Solar Pumps, Picking The Right One

Solar Pumps are an efficient and economic alternative to traditional pumping systems. This five step process will help you pick the right pumps for your application.

The solar inverter selection determines the solar pumping cost and efficiency. For example, for a given deep well pump you may find one inverter will need 5KW PV plant and the other may require 7KW PV plant – the difference 2KW costs approximately $4000. Here are 5 factors that affect solar pumps cost and performance you may consider to make the right investment in solar pumps.

Solar Pumps 5 Steps To The Right Pump

1. Solar Pumps with Specialized Pumping Inverter

Conventional solar inverters generate common power that can be used in any applications. They are not suited for Pumping -they will require additional investment in pump power and speed controller called Variable Frequency Drives (VFD). Without VFD, Solar inverters will require almost double PV power to supply the high current required to start-up pumps.

Solar Pumping inverters (SPI) have built-in VFD with soft starter. They gradually increase pumps speed (or RPM) to reach the maximum flow level with minimum power. SPI decrease PV plant size by 30%-to-50% compared to conventional solar inverters. Additionally, SPI can allow you to control flow rate and pump RPM.

2. Brand Independent Solar Pumping Inverter

Other Solar Pump manufacturers offer Solar Pump Controllers that can operate only with their brand of pumps. The pumps cannot be replaced by another commercially available three phase pump. These pumps are more expensive than commercial pumps to replace, and are 10-15% less efficient. Replacement may have wait time 4 weeks or more. It is essential to have solar pumping inverters in solar pumps that operate all pumps. Such universal Solar Pumping inverter are available in the market from reputable companies like Solaropia Inc. in USA.

3. Higher Efficient Pumps to Reduce PV Plant Size

Solaropia Solar Pumps (SPI) can operate any pump type, this feature will allow to select pumps with the highest efficiency for solar pumping. 10% difference in efficiency means decreasing the cost of PV system by the same amount. For guidance on pumping efficiency, make sure that manufacturer efficiency is the total pump and Motor efficiency.

Other pump manufacturers provide pump and motor efficiency rates separately. The total pumping system efficiency is lower than both. It equals their multiplication. It is recommended to have this total efficiency > 0.60 Higher efficiency pumps are more expensive than lower efficiency pumps. In Solar Pumping PV composes 75% of the total solar pump cost.

4. Solar Pumps that can operate in Hybrid Mode

Some solar pumps can also operate in conjunction with an AC source like diesel generators or even utility grid power. Make sure that SPI specification of the solar pump can operate in Solar and Hybrid mode. Also make sure that SPI in hybrid mode will use Solar as the main power source (higher priority) and AC as complimentary source so that SPI uses all power available from the sun and only portion of the AC power. Some SPI are equipped by Auto switching capabilities so that when solar PVs power is 100% available it switches OFF the Diesel Genset, and turns it back ON when solar PV power is shorter than what the pumps power requirement to run at specified RPM.

5. Select the Right PV Size Following your Pumping Application

Selecting the optimal PV size is vital for overall solar pumping performance. Higher PV size will increase the system cost, and lower PV size will decrease the pumping flow rate productivity. Customers can run easy engineering calculations by themselves to determine the right PV size for their pumping application. Here are few steps how to do it:

Starts with your pumping application and define the required flow rate (Q) in gpm, or m3/h and the pumping head (H) in feets or m or PSI.

Refer to pump manufacturer to select the pump model and its power for given Q and H . Many manufacturers offer on-line tools (like Grundfos Quick Pump Sizing Tools) that provides chart with the powers marked with P1:Max Pump Power, and P2: Operating power to deliver the required Q for given H.

Use 50% merging of the operating power P2 (KW) to calculate the required PV size. The 50% margin composes all loses in PVs operating under NOCT (Normal Operating Condition Test). See a typical PV sizing example for Solar Pumping below

Sizing Required PV power for Solar Pumps using Solaropia SPI

The solar pump is required to deliver flow Q= 100 gpm (≈ 25 m3/h) from well at total Pumping head H= 200’ (≈50 m).With these pumping parameters we refer to select submersible pump type from many available in the market. We will use the Quick Pump Sizing from Grundfos that generates the pump flow –power chart as the one shown below:

The flow –power chart provides P1= 7.5 KW and P2= 5.57 KW. The PV plant may be sized at 1.5 times factor when efficient SPI pumping inverter for 7.5 KW (SPI-7.5) like the ones supplied by Solaropia for this pump . The PV plant size is then will estimated at 9KW (PV= P2*1.5= 5.57*1.5 ≈ 9KW PV)