Energiebunker Hamburg—Ecological and Economic Sustainability of a War Relic?

Abstract

The Energiebunker in Hamburg-Wilhelmsburg used to be an abandoned and overgrown flak and air raid bunker—a relic of World War II. After the war, the interior of the bunker was blown up, and only the upper floors and the three- to- four meter thick concrete walls remained. For more than 60 years the building was not in use. The building was transformed during the International Building Exhibition in Hamburg between 2006 and 2013 and is now used to generate renewable electricity and heat for more than 1500 households. This adaptive reuse not only enabled the bunker to be integrated into the redesigned neighborhood, but also to fulfill certain societal needs. The historic building was conserved and given a new sustainable use. Nowadays it is a tourist attraction and a war memorial. This paper explores the economic and environmental sustainability of this transformation. Could this be a second use scenario for other bunker facilities from World War II?

1. Introduction

The adaptive reuse of the Energiebunker in the City of Hamburg in Germany is a unique project with flagship character. There are several examples of the adaptive reuse of old power plants around the world combining living, working, and consuming, e.g., the Gasometer Buildings in Vienna, Austria [1], Leipzig and Dresden, Germany [2] as well as the Battersea Power Station in London, UK [3]. The reuse of old buildings to produce energy, however, is rare. The Energiebunker in the city of Hamburg is a neglected defense facility reused as a plant to produce and provide renewable energy as electricity and heat for an urban neighborhood of the Wilhelmsburg district. The project offers a solution for the transition to renewables in cities and helps to raise public awareness for climate and environmental protection. In addition, it bestows a new life and purpose upon an important landmark and memorial, which would have otherwise fallen further into disrepair.

The Energiebunker was transformed as part of an energy efficient and social restructuring project in Wilhelmsburg that was initiated and implemented by the Internationale Bauausstellung (IBA; engl. = International Building Exhibition) between 2006 and 2013. Since 1901, the IBA has taken place irregularly in different cities around Germany [4]. The first IBAs outside Germany took place in Basel in 2010 and Vienna in 2016. The IBA is an exhibition for architecture and urban development to show new ideas and concepts for urban planning, however, nowadays also includes social, economic and ecological aspects. Each exhibition has a different aim and subject. As part of the IBA, projects were supported in the process of seeking funding from different sources and donors, e.g., the European Union. The city of Hamburg also financed a part of the IBA. Thanks to the success of the concept in Hamburg, the IBA Hamburg Ltd. is a 100 percent subsidiary of the City of Hamburg in Germany as an urban development company since 2014 [5].

The idea to reuse the bunker facility as a renewable energy plant is encouraged by the German Renewable Energy Law [6]. Overall, the German Energy Politics led to an increase in energy from renewable resources. In 2020 the share of renewable energy within Germany’s overall gross electricity production was 40 percent, ten times higher than in 1990 [7]. The new goal for 2030 is a 65 percent share of renewables on the overall gross electricity consumption in Germany [8]. The adaptive reuse of big buildings, like neglected industrial sites and defense facilities, is one possibility to bring renewable heat and electricity into dense urban neighborhoods, but also into rural areas, in order to reach this goal. There are hundreds of neglected and unused bunker facilities in German cities that could be given second lives as energy production sites. This article shows how bunker facilities can be reused to locally produce renewable energy and asks if this concept could be implemented in other German cities, too. For this research the authors used text analysis as well as on-site inspections in Hamburg-Wilhelmsburg. In addition, four semi-structured interviews with experts and representatives of the main actors were conducted.

The research activities of the authors are based on the activities of the ERASMUS+ Strategic Partnership for Higher Education LOTUS [9].

2. Materials and Method

This research is based on a literature review. An analysis of the relevant scientific literature, and literature detailing expert opinions, was conducted. However, as little has been published on the specific subject of the adaptive reuse of bunkers, the literature analysis was limited. The literature review concentrated on published informational material of the IBA and the literature on the conversion of bunkers to residential buildings. It was necessary to not only look into academic research but also consider newspaper articles, as well as websites of companies, like the IBA. Since there is not an official databank of existing bunkers, private communities of interest have to be considered for the analysis, too. Interestingly, the German National Committee of ICOMOS has not any guidelines published on the historic preservation of bunker or war facilities, and it seems like this is not (yet) a field of interest.

Besides the literature review, the authors were able to visit the city of Hamburg and the project area on a fieldtrip. Hamburg Energie organized a guided tour on the Energiebunker exclusively for the authors. Thus, the guide was able to show particularly interesting aspects of the technology and answer questions in detail. Furthermore, the authors visited the information center on the Energieberg, several other IBA projects in Wilhelmsburg and the IBA Dock in Hamburg Germany to experience the district themselves and obtain information in the field, besides that available in the literature.

In addition, several semi-structured interviews with relevant actors of the Energiebunker project were conducted from June to October 2022. Due to privacy restrictions, it is not possible to give more detailed information on the background of the interviewees. Since the project is very small, any further information on the interviewees would allow for backtracking. They prefer to remain anonymous.

3. Project Area

Hamburg is not only a city, but also one of Germany’s federal states. It therefore might have different opportunities than other municipalities to get funding for bigger projects like the Energiebunker. Hamburg is part of the Carbon Neutral Cities Alliance and signed the Chicago Climate Charter [10]. Thereby, the city committed itself to the national climate protection objectives, the goals of the Paris Agreement, and a proactive role within the city’s and state’s own possibilities. Hamburg aims for greenhouse gas neutrality in 2050 and an interim goal of a 55 percent reduction (or of 11,400 t) in the CO₂-emmisson in the year 2030 compared to 1990.

Hamburg is extremely vulnerable to the effects of climate change due to its geographical location. It is located in the North German Plain at only 6 m above sea level on average at the estuary of the river Elbe and its branches. The tidal range is visible although the city is located around 100 km from the North Sea. Tidal surges can impact the lower parts of Hamburg, especially in areas near the harbor in the north-west between Norder- und Süderelbe, such as the district of Wilhemsburg, and the areas next to the Elbe and its side branches [11,12]. As a result of climate change, heavy rainfall events will increase, which will lead to more flooding. Due to Hamburg’s location close to the North Sea at the estuary of the river Elbe, the rising sea level will be a serious problem for the city, its economy, and quality of life. For example, the expected increase in upstream sedimentation will affect the harbor and its routes, and lead to more maintenance work [9]. Between the years 1881 and 2013 the average temperature in Hamburg has risen by 1.4 °C [13]. Depending on the greenhouse gas emission the average temperature will rise between 1 to 5 °C by the year 2100 compared to the period of 1961 to 1990 [14].

Due to Hamburg’s vulnerability, climate change was one of three leading topics of the IBA that took place between 2006 and 2013. The City in Climate Change aimed for the ecological renewal of the city of Hamburg [15]. The spatial focus was on the Elb island of Wilhelmsburg, as this part of the city is especially impacted by climate change. Furthermore, it was a neglected district with numerous buildings in need of renovation. The neighborhood had the image of a transport corridor connecting the old city of Hamburg in the north and Harburg in the south. The district was especially interesting for the IBA and its projects due to its unique prerequisites. With 53,064 inhabitants in 2020 [16] it is the biggest district in the city, situated in the South between two wide arms of the river Elbe (Figure 1). In the Northwest, directly neighboring the district, lies the international seaport of Hamburg—Europe’s third largest container port. A lot of industry is located in this area, too. Furthermore, the district is very heterogeneous: around 32 percent of its inhabitants have non-German nationalities and 61 percent a migrant background [16]. Density and types of residential buildings vary in size—from single homes to apartment buildings—time period and architecture. In a study of the IBA Hamburg, 26 different types of urban space were identified in the district [17]. Wilhelmsburg is not an affluent district. Around 20 percent of the inhabitants received government support in 2020 and 10.4 percent of the residents between 15 and 65 years of age are unemployed [16]. The area of the Reiherstiegviertel, close to the Energiebunker, is characterized by poverty and aggravated living conditions [18]. Here the IBA project Weltquartier was implemented as a partnering project to the Energiebunker.

The Weltquartier project is located in the West of the Energiebunker and consists of several apartment buildings from the 1930s owned by the housing association SAGA [19]. These buildings were renovated, redesigned, and energetically modernized with the help of the IBA. In addition to these measures, the buildings are also connected to the heating network of the Energiebunker, reducing the energy costs even more.

The Energiebunker is one of more than 60 projects aimed at the production of renewable energy and education, implemented by the IBA in the Wilhelmsburg district [15].

4. The Energiebunker

4.1. Project Framework

The Energiebunker in Hamburg-Wilhelmsburg was a flak and air raid bunker built in 1943 by forced laborers [15]. It was built to protect civilians during air strikes, and also had four flak guns in the upper floors. The dimensions of the building are 47 × 47 m on a wider pedestal of 57 m × 57 m with an original height of 42 m. To build the bunker, 80,000 m³ of reinforced concrete were used. It is bigger than most bunkers from World War II and was not only used to protect the civilian population. Its main purpose was for the ground-based air defense of Hamburg as an important industrial site and the demonstration of defensibility of the German people. Of its nine floors, only the second and third were used as a shelter during its time of operation from 1943 to 1945. It included a military hospital and, on the roof-top level, four rotatable main guns. Although the bunker was hit by bombs several times although never severely damaged, the surrounding district of Wilhelmsburg and the harbor were destroyed heavily during the war. In 1947 the inside of the bunker was blasted by the British allies. The building, which was a particularly decrepit landmark of the district, was not in use for more than 60 years. The city of Hamburg, its owner, considered demolishing it. According to one interviewee, the costs of demolition would have been around 12 to 13 million Euros. Ideas of renovation and transformation to preserve this landmark grew. In 2001, the bunker was listed as a Denkmal to remember the war’s atrocities as well as a Mahnmal, urging to prevent tragic events like this [15]. Demolition was no longer an option. A first concept to reuse the bunker and its big roof top to produce solar energy, the so-called Solarbunker, was created by the urban development department of the city of Hamburg in 2004 and was refined with the help of the IBA in 2006. The renovation would not, however, have been economical at this point, especially as in 2008 it became known that the statics of the building were very fragile. The bunker was a danger to the only tenant, a beverages market, and the people living nearby. Eviction was necessary as well as the renovation. As part of the IBA and its various workshops the concept for the Energiebunker was developed until 2009. This was not an easy process, as the renovation and conversion had to follow the strict rules of the preservation order. This allowed only minimal intervention into the building’s outer appearance as well as requiring the concept to be economically viable and efficient.

After 62 years of vacancy and neglect, the ruin was redesigned as a project of the IBA [21]: dark and threatening in earlier times, today the building seems friendly and is an architecturally interesting historic site (Figure 2). The transformation of the building was accomplished by an architectural office from Hesse, Germany.

Altering the outline of the bunker was forbidden due to a preservation order [14]; therefore, the solar panels hover over the roof and facade of the building on a steel structure. The old outer walls had to be covered with shotcrete to prevent concrete spalling. Nevertheless, 600 m² of the old bitumen colored facade is still visible through vision panels. The large new window on the side of the building opens up the structure and demonstrates transparency. Not only did the preservation order have to be considered within this holistic development approach, and issues around conservation had to be taken on board as a bat colony was found to be living in the bunker and had to be protected. Civic participation was also an issue. Three project dialogues were organized to raise awareness and acceptance for the Energiebunker’s renovation within civil society and among local residents.

The architects’ concept did not only concentrate on the reconstruction of the building. Rather, the opening of the Energiebunker to the public was also a central interest. Besides enjoying the architecture, citizens and tourists can experience the Energiebunker by visiting a permanent exhibition on its history and on the energy concept. The roof-top café offers a wide view of Wilhelmsburg and the harbor and is a nice location to linger (Figure 3). Both elements have contributed to the site being developed as a tourist attraction.

The costs of investment were 26.7 million Euros for the renovation of the building and the installation of the renewable energy production systems as well as the local heating network [15]. One interviewee explained that the project was mainly financed by the city of Hamburg, which provided around 15 million Euros for renovation and conversion. Other donors were the local energy supplier Hamburg Energie and different funds. Hamburg Energie estimated that they would be able to refinance their cost of 9.8 million Euros within 20 years by producing and selling energy [15]. The goal of the partners was to build a plant for sustainable energy production as well as opening the building to the public as a memorial site. After five years of renovation and conversion the Energiebunker was put into use in 2015. Not all engines had, however, been installed yet. Today the Energiebunker generates renewable electricity for around 1000 households and renewable heat for around 3000 households in the neighboring Reiherstiegviertel [15,21].

The project was integrated into the IBA’s overall holistic development concept. To maximize the impact, an area in the southern Reiherstiegviertel close to the Energiebunker was not only connected to the heating network, but also insulated to upgrade the buildings’ energy performance. As part of the so-called Weltquartier project, a residential estate with over 600 apartments from the 1930s was redesigned and energetically refurbished by the housing association SAGA with support from the IBA [22]. Several new buildings were added to the area and the open spaces were redesigned as common areas and private gardens for the tenants. The investment volume for the whole project was around 103 million Euros [19]. The consideration of social aspects was also important for the holistic approach of the IBA. Before the construction work started, the residents were invited to join several workshops, where they could make suggestions to the renovation plan and directly express their wishes for the new apartments. All residents were offered apartments from the time of construction as well as the possibility to move into the renovated buildings after the Weltquartier was finished. The renovation of the existing buildings reduced the energy needed for heating and hot water from 165 kWh/m²/a to 60 kWh/m²/a. The 600 apartments of the Weltquartier were the first connected to the Energiebunker’s local heating network [15]. As a result, the inhabitants save around 0.40 Euro/m² in heating and CO₂ emissions are reduced by 75 percent [23].

The Energiebunker and the Weltquartier are part of the IBA project Erneuerbares Wilhelmsburg (Renewable Wilhelmsburg) for the protection of the climate. The goal of this project is a climate neutral Elb island Wilhelmsburg and should be reached by providing all energy needed in the district by renewables [24]. Around 50 percent of the households used locally produced renewable electricity in 2015. The plan is to reach 100 percent by 2025, and the required heat should be locally sourced by 2050. The strategy includes: (1) a high standard for new buildings to save energy; (2) high energy efficiency through local energy networks and cogeneration units; which use combined heat and electricity (CHP); and (3) the integration and participation of local citizens through workshops and economic incentives. Local self-sufficiency is not feasible due to fluctuating electricity and heat generation of sun and wind renewables over the course of a year. However, a study from 2013 showed that a 100 percent supply for Wilhelmsburg through renewable energies is achievable in annual sums [25]. The surplus of energy was estimated up to 158 MW, while the deficit is approximately 25 MW. A solution is several buffer storages, as seen in the Energiebunker, and both the use and production of renewable energy all over Hamburg.

4.2. Energy Production

The bunker has been converted into an electricity and heat plant and storage space. Figure 4 shows how different technologies work together within the Energiebunker. Water is heated, i.a., with a cogeneration unit fueled by biogas from Hamburg’s wastewater treatment plant in Dradenau [15]. The unit produces 2700 MWh/a of electricity and 3750 MWh/a of heat. In addition, when plans were made in 2000, the installation a wood chip boiler was imagined, which should have been fueled by locally sourced wood chips. According to one interviewed expert, with an output of 10,500 MWh/a, this would have been producing around 50 percent of the Energiebunker’s heat. Due to the long time period between planning and the implementation of the project, however, another solution emerged. As wood and timber prices had been rising, an economical operation was no longer possible. In 2010 the city of Hamburg and Hamburg Energie also started to explore the usage of geothermal energy as a possible energy supply for Wilhelmsburg and other parts of the city. Planners therefore decided to use a cogeneration unit fueled by natural gas during the transitioning period. Furthermore, inconveniences produced by traffic, noise pollution, and a decrease in the quality of life for local residents arising from the wood chip supply was prevented.

According to an interviewee, industrial waste heat of glycerin distillation from the nearby Nordische Oelwerke (the Nordic oil plant) in Hamburg, Germany has been fed into the grid since 2015. It adds another 4000 MWh/a to the system of the Energiebunker [15]. Due to the connection to the heat network, the company does not have to cool its heat through an energy intensive process and therefore can save energy costs. In addition, the Nordische Oelwerke becomes even more environmentally friendly as less energy and water are used. An interview showed that the company did not have to invest into the heat exchanger, the connection, or the pipelines, which were all financed by Hamburg Energie. As a result of an evaluation and their environmentally conscious strategy, the company decided to invest to improve the process of transferring the waste heat. On the downside, the heat supply is not steady. It depends on the seasons: Less waste heat is produced at colder ambient temperatures, than at higher temperatures.

Furthermore, information from an interview showed that Germany’s biggest solar thermal system on a roof of 1350 m² of solar panels produces up to 5 percent of the Energiebunker’s heat. The panels are installed with a 15-degree angle facing south to minimize shadowing and maximize the surface and heat production. The photovoltaic system on the facade is installed in a 36-degee angle and generates electricity for the machines inside the Energiebunker and the surrounding neighborhood on a module surface of 670 m². The photovoltaic system produces 78 MWh/a. To buffer peak loads and guarantee supply in case the combined techniques do not work, secure natural gas condensing boilers within the Energiebunker can be used to generate another 3570 MWh/a [15].

To store the heat, a buffer storage of 2 million liters of hot water has been created inside the building. The buffer storage tank is 20 m in height and 12 m in diameter [15]. The water has different temperatures with the highest being 90 °C in the upper layer, where the heat is withdrawn. In general, the water is heated during the night with excess heat, which then can be used during the morning peak consumption. In theory, the buffer storage could bridge the heat supply for 18 h, in summer even for several days. Due to the variety of systems producing electricity and heat, the Energiebunker is very resistant to disruptions. If one system is failing, the other systems are still able to work and provide electricity and heat for the surrounding neighborhood. The buffer storage is also claimed to be energy self-sufficient. Any surplus energy is distributed into the grid. This cooperation of different systems and the size of the buffer storage are globally unique. Therefore, the project is supervised and evaluated scientifically to improve the technology for other locations [26]. Overall, the building produces 22.5 million kWh of heat and 3 million kWh of electricity every year [15] and conserves 4600 tons of climate-damaging CO₂, as stated by an interviewed expert. The heat is fed into the local heat network through hot water. A heat exchanger in every building transfers the heat into the internal heating systems so that it can be used for heating [15].

5. Bunker Facilities in Germany

Bunkers, today largely unused, are scattered all over Germany. The type of construction and initial use of these bunkers is affected mainly by the advancement of warfare technologies. Four different phases of construction, occurring in a chronological order and at times overlapping, can be distinguished. The first phase begins with World War I. Bunkers were built within combat zones as a part of static warfare strategy and to fortify trenches, which demarcated the enemy camps. In the interwar period, continuous border fortifications with battle bunkers were built to defend a country close to the border. The Ostwall and the Siegfried Line [27] were built in the German Reich, the Maginot Line in France [28] and the Alpine Wall, also called Vallo Alpino del Littorio, in Italy [29].

In the second phase bunkers were built mainly in cities. This was a reaction to the first British air raids in World War II. In October 1940, the dictator Adolf Hitler ordered the construction of several tens of thousands air raid shelters, the so-called Führer-Sofortprogramm [30]. The areas within the immediate range of the Royal Air Force had to be protected: the North German Plain, the Ruhr area as well as Berlin. At first, underground air raid shelters were built mainly due to a shortage of space for construction inside the city centers. As the British air raids left more and more properties in ruins, above-ground bunkers were built on these plots as the construction costs were considerably lower compared to the underground air raid shelters while having the same degree of protection [31]. In November 1941, 237 bunkers with space for almost 60,000 people had been completed in the city, 53 for 21,000 people were under construction and another 100 for approximately 94,000 people had been planned [32]. Some months later, the program started suffering from permanent shortages of building materials. The goal of 2000 bunkers demanded by Hitler for Berlin was never reached [30] and this gigantic construction program was never fully implemented.

The planning, design, or construction of shelter buildings had been initially completely banned in Germany by the Allied Military Government after the end of World War II [33]. The Cold War finally led to the third phase: civil shelter construction was discussed again due to the new threat of nuclear attacks. As a first measure, the remaining bunkers from World War II were restored and converted into living spaces for long-term residence under a nuclear blast and radioactive fallout. Moreover, the German state relied mainly on private building projects that were publicly funded. Until the end of the Cold War, nuclear shelters were built for only 3 percent of the population in West Germany [34].

As late as May 2007, the Conference of Interior Ministers (IMK) decided to abandon the nationwide public shelter program and to stop all public financing [35]. This was the starting point for the last phase of use of bunker facilities. A building type had lost its originally intended purpose. The question was how to deal with it: demolition or conversion? The facilities were and still are permanently empty and unused due to the generally high costs of demolition. There are, however, some examples that are reused and being converted into living spaces [36], shopping centers, exhibition places, club locations [37] or, as in the case of the Energiebunker, as renewable energy plants.

To assess the potential of the concept of the Energiebunker in Hamburg-Wilhelmsdorf for sustainable energy supply for other German cities, it is necessary to get an overview of the existing bunker facilities in Germany. In addition, essential information such as the type of bunker, the volume of the structure, the distance to the nearest heating network or the current use is required for the assessment of the individual bunker facilities. A study by the Hamburg University of Applied Sciences (HAW Hamburg) shows that, given the current state of technology, only above-ground bunkers can be considered for the construction of heat storage facilities [38]. Underground facilities are poorly suited for the installation of heat storage systems. The horizontal tube shape of these facilities is contrary to the mandatory vertical installation of a cylindrical hot water storage tank with the necessary dimensions. In addition, gaining access to underground bunkers is much more difficult. In the city of Hamburg, the aforementioned study assessed 16 facilities with high potential and 18 facilities with a limited potential [38].

There is not any central record of the bunkers existing in Germany today. Some experts have taken on the task of compiling and documenting the shelter facilities of the World War II and the Cold War. The result is the publicly accessible Civil Defense Facilities Database, which documents civil defense facilities built or restored for public and semi-public use in the Federal Republic of Germany and Berlin between 1955 and 2007 [39]. For almost 2400 bunkers, the name, geographical location, year of construction, type, and photos are provided. However, this database is not yet complete.

In addition, various depictions of bunkers in German cities can be found in individual sources. On the wiki page of the city of Hamm, for example, 11 above-ground bunkers are documented [40]. For the city of Bremen, a total of 50 bunker installations are described [41]. Many of these facilities are unused and could potentially be converted and integrated into a sustainable municipal energy concept.

6. A Brief Evaluation: Is the Energiebunker a Lighthouse Project?

The analysis of bunker sites in Germany shows that there are several large cities with similar above-ground bunker facilities. The experience gained from the construction of the Hamburg-Wilhelmsburg Energiebunker is particularly interesting for these cities. There is definite potential for the use of this concept in other cities and their bunker facilities in Germany.

To assess whether this approach of heat supply can be transferred in an economically and ecologically sensible way, an analysis of the economic and ecological sustainability of the Wilhelmsburg project is necessary. Due to the available resources this analysis can, however, only be carried out incompletely. The total cost of the project was 26.7 million Euros [15]. It should also be considered that upgrading the district by demolishing the bunker complex would have cost 12 to 13 million Euros, according to one interviewee. A 2014 estimate by the IBA states that these costs should be amortized within 20 years through the sales of the energy produced by the Energiebunker [15].

This estimate could be verified by analyzing the energy production and consumption in the district of the Weltquartier before and after commissioning the Energiebunker. The publicly owned local energy provider Hamburg Energie, however, refuses to publish the necessary data. The company cites company confidentiality or a lack of data availability. This is even more astonishing as the original approach of the IBA, to set “new standards for the metropolises of the 21st century” with this pilot project [22] (p. 5), cannot be achieved in this way.

The withholding of data also impairs the analysis of environmental sustainability. Only estimates of sustainable energy production, based on statements made in the interviews conducted and not on any specific published data, are known. For an exact calculation of CO₂-savings, for example, it would be necessary to have an exact representation of the energy mix of the heat and electricity supply in the Weltquartier prior to the commissioning of the Energiebunker. Nevertheless, some data have been published that give an indication of the ecological value of the project. Around 70 percent of its electricity and heat production is produced by renewable sources and 30 percent comes from natural gas, according to an interviewee. The Energiebunker produces around 22.5 million kWh of heat and 3 million kWh of electricity every year. Thereby, it is possible to supply about 3000 households with heat and 1000 households with electricity. The analysis shows that, in addition to the economic aspects, the Energiebunker also has a positive environmental impact.

With a strengths and weaknesses analysis, the significance of the Energiebunker for the transferability of these kinds of projects into other large cities with similar bunker facilities in dense urban areas can be shown. Our analysis shows that some project aspects have a negative impact on the economic and ecological balance. We identified the following weaknesses of the overall project for a sustainable Wilhelmsburg:

The whole planning process of the project Energiebunker and the linked project Erneuerbares Wilhelmsburg is a top-down process supported by participatory elements. Citizen participation was primarily limited to information and surveying citizens’ wishes regarding the redesign of residential and living spaces in the Weltquartier. Citizens of other areas of Wilhelmsburg were not included. One possible result is that the sustainable urban development of the district is still limited to the Energiebunker and the Weltquartier. Large parts of Wilhelmsburg are still in need of rehabilitation (Figure 1):

• At the beginning of the project, an information campaign was conducted for Wilhelmsburg homeowners with the aim of renovating the building stock [42]. This approach was obviously not sufficient to mobilize private capital. There does not seem to be any further plans to involve private homeowners at the moment;
• Except for the supply of waste heat from industrial processes of the Nordische Oelwerke, the involvement of private investment capital does not seem to play a role in the project. The possibility of financial participation by citizens is completely absent;
• An ex-post evaluation of the project has not been published. Hamburg Energie refused access to data on the grounds of trade secrecy. Data on energy production and consumption in the Weltquartier and entire Wilhelmsburg are not accessible. This lack of data naturally prevents a more accurate assessment of the transferability of the project to other German cities.

Besides the above-mentioned weaknesses, the project and its developing process show the following strengths:

• The project achieves an extraordinary combination of the preservation of a World War II memorial in line with socially oriented urban planning, a touristic upgrading of the property, and the development of a sustainable energy supply for the urban neighborhood of the Weltquartier;
• The Energiebunker and the associated heating network make an important contribution to climate protection in the neighborhood: they supply 3000 households with heat and 1000 households with electricity. There are plans to expand the heating network with geothermal energy. Locally produced energy will be a solution to a higher share of renewables and the demand of energy, especially in dense urban areas;
• The whole project has generated a lot of public and scientific attention. This attention contributes greatly to raising public awareness of the local population and the local economy about the consequences of climate change;
• The Energiebunker project is being reproduced within the city of Hamburg. In the district of Altona, it is planned to transform another bunker to meet today’s needs and use. The KulturEnergieBunkerAltonaProjekt (KEBAP, or in English, the Altona project for culture and Energiebunker) is a project, initiated bottom-up by civil society, to produce energy locally and to create space for cultural displays inside a neglected bunker [43]. KEBAP is financed directly by citizens who have founded a cooperative. Also built during World War II, the bunker measures 50 × 12 × 19 m and comprises two buildings next to each other. Until 2020 the building belonged to the German state, when it was bought by the city of Hamburg for public purposes. The change of ownership was due to the engagement of the KEBAP-initiative for more than 10 years. In 2021 the KEBAP initiative got the exclusive right for planning for the next two years. The western part of the bunker should be transformed into an energy plant, focusing on heat, while the eastern part will offer space for cultural displays. The commissioning is planned for 2025 with an annual heat production of 10 GWh per year. Around 1000 households, as well as a nearby swimming pool and school, will be connected to the bunker via the district and local heating networks.

The last point in particular shows that the city of Hamburg appraises the adaptive reuse of old elevated bunkers for heat storage and energy producing facilities as successful. It can be assumed that the planning of the Altona Energiebunker KEBAP also includes data that is not available to the authors of this article.

7. Conclusions

The project shows, that it is possible to adaptively reuse neglected buildings while respecting the preservation order. The listing of the bunker permanently saved and opened up space for ideas to preserve the building. The Energiebunker still remains a historical monument as its defense architecture style of World War II is still visible. Furthermore, it can be experienced by the public through the exhibition on its history, the roof top café and the surrounding park.

The conditions and environment, in which the Energiebunker was transformed, are in some regards unique: the nearby industry, big apartment buildings close to the Energiebunker belonging to the only owner, SAGA, and the support of the IBA were three main factors contributing to the successful implementation of the project. In addition, the architecture and shape of the bunker was very well suited for transformation into an energy plant. The absence of inner structures left a huge space inside to hold the necessary technology for the renewable energy plant. Its height and the absence of higher buildings around it, offered the opportunity to use both the roof and the façade for solar power production. Despite these unique prerequisites, this project is an example of best practice for other neglected historical buildings and not only elevated bunkers. It maximizes resources while using the environment’s natural and economic capital. Instead of demolishing big historic structures like this bunker, the construction can be reused, leading to the saving of building materials and resources. This adaptive reuse for today’s needs also preserved the building’s history while being environmentally and socially sustainable. The concept can be transferred to other places and therefore be a flagship project of good practice for the sustainable use of unused industrial buildings and defense facilities.

The Hamburg-Wilhelmsburg’s Energiebunker project has sufficient economic and ecological potential to be used as a blueprint by other cities with elevated bunker facilities. Precise data, however, must be made available for specific planning.