Zero energy and onsite thermal energy storage
To bring the building community together, the DOE conducted an extensive process involving various industry stakeholders with the goal of reaching a common definition.
- Define zero energy building (ZEB)
- Describe ZEB boundaries and methodologies for verifying zero energy buildings' performance.
- Understand the impact zero energy buildings can have on the overall power grid.
- Understand how energy storage can be an integral part of zero energy building design.
What are zero energy buildings?
A ZEB (zero energy building) is defined by the U.S. Department of Energy (DOE) as an "energy-efficient building where, on a source-energy basis, the actual annual delivered energy is less than or equal to the onsite renewable exported energy." Prior to this definition, there were many ways to describe a zero energy building. To bring the building community together, the DOE conducted an extensive process involving various industry stakeholders with the goal of reaching a common definition. While a ZEB had been commonly referred to as a "net zero building," net zero energy and zero net energy, key feedback to the DOE concluded that "net zero" was confusing to consumers and held no substantive meaning. Therefore, zero energy building or ZEB is the currently accepted terminology under the common definition.
ZEBs are structures designed to be as energy-efficient as possible, reduce power system demand, minimize energy costs, and meet much of their energy requirements from local renewable energy resources. ZEBs should also be designed with other nonenergy environmental attributes in mind, such as good indoor air quality, water protection, and material-resource conservation.
In ZEBs, traditional primary energy resources, like oil, gas, utility-scale renewable fuels, or secondary energy resources, such as electricity, steam, and district heat and cooling, are delivered to the building site. These offsite energy resources are offset by onsite renewable generation which is designated for the building or exported to the grid.
Onsite renewable generation may also be sold as RECs (renewable energy certificates). Sellers of RECs relinquish ownership of the renewable energy aspect of their renewable energy resource. RECs may make sense for buildings in urban areas where it is not practical to install onsite renewable energy. For instance, in urban areas, RECS may be retained when it is not possible to have enough solar panels on a building to offset delivered energy.
Renewable energy certificate-zero energy building (REC-ZEB): An REC-ZEB is an energy-efficient building where, on a source-energy basis, the actual delivered energy is less than or equal to the onsite renewable exported energy plus acquired RECs.
There are many ways in which a building or a group of buildings may be identified as a zero energy building. Several projects have undertaken the challenge of ZEB, but few have succeeded. One problem is that there hasn't been clear, concise, and broadly accepted terms and definitions in the United States for ZEB. To some extent, designers, contractors, and property owners have not grasped the entire scope of what it means to be ZEB and its larger implications.
Years ago, the term would have invoked images of a small building in the middle of a desolate area with a solar panel on its roof to meet its energy needs. For many, zero energy meant self-sufficient and off grid. Today, a broadly accepted definition of ZEB boundaries and metrics with government and industry consensus will guide American building designers toward clearer design strategies and stimulate growth of the ZEB market.
The First Step: Defining boundaries for ZEB
To properly access a ZEB, a site boundary must be established, such as a property boundary. Here are a few different ways designers may define zero energy buildings depending on the boundary:
Zero energy building: An energy-efficient building where, on a source-energy basis, the actual annual delivered energy is less than or equal to the onsite renewable exported energy.
Zero energy campus: An energy-efficient campus where, on a source-energy basis, the actual annual delivered energy is less than or equal to the onsite renewable exported energy.
Zero energy portfolio: An energy-efficient portfolio where, on a source energy basis, the actual annual delivered energy is less than or equal to the onsite renewable exported energy.
Zero energy community: An energy-efficient community where, on a source-energy basis, the actual annual delivered energy is less than or equal to the onsite renewable exported energy.
Designers also need to think about the boundaries of onsite renewable energy. Will the onsite renewable energy be within the building's footprint or outside the building footprint?
Determining if a building is a ZEB
Once the boundary is determined, designers must assess whether the building, campus, portfolio or community meets the ZEB definition. Designers must account for energy use including energy for heating, cooling, ventilation, hot water, lighting and plug loads, vehicle charging, process energy, and transportation in the building. Then they must check if onsite renewable energy offsets the delivered energy used for the above-mentioned energy use.
Unlike conventional green-building calculations, site energy is not part of the new DOE guidelines used to access ZEB. Site energy is the most popular definition for building owners because it is easily verified with utility bills. Essentially, the grid is treated as a battery for buying and selling energy. However, this approach does not consider energy cost, availability of fuel, fuel differences, or emissions and is not effective for measuring both onsite energy renewables or cogeneration systems. Site energy doesn't consider if a more polluting peaker plant is providing the energy or if a wind turbine running at night is doing the same.
Site energy does not consider the values of the various fuels at the source. So electricity and gas are measured equally even though electricity is three times as valuable as gas when looking at the source (power plant). Similarly, regional time-of-day source-energy valuation like California's TDV (time dependent valuation) is not accounted for at the site. Site energy attempts to treat all energy the same, but energy is not balanced both day and night—and the utilities know this. During the day, when the most polluting plants come online, electricity is the most expensive. At night, with cleaner generation online, prices come down. Consumers across the United States see this effect by way of costly demand charges that make up 30% to 70% of electricity bills. As a result of looking at the source energy, ZEB strongly encourages energy efficiency and energy storage systems because ZEB looks at inefficiencies of the grid.
Source-energy calculations convert different types of energy into equivalent units of raw fuel. Energy used at the site then accounts for the true calculation of energy as it travels from its creation at the source to the building including energy used to extract, process, and deliver as well as any transmission and distribution energy losses. In using this methodology, designers are looking at what happens to energy that is delivered and exported while accounting for the inefficiencies of delivering and exporting energy to and from the grid.
Energy delivered from the power plant (source) is calculated using national source-site energy ratios (See ASHRAE Standard 105). To calculate, simply add up the Btus of all the energy types delivered, multiplied by the corresponding source-site ratio. Then subtract the Btus of all energy types exported, multiplied by the corresponding source-site ratio. If the result is less than or equal to zero, the building is ZEB.
Delivery of onsite renewables to the building are not calculated in the ZEB calculation. Instead, the onsite renewable energy acts as a demand resource similar to daylighting. The Btus of delivered offsite energy resources are multiplied by their corresponding national source-site energy ratio. Likewise, exported energy including exported renewable energy is multiplied by the same source-site ratio. What's great about this calculation is that it is possible to attain with cogeneration and fewer renewables. In other words, the building becomes the power plant and ultimately requires importing the least amount of offsite resources. Cogeneration efficiency can be improved via onsite thermal energy storage systems, which can use waste heat to run chillers off-peak.
Onsite energy storage is encouraged, too, if the building is designed to export onsite renewable energy. Exported renewable energy has a lower value than if the energy was used at the building. So, there is an incentive to store energy at the building. In other words, PV has more value if used instantly. If sold directly to the grid instead, the unused onsite PV must be exported at a source-site ratio multiplier, resulting in more onsite PV.