Comparsions of Solar Panel types

Collectors are perhaps the most researched and tested part of the solar thermal system. They have been subject to technical development to improve efficiency of the historical flat-plate collector and continuously evolved through the use of new materials, new designs and cost bases. The increased competition in Europe due to the importation of collectors from manufacturing areas in the East, has shifted the balance between flat-plate and evacuated-tube collectors and place new requirements for effective assessment of quality, given the lifespan of the use. European standards for the testing of collectors have been defioned for factory made collectors EN 12975, custom-made collectors EN 12976 and manufacturers solar systems EN 12977. These define tests for performance standards as well as test for durability and weatherproofing. In addition the Solar Keymark standard has been adopted to further authenticate collector quality. The collector testing is carried out at a number of centres throughout Europe, many of which will publish the result certificates online. It should be remembered however that it is system effectiveness in use that is most important and that some of variations in collector performance may not reflect the same changes in system performance.

Aperature & Absorber Collector Areas

In the interpretation of collector test certificates, manufacturer's datasheets or the outputs from system sizing, it is important to understand the area that is being discussed. The most important area is the collector aperture area. For a flat-plate, this is the area of the absorber plate that is open to the sunlight and is therefore capable of generating heat. The absorber are is the physical area of the collector plate part of which may be obscured by the edges of the collector.

In vacuum tube collectors, the aperature area is slightly bigger than the absorber area. The absorber is the flat plate inside the tube (in the high efficiency collectors) or the coated surface in the Sydney type vacuum collectors. The absorber area is the outside diameter of the glass tube.

The gross surface area is another measure used. This is the roof area that a collector will require. Vacuum tube panels have gaps between their tubes so for a given aperature area, they will require more roof space than a flat plate collector.

Flat Plate Collectors

Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes attached to the absorber plate. Absorber plates are commonly painted with "selective coatings," these coatings absorb UV and visible radiation, but are poor emitters of longer wave infrared so that they retain heat much better than ordinary black paint. Absorber plates themselves are made from copper welded to a copper pipe.

Sometimes non-selective coatings may be employed to increase heat losses at high temperatures and prevent a solar panel from boiling, The Solartwin system which deliberatley uses a non-selective coating to prevent overheating. Non-selective panels were more common in the past because of the increased cost of selective coatings. They are relatively uncommon in the marketplace, but may enjoy a renaissance as a cheap non-stagnating panel. However deliberatly decreasing the efficiency of the panel should be compensated for by increasing its collecting area.

Layout of pipe work

Serpentine

A Serpentine arrangement is where the same tube "snakes" its way from the bottom of the panel to the top of the panel. Efficiency is reduced slightly as the top of the panel tends to be hotter and therefore has higher thermal losses. The pumped pressure drop across the panel will be greater than for a parallel tube type arrangement, which will limit the number of panels that can be placed in series. Purging air from the panel generally poses no great problems as there are no possibilities for the antifreeze mix to take a parallel path, so generally air is easily removed when filling.

Parallel Tube

This is the most common pipework arrangement. Two manifolds, normally 22mm copper pipe are connected within the panel by a series of copper tubes (8mm or 10mm are the most common sizes). The colder water is fed into the bottom and the hot is drawn off the top. It is simple and cost-effective to manufacture hence its popularity.

Hybrid / PreHeat

This is a variation on the parallel tube type. The cold and hot feed are both on the top, but the top manifold is crimped to force the antifreeze mix to flow down the frist 3 or 4 tubes to the bottom manifold. This increases the temperature of the output of the panel.

Vacuum Tube Collectors

In essence there are two main types, the single clear walled tube with a convential plate absorber inside, and the second type, is made up of a double walled glass tube with the absorber coating sprayed onto the inside wall. Because this type of collector was developed in the University of Sydney, it is commonly referred to as the 'Sydney' vacuum tube or occassionaly as the 'Phillips' tube.

Common (High Efficiency) Tube

A clear single tube contains a selective absorber plate, the tube is evacuated down to the level normally seen in outer space. This virtually elimates the conduction and convection heat losses leaving only the radiation heat losses. These are minimised through the use of selective coatings which hinder the emmission of infra-red heat radiation. This makes this type of collector very efficient at retaining heat and hence it is very good for high temperature applications.

However, because there is only a single wall, the vacuum seal must be between a glass and metal surface. This has proved problematic in the past, with some models showing poor reliability. The glass metal sealling arrangement is a more complex manufacturing process than making a double walled glass vacuum tube, it tends to be more expensive.

The single glass surface surface generally gives it better a better optical efficiency than the Sydney tube, but because the glass is curved, the flat plate collector generally has a higher optical efficiency. As was discussed earlier, the thermal efficiencies are about the best of all the panels.

Sydney Tube

Each Sydney tube consists of two glass tubes made from borosilicate glass. The outer tube is transparent, the inner tube is coated with a selective coating (Al-N/Al) which absorbs the solar radiation and turns it into heat.

The top of the two tubes are fused together and the space between the two layers of glass is evacuated, giving a the tube vacuum jacket. The insulating properties are still excellent, though slightly less than the high efficiency tube.

The big advantage of the Sydney tube is its ability to passively track the sun. This gives it a more consistent output than any other collectore over the whole day.

CPC variation

This is a variation on the Sydney type where a curved reflector is added to the back of the collector. The CPC stands for compound parabolic collector. This shape of reflector is capable of reflecting the sun's rays onto the central tubular absorber, even as the sun tracks across the sky. The aperature area of the collector is now the area of the reflector, but it can only lose heat through the vacuum tube and the manifold. For this reason, the thermal characteristics of this type of collector are excellent.

However while the collector may stay relatively clean for a number of years, dust and dirt tend to collect quite quickly on the back of the vacuum tubes. The collector will require regular (at least once per year) cleaning to maintain its output. One of the principal advantages of conventional vacuum tube collectors is that the wind can pass between the tubes, however with a reflector, increased wind loading is inevitable. It is also very important to verify that the reflectors are very tightly connected to minimise any rattle.

Maintaining the Vacuum

Glass is very slightly porus to air molecules, so 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 that passes through the glass wall which maintains the vacuum. 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.

Extracting heat

There are two primary methods of extracting heat from the vacuum tube.

One, use a heat pipe or secondly pump the anti-freeze mix down into the vacuum tube. The second method is konwn as primary flow, or primary fluid. Using a heat pipe introduces an extra heat exchange surface but in practice there seems to be very little difference in efficiency between the two extraction methods.

Heat Pipe

Heat pipes are not exclusively found in solar panels but are commonly used in laptop computers and air-conditioning systems. The principle behind heat pipe's operation is very simple and surprisingly efficient.

A heat pipe is simply a copper tube with a small amount of heat conducting fluid inside, and evaucated. Because of the low pressure, the fluid inside the heat pipe starts to change state from liquid to gas once the temperature rises above 30C.

As the heat conducting fluid vaporizes, it raises the pressure within the heat pipe, preventing all the fluid from evoporating, so that more fluid evorporates the hotter it gets. The vapour rises in the heat pipe bringing heat with it (similar to convection) until it reaches the manifold. The heat pipe is pushed into pocket in the manifold so that so that heat passes easily to the antifreeze mix in the solar loop.

Top of heat pipe with conducting paste...

As the heat is lost at the condenser (top) to the manifold, the vapour condenses to form a liquid and returns to the bottom of the heat pipe to once again repeat the process.

Slim alumimum sections pushed up against the heat pipe are used to help with the heat transfer process.

Primary Flow / Primary Fluid

The alternative way to extract energy from a vacuum tube is by pumping the anti-freeze mix within the solar circuit into the inside of the vacuum tube. There are a couple of methods used. Pipe in Pipe, or U-Pipe.

The U-pipe method is self evident, a double manifold is used where the antifreeze mix is pumped from the cold manifold to the hot manifold via pipe extending down into vacuum tube.

The pipe in pipe system is similar except that the colder antifreeze mix is pumped down the inside of the inside pipe and as it is transferred to the outer pipe it picks up heat from the vacuum tube and is returned to the hotter manifold.

Primary Flow systems have several disadvantages, normally they must be pre-assembled with tubes before they are brought up to roof level, which makes installing more difficult. The second more serious drawback, is the anti-freeze mix flows into the middle of the vacuum tube. Under stagnation conditions this can lead to serious overheating of the polypropelene gychol mixure which if happens leads to a pre-mature breakdown of the anti-freeze. Thirdly, the narrow pipework is more prone to blockage, which can lead to some tubes being bypassed and overheating.

Maths

Imagine, the sun is directly overheat, and that the solar panel mounted along the line of the hypotenuse shown in the diagram. It stands to reason that the concentrated sunlight shing straight down will be spread out or shared over a larger surface area. If effect the shadow in the diagram is the same size as the adjectent side. The equivalent energy of this is shaed over the the larger hypotenuse surface.

This can be worked out with the following formula.

Radiation on Panel = Cosine of Angle (A) multiplied by intensity of radiation.

Example 1 : If A = 30 Degrees. Cosine 30 = 0.866, so if 1000 watts is shining straight down then the panel will only see 866 Watts.
Example 2 : If A = 45 Degrees. Cosine 45 = 0.707, so the incident radiation will be 707 Watts.

Incidence Angle Modifier

It has been the authors experience that the Incidence Angle Modifier (IAM) is poorly understood.

When Solar Panels are tested, performance measurements are normally taken with the solar insolation level measured perpendicular to the collector plane (i.e. facing the same direction as the collector). The IAM (Incidence Angle Modifier) values provides a performance factor, so that the output can be calculated when the light is not perpendicular to the collector. This Cosine modification as described in the maths above is NOT taken to account.

An IAM value of 1, means that at a particular incident angle, the output is directly proportional to the cosine of the incident angle multiplied by the full (perpendicular) radiation intensity.

However, at different angles solar panles reflect differing amounts of radiation. In the case of flat plate collectors more radiation is refelcted from the surface the more oblique the incident angle is, so the new IAM will be less than 1.0

In some vacuum tubes, particuarily the double walled type, because there are gaps between the tubes, as the sun tracks across the sky, it still sees a full tube so the output stays constant, even though the maths perdicts that the output should fall by the Cosine factor of the incident angle. In this case the IAM will be greater than 1.0. There are of course two types of IAM.

Horizontal - as the sun tracks from left to right

Vertical - as the sun tracks from low to high.

Some sample graphs below for comparsion.



The variation in IAM value is taken into account in some solar simulation programs such as TRNSYS and Polysun, where IAM tables can be entered and edited for each specific collector.

Optical Efficiency (Zero-loss coefficient)

Optical efficiency or the zero loss coefficient refers to the amount of radiation absorbed by the panel if the panel is at the same temperature as the ambient surroundings. i.e. it is not losing or gaining any HEAT through the walls/back of the collector. Simply speaking it percentage of light (over the whole sprectrum) emitted through the glass face.

Solar radiation when it arrives on earth is made up of different wavelenghts, from Ultra Violtet to Infra-Red. Plate (standard Window) glass is very good at letting through visible radiation, but reflects Infra-Red and Ultra-Violet. Because of this only about 60% the energy in the spectrum is transmitted through window glass.

Solar panel glass is normally made up with a low-iron content glass, this makes the glass far more transparent to Infra-red light as well as the visible radiation, this can increase the amount of energy transmitted through the glass up to about 80%. The energy passing through the panel glazing as a percentage of the overall energy striking the panel is known as the zero-loss coefficient or the optical efficiency.

Flat Plate panels normally have a higher optical efficiency figure than flat plates, but poorer thermal performance as the temperature of the collector rises significantly above ambient. Recent advances with new coatings on both sides of the cover glass has raised flat-plate optical efficiencies to around 82+%. This also improves the IAM curve by reducing the light reflected off the panel when the panel is not directly facing the sun.

An open swimming pool type collector without glazing would typically have an optical efficiency of greater than 90%. (less than 10% of the incident radiation is reflected). However heat is poorly retained in this type of collector and it is only suitable for low temperature applicaptions.

Thermal Efficiency

Thermal Efficiency As the panel heats up it begins to loose heat from the walls and the back as well as through the front face of the panel. The hotter the panel gets in comparsion to the ambient temperature the more heat it will lose. This reduces the efficiency of the panel. The amount of heat each meter sqaured loses per degree above the ambient temperature is known as the thermal efficiency. However this effect is not quite linear because as the panel heats up, the wavelenght of emitted heat changes, this changes the thermal characteristics of the panel. To cater for this mathematically, two loss coefficients are used. The first "a1", is by far the most important.

Loss Coefficient a1
Loss Coefficient a2

This is normally incorporated into the following formula which can be used to calculate the panel output.

Wind Chill

The rate of heat loss by a surface depends on the wind speed above that surface: the faster the wind speed, the more readily the surface cools. The effect of wind is to reduce any warmer objects to the ambient temperature more quickly. Because of the accelerated heat loss, the equivalent outside temperature is colder and the panel will perform below its predicted level. This has implications for siting panels in exposed locations, and in this situtaion a panel with a lower heat loss coefficient will suffer substantially less fall off in output.

Efficiency Curves of different types of Solar Panels - examples

A sunny spring day with and output graph might look like the following, the panel will start off in the morning at about 30C and get hotter as the day progresses, reaching a useful maximum of 75C as the thermal store becomes fully charged.

From the graph below, the Non-Selective Flat plate, the Selective Flat Plate, Sydney Vacuum Tube and the High Efficiency Vacuum Tube are all suitable for heating water. The unglazed collector will not be effective in heating water beyond 30C, which by lucky concidence is the upper temperature of a swimming pool!

On warm, sunny days, the performance of a flat plate collector will outperform an equivalent vacuum tube collector. But the vacuum tube panel will increasingly outperform the flat collector as the outside temperature decreases or as the system temperature rises.

The Sydney collector performs better than other collectors when the sun is not perpendicular due to its passive tracking nature. This highlights the effect of the Incident Angle Modification.

Advantages / Disadvantages of each collector.

Type	Advantages	Disadvantages
Unglazed Collector	Very low cost Easy Installation Large collector arrays normal and can lead to huge amounts of low grade heat collected. Very suitable for pool heating.	Not suitable for heating water to more than 30C
Non-Selective Flat Plate	Normally lower cost. Less prone to high temperature stagnation.	Lower Thermal Efficiency Can have lower optical efficiency depending on make. Wind Loading (in common with all flat plates). Larger panel required for given output.
Selective Flat plate	Efficient use of limited roof space. Very high optical efficiency. Lower stagnation temperature than Sydney or high efficiency vacuum tube	Higher cost panels. Wind Loading. More difficult to install than some types of vacuum tube panels. Higher stagnation temperature than a non-selective flat-plate
Large (>10 sq Meter) Selective Flat Plate	Efficient use of limited roof space. Very high optical efficiency. Thermal efficiency similar to Sydney Vacuum tube. Good for Space heating and commercial applications. Far fewer hydraulic connections	Crane required for install. Heavy anchor points required if flat roof mounting. Higher stagnation temperatures. System needs to be engineered for pipe and expansion vessel sizing.
Sydney Collector	Very good IAM leads to more consistent output over day. Very easy to install. Low wind loading. Heat pipe types may not require anti-freeze.	Larger roof space required for given aperture area. Prone to high stagnation temperatures.
CPC Sydney Collector (with reflector)	Low cost for given aperture area. Medium IAM performance. Very good thermal efficiency.	Underside of tube and reflector prone to dirt build up, which reduces efficiency over time. Reflector must be very well secured to prevent wind rattle. Higher Wind loading. Less adjustment than other vacuum tube panels so less easy to install.
Single Walled Vacuum Tube (high efficiency)	Most efficient use of limited roof space. Low wind loading. Higher optical efficiency than Sydney collector. Very high thermal efficiency.	High stagnation temperatures Higher cost.