Year to year variations are big

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.

 

Comments

[Bob Lemaire]

After two years of monitoring a mini-split, I discovered that there can be a significant increase in performance and efficiency when the outdoor unit is cleaned. My unit looked fine, but the output was dropping because it was defrosting more. Typically 14-16 times in a 12 hour period in typical seasonal conditions. After back blowing it with a leaf blower, the frequency dropped to about 10 and the output was noticeably warmer. There was no visual indication that the unit needed cleaning. Similar issues with HRV filters not being cleaned will drive up static pressures and power use in addition to reducing capacity. So in addition to setting expectations of variability, there should be an expectation that these houses need to be monitored and maintained to achieve peak performance.

[Marc]

Bob, when you used the leaf blower, did you see stuff coming out of the fins? Would a vacuum work better?

[Bob Lemaire]

It's located under a porch so I would have needed another person to see the discharge. The installer actually recommended hitting it with a hose, but after three freezing month I couldn't see adding to the glacier that it has created. I don't think a vac would do a better job than a leaf blower. When I do the math, the thing processes about 50-100 million cubic feet of unfiltered outdoor air during a winter season. Makes sense that it would need cleaning.

[Bob Lemaire]

To the vacuum question, when I jam a hose in the end of my leaf blower, it will support 24" of water - about 1 psi. When I put a vacuum gauge on my shop-vac, it will pull 5 psi. So with respect to putting more force on a clogged heat exchanger channel, the shop-vac appears to have greater potential. Better than either, I think I'll carefully hit it with compressed air this summer.

[Ben Hemberger]

Marc, did you analyze the uptick in heating load vs. heating degree days? Is that something that might explain the differences in the two years? I came across a neat web site the other day that monitored changes in winter temperatures by asking builders of back yard ice rinks to check in as often as they could with a simple question: is your ice rink skateable today? Smart and simple!

[Marc]

Ben, I think there are two components - lower efficiency at lower outdoor temperatures, and higher time spent at 70F setpoint instead of 66F setpoint