Solar Water Heaters

The principles:

Solar collectors absorb the energy from the sun and convert it into heat. A wooden frame with insulation and glass on top captures and keeps the heat, like a greenhouse. Without a frame, even a tiny airflow will blow the heat away. The collector can be from plastic, copper, aluminum, steel etc. Every material has it advantages and disadvantages.

  • PVC is UV-resistant, cheap and easy to make however it is not a that efficient as metal collectors. PVC is also limited by pressure because it gets soft with high temperature. Recommended is not to use more then 1 bar on a PVC collector.
  • Iron is more effective however good welders are needed and it corrosion sensitive.
  • Aluminum can transfer the heat more efficient then iron however is difficult to weld. A from the absorbing material to the water is essential.
  • Copper can transfer the heat very quickly which makes it an efficient material to use however expensive. Special soldering rods and equipment is needed for welding.

Find here a list of materials and their thermal conductivities = Thermal conductivity is the quantity of heat transmitted through a unit thickness in a direction normal to a surface of unit area, due to a unit temperature gradient under steady state conditions.

An solar collector with a warm water tank above is called a passive solar water heater system because it doesn’t need a pump. The system is also known as thermosiphon. The circulation from the collector to the hot water tank is driven by the difference in density of hot and cold water (or other fluid). The fluid being heated by the collector expands and becomes lighter. Then an upward movement is created. Therefore the hot water tank should always be positioned above the solar collector so that the water flows in the circuit from the solar collector to the warm water tank and back.

How to make the solar collector (absorber):

  1. Find here an instruction move how to make the PVC collector by EMAS
  2. Find in this PDF how to make the collector from metal by WECF, Solar Partners & RDCA
  3. From Metal by EMAS
  4. Find in this PDF how to make the collector from copper by WECF, Solar Partners & RDCA

Basic rules for PVC collector:

  1. DO NOT PUT THE COLLECTOR IN THE SUN WITHOUT WATER!
  2. DO NOT PUT MORE THEN 1 BAR (10 meter water column) ON THE SYSTEM AND DO NOT CONNECT TO A PUMP
  3. USE HOT WATER PIPES FOR CONNECTING

Sizing

One aspects to take into account regarding sizes is hygienic aspects. It is recommended to design the solar water heating system in that way that it reaches easily 63 degrees as this is the temperature to sanitize water for kitchen and shower purposes. Many calculations can be done but a rule of thumb for the sizes is: per square meter solar collector: 60 liter volume in temperate climates, 40 liters in cold climates and 80 liters in warm climates.

Installation without warm water tank:

Note that

  1. In this system there will be heat losses when the sun is not shining because the heat is not stored in a insulated tank. Therefore the water will be cold in the morning and during rainy days.

Installation with warm water tank and floater

Note that:

  1. the floater needs to be adapted in this system to the tank. This can be done by extending the floater with a cord or line with the length of the water level when the tank is full. Choose always a good quality floater, preferable from non corrosive metal.
  2. Iron tanks (like oil barrels) are possible if well coated in and outside to prevent corrosion. Better is aluminum, stainless steel, etc. Plastic is possible if heat resistant e.g. PP, HDPE but not PVC or PE.
  3. The floater can be installed on top as well (if the floater is heat resistant) however the cold water should always come in at the bottom of the tank. Therefore a extended outlet to the bottom is needed. (see picture below left tank)
  4. In order to get first the hottest water in the shower and then less hot. The outlet for the shower can be extended with an flexible hose and another floater so the water is always taken from the top. (see picture below right tank)

Installation with warm water tank without floater

Note tha:

  1. Experience has shown that it is difficult to close the hot water tank hermetically.

Checklist for optimal functioning:

  1. Is the bottom of the warm water tank higher then the top of the collector? (at least 50cm)
  2. Is the piping system correct? See schemes
  3. No loops in the pipes: hot water from the collector should always go up, cold water from the tank always down.

(source: WECF)

  1. No leakages of the warm water tank, solar collector, pipes. Check if the insulation material around the warm water tank and under the collector is dry.
  2. If condense on the glass, then open the collector during a sunny day so that the moisture can go out. A bit condense in the morning is normal.
  3. Is the warm water tank insulated well? Check the outside. Places which are warm indicate heat losses.
  4. Is the collector insulated well? Places which are warm indicate heat losses.
  5. Is the glass clean?
  6. Is the frame well watertight? Especially when there is a under and top frame.
  7. Are the hot water pipes well insulated?
  8. What is the temperature of the water. A recommendations is that the water should be once a day 63 degrees to avoid growth of pathogens.

References:

2010 Drexel, R. et al. Construction of solar collectors for warm water -Practical guide, WECF, Munchen, Germany. 

2010 Simon, J., Solar Collector Manual, Social Union Agerkesh, Bishkek, Kyrgyzstan

Additional information: