How Evacuated Tube Solar Collectors Work — and Why PV Solar Water Heating Is the Smarter Choice

Evacuated tube solar collectors have been a popular technology for solar water heating for many years. They work on a simple principle: sunlight heats a series of glass tubes, each containing a heatpipe that transfers the collected heat to a manifold, which then heats the water stored in a specific type of hot water cylinder with a the circulation pump and insulated plumbing to the solar collector and back.

🔍 Inside the Heat Pipe

Each evacuated tube contains a sealed heat pipe filled with a small amount of liquid. When the sun heats the tube, the liquid inside evaporates, carrying heat to the top of the pipe, where it transfers that energy to the manifold to heat the water. This hot water then need to be circulated with a small electric pump to your hot water cylinder.

However, this process depends entirely on the integrity of the heat pipe and the vacuum around it.

⚠️ Common Issues with Evacuated Tube Systems

1. Loss  of Heat Pipe Liquid
   If your system’s circulation pump fails and the system overheats, the liquid inside the heat pipes can boil away. Once that happens, that  tube can no longer transfer heat, rendering it useless.
2. Loss  of Vacuum
   Just like a thermos flask, evacuated tubes rely on a perfect vacuum for insulation. Over time, the vacuum can degrade. When that happens, the tube’s efficiency drops dramatically, and the collector loses its ability to retain heat. On a cold day, this could even cool down your tank costing you more money.
3. Overheating  and Leaks
   Thermal systems can easily overheat during summer, leading to excessive   pressure and water leaks. This is one of the most common service issues owners face with evacuated tube systems.

⚠️ Pump Failure

Pumps are mechanical of nature and due to wear and tear.These will often break down, causing overheating and damage to your system.  Regular maintenance will be necessary which can be costly over time. Pumps are grid powered and don’t work during power outages which can cause damage, leaks and destruction of the heat pipes, due to over temperature.

🌤️ Misunderstanding the Aperture Area

Many assume the entire surface area of the evacuated tubes collects sunlight — but in reality, only a narrow strip does.

• Each evacuated tube absorbs sunlight over roughly 2 cm × 1.7 m aperture area..
• Thus, a 20-tube system will only have about 0.4 m² × 1.7 m = 0.68 m² of actual sunlight collection area. On a very good day, the sun emits only 1kw of power per 1m² of aperture area. A 20 tube evacuated collector will then only give you a maximum of 0.68kw of power per hour on a perfect day.

Compare this to a solar electric water heating system,where 6 solar panels can provide 10.7 m² of active aperture area — over 15times more effective sunlight collection! On a perfect day, this system will deliver up to 2.7kw of power to your water.  In reality, both systems will only give you around 90% of this performance on a good NZ day.

☀️ Summer vs Winter Performance

Thermal solar systems often overheat in summer and under perform in winter. This seasonal imbalance means they generate the most heat when you need it least.

In contrast, PV solar water heating systems (like those offered by One Energy) work efficiently all year round. They:

• Maintain  up to 4× better performance in winter.
• Operate even on cold, clear days — as low as -25°C.
• Actually perform better in cooler temperatures, since photovoltaic cells produce more power when they’re cold.

🔋 Annual Yield Comparison

Because PV systems have a much larger aperture area and no thermal losses from overheating or circulation inefficiencies, their annual energy yield can far exceed that of evacuated tube systems, even up to x4 time in mid-winter— especially in New Zealand’s variable climate.

Evacuated tubes needs perfect sunny conditions to perform well, with high output in summer which can lead to overheating and very low efficiency in winter which gets even worst as the heat pipe fluid degrades. Photovoltaic water heating systems, using electricity generated by solar panels to power electric water heating elements and will be less affected by cold weather. Since photovoltaic (PV) panels work with light and not heat, they can continue to deliver strong performance year-round. PV panels also respond better to diffuse light, making them more reliable in cloudy or cold conditions.

🔧 Maintenance & Longevity

• Evacuated Tube Systems: Typically last 10–15 years with periodic maintenance, including checking pumps, replacing antifreeze fluid, and     repairing leaks or failed tubes.
• PV Solar Water Heating: Expected lifespan of 25–30 years with no maintenance required — no pumps, no moving parts, and no liquid to leak or evaporate.

🌎 The Future Is PV Solar Water Heating

PV-based systems represent the next generation of solar water heating — more durable, more efficient, and better suited to New Zealand’s climate. They deliver consistent performance all year, even in cold or cloudy conditions, without the maintenance headaches of older thermal systems.

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