Cost-optimal energy retrofit of a 1990’s residential building in Finland

On 16 December 2002, the European Union set the first version of the Energy Performance of Buildings Directive with an objective to improve the energy performance of buildings. The directive entered into force on 4 January 2003 but was later replaced by a recast on 19 May 2010. The new version of the directive focuses on nearly zero-energy buildings, cost optimality that meets the minimum energy performance requirements and improved policies. The directive applies to both new and existing buildings undergoing major renovation or retrofit of building elements such as roofs, walls and heating and cooling systems. The directive stated that Member States are to ensure that all new buildings owned by public authorities are nearly zero energy buildings by 31 December 2018 and 31 December 2020 and for all new buildings. In Finland residential buildings make up the majority of the building stock. A major contributor to carbon emissions are existing buildings with poor energy performance. An energy retrofit to improve the energy performance of these buildings will help reduce carbon emissions. The decision-making process in an energy retrofit can be complex especially when there are more than one objective. The objective of this study was to determine the cost-optimal retrofit solution of the case study residential building built in 1998 in Tampere, Finland with a nearly zero-energy building (nZEB) target. The scenarios were based on four different energy supply systems that are commonly used in Finland which are district heating, exhaust air heat pump, air to water heat pump and a ground source heat pump system. The methodology used was a simulation-based optimization (SBO) using the energy simulation software IDA ICE (Indoor Climate Energy, version 4.8) and the optimization tool MOBO (Multi-Objective Building Optimization). The base model of the existing building and the energy retrofit scenarios were created on IDA ICE. The life cycle and investment cost were calculated using the latest cost data. The MOBO tool was used to perform simulations to find the cost-optimal scenario with an nZEB E-value target for all energy retrofit scenarios. Based on the investment cost the results proved that the air to water heat pump (AWHP) system is the most cost-optimal solution although it did not reach the nZEB target. The investment cost for the AWHP is 123 €/m2 and the E-value is 103 kWhE/m2a. The investment budget was set at 147 €/m2 and all cost-optimal scenarios were under the budget. Based on the life-cycle cost, the ground source heat pump (GSHP) system is the most cost-optimal solution with an LCC of 153 €/m2 and E-value 97 kWhE/m2a. All scenarios were not able to reach the nZEB E-value target set by this study but achieved the minimum requirement level of 187 kWhE/m2a