Solar water heater performance in winter

Characteristics of Solar Water Heater Performance During Winter
The graph shows the first ten days of February 2024.

Contents

• Theory
• Graph Commentary
• Related Articles

Theory
The operation of solar water heaters in winter is a common question, as many people worry whether the reduced sunlight will affect their performance. Solar water heaters work normally in winter, but their performance is reduced compared to summer when there is more sunlight.

During the winter months, the water may not reach the same temperature as in summer, but it still heats up sufficiently for daily use. This is because, even on cloudy days, the sun produces radiation that is absorbed by the collectors. Modern solar water heaters come with backup solutions, such as electric resistances or central heating systems, to ensure a continuous supply of hot water, even when sunlight is limited.

In summary, solar water heaters can operate year-round, but on days with reduced sunlight, backup solutions are necessary to meet the hot water heating needs.

To demonstrate the performance of a solar water heater throughout the year, and especially in winter, the live monitoring project of the solar temperature was created. You can view the water temperature in the tank (DHW), the air temperature, which is the most critical factor for performance, and the heating of water in the solar collectors (apart from the presence of solar radiation), as well as the brightness based on an LDR sensor. In addition to the current values, there are graphs showing the historical data from the start of the data logger’s operation.

Graph Commentary
In Case A, we have a day where the ambient temperature was negative (-3.2°C), yet throughout the day, there was intense sunshine with no cloud interruptions, and a significant increase in the water temperature was observed, rising from 24.38°C to 52.19°C. A day that started with negative temperatures in early February saw the water “gain” about 28 degrees. This confirms that during the winter, solar water heaters perform very well, and when there is strong sunshine, the performance approaches that of the summer months, with the only difference being that the ambient temperature was much lower, with the maximum reaching only 11.5°C during the day. On the other hand, the sun’s rays are nearly vertical relative to the solar collectors at this time of year.

In Case B, we have a set of 3 days where there was a significant increase in the water temperature (20-22°C), with higher ambient temperatures. During these days, the temperature during the day ranged from 7°C to 14°C. However, the solar performance was lower, which can be explained by the presence of cloud cover. The LDR sensor graph proves that these days had unstable cloud coverage. In conclusion, solar heaters perform in winter, even on days with intermittent cloud cover.

In Case C, we have a day with heavy cloud cover in early February 2024. In the middle of winter, with intense cloud cover, the solar water heater gained 9°C, reaching 24°C from 15°C. While this may not be sufficient for a hot bath, 160 liters at 24°C can still be very useful for washing dishes and other domestic hot water uses, such as feeding a washing machine with water from the solar water heater. Read this article for more information on this topic.