Written by: Zoe Zakrzewska
Edited by: Justin Weir
The advancement and adoption of solar photovoltaic (PV) energy has undergone a meteoric rise in the last few decades. It has been the world’s fastest-growing energy source for eighteen consecutive years, while its total share of global energy generation has more than quadrupled over the last seven, increasing from 1.1% in 2015 to 4.5% in 2022 (Global Electricity Review, 2023). This growth is only beginning, and is not expected to stop anytime soon. The International Energy Agency projects that solar will exceed the power capacity of all other renewable and non-renewable energy technologies within the decade — by 2027, solar PV is expected to contribute to 22% of global power capacity, followed by coal at 21% and natural gas at 19% (Solar, n.d.). Finally, a recent study published by Nature Communications predicts solar PV will be responsible for over 50% of all global energy production by 2050, even without the implementation of additional climate policies (Nijsse et al., 2023).
This rapid increase in solar deployment can be attributed to several different causes. Private and public research and development efforts have significantly increased the efficiency of solar PV modules as well as their manufacturing processes. Economies of scale, which refers to the lowering of costs as the production of a good increases, has especially allowed the industry to expand in recent years (Kavlak et al., 2018). In other words, the continuous spread of solar creates conditions which accelerate that spread, in perpetuity. Government policies, subsidies, and tax credits have also supported these developments, with notable examples including Germany’s feed-in tariffs enacted in 2000 and China’s post-2008 financial crisis subsidies (Haley & Haley, 2013). Solar PV development is bolstered by a high learning rate, which describes the decrease in the cost of a technology as its cumulative installed capacity increases. This occurs through gains in knowledge acquired from the successful deployment and manufacturing of the technology. The learning rate for solar PV is higher than most other renewable energy sources, meaning costs will decline at a higher rate for solar PV than other clean energy technologies (Haas et al., 2023). These low and declining costs paired with solar’s short construction timeline are what researchers believe will give solar energy a competitive edge over other energy sources and ultimately lead to high rates of adoption (Ameli et al., 2023).
Despite these promising characteristics, solar PV’s widespread diffusion is still significantly held back by one key characteristic of the energy source: its intermittency. This refers to the fact that solar energy production varies due to external factors, such as the time of day, season, or weather conditions. This variability often leads to a mismatch in the demand and supply of solar energy. The problem is twofold. During peak energy demand periods, there can often be an undersupply of solar energy due to a low or non-existent solar energy supply, requiring alternative, mainly non-renewable, energy sources to be utilized to meet the demand. On the other hand, during off-peak periods, demand for energy may be low despite high solar energy production, leading to an excess supply (Pitra & Musti, 2021).
Solar intermittency poses a challenge across the globe. In the United States, notably in the state of California, solar intermittency leads to the phenomenon coined the “duck curve,” which refers to the shape of the graph displaying the difference between daily electricity demand and the availability of solar energy. The graph shows a low need for electricity generation from alternative, non-renewable sources during the day due to the large amounts of electricity provided by solar, but in the evening, generation from these alternate sources must be ramped up to fully meet the demand, as solar energy is no longer being produced (Pitra & Musti, 2021). In China, the variability of solar PV is one of the factors that leads to an oversupply of solar in the country’s northwestern provinces, where a limited population often has a low demand for energy despite the region’s high solar energy production. On the other hand, in periods of high demand, coal is used to supplement deficits of renewable energy power (Bradsher & Friedman, 2023; Xia, 2023). Similar situations are found in countries around the world with high shares of solar PV such as Chile, Australia, and Spain, and solar’s intermittency is increasingly expected to affect other nations' energy systems as they increase their solar capacities.
Solar Energy ‘Duck Curve’ Visualized (Bowers et al., 2023)
Grid stress and inflexibility is another area of concern related to intermittency. The implementation of significant amounts of intermittent renewable energy sources to the electric grid, especially solar, creates new challenges. Unlike readily-dispatchable energy sources, such as natural gas, coal, or nuclear, which can all adjust their power output at the request of power grid operators, solar energy generation is not controlled, meaning output must either be accepted or rejected from entering the grid. Yet solar is almost always accepted, as its zero marginal generation cost is more economical compared to other energy options (O’Shaughnessy et al., 2021). This means alternative dispatchable sources adjust their output to fill any gap present between solar supply and energy demand. However, the intermittency of solar PV means that dispatchable energy sources often must quickly ramp up or ramp down their energy production, such as in the evening when solar energy output drops or when cloudy conditions settle into an area (Fares, 2015). These steep ramp ups or downs can place stress on the grid and threaten to overwhelm the generating capacities of certain inflexible yet dispatchable power plants. Further, these traditional plants are not designed to vary their generation capacity significantly throughout the day, meaning operating them in such a fashion is excessively expensive. If the continued usage of such power plants becomes uneconomical, forcing the plants to retire without another dispatchable replacement, grid managers may have increased difficulties matching supply with demand (Bowers et al., 2023).
These challenges are compounded with the issue of oversupply. While solar energy is almost always accepted into the grid due to its zero marginal generation cost, there are situations in which there is an excess of solar energy produced. When this occurs, solar PV output actually needs to be curtailed — prevented from entering the grid — in order to maintain the balance between supply and demand. Although some curtailment is unavoidable, it jeopardizes solar energy’s financial viability at higher levels. Solar PV is best able to generate positive returns when its output is maximized, therefore curtailment reduces the quantity of energy sold and reduces profits. If significant enough, curtailment can impede investors’ ability to repay their loans and finance new projects (Golden & Paulos, 2015). Curtailment also drives up the average cost of solar, which can decrease the cost-competitiveness of solar PV in contrast to other energy sources (Ritchie, 2017).
Solar PV Curtailment Event (O’Shaughnessy et al., 2021)
Although solar energy’s intermittency poses a challenge to the energy industry, several different strategies have been developed to solve and address the issue. One type of solution can be described as demand response programs, and involve altering the behavior of energy users through price changes or other incentives. This strategy often aims to reduce energy consumption during peak hours, which makes it more feasible for grid operators to handle fluctuations in solar PV output. There are two main categories of demand response: incentive-based programs, where utilities offer payments to consumers who shift their energy usage upon request, and price-based programmes, where utilities increase the price of energy during periods of high demands in hopes of shifting their consumption patterns and moderating demand (Demand Response, n.d.). However, demand response programs have had a largely limited impact on energy demand, as their implementation is limited by, among other things, underdeveloped modeling and forecasting tools, limited pathways for information sharing, incompatible regulatory and business environments, and high investment costs (Cruz et al., 2018).
Increasing grid interconnection and transmission capabilities also mitigates the impacts of solar intermittency. When national energy transmission systems are comprised of disjointed regional grids with limited geographic dispersion, the solar PV systems that feed into them are exposed to uniform meteorological and daylight conditions, exacerbating the production disparities of intermittency. Energy demand also varies more on a smaller geographic scale, as people within a certain region and timezone often require energy at the same times of the day, such as in the early mornings or evenings. However, if transmission between different energy grids is expanded, both solar PV’s output and the demand for energy will become steadier — the peaks and troughs visible in energy supply and demand graphs will flatten (Cruz et al., 2018). Lack of transmission has been identified as at least partially responsible for high rates of solar energy curtailment in China, Chile and Germany (O’Shaughnessy et al., 2021). However, high costs, regulatory backlog, and geopolitical disagreements can limit transmission expansion and interconnection (Penrod, 2022).
Finally, energy storage systems (ESSs) are another key solution for intermittency. EESs allow solar energy to be stored when the demand is low and utilized when demand is high, or in other circumstances where energy needs to be quickly supplied to the grid (Cruz et al., 2018). EESs encompass a huge variety of technologies, each at different stages of development and characterized by their own advantages and disadvantages. Pumped hydropower storage, a mechanical system that generates energy by pumping water from one reservoir to another, accounts for the vast majority of global energy storage and continues to rapidly expand (Rojanasakul & Bearak, 2023). However, battery storage is becoming an increasingly competitive option due to its lowering costs, with new installations increasing exponentially with every passing year. Areas with higher solar energy penetration are especially looking to expand their battery installations: for instance, California’s battery storage capacity has grown from 0.2 gigawatts in 2018 to 4.9 gigawatts as of April 2023, with additional 4.5 gigawatts planned to be installed by the end of the year (Bowers et al., 2023). Other emerging technologies are also arriving on the market, often offering certain advantages that more commercially-widespread technologies lack. For instance, one Norwegian startup has developed a solid hydrogen storage device that is able to store solar energy interseasonally, which is a function most other ESSs are still incapable of (Billing, 2023).
As solar PV energy increasingly permeates global energy systems, intermittency remains one of the most complex problems the world will need to face if solar PV is to be scaled successfully. Of the solutions at hand, there is no panacea: each technology or mechanism has its own barriers to overcome. Solar energy intermittency will likely be addressed through the implementation of the various efforts described above, in a manner that maximizes efficiency, cost, grid stability and flexibility, and energy security.
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