Organic Rankine Cycle

An Organic Rankine Cycle (ORC) uses an organic working fluid that has a boiling point less than that of water to convert low-temperature heat into mechanical work. The mechanical work that is generated can then be converted into electricity.

Heat Engine

A heat engine is a simple engine that converts thermal heat into mechanical work. A heat engine operates by extracting heat from a hot reservoir and moving it over to a cold reservoir, generating work in the process. A drawing of a basic heat engine is shown below:

Heat Engine

In order to maximize the amount of work a heat engine can produce, the temperature of the hot reservoir needs to be increased as much as possible, whereas the temperature of the cold reservoir needs to be reduced.

Carnot Efficiency

An ideal (theoretical) heat engine operates at the Carnot efficiency. The Carnot efficiency is the maximum possible efficiency that any engine can achieve, regardless of size, complexity, money spent or the amount of time allowed for the engine to perform work. Carnot efficiency can be calculated by the expression:

Equation

Where:

is energy exiting the system as work, or in our case, electricity.

QH is heat put into the system.

TC is the absolute temperature of the cold reservoir (the cooling source temperature).

TH is the absolute temperature of the hot reservoir (the hot water supply temperature).

When calculating the Carnot efficiency, all temperatures are given with respect to absolute zero, i.e. for Celsius temperatures you must add 273.15 to convert to Kelvin scale.

Carnot efficiency is a theoretical efficiency that cannot be accomplished in reality.  In reality, the efficiency of any heat engine is significantly reduced by various factors such as heat loss, pressure drops, and frictional losses.