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Solar Solutions Photovoltaic solar circulator pumps


A


photovoltaic (PV) panel is basically a solar-pow- ered, 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 instal- lations, I have used a PV panel as the only means of con- trol 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 electri- cal 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 possi- ble 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 continu-


ous duty at high temperatures (e.g. 200 – 230 F), be free of corrosion or deterioration when in contact with gly- col/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 sta- tionary case (the stator) to the rotating shaft (the rotor). The rotor on a “brush” motor has a cylindrical set of elec- trical 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 com- mutated motors (ECMs), which are used in the Hartell,


Page 32/Plumbing Engineer


Bristol Stickney, chief technical director, SolarLogic LLC, Santa Fe, N.M.


Ivan and Laing examples shown in the Figures.


Unique design fea- tures 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 con- structed with mag- netic


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 com- monly 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 replace- ment 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 hemi- spherical rotor is balanced on a single ceramic ball bear- ing, 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 Continued on page 34


October 2011


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