Power Engineering - Massive Electricity Storage, Part 1

Renewable Power Needs MES
Electricity is generated and consumed instantaneously. Unlike other energy supply systems--oil, natural gas, or coal-–the electricity power grid or the generation plants that supply the grid have essentially no storage or "surge" capacity to smooth out peaks and valleys in demand or to provide reserve capacity during sudden spikes in demand.

Fortunately, coal-fired and nuclear power plants, as well as hydroelectric plants, have turbine generators that run continuously so that they deliver firm, continuous, and dispatchable power required by consumers. With such a steady baseload supply, power plants can meet demand shifts and daily cycles either by adding peaking generators or utilizing the available "spinning reserve," about 13 percent today for the U.S. power industry. Thus, the balancing of power supply and demand, critical for the operation of a safe and reliable grid, is now done "on line in real time."

Renewable power, whether generated by wind, photovoltaic, or solar thermal, is inherently intermittent, and there is no "spinning reserve." The sun generates power 10-12 hours a day at best, and clouds can block the sun at any time. The wind changes direction and velocity on a regular basis, and loss of wind can occur at any time. Yet, power must be supplied to match load demand using a mix of baseload and dispatchable generating plants. This is the Achilles' heel of renewable power.

Massive Electricity Storage (MES) is the critical technology needed by renewable power if it is to become a major source of baseload dispatchable power to eventually replace fossil/nuclear power plants. For system stability and load leveling, stored power of multi-MW capacity and multi-hours of application duration are needed to convert the intermittent and fluctuating renewable power that is generated into dispatchable power. Without sufficient
MES, accessible online, solar/wind power cannot serve as a stable baseload supplier; it can only piggyback onto baseload fossil/nuclear generators as a small incremental supplier.

State of MES Today
In 2006, the U.S. renewable power from non-hydro sources--mostly solar and wind--supplied 2.4 percent of our annual electricity consumption. Hydropower is a renewable power source and it supplied an additional 7 percent as dispatchable power; but this resource is essentially "maxed out". The 2.4 percent renewable power is simply piggybacked onto our fossil/nuclear power grid by direct connection. America's huge grid can absorb this small intermittent incremental amount without encountering grid instability.

However, as renewable power increases its grid penetration to 15 percent or more, grid instability will rise to a level that must be controlled. For smaller and weaker grid systems on islands, such as Hawaii or Ireland, which cannot be tied into large continental grids, the sensitivity to instability is much more acute.

No U.S. renewable power generator has MES today. Among battery storage technologies, only one is operating at above 1 MW power level, the 1.2 MW NaS (sodium-sulfur) battery system with 7 hours of storage capacity at an American Electric Power facility. The purpose for this installation is to reduce peak loads for improved distribution service, and not yet for renewable power stabilization.

In America today, there is an almost total absence of public awareness of the need for MES. The prevailing public view is that renewable power can replace fossil/nuclear as power sources if enough wind farms and solar generators are built. All attention and R&D support are focused on improving the cost and performance of wind/solar electricity generators. As a result, it is not surprising that MES is not even recognized as a top priority critical technology deserving sustained attention and support by both the U.S. Department of Energy (DOE) and Congress.

Because our power grid is the backbone of our society, its reliable and safe operation is absolutely essential to our nation's well-being. The MES devices to be deployed are very large, both in size and investment. More importantly, they must be tested on a commercial scale under a wide range of operating conditions and for extended periods of time before MES can be accepted and included in our grid system. This takes time, and we, as a nation, have yet to start down the development path.

For more information on the American Institute of Chemical Engineers and its specific involvement with power generation issues, visit the AIChE Energy Initiative at http://www.aiche.org/energy.

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