Concerns & Limitations of Evacuated Tube Systems

November 22nd, 2009 | Categories: News | Tags: evacuated tube, flat plate, solar water heaters

One of the most common questions that consumers who are interested in solar water heaters will ask themselves is whether they should purchase an evacuated tube system or a flat plate system. The basic difference is that while a flat plate solar water heater system allows the transfer of heat via an insulated glass covered box, an evacuated tube system allows the heat transfer to take place within a vacuum that is created within individual tubes. Both can heat water up efficiently and effectively that is sufficiently hot for domestic use however under certain conditions each one may have various advantages and disadvantages. In this post I will discuss a few of these advantages and disadvantages, however there are a few principals that we should keep in mind when making a comparison of this kind:

1. There is no substitute for good material and well manufactured product. Regardless of whether you choose a vacuum tube or flat plate, it is important to only purchase products that comply with various quality standards. In South Africa for example the South African Bureau of Standards have a detailed process that qualifies solar water heating systems for their standard approval.

2. Products may perform excellently but not in certain environments. Thus it is important to make sure you choose the right product for the right climate. For example vacuum tube systems may perform well in overcast, cold temperatures but if you do not live in a location with overcast and cold temperatures then you should consider using a flat plate system.

3. When choosing a product that is supposed to serve you for many years, it is important to ensure that the product and its manufacturers have the required experience and expertise since you will want a supplier who has a good chance of remaining in business for the many years that you use the product. This is important especially if we are talking about 10 years or more.

Here are a few specific reasons why evacuated tube systems are not appropriate for the South African market:

· In areas such as South Africa with good sunshine and a moderate climate evacuated tubes are not shown to provide any increased efficiency. In fact on warmer days, the insulation in evacuated tubes can cause water to superheat to temperatures in excess of 100 degrees Celsius, which can cause potential injury or damage to property.

· Evacuated Tubes rely on a vacuum seal to prevent heat loss, over time this seal can be lost, reducing efficiency and ultimately requiring tubes to be replaced.

· Evacuated tubes are better suited to high temperature applications such as industrial situations where steam is required.

· Evacuated tubes are made of lighter glass than flat plates, and can break if hit by a hard ball or rock. The more fragile frame also means that they need to be handled with extreme care.

· Flat plate collectors can be imbedded within roof structure to lie flush with tiles.

· Flat plate collectors are not pressurized and therefore far less likely to break or require maintenance.

· Many of the evacuated tube systems available in SA are manufactured in China without necessary expertise and lack SABS certification.

For further proof that flat plate is the most tried and tested, reliable solution for solar water heating, see the latest figures on page 5 of the European Solar Thermal Industry Federation. As you can see Vacuum Tube systems are a small fraction of the total installed base.

http://www.estif.org/fileadmin/estif/content/market_data/downloads/2008%20Solar_Thermal_Markets_in_Europe_2008.pdf

Please other source material:

1. http://www.homepower.com/article/?file=HP132_pg40_Mehalic

2. http://www.bsu.edu/web/capweb/ldi/projects/usbsc/studentwork/aliano/final_solar_collection.pdf

3. http://www.righthouse.co.nz/yk-files/708f7629e6a44151b7ddd6d1c975ecf9/Solar%20Flat%20Plate%20v%20Evacuated%20Tube.pdf

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