Efficiency drop of evacuated tubes on cloudy winter days?

Understanding Evacuated Tube Efficiency

Evacuated tube solar collectors are designed to capture direct solar radiation, but they can be affected by cloudy winter days. On these days, the reduced solar radiation and increased diffuse light can lead to a decrease in efficiency. This is because the evacuated tubes are optimized for direct radiation, and the diffuse light can scatter and be lost.

Mitigating Efficiency Loss

One way to mitigate the efficiency loss on cloudy winter days is to use a higher concentration of evacuated tubes. By increasing the number of tubes, you can capture more of the available solar radiation, including the diffuse light. This can be particularly effective if you have a larger roof or a more extensive solar array. For example, a 10 kW system might be reduced to 8 kW on a cloudy day, but a 20 kW system would only be reduced to 16 kW, maintaining a higher overall efficiency.

Additional Techniques for Efficiency Improvement

Another technique to improve efficiency on cloudy winter days is to incorporate a heat exchanger into the system. A heat exchanger can transfer heat from the evacuated tubes to a secondary fluid, such as water or glycol, which can then be used for space heating or domestic hot water. This can be particularly effective in systems with a high load, such as those in colder climates or with a high demand for hot water. By using a heat exchanger, you can maintain a consistent temperature and reduce the impact of efficiency loss on cloudy days.