Using Sand Inside Evacuated Solar Tubes for Direct Thermal Energy Storage and Free Heating

Abstract

This article explores an unconventional but promising concept: replacing water or heat-transfer fluids inside evacuated solar tubes with sand as a direct thermal storage medium. The approach enables solar collectors to function simultaneously as heat absorbers and thermal batteries, offering free and reliable heating, especially in cold climates.

1. Introduction

Conventional evacuated tube solar collectors rely on water, glycol, or heat pipes to transfer solar heat to an external storage tank. While effective, these systems suffer from heat losses, freezing risks, pressure issues, and system complexity.

A novel alternative is to fill the evacuated tube with sand instead of liquid, transforming the tube itself into a direct thermal energy storage unit. This eliminates liquid circulation, freezing problems, and pressure-related failures while enabling long-term heat retention.

2. Why Sand Instead of Water?

Sand is an ideal thermal storage material due to its physical and thermal properties:

• High temperature tolerance (up to 600–1000 °C)

• Chemically stable and non-toxic

• Extremely low cost and widely available

• No freezing, boiling, or expansion risks

• Long lifespan without degradation

Unlike water, sand does not require pumps, antifreeze additives, or pressure vessels.

3. How the System Works

System Principle

1. Solar radiation passes through the glass evacuated tube.

2. The absorber coating converts sunlight into heat.

3. Heat is transferred directly into the sand packed inside the tube.

4. The vacuum insulation minimizes heat loss.

5. The sand stores thermal energy for hours, days, or even weeks.

6. Heat is extracted later via:

   • Embedded metal rods

   • Heat pipes

   • Air or liquid heat exchangers attached externally

The evacuated tube acts as both collector and storage unit.

4. Thermal Storage Performance

Sand stores heat via sensible heat storage:

[Q = m \cdot c \cdot \Delta T]

Where:

• m = mass of sand

• c ≈ 0.8 kJ/kg·K (specific heat of sand)

• ΔT = temperature difference

A single sand-filled evacuated tube can store a significant amount of energy due to:

• High operating temperatures

• Excellent vacuum insulation

• Minimal thermal losses

When multiple tubes are combined, the system forms a modular solar thermal battery.

5. Advantages Over Water-Based Systems

✅ No freezing risk✅ No pumps or circulation loops✅ No pressure buildup✅ No leaks or corrosion✅ Works in extreme cold (-40 °C environments)✅ Extremely low maintenance✅ Long-term heat retention

This makes the system ideal for remote areas, cold climates, and off-grid heating.

6. Applications

• Space heating for homes

• Pre-heating ventilation air

• Greenhouse heating

• Seasonal thermal energy storage

• Off-grid cabins and rural buildings

• Arctic and sub-arctic regions

7. Challenges and Engineering Considerations

While promising, the concept requires careful design:

• Uniform heat distribution inside the sand

• Efficient heat extraction mechanisms

• Structural support for thermal expansion

• Protection against tube implosion

• Optimized sand grain size for thermal conductivity

These challenges are engineering problems, not physical limitations.

8. Comparison: Sand vs Water in Evacuated Tubes

Feature	Water	Sand
Freezing risk	High	None
Pressure	Yes	No
Maintenance	Medium	Very low
Max temperature	~100 °C	>600 °C
Storage duration	Hours	Days–Weeks
System complexity	High	Low

9. Conclusion

Using sand inside evacuated solar tubes instead of water represents a paradigm shift in solar thermal design. By combining heat collection and storage into a single component, the system offers free, reliable, and long-lasting heating with minimal maintenance and exceptional performance in cold climates.

This approach transforms solar collectors into true thermal batteries, unlocking new possibilities for sustainable heating without reliance on electricity or fossil fuels.