Glycol System Features for Solar Water Heating

Glycol-based solar water heating systems employ antifreeze solutions to provide freeze protection in climates where ambient temperatures drop below 32°F (0°C). These closed-loop pressurized systems circulate propylene glycol mixtures through the solar collector array, transferring thermal energy to potable water through a heat exchanger. Understanding the thermophysical properties, system behavior, and maintenance requirements is essential for reliable operation.

Propylene Glycol Properties and Selection

Freeze Point Depression

Glycol solutions depress the freezing point through colligative properties, where dissolved molecules disrupt ice crystal formation. The relationship between glycol concentration and freeze protection follows:

$$\Delta T_f = K_f \cdot m$$

Where $\Delta T_f$ is the freezing point depression (°F or °C), $K_f$ is the cryoscopic constant for water (1.86 °C·kg/mol), and $m$ is the molality of the solution.

For practical applications, propylene glycol concentration determines freeze protection levels:

ASHRAE Standard 90.1 recommends designing for the 99.6% winter design temperature with a 10°F (5.6°C) safety margin.

Thermophysical Property Changes

Glycol addition significantly alters fluid properties, directly impacting system performance:

Specific Heat Capacity Reduction:

The mixture specific heat follows a mass-weighted relationship:

$$c_{p,mix} = x_{glycol} \cdot c_{p,glycol} + (1 - x_{glycol}) \cdot c_{p,water}$$

At 50% concentration and 120°F (49°C):

• Pure water: $c_p = 1.0$ Btu/lb·°F (4.19 kJ/kg·K)
• 50% glycol: $c_p = 0.90$ Btu/lb·°F (3.77 kJ/kg·K)

This 10% reduction in heat capacity requires proportionally higher flow rates to transfer equivalent thermal energy:

$$\dot{m}{glycol} = \dot{m}{water} \cdot \frac{c_{p,water}}{c_{p,glycol}}$$

Viscosity Increase:

Dynamic viscosity increases exponentially with glycol concentration, directly affecting pumping power:

$$\eta_{mix} = \eta_{water} \cdot e^{(a \cdot x_{glycol} + b \cdot x_{glycol}^2)}$$

At 100°F (38°C), 50% propylene glycol exhibits viscosity approximately 3.5 times that of water, increasing pressure drop and pumping energy.

Thermal Conductivity Degradation:

Glycol solutions have lower thermal conductivity than water:

$$k_{mix} \approx k_{water} \cdot (1 - 0.25 \cdot x_{glycol})$$

This reduces heat transfer effectiveness in both collectors and heat exchangers.

System Architecture and Components

graph TD
    A[Solar Collector Array] -->|Hot Glycol| B[Heat Exchanger]
    B -->|Cooled Glycol| C[Circulation Pump]
    C -->|Pressurized Flow| D[Expansion Tank]
    D --> A
    E[Cold Water Supply] -->|Potable Water| B
    B -->|Heated Water| F[Storage Tank]
    F --> G[Domestic Hot Water Use]
    H[Pressure Relief Valve] -.->|Safety| A
    I[Fill/Drain Valves] -.->|Service| C
    J[Temperature Sensors] -.->|Control| K[Differential Controller]
    K -->|Pump Command| C

Critical Components

Heat Exchanger Requirements:

The heat exchanger introduces thermal resistance between collector fluid and potable water, reducing overall efficiency:

$$\eta_{system} = \eta_{collector} \cdot \epsilon_{HX}$$

Where $\epsilon_{HX}$ is the heat exchanger effectiveness, typically 0.80-0.90 for properly sized units.

Heat exchanger effectiveness follows:

$$\epsilon_{HX} = \frac{Q_{actual}}{Q_{maximum}} = \frac{C_{min}(T_{h,in} - T_{c,out})}{C_{min}(T_{h,in} - T_{c,in})}$$

For counterflow exchangers with equal capacitance rates, effectiveness depends on NTU (Number of Transfer Units):

$$\epsilon = \frac{1 - e^{-NTU(1-C_r)}}{1 - C_r \cdot e^{-NTU(1-C_r)}}$$

Expansion Tank Sizing:

Glycol solutions expand more than water with temperature. The expansion tank must accommodate volumetric changes:

$$V_{expansion} = V_{system} \cdot (\beta_{glycol} \cdot \Delta T)$$

Where $\beta_{glycol}$ is the volumetric expansion coefficient (approximately 0.0006/°F for 50% glycol, compared to 0.0002/°F for water).

For a 30-gallon system with 100°F temperature swing:

• Water expansion: $30 \times 0.0002 \times 100 = 0.6$ gallons
• 50% glycol: $30 \times 0.0006 \times 100 = 1.8$ gallons

The expansion tank must be sized for 3 times calculated expansion:

$$V_{tank} = 3 \cdot V_{expansion} \cdot \frac{P_{max}}{P_{max} - P_{min}}$$

Glycol Degradation Mechanisms

Thermal Breakdown

High stagnation temperatures in collectors (250-400°F / 121-204°C) cause glycol degradation through oxidation and thermal decomposition:

$$\text{Propylene Glycol} + O_2 \xrightarrow{\Delta T} \text{Organic Acids} + \text{Aldehydes} + \text{CO}_2$$

Degradation products lower pH, becoming corrosive to system components. The degradation rate follows Arrhenius behavior:

$$k_{degradation} = A \cdot e^{-E_a/(R \cdot T)}$$

Where degradation accelerates exponentially with temperature. Each 18°F (10°C) increase approximately doubles degradation rate.

pH Monitoring and Testing

Fresh propylene glycol maintains pH 10-11 with corrosion inhibitors. As degradation occurs:

1. Organic acids form, reducing pH below 8.5
2. Corrosion inhibitors deplete
3. Metal corrosion accelerates

Testing Protocol (ASHRAE Guideline 3):

• Test pH annually
• Replace fluid when pH drops below 8.5
• Test freeze protection with refractometer
• Visual inspection for color change (brown indicates degradation)

Maintenance Requirements

Annual Service:

• Measure system pressure (maintain 15-30 psi when cold)
• Test glycol concentration and freeze protection
• Test pH with calibrated meter
• Inspect for leaks at fittings and valve seals
• Verify pump operation and flow rate

Fluid Replacement Criteria:

Replace glycol solution when:

• pH < 8.5
• Freeze protection exceeds design temperature by 10°F
• Fluid appears dark brown or black
• System efficiency drops >15% from baseline

Expected Service Life:

• Non-stagnating systems: 5-7 years
• Systems with occasional stagnation: 3-5 years
• Frequent stagnation (>10 days/year): 2-3 years

Performance Impact Analysis

The efficiency penalty from glycol systems compared to direct water circulation includes:

Heat Transfer Reduction:

$$Q_{glycol} = Q_{water} \cdot \left(\frac{c_{p,glycol}}{c_{p,water}}\right) \cdot \epsilon_{HX}$$

For 50% glycol with 85% effective heat exchanger:

$$Q_{glycol} = Q_{water} \cdot 0.90 \cdot 0.85 = 0.765 \cdot Q_{water}$$

This represents approximately 23.5% reduction in useful energy delivery compared to direct water systems.

Pumping Energy Increase:

Higher viscosity increases pressure drop through piping and collectors:

$$\Delta P_{glycol} = \Delta P_{water} \cdot \left(\frac{\eta_{glycol}}{\eta_{water}}\right)$$

At typical operating temperatures, pumping power increases 150-200%.

Climate Suitability and Design Considerations

Glycol systems remain the preferred choice for:

• Climates with freezing temperatures (below 32°F / 0°C)
• Locations where drainback piping slope cannot be maintained
• Roof-mounted collectors with complex piping runs
• Systems requiring high pressure operation
• Applications where potable water quality must be isolated

Design Temperature Selection:

Select glycol concentration for the 99.6% winter design dry-bulb temperature with 10°F margin. For Chicago (99.6% = -7°F):

$$T_{design} = -7°F - 10°F = -17°F$$

This requires 40% glycol concentration minimum (freeze point -10°F, burst protection -40°F).

Operational Advantages and Limitations

Advantages:

• Reliable freeze protection independent of power supply
• Pressurized operation enables flexible piping configurations
• No minimum pipe slope requirements
• Suitable for all collector types and orientations
• Well-established technology with proven reliability

Limitations:

• Reduced thermal efficiency due to heat exchanger
• Regular maintenance required for fluid quality
• Periodic fluid replacement costs
• Higher pumping energy consumption
• Potential for leaks introducing antifreeze odors
• Environmental disposal considerations for spent fluid

Understanding these technical factors enables proper system selection, design, and maintenance for long-term reliable performance in freeze-prone climates.