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Configuration for an Off Grid Power Plant  RSS feed

 
Marcos Buenijo
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A configuration discussed here entails the daily operation of a small wood gas engine system for 3-5 hours, or however long the requirements and/or fuel availability permits.

First, let's assume that air conditioning is desired. I propose that a wood gas engine system be used to drive an alternator (for battery charging), but with most of the engine power used for driving an automotive a/c compressor for partially freezing a mass of water kept in an insulated tank. This cold source can be tapped for a mini chilled water system. BTW, these automotive compressors are inexpensive and durable. They can be used to make an a/c system with impressive performance by using large heat exchangers and cooling them well. Automotive a/c systems do not have high performance because the heat exchangers have to be very compact and they are not well cooled behind the fire wall of an engine compartment. Furthermore, this a/c system can be operated at night when air temperatures are lower for good condenser cooling. Also, it makes sense to do this at night since battery charging can be done as well, and this will minimize battery discharge since a modest solar array can be putting out during the day. Finally, since the heat from the engine system is not put to much use here, then a good way to use it is for fuel drying, water processing (i.e. pasteurization) and/or heating, and some heat might be used to regenerate a desiccant to help lessen the latent cooling load in humid climates.

Now, about heating. A heat exchange system can be devised to catch most of the heat from a wood gas engine system. A good example is Ken Boak's system (google Ken Boak, lister, powercubes). Water is pumped to catch the heat from the cylinders, wood gasifier, and engine exhaust and this heated water/steam can be used to heat the store of water that was used for cooling, then use a hydronic air heating system (i.e. using the chilled water system with hot water flowing through the coils instead of cold). However, since the output from the engine has to be fairly high to ensure enough air is pulled through the gasifier at all times, then this additional energy must be put to use (note: generally, a small wood gas engine system needs to operate at a rate of at least 5 hp to prevent having to use a very small gasifier that might require pellets or other highly processed particulate biomass). There is no a/c compressor here, and only very large battery systems can handle this kind of charge rate (would be on the order of 2500-3000 watts). So, the shaft energy normally applied to the a/c compressor could be used to generate more electricity for directly heating with electricity, and this would provide heat at a rate of about 1500-2000 watts, OR one can put a cheap modified sine wave inverter on the battery to power conventional space heaters to draw down the battery during charging and reduce the net charge rate, OR it's also possible to configure the a/c system for heating by placing a condenser in the home, and this would provide a lot more heat by transferring heat from outside to within this home. However, this latter option is not the best where outside temperature are particularly low. The idea here is that a condenser OR electric heat could provide a rapid heat up of a home while the engine system is operated each day, then the remaining heat from the engine could be stored up to provide heat over the rest of the 24 hour period.

In other words, a wood gas engine system can be configured to work exceptionally well in the off grid residential setting, but I don't recommend trying to adapt such a system towards powering a modern home in a conventional manner (i.e. primarily with electricity). One must put the heat to use where possible/practical. It would be wonderful if a wood gas engine system can be operated continually at low power, but this is really a very difficult if not impractical goal. Rather than force a square peg into a round hole, perhaps it's best to adapt a configuration that optimizes existing wood gas engine systems.

NOTE: On water processing with pasteurization, I was considering that passing the heated water (heated from the wood gas engine system) through a small thermal mass before it moves to other heating applications. Once the thermal mass reaches a preset temperature, then a thermostat can shut to energize the motor of a water pump and start sending water through this heated mass. Providing this thermal mass can simplify the water pasteurization process. BTW, I'm considering a setting where the home is situated near a body of fresh water that can be drawn upon as required. Pasteurization might also be useful for processing water from shallow wells. It might even be used as a preventive measure for processing all off grid water sources. After all, since one desires to heat the water anyway, then might as well jack up the temperature for a while to kill any bugs that might be lurking. Use heat regeneration to reduce energy requirements here, but the water can retain enough heat when stored in an insulated tank so that it would not require additional heating after the pasteurization process (instead of on demand heating of cool water when hot water is desired, try on demand cooling of hot water when cool water is desired... it's easier to cool water than to heat it, . So, store the water while it's still pretty hot. NOTE: Heat regeneration here entails the preheating of the incoming water with the water that is pasteurized to reduce the overall heat required to process a given mass of water. So, take the water to high temp with the thermal mass, then cool it down some by transferring some heat to the cool water before it moves into the heater.

NOTE: On desiccant regeneration, getting the heated water (used to harvest heat from the wood gas engine system) to the highest possible temperature, then passing this water through the desiccant solution FIRST (even before the thermal mass used for pasteurization) could be done. This will moderate the temperature somewhat before the hot water moves to the other applications. Also, since this is desired only when a/c is desired (i.e. summer months), then getting as much heat as possible into the desiccant should be done during these times. A desiccant can be very useful to help remove some moisture from the air in a humid environment and enhance the cooling effect of the a/c system.
 
Morgan Morrigan
Posts: 1400
Location: Verde Valley, AZ.
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you can also pump exhaust from an engine thru a water column filled with nickle/iron nanoparticles , capturing the methane gas that comes off.

I am working on designing a Loop Battery to use as a home scale energy storage well.

Taking a length of drip tubing, (looks like it is going to have to be at least 50 ft. long) filling it with a graphene covered wire, and filling the rest of the pipe with free floating graphene and water.

At the correct length , it should work as an inductor from ionosphere, and graphene will self assemble with water to act as a battery.

Calling it an Alfven Loop Battery.

Well see, just ordered some copper foil tape from a stained glass shop, and am researching uv lasers now.
 
Marcos Buenijo
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Morgan, my philosophy with respect to off grid energy is to be as practical and cost effective as possible. I appreciate any endeavor to develop new technologies, but I know from hands on experience that this is generally extremely costly. Therefore, if one is not developing such systems as a business venture, then I can't consider it as practical and certainly not cost effective.

So far, then only energy systems I've considered to be both cost effective and practical for off grid use include: (1) photovoltaics, (2) solar thermal water heating for thermal storage (heating applications), (3) solar air heating for direct space heating, (4) wind turbines (where average wind speeds are high enough), (5) small scale hydro (if someone is really lucky), (6) wood gas engine systems (where this fuel source is plentiful, and if the heat it put to good use), (7) biomass furnace (for heating applications). I encourage those who wish to devise an off grid energy system to restrict their considerations to these options, then work to determine the configuration that is ideal for their particular circumstance. However, I've considered the following systems as prospects for development that may not require excessive capital investment (I know of no suitable commercial systems available): These include various air conditioning systems powered by heat such as absorption and adsorption, steam jet cooling, or desiccant evaporative cooling, various geothermal configurations that might be used to provide space heating and some cooling, and small scale steam power systems fueled by biomass where the engine has reasonably high efficiency (which is very difficult to see in practice). See below (*) for one example of a relatively simple chilled water system powered by heat.

I mentioned a configuration centered on wood gasification mainly because few consider this option. In particular, I have not seen any mention of putting the heat from such a system to full use. I believe if most of the heat is put to use, then this option can be extremely efficient. However, it remains that this configuration consumes fuel. Therefore, I have also considered a solar configuration where a dc motor powered off a battery is used as a diversion load to power a chilled water system. This approach can provide cooling at a high capacity (well, high for a small efficient home) while requiring a modest battery system. This is significant because a battery suitable for providing the same cooling capacity would be very large indeed, and these large battery systems are very costly (especially over the long term). Heating applications can be met with solar collectors and hot water storage, or a small biomass furnace, and the water that is partially frozen for the chilled water system can be heated and used in the same heat exchangers normally used for air cooling. Optimizing these systems would be tough, but not nearly so difficult as engineering components from scratch. For example, these air coolers/heaters can be placed and operated efficiently by cooling only those spaces that are occupied (i.e. living room during day, and bedrooms at night).

(*) Most chilled water systems I've seen operate at a high vacuum to drive the rapid evaporation of a store of water. Chilled water flows through a heat exchanger contained in this store of water for continual cooling. Solid adsorbents like silica gel or zeolite are used in some systems to drive the evaporation. However, other systems use liquid desiccants. I believe a good system can be had using inexpensive aqueous calcium chloride here. This system can be more easily fabricated than most others I've considered. Heat can be used to regenerate the liquid desiccant after it drains by gravity into a heating vessel. The water vapor released can be condensed (with some heat recovery for useful purposes), and the cool condensate returned to the water vessel. The concentrated liquid desiccant is returned to the top of a packing column by a low power pump where the desiccant continually is exposed to the water vapor given off from the water vessel. The more dilute desiccant collects at the bottom of the packing column (after absorbing water vapor) where it drains back to the heater in a closed cycle. There is a slightly higher pressure on the heater side, so the fluid column consisting of the water vapor condensate and the desiccant pump (with check valve) are used to isolate this pressure. The pressure is also used to force the water vapor condensate back up into the water vessel. A compact system can be evacuated using steam to displace the air (to generate the high vacuum required).

 
Nick Raaum
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Marcos,
I'd agree with your general strategies on a gasification CHP/combine cooling power (CCP) system. If you don't mind I'll try to get a better ballpark on what the costs and fuel requirments for running such a system would look like. Using all power labs as an economic benchmark http://www.gekgasifier.com/products/product-overview you could get a small system at the upper end for $2/watt. If operated well you could get maybe 10,000hrs runtime before full rebuild on such an engine. Assuming one needs around 10kWhr/day You'd probably be Ok with a 3kW engine running on average 3.33 hrs per day assuming you had sufficient battery storage and inverter. Rebuild would come in around the 8 year mark on the engine.

Assuming 2.6lbs biomass/kwhr you'd need an annual supply of 9490 lbs of chipped biomass. Acreage needed for this supply would vary but most places could produce this comfortably on a 5 acre woodlot year in year out. A chipper would need to be added into the cost equation, assuming you rented one annually for a day or two for an annual cost of $250. Alternatively open market prices for chipped biomass would be between $332-$949 @$70-$200/ton.

Tallying up the costs for 8 years of runtime would be $6000 for original gasification system, a minimum of $2000 for fuel assuming you chip yourself), and say $1000 for maintenance. That comes out to around .31 cents per killowat hour, but of course thats only the electrical cost, when your heat and cooling would also be covered for that amount. Also the engine rebuild could be done for less than the cost of a new system presumably so long term costs could be considerably lower.

Lots of assumptions and plenty of room for error in the above just trying to get a feel for what operating such a system might cost.
 
Marcos Buenijo
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Hello Nick. Unfortunately your estimates on fuel consumption are off by a great deal. An efficient wood gas engine system like the APL unit can generate one KWh of ac electricity from 2.6 pounds of biomass, but this assumes (1) biomass with zero moisture and (2) the system is operated where its efficiency is optimal. In the real world biomass is harvested with moisture content on the order of 50%, and losses from part load operation, battery, and inverter are also roughly 50%. Multiplying your biomass harvest figure by a factor of four will yield a value that I believe to be much closer to reality. I realize you made a disclaimer at the end of your comment, but I've done the research to know what the real world efficiencies will be and how it will vary with the application. There is a lot of variation depending on how things are configured and how the system is operated. The configuration I outlined attempts to minimize real world losses by operating the system only where thermal efficiency is optimal, and by avoiding the losses in energy conversion associated with alternator, batteries, and inverters. There is also a great deal that can be done with the heat that should not be overlooked. In general, I believe there to be a myopic focus on electric power generation that stifles creative thinking in alternative energy.

The ideal off grid energy system in my mind for use in a modest residential setting would seek to minimize electricity use such that a solar array can provide all electricity except during periods of inclimate weather where a wood gas engine system might be useful for back up power generation only. A biomass furnace could be used for heating applications and space cooling with absorption, and with particular emphasis on making full use of the "waste" heat available from the system (for such applications as water pasteurization, water distillation, biomass fuel drying, etc.). A remarkably simple single effect one ton chiller can be devised to consume electricity at a rate of only 200-250 watts total using a 24 volt DC system (includes all pumps and fans) while also providing fairly high temperatures on the order of 150F at its condenser for heating applications (water heating, vacuum water distillation, and water pasteurization can be done at a high rate using heat regeneration by tapping the high temperatures at the furnace)... biomass fuel consumption would be on the order of 18,000 btu/hour. The system can be configured for space heating with about half the electricity load required (in space heating mode the system distributes heated water to fan coil units rather than chilled water).
 
Marcos Buenijo
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Marcos Buenijo wrote:
(*) Most chilled water systems I've seen operate at a high vacuum to drive the rapid evaporation of a store of water. Chilled water flows through a heat exchanger contained in this store of water for continual cooling. Solid adsorbents like silica gel or zeolite are used in some systems to drive the evaporation. However, other systems use liquid desiccants. I believe a good system can be had using inexpensive aqueous calcium chloride here. This system can be more easily fabricated than most others I've considered. Heat can be used to regenerate the liquid desiccant after it drains by gravity into a heating vessel. The water vapor released can be condensed (with some heat recovery for useful purposes), and the cool condensate returned to the water vessel. The concentrated liquid desiccant is returned to the top of a packing column by a low power pump where the desiccant continually is exposed to the water vapor given off from the water vessel. The more dilute desiccant collects at the bottom of the packing column (after absorbing water vapor) where it drains back to the heater in a closed cycle. There is a slightly higher pressure on the heater side, so the fluid column consisting of the water vapor condensate and the desiccant pump (with check valve) are used to isolate this pressure. The pressure is also used to force the water vapor condensate back up into the water vessel. A compact system can be evacuated using steam to displace the air (to generate the high vacuum required).


I have since experimented with calcium chloride and determined that the vapor pressure is simply too high to support an effective chiller (although it seems a good candidate for an open system to provide only air drying). However, I have been able to obtain lithium bromide fairly inexpensively. If a system can be devised to not require a large amount, then using lithium bromide should be considered for use in a chiller (silica gel is also a good candidate for an adsorption chiller). I also verified that I can draw a high vacuum using a conventional vacuum pump devised for use in evacuating a/c systems as long as the system is configured to allow water vapor to displace air from the system. I was able to chill a small volume of water to freezing using this method where lithium bromide absorbed water vapor from the system to chilled the water from 70F to freezing... in other words, if virtually all the air had not been evacuated, then the water could not have reached freezing. Furthermore, the cool down rate was constant from 70F to freezing. What I learned so far is that generating and working with a high vacuum is not difficult. Right now I am setting up to test various absorber designs for a small lithium bromide absorption chiller system. I'll share what I can when I can.

BTW, based on my research I am confident that I can build a very effective absorption chilled water system once I have a good absorber design. The absorber really is the heart of the system. I've sourced all the heat exchangers required of the system, and I've purchased and tested a fluid pump that works extremely well under a high vacuum. Once I come up with a cost effective AND effective absorber design, then the rest will fall into place easily. It's possible to vary the parameters in this kind of system easily, and the condenser temperature can be adjusted over a fairly wide range. I'm targeting 140-150F at the condenser for heating applications. I mentioned water heating and water pasteurization, but I am also interested in biomass fuel drying. Also, it would be easy to tap this heat for clothes drying if one desired to do this. I make these posts mainly to encourage others to thing along similar lines as very few people seem to engage in practical thinking when it comes to alternative energy, and when they do it's often associated with a poor understanding of the underlying physics involved. I hope to get practical thinkers interested in solving real world problems, and also to benefit from some outside the box thinking of others.
 
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