Kevin Dickson has come across an article about a high-performance house in Massachusetts that has got him wondering whether big photovoltaic systems are overtaking Passivhaus to become the next big trend in high-efficiency building.
The house is the work of R. Carter Scott and a design team that included Betsy Pettit and Joe Lstiburek of Building Science Corp., among a number of other experts. It was one of eight in Devens, Mass., that Scott’s company, Transformations Inc., was chosen to build for MassDevelopment, the state’s finance and development authority.
Among the many energy features of this net-zero energy home is an 18-kW photovoltaic array that completely covers the roof on the back of the house, including the porch and garage.
“Wow,” Dickson writes in a post at GreenBuildingAdvisor’s Q&A forum. “Does anyone agree with me that this is a stronger trend than Passivhuas requirements for new construction?”
Grid-tied PV is a standard part of net-zero designs, but at 18 kW, this system would dwarf most residential installations. The question at the heart of this Q&A Spotlight is whether falling PV prices and a variety of incentives such as Solar Renewable Energy Certificates have affected the way builders and homeowners make decisions about solar electricity.
Should PV arrays be designed solely to get a house across the net-zero energy threshold, or can PV serve another other purposes?
More power than a Passivhaus design would need
The PV panels on the Devens house should generate an estimated 10,200 kWh more electricity per year than it consumes, enough to power a Nissan Leaf or Chevy Volt for 30,000 miles, Scott has estimated.
As Mike Eliason points out, if the house were built to the Passivhaus standard and met its energy consumption requirement, it would consume no more than 6,828 kWh in a year.
So is the PV array overkill?
“Low PV prices, good PV incentives, and favorable feed-in tariffs allow homeowners to consider the installation of a very large PV array,” writes GBA senior editor Martin Holladay. “And the investment has a very quick financial return. If you can afford it, why not?”
“When it comes to Passivhaus certification,” Holladay adds, “it certainly seems optional or unnecessary if you already have a net-zero energy house. Why bother?”
Not a case of PV vs. Passivhaus
To John Semmelhack, Scott has found a “sweet spot” with “superinsulated saltboxes just a notch below Passivhaus efficiency.”
“At this point,” Semmelhack writes, “the giant PV systems here seem to be just as much a function of distorted incentives as low PV prices. The slow, regulated utilities (financed by Massachusetts ratepayers) are overpaying for the ‘solar attribute’ through high SREC prices and the state is kicking in unnecessary tax credits and rebates. Small, nimble businesses, such as Transformations, are stepping in to make some money while the gettin’ is good. I would do the same in their shoes.”
Semmelhack thinks the “relatively low” prices for the houses are made possible in part by the homeowner turning over all tax credits, rebates and SREC income from the PV system to the builder or, alternately, leasing the PV system from the builder.
“Brilliant strategy, I think. In addition, as a clever builder and business owner, Scott has integrated the PV installation into his own company, instead of outsourcing. Cheers to Carter.”
More PV, less conservation
Sonny Chatum’s answer to Kevin Dickson’s original question would be ‘yes.’
“Throwing up PV is a stronger trend than ANY sort of substantial, residential energy footprint reduction, I would bet, at least in the new residential/retrofit mainstream,” Chatum writes.
“For my state, I have personally and literally seen stack upon stack of applications for SRECs. There’s no way most of these people are substantially reducing their electricity demand (forget Pasivhaus) prior to silicon glazing their roofs. I would also bet that many of these people feel comfortable using MORE electricity after PV installation, since some of their previous bill has been offset by the PV.
“Sure,” he adds, “the PV helps the utilities meet renewable energy portfolio requirements, but it’s drops in a bucket with holes in it.”
An array as large as 18 kW also requires special approval from the utility, Dennis Heidner surmises, because it’s greater than the 10 kW normally allowed for a neighborhood transformer.
“There are 50+ panels on the house, probably 260w each, set up at different angles,” he says.
“Since this is larger than normally allowed net metering agreements, the house was designed…for the purpose of selling the power commercially back to the utility,” Heidner writes. “Perhaps taking advantage of good feed-in tariffs.
“Using the roof top as a source of revenue is actually a very clever and good idea,” he continues. “The solar on the roof was never about driving the consumption in the house to zero. It would be to compensate for the swimming pool, a car, and bring in extra revenue, etc.”
Our expert’s opinion
Here’s how GBA technical director Peter Yost views it:
I think pitting enclosure and HVAC efficiency against PV is inherently problematic. It’s not an either/or situation.
We all know that it’s Efficiency First, that efficiency dollars yield the quickest and best return on the investment, for the individual home and for the greater public. And we should have policies and incentives in place to promote and encourage efficiency as much if not more than renewables.
But if PV arrays don’t preclude subsequent efficiency work, and the opportunity for any size array presents itself, it should be squarely on the table. You can often interest a client in a non-invasive rooftop PV system over gutting the interior, but again, this is really an apples and oranges choice in my opinion.
Frankly, I favor centralized, typically ground-mount systems: large arrays (500 kW or more) perfectly situated and oriented for maximum gain, with three tilt-angle settings easily manually adjusted, seasonally. These community or large commercial arrays have much better economies of scale overall. Our town school system just signed an agreement for such a system, with land being leased from local landowners and investors with “tax appetite” financing the system to take advantage of tax underwriting that our (non-profit) municipality can’t.
So, residential roof top PV systems should be as big as the project and roof can bear, but for my money, let’s have sun harvesting more like what plants do: large fields that feed the grid more mightily than many tiny green PV roofs!