Sizing a Solar-Powered Irrigation Controller
A student team is designing a solar-powered drip-irrigation controller for a school vegetable garden. The controller must run a 12 V DC solenoid valve (holding current 0.45 A) for 30 minutes each morning and power a microcontroller that draws 40 mA continuously. The chosen panel is rated 12 V, 10 W at standard test conditions, and the battery is a 12 V, 7 Ah sealed lead-acid unit. The team plans to mount the panel flat on a shed roof in southern Victoria in winter, when usable peak-sun equivalents drop to about 2.5 hours per day and the panel's real output is roughly 70% of its rated figure. The teacher asks the team to justify whether the system can run indefinitely without mains charging, and to identify what would happen in a week of heavy overcast weather.
1. Calculate the daily energy demand and the realistic winter daily energy supply, then judge whether the system is self-sustaining.
Read the worked answer
Load energy per day
- Valve: 12 V × 0.45 A = 5.4 W; for 0.5 h → 2.7 Wh.
- Controller: 12 V × 0.040 A = 0.48 W; over 24 h → 11.52 Wh.
- Total ≈ 14.2 Wh/day.
Realistic supply
- Rated 10 W × 0.70 (derating) = 7.0 W effective.
- × 2.5 peak-sun hours = 17.5 Wh/day.
Judgement Supply (17.5 Wh) exceeds demand (14.2 Wh), so on a clear winter day the system has a small surplus of about 3.3 Wh - roughly 19% margin. That margin is thin. Any soiling, partial shading, or a cold snap extending valve run time erodes it. The conclusion is 'marginally self-sustaining in clear winter weather, not robust'. A defensible recommendation is to reduce controller sleep current, shorten valve time, or add a second panel rather than declare the design adequate.
2. Using the battery capacity, estimate how many consecutive overcast days the system could survive, and explain the practical limits of that estimate.
Read the worked answer
Usable stored energy A 7 Ah, 12 V battery holds 84 Wh nominally. Lead-acid should not be discharged below about 50% depth to preserve cycle life, so usable ≈ 42 Wh.
Survival time 42 Wh ÷ 14.2 Wh/day ≈ 2.9 days - call it under three days with no solar input.
Practical limits
- The 50% depth-of-discharge rule is a design choice, not a physical cliff; deeper discharge shortens life.
- Cold reduces usable capacity; heat reduces life.
- Actual controller current may spike during valve switching, and the solenoid's inrush current is higher than holding current, so instantaneous voltage sag matters even if average energy is fine.
- A week of heavy overcast would therefore flatten the battery around day three, leaving the valve unpowered and possibly the controller browning out mid-cycle.
Judgement The battery is undersized for the stated weather risk. The team should either accept a defined failure mode (controller shuts down safely, valve closes) or increase capacity and document the trade-off in mass and cost.