Modelling life cycle carbon impacts: general

8.7.1. Overview

The information collected during the monitoring period was used to construct and calibrate dynamic thermal models and embodied carbon models of each building. Alterations were made to the base models to simulate the interventions and refurbishment scenarios and the corresponding changes to operational and embodied carbon impacts were analysed. Models for equivalent new buildings were also developed that adopted modern fabric and system standards but retained the existing operational characteristics, for example occupancy profiles, heating and cooling temperatures and equipment and lighting use. The corresponding operational and embodied carbon impacts for the new buildings were determined for comparison. The results were compared with the data in the primary database for validation. The approach to constructing and calibrating the models, modelling the scenarios and accounting for analysis uncertainties is described in the following sub-sections.

8.7.2. Selection of modelling software

Operational carbon impact

To provide sufficient resolution for the analysis of building operational carbon impacts it was deemed necessary to use a dynamic thermal simulation (DTS) model. The IES Virtual Environment (IESVE) suite

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was selected as the most appropriate application for this purpose. In terms of validated performance, IESVE is understood to meet a number of international standards including CIBSE TM33¹⁵ and ASHRAE Standard 140¹⁶ and is also accredited for use to implement the UK National Calculation Methodology (NCM) (IES 2015). A number of similar dynamic thermal simulation applications exist, for example those offered by EnergyPlus, DesignBuilder, Hevacomp and EDSL (Tas), although IESVE offers a number of features collectively that were found to be beneficial to the analysis. These included the following: close reproduction of the existing building geometry, detailed breakdown of the energy results by end use and zone, and ability to external manipulate the model settings (construction and zone profiles) to facilitate bulk scenario analysis. The IESVE version used throughout was IESVE 2014.1.0.0.

Embodied carbon impact

As discussed in section 2.2, a variety of data sources, methods and tools exist for the purpose of calculating embodied carbon emissions. It was desirable that the tool selected for the study provided the following:

- A method and materials database compliant with the BS EN 15978:2011 standard, including standard outputs for determining impacts throughout the life cycle stages.

- A large, generic materials database sufficient to analyse options for a variety of building elements.

- Automatic calculation of material quantities from drawn geometry, including update following geometry changes.

- Direct link with a DTS to allow operational carbon impacts to be measured using the same model.

15 CIBSE TM33 2006: Tests for software accreditation and verification

16 ASHRAE Standard 140: Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs

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- Results breakdown to assess impacts by building element.

As seen in Table 2.2 (in section 2.2.8), a number of the available tools meet these criteria individually, however the EnviroImpact module of the IESVE suite was found to be an application that could offer them all. In combination with the DTS components of the IESVE, it was possible to assess the operational carbon impacts and embodied carbon impacts of material changes concurrently. As the module has been developed to meet the BRE IMPACT standard it is understood that the methods used and materials database meet the requirements of BS EN 15978:2011 and BS EN 15804:2012 respectively (BRE 2015). From initial testing it was found that the range of materials provided was adequate to assess a variety of options. The same IESVE version was used as that for the operational carbon impact, together with version 2 of the EnviroImpact materials database.

8.7.3. Model construction

Existing building geometry

The geometries of the existing buildings were constructed directly in the IESVE ModelIT module mainly by tracing over the respective CAD survey plans. For the Darwin building, floor heights were taken from the survey information; for other buildings, they were determined by site measurements. Glazing heights were also obtained either by site measurements or by measurement based on external images. The outline geometries of nearby buildings considered to have a potential shading effect were included for each building.

New building geometry

For the new-build scenario N1, the geometry used for the new building was identical to that of the existing building. For scenario N2, an alternative geometry was developed in accordance with the specifications set out in Appendix B1, although for direct comparison purposes it was assumed that the overall space use breakdown (in terms of the zones defined in Table II in Appendix A6) would

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remain the same. Furthermore, the new building footprint and overall height should not extend beyond that of the existing. This allowed a minimum floor to ceiling height of 3.3m to be maintained throughout. The main changes explored were to reduce floor depths where possible to improve the scope for natural ventilation and daylight penetration where it would be beneficial. A key criterion set was that all spaces for which natural ventilation would be generally appropriate – in terms of occupancy and casual gains and fresh air requirements - would have sufficient exposure to the building façade for this purpose. The detailed approach taken for the development of each new building is described in Appendix B2.

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Bentham House – existing Bentham House – new-build

Christopher Ingold – existing Christopher Ingold – new-build

Darwin Building – existing Darwin Building – new-build

Rockefeller Building – existing Rockefeller Building – new-build

1-19 Torrington Place – existing 1-19 Torrington Place – new-build

Figure 8.4 Case study buildings existing and new model geometries (images from the IESVE application)

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