Thriving On Low Carbon

PVs and credits and bills (Oh my!)

This morning we reached a new high for the credit we've built up in terms of net exported energy - 1,753 kWh. The previous high was 1,738 on December 3rd, then it dropped to a low of 1,477 on January 28th. This winter we were net importers in December and January, and I discovered to my dismay that even though I'd built up a credit of over $300 with Nstar, I was getting billed for the energy we imported. Looking closely, I learned that I had Nstar as my energy delivery company, and Coned Solutions, via Cape Light Compact, our municipal aggregator and default supplier, as my energy supply company. Having two separate companies is part of the deregulation process - you get a choice of who supplies the energy, but not who delivers it (owns the wires). It turns out that Nstar credit doesn't offset Coned supply charges. The simple solution, that doesn't make Cape Light Compact happy, is to switch to Nstar as the energy supply company. The rates are within hundredths of a cent per kWh, but that actually doesn't matter if we are net exporters - in fact, a higher rate allows us to build up a larger credit, which we could allocate to another meter.

Winter temperature data

We've been heating for the past year plus with a point source of heat - the Fujitsu minisplit heat pump in the living room. I've been logging temperature in the main space, in our second floor bedroom, the basement, and outdoors. You can see that the upper level varies more - it drops more at night, as it is heated by natural convection from the first floor, and it rises above the main level on sunny days, as it has proportionally more south glass and less shading. The basement drops through the winter as the ground cools (and the heat pump water heater removes some heat, while the freezer and PV inverter put some heat back in).

Average upper floor temperature was 68.1F, main level 69F, basement 52.6F, and outdoors 37.8F.

Professional Leadership Award

I had the tremendous honor to be given the Professional Leadership Award by the Northeast Sustainable Energy Association at the recent Building Energy conference. The award recognizes work of exceptional quality, integrity, and generosity in sharing knowledge. It's wonderful to be acknowledged by one's tribe...

Getting into hot water, Part 3

When the oil boiler went away the hot water tank did also, and this gave me an opportunity to re-locate the new water heater directly below the two bathrooms. This reduces the wait time to get hot water to the tap substantially, and the tank is now only half the distance from the kitchen as well.

When hot water leaves the tank it goes through a Caleffi thermostatic mixing valve (TMV). I installed this in case I decide to install solar DHW, because solar can make very hot water that carries a scalding risk. A TMV is set to a maximum discharge temperature, which it holds regardless of the inlet temperature. The TMV creates this mixed temperature by mixing hot water from the tank with cold water. Colleagues of mine have found that some TMVs, even when set at maximum temperature, still mix in some cold water. I tested the Caleffi and it doesn't do this, which is a nice feature in a solar DHW system - when the water temperature gets right down to the minimum usable temperature you don't want cold water mixed in, because that will cause the back-up heat to come on.

Once the mixed water leaves the TMV is goes to the distribution system. The DHW distribution system in House 5 is a home run system. There is a manifold for the hot water (and one for the cold water) that has a series of connections with shut-off valves, and each connection has a cross-linked polyethylene (PEX) tubing run that goes to an individual fixture. In my house, these are 1/2 inch, but if I were to do this myself from scratch they would be 3/8 inch for most fixtures. One of the advantages of a homerun system is usually shorter wait times for hot water than occurs with a traditional trunk and branch system. The smaller the tubing, the faster hot water gets to your tap. Having a shut-off valve for each fixture allows an individual fixture to be repaired or replaced without having to lose the function of any other fixture.

Here's the manifolds above the DHW tank, the cold manifold is behind the hot one:

Another efficient approach to getting hot water quickly to each fixture while wasting minimal water is to use an intermittently operating recirculation pump (Taco D'Mand). My very smart colleague Gary Klein has written extensively about the benefits of one of these systems (Gary Klein powerpoint) and here's a simple schematic he drew of one:

Notice that the hot water line continues back to the DHW tank after the last fixture branch. When you are in the bathroom, you push a button (it can also be set up with a motion detector, to work automatically) and the pump turns on, sending hot water to you at several gallons per minute. When it reaches the pump, the pump turns off, and you have hot water in a very short time, and almost no water went down the drain while you waited. If the water in the line is already hot from a recent draw, the pump doesn't come on. This clever system is a variation on the old continuous recirculation systems that waste both thermal energy (because the recirc loop is always hot) and electricity (because the pump runs all the time.)

The larger and more spread out a house is, the more benefit from this type of system. It really makes sense to look at each house and evaluate the best approach. My colleague Michael Chandler suggests that a modified home run system, where a single 1/2 inch line runs to a bathroom, and each fixture uses a 3/8 inch branch off of that, is the best, at least for a modest sized home. That approach means that once someone used hot water in the sink, for example, there is hot water available for the shower or second sink with no waiting.

Eight months of data - On the way to net zero

I've been traveling a lot for the past several weeks - teaching three days in the Passive House Consultant training, and about Deep Energy Retrofits and Zero Energy Houses in VT, IL, and WI. Next week is Building Energy 2012 in Boston (you ARE attending, right?) then I get to stay home for a while!

We now have eight full months of measured data, beginning in July 2011. The solar electric system came on line the afternoon of June 9th, and the commercial meter from NSTAR was installed May 25th, so I have our net energy usage from May 25th (we've exported a net of 1,590 kWh since then - I estimate we had 137 kWh on the meter when the PVs came on line) but I don't have sub-metered usage until some time in June,

It's been an astonishingly warm winter so far. We've had 73% of typical heating degree days. It has been slightly sunnier than last year (using PV production at the Eliakim houses as a surrogate). So this first year is unlikely to be typical in the long run (although I could get used to it!) As an example, in January last year we used 418 kWh in the heat pump, and this year it was 277 kWh. From July 1st to March 1st, we've used 2,639 kWh. the PVs have made 4,107 kWh, and the net exported is 1,468 kWh. I think we'll use less than 4,000 kWh this first 12 months, and I'm guessing the PV system will make at least 6,000 kWh. The surplus would power an electric car for 6,000 miles, which is more than our on-Island miles (Jill is driving 3-4,000 miles annually - her car stays here - and I'm in the 8-9,000 miles/year range, but almost all is off-Island).

Here's the Used/Produced/Net data by month:



It's clear that shading from trees hasn't affected us as much as I expected. It's most evident in December, as expected. Some people say that the PVWatts software we use to project production is quite conservative. Sunpower, the manufacturer of the PV panels South Mountain Company installs, says that there are important aspects of their panel technology that lead to higher production per watt than other products, and this may be part of what we're seeing. In the chart below, the predicted production is before any shading. Even with modest shading (one large tree to the south) the system has produced 13% more so far than the predicted unshaded output. Woohoo!

And here is the energy usage by end use (R/F is the refrigerator and the 15 ft3 chest freezer in the basement):

The heat pump is the most varying end use, peaking in January, the coldest month. There are other interesting trends. The energy used by the refrigerator and freezer drops from the summer to the winter, as the spaces they are located in get cooler. This both increases heat gain through the cabinet and also makes the compressor work harder. Conversely, the energy going into heating DHW increases, as the losses from the tank increase as the basement cools off, and the incoming water temperature drops. Cooking energy peaks in the holiday season - much more baking - and drops the last two months, partially due to all my traveling. Jill doesn't cook when I'm gone.

Here's a closer look at DHW usage and energy:

Again, usage dropped off when I was traveling, and energy in per gallon increases from the summer.

Le cheque du SREQue

SREC - solar renewable energy credit. The MA government under Deval Patrick set a goal to dramatically increase solar electric generation and jobs, and the increase has been dramatic - from under 4 MW in 2007 to 118 MW installed or in process currently. The state has used multiple tools, including tax credits and outright rebates from the MA Clean Energy Center, yet the one that will put the most green in my pocket is the SREC.

The utilities are required to have an increasing % of their generation from renewables and a portion of that from solar electricity. If they don't have it, they can purchase credits from those who do own systems producing solar kWh. Small systems like mine are pooled by aggregators (not to be confused with Republican presidential candidates, who are aggravators) who sell them into the open market quarterly at auction. The latest auction went for $540/mWh, or $0.54 per kWh (an SREC is 1,000 kWh or 1 mWh). That's almost three times what I pay for electricity from Nstar here on MV. My net after fees and such was $502 for my first SREC. My system should make 5-6 SRECs annually.

This golden goose doesn't last forever. I'm signed onto this for 40 quarters. Once the utilities hit the required targets the price will drop, but it's fairly likely that I'll be clearing over $250/SREC. So my SREC income over the next decade should be over $15,000. I should be installing solar electric systems wherever I can, it's a way better investment than any other I can make :-)

I'm not really a fan of subsidies. I wish for an economy where all subsidies are removed - if the effects of oil, gas, nuclear, etc. were accounted for, solar wouldn't need subsidies to compete. However, unlike a flat tax credit or a rebate, paying per unit of produced energy at least directs the subsidy towards lowering the cost of the technology - my motivation is to make as much energy as possible at the least cost, and that's the right form of incentive. If you live in MA, you should be putting solar electricity on your roof (or mounted on the ground) - get guaranteed dividends while cleaning up our energy supply.

A little scary but...

January 2012 will go down in the history books. Is this climate change? I feel as though I have moved to Boulder CO. Sometimes it's been cold, but mostly it's almost balmy. Last November, December, January heating degree days (base 65F) were 584, 978, 1,086, for a total of 2,646. This year the values are 429, 685, 823, for a total of 1,937, or 73% of last year's heating demand. I haven't once needed to wear my neoprene overbooties when riding a bike.

At the same time, January was extraordinarily sunny. The online estimating software called PV Watts estimates that my system, if completely unshaded, would make 330, 318, and 366 kWh in November, December, and January. The actual numbers, which include the effects of shading, have been 413, 303, and 468. December clearly has the most shading from trees to the south, and it's closest to the estimate. November and January have been 25 and 28% over the estimate. We at South Mountain think that Sunpower may be correct when they claim their panels outperform others watt for watt, but this also must be more sunshine.

In January, last year we used 418 kWh in the heat pump and this year it was 277. In fact, we only consumed 30 kWh more than the PVs generated. So it's scary but darn enjoyable.

Sunshine!

January so far has been incredibly sunny. PV Watts, the NREL software that estimates the output of PV systems based on location, tilt and azimuth of the array, and array rated power, estimates an average of 10 kWh/day for my location in January, assuming no shade. I have some mid-day shading from an oak tree. The first ten days this month have averaged about 17 kWh/day. This means that we've actually increased our net export credit, because our use of energy, in what has been a mild weather period, is below that number. Won't last - tomorrow is supposed to pour - but this must be what living in Colorado is like :-)

Cycling for pleasure rather than utility

Sometimes you gotta just have some fun, and when you get a 50F and sunny New Year's Day, riding up to Gay Head is one way to do it. Bring out the skinny-tire bike!

Here's the view to the south coast from Middle Road:

And Quitsa Pond and Stonewall Pond across State Road from one another:

I rode out past the Lobsterville Beach to the West Basin, looking across at Menemsha:

Then rode back out and up Lighthouse Road to the Gay Head Cliffs and Gay Head Light.

 

Six months energy data

Happy 2012!

Here's two graphs for July through December, showing our usage vs. PV production, and where the energy is going. In the first graph, one can see the usage increasing as we begin to use the heat pump at the end of October, and the solar electricity is dropping off as the days get shorter and cloudier. In December we go from being a net energy exporter to a net energy importer. Only by installing the woodstove will we be able to be energy exporters each month, which is my goal. Our shortfall of 183 kWh in December is caused by the 238 kWh the heat pump consumed.

In the graph below the energy usage is broken down by end use. Other includes the composter exhaust fan, lights, wireless router and modem, computers, and small kitchen appliances, as well as any power tool usage. I'm a bit confused why there are peaks in Other in August and October. Oh wait, in August we had an influx of guests, which also shows in the DHW energy usage. Maybe on MV that month should be AuGuest :-) October, that's still a mystery... One can also see the increase in cooking energy in November and December. Less fresh food straight from the garden, plus holiday cooking and baking, are the reasons. R/F are the refrigerator and the chest freezer, and the change in their energy consumption is addressed in this post.

I think we have a strong chance to achieve net zero for our first year. The utility meter read 100,137 kWh when the PVs came on line in early June, and it read 98,448 this morning. Since June 9th the PV system has generated 1,689 kWh more than we've consumed. I can imagine our January and February usage cutting that surplus in half, yet sometime in March I expect we'll get back to net exporting. Stay tuned...