What type of solar collectors and heat pump evaporators are usually used in solar air source water heaters

Choosing a Solar Thermal Collector: A Trade-off Between Performance and Scenario

The performance of a solar air source water heater (SAW) system depends largely on the type and efficiency of its core component: the solar thermal collector. In professional engineering practice, two mainstream technologies are considered: flat plate collectors and evacuated tube collectors.

1. Evacuated Tube Collector (ETC)

Evacuated tube technology plays a key role in SAW systems due to its excellent insulation performance.

Principle Advantage: Evacuated tubes utilize a high vacuum layer between tubes to significantly reduce convective and conductive heat loss. This means that even in low ambient temperatures or low solar irradiance (such as on cloudy days or in winter), ETCs can maintain high collection efficiency and heat the water to a higher level.

Applicable Scenarios: Due to its excellent low-temperature heat collection performance, the evacuated tube type is particularly suitable for northern China, cold climates, and commercial applications requiring higher outlet water temperatures. It more efficiently provides high-temperature preheating for air-source heat pumps, significantly reducing the heat pump's work load, thereby improving the system's seasonal performance factor (SPF) in winter.

Technical Considerations: Modern evacuated tube collectors often utilize heat pipe technology, enabling waterless operation and indirect heat exchange. This simplifies freeze protection and improves system reliability.

2. Flat Plate Collector (FPC)

Flat Plate Collectors are favored in some projects for their robust structure and thermal stability.

Structural Features: The FPC consists of a heat absorbing plate, a transparent cover, and an insulation layer. Its compact structure makes it easy to integrate with buildings (BIPV, Building Integrated Photovoltaics/Thermal).

Performance Considerations: Flat-plate collectors offer excellent instantaneous efficiency under high irradiance and ambient temperatures. However, they experience greater heat loss compared to vacuum tubes under large temperature fluctuations.

Application Scenarios: Flat-plate collectors are more suitable for southern China, areas with abundant year-round sunshine, or as a low-cost, large-scale system integration solution.

Professional Integration: In solar air source water heaters, flat-plate collectors often serve as a preheat source for the heat pump evaporator. The medium- and low-temperature hot water they generate increases the evaporation temperature and optimizes the heat pump's COP (Coefficient of Performance).

Heat Pump Evaporator Design: Matching and Efficient Heat Exchange

In a solar air source water heater system, the core of the heat pump is the evaporator, which absorbs low-grade heat energy from the environment. To meet the requirements of a complex system, the evaporator must meet high heat exchange efficiency and multi-mode adaptability.

1. Tube-in-Tube / Shell and Tube Evaporator

This type of evaporator is typically used in applications involving direct or indirect heat exchange between water and refrigerant.

Functional Positioning: In standard air-source heat pump (ASHP) mode, the evaporator absorbs heat from the air. However, in solar hybrid mode, the evaporator may be designed as a multifunctional heat exchanger.

Specialized Application: Some high-end Solar Air Source systems use an intermediate medium (such as antifreeze or circulating water) that is first heated by solar collectors and then transferred to the evaporator to supplement the refrigerant. This design requires the evaporator to have excellent flow resistance and heat transfer coefficient.

2. Finned Tube Evaporator

This evaporator is used in standard air-source heat pump outdoor units and absorbs heat directly from the ambient air.

Core Function: During off-season periods or auxiliary heating, the system primarily relies on the fin-tube evaporator to absorb air heat.

Defrosting Considerations: In low-temperature, high-humidity environments, fin-tube evaporators face the problem of frosting. Professional Solar Air Source systems utilize excess heat from solar collectors, even as a defrost heat source, improving defrost efficiency and reducing defrost energy consumption and heat pump downtime.

Combined System Integration and Optimization

The Solar Air Source Water Heater's expertise lies in the optimized control logic and thermodynamic cycle between the collector and evaporator.

Energy Synergy: The system uses an intelligent temperature control algorithm to accurately determine when to use solar preheating (to increase the water tank temperature or the heat pump evaporation temperature) and when to switch to pure heat pump mode. This dynamic switching ensures solar priority and maximizes the utilization of renewable energy.

Performance improvement: The preheating energy provided by the collector significantly increases the suction pressure of the heat pump compressor and reduces the compression ratio, thereby making the system's COP in the combined mode much higher than that of a pure air source heat pump, maximizing the marginal benefit of energy.