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Solar
Pumping: Dankoff Articles |
Tracking,
Inverter sizing, Cavatation, Pressure
settings
Solar Water Pumping: A Practical Introduction
How It Works by Windy Dankoff
Photovoltaic (PV) panels produce electricity from sunlight using silicon
cells, with no moving parts. They have been mass-produced since 1979.
They are so reliable that most manufacturers give a 10-year warranty,
and a life expectancy beyond 20 years. They work well in cold or hot weather.
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. 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.
A surface pump is one that is mounted at ground level. A submersible
pump is one that is lowered into the water. Most deep wells use submersible
pumps.
A pump controller (current booster) is an electronic device used with
most solar pumps. It acts like an automatic transmission, helping the
pump to start and not to stall in weak sunlight.
A solar tracker may be used to tilt the PV array as the sun moves accross
the sky. This increases daily energy gain by as much as 55%. With more
hours of peak sun, a smaller pump and power system may be used, thus reducing
overall cost. Tracking works best in clear sunny weather. It is less effective
in cloudy climates and on short winter days.
Storage is important. Three to ten days' storage may be required, depending
on climate and water usage. Most 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.
What is it used for:
Livestock Watering: Cattle ranchers in the Americas, Australia and Southern
Africa are enthusiastic solar pump users. Their water sources are scattered
over vast rangeland where power lines are few, and costs of transport
and maintenance are high. Some ranchers use solar pumps to distribute
water through several miles (over 5 km) of pipelines. Others use portable
systems, moving them from one water source to another.
Irrigation: Solar pumps are used on small farms, orchards, vineyards
and gardens. It is most economical to pump PV array-direct (without battery),
store water in a tank, and distribute it by gravity flow. Where pressurizing
is required, storage batteries stabilize the voltage for consistent flow
and distribution, and may eliminate the need for a storage tank.
Domestic Water: Solar pumps are used for private homes, villages, medical
clinics, etc. A water pump can be powered by its own PV array, or by a
main system that powers lights and appliances. An elevated storage tank
may be used, or a second pump called a booster pump can provide water
pressure. Or, the main battery system can provide storage instead of a
tank. Rain catchment can supplement solar pumping when sunshine is scarce.
To design a system, it helps to view the whole picture and consider all
the resources.
Thinking Small
There are no limits to how large solar pumps can be built. But, they
tend to be most competitive in small installations where combustion engines
are least economical. 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 it 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. The length
of piping has little bearing on the energy required to pump, so water
can be pushed over great distances as low cost. Small solar pumps may
be installed without heavy equipment or special skills.
The most effective way to minimize the cost of solar pumping is to minimize
water demand through conservation. Drip irrigation, for example, may reduce
consumption to less than half that of traditional methods. In homes, low
water toilets can reduce total domestic use by half. Water efficiency
is a primary consideration in solar pumping economics.
A Careful Design Approach
When a generator or utility mains are present, we use a relatively large
pump and turn it on only as needed. With solar pumping, we don't have
this luxury. Photovoltaic panels are expensive, so we must size our systems
carefully. It is like fitting a suit of clothes; you need all the measurements.
Here is a guide to the data that you will need to determine feasibility,
to design a system, or to request a quote from a supplier.
Next, we will 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.
Now, we 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.
Next, refer to our performance charts for the type of pump that is appropriate.
They will specify the size and configuration (voltage) of solar array
necessary to run the pump.
Copyright ©2002 by Dankoff Solar Products, Inc. Top
Solar Tracking for Solar Water
Pumps
By Windy Dankoff
A solar tracker is a PV rack that rotates on an axis to face the sun
as it crosses the sky. It is well known that solar tracking will increase
energy yield by 25-50%. For solar pumping, tracking offers even greater
gains and benefits that can greatly reduce system cost.
Optimum yield during the peak watering season
Tracking offers more water out of smaller, less expensive system by increasing
performance when the most water is needed - during long sunny days of
the growing season. This is most appropriate for agricultural and seasonal
summer uses.
Prevention of pump stalling
Many solar pumps experience a disproportionate drop in performance when
the sun is at a low angle (early morning and late afternoon). When the
PV array output is less than 50%, a centrifugal pump may produce insufficient
centrifugal force to achieve the required lift. By causing the pump to
run at full speed through a whole sunny day, tracking can often DOUBLE
the daily water yield.
Water distribution for PV-direct irrigation
Solar irrigation can be practical with NO storage device, in some situations
- The soil itself stores water during cloudy days! However, sun-tracking
may be necessary to achieve uniform water distribution. When water flow
is reduced, a sprinkler just makes a puddle. A trench or drip line feeds
only the first few plants, or the lowest ones. A tracking array minimizes
the periods of reduced flow. It makes solar-direct water distribution
an option for dry sunny regions. There will still be short periods when
distribution is not even, but our customers have proven that it can work.
Expediting the design process
The tracking decision as a handy variable in the design process. Often
you find a system that produces a little bit less than is needed, but
the next larger system costs much more. A tracker is a low-cost means
to increase the yield of the smaller system.
When NOT to use a tracker
Tracking is least effective during shorter winter days, and during cloudy
weather. If the need for water is constant during the year or greatest
in the winter, or if the climate is very cloudy, then it may be more economical
to design the system with more solar watts and no tracker.
Passive trackers
Dankoff Solar has sold passive Track Rackstm by Zomeworks, since 1983.
The tracking process uses no moving parts and no electrical parts, only
a fluid/vapor flow that tips a balance. An automotive type shock absorber
may need replacement about every 5 years.
Isn't there more to go wrong?
Zomeworks Track Racks rarely fail, even long after the 10-year warranty.
In a rare case of failure, it will hold at mid-day position and the pump
will still function, or it can be tracked by hand. Field repair is accomplished
easily by attaching outboard canisters.
Copyright ©2002 by Dankoff Solar Products, Inc. Top
Inverter Sizing for Submersible Pump Applications
By Windy Dankoff
One of the most vital uses of a home power system is to power a water
well pump. A pump can be a real power hog! Conventional pumps require
a high surge of current in order to start. The entire circuit, from batteries
to invert to pump, must be sized to handle the starting surge at the same
time as other loads. Otherwise, the inverter will shut down. Use the following
chart as a guide to inverter sizing.
Minimum continuous power rating
of an inverter
to start an AC submersible well pump (with no additional loads)
An inverter sized by these minimum guidelines will dip its voltage during
the starting surge. This is not harmful, but it will cause lights to dim.
Fluorescents may blink off, and computers are likely to crash. To eliminate
voltage dips, oversize the inverter by an additional 50% minimum plus
the watts capacity required to handle other household loads at the same
time.
Minimum inverter sizing is based on field experience with Trace inverters,
allowing ~25% voltage drop during startup. To eliminate noticeable voltage
dip, add 50% to the minimum size. Other brands of inverters differ in
their surge capacity relative to continuous rating. Exact starting capacity
is difficult to predict and inverter manufacturers are hesitant to specify
it. Dankoff Solar welcomes your feedback and will publish more information
as a result.
If a "modified sine wave" inverter is to be used and pump's
control box is labeled "solid state", then it must be changed.
Obtain a relay-type control box or a relay conversion kit, from any pump
supplier.
If the pump is a "two-wire" type (having no control box), oversize
the inverter by an additional 50%. A two-wire pump may not always work
on a modified sine inverter.
Most well pumps require 230 VAC. Either two stacked inverters, or an
inverter with 230V output, or a transformer must be used. (The Trace T-240
transformer will handle 2 HP max.). If all of this is too expensive for
your situation, consider replacing it with a lower power pump, carefully
selected for the best efficiency (watts per gallon). You can also consider
an intermediary storage tank with a DC pressurizing pump. The use of a
storage tank will relieve your well pump from the need to start every
time the pressure runs low (many times per day). You can pump into the
storage tank just once or twice per week, and then use a DC pump to supply
the water pressure as needed (or use gravity flow, if feasible). See DC
Pressurizing Pumps for Domestic Water Supply and Irrigation. You may also
be able to change to a lower power well pump, even a DC well pump, after
this step is taken because less pressure and less flow will be required
from the well pump.
Copyright ©2002 by Dankoff Solar Products, Inc. Top
Surface Water Pumps: Preventing
Cavitation and Noise
by Windy Dankoff
The most economical DC water pumps are surface (non-submersible) pumps.
They can reduce pumping energy consumption by half or more, compared to
conventional AC pumps. This article will help you to prevent a common
problem that all pumps are susceptible to. The problem occurs primarily
when a surface pump is mounted above the water source.
Excessive suction causes cavitation, which is the formation and collapse
of bubbles. When water pressure is reduced beyond a critical point, water
vapor and/or dissolved gasses are released, like when you open a carbonated
beverage. When a bubble reaches the pressure side of the pump, gas returns
to the liquid state. Bubbles collapse in sudden implosion. This causes
water to impact violently, like tiny hammer blows, against the working
surfaces of the pump. Cavitation causes loud noise and excessive pump
wear.
Cavitation is not the fault of the pump, but of the installation. To
prevent cavitation, follow these precautions:
* Refer to the pump's specification sheet and instructions, and observe
the limits of vertical suction lift.
* If feasible, eliminate suction lift by placing the pump below the water
source. Otherwise, minimize the vertical distance from the source to the
pump.
* Think of an intake line as a drain line in reverse. The water must flow
easily.
* Use large intake pipe (larger than the pump's intake port). This is
especially critical in cases of long intake piping. (See Pipe Sizing Chart
in SunPaper 1 or 2, or at our website.)
* Avoid 90o elbows. Use pairs of 45o elbows to reduce friction.
* Carefully choose intake screens or intake filters for low friction,
and make sure that they will be easy to clean.
* Work carefully to minimize the possibility of air leaks.
* Avoid humps in the intake pipe. They can trap bubbles that will restrict
the flow (like in a siphon). If a hump can't be avoided, install a pipe
Tee at the highest point, with a cap or a ball valve above it. When water
is poured in at the high point, it will displace all of the air to fully
prime the intake line. This may need to be done periodically.
Most plumbers work only with pressurized distribution and are not aware
of the critical nature of intake piping to a pump. Do not allow a plumber
to install intake piping unless s/he reads this article or the pump's
instructions.
Copyright ©2002 by Dankoff Solar Products, Inc. Top
Adjusting Water Pressure to
Reduce Energy Waste
Water well pumping can be the largest load on a remote home power system.
There are several ways to minimize its energy consumption (see SunPapers
1 & 2 or our website). Here is another technique that applies to any
pressurizing system, especially if it uses an AC pump.
If you look at the performance curve of any centrifugal-type pump (including
all AC submersibles and jet pumps) you will see that as the pressure increases
past a certain point, the flow drops drastically. Here is an easy way
to detect this situation at the job site. Release some water, just until
the pump switches on. Watch the system's pressure gauge and observe its
rate of rise (this indicates flow rate). Does it rise to a certain point
and then slow way down? If so, then that is the pressure at which the
pump "slips" and loses efficiency.
We had a customer in Colorado whose cut-out pressure had been set to
the typical 50 PSI. As the pressure got past 40, the flow rate slowed
way down. There seemed to be more than enough pressure at the faucets,
so we reduced the cut-out to 36 PSI. In doing so, we cut the energy use
of the pump nearly in HALF. The owner couldn't detect a change in the
water delivery but, as it was gardening season, she saw an immediate increase
in the amount of energy available from her PV power system!
Why do most Americans want more than 35 PSI at their home? It's because
of undersized plumbing! Most houses in the U.S.A. are plumbed to the legal
minimum requirements of the plumbing codes (1/2" and 3/4" pipe).
At the end of a long pipe run, the dynamic pressure may be diminished
by 30%. Where a house has not yet been plumbed, we recommend using one
size larger than minimum, for all cold water lines. Similarly, when using
garden hose, 3/4" hose will cause far less pressure drop than 1/2"
or 5/8" hose. When these measures are taken, a pressure setting of
25-35 PSI will please anybody. Where a house is already plumbed, observe
water delivery at the faucets. If water flow is satisfying without opening
faucets all of the way, then a reduction in pressure may be acceptable.
How to reduce water pressure
Pressure adjustments are made at the pressure switch. On a standard switch
there are two adjustment nuts, with a spring under each one. Turning counterclockwise
will lower the settings. You will see the result by watching the pressure
gauge as the pump cycles on and off. FIRST, loosen the nut on the LONGER
screw. This will reduce both cut-in and cut-out pressure. Set it for the
CUT-IN that you desire. Second, adjust the nut on the SHORTER screw. It
adjusts the CUT-OUT only. Cut-out pressure should be around 2/3 of the
cut-in pressure.
Once the pressure is set and everyone is satisfied, reset the precharge
air in the pressure tank. This will maximize its storage and minimize
on/off cycling. To reset the precharge, first make note of the cut-in
pressure. Now shut off the power to the pump. Release water until the
pressure gauge drops to zero. Measure the pressure of the tank's air bladder
using a tire pressure gauge at the fitting on top of the tank. Set the
air pressure to 2 or 3 PSI less than the cut-in pressure. Restart the
pump. Finally, write down the running time per cycle. Write it on the
wall, so the performance can be checked later to detect pump wear or other
problems.
Consider a DC pump for higher pressure
Our Flowlight Booster Pump, Solar Force Piston Pump and Lorentz PS Submersible
use positive displacement rather than centrifugal action. They don't "slip"
and lose efficiency at high pressure. They use 1/3 to 1/2 the energy per
gallon of an inverter/AC pump system. They are especially advantageous
for standard-plumbed homes that really do need 50 or 60 PSI.
Copyright ©2002 by Dankoff Solar Products, Inc. Top
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