Straight Green: Zero Energy Buildings
ZERO ENERGY BUILDING DEFINED
The term Zero Energy Building seems self-explanatory; it describes a building that uses no more energy than it creates, right? That is one possible meaning. It turns out that when it comes to defining zero energy, there are many meanings. For the Pearl River Tower project, Skidmore Owings and Merrill (the architect) has defined it as “a structure that does not require an increase in the community’s need to produce energy.” The Thirty-One Tannery building actually uses a combination of natural gas and electricity for power sources and defines itself as zero energy only with regard to the electricity used and produced. The Energy Plus office building also only considers electricity consumption with a goal to “consume no electricity other than that which it creates.”
The National Renewable Energy Laboratory (part of the U.S. Dept of Energy) presented a paper titled “Zero Energy Buildings: A Critical Look at the Definition” that defines a Zero Energy Building as “a residential or commercial building with greatly reduced energy needs through efficiency gains such that the balance of energy needs can be supplied with renewable technologies.” A building classified as Zero Energy using NREL’s definition includes buildings that do not produce all energy consumed, and can further be defined as “off-site ZEB”-a building that buys all its energy from a wind farm or other central location. NREL has settled on four sub-definitions for zero energy buildings:
Net Zero Site Energy: A site ZEB produces at least as much energy as it uses in a year, when accounted for at the site.
Net Zero Source Energy: A source ZEB produces at least as much energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to generate and deliver the energy to the site.
Net Zero Energy Costs: In a cost ZEB, the amount of money the utility pays the building owner for the energy the building exports to the grid is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.
Net Zero Energy Emissions: A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources.
In addition to defining what ZEB is, the paper includes seven examples of ZEBs that are thoroughly examined. Only one of the buildings has achieved the site and source ZEB goal and does so due to its small size and relatively large PV system. In fact, all seven examples are fairly small, ranging in size from 1,700 to 43,000 square feet and are one to two stories in height. In setting a goal to build a ZEB, as defined by NREL, size matters.
There is a noticeable absence of any example of a large commercial building more than two stories in height. It is also evident that the four definitions are not equally achievable from building to building, region to region. While small buildings will have an easier time meeting site and source energy goals, it’s also true that buildings powered by grid-source hydroelectric power will more easily achieve the zero energy emissions goal than buildings in regions with predominately coal fired power plants.
It is good that NREL has defined different possible types of zero energy buildings but it will not be easy to make apples to apples comparisons among the four.
GETTING TO ZERO
Depending on the definition being used, the type of building, the climate and the budget, there may be many ways to get to zero energy. In all cases, maximizing a building’s energy efficiency should be the starting point, and this is emphasized in the NREL paper. Design for energy efficiency should begin with site selection, climate-appropriate building orientation, massing, enclosure optimization, internal load minimization, and resource reuse and recovery. For the least environmental impact, these strategies must be implemented before consideration of more exotic and expensive strategies like raised flooring, dual wall facades, building integrated PV arrays and wind turbines.
In yet another NREL paper titled “Lessons Learned from Case Studies of Six High-Performance Buildings,” the authors examined the design process and measured performance of buildings included in its ZEB definition paper. Conclusions presented in this paper include the following:
Architectural features such as form, shading, space layout, envelope constructions, materials, sizing, orientation, and glass all have significant impacts on energy, lighting and comfort.
The envelope should be the first method of creating low-energy buildings; the mechanical and lighting systems should then be sized to meet any remaining loads.
Low-energy architecture is not effective if mechanical systems have to solve problems that result from the architectural design.
Plug loads were often greater than design predictions.
PV systems experienced a range of operational performance degradations. Common degradation sources included snow, inverter faults, shading, and parasitic standby losses.
Effective insulation values are often inflated when comparing the actual building to the as-designed building.
Energy consumption was higher and energy production was lower than simulations predicted.
THE EXCEPTION RATHER THAN THE RULE
There is no question that a Zero Energy Building, under any definition, is possible with the right amount of money and effort. But what about cost to build, operate, and maintain? It is estimated that the Enery Plus “greenest office building in the world” will cost between 25 to 30 percent more than an average office building built in Paris. The NREL paper found that when complicated zero energy equipment and systems failed, occupants simply used the built-in backup systems rather than commission and maintain the more energy efficient systems that stopped working properly.
The largest claimed Zero Energy building to date, the Pearl Tower, is just now being built but has already fallen short of its goal to be the first ZEB high rise. In the paper “Towards Zero Energy-A Case Study of the Pearl River Tower, Guangzhou, China,” the authors concede that the building will not actually achieve Zero Energy status, although originally conceived, and heavily marketed, to do so. Tens of thousands of dollars have been spent designing the very sophisticated building integrated PV and wind turbine systems, mechanical cooling system, and a dual-façade wall system in order to achieve ZEB status. What went wrong? In hindsight, wouldn’t it have been better for the environment if this money were invested in off-site wind or solar energy farms?
It is fascinating to read about these buildings and the efforts made to get them to zero but if they are relegated to the fringe and unable to enter the mainstream in short order, I fear their contribution to the betterment of the world will be negligible. We need something more to accelerate a movement toward ZEB, low energy, and near-zero energy buildings-and as soon as possible.
ZEB “X” PRIZE?
In 2006, the Northeast Sustainable Energy Association offered a $10,000 prize for the first building designed that could prove it consumed less energy than it created for one year of operation. In 2007, the award was bestowed upon David Pill and Hillary Maharam for a 2,700-square-foot home they built that consumed 6,094 kWh of electricity and generated 6,286 kWh in its first year of operation. The house uses electricity as its sole power source. Its site-generating equipment consists of one 10-kW wind turbine. The turbine was installed at a cost of $40,000, which was partially offset by a $12,500 utility company renewable-energy incentive.
This is an excellent example of an incentive that resulted in a building designed to maximize energy efficiency first, and then implement site renewable energy generation. The owners of the home should be applauded for designing a home that uses a fraction of the total energy consumed by the average U.S. home-without any site generation. The wind turbine is simply icing on the cake.
A $10,000 incentive might be significant for a small building, but would be a mere drop in the bucket for large commercial project. There may soon be something much more significant for large building owners and architects through the X Prize Foundation. You may be familiar with this organization, which recently paid $10 million to a company called Scaled Composites for a successful flight of a radically new aircraft that successfully flew into space and back. It has also established a new $10 million prize purse to be awarded to the first team that can build a car that exceeds 100 mpg. The X Prize Foundation is currently looking into establishment of prizes for clean fuels, renewable energy, energy efficiency, energy storage, carbon reduction, and sustainable housing. Could a $10 million prize be the incentive needed to spur more rapid development of a market rate, large commercial Zero Energy Building? It couldn’t hurt. W&C