Small Scale CHP Steam Engine Project

[buenijo]

NOTE: I had to abandon this project shortly after it began in late 2019 due to unforeseen circumstances. After several years, I was able to start again in 2023. See Post #21.

I started a long term project late last year to design and build a small scale combined heat and power steam engine system fueled primarily by wood (and other biomass fuels). I've been derelict on the project for several months now, and I am just getting back into it. So, I figured I would start a thread to document any worthwhile progress. Please note this is a LONG TERM project that will advance SLOWLY due to limited resources (including both time and money). I do not expect to actually have a test engine operating any time soon - if at all.

PURPOSE: To create a system that provides the same amount of heat from the same amount of wood fuel as compared to any highly efficient wood furnace, but ALSO provides all electricity demanded by the home during its operation. Design goals include (1) the use of wood fuel with minimal processing(*), (2) highly efficient heat recovery, (3) quiet unattended operation, (4) ease of service and repair by the end user, (5) reasonably compact design that can be transported, (6) efficiency equivalent to providing 1 KWh of DC electricity with no more than 5 pounds of air dry wood.

PIC OF TEST FURNACE OPERATING:

(*)I tested the furnace with small wood chunks (about 1" across), large wood chunks (about 3" across), and very small wood splits. It can't seem to tell the difference - but wood must be dry!

This crude test unit worked surprisingly well. There was no detectable smoke from the combustion chamber by sight or smell. I could place my face a few feet above the top of the combustion chamber with eyes wide open with no tearing of the eyes and no odor. I estimate the heat output at approximately 15 KW and with peak temperatures approaching 2000F. The basic design of the furnace is sort of a hybrid downdraft FEMA gasifier and rocket furnace. Using a gasifier allows for producing the fuel gas separate from combustion air. In turn, this makes possible greater control of combustion for higher combustion temperatures, a cleaner burn, and a wider turndown ratio - which I verified by operating the furnace for a long period at about 1/4 output (i.e. it still burned hot and clean at the lower output).

The crude furnace seen in the pic is scrapped. I am building a new furnace fundamentally identical, but with differences that include (1) it mounts on a heavy steel shelf that will accommodate the rest of the system, (2) a high quality 24vdc blower fan mounts under the shelf, (3) the combustion chamber is contained inside a insulated steel drum, (4) the furnace fire tube and the combustion chamber are made of a stainless steel alloy, (5) the drums I am using have lever locking lids for a positive seal, (6) the fuel gas and air are directed into the combustion chamber through a slightly different path.

Anyway, that's all there is for now. I don't expect another installment (if any) for several months.

 

[acptulsa]

Interesting.

The double use for both steam drive (electric generation) and heat sounds efficient, but will it be able to run the alternator efficiently without heating the house? Or is the firebox too big for that?

Will this be a reciprocal (piston) steam engine? Are you thinking of using steam pressure to create a draw through the firebox instead of using output electricity on the fan?

 

[buenijo]

Interesting.

The double use for both steam drive (electric generation) and heat sounds efficient, but will it be able to run the alternator efficiently without heating the house? Or is the firebox too big for that?

Will this be a reciprocal (piston) steam engine? Are you thinking of using steam pressure to create a draw through the firebox instead of using output electricity on the fan?

Hello acptulsa. I have good reason to expect the engine system can convert the lower heating value of wood fuel to DC electricity at an efficiency of 8-10%(*). The design maximum charging rate is 1 KWe. This corresponds to between 5 and 6 pounds of well seasoned wood consumed per KWh of DC electricity. Although, I argue the system makes the most sense where heating applications are the priority.

Yes, the engine uses a piston expander, and it is fully condensing. A small DC blower fan forces air into the base of the furnace.

(*) On efficiency, I must resort to gross estimates for now. The 8-10% figure is on the low end. However, a fully optimized system based on my design can show 15% conversion of fuel lower heating value to DC electricity (roughly 3.25 pounds of well seasoned wood consumed per KWh electricity - assuming steady state operation). My minimum target is 5 pounds. I would be very pleased with 4 pounds or less. However, I will not compromise system reliability and longevity for higher efficiency. One goal for a fully developed system is to make highly efficient use of the heat from the system. If this can be done, then high efficiency in generating electricity will be less of a priority.

 

[acptulsa]

Nice not to waste water. Clearly you don't intend to use steam exhaust to create a flow of exhaust in the flue.

Of course, your condensers are also a source of radiant heat. Steam radiators work well. Sometimes they work too well.

Can you create electricity without heating the space? Is it good for power in the summer?

 

[buenijo]

Of course, your condensers are also a source of radiant heat. Steam radiators work well. Sometimes they work too well.

Can you create electricity without heating the space? Is it good for power in the summer?

The condenser will heat water for hydronic heating (along with providing all water heating needs, of course) using a small DC magnetic drive circulating pump to distribute hot water to thermostatically controlled DC fan coil units. Yes, the unit can be operated without heating the space. It's just a matter of dumping the excess heat outside.

However, this system makes the most sense where winters are harsh and wood fuel readily available. Again, heating applications is the primary purpose. In that setting, the system could provide the same amount of heat from the same amount of wood fuel as compared to any highly efficient wood furnace, but ALSO provide all electricity demanded by the home during operation.

 

[acptulsa]

Well it sounds really interesting!

 

[buenijo]

Well it sounds really interesting!

It is surprisingly simple in design - including the control systems - and very few moving parts. However, the devils are in the details - as you know. The main headaches will likely be the steam generator and lubrication system.

 

[buenijo]

I considered another way to describe my system which might help some to understand my interest. I linked the following video in another thread on this forum. The purpose of my system is fundamentally the same as this "Cobber" system. However, my system would be A LOT smaller. Also, my system is simpler as there is no engine speed control. The design power output of my engine is constant up to about 1.5 KWe, but it can be adjusted lower by the operator. Another important difference is my control system is designed to intermittently stop/restart the system to meet demands. Time will tell whether or not this proves viable. The benefit is the ability to use a SMALL battery and SMALL thermal mass (possibly limited to the water contained in the hydronic piping system and heat exchangers). Notice the system in the video is designed to bulk charge a fairly large battery while simultaneously heating a large thermal mass. This is certainly viable. However, a system able to continue operating for extended periods shutting down and restarting as required to meet demands would be a lot less costly by avoiding the large energy storage systems (effectively it would use wood as a battery).

 

[buenijo]

Project Update

The new furnace and combustion chamber are mounted on one end of a 2' x 3' heavy steel shelf with the 24vdc blower fan mounted underneath. This shelf will eventually accommodate the rest of the system. Right now the shelf is on leveling mounts, but I will eventually switch to casters before it gets too heavy. I went through a few minor iterations before arriving at the current design that proved surprisingly easy to fabricate and assemble. I could build a second unit quickly - so hopefully it tests well. Incidentally, ease of fabrication and assembly is a major part of my basic design that seeks to minimize costs.

 

[Working Poor]

Looking forward to following your progress.

 

[buenijo]

Discussion of Basic Furnace Design

Some good news is I've been offered a practically unlimited supply of small wood splits including a lot of eucalyptus. For those who don't know, this wood is extremely energy dense at well over 30 million BTU per cord (nearly twice as dense as untreated pine lumber) - and burns really hot! Some advise against burning it in traditional fireplaces due to the high oil content, but it's ideal for a gasification furnace.

I will try to describe the basic design. Consider the basic Top Lit Up Draft wood gasifier (called a TLUD):

If you understand how the TLUD works, then you will understand how my furnace works. Roughly 1/3 the air (the primary air) is forced into the fuel mass. All free oxygen in the primary air is consumed in the "pyrolysis" section which produces hot wood smoke with carbon monoxide, tar vapors, and soot - all of which are combustible. The rest of the air is admitted in the annular space between the two vessels which heats the air. This preheated secondary air mixes with the smoke in the "burning gases" section. Pretty simple. My furnace does the same thing. However, it includes a sealed fuel hopper on the base furnace, and it shunts the hot fuel gases and preheated air separately into an attached combustion chamber. Pic of first test furnace operating: https://www.flickr.com/photos/184818924@N02/48845233148/in/photolist-f6n5E8-f5CVrH-f6bUD5-f6n5Ax-9DNCxB-AdxCX1-xxGhQA-51rBdW-wCZMRy-2biGfxU-wYUAVb-fAUWbW-zYmg7q-wjCASP-wZ9Pd8-xhntVK-xeGFwL-MCfHNc-euvHMR-2hqhA5d-xeffbY-z3qaBL-28C1w6L-wjKryA-tBFqfa-w5Z8aB-2hDmet6-2hYa1B7-2i3397B-7EwkBS-tyxXow-wZbkLF-r85jmE-sh3zub-amRRcg-wkkCBj-ocUDM6.

There are two main reasons for this configuration - plus a third side benefit:

(1) Conventional TLUD furnaces have limited fuel capacity. Whereas, the size of the fuel hopper for my system can be arbitrarily increased. The fuel chunks fall down into the fire tube as the fuel is consumed.
(2) I had to design the combustion chamber to accommodate the steam generator. The simplest way to build it was to attach a combustion chamber to the side of the furnace.
(3) The hopper can be opened and fuel added without any smoke escaping because the combustion chamber geometry provides a low natural draft when the blower fan is off.

 

[buenijo]

Furnace Update with Pic

Just sharing a pic of the new combustion chamber: https://www.flickr.com/photos/184818924@N02/50051990292/in/dateposted-public/

I can fire the unit now. However, I would like to first make a furnace hopper extension. The fuel capacity is rather low without a hopper.

I will share results when available. It will be a few weeks.

ADDENDUM: Just a comment to illustrate one aspect of my design philosophy. You can see the combustion chamber is simple. Indeed, "simple" was precisely my goal! A goal for any engineering project should be to avoid unnecessary complexities. Unfortunately, my shoestring budget makes this a time consuming process. Often an ideal component is simply too expensive. However, there is a fringe benefit. Namely, if the furnace works well, then I can replicate it both cheaply and quickly.

 

[jmdrake]

I mounted some heavy duty casters on the base and drilled the air supply holes to the fire tube. The only thing that remains before firing the furnace is to install insulation in the combustion chamber. This is relatively easy.

Some good news is I've been offered a practically unlimited supply of small wood splits including a lot of eucalyptus. For those who don't know, this wood is extremely energy dense at well over 30 million BTU per cord (roughly twice as dense as untreated pine lumber) - and burns really hot! Some advise against burning it in traditional fireplaces due to the high oil content, but it's ideal for a gasification furnace.

I will try to describe the basic design. Consider the basic Top Lit Up Draft wood gasifier (called a TLUD):

If you understand how the TLUD works, then you will understand how my furnace works. Roughly half the air (the primary air) is forced into the fuel mass. All free oxygen in the primary air is consumed in the "flaming pyrolysis" section which produces hot wood smoke with carbon monoxide, tar vapors, and soot - all of which are combustible. The rest of the air is admitted in the annular space between the two vessels which heats the air. This preheated secondary air mixes with the smoke in the "burning gases" section. Pretty simple. My furnace does the same thing. However, it includes a sealed fuel hopper on the base furnace, and it shunts the hot fuel gases and preheated air separately into an attached combustion chamber. Pic of first test furnace operating: https://www.flickr.com/photos/184818924@N02/48845233148/in/photolist-f6n5E8-f5CVrH-f6bUD5-f6n5Ax-9DNCxB-AdxCX1-xxGhQA-51rBdW-wCZMRy-2biGfxU-wYUAVb-fAUWbW-zYmg7q-wjCASP-wZ9Pd8-xhntVK-xeGFwL-MCfHNc-euvHMR-2hqhA5d-xeffbY-z3qaBL-28C1w6L-wjKryA-tBFqfa-w5Z8aB-2hDmet6-2hYa1B7-2i3397B-7EwkBS-tyxXow-wZbkLF-r85jmE-sh3zub-amRRcg-wkkCBj-ocUDM6.

There are two main reasons for this configuration - plus a third side benefit:

(1) Conventional TLUD furnaces have limited fuel capacity. Whereas, the size of the fuel hopper for my system can be arbitrarily increased. The fuel chunks fall down into the fire tube as the fuel is consumed.
(2) I had to design the combustion chamber to accommodate the steam generator. The simplest way to build it was to attach a combustion chamber to the side of the furnace.
(3) The hopper can be opened and fuel added without any smoke escaping because the combustion chamber geometry provides a low natural draft when the blower fan is off.

ADDENDUM: If the furnace performs well, then the next steps will be: (1) shape and install the steam generator, (2) assemble the combustion chamber lid, and (3) assemble the water feed pump. I hope to advance the project to this point by the end of the year and start testing the steam generator under pressure by driving the feed pump with a small DC motor. Engine assembly will begin after I know the system generates steam at the required rate, pressure, temperature, and efficiency.

ADDENDUM: I have also considered the system could power an absorption chiller based on a design I considered years ago. This would be a low priority after space heating, water heating, and water processing systems are developed. I did testing years ago that led me to believe my simple chiller design can work well. This would be the "holy grail" system in my mind: a compact biomass fueled unit that provides ALL off grid energy needs, in accordance with modern standards, and fully serviceable by the end user. No massive battery, no fancy electronics, no solar array required, no massive thermal storage systems, and easy transport if desired. Note the value of a chiller is much higher efficiency as compared to using my system to generate electricity for a convential a/c unit.

NOTE: Any development beyond the basic test engine requires outside funding which I would likely pursue via crowd funding.

How does the efficiency of your system compare with a rocket stove?

 

[buenijo]

How does the efficiency of your system compare with a rocket stove?

A gasification furnace is the most efficient way to burn wood. That noted, a properly designed and operated rocket stove is effectively functioning as a gasification furnace. The benefit of a gasification furnace design is superior control over the production of the fuel gas and supply of secondary air. This control makes it possible to show high efficiency over a wider power range and with a wider range of biomass fuel sources.

First test furnace at full power: https://www.flickr.com/photos/184818924@N02/48845233148/in/dateposted-public
First test furnace at roughly 1/4 full power: https://www.flickr.com/photos/184818924@N02/50059738728/in/dateposted-public/

 

[buenijo]

Discussion of Desiccant Evaporative Cooling

Just sharing some interesting videos. In previous posts somewhere in another thread I discussed desiccant evaporative cooling as a promising means to provide space cooling using heat from a small steam engine system. An engineer on YouTube made a very good video describing his test unit.



A follow-up video:



DISCUSSION: The testing was not under controlled conditions, so I do not consider the results meaningful. There is a lot of room for improvement. A fully optimized desiccant evaporative cooling system could be very impressive. In particular, the approach would be most effective in regions that are both hot and humid as desiccants can take humidy to much lower levels as compared to vapor compression systems. What I like most about this kind of system is the low tech (low voltage DC power can be used, no refrigerants, no special tools for repair). It can be simple and last indefinitely with infrequent low cost maintenance. However, to be practical, it must be effective while significantly reducing electricity consumption relative to vapor compression units.

People tend to understand humidity plays an important role in cooling. Warmer air holds more water vapor than cooler air. Therefore, warm air at a low relative humidity (meaning relative to how much water vapor can be held at that temperature) can feel more humid than cool air at a higher relative humidity quite simply because it may hold a lot more water! Enter the DEW POINT. This useful concept tells you how low the air temperature must go before the amount of water vapor in the air represents 100% relative humidity. It's a clever way of considering the actual amount of water vapor in the air. For example, at 30F and 100% relative humidity the dew point is obviously 30F. However, at 80F and 60% relative humidity the dew point is 65F and therefore contains a lot more water vapor. Again, it feels a lot more humid as well - because it really is. So, relative humidity is counterintuitive because at different temperatures its comparing apple to oranges so to speak.

A desiccant system can reduce the humidity more than vapor compression systems such that the dew point is significantly reduced. When this is done, then perspiration become more effective for cooling. Therefore, a desiccant dehumidifier can be effective in a hot and humid climate even without reducing air temperature. Yes, a sufficiently high temperature becomes oppressive. So, at a certain point evaporative cooling might be employed. Interestingly, if the temperature exceeds a certain point, then fans are no longer useful for personal cooling - even under very dry conditions. The reason is rather easy to understand. Sweat evaporates readily under hot and dry conditions to cool the skin. Blowing hot air over a person will only reheat the skin.

NOTE: With respect to air conditioning in the off grid setting, the superior alternative available today is a ductless unit ("mini split"). These have many qualities that are hard to ignore including the ability to operate over a wide output range (high cooling when an array is producing and low cooling when powered by battery), very high efficiency, a price that is declining in the U.S. market, a long history of use (it is a mature technology), many models come pre-charged with refrigerant which allows installation by the end user, many operate efficiently as heat pumps for space heating, and they have very low starting surge current (so an inverter with a surprisingly low power rating can be used). Operating such a system off grid on a solar array makes a lot of sense. Many models can be dialed down to about 300 watts power consumption while still cooling at a rate greater than 5000 BTU/hour!

ADDENDUM: The "inverter" compressor drive technology in most mini split systems is now available in window a/c units. The price is MUCH lower!
https://www.amazon.com/Midea-Invert...244-8a32-3e8c6816defc&pd_rd_i=B0866Y33PL&th=1

 

[buenijo]

Discussion of Absorption Cooling

I made the previous post primarily for education purposes. It is interesting. To be complete, I'll go ahead and describe another heat powered cooling system that might be configured with a small steam engine system. I did some very limited testing about 7 or 8 years back after acquiring a few pounds of lithium bromide from EBay. I tested the substance based on density and boiling point just to verify it was the stuff I bought, and it passed with flying colors. All I did with respect to testing was learn how to evacuate practically all air from a system by using a combination of cheap refrigerant vacuum pump and steam displacement. Take suction at a low point and start with warm water in the system. As the pressure in the system falls, the warm water boils. Water vapor is less dense than air, and so it rises to the top in the system to displace the air to the suction hose at the bottom. I verified the near complete absence of air by observing the lithium bromide chill the small mass of water down to 32F (after the system was isolated, the lithium bromide solution absorbed the water vapor in the system taking the pressure so low that the small mass of water continued to boil and cool to 32F). The other useful facts I determined is low cost DC mag drive pumps work perfectly under a high vacuum, and 6" schedule 40 PVC pipe holds a high vacuum well. My design is unique for chilling a mass of water DIRECTLY, then circulating the chilled water to a fan coil unit under vacuum. Why not? I mean, the pumps don't care. So, this eliminates a heat exchanger and lowers the chiller water temp - all else equal. The steam engine exhaust condenser would heat the lithium bromide solution that is under vacuum to boil off the water previously absorbed. This very low pressure steam is condensed by the lowest temperature ambient source available meaning air, but a body of water would be ideal. The liquid water from this condenser returns to the chilled water mass in the evaporator.

The main challenge is achieving a high rate of absorption and evaporation with a low cost and compact design. I have a design worked out, but it's not something I would pursue until after the test engine is completed.

The following video helps one to understand the absorption chiller. A vacuum pump is used to remove air from a chamber reducing the pressure so low that water boils. The pump continues operating to take the pressure lower and lower by removing water vapor. Since water vapor molecules are more energetic than those in the liquid form (on average), then the liquid water temperature decreases. In this video the water gets so cold it begins to freeze. An absorption chiller does the same thing in a different way by using a desiccant to absorb the water vapor. As long as all non-condensible gases are removed, then water is practically the only substance in the enclosure. So, the pressure can get very low indeed as the desiccant absorbs the water vapor!



NOTE: Again, this is all interesting, but powering a small vapor compression a/c unit as an opportunity load on a solar array makes a lot of sense. Optimizing the efficiency of my steam engine system and using it only for heat, backup power, wood drying, and perhaps water distillation is the rational alternative.

 

[buenijo]

I configured the furnace as a TLUD. It took a few minutes to heat up, but once hot it burned clean. However, the gases would not ignite when directed into the combustion chamber, and a tarry glaze was deposited. I am working on the solution.

 

[buenijo]

Discussion of Vapor Compression Chiller Configuration

Yet another installment describing an unconventional a/c system. The usual disclaimer applies: solar makes a lot of sense for powering conventional small a/c systems. This is just one way a stand-alone system based on my engine design might be configured.

It's simple (in principle): drive an automotive compressor off the engine via belt drive and use the evaporator to chill a mass of water circulated to the same hydronic fan coil units used for heating. The compressor drive is geared to consume most of the shaft power when engaged. The 24v compressor clutch solenoid (50 watts) is controlled by chilled water temperature feedback.

 

[buenijo]

Update

I've been brainstorming the problem with the furnace. I now believe the main problem is OPERATOR ERROR. The blower fan pushes too much air, and this is producing a weak fuel gas that is further cooled and diluted with excess secondary air in the combustion chamber. I purposefully selected a new blower fan I knew to be more powerful than required (I figured I would just dial down the output with a controller). However, I did not expect the fan to push so much air as to prevent furnace operation. I got lazy. I should have had the controller on hand.

I ordered a fan speed controller and purchased plugs for the furnace air supply holes. This will allow control of the air flow rate and the mixture.

ADDENDUM: Unfortunately, my very limited electronics knowledge got me in trouble. The blower fan is amazing, but both too powerful and apparently too sophisticated. It cannot be controlled with a conventional pwm speed controller, and I am not paying for a fancy unit. Also, the cut in voltage is too high for rheostat control. So, I had to get a new fan. Good news is the new fan is much cheaper. BTW, I got the fancy fan for free. Also, I arrived at an elegant way to install the blower fan to the furnace base that incorporates a damper. This damper will eventually be part of a control system that is critical to operation. The damper was going to be placed elsewhere, but I realized this new location is superior. In fact, it solves several problems. Yeah, I know things are moving slowly - but I expected these kinds of problems. The devils are in the details - and why R&D is so expensive.

 

[buenijo]

Update

The damper is installed under the base and the new blower fan is contained inside a small steel cap that press fits on the the damper. The purpose of this approach includes: (1) the fan can be easily removed by disconnecting the power supply cord and removing the cap that contains the fan, (2) closing the damper cuts off all air to the furnace while also isolating the fan from any smoke generated from the furnace. It seems to be a good basic design. Assuming the furnace ever tests well, and assuming I ever advance the project to install a steam generator, then the damper will be fitted with an extension spring and solenoid latch configured to shut the damper on high steam generator pressure OR high exhaust temperature (an automatic furnace shutdown feature).

ADDENDUM: There will be another long delay before any meaningful progress can be made. I attempted another test without success - and identified a couple problems in the process. Unfortunately, I must assemble another furnace. The basic problem is the fuel gas and air flow path I used for the existing unit is throwing off the mixture. Ironically, the original (crude) test unit got it right (the saying "if it's not broke, then don't fix it" come to mind).

ADDENDUM: Things are rocky (family drama), so the project is on hold for a while. I won't have anything worthwhile to share for some time. I have all hardware required to proceed with a new furnace build (including new steel drums that are slightly larger and heavier than the previous ones), but I cannot resume until this nonsense blows over. Oh well. This is the real world folks.