By Mark MacCracken
Conceptually, a Zero Energy Building (ZEB) design is simple to understand and dozens of projects have undertaken the challenge of becoming zero energy. Some of them have even “succeeded.” The reason for the partial success is that there hasn’t been a clear, concise broadly accepted term or definition of what ZEB means. As a result, to some extent, designers, contractors and property owners have not grasped the entire scope of what it means to be zero energy or its larger implications beyond the building.
To help bring clarity across the industry, the Department of Energy released its definition for ZEBs. The new definition states that a ZEB is “an energy efficient building, where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.”
With ZEBs, as with other buildings, off-site energy resources such as oil, gas, electricity, steam and district heat and cooling, are delivered to the building to power operations. What makes buildings “zero energy” is what happens on-site. At a zero energy facility, the energy generated by these off-site resources is matched or even exceeded by on-site renewable generation, which can be designated for the building or exported to the grid.
Since ZEBs rely less on fossil fuels and more on carbon free renewable energy resources, it is a great step toward even greater environmental design. However, for renewables to truly be able to dramatically reduce our dependence on fossil fuels, we need to address the other vital characteristic of fossil fuels: storage. Fossil fuels are a form of stored energy, while renewable energy is simply energy. In order to reach their full potential, renewable energy collection must be coupled with storage technology, either on the grid side or building side of the electric meter, so that it can be dispatched as required.
Understanding the Larger Implications of Zero Energy Buildings (ZEB) on the Grid
Along with the major benefits of ZEBs, the rise of ZEBs will present major challenges for the grid. The best way to explain this is by example. Assume for a moment you have a ZEB building that is not connected to the grid or what is called “off-grid.” In this scenario, to keep the building operating, the site will need many forms of energy storage connected directly to the building: battery storage for regular nighttime operation, thermal storage for heating and cooling and fossil fuel for backup generation when all other forms are depleted. However, most ZEBs do not install storage and instead use the grid for their storage needs. The problem is the grid essentially has no storage capabilities. Therefore, the grid needs to have instantaneous backup fossil fuel generation available for the ZEB, even though the ZEB, on an annual “net” basis, will not need any energy from the grid.
Consequently, the inconsistent nature of renewable energy resources creates problems for utilities as annual energy consumption dwindles, but peak demand (maximum energy consumed during a 15 minute period within a billing cycle) does not change. Utilities quantify the relationship between average annual usage and peak demand with a term called “Load Factor (LF)” or “Capacity Factor.” It is a simple number that shows how well their assets of generation, transmission and distribution, are being utilized. In the 1960s they were at almost 70 percent LF, but the increase in the use of air-conditioning has resulted in creating higher peak demands on summer afternoons so that the LF is now below 50 percent in the US. If we look at the LF for a ZEB, by definition, its LF is ZERO (average load for the year is zero divided by any peak demand). So for the grid, the ZEB is the worst type of load.
In order to remain profitable as renewable generation increases, utilities are identifying energy storage as key to their future success. Energy storage is being rapidly added to both the grid and buildings. Businesses that don’t manage demand and/or utility energy storage are charged at a higher penalty for peak demand energy consumption, which most often occurs on hot summer days when utilities are pushed to their maximum capacity. It’s during these peak demand periods that the utility’s most expensive peaking plants come online for just a few hundred hours a year.
Energy Spikes, Demand Problems and Stored Cooling
While commercial ZEBs are energy efficient buildings with low carbon footprints, they still have large peak demands that they get charged for. To avoid high peak demand costs, energy storage allows buildings to virtually become their own batteries, storing energy when it is at its lowest price and calling upon it when they need it most, which is also when it is most expensive. Thermal energy storage specifically stores what has caused the most demand problems for buildings and the grid, namely cooling.
To understand the impact of “stored cooling” let’s use the backyard barbecue as an analogy. Most would agree that it is a bad idea to wait to start making ice cubes until the guests start walking through the door. It turns out you need about 1 pound of ice per person for the party to keep drinks cold and make mixed cocktails. Your refrigerator would have to be giant to handle that kind of production, so most people would create ice the night before. To cool an office space for that same person it takes the equivalent cooling of 150-300lbs of ice per person. As ludicrous as it sounds, most buildings do not store their cooling ahead of time, and instead rely on the instantaneous creation of cooling, which has caused the majority of the electric grid’s problems. In fact, 35 percent of peak electric draw on summer days is due to the instantaneous creation of cooling.
A Major Key to Making the Business Case for Zero Energy Building
When coupled with thermal energy storage, renewable generation can become a more cost effective source of power. For instance, a ZEB can reduce its peak demand by making ice using excess grid energy generated during cheaper, off-peak hours when electricity is plentiful, or when the building has excess solar energy. The ice can then be melted the next day to cool building occupants during expensive peak periods, if the cooling demand cannot be met by onsite generation.
On the other side of the meter, using cool storage with a ZEB, allows the utility to lower its peak demand and improve the grid’s load factor. Additionally, this practice will help the grid more effectively utilize renewables and make the energy available when needed so that the full benefits of ZEB and renewables can be realized.
Mark MacCracken is the CEO of CALMAC.
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