Solar Water Pumping Module 3

Introduction to Solar Pumping System Design Considerations

The purpose of this module is to familiarise a potential user with some basic issues in the design of a solar water pumping system. It is not intended to enable an accurate and complete system design. This must be left to professional pumping designers and installers, who have extensive information on all required components that are required for a complete system design and successful installation. There are 9 general steps in completing a simplified solar water pumping system design:

Step 1 - Determine the available solar resource
Step 2 - Calculate the required daily water usage
Step 3 - Calculate the required flow rate
Step 4 - Establish general information on topography
Step 5 - Calculate the total "head"
Step 6 - Calculate the required pumping power
Step 7 - Estimate additional system losses
Step 8 - Determine the array size
Step 9 - Select a suitable pump and motor

Design Step 1- The Solar Resource

Solar Energy (the solar resource) needs to be established for the site of the pump – the map below indicates the range of solar conditions throughout Australia – Better resolution can be obtained from Bureau of Meteorology solar irradiation maps.

From the above data it is obvious that a fixed power solar water pump is going to pump far more water in mid Western Australia than in Tasmania. To complete a rough system design calculation, the first value you should have is THE AVERAGE NUMBER OF FULL (OR PEAK) SUNSHINE HOURS FOR THE MONTH OF DESIGN FOR THE LOCATION CHOSEN (a nearby figure is acceptable if the precise location data is not available). For an exercise use a figure of 6.0 peak sun hours.

Design Step 2 - Daily Water Requirements

The daily water requirement for the application is obtained by adding up all of the applications for water over a given period (usually 24 hours). There are charts available from Agriculture Departments etc. indicating the drinking requirements of both stock, crops and other users of water. The software chart below gives some indication of this activity

At the end of this activity, the required litres per day (24 hours) is known. For an exercise use a figure of 4300 litres per 24 hours .

Design Step 3 - Calculating the Flow Rate

Finding the flow rate (in litres per second) is required in order to calculate items such as friction loss in the pipeline and the capacity of the pump to be selected. Using the selected example, 4300 litres over 24 hours (this volume must be delivered during the pumping hours i.e. the solar peak sun hours). Therefore 4300 litres must be pumped in 6 .0 sun hours = 716 litres per hour = 0.2 litres per second.

Design Step 4 - Topographical Information

The topography conditions at the site are is an important factor in a good system design , and should be obtained as accurately as possible. The total head includes components in addition to the height that the water needs to be pumped. For example the system in the figure below shows a subsurface pump delivering water to a tank above ground level. The topography conditions at the site are a very important factor in a good system design, and should be obtained as accurately as possible. For this application, the topography can include such information as distances between system components and variation in terrain heights.

The Figure below is similar to the Basic Pumping System Layout from Module 1. For this example exercise the following values are used. K = the (effective added) friction loss head. J = 5m and is the sub-surface depth to water, I = 2m and is the drawdown of the resource, L = the length of pipe and the type of pipe to be used (1200m, class 9, 24mm), and H = 8m and is the head. (All values in the Figure are in metres)

Design Step 5 - Total Head Calculations

The total head for pumping = the static head plus the dynamic head (all in metres). The static head = 8 + 5 + 2 or 15 metres. The dynamic head is calculated for a pipeline of length 1200 m and 25 mm diameter, class 9. at the flow rate of 0.2 litres per second.

Using an appropriate pipeline friction loss chart * for these conditions , a figure of 0.41 m head to add per 100m of pipeline
i.e. for 1200m of pipeline, add 12 x 0.41 m = K Value 4.92m.

*Pipe frictions for Class 9, PVC pipe (metres head/100 m length of pipe)

Flow (litres per	Nominal pipe size (mm)
second)	25	32	40	50	65	80
0.10	.12
0.15	.25
0.20	.41	.17
0.30	.83	.27	.15
0.40	1.40	.46	.24
0.50	2.10	.66	.35	.12
0.75	5.00	1.06	.85	.29	.10
1.00	7.00	2.25	1.20	.41	.14
1.25	10.20	3.40	1.77	.61	.22	.10
1.50	14.30	4.78	2.50	.85	.29	.13
1.75	19.40	6.28	3.40	1.02	.38	.17
2.00	24.50	7.75	4.12	1.40	.48	.22
3.00		16.20	8.50	2.85	1.00	.45
5.00			21.80	7.30	2.50	1.13
10.00				26.00	8.60	3.85

Charts similar to these are available from Agriculture Departments and / or poly pipe manufacturers

From the table above a value of 0.41 m dynamic head per 100 m of pipeline is determined for the example system. As there is 1200 m of pipeline in the example system it is simple to calculate a value for dynamic head.
i.e. K = [(1200 m * 0.41) / 100 m ] = 4.92 m . The total head for pumping is now 15 m + 4.92 m or 20 m with rounding off. If rounding off is used always increase the value. This gives the design some "reserve".

The flow rate is 0.2 litres per second. Changing the type or length of pipe will affect the dynamic head - if for the example system we chose 32 mm pipe the dynamic head would be approximately 2 m. This is an important system design factor that needs to be considered.

Design Step 6 - The Required Pumping Power

Please note – this exercise is only a “ball park” design calculation – there are many other considerations that a total professional solar water pump design would include – items such as water temperature , pipeline pressure vs class of pipe, more accuracy in details given , pipeline pressure allowance , temperature variation with the PV array, possible module soiling and shading and electrical and heat losses, as well as motor characteristics.

The final design achieved by using this (simple) method if done correctly should be relatively close (around 15 %) to a professional design however , but should be used as a reference guide only for those wishing to understand the principles of solar water pumping.

Design Calculations

The total head for pumping is now rounded off at 20 m, and the flow rate is 0.2 litres per sec.

A simple formula (the ‘hydraulic” formula) can now be used to calculate the required power into the motor drive - this is not the true array power needed, as other considerations still have to be included – type of module temperature effect, losses in the MPPT and controls etc.

Formula : P (the power into the motor drive) = 10 x FR x TH / SE

Where:
FR = flow rate in l/s,
TH is the total system head in metres
SE = the pump/motor efficiency as a decimal

For our example, the power required in watts is = 10 x 0.2 x 20 / 0.4 = 100 Watts.

System Design Step 7 - Estimate Additional System Losses

These estimated losses include electrical losses, maximiser losses, typical PV deratings for temperature and other system design variables. There may also be some additional losses after the pump, due to poor topography detail being used etc., but this could occur even with a “professional design”.

System Design Step 8 - Determine the Array Size

Using the calculated figure for P ( watts ) of 100 , it would seem that a further 20 % should be added for the losses before the motor drive stage (estimate only), so a rated array power of some 120 Watts should meet the system requirements of the example pumping system.

System Design Step 9 - Select Pump and Motor

The final stages in system design would be to select a motor of around the power rating calculated (say 100 Watts or 1/8 HP) and to ensure that the pump selected would pump at around 0.2 litres/sec or 720 litres per hour when the motor is running at its rated speed and power. The need to match well here is critical.

It is essential to select components that have been designed to be used as a system. The use of components such as a MPPT would increase the daily output,as should a mechanical array tracker, but in a small system such as this example system, the costs may outweigh the benefits. However larger solar water pumping systems the use of mechanical trackers or MPPT's will reduce the number of required PV modules. Cost benefit analyses of these approaches need to be considered by comparing the additional costs of the extra components with the expected gains in water pumped.

In this example design other environmental factors have been ignored. These included factors relating to temperature and variable water resources. The drawdown is the amount of water that is available above the pump intake - in situations where the pump flow rate is greater than the natural flow of water into the well there is a rise that the well will run out of water and potentially cause damage to the pump.

In conclusion the overall learning outcomes of this approach to solar water pump design are :

For high volumes, use large diameter piping or shorter pipelines to minimize friction losses where possible
The static head should be minimized if possible
Pipelines should be buried in hot climates
Drawdowns must be considered for pump safety
The equipment used should be as efficient as possible
Most medium to large systems should use trackers and /or maximisers as long as they are economical
Getting the site detail as accurately as possible is critical
Getting the solar resource data as accurately as possible
Always err on the side of safety – do not underestimate!

This is the end of Module 3.

Click here to Return to the Solar Water Pumping Page to attempt the Module 3 Quiz.

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