(361d) Renewable Energy Storage Using Hydrated Lime

Providing sufficient energy storage capacity is a key to matching the energy generation profile of renewable energy systems with human use patterns. It is important to recognize that end use energy consumption takes the form of electricity and heat. As such, an energy storage technology that can deliver both forms of energy would be advantageous. Providing a low cost method for energy storage would help displace that oil as wind energy, a local resource, eventually gets built.

We propose a feasibility study on using lime (Calcium Oxide) as an energy storage medium by cycling between the hydrated and anhydrous states. The fundamental chemical equation is listed below. CaO(s) + H2O(g) ?\ Ca(OH)2(s) The reversible reaction has an enthalpy of formation of 100 kJ/mol and a free energy of formation of 0 kJ/mol at 520oC. Varying the partial pressure of steam can control the reaction temperature.

The primary energy released can be converted to 520 Watt-hour (Wh) per kilogram (kg) of lime, which can be compared to values of 130 Wh per kg for lithium ion batteries. The energy stored is in the form of thermal energy, as opposed to electrical, but it can be converted, using a Rankine cycle or Stirling engine, back to electricity. While the additional conversion steps reduce overall efficiency, the simplicity and multiple outputs of the proposed system more than compensate. Even with the efficiency losses, the valuable product in a renewable electricity grid is service and not electricity.