A parametric analysis of the energy savings predicted by different thermal models: implications for the UK Green Deal
Selin Yilmaz, Loughborough University
The UK government published The Carbon Plan in December 2011 that set out plans for reducing UK emissions by 80% from 1990 levels in 2050. It would be impossible to meet the 2050 objective without reducing emissions from homes. One of proposals brought by government to reduce the emissions from the house is the refurbishment of the existing stock. The UK government has introduced a new ‘Green Deal’ policy aimed at promoting the mass market refurbishment of the existing housing stock in order to help meet emission reduction targets. The scheme works by providing households with a loan that is paid for by the expected savings on their energy bills. Energy saving improvements include insulation such as loft or cavity wall insulation, heating, draught-proofing and double glazing.
Fully comprehending how an existing the building performs thermal is key to a successful energy efficient refurbishment. To understand the impact of this retrofit measures, several models are used to identify the energy demand of the buildings after the introduction of this energy refurbishment. Right now, Standard Assessment Procedure (SAP) is one of the models designed to be a simple but a reliable procedure for estimating the energy use of the dwellings in UK. SAP 2009 building energy model is used to predict the savings by the Green Deal Assessors. Dynamic thermal modelling such as Energy Plus which can predict the hour by hour space heating energy demands and internal air temperature of the buildings has been extensively used in the research and industry.
However, the models used for housing stock must accurately describe the complexities of the dwelling including the climatic, fabric of the house and the occupant’s behaviour. Accurately and robustly assessing the energy saved by of retrofit measures is a significant challenge and the literature demonstrates that savings are often less than predicted by models. Therefore, it is essential to test whether the models that are used to predict the effects of retrofit improvements to houses are reliable and capable of dealing with the variety of inputs to enable significant information about residential retrofits.
This study presents the energy modelling of a typical UK semi-detached house using the SAP 2009 model and the Energy Plus dynamic thermal simulation model. The inter-model comparison which included a parametric analysis is used to test the reliability of the SAP 2009 model and Energy Plus for a range of retrofit measures with varying occupancy schedules, construction data and orientation. Results are given for the base case house and 23 variations on the design, including installation of an insulation layer to the external walls and changes to the glazing type, orientation, and occupancy profile. The annual space heating demand and the energy saving predictions are compared and contrasted against a study carried out 20 years ago that used the same input sets with ESP-r, SERI-RES, BREDEM-8, and BREDEM-12 models.
This work intends to provide an insight into the algorithms and the features which Energy Plus and SAP 2009 model can provide for a particular design, providing a detailed data on design problems and limitations. These data will underpin future academic projects in UK as well as overseas on dynamic model inter-model comparison. The project will shed light on such questions such as “Is the same retrofit measure lead to similar values for energy demand of the dwellings and energy savings predicted by SAP 2009 model predict and Energy Plus?. The sensitivity analysis and inter-model comparison will help to test the reliability of the Energy Plus and SAP 2009 model. “If Energy Plus is used rather than SAP 2009 for its national energy demand policy formation, would the same recommendations be made for energy refurbishment?” The project will provide answers to these and other questions and be benefit to Green Deal providers, national and government officials, energy models, SAP and Energy Plus, developers giving them information and thorough analysis in energy refurbishment and the models used.