IMPERIAL COUNTY — Renewable energy production in the Imperial Valley is currently offsetting over 4.5 million tons of emitted carbon dioxide per year, according to statements made by Andy Horne, Deputy CEO of Natural Resources Development, Imperial County on Tuesday morning during the regular meeting of the Board of Supervisors.
Totaling over 9.3 million megawatt hours (MWh) in energy production from wind, solar, and geothermal within Imperial County, Horne concluded the annual carbon emission offset equaled roughly 4,650,465 tons of CO2.
To put this amount into perspective, Horne suggested that the offset could be compared to the amount of carbon dioxide consumed by trees.
“If we planted 193 million trees, that would equal the carbon offset of the amount of renewable generation we have here in the Imperial County,” said Horne.
Given the high amount of air pollution in the Valley, the prospect of additional power plants that emit far less carbon dioxide is welcome news.
“As we continue with renewable energy construction here in the Valley, we see the benefits,” said Supervisor Raymond Castillo. “Not only are we talking about job creation, but what we are doing for the environment is just as important.”
Geothermal in particular was highlighted for its consistent power output in comparison to wind and solar power.
“For geothermal, we have 725 MW –less than solar— but the megawatt hours are substantially higher,” said Horne, referring to a solar panel’s limitation of generating power only during daylight hours, a problem which is hoped to be solved by the installation of industrial batteries. Geothermal, however, is not depending on daylight hours or weather variations.
As a result, geothermal production in the Valley is over 5.3 million megawatt hours annually, compared to solar’s 3.1 million MW hours, despite solar having almost double the installed capacity (1,200 MW from solar and 725 MW from geothermal).
“We have repeatedly tried to explain to people the capacity factor, the operating hours during a year,” emphasized Horne. “For a geothermal project, I estimate it to be 85 percent capacity factor.”
According to the U.S. Geological Survey (USGS), emission rates for geothermal plants are far below traditional fossil fuel plants, and emit about 5% of the carbon dioxide, 1% of the sulfur dioxide, and less than 1% of the nitrous oxide emitted by a coal-fired plant of equal size.
Because geothermal resources naturally produce greenhouse gases (even before a plant is built), experts are still working to improve how emissions are measured at a plant site.
Accurate measurement demands being able to differentiate between natural geothermal emissions (which varies depending on the specific chemistry of the rock type and other factors) and those attributable to geothermal power plants themselves. Such variance between individual site locations makes it difficult to generalize emissions for the geothermal industry at large.
However, the overall emission output from a geothermal plant is still drastically less than fossil-fuel alternatives.
The type of plant can also impact the amount of emission and its composition. The power cycles of geothermal plants in production can be classified as either dry-steam, flash-steam, or binary. All three use noncondensable gases to power the plant turbines. Dry-steam and flash-steam discharge this gas content into the atmosphere or sometimes convert it (often to sulfur).
Alternatively, binary systems keep all the noncondensable gas in a closed loop, and inject the gases back into the reservoir after the brine by-product has been processed. As such, geothermal plants with binary systems produce near-zero emissions.
A new technology currently under development might further push the envelope. An enhanced geothermal system (EGS) injects liquid into a man-man reservoir where there is hot rock but little natural fluid saturation. This would allow the plants to be built outside of traditional hydrothermal areas.
Surprisingly, the liquid that appears to be the best candidate for injection is supercritical (liquid) carbon dioxide, which could also eliminate several production phases associated with steam water. Using carbon dioxide could eventually lead to not only the design of carbon-dioxide-free but carbon-negative geothermal plants, which would consume more CO2 than it produces.
EGS is still in its prototype-phase, and some concern exists about finding cost-effective piping and material that can resist the corrosiveness of supercritical carbon dioxide. The industry, however, is still optimistic and endorsement for pursuing a working model is still high.