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Difference: solar thermal energy and photovoltaics



Whether it's a new building or an existing building, more and more homeowners are installing a solar system on their roof. The technology helps to reduce CO2 emissions. It reduces energy costs and creates more independence. You can choose between solar thermal energy and photovoltaics – but what distinguishes them from each other? What do the technologies have in common and, more importantly, which one is more worthwhile on a rooftop?

Solar systems generate heat or electricity

Solar thermal energy and photovoltaics are solar systems that make free energy from the sun usable in the house. Solar thermal systems generate heat like a rain barrel in summer. When the sun shines on the container, the water inside heats up. Solar thermal systems use the principle in collectors and direct the water into the house, where it is used for hot water preparation and/or heating support.

Photovoltaics generates electricity by using small cells in solar modules (including solar panels) to use the photoelectric effect. In this process, the radiation from the sun releases electrons from a special material. If an electrical consumer is connected to both sides of the cells, current flows that can operate electrical devices.

The following video explains the difference between photovoltaics and solar thermal energy: Simple & understandable.

 

Overview of the structure and function of solar thermal energy

In order to be able to use the free heat of solar thermal energy in your own home, various components are required. Collectors on the roof catch the light of the sun. They consist either of tubes arranged next to each other (tube collectors) or boxes (flat plate collectors) through which pipes run. Both species have dark-coloured absorbers that heat up in the light of the sun. The heat is transferred to a solar fluid, which circulates continuously between the collectors on the roof and a storage tank in the house. The storage tank, which contains drinking or heating water, absorbs the heat and keeps it in the house until it is needed.

Overview of the structure and function of photovoltaics

Just like solar thermal energy, photovoltaics also have components on the roof that capture the light of the sun. These are called modules (also called panels) and consist of numerous small solar cells. The latter obtain direct current from the sun's radiant energy, which an inverter converts into alternating current in the next step. This is necessary in order to be able to consume the electrical energy in one's own house or feed it into the public grid. While the components are already sufficient for the function of photovoltaics, another is becoming increasingly important: the solar power storage system. It absorbs electrical energy during the day and makes it possible to consume the self-generated solar power at night.

Tip: Find out more about "Is photovoltaics worth it?"

Solar thermal energy, photovoltaics or both?

Solar systems cover their own energy consumption from renewable sources. They save money, reduce the CO2 footprint and create a high level of energy independence. Since solar thermal and photovoltaic systems work best on the roof, competition arises there. This is because the available space is limited and both systems require sufficient space. So the question arises: "Solar thermal or photovoltaics: Which is more worthwhile on my roof?".

The following table shows similarities as well as differences and helps to make a decision.Solar thermal energy or photovoltaics: Differences at a glance

Solar thermal systems get by with just a few square meters

If solar thermal energy is used to heat water, a roof area of 1.0 to 1.5 square metres per person is sufficient. In addition, there is a storage tank that stores 100 liters of water for each roommate. For four people, 4 to 6 square meters of collectors and a 400-liter tank are required. Under favorable conditions, the technology saves around 60 percent of hot water and 5 percent of heating costs. The costs range from 3,500 to 5,000 euros.

If the system is to support the heating, a collector area of one square metre per 10 square metres of living space is required. In addition, there is a buffer tank with 50 to 70 litres per square metre of collector area. A 140-square-metre house therefore requires around 14 square metres of collector area and a storage tank with a capacity of at least 700 litres. Heating costs can be reduced by about 20 percent in this way. The costs of solar thermal energy for heating support are 8,000 to 12,000 euros.

Photovoltaic systems cause higher acquisition costs

In the case of a photovoltaic system, the size depends on the electricity consumption. For every 1,000 kilowatt hours per year, a system output of one kilowatt peak is recommended. This requires an area of 6 to 8 square meters. Together with a storage system that stores one kilowatt hour of electricity per kilowatt of power, photovoltaics saves around 60 percent of electricity costs (20 to 30 percent without electricity storage). The costs are around 8,000 to 9,000 euros without and 12,000 to 15,000 euros with electricity storage.

On large roofs, the combination of the solutions is possible

If the roof area is large enough, homeowners can combine photovoltaics and solar thermal energy well. For example, a solar system for hot water heating can be combined with photovoltaics and a heat pump to achieve a high degree of independence.

For smaller roofs, the space in the house (can a water reservoir be installed?) and/or the economic efficiency are decisive. Energy consultants and specialist tradesmen help to compare expenses and savings with each other in order to be able to make a decision.

If the roofs are shaded or north-facing for large parts of the year, neither solar thermal energy nor photovoltaics are worthwhile. In these cases, the solar irradiation is usually too low to generate sufficient energy.

 

Heating with the sun: solar thermal energy and photovoltaics possible

Heating with the sun is in vogue – sustainable concepts can be implemented with solar thermal energy and photovoltaics. The following overview shows solutions that have proven themselves in practice:

  • Solar thermal energy and gas heating: Solar thermal energy provides warm drinking or heating water here. If the free energy supply is not sufficient, the efficient gas condensing boiler switches on. It automatically ensures a high level of comfort and that there is never a lack of warmth. The solution is interesting for builders and renovators alike. The latter can also combine an existing gas heating system with solar thermal energy.
  • Solar thermal energy and pellet heating: Pellet heating systems work fully automatically and are also suitable for combination with a thermal solar system. While the latter is used for heating and/or hot water, the pellet system compensates for the lack of solar energy. Here, too, it is possible to combine new and existing pellet heating systems with solar thermal energy.
  • Solar thermal energy and heat pump: In this combination, the thermal solar system provides warm drinking water. The heat pump works longer at lower temperatures and thus consumes less electricity. If the solar energy is not sufficient, the solar heat pump also heats the drinking water.
  • Photovoltaics and heat pumps: The solar system generates electricity, which the heating system uses to make environmental energy usable. For the combination of heat pump and photovoltaics to work, the photovoltaic system must be as large as possible.
  • Photovoltaics and electric heating: With photovoltaics, electric heating is also worthwhile again. For example, more and more builders are opting to heat energy-saving efficiency houses with infrared heating panels and solar power. The advantage lies in lower acquisition and maintenance costs.
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