Solar Energy at Night

The energy emanating from the sun can be converted into electricity by using:

• Photovoltaic Cells where semiconductor materials absorb photons and releases electrons, or
• Solar-Thermal Turbines that generate steam from the sun’s heat, spinning a turbine to produce electricity.

Since cloudy days and nighttime limit the practical conversion time of the sun’s energy, traditional applications of solar power have proved inefficient, variable, and expensive. This is particularly limiting given that during the early evening, when power demand is normally near its peak, solar power is declining in intensity.

Potential Solution – Storing Solar Energy

Until recently, the possibility of storing solar energy has not been feasible. Methods involving water and compressed air  will lose at least 20 percent of the energy received from the sun and batteries are inefficient and too expensive to be a viable option. By recognizing the role that stored heat can play in generating steam that turns a turbine, the industry has developed a solution that is now currently in operation at the Andasol 1 power plant in Grenada, Spain. The key discovery was the applicability of salt as a medium to assist in the storage of heat:

• Salt vaporizes only at extremely high temperatures, above the required level estimated at 750 degrees F.
• Its availability is virtually unlimited and it is relatively low cost.
• Energy loss is less than 7 percent.

Though table salt will suffice, the initial prototype Andasol 1, uses 30,865 tons of sodium and potassium nitrate.

Andasol 1 is a Proven Application

Starting operation in November 2008, Andasol 1 operates with or without sunlight:

• As long as the sun is shining, it operates like a typical solar-thermal power plant. The sunlight strikes a field of parabolic mirrors (solar panels) oriented toward tubes filled with oil which typically heat up to 400 degrees F. The hot oil boils water which converts to stream and spins a turbine.
• The plant’s capacity to collect the sun’s energy is equivalent to almost twice as much as is required for normal solar operation during sunlight hours. This results in extra heated oil that relies on a heat exchanger that heats the molten salt at 752 degrees F, stores it in a vat, and pumps it back into the system when sunlight is no longer available.

In adopting this approach to storing the sun’s heat, the Andosol 1 power plant can:

• Operate almost twice as long as the more traditional solar power plants,
• Generate 50 percent more energy – 178,000 MWh of electricity, and
• Rival the cost of natural gas power.

Other Options Under Consideration

Other designs and approaches are considering more direct approaches by skipping the oil, collecting and storing the sun’s heat in salt or sand. Also, the molecular effects of photosynthesis, where the sunlight splits the water molecules into hydrogen and oxygen to be put back together in a fuel cell offers yet another technological approach.