The 2023 ASHRAE Lowdown Showdown Competition recently took place in Austin TX at the annual Building Performance Analysis Conference. The highlight of the conference is the modeling competition. This year, the competition asked teams to envision a new creative reuse of the Houston Astrodome to provide urban amenities and to do so within the constraints of reduced fossil fuel use and reduced carbon emissions. 240 building industry professionals voted in favor of the Stantec Team’s submission: the CO2 Crew. This article highlights some of the design features using IESVE Software.
The building was designed to accommodate the extreme climate conditions of Houston, including humidity, high rainfall and flooding potential, and extreme heat.
Energy Performance & Design Optimization
The design for the Astrodome has undergone a complete optimization process that was influenced by many factors concerning the operations and energy performance of the building. Substantial optimization into the geometry, building orientation, and fenestration location was done to maximize daylighting opportunities to reduce reliance on lighting systems while also reducing glare and overheating through external shading devices and Low-E glazing products.
Opaque assemblies and fenestration thermal resistance are optimized with the balance of economically feasible and reductions in annual heating and cooling consumptions.
Indoor setpoints for temperatures and humidity levels were chosen specifically for Houston’s climate to maximize thermal comfort but minimize impact and strain on the mechanical systems. Lighting systems implemented use efficient LED fixtures controlled via daylight harvesting sensors and occupancy sensors for all space types and modeled with the expected absence of occupancy rates for all spaces.
Robust optimization and parametric analyses were completed to determine optimal design parameters to minimize energy usage while maximizing occupancy comfort. Several different tools were used to complete the analysis. The concept architectural design as completed in Rhino. Once the design was ready, Pollination was used to export the model geometry to IESVE-2023. Detailed modelling in IESVE was completed utilizing the full suite of in-built tools; APACHE, ApacheHVAC and RadianceIES.
As part of the design approach, the roof of the astrodome was removed, exposing the once interior space to the environment. Thus, the superstructure of the astrodome, including the lamella structure, was imported as a shading surface, totaling over 3,000 different shade geometries.
Mechanical Design Strategy
The proposed HVAC system harnesses natural heating and cooling strategies and tools to improve energy efficiency and resiliency. This is accomplished through night sky radiation cooling to supercool the borehole thermal storage field, separating HVAC performance and capacity from outdoor temperatures, especially important for warming temperatures in future years.
The complex mechanical system as detailed below was created in ApacheHVAC and no work arounds were required, utilizing the full extent of the central heat pump heating/cooling sources as well as the customizable air-side configurations.
The hot humid climate results in very high latent loads. Applying a liquid desiccant dehumidification system allows the latent loads to be decoupled from the sensible loads. The regeneration of the liquid desiccant requires thermal energy input in the range of 150-175°F. This heat comes in part from the rejected energy of the vapor compression cycle that provides sensible cooling. Any additional input heat comes from PV/Thermal collectors. The liquid desiccant system allows the air to be dried even further than is required for the latent loads of the spaces. This allows direct evaporative cooling to deliver 33% of the sensible cooling demand thereby reducing the work required from the chiller. High occupancy zones with larger sensible loads can be cooled directly from the bore field at the beginning of the cooling season or actively using the chilled water loop delivered to fan convectors or chilled beams.
The cool dry air is delivered to the spaces using a displacement ventilation air distribution system. This allows the cooling to be delivered only where needed based on demand control signals from CO2, humidity, and temperature, and only to the occupied height of the zone. The displacement ventilation system is particularly efficient at extracting moisture while delivering fresh air where needed.
Thermal Comfort & Indoor Air Quality (IAQ)
Temperature and humidity setpoints were designed based on acceptable ranges for the given climate, resulting in comfortable space conditions for building occupants for the activity types. An unmet hour analysis was completed to analyze indoor comfort and verify equipment is properly sized and able to provide the required heating and cooling loads to maintain comfortable indoor conditions. Demand control ventilation strategies were considered to optimize ventilation requirements to occupied spaces, with displacement ventilation to provide the necessary cooling and IAQ requirements to the occupied zone levels only.
The facility utilizes a fully electric heating and cooling system, resulting in no on-site fossil fuel combustion. Operational carbon is fully offset using on-site renewable energy generation, located on the Astrodome glazing and surrounding perimeter greenspace. This results in a reduction in operational carbon emissions by approximately 803 tCO2e per year, achieving net zero carbon.
The proposed design has approximately 3,929 tCO2e (lifecycle of 60 years) of embodied carbon. It was important for the project team to create a sustainable facility in all aspects of design, including offsetting embodied carbon. As such, local food production within the Astrodome is expected to reduce transportation-associated emissions compared to traditional produce supply chains. The Astrodome is estimated to produce 1.4 million kg of food per year, offsetting the long-haul transportation of 40 fully loaded tractor-trailers to local grocery stores, restaurants, and residential homes throughout Houston. This is expected to save 227 tCO2e per year, requiring 17 years of facility operation to offset the initial embodied carbon emissions.
A biophilic design was inherently incorporated throughout the process through the creation of the Agridome. A mass timber inner pyramid construction was selected due to the low embodied carbon aspects, while also providing a warm ambiance to the interior areas and impersonating natural landscapes. The mass timber lattice structure was created to mimic and tie in the existing dome lamella structure of the roof, while providing a place for low-maintenance natural shades (such as ivy) to grow, improving temperature control, humidity regulation, and pollution filtration.
The vegetation throughout the dome is apparent in farming areas, with botanical vegetation woven throughout the common lobby space, corridors open gathering space, and exterior greenspaces and development site areas.
Water features were also considered throughout the design, including the central opening in the inner pyramid, providing a waterfall effect during precipitation. Additionally, the open lamella structure provides substantial fresh air and natural lighting throughout the facility.