Solar Water Pumps
A solar water pump is simply any water
pump that uses solar energy for its power source. Solar water
pumps are specially designed to utilize DC electric power from
photovoltaic panels. They must work during low light conditions at
reduced power, without stalling or overheating. Solar water
pumps have one significant advantage over other types of pumps, they do not require the
presence of an electric line in order to operate. This makes
them extremely useful in rural locations such as ranches and farms, or
in the developing world where electricity is often not available.
In many parts of the world the cost of running traditional water
piping or an electric line for a pump is cost prohibitive. A solar
water pump is a perfect solution to this problem. Because of this they
have been used extensively in places like India, Africa and Asia both for pulling water
from wells, providing water to livestock, and for irrigation.
pumps have plenty of application in the U.S. as well. They have
become extremely popular for rural and agricultural well pumping and
are starting to replace many pumps that were originally operated by
wind mills. Many homes in the U.S. rely on well water and solar pumps can be a
convenient approach for keeping their water tanks full. Solar
pumps are also being used extensively in landscaping. Because they do not require any type of wiring they allow
landscapers tremendous freedom in where to place water features such
as fountains, streams and waterfalls.
Solar pumps are friendlier to the environment and cheaper to
operate than conventional AC pumps. By harnessing the energy of the sun they eliminate the
need to use traditional fuel sources such as oil or coal to generate
the electricity they need to operate. This saves money as well,
particularly in developing countries where electricity is both scare
and expensive. Also, because they eliminate the need for digging
ditches for electric lines or putting up power line poles, they are
less disruptive to the natural environment. Another
advantage of solar water pumps is that they are not dependent upon the
grid for electricity. That means you can pump water even when the electricity goes out!
For those homeowners who rely on well water this can be a big
Types of Pumps
Solar panels generate DC current and so the vast majority of solar
pumps use direct current rather than alternating current so no current
inverter is needed. Most of the pumps are packaged without
batteries so they really only run while the sun is shining.
However, for most typical uses this is perfectly acceptable and it has
the advantage of being a very simple setup that requires little
maintenance. There are different kinds of pumps depending upon
what you are trying to do:
- Circulating Pumps: These types of pumps are often used in
solar water heating
applications. They can be used to move either
water or a thermal transfer fluid such as antifreeze (glycol) through a home hot water heating system.
These same pumps can also be used for a pool heating system. They are also frequently used in landscaping applications for waterfalls or fountains.
Most circulating pumps are relatively small and many have the option
to run on AC or DC power.
- Submersible Pumps: Submersible solar pumps are generally
used for pumping from wells, and are designed to fit inside the well
casing in a drilled hole. The most common casing size is 6", but 5"
and 4" are also common. Some older wells drilled for
less than 4", and most solar submersible pumps will not fit - for
that the best thing is to replace the windmill with an ERA jack
pump. Submersible pumps can lift up to 650 feet and are used when
the water supply is more than 20 feet from the surface. The well may
be anywhere from a few feet to 1800 feet deep - but the deeper the
well, the more expensive the system. Depths over 400 feet will generally cost quite a bit more. Solar powered well pumps are seldom
suited for large amounts of water, such as irrigating larger fields
if you have AC power available. The largest pumps generally
available are 1/4 to 2 HP.
- Surface Pumps: These types of pumps use a suctioning
approach and are good for applications with shallow wells, ponds, streams or storage tanks.
They work best if the water supply is 20 feet or less from the surface.
Whenever you should try to minimize the suction lift to
just a few feet. The maximum possible suction for any pump is about
20 feet but if you push them to lift more than a few feet they will
become noisy and come under more strain. As with other solar pumps
the solar panels for the pump can be remote from the pump itself which
allows a great deal of flexibility in placement. In some landscaping
systems designed for pools and ponds the whole system floats with the
solar panels lying just below the surface of the water.
- Delivery pumps:
- These pumps are used to move water from
one place to another. Some are capable of high pressure while others
are intended mainly for moving large volumes at low pressure (such
as moving water from a cistern to a stock watering tank). Flows can
be small (1/2 gallon per minute) up to 30-40 gpm.
One disadvantage of DC solar pumps which do not use batteries is
that the energy they generate varies throughout the day based upon the
available sunshine to the solar panels. Therefore most DC solar
pump kits include a device called a linear current booster (LCB), also called
controller, which acts like an automatic transmission, helping the pump to
start and not to stall in weak sunlight. An LCB also allows the pump motor to run longer during the day by translating additional voltage to necessary current during periods of low sun.
Solar Panels for Pumps
Solar pumps get their power from solar panels. The type of solar
panels you would use is no different than the kind that might be put
on a roof in a residential solar electric system. Solar panels come in
many different sizes. Generally the larger they are the more
watts of electricity they can generate. The number of watts you
will need to generate, and therefore the number of solar panels you
will need for your pump, depends upon the size of the pump and what
you are going to use it for. For example, when using solar
panels with submersible well pumps the number of panels you will need
depends greatly upon the depth of the well -- the deeper the well, the
more power needed, and therefore the more solar panels needed. When
using delivery pumps the number of panels you will need is a function
of the volume of water you need to move (usually measured in gallons
per minute) and the water pressure you must generate.
Most DC pumps are fairly small and only require a single solar panel.
pump kit manufacturer will have properly matched the solar panels
output to the power requirements of the pump. However, if you
find your pump requires more energy you can usually add on another
panel since most panels are designed to be connected in strings or
modules. The main thing is to be sure that you don't exceed the
maximum voltage the pump can handle. If you are not sure check
with an electrician or the contractor that provided you the pump.
Low volume pumps use positive displacement (volumetric) mechanisms which seal water in cavities and force it upward. Lift capacity is maintained even while pumping slowly. These mechanisms include diaphragm, vane and piston pumps. These differ from a conventional centrifugal pump that needs to spin fast to work efficiently. Centrifugal pumps are used where higher volumes are required.
Most solar pumping systems use water storage rather than batteries, for simplicity and economy. A float switch can turn the pump off when the water tank fills, to prevent overflow.
Compared with windmills, solar pumps are less expensive, and much easier to install and maintain. They provide a more consistent supply of water. They can be installed in valleys and wooded areas where wind exposure is poor. A PV array may be placed some distance away from the pump itself, even several hundred feet (100 m) away.
The smallest solar pumps require less than 150 watts, and can lift
water from depths exceeding 200 Feet (65 m) at 1.5 gallons (5.7
liters) per minute. You may be surprised by the performance of such a
small system. In a 10-hour sunny day a solar pump can lift 900 gallons
(3400 liters). That's enough to supply several families, or 30 head of
cattle, or 40 fruit trees! Slow solar pumping lets us utilize
low-yield water sources. It also reduces the cost of long pipelines,
since small-sized pipe may be used.
Determining the Type of Pump You Need
One of the first things you will need to do in most water pumping
scenarios is determine whether a submersible pump or a surface pump is
best. This is based on the nature of the water source. Submersible
pumps are suited both to deep well and to surface water sources.
Surface pumps can only draw water from about 20 feet (6m) below ground
level, but they can push it far uphill. Where a surface pump is
feasible, it is less expensive than a submersible, and a greater
variety is available.
Once you have decided the type of pump you will need you will need
to determine the flow rate
required. Here is the equation, in the simplest terms: Gallons (Cubic
Meters) per Hour = Gallons (Cubic Meters) Per Day / Available Peak Sun
Hours per Day Peak Sun Hours refers to the average equivalent hours of
full-sun energy received per day. It varies with the location and the
season. For example, the arid central-western USA averages 7 peak
hours in summer, and dips to 4.5 peak hours in mid-winter. In most
cases, to be safe, you will want to talk to your well contractor or a
local pump provider to ensure that you have correctly estimated your
requirements but inevitably whoever you talk to is likely to ask one
or more of the following questions, so be prepared:
- How deep is your well or alternative water source?
- What is the static water level in the well (the static level is that height in the well to which the water rises under it’s own pressure)?
- How many gallons per minute does your well produce?
- How many gallons per day do you need?
- Will you pump to a non-pressurized holding tank or to a pressure tank?
- How many feet above the well head is the tank located?
- If a pressure tank is used, how many pounds of pressure will you ask the pump to create?
- How many feet from the solar array to the well head?