Alternative Energy Projects

[buenijo]

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

[buenijo]

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.

[buenijo]

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

[buenijo]

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.

[buenijo]

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.

[buenijo]

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).

[buenijo]

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.

[buenijo]

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.

3. Use some heat from the engine exhaust and gasifier outlet to regenerate a desiccant. This desiccant is used to dry the air in the home and remove much of the latent heat load from the air in the home so the sensible air cooling from the ice will be a lot more effective.

4. Use heat from the engine cooling fan to dry the next batch of wood chip fuel.

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

6. Operate the system during evening or morning when the outside temps are lower. If near a body of water, then use water to cool the condenser. 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.

NOTE: In principle, it is possible to harvest a lot more heat from a wood gas engine system than what's available at the engine exhaust. In fact, there's almost the same amount of heat at the cylinder. The problem is in harvesting all the heat at a high temperature to make it more useful. The temperature at the engine exhaust and gasifier outlet are quite high, and this high temperature should be used to heat water to high temperatures in a pressurized circuit for desiccant regeneration. In fact, generating some steam here would achieve good results. The heat from the cylinder is provided with small engine blower fans (many models provide this) that can be used in wood fuel drying.

[buenijo]

Good YouTube channel for solar thermal projects. http://www.youtube.com/watch?v=WP8H5IOTwYU
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.

[buenijo]

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.