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Solar energy in the winter: the decisive factors
The sun represents the source of energy for your solar power system. The greater the number of rays of sun that land on the solar modules of a photovoltaic system, the more electricity it generates. The amount of usable solar power produced is called the solar yield. However, this yield depends on other factors as well:
- The closer the system is to the equator, the greater the global radiation. This is the proportion of solar energy that actually reaches the earth’s surface. This is because the rays of sun have to cover the lowest distance to the earth’s surface at the equator.
- The higher the solar power system is positioned, the greater the global radiation. The path of the sun’s rays is shorter than to a system located in a valley, for example.
- The steeper the angle of the sun above the solar power system, the greater the global radiation. The potential energy production is therefore highest at noon, but lower in the morning and evening. What’s more, in the northern hemisphere the sun is generally lower in the sky in the winter than in the summer.
There are two common types of solar power systems available to private households:
- A photovoltaic system has solar modules to convert solar energy into electricity that can be used for any purpose. The colder months of the year account for just 25 to 30 percent of total yield.
- The situation is similar for a solar thermal system. This type of installation has collectors that convert solar energy into heat. This heat can then be used to provide hot water for your household needs. With the appropriate output, this system can fully supply a single-family home during the warmer months of the year, i.e. from May to September. The colder it gets, the less suitable a solar thermal system as a primary source of energy.
Solar power production: a calculation example for Switzerland
Here in Switzerland, the topographical conditions vary a great deal: we have shady valleys, hilly midlands and high alpine areas. The potential for harnessing solar energy differs depending on the location. On average, the figure stands at 1,100 kilowatt hours per square meter (kWh/m2) per year. At extreme altitudes, an average of up to 1,600 kWh/m2 is possible – roughly the same as in sunny Spain, for example.
The following is a theoretical comparison of the expected electricity production between Winterthur on the Central Plateau and Jeizinen in the Valais canton. It is based on a sample calculation by WWF Switzerland. The values are given in kilowatt hours (kWh) and apply to a photovoltaic system with a capacity of one kilowatt peak (kWp), which in private households corresponds approximately to a roof area of seven to ten square meters equipped with solar modules.
- 30 kWh
- 90 kWh
- 50 kWh
- 85 kWh
- 80 kWh
- 115 kWh
- 100 kWh
- 130 kWh
- 110 kWh
- 115 kWh
- 115 kWh
- 105 kWh
- 125 kWh
- 105 kWh
- 120 kWh
- 120 kWh
- 85 kWh
- 115 kWh
- 70 kWh
- 125 kWh
- 30 kWh
- 115 kWh
- 25 kWh
- 100 kWh
- 940 kWh
- 1320 kWh
As the temperature falls, production in both regions decreases. In Jeizinen, however, the overall yield is around 40 percent higher. In addition, the loss fluctuates much less during the cold season.
By way of comparison, an average four-person household in Switzerland has an annual electricity consumption of approximately 5,000 kWh. This household would therefore need a good five times as many solar modules as in the Winterthur example above. The system would therefore need a capacity of at least five kWp.
Solar power systems: optimal orientation
Solar power systems can be oriented by means of an adjustment screw to ensure efficiency – particularly in the winter. A distinction is made between two different types of solar radiation in a photovoltaic system:
- Direct solar radiation is the most efficient. It consists of rays of sun that land directly on the modules of the system.
- Diffuse radiation consists of rays of sun that are scattered before they hit the modules of the system. The rays may have to pass through dust, for example, or through water vapor present in fog and clouds. Diffuse radiation is still used by the system, but the yield is lower.
In order to make effective use of direct solar radiation at all times, in theory the angle of inclination of the solar power system should be modified in different seasons – for example via adjustable solar modules. This is because the sun is lower in this part of the world in the winter. The ideal year-round angle of inclination, i.e. a compromise for summer and winter, is 30 to 35 degrees. The system makes very efficient use of the sunlight falling at a steep angle in the summer months, but also makes sufficient use of the weaker, more horizontal radiation in the winter months.
The higher up you live, the more you have to tilt the solar modules to achieve the highest yield. An angle of 55 to 65 degrees is recommended for locations in the Alps – such as Jeizinen in our example, which is located at around 2,000 meters above sea level.
A different approach to alignment is required for a solar thermal system. Unlike electricity from a photovoltaic system, which you need all year round, the heating warmth of a solar thermal system is only required in the winter. You should therefore optimize the collectors for the winter months. And this in turn means that you should always set the system at a steeper angle of inclination, i.e. 60 to 70 degrees.
The influence of snow
Given that Switzerland in general is at a higher altitude than other European countries, many people in Switzerland are affected by greater quantities of snow. It could be assumed that a layer of snow would damage a solar power system. But this is not normally the case.
Light snow does not affect the stability or functionality of a photovoltaic system. The sun’s energy-supplying UV rays easily pass through a 15-centimeter thick layer of snow. You will only notice a slight production loss in the single-digit percentage range.
On the other hand, large quantities of new snow and changing old snow are a danger. For example, ten centimeters of powdery snow weigh about ten kilograms per square meter. Depending on atmospheric conditions and age, snow can become wet over time. The same layer then weighs 40 kilograms. If it freezes, the load could be as high as 90 kilograms. This can quickly become too much for the sensitive solar modules. Depending on the construction, this could also be a critical value for your whole roof.
So, depending on the weather conditions, judge for yourself whether you should relieve the solar modules of the load.
Attention: never scrape ice from the surface, as this could damage the sensitive technology. It will melt by itself in strong sunlight.
Owners of a solar thermal system don’t need to worry about snow. The resulting energy loss is also only marginal. However, after extreme snowfall, you should remove the snow from the collectors.
The most important questions and answers at a glance
Can I continue to operate my solar power system in the winter?
You should definitely operate your solar power system in the winter, as it can still harness the sun’s energy during the cold months and convert it to meet your requirements.
Does the sun shine as strongly in the winter as in the summer?
In this part of the world, the sun is lower in the sky in the winter than in the summer, so the sun’s radiation is lower.
Does a solar power system produce as much energy in the winter as in the summer?
Due to the lower solar radiation, the system will yield less energy. In low-lying regions, the yield of a photovoltaic system in the winter can be as low as a quarter of the yield in the summer.
Can I improve the performance of my solar power system in the winter?
The angle of inclination is the crucial factor when it comes to performance. For a photovoltaic system made up of solar modules, a compromise value suitable for both the summer and the winter is 30 to 35 degrees. The solar collectors of a solar thermal system should always be set at a steeper angle to ensure the most effective absorption of the rays of sun, since the sun is lower in the sky in the winter. This will allow it to produce the heat urgently needed for heating.
Should I remove snow from my solar power system in the winter?
After a light fall of snow, you don’t need to do anything because UV rays can penetrate the layer of snow. However, large and wet masses of snow should be removed to relieve the load from the solar power system in relation to the roof.
Conclusion: use solar energy all year round
Whether it’s cold or warm outside, whether the sun’s shining or it’s snowing – photovoltaic and solar thermal systems will both continue to perform well, even in the winter. Depending on how low or high you live above sea level, it’s worth adjusting the angle of inclination of your solar modules or solar collectors.