Blog

Wednesday, February 12, 2014

Solar hot water evacuated tubes

Evacuated tube collectors

Solar Hot Water Heating Evacuated Tube Collectors are the latest in solar hot water technologies. They are composed of a few basic elements but when combined make a very simple and efficient form of heat source.

Evacuated Glass tubes are made from borosilicate glass, a very high resistant glass (as an example Pyrex dishes are made from borosilicate glass). Each tube has two layers of glass and a vacuum (negative pressure) between them. You can relate this to a coffee thermos; because of the vacuum between the two layers of glass heat is trapped inside the second inner tube. Once the heat is inside the tube it is trapped and cannot come back out. This is used to heat the heat rod. If you took a thermal image of a flat panel during a hot day and a bank of evacuated tubes there would be a large amount of thermal coloring around the flat panel indicating heat loss and none near the evacuated tubes. Evacuated tubes do not have heat loss but they can get extremely hot.

Most solar products have some form of glass associated with their everyday workings. The fact the tubes are cylindrical adds to their strength giving them deflection when materials like hail come into contact with them. It also makes them more efficient through passive tracking of the sun’s U.V. rays around the surface of the tube for maximum efficiency. The inner tube has an absorption coating, which attracts U.V. into the tube, without this the sun’s U.V. would simply pass through the tube. The U.V. is trapped inside the tube heating the sputtered aluminum lining on the inside of the tube. The inside temperature of the tubes can easily reach close to two hundred degrees Celsius on a hot day.

If a tube was ever to be broken it can simply be replaced without shutting down or affecting the whole system. Also there is not water loss what so ever as water from the system does not pass through the tubes.

Unlike Flat Plate Collectors, Evacuated Tubes are heated from the sun’s U.V., not the ambient air temperature. , This means that your system will continue to heat even on cold and overcast days. A good example of the effect of UV is that people still get burnt on a cold overcast day.

he Heat Rod is supported inside the evacuated tube by the sputtered aluminimum towards the top of the tube for maximum efficiency. It is a simple design naturally heat rises. The Heat Rod is made from copper with a small amount of demineralized water and anti freeze inside the rod. It too also has negative pressure inside the rod in the form of a vacuum. When you add negative pressure to liquid it lowers the boiling point allowing the small amount of liquid inside the rod to boil around 30 degrees Celsius, instead of 100 degrees Celsius such as occurs in a boiling kettle. Therefore each evacuated tube is basically its own individual heating device, with its liquid boiling turning to steam to heat the bulb part of the heat rod before re-condensing and starting the process over again, continuing this cycle repeatedly during the day. Combining the evacuated tube’s ability to rapidly absorb heat through U.V. with its heat rod’s low boiling point of 30 degrees Celsius, a fully assembled evacuated tube can reach temperatures above boiling point in a very short period of time.

Heat rods are used to heat a manifold through a basic induction process; water from the storage tank is passed through the manifold heating it up to high temperatures throughout the day. Evacuated tubes reach high temperatures in a very short time, they hold the heat for long periods of time and this assists them in overcast conditions. Remember, because of their design evacuated tubes can work in the coldest of environments. Frost, snow, ice are not a problem. The one thing most people tend not to think about when purchasing an evacuated tube solar hot water system is regular maintenance. Evacuated tubes require more regular maintenance the flat panels sensors and pumps do sometimes only last half as long as sensors and pumps on flat plate systems

Material such as stainless steel, aluminum, and borosilicate glass are all lightweight and long lasting in our environment. The fact that the products are of lightweight means there is minimal concern on roof load rating. A 30-tube collector can weigh as little as 95 Kilos and be spread over 4 square meters. Because of how the collector is designed, air can easily flow through the collector eliminating drag or wind load concerns. This also minimizes corrosion issues between collectors and your roof materials. The lightweight means the mounting options for evacuated tubes are endless, they can be mounted on any direction roof with the tubes facing north, and they can be mounted on the ground or on a wall.