Recent energy storage research findings
A new report, ‘Simulating the value of Concentrating Solar Power with Thermal Energy Storage in a Production Cost Model’ (8 January 2013) from the US Department of Energy’s National Renewable Energy Laboratory (NREL) has found that there are considerable benefits to be had from the utilization of CSP and available energy storage methods for an organisation’s optimal mix of energy sources.

The report states that: ‘CSP with TES, with an ability to store thermal energy in, say, molten salt, can use its heat energy to drive turbines at power plants over much longer stretches of the day.’
Expanding on this, Marissa Hummon, one of the authors of the NREL report, says: “CSP with thermal storage can continually reduce peak demand as the peak moves into evening. It continually maintains a high operational value and high capacity value.”

Mark Mehos, Manager of NREL’s CSP programme, comments: “We’ve known for a long time that CSP with storage adds significant value, however, we are now able to quantify this value in the language utilities understand.”

Latest developments in SA regarding CSP storage
In South Africa a great deal of research is being carried out by the Centre for Renewable & Sustainable Energy Studies at Stellenbosch University. Professor Wikus van Niekerk, Director of the Centre for Renewable & Sustainable Energy Studies, states that in the South African context, storage is of particular importance for two reasons – the country’s early peak in national energy demand, and the thermal inertia (measurement of responsiveness to variations in temperature) the CSP plants will provide to stabilise the grid.

“In South Africa we have an early peak in our national demand, and during this time any plant that can provide electricity to the grid is much more valuable. This is perhaps as high as R 5/kWh if one considers that the alternative is the open cycle gas turbine (OCGT) that Eskom [South Africa’s Utility] needs to fire up.”

On the issue of inertia that CSP plants will provide, Professor van Niekerk explains: “In the Northern Cape, where most of the solar energy plants will be installed, the grid is not very strong. Having thermal plants with inertia online will assist to stabilise the grid in this area and overcome intermittency.”

In South Africa ACWA Power is commissioning a 50MW parabolic trough with nine hour storage capacity, and Abengoa is commissioning its Khi Solar One 50MW power tower plant with two hours of storage, and its Kaxu Solar One 100MW parabolic trough with three hours of storage. Professor van Niekerk says: “These three plants will serve as pilot plants to show the various stakeholders in South Africa that CSP is possible, reliable and can make a difference to our energy mix.”

Supporting this viewpoint is Paul Gauché, Senior Reseacher and Director for the solar thermal energy research group (STERG) at Stellenbosch University, who comments: “The ACWA Power parabolic trough will likely be the first South African plant with significant storage hours to begin proving the value proposition of CSP technology here.”

He continues: “To me, the most important value of these three plants will be the demonstration of early localization and the quality of electricity in terms of dispatch and stability.”

Factors that affect how many hours of storage capacity a CSP plant can offer seem to be a question of need and cost.

Gauché says: ”Regarding the need, storage can be optimized for peaking power in the earlier years and can be cost competitive now. We believe this is the case in South Africa. Ultimately CSP storage will be optimized to dispatch when other renewables can’t provide energy and we think this will be a smaller storage than for a baseload plant.”

Professor van Niekerk believes that cost affects hours of storage capacity tremendously and states: “The Government should consider awarding a higher ‘commercial rate’ in the REIPPPP for solar energy that can be supplied to the grid after 17h00. This will then assist the IPP to justify additional storage on future CSP projects.”

When asked about the technological advances in storage capacity of CSP plants, in particular those to be commissioned in South Africa, Gauché says: “We are looking at several alternatives with the goal of reducing levelised cost of energy (LCOE) by different methods. One is using extremely low cost material such as rock. The other is using advanced materials such as high temperature phase change metals and liquid metal heat transfer fluids to smartly reduce the bottlenecks of the technology.”

Gauché concludes that storage research and advances are ongoing and people around the world are turning over every stone to reduce LCOE for dispatchable CSP.