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Article # 0060
Solar Submersibles: Design considerations for a remote, multi-well installation
Greig S. Latham, P.E., Solatex, Allen, TX
Abstract—Exceptional Drought and dire need by wildlife forces the decision to install a water pumping solution in a remote, mountainous region after windmills fail. Custom features driven by a clear mission result in a robust, yet simple solution. Combining off-the-shelf components in a carefully integrated manner addresses reliability and maintainability concerns for an installation several hours away from the nearest ‘civilization’
Extreme and Exceptional Drought continues over much of the southwest. Perhaps nowhere is the drought more acute than in the mountains of New Mexico. The state has just marked the driest two-year period in nearly 120 years of record-keeping.
 Several years on with no relief in sight, many are taking steps to save livestock and wildlife in conditions that genuinely threaten their survival. These remote locations pose challenges for any technology, including windmills that helped settle the west. The challenges are especially acute for things electrical. This paper examines elements of the mission and the solution when solar-powered submersibles replace windmills in a remote, mountainous environment.
With abundant wind, one easily wonders why windmills or even wind-powered generators are not the first choice for water producing power plants in remote mountain climes. It is not an easy call and different opinions certainly exist. However, there are a few considerations that at least put solar in the game.
The subject ranch has well over a dozen wells that are candidates for water pumping. Before the installation of the pump solutions described in this paper there were no operating pumps. Further, as an indicator of the acute need, there was no surface water on this large ranch. The land owner had only a few mission requirements. Aside from a low-cost solution, the owner’s main concern was to conserve the limited water resource. The third requirement, reduced maintenance, was driven by location – the install site is almost two hours from the nearest town with most of the time spent driving over rough mountain roads. A fourth requirement was for a heater element to prevent freezing during low temperatures; this design feature is untested and viewed as an applied experiment.
Solatex normally installs wells and primary lighting for humanitarian purposes. This rugged and remote location (yet, still in the United States) affords the company an opportunity to evaluate some design features described here.
The windmills on each of the four wells selected for retrofit had been overhauled or otherwise serviced last year and had produced water; however, the persistent, high winds had taken their toll on the machines and rendered the windmills not mission capable. With the stalwart windmill out of the picture, the remaining choices were solar and wind powered submersible pumps.
Given the debate about wind vs. solar, it is dangerous to take a position as it can be hazardous to one’s health around devotees from the opposing camp. Many studies show that hybrid systems are better than either technology alone. However, given the mission profile in this installation, the choice was relatively easy. The land owner wanted to preserve the windmill installation in case of future consideration. Not being able to use the tower for a wind turbine increased the infrastructure cost for wind.
While water production in the winter months was desired, the most critical time was summer. There is, of course, plenty of summer sunshine compared to wind at the installation location.
Older windmills produce water whenever they run; if the tank overflows, the water is essentially wasted having been lifted from the reservoir only to soak back in the ground. In the grand scheme, the water has not been destroyed and it will find its way back to the reservoir, but for practical purposes such a solution is not optimal.
There is no clear advantage of solar over wind when it comes to controlling when water is produced; both are easily switched and controlled.
The control solution presented later illustrates the measures taken for this installation to conserve water. High and Low Level float switches were installed for both the trough and the well. The trough calls for water when at low level; the call is dropped when the trough reaches the high level. The well is monitored to only provide water when it has sufficient water to warrant cycling the pump and ceases supplying water when the well reaches a low water level. Having positive control over the demand and supply of water directly addresses the concern of conserving the water resource.
Wind-powered generators can supply switched power to pumps, but they require dump loads to dissipate the generated power when not used by the pump. As the pump run time is generally a small portion of the generating time, there is considerable wear and tear on the hardware.
In contrast, solar power, in the form of photovoltaic modules, has no moving parts, requires no batteries if the design accommodates this choice and has no special dump load requirements.
The level float switch arrangement discussed earlier represents a significant maintenance savings. By limiting calls for the pump to times when the trough demands water and the well can supply water, there is no short cycling of the pump. Often times, submersible pumps have no such protection and may run dry. Some installations employ a well low level switch that prevents the pump from running if the water level gets too low. However, there is little to no hysteresis in this arrangement as the pump runs when the well recovers to the point where the level switch indicates there is water in the well. By using a high level switch, pump cycles are greatly reduced without significant impact to water production.
Most prior installations used the Grundfos SQ Flex family of pumps. The SQ is an impressive design; has integral, “dry-running” protection and will accept input power from 30-300Vdc and 90-240Vac sources. However, Grundfos and/or Federal Reserve policies have priced this pump out of our typical installation – the price has doubled over recent years. By way of illustration, the price of the pump we would normally have used now exceeds the total cost of all components for the subject install.
After researching alternatives, the Ranch Pump, RP2, from Advanced Power Inc. (Robison Solar Systems) was selected due to its value, design and warranty.
The pump is switched by a dedicated control relay to prevent carrying the pump current through low-rated switch contacts in the event the float switches were ever changed in the field.
Trade and tariff issues surrounding the manufacture of photovoltaic modules continue to affect the market and make news. Avoiding the nationalistic debate and viewing PV modules as a commodity, Renogy 100W PV modules were selected based on warranty, performance specifications and price.
Two PV modules were allocated for each well. One PV module is dedicated to water pumping; the other module is switched to pumping in the summer and powering the trough water heater in the winter.
Madison manufacturers float switches that fit the bill for this installation. For the trough, tilt float switches (http://www.madisonco.com/special_models/tilt_float.php) were selected for their durability; specifically, the M4546 model was employed due to its cost and availability.
The well level switches were more difficult to source given the space constraints required of the 5-inch to 5.625-inch wellbore diameter. The Madison model 8000 plastic, miniature, liquid level float switch was selected due to its availability, size and price. The 1/8-inch NPT fitting was paired with a 1/8-inch FIP to 1/2-inch MIP fitting for ease of mounting (see photographs below). A reverse-biased diode was installed to protect the magnetic reed switch used in these small float switches from voltage spikes resulting from the collapsing magnetic field of the associated control relay – even though the draw of the relay is only 0.7W.
Cable guards were installed in a special PVC pipe fixture mounted above and below the well float switches to ensure the arrangement was centered in the wellbore. This setup, for the high level float switch, is depicted in the below photograph.
Control relay CR4 (see later diagram) could have been saved and the pump driven by CR3 if CR3 was a two-pole device; however, by keeping it a one-pole device all relays are one-pole and therefore interchangeable This, of course, reduces spare parts stocking requirements.
All manual pilot devices are of the same style and share contact blocks.
By considering the mission across all wells, depths range from a few feet to hundreds of feet, components are interchangeable from well to well.
Pump manufacturers often sell a controller specifically for their pump. Because of the unique demands and mission for this installation, a simple, custom controller was fabricated. The control diagram appears below.
An external switch, SW1, enables the complete system and allows for easy shutdown. SW2 selects the second PV module for pumping or heating duties.
PB2 allows the user to manually operate the pump as a part of maintenance checks without having to open the controller. A maintained selector switch here would allow unattended bypass of the pump, say to fill a portable tank; however, the conservation mission dictated a momentary operator for this function.
PB1 allows for manual operation of the trough heater.
Note that all field devices are brought back to terminals in the control panel. This increases the field wiring, but it allows for 100% troubleshooting of field devices at the terminal blocks. Having all field devices on terminals also allows for bypass wiring in the event of critical or emergency needs.
As a nod to ease of maintenance, a 12Vdc indicator lamp was mounted in the control panel and wired to a test probe. This feature allows for easy troubleshooting without having to have a separate test tool. A copy of the control diagram appears on the inside of the front cover for easy reference.
An hour meter is an inexpensive addition that affords several capabilities of interest in this installation. Paralleled with the pump, the hour meter keeps track of the total time on the pump. This can be helpful for maintenance scheduling and documenting the runtime on the pump for repair or warranty claims.
Knowing the diameter of the wellbore and that float switches all but guarantee that water is produced when the pump runs, it is possible to calculate the total volume of produced water without the need of an expensive flow meter.
The installation took place over the interval of 30 May to 02 June 2013; each well took about one day and included fabrication of PV module mounting, pump configuration, trough configuration, controller mounting, field wiring, testing and erection of protective fencing.
All piping and controller subassembly fabrication was accomplished before travelling to the field.
One consideration for remote installations is the relative inability to obtain parts if not included in the load plan. Every aspect of the system should be tested prior to departing for the field. Full-scale, complete systems tests are necessary in order to avoid trips to town that consume an entire day – in this case, the round trip time to the nearest supply store was more than six hours. Over the course of four wells, there was only one part broken during assembly and installation; fortunately, a spare was included in the estimate.
In the first revision of the design, a control relay (CR1) was used to switch PV2 between pumping and heating based on the state of the thermostat. However, during the full-scale test it was discovered that the heater element pulled the voltage of the PV module below the must turn off threshold of the control relay. Interestingly, the potential for relay chatter was included in the design; but, only if the second PV module was lost. Unfortunately, this foresight was not continued to the switching of CR1. Not having a constant-voltage source is a natural byproduct of normalcy bias in mainstream designs; however, remote, off-grid considerations require a more thorough analysis. The solution was to use the second selector switch as a fixed setting for the function of the second PV module; CR1 was relegated to a ready spare part.
Testing of the first well uncovered a problem with the well level float switches. After anomalous behavior, the pump fixture was pulled and inspected. There had been little to no activity in the wellbore for an extended period of time. The scraping of the wellbore sides by the cable guards during installation knocked a considerable amount of rust and scale from the inside face of the well casing. This ferrous material was attracted to the magnet inside the switch float. For a time it seemed this discovery would force the abandonment of this approach. However, on further investigation, it was determined that if the well casing was descaled or swabbed, there was little ferrous material left in suspension. Once cleaned, all remaining pump switches performed as expected. Repeated inspections of the switch did not reveal further contamination.
Field wiring proved tedious; incorporating connectors would have allowed prefabricated subassemblies. Nothing was known of the surface or subsurface conditions; not knowing the physical arrangement prevented the use of devices to reduce the field wiring time.
Accurate surface and subsurface observations, dimensions and conditions are important for proper planning. Plans should include as much information and documentation as possible. Surface photographs would have significantly improved the subject installation.
It was discovered that trough float switches, in practice, are located fairly close to the same height as there is a good deal of switch travel on tethered, tilt switches – especially for relatively small tanks. An alternative design might use one switch with a timing relay to create the desired hysteresis between switching points. However, given the cost of timing relays compared to another float switch and considering power cycles from nighttime or sun obscuration makes the two switch arrangement a solid design choice.
The wellhead arrangement included a union for ease of disassembly and a capped extension above the flow line tee to the trough. The cap and pipe section allow for future design changes that might make use of the flow line. However, it became apparent, soon after installation, that a valve is needed in the flow line to divert water for local use. Ranch workers were able to quickly identify several uses for such an arrangement.
The efficacy of the trough heater will remain in question until at least the first winter season; too, the choice of troughs impacts heat retention, solar heating and geothermal effects. Otherwise, the installation proceeded as planned and each well has produced enough water to fill its trough on the same day as installation. Birds found the water almost instantly and were drinking alongside installers – clearly more worried about thirst than danger from humans. Other wildlife had found the water within a day. The solution was based on clearly defined objectives and analysis of trade-offs found in any engineering endeavor. The focus on reliability and maintainability seems well placed and is likely to provide years of trouble-free service in a remote, mountainous environment. The installation serves as a good model for design analysis that will ultimately benefit humanitarian installations in remote locations throughout the world.
Greig S. Latham is a Managing Member of Solatex, LLC, Allen, TX 75002, USA; 972-881-2349; anti-spam e-mail: golf-romeo-echo-india-golf at sierra-oscar-lima-alpha-tango-echo-xray dot charlie-oscar-mike).
 "New Mexico setting new records with drought", Las Cruces Sun-News (21 May 2013), http://www.lcsun-news.com/dona_ana_news/ci_23290815/new-mexico-setting-new-records-drought (accessed 27 May 2013)
Article # 0000
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