Zen ZNE Office
Multiple Modes of Energy Capture Create ZNE Office Building
A Case Study from Davis, California
By Jonathan Hammond
We wanted the design to reflect our mission statement: Integrating art, architecture and ecology in order to put people in touch with the environment, each other and their spiritual and cultural aspirations.
Art and architecture: the building reflects the personal sculptural vision of the architects, using sensuous curving strawbale walls clothed in luminous white lime plaster with those mid-century gull wing glulam roof beams juxtaposed with austere corrugated metal boxes as well as sculpted columns, guardian cats and furniture from the hands of the architects…
Ecology: the building is adapted to the climate to capture 90% of its heating and cooling energy from its environment. In the summer, no direct sunlight enters the windows. A natural ventilation system brings cool breezes into the building to cool heat sinks. Natural light provides all necessary lighting during daylight hours (more on this below)…
In touch with the environment and each other: occupants have direct access to inviting, usable outdoor spaces designed for working, eating, and socializing. The spaces are shady in the summer while sunny and protected in the winter…
Spiritual and cultural aspirations: in the world today there is a convergence of religious and secular aspirations to care for the environment and climate. This building reflects our aspirations by having a low carbon footprint and further celebrates them by focusing on creating a beautiful merging of art and architecture.
Our design philosophy is based on a bio-regional understanding that each building needs to be adapted to the specifics of the local climate and site; that is, we “design with climate.” We design so that the building extracts energy from the environment in a benign way. Our goal was to create a “Zero Net Energy” building. Thus, while we designed to reduce the need for off-site energy, we were passionate about creating a beautiful studio environment where we would be inspired to do creative work. Therefore, we worked to integrate architectural and sculptural beauty with the sustainability/energy harvesting features into a harmonious whole where beauty and sustainability mutually reinforce each other.
Our natural and solar energy capture systems include natural light, replacing electrical light; natural ventilation, replacing forced air; passive solar heat gain in winter, reducing heating needs; and cool air night flushing, thermal mass storage for heating and cooling, and solar heat gain avoidance in summer by orienting the windows to the south and the north and making sure they do not act as solar heat collectors.
Another strategy to reduce unwanted summer heat gain and winter heat loss was the utilization of thick, highly insulating strawbale walls. The baled rice straw is a local agricultural waste product that I have been using for almost 35 years to create super-insulated buildings. The walls are encased in a lime/cement plaster that protects the bales from the elements and permanently sequesters the straw. Not only can the strawbale wall system be curved and sculpted, but the thick straw walls also provide excellent (R-40) insulation that reduces the heat flow through the walls to a trickle.
Our natural light strategy is designed to eliminate the need for artificial light on all but the darkest days. We employ small distributed skylights with clear prismatic diffusers; since skylights are thermal weak points, we designed them to provide the optimum amount of light without overheating the space in the summer. We supplemented the natural lighting with the north- and south-facing windows. In addition, when we looked for interior paint and materials, we carefully considered light reflectance, especially for places such as the skylight light wells. We found a highly reflective and diffusing white paint with 98% reflectivity. The backup lighting system employs all LED light fixtures; however, we seldom need them. Thus, natural light represents a major capture of energy and a very powerful way that we have minimized our electrical energy use.
Natural ventilation is provided by operable windows and automatic louvers on the north and south sides of the building. The louvers are programmed to open automatically on summer nights, as soon as the outside temperature becomes cooler than the inside temperature. Since the “sea breeze” is from the south, the air flows through the building exiting on the north side, purging heat from the building at night. The same south windows that capture (cooling) energy in summer also capture energy in winter by allowing solar heat gain.
We moderate the passive heating and cooling of our building by thermal mass in the form of 26 vertical 18” diameter water columns, which together hold 22,000 pounds of water. During a cool summer evening the water is cooled from 75° F to 70° F. This is the equivalent of 110,000 BTU or nine tons of ice melting. Since our design has minimized summer heat gain, the water mass and the cooling potential of the slab floor are plenty to keep the building cool on most summer days. This same mass is available to store solar heat on sunny winter days when the low sun comes into the south-facing windows. Water is particularly good at storing passive heat gain since it is a fluid. When the temperature changes at the surface of the tank, the cooler water thermosiphons out of the way, allowing heat transfer throughout the water by both conduction and convection.
Finally, an air-to-water heat pump provides both backup heating 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 summer 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 precooled 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 heating 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 capacity 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