By Andrew Williams

Concentrated solar power has a major advantage over other renewable energy sources in that it is the only renewable energy that can provide dispatchable power. In other words, by integrating thermal storage into the plant design, a concentrated solar power (CSP) plant can generate energy day and night, with or without clear skies.

The other option on the table is to either back up CSP with coal or gas, or vice-versa. Bolting CSP to gas or coal-fired plants helps hybrid plants to lower costs and continue operating during periods of cloud cover.  Hybrid systems can also be very efficient in terms of solar-to-electric conversion, although this depends on the type of integration. 

For example, integrated solar combined-cycle (ISCC) systems allow operators to integrate a relatively small fraction of CSP (approximately 10%) without having to pay for a separate power-block, transmission, or grid interconnects.  Other gas turbine/CSP hybrids, of the type proposed by the US-Based National Renewable Energy Laboratory (NREL), are greenfield designs that are approximately a 50/50 mix of solar and gas.

“These schemes have good solar-efficiency and low gas heat-rates,” says Greg Glatzmaier, Project Leader of the CSP Heat-Transfer Fluids & Thermal Storage Project at NREL.

Thermal energy storage plants can collect energy all day long and store it until needed – including at night.  This is particularly attractive for utilities requiring a combination of peak-shaving and load-shifting capabilities - in addition, storage-based plants can also be configured as a peaker-unit.

“So, instead of generating at 100% capacity for, say, twelve hours per day, it can generate at 200% capacity for half as long.   This peaker configuration is useful in locations with time-of-day pricing or pronounced system peaks,” explains Bill Gould, Chief Technical Officer at SolarReserve.

Cost and performance

According to Craig Turchi Senior Engineer at NREL, CSP thermal storage systems have higher initial material and construction costs compared to hybridization.  On the flip side, hybrid plants have higher operational costs because of their dependence on fossil fuels. 

Hybrid plant operators are also required to monitor and report on emissions, which is not the case for thermal storage plants. 

In terms of O&M costs, a fossil fuelled boiler and its steam generator incurs additional costs – although molten salt-based thermal storage plants also have salt pumps, which would require periodic maintenance.  

“Hybridization will have higher O&M costs because of natural gas consumption. However, O&M labour costs per MWh for hybrids will be lower due to sharing staff between the solar and fossil components,” says Turchi.

In his view, although life expectancy costs are comparable, the decommissioning of hybridization plants will cost ‘somewhat less’ than thermal storage plant decommissioning.  However, Gould argues that scrap metal and molten salts are ‘recoverable assets’ that could help to mitigate the decommissioning costs of some types of thermal storage plants.

As far as performance is concerned, Turchi says that both designs have good reliability and capacity value – but whereas CSP with storage will typically be able to deliver its full nameplate capacity for the duration of its storage system, hybrids may only deliver a fraction of their nameplate rating if the solar component is not operational.  However, the operating duration of hybrid systems is not limited by storage capacity.

In terms of environmental performance, hybridization will produce CO2 due to the combustion of natural gas.  In contrast, CSP plants using thermal storage have the benefit of being 100% solar and, after start-up, will not generate emissions.

On the flip side, storage-based CSP will cover a greater land area than hybrid plants because the various types of CSP solar technology require a larger amount of land per plant than natural gas plants. As such, “CSP with storage will have a larger plant footprint per MW due to its 100% solar fraction,” says Turchi.

Best long-term solution?

Gould points to a 2010 ESTELA report, which concluded that plummeting PV prices will mean that those CSP plants that are unable to shift or boost plant output will inevitably lose out to PV plants – largely because they only offer ‘run-of-sun’ energy and their energy cost is higher than that of PV plants.  His view is that thermal energy storage plants offer both an energy and a capacity solution - and he believes that it is for these and other reasons that most tower and trough developers have announced their intention to provide thermal energy storage in the future.

Glatzmaier argues that both approaches are effective ways to implement dispatchable solar electricity.  For him, hybrids are low-risk and do not require as large a solar-field for a given plant capacity, whereas thermal storage technologies are still quite new and add cost to CSP plants.

“For these reasons, hybrid plants are a good near-term solution for deploying efficient solar energy,” he says.

“However, as thermal storage matures and costs decrease I expect dedicated, 100% solar systems will dominate CSP deployment.  ISCC systems will remain a good option, but are limited to sunny sites with big gas combined-cycle plants,” he adds.

Both hybridization and thermal storage options continue to offer a contrasting range of advantages and disadvantages - but as fossil fuel prices go up and PV prices continue to fall, fossil fuel resource scarcity is likely to place an upward pressure on hybrid system energy prices.

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Rikki Stancich: rstancich@csptoday.com