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101 renewable - solar water pump systems

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Last Updated
13th of October, 2018

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water pumps tree.gif

There are really only two basic types of solar pumps; positive displacement and centrifugal and these can both be subdivided into surface mount and submersible categories. Your water source will dictate whether you need to use a surface mount or submersible pump while your daily volume requirement and total dynamic head (TDH) will determine if you need a positive displacement pump or centrifugal pump. In general, positive displacement pumps (diaphragm, piston, helical rotor, etc.) are best used where the TDH (total dynamic head) is high and the daily volume requirement is low. Positive displacement pumps usually require less power to operate than a centrifugal pump and they will produce some water even in cloudy conditions where a centrifugal pump might not. Centrifugal pumps are good in situations where the TDH is low and the daily volume requirement is high.

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Positive displacement pumps are further classified based upon the displacement: Reciprocating pump if the displacement is by reciprocation of a piston plunger. Reciprocating pumps are used only for pumping viscous liquids and oil wells. Rotary pumps if the displacement is by rotary action of a gear, cam or vanes in a chamber of diaphragm in a fixed casing. Rotary pumps are further classified such as internal gear, external gear, lobe and slide vane etc. These pumps are used for special services with particular conditions existing in industrial sites. In all positive displacement type pumps, a fixed quantity of liquid is pumped after each revolution. So if the delivery pipe is blocked, the pressure rises to a very high value, which can damage the pump. In principle, any liquid can be handled by any of the pump designs. Where different pump designs could be used, the centrifugal pump is generally the most economical followed by rotary and reciprocating pumps. Although, positive displacement pumps are generally more efficient than centrifugal pumps, the benefit of higher efficiency tends to be offset by increased maintenance costs. Centrifugal pump has to operate at a high enough rpm to push the water all the way out of the well. If it is cloudy and the solar array is not producing enough power, the pump/motor may be turning but not fast enough to do this.

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The solar pumps are equipped with a motor. This can be DC or AC. If an AC motor is used then an inverter is needed. AC motors are more widely available. Inverters have become cheap and efficient and solar pumping systems use special electronically controlled variable-frequency inverters which will optimize matching between the panel and the pump. A typical AC system would also need batteries which require maintenance and add to the cost as the system is less efficient and would need a larger array. The most efficient type of DC motor is a permanent magnet motor. CD motors may have carbon brushes which replacing when they wear out, If a brushed dc motor is used then the equipment will need to be pulled up from the well (approximately every 2 years) to  replace brushes.  Majority of solar pumps applications (irrigation, stock water feeding, etc.) are referring to water wells and therefore to submersible pumps. It is very important to know the main elements of a water well and the way solar pumps are placed into well casing. The well pumps are either submersible pumps or jet pumps .

Submersible Pumps vs. Jet Pumps:~

Submersible and jet pumps are both used in domestic groundwater systems. When high flow rates and pressure settings are required at high operating efficiencies, submersible pumps are generally preferred. Submersible pumps have the advantage of performing well both in shallow well applications as well as at depth to several thousand feet. An extensive range of submersible pump models is also available allowing a precise match to exact system requirements.

aquifer

About the water wells:~

Solar water systems are using drilled wells equipped with submersible pumps. The water wells are either shallow wells (relative close to the surface) or deep wells. Wells are drilled in an area called "Aquifer". An aquifer contains water with a low conductivity actually a sandwich of layers soaked with water. At a certain deep called water table, the water accumulate and for a certain depth it becomes consistent and ready to be lifted to the surface. The water table sometime recedes. The difference between the water table levels (min and max depth) it is called "Draw down". This value it is very important to be known and allows us to place the water pump in an area where is secure to operate (under the water table plus the drown down deep length) . The aquifer is a living entity and it has own balance.

Wells are drilled and cased respective sealed to remain septic during the operation. The case runs up to the surface of the water table . The casing it is mainly made of clay , bentonite clay or concrete. The deep well continue from the water table level with a 15" in diameter drilled hole up to the confined aquifer level. For very deep drills the electric motor it is placed at the surface on the top of the well shaft for easy maintenance purpose.

THD graphic

Pumping system characteristics:~

Resistance of the system: head:-

Pressure is needed to pump the liquid through the system at a certain rate. This pressure has to be high enough to overcome the resistance of the system, which is also called “head”.

The total head is the sum of static head and friction head:-

Static head:-

Static head is the difference in height between the source and destination of the pumped liquid. Static head is independent of flow. The static head at a certain pressure depends on the weight of the liquid and can be calculated with this equation:

Head (in feet) = Pressure (psi) X 2.31 / Specific gravity(SG) = hs + hd or H = [Pd-Ps] x 2.31 / SG  

Static head consists of:  

Static suction head (hS): resulting from lifting the liquid relative to the pump center line. The hS is positive if the liquid level is above pump centerline, and negative if the liquid level is below pump centerline (also called “suction lift).

Static discharge head (hd): the vertical distance between the pump centerline and the surface of the liquid in the destination tank.

Friction head (hf):-

This is the loss needed to overcome that is caused by the resistance to flow in the pipe and fittings. It is dependent on size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid. The friction head is proportional to the square of the flow rate.  A closed loop circulating system only exhibits friction head (i.e. not static head).

operating point

Pump performance curve:-

The head and flow rate determine the performance of a pump, which is graphically shown as theperformance curve or pump characteristic curve. The figure shows a typical curve of a centrifugal pump where the head gradually decreases with increasing flow. As the resistance of a system increases, the head will also increase. This in turn causes the flow rate to decrease and will eventually reach zero. A zero flow rate is only acceptable for a short period without causing to the pump to burn out.

How to calculate pump performance:-

The work performed by a pump is a function of the total head and of the weight of the liquid pumped in a given time period.

Pump shaft power (Ps) is the actual horsepower delivered to the pump shaft, and can be calculated as follows:
Pump shaft power Ps = Hydraulic power hp / Pump efficiency ηpump
or:
Pump efficiency ηpump = Hydraulic power hp / Pump shaft power

Pump output, water horsepower or hydraulic horsepower (hp) is the liquid horsepower delivered by the pump, and can be calculated as follows:

Hydraulic power hp = Q (m3/s) x (hd - hs in m) x ρ (kg/m3) x g (m/s2) / 1000

Where: 
Q = flow rate
hd = discharge head
hs = suction head
ρ = density of the fluid
g = acceleration due to gravity

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Type of solar pump applications:~

Array-Direct to Elevated Storage:-

The most efficient solar water-pumping systems are PV-direct without batteries. This classic off-grid pumping solution connects a DC submersible pump directly to the PV array. One additional component, either a controller or linear current booster (LCB) wired between the PV array and the pump, optimizes the relationship between array voltage and current to maximize the amount of water pumped under varying sunlight conditions. When the sun is shining, the pump moves water to a tank located above the water’s point of use. (For potable water, use only drinking-water-grade tanks.) For each 2.31 feet of elevation, there will be 1 psi of water pressure. Many household water supplies operate at about 40 psi (though down to 20 psi can often work), so locating the storage tank about 100 feet higher than the point of use will provide adequate water pressure.

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Array-Direct to Storage Tank with Pressure Pump:-

If you like the efficiency of DC pumping and want stored water at your disposal, but don’t have enough of an elevation change on your property for a gravity system, there are still options. A PV-direct DC pump can be configured to fill a storage tank near its point of use, and an additional DC or AC booster pump can be installed at the tank to pressurize the water. This solution gets the heavy lifting (from the well to the storage tank) done whenever the sun’s shining, and limits energy use during cloudy periods to a smaller pressure pump that activates via a pressure switch when water is used. While this configuration gives you more flexibility to choose when you consume the energy required to pump well water, the up-front costs of the storage tank and additional pressure pump add to overall system cost and installation complexity.

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