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.