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photo of SP-20 coaxial evacuated tube collector

optional 60 degree mounting frame

Evacuated Tube Heat Pipe Principles:

The multiple deposition selective surface absorbs the full visible and infrared light radiation wave lengths from 0.3 to 1.3 u. First deposition is a copper metal layer, M-AL-N / CU which has low emission ratio and high thermal transmission through inner glass wall to the refrigerant heat pipe. Effective work range of 572 to 662 deg F
Second deposition is layer of AL2 O3 film to prevent cross migration. This entrainment allows the copper molecular layer to be very stable at temperatures in excess of 750 deg F.
Third deposition is a cermet aluminum nitride AL-N/M-AL-N sputtered simultaneously in gas mixture of argon and nitrogen to produce which absorbs solar radiation at very low emissivity, a = 95% ; e = 5%.

Operating principles of double wall vacuum tube with selective surface coating Glass draw-power-W.jpg - 24075 Bytes evac-tube.jpg - 13865 Bytes(c) 2003
The key component of the solar collector is the double wall vacuum tube. It is made by two concentric transparent borosilicate glass tubes (Pyrex) able to resist impact from hail up to 25mm in diameter. The inner glass tube is coated with a selective coating of Al-N/Al, which absorbs and converts the maximum of solar radiation & infrared light into heat and reduces emissivity. A barium getter is used at bottom of the inner evacuated tube. This barium layer actively absorbs all CO, CO2, N2, O2, H2O The "tube" is one continuous piece with no metal embedded which makes the vacuum tube have longer service life time.

Vacuum tube with heat pipe inserted vacuum tube-heat pipe.jpg - 4711 Bytes(c) 2003


The heat pipe evacuated tube system consists of the glass evacuated tube and a copper heat pipe which is installed within the glass tube. The copper heat pipe conducts from within the evacuated tube to its tip (condenser) which is plugged into the header pipe contained in the manifold. As water runs through the header pipe heat is transferred from the heat pipe condenser to the water. Collector conversion efficiency at 72% in full bright sunlight regardless of wind or freezing temperatures.

The drawing on the right illustrates the phase change operation of a refrigerant heat pipe.

For a good explanation in general of types of solar thermal collectors go to this US DOE Energy Star web page: Energy Star solar water heating

A heat-pipe is a sealed tube containing a small quantity of a volatile liquid with no air or other "permanent" gas present.

When it is placed vertically and the lower end is heated, liquid will evaporate and the vapour so formed will travel to the cooler parts of the pipe where it will condense and give up its latent heat to vaporisation. The condensate will then run back to the heated end where it can re-evaporate. This is illustrated above, right.

Because the heat transfer within the pipe comes from boiling liquid and condensing vapour, both of which processes have inherently very high heat transfer coefficients, and because the amount of material which has to move from one end of the pipe to the other is small, the effective thermal conductivity of the heat-pipe is very large.

To illustrate the magnitude of these quantities imagine that the heat-pipe is transmitting one kilowatt using refrigerant as the working fluid. The mass flow would be just under 0.5 g/s. At a temperature of 100 C in a 20 mm diameter pipe this would correspond to a vapour velocity of about 2.5 m/s.

In more sophisticated versions, the heat pipe contains a capillary wick to assist the return of the liquid from the condenser end to the evaporator end. Such pipes will work without the aid of gravity, for example in spacecraft. However, for terrestrial applications it is far cheaper and simpler two-phase thermosiphon, as the gravity return heat-pipe is usually known, is adequate.

The maximum operating temperature of a heat pipe is the critical temperature of the used heat transfer medium. Since no evaporation/condensation above the critical temperature is possible, the thermodynamic cycle interrupts when the temperature of the evaporator exceeds the critical temperature.

The main useful characteristics of the two-phase thermosiphon are:

(1) the thermal conductivity is extremely high: about a thousand or more times that of copper,

(2) the thermal conductivity is almost independent of the metal that the heat-pipe is made from.

(3) the device acts as a thermal diode. That is, the conduction is very high in one direction (upwards) and very low in the other (downwards).

These characteristics make heat-pipes useful wherever a large amount of heat needs to be conducted through a small cross-section. They have been used in cooling space-craft components, in cooling plastics-forming dies, for the construction of air-to-air heat exchangers for industrial and domestic energy recovery, and in cooling electronic components mounted in confined spaces. Or, as the first heat exchanger in a solar thermal collector for domestic applications.

Infra red camrea photo of flat plate collector vs evacuated tube collector. Under cold weather conditions a flat plate collector will reradiate energy absorbed to sky.

Technical Data

Typical performance for 20 tube array with 7 hours of sunlight 2 produces more than 20,000 Btu's which will raise 50 gallons of water by 60 deg F. Paybacks of system costs as soon as 5 years of usage.

1 kW solar vacuum tube collector for 95 cents per rated watt, balance of system components described on this web site.

vacuum tube:

Length Outside dia. Inside dia. weight material
1800mm (70.8") 58mm (2.28") 47mm (1.85") 2 kg (4.4lbs) tempered glass

Selective surface Absorbtance ( a ) Emissivity ( e ) vacuum thermal expansion heat loss
Cu / SS / AL N
triple deposition
+95%@AM 1.5 5%@80degC P=5x10-3PA 3.3x10-6degC 0.8W/m2 degC

hail resistance Warranty
25 mm 10 years

heat pipe:

dimensions efficiency heat transfer fluid
8 x 1750 mm + 95% non-toxic inorganic

20 vacuum tube collector:

length width height gross area net aperature
1980 mm ( 77.9" ) 1.65 M ( 65.25" ) 170 mm ( 6.6" ) 3.07 M2 ( 33.1 sq ft ) 2.01 M2( 21.64 sq ft)

fluid capacity flow rate pressure rating aperature efficiency power: watts ( Btu/hr)
1,300 ml 1 - 1.5 gal / min 65 psi max working + 90% 1,500 W * at STC =
1,000 W/m2insolation,Ti -Ta = 10 degC

Collector assembly manual in PDF format: Assembly manual for SP-20 manifold collector.

Annual performance chart in Btu hours and 2006 payback guide in PDF: annual performace chart btu hours offset.

Michigan insolation values in PDf: insolation data for sites in state of Michigan.

The Solar Patriot (tm) collector has been tested by an independent laboratory. Here is the complete Canadian National Solar Lab report ( requires Adobe Acrobat free reader program installed on your computer ) :

Canadian National Solar Laboratory test report

The SP-20 collector has been certified in the USA by the Solar Ratings Certification Corporation ( SRCC ). BTF Ltd is a participating member of the Florida Solar Energy Center's SRCC.

Solar Collector Certification and Rating

Solar Patriot™ test data from July, 2008 at the Madison Area Technical Center, Madison, Wisconsin

Comparison of flat plate collector vs evacuated tube collector by BC Hydro, Canada

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