3.6. Solar pump control

Greg Lawson edited this page Jun 2, 2011 · 4 revisions

Running a pump (and probably any motor or electrical device) on solar power presents some challenges. (see 3.6.1.-Pump Specs and 3.6.2.-Solar Panel Specs)

  1. Solar production varies substantially by season and time of day.
  2. Solar panels are relatively expensive, so minimizing the cost is a major goal,
  3. Battery backup and AC inverters are relatively expensive and require multiple replacements during the lifetime of other components,
  4. Most consumer electrical products run on fixed voltage AC or DC power, but solar panels produce varying DC power,
  5. Most motors have low resistance (e.g. 2 to 3 ohms) leading to relatively high currents and lower voltages while most solar panels are designed for higher voltages and lower currents. (3.6.5.-this is not inevitable)

Voltage and Current Characteristics

3.6.4.-Voltage and Current Characteristics of Solar Panels and Pumps

Solar Hot Tub Control Systems

These challenges lead to considering designing some sort of control system. The following solar heating pump control types have been experimented with for a solar hot tub heating system:

  1. totally passive - connect pump directly to solar panel. Produces water temperatures incompatible with human comfort. Too hot (up to 130F in summer) and too cold (below 50F in the winter),
  2. solar maximizer - $20 electronic kit previously available from Futurlec Electronics. Uses 3.6.3.-Pulse Width Modulation to boost voltage to 12 or 24 volts by switching power on and off rapidly. I found this system undesirable for two reasons:
    1. The device only operated for a few weeks, and
    2. it was loud, probably because:
      1. the high voltage runs the pump much faster and much louder and
      2. the switching frequency was probably in the audio range adding to the sound.
  3. thermostat - simple on/off switch installed by ABC Solar. This solves the problem of the water getting too hot in the summer. In the hottest weather the pump was off most of the time. Unfortunately since I was using the pump to also circulate the water past a UV disinfecting light, the hottest weather produced the smelliest biological growth. ABC Solar solved this problem by installing a second pump to circulate past the UV light. Also since the second pump was installed in parallel, the voltage / current mismatch was made worse. Having just realized this I'll have to schedule an experiment with the pumps wired in series.
  4. two-way switch - manually switch between modes based on slowly changing criteria such as season or recent filling with cold water. Experimented with heat-only mode and thermostatic mode.
  5. relay - Two way switch controlled by either solar cell or thermostat. Relay burned out prematurely, possibly because long mornings and afternoons of solar power provided not enough current to switch relay, but enough to heat relay coil.
  6. data logger - specialized logomatic micro-controller that writes multiple voltages to SD memory card. Design and implementation in progress. Current hurdle is excessive 60 Hz AC pickup in sensor wiring. Hope to use twisted pairs of Cat-5 Ethernet cabling to reduce this.
  7. motor control full bridge driver - allows low power digital control of higher power motor drive current. (see interface board or 4A L6203 full bridge chip)
  8. motor control PWM chip - combines PWM controller with H-bridge - requires custom circuit board for heat dissipation.Allegro 3959 chip
  9. micro-controller - the ultimate smart controller - requires data from successful previous experiments. AVR
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