Mastering Thermal Storage: The Key to Efficient Solar Water Heating

Thermal Storage in a Solar Water Heating System

In a solar water heating system, a water storage tank, sometimes also called a heat storage tank, is used to store the heat generated by the solar collectors. Using liquids (particularly water) for thermal storage is the most mature, technically sound, and widely used of various thermal storage methods. It is generally desirable for the liquid to have not only a high specific heat capacity but also a high boiling point and low vapor pressure. The former is to avoid phase change (to a gaseous state), while the latter is to reduce the pressure on the thermal storage vessel. Among low-temperature liquid thermal storage media, water offers the best performance and is therefore the most commonly used.

Advantages and Disadvantages of Using Water as a Thermal Storage Medium

Advantages

① Its physical, chemical, and thermal properties are very stable, it is well understood, and its application technology is mature.

② It can serve as both a thermal storage medium and a heat transfer medium, eliminating the need for heat exchangers in thermal storage systems.

③ It has excellent heat transfer and liquid properties. Among commonly used liquids, it has the highest specific heat capacity, the lowest thermal expansion coefficient, and low viscosity, making it well-suited for both natural and forced circulation.

④ The temperature-pressure relationship at liquid-gas equilibrium is very stable, making it suitable for flat-plate solar collectors.

⑤ It is abundantly available and inexpensive.

Disadvantages

① As an electrolytically corrosive substance, the oxygen produced easily corrodes metals. It is also a solvent for most gases (especially oxygen), making it susceptible to corrosion of containers and pipes.

② When solidifying (freezing), its volume expands significantly (up to approximately 10%), which can damage containers and pipes.

③ At temperatures above moderate (over 100°C), its vapor pressure increases exponentially with increasing water temperature. Therefore, when using water for heat storage, both the temperature and pressure must not exceed its critical point (373.0°C, 2.2×10 Pa). For example, the cost of heat storage at 300°C is 2.75 times higher than at 200°C.

When using water as a heat storage medium, heat storage containers can be made from a variety of materials, including stainless steel, enamel, plastic, aluminum alloy, copper, iron, reinforced concrete, and wood. The shapes can range from cylindrical, box-shaped, to spherical. However, the corrosion resistance and durability of the materials used should be carefully considered. For example, when choosing cement and wood as heat storage container materials, their thermal expansion must be considered to prevent cracks and leaks from long-term use.

A hot water storage tank is a device that can store both heat and cold. It was developed as a component in systems that supply hot water, heating, and air conditioning to buildings. Its primary function is to adjust the imbalance between energy consumption and energy consumption, thereby improving the system's thermal efficiency and meeting the required heat load.

Hot water storage tanks can be categorized into various types based on their heat release characteristics (full extrusion flow, full mixing flow, and partial mixing flow), pressure state (open or closed), number of tanks (single or multiple), installation method (vertical or longitudinal or horizontal or transverse), structural materials, and intended use. The following focuses on the first two types.

Hot water storage tank heat release characteristics

Based on the heat release characteristics (or the mixing characteristics within the tank), hot water storage tanks can be classified into three types: full extrusion flow, full mixing flow, and partial mixing flow. If υ represents the water flow velocity, L represents the length of the water tank, and E represents the mixing diffusion coefficient, then the above three categories can be classified according to the degree of mixing of the water temperature in the tank or the value of the mixing characteristic M=υL/(2E).

(1) Complete extrusion flow

Also known as piston flow, that is, the flow in the water tank is completely piston-like, and there are two hot and cold water areas in the tank. The interface between the two is very clear, indicating that there is almost no mixing. At this time, it can be considered that E→0 or M→∞. When the hot water storage tank releases heat (cools), water flows in from the bottom (top) and all the heat can be utilized. This is an ideal state, as shown in Figure 2-11. Assume that there is 100L of hot water with a temperature of 80℃ in the hot water storage tank, and then 20℃ cold water is slowly injected from the bottom inlet A, and all the water flowing out at the outlet B is 80℃ hot water. But as soon as the amount of water flowing out exceeds 100L, the water temperature immediately drops to 20℃.

(2) Completely mixed flow

The temperature in the water tank is completely uniform, indicating that the mixing is very thorough. In this case, E→∞ or M→0 can be considered. Under normal circumstances, this can only be achieved when a powerful mixer is installed in the hot water storage tank and cold water is slowly injected while stirring. At the beginning, the water temperature flowing out of outlet B is 80℃. Then, as time goes by, the water temperature decreases in the form of an exponential function. When the outflow of water just reaches 100L, the water temperature has dropped to about 80×e≈29.3℃.

(3) Partially mixed flow

Also known as temperature stratified flow, it indicates that the temperature distribution in the water tank is uneven and stratification occurs. In this case, the E value can be considered to be finite, that is, 0.

The pressure state of the hot water storage tank

According to the pressure state of the hot water storage tank, it can be divided into two categories: open and closed. Under normal atmospheric pressure, the form of space to be taken depends on the actual situation.

(1) Open type

Because the water tank is open to the atmosphere, it is subject to less pressure, but is easily corroded by acid. Since oxygen is easily soluble in water, the corrosion resistance of the container is required to be high. In addition, the head required by the system is also required to be high. It is generally used in large solar energy systems.

(2) Closed type

Because the water tank is full of water, an expansion tank should be installed on top to avoid damaging the heat storage tank. Its advantages are simple piping system, small head of water pump required, and less power consumed by the circulation pump; its disadvantages are relatively large static pressure, high pressure resistance requirement for heat storage tank, and high equipment cost of pressure-resistant container. It is generally used in small solar energy systems.

In actual applications, the building's hot water supply system and rooftop heat storage tank (used in conjunction with the natural circulation hot water system) are mostly open type; in addition, using the space of the foundation beam as a heat storage tank and using a separate heat storage tank made of concrete are also open type. Conversely, when the system operating temperature exceeds 100°C, the hot water storage tank must be enclosed unless a special heat transfer medium is used. Furthermore, hot water storage tanks in ground-mounted forced circulation hot water systems are generally enclosed.

Open-type hot water storage tanks are often constructed from galvanized steel, stainless steel, and fiberglass, while enclosed types are often constructed from enamel, stainless steel, and fiberglass.

Hot water storage tanks are often cylindrical in shape. Firstly, they are easy to manufacture and seal, making them economical. Secondly, they offer better heat dissipation and minimize dead water areas. Thirdly, they offer better pressure resistance (under constant internal pressure, the tension acting on the cylinder wall is proportional to its radius).

Thermal dynamic characteristics of heat storage tanks

(1) Main parameters of thermal dynamic characteristics

① The size of the dead water area in the heat storage tank;

② The size of the mixing characteristic M value determined by the degree of mixing of water at different temperatures in the heat storage tank;

③ The temperature gradient inside the heat storage material;

④ The heat capacity of the heat exchanger;

⑤ The heat capacity of the pipe system connected to the heat storage tank;

⑥ The heat capacity of the heat storage tank itself and the surrounding environment in contact with it (applicable to heat storage tanks buried underground).

For heat storage tanks that use water as a heat storage medium, since a heat exchanger is not required, the above two items ③ and ④ can be ignored.

(2) Factors affecting thermal dynamic characteristics

① The mixing state of the fluid in the water tank - In the actual use of heat storage tanks, the water flow line may form a non-complete piston flow form, which not only fails to fully store heat, but also makes the stored heat unable to be fully utilized.

② Water tank structure and circulating water volume—primarily referring to the number and configuration of baffles within the tank, the number, diameter, and location of connecting pipes, as well as the tank's shape and circulating water volume.

③ Heat loss and gain—Because the water tank itself has a protective surface, heat loss and gain are inevitable. For short-term heat storage tanks designed to mitigate transient peaks in heat demand, burying them underground and insulating them can actually negatively impact their thermal dynamics. This is because the soil, with its heat capacity, can also serve a certain purpose in storing heat.

④ Heat storage and heat withdrawal temperatures—The heat storage temperature refers to the average water temperature within the tank at the end of heat storage; the heat withdrawal temperature refers to the outlet water temperature at the time heat is withdrawn from the tank. Whether heat can be fully utilized and the operating life of the heat storage tank are closely related to how these two temperatures are measured.

Transient Response of Heat Storage Tanks

When using a heat storage tank, the fluctuation of the outlet water temperature is crucial for determining the heat load. Theoretically, the functional relationship between the input temperature and the output temperature (commonly known as the inlet and outlet temperatures) can be derived by calculating the water temperature distribution within the tank. However, this requires solving the three-dimensional continuity equation, the momentum conservation equation, and the energy conservation equation, which is a complex process and requires a long computational program.

In practical design, it is not necessary to directly understand the water temperature distribution within the tank. Instead, it is sufficient to know the temporal variation of the input temperature and heat input, and to be able to calculate the temporal variation of the output temperature. Currently, the most commonly used method is the "transient response method," which treats the entire tank as a single system. If a linear relationship is assumed between the input and output (which can be approximately assumed when the inlet and outlet water temperatures are similar), then the change in output temperature can be calculated for any change in input temperature using a convolution integral.

In summary, using heat storage tanks as small-scale, short-term heat storage devices for hot water, heating, and air conditioning systems plays an important role in solar thermal utilization and has found a range of practical applications. If large-scale and long-term heat storage across seasons is required, some countries have begun to study underground aquifers as an effective heat storage and energy-saving measure in the past two or three decades.