Backyard Aquaponics
A place to discuss aquaponics

Yes, it will be interesting to see how the different energy sources compare after a national carbon trading system starts in Australia.

Current hinds (from the Fed. Govt) look like a minimum price of $40 to $80 AUD per tonne.


Gnoll110

Stirlings are the perfect system for power generation from solar concentrators.... Err... Until you start to look at how long they will last in operation. Then they suck. There are lots of starry eyed people out there who just KNOW that Stirling engines will solve all the problems facing man-kind today. And yet, there is not ONE stirling power plant left working after even a few years. I truly wish someone would prove me wrong on that point.

PV systems, in contrast, are not terribly efficient, cost more for the same output, and can not be built in the common home hobby machine shop. On the other hand, they work for 20 years or more.

Care to go into detail.

I personally feel some design and/or materials innervation are need to improve applicability.

I think I've said before, invent a compact low temp. differential (LTD) high output stirling and you would go down in history with the inventors of the electric airliner & cold fusion!


Gnoll110

Seals fail, working fluid corrodes cylinders or escapes, bearings crack due to temperature fluctuatoins, etc...

It is easier to show emperically that Stirlings have reliability problems: There are no long lasting installations. Even hobby use engines fail far faster than other technologies in the same power / efficiency class.

There are stirling designs that last for long periods of time, but they are low efficiency or very expensive to make.

I agree that the person who comes up with a workable design will go down in history, but how many attempts should be made when other, more viable technologies hold some, if not as much, promise?

Personally, I've offered to support development into solar ranken cycle engines or internal expansion steam.

http://techref.massmind.org/techref/mem ... urbo-1.htm

http://techref.massmind.org/techref/ide ... inject.htm

Hi,

The Swedish Navy operate submarines powered by Stirling engines.

The principle benefits of Stirling Engines over solar panels is that they are able to run 24 hours per day and they use a wide variety of fuel sources.

Some of them are designed to run in conjunction with solar heating devices.

I'm under the impression that Stirling engines (particularly at the backyard level) are generally ineffient but my understanding could be dated.

Stirling engines and PV panels are both examples of what we call heat engines. They convert heat into mechanical (or equivalently, electrical) energy. PV have a number of advantages - they are solid state, long lived, have a huge operating range and convert directly to electricity.

Stirling engines also have a number of advantages - they are robust, can be designed without any high temperature seals, they are efficient, they operate with low temperature differentials.

High efficiency heat engines such as the stirling engine allow lower temperature Th which means you can use stored energy and lower efficiency collectors. Storing as chemical energy, such as the solar oasis ammonia recombination project is practical with such engines (imagine trying to run solar panels off the light from a gas flame!).

When we compare heat engines we often refer to a theroretical upper limit for efficiency, the carnot efficiency. This was derived based on Carnot's observations of water engine efficiency and tells us the best possible useful (gibbs free) energy to be extracted from the source. It is easy to work out :

(Th - Tc)/Th

For a stirling engine working with a hot of 100C and a cold of 30C we have a maximum efficiency of 70K/373K = about 19%. For a solar panel with a hot side of 6500K and a cold side of 80C we have about 95% upper limit. So the stirling engine that is 10% efficient in practice is over 50% of the theoretical efficiency, whereas the 10% efficient solar panel is in fact only 10% efficient.

Furthermore, a highly efficient engine has to be very big (the reasons for this are subtle, but basically you need to make lots of very tiny temperature changes to get maximum efficiency) and small heat differences mean lots of working fluid, and hence even bigger. As a result, stirling engines are poor performers due to their large size, slow response rate and complex mechanics.

So I would personally not expect stirling engines to save the day. I think the current best bets for large scale solar electricity are, in order: solar oasis (mostly because it can efficiently store energy on a large scale), water cooled concentrating PV (waste heat is useful for desal) and finally solar preheat for existing coal power stations.

Theres lots of interesting stuff going on thats for sure , while stirling many not fit the bill for every application as Gary pointed out Kockems built the Stiring sub, Sunpower have had their solar array powered stirling churning out power for over a decade with minimal maintenance(by the way stirling solar currently holds the record for efficiecy beating pv) tiny stirlings live inside night vision goggles to cool them and acoustic free piston stirlings run freezers in the space shuttles. they do have problems as in self starting, throttling etc but best of all there great to build ! Having said that James im also abig fan of ORC'S (organic rankine cycle) and do honestly believe that more and more we will be seeing these popping up in our lives. In fact These systems have been operating reliable for decades.
Cheers Tim

TimB, do you mean stirling engines or heat pipes. The NV gear I have used gen II and III, do use heatpipes to move heat from the intensifier tube to the heat sinks located on the main body of the unit. They are little more than copper tubing with strands of copper and a refrigerant gas inside. My PC has them also. I think the term stirling engine refers to a machine that can generate movment using the difference in temperature of two parts of the machine.

Heres a question: can propane be used to run an air pump?... Ie. Solar Heater, with copper pipe grid Where propane expands and rises to a hight chamber, with another copper grid submerged in water. The gas is then cool to a liquid and goes down to the heat element through a pump that is driven by the weight of the LP.. Any thoughts? I will get onto a diagram..

TimC, yes, LPG is an excellent refrigerant. I have gased R22 systems with LPG without any problems (indeed they seem to run better), google for propane refrigerator or similar. You can get a new r22 air con from bunnings for about $300.

lpg tends to be a better replacement for the old r12 systems. Use only 1/3 the weight charge, has higher heat transfer capability and efficiency. Best to use a refrigerant quality gas as it is guaranteed pure. Hydrocarbon R-12 replacement is mixture of butane / isobutane

There is one for r-22 it is a propane / something else mix.

See hychill.com.au

IMHO HC R-22 replacemnt will become popualr becasue as far as i understand it R-410a is still proprietary, and all the $300 chinese bunnigs units are still on R-22 for this reason. R-22 is being phased out, are there are a HELL of a lot of R-22 splits in operation.

Don't evben get me started on this R-410a bullshit, It was specifically created as well as its new oil as the "ozone friendly more efficient gas"

Sure it has a low ODP (ozone depleting potential) but it GWP (global warming potential) is huge, not to mention it runs at higher pressures, requires more dilligence in ensuring the system is moisture free, and if you have ANY leak then you may as well reclaim the complete charge and start again as its a blended gas. And then there is the OHS concerns with the oil.

HC refrigerants have an ODP of Zero and a GWP of 1( i think, maybe 10, but the HFC 410 is in the thousands)

And don't belive the hype on flamability concerns, uni NSW did test on it, Besides how many cars do we have on LPG?

Thats my 20c

Steve

Oh tim, you'd still need a refrigerant "pump" to transfer heat, otherwise the refrigerant would just condense out at the coolest pointt

Id definatly go refrigerant based for heating, you get 2.5 -3.5 : 1 heat to electrical energy.

EG an electrical heater will consume 1Kw energy and evolve 1KW heat energy.

A refrigerant based system will consume 1kW energy and might evolve 3.5kw heat

Seems like over unity at first glance, but what is happening is that 1kw is being used to run the compressor "pump" and the refrigerant is extracting and relocating 3.5 kw of heat from ambient to your heated space.

Anything above absolute zero has "heat" so just becasue it 2C outside doesn't mean there isn't "heat".

There you go, refrigeration 101

Steve

What you just described is a heat pump Steve , like you said they dont make heat just move it against a temperature gradient. Theres a guy Jeff Stirling (nothing to do with Stirling engines) his web site is www.matteranenergy.com it has an absolutley brilliant animation of his one kw ORC (basically a steam engine running on refrigerant) really worlth checking out.

bloody hell, two freaking valves, thats all i was missing! was really keen on the same concept when i first got into refrigeration. I never thought of using two shut off valves build pressure in the reurn vessel and then open the return valve to get the refrigerant back!

That was always the problem, getting the refrigerant BACK!

Good on him! That technology ROCKS.

Easy Easy to backyard make too. Thanks heaps for the link TimB, little dissapointed i couldn't work it out, but happy someone did!

Steve

Steve theres another system (actually i think theres heaps of home brew systems outther) by caltec or possibly stanford Uni using trough collectors vane type car powersteer pump for the expander punching out over a kw they even went over to Africa And set it up for an aid thing if i can find the like i will post. This is what really gets me going hope to build a proof of concept in the next couple of years for a patent application im putting together. its not rocket science this stuff it really works and were going to see a lot of it mark my words

good stuff! I'ts a running joke between me and the boss that i'm about 1 year behind current patents, there have been atleast two designs for things i've done, with no inkling that anything was around, and then found that it had been patented 12 months earlier. Doesn't matter, the dollar doesn't mean as much to me as it once did. Hurry up and patent your concept so then you can release the details and we can build back yard versions of whatever it is