Solar thermal energy: from single family homes to multi-megawatt systems

A key technology for the transition of the heat market to renewable energy

When people think about solar energy in Germany, they usually think about electricity generated from photovoltaic solar panels (PV). In the media, heat generation from solar collectors tends to be of secondary interest, despite the fact that on a global scale solar thermal energy contributes much more for reducing CO2-emissions than PV. Systems for converting district heating grids and major heat generation plants to solar thermal energy will play a key role in the transition to renewable energy, the so-called ‘German Energiewende’, in the heat market. The Steinbeis Research Center for Solar and Sustainable Thermal Energy Systems (Solites) is involved in a variety of national and international research and market development projects.

The average German household uses 57 % of its energy for heating, 33 % for private transport and only 10 % for electricity (source: BMWi 04/2014). This underlines the significance of the heat market, as a “sleeping giant”.

Around 95 % of the solar heating systems sold in Germany are still mounted on one- or two-family houses. However, the market has been in gradual decline in recent years, partly also because of the increasing competition with PV installations. Today the prices of PV installations are close to the prices of solar heating systems. Under the German Renewable Energy Act (EEG) one can receive much more subsidies for the installation of PV systems than for solar heating systems. However, over 50 % of the potential of solar heat lies in the multi-family-building sector of the housing market. Also trade firms and industrial enterprises as well as district heating providers have an increasing interest in solar heat: The integration of solar heat into district heating systems can even help to supply whole quarters, villages and cities with heat from renewable energies and from other highly efficient energy sources. Also on a local level, there are many opportunities for municipal utilities or energy cooperatives to implement models of citizen participation and citizen finance. In whole Europe the interest in German technical knowhow is increasing: In the frame of EU-projects (such as Pimes, Einstein and Pitagoras), first pilot plants are in realization in Poland, Spain, Norway, Hungary, Italy and Austria. All of them are based on the technology that was used in the mid-1990s, when in Germany pilot plants for the use of solar district heat with seasonal heat storage were built.

Industrial enterprises and in particular district heating providers frequently operate combined heat and power plants (CHP). So they typically generate electricity from fossil fuels. The waste heat is usually used for their own heating supply or for district heating purposes. If a solar thermal plant provides solar heat and therefore reduces the operation time of a CHP plant, less electricity is produced. This shortfall then has to be covered by electricity from the German grid related to higher CO2-emissions. In some cases this can lead to the effect that the installation of solar thermal systems does not help to reduce CO2-emissions.

Despite this, the transition to renewable energy and thereby the large scale expansion of electricity generated from renewable energy sources (e. g. wind, photovoltaic and biogas) reduces the CO2-emissions caused by electricity generation in Germany in the medium term.

Further on, the summertime-use of CHP plants operated with fossil fuels is becoming more and more uneconomic because the generation of electricity gets increasingly supplied by renewable energies. Under German law, electricity from renewable sources has priority over electricity from fossil fuels. If CHP plants operated with fossil fuels can no longer supply the grid with electricity on an economically feasible base, they are switched off. The heat generation then has to be taken over by other sources.

For this, fossil fuel gas boilers as well as renewable energy systems can be used, especially biomass boilers and solar thermal plants. But from a strategic point of view, biomass will increasingly be needed to produce fuels and basic materials. So, just the production of solar heat remains, assuming it is more economic than to operate fossil fuel gas boilers. Also heat generated by geothermal energy systems has a high market potential in the long term. At present, detailed research is underway in this area. Managed by Solites and financed by the Baden-Wurttemberg Ministry for the Environment, experts are examining quality standards relating to the production of geothermal probes.

Compared to Germany, Denmark already has a much higher share of electricity from renewable wind energy in its electricity grid. Especially on windy days in summer, the entire Danish electricity demand can be met with renewable energy. As a result, fossil fuel-fired CHP plants have to be switched off and the numerous Danish district heating systems require a replacement heat generation. The most economical heat source in Denmark, delivering today heat at less than 50 Euro/MWh, are solar thermal plants in the megawatt range. Until now, 82 large solar thermal plants (with an output of more than 1 MWth) have been built in Europe. The installed capacity of large solar heat plants throughout Europe is 433 MWth (status Dec 2013). In 2013 alone, there was a 31.6 % increase in capacity.

Large multifunctional heat stores combined with such plants can store solar heat from the summer until the winter. In addition they can be used for storing waste heat taken from CHP plants, which are today operated by following the ups and downs in electricity prices. A further option is to store heat from overproduced renewable electricity following the power-to-heat principle. With the support of experts of Solites, first pilot plants using such “smart” district heating systems have been realized, for example in Marstal on the island of Aro and in Bradstrup (Denmark).

The Steinbeis research institute Solites is working on a variety of projects related to the German Energiewende, not only in electricity sector but especially in the field of thermal energy. This includes: basic R&D related to system development; the simulation-based development of technical and economic concepts, focusing on solar and flexible heat and power generation systems; support to the realization of pilot plants; knowledge transfer; market development projects; consulting of politicians and associations. These are examples of areas in which Solites can contribute with expert know-how on national and international level.


Dirk Mangold leads Solites – the Steinbeis Research Center for Solar and Sustainable Thermal Energy Systems. Together with Thomas Pauschinger he has been working on the research, development and application of solar heating systems since 1990, as well as on large solar thermal plants, energy-efficient buildings, solar district heating systems and seasonal heat store technologies.

Dirk Mangold, Thomas Pauschinger
Steinbeis Research Center for Solar and Sustainable Thermic Energy Systems (Solites) (Stuttgart) |

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