One of the biggest issues that detractors can use to argue against the increased use of solar power is its intermittency. By its very nature, solar power is diurnally-based. When the sun isn't shining, the system isn't generating power. Additionally, the system's power capacity fluctuates according to changing weather conditions: passing clouds or cloudy days can substantially reduce the available power output from PV panels, causing spikes or lulls in energy production. This means that a solar PV system by itself requires a backup power source to ensure a smooth energy distribution curve, increasing the cost of electricity to homes and businesses alike. Let's take a look at this graphically to understand this a bit better:
As you can see, solar power's production potential is typically highest during a time of the day when energy use is rather low, and vice versa. However, this fact presents opportunities as well. If the excess energy generated by PV panels during the middle of the day could be stored, then that energy could be spread over a longer time period and provide even more benefit to consumers while simultaneously reducing the use of expensive backup power sources.
It is this concept that has led to advancements in solar thermal plant design. Instead of using photovoltaics to generate electricity, a solar thermal plant instead uses the sun's rays to generate heat. The Arizona solar plant utilizes a relatively new application of an older technology, molten salt, to help store the excess energy produced by the sun. Molten salt is a combination of potassium and sodium nitrate heated to very high temperatures. The reflected energy from the sun is used to heat this salt to approximately 1,000 degrees Fahrenheit, and that heat is used to create steam to turn turbines and generate power. If left by itself the salt loses less than 1% of its heat overnight, which means that the system can remain hot for 2 months even if all of the solar panels suddenly went offline! However, the designers of the system anticipate using the heat from the salt to generate enough energy so that all the heat will be used up overnight.
Source: treehugger.com
The use of molten salt in the solar tower means that there is enough energy for an additional 10 hrs worth of energy production from the solar facility even after the mirrors have stopped reflecting sunlight for the day. This allows the solar system to be more flexible in its energy generation, depending on the needs of the utility. Since Las Vegas (the city that the plant will service) is by its very nature more active at night, this plant can help to alleviate some of the greater energy usage by its denizens at any hour of the day.
Energy storage is an important technology for making sure solar power can provide an efficient, steady flow of energy. As new advances in storage, like molten salt, become economically feasible, they can provide a way for solar power producers to match their distribution curves to typical energy use patterns. I imagine that PV panels could be used to heat mini salt storage systems as well: the excess electricity generated by panels during the daytime hours can be used to heat the salt, and then the heat from the salt generates electricity at night. It's an exciting technology development, and hopefully it ends up being one that can become widely used and accepted as well.
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