Thriving On Low Carbon

One year energy usage

One year ago today NSTAR installed a commercial meter here at my request so I could track energy delivered to my house and energy exported to the grid, in addition to knowing what the overall net energy was. When the meter was installed, it read 000,000 kWh. On a meter without a grid-tied power source, this number would only go up. Here, it can go either way, depending on whether we use more than the PV system produces, or not.

After one year, the meter reads 97,409. Either we've used a hell of a lot of energy, or we've sent more energy to the grid than was consumed on site. I think it's the latter. The PV system didn't go operational until the afternoon of June 9th, so this net export of 2,591 kWh includes about two weeks where the energy flow was only incoming.

The stats:

- 3,813 kWh consumed on site

- 2,752 kWh imported

- 5,343 kWh generated and exported

- 6,403 kWh generated

- 1,060 kWh generated and consumed on site

This year's usage can be compared with program benchmarks such as Passivehouse (PH) and Thousand Home Challenge. My allotment of site energy usage according to the Thousand Home Challenge (see post Finding the Right Target, Part 2) is 5,375 kWh, so we used 71% of our allotment. Of course, it has been a very warm winter, so it's not quite as rosy as that looks. But we've met the THC quite handily.

The most meaningful Passivehouse criterion is Primary Energy (PE, see post Primary Energy). Primary energy is the energy consumed to get the energy to the site as well as the site energy. For electrical grids it's about three times higher than the site energy (the Passivehouse software used 2.7 as the primary energy factor in Germany - various sources say it's over 3 here in the US). The PH PE criterion is in kWh per square meter of Treated Floor Area (TFA, following the German convention for calculating usable floor area). This house would have about 150 square meters of TFA. The criterion is 120 kWh/square meter of TFA, so the limit here would be 18,000 kWh/year of PE, or, using a PE factor of 3, 6,000 kWh of site energy usage. We managed to be comfortably below that limit this past year.

I have proposed an amendment to the PH standard for New England (see post New England PH amendment), where I propose that the PE limit be set according according to number of bedrooms rather than according to floor area. How did we do according to the amendment? A three bedroom house is permitted 13,600 kWh/year, a good bit lower than permitted under the standard as is. With a PE factor of 3, the site energy limit would be 4,533 kWh/year. We squeaked under with 3,813 kWh this past year, but a really cold year might yield a different result!

6000 kWh

After a stunning amount of sun in April, May has been cloudy and has brought much-needed rain. Yesterday at Island Cohousing Lynn Weber, our garden guru, marshalled a dozen of us to do the semi-annual move of the hoop house from the winter location where it protected greens to the warm season location where the seedlings now in the cold frames - tomatoes, eggplants, peppers, cukes - will get a head start on the season. The compost bin was dug out and new piles of compost built. The last of the collards got pulled and it took me close to an hour to pull leaves off the plants - ending up with a heaping wheelbarrow of collard leaves (chiffonade the collard leaves, saute garlic in a pan, remove, saute collards, throw the garlic back in, eat). It was cool and cloudy, a great day for garden work.

Meanwhile, the House 5 PV system quietly passed the 6000 kWh output milestone. With 5 weeks left to go in year one of operation, we may exceed 6,500 kWh. This would be 1.37 kWh/W of DC rating, an exceptional output, especially with the mild amount of shading present.

Habitat for Humanity MV, energy, and affordability

The Town of West Tisbury recently passed the Stretch Code at Town Meeting. It's a more stringent building code and in essence it speeds up the adoption of the next iteration of the International Energy Code. Debate was spirited. Aginners (as in, I'm agin it) made impassioned pleas about how it was just another gubmint imposed tax on those that could least afford it, the working folks. My opinion is exactly the opposite - Martha's Vineyard's tax on the working people is $4+ per gallon oil and propane, and almost $0.20/kWh electricity. A tougher energy code is the worker's friend in the long run.

Next to those designed and built by South Mountain Co., the most energy efficient houses on MV are being built by Habitat for Humanity. They are dedicated to permanent affordability for their owners. In the past, HfH often was the recipient of donated products that were sometimes on the low end of energy efficiency, such as inexpensive windows and low efficiency heating and hot water equipment. On MV, the forward thinking folks who operate HfH are, with some detailing help from SMC, building super insulated homes that are very airtight. I help with QA, testing the houses with blower door and fog machine or IR camera. The first house at Bailey Park Rd. was 175 cfm50 at the stage of windows and doors installed and most of the roughing done, but no drywall. In the next month they'll be ready for a final test. Today we tested the shell at the second house they are building there, with window and door openings poly'd off. It was just about 300 cfm50 with the air barrier being one layer of taped polyiso foam on the walls and roofs. The house is a kit, which is all T&G pine inside, a potentially very leaky strategy.

With volunteer workers, they're building a house tighter than almost all the pros do here. The beneficiaries will be the owners. The mechanical system is mini split heat pumps, far less costly to operate than fossil fuel equipment. Thanks, Habitat, for leading the way to permanently affordable housing.

 

 

 

Monthly energy balance and end use consumption

Sunny April

I've mentioned before that our Sunpower systems here at SMC seem to be significantly out-producing the production estimates created by the NREL web-based tool called PV Watts. One never knows if this is the better Sunpower technology or merely more available sunshine than normal. When we see that the systems are out-producing the estimates almost every month, we think that the Sunpower claims may well be valid, and we're looking for data from other manufacturers' systems to understand this better.

However, April 2012 was just plain sunny as all get-out - no April showers here. The PV Watts April estimate for an optimally oriented 4.76 kW array is 538 kWh. In April, our system made 802.5 kWh! Here's the graph of actual output vs. estimate:

In the depth of winter we have shading from trees to the south, so making less than the optimized estimate makes sense. It's interesting to see August and September being so close, and then how things shift in 2012. There's been a drought here, and we need more rain, yet if we have to have a drought, it's nice to have a clear benefit as well.