Solar glossary

Aperture: The part of the collector through which light enters. For evacuated tubes this refers to the cross-sectional surface area of the outer clear glass tube measured using the internal diameter, not the outside diameter.
(Eg. 0.0548m x 1.72m = 0.094m2). 1.72m is the exposed length of the evacuated tube.

Absorber: The part of the collector that actively absorbs the light rays. For solar tubes this is defined as the cross-sectional area of the inner tube (selective coated) measured using the outside diameter. (Eg. 0.047 x 1.72m = 0.08m2) This value is used when calculating efficiency values. For solar tube collectors with reflective panels, the entire circumferential surface area of the inner tube is often used when calculating absorber area, as the reflective panel is supposed to reflect light onto underside of the evacuated tube.

Collector - A solar collector is not really a solar water heater. A solar water heater is a system which may include a tank, pump, controller and solar collector panel. A solar collector is that part of the system which absorbs the sun's energy and converts it into heat.

Delta-T Controller: Delta-T refers to the difference in two temperatures. This term is often use in relation to a solar controller. In such case the Delta-T is the difference between the solar collector temperature and the temperature of the water in the solar storage tank. A Delta-T controller can be configured to turn on the pump when the Delta-T difference exceeds a certain level (Eg.7oC / 12.7oF) and off again when the temperature difference drops below another setting (Eg. 2oC / 3.6oF). The controller turns on the pump when there is heat potential in the manifold. A Delta-T controller can also be used to provide freeze protection by circulating warm water from the tank through the manifold when the manifold temperature drops below 5oC.

Efficiency: Solar collector efficiency is usually expressed as a percentage value, or in a performance graph. When assessing a collector's performance make sure it is based on the correct surface area values. Eg. If performance values are based on gross area, then the gross area must be used when determining total heat output. IAM values have a significant influence on actual heat output throughout the day, and should be considered. Looking at just the percentage efficiency value will not give a true indication of daily heat output.

Efficiency testing is usually completed by testing bodies such as SPF, SRCC and other government approved testing bodies.

Tm* is the x axis value on performance graphs for solar collectors.
Tm* is calculated as:
(water temp - ambient temp)/Insolation
Eg. (44oC - 20oC)/800Watts = 0.03

Flow Rate: The volume of water flowing through plumbing in a given period of time. Usually measured in volume/minute or volume/hour. 1 Litre/min = 0.264 US Gallon/min

Gross Area: The total surface area of the collector including the frame, manifold and absorber. This area is often used when comparing collectors, but a better comparison to use is value for money. Roof size is not usually a limiting factor for domestic solar water heating installations, so the size of the collector is not really that important.

Heat Pipe: An evacuated rod or pipe used for heat transfer. Click here for more information.

Insolation: Don't confuse this with insulation - the one letter change makes a big difference. Insolation refers to the amount of sunlight falling on the earth.

Insulation: The ability to protect against transfer of heat/cold. Solar collectors use compressed glass wool to insulate the header from heat loss. Glass wool has excellent insulation properties, is very light and can withstand high temperatures, making it an ideal choice for a solar collector. It is made from a least 80% old glass bottles and can be recycled so is very environmentally friendly.

Incidence Angle Modifier (IAM): refers to the change in performance as the sun's angle in relation to the collector surface changes. Perpendicular to the collector (usually midday) is expressed as 0o, with negative and positive angles in the morning and afternoon respectively. Collectors with a flat absorber surface, which includes some types of evacuated tubes, only have 100% efficiency at midday (0o), whereas solar tubes provide peak efficiency mid morning and mid afternoon, at around 40o from perpendicular. This results in good stable heat output for most of the day.

Pressure: Refers to the water pressure in the system. The conversions for the most commonly used units are: 1 bar = 1.02kg/cm2 = 14.5psi = 100kPa = 0.1Mpa = 10m water head

Evacuated Tubes/Vacuum 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. Apricus 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.

Heat pipe

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 AP 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 Apricus 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 Apricus heat pipes are not installed horizontally. Heat pipes can be designed to transfer heat horizontally, but the cost is much higher.



The heat pipe used in Lisheng 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.

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 Lisheng heat pipes still freezes in cold conditions, but it freezes in a controlled way that does not cause swelling of the copper pipe

U shape heat pipe

The U-pipe is made by a straight copper pipe which is bended at certain point. The two segments are parallel, and welding on the two main pipes. Several U-pipes and two main pipes build up a whole water-circuit system. The U-pipe is inserted to the vacuum tube with the aluminum fin. When the solar energy is absorbed by the vacuum tubes, the energy is transferred to the U-pipe crossing the aluminum fin. then the U-pipe is heated.

When the water or other medium flows across the main pipe, they have to flow across the U-pipe because the two main pipes are not linked directly, they are separated. So, the water or medium is heated by the u-pipe and rised a high temperature.

The heated water or other medium makes water in the tank hotter and hotter by walking across the coils in the tank.

Copper Header Pipe

The LC solar collector's header is designed to providing excellent heat transfer and corrosion resistance while using a simple "plug in" installation method.

The key features are as follows

1. Heat pipe ports provide simple plug in installation while still ensuring tight contact with the heat pipes for optimal heat transfer. Thermal heat conduction grease is applied to the heat pipes condenser prior to insertion to further enhance heat transfer. Given the high temperatures that the manifold is exposed to, the expansion of the heat pipe condenser and "setting" of the heat conduction paste results in the heat pipe being firmly held in place. This ensures excellent heat transfer for the life of the solar collector. As the heat pipe is extremely reliable and durable, there is no need to ever remove or replace the heat pipe, even if changing a solar tube.

2. The twin header pipes are molded to match the shape of the heat pipe ports in order to maximised contact area. In addition, the heat pipe ports are brazed to the twin header pipes providing a direct metallic connection.

3. The "contoured" header pipe design produces turbulent water flow enhancing heat transfer.

4. The header pipes are brazed using Ag45CuZn, lead free brazing rods, which are suitable for potable water and provide a strong, quality joint.

5. Available in rear port or end port inlet/outlet configuration.

6. 8mm ID copper temperatures sensor ports at both the inlet and outlet which are brazed directly to the header pipe for accurate temperature measurements.

Glass Wool Insulation

Glass wool is a very popular insulation material, used throughout the world in many high temperature insulation applications. Glass wool is also non-flammable, and so an excellent choice for a high temperature solar thermal solar collector. One key advantage of glass wool is that it can be molded into any shape. Via a process similar to baking a cake, the glass wool is "cooked" at high temperatures matching perfectly the shape of the header and the evacuated tubes.

Glass wool is:
- An excellent insulator K = 0.043W/mK
- Non-flammable (can withstand temperatures up to 300oC / 572oF)
- Made from 90% recycled glass
- Very Lightweight (~70kg/m3 density - 4.36p/ft3)

Manifold Casing

The manifold casing serves two main purposes, protecting the header and glass wool insulation from the elements, and making the collector attractive and neat. The casing is made from corrosion resistant grade aluminium and is available in a matt black or silver finish.

Mounting Frame

The Lisheng Solar Collector can be installed on most roof surfaces, and a full range of roof angles. A standard frame is provided with all collectors, and additional frame kits are available to suit most common installations. The various frame components can also be used to install on most other non-standard surfaces.

The frames are designed to withstand high speed winds; the tubes provide minimal resistance due to the round shape. Attachment points must also be strong enough to withstand significant pull forces that will occur during strong winds.

Standard Frame

For flush installation on a pitched roof, the standard mounting frame is used. The front tracks are secured to a tiled roof using the stainless steels straps, rubber pads or round feet, depending on the roof's surface.

Low, Mid & High Angle Roof Frames

Frame kits are available which increase the angle of the solar collector, from as little as 12o to as high as 53o, allowing the collector to be installed at an optimal angle. The frames kits even allow mounting on a wall.

All frame components, including bolts, washers and nuts are made from stainless steel. The frame kits are already partly assembled, making installation very quick and easy.