Operating at Net-Zero Energy
Net-zero strategies for new houses are well established, but retrofitting an existing house is another story
A few months ago we completed a deep energy retrofit of a house that we hope will be net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. — in other words, that we hope will produce as much energy as it consumes on an annual basis. If we succeed, this will be our first net-zero project.
There are two key strategies for designing a net-zero-ready home: you minimize the amount of energy the house needs to operate; and you maximize the amount of energy that the house can produce onsite, usually with a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. system (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.). In short, you create an energy budget that balances consumption with production.
Implementing these strategies on a new home is relatively straightforward. In fact, leaders in the high-performance building industry have been building net zeroProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. Calculating net-zero energy can be difficult, particularly in grid-tied renewable energy systems, because of transmission losses in power lines and other considerations. ready homes for years now, and some are even doing so at market rate. (To learn more about one Massachusetts company, Transformations Inc., that is building market-rate and affordable net zero energy homes, click here.)
Existing houses are a bigger challenge
The same can’t be said for existing homes. On the contrary, retrofitting a home to operate on a net-zero basis is a pretty audacious undertaking. Most of the homes we work on push back hard against such ambition. There are several obstacles, including our relatively harsh climate; the older, architecturally complex character of our housing stock; the frequency with which homes have been renovated without attention to efficiency; and the prevalence of smaller lots, particularly in urban settings.
But with this project we had two things going for us: a relatively accommodating building, and highly motivated homeowners (not to mention our top-notch planning team, ledLight-emitting diode. Illumination technology that produces light by running electrical current through a semiconductor diode. LED lamps are much longer lasting and much more energy efficient than incandescent lamps; unlike fluorescent lamps, LED lamps do not contain mercury and can be readily dimmed. by David Foley of Holland and Foley Architecture).
The home’s relatively simple form made it possible for us to superinsulate the building shell and drastically cut air leakage. This drove the heat loss down low enough that we were able to significantly downsize the mechanical system, replacing the existing gas boiler with three minisplit heat pumps for both heating and cooling. (Heat-recovery ventilation(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ensures a steady supply of fresh air.)
Equally important is the home’s unshaded south-facing roof, which supports an 11.7-kW PV system projected to produce 15,000 kWh per year, or 120% of what the house is modeled to consume. (For context: according to the Energy Information Administration, the average Massachusetts household consumes roughly 32,000 kWh annually, or two and a half times as much as this house).
It's up to the homeowners to make it work
Now it’s up to the highly motivated homeowners to actually make it to net zero — i.e., to live within their energy budget. We are currently in the process of gathering monthly usage and production data to determine how close we get to this goal. We are also monitoring at a more finely grained level, increasing the likelihood that we will be able to get there.
With the help of the SiteSage eMonitor system, we are collecting circuit-by-circuit usage data. Monitoring this closely has several purposes, not least of which is it tells us whether equipment is working properly. When a usage pattern looks atypical, we can sometimes use the eMonitor to help us troubleshoot in real-time.
We can also use circuit-by-circuit monitoring to help the homeowners develop net zero “habits” — to give them the feedback they need to live within their energy budget. We can let them know when they’re at risk of exceeding their budget and overdrawing their “energy account.”
While in many respects living in a net-zero energy home is easier than living in a standard home (for one thing, a well-designed net-zero energy home is far more comfortable from a thermal standpoint), net-zero living does take some adjustment. Much of the conventional wisdom about operations and maintenance, particularly in regards to mechanical equipment, doesn’t translate well to low energy homes.
For example, minisplit heat pumps operate most efficiently when they are maintained at a fairly constant temperature. The eMonitor data can help us identify whether the homeowners are laying down a net-zero habit of “setting and forgetting” their minisplits, or are falling back into a habit of using setbacks.
It remains to be seen whether we will achieve our goal of net zero energy. If we do, there’s no question that thoughtful operations on the part of the homeowners, supported by monitoring on our part, will have played a key role in getting us there — at least as key as lucking into such an accommodating house in the first place.
Paul Eldrenkamp is the owner of Byggmeister, a design/build firm based in Newton, Mass. Rachel White is the company's performance manager. This blog, the third in a series of posts about the project, originally was published at the Byggmeister website. The first and second posts are Preparing a Historic Home for the Next 100 Years, and Planning for Net Zero Energy.
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