Thermal
Warmth from Below
The Bullitt Center is heated (and cooled) by a dense system of veins, or hydronic radiant tubing that coils a few inches beneath the concrete overlay of each floor. Formally known as “cross-linked polyethylene”, the tubes are commonly called PEX. Inside the tubes, a special mix of water and glycol run quickly, warming or cooling the concrete slab, which efficiently radiates into the occupied spaces.
The source of the heat for the radiant system actually starts 400 feet below the Bullitt Center, where twenty six geo-thermal wells dive 400 feet below the building. This ground-source heat pump uses a closed-loop tube containing a mixture of water and glycol that receives and gives off heat quickly to the surrounding soil and groundwater, which remains at a constant temperature of 53 degrees F.
When heating the building, the glycol mixture absorbs the ground’s warmth, before it is pumped back up and run through heat pumps in the mechanical room that warm 53° F fluid into 90° F fluid.
In the winter, the system removes heat from the ground. In the summer, the system can be run in reverse, restoring this heat back into the ground. In this sense, the ground is used as a “battery” for heat.
The geothermal well field is under the west side of the building, in an area with a constant lens of groundwater moving slowly toward Puget Sound. As a result, any “heat pollution” from the wells dissipate within approximately 12 inches.
Since the Living Building Challenge requires all of the energy used at the Bullitt Center to be generated onsite, it was critical to use an energy efficient heating and cooling system.
While electric resistance heating is the cheapest and most direct way to heat a building with electricity, it is incredibly inefficient. In contrast, the hydronic system uses relatively little electricity – which is consumed mostly to run a compressor – to exchange heat from one place to another with amazing efficiency.
High-Performance Envelope
The building envelope greatly exceeds the Seattle Building Code requirements, using a triple-glazed curtain wall system that originated in Germany and is now produced locally. The well-insulated walls have been designed to eliminate thermal bridging and dramatically reduce air infiltration. Building massing and orientation, as well as glazing selection, control heat gain. To the extent possible on a compact, five-sided urban site, major glazing areas face south and north to improve daylighting and solar control. The building's windows, which open and close automatically in response to conditions outside, were selected for optimal control of heat loss and solar gain, while maintaining superb visibility for daylighting. Analysis shows that increasing the thermal performance of the envelope beyond current levels would have little overall impact on energy use in the proposed building.
Radiant floor
Curtain wall
Quantity Analysis
Annual Energy Use by Fuel
Electricity: 236,400 kWh (from solar array)
Gas: n/a
Fuel Oil: n/a
Biomass: n/a
Other fuel: n/a
Total: 230,000 kWh
Annual Energy by End Use
Heating: 6,000 kWh
Cooling: 5,600 kWh
Fans & Pumps: 33,000 kWh
Lighting: 53,000 kWh
Domestic Hot Water: 7,800 kWh
Plus Loads & Equipment: 131,000 kWh
Other End Use: n/a
Annual On-Site Renewable Generation
PV: 257,800 kWh
Total: 257,800 kWh
Peak Use
Peak Electricity Demand: 40kw
Peak Natural Gas Demand: n/a
Peak Cooling: 1,000 ft²/ton
Connected Lighting Load: 0.4 W/ft²
The right diagram compares energy distribution at the Bullitt Center, which is expected to have an annual energy use intensity (EUI) of 16-16,000 Btu per square foot, with that of a typical Seattle office building, which has an EUI of 92.
The right diagram compares energy distribution at the Bullitt Center, which is expected to have an annual energy use intensity (EUI) of 16-16,000 Btu per square foot, with that of a typical Seattle office building, which has an EUI of 92. (Photo courtesy of Urban Ecology Partnership [UEP])
Energy consumption
Compare energy usage between typical building and Bullitt center