Last post, I described some of the details on the 20 homes that were included in my research project on net-zero and near net-zero energy homes in New England. Today, I’ll describe some of the common features of these homes, and also explain how I collected the data I used in my research. I’ll try not to get too bogged down in details here, but forgive me if I do, please!
Insulation levels: Most homeowners understand that the more insulation they have in their home, the lower their energy bills ought to be. Most localities have building or energy codes that specify minimum levels of insulation for walls, roofs, floors, etc. Typical new homes are built to code, and hence meet these minimums. However, homes designed to minimize HVAC energy loads will have high amounts of insulation in their walls or “envelope”. This of course includes homes designed to be net-zero or near net-zero. Table 1 below shows the average wall and roof R-values (in F-ft^2-hour/Btu) for the homes in my research project.
|House ID||R-Value Walls||R-Value Roof|
Tight envelope and air “sealing”: Buildings–homes included–typically “leak” air between the interior and exterior. If they have a lot of leaks, they’re labeled “drafty”. For instance, if you’ve ever sat next to a single pane window in an old colonial or Victorian style house on a New Hampshire winter day, you’ve probably felt a draft coming from that window. Newer, better-built homes won’t feel drafty, but they still leak. For example, we live in an Energy Star certified home with above-code insulation. However, on a cold windy day, I can still feel cool air coming into my house through some electrical outlets mounted in exterior walls. The wind is pushing air through small gaps in the exterior sheathing, and that air works it’s way through the various layers of the wall until is finds its way into the interior, via the slots in the outlets.
One metric of how well a home is sealed against air “infiltration” is called air changes per hour (ACH). An ACH of 1.0 means that there is enough air moving to replace the entire volume of air in the house in just one hour. That would not be good from an energy consumption perspective, since the heating system would have to keep warming up the cooler air as it moved into the house. Of course, you need some fresh air entering the home otherwise the air quality would deteriorate. So, while a low ACH will reduce load on a home’s HVAC systems, you don’t want it to be zero.
There is a tremendous amount of variability in ACH rates across the housing stock in the US, depending on age, location, style, etc., etc. However, having a well-sealed envelope is a basic design parameter in a net-zero home, and, based on the ACH rates listed below, all the homes in my research were very well-sealed indeed:
|House ID||Natural ACH (heating)|
Windows: Like high amounts of insulation and excellent air-tightness, having high-quality windows is nearly a prerequisite for a net-zero energy home. A lot of energy can be lost through windows–not just through leaky frames but passing through the window pane and frame themselves via conduction and convection. The U-value of a window is used for comparing the heat transfer parameter of a window (U-value = 1/R-Value). The lower the U-value, the less heat will be lost through that window. I can hear you now–“I bet the homes in Walter’s project had really low U-values, too”. Well, you’re right! Here they are:
|House ID||Average Window
Keep in mind these were average U-Values. Every home used multiple styles of windows, and each would have had a different U-value.
Lighting: All but one of the houses used at least 85% LED or CFL lighting. This is pretty straightforward, in that LEDs and CFLs typically use 70-80% less energy than a comparable incandescent light, hence they are a natural feature for any net-zero or near net-zero designed home.
Appliances: All the homes used Energy Star certified appliances. Many used very efficient appliances, electric or gas.
Passive solar design: All the homes’ long axes ran east-west, meaning they had long south-facing walls. In New England, where heating load is far greater than cooling, long south walls mean more solar gain in the winters, which will reduce heating load. This heat gain comes mostly through windows in the south walls. The homes had interior shading devices (curtains, blinds, etc.) to cut the solar gain during the summer, to lower cooling load when necessary. Many of them were designed to maximize natural or “day” lighting, too, to further reduce lighting energy load.
I think you get the idea. All of these homes were designed to minimize energy consumption, and hence they all had high amounts of insulation, were very well-sealed, used excellent windows, CFL and/or LED lights, and highly efficient appliances, and were built with their long axis running east-west to maximize solar gain. These are all features common to net-zero homes, and it is no surprise they were included in these homes built in New England.
Okay, just a little bit on my data collection methodology. Once a homeowner signed on to participate, I sent him or her an “initial survey” to collect basic construction information, occupant statistics, cost information, HVAC systems make and model number, etc. This came to about 50 questions. Some of the owners sent the completed survey back within a week or two, others took months and months. Beginning with September 2011, I collected energy consumption and production measurements in one of three ways: homeowners sent the information directly via email; homeowners sent a copy of their actual utility bills, from which information was collected; or the information was read off production/consumption reporting websites to which homeowners had granted me access (e.g., Solectria Renewables’ “Solren View” website). Depending on availability and applicability, data included: billed electrical and natural gas usage; monthly cost for electricity and natural gas; quantity and cost of any other fuel purchased or consumed that month (e.g., gallons of propane); and energy production by renewable generation systems. Eventually I collected 12 months of energy data on 17 out of the 20 homes, with three homes providing fewer than 12 months because they started a few months late. I also gathered energy cost information from 16 out of the 20 homeowners.
The surveys and monthly data gathering was just the tip of the data iceberg, so to speak. I also placed temperature and humidity data loggers inside and outside 13 of the homes, to record those parameters during the project. I installed eMonitors in two of the homes to monitor circuit-level energy consumption (had I had any funding, I would have installed these great devices in all the homes). Two other homeowners installed eMonitors on their own and provided me with their data. Then I went to most of the homes and, using a Solmetric SunEye device, measured the loss due to shading of their PV systems. I also downloaded lots of weather data from the NWS and other providers to use in my modeling. Finally, I downloaded Typical Meteorological Year (TMY) data sets from NREL’s site to use in my PV production models. Lots and lots of data.
That’s clearly enough for now. Next post, I will get into my energy modeling, starting with PV production. After that, I’ll get into the consumption modeling, and then start presenting some results. Looking further ahead, there’s the economic results to discuss, as well as the carbon dioxide emission simulations I performed, then my conclusions and recommendations. So, lots more to come people. I hope I am keeping it interesting for you!