Net-Zero Energy Use
To achieve Net-Zero status, Unity house utilizes a solar panel array to capture the sun's energy. In contrast to photovoltaic systems that store their captured energy in stacks of batteries for use on cloudy days, or at night, the PV (photovoltaic) system on Unity is connected to the public electrical power grid. When the system is producing surplus energy beyond the electrical load requirements of the house, it feeds electricity back to the grid. Then, when no solar power is available, the house can use conventionally generated power off the grid. With its electrical credits offsetting its grid requirements, Unity achieves Net-Zero energy use.
The public power grid effectively acts like a large battery and buffer to level the spikes of the PV power output. A sophisticated monitoring system is located in a central location to inform the homeowner. Getting real time feedback about power usage helps to improve user behavior.
In a super-insulated house, the need for heating and cooling is relatively small. With such low heating and cooling loads, it makes sense to use the PV generated electricity for keeping the home at a comfortable temperature as well. This eliminates the need for other heating systems requiring fuel.
What is Net-Zero?
- Net-Zero-annual site energy: Defined as a building that offsets any imported energy by exporting an equal amount of site-derived energy. In this instance, on site is limited to within the building's footprint.
- Net-Zero-annual source energy: Based on energy used offsite to generate and transport the energy used at the building. This definition tends to favor the use of fossil fuels (it takes approximately one unit of site generated electricity to offset three units of imported natural gas).
- Net-Zero-annual energy cost: The amount of money the utility pays a building owner for the energy that building exports to the grid equals the amount it charges the owner for the energy imports from the grid. The amount of electricity that has to be generated on site and exported to offset imported electricity, depends entirely on the relationship between the purchase price and the price at which the power is sold to the grid. Variations in the utility grid can have a huge effect from year to year.
- Net-Zero-annual emissions: A zero-emissions building offsets emissions equivalent to the amount emitted through the source energy that powers the building. Calculation is often limited to greenhouse gas emissions, in this case a zero-emissions building can be called climate neutral. This can be achieved either through on site electricity production or through the purchase of RECs or green tags (even the most inefficient buildings can achieve climate neutrality).
Energy Modeling
A key tool in developing a true Net-Zero home is the energy model.
Using information, including projected electrical loads, heating requirements, the insulation values of the structure, and the different types of energy (electric, gas, solar), a profile of use is created. With this profile, designers can determine how best to supply the necessary energy, but also how to balance the needs and types of energy. Once these things have been determined, the appropriate systems can be developed.
For Unity, the energy model provides important information that will determine the size and placement of the photovoltaic rack. Based on the geographic location of the home, additional sources of power may need to be considered, strengthening the decision to keep the home "on the grid." Additionally, the model will provide data integral to the development of the heating system.
The results of this energy model, specifically the electrical loads, have been sent to the solar/photovoltaic consultant, for their review.
A Word About the Windows
In most homes and commercial structures, doors and windows represent the largest inefficiencies in an energy model. To build a Net-Zero home, the insulating properties of the windows must be as high as possible, given the cost considerations.