Pump

- input data - pumping systems - pump weight: 30 title: Pump

Definition of a pump

The pump device is considered a black box, with Current and Voltage inputs on the electrical side, and Head and Flow Rate values on the hydraulic side. Detailed technical specifications of the motor-pump assembly are not required.

In PVsystBasic, many pumps are associated with a power converter, which must be included in the definition of the pump device. In these cases, the electrical input variables are those of the power converter.

Pump Technologies

There are two main classes of pump technologies:

Centrifugal Pumps

Water is set in motion by an impeller rotating at high speed. The pump must reach a sufficient rotational speed to provide the head required by the external system. Efficiency is primarily related to flow rate. It follows an increasing curve (with zero efficiency at zero flow) up to a maximum, which generally does not depend much on the head. Beyond this maximum, the drop in efficiency is more pronounced for lower heads.

Flow rate exhibits a quadratic relationship with power, with a power threshold dependent on the head; this threshold corresponds to the minimum speed required to reach the head of the external system.

Centrifugal pumps are suitable for systems with relatively low heads and high flow rates.

To extend the head range, many pumps use a multistage design, consisting of several impellers mounted in series on the same motor shaft, each providing a portion of the total required head.

Positive displacement pumps

In a positive displacement pump, water is enclosed within a sealed, moving chamber, either by means of valves or through moving parts with specific geometries. Thus, as soon as the pump rotates, a certain amount of water is displaced, and the flow rate is directly proportional to the pump’s rotational speed. The power threshold is due to electrical losses in the motor before sufficient force is reached to overcome the resistance torque.

In many pumps, the starting torque is greater than the steady-state torque (since friction losses are higher when the pump is stationary), which requires a surge of current at startup.

There are several technologies:

• Piston pumps, where a reciprocating piston in a cylinder draws water in from the inlet or pushes it out to the outlet using check valves.
• Diaphragm pumps, which operate similarly, but where the piston seal is replaced by a movable diaphragm.
• Positive displacement pumps, using a special screw-shaped rotor in a cylinder, which traps a volume of water in the inlet chamber and pushes it along the tube to the outlet.
• Rotary positive displacement pumps, consisting of a rotor resembling a vane wheel rotating in a cylinder equipped with inlet and outlet ports.

Positive displacement pumps are well suited for systems with high head. Their efficiency is generally fairly constant across different flow rates.

Surface pumps and submersible pumps

"Conventional" surface pumps consist of a motor-pump unit, which is not necessarily integrated into a single housing, allowing different types of motors to be coupled with different pumping devices. The pump must be installed within a limited distance from the water source (and at a maximum head of approximately \(5 \mathrm{m}\) of water to avoid cavitation issues). Accessibility for maintenance is not an issue. However, being positioned above the water level often requires a priming procedure, as well as certain precautions to prevent air ingress.

For boreholes, submersible pumps must be installed at the bottom of the well. These must, of course, have a cylindrical shape adapted to the borehole’s diameter, and the electrical components must be completely watertight throughout the equipment’s service life. The technical requirements are more stringent and the quality must be much higher, as maintenance is difficult. Consequently, the price of these pumps is generally much higher than that of surface pumps.

Furthermore, it is technically very difficult to install multiple pumps in a single borehole.

Nevertheless, there are now highly sophisticated submersible solar pumps on the market, some of which even incorporate the power converter and accept a very wide range of input voltages. These significantly simplify system design.

Pump Data

General

Pump Device Identifiers

• Model and Manufacturer are identifiers that will appear in the pump selection lists.
• Data source generally refers to the primary source of the parameters (most often the manufacturer, but it can be an independent institute or your own measurements).
• File name: it must have the ".PMP" extension. You can create a new pump device by changing the file name.

Electrical Side

Definitions related to the pump device input, treated as a black box:

• Define the motor type, specifically whether it operates on alternating current (AC) or direct current (DC).
• Define the power converter type, if applicable.
• Define the nominal voltage (at nominal head) and the nominal (or maximum) power ratings for the three head values specified in the hydraulic panel.
• These nominal or maximum power ratings are not always clearly defined. They will be used in the simulation as nominal values (the absolute maximum limits must be defined in the “detailed” parameters). Some technical data sheets list nominal values and maximum values only for exceptional conditions.

!!! "Please Note" Info These voltage and power values refer to the pump motor itself in the absence of a converter, but to the converter inputs if one is present.

Hydraulic Side

You must define:

• The pump technology, specifically whether it is centrifugal or positive displacement
• The pump configuration
• The minimum head: generally the minimum head for which the manufacturer provides data.

• The maximum head: generally the maximum head for which the manufacturer provides data. It is not an absolute limit: if the system’s operating conditions require higher heads, the model’s results will extend up to the required value.
• Nominal head: this value is not precisely defined. It must correspond to the head most suitable for the use of this pump. For centrifugal pumps, it can be chosen as the head corresponding to maximum efficiency. Otherwise, it can be an intermediate value (tending toward the higher values) between the minimum and maximum heads.

Once the model is fully defined, the dialog box will display the flow rates and efficiencies corresponding to these heads, as well as their corresponding rated power, as defined in the electrical panel on the left.

Performance Curves

You can first select the units for manometric head and flow rate, based on your original data.

Next, for each curve, you must select the parameter (Pressure or Head).

To construct a curve, it is recommended to place the data points at their approximate positions using right-clicks, then specify their exact values in the edit fields.

Since the curves are fairly linear (and due to cubic interpolation), increasing the number of data points does not provide greater accuracy. In general, 4 or 5 points per curve and 3 or 4 curves are more than sufficient.

In some cases, the power thresholds (for flow rate production) are naturally defined by extrapolating the data curve (for example, in the curves \(\mathsf{Flow Rate} = f(\mathsf{Power})\)). In other cases, they are instead defined by the voltage threshold and the \(I = f(U)\) characteristics. Determining the thresholds and the model’s behavior in their vicinity is one of the most delicate aspects of the pump model.

Detailed Parameters

This is a set of additional parameters.

Electrical Specifications

• Motor type: Recall the selection made on the previous sheet.
• MPPT or DC converter: Requires the model name (for informational purposes only; this does not refer to a device in the database).
• Nominal voltage: a reminder of the previous sheet. With a DC converter: input voltage. In other cases: the most relevant operating voltage, often mentioned in the technical data sheets, even when the voltage characteristic is not provided.

Other variables depend on the configuration:

• MPPT min/max voltage: voltage range of the MPPT converter.
• Absolute maximum voltage, absolute maximum current, absolute maximum power: absolute maximum limits at the device input (pump or converter), which must never be exceeded during the simulation. The corresponding protections must be defined in the control unit.
• Maximum and European efficiency values, from which an efficiency profile will be constructed.

Hydraulic Side

Selection of the data set available in the technical data sheets.

This selection will determine the type of model used by PVsystBasic to simulate the pump’s behavior and will make the corresponding data entry sheets available.

Pump Motor Technology

Motor technology is not a determining factor in PVsystBasic. This specification is mentioned in certain output results. Brushless DC motors appear to offer the highest efficiencies.

The only information actually used by the program is whether the motor is AC or DC.

Current Thresholds

With positive displacement pumps, and in the absence of an integrated power converter, the motor requires an inrush current before it begins to rotate.

This panel allows you to define these inrush currents for the minimum head, the maximum head, and an intermediate head located halfway between the two. The final value for any head in the model will result from linear interpolation.

The threshold voltage must also be defined. This is the voltage at which the pump (i.e., flow production) stops. It generally corresponds to the “kink” in the measured Current/Voltage curve.

This kink is not always clearly defined (and is not always provided in technical data sheets). The model selects it below the lowest specified operating point. The behavior \(I = f(U)\) between the last significant point and the origin \((U = 0, I = 0)\) is approximated by a quadratic curve to ensure the model’s completeness, but its exact values are not of great importance during the simulation process.

Pumping Systems

Stand-alone Pumping Systems

The “pumping systems” in PVsystBasic refer exclusively to stand-alone pumping systems, which operate based on solar availability, without electrical storage. Such a system consists of a pump, a photovoltaic array, and a controller/power conditioning unit.

Implementing these systems requires a detailed definition of the hydraulic circuit (system type: deep well, pumping from a lake or equivalent, pressurization system), as well as user requirements: the head (depending on flow rate and possibly other parameters), water demand, and storage in a tank. Other constraints may also be taken into account (maximum drawdown in a deep well, full tank, etc.).

Since the operating mode depends on solar availability, the pump operates at a power level dictated by the maximum power output of the PV array at any given moment. Because the head is determined by external conditions (level difference, pressure losses in the hydraulic circuit, drawdown in a deep well, etc.), the resulting flow rate is directly linked to the instantaneously available power.

Consequently, the simulation requires a comprehensive model of the pump’s behavior, enabling the determination of the resulting flow rate for any combination of power and head. The operating point, which depends on variations in total head as a function of flow rate (pressure drops in the pipes, drawdown level), will be evaluated through successive approximations.

The main advantage of off-grid pumping systems is the absence of batteries and the associated maintenance costs (replacement, etc.). Storage is effectively provided by the accumulation of water in the tank. On the other hand, this requires a pump capable of operating over a wide range of power levels.

Conventional Pumping Systems

Conventional pumping systems, powered by an electrical grid (or possibly by a large off-grid system such as a village mini-grid), operate at the grid’s specified voltage. The operating power is fixed and assumed to be available at all times. The system operates in “on/off” mode, depending on user needs and the control system. A smart energy management strategy may prioritize pumping during daylight hours when solar energy is available.

Consequently, a pumping system as defined in PVsystBasic cannot be integrated with another photovoltaic system, even an off-grid one. It must remain completely independent of any other electrical system.

Static Level

The static depth is defined in the parameters for deep well systems.

It represents the depth of the water table, which can vary throughout the year. It is therefore possible to redefine it with seasonal or monthly values.

However, the sizing calculation cannot account for these variations and will be based on the annual average value.

Of course, the detailed simulation will be based on the values specified at each time step.

Water Demand

Water demand (volume of water pumped) can be specified on an annual basis (constant value), or as monthly or seasonal values.

Defining requirements in terms of hourly values (daily distribution) is meaningless, since in most cases the pumping system includes storage covering at least one day’s consumption.

Units of Head and Pressure

In solar pumping systems, head is generally expressed in units of level difference [meters or feet]. The pressure at the base results from the weight of the water column.

From a physical standpoint, converting to pressure units involves multiplying the height by the density of water (\(1000\ \mathrm{kg/m³}\)) and by the gravitational constant (\(g = 9.81\ \mathrm{m/s²}\)). To obtain bars, one must then divide by \(100,000\ [\mathrm{Pa/bar}]\).

In summary, we obtain the following equivalences:

• \(1\ \mathrm{Pa} = 1\ \mathrm{N/m²}\) (base unit of the MKSA system)
• \(1\ \mathrm{bar} = 100\ \mathrm{kPa}\) (definition of \(\mathrm{bar}\))
• \(1\ \mathrm{bar} = 10.19\ \mathrm{mWS}\) (meters of water column)
• \(1\ \mathrm{bar} = 33.44\ \mathrm{ftWS}\) (feet of water column)
• \(1\ \mathrm{bar} = 2,088\ \mathrm{lb/ft²}\) (pounds per square foot)
• \(1\ \mathrm{bar} = 14.504\ \mathrm{PSI}\) (\(\mathrm{PSI}\) = pounds per square inch)
• \(1\ \mathrm{bar} = 0.987\ \mathrm{atm}\)
• \(1\ \mathrm{bar} = 750.1\ \mathrm{torr}\) or \(\mathrm{mmHg}\)
• \(1\ \mathrm{mCE} = 0.0981\ \mathrm{bar}\)
• \(1\ \mathrm{ftCE} = 0.0299\ \mathrm{bar}\)
• \(1\ \mathrm{PSI} = 0.069\ \mathrm{bar}\)

Sizing a Pumping System

When sizing a PV pumping system, the basic constraints are the availability of solar energy throughout the year and meeting the user’s water needs. The problem to be solved is optimizing the size of the photovoltaic generator and the pumps, taking into account the head and the electrical compatibility between the PV system and the pump, as well as the chosen system configuration.

Pump Sizing

We begin by sizing the pump.

First, we determine the hydraulic energy requirements over a single day, assuming that the flow rate and head remain roughly constant throughout the year (otherwise, the day-by-day simulation provided in the pre-sizing tool is essential).

As a rule of thumb, we can assume that, on fairly favorable days, the pump operates at its “full power” equivalent for about 6 hours; that is, it delivers a flow rate \([\mathrm{m³/h}]\) of approximately: daily water production \([\mathrm{m³}] / 6\ [\mathrm{h}]\).

Assuming a pump efficiency (generally around 50% for positive displacement pumps, or 35–40% for centrifugal pumps), we can then determine the nominal electrical power of the pump suitable for these clear-day conditions.

At this point, it should be noted that the system architecture has a significant influence on the pump’s rated power, particularly when head-related losses are significant, as is the case in simple “direct-coupled” systems.

PV Array Sizing

As a general rule of thumb, the nominal PV power (STC) can be set at approximately 20 to 30 % above the pump’s nominal power. Oversizing the PV array will result in unused energy during sunny weather. Undersizing will cause the pump to operate at lower power levels, where its efficiency may drop, or where head losses can significantly affect production in cloudy conditions, or in the morning and evening.

Tank Sizing

The size of the tank is simply determined by the required runtime, based on the daily consumption defined by the user, assuming no water production.

Other secondary characteristics of the pumping system must be determined in a second step: cable cross-sections between the PV array and the pump, sizing of the hydraulic circuit, etc. These are addressed in the detailed simulation process.

Furthermore, sizing may be subject to criteria whose relative importance depends on the application:

• Reliability of supply and consequences of periods without supply (which can be offset by a backup generator),
• Investment and maintenance costs, to be considered taking into account the cost of the PV generator, the pump(s), system control, and maintenance. For systems with a battery buffer: initial cost of the batteries, as well as their maintenance and replacement.
• Durability: quality of pumps and controllers, ease of maintenance and replacement, specific wear conditions (sand or impurities in the water, etc.).