LIVING TINY OFF-GRID HOUSE IN WINTER
Photo courtesy of Stock Cake
The second law of thermodynamics states cold does not inherently exist but is simply the absence of heat. When the body experiences, potentially deadly, hypothermia heat is escaping from inside the biological furnace of the body out to the cold environment. The inverse is not possible; since the rules of physics does not allow for the concept of cold to manifest a physical form that can tangibly transfer in to a warm body or environment. The biophilic design of the Tiny Off-Grid House counters the cold using the same rules of physics as one would use to help keep the human body warm.
The Tiny Off-Grid House is designed for the occupant(s) to live comfortably in 4 seasons climates. To achieve this it requires energy to power the environmental systems. “Primary” energy is obtained from a rooftop solar array and the wattage is deposited in a lithium battery bank to power the electrical loads. The solar array wattage and battery bank capacity are adequately sized to handle the electrical loads during seasonal decrease in sun hours during fall & winter. “Secondary” energy is obtained from a ground mounted solar array, wind turbine and micro hydro energy with the energy deposited in to the battery bank. There are some winter maintenance for the solar arrays which requires removal of snow accumulation to maximize photovoltaic productivity; along with elevating the ground mounted solar array to prevent it from being buried in snow. Passive solar thermal radiant heat is provided by sunlight shining through the windows warming the ambient air and solid surfaces. As a consolation, in winter, nature provides a balance and the wind turbine performance peaks because it benefits from stronger, more persistent, winds.
Since the battery bank is located in the engineering section of the Tiny Off-Grid House it benefits from the indoor heating & insulation. In the indoor engineering compartment the battery bank is able to operate comfortably shielded from the impacts of inclement weather conditions; compared to an outside uninsulated battery bank compartment.
The Victron Energy, Lithium (LiFePO4) Next Generation (NG) 48 Volt battery bank has a charging temperature range of 41 °F to 50 °F / 5 °C to 50 °C; with a discharge temperature range of -4 °F to 122 °F / -20 °C to 50 °C. The operating temperature ranges for the battery bank is definitely achievable if the indoor ambient temperature remains at a comfortable minimum of 68 °F - 72 °F / 20 °C - 22 °C; in winter. The warmth of the ambient indoor air negates the need for battery warming pads.
Indoor heating of the Tiny Off-Grid House has a two prong approach: Heating of the indoor space & insulation (Thermal resistance) of the structure to generate then trap the heat inside.
Mechanical heating is provided by a wall mounted mini split while heat recovery ventilators (HRV) maintain heat recovery of the conditioned indoor air while providing fresh ventilation.
A mini split, aka heat pump, has a condenser that is located outside the Tiny House while the evaporator is located inside with vents that provide heating or cooling. A mini split does not inherently “generate” heat; it works by the “transfer” of heat from one space to another. In summer, the mini split refrigerant absorbs the indoor heat expelling it outside; while in winter, the reverse chemistry happens and the refrigerant absorbs latent heat from the outside to warm the indoor space.
A pair of Lunos e²60 HRV’s operates continuously to provide balanced mechanical ventilation at up to 22 CFM (Spaces under 1000 SF). Each alternating between positive or negative air flows with a 90.6% heat recovery efficiency; while the short 6.3” model (For walls thinner than 11 3/4”) has a 85% heat recovery efficiency. Each alternating Lunos e²60 will increase the delta difference between the in / outlet air temperatures significantly to maintain a comfortable conditioned indoor air temperature. The HRV’s does not sabotage the work performed by the mini split and the energy expended to generate the conditioned air hot or cool temperatures. For instance, if the outside temperature is 20 °F the outside colder inlet air is conditioned in the Lunos e²60 by the warmer exhausted indoor air causing the final inlet air temperature to be raised to approximately 60 °F; resulting in a 40 °F delta difference in temperature (40 °F Δ T). Even in cold temperatures, down to -32 °F, the Lunos e²60 can remain operational; requiring no frost protection.
In colder seasons, the regenerative honeycomb ceramic core is heated by heat captured from the indoor exhausted air, then as the dual fan blades reverse the cool inlet outside air is heated as it passes through the heated core. The two air streams never physically touch each other. This cycle continues every 70 seconds (250mm) for the standard model and every 50 seconds for the short model with a max fan setting of 38 CSM (The velocity of how fast air flows measured in cubic feet per minute).
The majority of the water and drain pipes are located inside the insulated Tiny Off-Grid House; with only a very short insulated section of drain pipe attachment exposed under the trailer for grey water discharge.
An air-tight insulated structure prevents the escape of warm conditioned indoor air during the cold season and maintains cooler conditioned indoor air during the hotter season. This reduces the electrical demand needed to maintain a comfortable living environment in the Tiny Off-Grid House. The structure has to be air and vapor barrier tight with adequate mechanical conditioned ventilation that provides adequate air exchanges while also filtering out dust and allergy triggers from the outside air.
To achieve a maximum air tight, moisture resistant, closed building envelope nonpermeable “closed” spray foam (spray polyurethane, SPF) insulation and the Huberwood ZIP System® will be used as whole house insulation and vapor barrier in the Tiny Off-Grid House. The hydrophobic insulated shield and the indoor heating provided by the mini split, along with the HRV counters the 4 physics of cold: conduction, convection, evaporation & thermal radiation.
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