Residential PV’s Inherent Limitations


Author’s note: This is a beta format, temporary post. 7/23/14 I find myself needlessly having to explain these limitations countless times, effectively wasting time explaining the reality of PV to those unfamiliar with these real world limitations. Under current implementation trends, and infinite variability, residential solar rarely breaks a threshold of 30% nameplate capacity (adjust for region and day of the year) that can be considered baseload, reliable input to the grid.

Current trends, especially in the U.S., of PV’s effective implementation, are proceeding down a pathway that are counterproductive to maximizing our investments with the overall goal being to significantly reduce our release of carbon dioxide into the atmosphere.

Typical Colorado Neighborhood
Is a focus on residential the wisest, when existing construction, layout and shading magnifies the reductions in the overall effectiveness of the systems?

If we do not do this second part with an altruistic twist, those less fortunate will attempt to come up to our standards through whatever means possible. As seen in China, those “whatever” means will be through “cheaper” means, most likely conventional fossils fuels or enormously expensive large scale hydro.

Residential Solar Libertarianism (Part ?)

Angle of Incidence (AoI) – the angle off from square of the sun from the PV panel, it can be above or below, right or left. 90 degrees (like shooting a bullet straight on to a flat target) achieves maximum energy collection/electricity output, the 200watts in the example above.

[note: Colorado, sun blessed with 300 days of sunshine a year, receives about 1000 watts/hour per square meter of surface area on a typical averaged day. Your run of the mill aka cheap PV panels are able to convert approximately 20% +/- 3. Much more expensive “stacked” layer PV panels are in the 25-30% range. There is a theoretically limit around 35% maximum conversion rate. As the 1000 watts is measured across the entire electromagnetic spectrum, PV only operates within a narrow section of it in the conversion to electricity.]

The further off from either of those angles, just like skipping a rock across the water, the less electricity collected.

For simple purposes, the sun moves right to left on a south facing panel (northern hemisphere) and up (summer) to down (winter) over the year… this is why panels tend to be angled.

Throughout the country, TARP funding was used to install large scale PV farms on government property like this facility at Buckley Air Force Base in Colorado.
Throughout the country, American Recovery Act funding was used to install large scale PV farms on government property like this facility at Buckley Air Force Base in Colorado.

Flatter, south facing arrays tend to reduce the amount of “skipping” in the mornings and evenings, and favor longer solar days in the summer, significantly increasing production in contrast to attempting to increase production during short winter days with higher angle tilts to maximize shooting bullets of a down sun.

This is why PV & batteries alone cannot provide ‘autonomy’ during the winter months…. They require backup; fossil, nuclear, wind or hydro (the last of which is also down in winter) all requiring grid because those generators tend to be distant (where there is cheap, available space and/or NIMBY applied to large generators’ construction).

So shooting bullets = max production, 200 w/hr in our example.

Skipping rocks = minimum production, down to 0 watts per hour.

Outside 2 days a year at midday are you ever shooting perfect bullets, everything else is some wattage less towards zero.

Clouds, shading or snow also take collection down towards zero as well.

And now for the biggie…. Residential rarely has maximized rooves pointing south, often with smaller roof sections pointing to the right or left.

If they are pointing east, normal skipped rocks in the morning become bullets until shortly after midday, at which time they become nothing. West facing are zero production until just before midday moving towards bullets as the sun is setting.

PV panels also lose efficiency the higher the temperature of the panel, during the later half of the day when ambient air temps are higher and a large portion of the other 80% of the incoming sun’s energy has been absorbed on darker surfaces as ‘waste heat’.

Things like dormers or peak lines, running south can shade out south facing panels for a good part of the day as well, usually at the earlier & later parts or either side of winter when the sun is down. Adjacent buildings or structures, like chimneys or neighbors, also contribute to partial/complete shading in dense communities.

Neighbor’s trees, providing cooling or aesthetic benefits to the neighbor, likewise decrease system efficiencies through shading in increased density developments, say like suburbia. [Author’s note: I new system in my neighborhood, mid day, summer, their own tree blocks out the entire PV system for 2-3 hours, partial on either side of that!]

Building codes, especially in the US, and increasingly with newer homes, often require a dedicated vent per every plumbing fixture, i.e., per toilet, bath or sink, as does fossil heating elements, for example, water heaters, furnaces, space heaters. Only stacked structures (high rises) can use common venting for fixtures stacked one directly on top of another. These vents, particularly in single family dwelling, riddle structure rooftops with protrusions, which de facto PV installations cannot or do not over build, thus significantly decreasing available space for panels.

But all these different roof aspects & variables do not stop the Distributed Generation – Microgrid model proponents from installing panels on all roof surfaces, except for where layout meets roofline angles or venting.

Then the claim is, the nameplate (maximum production shooting bullets per panel) capacity of the system is x, y or x… Number of panels times panel nameplate capacity. For example, 3,000 watts (20 x 150 watt panels) is a typical residential installation. Collectively, the argument goes, 1.5 MW (megawatt, 1.5 million watts) of nameplate capacity could be installed in a neighborhood over 500 homes, yet given variables detailed above, we might only see 750,000 watts produced at any one peak time.

The cost is the same for that nameplate capacity of the panels regardless of how many bullets will be shot. The cost of install will be higher due to losses in economy of scale. Inverter costs, what changes PV’s DC current to AC current of the grid, is roughly the same regardless the size of each typical residential system, and newer panels even have a micro inverter on each one, resulting no need for a central inverter per system.

If we break the typical residential system up, 6 east, 6 west & 8 south panels (a common occurrence), is it ever possible on those 2 perfect days & perfect time of the day to hit nameplate output? The simple answer is no & it doesn’t matter how much it cost to install the system.

Now take those 20 panels, put them on a warehouse with a flatter roof, orient them as close to south as you can, add a small tilt. Can you hit nameplate capacity on the ‘system’ here? Yes you can. Is the production curve, the output over the day, smooth in reaching peak and consistently maximizing output throughout the daily cycle? Passing cloud cover tends to be the only variable that affects production in this design to any notable degree.

Multiply this out 10, 20, 30 times for a single warehouse in contrast to 10, 20 or 30 separate houses, reduce your install cost thru economies of scale, while increasing your maximum output.

The City & County of Denver, owner of DIA, one of the largest by area airports in the world, have invested in large single axis PV farms.  They continue to expand.  Neighboring business have likewise installed PV systems as noted by arrows.
The City & County of Denver, owner of DIA, one of the largest by area airports in the world, have invested in large single axis PV farms. They continue to expand. Neighboring business have likewise installed PV systems as noted by arrows.

Consider how much collective money (taxes) was spend on subsidies for lower efficiency, private home PV systems, particularly in the US.

Larger plant sized systems can also incorporate single axis trackers which turn the panels like BBQ skewers to follow the sun (usually right to left).

Pole mounted arrays can use dual axis trackers to mimic the right-left and up-down daily & seasonal changes of the sun.

Both single & dual axis trackers increase system efficiency accordingly. But due to increased costs, that can be mitigated in large applications or simple physical constraints, they are rare in residential installations.

In the US significant ‘American Recovery Act’ money was spent subsidizing private residential PV, upwards of 50% of system cost was covered if the individual residential customer could finance or afford to pay the remainder. Given the financial struggles at the time, one can easily surmise wealthier individuals were able to take advantage of this. And arguably, this did lay the seed for PV’s increased & growing application in the US.

At the same time, significant money was invested by cities, towns, municipal entities and the federal government, notably at military installations, installing plant-sized PV system on the order of one-half MW and greater arrays, mostly ground based simple title, but single tracker arrays are common as well.

As seen here, significant rooftop real estate continues to go without PV installations.  How much PV could be installed here?
As seen here, significant rooftop real estate continues to go without PV installations. How much PV could be installed here?

Last I checked, batteries are not free and require significant physical space, the last of which can be a problem in houses. Conversion of electrical energy to chemical energy for storage in batteries is defined by laws of nature, resulting in usable energy losses in every conversion. The losses significant, and most research investment in batteries is focused on minimizing these losses or inclusion of complex systems to attempt to utilize waste energy in a beneficial manner. Laws of nature are what they are, and the economic Law of Diminishing Returns, specifies every marginal increase in efficiency will cost more to realize on an exponential growth curve.

And because daily deficiencies (production less than demand) of solar PV can extend for weeks or months during the winter, one has to significantly over build the system, if there is enough space & money, and still won’t achieve autonomy. However, this “is acceptable” because distributed generation/microgrid/battery proponents have no problems advocating for building an entirely new electrical interconnection system of dedicated power lines from house to house or from some new, radical power generator of the future.

Or to provide an analogy, like Samwise Gamgee nobly offered Frodo Baggins in reducing his burden of being the sole Ring Bearer, ‘Share the load’ in the epic battle between good and evil in The Lord of the Rings. However, existing Grid considered the Eye of Sauron, or the growth of large scale utility or municipal solar PV, wind farms or natural gas generators are no less evil than Orcs, Goblins or Trolls, or are not to be trusted akin to the differences amongst races of elves, dwarfs and man.

They also have little qualms about selfishly defending their own people at Helms Deep with personal fossil fuel electric generators, regardless that there are others equally invested in risking all for the good fight.

This wind farm in Wyoming, was one of the earliest installed.  It continues to expand as the technology advances.
This wind farm in Wyoming, was one of the earliest installed. It continues to expand as the technology advances.

If the ultimate goal is to reduce carbon dioxide emissions, while meeting humanity’s needs, through producing the most renewable or reduced carbon dioxide releasing electricity, at the highest, most cost effective technological implementations as possible, to shutter coal or nuclear generators, is the residential Distributed Generation – Microgrid Model the best, most cost effective, highest efficiency way to do this?

My personal opinion, based on the realities I see or have experience with around the world, the US Solar (Residential PV) model as presented, is an otherwise self-serving, energy Libertarianism.

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