Solar Direct Water Systems

Water pumping is the most common application of direct drive. While most commonly used for irrigation, any residential or commercial water system can be powered by a solar direct pump, with additional storage tanks added on to maintain pressure when the sun is not shining.

Included in this document:

  • Water source
  • Types of pumps used in solar direct systems
  • Lift, head and flow
  • Where to buy a solar direct pump
  • Electrical supply and controls
  • Storage
  • Notes and resources


Water Source

We’ve built solar direct water systems that use a variety of water sources including rainwater, wells (both bored wells and modern deep wells), and surface water like ponds, creeks, springs, etc. If you have a well or can afford to drill one, this is probably your first choice because the water usually has little grit/debris, is potable and resistant to droughts. Also, the pump and plumbing is not impacted by rain events, and can be more easily protected from
freezing.

Rainwater is generally cleaner than surface water, but depending on your climate, it may be difficult to store enough for high-volume applications like irrigation and flush toilets. Surface water is typically more drought resistant than rainwater, but has more debris and grit, so any pump used for surface water must be able to tolerate this. Diaphragm pumps and helical rotor pumps are NOT good for surface water. A system that pumps surface water is going to be
higher-maintenance than a well or rainwater system, because the pump, input line, and other plumbing is likely to be disturbed by rain events and needs to be checked regularly.

Types of pumps used in solar direct water systems

Centrifugal

Sometimes called impeller pumps. Most conventional (not solar) water pumps are centrifugal. The mechanism that moves the water is an impeller that spins at high speed. Impellers come in many designs. One common design looks like the spiral cut out a snail shell. The water enters at the center of the spinning spiral. As the water is flung outward in the spiral, the centrifugal force pushes the water out the outlet of the pump.

The number of “stages” refers to the number of impellers. Single stage centrifugal pumps have only one impeller and are low-pressure pumps. Multi-stage centrifugal pumps use many impellers stacked on top of each other and are capable of more head (pressure).

Centrifugal pumps tolerate dirt, grit, or trash as long as the particles are not so large as to clog the impeller. This is partly why they are so popular. Some centrifugal pumps have impellers with widely spaced blades specifically so they can pump dirty water without clogging.

While solar direct centrifugal pumps don’t last forever, they have proven durability. Their biggest weakness in solar applications is that, unlike positive displacement pumps, the motor must spin at a minimum speed to pump at pressure. This means that if your well is deep, the water may not make it to your tank in heavy overcast conditions. A residential system with a deep well and a centrifugal pump must have sufficient storage to last through cloudy spells and/or be willing to conserve water in cloudy weather.

Positive displacement

A general term that refers to a number of pumps which can pump
pressure at low power. These are the two typically used in solar water systems are diaphragm
pumps and helical rotor.

  • Diaphragm: have very good pressure output and tolerate a wide range of motor speed. Their big weakness is that they do not tolerate any dirt or grit, and generally do not last as long as centrifugal pumps. We typically don’t recommend these pumps for a primary water source, but they can have a place in intermittent use applications or with very clean water.
  • Helical rotor: Also called a screw pump, helical rotor is becoming common for solar applications. Dirt/ grit resistance is probably not as good as centrifugal, but it’s better than a diaphragm pump. They are typically low flow pumps, better for residential applications than irrigation. Unless there are concerns with grit in the water, we usually recommend this pump for residential systems because pressure performance will not be impacted as much by clouds as it will with a centrifugal pump.


Lift, head, and Flow


All water pumps push much better than they pull, which is why submersibles have become standard. If you can’t use a submersible, and your pump must be above the water source, the distance from the water to the pump is the lift. Almost all pumps have very limited lift, so if you must use a surface pump, put the pump at the lowest point in the system.

The distance from the pump output to the maximum height that a pump will push water is the head. Head converts to pressure at the output of the pump at one PSI = 2.31 feet of head. Chinese pump companies use meters of head.

The volume of water that a pump can move is called the flow, measured in gallons per minute, or cubic meters per hour in metric systems. Flow and head trade off against each other, as demonstrated in a pump curve. A solar pump curve should show head and flow at different power inputs. The curve below is for a Grundfos SQFlex helical rotor pump.

Where to buy a solar direct pump

You have many options of companies that will sell you a direct drive pump. Where you should go depends on your budget and need for technical support.

  • Highest quality, high price: Buy direct from a US and European manufacturer. Sun Pumps, Grundfos and Lorenz are in this category. Their pumps cost over $2,000. Sun Pumps has the best technical support, but Grundfos has the most electrical flexibility with the SQFlex line. If you can afford it, we recommend going with one of these companies.
  • Moderate quality, low price: Buy direct from a Chinese manufacturer. We recommend Feili and Ningbo Cheer. Cost is in the $500 range. Companies do not sell “kits” or offer much technical support.
  • Moderate quality, high price: Buy from a US supplier of Chinese made pumps. In this category is RPS and Workhorse pumps, there may be more. You will pay two or three times as much as buying direct from China, for similar quality. You are paying for the customer service and technical support.

Electrical supply and controls

Look carefully at the pump curves and choose a pump with enough power draw (AKA wattage or horsepower) to meet the head and flow needs of your water system with some buffer. When choosing your pump you may also want to consider the power and voltage requirements of any other loads you will power with the same solar panels. We usually recommend a voltage around 90-100V for direct drive motors with high power needs, like water pumps. This voltage will allow you to power 90V PMDC and 120V universal motors with the same solar panels that you use for your pump. Additionally, we have found that high voltage pumps will perform better than low voltage pumps with the same power draw. So if you have a choice between a 48V pump and a 90V pump, go with 90V.

A rule of thumb is to build a PV supply at least 1.3X the power input recommended for the pump. It doesn’t hurt to go higher, depending on other loads on the system and expected cloud cover. However, you must decide carefully whether to wire the panels in series or parallel, depending on the voltage tolerance of the controller. Most, maybe all, decent quality solar pumps come with MPPT controllers that tolerate a range of voltage input. While it’s typically best to aim for the high end of this voltage range, to minimize voltage drop, every controller has a hard upper limit of voltage tolerance that must not be exceeded.

Most pump controllers can be wired with sensors to prevent running the pump from running dry, and also to turn off the pump when a tank is full, or the system comes up the pressure. A float valve or “reverse action” pressure control switch may be used for this function. Or, a standard pressure control switch may be used between the power supply and the controller. If you do this, be sure to put snubbers across the switch.

Storage

If your solar direct water system is used for irrigation only, no storage may be needed- just irrigate when the sun shines. However, for residential systems, most people appreciate having water pressure all the time. There are several options for water storage that can maintain pressure in your system at night and during cloudy spells.

Gravity

Gravity is the cheapest, most low-tech solution for water pressure, already in common use around the world. For example, in Puerto Rico, almost everyone has a cistern on their roof to make up for gaps in supply of municipal water. This works well because most people have flat, concrete roofs, and freeze protection is not an issue.

It’s important to note that people in the global south are accustomed to low water pressure much more than we are in the US. Gravity works, but it is a wimpy source of pressure. Remember that 2ft of head is about the same as 1 PSI. This works in reverse: a 10’ roof will give you 5 PSI at the faucet. This feels pretty weak compared to the 30 PSI shower that Americans are used to. So yes, you can put a cistern on your roof, but it will need to be drained seasonally if you don’t live in the tropics, and the pressure will be weak.

If you live someplace hilly, a good option is to place a water tank uphill of your house. Remember that you need a lot of rise. For example, you need 40-50ft vertical lift to give you 20-30 PSI at the faucet. Also remember to bury the plumbing, and all or part of the tank itself (depending on the severity of the winters in your region) to protect the system from freezing.

Pressure Tanks

For most residential water systems outside the tropics, we recommend well pressure tanks for storage. These tanks are commonly used in rural water systems, therefore widely available and familiar to most plumbers. They are relatively easy to install, and can be put indoors to protect from freezing. They contain an air bladder which is compressed by the water when the pump is running to create pressure. On a conventional system, a pressure control switch will turn the pump on every few hours, whenever pressure in the tank falls below a certain threshold (typically 20-30 PSI). In a solar water system, it works the same way but the pump turns on less frequently- perhaps every 12 hours- because the tanks are bigger and the pump can only run when the sun shines.

The biggest issue with these tanks is the cost, which is around $10 per gallon on water storage. (Keep in mind that these tanks are half air by volume, so a 86 gallon tank can hold 43 gallons of water.) Depending on water needs, this can add up to thousands of dollars worth of tanks. On the up side, tanks can be installed incrementally. You will only need one to get started, and you can always add more later.

Well pressure tanks can be steel or fiberwoven. Steel tanks are more common, but we have seen the liners fail and put rust in the water, so we prefer fiberwoven, especially if your water is corrosive. The cost is similar for steel or fiberwoven. When sourcing tanks, check the pressure rating and make sure it’s sufficient for your system. This is another way we have seen these tanks fail.

Installing and managing storage

How much water storage to install? There’s no such thing as too much, but you’ll probably be limited by budget, and maybe physical space. How much you need depends on your solar conditions, type of pump, and of course, your water use. Every system is different.

Here’s an example for your reference. At Living Energy Farm we have a centrifugal deep well pump, which is not a positive displacement pump and requires a minimal power input to pump at pressure. (We chose this pump instead of helical rotor because we have a need for high flow for irrigation.) We live in a climate that experiences heavy cloud cover, sometimes for days at a time. Therefore we need at least two days of storage. Our community population is 8-10 people.

Over the years we have installed three (3) 120 gallon well tanks plus one 60 gallon tank, for a total of 210 gallons of pressurized storage. This is sufficient for our needs, but we are very conservative with water during cloudy spells. We also supplement with rainwater for nonpotable uses to stretch out our storage. And we do not use conventional flush toilets. We do have a low flow toilet that feeds our biodigester, but we use rainwater to flush when it’s cloudy.

If you use a helical rotor pump instead of centrifugal, this will improve your pressure on cloudy days and you may need less storage or redundancy in the system (like using rainwater).

Redundant storage on a water system can be helpful to prevent a complete loss of pressure, and also improve community harmony. For example, if one pump supplies several households, each households’ storage tanks may be kept separate with check valves to prevent backflow. In this system, each household manages their own storage, providing better user feedback and less hard feelings when one mistake drains the whole system.

We generally recommend against flush toilets, but if you must use them, consider installing dedicated storage to supply the toilets, like an elevated cistern. Low water pressure is not an issue for flush toilets.​

Notes and Resources

  • ​Water pumps should never be exposed to freezing temperatures while full of water, or run while dry. Design your system to prevent this.
  • While there are lots of cheap 12VDC pumps on the market, performance and durabilityis usually poor. If you have a lot of head on your system, go for 90V or higher. For lower head systems, 24V or 48V is recommended.
  • A good source for fiberwoven pressure tanks is Aquascience. Steel pressure tanks are sold at most plumbing supply houses and big box stores, although you may have to special order the big ones. Keep an eye on Craigslist, as smaller used tanks are sometimes listed for free or cheap.
  • Recommended pump companies: Sun Pumps (https://www.sunpumps.com); Grundfos, especially SQFlex (you’ll have to find a US supplier); Feili (http://www.feilisolar.com); Ningbo Cheers (http://www.pumps-in-china.com). You can’t reach these Chinese companies over the phone, but they will talk to you through whatsapp or Ali Baba.