Integrating active thermal mass strategies in responsive buildings

Abstract

Thermal mass can be used in buildings to reduce the need for and dependence on mechanical heating and cooling systems whilst maintaining environmental comfort. Active thermal mass strategies further enhance the performance of thermal mass through integration with the Heating, Ventilation and Air Conditioning (HVAC) systems. For the design of new buildings to include active thermal mass strategies, experience from operational projects and design guidelines are normally used by engineers. However, dynamic thermal modelling is required in most cases to accurately determine the performance of its integration with the environmental systems of the building. Design decisions made in the preliminary stages of the design of a building often determine its final thermal characteristics. At this stage, reasons for not integrating active thermal mass strategies include the lack of knowledge about the performance of previous buildings and the time and resources required to carry out detailed modelling. In this research project a commercially available dynamic building thermal program has been used to construct models for active thermal mass strategies and compare the results with monitored temperatures in buildings incorporating the strategies in the UK. Four active thermal mass strategies are considered (a) hollow core slabs (HCS), (b) floor void with mass, (FVWM) (c) earth-to-air heat exchanger (ETAHE) and (d) thermal labyrinth (TL). The operational strategies and monitoring are presented and their modelling is described in terms of geometrical configuration and input parameters. The modelling results are compared with the measured parameters successfully. Using the calibrated model, an excel based tool (TMAir) was then developed that can be used at the concept design stages of a typical office building to determine the benefits of integrating an active thermal mass strategy. Key design parameters were identified for each system. These parameters can be split into two categories; fixed parameters and user selected parameters. The fixed parameters are pre-selected for the design tool and have to be a fair representation of the projects that the tool will be used for. The user selected parameters are chosen by the user to represent the way the building will be used, and to look at the effect of key design decisions on the performance of the building. The tool has an easy-to-use interface which allows direct comparison of the different active thermal mass strategies together with the effects of changing key design parameters. Results are presented in terms of thermal comfort and energy consumption. TMAir has then been used to carry out a series of parametric analyses. These have concluded the following:  There is only a benefit in integrating a HCS strategy when night cooling is introduced  There is no benefit in integrating a FVWM strategy when only one parameter is improved  An ETAHE and TL strategy will always provide a benefit, although the benefits are greater when night cooling is introduced, solar and internal gains are reduced and when the air change rate is increased. When all of the parametric improvements are applied to the test room the results show that all of the active thermal mass strategies can provide a reduction in annual overheating hours when compared to the Standard Strategy. Only a small benefit is found for the FVWM Strategy, however around a 25% reduction is found for the HCS Strategy, over a 50% reduction for the TL Strategy and nearly a 75% reduction for the ETAHE Strategy. This demonstrates the importance of applying a low energy, passive approach when considering the application of active thermal mass strategies. The key results have shown that when comfort cooling is provided, adding a HCS or FVWM strategy always results in an increase in the annual cooling load. This is as a result of the temperature of the air being supplied into the cores or floor void being higher than that of the internal surface temperatures of the cores or void. This results in the supply air being heated, and less cooling provided to the test room per cooling energy delivered. Due to the pre cooling effect of the ETAHE and TL strategies, these strategies always result in a reduction in the annual cooling load. The key results have shown that the annual heating load is reduced by a small amount for the HCS and FVWM strategies unless the solar gains or internal gains are reduced, whereas the ETAHE and TL strategies always result in a around a 10% reduction in annual heating load as a result of the preheating effect these strategies have on the supply air.