After working through several projects to retrofit an existing home to make it more energy efficient, it's hard not to think about what could be done by starting from scratch. A clean slate isn't something that everyone will get when it comes to living in an energy efficient dwelling but those that decide to build their own home should consider building a Zero Net Energy(ZNE) Home.
A home is considered ZNE when it produces more energy than it uses, typically, measured over a year. This doesn't mean that the home is "off the grid" or doesn't use grid energy. Actually, the home is connected to the local utility and pushes any excess power out to the grid for others to use. For example, a solar PV array will generate electricity during the day but is dormant at night. However, the array is usually sized to produce more than enough energy to offset the amount of grid power used at night or when energy is not being produced. The net result is more energy flowing into the grid from the home than is used over a year.
There are two primary considerations when building a ZNE home. First, you must have an energy generation system installed such as a solar array or wind turbine. Second, you need to use building techniques that optimize the energy efficiency of the home with the goal of reducing your energy footprint to fit within the power generation capability of the home.
How do you build a Zero-Net Energy Home? Below are ten steps to get you there.
- Location, Location, Location. Do some research on the NREL site to find out if your region is good for solar or wind. If solar works in your area, make sure you pick a lot that has great sun exposure and that will allow you to position your roof in a "true solar south" orientation. Trees provide cool shade for humans but greatly reduce the energy output of a solar array. If wind is an option, identify the optimum height for your wind tower and select a location within reach of your building site. Selecting a location that is near your work and amenities will help you save on transportation energy costs.
- Insulated Concrete Forms (ICF) Consider building your home with insulated concrete forms. This is accomplished using ICF blocks that are assembled and interlocked to form walls. Once the walls are assembled, concrete is poured into the center of the insulated blocks. The blocks have mounting mechanisms built in for drywall, siding, etc. to make for easy construction. The resulting structure is a super-insulated envelope that will last 200-300 hundred years.
- Foam Insulation Icynene spray foam insulation is a great choice for insulating the under roof and attic space in your new home. The goal is to effectively seal the building envelope and then control the entry and exit of air to/from the home.
- High Efficiency Windows. Use triple-paned, krypton-filled windows or equivalent for best efficiency. Orient most of the windows south to take advantage of passive solar heating and cooling effects, while ensuring plenty of light on all floors. This completes the process of properly sealing the building envelope.
- Heat Recovery Ventilator (HRV) An HRV controls the exchange of incoming and outgoing air from the home. If your building envelope is sealed using the products listed above, the HRV should provide the only exchange of air when all doors and windows are closed.
When air enters and leaves your home, the new and old air are forced through a heat exchanger that transfers the energy to/from the incoming and outgoing air to help reduce the need to recondition the home. In the winter, the heat from the outgoing air is transferred to the colder incoming air. In the summer, the heat from the incoming air is transferred to the outgoing colder air to reduce the amount of energy required to recondition the air entering the home.
HRVs can recover up to 80 percent of the energy from outgoing tempered air that is completely lost in conventional homes. The system also allows for superior air quality and serves as a mechanism for removing humidity from the home.
- Climate Control System - Ground Source heat pump(GSHP) A GSHP provides heating and cooling using the constant temperature of the earth several feet below the surface. As in caves, the temperature below the surface of the earth is warmer than surface air in the winter months and colder in the summer months. GSHP systems exchange the energy between surface air and the ground using an underground loop of piping. GSHPs are much more efficient than Air Source Heat Pumps, last much longer and require very little maintenance. They can also provide some hot water via a pre-heat tank.
- Electricity Generation - Solar Array/wind turbine One of the key factors of building a ZNE home is to find a way to generate electricity at the point of use. Typically, we assume that the fuel source used comes from renewable sources such as wind and solar. Choosing this fuel source depends on your location. If you have access to plenty of sunlight, solar is your best choice. If you are in an area with a good source of steady wind, a turbine is a great choice. If you have access to both then you have done your homework and have picked a great location for a Zero Net Energy home. Congratulations! You can choose either or both sources for power generation. Typically, you will want a system with an output of 8-10 kW to meet the demand of a 2500-3000 square foot home with a family of four using the building technologies described in this article.
- Hot Water System - Solar Hot Water(SHW) If you have sun and a good south-facing roof area, add SHW collectors to your roof and a pre-heat tank to the hot water system in your home. SHW uses the radiant heat of the sun to heat water. This is much more efficient than solar photovoltaic systems that require conversion of photons into electricity. They are also more forgiving with partial shading of the collectors and will provide energy in environments that would disable a solar electricity system. If you are also using a Ground Source Heat Pump (above) you can use some of the waste heat to supplement the SHW.
- Energy Star Appliances When choosing appliances, make sure to visit the DOE Energy Star site to pick the most efficient appliances for your home. Appliances are some of the most energy intensive devices in the home so you can save on efficiency by making good appliance choices.
- EV Charging Station This is an article on home efficiency but you can't overlook ways to improve your transportation efficiency. If you picked a location that doesn't require a large commute to work and to local amenities consider buying an electric vehicle (EV) as your primary source of transportation. If your budget permits, you can keep a fuel effieient gas or diesel vehicle for road trips or consider renting a car for trips that won't allow for electric transport. Install an EV charging station in your garage to refuel your EV. The J1772 charging standard has been adopted by most EV manufacturers. However, your best bet is to pick an EV and buy their charging station. Many states offer incentives for buying and installing an EV charging station. Make sure to check the DOE's state incentive finder site to see if your location qualifies.
DOE site on Electric Vehicles: http://www.fueleconomy.gov/feg/evtech.shtml
Solar site selection: http://www1.eere.energy.gov/solar/want_pv.html
Wind Site Maps: http://rredc.nrel.gov/wind/pubs/atlas/maps.html#2-12
DOE site on insulation: http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11320
DOE site on High Efficiency Windows: http://www.energysavers.gov/your_home/windows_doors_skylights/index.cfm/mytopic=13330
DOE site on HRV/ERV Systems: http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
DOE site on Space Heating: http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12640
DOE site on Solar Hot Water: http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12490
DOE site on Energy Efficient Appliances: http://www.energysavers.gov/your_home/appliances/index.cfm/mytopic=10020
Here are some helpful links for more information on building Zero Net Energy Homes:
Link to DOE site for Zero Energy Home Design: http://www.energysavers.gov/your_home/designing_remodeling/index.cfm/mytopic=10360
Link to Solar Today article on a Functioning ZNE Home: http://www.solartoday-digital.org/solartoday/20100708#pg48
Ground Source heat pump manufacturer - http://waterfurnace.com
Foam insulation - search for local installers: http://icynene.com
Find an ICF manufacturer - http://www.forms.org
Heat Recovery Ventilator Manufacturer: http://lifebreath.com
An Example of a ZNE Builder in New Paltz, NY: http://greenhillcontracting.com
Green Acres ZNE subdivision in New Paltz, NY: http://greenacresnewpaltz.com
Online courses offered by the Florida Solar Energy Center on building ZNE Homes: http://www.fsec.ucf.edu/en/education/cont_ed/bldg/zero.php
- The Mapawatt Team