Agricultural and Biosystems Engineers Licensure Examination Renewable Energy Sources (Solar, Wind): practice questions

C. Wind

Wind is continually replenished by solar heating of the atmosphere. Coal, natural gas and uranium are stocks laid down over geological time and used far faster than they form.

C. Net consumption: energy imported minus energy exported over the billing period

Under net metering the meter effectively runs backwards on export, so the bill is for net import. Selling all generation at a fixed price and buying all consumption separately is gross metering (a feed-in tariff). Exact rules and settlement prices are set by each national regulator.

A. 5 years; it ignores the time value of money and savings after payback

Simple payback = 300,000 / 60,000 = 5 years. It treats a unit of money in year 5 as worth the same as today and ignores everything after year 5, so it is a screening tool; NPV or LCOE are used for the decision. 0.2 is the inverse ratio (60,000/300,000).

A. 84.85°

At noon θz = |φ − δ| = |28.6 − 23.45| = 5.15°, so altitude α = 90 − 5.15 = 84.85°. Taking θz = φ + δ gives 37.95°, a low sun wrong for midsummer; 5.15° is the zenith angle, not the altitude; 61.4° ignores declination (equinox value).

B. 11:41.8

Solar time = standard time − 4(Lst − Lloc) + E for a site west of its standard meridian in the eastern hemisphere: 12:00 − 4 × (82.5 − 77.2) min + 3 min = 12:00 − 21.2 + 3 = 11:41.8. Adding the longitude correction gives 12:24.2 (or 12:18.2 if E is also subtracted), and subtracting E gives 11:35.8. The site lies west of the standard meridian, so the sun reaches it later than the clock suggests.

A. Pyrheliometer

A pyrheliometer is a collimating tube pointed at the sun on a tracker, so it sees only the sun's disc and a narrow ring around it. A pyranometer sees the whole hemisphere (global radiation); the Campbell–Stokes recorder gives sunshine hours; an anemometer measures wind speed.

B. 857 Wp

Array = daily energy/(PSH × derating) = 3000/(5 × 0.7) = 857 Wp. 600 Wp ignores losses; multiplying by the derating gives 420 Wp; 2100 Wp is 3000 × 0.7, a confusion of energy with power.

B. Disconnects the inverter when the grid fails, so the system does not energise lines being repaired

An 'island' is a section of network kept live by local generators after the grid has gone down; it endangers line workers. Grid-tied inverters must shut down on loss of grid, which is why an ordinary on-grid system without battery backup gives no power in an outage.

C. Water in an elevated tank or reservoir

Storing water is far cheaper and longer-lived than storing electricity, so most solar pumps are battery-less and directly coupled through a controller, with a tank or the soil profile buffering supply.

D. Turns the rotor partly out of the wind (usually through the tail vane) to limit speed and loads

Above the design speed a windmill's rotor could overspeed and overload the pump rod. Furling swings the rotor edgewise to the wind, reducing its effective area. It does not change the pump's action.