Stirling Engines for Combined Heat and Power

[JF]

Dear Friends

I have noticed several recent remarks and comments concerning Sterling engines.

Please check-out the following bio-mass [wood-chips] powered sterling engine mainly intended for small/medium-scale local heating and power [a cluster of houses or houses on a small street etc]:

http://www.stirling.dk

This project is a result of a long development process by Prof Carlsen and colleagues at the Danish Technical University near Copenhagen. I have several times visited and seen the hardware and discussed with the developers of this project. These models have perhaps the best track-record during long-term pre-production testing. And in my thinking perhaps offer the possibility of a real break-through in wider-scale aceptance and d1semination of this most useful RE technology.

Among other recent "small-scale production" deliveries - a 35 kW [el] and 130 kW [heat] model is soon to be delivered for the entire heat and power requirements of about 83 housing units and about 200 people in a small community near Aarhus on the east coast of Jutland. Other models have also been delivered to Sweden and Austria.

With best wishes - JF

[Gibby]

Do you know of a single home unit that will produce 3kw/hr.

By the way this claims a 9kW generator for use in small houses...  When they say they produce 9kW's  is that over a period of time or how is that calculated.  The reason I ask is my home uses around 2.5kW per hour, on the average.  I would assume the 9kW would work but then the claim it is for small houses.

Please explain,

Thanks,

Gibby

[jimovonz]

A kW is an instantaneous measure. A kWh is the equivalent of producing 1kW for 1hr (or 0.5kW for 2Hr etc). To talk of kW per hour does not make sense. If a generator is rated at 9kW then it should produce 9kW continuously. Using 2.5kW on average means you use 2.5 x 24 = 60kWh/day. This is a very large amount of energy! I have a 2800ft^2 home with all the mod con's (taking care to conserve energy where possible) and use 5kwH/day. A 9kW generator should meet your requirements depending on your peak loading. You could easily top 9kW if you turned on the stove, toaster, jug and the hot water cylinder kicked in...

[Gibby]

I downloaded the past 18 months bills from Alliant Energy's website and the average daily use is 51kWh per day.  This would be 43kWh per day during the summer and 61kWh per day during the winter.

I don't know how you can get down to 5kWh per day but would really like to.  That would say your electric bill is appoximately 15.00 per month???

I know mine is high but that is realllllly low.

At any rate, I like your explination and believe it is on the right track but am still a little confused.

If my peak never goes over xkW than I may need to have a generator rated at xkW?

Say I want to cover the majority of the usage and automatically switch to grid power when the usage is higher or when the output of the generator is to low.

And this gets to Amperage used as well, does it not?  Right now my meter is a 200Amp meter and I have a 200Amp breaker box.  I obviously am not using 200Amps but I could very easily be using 40Amps at peak.

I'm trying to get to a point that I can purchase a alternator that will produce the needed energy.  I want to create the mechanical drive portion of a generator using Solar produced steam.

Any suggestions?

[jimovonz]

5kWh/day is towards the lower end of the scale but I believe that it easily achievable. I have built my house (myself) to be as efficient as practical (read as 'as efficient as my wife will withstand'). The house is 'super' insulated, with straw walls, insulated concrete floor slab and 12" shreded wool in the ceiling. All of my heating is 100% solar (hydronic radiant floor). I use CF lighting exclusively, my oven/hobb is gas and I have a super efficient refrigerator and freezer (Vestfrost). My monthly bill is more like US$35/month as we have high fixed supply charges (as well as the fact we pay approx US$0.14/kWh)

Your generator does have to be sized to supply your peak demand, however most generators have a surge rating as well as a continuous rating. If you only operate at your peak for a short time, you will be able to get away with a lower rated generator. Using grid power as a backup is possible in theory, however it would require that the generator is sync'd to the grid when both are required. It would also have to be disconnected from the grid when the grid was down so as to not endanger the linemen working on the line. I don't know if this would be allowed where you are, or of any controller that would do this for you.

Solar steam on a small scale is very inefficient. I have been down that road. I currently have a 5Hp steam engine gathering rust in my paddock. The remains of the polished stainless sheets that I was going to make into a parabolic trough lie out there too (now much dented from the cows treading on them)... You can probably count on obtaining 5-10% efficiency from such a setup (that equates to 50-100W/m2 of collector peak sun - you would need a collector at least 100m2 plus storage to supply your current energy needs!) If you still want to persue this path I can give you more detailed info.

[Gibby]

I have to ask: why is steam power efficient on the large scale but becomes inefficient on the smale scale?

I am interested in pursuing this further.  I'm hardheaded and can't give up until I prove myself wrong.  Any detail you can provide that will assist me would be greatly appreciated.

What are your numbers based on?

Thanks,

Gibby

[Gibby]

I have another question I just thought about:

You were using a parabolic trough system.  I know steam is steam but woould it be different if you were using a parabolic concentrator?  I understand that even in small concentrators they are reaching temperatures of over 1200F.

Thanks!

[jimovonz]

Hey Gibby,

You are right to be 'hard headed'. You wouldn't achieve much in life if you let every 'nay sayer' put you off. What ever the end result, it will teach you a lot! I don't mean to dampen your enthusiasm, just to help you with what you might expect from such a system. I aborted my mission before it was ever complete - the numbers just didn't stack up for me. For what would probably be less effort, I decided to derrive the largest portion of my power requirements from wind. I still may make use of my steam engine at some stage in the future - just for the experience.

Electricity generation using steam on a large scale becomes more efficient through the added complexity of high speed, multi stage turbines along with various heat reclamation processes. Also factored into the equation are the availability of local fuel (some waste product such as wood chip) and any potential use for the copious quantities of low grade heat that results. The 5-10% overall efficiency (energy in vs usuable energy out) figures that I quoted are readily verifyable through a simple Google search. The system efficiency improves somewhat if you have a use for the waste heat for water/space heating etc. The amount of energy available for you to capture obviously varies with weather conditions etc. To get an idea of what is available to you in your specific area, check out this story I posted a while back:

http://www.fieldlines.com/story/2004/10/6/164945/703

This site I link to here will give you (among other things!) the averge solar insolation for your area. It gives figures for the energy falling on a horizontal surface as well as various orientations including optimal. The collector sizing figures I quoted above were roughly based on the solar insolation figures for where I live (Bay of Plenty, New Zealand) and may vary considerably for your location (though for your current electricity usage your collector would still be very large!) Visit the site and download the data for your region. You can then use the 5-10% total system efficiency figure to calculate how big your collector area would have to be to produce a given electrical output - be sure to factor in seasonal variations as your collector area would have to be sized to produce the minimum required output during the least sunny part of the year! If you have any problems with the figures, let me know where you live and I will look them up for you.

A parabolic trough is a parabolic concentrator so I presume you are refering to a parabolic dish that has a point focus rather than a line focus. Using a dish will only give you a (potentially) hotter focal point, not capture any more energy. The time taken to produce steam of a given temperature from a given volume of water is directly related to the collector aperature size (the area directly facing the sun). There are problems with having a focal point that is too hot. The efficiency of the heat capture system is inversely proportional to the collector temp. As it gets hotter, more energy is re-radiated back into the environment and lost. Use the minimum concentration requiured to achieve your desired results. Trough concentrators also have the advantage of perfroming well while only requiring tracking in one axis (and with the size of collector we are talking about, two axis tracking becomes a challenge...)

Keep the questions comming - I am happy to help