Due to the falling price and manageable size of silicon solar cells, the sight of solar panels on household rooftops has become familiar.
However, solar energy, like many other sustainable forms of energy, is intermittent.
At peak times, such as on sunny summer days, the grid is flooded with solar energy, while during the evening and winter, when energy tends to be most in demand, supply is lower.
To secure energy availability on demand, storage needs to be integrated into the grid.
The pumped hydroelectric storage plant is one example of widespread technology for storing energy by pumping water from a lower reservoir to a higher one. Electricity is generated on demand by letting the water flow back through turbines.
The expansion of these plants, however, is limited by the number of suitable geographic sites.
A technology that does not face these constraints is the electrochemical battery. Lithium-ion batteries used in everyday electronics are examples. On a larger scale, companies are starting to invest in large batteries that support the grid. The dominant technology for such large batteries is currently lithium iron phosphate (LFP).
A report by the International Energy Agency (IEA) predicts lithium demand to multiply by more than 40 times in 2040 compared with 2020, largely because of this need. Lithium mining, however, comes with considerable geopolitical and humanitarian concerns.
If lithium mining has a significantly negative impact on local health, water supply, biodiversity and soil, can we maintain that it’s helping us move toward a sustainable future?
Thermal storage as an alternative to electrochemical batteries
Another alternative is thermal energy storage, in which energy is stored in the form of heat.
Due to the high energy density, abundance and low cost of storage materials, thermal storage seems promising for longer-duration storage. This is the principle behind solar water heaters. On a larger scale, it is mainly used in combination with concentrated solar power plants (CSP), such as the Crescent Dunes Solar Energy Project in the U.S. or Andasol Solar Power Station in Spain.
In these installations, the first step is to convert solar energy to heat using large mirrors, either parabolic or smaller flat ones tracking the sun, which concentrate sunlight and heat a large reservoir of molten salt. The second step consists of converting heat into electricity: The heat stored in the salt powers a turbine.
Thermal storage implemented in this way has remained marginal because the turbines used in CSP become more efficient as they become larger. This means the concept is generally not cost-effective at smaller scales and therefore requires very large plants, involving high startup investment and maintenance costs. Even though smaller CSP plants with integrated thermal storage exist, such as the linear Fresnel plant at Llo in southern France, they remain even more uncommon.
The efficiency of thermal storage poses a challenge because heat is notoriously difficult to convert to “useful” energy, including electricity: The efficiency of this process is fundamentally limited by what is known as Carnot’s law. For example, at 300°C (572°F), the theoretical maximum conversion efficiency is about 50%, meaning the actual efficiency will be even lower. In comparison, lithium-ion batteries, which are not limited by the same physical law and have been optimized for decades, surpass 90%.
While the efficiency of heat-to-electricity conversion would improve at higher temperatures, turbines can hardly withstand such elevated temperatures.
Using photovoltaics to convert heat radiation to electricity
However, there is another way to turn heat into electricity, namely via radiation: This is the principle behind the solar cells found on rooftops and solar farms. Typical solar cells convert energy from the sun. In contrast, the idea behind so-called thermophotovoltaic devices (TPV for short) is to harvest the (mostly infrared) radiation from a very hot object, where energy is stored, and turn it into electricity.
For example, silicon melts at 1414°C (2577°F), and graphite can be heated to more than 2000°C (3632°F). At these high temperatures, the Carnot limit is pushed up to 83% and 87%, respectively. In practice, experiments at MIT and the University of Michigan have now pushed conversion efficiency beyond 40%.
Different device designs
The most recently developed device, which is now being introduced on the market, features a storage medium in the form of a block of graphite or silicon, for example, heated by electricity. It is therefore similar to a lithium-ion battery, but it operates through successive energy conversions from “electricity to heat to electricity” instead of from “electricity to chemical energy to electricity.”
However, it would be interesting to heat our storage medium directly with the sun, so that the device becomes a direct source of renewable energy.
In our setup at PROMES laboratory, conveniently located in the sunny south of France, the storage medium (graphite during our first tests) was heated with concentrated radiation from the sun, using a parabolic mirror and a tracking mirror, and can be dubbed “solar to heat to electricity” or storage-integrated solar TPV (SISTPV). It’s the first prototype of a SISTPV system. An important advantage compared with CSP with molten salt storage is that the TPV battery could be cost-effective at a much smaller scale.
Although substantial work remains to improve both the efficiency of TPV cells themselves and thermal batteries’ overall efficiency, SISTPV offers a promising avenue for supporting the transition to renewable energy.
By combining energy collection, storage and electricity generation in a single, compact and potentially cost-effective device, it transforms an intermittent energy source—the sun—into a firm one, making solar energy available on demand.
Written by Vera Moerbeek, The Conversation
