Question

You have access to two sources of energy that can be used to turn a steam turbine in a heat engine (a steam turbine is a device that converts thermal energy into mechanical work):

1. Superheated steam at 400°C and 20 bar
2. Superheated steam at 600°C and 20 bar
Which of these two energy sources can be converted into more mechanical work per unit mass of flowing steam in a heat engine? You may assume that each steam source will leave the turbine at 200°C and 1 bar.

Answer 1

Option 2.

Step-by-step explanation:

A turbine produces mechanical work at the expense of fluid energy, that is to say according to the First Law of the Thermodynamics:

`w_(out) = h_(in)-h_(out)`

The conditions at inlet and outlet are, respectively:

Inlet - Option 1 (Superheated Steam)

`h=3248.4\,(kJ)/(kg)`

Inlet - Option 2 (Superheated Steam)

`h=3690.7\,(kJ)/(kg)`

Outlet (Superheated Steam)

`h = 2875.5\,(kJ)/(kg)`

The mechanical work per unit mass of each option is determined hereafter:

`w_(out, 1) = 3248.4\,(kJ)/(kg) - 2875.5\,(kJ)/(kg)`

`w_(out, 1) = 372.9\,(kJ)/(kg)`

`w_(out, 2) = 3690.7\,(kJ)/(kg) - 2875.5\,(kJ)/(kg)`

`w_(out, 2) = 815.2\,(kJ)/(kg)`

The second option offers more mechanical work per unit mass.