Earth RoomIn addition to capturing solar gain, Pura Vida takes advantage of another source of free heat (and cooling): from the relatively constant temperature of the ground. In Europe, earth tubes are often buried around the foundation of homes to provide tempered fresh air, but in the United States, there has been concern that earth tubes can grow mold or mildew. With Pura Vida, and subsequent buildings I have designed, an “earth room”—a modified approach to earth tubes—has been successful.
In Pura Vida, the earth room lies below the front porch. A short, 12-inch-diameter tube brings fresh outdoor air into one end of the earth room. The air flows the length of the 48-foot-long room where it is preheated (in the fall and winter) or precooled (spring and summer) through direct contact with the concrete walls prior to entering a heat recovery ventilator (HRV). The earth room is like a large thermal battery, storing heat in the summer for use in the fall and winter and storing “coolness” in the winter for use in the spring and summer. The earth room eliminates the need to use a conventional mechanical heating system for about two months of the year (October and November) and eliminates the need to run a cooling system from mid-May to mid-June. Throughout the rest of year, the earth room significantly reduces the heating and cooling loads.
After passing through the earth room, the air enters the Nu-Air Ventilation Enerboss, a complete heating, filtration, and HRV system. An efficient fan constantly pulls air from the bathrooms and kitchen, which is exhausted, while the same amount of fresh air is evenly distributed throughout the home by an airflow-balanced high-velocity duct system. The system has operated flawlessly for the past five years. The Enerboss system requires an external source of hot water for heating the air to be distributed. In Pura Vida, we use a 4.5 kW Marathon water heater to provide hot water to the heat exchanger coils within the Enerboss. Air conditioning is provided by a 3-ton, 16 SEER Lennox Elite. The evaporator for the AC unit is mounted on top of the Enerboss.
Visitors to Pura Vida often comment how fresh the air is and how quiet it is within the home. The comfort provided by constantly filtered fresh air moving throughout the home makes it difficult to go back to living with a conventional HVAC system.
Domestic Hot WaterA small Nyle Systems air-to-water heat pump mounted on a wall in the earth room provides domestic water heating. A timer is programmed to turn it on in the evenings when the time-of-use electricity price is $0.02 to $0.03 per kWh. The hot water is stored in a well-insulated 105-gallon Marathon water heater for use during the day. The average monthly cost for domestic hot water is about $5. (The timer is bypassed when we have guests or need to use more hot water.)
We also have a GFX Technologies wastewater-to-water heat exchanger. Although the concept of recovering the energy in hot water going down the drain is an interesting one, the high cost of copper makes this technology too expensive to be cost-effective for the amount of preheated water the unit provides.
Demonstrated PerformanceSix Lascar temperature and humidity sensors were placed throughout the home, outside, and in the earth room. Data collected from these sensors every 30 minutes for the past five years—along with energy use data collected from several TED (The Energy Detective) units and our utility bills—have validated the home’s performance.
The all-electric home’s advantage is that we can measure and directly compare the energy use for every appliance and system. Having collected data on the efficiency of the air-to-water heat pump in the earth room for providing domestic hot water, I plan to modify the heating system to include a second heat pump, instead of the existing water heater, for space heating. This should reduce the electricity demand for space heating by at least 60%.
When the home was built, we also decided to sign up for time-of-use utility metering. This utility billing method provides us with cheaper energy during off-peak times, and more expensive energy during peak times. The risk we took with this decision was that if we needed to use large amounts of energy during peak times—like running air conditioning midday during the summer—the cost could be significantly higher. The table (upper right) shows our average annual energy use for the various portions of the property along with an approximate annual cost for each.
Because of the relatively high upfront cost of renewable electricity systems, we felt it was important to first design and build a home that is as efficient as possible—and then get a good understanding of the home’s energy demands. A year after the home was completed, we applied for and received a state grant to measure the effectiveness of residential small wind systems.
We installed a 2.4 kW Southwest Windpower Skystream 3.7 turbine on a 60-foot tower (at the time, we did not understand that height is too short for nearly all applications). The utility account was converted to net metering so that we could sell our excess energy back to Commonwealth Edison. The energy production from the Skystream was monitored through a Zigbee data system and logged on a computer.
We believed we had a wind resource that was sufficient to justify a wind-electric system. Our chosen turbine, the Skystream, is predicted to produce about 2,600 kWh per year given an 11 mph resource, and about 900 kWh per year in 8 mph winds. After several years of operation, the system has produced roughly 500 kWh per year, or the equivalent of $50 of energy, which is expensive electricity when you consider the $21,000 installation cost.
It is clear now that the information we started with was not sufficient to justify using wind energy at our site. Factors contributing to our turbine’s lower-than-anticipated production include a tree line that sits about 800 feet to the west, resulting in turbulent wind from the prevailing wind direction. A taller tower would undoubtedly help with this problem, because the farther you get away from the earth and its obstructions, the more wind there is. As a well-known small-wind expert has said, we unknowingly chose expensive energy over an expensive tower.
Over the past several years, the cost of photovoltaic systems has come down dramatically. During that time, we collected information on the energy demand for Pura Vida. We found that during the fall and spring, when there was no energy use for heating or cooling, the average demand was between 600 and 700 kWh per month.
In 2011, we installed a 4.3 kW grid-tied PV system that should average about 400 kWh per month. Twenty-four batteries provide backup for power outages and provide enough energy to live without the grid periodically—assuming that the backup heating would be provided by the wood heater instead of electricity. Performance data is being collected by the OutBack MATE3 energy monitor and we should have an accurate performance picture by the end of 2012.
Future of APHWe believe that building super-energy-efficient homes will have a significant impact on U.S. energy use. If the building cost of an APH is within 15% of a conventional building method, a conservative energy cost savings estimate is more than $1,000 each year per home.
One outcome of the recent economic downturn has been that the American consciousness has been awakened to the importance of common-sense solutions to important challenges. For families paying to heat and cool their homes—and as a nation that needs to use less fossil fuel—emphasizing energy efficiency in homebuilding is a simple yet promising way to address these challenges.