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Indian scientists have conducted a comprehensive review of the entire direct expansion solar heat pump technology in an effort to expand its residential adoption. This system can produce hot water from 15ºC to 60ºC with an average coefficient of performance of 1.5 to 4.5.
A group of researchers from MIT World Peace University in India has conducted an extensive review of the entire direct expansion solar heat pump (DX-SHP) technology for domestic heating applications such as water heating, solar drying, space heating, and industrial process heating.
These systems can reportedly produce hot water from 15ºC to 60ºC with an average coefficient of performance (COP) of 1.5 to 4.5 and can operate optimally in freezing conditions.
“This work presents models to analyse the energy and exergy performance of the components of a DX-SHP system,” the academics stress. “The research shows that for optimal solar heat pump performance, solar radiation ranges between 350 W/m2 and 700 W/m2, wind speed between 0.5 m/s and 2.5 m/s, and ambient temperature between 5 ºC and 35 ºC.”
They generally consist of a solar collector-evaporator, a compressor, a condenser and an expansion valve operating within a heat pump cycle. The solar collector, which can be based on a solar thermal unit alone or on a photovoltaic-thermal (PVT) panel, is defined by the research group as the crucial component of the system, as it directly expands the refrigerant and collects heat from both solar and ambient air, the latter prevailing on cloudy days.
“The solar collector-evaporator is directly exposed to solar radiation,” he explains. “The refrigerant flowing through the tubes absorbs heat energy from solar thermal conversion and ambient air. The refrigerant then pumps heat from the evaporator to the condenser via the compressor.”
In all system configurations, waste thermal energy, direct heating or heated waste water are used as thermal energy inputs to the evaporator, which is crucial for capturing solar thermal energy and transmitting it to the refrigerant.
The researchers stressed that finned tube collector-evaporators should be the preferred choice as they perform well in all weather conditions and their use is not affected by temperature or operating mode. “As temperature and solar radiation increase, COP increases and warm-up time decreases,” they added. “Increasing compressor frequency decreases warm-up time and energy consumption, while increasing evaporator heat gain.”
They also recommended adopting environmentally friendly refrigerants such as R290, R600, R1234ze(E), R1270 and R1234yf, as well as using nanofluids to improve the thermal and electrical performance of PVT collector-evaporators.
“Future research should explore the use of heat transfer fluids in roll-on and air-source evaporators as indirect expansion heat exchangers instead of refrigerants,” they concluded. “This includes performance evaluation, optimization of various fluids, and investigation of system integration and sustainability for broader deployment.”
Their review was presented in the article “ Recent Advancements in Solar Collector-Evaporator for Direct Expansion Solar Heat Pump ” published in the International Journal of Thermofluids . |
