Solar Water Heating Systems: A Comprehensive Guide to Types & Operation

Introduction to Solar Water Heating Technology

Non-Powered Circulation Instantaneous Solar Water Heating System

System components: Evacuated tube collector, connectable water tank, adjustable bracket, and heat exchanger.

Operating Principle of the Non-Powered Circulation Instantaneous Solar Water Heating System: Water within the evacuated tube begins to heat up when exposed to sunlight. As the water heats up, its density decreases, and it naturally circulates to the tank, gradually heating the water. The heated water is then stored in a tank insulated with polyurethane foam. Cold indoor water flows through a fixed bellows channel within the tank, raising the pressurized tap water to a temperature nearly identical to the water in the tank (with a temperature difference of less than 2 degrees Celsius). This results in stable, pressurized, clean hot water.

Natural Circulation Solar Water Heating System

A natural circulation solar water heating system utilizes the temperature difference between the collector and the water tank to create a thermosyphon head, which circulates water through the system. Simultaneously, the useful energy gained from the collector is continuously stored in the tank by heating water.

During system operation, the water in the collector is heated by solar radiation, increasing its temperature and decreasing its density. The heated water gradually rises within the collector, flowing through the collector's upper circulation pipe into the upper portion of the water storage tank. Simultaneously, cold water from the bottom of the water storage tank flows through the lower circulation pipe into the bottom portion of the collector. Over time, distinct temperature stratification forms in the water storage tank, with the upper layer of water reaching a usable temperature first, and this continues until the entire tank is fully usable.

There are two methods for obtaining hot water. One uses a make-up tank, which replenishes cold water to the bottom of the storage tank, pushing the hot water from the upper layer out for use. The water level is controlled by a float valve within the make-up tank. This method is sometimes called the top-up method. The other method, without a make-up tank, allows the hot water to fall from the bottom of the storage tank by gravity. This method is sometimes called the drop-down method.

Forced Circulation Solar Water Heating System

A forced circulation solar water heating system uses a water pump installed in the pipeline between the collector and the water storage tank to circulate the water in the system. Simultaneously, the useful energy gained from the collector is stored in the water storage tank by heating the water.

During system operation, the activation and deactivation of the circulation pump must be controlled, otherwise both electrical energy and heat energy are wasted. Temperature differential control is generally the most popular method, and sometimes both temperature differential control and photovoltaic control are used simultaneously.

Temperature differential control utilizes the temperature difference between the water temperature at the collector outlet and the water temperature at the bottom of the water storage tank to control the operation of the circulation pump.

After sunrise in the morning, the water in the collector is heated by solar radiation, gradually increasing in temperature. Once the temperature difference between the collector outlet and the water at the bottom of the storage tank reaches a set value (generally 8-10°C), the temperature controller generates a signal to start the circulation pump, and the system begins operation. During periods of cloud cover or in the afternoon before sunset, solar irradiance decreases, causing the collector temperature to gradually drop. Once the temperature difference between the collector outlet and the water at the bottom of the storage tank reaches another set value (generally 3-4°C), the temperature controller generates a signal to shut down the circulation pump, and the system stops operating.

Solar Water Heating Systems

There are also two methods for obtaining hot water: top-flow and drop-flow.

The top-flow method involves adding cold water (tap water) to the bottom of the storage tank, pushing the hot water out of the upper layer for use. The drop-flow method relies on the hot water itself to fall from the bottom of the tank due to gravity. Under forced circulation, the water in the tank is thoroughly mixed, preventing significant temperature stratification. Therefore, both the top-flow and drop-flow methods provide hot water from the outset. Compared to the topwater method, the advantage of the topwater method is that the pressurized hot water spray improves user comfort, and there's no need to worry about refilling the water tank. However, the disadvantage is that cold water entering from the bottom of the water tank can mix with the hot water in the tank. While the topwater method has the advantage of preventing hot and cold water from mixing, it also has the disadvantage that the hot water falls by gravity, affecting user comfort, and the need to refill the water tank daily is a must.

In a dual-circuit forced circulation system, the heat exchanger can be either an immersion heat exchanger placed within the water tank or a plate heat exchanger placed outside the water tank. Plate heat exchangers offer many advantages over immersion heat exchangers: First, plate heat exchangers have a larger heat transfer area, smaller heat transfer temperature gradients, and less impact on system efficiency. Second, plate heat exchangers are placed within the system piping, offering greater flexibility and facilitating system design and layout. Third, plate heat exchangers are commercially available and standardized, making quality assurance and reliability easier.

Forced circulation systems are suitable for large, medium, and small solar water heating systems.

Flow-through solar water heating systems heat water to the desired temperature in a single pass through the collector, with the heated water then being released into the storage tank in a continuous stream.

During system operation, a constant-temperature discharge method is typically used to ensure hot water meets user requirements. The collector inlet pipe is connected to the tap water line. The water in the collector is heated by solar radiation, gradually increasing in temperature. A temperature sensor is installed at the collector outlet. A temperature controller controls the opening of an electric valve installed at the collector inlet. This temperature controller adjusts the collector inlet flow rate based on the collector outlet temperature, maintaining a constant outlet water temperature. The reliability of this system depends on the performance of the variable-flow electric valve and controller.

To avoid stringent requirements on the electric valve and controller, some systems install the electric valve at the collector outlet, and the valve has only two states: open and closed. When the collector outlet temperature reaches a set value, a temperature controller opens the electric valve, allowing hot water to flow from the collector outlet into the water storage tank. Simultaneously, cold water (tap water) is added to the collector until the collector outlet temperature falls below the set value. The electric valve closes, and the process repeats. While this constant-temperature water discharge method is relatively simple, due to the hysteresis in the electric valve's closing, the resulting hot water temperature may be lower than the set value.

A direct-flow system has many advantages: First, compared to forced circulation systems, it does not require a water pump; second, compared to natural circulation systems, the water storage tank can be located indoors; third, compared to recirculating systems, usable hot water is available earlier each day, and a certain amount of usable hot water can be obtained as long as there is a period of clear weather; fourth, it is easy to implement a design for draining the system at night to prevent freezing in winter. A disadvantage of a direct-flow system is that it requires reliable variable-flow electric valves and controllers, which complicates the system and increases investment.

A direct-flow system is primarily suitable for large-scale solar water heating systems.