More Accurate Solar Cost Accounting

My previous post updating the results of our ten year struggle to reduce our carbon footprint mentioned that I was not satisfied with the cost accounting for our previous and current solar PV system. The percent reduction chart showed a 33% reduction in our carbon footprint between 2003 and 2004 due to the 2.5 kw solar PV system, and a per kilogram lifetime carbon reduction cost of $0.43 over the projected 30 year lifetime of the system (though we only used the system for 6 years). For the 7.05 kw system we are currently installing, the projected reduction is shown as only 0.05% and the lifetime per kilogram cost is shown as $2.10 over the projected 30 year lifetime of the system. The calculations were based on measured or projected reduction in electricity use from the year before to after the system was installed. These results made little sense to me, because the cost of solar PV has come down dramatically in the last 10 years, even though the subsidy has dropped even more.

The problem with this calculation is that the original system was installed along with other improvements, including removing the pool, replacing many of the lights with CFLs, and replacing the washer and dryer with more energy efficient models. The reduction due to these measures was not broken out. For the new system, as mentioned, we still are projected to draw around 2000 kwh/year from the grid, but the actual amount is slightly less than we draw now. That accounts for the 0.05% reduction between before and after the system is installed, but the lifetime reduction cost did not account for the fact that the new system will also be offseting the same electricity use as the old. 

Unfortunately, I don't have actual figures for the amount of electricity generated by the old solar PV system, since I did not write down the production amount recorded on the SonnyBoy inverter before it was decommissioned, and the PG&E bills (upon which most of my measurements are based) do not break out total usage from usage net of solar production. But based on the carbon calculator at Altstore.com, I calculated the expected energy production for the 2.5 kw system at 3780 kwh/year, which amounts to 1.12 metric tons / year of carbon using the carbon calculator at Cool-it.us. Here, again, there are some inaccuracies. The Altstore.com  calculator might be too optimistic, and the Cool-it.us calculation is based on the current mix of energy sources in the California grid. Since the grid has become greener over the last 10 years, the actual amount of carbon eliminated could be more. The other measures we took in  2004 - removing  the pool, CFLs, and  appliances - then account for 1.193 metric tons of eliminated carbon, slightly more than the solar PV.

The effect of this change on the lifetime cost of our current PV system is then rather dramatic (and more likely to be correct) as the following graph shows:

This graph shows the cost of the 2004 system, $0.54/kilo,  as slightly greater than  the cost of the 2007 system, $0.46/kilo.  The cost of the other measures is around $0.34/kilo. For the other measures, I've estimated the lifetime as 30 years, even though the appliances and CFLs will have a shorter lifetime. The pool removal probably contributed the largest amount to the other measures, and its lifetime is essentially the lifetime of the house (actually probably more, it will be permanent) which is around 30 years.

Note that in the graph, the reduction in backup hot water carbon emissions is attributed to the "Reinsulate + on demand electric hw" rather than to the solar, even though the solar will be contributing the renewable power to offset the electricity used for backup hot water.  But apportioning the costs between  the two would be difficult, and since the backup hot water reduction would not be possible without the electric on-demand  heater, the breakdown seems fair.

Anyway, the projected results from our current system remodel are mostly theoretical. It will be interesting to see over the next couple years how the improvements we've made actually impact our carbon footprint. The Tigo energy maximizer we'll be installing on the solar PV system has included a real time measurement package that records the amount of power generated on a web site, so we should now have much more accurate data in an easy to access form. The gas usage data will unfortunately be not much changed than before,  since it will come from the PG&E bill, though we now have a SmartMeter for our gas meter which gives accurate, daily gas usage information on PG&E's web site.  Perhaps if I get ambitious, I might try to do some Ajax hacking to consolidate this information in one place so I can display real time information about our energy usage and carbon footprint.

Ponzini Comes Through

Last weekend, I sent Paul a plan with places marked that had problems with cracks and such as described in my post on insulation a week ago. I also sent him two zip files with photos, but, unfortunately, they were too big and didn't get through, and I didn't want to spend the whole evening last Sunday uploading them to Flickr.

Ponzini came by on Monday and fixed some of the problems, but not all of them. In particular, they finally sealed off the gap under the floor in the chase upstairs where the HRV is located. There were still a lot of cracks, though, and I did a walkthrough with Ponzini on Thurs. when Paul came by for the weekly meeting.

On Fri. they were out again, and this time they seem to have fixed most of the problems. They've mostly filled the cracks with pink foam. Here you can see how they fixed the small holes at the end of the beams in the ceiling, and the cracks between beams:

Paul says that the holes at the end of the beams and the gaps between two studs placed next to each other come from shrinkage in the studs and beams as they age. When they were originally placed, there was likely no gap.

Below, you can see how they've used pink foam to fill in a crack between two studs that were placed next to each other:

And here you can see  how they filled out the  foam along a header so it is level with the ceiling instead of only at the level of the wall:

This is important because the ceiling must be to R-30 whereas the walls are R-19. Heat leaks along headers are common if they are not properly insulated.

Finally, the cracks on  the big headers over the doors were also filled in:

You can even see where they used acrylic caulking compound in the places where the cracks are smaller than 1/4". The foam can only be put in if the cracks are larger than 1/4".

There is still one place Ponzini seems to have missed, in the new hot water heater closet upstairs, but, by and large, they seem to have done a thorough job of fixing the problem areas. It is somewhat puzzling, though, that they didn't just do these without my having to hassle them about it. Are most of their customers not as picky? Do they just not see these problems and need me to point them out? They are not, by any stretch  of the imagination, a "green" contractor, but they seem to know what to do when they are told where to do it. So I'm happy with the work they've done.

Update On Projected Carbon Footprint

Since my last post on the estimated carbon footprint after our current system remodel and projected purchase of a Nissan Leaf, a few things have changed:

• The design for the solar PV system was finalized,
• Figures for yearly comparative gas usage with and without solar thermal hot water are available,
• A more accurate cost estimate is possible.

So this post provides an update on the projected carbon footprint and cost per kilo carbon eliminated for our direct carbon emissions (except for flying) after the current system remodel is complete and after the Nissan Leaf.


Perhaps the biggest impact on our projected carbon footprint - upward unfortunately - came from finalizing the solar PV design. We now anticipate that the solar PV system will generate around 7,024 kwh per year. This is around 2000 kwh per year less than our anticipated utilization, considering the new HRV system, the new electric hot water backup, 8000 mi/year on the Nissan Leaf, and our existing utilization, including the plug-in Prius. Interestingly enough, 2000 kwh per year is almost exactly where we fall short of net zero today, though we will be offsetting considerably more carbon sources through  the new system than we currently do. Since our solar resource will be maxed out with the new system,  we have no more room to expand, short of new technology that generates more power per square foot.

On the positive side, the impact of solar thermal hot water on our hot water usage was considerably better than anticipated. With the current gas-fired tank as a backup, the solar thermal hot water system eliminated 23% of our yearly gas usage, rather than 15% as previously assumed. With the installation of the new electric on-demand hot water backup, the additional amount of gas consumed to heat water during the winter will be eliminated, reducing total gas consumption by 31%.

The assumptions in the projections are about the same as before. The result modifies our per cent carbon reduction graph as follows (please click on figures to get full picture):

Our projected carbon footprint reduction has decreased from 90% to 78%. This is primarily due to the reduction in expected solar power from the PV system. Previously, we had expected to completely eliminate our grid draw,  and have a a 15% surplus. Now, we expect to draw around 2000 kwh per year from the grid.

The contributions from each component to our annual carbon footprint can be seen in this graph:

Since we are projected to use all the solar PV we generate, we no longer have a negative footprint due to generating more solar PV than we use. The expected size of our carbon footprint is around 2.8 metric tons per year. This is over twice the size of the carbon footprint (around 1 metric ton) that this Swedish family is shooting for. In our case the deciding factor is our plug-in Prius, which gets around 80 mpg but still generates around 1.1 metric tons of carbon per year. The Swedish family has a plug-in Volvo but no second car, since in Sweden, the public transport system is adequate for transportation around town if you don't need to haul anything. In addition, the Swedish electrical grid is 52% carbon free, due to hydropower and nuclear energy, whereas in California, it is considerably less (but better than many other states which generate electricity using coal), maybe around 30% including large hydro and nuclear.

Finally, the cost per kg carbon eliminated over the life cycle of each technology employed:

I've included more technologies here than in the original post, in fact, every technology we've employed since we moved into the house in 2003. Note, however, that I still do not have an accurate partitioning for how much the reduction in 2003 was due to the original solar PV system and how much was due to eliminating the pool. I'm working on estimating that. For this reason, the original solar PV system looks considerably cheaper than the current system, but it is probably about the same price or somewhat cheaper, considering that the cost per watt has decreased and we are installing more capacity, and that the grid is getting greener. Another point to note is that the price of the 200 amp upgrade has been  distributed between the Nissan Leaf and the on-demand electric hot water heater, both of which require 200 amp service. I've also included the price of carbon offsets to make up for the GHG emissions during closed cell foam installation. Otherwise, the assumptions are the same as in the previous post.

So we're projected to fall somewhat shorter of our net zero goal than originally planned, but we are projected to achieve 78% carbon reduction, from the 2001 baseline. This is just 2% short of the general policy goal expressed by many analysts of reducing carbon by 80%. Time will tell whether we can do better. Through efficiency, like bicycling in summer instead of taking the car to work, perhaps we can reduce even more, and some of the technologies, like the closed cell foam insulation and HRV, may turn out to be more effective than originally thought. I'll be monitoring our gas and electricity usage to see how much reduction actually occurs.

Insulation Problems

Well Ponzini finished the insulation this week, for some definition of the word "finished". I went through the house and made a close inspection of their work, and I have to say I am Not Pleased. They left lots of cracks and gaps, some of them  so obvious that it is pretty hard to see how they could have been missed.  

Here's some pictures to show what I mean. Every window and door has an uninsulated crevice around it like this one:

We also have a lot of places where there are two structural members right next to each other with a narrow crack between them. In all of these cases, they neglected to seal up the crack, like this crack in the corner next to a window:

The same goes for cracks between headers  and other structural members over the doors:

And in the Bedroom3 chase upstairs, there is even a place where I can shine a flashlight through the crack. It's the blurry light in the middle (sorry, my old point and shoot doesn't have image stabilization in it):

There are gaps between the roof rafters and the main rafter running along the spine of the house:

And in a couple cases, the insulation on the 2x6 walls doesn't look like it's R-19:

Finally, that place in the HRV chase where the old forced air ducts dive under the floor was not properly done:

They will  probably need to staple in a piece of plastic and  foam over it to seal the hole against air penetration.

Now, I have to say,  if I was building a house, I would never build it with these kinds of gaps and cracks. Most houses today aren't built this way, so they are probably easier to insulate. But we have to work with the house we have. Ponzini did a walk through before they bid the job, and we warned them we wanted all the cracks sealed. It's not like they took the job when the drywall was on, then were surprised when they faced a lot more work than they anticipated when the drywall was taken off.

Ponzini is supposed  to come back with Paul tomorrow, and I've sent Paul an annotated  house  plan with locations of all 45 pictures I took, plus 2 zip files with the pictures (43Meg of pictures). We'll see what they do.

Making Closed Cell Foam Green

If you recall this post, I talked about how the process of insulating our house with closed cell foam would result in the emission of HFC-245fa, a Green House Gas (GHG) with around 1000x the warming potential of carbon dioxide, but which decays from the atmosphere very quickly, in 7 years instead of 400. My solution to reducing the GHG footprint is to track the amount of the B chemical used, which contains the GHG, to calculate the carbon dioxide equivalent released, and then to buy carbon credits of a variety that will take carbon out of the air for up to 80 years, long after the HFC-245fa has decayed.

But carbon credits are always a last resort, when you don't have a choice: flying, whatever part of your gasoline usage you can't get rid of through an electric or hybrid car, and, as in our case, fossil gas and electricity drawn from the grid that can't be offset with renewables or eliminated by efficiency. It would be really great if we could figure out some way to make closed cell foam more green and, in addition, make it more accessible to the average home owner. So here are some thoughts along those lines.

Closed cell foam is a really super insulating material. It's positive properties are:

• Inexpensive relative to even better insulators, such as aerogel
• Nontoxic
• Best insulating value of any affordable material (R-6/inch, approximately)
• Easy to install
• Functions as an air and vapor barrier
• Can be partially made with recycled and plant-based  material

It would be really great if we could figure out some way to reduce its negative properties:

• Costs 2-3x as much as fiberglass batt, which is a lousy insulator
• Uses petroleum for the part that is not recycled and plant-based, unlike alternatives such as blown cellulose, which is completely made of recycled material
• Installation releases HFC-245fa, a powerful GHG

Some folks would include the fact that it burns when exposed to flame as a negative, but I don't necessarily see that. Fiberglass batt doesn't burn, but the paper casing will, and of course so will the studs and other structural materials in a house. As long as there are no additives that produce toxic fumes, the foam itself will simply combust to carbon dioxide and water. Most houses these days have far more stuff in them that will produce toxic fumes when burned.

Let's walk down this list one by one.

First off, cost. I frankly have no idea why closed cell foam is so expensive. Cost for a particular product or  service is generally the sum of the cost of materials, cost of equipment (amortized over some period of time), and the cost of labor.

Is it because the materials that go into closed cell foam are more expensive than fiberglass batt or blown cellulose? I can imagine that they might be somewhat more expensive because they are specialized chemicals, but 2-3x more expensive?

Is the equipment more expensive? Ponzini parked a trailer in front of our house that contained the spray equipment.  Though I didn't see their equipment, I've seen it before when we had open cell foam sprayed into the garage and attic wall. It is obviously more expensive than a staple gun, but I wonder if it is more expensive than the spray device they use to  install blown cellulose? And, in any event, they can amortize it over a lot of jobs.

Is the labor more expensive? It took Ponzini maybe 6 days, 10 AM to 4 PM, to finish our house (well, let's put it this way, they think they are finished, the thermal imaging test will tell if they really are). I can't imagine that it would have been any more or less time than if they had installed blown cellulose or fiberglass batt. Actually, I think it probably would have been more time with either because both require that the installers actually get up to the walls and staple something on: the batt itself for fiberglass batt or netting to keep the cellulose in with cellulose. With foam, they just need to aim the spray gun and fire. It's certainly messier, bits of foam end up everywhere (including in our front driveway where they would pose a menace to aquatic wildlife if they got into the bay) but I'm sure fiberglass batt and blown cellulose installation have their unpleasant aspects too.

Aside from a possible slight difference in cost of materials (and of course absent any hard data),  my conclusion is the exaggerated price difference between lousy insulators such as fiberglass batt and closed cell foam is simply because closed cell foam is a "premium" product and therefore must command a "premium" price. In other words, you get what you pay for. To get the same air and vapor barrier function as closed cell foam with batt or cellulose requires all the cracks in the building's thermal shell to be sealed and the installers to pay close attention to how they are installing the material, all of which would make batt and cellulose more expensive too. Such measures won't increase the R-value of batt or cellulose, of course. In any case, I'd certainly like to find an insulation contractor who wouldn't mind someone taking a look at their cost structure and really getting some hard data on the topic. After all, if the computer industry pricing and cost structure worked the way the building industry seems to work, we would all still be using PCs with 386s and 640K of RAM because the fast Pentiums would be way out of our price range.

Next, use of petroleum. I'm no chemical engineer, but I think that there should be a way to increase the recycled and nonpetroleum content of closed cell foam. Demlec already advertises that it uses soy oils and partially recycled content in its product. If I were an academic with a chemical engineering background, I'd be writing grants to EnergyARPA and the Energy Dept. to do research on the topic. And, let's put it this way, the long term future of petroleum-based products is pretty much known: they will shortly be gone. According to the International Energy Agency, we'll be almost out of oil by 2050.

Finally, GHG emissions. On the face of it, this seems like it might be the most difficult point, but actually, its the easiest. HFC-245fa is used because it's nontoxic, much heavier than air (so the bubbles expand more slowly),  and relatively inexpensive. It's a good choice, except for the greenhouse gas problem. If there were some way we could keep it out of the air...hmm...this brings to mind something that happened when we first bought our house.

The roof had termites. So we had to have it tented. During tenting, the pest control company encases the house in a large, closed tent and fumigates with sulfuryl fluoride, which is extremely toxic to anything that's alive (we had some plants damaged when we had it done) and is itself a green house gas 4800x as powerful as carbon dioxide. There are now more environmentally benign alternatives, we didn't investigate them at the time, since they probably didn't exist. 

Why not enclose a house undergoing closed cell foam insulation in a large tent, sealed as tightly as possible, just like when tenting for termites? While the foaming is going on, a blower can pull air out of the house containing the HFC-245fa and exhaust it through a system that recovers the GHG for reuse. It might also be burned, but the fluorine in the HFC could end up as hydrofluoric acid, which is really nasty stuff. Left behind is the HFC in the foam bubbles, some of which will outgas over time, but the amount that's released during such aging should be much less than during installation. This could then be offset using carbon credits.

Tenting will add something to the cost, but if my suspicions about why closed cell  foam is so  expensive are true, the cost could still be reduced enough that closed cell foam should become  much more competitive with fiberglass batt.


So there's some ideas about how to make closed cell foam cheaper,  less impact on non-renewable resources, and less GHG-polluting.

Skylight Shade Problems

Things seem to be coming along. REC removed the old solar panels and my friend Bill took them up to Mendocino County. It felt like when I traded up my first PC, a grey no-name PC-AT clone with an upright black and white Xerox monitor, for a Pentium with a color monitor: a bit sad to see an old friend go but at the same time exciting that I'm getting new technology that costs less and performs better. All that is left is a pile of mounting rails:


 We'll take those up to Bill's in April or May on a weekend trip to Mendocino County.

The roofers fixed the roof where the old solar panels were removed, since the rails used back in 2004 when we had our system installed were not as well designed as today. We had some leaks in the roof, one of which caused a mold problem that we only finally got fixed in 2009. There were a couple leaks after the roofers were done, but we found them in the leak test. Ponzini has started insulating the interior, they have the Buddha room almost done and have started on the living room.The big mystery right now is: where's PG&E? They seem to have disappeared over the holidays. But we were told this week we will get a date for the 200 amp upgrade on Jan. 17. Hopefully, the upgrade date will be soon. I'd really like to get back into the place by the end of March.

Better news: I got to order my Nissan Leaf! The process was fairly painless,  through a form at the Leaf web site on which I have an account due to my registration. I opted for the 440V fast charger, in addition to the 220V and 110V chargers, since I'd like to be able to take it up to Pt. Reyes or down to Monterey some day when the charging infrastructure is in place. Today, the dealer called and asked a couple questions. He told me there have been some problems in the initial distribution (which I had read about on plugincars.com). In December, Nissan reconfigured part of a factory in Japan to build Leafs (Leaves?) because demand  was running much higher than expected, and that pushed out the deliveries. The dealer told me that the car won't sell like a regular car - where the dealer has inventory and you go in and buy out of that - for another 2 years. This is very encouraging for electric cars, they seem to have much more demand than I originally had thought. I'll have to wait 4 to 6 months for the car, and in March they'll have a demo model at the dealer which I can try out.

But on to the subject of this post: skylight shades. Part of the work we were intending was to install motorized shades in the skylights that adorn the ridge of our roof. They are at the top of the cathedral ceiling in the hallway, which is where most of the warm air collects. In summer this is great, because we can open the middle skylight and all the hot air simply exhausts itself out of the house. But in winter, it's not so good because we lose a lot of heat through the windows. The HRV exhaust ducts will reduce the amount of warm air by redistributing it downstairs and to rooms upstairs, but the skylights will still act as a heat radiator on cold winter nights. I would like to have some motorized insulating shades on a timer that will close after sunset in winter and open at sunrise.

Here is a picture of one of our skylights:

The yellow cable you see hanging down is intended for the motor. Each skylight has one now, replacing the useless ceiling fan we had in place before.

The geometry of our skylights is a bit peculiar. The skylights form a triangle, as does the bottom of the frame box onto which the skylight is installed. So there is no part of the frame that is square where we can install a shade motor.

On top of this, the products on the market seem mostly to be for skylights set at an angle and not horizontal as ours is. Most of the companies we talked with seemed to want us to have two shades that ran directly against the glass. But this wouldn't work for the middle skylight, it opens. Even for the others, there is no real place along the spine that would be big enough for a shade track that sealed. The final problem seems to be that Hunter-Douglas, apparently the largest manufacturer of shades of all sorts, discontinued a certain motor that would have been the right thing for us.

Aside from the geometry, I'm having a bit of a problem seeing why nobody makes a product that would  fit our case. It seems fairly straightforward. At any rate, we've pulled Christine off the trail since she was spending a lot of time coming up short, and it was costing us money. She has one more contact to follow up, a guy who makes his own motors. If that doesn't pan out, I'm going to look into what I can find, maybe ultimately I'll end up with a DIY solution if I can't find a product that works.

Solar Tradeoff

After writing my last post about the new solar PV design, while I was riding my bicycle on my usual Saturday trip to the Palo Alto library, I got to thinking about the tradeoff we implicitly made in our use of solar. We have a fairly limited solar resource due to the large redwood trees in our neighbor's lot on the south side of the house, and due to the orientation of the roof axis, which runs more or less north-south instead of east-west. The redwood trees are great in summer, since they allow us to have a glass sun room on the south side of the house where we can enjoy our beautiful California native plant garden (maintained by The Lovely Wife). If the trees were not there, the sun room and in fact the south side of the house would be much too hot in summer. However, because the south side is so heavily shaded, it does not represent a particularly good solar resource.

Most of our solar resource is therefore on the north side of the roof. The orientation of the roof axis means the roof slopes to the southwest and northeast, causing the solar panels on the north part of the roof where the solar resource is to only receive full sun during part of the day. The southwest slope has sun in the afternoon and the northeast slope has sun in the morning. In summer because of the high sun angle, there is enough sun that the panels are fully illuminated for most of the day, but in winter the low sun angle means they only get sun for maybe 4 hours on the east and 5 on the west. If the roof axis ran east west and there was no shade on the south, we could in theory get 100% sun all day in summer and winter and we would have a much better solar resource.

So given that limited solar resource, we've chosen to utilize it by having a 2 panel solar thermal hot water collector near the peak on the northeast slope, with 10 solar PV panels below, and to populate the southwest slope with 20 solar PV panels. I actually didn't run a complex software program to calculate the tradeoff, or even give it much thought; it just evolved as we evolved our renewable energy systems over the years since we bought the house. We put the 2.5 kw solar PV system in place when we moved in in 2004 because residential solar PV was just getting started and I have wanted a solar PV system for years to get started on reducing our carbon footprint. It was right-sized for our usage at that time, and about all we could afford given that solar PV was expensive. We put the solar thermal system in last year because reducing our carbon footprint even more through reduction of natural gas usage seemed like the next step. And we are putting the larger solar PV system in now because solar PV has come down radically in price and the performance has almost doubled since we put in our original system, just like PCs in the 80's. But we could have done things differently if we had designed the system to utilize our limited solar resource in a single step, rather than installing it incrementally.

What if, instead, we had populated the entire roof with PV panels and not installed the solar thermal hot water collector?

There are of course a couple ways one can look at this tradeoff. The most obvious way is to ask how much cost reduction in energy use the solar thermal provides in comparison to the solar PV. But this is actually the least useful comparison. Natural gas is cheap and electricity is much more expensive per unit of energy delivered, which might argue for having more solar PV instead of solar thermal. But, on the other hand, because the net metering tariff rates pay us 3x for power we generate on summer afternoons compared to power we use at night and in the winter, our new design with 20 panels on the southwest slope and 10 on the northwest should ensure that, as is the case today, we don't have a bill even though we may end up drawing more power from the grid than the solar PV system generates.

A better way to look at the tradeoff is to compare the amount of carbon eliminated by the solar thermal hot water system compared to the amount eliminated by the extra panels that we could have installed. To do that, we must calculate the amount of carbon eliminated by the power the extra panels would generate, and compare that to the amount of carbon eliminated by the solar thermal hot water system.

The solar thermal system take up about as much space as another row of 5 panels. How much extra power would 5 panels generate? The existing 10 panels on the northeast slope will be generating around 2462 kwh/yr or 246.2 kwh/panel/yr. So an additional 5 panels should generate 1231 kwh/yr. This isn't enough to net out our estimated usage, unfortunately. My calculations show that our new design will be around 1807 kwh short as reported in the last post, which is power we will have to draw from the grid if we use it. However, those panels would certainly make it more likely that we might actually end up zeroing out our carbon footprint for electricity.

How much natural gas does the solar thermal hot water system actually save? Well, from 8/2007-7/2008 we used 465 therms of gas. That was the last full year before we installed the solar thermal hot water system. From 8/2009-7/2010, we used 356 therms of gas, for a reduction of 109 therms. That is around a 23% reduction. Now, this is only one year and our gas usage tends to fluctuate quite a bit from year to year, much more than the electric usage. In an El Nino year, the winter might be warmer and we use less gas. In a La Nina year, like this one, the weather is colder and we use more. With the electricity, the stuff we do - like using the computer or charging the plug-in hybrid - is pretty much the same year to year, with seasonal variations. In winter the lights are on longer and the pumps for the hydronic heating system are on, while in summer these sources of electricity demand are absent.

Calculating the carbon savings from these two alternatives shows that we really made the right decision. 1232 kwh/yr of electricity generates around 802 lbs/yr of carbon in California (your mileage may vary depending on the carbon footprint of your local grid). 109 therms/yr of natural gas generates 1276 lbs/yr of carbon regardless of where you are. California's grid is pretty green and will get greener now that Proposition 23 was shot down. Natural gas, on the other hand, will not get any greener, it is fossil carbon and will always bloat out your carbon footprint.

If offsetting our natural gas from hot water heating reduces our carbon footprint more than the electricity from solar PV, what about if instead of the solar PV, we install two more rows of solar thermal collectors and use the hot water for the hydronic heating system? Would that have been a better alternative than the solar PV? Unfortunately, no. Most of our sun comes in the summer, and the heat generated during the summer from a solar thermal system for space heating needs to be discarded somehow. The electricity generated from the solar PV in summer goes into the grid and helps reduce the carbon output from peaker gas power plants that power the air conditioners in the Central Valley. Heat from our hot water solar thermal system is used year round (in fact, we turn off our gas hot water heater in summer). In addition, the amount of sun we get on the roof in winter and when it comes is not enough to contribute any meaningful amount to reducing our winter heating carbon output. We need heat when it is cloudy, cold, and raining, exactly when there is no sun. So the solar PV is the right choice for rounding out the utilization of our solar resource.

Update on the Solar Design

Lauren at REC sent me the final numbers on the three design options.

The ten panels on the northeast slope have 87% solar access and should generate an estimated 2,464 kwh per year.

The three design options for the southwest slope result in the following:

• With the four panels strung out along the south side, the solar access is 80% and the annual power generation from the southwest slope is 4,245 kwh,
• With the four panels stacked as 2 rows of 2 along the south side, the solar access is 80% and the annual power generation from the southwest slope is 4,290 kwh,
• With the four panels on the top of the dormer, the solar access is 84% and the annual power generation from the southwest slope is 4,560 kwh.

So for the dormer option, the total power should be 7,024 kwh/year, about a 4% improvement over the original design.

The inital estimate based simply on the panel size and number of panels was 8,722 kwh. That means we will have 1,698 kwh/year less than originally planned, around 20%, primarily due to shading. There is nothing we can do about this (except maybe regularly trim the Polycarpus trees in our neighbor's yard that shade the southwest slope panels). I don't know if REC is including the effect of the Tigo MPP balancing system in the calculation, that could possibly increase the power production somewhat.

My original calculation for how much power we would need including the new electric car, the new hot water heater, the new HRV, and 2000 kwh/year we are currently drawing from the grid came out to around 5,000 kwh/year. The old 2.5 kw PV system was supplying around 3,831 kwh/year estimated. The total that I was trying to design for was 8,831 kwh/year. So we will be around 20% short of carbon neutral, but for that, we will be covering a lot of additional energy uses. That's better than the old system, where we were around 50% 34% short. In any case, we should still have our electricity bill covered through net metering, since we'll be generating the most power during the afternoons in summer when the tariff is 3x at night, when we will be drawing power out of the grid for charging the car.

With this system we will have essentially maxed out our easily harvested solar resource. If we want to put any more solar PV up, we will likely get marginal return (both in terms of carbon reduction and financial) because the panels will be shaded much more, maybe even more than 50%. We will have to see how the system performs. Stay tuned.

Solar Design

Our roof slopes in two directions: a southwest facing slope that gets most sun during the afternoon and a northeast facing slope that gets most sun in the morning. The roof spine runs on approximately a north/south axis. Our neighbor on the south side has a hedge of large redwood trees along the property line. In summer, the trees only shade the south side of the roof, but by late October, the sun is far enough down in the sky that half the roof gets almost no sun at all for most of the day. On the winter equinox, the northeast slope on the north side gets only 4 hours of sun in the morning, the southwest slope gets a bit more in the afternoon. In the summer, both sides get lots of sun for the whole day. Overall, our solar resource is adequate if not overwhelming, but these are the rough parameters we have to work with.

Our system has a total of 30 panels, and we need to optimize the amount of power the system will generate, given the constraints of our roof. The directional and large scale shading constraints above are one set. Another set is stuff already on the roof. This includes:

• A solar powered attic fan to keep the attic cool,
• Various vents from the kitchen and laundry room fans on the southwest slope,
• The 2 panel solar thermal hot water system on the northeast slope we had installed last year,
• Shading from the two dormers, one on either slope of the roof,
• The skylights, which are actually along the spine running from the north side to central roof but do project slightly down the two slopes.

The current system consists of 18 panels on the southwest facing slope, but some of that space won't be usable with the new system. The new system will be slightly down from the ridge line since there are new building codes requiring a slight offset from the ridge so a fireman can chop his/her way into the house through the ridge in case of a fire. In addition, REC has calculated using their software system that panels which are currently in the bottom row on the southwest slope will be shaded too heavily by the west side neighbor's trees. These trees are pruned down heavily usually on an annual basis, but they grow back quickly and if we miss a year, like we did this year, they do end up shading more of the roof. So REC has figured that only 16 panels can go on the southwest slope where the current system is, two less than currently.

On the northeast facing slope, the solar thermal panels take up prime real estate on the top of the slope near the ridge. Having the solar thermal panels there is better than having PV, since this is the area that only gets 4 hours of sun in winter and the solar thermal panels won't heat up much if they don't get any sun. REC doesn't want to put any panels on top of the feed lines leading from the solar thermal panels to the house, so that leaves just the space on the bottom of the slope below the solar thermal panels. Space for 2 rows of 5 is available below the solar thermal panels, 10 panels in all.

That leaves 4 panels that still need placement. Since PG&E pays the best rates for power during summer afternoons, it makes the most sense to place the panels on the southwest slope. Given that, though, we want to optimize the amount of electricity generated regardless of the tariff, since we are constructing the system primarily to offset our carbon footprint.

So the first design REC proposed was with the 4 panels strung out along the south side of the southwest slope (the panels are outlined in magenta):

According to their calculations, the array on the southwest side has 80% annual sun and 20% shade while the array on the northeast side has 87% annual sun and 13% shade.

I asked them if they could do better, so they proposed two additional configurations. One has the 4 panels on the south side of the southwest slope, but stacked instead of strung out:

The other has the 4 panels on the dormer roof on the southwest slope:

REC doesn't have any calculations on top of the dormer yet, but they gave me estimates of the improvement. The stacked configuration would increase annual power production by 50 kwh/year while the dormer configuration would increase power production by 250 kwh/year. This amounts to around .6% for the stacked configuration and around 3% for the dormer configuration, figuring on 8722 kwh/year from the original REC calculation. 3% isn't much but it is something, so unless we can come up with some other configuration that does better, I will probably go with the dormer.

Tomorrow REC will remove the old solar panels and at that time they'll take a shade measurement on the dormer to get an exact value, so we should know shortly. Once the final design has been decided upon, REC will send it to the city and they'll take 2-3 weeks to consider sign off, provided there are no  problems. Then comes the installation. After the installation, PG&E must certify the system (they don't have to install a dual ported meter in our case, we already have one) which should take a day or two (or three, PG&E has already taken 2 months with our request for a 200 amp service upgrade) and then we can turn on our new system.

Under Floor Insulation is Done and Skylight Shade Problems

Last week, The Lovely Wife and I had a short but relaxing vacation in southern California. Seeing as it was the week between Christmas and New Year, I had every expectation that we would get back and not a thing would have been done during the week. After all, even during normal work weeks, very little seems to be happening. But Ponzini suprised us: they completed the insulation under the house and in the shelf under the living room windows.

Here's a kind of blurry picture of what the underside of the floor assembly looks like after Ponzini finished the spray foam, taken through the crawlspace access:

The pinkish white at the top is the foam under the floor, including a wire that got foamed. On the bottom you can see the bits of foam on the dirt. The bright white dots in the background are disco lights. Our crawl space is outfitted with disco lights so anybody who is working in it doesn't need to set up lights or try to use a flashlight. These were put in when the stapleup radiant heat system was installed.

I'd like to check out the job more closely but I try to avoid going into the crawlspace because it aggravates my allergies. I'll only go in if I really have to. Paul is going to check out the job Ponzini did to make  sure they've sealed up all the cracks.

The space under the windows in the living room looked like this before Ponzini reinsulated it. Remember this picture from the Holes post:

Here you can see what passed for insulation in the shelf under the window: an inch of fiberglass batt with gaps where you can see the electrical wiring and drywall. Here's what it now looks like after Ponzini finished with it:

Because the cavity is so deep (at least a foot and a half) they didn't fill it entirely with foam. Instead, they stuffed it full with fiberglass and foamed over the outside of the fiberglass to seal it. The foam provides additional insulation as  well as vapor, moisture, and air barrier. Unfortunately, Ponzini left little bits of foam in the garden next to where they foamed the area under the living room windows, which somebody needs to clean up.

So, going into 2011, progress is slowly being made. Right now, the completion of insulation on the interior is being held up by completion of  some electrical work so the building inspector can certify the open walls. We are having a hard time finding motorized, insulating skylight shades that we will fit our skylights, so the wiring for them didn't get completed  before the inspector's first visit. There were just three wire coils where the skylight shade connections should be. These need to be put into junction boxes until the skylight shade situation is clearer. It seems difficult to find insulating skylight shades that install horizontally and achieve a good seal so air doesn't penetrate the shades when closed. Most motorized, insulating shades are designed to be installed vertically, or, at least, at an angle less than 90 degrees to vertical. Such shades would fit on  our skylights, but we would  need two  shades per skylight because the skylights are triangular. I don't think they would achieve a real  airtight seal. Anyway, I am becoming increasingly skeptical that we will find shades to meet our requirements, but I want the electrical connection  installed anyway. Appropriate shades might appear in the future, or we could ultimately end up using the connections for lights inside the skylight.

The other hold-up right now is PG&E and the 200 amp service installation. According to the Web site record they sent me, they were supposed to start on Dec. 24, naturally they didn't. So I now have no idea when they will start.

Finally, REC is supposed to come on Thurs. this week  if the weather is OK and remove the old solar panels. After that, the roofers are  supposed to come and fix the three holes in the roof that the old solar installer punched in 2004 when the first solar panels were installed. Considering that it has been  raining off and on, with wind and cold since the beginning of December, I think the probability is high that this work may also get delayed, but we'll  hope for the best. A week of sunny weather at this point would help the job along.