Lab exam answers

1. 1. Short circuit current: The short-circuit current is the current through the solar cell
when the voltage across the solar cell is zero (i.e., when the solar cell is short-
circuited).
Open circuit voltage: The open-circuit voltage, VOC, is the maximum voltage
available from a solar cell, and this occurs at zero current.
2. At very low speeds, there is insufficient torque exerted by the wind on the turbine
blades to make them rotate. However, as speed increases, the wind turbine will begin
to rotate and generate electrical power. The speed at which the turbine starts to rotate
is called startup speed, and the speed at which the turbine first starts to rotate and
generate electrical power is called the cut-in speed.
3. Tachometer: Tachometers are generally used to measure the rotation speed (angular
velocity) of a shaft or disk, as in a motor or other machine.
Anemometer: Anemometer is a device used for measuring the speed of the wind.
4. The relationship between the wind speed and the rate of rotation of the rotor is
characterized by a non-dimesional factor known as the tip speed ratio (TSR). The
TSR for wind turbines is the ratio between the tangential speed of the tip of a blade
and the actual velocity of the wind
5. 𝜆 =
𝜔𝑅
𝑣
=
2𝜋∗𝑅𝑃𝑀∗𝑅
60∗𝑣
=
2∗3.141∗720∗0.6
60∗2.5
= 18.095
6. Flash Point: The flash point is the lowest temperature at which the vapours of a
volatile material ignite when we provide an ignition source. This temperature does not
depend directly on the temperature of the ignition source. However, usually, the
temperature of the ignition source is considerably higher than the flash point.
Fire Point: The fire point of a fuel is the lowest temperature at which the vapour of
fuel can continue to burn for at least five seconds after the ignition has started. It
means; the term fire point describes it as the lowest temperature for a substance to
hold the combustion for a small time period after the ignition by an open flame.
Usually, the fire point of a substance is about 10°C higher than the flash point of the
same substance.
7.

2. 8. LMTD: The log mean temperature difference (LMTD) is used to determine the
temperature driving force for heat transfer in flow systems, most notably in heat
exchangers. The LMTD is a logarithmic average of the temperature difference
between the hot and cold streams at each end of the exchanger. The larger the LMTD,
the more heat is transferred. The use of the LMTD arises, straightforwardly from the
analysis of a heat exchanger with constant flow rate and fluid thermal properties.
9.
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