Sunday, March 2, 2008

Redwoods vs solar panels

A CNN article on the conflicts between growing more trees and getting lesser sunlight for solar panels was interesting. Some bare facts:
  1. Mark Vargas, who owns solar panels (an investment of 70,000 (2001 USD)) contends that his neighbour, Treanor's redwood trees are blocking sunlight for his solar panels
  2. After 6 years, a judge cites CA's obscure sunlight availability law to rule in favor of Mark Vargas.
  3. As alternate energy sources become more mainstream, many experts predict that conflicts similar to this case, will happen.
In response to Treanor's claims that the redwoods help capture CO2, Vargas "counters it would take two or three acres of trees to reduce carbon dioxide emissions as much as the solar panels that cover his roof and backyard trellis."
Legal issues aside, this case represents an interesting conflict. For starters, if we assume that the solar PV panels indeed help prevent more CO2 emissions than the redwoods (which turns out to be valid, see the appendix for back-of-the-envelope calculations), the question for environmental economists would be which option would generate the least negative externalites ? In other words, if we could put a price/ton on every pollutant (including CO2) evolved, which option would result in a lower price? Can the amount of services provided by redwoods be quantified in monetary terms?

Analysis of net CO2 emissions prevented by using solar PV and amount of CO2 fixed by redwood trees over the course of a year:
The data on photosynthetic CO2 fixation in coastal redwoods was taken from this paper (Osborne and Beerling, 2003). The authors of the above study used 3 year old trees to measure photosynthetic rates in a controlled atmosphere. Only results for coastal redwoods at pCO2 (partial pressure of CO2)=40 Pa (pascal) (corresponding to 395 ppm CO2 in the atmosphere) will be used in this analysis.
The PVWatts website was used to estimate costs and energy production from a single solar PV panel. San Francisco was chosen as the location (due to its proximity to Sunnyvale). The DOE web page puts the amount of CO2 emitted/unit of energy produced for coal power plants at 2.12 lb/kWh.
In Fig.7 (Osborne and Beerling, 2003) show the average rates of photosynthetic fixation (milli moles of CO2/plant/day) for different species. The area under the curve (in Fig.7 of the paper) for coastal redwoods gives the total CO2 photosynthetic fixation over the entire year. It comes to ~1650 mmol CO2/plant/year. This amount is however, for 3 year old plants. Let us assume that the amount of CO2 fixed increases linearly with the leaf growth and the plant age. If we consider a 12 year old tree, ~3350 mmol CO2/year are fixed by a single redwood tree. This amounts to 0.15 kg CO2/tree/year.

In comparison, Vargus installed 70,000 $ worth of solar PV panels. I assumed the average 2001 costs of solar PV panels to be 0.2$/kWh (current costs from the PVWatts website are 0.125 $/kWh). Per a 4 kW panel, the energy produced/year (from the PVWatts wevsite) would be ~5800 kWh. This represents an investment of ~1150 $/4kW panel. Since Vargus invested ~70,000$, he probably has 60 solar PV panels installed.
In a year, the AC electricity produced from all the panels (from the PVWatts website) would be 464000 kWh.If we assume that only 70% of the panels are operational at any given time, the figure becomes a little lower, ~330,000 kWh/year. I took electricity from thermal power plants as the basis for comparing CO2 emissions. Using the value of 2.12 lb CO2/kWh generated (and not accounting for transmission and distribution costs), I calculate that emissions of ~312 T of CO2/year will be avoided with the 60 solar PV panels.

Comparing the above two values (CO2 fixed/plant/year and CO2 avoided/year), we can clearly see that there is orders of magnitude difference between these two quantities. 60 solar PV panels prevents more CO2 emissions than ~1000 redwood trees. However, this is not the end of the story.

Solar PV panels are produced in a energy-intensive process which leads to CO2 emissions. For a complete CO2 emissions analysis, the emissions for solar PV manufacturing also need to be taken into account. However, this will not change the results here by very much as there is clearly differences in orders of magnitude between solar PV panel and a redwood tree.

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Nari said...

This looks good, Why dont you post this on nature network?

Nari said...

Another issue to consider is if the redwoods existed before the solar panels were ever installed by Vargas on his property. Additionally, the redwood trees provide passive cooling during summer for the owner (so there is a reduced need to run air conditioners and corresponding usage of coal based electricity. I am not sure if the redwood owner presented this argument at all. I dont have any analysis on passive solar cooling, but just to consider solar replacing coal electricity is a very narrow point of view.

Pradeep said...

Thanks, may be I will.

Good point with the passive solar cooling..
The trees existed before Vargas installed solar panels on his property

Anonymous said...

There is a nice solar estimator at It uses PVWatts for some analysis, but also applies other data to help analyze solar energy production, costs, greenhouse gas reductions, etc.

Edward said...

Residential solar panels of the same size may produce variable amounts of electricity, this discrepancy is caused from variations in the functioning age of the solar cells, the quality of the solar cells, and with what technology was used to produce them.

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