Solar Southwest Initiative

While energy consumers rarely think about it, the power grid upon which we depend for our energy is one of the world's largest machines and has a number of different parts. On the electrical energy generation side, grid managers make three main distinctions between types of generators according to the type of power they output: baseload, load-following, and peaker generators. In addition, power engineers value a generator more if it is dispatchable, meaning it can generate more or less power on-demand or on a human-defined schedule.

A baseload generator can generate power constantly day and night at an even level. Usually the per kilowatt-hour cost of baseload generators are low. Coal and nuclear plants are commonly used as baseload generators but also large natural gas and hydroelectric plants are used for baseload power.

A load-following generator gradually ramps up and down its power output to respond to scheduled changes in power demand over the course of a day. Gas, pulverized coal, and hydroelectric generators are commonly used to follow the load. Solar photovoltaic or CSP without storage can approximately follow the load on sunny days, when peak demand is around mid-day.

A peaker plant responds rapidly to changes in power demand that baseload and load-following plants do not; often within less than a minute. Natural gas turbine and hydroelectric plants are used as peakers. Peakers are the most expensive to operate but produce the least amount of power over the course of the year. Some newer types of energy storage, like batteries and flywheels, can also function as peakers, though they are still in the early stages of commercialization and are not yet cost competitive with fossil peaker plants.

To reduce and eventually eliminate the carbon footprint of the electric grid, we will have to develop renewable generators that can output power in ways that partially or completely occupy these roles. While hydroelectric facilities with storage (dams) can occupy all three roles, they are limited by geography, water availability, and competing natural and societal uses for water and river valleys. Geothermal wells work well as baseload power, though are currently limited to specific hot spots. In decades to come we will see more geothermal baseload power as what has been called Enhanced Geothermal Systems are developed.

Currently, to use the strongest renewable resource we have, the sun, to fulfill these roles, CSP with Storage is our most economical and scalable alternative. Paired with enough thermal storage and solar collectors, a CSP plant can operate as baseload power. Alternatively a CSP plant with storage can be built to scale up and down to follow the load, peaking its power output in mid-afternoon when power demand is highest on summer days. With storage and with a steam generator and turbine already warmed up, a CSP plant with storage can be dispatched to meet peak demand within a few minutes.

The usefulness of CSP with storage as a coal and natural gas power plant replacement is then clear in an era of carbon-constraint and concern about climate stability. Coal moratoria and other measures to curtail the use of coal and natural gas generators should to be paired, along with increased energy efficiency, with a promotion of sustainable replacements for these generators. Furthermore, the emergence of plug-in electric vehicles also will eventually increase the demand for clean nightime power, which CSP with storage as baseload can help provide.

Almost everyone wants clean renewable energy to power our future economy. Solar and wind energy are, for most people concerned about the planet's future, our next energy system. But these devices, solar and wind generators,are not the solutions by themselves. To phase out the use of fossil fuels and actually mothball or shut down coal and gas fired power plants, we will need new clean energy storage in addition to renewable generators.

The Critical Role of Energy Storage

The way the electric system works is that electrical energy gets generated and released to the grid just fractions of a second before it is used. Energy consumers like ourselves have structured our lives and work to use electrical energy on demand and in order to receive that energy when we need it, some portion will need to have been stored and ready for use. Fossil fuels, like coal, gas and oil, and fissionable fuels like uranium, it turns out, function as very extensive energy stores, allowing energy to be released when it is needed. A grid run largely on renewable energy represents a new technical and organizational challenge for grid operators, increasing the value of energy storage that can be released rapidly to serve the energy demand of grid customers.

Solar and wind energy alone are intermittent and do not come naturally with energy storage. Solar thermal electric has the potential for storing the sun's energy as heat. One of the least expensive energy storage mechanisms is to store thermal energy in a high heat capacity fluid, such as molten salt. Molten salt is a combination of chemical salts, usually a combination of sodium nitrate and nitrite, that can cycle between two fairly high temperatures, releasing its heat at the higher temperature but remaining liquid at the lower temperature. Currently molten salt storage costs $50/kilowatt-hour, substantially less expensive than lead-acid batteries. Some thermal energy storage solutions suggest using pressurized water, though this solution has not yet been fully implemented.

Integrated with thermal storage, solar thermal power plants can tap into the essentially limitless power of the desert sun and release that power when it it needed by the grid. In the 1990's, using a molten salt storage system, Solar Two, a demonstration plant funded by the US Department of Energy, generated power for 7 days continuously on solar power alone. Commercializing this technology will allow solar power plants to replace the output of coal and natural gas power plants, which in combination of energy efficiency and a leveling of power demand, effectively reduce our greenhouse gas emissions.