To cut human-generated carbon dioxide emissions by 80 percent, an oft-quoted goal, the electricity sector might have to reduce its own emissions even more because other sectors like agriculture or aviation could find 80 percent impossible. Many states have quotas for reliance on renewable electricity, but the highest is California’s at just 33 percent.
Yet how much renewable energy can an electric grid tolerate?
A lot more than is generally assumed, according to a new report commissioned by an environmental group, HEAL Utah.Yet the group takes a different tack from that of most other organizations that envision a low-carbon future; it wants to forswear both nuclear power and coal-fired power with carbon dioxide capture.
Utah has no renewable energy quota. But the report, released on Tuesday, proposes a system that would be nearly entirely based on solar, wind and geothermal power and the mass deployment of two technologies that are still in their infancy: compressed air energy storage, and a smart grid that takes control of customers’ appliances.
The report was prepared by Arjun Makhijani, the head of a Washington-based anti-nuclear group, the Institute for Energy and Environmental Research. Dr. Makhijani, who has a Ph.D. in nuclear fusion, pointed out that one of the main costs of renewable energy is the amount that is “spilled,” or produced from intermittent sources at a moment when there is no use for it.
Some utilities are experimenting with large-scale battery storage, but the only two mass storage technologies available are pumped hydro and compressed air, he said. His report envisions an energy system in which the only fossil fuel is natural gas, used in conjunction with compressed air systems. A utility digs a cavern underground and, when surplus electricity is available, uses it to pump the cavern full of air at high pressure.
When renewable energy is not available, the air is tapped and mixed with a small amount of natural gas and then run through a gas turbine to make electricity. The technology is similar to existing gas-fired generators, except that in a conventional gas generator, some of the energy from burning gas is used to compress air to make the process more efficient. In Dr. Makhijani’s vision, the wind blowing at off hours accomplishes some of the work ordinarily done by gas.
The storage proposal poses a variety of challenges. One is that what it stores is energy, but not always renewable energy. The one commercial compressed air storage project now in service, in Alabama, stores energy that is made late at night by burning coal. One reason is that the goal in that case is not to reduce carbon dioxide emissions but to save money: off-peak coal power is very cheap.
The study is based largely on costs that are estimated or projected and thus could be inaccurate. Still, it reaches some noteworthy conclusions.
One is that the concept of peak demand may have to change. Today, electric systems are designed to cope with peak demand, the hours when demand is highest, usually in the summer. But if a great deal of solar power is installed, summer demand may not be as challenging as the hours when demand is high and renewable supply is low — for example, in the dead of winter, when it could be quite cold but the wind has died down, Dr. Makhijani suggests.
In some states, like California, companies are exploring whether they can store large amounts of energy as heat by using the sun to heat molten salt. When the sun goes down, the salt can be used to make steam that is converted to electricity. But in a Utah winter, “molten salt doesn’t help you a lot,’’ Dr. Makhijani said, because there is not enough at any given hour to make much of a surplus.
Aside from that that challenge, far more adaptation would be needed in the grid beyond the notion of smart meters, which could manage functions like changing the price of energy at different times of day and informing homeowners instantaneously about their consumption and their energy spending.
“We need a system in which the load is responding to the state of generation,”
Dr. Makhijani said. For example, in the morning a residential customer could put soap in the dishwasher, push a button indicating that the machine is ready and leave for work. When the grid had excess electricity, it would turn on the dishwasher.
Today, utilities offer supply to meet demand, but in the future, demand will partly tailor itself to supply, Dr. Makhijani said.
By MATTHEW L. WALD/NYT