Off grid electric cooking and opportunity loads

mcgivor

Needed to find a way to use excess energy, my system is in float by 11:30 on a good day, the array still has a potential 45A+ with nothing to do but keep the battery on float and power the refrigerator running 6.5A at 24V nominal. Got a water cooler, very handy but not much of a load even with a hot water supply, can only use and drink so much water. As hot water for showers is not needed, hot climate, had to think of alternatives and cooking rice came to mind, we grow about 15 tons each year and is the staple for both humans and animals, 7 dogs consume a lot of rice. So went to buy a large rice cooker but ended up with something much more versatile, a rice cooker,steamer, slow cooker and oven, yes you can bake bread and cakes, see link. This is the most versatile appliances ever for utilizing excess power, the one I have is a 750W 230V, there are smaller 500W available, here's my findings electrically.

The demand is not a constant, initially it draws 750W until the cook temperature is reached, I'll use the slow cooker as an example as it needs more time, once the preheat is complete the unit cycles on for 7 seconds, off for 20 seconds, my observations were achieved using a clamp on ammeter and DVM on the battery side. Even when the sun was going down, the short on cycles, although greater than the array output, were short enough to not pull the controller out of float, sometimes dropping slightly below the 25v threshold but not long enough, 7sec, of the minute required to prompt an absorption-cycle. Over the last week I've attempted to see how much my 1.5KW array can supply, so far the highest is 5.5Kwh in a day, 3Kwh more than regular consumption/production, seems a lot to just throw away, would be significantly more with a constant load but passing clouds could be a problem. So if food is a priority this is a great way to utilize that otherwise wasted energy and although the price here is half of the US price, it would still be a worthwhile consideration.

http://shop.panasonic.com/microwave-and-kitchen/kitchen-appliances/rice-cookers/home-rice-cookers/SR-ZG185.html

jonr

In a hot climate, I'd consider an air to water heat pump (like Chiltrix) and using it to cool a very large tank of water (like 2000 gallons).  Then use it for space cooling.   But perhaps this is much more complexity than you want/need.

mcgivor

jonr said:

In a hot climate, I'd consider an air to water heat pump (like Chiltrix) and using it to cool a very large tank of water (like 2000 gallons).  Then use it for space cooling.   But perhaps this is much more complexity than you want/need.

One thought was to use a small refrigerator compressor to cool water in which the battery sits, a water bath, insulated with styrofoam, with a set-point around 25ºC, thereby not subjecting them extreme temperature and extending their life, always thinking of ways to improve upon the efficiency, thanks for the suggestion.

jonr

Opportunistic battery cooling makes sense to me (in a hot climate).   Would be interesting to know exactly how many btu batteries dissipate.  I suppose that even without a compressor, a large water bath might help cool batteries (more effective conduction and convection plus some evaporation).

BB.

10 to 20% off the charging current is dissipated as heat.

Below 80% state of charge, only a few percent is lost as heat.

Above 95% state of charge, something like 50% of charging current is lost as heat (equalization/gassing).

During discharge, probably not much lost as heat (I^2R heating where R~0.005 to 0.010 ohms per 12 volt lead acid storage battery).

-Bill

Photowhit

You live in a hot climate, there is always a good old air conditioner!

jonr

In a hot climate with lead acids, would it improve battery life (by reducing heating) to only charge to 80% on most days and to 100% once every week or two?

mcgivor

Photowhit said:

You live in a hot climate, there is always a good old air conditioner!

Without getting into physics too deeply, water is about 1000 times more dense than air and for that reason the heat transfer between a heated body, the battery in this case, is around 4 times greater. Rational, a person can be comfortable in 24°C air, but can suffer hypothermia in water of the same temperature. Being more dense, water is also able to retain it's temperature for a longer period. Cooling the entire room would be less effective due to losses, in this case uninsulated concrete walls, lining the water bath with styrofoam and covering the  surface of the water as well, would be quite efficient in terms of losses.

My battery temperature varies over a 24 hour period between 25 and 30°C, unless ambient air rises above 45°C for a week, it rises to 40°C, so keeping it stable at 25 would greatly increase the life expectancy. In a cold climates perhaps a warm water bath could be utilized to increase capacity otherwise lost to the cold, the tradeoff would be a lower life expectancy.

Photowhit

I was more concerned with an opportunity load that would provide "you" with comfort.

You may be more concerned with your batteries comfort than your own, but at about 85°F degrees, my comfort wins out! Actually my batteries remain reasonable the same temps during the day, just living in the shade of my array. There is pretty much no load on them running the A/C during the day, and during the night they will warm up a bit with the load!

mcgivor

Photowhit said:

I was more concerned with an opportunity load that would provide "you" with comfort.

You may be more concerned with your batteries comfort than your own, but at about 85°F degrees, my comfort wins out! Actually my batteries remain reasonable the same temps during the day, just living in the shade of my array. There is pretty much no load on them running the A/C during the day, and during the night they will warm up a bit with the load!

Today was warm 104ºF / 40°C, battery temp 89°F / 32ºC, will in the near future build a system to provide a refuge room in the house with air conditioning,  for when it gets hot, the battery room is independent and shaded after 9 am. so stays cooler. My figuring is if I take care of the batteries, they will take care of me, for a little longer at least.

mcgivor

jonr said:

Opportunistic battery cooling makes sense to me (in a hot climate).   Would be interesting to know exactly how many btu batteries dissipate.  I suppose that even without a compressor, a large water bath might help cool batteries (more effective conduction and convection plus some evaporation).

BB. said:

10 to 20% off the charging current is dissipated as heat.


Below 80% state of charge, only a few percent is lost as heat.

Above 95% state of charge, something like 50% of charging current is lost as heat (equalization/gassing).

During discharge, probably not much lost as heat (I^2R heating where R~0.005 to 0.010 ohms per 12 volt lead acid storage battery).

-Bill

My plan is to use the auxiliary output of the CC using the BTS to trigger the cooling , or an alarm, still in the thinking stage of things, personally I've  always been a manual type, automation can often lead to unwanted supprise, Bill has often mentioned this in reference to AGS. Used ground water 24°C last year but my pump is too large, and in storage the temperature rises so for less than $80 I could buy a water cooler, gut it and use that as a source for refrigeration, 90w compressor, but in the end I'm just entertaining myself and thinking outside the box. Ideas always welcome !

jonr

A 90W compressor seems about right to opportunistically cool water surrounding a set of batteries in an insulated box.  Evidently reducing lead acid battery temp from 33C down to 25C can double life.  That's significant.

Estragon

90w sounds a bit light to me. Say 2000w charging with 10% going to heat is 200w plus losses through box insulation. Current drops in absorb, but more lost to heat. Still better than nothing though.

Maybe get multiple coolers and use in a hydronic loop for respite room as well?

jonr

90 watt compressor with COP=3 is 270W of cooling.

mcgivor

Estragon said:

90w sounds a bit light to me. Say 2000w charging with 10% going to heat is 200w plus losses through box insulation. Current drops in absorb, but more lost to heat. Still better than nothing though.

Maybe get multiple coolers and use in a hydronic loop for respite room as well?

 When the ambient outside is 35ºC, the battery room would be in the high 20's, a shaded concrete room, never in direct sunlight, cooled overnight. When temperatures are in the 45°C+ range, for a week then that's another story, but generally 35ºC is the normal high and 24°C the low. Currently it's normal, so morning battery temperature is 25°C , when transition to float occurs it's 29°C +/- a degree, so my thinking is, by using a 90w compressor setup,  starting after sunrise, should cool the water before the battery begins to warm. By 9 am. the array is producing around 750W, by noon it is 1150W (potential ) , from a 1500W array. So using a fraction of the power as sort of preemptive cooling would be benificial, as the array, on a good day, can actually produce 3.5Kwh excess if pushed to the limit. The system reference is to the system 1 in my signature, so battery is sized for the loads, 30% DOD, never need a generator as even on an overcast day the system reaches float, albeit a little later, 2 pm, winter or summer, there is very little difference.

706jim

I never knew that dogs would eat rice.

mcgivor

706jim said:

I never knew that dogs would eat rice.

When you consider dog food is probably 90% grain, corn, wheat, rice whatever is cheaper, with a little fat and flavor that's much the same thing as feeding them rice, with chicken carcass, chicken  blood soup. Dog food is costly, I grow the rice and since selling it at low current prices is not an option, I use ti to feed the dogs. The uncooked rice is fed to the chickens converting it to protein, the chicken carcass and blood  is fed to the dogs, it's all a means to be as self sufficient as possible.