Achieving ‘Net Zero Energy’ Performance Through Retrofitting of Residential Buildings in Ireland.
Authors:   ,   Cork Institute of Technology, Costello M. Alan 
Published: September 2024

"A Net Zero Energy Building (nZEB) is a residential or commercial building that consumes net zero energy over a set period of time, usually taken as one year. In short, for every unit of energy the building consumes over a year, it must generate a unit of energy to maintain an overall net balance. This is achieved by reducing the operational energy usage to a very low level through energy efficient design, and then providing renewable generation technologies on site to cater for the remaining energy demand. Energy use in all sectors of society is under scrutiny at present, largely due to the effects of climate change. In order to limit the release of carbon emissions and promote energy savings, the EU has set targets for Member States in relation to energy efficiency and renewable energy sources. As the residential building sector represents over a quarter of the energy usage within the Irish economy, the government has committed to improving building standards for new dwellings to enhance overall energy performance. While this will improve future energy trends within the sector, it is also necessary to improve the energy characteristics of current building stock which stands at almost 2 million housing units (based on 2011 figures). The research aims to establish the level of retrofit required to achieve net zero energy performance and investigate the effect of site location and terrain on the energy consumption and generation potential of a dwelling. In addition, the research examines the optimum phasing strategy for retrofitting a dwelling to achieve net zero energy performance and the economic viability of retrofitting dwellings to achieve this standard. To assess net zero energy performance for existing buildings in the Irish context, a bespoke software tool has been developed by the author for the purpose of analysing building energy performance in the pre- and post-retrofit condition. The Net Zero Energy Retrofit Calculator (NetZERO) is a Microsoft Excel based software program which estimates the existing energy consumption of a variety of dwelling types and assesses the level of retrofit required to achieve nZEB performance by implementing a range of energy efficiency upgrades and providing on site micro-generation. Overall energy usage is calculated by estimating the demand for space heating, hot water heating, lighting, auxiliary power and household appliances for the pre- and post-retrofit scenarios. The effect of dwelling occupancy and heating schedules is accounted for within the NetZERO calculations, together with the influence of local site climate. On-site generation is estimated using manufacturer data for a range of domestic wind turbines and solar PV arrays. The configuration of the overall micro-generation system can be amended based on user preference and site conditions. Using NetZERO, a total of four case studies are assessed to determine the potential of achieving nZEB performance for existing dwellings in Ireland. The selected case studies represent various combinations of dwelling type, site location and terrain, retrofit specification and micro-generation systems. Case study 1 examines a two-storey semidetached dwelling in an urban setting located in the Dublin region. Case study 2 consists of a one-story detached dwelling in a rural setting located in the Mid-West region. Case studies 3a and 3b comprise a one-story detached dwelling in the South-East region; with 3a representing a sub-urban setting and 3b representing a rural site. A net zero energy balance over one year is achieved for case studies 2, 3a and 3b while case study 1 does not achieve nZEB performance. The deep level energy efficiency retrofit applied to each dwelling is in accordance with S.R. 54:2014 Code of Practice for Energy Efficient Retrofit of Dwellings and yields energy savings of between 54% and 61%. The greatest potential for energy savings is associated with space heating which represents between 56% and 62% of the total energy consumption. The ability of each site to meet the residual energy demand using on-site generation varies according to site climate, location and terrain. For case studies 3a and 3b, a wind turbine provides a significant portion of the residual energy demand in combination with a solar PV array to achieve nZEB performance. For case study 2 a single wind turbine supplies the entire residual energy demand following a deep energy retrofit of the dwelling fabric and systems due to the high wind resource available on site. As case study 1 cannot facilitate the installation of a wind turbine due to poor wind resource and restricted site area, the dwelling does not achieve a net zero energy balance. Accordingly, the micro-generation system comprises a solar PV an'ay which only provides 24.5% of the residual energy demand. The export of excess electricity is important to the economic viability of a nZEB project as the export tariff gained offsets the capital investment over a shorter period. This is particularly evident for case study 2 where the calculated payback period is 15.7 years due to high levels of excess electricity at the site and the relatively low capital cost invested. The remaining sites which achieved nZEB performance have payback periods of 23.8 and 22.0 years respectively."