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Basic overview

Getting Started with PVsystBasic

The "Pumping Systems" in PVsystBasic refer exclusively to off-grid pumping systems, which operate based on sunlight availability, without using electrical storage or being connected to the grid. These systems generally consist of:

  • A pump
  • A photovoltaic array

To properly implement such systems, it is essential to precisely define:

  • The hydraulic circuit (whether it involves wells, boreholes, pumping from a lake, or a pressurization system)
  • Water requirements
  • The head, based on flow rate and other potential parameters
  • A storage tank.

Other constraints may also be taken into account, such as the maximum drawdown in a deep well or the management of a full tank.

System Operation

The system operates by adjusting the pump’s power output based on the available power from the photovoltaic generator at any given time. The head (or drop) is determined by external conditions such as the difference in elevation, pressure losses in the pipes, or drawdown in a deep well. Consequently, the water flow rate will be directly related to the instantaneous power supplied by the PV generator.

Modeling in the Simulation

The simulation requires a complete model of the pump to determine the flow rate based on power and head conditions. Since the total head varies depending on the flow rate (particularly due to pressure losses in the pipes or drawdown), the operating point is evaluated through successive approximations.

Advantages of off-grid systems

One of the main advantages of these systems is the absence of batteries, which reduces maintenance costs associated with battery replacement. Storage is achieved by accumulating water in a reservoir, eliminating the need for batteries. However, this requires the use of a pump capable of operating over a wide power range.

Pumping Tests

Before drilling a well, it is essential to ask a few key questions, such as: “How much water will I be able to pump in the short and medium term, and what will its quality be?” To answer this, a pumping test must be conducted.

What is a pumping test?

The principle is simple: water is extracted from a well or borehole, which lowers the water level. During this operation, the water level and flow rate are measured over a given period, while also observing other parameters. Analyzing variations in the water level allows us to determine the performance of the borehole and the hydraulic properties of the aquifer.

There are various types of tests, whether intermittent or continuous, short- or long-term, with low or high flow rates. One of the major challenges in studying groundwater is that the aquifer is not directly visible. Information about the borehole and the aquifer can only be deduced by observing how the water level reacts to pumping.

Objectives of Pumping Tests

Pumping tests are used for various reasons, including:

  • Evaluating the long-term performance of a well: This helps determine whether the well is effective and how many people it can serve.
  • Evaluating the hydraulic performance of the well: Flow rate and drawdown characteristics are analyzed to better understand how the well operates.
  • Determining the hydraulic properties of the aquifer: The pumping test is the primary method for evaluating the aquifer’s transmissivity and storage coefficient, or for detecting potential hydraulic boundaries.
  • Testing equipment: We verify that the pumping and monitoring equipment is functioning properly to ensure safe and efficient operation.
  • Analyzing effects on neighboring wells: The test allows us to assess the impacts of pumping on other wells, particularly potential interference.
  • Assess the environmental impact of extraction.
  • Obtain information on water quality.
  • Anticipate potential problems, such as pumping up brackish or polluted water after prolonged periods.
  • Determine optimal operating conditions: select the most suitable pumping station for long-term use, and estimate potential pumping or treatment costs.
  • Determine the ideal depth for installing the pump in the well.

Different Types of Tests

  • Step test: This test aims to establish the short-term relationship between flow rate and drawdown in the well. It involves pumping in successive stages, gradually increasing the flow rate at each step, until the estimated maximum yield of the well is reached.
  • Constant-flow test: This test involves pumping at a constant flow rate over a longer period than in a step test. Its primary objective is to provide data on the hydraulic properties of the aquifer. Information on the storage coefficient can only be obtained if appropriate wells are used for observations.
  • Drawdown test: After pumping stops, this test measures the drawdown of water levels. It is often performed after a constant-rate or step test and serves to validate the observed characteristics of the aquifer. However, it is only valid if a check valve is installed to prevent water from flowing back into the well.

These tests can be performed separately or in combination. Generally, a complete series begins with a step test to determine the flow rate for the constant-flow test and ends with a rise test. The process can be adapted based on the size of the wells, with variations in pumping rates, test duration, and the complexity of the observation systems.

In PVsystBasic, the reference is taken at ground level, so we have (see fig): $$ \mathsf{HT} = \mathsf{HG} + \mathsf{HS} + \mathsf{HD} + \mathsf{HF} $$ where:

\(\mathsf{HG}\) = Height of the water column between the ground and the top of the reservoir

\(\mathsf{HS}\) = Static head due to the depth of the water level in the well, in the absence of any pumping.

\(\mathsf{HD}\) = Dynamic "drawdown" head: in a well, the effective water level is dynamically lowered by the extraction of water flow (see below). It depends on the flow rate at any given moment.

\(\mathsf{HF}\) = Friction losses in the piping system, which depend on the flow rate.

Diagram of a well pumping system

Pumping Systems from a Lake or River

Pumping systems from a lake or river operate similarly to those for boreholes or wells, but with certain technical simplifications:

The pump can be installed near the source, at a maximum distance of 5 m above the water surface. This distance must be reduced even further at higher altitudes to avoid cavitation problems.

It is not necessary to use a submersible pump, which makes the system less expensive and facilitates pump maintenance.

It is important to note that the pressure or manometric head depends primarily on the difference between the water inlet and outlet levels. The pump must be capable of delivering a total head resulting from several combined factors.

In PVsystBasic, the reference is taken at ground level; we have (see fig): $$ \mathsf{HT} = \mathsf{HG} + \mathsf{HS} + \mathsf{HF} $$ where:

\(\mathsf{HG}\) = Manometric head due to the height of the outlet pipe above ground level (assuming that the outlet pressure is negligible).

\(\mathsf{HS}\) = Static head due to the depth of the water level relative to ground level.

\(\mathsf{HF}\) = Friction losses in the piping system, which depend on the flow rate.

Diagram of a lake or river pumping system

For this system, in the section “Hydraulic Characteristics”, you will be asked to specify:

  • The level of the lake or river relative to ground level.
  • The depth of the pump. It must be strictly less than \(5\ \mathrm{m}\) above the depth of the source, but may also be submerged.