Power for a rural vaccine clinic
A rural clinic needs 4.0 kWh of electricity every day, and its vaccine fridge must never rise above 8 °C for more than 30 minutes. Option A is a 1.5 kW solar array with a 6.0 kWh battery bank; it delivers about 5.0 kWh per day in the dry season but only 1.8 kWh per day during the three-month rainy season. Option B is a 2.0 kW diesel generator that runs 4 hours a day and uses 0.9 dm³ of fuel per hour; fuel costs $1.20 per dm³ and is carried 30 km by motorbike. The clinic has a $900 budget for the year and no grid connection.
1. Calculate the daily and yearly fuel cost of Option B and check it against the $900 budget.
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Fuel per day = 4 h × 0.9 dm³/h = 3.6 dm³ Cost per day = 3.6 × $1.20 = $4.32 Cost per year = $4.32 × 365 = $1576.80
That is well above the $900 budget, and it excludes buying and servicing the generator, oil, and the cost of carrying fuel 30 km - transport alone would add a substantial amount.
If the generator ran only during the rainy season (about 90 days), fuel would cost $4.32 × 90 ≈ $389, which is affordable. Running it all year is what breaks the budget.
The calculation shows that running cost, not purchase price, decides the issue. Any comparison that looked only at capital cost would give the wrong answer.
2. In the rainy season the array delivers 1.8 kWh per day against a 4.0 kWh demand. Work out the deficit and decide whether the 6.0 kWh battery saves the clinic.
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Deficit = 4.0 − 1.8 = 2.2 kWh per day.
The battery holds 6.0 kWh, but a lead-acid bank should not be flattened regularly; perhaps 4.5 kWh is usable. 4.5 / 2.2 ≈ 2 days.
So the clinic can ride out about two heavily overcast days in a row. On a third day the fridge loses power. Since the fridge must not exceed 8 °C for more than 30 minutes, and an unpowered fridge warms through its insulation in a few hours, the risk is serious rather than theoretical.
The honest conclusion is that the battery covers short cloudy spells but not a run of bad days, so solar alone is unreliable in the rainy season. Extra panels, or a small generator switched on only when battery charge falls below a set level, would be needed.
3. What further information would you collect before advising the clinic, and why does each item matter?
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First, the longest run of consecutive cloudy days in a bad rainy season: battery size depends on the worst run, not the average.
Second, the fridge's real consumption and duty cycle. A vaccine fridge may use 1.0–1.5 kWh per day, so the 4.0 kWh total may be dominated by lighting and equipment that could be trimmed.
Third, how long the fridge actually holds below 8 °C with no power, which sets the acceptable gap.
Fourth, the delivered output of the 1.5 kW array rather than its rated figure, since dust, tilt, shading and age all reduce it.
Fifth, fuel supply and maintenance reliability: a generator that cannot be serviced or refuelled when the road floods is not a backup at all.
The decision should rest on the worst case, not the average day.