Urban development questions present complex decision contexts concerning ill-defined problems, in particular when integrated sustainable functioning is pursued [32,33]. Therefore, environmental assessments should in this context be complemented by an evaluation in the social, economic and policy/process realms, e.g., [5,6,7,8].

For the present discussion, we will group these dimensions together into a second step that aims at putting sharp the picture of integrated sustainable functioning. As a detailed analysis of sustainable urban development evaluation is beyond the scope of this article, we summarize a methodological approach based on multimodal system analysis to frame the present discussion [7,8,34,35,36]. The multimodal system theory states that reality manifests itself in human experience through independent modal aspects or law-spheres. In fact, these modal aspects can roughly be considered as knowledge fields. Herman Dooyeweerd, founder of the theory, identified 15 such spheres that logically succeed each other as follows: the numerical, the spatial, the kinetic, the physical, the biological, the sensitive-psychical, the analytical-logical, the historical-cultural, the linguistic-communicative, the social, the economic, the aesthetic, the jural, the ethical and the credal law-sphere [34]. Sphere sovereignty on the one hand and functional links between the law-spheres on the other hand, are two important characteristics of the modal aspects. For example, a law-sphere cannot exist without its ‘substrate’ of preceding spheres, but at the same time it is fundamentally different from the latter. Hereby the character of the modal aspects evolves from determinative (ruled by the laws of mathematics, physics or biology) to normative (regulated by human convention). Dooyeweerd’s theory has been successfully applied in, among others, system sciences and sustainable urban development studies [7,35,36]. For the present analysis we thus propose an aspectual lecture of reality that has been interpreted in a way to formulate a series of principles for sustainable (urban) development (SD) [8]:

Based on the research mentioned higher, we opinion that a method considering these principles of extended sustainability assessment is beneficial for setting up a well-balanced design or planning process. In particular, the method allows situating questions of energy provision or intervention in the built environment in a wider landscape of mutual interferences. It facilitates structuring the question at hand through in-depth analysis, and thus helps to pave the way for informed assessment, design, and decision making.

When considering the impact of urban energy use in buildings, several contributing factors may be distinguished as represented in Figure 1, based on [40,41]. The impact is determined by the quantities of the different energy uses multiplied by the environmental impact of every used energy type. When looking at the typical ranges of difference between good and bad performance, the factors displayed in Figure 1 appear. It is to be noted that simply multiplying the separate limit values for obtaining the overall variation range is not correct, since the contributing factors influence each other. Rebounds, for example low energy bills, may cause inhabitants of a passive house to waste more energy through lighting and household appliances.

Among the above factors, urban morphology and building design should be considered as the pre-eminent passive design variables that can make a difference with regard to sustainable performance.

Estimations on the role of urban morphology in terms of building textures vary considerably. Ratti, Baker and Steemers arrived at variations in the order of magnitude of 10%, considering heating and cooling, lighting and ventilation [41]; whereas Salat identified 80% differences for heating energy and concludes, more generally, that urban morphology can make up for a factor 2 in energy use variations [40,42].

In addition, this analysis does not yet include higher order design parameters concerning location choice and the related effects of site, relief, vegetation and local climate. This is in fact a parameter that has been historically important, but later ignored, especially in the era of technological innovation and urban expansion that has come with the industrial revolution. If any, the focus in the past was on energy conservation through careful site planning, climate-responsive building techniques and proper use of vegetation and water structures [43,44,45] (Figure 2). A renewed interest in these aspects can now be found in studies inquiring the influence of vegetation on the urban climate, and in particular the urban heat island [46,47,48]. In a similar way, vegetation and water structures are gaining wider interest for their potential contribution to climate adaptation strategies within cities.

Increasing urban compactness must hereby not become an absolute principle. There are quantitative as well as qualitative reasons for restricting the compaction to certain limits. First, energy consumption in compact and deep structures will increase by the rising need for artificial lighting and mechanical ventilation ([41], pp. 772–773). Second, qualitative requirements like the access to daylight in living and working spaces or the sufficient availability of open, green areas within the city put equally a limit on the compactness of the urban texture. Consequently, defining optimum morphology and compactness is an extremely difficult and location-specific task, regardless of the energy or sustainability parameters under consideration.

While urban morphology is an aspect that has until recently remained undervalued, the influence of the individual building design is better known. Orientation, glazing ratios, building envelope characteristics and thermal capacity define the major passive impacts on energy use [23,49].

When considering the influence of urban morphology on energy needs for transport, general propositions such as the hyperbolic correlation between urban density and personal transport energy derived by Newman and Kenworthy [50] may be challenged in particular local situations, not least because transport user choices and behavior are fairly unpredictable. Urban compactness appears to facilitate, but not necessarily to guarantee, reduced transport energy demand. The estimation of related transport impacts, for example at the neighborhood scale, thus becomes a daunting task. Surveys have proven to be a way of monitoring the real outcomes of the many mutually interfering aspects of urban transport. Examples hereof can be found in some flagship sustainable neighborhood projects, e.g., Hammarby Sjöstad (Stockholm) ([51], pp. 6–7) and BedZED (London) ([52], pp. 26–28). For Hammarby, a compact brownfield redevelopment with good public transport connections to Stockholm’s city center, CO2-emissions related to private car use were found to be 50% lower than for reference mobility patterns. In a similar way, private car mileage in BedZED was estimated to drop some 65% under the British average.

From these observations, we may conclude that the general principles of passive urban design appear as relatively robust, including transport related issues, but that detailed studies are essential when intervening in particular contexts. These detailed studies will depend upon proper methodologies and data sets at the planning stage of an urban project. Apart from environmental and cost efficiencies, a number of qualitative parameters have to be taken into account, as discussed in Section 2.2.

It should be noted that compact urban and building structures also facilitate savings in the required amounts of construction materials (e.g., [53]). Research on building insulation has indicated that the energetic pay-back times of insulating materials are relatively short [16]. In conclusion, these types of passive energy-saving measures can concur well with the overall goal of reducing total environmental impact, and therefore the first action of the stepped strategy remains principally uncontested.

As argued above, it is essential to differentiate between energy qualities when considering building related energy streams. Taking into account the observations made in Section 2, a fairly new branch of research called energy potential mapping investigates the opportunities for optimized exergetic use of available energy sources at the urban and regional scale levels [20,24,31,54]. Hereby the spatial aspect of energy potentials becomes critical for two reasons.

First, electricity and heat (or cold) are characterized by different transportation and storage possibilities. In basic terms and considering the present state of affairs, this may be resumed as electricity being easy to transport and difficult to store, while the opposite applies to heat and cold [19]. As a result, local heat and cold applications are feasible within the built environment, while the generation of electricity can be delocalized at larger distances from the consumers where appropriate. Some nuances must be made, however. In the future, electricity storage may become more feasible, for example by using car batteries as a storage medium within smart grids (e.g., [55]). Exploiting beneficial interferences between electricity and heat/cold applications is another aspect that gains importance (e.g., [56]). The upscaled storage of heat and cold can be achieved with buffer tanks of increasing size, e.g., concrete or polymer tanks containing up to several thousands of m³ of water (e.g., [57], Figure 3), but also by using the underground as a free reservoir through the different types of geothermal storage (aquifer-, soil-, or cavity-based).

This leads to a second aspect illustrating the importance of spatial planning in the energy equation: all of the building related energy infrastructures must find their place either on or below the ground. Spatial arrangements and distances between applications thus become an important consideration. Moreover, as it was the case for the reduction of the energy demand, an optimization in the environmental sense alone will not be sufficient to arrive at sustainable solutions. Spatial planning involves many other social, economic, juridical as well as historic, cultural and aesthetic aspects having a decisive influence on the feasibility of any proposed intervention. This complex exercise leads to urban or regional energy plans such as worked out, for example, in the Dutch SREX and Lowex research projects (e.g., [31], Figure 4).

Two types of application, waste heat recovery and exchange of heat and cold, deserve particular attention. Waste heat recovery is most feasible from high temperature industrial and electricity generation processes, and enables the cascading of heat through low temperature applications in the built environment. Heat exchange between building programs, on the other hand, is possible between net suppliers and demanders. Another optional but related strategy is to store excess heat or cold until it can be used. Here, the more known shallow geothermal applications come in view and perhaps gain more momentum in the near future. One example is to manage the demand and supply of both heat and cold on the neighborhood or district level.

District heating and cooling networks are defined by similar operational conditions. The heat or cold demand should be sufficiently high and spatially concentrated to allow for satisfactory efficiency, both in environmental and in economic terms. Regarding the important question if district heating remains interesting in combination with low energy building districts, a Danish case study points out that this can be the case if transmission losses in the network are reduced to the maximum, combined with the use of heat storage tanks in the serviced buildings [18]. The opposite situation appears in historic town centers, where it is difficult to upgrade all of the building patrimony to low energy or passive standards. Here it may be efficient to invest in district heating and cooling, while at the same time topping the efforts in building renovation to levels that are environmentally responsible, yet culturally acceptable and economically feasible.

Therefore, first of all, an environmental evaluation should allow formulating adequate and sufficiently precise trade-offs between the available applications. Which scenario delivers the best energy/exergy/LCA-scores? Economic, social and other qualitative assessments should then build further on the outcome of this environmental evaluation.

In practice, a deeper level of restructuring and life cycles must be addressed as well. Since the built environment has a high permanence, questions regarding the very long term (more than 100 years) should be considered. This is linked to the fact that the rotation in the built environment is typically around one or a few per cent per year (e.g., [2], p.59). How shall the building stock evolve to realize increasing sustainable performance? This means that, for example, the following type of problem needs an address: is renovation the best option; is it demolition and rebuilding; or is it demolition and building at another location? A similar reflection should be made concerning energy infrastructures whose life cycle may span across even longer periods of time [25]. Each of these options may deliver other preferable energy systems.

The fact that both the energy and exergy efficiency of the current transport sector are poor (e.g., [28]), further emphasizes the need to address mobility as well, and this at all levels (demand, modal switch, technology).

Renewable energy provision may take place at three scale levels, which can be conceptualized as follows:

In theory, a purely technical trade-off could define which applications are most favorable in a given design context. As mentioned for the other constituents, there are however many more factors to be considered. This results in a complex decision making process, especially for urban planning concerned with long term development. In how far may one, for example, expect that a smart grid or super grid will be available within a few decades?

At the supranational scale, research has indicated that Europe and its neighbors can have electricity based for 100% on renewable sources under the condition that a super grid is created throughout Europe and its neighboring territories (in particular North Africa and parts of Russia) [58]. Therefore, technically spoken, this option delivers an important potential for filling in urban demands that cannot be met locally or within the proper regional context.

In a case study from Austria, Wächter et al. stress that “Critical issues are region-specific production of energy and its use, settlement and regional structures and values and role models, which all have a determining influence on energy demand.” ([59], p. 193). Energy-intensive land use patterns (urban sprawl,…) are a hard nut to crack when it comes to rendering these structures more sustainable: both settlement structures with their mobility patterns, and energy infrastructures, should be profoundly transformed over the coming decades.

In another case study for a renewable energy system in the region of the Peat Colonies in the Netherlands, a strategy was proposed where areas with high renewable energy potential will have to invest less in energy savings whereas areas with small renewable energy potentials will have to focus on savings even more [60]. This proposal—to determine ambition targets for both renewable energy provision and energy savings based on the local energy potentials—once again stresses the need to map energy potentials prior to any decision with regards to alternative energy systems.

Diversity is another relevant criterion to be added for robust energy systems at all scales [61]. It implies that sources and technologies replace each other when failures appear or shortages occur. In addition, a diversified energy system can deal with intermittencies between energy supply and demand.

From this discussion it becomes clear that the third step—renewable energy provision—goes beyond individual interventions at the building or urban scale. Urban energy problems should, at all times, be considered within their regional energy landscapes. Amongst other benefits, this accommodates for the low density (or high demand for land) of most renewable energy sources. Cities should, however, also be enabled to connect to future super grids. This is especially important as macro-scale electricity generation presents itself as an environmentally efficient means of energy provision that can compensate for the lack of resources within cities and towns.