Solar Design

Sizing Pipework following IDHEE methodology

Minimizing pipework diameters helps increase the efficiency of solar systems, by

• Reducing the thermal inertia of the solar system so that it better react during intermittently cloudy weather.
• Reducing the surface area of the pipework so that lower thermal losses ensue.
• For smaller diameter pipework, it is possible to economically insulate beyond the "wall thickness > internal pipe diameter" guideline, further reducing thermal losses.
• Minimising the amount of liquid held in pipework that undergoes a cool/heating cycle at least once per day when the panel is working.

However consideration must be given to the following conditions;

• Whether sufficient flow rate is possible to transfer heat from the panel during maximum insolation conditions efficiently.
• Polypropylene Glycol is by its nature more viscous and leads to higher pumping losses, so the percentage of Polypropylene Glycol used in the solar loop should be considered. Polypropylene Glycol also has higher insulating properties so a solar coil/heat exchanger performs less well as the percentage of Glycol increases. Designing a solar system for Ireland to deal with the worst expected German winter conditions leads to a more expense and lower performance.

REIA Guidelines.

A correctly designed solar system should lead to a 6 to 15C rise in temperature during full output. Many panel manufacturers suggest reducing the flow rate on the pumping unit to between 0.5 and 1.0 litres/minute per aperture m2 of panel.

Example:

A panel has 3kW (typical 6 meters squared flat plate) output during full sun conditions. To meet the REIA guidelines the flow rate can be calculated as follows;

3 kW = 3000 Joules of energy per second or 180,000 Joules of energy per minute.

It takes approx 4200 Joules to heat 1 litre of water by 1 C. So one litre of water flowing through the panel per minute would increase in temperature by

180,000 / 4200 = 42.85 C.

3 litres of water flowing through the panel in one minute would lead to the following temperature rise.

180,000 / (4200 x 3) = 14.28 C which is near the upper end of the REIA guidelines

while 6 litres per minute gives the following;

180,000 / (4200 x 6) = 7.14 C, which is very close to the lower end of the REIA guidelines.

This ties up nicely with the rule of thumb given by many panel manufacturers of allowing 0.5 and 1.0 litres/minute per aperture m2 of panel.

Using pressure loss tables to calculate pipe sizes.

The picture below is a screenshot of the pressure loss tables for 8mm, 10mm and 15mm copper tube. The column on the left denotes the flow rate in kg/s which roughly equates to litres/ second. The right hand column denotes the pump head loss for each meter of tube in the circuit.

e.g. Taking the 10mm column, at a flow rate of 0.55 kg/s each meter of tubes offers resistance of 0.139 meters head per meter length. 0.55 kg/s can be converted to kgs/minute by multiplying by 60.

= 3.3 kg/minute = 3.3 litres / minute

Pump Characteristic

A typical pump characteristic is shown below, solar systems always have MUCH lower flow rates than heating systems that plumbers are more familiar with.

As a result, the pumping lines are very near vertical where a solar system operates. Changing the pump setting from 2 to 3 only results in a very small increase in flow rate.

A red line has been added at 0.55 kg/s. If the pump is set to setting no. 1 then the pump is capable of pumping 0.55 kg/s at a 2 meter head loss. Looking at the table above again each meter of tubes offers resistance of 0.139 meters head per meter length at this flow rate.

Dividing 2 by 0.139 gives the equivalent pipe length possible. 14.38 meters (or 7 meters flow + 7 meters return).

At setting no. 2 then the pump is capable of pumping a head of about 4.25 meters, giving a equivalent pipe length of 30.57 meters.

Reducing the flow slightly.

Reducing the flow rate slightly can lead to surprising increases in acceptable pipe runs. This is because pipe resistance increases with the Square of the water velocity. (the red line above shifts very slightly to the left, but this is ignored here)

Looking again at the table above and taking a 3 litre/ minute as a design point.

This has a head loss of 0.118 meters per meter, giving the following pipe runs.

For pump setting 1

Dividing 2 by 0.118 gives. 16.94 meters (or 8.5 meters flow + 8.5 meters return).

For pump setting 2

Dividing 4.25 by 0.118 gives. 36.01 meters (or 18.0 meters flow + 18.0 meters return).

Following the pump curves.

Normally, the design point will fall between two curves as in the example below. 3.2 meter head at 3 litres/minute (or a total equivalent pipe run of 27 meters).

To find out what the real flow rate will be at the various pump speeds it is necessary to draw a new pump curve as parallel as possible to it's nearest neighbour and follow the curve up to the pump speed line intersects

Because of the steepness of the curves the flow rate will only increase by a few percent and there is very little difference between speed 2 and speed 3. However speed setting 3 can use significantly more electricity.

Putting the pump on speed setting 1, and by following the blue line down to the intersect, we can see that the flow rate will reduce by about 1/3.

Allowances for Pipe Fittings.

In heating systems with elbows etc. distances are reduced by 25% which actually ties up quite well with experience. In solar systems, keep sharp bends and fittings to a minimum, if any are present then it is recommended that the 25% pipe-lenght reduction is considered during design.

Allowances for Polypropylene Glycol.

One forgiving feature of Polypropylene Glycol is that the viscosity reduces and it becomes easier to pump at higher temperatures. At 60C a 20% mix of Polypropylene has the same pumping resistance as water at 20C.

However more importantly a higher portion of Glycol gives lower heat exchanger efficiency by virtue of its lower thermal conductivity.

Freezing protection should take this feature into account and perhaps a combination of a lower concentration of Glycol with the frost protection setting on the controller offers the best compromise.

   

Ethylene Glycol is used in car radiators as an anti-freeze, however the ingestion of just 2 tablespoons of Ethylene Glycol can be fatal in adults, and unfortunately because of its sweet taste, animals or children may consume large volumes if exposed to it. DO NOT USE IN ANY CIRCUMSTANCES