1. Introduction and Motivation
The existing built environment embodies a large potential within the German governmental energy policy goals of halving primary energy need and increasing the ratio of renewable energy to gross final energy consumption to 60% by 2050 [1
]. Buildings represent almost 40% of primary energy consumption in Germany, of which almost 70% consists of thermal energy. Most of Germany’s current stock of 19 million residential buildings is, partially or not, energetically renovated. While up to 80% of their energy need could be offset [2
], building renovation rates have not reached targeted goals for many years. Time-consuming planning due to cost-intensive accompanying measures and complex boundary conditions required to comply with the strict normative for energy savings (“Energieeinsparverordnung”, in short EnEV) play a role in this [3
]. The current ordinance allows a primary energy balance for renovated buildings of 140% compared to a reference building for a new construction [4
]. The calculation of primary energy need takes into account both the quality of the thermal envelope and the efficiency of the energy supply chain, from generation to storage, conversion, and distribution. Building installations and the choice of energy systems thus play a decisive role in the evaluation of renovation concepts.
An array of different energy technologies for renewable as well as decentralized energy generation, efficient energy storage and distribution are both available on the market and being currently developed. There is, nevertheless, no integral concept for their physical connection and networked operation as a building energy system. Improving the thermal envelope, on the other hand, has proven controversial due to aesthetic limitations, problematic end-of-life scenarios (insulation creates waste with no residual value, which is also difficult to separate and recycle [5
]), incompatibility with historical preservation, fire hazards, and the need for extensive modification of construction details (i.e., roof projections).
The approach developed in SWIVT, an acronym for “district energy modules for existing residential areas—impulses for linking energy efficient technologies” (in German “Siedlungsbausteine für bestehende Wohnquartiere—Impulse zur Vernetzung energieeffizienter Technologien”), focuses on entire districts instead of single buildings in order to exploit stochastic and synergetic potentials of an efficient local energy generation and distribution system. The developed approach combines interventions to reduce the energy demand of the current building stock with its physical and operational connection within a local heat network and power micro-grid equipped with energy generation and storage technologies. The district’s operation is regulated through an energy-management unit called SWIVT-Controller, which optimizes energy flows through a control strategy. Thanks to the flexibilities offered by the components and the control strategy, the system can react to volatility in renewable energy generation and consumption, while optimizing energy efficiency and operational costs. The joint project, led by TU Darmstadt and developed in collaboration with University of Stuttgart and AKASOL GmbH, a supplier of lithium-ion batteries, sets the following goals: the use of technological innovations in energy efficiency for the built environment, the linking of different disciplines and actors, and the development of new methods for monitoring and energy management. Figure 1
shows a diagram of the SWIVT concept.
Buildings with different thermal qualities can be connected within a district heating distribution system and supplied efficiently at different working temperatures, thus minimizing the need for renovation measures. Roofs and cellars, currently excluded from profit generation, can be rented out to the district operator for the placement of energy system components. The developed control strategy can maximize self-consumption of locally generated energy and avoid generating any unused heat, thus minimizing overall system losses. Other than selling energy to tenants, the investment can be supported by offering flexible energy supply and capacity in power markets. Thanks to better resilience of the system to fluctuations in energy availability, the share of renewably generated energy within the power distribution network can be increased without expensive conversion measures. Thanks to clustering flexibilities at the distribution level, the necessary computing power to address complex forecasts of balancing energy capacities at the power transmission level can be reduced. By introducing a new actor who sets-up, operates, and maintains the energy system of the district, energy providers can remain in the value-chain while housing companies can focus on their physical assets. The creation of a feasible investment model can help increase market penetration of innovative energy technologies, such as storage systems.
A successfully realized example of energy efficiency in existing residential buildings is the project Lichterfelde in Berlin. The buildings were renovated and equipped with energy generation and storage components, as well as an energy management unit. The system delivers a net balance of heating supply from renewable sources [6
]. However, by addressing single buildings, the concept misses essential synergies, especially from the perspective of electrical energy. The role of smart districts [7
] within a power network supplied by a high share of renewable energy has been explored by different research projects. In particular, the model developed for the city of Mannheim in the project “moma” foresees clusters of buildings serving as flexible energy units within greater balancing regions [8
]. The essential role of transparent but still secure digital communications between flexible energy units was one focal point of the project Flex4Energy, which develops an open platform for regional energy markets [9
]. The district envisioned in the SWIVT project would find here its ideal application.
This paper presents the approach to building renovation developed within the SWIVT concept. Interdependent parameters between renovation measures, the addition of new, energy-efficient living area, and different thermal and electrical systems on district scale are explored through three scenarios—High-Exergy, Mid-Exergy, and Low-Exergy—developed for an existing residential area in Darmstadt, Germany. Findings support the implementation of an energy system on district scale as a more resource-efficient solution for building renovations than the improvement of the thermal envelope of individual buildings. The primary energy balance of existing residential areas can be improved by more than 30% compared to a standard renovation through minimal intervention measures on the existing building stock, thanks to an efficient energy supply system on district scale and the connection of its components within an operational strategy.
2. Documentation of Current Building Stock on Project Site Moltkestraße 3 to 19 in Darmstadt
The project site is located in the district Bessungen in Darmstadt and consists of five multifamily social housing buildings property of the consortium Bauverein AG, with a total living floor area of 3.630 m2
on a 10.453 m2
plot. The northeast to southwest oriented linear apartment blocks were built in the years 1949–1952 and consist of two- to three-floor-high lightweight concrete constructions with saddle roofs and cellars. The small residential district has a total of 87 apartments with an average of 23.4 m2
per resident. The lot is suitable for densification measures and for the introduction of new communal functions in the currently underused green areas between the buildings [10
]. The original plans divide the buildings in three categories: Ledigenheim I and II (literal translation: home for unmarried), Types E/F, and Type P. Ledigenheim I and II (LI and LII) consist of 1-person apartments accessed through an open gallery on the northeast side. The two twin constructions of Types E/F consist of 2–3-person apartments spanning the whole width of the construction and connected vertically through three stairwells per building. Both categories have small southwest balconies. Type P consists of a 2-floor construction with four 3-person apartments and no balconies, with a northwest to southeast orientation. This categorization is clearly recognizable in the floorplans of the buildings, shown on site in Figure 2
The buildings are not renovated except for a window replacement in the 1980s and the application of roof insulation where the living area was extended to the attic floor, which is the case for two apartments per block in buildings of Types E/F. The building envelope presents poor thermal quality, leakage, and condensation problems. Ceilings, staircases and outer doors are insufficiently acoustically insulated and necessitate fire-proofing measures, while apartments lack barrier-free access. Deficiencies of the façade were determined through a detailed inspection with an infrared camera performed at 5 a.m. on 4 February 2015. Strong losses through the outer walls were observed together with thermal bridges, especially pronounced along the balconies, the window lintels, and the cellar floor. While helping with the analysis of site-specific deficiencies, these images, shown in Figure 3
a,b, show a typical picture of the conditions of the non-renovated building stock from the 1950s to the 1960s.
Thermal energy is supplied through a floor gas heating system. LI and LII have significantly higher gas consumption. LII supplies LI through two constant temperature boilers located in the cellar. The pipes are in good condition and largely insulated, but the control is not efficient. Both boilers are always running together and working temperatures are fixed, so that high amounts of unnecessary heat is produced. The heat-delivering elements are large-volume radiators with manual control valves. Type P’s high heating demand can be justified by a larger surface area to its volume. Figure 4
shows the energy consumption for the five buildings according to the evaluation of project partners ENTEGA AG (Darmstadt, Germany) the energy provider, and of Bauverein AG, (Darmstadt, Germany) the housing company. The district’s energy demand balanced on net floor area is 211 kWh/m2
for heating and warm water, which corresponds to the typical consumption for buildings of the same typology and location based on EN ISO 15316 [11
] and 49 kWh/m2
for power demand.
3. Setup and Nature of the Study
SWIVT develops its specific approach to integral planning through a design methodology based on scenarios consisting of four modules: energy load, energy generation, energy conversion, and energy storage. Involved stakeholders agree on the requirements for each module at the start of the concept phase, taking into account mutually influencing parameters in order to create different design scenarios. This enables disciplines to develop specific approaches and solutions, while collaborating to the same overall goals. Three design scenarios were developed: High-Exergy (S1), Mid-Exergy (S2), and Low-Exergy (S3). These are evaluated against two reference scenarios: current stock (R1) and standard renovation (R2). Energy demand in R1 and R2 are benchmarked on measured data provided by ENTEGA AG. R2 represents the renovation of four buildings in Moltkestraße 27–37 performed in 2009, which achieves a total primary energy balance of 56 kWh/m2
]. Each design scenario should reach a 30% lower energy balance than the traditional renovation approach benchmarked by R2. An optimized implementation scenario, which will be built on site in the follow-up pilot project SWIVT II, projected to start in 2018, will be designed based on the results collected through the evaluation of the three design scenarios. The five scenarios and the requirements set within the modules are shown in Table 1
The parametric study presented in the manuscript is carried out within a mathematical model developed in Microsoft Excel. The model integrates parameters from the building concept with parameters from the energy concept to show how different thermal loads for the renovated surface affect primary energy need on a district level. Within the model, it is possible to evaluate the interaction between differently renovated and new, energy-efficient living areas. Selected variables are the extension of each type of area and the final thermal load for the renovated portion, while the thermal load per square meter of new living area is kept constant. The next step will be a modeling study with the software Hottgenroth Energieberater 18599. The study will evaluate the necessary building refurbishment measures to meet thermal loads for each type of renovated living area according to the implementation scenarios. The overall aim is to prove a more efficient strategy for investing economic and ecological resources through minimal renovation and the use of renewable generated energy on site, against a standard renovation concept based on façade insulation.