Experimental and theoretical study of the dynamic response of a small size CO2 direct expansion solar assisted heat pump

• Autor(a) principal: Tiago de Freitas Paulino
• Data de Publicação: 2019
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Repositório Institucional da UFMG
• Texto Completo: http://hdl.handle.net/1843/42514; https://orcid.org/0000-0002-2446-0373
• topic: Heat pump; Solar assisted; CO2; Dynamic response; Direct expansion; Engenharia mecânica; Dióxido de carbono; Energia solar; Bombas de calor

Providing adequate and low-cost control is an important step in the use of direct expansion solar assisted heat pump (DX-SAHP) in replacement of the electric showers in the Brazilian residences. In that way, design a low-cost controller to regulate the hot water temperature in 60°C (to storage) and 45°C (to direct consumption) is the first goal of this study. Experimental tests demonstrated the capacity of the control system to keep hot water in the correct temperature in different environment and system conditions. The second goal of this study is to evaluate the expansion valve opening. The impact of a fixed aperture in the functioning and on the performance of these systems is not well known, especially for small-size systems. In this context, it is presented an experimental analysis of the influence of the expansion valve opening on the performance of a carbon dioxide (CO2) DX-SAHP. Experimental tests were carried out considering solar radiations about 48 W/m² and 715 W/m² in which the coefficient of performance (COP) observed were 2.21 and 2.58, respectively. The value of the expansion device opening that leads to the maximum COP was similar, regardless the solar radiation. In consequence, for a small CO2 DX-SAHP, using a basic cycle configuration (where the expansion device is used to control the optimum high pressure and consequently the COP), a low-cost embedded system water temperature control and a static expansion device would be suitable for proper control. Finally, the third goal of this study is to evaluate the effects of the sudden variations in the solar radiation on the superheat. The mathematical model was validated using experimental data and employed to perform several simulations. The results obtained with the mathematical model revealed that a small variation of the solar radiation leads to a significant variation in the superheat, therefore requiring an immediate action of the expansion device. In conclusion, an electronic expansion valve would be better suited to meet the needs of rapid interventions on the mass flow rate at the evaporator inlet and also the DX-SAHP could operate in a continuous transient condition in some seasons.