Photovoltaic solar circulator pumps

By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.

A photovoltaic (PV) panel is basically a solar-powered, direct-current (DC) battery. It provides DC electricity when exposed to the sun and acts like a “dead battery” after dark. This provides an elegant control strategy for solar equipment that is required to run during the day and shut off at night. In many solar heating installations, I have used a PV panel as the only means of control to operate the glycol circulator pump. In most cases, this has been accomplished when a properly-sized PV panel is hard-wired to a matching DC circulator.

When the sun shines, the pump runs, circulating glycol through solar collectors to gather heat. When it is partly cloudy, the pump runs slower, which is a good match for the needs of a solar thermal collector. When the sun goes down, the pump stops. As an added bonus, this type of system will continue to circulate solar heat even during daytime power failures on the conventional AC electric grid. This provides an extra measure of protection against the collectors’ overheating during a grid power failure.

For this approach to work reliably, the pump motor must be able to handle a wide range of voltages during normal operation. PV panels are commonly manufactured to produce 0 – 17 volts DC when connected to an electrical load, with an open-circuit voltage of around 21 volts DC in full sun. These voltages are ideal for charging a 12-volt battery, and I suppose that is why PV panels with these characteristics are so common. Not all “12-volt nominal” motors, however, can handle 17 volts or possible switch-contact surges of 21 volts. So I have always been on the look-out for circulator pump motors that are compatible with PV-direct wiring.

A good PV solar circulator must also provide continuous duty at high temperatures (e.g. 200 – 230 F), be free of corrosion or deterioration when in contact with glycol/water heat transfer fluid and have enough pumping power to start up in the morning on a cold day, when the glycol is at its thickest. Figure 39-1 and Figure 39-2 show pictures of four examples that meet these requirements that I have used in recent years.

Electronically commutated motors (ECM)

The earliest DC pump motors have always employed “carbon brushes” to transfer electric power from the stationary case (the stator) to the rotating shaft (the rotor). The rotor on a “brush” motor has a cylindrical set of electrical contacts built into one end called the “commutator.” The brushes are rectangles made of graphic that rub against the spinning commutator and are slowly worn away with normal use. While brush-type motors are still available (e.g. Figure 39-1, March model 809), replacing the carbon brushes every two to five years has proven to be a costly maintenance headache. This headache has been eliminated by the development of DC electronically commutated motors (ECMs), which are used in the Hartell, Ivan and Laing examples shown in the Figures.

Unique design features

All the pump motors shown here use a magnetically coupled impellor, which eliminates the need for a physical connection between a motor shaft and the impellor. The impellors are constructed with magnetic material embedded behind the impellor vanes. The Hartell and March versions use a rotor shaft to spin a circular magnet that causes the impellor to spin in response to the moving magnet. The Hartell MD10HEH model, shown in Figure 39-1 is a brushless ECM circulator that has been around for decades. I have installed these on many PV-pumped solar hot water systems over the years and found them still running normally a dozen years or more later.

The Ivan and Laing examples use electronic controls to create a rotating magnetic field surrounding the impellor enclosure, which causes the impellor to spin. Since this magnetic field-generator is not really a motor as we commonly know it, Ivan Labs calls it a “static impellor driver” (SID), which is where the name “El SID” comes from in their line of circulators. Laing just refers to the motor-end of their circulator as “the stator” since it has no moving parts.

The Ivan SID motor was designed to fit on the March pump body, which can be seen in Figure 39-1. The March impellor enclosure is designed to allow motor replacement without breaking into the plumbing. A motor replacement can be made without leaking any glycol or glycol pressure, and it is possible to replace a March 809 brush motor with an Ivan SID motor (or vice versa) using only a screw driver.

The Laing D5solar circulators are perhaps the most innovative and unusual, as seen in Figure 39-2. A hemispherical rotor is balanced on a single ceramic ball bearing, and magnetic material embedded in the hemisphere moves in response to the field generated by the stator. The stator/pump motor screws onto the pump body with a threaded ring that resembles a large O-ring union that is very easy to handle. This is the same motor attachment that they have used on their hot water re-circulator pumps for decades, so the D5solar pump motor is interchangeable with other motors from this line of Laing circulators. In some installations, we have swapped the D5solar DC motor with the E1 or the E3 AC motor to provide more pump flow when more collectors were added or when an AC control system was retrofitted.

The Laing D5 is not just an ECM device; it is also endowed with “brains” built into its integrated circuits. It features a soft start, maximum power point (MPP) tracking and an over-temperature protection shut off capability. Most models also include a selector switch to allow the user to choose a speed or a startup voltage (depending on the model). This is the most commonly used PV solar pump used in our region of northern New Mexico today.

Pumping power

Figure 39-3 shows the pump curves for some of the most common DC solar circulators. As you can see, these are small circulators that typically run on 20 to 30 watt PV panels. The larger circulators are more popular and typically produce around two to four gallons per minute of flow through a solar collector loop. They are most easily applied to water heater systems where there are only a few collectors and where the pipe runs are not very long. I have also used them on larger banks of collectors (six to eight in parallel) successfully, as long as the pipe resistance, the size of the collectors and the PV panel are carefully matched to the capabilities of the circulator.

Control options

Early morning startup, when the solar collectors are still cool, can sometimes be an issue with PV-direct circulators. Sometimes the owner or installer will want to delay startup until the solar heat collector reaches a certain high temperature before allowing the circulator to turn on. The optional voltage knob on some versions of the Laing D5solar offers one solution to this issue. It can be adjusted to wait until a certain minimum PV voltage is generated before allowing the pump to start up. Also, on some systems, I have added a snap-disk or cap-tube temperature switch to the hot pipe on the collector to prevent the pump from running until a minimum temperature is available. There are also several PV-direct differential temperature controllers available that allow precise electronic control of the circulator pump (e.g., Eagle D2E1).

In some cases, the solar circulator is used for heat dissipation functions that require it to run at night occasionally. This is used, for example, to automatically cool an overheating solar storage tank when the occupants are on vacation. In these cases, we have used an AC to DC power supply to run the pump at night, wiring it in parallel with the PV panel so that it has two power sources (e.g., Tripp-Lite PR-7b).
These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned only to provide examples for illustration and discussion and do not constitute recommendation or endorsement.

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.