One thing I learned many years ago following the energy usage of buildings I designed was that as the energy needs of the building are reduced, and the fraction of those needs supplied by solar energy increases, the variation in back-up energy from year to year increases. Let's look at House 5 this winter and last winter.
First, the variation in heating load - this winter (Nov-Feb) is 20% colder:
We've used 23% more energy over these months, and here's how it breaks down between the heat pump and all other uses:
Heat pump energy used is up 32%:
As it gets colder outside, the heat pump efficiency drops, so it makes sense that heat pump energy goes up faster than heating demand. However, another difference in what we use is that Jill now works 3/4 time and is home Mondays and Fridays, so I'm guessing the average thermostat setting has gone up a tad, and there are more kWh going to lights, computer, music, etc. All other uses are up 15%, with the biggest bump in January:
Meanwhile...it's been noticeably cloudier this winter, so PV production over the four month period is down 22% over last year:
Consequently, our net imported energy last year over this period was 18 kWh, and this year is 45 times higher at 804 kWh!
If you design or build zero net energy buildings, it's important to communicate to the owners that these year to year variations are significant. Manage expectations! How much they use for plug loads/appliances/lighting is under their control beyond a certain baseline. How much they use for hot water is similar. At a given thermostat setting, how much they use for heating and cooling is dependent on the weather and the amount of sun available, and how much a solar electric or solar thermal system generates depends on solar availability and how much of the time the collectors are covered with snow. If you want to be pretty certain that a building is net zero every year, the solar electric system probably needs to be oversized by close to 30% (a SWAG). Driving the building's heating load down will make this variation smaller, as it takes one weather-dependent factor and makes it smaller.
I tried an experiment this week during our cold snap. We've kept the door closed to the first floor ell (bedroom and bath) and let it run cold, because the Fujitsu wasn't sized to heat that space too. I opened the door early in the cold snap, and let the heat pump go, leaving it set on 70F. What I found was that overnight the main space went to 66F, and the upstairs and back bedroom were 3-4F lower.
My calculated heat loss in these conditions is about 24,000 BTU/hour, and the heat pump is rated at about 17,000 BTU/hour at about 10F. You'd think it would not be able to keep up. My heat loss number may well be too high, and the rated output of the unit may be quite conservative.
One other thing I wonder about is that even though the room was not at the setpoint it seemed that the unit didn't run on full output much. My system has the temperature sensing built into the wall cassette, so it may be sensing a higher temperature than out in the room. It may make sense in severe weather to set the thermostat up to 72F instead of our normal 70F. Unlike a boiler, these variable speed units taper off the output as the space approaches the setpoint instead of always running at full bore, so that may be a disadvantage of a smart unit - it's trying to stay at a more efficient operating point instead of making me as comfortable as possible.
Anyway, comfort trumped further experimentation and we closed the door again to the ell.
Habitat for Humanity of Martha's Vineyard is really getting the superinsulated, energy efficient home down pat. Last Saturday, accompanied by job super extraordinaire Lee Taberner, I did a final blower door test on the second house they've built at the end of Bailey Park Road. The result - 203 CFM50. Real tight, well under 1 ACH50. Along with this, the HVAC system is a minisplit heat pump, a heat pump water heater, and a heat recovery ventilator.
HfHMV has made the connection between energy efficiency and permanent affordability. So has the Island Housing Trust (South Mountain is currently building two new homes for IHT, also in West Tisbury). It's heartening to see the two most significant affordable housing providers on MV so committed to high performance homes.
In a workshop, it’s difficult to ensure that all the students are getting the concepts; there’s not much time for them to practice what is being taught; there is a limited time for questions; and variation in students’ personalities and learning styles means that some students can dominate the discussion. Online, a course is spread out over a number of weeks, and students engage on their own schedule and spend as much time as they need to master the concepts. In addition to the videos created by the instructor, there are supplementary readings from books and the Web, and most importantly, there are homework assignments where the students apply what they have learned, to see if they’ve learned it. An online message board allows each student to post questions or comments at any time, and post their homework solutions so I and others can comment and discuss.
My mental model comparing the workshop format with the online format is that the workshop is a menu, and the online course is a meal. We incorporated a final project in which students are asked to design a zero net energy house, submitting plans and elevations, a wall section, an HVAC plan, R value calculations, and an annual energy model. The calculations are done in a series of simple calculators I put together in Excel. 60% of the students registered for the course have completed this final project, a high number for an online course. It appears that they do more, the more you ask them to do – amazing! And the feedback from the students has been overwhelming – they are so excited to be able to assess how their choices affect performance, with quantitative tools to answer their what-if questions. It’s a “how-to” experience that lifts the student to a significantly higher level of practice.
I’ve found it incredibly satisfying to guide this group of professionals through this course. As you may imagine, I’ve learned a lot through teaching it, too. It was a lot of work to assemble the course – I made twelve hours of videos (narrated slide shows) as the backbone of the course – and then of course there is monitoring the class message board for questions and looking at the homework assignments the students upload. The students get the value of others looking at their solutions and asking questions.
My plan over the next year or so is to add a course on Deep Energy Retrofits; Mechanical Systems for Low Load Buildings; and Fundamentals of Solar Energy. I'm also trying to recruit other instructors who are truly masters in their fields to join me.
I'd like to list the graduates of this first course (those who finished all the homework including the final project - quite a few other folks completed everything except the final project):
I am reluctant to have anything in the house that uses energy 24/7. With 8,760 hours per year, small usages add up. One such is the exhaust fan on the composting toilet - at about 20W it uses 175 kWh/year, or 4-5% of our energy usage. Another is the cable modem for internet access we have from Comcast, and the Apple Airport Express wireless router. I've been measuring both. The router uses just over 3-1/2W, the modem 6-1/2W. Together they are 88 kWh/year. I imagine that if we were off-grid we'd unplug them in the winter when solar power is short. Being grid-tied, we're just lazy about it.
What's using power 24/7 in your house?
Last year, during November and December 2011, our net energy with the grid was 98 kWh - we imported 98 kWh more than we used. This year during the same period that figure was three times higher, 299 kWh net import. Why so different?
For one thing, it was cloudier. The PV system made 68 kWh more last year during this time. The larger difference is that we used more - 947 kWh this year, vs. 814 kWh last year. How come? I can think of three things:
- It was colder this year - 1,580 heating degree days (base 68F) vs 1,296 in the same period last year, mostly in a colder-than-average (yes, we still have those occasionally!) November. So the heat pump ran more - 98.5 kWh more, in fact. That's the big one.
- Jill is working three long days a week instead of going to work five days/week, so she's home more, using lighting, a higher thermostat setting, the computer. A small impact, but it's likely real.
- Jill's Christmas cookie marathon, in which she made 16 different types of cookies. Lotta oven usage, plus kitchen appliances and lighting, plus dishwasher, plus more hot water cleaning up. This extra usage of energy is very well appreciated by many cookie recipients!
You can see that the big difference is weather-related - colder and cloudier. The lower energy use a house is, the larger these year to year variations will be, on a percentage basis. Yet the benefit of solar-driven, energy efficient building is that the percentage variations may be high, but the absolute difference in dollars is small. So the difference in our net energy flow over those two months year to year is about $37 total.
With the exception of one week in February 2011 where I switched back to the oil boiler to take some data before it went away, the Fujitsu 12RLS has now been heating the house for two years. The meter reads 2,584 kWh. So, about $250/year to heat House 5, in mostly milder-than-normal weather. This is about 1/4 the cost of operating the oil heating system.
Most houses in the northeast have a boiler and forced hot water heating, and most of the rest have a forced air furnace - both are central heat systems. Without some energy retrofit work, most houses can't be converted over to a single zone minisplit and have adequate heat throughout the house. In cases where the central heating system is due for replacement, a multizone minisplit may be worth considering. We've done just that at SMC, for a client with a 30 year old boiler and a poorly designed distribution system. That system cost over $20K installed, though.
A single zone minisplit costs about $4K installed. In cases where the entire house doesn't need to be fully heated, or houses in which a point source heater can carry the load of the house in mild winter weather, a minisplit can be a great retrofit. In the Pacific Northwest a major study has been conducted using a single zone minisplit as a retrofit to the many electrically homes there (http://www.bpa.gov/energy/n/emerging_technology/DHP.cfm). On average they have shown a 40% reduction in heating energy, with some homeowners experiencing much higher savings (the ones most likely that kept the doors to the bedrooms open!) The electric resistance heat is still in place, to be used as needed. It's very possible to consider a similar approach in fossil fuel heated homes. The best candidates are houses with open plans, so the heat pump can heat a good portion of the kitchen/dining/living space, and houses where the other rooms are located where natural convection (warm air rising) can transport heat to them. It would be best for the existing heating system to be one that has more than one zone, so that the zone(s) not well heated by the heat pump can still be heated by the existing system.
Best suited might be houses where a number of the rooms are not occupied - the large house with a single occupant, who needs a bedroom, bath, and the public areas heated, not the other four bedrooms and two baths. In essence, it's going back to the days when a central hearth kept the public spaces warm and the peripheral spaces were much cooler.
These changes will likely be driven by fuel prices, so they are more appropriate where there isn't natural gas - houses where oil, propane, and electric resistance are the primary heating fuels. As of the 2009 EIA energy use surveys, there were almost 9 million households in the northeast (New England and mid-Atlantic states) using those fuels as the main heating source. That's a significant opportunity.