Associating Solar Energy and Combined-heat-power (CHP) Systems in the Urban Environment

District heating (DH) systems represent a major contribution to local energy production, especially considering countries where heat energy play an essential role in ensuring comfortable conditions for urban activities. Generally, in the urban environment DH becomes part of a regional or local CHP network that is combined with electricity production in CHP plants. These facilities also tend to be commonly fuelled by biomass. Inside the smart-grid, the local CHP function as a member of the aggregator actors, who are responsible for the optimal production of power and for connecting the various scales within the energy structure. In this context, heat is produced for the local DH grid, while the aggregator distributes electricity as a by-product to national or continental power grids. Yet, this does not determine that municipalities are not able advocate for their say in the system over the aspects that are most impactful for the local communities and producers.

Though, usually driven by residential consumption, the development of CHP systems can also be motivated by industrial processes that demand high heat consumption, as an example from the glass houses and refineries in the Netherlands. As a result two common types of CHP seem to stand out : a residential one guiding heat production for household purposes and an industrial one that generates surplus heat from industrial processes. But with the further development of RES technologies such as heat pumps and solar heat panels, with the latter being more widely broadcasted and adaptable to most modern urban sites, the heat energy originated from RES can become more integrated to the energy structure also via CHP.

When it comes to solar power, one can find two types of energy harvesting equipment: photovoltaic cells (PV) and solar thermal panels. The PV cells are usually more expensive as they are able to convert sunlight directly into electricity, while the thermal panels are usually associated with residential and district heating systems. Both devices are wide spreading in countries and cities where SET policy measures support the development and implementation of such technologies. At a large scale PV cells and solar thermal panels have high relevance for energy production, turning large size sites into solar farms. An example of this potential is pointed by Groth et al. : “one of the world’s largest reservoirs of warm water heated by solar panels has been established in the Danish town of Vojens. The reservoir has a capacity of 200 mil litres of water, heated by 4,166 solar panels with a joint surface of 52,500 m2. The reservoir was established in a former gravel pit. The solar panels are added to 17,500 m2 panels that are already established, extending the total surface up to 70,000 m2. The system is going to supply energy for 2,000 households in the city.”[1]

In the context of dense urban environments, the extensive land requirement for solar energy generation created the need for finding alternative location where this resource could be harvested, calling the attention to surfaces inherent to the built environment such as building’s roofs and facades. In the Netherlands, another kind of surfaces that is currently being tested are roads and bicycle lanes in a project called ‘Solaroad’ that gathers multi-scale actors such as research institutions, technology industries and local and national governments.


The case of Freighburg

Differently from the development trajectory of most cities in the post-war period , Freiburg was a pioneer of the concepts of ‘eco-city’, incorporating sustainable development principles in its urban development planning [2]. Following the oil crisis and the increasing awareness of the risks posed by unsustainable energy resources such as nuclear and fossil fuels, the  energy agenda of the city started being developed in the mid-1970s, nurturing the momentum that catalysed the local SET process that started in the following decade. With its own ‘energy supply concept’ , create by the city council in 1986, created its own CHP system that already prioritized energy production from renewable resources, together with the compromise towards eliminating nuclear and fossil-fuel based energies [3]. Alongside strong municipal strategies focused or RES incentives, investment has also been based on resident’s private financing, “which has helped to realise the potential of local solar and wind alongside other renewable forms, ultimately making it a leading German eco-city.” [4].

In modern Freiburg, the developed environmental policy integrates solar technology, sustainability and climate protection, which become impactful drivers of economic, social and political growth promoted through urban planning. The city also intensively invests in solar research and technology improvement as it hosts the Fraunhofer Institute for Solar Energy Systems ISE, which is the largest of this research field in Europe. The Institute develops materials, components, systems and methods for energy efficiency, generation, distribution and storage with the aim of delivering efficient and environmentally-friendly energy. Moreover, alongside the pilot projects of three solar power plants (Eichelbuck, Messe Freiburg and St. Gabriel) and many CHP systems and plants, the concepts of energy-saving and solar power optimized harnessing are incorporated in architectural designs and plans at initial stages or by the application of the mandatory Freiburg “Energy-efficient Housing Standard”. These various factors pushed for the installation of PV and solar thermal systems that harness the energy from the sun in many urban locations such as households, schools and public buildings. The bigger goal for Freiburg is that by the year 2050 100% of the energy utilized in the city needs to come from renewable resource.

An example of a successful integrated renewable resource approach is pointed in a document created by the city of Freiburg that provides an overview of its ecological competence: “ Freiburg‘s largest solar installation was erected there in 2011, with a total capacity of 2.5 megawatts peak (MWp), and therefore meets the annual electricity needs of around 1,000 households. And the landfill gas from the 50-metre high, former waste mountain is also used: mixed with biogas from the Reterra biogas plant, it is supplied to a co-generation plant in the Landwasser district and is used to generate electricity and heat. Since mid-2014, an innovative wood gas combined heat and power plant has also been in operation, increasing the proportion of renewable energy in Landwasser‘s total energy supply by c. 15 %. The CHP plant together with the two existing biogas and landfill gas motors means that a total of around 3,600 households can now be supplied with green electricity, and 780 households can also be supplied with heat. The wood gas generation module reduces CO2 emissions by around 750 t annually compared to conventional power plants and separate power generation. The annual CO2 savings compared to generation with conventional natural gas combined heat and power plants are around 330 t.” [5].


By Felipe Marcelino


Work referenced.

  1. Groth, N. & Fertner, C. & Große, J. (2016). Urban Energy Generation and the Role of Citie Journal of Settlements and Spatial Planning.
  2. Daseking, W. Freiburg: Principles of sustainable urbanism.
  3. Lange, J.; Ufheil, M.; Tanner, C. (2010). Expansion of cogeneration in the city of Freiburg.
  4. Sait, M. & Chigbu, U. & Hamiduddin, I. & De Vries, W. (2019). Renewable Energy as an Underutilised Resource in Cities: Germany’s ‘Energiewende’ and Lessons for Post-Brexit Cities in the United Kingdom.
  5. City of Freinurg (2018). Freiburg GreenCity – Approaches to Sustainability

Author: GEN