On paper, a megawatt-hour is just a megawatt-hour, it will charge your phone or run your dishwasher the same way, regardless of the source. However, the value to, or impact on, the environment is not fixed. It depends almost entirely on what power plant it replaces. A new wind turbine only helps the climate and local air quality to the extent that it pushes a more polluting plant offline. If instead it is just competing with another clean energy source, the environmental benefit effectively disappears.
Think of it like carpooling. Taking one car off the road does a lot more good during a gridlocked rush hour than it does taking it off an empty highway at 3 AM. Same action, but vastly different impact, depending entirely on what else is going on in the system. In many cases, renewable energy works the same way. A wind turbine spinning at 3 am in Indiana is like carpooling during rush hour. It is likely pushing a coal plant offline, cutting real emissions. That same turbine spinning at 3 pm in California, when the grid is already flush with solar-generated electricity, is more like carpooling on an empty road; the energy is clean, but it is not actually reducing emissions.
To get a more accurate picture of the environmental value of energy and how it changes across time and location required significant analysis.
"You can have a technically identical megawatt-hour of energy from a wind farm or a solar plant," explains Kaffine. "But the value to society of that megawatt-hour, from an environmental standpoint, can be very different."
Kaffine and Fell show that the magnitude of a renewable energy source's environmental value hinges on which "marginal" power plant it displaces, and that this displacement can actually follow predictable patterns. The paper that appears in The Energy Journal , explores the different factors and implications at play in this determination. The team explored a range of different scenarios. The analysis included how location and energy transmission constraints impact which generators are offset, it looked at how the timing of renewable production plays a role and how policies such as renewable production subsidies and regulations around green hydrogen impact the energy value of new projects. Through this detailed analysis, some headline findings emerged.
The time of day matters. Displace power at night, and you are more likely to displace a coal plant. Displace it midday, and you are most likely competing with gas, or worse, another renewable source.
Location is key. Wind in the coal-heavy upper Midwest is likely offsetting coal near population centers, delivering real air-quality improvements. Wind or solar in California is more often competing with the state's already abundant renewable sources.
Understanding that the value varies is essential for building policies that drive change in the grid. In California, wind turbines that turn in the middle of a sunny day often don't displace fossil fuels at all. Instead, it forces solar generation to be curtailed.
"You don't want to be subsidizing a wind generator that is just bumping off solar in the middle of the day," explains Kaffine.
Until we recognize that we need to take the environmental value into account, and install renewable generators in locations where they can displace the most polluting plants, then we are not maximizing the possible change.
Currently, most environmental policy treats renewable energy as fungible: a megawatt-hour is a megawatt-hour. Production tax credits pay a flat rate per MWh of wind or solar, with no consideration of when or where it is generated. Many renewable portfolio standards just require a fixed percentage of power to come from renewables, with little attention to their environmental value. Renewable Energy Credits (RECs), which are the tradable certificates that are woven into many of these programs are priced simply on energy output, and no concern about environmental value.
This kind of flat-rate logic assumes that all renewable energy generation delivers roughly equivalent environmental benefit. Kaffine and Fell's work shows how this assumption breaks down when you look deeper into the details.
Two ideas emerge from this study as the researchers look forward. First, you can only manage what you can measure. Regulators need something close to real-time data on which power plant is getting displaced at any given moment. Historical estimates are not good enough. The infrastructure required for such measurements is starting to emerge. For example, grid operators in the upper Midwest have begun piloting dashboards that track local marginal emissions, using the same framework that electricity markets already employ to track local marginal prices.
Second, integrating this data into policy tools. Currently, subsidies don't distinguish quality. A wind farm gets the same subsidy whether it is offsetting a 40-year old coal plant, or just curtailing a neighboring solar array. Think about how food labels distinguish an organic, farm-identified chicken breast from a generic one. This gives buyers—in the case of energy, the regulators and utilities—a way to tell higher-impact renewable energy apart from lower-impact renewable energy, rather than treating every megawatt-hour as interchangeable.
"We know clean air is valuable to people. We know that the environment is valuable to people," says Kaffine, "But there's no market for it, so we don't have a price for it. How do you put a number on that?" Kaffine sees the work as ultimately in service of a simple goal.
"At RASEI, the end game is always a better environment," he says. "All the technical research is in service of making a cleaner environment. It is really cool to be able to trace these advances all the way through to the end impact."
