ing and cooling. The heat pump is linked to a heavily insulated 1,250-gallon water tank, which provides an additional 100,000 lbs. of thermal storage. Because of this thermal mass and other thermal mass in the building, we have a sufficient buffer so that the heat pump needs to come on only during the most favorable time of day. For example, in the sum-mer the heat pump cycles on in the early morning hours when the air temperature in Davis is 60° F or less, allowing it to work extremely effectively. The stored pre-cooled water is then pumped throughout the building’s radiant heating/cooling system, cooling the building’s slab and the water collectors. The average California office building has an energy utilization intensity (EUI) of 15-20 kWh per square foot per year. Since our natural energy capture systems provide light and space heat-ing and cooling, our building, which is all electric, has an EUI of 3.7 kWh per square foot per year. This means that our recently installed 11 kW photovoltaic (PV) system will have plenty of capacity to make our building Zero Net Energy (ZNE) and charge one or two electric cars to boot. Optimizing the building’s capac-ity to tap naturally occurring energy sources, combined with a very efficient radiant back-up heating/cooling system and a small PV array, demonstrates the feasibility of creating very low-cost ZNE commercial office buildings. ST The conference room shares the thermal mass water wall and polished concrete floor, designed to store heat and cool. 30 SPRING 2017 SOLAR TODAY Copyright © 2014 American Solar Energy Society. All rights reserved.