Regarding Sunda evacuated tube collectors

• Why is this a better collector than a flat plate?
  There are various reasons:
  1.  Higher yearly efficiency due to the superior insulation provided by the vacuum. This makes vacuum tube collectors much better for winter space heating applications as well as all year DHW.    
  2.  Easier Installation & Servicing – tubes are installed/removed individually without special tools. You don’t have to shut the system down if a tube is removed due to the dry connection to the heat exchange manifold (no leakage potential)    
  3.  Longer Life – absorber surfaces are protected from environmental degradation by the hermetically sealed glass tube.    
  4.  Tubes Rotate – at installation the tubes can be adjusted on axis to compensate for non-south-facing roofs (flat plates require building a frame to tilt the array)
  
   
• How do Sunda collectors compare to and so called “double glass wall” products?
  In Sunda evacuated tubes the absorber surface and heat pipe reside in the vacuum environment, which prevents any deterioration in long-term performance. This is not the case with the double glass wall tubes in which both absorber and heat pipe are open to ambient air. Being exposed to air, the copper to aluminum interface between absorber plate and heat pipe is subject to oxidation which may lead to degraded performance over time. Double glass wall tubes are also subject to collection of water from condensation which could freeze and break glass tubes in frost prone environments. These concerns are not applicable to the Sunda design.
   
• Will the glass break in a hailstorm?
  The tubes are manufactured and tested to withstand 35mm (1.38 inch) diameter hailstones. The glass is low-iron tempered glass that is 2.5mm thick.
   
• How can you tell if you have lost the vacuum?
  All tubes now being delivered have a silvery coating on the inside at the bottom of the tube. When vacuum is lost, this coating depletes and the tube becomes clear. Older style tubes did not have this feature and you had to rely on observing a build-up of condensation inside the tube as an indicator of vacuum loss.
   
• How many tubes do you need and how much space is required?
  For DHW we typically use 4 tubes per person, which assumes 20 gal/person/day usage rate and raising the water temperature from say 55 F to 120 F at a minimum. A 2-person family would need a Seido 1/5-8 tube collector and a 50 gal storage tank. A 4-person family needs a Seido 1/5-16 tube collector and an 80 gal tank. The 16-tube collector is just over 6ft wide (75.6”) and about 7 ft long (83”).
   
• Can I heat my house with these?
  Yes, provided that the home is well insulated, thereby having a low heat loss to the environment. There are no rules of thumb for sizing a space heating array based on square footage of a house. You have to know the heat loss, design temperatures, weather data, and solar radiation available to estimate the size. It will be larger than the DHW component alone.
   
• Is there a potential of overheating?
  Yes, any solar thermal collector can overheat if the circulator is off during peak sun hours for a long period of time. The pressure relief valve guards against damage to the system components. The tubes will not “blow-up” or break due to high temperatures.
   
• What is the warranty?
  Six full years unconditional on the tubes. If you lose a vacuum seal during the first six years after purchase, you receive a replacement tube free (shipping costs extra).
   
• How heavy is the collector?
  Only 110 lbs for the 8-tube, and 220 for the 16-tube when fully assembled.
   
• How much do they cost?
  Depending on the model, if you include the tubes and the rest of the collector (frame, header, nuts & bolts)
   
• What is the payback or ROI?
  Easily half as long as PV but and on the order of 5-7 years when compared to heating water with electricity. It depends on usage and cost of energy. Ask yourself, “what is the payback on a conventional water heater from the local home center?” Answer, “none” – you just pay and pay and pay from the moment you install it. What is your energy independence worth to you? It should be “priceless”.
   
• What is the life of a tube?
  The statistical “mean life to failure” is 15 years.
   
• Are these new?
  No, the technology is about 20 years old. The Sunda tubes were developed in Germany by Daimler-Benz Aerospace in cooperation with Sunpu, a Chinese solar energy research company.
   
• Why does one tube have a straight absorber and the other a bent one?
  Seido 1 tubes have a straight absorber. These tubes can be rotated on axis to compensate or non-south-facing roofs. The Seido 5 tubes have a convex absorber providing 20% more surface area. They produce more heat per day than the Seido 1’s but cannot be turned on axis and should therefore be used only for true south-facing roofs (+/- 10 deg).
   
• Which side of the absorber in the tubes should face to the sun?
  Regardless of which Seido tube is used, always face the blue side to the sun. It is treated with a selective coating which absorbs the most energy from the sun’s rays as possible.