient temperatures, representing a significant challenge for developing an energy-efficient system. Thus, one of the biggest challenges for the future is to produce green electricity and invest in highly efficient heating and cooling systems. Increasing the variety of heat sources and their combinations can increase the overall energy performance and reduce greenhouse gas emissions. In this thesis, an innovative compound photovoltaic thermal (PV/T) waste heat¿driven ejector-heat pump system was developed for simultaneous cooling and heating. The system arrangement combines five promising environmentally friendly technologies: heat pumps, ejectors, PV/T panels, waste heat recovery, and low-global warming potential (GWP) refrigerants. A comparative study of various low-GWP refrigerants was conducted to determine the performance of the proposed system under different conditions and waste heat levels. Initially, the system was combined with different waste heat sources, PV/T systems, milk pasteurisation processes, and data centres, and finally, it was combined with a geothermally actuated organic Rankine cycle to produce a trigeneration system with high performance. Heat pumps are a promising technology owing to their high performance. However, their performance is limited by the operating temperature. To identify these problems and produce radical solutions that are economically viable, an experimental comparison of low-GWP refrigerants (R513A, R516A, and R1234yf) used in vapour compression cooling and heating systems was conducted to evaluate their use as drop-in refrigerants and to replace the hydrofluorocarbon (HFC) R134a under different steady-state conditions: for the cooling mode, the evaporating temperature was -5 ¿, -10 ¿, and -15 ¿ and was combined with two condensing temperatures (32.5 ¿ and 40 ¿). Furthermore, in the heating mode, the evaporating temperature was 7.5 ¿, 15 ¿, and 22.5 ¿, with five condensing temperatures (55 ¿ to 75 ¿; in increments of 5 ¿). For all tested refrigerants and teasing modes, an increase in the condensation temperature negatively influenced the system¿s performance; however, an increase in the evaporation temperature augmented the system¿s performance. In addition, under different operating conditions, the predicted compressor formulas had maximum model deviations of ±5% for the volumetric efficiency and ±3% for the isentropic efficiency. The main conclusions of the first arrangement of the proposed system (including the ejector pump and all system components) showed that the Coefficient Of Performance (COP) of the cooling system improved by 7% using R450A compared with a conventional vapour compression system using R134a. Furthermore, the condenser¿s waste heat recovery enhanced the system¿s COP from 3.7 to 4 without increasing the solar intensity. However, the results of the heating mode showed that the system with PV/T waste heat pump and using R450A demonstrated an increase in the COP over the adopted solar time, ranging from 1% to 5%. The second arrangement of the proposed system demonstrated the novelty of using a PV/T waste heat pump with an evaporative-condenser as the driving force of the ejector. The advanced exergy analysis results indicated that further design optimisation could prevent 59.4% of thermodynamic inefficiencies in the whole system. The compressor had the highest contribution to the avoidable exergy destruction rate (21%), followed by the ejector (18%) and condenser (8%). Moreover, the advanced exergoeconomic results proved that 51% of the system¿s costs are unavoidable. The sensitivity analysis results showed that an ejector efficiency of 85% represents the optimum value for the best overall performance of the system and was adopted for all analyses. The arrangement showed a remarkable enhancement in the overall performance for all the investigated refrigerants in both modes. R515B (54% enhancement in the cooling system¿s COP and 49% enhancement in the heating system¿s COP enhancement) exhibited the most pronounced COPC enhancement, followed by R515A and R1234ze(E). Regarding the exergy analysis, R515B performed better than all investigated refrigerants. Furthermore, R515B demonstrated the highest reduction in electricity consumption at 84.1 MWh per year. The system improved the data centre¿s Power Usage Effectiveness index from 10% to 19%. From a financial perspective, R515B had the shortest payback period (6.3 years), followed by R515A and R1234ze(E). Combining the organic Rankine cycle with a compound ejector multi-evaporator vapour compression system for power generation, cooling, and heating purposes demonstrated an overall improvement in the proposed system in both the single- and dual-working fluid cases. Therefore, the dual-working fluid system exhibited the highest overall energy performance, and the combination of R1234ze(Z) and R1234ze(E) resulted in an increase in power generation from 21% to 75% at high and low geothermal temperatures.