Mark Mehos manages both the Thermal Sciences R&D group and the Concentrating Solar Power (CSP) Program at the National Renewable Energy Laboratory (NREL). The emphasis of NREL’s CSP program is the development of low-cost, high-performance, and high-reliability systems that use concentrated sunlight to generate power. Mark is also the leader for the International Energy Agency’s SolarPACES “Solar Thermal Electric Power Systems” task.


Recently, there seems to be increasing interest in grid flexibility. Why is grid flexibility important?

[Mehos, Mark] Grid flexibility is important for any grid system given the need to continuously balance load and generation. However, this becomes even more important, and more difficult, as greater amounts of variable generation technologies like wind and PV are added to a regional system.  Greater flexibility – fast ramping, high turndown ratios while maintaining high efficiency, and increased connectivity between regions – allow for greater penetration of wind and PV. Without a flexible system, these technologies would see increasing curtailment which would drive up the cost of generation while also increasing the financial risk to the project lenders.

Can Concentrating Solar Power (CSP) with thermal storage help enhance grid flexibility?

[M.M.] Yes. CSP has a number of advantages over variable renewable technologies. Today’s CSP systems, especially those that use the same fluid for both solar field heat transfer and thermal storage – think molten salt power towers – are completely dispatchable. Once the system has begun to operate for the day, the steam turbine acts very similar to a conventional gas-fired plant. Relatively fast ramp rates allow the operators to respond to utility demands for power. CSP systems are also designed to run efficiently at high turndown ratios. When integrated with thermal storage, this allows CSP plants the ability to provide additional headroom for PV in the middle of the day while storing the solar energy for dispatch during evening peak demand. Lastly, in theory at least, CSP plants could be configured with governor controls allowing CSP plants to provide primary frequency response. I alluded to this in my presentation during the MENA CSP KIP workshop in Dubai. NREL will release a report in the next month or so that refers to a detailed study on the ability of CSP plants to provide inertia and governor response.

Figure 1. Power block and Thermal Energy Storage System at La Africana CSP Plant in Cordoba, Spain (Source: Grupo Ortiz)

In terms of grid flexibility, how does CSP compare with fast-ramping natural gas plants?

[M.M.] CSP and frame-type CTs are very similar in terms of their ramping capability (roughly 10% of full capacity per minute). Aeroderivative turbines have a faster ramping rate albeit at a lower capacity rating.

What other technology options could a power-system planner deploy to make the grid more flexible?

[M.M.] I don’t have any direct information on other technology options. Clearly coal and nuclear, as designed, offer limited flexibility to the system. Natural gas fired technologies, especially CTs and CCs, offer more flexibility but at the cost of carbon emissions or price risk.  Hydro is a clean flexible generation option but can be constrained geographically. Of course, demand response and increased regional trading are non-generation options that should always be considered.