SOLAR PUMPING- How to size and select

So how do you size your solar pumping system? The key parameters that one needs to look at in designing a solar pumping system include the following [1]:

1. Water Demand and Design Flow-rate

2. Water Storage Requirements

3. Total Pump Dynamic Head

4. PV Panel array size

5. Pump Controller

Water Demand: this is a measure of the amount of water needed per day. This can vary with use. For example in a urban domestic house it can be 100litres/person/day.. or can be 5000litres per day for irrigation at a farm. So an urban family of say 8 people would need 100x8= 800litres of water per day. Since solar pumping systems use the sun as a source of power, the flow-rate of the required pump will come from total daily demand divided by pumping hours. The pumping hours used should be average sunshine hours (ideally winter averages which are worst-case). So for example assuming our case study domestic household is from Harare, with winter effective sunshine hours of 5 hours, the design flow-rate of the pump= 800/5= 160l/h. PLEASE NOTE THAT WATER YIELD CALCULATIONS ALSO HAVE TO BE DONE TO ENSURE THAT THE WTAER SOURCE (e.g borehole or well) EXCEEDS THIS REQUIRED FLOWRATE.

Water Storage Requirements: Water Storage should usually be water adequate enough for 2 to 3 days. So for our Harare family, they would need a storage tank of size =800 x 3= 2400litres.

Total Pump Dynamic Head:

It is not enough to have a flow-rate (amount of water to be pumped), the pump must be able to “push” the water up into the tank against resistance of gravity, fittings and pipe friction, This “resistance” is what is called Pump Head. The head is determined from a combination of pumping head, discharge head and friction head (as shown above). The pumping head/static water head is the height from the water level (average lowest, NOT depth of borehole) in the well to the surface. Discharge head is the height from the surface of the ground to the inlet of the pipe into the tank (NOT height of tank-stand). Friction head is a combination of pipe friction losses and losses at different fittings along the pipe run, these are calculated from graphs and fitting friction loss charts/tables.

Pump Selection: Once you are “armed” with the flow-rate and Total Pump Dynamic Head, you can now go to the pump catalogue and select the appropriate pump. Ideally this should be a pump that operates at a high efficiency, and with the lowest possible power consumption (to limit solar panel requirements). Look at the different pump curves (see typical below), and select the pump within the required duty point (flow-rate and head) and operating at a good efficiency.

Solar Panels Sizing: Solar panels should now be selected to match the pump’s power consumption (panels slightly oversized by 25-30% to allow for bad weather days) and should also match the pump voltage

Pump Controller: From the pump controllers’ catalogue, you simply select a controller that matches your pump and PV array specifications. Safest way is to look at the pump manufacturer’s catalogues and use their recommended controllers. The means of pump actuation (turning on and off) will vary from using a float switch, pressure tank, level probe etc

For FREE sizing and selection guidance, please get in touch with our team. The range of solar pumps we have can be found on our online shop https://www.rentechsyst.com/shop , or whatsapp catalogue https://wa.me/c/263713229721

Sources:

1. World Bank Group- Solar Pumping The Basics

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About Author:

Lovewell Chitiyo is a Mechanical Engineer with 15 years experience in the building services Engineering consultancy, renewable energy systems and sustainable design fields. He is currently studying Masters in Renewable Energy at Stellenbosch University in South Africa

Lovewell is an Accredited Professional with the Green Building Council of South Africa for Existing Buildings Performance and New Buildings; Interiors, holding several qualifications and certification in Renewable Energy and Sustainability.

Lovewell also has vast experience in design, supervision, installation and commissioning of several projects locally and regionally in the last 15 years.