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Developments in Solar Energy Storage Solutions

published: 2011-09-27 8:31

Solar power systems capture the sun's radiation in two ways: Photovoltaic (PV) and concentrating solar power (CSP). PV converts sunlight into electricity. CSP, also called solar thermal, focuses the sun's energy on pipes filled with water. The method generates high temperatures, which boils water and creates steam to power a turbine and generate electricity. The thermal system also provides heat for home heating or hot water.

One issue that has plagued the thermal solar industry has been how to store energy captured from sunlight - the most abundance source of energy in the world, to provide energy when sunshine is not an option. Regardless of the method of generation, it is just plain difficult to store electricity to meet energy demands on dreary overcast days or at night.

The industry seems to be making progress by focusing on old as well as new technologies to balance power supply and the demand side of the equation. Utility company managers salivate at the thought of a solar thermal power plant that stores excess output, which enables them to sell electricity based on an expected price.

New Molten Salt Design

Parabolic troughs make up about 90 percent of all solar thermal concentrator systems. The system, which consists of long parabolic-shaped mirrors that reflect sunlight on tubes filled with liquid, relies on molten salt to provide energy storage. Molten salts consist of a mixture of sodium and potassium nitrate. Two products commonly used in the agricultural sector as fertilizers. The substances allow storage of heat. The sun's rays heat the salt, usually close to water in some type of heat exchanger.

Salt melts at extremely high temperatures. Take table salt, temperatures must approach 1472 degrees Fahrenheit (800 C) before the salt starts to melt. The temperature must reach an even higher range for the salt to vaporize.

One of the primary characteristic of molten salt is the efficiency of the storage solution - most of the solar energy absorbed does not dissipate in the process. The molten salt technology allows utilities to plan and manage electricity production and distribution.

However, several solar power developers have employed this technology in power towers because it allows for even more energy storage capacity. Power tower systems concentrate mirrors on a central tower. Power towers have the capacity to produce higher temperatures. As a result, it stores more energy in molten salt when compared to other solar thermal designs.

The CEO of BrightSource, John Woolard states the power tower “is a much more efficient system and much more cost effective,” the technology requires a smaller quantity of salt. According to research from the National Renewable Energy Laboratory (NREL), storage from power tower technology has the potential to reduce costs 25 to 30 percent per kilowatt-hour.

Indefinite Storage of Solar Energy

The Massachusetts Institute of Technology (MIT) currently works on a solar energy storage solution that captures the sun's energy in the form of certain molecules. The molecules release the store energy and provide usable heat. This thermo-chemical method causes the molecules to change its makeup when it absorbs sunlight.

The transformation pushes the molecules into a “higher-energy” state. A key attribute of this molecule - it remains in this expanded state indefinitely. Application of a just a minute amount of heat, or other catalyst, stimulates the molecule to regain its initial configuration and release heat.

Researchers have known of the potential of this fuel since the decade of the 70s. The obstacle has been to find a chemical to transverse the two states: absorb sunlight and transform to the “storage state,” and release heat after re-conversion to the original state.

According to the Jeffery Grossman, an associate professor the Carl Richard SoderbergAssociate Professor ofPower Engineering in the Department of Materials Science and Engineering, an intermediate step between the original state and the transformed configuration, helped the researchers to understand the stability of the molecule and the qualities that make the process reversible.

Grossman states, finding makes it possible to produce a “rechargeable heat battery,” with the capacity continually to store and release heat collected from sunlight and other sources. The research relies on fuel that uses a rare molecule called fulvalene diruthenium. Fulvalene diruthenium stores heat that reaches a temperature of 200 degrees Celsius (392 degrees Fahrenheit) when released - enough to heat a home or run an engine to generate electricity, says Grossman.

To move from theconcept phase to roll out of a reliable storage solution, researchers hope to identify inexpensive, plentiful molecules with chemical attributes of fulvalene diruthenium.

Super Capacitor

Voltage sags, demand spikes, or cloudy weather requires energy storage solutions for utility-scale photovoltaic power plants. The proper storage system helps operators manage voltage fluctuations in the grid. Many PV plants require low-energy storage systems. The system includes lithium-ion batteries, advanced lead acid batteries, and super capacitors. The sizes range from 500 kilowatt to one megawatt or larger with 15 minutes to one hour of storage capacity.

Batteries are highly inefficient and have a limited capacity for storage. A capacitor is a simple version of a battery, but it only stores energy. A super capacitor, which is comprised of nano porous carbon, holds a significant amount of charge - 1,000 to 10,000 times the amount of a regular capacitor.

Some industry experts expect super capacitors to become the dominant technology and could   make battery storage obsolete.


Small solar energy storage solutions provide a backup power source, which helps customers through short-term power outages until the restoration of primary power.

Developing large solar storage solutions helps lower capital costs. In addition, utility companies will pay electricity producers a premium because storage capacity eliminates the need for back up power to address power fluctuations in the grid or unanticipated increase demand for electricity. More critical for solar thermal system plant developers, effective storage solutions help solar thermal to compete with PV solar power plants.

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