THE DIFFERENCES OF VACUUM TUBE SOLAR ENERGY SYSTEMS FROM SHEET GLASS COLLECTORS

*Our system consists of two pieces of telescopic tube. The heat dissipation is minimum, because there is a vacuum among two tubes. The external glass is resist and against the weather conditions. The internal tube is made of a specific combinated glass, which is transparent and resistant, and covered with a specific surface in black. This selective surface absorbs %93 of sunbeams and turns it into the heat and minimizes the reflection of sunbeams (%7)

*Because there is a vacuum among two tubes, the heat dissipation occurred by transmission and moving is minimized. The heat dissipation inside of the tube is prevented by the vacuum tube during cold, rainy, snowy, and windy times. That's why the system van be run along 12 months.

There is no vacuum in old styled collectors; therefore the heat dissipation is very high. This situation minimizes the productivity and prevents using along the whole year. 

Old styled collectors require antifreeze in case of freezing of water. This kind of systems don't run so good under 0 C.
   

*The metallic component of our system is made of stainless steel(chrome-steel). The water is drinkable.

The old styled solar energy system has some problems like an oxidization, calcification or obstruction. And also the water is not drinkable.

*The 50 mm polyurethane, whose heat transmission coefficient is very slow, is used for the hot water tank isolation in our system.

The fiberglass is generally used for the old styled solar energy system boilers. The heat keeping degree of it is lower than the polyurethanes. 

*Our firms cylinder vacuum tube receives the sunbeams vertically and the reflection minimizes, because it has a special cover. Thus the temperature of water can rise up to 9500 C during summer, 5500 C during winter.

Old styled solar energy systems receive the sunbeams vertically only on midday. And the heat degree is always lower than our new systems.

Solar Efficiency

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. Absorber area = 0.6m2, gross area = 1.1m2. If performance variables are provided for gross area, multiply by 1.83 (1.1/0.6 = 1.83) to obtain absorber area values. The smaller the area used, the higher the performance variable values.

The three performance variables for the AP solar collector as provided by the SPF testing laboratory in Switzerland (SPF report C632LPEN) are as follows (for metric calculations - absorber area):

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.

How to use the formula?

Based on the ambient temperature, average manifold temperature and insolation level firstly calculate the value for x.

Eg. At 2:00pm, the ambient temperature is 25oC (77oF), and the average water temp [(Tin+Tex)/2] is 50oC (122oF). The insolation level is 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*0.03125*0.03125)

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 AP solar water heater 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. As can be seen conversion efficiency is highly 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. Your local ApricusTM distributor can provide estimates of average monthly and annual performance, heat output and thus solar contribution for your location.

One factor which is not considered in the straight performance calculations outlined above, is the affect of transversal IAM values (Incidence Angle Modifier) on solar collector output throughout the day. Please read the following section to learn more about IAM

A solar collector that is mounted on a device to track the sun from sunrise to sunset (as commonly use in PV applications) will maintain a IAM value of 1 throughout the day, as the collector is always facing the sun. A collector that is installed at a fixed angle (generally equal to the location's latitude), will experience decreased radiation levels (IAM value < 1) in the morning and afternoon when the sun is not perpendicular to the absorber surface.

For most solar water heaters currently on the market, IAM is not an important consideration when comparing performance. This is because flat plate collectors, evacuated tube collector with a flat absorber, or those that using reflective panels usually have a fairly similar set of transversal and longitudinal IAM values. The value of most concern for fixed angle collectors is transversal IAM, as this reflects the solar radiation throughout the day. Longitudinal IAM is useful when looking at installation angle, and the changes in heat output throughout the year as angle of the sun above the horizon changes between winter and summer.

The longitudinal and transversal IAM values for the AP solar collector are as follows:

0 0o 10o 20o 30o 40o 50o 60o 70o 80o 90o
Kq (longitudinal) 1.0 0 0 0 0 0.93 0 0 0 0.0
Kq (transversal) 1.0 1.02 1.08 1.18 1.37 1.4 1.34 1.24 0.95 0.0

SPF report C632LPEN)

Note: 1o = 4minutes of time. So 1hour = 15o

The following graph displays the transversal IAM values for the AP model, a leading flat-plate and leading evacuated tube reflective panel solar water heater.

As can be seen, the AP solar water heater has a curve which is quite different to the other two solar water heaters. This is due to the cylindrical absorber area, which passively tracks the sun throughout the day. At 40-50o there is no light lost between the tubes, and no tube overlap, hence a peak in relative performance. This is ideal, as during this period (mid morning through mid afternoon) solar isolation levels are quite highest. The peak at 70o provided by the ET-reflect is of little benefit as this angle corresponds to early morning or late afternoon when solar insolation levels are very low. The flat plate collector's IAM values drop away from 1 as the angle increases, and as such solar conversion efficiency is only at peak levels at midday.

In order to accurately assess these values, they must be cosine adjusted, to account for the change in solar radiation levels on a given surface area as the sun passes across the sky each day.

Cosine Adjusted Transversal IAM Values (IAM Adjustment)

Because of the round shape of the solar tubes, the absorber passively tracks the sun from 40o either side of midday (9:20am to 2:40pm). The cosine adjusted IAM values confirm this, as the collector maintains a value of ~1 up until 40o, beyond which the tubes start to overlap and the relative surface area decreases. Flat plate collectors, and other collectors with flat absorbers display a fairly standard bell curve, only peaking at midday.

To understand how the tubes passively track the sun throughout the day, refer to the diagram to the left.

When looking at the tubes from above (0o) each tube's surface is clearly visible, and therefore exposed to the maximum amount of sunlight. At this angle however some light is lost between the tubes, and therefore because this is used as a reference point for the IAM value of 1, when the gaps close up, the IAM value with actually increase (a greater % of light shining on the collector is actually being absorbed).

When the sun reaches an angle of 40o which correlates to 2h 40min before or after midday, the solar tubes are still fully visible with no gaps between, and no overlap. It is at this point that the pure IAM values reach their peak. The tubes are exposed to all the sunlight shining towards them, and all the tubes are still perpendicular to the sun. This is why even at this point the cosine adjusted IAM is still 1.

As the angle increases, the tubes start to overlap, shading each other. They are still facing the sun, but the actual surface area of absorber exposed to the sunlight is reduced. Only a small amount of sunlight falls beyond 40o (early morning and late afternoon), and so this decrease in surface area has minimal influence on the total daily energy output of the collector.

IAM Adjustment

When calculating the heat output of a collector, the cosine adjusted IAM value should therefore be included in the formula.

Heat Output = Performance x Cosine Adjusted IAM value x Insolation x Absorber Surface Area

Example:

Performance = 66.3%
Cos Adj IAM at 30o = 1.02
Insolation = 800 Watts/m2
Absorber Surface Area = 2.4m2

Heat Output = 0.663 x 1.02 x 800 x 2.4 = 1298.4Watts

So the collector will provide 1298.4 Watts of heat output.

Simplifying IAM Adjustment Calculations

The calculation completed above is only for a specific point in time, and does not give an indication of the the actual performance over an entire day. Using performance modeling software, hour by hour calculations can be made taking into consideration average daily temperature changes, cold water temperatures, hours of sunlight, solar insolation levels in addition to collector performance variables and cosine adjusted IAM values. Monthly and annual average performance values may therefore be estimated.

To complete a simple single day calculation for the purpose of comparing collector performance, an average IAM value can be use, along with an average Watt/m2 value. Although this won't give a completely accurate indication of the heat output for the day, it allows a comparison between the two collector to be made.

As the majority of useful solar radiation falls during the middle 6-7hours of the day, an average of the IAM values during this period can be used. If 1 hour corresponds to 15o then 7 hours corresponds to 50o either side of midday. The average cosine adjusted IAM for the AP solar collector for this period is 1, and a flat plate collector is 0.83 (see table here). These factors can therefore be used in the performance formula. See the following section for more details.

Putting it all together
How do I compare the performance of different collectors?

When comparing collectors, it is better to use efficiency values from the normal operating range rather than peak efficiency levels, as this will better represent average annual performance. The "normal operating range" refers to the normal Delta-T range (Tm - Ta) that the collector is exposed to. For domestic water heating an average value of 30-40oC is common.

Every region has different ambient temperatures and different insolation levels, but for the purpose of a comparison we can use a "standard" set of environmental conditions.

In a moderate climate, an "average" intermittently clouded day in Spring can provide an insolation level of 3.5kWh/m2/day. The solar radiation distribution throughout the day from sunrise to sunset is displayed in the following graph.


It can be seen that 90% of the radiation falls between 9:00am and 4:00pm with an average insolation level during this period of 400W/m2.

We now have a full set of factors in order to do a comparison:

1. Insolation Level = 400Watts/m2 (G)

2. (Tm-Ta) = 35K
3. (Tm-Ta)/G = 0.0875(x)
3. Apricus AP:
- Performance variables: h0 = 0.717, a1 = 1.52, a2 = 0.0085 (SPF)
- IAM adjustment = 1.0(M)
4. Leading Flat Plate:
- Performance variables: h0 = 0.8, a1 = 2.99, a2 = 0.023 (SPF)
- IAM adjustment = 0.83 (M)

Remember the formula from earlier? To the end we just need to add the IAM adjustment (M).

The calculations for the two collectors are therefore as follows.

AP: Performance = 0.717 - (1.52 x 0.1) - (0.0085 x 400 x 0.0875 x 0.0875) x 1.0= 53.9%

FP: Performance = 0.8 - (2.99 x 0.1) - (0.023 x 400 x 0.0875 x 0.0875) x 0.83 = 35.8%

Given the set of variables used, the AP solar collector provides 33.6% greater heat output for a given absorber area.

The same calculation can be completed with other collectors using performance variables and IAM values

SPA: Active Closed Loop Solar Water Heating Systems

Closed Loop Solar Heating Systems are suitable for single and multible solar heating application systems,e.g.domestic solar water heating,solar water heating hot tub,solar swimming pool heating or solar space heating systems.The Closed Loop Solar Systems are suitable for areas with questionable water quality and all climate conditions.The Closed Loop Solar Heating Systems are the preferred option for extremely cold areas.

Operation
- The controller(18) will switch on the pump when the temperature at the collector sensor TC is higher than the return temperature TR by at least the pre selected(delta T)amount.
- The pump circulates a heat transfer liquid around the loop.
- Heat from the collector is transferred to the domestic water through the heat exchange coil in the tank.
- With the pump running,if delta T is not met,the pump will switch odd.
- When a present tank temperature is reached at Tmax,the controller switches off the pump.
- The check valve(non-return valve)prevents heat from the tank fising towards the collectors should the tank be warmer(e.g.at night).

Place your mouse on the pictures above to see what each represents
PO-D solar water heater is not designed for mains pressure. The maximum pressure rating is 0.4 bar, which is equivalent to 4m of head. For this reason the system should be supplied with water by a small gravity feed tank located no more than 4m higher than the collector automatic filling valve is available which can provide the tank with automatic water top-up with pressure from a mains water supply.
PO-D solar water heaters are suitable for open flow configuration. It could also be used as closed loop, supplying heat via a heat exchanger, but a circulation pump would be required as mains pressure pre-heated water is required, the SPHE-C solar collector may be a better choice.
A circulation pump is not normally required.
The only reason you would need a pump is if:
1. The mains cold head pressure is insufficient for the cold water to reach the solar water heater or feed tank a small pump controlled by a water level switch in the feed tank can solve the problem.
2. The height from the solar water heater to the hot water tap is not enough to provide pressure hot water. In this case a small pressure pump with built in flow switch on the hot line would provide higher pressure hot water(may need a thermostatic mixing valve prior to pump as some pumps cannot handle temperatures above 60?C).
3. The system was used in a closed loop-see the section above.

Freezing is a concern for any solar water heater, but with the PO-D, mild sub-zero temperature is not a major problem. The solar tubes are very well insulated (vacuum), as is the tank (5 polyurethane). Furthermore, the tank is full of warm or hot water. It would take several days of weather day and night to freeze the water. Also ice floats and so would not form in the evacuation tube, but on the surface of the water in the tank.
As long as there is some sunlight during the day, the water temperature would increase and preventing freezing at night. If you have sub-zero temperatures day and night and virtual sunshine, then it might be necessary to empty the system of water (including the water in tubes) for such periods each year. Glycol can be used in the system to prevent freezing, but such the system could have to be a closed circulation loop. In such case it would be better to use the sun power solar water heater.
The advantage of solar tubes is that they insulate the inner tube form heat loss. This means once heat is absorbed, it is transferred to the water in the manifold, and not lost to the out environment. Because the water is in direct contact with the inner glass tube the heat transfers very well. The performance of the tube itself can reach 80%, but because of the heat losses of the tank, the total system efficiency is around 70-75%(depends on ambient temperatures)

The AP solar collector is designed to be used with pressure up to 8 bar/116psi. This means it is compatible with all low pressure, and most mains pressure domestic hot water systems. In closed loop or sealed (dead water) thermal store systems an expansion vessel is often used to prevent pressure buildup as the water expands. A pressure release valve should also be used as a safety backup.

In areas where freezing is not of concern, open loop systems are often used. Open-loop systems are also appropriate for cold regions when used in combination with a Delta-T controller that incorporates a freeze protection feature. Closed loop systems usually incorporate the use of a heat exchanger, either inside or outside the hot water storage tank. Apricus solar collector are suitable for both open or closed systems, as long as pressure, heat and freezing are controlled.

The AP solar collector does not have a built-in tank, in fact the manifold of the 20 tube solar collector only contains about 510ml/1pint of water. A circulation pump is required to circulate the water through the manifold and back to the solar storage tank. Generally a Delta-T controller is used to control the pump. A flow rate of only 2L/min is required for most domestic installations, and therefore a low wattage pump is sufficient. Larger pumps are only necessary when several solar collectors are connected in series, or when the pump is required to overcome head pressure. The pressure drop at low flow rates is very minimal, only 700 Pa @ 3.3L/min for 20 tube manifold, and so is not a major consideration when sizing pumps.

Thick glass wool surrounds the AP solar collector's copper header, providing excellent insulation. The piping to and from the collector are however still susceptible to freezing, and therefore traditional freeze protection should be employed (low temp controller setting, or glycol/water closed loop). Solar tubes and heat pipes are able to withstand extremely cold conditions without being damaged. (click here for heat pipe details). Top of page

The advantage of solar tubes is that they insulate the inner tube from heat loss. This means that once heat is absorbed, it is transferred to the water in the manifold, and not lost to the outside environment. This is the key difference between solar tubes and flat plate solar collectors: the insulative properties. Combined with the heat transfer efficiency of the heat pipe, the AP solar collector can deliver excellent heat output all year round. The IAM (Incidence Angle Modifier) values of the AP are also very different to solar collectors with flat absorbers. The positive (>1) IAM values mean that the solar collector actually performs best mid morning and mid afternoon, resulting in a more stable heat output throughout the day. Click here to learn more about efficiency.
When installing a solar collector on your roof, how it looks is certainly important. The AP solar collector is designed to be low profile, sitting close to the surface of the roof. The tubes are black and so blend in nicely with most roof colours. The manifold is available in black, dark brown, or silver powder coated aluminium, and with either rear (R) or end (E) port models. The rear port manifold allows the plumbing to be hidden behind the solar collector manifold. In addition by using rear ports, two or more solar collectors may be connected side by side without a gap in between. End ports may be preferred for large scale applications for ease of connection in series, and reduction of pressure drop through the piping. Click here to see some installation photos.

Corrosion is always a consideration for any system that involves water and high temperatures. In warm environments, heavily chlorinated water can become a strong corrosive agent. In order to provide maximum corrosion resistance, the AP solar collector uses high purity (99.93%) copper piping and silver braze for the header. Copper provides excellent corrosion resistance and is commonly used in household plumbing. If corrosive liquids are to be used in the system, then a closed loop is highly recommended, thus allowing a non-corrosive liquids to be used in the solar collector loop.
If installed in open flow with a dead water thermal store style tank, corrosion and scale are almost eliminated as the system accepts almost no fresh water supply.Top of page

The high cost of solar tube style collectors, and in fact all solar collectors, has been a major obstacle to their popularity and wide scale use. The AP solar collector is a high quality system that provides excellent heat output and reliable operation. As a result of clever product design and low manufacturing costs, AP solar collectors are now very affordable, providing fast payback times.*
Please contact you local agent for retail pricing in your area.
*Depends on factors such as total system cost, energy costs and solar insolation levels.

Scale formation is an issue in many regions, as it gradually blocks up plumbing particularly in hot water systems. With the high temperatures that the AP solar collector produces, scale formation in the manifold may occur. If water supply is very hard there are two main options:
1. Use an electric or magnetic water softener on plumbing
2. Use a closed loop system
3. Use an dead water thermal store configuration. Option 1 may still be required to protect the rest of the system.
A closed loop requires a more complicated system design and added cost. If there is no other reason to use a closed loop than to avoid scale, then it is advisable to use one of the widely available water softening devices.
AP solar collectors are ideal for large scale solar water heating applictions, able to be used in hotels, airports, apartment buildings or anywhere where hot water or heating is required. The economics of large scale applications are generally more favorable than domestic, as instead of having a pump and tank for every one or two solar collectors, a single tank and pump can be used for 50 solar collectors. AP solar collectors can accept mains pressure, are corrosion resistant and can be installed in series and/or parallel, thus are suited to a wide range of large and small scale applications.

The cost involved in repairing a household appliance is increasing all the time, not due so much to spare part costs, but rather the labour cost involved in having a technician visit. With this in mind the AP solar collector is designed to be maintenance free, but if for some reason a solar tube should ever be damaged, it can be replaced quickly and easily by any person possessing basic trade skills.