At eight years old, Xóchitl Guadalupe Cruz López built a solar water heater to address the shortage of affordable hot water affecting her family and others in her community. The girl from San Cristóbal de las Casas, Chiapas, used discarded PET bottles and a length of hose, along with wooden planks and glass doors salvaged from a broken refrigerator. Her device was later recognized for its ingenuity and impact.
Xóchitl named the device “Baño Calientito.” It was designed to provide hot water without costly equipment or firewood. The National Autonomous University of Mexico (UNAM) highlighted its use of recycled materials and solar energy, which is abundant in the region.
Turning Waste into Practical Solutions
With her father’s help, Xóchitl assembled the prototype from ten PET bottles and a hose, adding wooden supports and glass panels. They installed it on the rooftop to capture sunlight efficiently. Painted black, the bottles absorbed the sun’s heat; the glass helped retain warmth. So, water inside the hose reached temperatures as high as 44.5°C, significantly higher than in uninsulated alternatives.
El País reported that the materials cost between 180 and 200 Mexican pesos, putting the invention within reach of families with limited resources. Over time, the heater’s capacity grew to hold up to 50 liters of water, enough for basic household needs.
Reducing Reliance on Firewood
For many families in Chiapas, heating water means burning firewood. The practice contributes to deforestation and exposes people to harmful smoke. Research from the Universidad de Ciencias y Artes de Chiapas (UNICACH), reported by Alerta Chiapas, classifies 54 municipalities as having very low energy poverty. The report puts 37 in the low category and 22 in the medium category; eight have high energy poverty, while three have very high levels. Its findings show that access to electricity alone does not guarantee enough energy for cooking and heating, or for preserving food, particularly in the colder highland region of Los Altos.
Data from Mexico’s 2020 Population and Housing Census, cited in the same reporting, illustrate the gap: in Mitontic, 99.2% of homes had electricity, but only 7.9% had a refrigerator. Firewood and charcoal use is also concentrated in rural areas. Portavoz Chiapas reported that in 2025, 73.4% of homes in rural localities of Chalchihuitán cooked with these fuels, compared with 3.3% of homes in cities with populations of at least 100,000. Alerta Chiapas cited a separate state-level figure: 43.4% of Chiapas households cooked with firewood or charcoal. These figures concern cooking fuels, not water-heating practices, but show the wider reliance on solid fuels. In an interview with La Jornada, Xóchitl said she had seen families cut down trees to cook and heat water, a cycle that harms the environment and public health.
The project drew recognition beyond its technical merits. In 2018, Xóchitl received the ICN Recognition for Women from the Institute of Nuclear Sciences at UNAM, highlighting the value of giving children early access to science and technology. She also took part in the PAUTA program, which encourages young talent in mathematics and science. Her participation further highlighted the importance of nurturing curiosity and problem-solving skills from a young age.
Simple Physics, Real Impact
The “Baño Calientito” system uses a simple design: black-painted bottles absorb heat, while the glass enclosure limits heat loss. Water circulates through the hose and gradually warms before use. The approach saves money and cuts the need for firewood, offering a cleaner, more sustainable way to meet daily needs.
Home solutions like Xóchitl’s reflect a wider interest in practical, low-cost ways to improve comfort and efficiency. For example, this guide to cooling rooms without air conditioning looks at how simple adjustments and resourceful thinking can make a difference in everyday life.
Encouraging Grassroots New idea
Xóchitl Guadalupe Cruz López’s solar water heater addresses an urgent need in her community, showing how accessible technology and creativity can lead change. As more families seek affordable options that use sustainable energy, grassroots inventions like this could influence household energy use and environmental stewardship.
