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A group of Chinese scientists has designed a residential heat pump system with a potential coefficient of performance of 4.271 at -5ºC. The system is capable of adjusting the necessary pressures depending on the intensity of solar irradiation and ambient temperature.
Scientists from Chinas Chongqing University have designed a solar-air dual-source heat pump system to heat water in self-built rural houses in Chinas Beijing-Tianjin-Hebei region.
“Our system can maintain stable heating performance even in adverse weather conditions, with low temperatures or limited solar radiation,” Jun Lu, corresponding author of the research, told pv magazine .
The researchers described the proposed system as an enhanced ejector-solar air composite dual-source heat pump (EDHP) using photovoltaic thermal collectors (PVTs). The heat pump initially uses a compressor to convert low-temperature, low-pressure steam into high-temperature, high-pressure superheated steam.
This vapor is then distributed between PVT collectors and an air source heat exchanger while passing through expansion valves that adjust the necessary pressures based on the intensity of solar irradiation and ambient temperature. “The ejector takes advantage of the high-pressure coolant as the main driving current to increase the pressure of the low-pressure coolant,” the academics explain. “Then the mixed refrigerant from the ejector outlet returns to the compressor.”
The system also uses an inverter to store the electricity generated by the PVT modules in the batteries or to convert direct current (DC) to alternating current (AC) which can be used to power the compressor or injected into the grid.
The heat pump operates in three operating modes: in solar-assisted mode, in which the heat necessary for the heating process is provided by the PVT collectors; with insufficient solar radiation, when both the air-cooled evaporator and the solar collector/evaporator operate simultaneously; and with the use of ambient air as the primary heat source, which supposedly guarantees continuous operation of the heating.
The academics performed a series of simulations to evaluate the performance of the heat pump system and analyzed, in particular, the thermodynamic properties of the working fluid, the environmentally friendly refrigerant R134a. “The ejector plays a crucial role in improving the circulation performance of vapor compression cycles,” they stated. “As the ejector primary flow pressure increases, the coefficient of performance (COP) increases, while the decrease in COP occurs simultaneously with the decrease in the ejector secondary flow pressure.”
They assumed that the system operated at a temperature of -5 °C and an irradiance level of 250 W/m2. They also considered using 20 m2 of PVT collectors for a 5 kW heating supply.
The evaluation showed that the heat pump was capable of achieving a COP of 4.271, which academics say is more than 25% higher than that achieved by conventional aerothermal heat pumps. “The reduction in electricity demand from the network can reach up to 51.96% by incorporating the electricity generated by photovoltaic panels,” they added.
They estimated the payback period for the system between 10 and 12 years.
The system is described in the article “ Thermodynamic and economic analyzes of modified ejector enhanced solar -air composite dual-source heat pump system in residential buildings ” with modified ejector in residential buildings), published in Energy and Building s. |
