Direct Use vs. Indirect Use
The sun is by far the most important source of energy on Earth. It creates a favorable environment for all forms of life and also provides the basis for technological energy production. In a sense, solar energy is even stored in fossil fuels. When it comes to technological energy production, it is not only important that solar energy is utilized, but also how it is utilized.
From a physical perspective, a simple principle applies:
The more direct the utilization, the greater the efficiency - or, in other words, direct utilization is the most efficient.
Every conversion involves losses and therefore reduces the amount of energy that can actually be used.
Direct utilization primarily refers to solar thermal energy - that is, the direct conversion of solar radiation into heat. Depending on the system and operating conditions, it can achieve very high levels of efficiency and is particularly well suited to water heating, space heating, and process heat applications.
Why thermal utilization plays virtually no role in Germany - the self-proclaimed “land of the energy transition” - remains a mystery. Could this possibly be related to the difficulty of regulating and taxing the direct utilization of thermal energy?
Passive utilization also plays an important role. Window areas, building orientation, thermal mass, and architectural design can make solar heat directly usable without requiring any technical conversion at all.
Solarthermie
Solar thermal energy uses the sun’s energy directly to generate heat. Unlike photovoltaics, it does not involve conversion into electrical energy. This offers several physical and energy-related advantages, as well as very high efficiency - up to 90%, depending on the type of collector:
- Flat-plate collectors: approximately 50–70%
- Evacuated-tube collectors: approximately 60–80%
- Low-temperature applications: up to approximately 90%
Solar thermal energy is considered one of the most environmentally friendly and, from a physical perspective, most efficient ways of using solar energy.
It is ideal for:
- water and space heating
- local and district heating
- industrial process heat
Existing roof and building surfaces are well suited to the installation of solar collectors.
Solar thermal energy offers a high power density, can be used locally, and can be readily stored in the form of heat.
Photovoltaics (PV) — flexible, but with significantly lower efficiency
Photovoltaic systems convert sunlight into electrical energy. However, their efficiency is significantly lower than that of direct solar thermal utilization. A large proportion of the incident solar energy is converted into heat rather than electricity or is lost through other system-related processes.
For this reason, photovoltaics is particularly advantageous in locations where a significant amount of electricity is actually required. If the electricity generated is subsequently used to produce heat, this is considerably less efficient from an energy perspective than direct solar thermal utilization.
Typical efficiency levels
- Standard modules: 18–22%
- High-efficiency modules: 22–24%
- Complete systems, including losses: 12–18%
The greatest benefit—and the most efficient use of solar energy—can be achieved through a decentralized combination of solar thermal and photovoltaic systems, including electricity storage, installed on existing roof and building surfaces.
Ground-Mounted Photovoltaic Systems
Ground-mounted photovoltaic (PV) systems enable the large-scale utilization of solar energy. At the same time, however, they have a considerable direct impact on the landscape, energy flows, and land use. An objective assessment must therefore also consider their physical and systemic effects.
The risk of total system failure due to storm or hail damage, lightning strikes, or fire—as well as the resulting potential for soil contamination—must also be taken into account.
Low Power Density and High Land Use
A key factor is the comparatively low power density of solar radiation. Taking into account:
- the sun’s daily and annual path
- cloud cover and shading
- the spacing between rows of modules
the average power density of ground-mounted PV systems is generally only about 5–20 W/m².
Impact on the Local Energy Balance
Ground-mounted PV systems alter the distribution of energy at ground level through:
- less direct solar radiation reaching the ground
- altered heating of the ground surface
- significantly greater heating of the air above the modules
- changes in evaporation conditions
- altered air movements near the ground
A portion of the incident solar radiation is converted into electrical energy and removed from the local system. Most of the remaining energy is converted into heat and re-radiated, thereby changing the local radiation and heat balance.
Systemic Analysis
Ground-mounted PV systems generate electricity only when sufficient sunlight is available. This results in:
- significant fluctuations over the course of the day and throughout the year
- low electricity generation during winter
- substantial storage requirements
- additional grid expansion and extremely high associated costs
These factors reduce the proportion of the generated energy that can actually be used.
Alternative: Utilizing Existing Areas
It is often more efficient to use areas that have already been developed or sealed, such as:
- roofs and façades
- parking lots and industrial sites
- noise barriers
This approach avoids additional land consumption.
Conclusion
Ground-mounted photovoltaic systems can contribute to the energy supply but are associated with several significant disadvantages:
- low energy yield per unit area and consequently high land requirements
- impacts on the landscape and land use
- changes to the local energy balance, with potential climatic effects
- highly variable electricity generation
- substantial storage requirements
- risk of damage from storms and other extreme weather events
From an energy perspective, it is therefore often more sensible to prioritize the use of existing roof and building surfaces in combination with storage systems, as well as the most direct possible thermal utilization of solar energy, rather than pursuing the large-scale expansion of ground-mounted PV systems.
