- With the right policies, renewable energy and natural gas can lead to an energy system with low environmental impacts.
- Compared to coal, natural gas has lower CO2 emissions, air quality, and water impacts while renewable energy has lower environmental impacts than either coal or natural gas.
- In the short term, natural gas can reduce emissions while renewables scale up.
- In the long term, renewables can limit natural gas consumption to only that necessary to balance renewable intermittency.
NG and RE are Environmental Complements as Well
Part 1 of this analysis described how renewable energy and natural gas make good allies from a financial perspective. In short, high capital cost, low marginal cost renewables balance well with low capital cost, high marginal cost natural gas. Renewables provide a key hedge against natural gas price volatility while natural gas’ ability to dispatch enables higher levels of renewable energy.
In this second part of the analysis, I focus on how natural gas and renewable energy work well together from an environmental perspective.
Very simply, renewable energy and natural gas have better environmental profiles when compared to coal. Although renewable energy is better than natural gas environmentally, both lead to significant reductions in air emissions and power plant water withdrawals.
From a systemic perspective, the combined environmental attributes of a joint renewable energy and natural gas system are much higher than systems that are dominated by either type of energy individually.
Renewables Have No Significant Air Emissions
From an environmental perspective, wind and solar are probably the lowest impact energy sources available.
They do not emit any toxic air pollutants or greenhouse gasses during operation.
The absence of traditional air pollutants (SO2, NOx, mercury) is especially notable. By reducing these air pollutants, renewables from RPS policies provided $2.6-9.9 billion in health and environmental benefits in 2013 alone.
When accounting for upstream manufacturing, the median lifecycle greenhouse gas emissions of renewables are also minuscule compared to fossil fuel sources:
- Solar photovoltaics: 44 g CO2e/kWh
- Onshore wind: 12 g CO2e/kWh
- Natural gas combustion turbines: 670 g CO2e/kWh
- Natural gas combined cycles: 450 g CO2e/kWh
- Coal: 979 g CO2e/kWh
Renewables also have smaller variations in their lifecycle greenhouse gas profiles compared to natural gas, coal, or oil. Different plant efficiencies and upstream emissions for fossil fuels can lead to significant variations in lifecycle carbon intensity, posing distinct plant-level and upstream greenhouse gas management challenges for different countries.
Lifecycle Greenhouse Gas Emissions for Electricity Generation Sources
Solar and Wind have Minimal Water Needs
Solar photovoltaics and wind are also great resources from a water perspective.
Traditional thermal power plants (coal, natural gas, and nuclear) with once-through cooling withdraw 20,000-60,000 gallons of water per MWh. In total, thermal power plant water withdrawals accounted for 161 billion gallons per day in 2010, 45% of total water withdrawals in the U.S.
Comparably, once built, solar PV and wind do not withdraw or consume water.
This has two major implications. First, thermal plants and hydro facilities can be heavily impacted by drought, which can reduce their output significantly and impact grid reliability. Second, thermal plants and hydro facilities can impact surrounding watersheds, aggravating the impacts of drought on both human and natural systems.
With climate change potentially worsening droughts worldwide, the drought resilience of solar and wind are valuable environmental attributes. State RPS policies, primary drivers of renewable energy growth to date, reduced power sector water withdrawals and consumption by 2% in 2013. Critically, the largest reductions occurred in California and Texas, especially water stressed regions.
Land Use Concerns Could Delay Renewable Growth
While very good on the air and water front, solar and wind do have distinct land use issues. Both energy sources require large amounts of land to produce electricity and their resources have regional constraints.
Wind projects frequently run into local opposition worried about the impact of the turbines on view sheds and property values. At its most extreme, this NIMBYism was responsible for more than a decade of challenges to the flagship offshore Cape Wind Project.
Utility-scale solar projects also face notable land use challenges. A recent study in the Proceedings of the National Academy of Science found that the majority of solar power plants in California were sited in natural environments and were close to protected areas.
Opponents of renewable energy often play up these land use issues while proponents often dismiss them. In reality, the situation is pretty straightforward – land use issues do not make renewable energy an unattractive energy source but they are legitimate concerns.
Better planning and community outreach can address many land use challenges. Nevertheless, land issues can delay projects, increase project risk, and could become larger impediments to renewable energy growth if wind and solar continue to scale up rapidly.
Natural Gas Enables Greater Renewable Buildouts
Although land issues can delay renewables, they are not the main challenge with using wind and solar to green the power sector. Rather, the primary environmental challenge for solar and wind is the time it will take to build enough renewable energy to replace dirtier fuels.
As discussed in Part 1, the capital intensive nature of solar and wind make them less scalable than natural gas in the short term. Even in best case deployment scenarios, solar and wind will take years to fully replace coal.
In this context, there are three primary ways that natural gas can enhance the environmental benefits of using renewable energy: reducing coal generation in the short term as renewables are built out, balancing intermittency to allow more renewables to be used in the mid-term and beyond, and by pairing renewables with natural gas employing carbon capture in the long-term.
During the last several years, increased natural gas generation brought significant environmental benefits by reducing coal generation. Since the shale revolution began in the late-2000’s, coal generation has fallen as natural gas generation rose. This has brought significant environmental benefits, as discussed further below.
Monthly U.S. Electric Power Generation, by Fuel Source
As the chart above indicates, natural gas is now passing coal as the primary electricity source in the U.S. Despite the recent rapid gains in wind and solar, their overall generation share remains low. Natural gas can quickly reduce air pollution and water impacts from coal during the next 10 years, buying time for renewables to grow to their full potential.
Natural gas can also play a critical role in balancing out the intermittency poised by wind and solar generation. While this balancing is a key factor behind the financial synergies of renewables and natural gas, it is also beneficial for their environmental synergies. Natural gas power plant dispatch is more flexible than coal or nuclear, so it can support higher levels of renewables than other types of grid configurations.
Finally, carbon capture can make natural gas a very low carbon power source. An electric system balanced between nuclear, hydro, solar, wind, and natural gas with carbon capture can provide an optimal low-cost, reliable system with very limited climate or environmental impacts.
Natural Gas has Moderate Environmental Impacts
Although renewable energy is better, natural gas still has an attractive environmental profile compared to coal.
Perhaps the most heralded environmental characteristic of natural gas is its lower carbon emissions compared to coal. On an energy basis (per MMBtu), natural gas is 43% less carbon intensive than coal.
Moreover, most natural gas is burned in power plants that use efficient combined cycle technology. On an electricity generation basis (per MWh), natural gas combined cycle power plants can thus be around 57% less carbon intensive compared to existing coal plants. This lower carbon intensity explains the major role natural gas has played in the U.S. reducing power sector carbon emissions more than 15% since 2005.
Recession, Energy Efficiency, Natural Gas and Renewables Reduce Carbon Emissions
While the carbon benefits of natural gas get the most attention, the air pollution benefits compared to coal are potentially more important. Natural gas facilities emit negligible amounts of SO2, particulate matter, and mercury. Although natural gas power plants do still emit NOx, they emit significantly less than coal plants.
Coal plants face increasingly stringent environmental regulations due to their air pollution. Low natural gas prices have led to coal plants retiring instead of retrofitting with pollution controls (which still have notable emission levels).
The U.S. is now reaching decade lows in NOx and SO2 emissions. Low natural gas prices played a critical role in achieving power sector reductions since 2008. One estimate found that natural gas alone was responsible for reducing SO2 and NOx emissions in the power sector by 40-44% in the last decade.
Water Impacts Better than other Thermal Power Plants
At the power plant, natural gas is also good on the water front.
When using similar cooling technologies, natural gas withdraws and consumes less water than coal or nuclear. However, the timing of the natural gas build out in the United States has led to very different cooling technology properties for the natural gas fleet.
Thermal Power Plant Cooling Technology by Plant Operational Year
Almost all nuclear and coal power plants in the U.S. were built decades ago, meaning they primarily use water withdrawal-intensive once-through cooling. Most existing natural gas combined cycle capacity has been built since the 1990’s when recirculating cooling technology became more widely used. As a result, most natural gas power plants withdraw a fraction of the amount of water that nuclear or coal units do, with similar consumption levels.
More importantly, natural gas power plants are still being built in the U.S. – these new builds are able to take advantage of dry or hybrid cooling technologies which virtually eliminate water withdrawals and consumption. These systems do bring efficiency penalties, but are ideal in drought-prone areas like Texas.
Upstream Impacts are Major Environmental Question for Natural Gas
While the air quality and water impacts at the power plant level are limited, the upstream environmental impacts of natural gas are potentially troubling.
Hydraulic fracturing, now responsible for the majority of U.S. natural gas production, is a water intensive process. It has caused competition for water resources in water-stressed regions. In Oklahoma, the use of waste water injection wells has also caused a significant increase in earthquakes.
This water usage intensity leads to water quality concerns. A major study by EPA on the water impacts of hydraulic fracturing recently found no evidence “widespread, systemic impacts on drinking water resources in the United States.” However, its Science Advisory Board has since questioned that conclusion, noting contradictions between high level findings and the body of the report, as well as challenges from a distinct lack of data.
These are major issues that need more scientific study to fully understand their severity and, importantly, how policy can be used to mitigate their impact.
Methane Leakage is a Key Uncertainty
Perhaps no area is more contentious in the debate about the sustainable use of natural gas than methane leakage.
As methane is a stronger greenhouse gas that carbon dioxide, relatively small leaks in natural gas infrastructure hurt the lifecycle carbon benefits of natural gas over coal. The best meta-analysis of these numbers to date, however, indicate that leakage is not high enough to completely eliminate the climate benefits of natural gas compared to coal, particularly over long time scales.
Indeed, the relative youth of the U.S. natural gas fleet compared to the coal fleet impacts the equation. Inefficient coal units are being replaced by very efficient natural gas combined cycles. This reduces the negative impacts of methane leakage on natural gas lifecycle emissions greatly.
Nevertheless, methane leakage is an environmental issue that needs to be addressed. Well-designed regulations can reduce leakage while minimizing impact to industry.
Longer term, carbon capture for natural gas can reduce lifecycle emissions of natural gas significantly, minimizing the importance of leakage. At leakage rates of 1-3% (likely levels following leakage regulation), carbon capture can reduce lifecycle greenhouse gas emissions of natural gas by 56-70%.
Lifecycle Emissions from Natural Gas with and without CCS
Renewables Limit Natural Gas Consumption
In conclusion, key environmental characteristics make grid systems with a mix of renewable energy and natural gas ideal. In the short term, natural gas generation can quickly ramp up to replace coal use, driving reductions in CO2 emissions, plant-level water impacts, and toxic air pollution.
Long term, solar and wind can grow into roles as dominant energy sources, virtually eliminating many environmental impacts in the process.
The long term replacement pattern that this puts forth likely means that the environmental negatives for natural gas should have increasingly lower effects over time, as consumption decreases.
For more information:
- The following publications from NREL and JISEA have a more in-depth look at many of the issues present here: http://www.nrel.gov/docs/fy13osti/56324.pdf, http://www.nrel.gov/docs/fy15osti/63904.pdf
- An in-depth report focusing on water consumption at power plants: http://www.ucsusa.org/sites/default/files/attach/2014/08/ew3-freshwater-use-by-us-power-plants.pdf
- An overview of how to address renewable intermittency issues: http://www.nrel.gov/docs/fy13osti/60451.pdf