Center for the Built Environment

Thermally Activated Building Systems (TABS) is a recognized low-energy HVAC system. Sizing of these systems is complex due to their slow thermal response. Limited cooling capacity of these systems and inadequacy of conventional sizing method, that assumes high factor of safety, is preventing early adoption of these systems. TABS, however, is proven to be energy-efficient and capable of preserving comfort in several commercial buildings of Europe. There is, however no comprehensive case study report on comfort performance of TABS in the US. With this being the background, my dissertation aims to identify and recommend a design method for TABS that balances between accuracy of multivariable complex design models, high computational cost of models requiring an iterative approach and computational ease of simple single to bivariate linear design models. The dissertation work involved: 1) a systematic qualitative review of seven TABS design models from the literature, and 2) a simulation based quantitative comfort performance assessment of three shortlisted design models. I reviewed seven design and control models and characterized them systematically with an aim to investigate their applicability in various design scenarios and at different design stages. All of these models size water supply temperature (WST) as this parameter will be used for selection and sizing of the cooling plant or the condenser unit. The design scenarios include variable internal heat gain, different building thermal mass, varying pump operating hours and varying solar gain due to orientation. Other parameters affecting cooling load and thermal performance of TABS that were held constant in this study included window-to-wall area ratio, zone volume, construction insulation, supply air temperature and volume flow rate of the ventilation system, external shading, location, TABS mass flow rate, pipe layout, active surface configuration and TABS thermal properties. I considered three design stages: feasibility study, early design decisions, and detailed design sizing and the selection criteria are reliability and ease of implementation. Results of the qualitative analysis indicated that based on the above-mentioned criteria, a hybrid model recommended by ISO 11855 is the best candidate for detailed design and sizing of the cooling plant. An outdoor temperature (Toa) compensated model, a zone operative temperature (OT) feedback based model and the hybrid model from ISO 11855 were isolated for transient simulation based quantitative evaluation in terms of a novel comfort exceedance metric. This metric accounts for both duration and severity of discomfort and is weighted by instantaneous occupancy. For comfort analysis in terms of zone OT, zone RH was maintained using humidistats. TABS was the only cooling system in the building. Twelve simulations were carried out in a standard 5 zone small office building for CZ03 in EnergyPlus v7.0 under 2 different heat gains and 2 construction types.  Results of the simulation study indicated that both the Toa compensated model and zone operative temperature feedback based model provided equally good comfort in 14 out of 20 design scenarios including zone orientation. However, the zone OT feedback model responded better to the heat gain and thermal mass conditions as expected, and is therefore recommended as a more robust model for early and detailed design phase implementation.  The hybrid model recommended by ISO 11855 resulted in comfort exceedance of 10% to 48%, while the recommended threshold exceedance for this study was 3-5%. This model also resulted in significantly reduced discomfort using 24 hours hydronic cooling energy of TABS instead of the design day 24 hours cooling energy of convective system.