Regenerators for Stirling Engines

Introduction

What is a Stirling Engine?

• A heat engine invented by Dr Robert Stirling in 1816

• The temperature gradients in the hot and cold regions of the engine cause the working fluid to expand and contract, generates the mechanical work.

•The Stirling engine is an external combustion engine which works on the principle of thermal expansion and contraction of the working fluid.

Introduction

How this might actually be arranged:

Gas is in hot cylinder and expands, driving pistons in

Expansion Stage

Introduction

How this might actually be arranged:

Flywheel momentum moves gas to cold cylinder

Transfer Stage

Introduction

How this might actually be arranged:

Gas is in cool cylinder and contracts, pulling pistons out

Contraction Stage

Introduction

How this might actually be arranged:

Flywheel momentum moves gas to hot cylinder

Transfer Stage

Introduction

But many configurations are possible:

Introduction

Advantages: Disadvantages:

• High efficiencies of 30-40%

• Quiet operation

• Can be run in reverse

• Low maintenance

• Can run from any heat source

• Relatively Expensive to produce• Requires double the

cooling capacity of an equivalent diesel or petrol engine

• Slow response time

A 25 kWe solar dish Stirling

Introduction

High performance regenerator

Large specific

surface area

Low dead volume

High thermal capacity

High thermal

conductivity

Low pressure

drop

The regenerator is a key component of a Stirling engine.

Any improvement will have a knock on effect on the overall engine efficiency

Introduction

Regenerator Requirements:• High heat capacity » maximize stored energy• High thermal conductivity » fast temperature distribute

• Large surface area » good heat transfer between solid and working fluid

• Variable porosity» small dead volume

• Variable pore size» low pressure drop

• Cheap manufacturing » reduce costs

Would a porous metal foam be suitable?

Other Advantages:

• Easily variable structure » tailor properties

In Stirling engines, it is very easy to spend more energy to overcome friction power than to transfer heat energy.Heat transfer increases by less than the first power of the

velocityThe friction power expenditure increases by as much as the

cube of the velocity.Increasing the pore size reduces the thermal

performance but it is much lower than the friction power reduction.

How Best to Optimise?