Lloyd Energy Storage Graphite Block Thermal Storage
As I discussed in a previous post, a practical demonstration of molten nitrate salt as a thermal storage medium for concentrated solar power (CSP) has already been achieved. However a variety of different storage media have been proposed. A company called Lloyd Energy Storage has developed a system based on high purity graphite blocks. Some details about this system taken from their web site are given below:
At the heart of the Lloyd system is the Graphite Block Energy Storage System. High purity graphite has unique properties that make it ideal as an energy storage medium namely:
High melting point (over 3500°C)
High specific heat (capacity to hold heat energy)
Low emissivity (does not radiate heat readily)
The storage capacity of high purity graphite ranges from around 300kWh (thermal) per tonne at a storage temperature of 750°C to around 1000kWh (thermal) per tonne at 1800°C.
The energy density produced by the Lloyd Energy Storage System is the highest, when related to capital cost and storage durations, of any energy storage system known. The extensive R & D carried out on graphite purity, heat transfer and OTSG technologies enable Lloyd to size the storage system for all grid and off-grid applications.
The heat storage technology accepts heat in any form as it stores it in high purity graphite. Due to the unique properties of high purity graphite, the heat when stored remains available for use for many days/weeks depending on the rate of energy extraction to the rate of energy replacement.
The storage blocks can accept energy from the grid, wind, wave or thermal systems and convert this electrical energy to heat energy to heat up the block.
The block once heated can then transfer the heat energy to a conventional steam raising system by the embedded heat exchangers. The direct contact method of heat transfer employed is very effective giving rise to one of the most efficient steam raises yet designed.
Lloyd Energy Systems has developed and patented a low-cost method of refining low quality graphite to create high quality crystalline graphite for the manufacture and use in the graphite heat storage block. This purification process enables the production of graphite of the required purity at a low cost relative to the cost at which high purity graphite can be acquired and insulates Lloyd from the price fluctuations that may occur in the world high purity graphite market.
Lloyd does not appear to have sold any of their energy storage blocks. They have developed plans to build complete CSP power tower systems which incorporate graphite energy storage at Conclurry and Cargelligo Australia. Some details of these projects taken from presentations on their web site are given below:
Solar energy is collected by tracking heliostats (mirrors) focusing the sun’s rays directly into Lloyd
storage boiler unit
The thermal energy is stored in high purity graphite inside the unit with low losses
The thermal energy is turned into electricity on demand by passing water through tubes embedded in the graphite to make steam
The steam drives a turbine to generate electricity
The plant will utilise a Once Through Steam Generator (OTSG) to produce superheated steam . This
steam will run through a condensing Rankine Cycle steam turbine with an air cooled condenser.
Characteristics of the components are:
Once Through Steam Generator (OTSG) – doesn’t have a boiler drum and consequently
doesn’t have the blowdown requirements of a normal steam boiler.
Air Cooled Condensor – doesn’t use evaporation of water for cooling effect, therefore
minimising the use of water in the process.
Turbine – superheated Rankine Cycle steam turbines are robust and well established in the
Conclurry Project Hardware:
mirrors – total 60,000 square metres
54 modules, 540 tonnes, 80,000 kWh (electrical)
Electricity Generation Capacity: 10,000 kW for 8 hours
Conclurry Project Cost:
Lloyd and Financiers $24,000,000
Queensland Gov’t Contribution $7,000,000
Cargelligo Project Hardware:
2200 mirrors – total 16,000 square metres
16 modules, 160 tonnes, 16,000 kWh (electrical)
3,000 kW for 5 hours (winter peak)
9 hours (summer peak)
Carelligo Project Cost:
Aust. Gov’t AEST Grant $5,000,000
As is clear from the above description as well as the pictures given in the presentations the graphite storage blocks will be mounted directly on the central power towers where they will be heated by the field of heliostat (sun-tracking) mirrors. Because of this direct heating a heat transfer fluid is not required. Water will be passed into the graphite blocks through tubes and be converted into steam to run the generator turbines.
If we assume 40% conversion efficiency the advertised energy storage density of 300kWh thermal per tonne (2000kg) at 750şC translates to 150Wh/Kg electrical. This number can be compared to 25Wh/Kg for lead acid batteries or the value of 12Wh/Kg for nitrate salt storage which I calculated in a previous post. The question of course is how expensive the high quality crystalline graphite blocks will be. Llloyd Energy says that they have developed their own process for producing this material out of low quality graphite.
Although Llloyd does not specifically give the cost of the energy storage system they do give projected total costs for their planned power tower system which incorporates energy storage. For example the Cargelligo project is planned to have 3000kW of generation capacity for 5 hours in the winter and 9 hours in the summer. If we take the average to be 7 hours then the average capacity factor is 7/24=0.29. This capacity factor is comparable to wind energy and can also be compared to an average capacity factor of .55 to .6 of fossil fuel plants. If we assume a plant lifetime of 20 years and ignore operation and maintenance costs and the cost of interest the we can calculate a cost per kWh:
These are Australian dollars of course. At current exchange rates this cost converts to US $0.058/kWh. In reality maintenance costs will not be zero and the plant will have some amount of down time so that cost will be higher. Of course Lloyd energy has not yet delivered one of these plants so that the projected capital costs are still conjectural.
You may wonder at the fact that I ignore interest, something which certainly cannot be done in the current economic environment. However, my thinking is that the day is rapidly approaching when the assumption that if we were not investing our resources in valuable infrastructure we could just dump them into toy factories and have our wealth grow at 6% a year forever will be proven to be complete and utter nonsense.
The 9 hour summer capacity compared a 5 hour winter capacity projected for the Cargelligo project highlights one of the limitation of solar energy; Either we have to accept seasonality in energy use or we have to find some other energy source to complement solar.