Solar Expansion Tank Sizing

Calculating the size of a solar expansion tank for a pressurized glycol solar hot water system requires understanding various factors related to the thermal expansion of the glycol solution, the characteristics of your system, and the pressures it will face. The steps to determine the right size of expansion tank generally include:

1. Calculate the volume of your solar liquid (a mixture of water and glycol).
2. Calculate the thermal expansion of the solar liquid.
3. Factor in the initial and maximum system pressure.
4. Factor in the acceptance factor for the expansion tank.

The general formula to calculate the size of a solar expansion tank is as follows:

Size of Expansion Tank = (System Volume x Coefficient of Thermal Expansion x (Max System Temp – Ambient Temp) x Safety Factor) / (Max System Pressure – Initial System Pressure)

The following parameters are involved in this calculation:

1. **System Volume (V)**: This is the total volume of liquid in your solar system, including both the piping and the solar collector. This is often determined by the system’s specifications or can be calculated by adding up the volumes of individual system components.

2. **Coefficient of Thermal Expansion (β)**: This value depends on the type of glycol solution being used and its concentration. The thermal expansion coefficient for propylene glycol, for example, is approximately 0.0004 per degree Celsius at 20°C. This value increases with temperature.

3. **Max System Temp (Tmax) and Ambient Temp (Tamb)**: Tmax is the maximum temperature the solar liquid can reach during operation. Tamb is the ambient temperature or the starting temperature of the system. These temperatures are often in degrees Celsius.

4. **Safety Factor (S)**: This is a factor of safety that’s used to account for unknown or varying conditions. A common value is 1.1 to 1.3, but it may vary based on system design and local regulations.

5. **Max System Pressure (Pmax) and Initial System Pressure (Pinit)**: Pmax is the maximum pressure your system is designed to handle. Pinit is the pressure in the system when it is cold and at rest, before solar heat is applied. These pressures are typically given in bars.

6. **Acceptance Factor (Af)**: This is the ratio of the amount of water the tank can hold compared to the total volume of the tank. For most expansion tanks, this value is usually around 0.5 but can vary depending on the design of the tank.

Now, let’s use a hypothetical example:

Suppose you have a system volume of 200 liters and you are using a propylene glycol solution with a thermal expansion coefficient of 0.0004 per degree Celsius at 20°C. Let’s assume the max system temperature is 120°C and the ambient temperature is 20°C. You’re using a safety factor of 1.2. The max system pressure is 6 bars and the initial system pressure is 1 bar.

Here’s how you would use the formula:

Size of Expansion Tank = (200L x 0.0004/°C x (120°C – 20°C) x 1.2) / (6bar – 1bar)

The result will give you the required volume of the expansion tank in liters. Note that the actual size of the tank you purchase should be larger than this because the acceptance factor is less than 1.

Remember that this calculation is an approximation and other factors such as local regulations, manufacturer guidelines, and system dynamics can influence the appropriate size of an expansion tank. It’s always recommended to consult with a professional when designing solar hot water systems.

The system volume needed to calculate the size of a solar expansion tank in a pressurized glycol solar hot water system is specifically the volume of fluid in the system that is subject to thermal expansion. This will usually include the volume in the solar collectors and the associated piping, up to the location of the expansion tank.

The exact location of the expansion tank can vary in different systems. In most cases, the expansion tank is typically installed near the solar heat exchanger or the solar storage tank, and on the solar loop side. Any fluid downstream of the expansion tank in the system (such as in the domestic water portion of the system or on the “house” side of a heat exchanger) is typically not subject to the same degree of thermal expansion and thus is not included in the calculations.

To determine this volume, you would add the volume of the solar collectors (usually provided by the manufacturer) to the volume of the solar loop piping. The volume of the piping can be calculated using its internal diameter and length. Remember to account for all the piping that connects the solar collectors to the expansion tank.

Use the table of piping volumes and the length of your pipe runs to determine the volume of your solar loop.

Sizing the solar expansion tank:

1. The volume of the fluid inside the solar collectors.
2. The volume of fluid in the piping between the discharge side of the circulation pump and the connection point to the solar collectors.