In densely built residential areas, it is worth generating renewable energy at local level. A report by VTT shows that this increases energy efficiency and sustainable housing construction, and leads to the production of ecological and optimally self-sufficient energy solutions. Seasonal thermal energy storage (or STES) can markedly reduce emissions from the heating of localbuildings.
All new buildings must be close to the level of zero-energy buildings by late 2020. The related challenges lie in the potential uses of renewable energy and achieving greater energy efficiency in new construction and housing. Local production of renewable energy brings considerable advantages, without placing unreasonable demands on individual buildings.
VTT's study presents options for energy generation based on local energy systems. An analysis of energy needs and production in Vartiosaari, Helsinki – based on ecologically, socially and culturally sustainable construction – was drawn up as a case study. A compact city district will be designed for 5,000-7,000 residents in an area of around 82 hectares. The overall floor area is about 300,000-350,000 square metres. The results of the study are widely applicable.
Energy needs and streams were analysed for two sets of building stock, comparing buildings erected in accordance with the 2012 building regulations and with the SunZEB construction model (a possible concept for energy-efficient construction in the future). The analysis suggests that the calculated heat requirement is only around 50% of the heat consumption of newer residential areas in Helsinki. A special feature of the concept lies in the fact that, when being cooled, the buildings act as a heat source for the heat pump. During cooling in the summer, renewable energy accumulating in the buildings is recovered and recycled back into the local district heating network.
The project studied the impact of introducing solar thermal energy on local self-sufficiency in heating energy, if excess solar heat in the summer is stored using BTES (borehole thermal energy storage) or TTES (tank thermal energy storage) for use in the winter. In both solutions, the optimal surface area of solar thermal energy collectors was 5% of the total floor area of the buildings included. Emissions from the heating of local buildings can be markedly reduced when thermal storage solutions are used. For example, when using BTES (borehole thermal energy storage), carbon emissions fall by more than 50% in an area built in accordance the 2012 regulations and around 40% in a SunZED-type area. Sulphur dioxide and particulate emissions can be reduced by up to 70% by using TTES (tank thermal energy storage). Self-sufficiency in electricity for buildings was examined in relation to the number of photovoltaic panels. The most efficient option when introducing solar electricity involves meeting 25% of the buildings' electricity needs using solar panels. This reduces emissions by around 30% in a 2012-level area and 45% in a SunZED area.
Seasonal thermal energy storage (STES) of solar energy on a local basis is rare in Finland, despite demonstrations by international studies that the utilisation rate of solar energy can exceed 50% of the annual local heat requirement in similar climate zones. STES also enhances the use of various kinds of waste heat. The decreasing price trend in renewable energy is another supporting factor. To increase its energy self-sufficiency and use of renewable energy, Finland needs to catch up with the local use of STES by other countries with a similar climate, such as Sweden, Denmark, Germany and Canada – where district heating is connected to local STES solutions. This would require e.g. system development and the piloting of various STES and local energy solutions.VTT kicked off its project in April 2015 and completed it in October 2015. The project was funded by the Housing Finance and Development Centre of Finland (ARA), Helen Ltd and the Helsinki City Planning Department (KSV).