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Lifestyles & Appliances 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