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January 21, 2010

EMC Solar Claims Calcium Hydride Has Ten times the density of conventional molten salt solar storage


A cheap and effective way to store solar power is needed to increase the usability and adoption of solar power. Molten salt storage has been seen as one of the best ways to create solar power storage that is scalable to megawatt hours or more.

EMC Solar and partners are trying to achieve continuous, cost competitive, and modular configuration for large scale solar power production, in GWatt quantities.

Calcium hydride is chosen due to its ability to be broken apart using a thermal heat source, such as the sun. Calcium hydride provides up to 0.90 kW-hr/kg of heat when it converts to calcium and hydrogen.

Hydrogen is stored in a separate low temperature hydride tank. Two heat exchangers are used to extract the thermal energy from the hydrogen before storage in the low temperature hydride. The heat exchangers are required to extract the remaining 20% of the total system energy still in the 1100 C hydrogen before it is cooled to near room temperature.

A central triple walled reaction chamber holds both Calcium and Calcium hydride as liquids between 1000 C and 1100 C. Heat is extracted from the reaction chamber to drive one or multiple 100 kW high temperature Dual Shell Stirling engines operating at 50% conversion efficiency.

The thermal storage costs are substantially lower than a nitrate salt system and reflect both the simplicity of the calcium hydride system and the significant increase in power density. In the calcium hydride system the two liquids, calcium and calcium hydride, remain in the central reaction chamber. Only hydrogen is pumped between tanks

Proposed 100 kW solar system:

* Store 18 hours of thermal energy
* Down mirror focuses sunlight from heliostat field
* 4,690 kg Calcium
* 234 kg Hydrogen
* Reaction chamber insulated with a quartz window for solar heat input
* Two tank boron oxide high temperature heat exchanger for hydrogen
* Two tank nitrate salt low temperature heat exchanger for hydrogen
* Low temperature Sodium aluminum hydride tank holds 5% hydrogen by weight




The system uses a new low cost wire braced heliostat field with 50 square metres per panel at $100/metre squared in production. The new heliostat configuration eliminates the cosine effect, common with power tower designs, by utilising a parabolic mirror aligned with the sun throughout the day. The parabolic mirror is integrated with a quartz lens and side mirror which provides a 0.1 metre constant diameter focused light beam. The heliostat design has the added advantage of eliminating the power tower and replacing it with a small down mirror located directly above the reaction chamber.

Sunlight is focused through a quartz window, into the reaction chamber, onto an inverted molybdenum cone submerged in the liquid calcium which absorbs the solar energy. Major cost reductions occur due to the use of a down mirror system which allows the power head to be immersed within the reaction chamber inside the liquid calcium. This allows a significant increase in heat transfer capability. An insulated cover is placed between the quartz window and power head at night minimizing thermal losses.

The advantage of this system is that it is a completely reversible closed cycle. The intermittent sunlight can be chemically stored and released at a controlled rate for electric power production. The system uses materials which are low cost and provide a competitive electrical production facility for very large scale application.




RELATED READING
An 11 page paper on thermal energy storage and phase change materials by an unrelated researcher (2006)

The storage of thermal energy in the form of sensible and latent heat has become an important aspect of energy management with the emphasis on efficient use and conservation of the waste heat and solar energy in industry and buildings. Latent heat storage is one of the most efficient ways of storing thermal energy. Solar energy is a renewable energy source that can generate electricity, provide hot water, heat and cool a house, and provide lighting for buildings. Paraffin waxes are cheap and have moderate thermal energy storage density but low thermal conductivity and, hence, require a large surface area. Hydrated salts have a larger energy storage density and a higher thermal conductivity. In response to increasing electrical energy costs and the desire for better lad management, thermal storage technology has recently been developed. The storage of thermal energy in the form of sensible and latent heat has become an important aspect of energy management with the emphasis on the efficient use and conservation of the waste heat and solar energy in the industry and buildings. Thermal storage has been characterized as a kind of thermal battery.




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