1. A very nice rocket mass heater/stove: www.zaugstoves.com

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3. Study measuring the performance of a wood furnace with thermal mass (a masonry wood stove). This system works on the same fundamental principle as a "rocket mass heater". http://pages.uoregon.edu/hof/W09HOF/...Heater_ppr.pdf

The performance is impressive. An average wood fuel consumption of 35 pounds per day over a week is used to maintain a temperature difference between inside and out of roughly 20F. Outside temperatures averaged in the mid-high 40's F during the period. The home is 1700 square feet. Total fuel consumption represented about 8700 btu/hour. However, it is estimated that 80% of this heat was retained in the home, so that's about 7000 btu/hour provided by the furnace.

I find this study to be interesting because it confirms some research I had done before on the average heat loss from homes. I was considering cooling at the time, but the same principles apply. This study suggests that cooling the same home by 20F lower than average outside temperatures will require about 7000 btu/hour averaged over a 24 hour period. I realize this is only an estimate, and certain heat gains (especially solar gain through windows and attics) should be minimized when cooling. Anyway, let's consider an average outside temperature of 85F during summer. This is about right for many hot regions in the south. It might be 95 during the day, then drop to 75 at night for an overall average in the mid-80's. Now, an average of 75F in the home is good enough. So, this suggests that the cooling rate might be as low as 3500 btu/hour averaged over a 24 hour period. Well, this would consume only about 9 KWh of electricity for standard window a/c units. This suggests that perhaps the idea I mentioned elsewhere in using a large solar array to power window units as the loads for battery diversion charge controllers might work rather well. Remember, the Achilles heel of off grid power systems is the BATTERY. This cooling system does not require much of a battery system, and it can avoid a lot of battery losses.

This low cooling load also suggests a modest desiccant evaporative cooling system can be effective when operated 24/7. So, a system devised for space heating during the winter months to provide a continual heat on the order of 10,000-15,000 btu/hour (which should be plenty where winters are modest as suggested by this study) can also provide enough cooling for the same home during a hot summer. In that case a desiccant evaporative cooling system should provide on the order of 5000-7500 btu/hour cooling when such a heat source is provided. Please note that I realize these are estimates, and they are for illustrative purpose only. However, when considering a more modest off grid home on the order of 1000 square feet and well designed, then this does suggest impressive performance is possible for the cooling systems I had proposed earlier.

4. Great site, check it out: http://www.frugal-living-freedom.com/

5. Thoughts on water purification: There are many different methods. I prefer the one that is cost effective, efficient, simple, does not require specialized equipment, and that can process a large volume of water quickly and efficiently. Therefore, I prefer pasteurization. Any controllable source of heat at a high enough temperature will work well. Steam would be ideal, but a small furnace operated at a constant low output would be fine. Make sure to use the heated (pasteurized) water to preheat the cool (unpasteurized) water before it moves to the heater. A copper heat exchanger has fantastic heat transfer properties, so go with copper tubing. For the final pass take the water through a filter of sterilized sand followed by charcoal. Everyone says "activated" charcoal, and yeah this is best, but plain crushed charcoal is a lot simpler to make and works well enough. You're just wanting to remove nasties that will make it taste bad and be a bit more susceptible to re-establishing a culture of pathogens. The heat regeneration provided by preheating the water will increase the efficiency many fold because most of the heat placed into the water when the system first starts to heat up can be transferred to the cool water before it reaches the heater (*). This means you need a lot less heat for the same flow rate, or you can have a much higher flow rate for the same heat input rate. I say go with the higher flow rate. So, in this case you just admit water slowly during start up to get really hot water leaving the heater, then you can speed things up to a constant level once you have the preheated water moving through the system. A thermostatic valve would be awesome here. Make sure to flush the lines with super hot water on first start up.

(*) Note that the efficiency gains possible by preheating the water in this manner is not trivial. Expect to increase the yield on the order of 5 fold using this process. More is possible.

ADDENDUM: I realized recently after talking with a friend of mine that pasteurizing water using heat regeneration may not be so easily understood by many. So, consider the following scenario. Water can be pasteurized by heating to 160F for 15 seconds. If the temperature is higher, then it can be held at this higher temperature for less time. So, let's say you put a pot of water on a burner and heat the water to 200F. It is now well pasteurized. The harmful microorganisms in the water are dead or neutralized. So, you don't need the heat in the water anymore. Rather than wasting this heat, use it to preheat the next batch of water to be heated. Of course, the most efficient process would be the one that sends water through continually. Consider the two scenarios:
(1) You send 50F water at 1 gallon per minute through a heater to take the temperature to 200F. It is well pasteurized.
(2) Before the 50F water gets to the heater it first passes through a heat exchanger to be heated by the 200F water leaving the heater. The 50F water gets heated to 170F, and the 200F water is cooled down to 80F (actually, it's a little cooler due to thermal losses). Therefore, without increasing the output of the heater, you can now increase the water flow rate to 5 gallons per minute and get the 170F heated to 200F. The water is heated to greater than 160F for what is likely a lot longer than 15 seconds, and it reaches 200F. It is well pasteurized.

6. I was just wondering that it may be practical to heat a water pasteurizing system with solar energy using photovoltaics by using the heat regeneration scheme discussed in the previous post. Before one rejects this notion completely (and I might have done just that in the past), consider the details. Sure, there is a lot of energy in the available sunlight that is not accessed with photovoltaics vs. using the solar energy for direct heating. However, it's not so easy to catch and hold most of this energy, particularly when higher temperatures required for water pasteurization is desired. Conversely, with photovoltaics the energy can be delivered to a very compact water heating vessel that can be easily highly insulated. Furthermore, the system can be tightly controlled.

If we assume that the cold unpasteurized water is preheated by the hot water leaving the heating element to within 10F of the peak temperature, and neglecting thermal losses and the pump load, then one KWh of electricity should be able to pasteurize on the order of 40 gallons of water. This is not trivial. Most areas in the U.S. provide at least 2.5 sun hours each day even during winter days. A 1 KW array should process more than 100 gallons of water under these conditions. In principle, it's possible to control flow through the system with feedback from a thermostat that can be used to operate a small pump. In my opinion, with the high reliability of PV panels, this option should not be discounted.

ADDENDUM: I checked out the specs on a commercial solar water heating panel (not evacuated tube design, but flat plate). It turns out that heating the water to a level sufficient for pasteurization results in substantial thermal losses. More important, these losses rise with decreasing solar irradiance. Considering the losses here, it turns out that the actual mass of water that can be pasteurized by this water heating panel is LESS than what's possible from a PV array of the same price. Very interesting results indeed. Of course, one could just go with a small furnace fueled by wood chips and call it a day. However, it sure is an interesting prospect to have a fully automated water processing system that consumes no fuel.

7. Another wacky idea for air conditioning in the off grid setting. Drive an automotive ac compressor with a dc motor powered by a solar array. This seemed a bad idea when I first considered it, but these compressors are durable and efficient despite the poor performance of automotive a/c systems. The reason for this is not the compressor, but the small heat exchangers and higher air temperatures available for the condenser. A system could be optimized for efficiency by using large heat exchangers and providing good cooling. Performance could be further enhanced by reducing the evaporator temperature, and this can yield good results with dry air (I am considering air dried with a desiccant, of course).

8. Here is an interesting approach to space cooling for those who desire a simpler system and suitable for a modest dwelling. I was considering a system specifically to minimize electricity requirements. Use a small biomass furnace to regenerate a calcium chloride desiccant solution to high concentrations. This solution must be cooled and pumped to the air dryer. This might be a vessel with packing material (can't obstruct air flow much) over which the concentrated calcium chloride solution continually flows and must be distributed evenly over the packing material (I speculate here, but one might use rocks or maybe even large wood chips). A large insulated duct is connected to the top of this vessel that extends vertically over a good distance. Since the calcium chloride solution increases in temperature as it absorbs water vapor, this air moving through the vessel will be heated. I speculate that retaining this heat with insulation on the vertical ducting, then transferring the dry hot air through a long uninsulated horizontal section of duct for cooling to return to the home might induce enough differential pressure through natural convection to dry the air in a small home efficiently.

As long as the air is dry, then an evaporative cooler can work well. A small portable commercial unit might be used for spot cooling as these use low power fans. It may be possible to provide an evaporative cooling effect by using a low volume, high pressure water pump to send water through atomizing nozzles, and this would consume the least electricity.

9. I'm expanding a bit on the the idea of using a small wood gas engine system to power an automotive a/c compressor for air conditioning. I considered the idea a couple years back (well, actually longer ago than that), but I didn't really consider it seriously until more recently. First of all, let me emphasize that the desiccant system is particularly promising for humid regions, and it should be used at least in tandem with a vapor compression system. If this is not done, then the vapor compression system will have to work harder harder. The good news is that some heat from the wood gas engine system can be used to regenerate the desiccant. Furthermore, it is possible to devise the system to freeze a large store of water that can be drawn upon over the following 24 hour period. All in all, I think this configuration has merit. I will summarize here:

1. Operate a small wood gas engine system at a constant output at roughly 5 hp to drive an automotive a/c compressor (or whatever minimal output can be reliably and efficiently maintained, and this is often about 5 hp for a wood gas engine system). How long you operate the system depends on the cooling load and how much fuel you want to burn.

2. Use the evaporator coil to freeze water contained in an insulated vessel. This latter configuration might work well particularly if the water were never fully frozen (just nearly so - make a slush) because ice tends to insulate. Sizing the water vessel properly would make this simple. Use this cold water for air cooling in a mini chilled water system. Note that it may be possible to cool a heat transfer fluid like a water/glycol contained in the insulated vessel, then place smaller water vessels inside the vessel. This will prevent ice formation on the evaporator tube while also freezing water. The water/glycol can be used directly in fan coil units for cooling.

3. Use the engine cylinder cooling blower along with engine exhaust to regenerate a desiccant. This desiccant can be used for both space cooling (see desiccant cooling) and space heating (see desiccant heating).

5. Use heat from the gasifier fuel gas cooler to dry the next batch of wood chip fuel.

5. Use the steam emitted from the heated desiccant to heat a store of water - waste not want not!

6. Operate the system during evening or morning when the outside temps are lower. Anything that gets the condenser temps down with increase efficiency.

7. May also drive an alternator with the engine for battery charging at this time.

Videos show very simple designs for water and air heating systems using solar energy including a hot water storage tank and hydronics heating system used to heat the floor above the basement. This is about as simple as it gets, but very effective.

There is a lot of potential in solar thermal and photovoltaics at the residential scale. I've become increasingly interested in making use of direct solar energy over the last year or so in order to minimize fuel consumption. I can think of several principles that can dramatically increase efficiency in these systems. With solar thermal, optimizing efficiency is all about increasing solar capture and minimizing thermal losses. Certainly if someone desired to provide most of their space heating and water heating with solar, then a large thermal mass is necessary. An ideal system would store heat in a phase change material contained in an insulated enclosure, then tap this heat for all space heating and water heating needs. However, good old fashioned water is really the only practical and cost effective thermal mass for this particular application. You know, 1000 gallons of water is not too imposing, and yet it weighs more than 8000 lbs. Take the temp of that up by 50F with solar heat, and you'd be storing 400,000 btu of heat energy. A modest well insulated home can get by on this, and a simple wood furnace can heat this water via thermosiphon when solar is insufficient.

11. See how easy it is to convert a small window a/c unit for water heating using the condenser: http://www.youtube.com/watch?v=GBlrewwpt8M. Note the energy recovered at 7500 btu/hour! This represents the energy removed from the air at the evaporator and most of the energy consumed by the compressor motor. That's a lot of energy at more than 2 KWh per hour (i.e. 2 KW for a roughly 500 watt compressor motor). Operating this unit over one summer could fully pay for itself with the electricity savings alone. Kinda seems absurd to consume energy in operating an a/c system while consuming additional energy to heat water, doesn't it?

Imagine placing a small pulley on the shaft from which the condenser fan is removed, then using this shaft to drive a small water pump. This water could then be used to water cool the condenser. If geared properly, then it should provide optimal heating with a single pass of the water provided the temperature of the supply is within a certain range. What I'm thinking here is replacing the cover on the unit, but placing the condenser in an insulated vessel that is secured to the outside of the unit. The hoses connected to the pump would penetrate the cover of the unit. This should restore the convenience of the unit and make it compact and attractive. Plus, water cooling the unit would provide a lot of versatility with respect to placing the unit as it would no longer have to be placed in a window for cooling. Perhaps this latter advantage would make this retrofit worthwhile even if the heat is not put to use. For example, perhaps the heated water could be distributed to a heat exchanger (or large water tank) placed outside to cool the water, then the water would return to the pump in a closed circuit. Being able to place a small a/c unit precisely for spot cooling would allow for consuming a lot less energy (why cool an entire room if you don't have to?). Also, this approach might conceivably be used to circulate water through a heat exchanger buried in the ground (a mini geothermal heat pump). On that note, perhaps a unit could be retrofitted to cool the evaporator with the evaporator fan removed and the shaft used to drive the pump. Then, a geothermal heating system could be had. Seriously, if one were living in a very small and well insulated cabin, then something like this might be effective for heating and cooling.

12. Excellent discussion on an experimental off grid home with a series of short articles discussing the system:

http://howtousesolar.com/our-off-gri...-it-all-began/

13. I found a 5 ton ammonia absorption chiller designed for residential applications. I'm looking into it, but here it is: http://www.firechill.com/products/ac500/ Too big and too much electricity for off grid use, but interesting nonetheless.

14. My opinions on a suitable battery system for an off grid power system:

I consider overall (long term) costs to be the single most important factor. Getting the best value for the battery is paramount. I write "value", not price. What kills a battery is excessive discharge and temperature extremes. Overcharge is also a killer, but it's not so much a problem as the other two. I say go with a new forklift battery. Size the battery such that a full charge on the battery will provide you with all the electricity you need over a 24 hour period without dropping the state of charge below 60%. You never want to drop below 50% state of charge. The bigger the battery, the longer it will last all else equal. The charging efficiency of a battery is very low while it's at a high state of charge. However, with the price of solar panels down so much I consider it preferable to choose a larger battery that stays at a high state of charge and buy extra panels. The overall efficiency can be improved dramatically by using most of your electricity while the panels are producing as this effectively bypasses the battery charging. One example would be placing a thermal mass in freezers and refrigerators and putting them on timers to operate during the day while the panels are producing. Another example is using any high power electrical appliance only when the panels are producing.

Forklift batteries are hands down the best value I've seen in an off grid battery system. You're looking at about \$130-\$150 per kilowatt hour of rated storage capacity (80% discharge at 20 hour rate). So, a 1050 pound 24 volt forklift battery will cost you about \$2500 delivered in the 48 States (see www.giantbatteryco.com), and will provide 19 KWh electricity at rated capacity. In practice, if you limit discharge to no lower than 60% state of charge and considering inverter losses, then a fully charged battery will provide about 8 KWh of AC electricity, and this is more than most off grid home need each day. Keep a wood gas engine system around for backup charging, and keep a large solar array to minimize wood fuel consumption for this purpose.

As far as the solar array goes, I recommend using two parallel arrays each with a separate controller. These can feed a common battery. I've not found any 48 volt forklift batteries that are not truly massive (way overkill for most off grid set ups). Unfortunately, most solar charge controllers are limited to about 80 amps. At 24 volts this limits the array to no more than 2000 watts. If you're good with what a 2 KW array provides, then fine. If not, then I recommend two parallel arrays, but you'll have to get two controllers. This isn't so bad as it provides redundancy. In my particular case, I'm planning on a 3 KW array to provide a net 6-8 KWh per day in east Texas. I'll get a wood gas engine system at about 2500 watts bulk charging rate for the battery for use only when required to prevent excessive discharge on the battery (bulk charge during early morning, then let the solar array take the battery the rest of the way. Personally, I wouldn't break out the gasifier until the battery voltage drops to under 50% state of charge.

Well, that's enough ranting for now. Later!

15. Originally Posted by buenijo
My opinions on a suitable battery system for an off grid power system:

I consider overall (long term) costs to be the single most important factor. Getting the best value for the battery is paramount. I write "value", not price. What kills a battery is excessive discharge and temperature extremes. Overcharge is also a killer, but it's not so much a problem as the other two. I say go with a new forklift battery. Size the battery such that a full charge on the battery will provide you with all the electricity you need over a 24 hour period without dropping the state of charge below 60%. You never want to drop below 50% state of charge. The bigger the battery, the longer it will last all else equal. The charging efficiency of a battery is very low while it's at a high state of charge. However, with the price of solar panels down so much I consider it preferable to choose a larger battery that stays at a high state of charge and buy extra panels. The overall efficiency can be improved dramatically by using most of your electricity while the panels are producing as this effectively bypasses the battery charging. One example would be placing a thermal mass in freezers and refrigerators and putting them on timers to operate during the day while the panels are producing. Another example is using any high power electrical appliance only when the panels are producing.

Forklift batteries are hands down the best value I've seen in an off grid battery system. You're looking at about \$130-\$150 per kilowatt hour of rated storage capacity (80% discharge at 20 hour rate). So, a 1050 pound 24 volt forklift battery will cost you about \$2500 delivered in the 48 States (see www.giantbatteryco.com), and will provide 19 KWh electricity at rated capacity. In practice, if you limit discharge to no lower than 60% state of charge and considering inverter losses, then a fully charged battery will provide about 8 KWh of AC electricity, and this is more than most off grid home need each day. Keep a wood gas engine system around for backup charging, and keep a large solar array to minimize wood fuel consumption for this purpose.
What is the expected lifespan of those batteries?

16. Originally Posted by klamath
What is the expected lifespan of those batteries?
It depends on how they're used. In their designed application (i.e. forklift duty) they are often near fully discharged during a work day, and fully charged before the next duty cycle. Often they see at least one full duty cycle each work day. You'll have to verify, but I recall a typical life time of roughly 6-8 years under this heavy industrial usage. However, they are not discarded at this point. Rather, they are often sold second hand (used). The cells at this point might not be able to take a forklift through a full work cycle, but they often work very well in light duty applications (like off grid). In the off grid application, they last 15-20 years by most accounts I've seen. I know of one individual who claimed 10 years on a USED forklift battery, several claimed 15-20 years for a new battery, and one claimed 33+ years for a new battery. Note that this represents information I've mined over the last 5 years or so. NOTE: Personally, I don't recommend that anyone consider a used battery.

http://www.sustainablepreparedness.c...newable-energy

NOTE: See post #3 in this thread.

ADDENDUM: BTW, the much lauded Rolls-Surrette battery has a 6 volt, 400 amp hour, 127 pound battery that I've seen for sale at a good price of \$340 (see www.wholesalesolar.com). This price is on par with the price I've seen for forklift batteries. However, there is likely to be a hefty additional charge for delivery. The main advantage here is that the 127 pound battery is much easier to handle than a single massive forklift battery. This seems a value worth the extra cost. By all accounts I've seen, the Rolls-Surrette is among the best battery one can choose for off grid use.

ADDENDUM: The company that I linked provides forklift batteries at the listed prices without delivery charge. Also, they pick up a discarded battery with no charge. This is interesting as one common complaint on forklift batteries is their massive size. Well, if you can have a new battery delivered with the old battery carried off, and all with no charge beyond the cost of the new battery, then that seems quite a deal.

17. I started a project recently to develop a micro absorption chiller. I don't yet have anything worthwhile to share, but I'll share what I can when I can. The system is to use lithium bromide as the absorbent. I've acquired the lithium bromide as well as vacuum equipment, but I have yet to get suitable fluid pumps. Note that one must take time to prevent going through precious capital too quickly.

FYI, while I am doing only basic testing for now, if the results of my testing is positive, then I will assemble a complete test unit. The system I have in mind is to operate at a cooling capacity of 1 refrigeration ton (or 12,000 btu/hour), and I believe this is sufficient for a modest off grid home - even a fairly large home provided the unit is designed for continual operation at this rate and the cooling is split. A small wood gasifier is the desired heat source, but there should also be a provision for using natural gas. The idea for now is to design the system to run at a constant output with chilled water distributed to small fan coil units placed in the home. It's possible to vary the output over a limited range while keeping high efficiency, but one of the best means to reduce fuel consumption would be to split the cooling (for example, send chilled water to the fan coil unit in the main living area of the home during the day only, then send chilled water to small fan coil units in sleeping quarters at night - of course, a proper off grid cabin could use a single fan coil unit). This is being designed specifically with off grid living in mind. There will also be a provision for heating water with the condenser of the system. It's also possible in principle to configure the system to pump heated water to the fan coil units for space heating. It's also possible to configure the system as a heat pump for space heating, but this requires a source of heat at a moderate temperature such as geothermal or a body of water at least 50F. Right now I'm taking things one step at a time. This is a long term project that will be slow going.

18. I performed an interesting experiment today. I was able to draw a high vacuum within a vacuum chamber with a poor vacuum pump by using water vapor to displace the air. I placed hot concentrated lithium bromide solution on the bottom of the vessel, then placed hot water in an insulated container above the solution. I then drew on the chamber with the vacuum pump with a suction line that extended down into the vessel just above the solution. The vacuum pump was able to draw down the vacuum enough to boil the hot water down to about 70F. The idea here is that the water vapor that is less dense than air will displace virtually all the air in the vessel and force it out the vacuum pump suction since this suction is placed at a low point in the system. Once the vacuum pump could no longer boil water to a lower temperature, I then shut the valve and disconnected the pump. I then placed the vessel in a shallow water bath to cool the lithium bromide solution at the bottom of the vessel. Shortly after placing the vessel in the water bath the water within the vessel began to boil and the vacuum gage showed a reducing pressure and the water thermometer showed decreasing temperature. The rate of temperature drop was not fast, but was steady. A gentle swirling of the vessel increased the rate of water evaporation as expected. The temperature fell at a steady rate to an indicated temperature of -1.1C... let's just call it freezing. However, the water showed no obvious signs of actually freezing, it's just really freakin cold. I haven't broke vacuum yet, but once I do I'll check for any signs of ice. Note that the main purpose of this test was to verify that a combination of crappy vacuum pump and water vapor displacement of air can get all the air out of a vacuum vessel. Test is successful.

ADDENDUM: I broke vacuum. No ice, just ice cold water at 33F.

19. The latest on my chiller project (beyond being very slow going AND expensive) is that I've sourced and tested the pumps I need, and they work perfectly. I am very pleased. This was one of my major concerns. So, I have everything in place for the next phase of testing which will be the most important: absorber design. I'll post more when I have it.

20. I had an independent thought a few days ago on how to go about building a flat plate solar collector while also keeping the interior under a high vacuum for insulation. The purpose is to generate high temperature water and/or steam without resorting to concentration or tracking, and to minimize thermal losses because most flat plate collectors designed for water heating see very poor efficiency at high temperatures. Well, just a few google clicks showed this to have been done before. I believe I can build a unit for about \$100 per square meter, and I don't know what the commercial units cost (not yet). Here is one example I stumbled on: http://www.srbenergy.com/pages/carac...s-del-colector . This unit and others are characterized by achieving high temperatures without concentration and with minimal thermal losses to ambient air as expected. This kind of system seems ideal for generating high temperature saturated water under pressure for driving an absorption chiller efficiently. I'm liking the idea of tapping solar heat when its available for space heating, water heating, and space cooling... but then using a biomass gasifying furnace when solar is not available. Use photovoltaics with battery storage for the relatively small amount of electricity required, and one may also use a small wood gas engine system for battery charging on rare occasions when solar is insufficient (may even use the same furnace). Lots of possibilities come to mind.

21. Originally Posted by buenijo
I started a project recently to develop a micro absorption chiller. I don't yet have anything worthwhile to share, but I'll share what I can when I can. The system is to use lithium bromide as the absorbent. I've acquired the lithium bromide as well as vacuum equipment, but I have yet to get suitable fluid pumps. Note that one must take time to prevent going through precious capital too quickly.

FYI, while I am doing only basic testing for now, if the results of my testing is positive, then I will assemble a complete test unit. The system I have in mind is to operate at a cooling capacity of 1-2 refrigeration tons. A small
wood gasifier is the desired heat source, but there should also be a provision for using natural gas. The idea for now is to design the system to run at a constant output with chilled water distributed to small fan coil units placed in the home. It's possible to vary the output over a limited range while keeping high efficiency, but one of the best means to reduce fuel consumption would be to split the cooling (for example, send chilled water to the fan coil unit in the main living area of the home during the day only, then send chilled water to small fan coil units in sleeping quarters at night - of course, a proper off grid cabin could use a single fan coil unit). This is being designed specifically with off grid living in mind. A first test unit will be single effect. If the results are good, then a double effect system will be configured. There will also be a provision for heating water with the condenser of the system. It's also possible in principle to configure the system to pump heated water to the fan coil units for space heating. It's also possible to configure the system as a heat pump for space heating, but this requires a source of heat at a moderate temperature such as geothermal or a body of water at least 50F. Right now I'm taking things one step at a time. This is a long term project that will be slow going.
I am working on an off grid walk in freezer. If you have an interest I have a thread going here. http://hvac-talk.com/vbb/showthread....ment-questions

22. Originally Posted by klamath
I am working on an off grid walk in freezer. If you have an interest I have a thread going here. http://hvac-talk.com/vbb/showthread....ment-questions
I checked out the link briefly. There is some good advice there. Using two or three small inits for redundancy is a good idea. Going with urethane insulation and being generous with its application is good. If you can go with water cooling the condenser then definitely do it as this will increase efficiency. One comment on the link suggested that water cooling is not a benefit if the water is not much cooler than the air, but that's wrong. It takes a lot less enegy to flow water over a condenser than air, so water cooling will reduce energy required even if the condenser temperature is constant... but in practice water cooling often reduces the compressor load significantly as well. If your goal is to reduce electricity consumption, then an ammonia absorption system would be ideal. Unfortunately, this would require a lot of development work. If you have the gumption, then I'll discuss it further. However, vapor compression does seem more practical in this particular case mainly because it's possible to charge the freezer and let the insulation and thermal hold the temps until the following day. I believe photovoltaics will be useful here. If you can find a suitable compressor that ismechanically driven as opposed to a hermetic electric motor drive unit, then consider driving such a compressor with a solar array and modest battery system used to power a dc motor. Yeah, it's a big job but it does seem an ideal configuration. What I'm thinking is to configure the system to run only when the solar array is producing as this will not require any significant battery discharge. This would also avoid inverter losses and it would be a lot simpler to get this to handle the motor starting current.

23. Originally Posted by buenijo
I checked out the link briefly. There is some good advice there. Using two or three small inits for redundancy is a good idea. Going with urethane insulation and being generous with its application is good. If you can go with water cooling the condenser then definitely do it as this will increase efficiency. One comment on the link suggested that water cooling is not a benefit if the water is not much cooler than the air, but that's wrong. It takes a lot less enegy to flow water over a condenser than air, so water cooling will reduce energy required even if the condenser temperature is constant... but in practice water cooling often reduces the compressor load significantly as well. If your goal is to reduce electricity consumption, then an ammonia absorption system would be ideal. Unfortunately, this would require a lot of development work. If you have the gumption, then I'll discuss it further. However, vapor compression does seem more practical in this particular case mainly because it's possible to charge the freezer and let the insulation and thermal hold the temps until the following day. I believe photovoltaics will be useful here. If you can find a suitable compressor that ismechanically driven as opposed to a hermetic electric motor drive unit, then consider driving such a compressor with a solar array and modest battery system used to power a dc motor. Yeah, it's a big job but it does seem an ideal configuration. What I'm thinking is to configure the system to run only when the solar array is producing as this will not require any significant battery discharge. This would also avoid inverter losses and it would be a lot simpler to get this to handle the motor starting current.
All good points. Solar is still out of my budget as far a costs but it is getting closer. The EPA regs restrict a lot of experimenting on refrigeration systems. Without a license I don't even believe it is possible to buy refrigerant.

24. Originally Posted by klamath
All good points. Solar is still out of my budget as far a costs but it is getting closer. The EPA regs restrict a lot of experimenting on refrigeration systems. Without a license I don't even believe it is possible to buy refrigerant.
I didn't read the link well at all. If you have 6 KW from a hydro turbine, then you can make this work. Sorry for recommending solar as you simply have no use for it with a hydro resource like that. You're in a great position with that resource.

I think the heat influx will be significantly less than 12,000 btu/hour based on the size of the freezer and the insulation you discussed in the link. With that kind of insulation I expect a value roughly half this figure at about 6,000 btu/hour and possibly less. Here is my reasoning: an 8 cf deep freezer will consume roughly 1 KWh of electricity over a 24 hour period. If it were super insulated like your freezer, then this would easily drop to about 1/2 KWh per day. This is confirmed by looking at the specs on super insulated "solar" freezers with 4" urethane. The surface area of an 8' by 8' by 8' freezer is about 16 times that of an 8 cf freezer (assuming 2' by 2' by 2'), so the larger freezer would see a heat influx of roughly 16 times that of the smaller freezer, and therefore consume roughly 16 times the electricity, or 8 KWh per day. Yes, a very rough estimate. I do KNOW that superinsulating a freezer with several inches of urethane will dramatically reduce heat influx as I've read accounts of this being done to augment the insulation on existing large units with a resulting dramatic reduction in electricity consumption. This suggests a compressor at only 333 watts could cool the unit (assuming a continual operation). The compressors for large chest freezers might actually work here if running continually of course. If possible, then securing several such systems could be done for both redundancy and operating two in tandem if required. Furthermore, this approach would allow for reducing the starting load, and such a small compressor would not present any problems. It seems a big job to engineer this right, but I don't think it needs a large compressor.

25. Originally Posted by buenijo
I didn't read the link well at all. If you have 6 KW from a hydro turbine, then you can make this work. Sorry for recommending solar as you simply have no use for it with a hydro resource like that. You're in a great position with that resource.

I think the heat influx will be significantly less than 12,000 btu/hour based on the size of the freezer and the insulation you discussed in the link. With that kind of insulation I expect a value roughly half this figure at about 6,000 btu/hour and possibly less. Here is my reasoning: an 8 cf deep freezer will consume roughly 1 KWh of electricity over a 24 hour period. If it were super insulated like your freezer, then this would easily drop to about 1/2 KWh per day. This is confirmed by looking at the specs on super insulated "solar" freezers with 4" urethane. The surface area of an 8' by 8' by 8' freezer is about 16 times that of an 8 cf freezer (assuming 2' by 2' by 2'), so the larger freezer would see a heat influx of roughly 16 times that of the smaller freezer, and therefore consume roughly 16 times the electricity, or 8 KWh per day. Yes, a very rough estimate. I do KNOW that superinsulating a freezer with several inches of urethane will dramatically reduce heat influx as I've read accounts of this being done to augment the insulation on existing large units with a resulting dramatic reduction in electricity consumption. This suggests a compressor at only 333 watts could cool the unit (assuming a continual operation). The compressors for large chest freezer might actually work here if running continually of course. If possible, then securing several such systems could be done for both redundancy and operating two in tandem if required. Furthermore, this approach would allow for reducing the starting load, and such a small compressor would not present any problems. It seems a big job to engineer this right, but I don't think it needs a large compressor.
I have been researching the cold plates the one guy mentioned. So far I can only find cold plates the hold the box at 0 degrees. I am pretty set on the -15 temperature. For really preserving food the colder the better. My family did canning for years but I love freezing as almost everything is better about freezing.
As far as load calculation you can download a free program here. http://www.keepriterefrigeration.com/node/213 It is a Good program and it shows about what you said as far a BTU's per hour. All the variables can be figured in from the type of food to the light wattage.

26. Originally Posted by klamath
I have been researching the cold plates the one guy mentioned. So far I can only find cold plates the hold the box at 0 degrees. I am pretty set on the -15 temperature. For really preserving food the colder the better. My family did canning for years but I love freezing as almost everything is better about freezing.
As far as load calculation you can download a free program here. http://www.keepriterefrigeration.com/node/213 It is a Good program and it shows about what you said as far a BTU's per hour. All the variables can be figured in from the type of food to the light wattage.
Thanks for the resource, I will bookmark it. If my figures are close, then it's a testament to the value of estimation when it has a solid foundation. I've found it very useful to check my estimates against the claims of others, even professionals... after all, everyone can be wrong at times.

27. I recently became aware of the Xantrex XW inverters. These are very sophisticated inverters designed for integrating multiple power sources with a battery system in the off grid setting. One system I am aware of uses a 6 KW solar array, 7 KW wind turbines, and a generator to power an off grid all electric home (along with a 7000 pound battery system). The inverter can be programmed to automatically start the generator on low battery voltage or high inverter load. It's the latter feature that I find particularly interesting. Since the efficiency of a constant speed generator varies widely over its power range with the optimal efficiency near the highest rated power, then the system is ideally designed such that the generator is always loaded to its optimal output. Well, that's precisely what this inverter can do! So, let's say the inverter is rated for 40 amps continuous, but the demand load climbs above this value. It's possible to program the inverter such that the generator starts, warms up, and then goes online. Now, let's say the generator is most efficient at 30 amps output, and let's say the demand load is 50 amps. Well, the system is designed to allow the generator to provide 30 amps, then let the solar/wind/battery system to provide the balance of 20 amps. Also, if the battery is not full charged, then any excess electricity provided by solar/wind is diverted to the battery for charging. Also note that the inverter stays in phase with the generator at all times. Folks, this performance is quite extraordinary. The system makes it possible to power an off grid home as if it were grid tied. As long as the solar/wind system can provide the bulk of the energy consumed, and as long as the loads are managed reasonably well and energy is conserved, then fuel consumption can be minimal - especially when considering the ability to operate the generator where thermal efficiency is optimal. A Diesel engine can provide ac electricity at an overall efficiency well over 20%, and this corresponds to 8-10 KWh of ac electricity per gallon of fuel consumed (around 40 cents per kwh not including cost of hardware). I don't suggest a Diesel or other generator be used as a primary source of electricity in the off grid setting, but it can be very useful to buffer the solar/wind system along with protecting a battery from excessive discharge. Put the heat from the generator to use in water heating, and the value of the fuel can be further leveraged. Consider that catching half the heat otherwise wasted will triple the energy harvested from the generator system.

All of this hardware is expensive. However, I'm now thinking that a modest and well designed off grid home can be powered reliably and efficiently with a solar/generator hybrid system. If the home is very well insulated, then I believe a more or less conventional electric motor driven vapor compression a/c system is a viable candidate for space cooling mainly because of how this inverter can manage the loads. People are doing it today... although, not without a hefty price tag along with dependence on refined fuels. However, it seems the most practical off grid solution. In particular, I think it's a practical configuration for powering a small home in a remote site where grid power is not available.

I continue to put a premium on configurations that minimize both the battery size and discharge, and that minimize the consumption of refined fuels. Under these conditions a/c would have to be provided almost exclusively during the day when the solar array is producing, and electricity should not be used for any heating applications (unless it were a dump load to prevent excessive battery charge rates). It makes sense to me to use a small biomass furnace for all heating applications. I still consider a micro absorption chiller with heat recovery and fueled by biomass as an ideal solution for all but electricity generation, but these are not available.

NOTE: This individual who posted the video owns the system I described earlier (6 KW solar and 7 KW wind). Hot summer months require the use of his small Diesel generator to provide sufficient cooling, but the system dramatically reduces fuel consumption that would otherwise be required. He says his system would cost about 70K if installed today, but much of that cost is the wind turbines.

28. I was killing time recently doing research to figure what the average household electricity consumption is in Texas for providing a/c. I can't come up with anything other than estimates as the data is limited, but the figures are pretty much what I expected. A typical apartment from 750-1000 s.f. and modern construction will see an average of 500 KWh electricity per month for air conditioning alone in east Texas. The numbers vary widely depending on many factors, but this is a good round figure.

Personally, I consider this square footage to be suitable for a modest off grid home. With good insulation, radiant barriers, shading, etc. I expect this figure to apply for such a modest home. A typical vapor compression a/c system consumes about 1 KWh for every 10,000 btu of cooling provided. Therefore, this home should require about 500/30 = 16.7 KWh of electricity for air conditioning to provide 167,000 btu of cooling.

A typical single effect absorption chiller will see a COP of 2/3. This corresponds to about 250,000 btu of heat required to provide the aforementioned cooling capacity, or roughly 31 pounds of commercial wood pellets (assuming 8000 btu/pound). If purchased in bulk, then this quantity of wood pellets will cost about \$4 for a total monthly fuel cost of \$120 for air conditioning (along with all the heating applications provided at the condenser). The system can also be configured for space heating during winter months, and the same fuel consumption can support space heating and all other heating applications where winters are mild (as in east Texas). Now, one need not rely on commercial pellets, but I'm considering it only for sake of interest. If one were harvesting their wood, then know that green wood has roughly 4300 btu/pound, so one would need to harvest and process about 58 pounds of green wood for a day of operating such a system. The electricity consumed by such a system would be right at 1/4 that of a typical vapor compression system with the same cooling capacity, and this can be supported by a 1.2 KW solar array.

The reason I'm going over this line of reasoning again is to reinforce my argument that a micro absorption chiller fueled by biomass could be one of the most useful off grid tools ever devised. Most important: it need not exceed a one ton capacity. You get space cooling, space heating, and heat for all manner of applications (with essentially free heat while in cooling mode) for water heating, potable water processing (pasteurization and/or distillation), and even biomass fuel drying for those who wish to process their own fuel. Also, the system is fundamentally simple. Therefore, if properly designed, then it can be made easy to service by the end user. The value of such a system has to be considered in light of this holistic perspective.

29. I believe a modest and very well insulated off grid home can be cooled efficiently and effectively by powering a properly sized inverter air conditioner with a photovoltaic array. These units are a lot more efficient than conventional units. The compressor motor is a variable frequency ac motor with permanent magnet rotor. This reduces electricity consumption for two main reasons: (1) there is no electricity consumed in the rotor as is required in conventional motors, and (2) the system can adjust the frequency of the drive (and the speed of the motor) to match cooling demand. This feature makes the size of the heat exchangers for the unit relatively large during part load operation. The result is very efficient cooling of the heat exchangers at these lower outputs which drives down the differential pressure across the compressor which in turn reduces the load on the motor. The performance based on my limited research is an overall COP of 4.0 in many models at part load operation, and even higher in some cases (up to 5.0 in some models). This is ideal for a PV array since the cooling capacity needs to be highest during the day when the solar array is producing, and this will avoid many battery losses by diverting more electricity directly to the compressor motor. However, at other times the cooling demand is likely to be lower, and the unit will operate most efficiently at part load during this time to minimize battery discharge. I've seen units that are programmable with timers that allow for adjusting the thermostat setting automatically at set times throughout the day. A low thermostat setting might be used during the day when the solar array is producing to cool the home rapidly as a thermal mass while also minimizing battery charge rate (and resulting battery losses), then raise the thermostat setting at other times to reduce the output of the unit for optimal efficiency and minimal battery discharge. The unit can also be programmed to shut off at a certain time and restart automatically (at least the unit I reviewed had this feature) - or a timer might be used. This seems a practical solution. Also, I understand that these compressor motors see much lower starting current than conventional units since they increase frequency to the motor incrementally on initial start up (and this is great for powering these units with power inverters off a battery system). While these units are sophisticated and more expensive, they have been mass produced for a long time now. For example, I understand that most if not all a/c units in Japan are the inverter type.

This approach does seem a practical way to achieve air conditioning in the off grid setting. A large PV array and large battery system would be required. Let's say a modest off grid home uses an inverter a/c system rated at 1.5 ton. This system would draw about 1750 watts of electricity when operated at full load (operate at full load by lowering thermostat setting), and this full load would be maintained whenever the large solar array is producing. At other times the unit is operated at a low part load of roughly 1/2 ton, and here it would draw only about 400 watts of electricity for a total electricity consumption of only 18 KWh over a 24 hour period. Most important is only about 7.2 KWh of this is taken from the battery, so a modest battery can handle this without excessive discharge (modest by off grid standards). How big does the solar array have to be? Well, let's consider average solar insolation of 6 KWh per square meter per day (summer time in east Texas). According to my estimates and calculations, a solar array rated at 4670 watts would be required to support this unit, this includes all losses. Of course, one would require additional solar panels to support other electrical loads. A solar array on the order of 6000 watts would be required here. Good news is that the unit cost of solar panels is remarkably low when purchased in bulk.

By comparison, a micro absorption chiller that provides the same cooling capacity would consume 3-4 KWh of DC electricity (no large inverter required). The battery could be much smaller and still see less discharge. However, the fuel consumed each day would be equal to 43 pounds of wood pellets. Of course, all water heating needs are easily met (and other heating applications can be had like clothes drying, water distillation, biomass fuel drying, etc.). The chiller can also be configured for space heating. However, note also that the inverter a/c unit is generally configured to operate as a heat pump, so it will also provide heating during the winter months. If the winters are mild, then this is where these heat pumps do particularly well. There would be only about 3.5 KWh per square meter per day of solar insolation during the winter months in east Texas, but the large solar array used to provide a/c will provide quite a bit of space heating as well that should equal a bit more than half the cooling capacity during the summer months (so roughly 125,000 btu/day... equal to about 20 pounds of wood pellets, or about \$1.50 of natural gas when on considers 80% furnace efficiency).

In short, a modest off grid home with excellent thermal characteristics, a large PV array, a modest battery with excellent battery management, good inverter, and a few strategies to optimize things should make it possible to provide all the modern comforts in a totally off grid home. Gonna need a small biomass furnace to supplement heating applications, and a back up generator is necessary as well.

So, which is better: (1) large PV supplemented with small biomass furnace, or (2) biomass fueled micro absorption chiller supplement with small PV? I guess it depends mainly on whether or not biomass fuel is inexpensive and readily available, and the average solar insolation in one's region. Of course, one could always do both (large PV supplemented with biomass fueled micro absorption chiller). I kinda like this latter idea for disconnecting a typical suburban home from the grid (no bills!... well, you still have the property extortion, err "taxes"). Actually, this kind of system is suitable mainly for a remote/off-grid location.

30. My chiller project has required me to work with vacuum equipment, and it's quite easy to work with a high vacuum when the size of the vessels are not large. I've considered different applications for vacuum including water distillation, and also ETHANOL distillation at reduced temperatures. I haven't yet done any serious research into the latter, but here is a site I stumbled on: http://homedistiller.org/equip/designs/vacuum .

According to the claim, it's possible to achieve near 100% ethanol in a vacuum distillation apparatus. More important, the temperatures required for this distillation are so low that many sources of heat such as solar energy can be used efficiently. If someone is interested in fuel ethanol production, then this prospect should be researched. Getting high alcohol percentage (over 90%) with a single pass from a simple still would be very interesting. Of course, the main problem here is finding an inexpensive source of fermentable sugars. There is also the problem of optimizing an engine for ethanol (one possible solution is to vaporize ethanol and admit the vapors with the intake air to a Diesel engine - ethanol can tolerate very high compression). Also, I sure like the idea of placing the condenser of such a unit inside the home during winter months - may as well get some free heat if you're doing something like this.

31. I've been seeing real world practical difficulties in my absorption chiller project, but it's nothing that I had not expected. Developing something like this from scratch is a real pain in the ass. It's one thing to understand clearly the physics of a process, and another thing entirely to build a working system... especially with a limited budget of time and money, and few fabrication skills. I'm shifting gears as I have a new idea on how to go about building the absorber and evaporator. I can't discuss specifics, but the design makes fabrication simpler, and it's modular to allow for easily enlarging capacity. I'll share meaningful results as they come. Right now my hope is to test a very small version of the system and get data to support a conclusion that scaling the system up can achieve the desired capacity of 12,000 btu/hour. So, let's say I can show a low cooling rate with a small system that should scale to provide the desired higher output without becoming too large or expensive to fabricate. I would consider that sufficient reason to build a larger test unit. Until that time I am keeping things small.

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