The international standard ISO9459 proposes a scientific classification method for solar water heating systems, which is classified according to the 7 characteristics of solar water heating systems. Each feature can be divided into 2~3 types, thus forming a rigorous solar hot water system classification system, as shown in Table 1.

1. The first feature represents the relationship between solar energy and other energy sources in the system

(1) Separate solar system: solar water heating system without any auxiliary energy.

(2) Solar preheating system: A solar hot water system that preheats water before it enters any other type of heater.

(3) Solar energy belt auxiliary energy system: a solar hot water system that uses solar energy and auxiliary energy in combination, and can provide the required heat without relying on solar energy.

2. The second feature indicates whether the heat transfer working medium in the collector is hot water consumed by the user

(1) Direct system: A system in which the hot water that is ultimately consumed by the user or circulated to the user flows directly through the collector, also known as a single-circulation system or a single-loop system.

(2) Indirect system: The heat transfer fluid is not the water that is ultimately consumed by the user or circulates to the user, but the system in which the heat transfer fluid flows through the collector, also known as a dual-circulation system or a dual-loop system.

3. The third feature indicates the contact of the heat transfer medium of the system with the atmosphere

(1) Open system: A system in which the heat transfer medium is in contact with the atmosphere in a large area, and the contact surface is mainly on the open surface of the heat storage device.

(2) Opening system: The contact between the heat transfer medium and the atmosphere is limited to the free surface of the supply tank and expansion tank or the system with the opening of the exhaust pipe.

(3) Closed system: a system in which the heat transfer medium is completely isolated from the atmosphere.

4. The fourth feature shows the condition of the heat transfer working medium in the collector

(1) Full system: A system that is always full of heat transfer working fluid in the collector.

(2) Reflux system: As part of the normal working cycle, the heat transfer medium flows from the collector to the heat storage device when the pump stops running, and then flows into the collector when the pump is restarted.

(3) Discharge system: For the purpose of anti-freezing, water can be discharged from the collector and no longer used.

5. The fifth feature indicates the type of system circulation

(1) Natural circulation system: A system that only uses the density change of the heat transfer medium to realize the circulation between the heat collector and the heat storage device (or heat exchanger), also known as the thermosyphon system. – Natural circulation solar water heating system

(2) Forced circulation system: a system that uses a pump to force the heat transfer working fluid to circulate through the collector, also known as a forced circulation system or a mechanical circulation system. – Forced circulation solar water heating system

6. The sixth feature indicates how the system operates

(1) Circulation system: During operation, the heat transfer working medium circulates between the heat collector and the heat storage device (or heat exchanger).

(2) DC system: The heat transfer working fluid to be heated once flows through the collector, and then enters the heat storage device (storage tank) or enters the system using auxiliary energy heating equipment, sometimes also called the constant temperature water discharge system.

7. The seventh feature indicates the relative position of the collector and the water storage tank in the system

(1) Split system: a system installed at a certain distance between the water storage tank and the collector.

(2) Compact system: a system where the water storage tank is installed directly adjacent to the collector, usually also called a compact solar water heater.

(3) Integral system: a system that uses a collector as a water storage tank, usually also called a sun-dried solar water heater.

In fact, the same solar water heating system often has each of the above seven characteristics at the same time. For example, a typical solar water heating system used in a building can be a solar auxiliary energy system, an indirect system, a closed system, a full system, a forced circulation system, and a split system at the same time.