Exhaust Heat Recovery - Steam Power

[Patrick]

Originally Posted by Daox

The implementation is relatively simple, you weld a sleeve around the exhaust pipe (depending on the pressures you may need a thicker pipe). Fill it with water, and route a line to something that will do work for you. I was thinking of routing the outlet to a turbine that spins an alternator. All the benefits of an alternator delete, but no plugging in to recharge. Depending on the efficiency, you very well may even have excess power. I haven't even really thought of how else to use this yet.

It might work on a hybrid to partially recharge the batteries. But with steam you'd have to condense it, which would require a heat exchanger (adding to the complexity); or, you could vent the steam to atmosphere, but then you'd have to replenish the water periodically (and haul around the weight of the water in the meantime).

[Daox]

Quite true. I'm also unsure of how much water you would go through to produce X amount of power.

[320touring]

Originally Posted by Daox

Sorry, that would not work. Turbochargers spin incredibly fast. Smaller turbos can spin over 100,000 rpm, and some up to 200,000 rpm. Gearing it down is a big pita, you'd have to make your own journal bearings to take that kind of rpm input. Then you have to find something that will spin that fast without blowing up. It gets complex really fast.

this is true, however, there must be a way to use the exhaust gasses to drive something that will generate sufficient elec to do the alternator's work?
Thinking behind this would be similar to a wind turbine with adjustible blades..

Is it possible to place something that is driven by the exhaust gases inside the exhaust?

[user removed]

Since exhaust temps are so high, why not use the type of system they use in solar setups.

A synthetic oil that can be heated to 800 degrees that transfers heat to a boiler in a small well insulated steam engine.

regards
Mech

[Daox]

Yeah, that would be more efficient, but again it would be a lot more expensive, and complicated. I'd like to keep it as a cheap DIY mod.

[Wonderboy]

A quick google search tells me pretty varied results on exhaust temperatures (anywhere between 500F and 2000F). Anyone here know a real live accurate number? Depending on how high it is (for this application, the higher the better) a stirling engine of this size may be an idea. The guy who makes that video makes a bunch of really cool DIY alternative energy generation videos, btw. Check out his youtube channel.

I'm thinking a beefy stirling engine with a heatsink affixed directly to the bottom of the manifold or as high up on the pipe as possible (wherever there is space enough) could constantly power an alternator, and actually run faster the faster you go. Would be particularly effective on long highway trips, and would only take the initial investment to get the stirling engine - everything else is cheap and easy to cobble together. BTW, here's a video demonstrating the ballpark of the temperature on the business end of that engine generated by a big fresnel lens. Roughly 1000F. Doesn't necessarily need to be that hot, but like I said the hotter the better.

[Daox]

Well, I did an exhaust temperature test a while back, the manifold just idling for I think 8 minutes got up to 500F.

Where can you buy off the shelf stirling engines?

[RobertSmalls]

BMW have experimented with turboalternators and thermoelectric devices as exhaust waste heat recovery. They worked in the lab, produced modest MPG improvements (I want to say 5-10%), and proved far too expensive to implement in a BMW.

The other sector where WHR has had some success is heavy duty engines. Some of these engines see ten times as many hours in a year as a passenger car, so really expensive WHR schemes can still pay off.

The DoE are putting some money into thermoelectrics. Efficiency is low right now, but if they come up with cheaper or more efficient thermoelectrics, I expect it to become standard equipment on Class 8 trucks.

Vehicle Technologies Program: Annual Progress Reports contains some information on what your tax dollars are up to in the exhaust waste heat recovery field. There's also discussion of rankine cycle heat recovery, using R245fa as a working fluid, as well as a turbogenerator.

I think the best use of waste heat in a small car is to heat and cool the cabin.

[RobertSmalls]

Originally Posted by Wonderboy

A quick google search tells me pretty varied results on exhaust temperatures (anywhere between 500F and 2000F). Anyone here know a real live accurate number?

bwilson4web has read his Prius' cat temperature sensor in real time, and it was 600-700°C: http://ecomodder.com/forum/showthrea...ius-13288.html

I've measured my own catalytic converter's housing with an IR thermometer at 800°F, about 30s after shutting off the engine. Yay, mixed units!

Stirling engines are fun because they can operate with a very small delta-T, but I haven't seen a compact, inexpensive, powerful Stirling engine. Come to think of it, I haven't seen a Stirling engine do any work aside from rotating its own shaft. Rankine is probably more realistic.

[jdgFirefly]

The cheapest DIY way I can think of to make a recovered heat engine is to modify a air conditioning to basically run in reverse as an engine.

You would have to pick a system that has a compressor that can be used as a motor when under pressure (or one that can easily be modified to work). The expansion valve will have to be replaced with a small positive displacement pump to pump the condensate into the boiler. I would use the cooling system to preheat the condensate before it enters the boiler to also recover some of that heat. I would also double up of the condenser to keep system pressures/temperatures under control. Actually in this situation the condenser will probably be the component that dictates how much power can be made.

As for the suggestion to just go with the turbo or turbo generator, I think its a great Idea but not exactly the same thing. In my mind the turbo recovers waste pressure not heat, so ideally you could benefit from having both at the same time. You have potential for bigger gains from heat recovery engine but its a lot harder to do and likely a lot bulkier for what you will get out of it.