The content of greenhouse gases in the atmosphere is increasing resulting in climate change and efforts to stop the negative trend need to be intensified. The energy use in the Swedish residential and service sector constitutes 40 % of the total energy use of 378 TWh in the country. Nationally there is a target to reduce the energy use per heated area with 20 % to 2020 and 50 % to 2050. Energy renovation of buildings from the Million Programme is foreseen to be able to contribute to achieving the targets owing to the large building stock and energy efficiency potential. In the master thesis cost optimal energy renovation strategies are investigated for two multi-family buildings in Linkoping built during the Million Programme, one with an unheated attic and one with a heated attic. The thesis is carried out by using life-cycle cost optimisation (LCC-optimisation) by utilising the software OPERA-MILP, developed at Linkoping University. The aim of the thesis is to obtain the energy renovation strategy that is optimal from an LCC-perspective and to investigate the energy reduction and LCC. Optimal energy renovation strategies are also investigated for energy renovation to levels of the Energy Classes of the National Board of Housing, Building and Planning in Sweden and the stricter limits for nearly zero-energy buildings (NZEB) that will likely come into force in 2021. Greenhouse gas emissions and primary energy use are also investigated for the different cases with the purpose of putting energy renovation in relation to climate impact. Local environmental factors are used for district heating while electricity is assigned values based on the Nordic electricity mix and Nordic marginal electricity respectively. The current LCC and annual energy use is 2 945 kSEK and 133 MWh for the building with an unheated attic and 3 511 kSEK and 162 MWh for the building with a heated attic. The result shows that LCC can be reduced by approximately 70 kSEK and 90 kSEK respectively. The optimal solution constitutes of a window change from windows with U=3,0 W/m2°C to windows with U=1,5 W/m2°C and results in a reduction of the energy use by 13 % and 15 % respectively. LCC increases with 240 kSEK for the building with unheated attic and decreases with 18 kSEK for the other building when Energy Class D is reached. Energy Class C is attained through an increase in LCC by 300 – 590 kSEK and Energy Class B through an increase by 1610 – 1800 kSEK. It is not possible to reach Energy Class A or the future requirements for NZEB (55 kWh/m2Aheated) with the energy renovation measures that are implemented in OPERA-MILP. The largest energy reduction that can be attained is approximately 60 %. The most cost optimal insulation measure is additional insulation of the attic floor/pitched roof followed by additional insulation of the ground concrete slab. It was shown to be more cost efficient to change to windows with U=1,5 W/m2°C in combination with additional insulation compared to changing to windows with better energy performance. For greater energy savings additional insulation on the inside of the external wall is applied, while insulation on the outside of the external wall is never cost optimal. To reach Energy Class B installation of HRV is required which gives a large increase in cost. Less extensive energy renovation is needed to reach the energy classes for the building with heated attic compared to the building with unheated attic. The annual use of primary energy in the reference case is 22 MWh for the building with an unheated attic and 26 MWh for the building with a heated attic. The emissions of greenhouse gases are 18 tonnes CO2e and 22 tonnes CO2e per year respectively when the emission factor of the Nordic electricity mix is applied and 20 tonnes CO2e and 25 tonnes CO2e respectively when the Nordic marginal electricity is applied. The yearly primary energy use can be reduced with up to 7 MWh through energy renovation. When the energy renovation leads to an increase in electricity use the primary energy can however increase with up to 12 MWh. The yearly greenhouse gas emissions can be decreased with up to 14 tonnes CO2e. When Nordic marginal electricity is applied to estimate the emissions of greenhouse gases for an energy renovation strategy that leads to an increase in electricity use the result is less beneficial from a climate perspective compared to when Nordic electricity mix is applied
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