Wednesday, August 26, 2009

Some Sterling Stirlings


Some Sterling Stirlings

What started me reading up on the Stirling engine was research reading I was doing on some new solar energy projects. one of these projects uses a Stirling engine to convert the sun's energy into rotary motion and thus into electricity. I will write about this later on.

Lots of Modern uses for the Reverend Stirling's Invention

A variation of the Stirling engine can be used as a heat pump for refrigeration or air conditioning, when it is driven by an electric motor. There is also a machine called the "Stirling Cryocooler" which is used to liquefy air and other gases.

Wikipedia has a very interesting entry for Stirling engine uses:

"Marine engines:

The Swedish shipbuilder Kockums has built 8 successful Stirling powered submarines since the late 1980s.

They carry compressed oxygen to allow fuel combustion whilst submerged which provides heat for the Stirling engine. They are currently used on submarines of the Gotland and Södermanland classes.

They are the first submarines in the world to feature a Stirling engine air-independent propulsion (AIP) system, which extends their underwater endurance from a few days to two weeks. This capability has previously only been available with nuclear powered submarines.

Tiny Stirlings

There is a market in model Stirling engines, and quite a few interest groups around the world. Some of the more spectacular at demonstrating the principle are the little low pressure low temperature engines which can work from the heat of your hand or happily run for months on the heat of an Internet router. The computer component company MSI have developed a Stirling engine that uses the heat of a CPU (Central Processor Unit) to drive the fan which help cools the chip. I does not appear to have reached the shops though, I have been trying to locate a unit to play with and have failed to find one!!

The Stirling principle is used in this toy low-pressure engine

Here is a very novel and practical application of a low pressure Stirling engine
made by MSI
used as a CPU cooler fan. (Not available in the shops!!!)
(OOps just spotted this is a NorthBridge cooler not a CPU cooler)

Stirling Engine based CHP units

Micro-Generation is a term which is becoming more popular in recent times. Previously it referred almost exclusively to small domestic wind generators. Recently there is a trend to use heat in domestic gas boilers to generate electric power. The companies developing these CHP units claim that it is a a far more economical way of generating electricity than the large scale central model currently in use.

CHP Micro-Generation Efficiency Figures published by Disenco UK

Here are two examples of (CHP) combined heat and power using a modern reincarnation of the Reverend Stirling's engine. The first example is from a UK based company called Disenco.

Top above is test model of the Disenco CHP gas boiler
Below it is an artist impression of the unit

IMHO CHP units would not be of much interest as yet in Ireland , not until smart meters are introduced, where micro-generating units like the above can feed power back into the electricity network and get paid or credited, at a reasonable rate, for the power generated.

Pictured below is a very very similar unit from a New Zealand based company called Whispergen, the idea seem to be catching on in several places at once.

Stirling application by New Zealand company Whispergen

Whispergen are a Zealand based outfit. They do have agents in Europe etc. check out their site, link below. Thanks to Jim O'Riordan for the heads up on this one.

The layout of the CHP unit:
A) Gas burner: flame heats the four cylinder heads of the engine
B) Exhaust heat recovery unit: uses the exhaust heat to heat the water
C) Stirling engine: uses the heat from the burner to push pistons back and forth
D) Water heating: water is further heated as it passes through the engine
E) Wobble Yoke (inside): converts the linear motion of the pistons into
rotating motion for the alternator
F) Gas valve: supplies and regulates the gas to the burner
G) Rotary alternator (inside): generates electricity
H) Water pipe connections: delivers the hot water to the home’s hot water
cylinder and heating system

In the next Stirling post I will publish some photos of solar generators using Stirling engines.


Tuesday, August 25, 2009

Variations on a Theme of Stirling



Variations on a Theme
of Stirling

There are three fundamental variations of the Stirling engine. All three have two pistons but these work in different ways in the variations.

The Alpha Stirling Engine

The Alpha type Stirling engine has TWO power generating pistons. The two pistons are in separate cylinders, one hot and one cold. The Alpha configuration has a high power-to-volume ratio. It also has problems with seals and lubrication due to the high temperature of the hot piston. The hot piston generally has a hefty insulating head which adds inefficiency.

The Beta Stirling

The Beta type Stirling has only a single POWER piston. This is fitted in the same cylinder on the same shaft as a second piston called a displacer.

The displacer piston fits loosely in the cylinder and does not generate any power. It's sole purpose is to shuttle air / gas from the hot side to the cold side.

When the working gas is pushed to the hot end of the cylinder it expands and pushes on the power piston. When it is pushed to the cold end of the cylinder it contracts and the momentum of the machine, assisted by the flywheel momentum, pushes the power piston the other way to compress the gas. The beta engine avoids the difficulties of hot moving seals.

The Gamma Stirling

The Gamma Stirling is a Beta Stirling with the power piston mounted in it's own separate cylinder. The hot air/gas flows between the two cylinders as a single body. The Gamma configuration produces a lower compression ratio but has the advantage of being simpler and is often used in multi-cylinder engines.

Another diagram of the Stirling Gamma engine
close enough a layout to the working model below

Another toy model clearly showing the principle of the above diagrams

In the next post on Stirling, I will be looking at some of the interesting applications and variations of the engine.


Monday, August 24, 2009

The Reverend Stirling's Engine



Robert Stirling's Unique Engine
2nd post on the Reverend Stirling's Engine

Beautiful German model of a gamma Stirling engine
clearly showing the 2 piston configuration

(note the cooling fins on the power piston)

It is almost 200 years since a minister of the Church of Scotland invented a unique type of engine. His engine employed radically different techniques to anything before and much since. It took science some time to explain the principles at work, even today there are doctorates being written on aspects of the Reverend Stirling's engine.

The Stirling Engine was not Popular

The engine never really 'caught-on'. It was used mainly to drive water pumps and for some industrial purposes but compared to the steam engine, it remained little more than a curiosity.

There are two main reasons for this:

1. The engine does not produce the massive power that steam engines can produce.
2. The engine uses a direct external heat source and does not have the store of power like steam built up in a boiler. Because of this, it cannot easily or quickly accelerate, nor can it slow down quickly, due to heat lag. The Stirling engine prefers to run at a steady speed and was therefore not great for automotive purposes.

Has Stirling's Time come?

It would appear that the time have arrived come for a second coming of Robert Stirling's engine. Today we need engines to drive electricity generators and for this purpose the Stirling engine is just the biz because with electrical generator what you need is an economical motor with a steady speed and output, there is no need for acceleration.

The Stirling Design Concept

Reverend Stirling's engine directly converts heat into mechanical motion, and it does this very efficiently. Given the times 1818, almost 200 years ago, Robert Stirling's engine was very special indeed in both its concept and design.

The main advantages of the engine were (a) economy, (b) quietness of operation, (c) the ability to use just about any fuel or heat source, and (d) safety.

Wikipedia says of the Stirling Engine:

"The Stirling engine is noted for its high efficiency, quiet operation, and the ease with which it can utilize almost any heat source. This compatibility with alternative and renewable energy sources has become increasingly significant as the price of conventional fuels rises, and also in light of concerns such as peak oil and climate change. This engine is currently exciting interest as the core component of micro combined heat and power (CHP) units, in which it is more efficient and safer than a comparable steam engine. Widespread adoption of CHP could have a significant effect upon worldwide energy utilization."

In the next post I will have a go at understanding how the Stirling engine works and a look at some of the main variations of the design.


Sunday, August 09, 2009

Gerkros Boiler Woes


Beware of
Wood Pellet Technology!
Especially in Ireland!

The following piece came in in the form of a comment on an older post on the Gerkros Wood Pellet boiler from "Ken" who remains unidentified. It more or less echoes what I have been saying about the Irish woodpellet heater industry. I decided to post it rather than leave it be lost as a comment on a old posting.

Many design features currently found in wood pellet boilers are nothing short of "Heath Robinson" affairs. Designs are still in early development and have a long way to catch up on what is now very reliable, standard+ interchangeable, and reasonably priced hardware used in oil fired boilers. It does not surprise me in the least that Gerkros went into liquidation. Many more Irish peddlers of 2nd rate and over-priced "sustainable energy" hardware are heading for the same route. Wood pellet boilers at around €6000 a pop are still extremely bad value for money.

With a little extra manual tending, simple lumpwood boilers at around €2000 would be a much better investment, especially if you have access to timber.

Come to think of it, a lot of the hassle Balcas got about woodpellet quality in the last couple of years was probably from Gerkros owners!!

I am so glad I decided to stay away from wood pellet heating for the time being. I am keeping an eye on a couple of manufacturers who have offerings in the pipe-line. One in particular has a very good engineering history. I am hopeful for the future.

The Letter

Ken has left a new comment
on your post:
"Gerkros Boilers":


Just a note on the Gerkros 15 kW pellet boiler. I have a 2008 model of this boiler. On the surface it looks a decent enough boiler. But look a bit closer and you will find it has been made by a blacksmith with poor design and engineering. Although it is meant to have a self cleaning burner you will find it has been disabled in the computer because of poor design the ash gets behind it and jams it out.

If you own one you will also know the auger fills up with sawdust at the bottom and then the pellets cant enter it. This is not from dirty pellets. In there wisdom Gerkros have put a 50 mm auger coil in a 90 mm tube. THIS auger is continually grinding over the top of the pellets that it is not moving until they are dust .IF the auger even sat in the bottom of the tube at least it would sweep the auger clean but it runs down the centre grinding over the top of the pellets until the boiler cuts out. Then try cleaning it out they Didn't even think of putting a slide in the bottom of the hopper to stop the pellets falling in . TRY getting the auger back into the hopper when its full of pellets.

Then there is the little problem of removing the fireproof board behind the ash pan for cleaning. You can’t remove it in one piece without unbolting the burner because someone did not think about it when making it. I am slowly working through these problems but its no surprise they went bust, these problems were coming back to hunt them .If someone with a little bit of knowledge and care had studied this for one day things may have been different.


Friday, August 07, 2009

Story of a Scotsman's Invention Re-incarnated


A Series Outlining
The Re-Incarnation
Robert Stirling's 1818 Engine

It was a Scotsman that gave us TV and another Scotsman gave us one of the most fundamental laws of electro-magnetism; yet another Scotsman gave us an engine, which is experiencing a re-birth in a new incarnation after more than 100 years in mothballs.

I recoloured this photo of Robert Stirling to give it new life

Robert Stirling was born in Perthshire in Scotland. Like his father, he had an interest in engineering. However, much of his energies went in a very different direction for part of his life. He studied divinity and became a minister of the Church of Scotland with a parish at the Laigh Kirk of Kilmarnock in 1816.

The engineering ducas was in the blood nevertheless and would not remain buried for long in sermons and parish work. The Stirling ducas was duly passed on to at least two of his seven children who became locomotive engineers. They were Patrick Stirling and James Stirling.

Robert Stirling died in Galston, East Ayrshire in 1878. He left us quite an engineering legacy, his technology is being re-born in this post industrial age, it is even being considered and talked about using one of his engines on the moon!!

The Stirling Engine

Robert Stirling invented what he called the Heat Economiser also called the regenerator. He filed a patent for the economiser and an engine that utilised the principle in 1816.

An Original Prototype made by Robert Stirling

His engine had several major advantages: 1. It could not explode, because of its lower working pressure, 2. It would not cause steam burns and 3. It was very efficient and economical. It also had a few disadvantages which is why the industrial revolution left it behind. In 1818 he built the first practical version of his engine, used to pump water from a quarry.

In the next week or so, I will take a look at the development of the stirling heat engine, and then detail it's newest manifestations and uses.