1. Evacuated Tubes:
Evacuated tubes are the absorber of the solar water heater. They absorb solar energy converting it into heat for use in water heating. Evacuated tubes have already been used for years in Germany, Canada, China and the UK. There are several types of evacuated tubes in use in the solar industry. Beida HuaFeng collectors use the most common "twin-glass tube". This type of tube is chosen for its reliability, performance and low manufacturing cost.
Each evacuated tube consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating (Al-N/Al) which features excellent solar radiation absorption and minimal reflection properties. The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperatures. This "evacuation" of the gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes.
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Why a vacuum? As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. This is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat, we don't want to lose it!! The vacuum helps to achieve this. The insulation properties are so good that while the inside of the tube may be 150oC / 304oF , the outer tube is cold to touch. This means that evacuated tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss (during high Delta-T conditions).
In order to maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture of the evacuated tube this getter is exposed to high temperatures which causes the bottom of the evacuated tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 out-gassed from the evacuated tube during storage and operation, thus helping to maintaining the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is ever lost. This makes it easy to determine whether or not a tube is in good condition. See picture below.
The Getter is located at the bottom of the
evacuated tube.
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Left Tube = Vacuum Present
Right Tube = Faulty
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Evacuated tubes are aligned in parallel, the angle of mounting depends upon the latitude of your location. In a North South orientation the tubes can passively track heat from the sun all day. In an East West orientation they can track the sun all year round.
The efficiency of a evacuated water heater is dependent upon a number of factors, one important one being the level of evacuated radiation (insolation) in your region.
Evacuated Tube Basic Specifications
Length (nominal) |
1500mm /1800mm |
Outer tube diameter |
58mm |
Inner tube diameter |
47mm |
Glass thickness |
1.6mm |
Thermal expansion |
3.3x10-6 oC |
Material |
Borosilicate Glass 3.3 |
Absorptive Coating |
Graded Al-N/Al |
Absorptance |
>92% (AM1.5) |
Emittance |
<8% (80oC) |
Vacuum |
P<5x10-3 Pa |
Stagnation Temperature |
>200oC |
Heat Loss |
<0.8W/ ( m2oC ) |
Maximum Strength |
0.8MPa |
2.
Heat pipes might seem like a new concept, but you are probably using them everyday and don't even know it. Laptop computers often using small heat pipes to conduct heat away from the CPU, and air-conditioning system commonly use heat pipes for heat conduction.
The principle behind heat pipe's operation is actually very simple.
Structure and Principle
The heat pipe is hollow with the space inside evacuated, much the same as the solar tube. In this case insulation is not the goal, but rather to alter the state of the liquid inside. Inside the heat pipe is a small quantity of purified water and some special additives. At sea level water boils at 100oC (212oF), but if you climb to the top of a mountain the boiling temperature will be less that 100oC (212oF). This is due to the difference in air pressure.
Based on this principle of water boiling at a lower temperature with decreased air pressure, by evacuating the heat pipe, we can achieve the same result. The heat pipes used in BeiDa HuaFeng solar collectors have a boiling point of only 30oC (86oF). So when the heat pipe is heated above 30oC (86oF) the water vaporizes. This va pour rapidly rises to the top of the heat pipe transferring heat. As the heat is lost at the condenser (top), the va pour condenses to form a liquid (water) and returns to the bottom of the heat pipe to once again repeat the process.
At room temperature the water forms a small ball, much like mercury does when poured out on a flat surface at room temperature. When the heat pipe is shaken, the ball of water can be heard rattling inside. Although it is just water, it sounds like a piece of metal rattling inside.
This explanation makes heat pipes sound very simple. A hollow copper pipe with a little bit of water inside, and the air sucked out! Correct, but in order to achieve this result more than 20 manufacturing procedures are required and with strict quality control.
Quality Control
Material quality and cleaning is extremely important to the creation of a good quality heat pipe. If there are any impurities inside the heat pipe it will effect the performance. The purity of the copper itself must also be very high, containing only trace amounts of oxygen and other elements. If the copper contains too much oxygen or other elements, they will leach out into the vacuum forming a pocket of air in the top of the heat pipe. This has the effect of moving the heat pipe's hottest point (of the heat condenser end) downward away from the condenser. This is obviously detrimental to performance, hence the need to use only very high purity copper.
Often heat pipes use a wick or capillary system to aid the flow of the liquid, but for the heat pipes used in Beida HuaFeng solar collectors no such system is required as the interior surface of the copper is extremely smooth, allowing efficient flow of the liquid back to the bottom. Also BeiDa HuaFeng heat pipes are not installed horizontally. Heat pipes can be designed to transfer heat horizontally, but the cost is much higher.
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The heat pipe used in Beida HuaFeng solar collectors comprises two copper components, the shaft and the condenser. Prior to evacuation, the condenser is brazed to the shaft. Note that the condenser has a much larger diameter than the shaft, this is to provide a large surface area over which heat transfer to the header can occur. The copper used is oxygen free copper, thus ensuring excellent life span and performance.
Each heat pipe is tested for heat transfer performance and exposed to 250oC (482oF) temperatures prior to being approved for use. For this reason the copper heat pipes are relatively soft. Heat pipes that are very stiff have not been exposed to such stringent quality testing, and may form an air pocket in the top over time, thus greatly reducing heat transfer performance. |
Freeze Protection
Even though the heat pipe is a vacuum and the boiling point has been reduced to only 25-30oC (86oF), the freezing point is still the same as water at sea level, 0oC (32oF). Because the heat pipe is located within the evacuated glass tube, brief overnight temperatures as low as -20oC (14oF) will not cause the heat pipe to freeze. Plain water heat pipes will be damaged by repeated freezing. The water used in Beida HuaFeng heat pipes still freezes in cold conditions, but it freezes in a controlled way that does not cause swelling of the copper pipe
Solar water heater performance is often presented as a graph, or set of three performance variables. Values may be provided based on gross area, aperture area or absorber area. In Europe, aperture or absorber is often used, in the US, gross area is often used. It doesn't really matter which values is used, as long as you use the correct value. ie. Don't use absorber area when using performance values based on gross area.
To adjust from one to the other, multiply by the size difference.
ie. If absorber area = 0.6m2 & gross area = 1.1m2 then (1.1/0.6 = 1.83), so multiply the performance factorsby 1.83 to convert from gross to absorber.
Conversion Factor: h0 = 0.717
Loss Coefficient: a1 = 1.52 W/(m2K)
Loss Coefficient: a2 = 0.0085 W/(m2K2)
As well as the three performance variables shown above, insolation level (G) in Watts/m2, ambient temperatures (Ta) and average manifold temperature (Tm) must be know. These values give the value x, also sometimes presented as T*m, used in the formula below.
(other slightly different forms of this formula are used, but provide the same result)
How to use the formula?
Based on the ambient temperature, average manifold temperature and insolation level, firstly calculate the value for X.
Eg. At 2:40pm; ambient temperature of 25oC (77oF); average water temp [(Tinlet+Texit)/2] of 50oC (122oF); insolation level of 800Watts/m2 (252Btu/ft2).
x = (50-25)/800 = 0.03125
Now enter all the values into the formula:
h(x) = 0.717 - (1.52*0.03125) - (0.0085*800*0.031252)
h(x) = 0.717 - 0.0475 - 0.0066 = 0.663
The solar conversion efficiency for that specific point in time and set of environmental conditions is 66.3%. That is: 66.3% of the energy provided by the sun is actually used to heat the water.
Based on the assumption that those three environmental factors (G, Tm and Ta) are stable for a period of one hour, then 800 x 0.663 = 530.4 Watts of energy per m2 of absorber area will be used to heat the water (168Btu/ft2). 530.4Watts is equivalent to 456kcal, which could heat 100L of water by 4.56oC (20 Gallons by 10.9oF)
Below is a graph showing the performance curves for the BDHF solar collector at three different insolation levels, from 0 to 80oC delta-t. In most cases the delta-t values will be in the range of 20-50oC, with higher values present for high temperature heating such a for absorption cooling applications, or during very cold weather. Except for when the delta-t is zero, conversion efficiency is dependent on solar insolation levels, with higher insolation yielding greater levels of solar conversion.
In reality ambient temperature will fluctuate, and the manifold temperature will gradually increase as the water is heated. Furthermore insolation levels may fluctuate with intermittent cloud cover. In order to more accurately calculate energy output per day/month/year a more complete set of environmental data must be considered and many (hourly) performance calculations throughout the day taken.
One factor which is not considered in the straight performance calculations outlined above, is the affect of transversal or longitudinal IAM values (Incidence Angle Modifier). Considering IAM is important as for BDHF solar collectors it accounts for as much as an additional 25% in total daily heat output values.
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