Innovative energy solutions in plus-energy buildings

Municipal housing company Stockholmshem built two plus-energy buildings in Stockholm Royal Seaport, with occupancy from 2019. The two buildings, with 43 rental apartments are designed to be net energy producers with the help of solar panels, geothermal heat, efficient insulation and ventilation, and recycling of excess heat from wastewater. The architecture features balconies built into the facade and angled roofs to maximize solar energy usage, offering a unique design. In this cutting-edge project, there is an innovative combination of about twenty energy solutions.

Buildings account for approximately 40 per cent of global energy use and also consume significant amounts of water, building materials, and other resources. Designing residential buildings that generate more energy than they consume represents a cutting-edge solution.

• Local renewable energy meets a large part of the buildings’ energy needs, and together with energy-efficient architectural design and heat recovery, these buildings qualify as plus-energy buildings.
• Technological solutions allow residents to minimise electricity and hot water usage.

Implementation and technology choices

Stockholm Royal Seaport was identified in Stockholm’s environmental program in 2008 as the city’s next sustainability-profiled area. In 2010, the overarching policy document was adopted by the city council with objectives including transforming the district into an attractive, resource-efficient, fossil-fuel-free area and an international model in sustainable urban development. The area is a testbed for innovative tools, methods, and work processes that can later be scaled up to other parts of the city.

Land allocation competitions are one of the innovation-driving tools for achieving these goals. In 2014, Stockholmshem won a land allocation competition in the Brofästet phase in Stockholm Royal Seaport. The focus of the competition was to create plus-energy buildings through smart energy solutions and a functional architecture. Out of 16 participating housing companies, Stockholmshem scored the highest overall points. The project was awarded for its “holistic approach to energy efficiency and good living environments with interesting and exciting architectural expression”.

The buildings are designed with angled roofs to maximize the benefits of the sun as an energy source, balconies that are built into the facade, and cutting-edge energy and environmental performance. Stockholmshem gathered experts from various fields and combined solutions from previous development projects, which were further developed and combined in new ways to create synergies in a new comprehensive solution.

Stockholmshem has collaborated with, among others, the architectural firm Dinell Johansson, the energy engineer Incoord, the structural engineer Kåver & Mellin, and Tema landscape architects, focusing on sustainable building and good living environments.

To meet the requirements for a plus-energy building, the building’s energy use needed to be minimised and the technology optimised. Energy calculations show that the building uses approximately 12.3 kWh/m² per year (according to BBR), including heating, hot water and building electricity. The solar cell production is estimated at about 18.2 kWh/m² per year, meaning the building will produce more energy over a year than it consumes.

Architecture and building design

The compact design of the plus-energy buildings with tight climate shells and adapted for a range of energy-technical solutions are fundamental for meeting the energy requirements. Together with a wastewater heat exchanger and geothermal heating, this forms a functional whole.

The buildings have an optimal shape factor with angled roofs to maximize electricity production and small areas exposed to weather and wind, thereby minimising energy losses.

The exterior walls are straight without niches and angles. The buildings’ roof ridges have been rotated to provide directly south-facing roof surfaces, optimised to capture the sun's rays. The roof angle of 30 degrees is within the optimal range for high-efficiency solar cells with silicon technology. On the opposite side, the roof is covered with sedum plants that delay stormwater’s path to Husarviken through a so-called rain garden – a shallow depression in the landscape with a draining system and a filter material covered with plants.

Sun-facing gable facades are clad with solar cells using less shade-sensitive thin-film technology. North-facing gable facades have trellis panels for climbing plants.

The actively heated living area is minimised but supplemented with an unheated living zone in the form of glazed balconies along the long facades. This provides a compact, functional residence during the winter half of the year which is significantly expanded during the summer half with areas heated naturally. By using sunshades such as curtains along the balconies, heat can be let in during the winter and stored in the balconies’ concrete structure, but reflected away during the summer.

Plus-energy building energy production and usage

The installation consultant Incoord, who designed for Stockholmshem, primarily worked to minimise energy losses, secondly optimised internal heat loads, and finally addressed the remaining energy needs with technically energy-efficient solutions. Detailed engineering was required to minimise energy use.

The heat from cooling the space with the solar cells' inverters is used to preheat the heat pump and, in the best case, energy is stored in the rock.

The house produces its own energy

About 30 per cent of the solar cells' electricity production can be used directly in the building's electricity needs for lighting, heat pumps, and fans. The remaining electricity production is sold on the grid and balanced against purchased electricity during the cold season. Over the year, the solar cells produce more energy than the building needs.

The tenants' electricity subscriptions are initially signed by Stockholmshem with Bra Miljöval (Good Environmental Choice).

Heat storage

Internal heat loads are optimised by utilising the heat generated by residents and technical equipment. Here, the thermal storage capacity in the buildings’ concrete structure provides a good condition.

Surface-efficient apartments relative to the number of residents have been a goal in the effort to maximise the effect of internal heat loads. This has resulted in spatially well-utilised four-room, three-room, and two-room apartments. Optimised airflow and thus smaller air volumes to heat make it easier to utilise internal heat sources.

Reuse of heat

Heat exchangers for heat recovery from ventilation air and wastewater play an important role in utilising waste heat. Additionally, the glazed concrete balconies create a kind of 'microclimate zone' where the concrete stores the heat from the sun's rays at night.

Not just technology – tenants also need to know what to do

For a plus-energy building to function as intended, tenants must understand how to ventilate and what it means to be away during the cold season. Therefore, Stockholmshem actively engages with its tenants about what is required to live in a plus-energy building.

Morten Johansson, architect at DinellJohansson, and Torbjörn Kumlin, project manager at Stockholmshem, discuss the plus energy buildings in Brofästet. (Swedish only)

Potential and future scaling opportunities

Building energy-efficient residential buildings that also produce most or more of their own energy needs is a challenge. Experiences from this project can be spread and facilitate for other actors in similar upcoming projects. The plus-energy buildings are important milestones where energy-smart solutions are combined with exciting architectural forms.

Land allocation competitions have proven to be an important tool for challenging developers to perform even better through collaboration of various disciplines at early stages. At least a hundred people from contractor, consultant, and client sectors engaged creatively to develop competitive proposals.

The buildings' innovative character had already made a significant impact in national media and trade press for urban development, building, property management, and energy issues before occupancy.

The ambition for the plus-energy buildings is set high, and monitoring the energy performance is an important part to ensure they meet expectations and draw lessons for future development and requirements setting in Stockholm Royal Seaport and the City of Stockholm.

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Article Published: 10/05/2023 Updated: 27/06/2024