Techno-economic investigation of the potential for energy efficiency measures of multi-story apartment buildings at cluster level using different district heating tariffs

 

1. Introduction

Reducing the energy use of the building sector in the European Union plays a central role to achieve projected energy and climate goals. About 35% of the existing building stock is more than 50 years old and roughly 75% is energy inefficient compared to today’s performance requirements. For this issue, a concept of cost-effectiveness has set energy performance requirements for existing buildings subject to major renovations, where at least a cost-optimal balance between investments involved and energy costs saved should be achieved throughout the life cycle of a building. To accelerate and scale up building renovations, the amended energy performance of buildings and energy efficiency directive address measures supporting investment activities and strengthening long-term strategies on national levels with identified cost-effective approaches relevant to building type.

Apartment buildings are significant segments of national residential building stocks. While new buildings’ energy use follows modern code requirements, older stock consists mainly of low-performance buildings, for instance, constructed in the 1960s and ’70s before the appearance of performance requirements and with unrealized potential for energy savings. Many of these buildings are characterized by standardized solutions with industrial prefabricated constructions including uniform block designs with horizontal lines of windows or balconies, multiple stories and dwellings, and produced in neighborhoods with the same type all over a country. Besides commonly used age and size classes to describe residential building stocks the characteristics and occurrence in neighborhoods also enables categories (or typologies) with regard to plan design of apartments or urban zones of cities .

District heating (DH) is the preferred energy carrier for apartment building stocks in many European countries with cold climate conditions . The concept of DH with heat distributed via a network of pipes to substations and final use in buildings (e.g., space heating) provides a variety of heat supply options, such as combined heat and power (CHP) plants, industrial heat supply, or heat-only boilers In Sweden, where the studied buildings are located, the apartment building stock used 27.7 TWh in 2016 for space heating and domestic hot water (DHW) and DH covered 91% of the energy use .DH is characterized by local system conditions and different price models with tariffs for fixed component and variable load and energy components

2. Selection of buildings and studied cases

The buildings used to investigate the techno-economic potential for EEMs were selected from the time period 1965–74, since it is assumed that these buildings will be renovated over the next ten-year period as part of Sweden’s national strategy for energy efficient renovation. Known as the “Million Homes” program, about one million dwellings were built during that time period using standardized solutions mainly in residential apartment buildings (66%)  which today embody almost 20% of the existing Swedish dwelling stock. As shown the dominant building type with a proportion of about 85% was slab blocks constructed in various numbers of stories. More than half of the dwellings were built in three-story slab blocks and six to eight-story tower blocks (higher slab blocks). Since many of these new buildings were immediately connected to DH systems, the Million Homes program is emphasized as an important driving force for the expansion of DH in Sweden

Buildings were studied in the municipalities of Gävle and Sandviken with a population of over 130,000 inhabitants, located in Gävleborg County in east-central Sweden. Only climate data for Gävle (latitude 60.679, longitude 17.183) with an average annual external temperature of 5.8 °C was considered for both municipalities, due to short geographical distance. Buildings were treated as overall building stock and clustered to facilitate decision-making regarding slab and tower blocks with various numbers of stories. It was assumed that existing buildings would undergo inevitable renovations, but not with focus on energy renovation to be able to study the full potential for EEMs.