Working Princible of Pressure Systems

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.

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