Concentrated Solar Power: Bypassing the Duck Curve

Introduction

Global warming poses an existential threat to life on Earth as we know it. Two hundred years of releasing CO₂ and other greenhouse gases into the atmosphere by burning fossil fuels now threatens to uproot the world’s energy infrastructure. World leaders know that if they do not invest, together with other countries, in the research and development of new, carbon-neutral energy sources, the consequences of global warming will have a devastating impact on their territories and economies (Intergovernmental Panel on Climate Change [IPCC], 2023). In fact, many countries have incurred, and are still incurring, massive economic, human, and material losses due to climate change. As a result, the market for renewable energy sources is growing rapidly, with new technologies and applications being developed every year in all parts of the world. Investments from the private sector and heavy government subsidies for carbon-conscious energy production technologies amplify demand in the market, with a focus on economically sound and feasible renewable energy options for the average person. Simply put, if renewable energy is not made reliable and inexpensive relative to its fossil fuel alternatives, most people will not switch over to it, even with its environmental benefits. It follows, then, that to have a measurable effect on curbing worsening climate change, we need to find renewable energy sources and make them financially accessible to the masses.

Present Constraints

For years, photovoltaic (PV) solar power has been seen as the gold standard for affordable and feasible renewable energy. Aside from the high installation cost, PV solar power is among the cheapest energy sources per kWh, not just among renewables. The International Renewable Energy Agency (IRENA) quoted PV as generating energy for approximately $0.04/kWh in 2024. Photovoltaics’ low Levelized Cost of Energy (LCOE) of $0.04/kWh means that the general public can offset the initial cost of installation through the savings they will make on cheaper electricity bills within 10 to 15 years (International Renewable Energy Agency [IRENA], 2024). Once the cost of these panels has been recouped through electricity savings, we start to see their positive long-term economic benefit. However, the criteria for a good renewable energy source are not simply economic feasibility. If the energy source is cheap but cuts out at crucial times of the day, that also acts as a strong deterrent for most people. In fact, this is one of the main drawbacks of PV. Power generation tapers down and cuts out from sundown until sunrise, and sometimes depending on weather conditions (clear skies are conducive to more power generation, as opposed to dusty or cloudy conditions, where power generation is greatly inhibited). However, energy demand is highest at sunrise, when people wake up and turn on appliances, and at sundown, when they return home and turn on TVs, air conditioning, and other devices. During these time slots, solar PV power generation is negligible or even nonexistent. By contrast, solar PV power generation peaks around midday, right when energy demand is at its lowest, resulting in one of three unfavorable scenarios: let the energy dissipate as losses, pay neighboring power plants to take it, or install costly, critical-mineral-heavy lithium batteries to store the unused energy. This problem is modeled by one of the most important graphs in renewable energy research: the Duck Curve. Pictured below, it represents the energy demand as a function of the time of day. The area where the curve is at its minimum, the “flat belly” region, coincides with the period of maximum power generation from solar PV, while the periods of high demand are left unfulfilled (U.S. Energy Information Administration [EIA], 2023).

Fig. 1: Duck Curve

Source: U.S. Energy Information Administration, 2023

This is the bottleneck of solar PVs, and one of the main reasons it cannot, in its present state and on its own, be the sole energy source for a large portion of people.

Concentrated Solar Power (CSP)

Another facet of solar energy collection promises to avoid this issue. Concentrated Solar Power (CSP) is fundamentally different from solar PV. Instead of specialized panels that capture photons’ energy to generate an electric current, CSP uses strategically placed mirrors on motorized ball-and-socket joints to concentrate sunlight throughout the day on a single focal point. This heats a Heat Transfer Fluid (HTF) that can store energy for long periods and continuously heat water to produce steam, which drives a turbine that in turn generates electricity (Rankine cycle). This process occurs mainly in two distinct ways. 

Parabolic Troughs

Curved mirrors track the sun on a single axis, concentrating light onto a stationary receiver tube running along the trough's focal line. The system pumps synthetic thermal oil through these tubes to act as the Heat Transfer Fluid (HTF). Because it is a legacy technology dating back decades, parabolic troughs are highly bankable and carry lower technology risk for investors. However, the synthetic oil degrades at temperatures exceeding 400°C (U.S. Department of Energy, n.d.). This thermal ceiling strictly limits the efficiency of the steam turbine (Rankine cycle), ultimately capping the plant's maximum power output.

Solar Tower (or Point Focus)

Thousands of flat, independently tracking mirrors (heliostats) use motorized dual-axis drives to focus sunlight onto a single receiver at the top of a central tower. Rather than using oil, these systems pump "Solar Salt,” a high-capacity mixture of sodium nitrate and potassium nitrate, directly through the tower's receiver to absorb the concentrated heat. Because molten salt remains thermally stable up to 565°C, solar towers push the steam turbine to much higher thermodynamic efficiencies (U.S. Department of Energy, n.d.). This yields significantly more electricity per unit of heat, though the extreme thermal cycling introduces higher operational risks.

Constraints of CSP

Constructing a CSP plant comes with its own set of challenges compared to other types of renewable energy sources. Unlike PV-Solar, which can be assembled in batches and completed within a few months, CSP requires large, centralized infrastructure. This demands a steep initial Capital Expenditure (CAPEX) for its lengthy, multi-year construction, which significantly delays the return on investment (ROI), as building steam turbines and assembling an entire thermal power plant makes for long lead times.

Additionally, CSP depends solely on Direct Normal Irradiance (DNI) to produce energy, which means that it relies purely on direct sunlight. If conditions are unfavorable (clouds or dust), power generation halts. This geographically limits CSP to the Sunbelt (the MENA region, Chile, Australia, Southwestern US). Furthermore, CSP plants use the Rankine cycle, meaning the water vapor that exits the steam turbine needs to be cooled down and condensed. There are two main cooling methods: wet cooling, which requires millions of gallons of water to be pumped to what are mostly desert areas, and dry cooling, which uses large fans but significantly reduces the efficiency of the plant (Mehos et al., 2020). In addition, CSP plants, especially troughs, require large, perfectly flat plots of land, which can lead to massive civil engineering costs.

Operationally, the highest risk comes from thermal fatigue. This refers to the material stress the Solar Tower incurs from pumping molten salt up and down the tower, which causes its materials to constantly expand and contract. Historically, this has led to massive leaks, such as at the Crescent Dunes plant in Nevada, which ultimately filed for bankruptcy. However, the main operations and maintenance (O&M) cost comes from the sensitive nature of the mirrors and the many moving parts involved.

Case Study of Morocco’s Noor I/II and Noor III

Despite all the risks involved, the need for dispatchable renewable energy is so urgent that countries like Morocco are willing to absorb these risks. Located on the edge of the Sahara Desert, Morocco’s Noor CSP plant is among the largest in the world. This project is part of the Moroccan Solar Plan, which aims to produce 52% of the country’s energy from renewable sources by 2030. Therefore, these thousand-acre CSP plants are vital for achieving this sustainability goal and helping Morocco move closer to energy independence in a world of unpredictable energy and oil markets. It is important to note, however, that the Noor plants utilize both types of CSP: Parabolic Troughs (Noor I/II) and a Solar Power Tower (Noor III) (Moroccan Agency for Sustainable Energy [MASEN], n.d.). This highlights the versatility of both systems, which are able to operate in close proximity to each other. The main benefit of operating both systems is the merging of mature trough technology to provide a reliable energy production baseline with newer, higher-efficiency tower technology.

Image: Noor III Solar Tower

The Dispatchability Advantage

While CSP plants are fundamentally more expensive to build than PV power plants, their advantages over the latter go beyond their different energy production window. Building CSP plants involves a steep initial CAPEX, especially compared to mass-produced silicon photovoltaic panels. However, there are two main characteristics of CSP that overshadow the high CAPEX and make it justifiable.

Grid Inertia

First, the duration of time between a disruption of the power grid and the resulting power outage differs for each energy source. PV-Solar cuts power off immediately, just like removing a battery from an electronic device. This makes the grid fragile and susceptible to frequent power outages, as engineers do not have time to fix the root cause before the power cuts out. Other energy sources that rely on massive spinning turbines, such as coal burning, nuclear fission, and CSP, have a grace period as the remaining rotational kinetic energy of these turbines continues producing electricity even if there is a disruption of the power grid. We call this concept Grid Inertia, and it is crucial for maintaining the integrity of the grid and reducing frequent power cuts or blackouts (Khan et al., 2024).

Thermal Energy Storage

Second, CSP plants produce energy using Thermal Energy Storage (TES), wherein the HTF is heated by the Sun and stored in large, heavily insulated tanks to be used later to turn water into steam, which then spins the turbine and produces electricity. This decoupling of electricity production means that these plants can capture the Sun’s thermal energy all day, store it, and use it whenever needed to produce electricity even long after sunset. Furthermore, TES is fundamentally easier and cheaper than storing energy in expensive, critical-mineral-heavy lithium-ion batteries. More importantly, this power decoupling completely solves the duck curve problem that bottlenecks PV-Solar’s future: this is the Dispatchability Advantage. A CSP plant can sit idle during midday at the “flat belly” as energy demand is low, then dispatch the thermal energy it collected and stored throughout the day to spin the turbine at “the duck’s neck” where energy demand spikes. Contrast this with PV power plants selling the energy they produce at midday for free or even paying neighboring plants to take some of that energy, while at the same time being unable to provide energy when people actually need it, and the benefits of CSP are clear. Since CSP generates revenue during highly lucrative time-blocks, its Net Present Value (NPV) will eventually outweigh the steep initial CAPEX (Khan et al., 2024).

Market Validation: Cerro Dominador

There is a real-life manifestation of the economic benefits of CSP outlined above in Chile’s Atacama Desert. The Cerro Dominador complex pairs a 110 MW Solar Power Tower with 17.5 hours of molten salt storage (SolarPACES, n.d.). This allows the facility to operate around the clock, unlike intermittent PV plants. This plant powers Chile’s mining sector with clean, uninterrupted renewable energy. It is a foundational example of the economic benefits provided by CSP’s power decoupling, and it proves that even with a steep initial CAPEX, building CSPs can be economically sensible.

The Future of CSP: Hybridization

With all the benefits of CSP mentioned, the future of renewable solar energy is likely a hybridization of both PV and CSP rather than competition for market share. If we look at their strengths and weaknesses, they are almost complementary: PV-Solar is among the cheapest daytime energy sources available, while CSP excels in TES and Grid Inertia for nighttime energy demand. A symbiotic relationship between these two technologies could enhance the economies of both.

A CSP plant requires a large amount of electricity to operate itself, from pumping heavy molten salt to rotating motorized mirrors and heating the molten salt so it does not freeze. Most of that energy demand comes during daytime, when the plant is actively harvesting the sun’s energy. Therefore, a smart approach would be to build a hybrid plant in which PV-Solar provides most of the daytime energy needs for the CSP plant so that it can provide constant energy at peak hours. By combining these two energy sources, developers can achieve a “blended” LCOE that competes with traditional fossil fuels while still offering 24/7 dispatchability.

An interesting case study relating to this proposal is the Noor Energy 1 hybrid CSP and PV plant in the UAE’s solar park in Dubai. It is the largest single-site investment in CSP globally. With a combined capacity of 950 MW (600 MW from parabolic troughs, 100 MW from a solar tower, and 250 MW from PVs) and 15 hours of molten salt thermal storage, this project is the most ambitious of its kind. The CSP component secured a Power Purchase Agreement (PPA) of 7.3 cents/kWh, proving to markets that hybridizing PV and CSP at large scales can achieve cheap, dispatchable renewables (ACWA Power, 2023).

Conclusion

When looking at different renewable energy sources, it is important to note that LCOE must not be the sole deciding factor for one source over another. From the duck curve and the severity of the energy availability problem at peak usage times, it becomes clear that the critical metric for measuring an energy source’s viability is its dispatchability, i.e. whether it can deliver clean, renewable energy when demand peaks. While CSP excels at dispatchability, it is best used synergistically with PV-Solar to achieve a competitive LCOE with 24/7 available energy. Even if initial CAPEX is steep and the operational risks involved are important to consider, the cost of grid instability, as seen with standalone PV-Solar, and the cost of worsening climate change will far outweigh them. With consistent funding for CSP mega-projects like the UAE’s solar park, CSP will move down the cost curve. With climate change posing an existential threat to our economies and to billions of lives, if countries do not find cheap and dispatchable renewable energy sources, they risk incurring massive economic, material, and human losses. This is why hybridized PV-CSP, capable of delivering 24/7 renewable baseload power, is so important.

Bibliography

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