The Winter 2014 issue of Higher Performing Buildings has a great article on the new Bill & Melinda Gates Foundation Headquarters, built in Seattle in 2011. Since the Foundation intends to occupy the building for the long term, the major heating, ventilating and air conditioning system design decisions were based on a total cost of ownership analysis. It’s interesting to see how this analysis informed the design of the central cooling plant. A number of configurations were analyzed and, surprisingly, the one that came out on top is an air-cooled chiller plant connected to a thermal energy storage system. This result is surprising for two reasons: 1) electricity costs in Seattle are low and do not include peak demand charges or time-of-use rates and 2) air-cooled chillers tend to underperform water-cooled systems that use a cooling tower for heat rejection.

The thermal energy storage system consists of a 600,000 gallon (2.3 million litre, 8,750 ton-hour) underground storage tank that allows the chillers to store cooling capacity in the form of chilled water during hours when the cooling load is small. When the cooling load rises, chilled water from the storage tank is used to cool the building and reduce the load on the chillers. Thermal energy storage systems are often used in this way to shift some of the electricity consumed for space cooling from the daytime to the nighttime when time-of-use rates are lower. By sharing the peak load between the chillers and the thermal energy storage tank, the peak electrical demand is also reduced, saving money on peak demand charges. Neither of these cost-saving strategies come into play for this building in Seattle, so how does this design lower the total cost of ownership? It turns out that the air-cooled chillers operate primarily at night when they are more efficient due to Seattle’s cool nights. By operating at higher efficiency, the chillers are able to use less electricity. On an annual basis, the design team found that the combination of air-cooled chillers and thermal energy storage uses the same amount of electricity as a standard water-cooled plant, but with the advantage that water use from the cooling tower (2.6 million gallons or 9.8 million litres per year) is eliminated. The key advantage from a total cost of ownership perspective is that thermal energy storage allows the chiller plant size to be reduced by 40%. On the whole, the air-cooled chiller system has lower up front purchasing costs, reduced space needs, and lower maintenance and replacement costs than a standard water-cooled plant. The air-cooled plant was also compared to a water-cooled plant in combination with thermal energy storage. However, the higher upfront costs for the water-cooled plant plus thermal energy storage led to a higher total cost of ownership.

In short, air-cooled chillers plus thermal energy storage won out for these reasons: higher night-time cooling efficiency, lower water consumption, reduced chiller plant size, and lower maintenance costs.