Uses of Solar Energy From Ancient Rome to Modern Italy

By Reagan Peterson

Ancient Romans are remembered for their ingenuity. They cleverly planned out projects to most effectively use the natural world around them, and they were able to build countless awe-inspiring structures–some of the most notable being the public baths. Not only were ancient Romans able to construct these bath houses without electricity, but they could heat them as well. Romans used fire to heat their baths, but they also cleverly oriented these bathhouses to use the sun’s energy to help heat the hot rooms and baths (Ring, 1996). Much like ancient Romans, modern Italians are harnessing the power of the sun for their advantage. Italians have embraced using photovoltaics and are actively developing new technology to improve the efficiency of these devices as well as developing smart grids to more efficiently distribute electricity.

Solar Energy in Ancient Rome – The Baths!

Roman bathhouses featured rooms with pools of various temperatures: the frigidarium (cool water), the tepidarium (warm water), and the caldarium (hot water). Water would flow into the bathhouses from aqueducts and would be heated by the sun. I visited several ancient Roman baths during my time in Italy, and each of them utilized south facing windows and hypocausts.

Hypocausts are open caverns beneath the bathhouse where hot air from fires can be distributed throughout the room. Many bath houses have hollow bricks lining the walls to allow the hot air to rise around the room. This allows the hot rooms and baths to be heated from beneath, along the walls, and through the windows.

Ancient Romans oriented their bath houses to have large glazed–although this is disputed–windows facing southwest to maximize the amount of sunlight the caldarium (hot rooms) was exposed to (Ring, 1996) which can be seen in Figures 1–3. This helped heat the water in tandem with hypocausts; however, hypocausts required a lot of wood, so south facing windows were more effective and cost efficient.

Baths of Caracalla

Baths of Trajan

The Baths of the Forum in Ostia

While the baths had hypocausts designed to heat the caldarium, it was more economical to use only the sun’s energy to heat the room through south facing windows. Eventually, wood, and other resources that could be burned, became more scarce, so it was up to the sun to keep these baths warm. Fortunately, on sunny days that is all that was necessary, but there have been debates about whether the bath house windows were open or if they had glass. According to James Ring, heating the baths to a comfortably hot temperature was only possible if the Romans used glass in their windows. He calculated that on sunny days, especially during the summer, the sun alone could provide enough energy to maintain internal temperatures of 100°F (Ring, 1996). Glazed windows help to keep heat inside the hot room rather than allowing it to escape if there were no glass. Ring calculates that with open windows approximately 530,000 British Thermal Units flow into the hot room per hour but 5,290,000 BTU flow out in that time. If this were the case, indoor surface temperatures would cool toward 30°F (Ring 1996). With glass windows, less thermal units flow out of the caldarium than flow in, so the room gains heat and stays quite warm.

Figure 4 depicts what was likely a steam room in the Baths of the Forum in Ostia. It was likely there was not a hot pool in that room. The photo is taken from the side the window would have been hence why the room is completely filled with afternoon sunlight. Figure 5 depict the hollow bricks–called tubuli–that lined the walls of the hot rooms (Ring 1996). All surrounding walls were lined with these hollow bricks as seen which would allow heating from all sides. Figure 6 provides a good visual of the insulation of these walls and shows the channels where hot air would travel when the hypocaust was being used.

The Stabian Baths at Pompeii

The Stabian Baths in Pompeii demonstrate glazed window heating very well, as the room was far warmer than the outside air. The open floors also provide a great view into what hypocausts looked like beneath the flooring. The hypocaust flooring can be clearly seen in Figures 7-9. Other bathhouses I visited were the same temperature as the area around them because so much of the original structure had been destroyed and they were merely open air; however, the Stabian Baths in Pompeii remained in tact enough to demonstrate just how much warmer these bath houses can be than their surrounding area. The Stabian Baths validate Ring’s argument because they are the only baths I visited with windows rather than open air, and because of this, they were the only baths with a far more notable temperature difference. Even without the hypocaust, this room was at least 15°F warmer than outside.

Da Vinci’s Contribution – Concave Radial Mirror

Leonardo Da Vinci, as seen in the statue in Figure 10, theorized about a Radial Concave Mirror intended to concentrate sunlight and use it to heat water among other uses. It is unclear whether or not his invention was created or simply theorized about; however, the principles he investigated remain useful today and could further improve solar technology.

Theory Behind Da Vinci’s Concave Radial Mirror

Leonardo Da Vinci was not the first to consider sun ray concentration, as several ancient societies including the Greeks and Romans have art depicting the concept way before his time. However, Leonardo was presumably the first to consider using sun ray concentration for various helpful tasks such as heating water (Szabó, 2017). Leonardo Da Vinci theorized that a curved mirror like the one in Figure 11 could be used to localize sun rays in one spot. He proposed several variations of this parabolic mirror, and some of them have several layers of mirrors.

Figure 11. Sketch of Leonardo Da Vinci’s Concave Mirror Solar Concentrator with labeled diagram (Kulowski, 2022).

The curvature of the mirror creates a focal point for all of the sun’s rays to concentrate (Kulowski, 2022). This high concentration of energy creates thermal energy which has various possible applications such as heating water or even melding metals.

Modern Applications – Dish Sterling System

It is unclear whether this idea came to fruition during Da Vinci’s lifetime; however, there are many modern studies featuring the principles Leonardo used in hopes of improving our ability to harness solar energy. Current studies are utilizing these same principles to create a focal point where a maximum amount of solar energy can be collected. The diagram in Figure 12 demonstrates this idea in action in a form of solar power called the dish sterling system.

This solar device differs from other solar panels because it is thermodynamic rather than photovoltaic. The dish concentrates the suns rays and the solar energy receiver collects thermal energy which is then converted into electricity. Temperatures at the solar energy receiver can reach around 750-1,112°F which provides tremendous potential for energy collection (Palazza, 2022). This type of solar panel is more efficient than photovoltaic cells, and it is currently being developed in Sicily and other regions around the globe.

Solar Power in Modern Italy and Vatican City

Current Use of Solar in Italy and Vatican City

Vatican City is currently implementing solar into their power infrastructure. Figure 13 describes one of the current developments for solar at the Vatican Museum, and Figure 14 shows a glimpse into its actual construction.

Vatican City has a goal of zero emissions by 2050, and in pursuit of this goal, Pope Francis has commissioned for the development of an agrivoltaic system (Glatz, 2024). An agrivoltaic system is a system of solar panels that exist among crops or livestock. The panels are situated in a way that crops or animals can still be farmed around them. Another part of this development is implementation of solar at the Vatican Museum as seen in the images above.

Solar power is currently being used throughout Italy on a large and small scale. Figures 15-17 show smaller scale implementations of solar power in and around Naples.

The Intersection Between Modernity and Antiquity – Sun Dial & Invisible Solar

There is a fascinating intersection between modernity and antiquity throughout Italy and Figure 18 demonstrates one small example of this with the sun dial located in Capri. The sun dial was added in 2015, and it serves a more decorative role than it would have in ancient times, but it is fascinating to see the lasting impression ancient technology has made on modern times.

Another intriguing example of the intersection of ancient and modern technology are the invisible solar panels at Pompeii. A small company named Dyaqua has invented a photovoltaic terracotta tile intended to discretely implement solar power at historical sites without obstructing the view (Dyaqua Invisible Solar). The House of Vettii, seen in Figure 19 , at Pompeii is among the first historical sites to use Invisible Solar.

How it Works – Invisible Solar

Dyaqua Invisible Solar uses mono crystalline silicon cells like many other PVs; however, invisible solar is then covered in a polymer that looks like terracotta. This polymer is specially created to encourage the absorption of photons (Dyaqua).

Southern Italy appears to have a greater number of solar powered devices being put to use on smaller scale items. This is likely due to the greater intensity of sun closer to the equator and many national parks often try to use clean energy whenever possible.

The Current Solar Industry in Italy

Most of Italy’s solar consumption is converted to electricity, but a small percentage draws from Leonardo Da Vinci’s early ideas and uses thermal energy to heat water. In 2020, solar power produced 24,942 GWh of electricity which only accounts for around 8.2% of the energy consumed that year (Palazzo, 2022). This means Italy produced around 304,170 GWh were produced in total. Most of Italy’s current supply of solar power comes from Northern Italy. Interestingly, this is region has a lower PV Power Potential than Southern Italy.

Current Solar Potential in Italy

Southern Italy holds the greatest solar potential as displayed in Figure 20, as it is predicted to nearly produce nearly double the kiloWhatthours per installed kilowhatt-Peak (the highest amount of energy a given installation can collect) as the Northernmost areas of Italy (Solaris, 2021). Southern Italy is a hub for solar research and innovation in solar collection. As of 2022, Sicily is a leading region for the development of thermodynamic solar panels which are even more efficient than photovoltaic panels (Palazzo, 2022). An example of a thermodynamic solar panel is the Dish Sterling System mentioned above in the section about the modern applications about Leonardo Da Vinci’s theories.

Current Research on Solar in Italy

The following photos in Figure 21 are posters of four research projects regarding PVs and energy collection being presented at the Sapienza Universida di Roma 2024 Nano Innovation conference. The convention focused on innovation in nanotechnology, so most of this research is geared toward improving the efficiency of absorption of solar energy of solar cells at a chemical level. While solar has come a long way from the Roman’s baths, there is still plenty of research and improvements to be made on this form of clean energy. Efficiency is always something engineers strive to improve; however, storing solar energy continues to be one of the main challenges. One way to alleviate the issue of energy storage is more efficiently distributing it when and where it is needed.

Smart Grids

Italy is currently implementing smart grid technology to track energy usage and demand throughout the country in order to most efficiently distribute energy when and where it is needed. This will also allow for Italy to utilize clean, renewable energy sources more efficiently (Fraschini). One of the main challenges with solar power is storing electricity. Often when energy demand is high, solar availability is low. For example, at night more people use their lights and heating and cooling systems, but by then, the sun is down. Fortunately, energy storage already exists which allows people to use energy stored from the day; however, these systems require greater efficiency because a lot of energy is lost. Smart Grids implement a very innovative solution to this problem! Rather than simply storing the energy, smart meters are implemented to determine where electricity is needed in real-time. Knowing exactly when and where power is needed is a smart way to ensure available electricity at a given time is being put to use. Some of the excess power will be used to fuel the grid itself (Fraschini, 2022). Smart grid technology aims to decentralize energy distribution which can also allow customers to be part of the energy production and distribution process because the main goal is to connect all energy sources within a multidirectional distribution system in efforts to reach 100% renewable energy. Progress in Italy is certainly moving in the right direction because “according to the figures reported by Juniper Research, smart grids will permit annual energy savings of 1,060 terawatt-hours by 2026, almost triple the 316 terawatt-hours of 2021” (Fraschini, 2022). Smart grids have a bright future and provide a clever solution to a large problem that often hinders the use of renewable energy. Transitioning to smart grid technology would be a great investment for Italy.

Looking Forward

The sun is one of our most valuable resources, and Italy is really paving the way with some of their newly developed technology like smart grids and thermodynamic solar panels. This technology is particularly useful because it can work in tandem with various types of renewable energy.

We only have one planet, and the more we invest in clean energy and developing a healthier relationship with earth, the brighter humanity’s future is. Fortunately, Italy and Vatican City are committed to these goals and are invested in developing and implementing solar technology–among other green initiatives–and are working toward a zero emission future.

Italy’s unique history with solar has helped pave the way for Italians and people around the globe. From early examples of harnessing solar power to heat Roman baths to modern ingenuity of developing and implementing smart grid technology, solar has experienced a fascinating evolution in Italy.

References

Dyaqua Invisible Solar. (n.d.). What Is Invisible Solar. https://www.invisiblesolar.it/tecnologia/

Fraschini, S. (2022, February). Smart grids for energy and the smart cities: a vital combination. Infra. https://www.infrajournal.com/en/w/smart-grid-for-energy-and-the-smart-city

Glatz, C. (2024, June 26). Pope launches project to get Vatican to run solely on solar power. Earthbeat. https://www.ncronline.org/earthbeat/science/pope-launches-project-get-vatican-run-solely-solar-power

Hossain, S. Rahat, A. Khan, S., Salehin, S., Karim, R. (2023, March 8). Solar-driven Dish Stirling System for sustainable power generation in Bangladesh: A case study in Cox’s Bazar. Heliyon Vol. 9 Issue 3. https://doi.org/10.1016/j.heliyon.2023.e14322

Kulowski, A. (2022, July 5). Do Leonardo da Vinci’s drawings, room acoustics and radio
astronomy have anything in common?. Herit Sci 10, 104. https://doi.org/10.1186/s40494-
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Palazzo, B. (2022, April 30). Solar energy in Italy: where do we stand?. Eniscuola. https://eniscuola.eni.com/en-IT/articles/2022/energy/solar-energy-ita.html