Solar water Pumping – FAQ’S

  • Why are daily water requirements important to know, and not necessarily flow per hour?

Unlike conventional electric pumps which pump at a constant rate, once switched on, flow rates for solar pumps are variable over the course of the day. Therefore solar direct pumps start pumping slowly in the morning, with peak flow rate over midday, then dropping off in the afternoon.

This variable flow rate is represented in the illustration below, resulting in an average pumped volume per day, which repeats itself on a daily cycle ( sun permitting). In order to optimise solar pump sizing, daily water requirements, and not ( only) hourly flow rates should be known.

 

It is possible to extend peak hourly flow rates for more hours around the noon period, as illustrated in the graphic below. This may for example be desirable in pressure irrigation applications, where the sprayer or sprinkler nozzles require constant flow and pressure for longer periods per day, to operate effectively.
This is done by adding more solar panels in order to getting maximum power to the pump earlier in the morning and till later in the afternoon. The downside of this is only a marginal increase in overall daily volume, relative to the increase in solar power. This is due to a solar overcapacity at midday which the pump cannot absorb, since the solar pump only draws the power that it requires for the given parameters in which it operates – much the same as a conventional electric pump, which does not draw all the available power which may be available at the plug point.(see diagram)

  • Why is the borehole depth, and pump depth, of minor importance in solar submersible pump selection and sizing?

The important parameter is vertical lift which is NOT determined by the depth of the borehole, or the depth of the pump setting. It is determined by the depth of the water level in the borehole, plus lift ( height or elevation) above ground to the water delivery point.

Please note that the water level depth refers to the so called dynamic water level. This is the level the water drops to when the pump is in operation, that is the static level ( pump not operating), plus the draw-down when water is being pumped. In weaker boreholes with low yields the draw down can be significant, where as in strong boreholes the water may hardly drop from the static level, especially if the pumping rate is well below the maximum yield of the borehole.

The borehole depth and pump installation depth would determine only the pump cable length and perhaps a worst case scenario. However if system sizing is done on borehole depth this may lead to a wrong pump selection and /or major over design and unnecessary cost. For example say a borehole is 100m deep, but the water level is at 50m: if we design for a 100m ( at a given volume per day) we would end up with a wrong pump selection, and at twice the system cost than required.

  • Why are Solar Water Pumps not specified in Horsepower, Watts or Kilowatts?

Although solar pump motors and controllers have nominal power ratings, the power at which solar pumps operate are variable. This  depends on the solar array size,  and time of day ( sun radiation intensity varies over course of the day),  and factors such as temperature, pump geometry and dynamic head of  the application.

It is therefore not possible to make direct comparison between conventional electric pumps and solar pumps, in terms of horsepower or Kw rating. A quotation request to replace an  xxx KW electric pump with a solar pump is generally not possible, unless the required pumping parameters are specified. ( volume per day and pumping head)

  •  Are solar pumps cheaper than Diesel pumps?

Although the initial capital cost of diesel pumps may be lower than solar, the operating and maintenace cost for diesel accummulate quickly so that the break-even point for solar can be reached after a fairly short time, after which further savings accumulate, making solar a far cheaper option over the solar pump’s life time.

To illustrate this, a Solar vs Diesel calculator is available which you can access by clicking on: Solar vs Diesel Calculator