Long post follows . . .
Okay, I did some back-of-the-envelope calculations that I think are interesting. The objective was to find a way to keep 2 people comfortable and cool as they sleep without need for a generator
- A sleeping person gives off about 250 BTU/hr in heat. So, two people = 500 BTU per hour in cooling is needed just to keep them from heating up the sleeping area.
- Heat gain from the outside: If we assume it is 90 deg F outside and we want it to be 70 deg F inside, an R-10 wall (2" of rigid foam insulation) will pass 2 BTU per sq ft per hour. If we imagine a sleeping "capsule" with a queen-size bed and 4' walls all around, that would be 148 sq ft, or 300 BTU required. But even in Dixie we can expect it to cool off a bit overnight, so let's assume that it will be 80 degrees outside in the morning (8 hours later), and at that point the heat gain will be just 150 BTU/hour through walls, ceiling, and floor.
So, we'll figure an average of 225 BTU/hour gain through the walls for the whole night, plus 500 BTU for the people. We'll round up to 750 BTU/hr on average, total. 8 hours of sleep = 6000 BTU of cooling required for the whole night. Note that there's not much point in going for "superinsulated" walls--the 2" of foam gets us to the point where the majority of the heat we're removing is coming from the two people, it's not heat gain from outside.
Now, that's not a huge amount of energy, and might make generator-free cool sleeping feasible, even for those on a budget. Examples:
Using ice: It takes 144 BTUs to melt a pound of ice, and approx 40 more BTUs to warm up the resulting water from 32 deg F to 70 deg F. So, each pound of ice gets us 15 minutes of sufficient cooling. A gallon (8 lbs) of frozen water buys us 2 hours of comfort, and it takes 4 gallons to get us through the 8-hour night. Bring a mid-sized cooler with 4 one gallon jugs of frozen water/iced tea, etc and we'd be good to go (they make simple rigs like this for cooling small airplanes--it's not much more than a cooler with holes int he lid for 4" ducting in and out, plus a small fan. We'd want a thermostat on the incoming air for our sleeping chamber. As a bonus, you can use the water for drinking, washing dishes, flushing toilets, etc. the next day. Obviously, the big downside is needing to get 36 lbs of ice for each night of comfort, but it might be ok for single overnight away from AC power.
Using a small A/C unit: A 5000 BTU/hr AC unit draws about 500 watts when it is running, which is a lot. But we need only 750 BTU/hr of cooling, so the unit will only need to run 9 minutes per hour, and in that time it will draw only 75 watt/hours. For the whole 8 hour night, that amounts to 600 watt/hours. For a 12V battery
, that's 50 amp-hours, or about what a fully-charged Group 27 deep cycle battery
can yield if drawn down to 50% of capacity. Now, there will be other losses (12VDC to 120VAC conversion, etc), but some of these could be addressed by using a DC air conditioner. This approach has a lot of advantages--much less hassle than using ice, the 5000+ BTU A/C unit can be used to cool the whole egg during the day using a generator or shore power, the nighttime batteries can be recharged from the generator during "generator hours" each day, etc. And a major "+" for this approach is that it will constantly be removing moisture from the air, which can make even higher temps much more comfortable.
: The small power demands of the "cool chamber" approach might even make it possible to keep cool with PV-produced electricity. Even dropping the nighttime temp by 10 degrees, together with the removal of humidity and accompanied by some air movement with fans, might be a big victory for the devotees of silent solar power
Other options--a regular home fridge
can produce about 1200 BTU/hour of cooling, so if somebody wanted a science project, the "guts" of one could probably be adapted to provide adequate cooling to a well-insulated sleeping area. I don't know if it would be more energy efficient (due to the longer duty cycle) than just using a regular 5000 BTU air conditioner at a shorter duty cycle. Also, maybe Peltier (solid state) cooling might work, but last time I checked on these units they were pricey and not especially efficient.
Sorry for the lengthy post, thanks for indulging my number-crunching.