Engineering Rome

Time Tested Applications of Solar Energy in Rome

kellykobashigawa Sep 16, 2013


The Roman Empire was known for its efficiency and skill in engineering technologies. As their society became more lavish and populated, a sophisticated culture began to complement their technical abilities. One of their major structural accomplishments was the construction of the various baths around Rome. These bathhouses were some of the first places in which vaults, domes, and large windows were found. By the fourth century A.D., there were more than 800 baths in the city of Rome (Borrelli & Targia, 2008). Not only were these buildings magnificent in structure, they were exemplary in solar design.

Romans were proficient in expanding upon and improving the ideas of others as seen in their use of Greek architecture techniques. The Greeks used many solar energy technologies that were adopted by the Romans. The Baths of Caracalla and the Forum Baths at Ostia will be used as examples of how the ancient Roman Empire used passive solar energy and radiant heating. The process of these techniques will also be analyzed. The longevity of passive solar energy and radiant heating is evident as they can still be integrated into the technology that is used today. Just as the Romans improved on past technologies, society today uses Roman ideas as a basis for design and engineering. There has been immense growth in the field of renewable energy and green technology in recent years. This essay aims to examine the transformation of passive solar energy and [[#|radiant heat]] from ancient to modern times, as well as determine what the present state of renewable energy in Rome is today.

Emergence of Public Baths in Rome

The society of Rome placed importance on all aspects of life and maximized the time spent on any task, including undertakings as basic as hygiene. For this reason, the public baths were built on a large and luxurious scale. The baths had a social as well as a hygienic function. After a long day in the hot Roman sun or cold winter, you could bathe, refresh, and lounge about with companions. It is difficult to imagine the scale of these baths as most have deteriorated to ruins, but the ancient Romans spared no expense for these grand meeting places.

The Baths of Caracalla, officially known as the Thermae Antoninianae, is one of the largest bathhouses in Rome and can still be viewed by archaeologists, scientists, and tourists today. These baths were erected on the outskirts of the city in 212 A.D. and completed in 235 A.D. The actual construction took 9000 workers and five years (Borrelli & Targia, 2008). A new aqueduct, the Aqua Nova Antoniniana, was built to supply water to the baths. Since the Baths of Caracalla was a public place, people of all economic standings and backgrounds had access to this extravagant building. The baths covered about 50 acres and included swimming pools, exercise yards, a stadium, steam rooms, libraries, meeting rooms, fountains, and other amenities, all of which were enclosed in formal gardens known as xystus. See Figure 1. Thick concrete was used to build the infrastructure of the baths. This foundation was ornately decorated with mosaics on the floors, marble on the walls, gilded stucco and glass mosaics on the ceilings, and marble columns (Borrelli & Targia, 2008). See Figures 2 and 3.

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Figure 1. Floor plan of the Baths of Caracalla. Source:
Figure 2. Use of Domes and Arches.

Figure 3. Original mosaic pieces used to decorate the Baths of Caracalla

These enormous structures could hold 1,600 bathers at a time and about 6,000 to 8,000 per day (Borrelli & Targia, 2008). Men and women could use different parts of the facilities or were allotted separate times for bathing. They would enter the baths visit the changing rooms, swim in the large pools called natatio, and then move from three different baths that increased in temperature. The three baths included a cold-water bath (frigidaria), a mid-temperature bath (tepidarium), and a hot bath (caldarium). To gain a perspective of the size of these rooms, the circular, domed caldarium at the Baths of Caracalla was nearly as large as the Pantheon (Borrelli & Targia, 2008). The Baths of Caracalla were abandoned in 537 A.D. during the siege of Rome by Vitiges the Goth. The majority of the population fled to the center of the city where more protection could be offered to them. This left the Baths vulnerable to deterioration and theft. Starting in the 12th century, valuable materials began to be stripped from the building. Columns, marble, brick, and metal were some of the coveted items. These goods were then dispersed across Rome in various buildings. Some of the columns from the Baths ended up in famous churches such as the Santa Maria in Trastevere. The Baths also contained many priceless pieces of artwork from sculptures to frescoes. The Belvedere torso, which was a piece that profoundly inspired Michelangelo, was found in the Baths of Caracalla and now resides at the Vatican Museum (Borrelli & Targia, 2008). Although not much remains of the Baths of Caracalla today, visitors can still get a sense of the grandeur that the Baths once exuded.

On a slightly smaller scale were the Forum Baths at Ostia. These baths were more for the elite and financed by the imperial government early in the second century A.D. There are many inscriptions around the Forum Baths that pay homage to the various contributors and financers of the structure. The Forum Baths at Ostia had the same basic layout as the Baths of Caracalla and was the largest public bath in Ostia. The various rooms at the forum baths included the standard frigidarium, tepidaria, and caldarium, but also had rooms for sun-bathing (heliocaminus) and a sauna (sudatorium). See Figure 4 and 5. The Forum Baths received water through an Ostian aqueduct, which was then channeled into a large cistern and transferred to a waterwheel operated by slaves (“Regio I”, 2006). Today, visitors can experience the exposed brick and concrete that laid the foundation of the Forum Baths. See Figure 6. Identical to the Baths of Caracalla, the Forum Baths were ornately decorated with marble and black and white mosaics. There are still cavities that remain in the walls where various terracotta and marble sculptures were housed. See Figure 7. It is believed that many of the decorative aspects of the Forum Baths were composed of reused material and it is unsure which statues found in the building originated there. After Ostia was abandoned, materials were taken from the bath in the early 19th century and mostly likely at previous times as well. The major excavation of the Forum Baths at Ostia took place from 1920 – 1941 (“Regio I”, 2006).

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Figure 4. Floor plan of the Forum Baths. Source:

Figure 5. Window of the caldarium.

Figure 6. Bathing area at the Forum Baths
Figure 7. Cavity to hold a sculpture at the Forum Baths.

The Green Technology of Ancient Rome

Heating structures that were on the scale of the baths was a difficult undertaking. Being such an efficient society, Rome relied on direct heat from the sun and radiant heat to create the temperature variations within the baths. For the warmest rooms, it is estimated that the floor and inside wall surfaces were held at about 38°C through convection and radiation during the winter (Ring, 1996). In his article, Ring also states that the heat of the walls had to be greater than that of normal skin temperature which is around 34°C. If the temperature dropped below 34°C, the bather would radiate heat to the walls, as well as lose heat by convection should they be in contact with the surfaces of the bath. In this case, the bathers would not be able to retain their body heat and the caldarium and sweat rooms would not have their heating function. The design of the Baths of Caracalla and the Forum Baths at Ostia were such that there were many large south-facing windows so that the sun could heat and provide natural light to the warm sections of the baths. See Figure 8. Glazing the south-facing windows helped trap heat and the hollow tiles used in the floors radiated heat. The facades of the hot rooms received sunlight from early afternoon to sunset, which were the most popular hours for Romans to bathe. The thick walls and floors had the ability to store thermal energy so that the Baths wouldn’t cool down too substantially at night. It was also important that there were large wooden shutters over the window areas to retain heat. Using the sun to heat the baths helped to reduce fuel usage, even during the winter. In the summer months, the structure naturally provided shade to maintain the correct temperature of the cooler rooms (Ring, 1996).

Figure 8. Remains of the caldarium at the Baths of Caracalla

Passive [[#|solar heating]] was only a supplemental resource to the hypocaust system used by the Romans. Hypocausts were first used during Hellenistic times around the fourth and fifth century B.C. The Romans improved the hypocaust system through more conductive materials and by channeling the flow of the heat. The Roman hypocausts were hollow floors that consisted of brick and limestone supports, called pilae. The height of these supports was usually tall enough for a person to walk through for any maintenance issues (Basaran & Ilken, 1998). The floor, or suspentura, that covered the system was made of brick and mortar. Flue gas was produced by burning charcoal or wood in a furnace known as the praefurnium. See Figure 9,10, and 11. The flue gas had the ability to provide heat to many of the amenities in the bath because of its transfer ability and radiation. This gas heated water in large copper or bronze tanks. Chimneys were also used to create a flow of flue gas to heat the pools. The hypocaust system heated the floors easily as the gas rose. The gas then provided heat to the walls through tubuli which were small gaps or tubes that were inserted in the wall. The tubuli helped to sustain an even temperature and prevent condensation on the walls. See Figures 12 and 13.

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Figure 9. Parts of the hypocaust. Source: Ring (1996).

Figure 10. Remains of a hypocaust at the Forum Baths at Ostia

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Figure 11. How heat flows through the hypocaust. Source: Ring (1996).

In an academic article, Basaran & Ilken (1998) explains the various studies that have been done to analyze the effects of heating the Roman baths using the hypocaust system. Kretzschmer and Hüser performed experiments on a reconstructed bath that was similar to baths during Roman times. Jorio and Rook used results from Hüser’s experiments to calculate conditions in the Stabian Baths and Welwyn Villa Baths. In Kretzschmer’s experiment, he observed a hypocaust system in the bath and recorded various temperatures in the furnace. The temperature of the room was 21°C and the flue gas was at 60°C – 63°C. Hüser found that the average floor temperature was 18°C and the floor temperature closer to the furnace was 38°C. Jorio concluded from his experimental data that it would require 7 kg of wood per hour to achieve a temperature of 35°C in the 114 square meter caldarium of the Stabian Baths. Rook found for the Welwyn Villa Baths it would require 13 kg of wood to keep the caldarium at 70°C and the tepidarium at 55°C, but only over an area of 15 m2. Through studies of Turkish baths with Finnish saunas, Brödner hypothesized temperatures ranging from 23°C – 25°C for the tepidarium, 32°C – 33°C for the caldarium, and 37°C for the sudatorium. These experiments give a rough idea of what temperatures might have been produced by the hypocausts. Jorio and Rooks experiments show that although hypocausts were efficient, it would have taken a substantial amount of fuel to heat the gases. As fuel costs were increasing during this time in the Roman Empire, there would have been an impetus to rely more on passive solar energy and an aim to limit the use of the hypocausts.

Figure 12. Exposed tubuli in the wall.
Figure 13. Tubuli inlaid in the foundation of the wall.

Experiencing Roman Baths

  • Observations and Inferences

  • While visiting both the Baths of Caracalla and the Forum Baths at Ostia, it was interesting to experience firsthand something that was so integral to Roman culture. At the Forum Baths at Ostia, I was able to feel the warmth of the sun on the bricks and the differences between the surfaces that were in the sun and shade. Through the gradient of temperatures, I could tell which areas had been exposed to the sun for a longer period of time. I was able to sit in the sudatorium at the Forum Baths and confirmed that this room received a lot of sunlight and warmth. The circular structure of the sudatorium was ideal for sitting and socializing. The windows of the caldarium at the Forum Baths probably covered a third of the wall. There were three adjacent windows about 6 meters tall and 3 meters wide and would have captured plenty of sunlight. The planning of the baths seems to have been extensive and well thought out.

  • The Baths of Caracalla far exceeded my expectations on the size and scale of the complex. It is surprising that its construction only took 5 years. The skeleton of the building alone contained many layers of brick and mortar, so to have a layer of marble on top of that foundation is hard to imagine. I did not expect the walls of the baths to be so high, estimates of the height of the walls are around 40 to 45 meters. The Baths of Caracalla are exemplary of the idea that Rome is full of contradictions. For a society that strived for efficiency and advanced engineering techniques, they were also very concerned with aesthetics and somewhat excessive decorations. The overarching vaults and high domes were unnecessary towards the function of the Baths. The caldarium at the Baths of Caracalla had a high-ceilinged dome that wouldn’t aid in retaining heat as any heat in the air would rise. The vastness of the walls also expended materials when they could have reduced the impact and cost of the baths. The excess aspects of the Baths demonstrated the wealth of the upper echelon of Rome. Surprisingly, with the amount of money funneled into the Baths of Caracalla, all inhabitants of Rome had access to the facilities free of charge. This is telling of the value that was placed on Roman society and community. In a sense, Rome strived to exemplify a society that was more evolved than that of the people they conquered. Below are the Baths of Caracalla and the Forum Baths at Ostia located on a map.