Assuming that the cabin needs electricity, clean water, heating and cooling, cooking and bathing facilities and refrigeration to keep food fresh or frozen, we have to solve some technical issues. I will give more detailed information about how I addressed these issues, but to get an appreciation of the challenge, let me describe this challenge in some more detail.
Electricity. So, if we are not connected to the grid, and have to produce our own electricity, it becomes very important to keep our needs for electricity as small as possible and to have enough in storage that we can go several days without generating electricity. We can generate the electricity through a generator or PV panels (solar) and store it for later use in batteries. The sizing of the electric system should be based on the anticipated needs for electricity, both the required and optional loads and the anticipated number of days without electricity production. It’s important to know exactly how much electricity is consumed in a day, or how much KwH (kilo watts/hours) we use. Devices that use electricity are water pumps, lights, phone chargers, computers/tablets, fridge, stove, toaster, microwave, fan, air conditioner, hot water heater, etc. To keep the total electricity consumption, or Watt/hours low, it helps to have only low voltage and low current devices, and use high Watt items like microwave and toaster and AC only when we have enough electricity available when either the generator is running or the sun is shining. My designs is based on using 1.5 KwH/day of non-optional devices, a battery bank that can go three days without charging, solar system that can replenish 1 day of electricity use with 90 minutes of sun shine, and that can run optional loads of 1 kW continuously while the sun is shining. All the high load devices are carefully selected for energy efficiency so that the use of a backup generator is only needed when there are more then three days of continues rain and overcast, or when extended time of optional loads are preferred even when the sun is not out.
Details:The 3 solar panels (Sunpower) have a capacity of 435 W each, or combined about 1 kWh for each hour of sunshine, with about 6 hours of usable sun in the summer and 3 hours in the winter, assuming that we tilt the panels in spring and fall. A 30A MPPT Midnite Kid is employed to harness the solar energy and store it in the 48V battery bank. With a battery bank of 8 deep cycle golf cart batteries (6V each) at 225 AH, there is at 50% State Of Charge about 5 kWH available for use or 3 days of 1.5 kWh per day. The split level AC/heat pump (9000 BTU) can operate at a low setting and with a SEER rating of 33.5 will only need about 200 W to maintain the temperature which in the summer is enough to maintain a comfortable temperature throughout the day. In the winter, it is one of the heat sources available during the day, while at night the radiant floor heating will be primary. As it will increase the daily electricity use with 2.5 kW, assuming that we keep it running for 12 hours a day, the battery reserve will only be enough for 1 day. In the winter, we may need to use the generator for 1-2 hours a day to recharge the batteries. The thermal solar panels is expected to produce about 10 kBtu per day, enough for the hot water needs and 30-50% of the heat during very cold nights, with the rest of the heat supplied by the propane hot water heater.
Water. As we are not connected to a water source, we need to produce our own water through collection from rain water, specialized devices that create water from humid air (but require a lot of electricity) or hauled in by other means. We have water tanks to keep water in storage but the capacity of these tanks is limited as water is heavy. So again we look to conserve and reuse water if possible. We don’t use water to flush the toilet but instead use a composting toilet (see later). Faucets and showers are designed to regulate water easily so that no water is wasted. The anticipated water use is an average of 5 gallons/day, or a week of use with the 40 gallon on board water tank. So we need to have a system to collect and treat the rain water for consumption or if needed, recycle gray water. A calculation of the average rainfall locally, about 36 inches a year, and a roof surface of 176 sf. produces about 4200 gallon of water a year or 11 gallons/day. By collecting and treating the rain water, we can meet the water needs. The same system that treats the rain water would be able to also treat the gray water but many people find this still too yucky. A separate filter system will provide the potable water.
Details: The rain water collects in 2 barrels of 55 gal each that has a small amount of bleach added. The water is prefiltered by a sand filter when it enters the barrel and before it is pumped by a submersible pump through a carbon house filter of 5 micron into the 40 gal water tank. If needed, the water can recirculate through the sand filters or the house filter, and as the water gets heated by the thermo solar panels up to 135 F, the risk for contamination is greatly reduced. Potable drinking water is obtained using a high quality filtration and is only available at a dedicated faucet.
Heating and cooling. We can limit the use of heating and cooling by having a well insulated space. As we only have 3.5″ walls, and 5.5″ roofs and floors, we can only insulate at an r-21, and r-32 using the best foam board and energy-star windows. Again, having a small space volume is an advantage, and we can probably keep the space warm (and cool) with 3500 BTu/hour if the outside temperature stays above zero. By covering the windows and doors at night with insulating curtains, the need for heat can be even further reduced to 2500 BTU/h during the nights.
For heating a combination of radiant floor heat, and an electrical heat pump is used. Water for the radiant floor is preheated by the thermal solar panel and if needed further heated to 130 F by a LP gas water heater that has a capacity of 6000BTU/h . Radiant floor heating, the circulation of hot water through tubes under the floor, produces enough heat to maintain a comfortable temperature. A solar thermal panel can heat up 40 gallons of water from 50F to130 F in two days, and has an expected heat loss of 5 degrees F per day if not used for circulation. The heat pump, powered by the PV solar panels can further supplement the heat, so by relying on the sun, we have a comfortable, steady, quiet and non-polluting heating source. Electronics regulate the temperature of the water tank and stops the heating when a max. temperature is reached or send water for heating if the sun is out and the water temperature has dropped below a certain level. Of course, this assumes that there is enough sun in the winter, but a propane backup water heater can take over on extended days of overcast. It is expected that the thermal solar panels will substantially contribute to the heating in the months of Oct, Nov and March, April but for the coldest winter months the propane water heater and heat pump will provide the most of the heat.
In the summer, cooling can be promoted by open and closing the doors and windows, but when the outside humidity raises above 80% and the temperature raises above 90 F, air circulation from fans and windows have reached the limit. By closing the space early enough, a comfortable temperature can be prolonged with several hours, and for as long as the sun is out, the AC can easily keep the air comfortable. In the evenings and nights, the temperature usually drops below 80 and fans can take over to keep the space pleasant or the porch can be a comfortable place to sleep.
Cooking, refrigeration, bathing, toilet. Well there is a shower with hot water only, the right temperature for quick showers is preset by the water heater. It is expected that showers will be kept short (2-3 gallons) unless there is ample water available. Biodegradable soap is provided so that the water can be reused if needed. There is a (solar) refrigerator/freezer for storage of fresh food and frozen meals and a stove/oven for food preparation. Appliances like microwave, toaster etc. that require 110 V AC are available, but should only be used if there is a surplus of electricity. The composting toilet is designed to separate the liquid from the solid waste and both men and women are expected to sit on the toilet seat. For the solid waste, a small bag of saw dust is added to the waste to promote the composting. Once a day, the urine bottle is emptied and replaced while the compost bucket is closed with a lid and exchanged for a new one when all the saw dust has been used, usually after a few weeks.
As you can see, a tiny retreat cabin can provide most of the conveniences of a comfortable home with only minor adjustments. Will see if this research will resemble the real life performance. The detailed description of the implementation with pictures.