Zero Energy Models and definitions influence design outcomes
Net zero-energy buildings are an exceptional concept in that such buildings consume very little energy while using renewable energy sources to feed power back to the grid. In other words, these residential and commercial buildings give back the same amount of energy they take in, if not more. The term “zero energy,” however, is somewhat unclear, and there is little agreement about what it means for a building to be defined as such.
A clear definition of zero energy is necessary because the way we think of net zero-energy buildings affects that way that designers go about attempting to accomplish this goal. Depending on how the definition is stated, zero energy buildings can be approached from the point of view of the consumer or the supplier. We will look at the advantages and disadvantages of four commonly accepted descriptions of low-energy buildings—net-zero site energy, net-zero source energy, net-zero energy costs, and net-zero energy emissions; hence a major change in the actual definition as specified can influence the design, construction, or energy retrofit of a zero energy building.
Energy usage in buildings occupies about 40% of the total energy consumption in the country, including 70% of the total electrical use. Energy use in buildings continues to grow each year, as new buildings continue to develop before older buildings end their lifespans. In fact, the use of electricity in commercial buildings alone doubled over 20 years in the year 2000, and will continue to grow by about 50% within the next twenty years. The demand for energy and consumption of fossil fuels to power buildings will only continue to grow unless designers find a way for buildings to give back as much energy as they consume. The Department of Energy is already actively pursuing this goal, gathering the necessary research and developing technology to develop economically-sound zero energy homes and commercial properties by the year 2025.
Although we use the phrase “zero energy” to describe these buildings, we only have loose definitions to describe what this term really means. In theory, net zero-energy buildings should allow for a balance between the amount of energy used and the amount gained from renewable energy sources, but there is little understanding or consensus on how this can happen.
Microgrid Energy focuses on the way that low energy buildings can save additional energy, we look at how zero energy buildings may be designed, in order to effectively meet the goal of sustainability. Setting goals and finding working definitions for zero energy buildings affects the way that such buildings and their energy systems are designed. Setting design goals is vital to ensuring these buildings are highly efficient and still functional, which is why it is so important that we focus on the definition of the zero energy goal so that we can learn which renewable sources and materials are most desirable.
Zero Energy Design
Ideas behind zero energy buildings emphasize that any energy needs may be met from renewable energy sources which cost once to acquire and have a minimum impact on the environment. A zero energy building should produce just as much energy as it uses each year, or potentially return energy back to the grid. Some of the most commonly-accepted concepts and modelling about the zero energy goal are detailed below in order to establish a closer definition.
Grid Tie Buildings for Grid 2.0
In general, zero energy buildings still use conventional fuel sources to run when renewable energy sources and pre-generated energy are not enough. If there is more energy generated that the zero energy building needs, leftover energy feeds back to the grid. The utility grid serves as a way of keeping track of how much energy is offset throughout the year. Although going off the grid entirely seems ideal, this would be tough to accomplish, as current renewable technologies are not capable of storing a lot of excess energy. Even for homes and commercial spaces that are off the grid, alternative fuel sources like propane are used to power ovens, heaters, and water heaters. And because such homes have no way to give back any excess energy the building produces and cannot always store it, this energy is essentially wasted. Energy surety comes from Microgrid designs that allow for the new grid 2.0 to arrive where buildings can jump on the grid- grid tied; and also during certain times disconnect and run off the grid commonly known as islanding. This is where we are at the intersection of the future with smart grid, communications, coupled with onsite generation from solar and some cases engines, backed with energy storage all topped off with analytics to optimize the microgrid and zero energy building.
While ideally, any additional energy that is generated would be fed back into the grid, this may not always be possible, and in some cased feeding back into the grid during crisis needs could provide a more well balanced grid. If there were enough buildings generating energy through renewable sources in a single area, any energy left over would need to be stored and could be discharged or used to provide uninterruptable power, or dis-charged during peak time of use periods depending if utility has that provision in an effort to reduce or peak shave consumption during highest peak energy hours.
Distributed On-Site Generation is optimal when applicable
There are a number of renewable sources available for zero energy buildings. Many are familiar with the conventional PV solar panels, as well as wind and hydroelectric power. These choices are more energy-efficient and environmentally-sound than natural gas and other common fuels used in homes and businesses today. When deciding which renewable technologies are most desirable for use with zero energy buildings, it is useful to take the following considerations into account.
- The technology has a minimal effect on the environment as it leads the way towards more efficient residential and commercial building designs.
- The technology will be locally available and will not die out within the lifetime of the building.
- The technology will encourage future designers to use similarly efficient building designs with this renewable technology and optimal microgrid design.
The most preferable renewable energy sources are those which can be integrated into the building design on-site and replicated on a larger scale for future zero energy building designs. Rooftop PV panels, for example, and solar water heaters are high on the list when it comes to the most ideal renewable technologies for widespread use in zero energy buildings. Renewable energy sources such as wind, which is usually available only off-site, may be a more restricted source of energy on certain occasions wind may work as onsite energy. Wind and solar energy in in certain instances may be more viable when found onsite in a parking lot structure. Note: Those buildings that obtain most of their renewable energy from off-site sources, however, should be defined differently than other buildings with zero net consumption.
“Zero Energy” Definitions Influence Design
How each definition is employed has an impact on the way we perceive the design of zero-energy buildings. Depending on the interests of the person defining the zero energy building, it can be viewed in several different ways. One group of people might define a zero net energy building according to how little pollution results from carbon dioxide emissions, for example. The designers, on the other hand, may primarily concern themselves with meeting the energy specifications laid out for them. While the Department of Energy is looking at how a zero energy building reduces the national energy consumption average, the owner of a building is likely to be more aware of the cost. These differing values and viewpoints lead us to four commonly accepted definitions which we will define below.
Each of the four terms calculates net energy usage using the grid as a reference point. While the terms still work for homes and commercial buildings that are off-grid, these off-site zero-energy buildings require the purchase of off-site renewable energy. Therefore, for our purposes and in order to support the zero energy goal of the Department of Energy, we only look at those buildings which get renewable energy on-site. These definitions assume that the renewable energy source used will continue to be around as long as the building is. Any zero energy building that receives energy away from the property is an off-site zero-energy building.
- A net-zero site energy building generates the same amount of energy it consumes over the course of a year, if not more, according to on-site calculations.
- A net-zero source energy building also generates at least the amount of energy consumed, but this is calculated at the renewable energy source. Both on-site and off-site energy sources are taken into account in order to look at the total amount of energy used to power the building and the total amount of energy generated.
- A net-zero energy costs building calculates energy usage and generation by cost. How much the building owner takes in for generating energy and feeding it back to the grid must be the same as the amount paid for energy over the year.
- A net-zero energy emissions building looks at energy consumption from an environmental standpoint. The amount of emissions an energy source generates must be offset by the amount of no-emission renewable energy the building uses.
Definition: Net-Zero Site Energy Building
Net-zero “site” energy buildings use the same amount of energy that they produce, using the grid to verify this amount. Some of the most common on-site sources that can account for zero-energy buildings include rooftop PV panels and solar water heaters. Others include additional power sources still located on the property but not directly on the building, such as photovoltaic solar parking lots, small wind turbines, and some hydroelectric sources.
One potential disadvantage to this definition is that there is no differentiation between the value of one derived fuel over another. Although we know that electricity is more valuable than natural gas, they are accounted for equally by the net-zero energy building definition. Therefore, by definition, a site zero-energy building is the same thing as a source zero-energy building when that building is powered solely by electricity. But a building that uses a lot of natural gas will need to produce a lot more on-site energy if it is defined as a site zero-energy building than if we define it as a source zero-energy building. The building must accommodate a far more efficient design in order to offset gas use at the same rate as it offsets electricity use.
One advantage of the net-zero site energy definition is that the energy use and production is easy to measure. While other definitions involve more complex calculations, the energy use of site zero-energy buildings can be measured directly on site. This measurability should be an important factor in achieving the zero energy goal.
Definition: Net-Zero Source Energy Building
Net zero “source” energy buildings also generate the same amount of energy they consume, but this amount is calculated at the energy source. In order to arrive at this measurement, we multiply the energy used and consumed by site-to-source energy considerations. Therefore, we must also look at factors such as how much energy is lost in transmission. Because the conversion rate of electricity to natural gas is about 3 to 1 (3 units of natural gas for about 1 unit of electricity), the net zero source energy definition may urge businesses and homeowners to use natural gas in place of electricity wherever possible as a means of benefiting from the potential of switching fuels to help meet the zero energy goal.
One issue is that measuring the source energy use is based on the national average, which does not represent the fact that the source energy may vary at peak hours or that the heat rate may vary at the power plant. For example, using electricity late at night will have less of an effect on the source energy than using it during the peak hours of the day time. This would likely require time-dependent variations in different regional zones, something that has already been developed in California in order to examine hourly variations in source energy for 16 different regions or known as Time of Use demand pricing. California utilities will eventually go to real time pricing as the form of time of use or TOU. Another limitation of the source zero-energy building is that cost is not accounted for. Even as a source zero energy building, there may be few cost savings, but this may be reduced more effectively if the source zero energy building definition takes peak hours and other Time of Use variations into consideration.
Definition: Net Zero-Energy Cost Building
With a net zero-energy “cost” building, the cost of energy usage is the same as what the building owner gets back in financial credit for producing energy. The financial credit must make up for the total spent in energy costs, including peak variations and taxes in addition to the meter charge. This definition allows for us to look at fuel sources in a way that is relatively accurate and even, so the cost of energy and its availability helps determine the best way to design the zero-energy building. On the other hand, fuel costs can vary significantly from year to year and even from month to month. So this creates sort of a moving target while a building may meet the zero-energy goal by this definition in one year, it may not the following year.
In order to achieve the zero-energy goal on a large scale, the net-zero energy cost building definition may not be sufficient. Utility costs can vary drastically between regions, at different times of day, and throughout the year. As more and more low-energy buildings are constructed in higher concentrations in a given area, the rate of incentive may reduce. Any electric utility has a lot of costs to maintain—purchasing fuel, for example, and remaining profitable for shareholders’ interests—and this results in rate variations that would not allow for a fixed incentive for those in cost zero-energy buildings. As there are more and more efficient buildings in an area, a utility is unable to offer the same incentives it once could, and the cost of purchasing the utility may increase as well. This would force utilities to raise rates, making it more difficult to maintain a cost zero-energy building.
Achieving the cost zero-energy building goal is most difficult in commercial applications, as cost depends largely on how a utility charges and credits production. Cost zero-energy buildings may be achievable if the rate of utilities were not fully dependent on demand. However, all of the following conditions would be required for a cost zero energy building to be maintained:
- Low energy cost.
- Photovoltaic panels are able to offset energy costs based on demand. At peak hours, photovoltaic panels do little to reduce cost, but lower demand would allow the zero energy building to be eligible for a smaller rate.
- Utility charges which emphasize amount of energy used rather than peak changes. Shifting charges towards amount of energy usage would help divide costs more evenly among peak demand and energy spending, which would be better for a cost zero-energy building.
- An agreement between the utility company and the building owners to financially credit any additional energy produced without placing limitations on the total size of a photovoltaic system. In most cases, a utility compensates a building owner based on the amount of money the utility saves by not producing this electricity, commonly known as avoided cost. What would be more ideal for cost zero energy buildings would be for the utility to credit the building owner at the standard rate. This additional energy credit should be allowed for use paying off other energy-related fees, including taxes and peak charges.
A cost zero-energy building is not feasible unless this energy credit can go toward additional related energy charges, regardless of the capacity of the photovoltaic panel system, the savings during peak demand, or the balance between energy and non-energy charges. The net-cost zero-energy building is tough to maintain unless demand costs make up a large part of the total utility bill.
Where this type of net metering is not possible, cost zero energy buildings should store energy created on-site in order to maintain peak demand costs. The definition of the cost zero-energy building should take this into account, as demand costs can be reduced by releasing stored energy rather than giving this energy back to the grid.
Definition: Net-Zero Energy Emissions Building
The net-zero energy “emissions” building is designed to use only the same amount of carbon emissions-generating energy sources as it produces no-emissions energy from renewable sources. It does so by using photovoltaic panels, Thermal solar technologies, wind power, some hydro sources, and other possible on-site renewable energy. A building already qualifies as no emission when it meets all of its energy needs from off-site renewable sources, and this building would not have to produce any additional energy in order to qualify as a net-zero energy emissions building. But if any other fuel source is used for any purpose—a natural gas boiler, for example—then it would have to give back as much renewable energy as is necessary to offset this use. The building owner could also buy a source of offsetting credits which could be in the form of Carbon Credits, Renewable Energy Credits, White Tags, Solar Renewable Energy Credits SREC’s or similar in order to become an off-site net zero energy emissions building.
The net zero energy emissions goal would depend largely on which type of energy source is utilized for generation. Emissions from nuclear sources differ greatly from emissions of coal, for example, and the source from which the energy is produced has a major impact on whether a building is actually offsetting the energy use. Coal produces greater emissions than hydro, so a building located in a region that depends primarily on hydro sources for energy production would need to generate less energy than one in an area that obtains most of its energy from coal. The main issue with this type of definition is that measuring whether a building is truly a zero emissions building is somewhat difficult because it is not always easy to tell which type of energy is used to produce the electricity consumed by the building unless you know the mix the utility is using to provide power. Just like with source zero energy buildings, it is necessary to understand more about the source of the electricity and how it is delivered to the building for an accurate record of emissions from each energy source used.