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PET Solar Heating System

PET Solar Heating System

A major part of our preparation work for the Summer School program is to explore, experiment and play with sustainable and low cost solutions we’ll implement in the refurbishment of socially relevant spaces. Our goal is to collect experiences from experts in different fields, improve the living quality of the inhabitants of these spaces and share this knowledge with our students. After tackling heating with the rocket stove, our prototype which can heat 70 m2 room burning scrap wood, we are ready to dive into water heating systems.

Portugal is facing a paradox of high winter mortality in a southern country. Even with mild winters, Portugal has more deaths during this season that any North European countries. Currently it is the second European country with the most death from the cold. Many portuguese houses are not well insulated and gas, electric and oil heating systems are not affordable for everyone. We want to fight this issue and work with realistic solutions to quickly improve lives of the most vulnerable.

So, how can we connect easy accessible sources into a system that is not only affordable to everyone but also sustainable in maintenance?

Man standing by Solar Heater

After building a water solar heater in our first summer school edition, we want to explore now  a model based on re-used PET bottles. As in any DIY solution, there are plenty of different manuals on how to do this all over the internet, but we’ll focus on a method used in Brazil since 2002. Designed by José Alano, a retired Brazilian mechanic, this heater recycles PET bottles and tetra-pak cartons. This easy to make and affordable water heater quickly spread throughout Brazil.

“More than 7,000 people are already benefitting from the solar heaters in Santa Catarina state alone. There are two cooperatives, one in Tubarão and other in Florianópolis, the last producing 437 solar heaters to be installed in council houses. In Paraná state, the number of solar heaters had reached 6,000 in 2008, thanks to the DIY leaflets and workshops that the governmental body SEMA organised there” says Alano to theecologist.org.

Providing hot water for over 50 soldiers, the biggest PET heater was built in the state of Paraná. It uses 1.800 PET bottles and 1.500 tetra-Pak cartons. It recovers more than 100kgs of plastic material and saves up to 35% of electricity.


PET is the short term for Polyethylene terephthalate. It’s a thermoplastic polymer of the polyester family and it is used for clothing, thermoforming and most common as packaging for food and liquids. Basically every bottle you see is made out of PET.


  • Physical Character: Melting point at 260°C ; it keeps its shape until 80°; does not react with foods, is resistant to attack by microorganisms
  • Safe, non-toxic, strong, lightweight, flexible material that is 100% recyclable
  • PET is resistant to various bio microorganism:  it takes at least 450 years to biologically degrade
  • Biological inert: very stable, soluble and almost independent of mineralization micro-plastic particles can be reduced continuously, but not completely
  • Recyclability: PET is the most widely recycled plastic in the world
  • In Europe 26% of all plastic produced in 2012 was recycled. In the US, only 9% (worldwatch.org)
  • In Europe 36 % of all plastic produced in 2012 ended in Landfill (PlasticsEurope)
  • The 5 biggest soft drink companies produce together 2 million tonnes of plastic bottles every year
  • Only 7% of throwaway plastic bottles of the 5 of the 6 biggest soft drink companies are made from recycled materials. (Greenpeace 2017)
  • Currently there are five trillion pieces of plastic in the oceans – enough to circle the Earth over 400 times (Greenpeace 2017)


The logic behind PET Solar heating systems is based on simple physical rules and the most powerful natural source – sunlight. Cold water runs through plastic pipes which are located in a construction of PET bottles. The PET bottles work as a lens to concentrate the sun rays, convey the heat and prevent the reflection of the rays. Thanks to the physical nature of plastic and unlike glass, PET allows UV rays, emitted by the sun, to pass through, which makes it useful in the heating process of water. This way the heater takes advantage of the greenhouse effect inside the bottle. This effect can be increased by using black painted Tetra-Pak cartons, which are useful to absorb the solar radiation.

Thanks to another physically character of fluid, the thermosiphon principle, there is no need of a water pump for circulation. Hot water has less density than cold water, which means that hot water goes up while cold water flows down. This principle applied to the PET solar heater guarantees a natural circulation.

At the beginning, cold water flows, in virtue of its high density, down into the system. Through the solar insolation, the water heats up, changes its density and flows back, up to the tank. Basically the same principle is applied inside of the tank: cold water stays on the lower part of the tank, from where it’s constantly feeding the circuit. On the other side, the hot water remains on the upper part of the tank, from where it flows down into the apartments/houses by gravity.  This allows the integration of the PET systems into buildings even If they don’t have electricity or the resources for obtainment.

Notice that it is recommended to use two separate tanks to benefit from the highest efficiency of the system, if you already have a water tank on your house. It is though possible to integrate the system to an existing water tank.  In regions where the water supply is provided through the local operator and there is no risks of service interruptions, one water tank dedicated to hot water is sufficient.



The whole system must be placed on the roof or a terrace exposed at south (northernhemisphere) or north (southern hemisphere), paying attention to the angle – it must be 10° more than the geographic latitude –  and at the slope – which must be around 2% toavoid air bubbles inside of horizontal pipes.


  • PET Bottles (60)
  • Tetra Pak Cartons (50)
  • 100mm PVC pipe (70 cm)
  • 20mm PVC pipe (11.7m)
  • 90-degree 20mm PVC elbows (4)
  • 20mm PVC T-connectors (20)
  • 20mm PVC end caps (2)
  • PVC glue
  • Water tank (tank for 60L)
  • 19mm water resistant tape
  • Matt black paint resistant to heat
  • T junction



  • collect 2L PET Bottles, transparent, same shape;  1 bottle will correspond to 1 liter of hot water. For example, if a Family needs 200L water > they will need 200 Bottles

***To avoid leaks in the system due to thermal expansion, use hourglass shaped bottles***


  • Unglue all 4 ends (a) and press it in the middle to make it a flat surface
  • Cut either top or bottom to have a total height of 22,5 cm (c)
  • Fold both sides in (b)
  • Fold the top and bottom edges diagonally to fit the form of the bottles (d) and make a7cm cut in the middle of the package (e) for the fitting with the next bottle
  • Paint all tetra-packs in matte black paint for outside use (iron or wood).

*** make sure to let them fully dry to avoid proliferation of microorganism ***


  • insulated tank to keep the water flow, covered by a wood box filled with insulating material to avoid losing temperature
  • you can either buy a fiberglass tank or re-use a container which can hold the amount of water according to your consumption. If you reuse a container that holded toxic fluids, make sure to clean it thoroughly.


  1. Cut the bottom of the bottle. Cut the pipes according to the total height of the bottles.
  2. Use the 100mm PVC pipe as a mold
  3. Cut the 20mm PVC pipes into 10 x 1m and 20 x 8.5 cm pieces, after assemble with the T-junction
  4.  Protect the edges of the pipes with a 19mm wide tape before painting it, this will help the connectors to fit in more easily
  5. Stack the bottles around the pipe, with the top side up. After putting each bottle, place the black-painted tetra-pack inside.
  6. After the bottle row is completed, finish the bottom section of the system. Make sure to use water resistant tape to close the circuit and avoid leaks.
  7. Place an exit on the bottom of the tank for the cold water to reach the system.
  8. Place an entry on the upper part of the tank for the hot water to come back from the system.
  9. Connect the tank with a pipe on the upper part to the households.


Studies conducted by the UERJ – Universidade do Estado do Rio de Janeiro and UFSC – Universidade de Santa Catarina in Brazil demonstrated that in three cycles water can reach temperature up to 55°C in summer and 37.5°C in winter, starting from 22°C and 15°C. It takes about four hours to heat a 200L tank.

Moreover UV ray and high temperature kill bacteria through a process called photo-oxidation, which is an advantage of the systems, especially in poor areas.

Still, since PET heating systems are dependent on solar energy and can only provide a limited amount of hot water, they do not have the same performance as traditional electric systems. In terms of maintenance, José Alano recommends to exchange the bottles every five years with new bottles:” Over time, the plastic becomes opaque, which reduces the heat caption, while the black cartons can be repainted”. Out of an affordability perspective this might not be a problem, since plastic bottles are easy accessible, but from the sustainability perspective this could be the biggest issue of the system. If the PET solar heater only recycles plastic bottles for five years what is going to happen with the opaque, used pet bottles afterwards? Even if the PET Solar Heater contribute to the short time reduction of plastic waste, we need to think further; finding a longterm solution how to reuse this plastic waste.

A dutch initiative called Precious Plastic launched machines which can transform plastic waste into useful materials and objects. Through publishing free manuals, Precious plastics approach ensures people around the world to re-build their machines with basic tools and materials. A combination of both systems; the PET solar heater and precious plastics –  might be a realistic contribution to a long term reduction of plastic bottles; taking advantage of its physical characteristics and reusing its properties sustainably.

Finally PET Solar systems can meet the usual hot water needs and does not require any advanced tools or knowledge. Especially in low budget households and poor regions, it has the potential to rise the awareness of plastic waste and show an alternative to electric heaters.

Critical Concrete currently is working on partnerships to fund the construction of the Precious Plastic’s machines for the production centre Co-Lateral in Porto.

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