Passive Solar Heating a Home in a Cold Climate
Part of our 850-Page eBook + 3,000 presentation slides on our ZED E-Book /CD-ROM
The south portion of the double-shell convection airflow loop is the most dramatic. It has a large quantity of glass that creates a warm, inviting sunspace in the Winter, and a year round Thermal Buffer Zone (TBZ), day and night. In most ZED double-shell homes, the beautiful sunspace is both a tropical greenhouse, and a central focal point for every room in the interior visual design. It is NOT just a solarium afterthought on a poorly-designed conventional home. The entire thermal and aesthetic package is comprehensively integrated with Holistic ZED Systems Design.
For our friends in the southern hemisphere it would be the "north portion of the double-shell". In the winter down under the sun is mostly in the north which is reversed in the northern hemisphere.
Our ZED floor plans consider the view when you first enter a room, and we add interior glass (and sometimes wall-sized mirrors) to greatly enhance the visual appeal, beyond what is even possible or legal with less-efficient single-shell design. The “two small Delta T” ZED double shell TBZ concept allows us to use far more glass than building codes would even permit a conventional architect to work with. AND, our ZED solarium glass actually contributes to near-zero-energy thermal performance, whereas a large area of single-shell glass would make thermal performance much worse.
During a cold Winter day, the low Winter sun enters our vertical south facing glass at close to perpendicular, and warms the air in the sunspace. The solarium temperature frequently exceeds eighty degrees Fahrenheit, when it is below freezing outside. Warm air is less dense and rises to the top of the sunspace, just like a hot air balloon rises. The sunspace ceiling is designed to allow the warm air to freely flow into the insulated attic space between the ceiling and the roof. In a conventional house, the roof is NOT insulated and the attic is usually ventilated to the outside year round (to reduce humidity build up in the ceiling insulation).
In contrast, the roof of a double-shell ZED home is VERY well insulated and sealed from the outside during the Winter. This double insulation significantly reduces heat loss through the ceiling and roof. Humidity is automatically controlled with the design of the TBZ convection airflow loop.
The ZED sealed attic requires careful design of the double envelope to eliminate the possibility of condensation - discussed later. Caution: Indiscriminate sealing of a conventional attic can cause condensation and serious mold damage to the insulation AND to the structural components of the roof and ceiling. Trapped humidity lowers the thermal resistance of ceiling insulation, like a wet sponge, but this does NOT happen in a double-shell convection airflow loop, which automatically equalizes temperature, air pressure, and humidity with continuous natural convection airflow.
The north portion of the double-shell natural convectional airflow loop is a set of un-air-conditioned Zero Energy Design Thermal Buffer Zone (ZED TBZ) usable spaces, like the laundry room, double-door airlock entry, long-term storage closets, double bay window plant area, etc. The ZED TBZ typically ranges from 65o to 85 o most of the year in most American (and similar) climates. These highly-insulating usable TBZ spaces are open to airflow from the well-insulated attic down to the crawlspace under the raised floor of a double shell ZED house.
On Winter days, the north wall TBZ rooms are naturally heated by high-volume natural convection airflow from the south sunspace (with no fans or anything that uses any type of electricity). On Winter nights (when the sun is not shining), and in the Summer (when the sun is mostly on the NORTH side of the house), the temperature of the TBZ air is influenced by the ambient temperature of the earth in the crawlspace (about 72o F year round in Central Florida, and a bit cooler further north). Ambient temperature earth is why water pipes buried below the “frost line” do not freeze, even during frigid Canadian Winters.
The coldest Winter days occur when Arctic cold fronts sweep down and rapidly cool the air. This causes outside air to lose its capacity to hold moisture, which precipitates out of the skies (in the form of rain, ice, sleet, or snow), leaving the sky cold and clear blue. On the very-coldest sunny Winter days, solar-heated warm air from the ZED solarium sunspace rises and enters the double-insulated sealed attic. When the warm air is exposed to the insulated roof, a small amount of its heat is lost.
On windy cold days, turbulence (wind chill) on the north side of the house draws heat from the insulated exterior north wall (which may have windows in the outer shell). This makes the air in the north portion of the whole house convection airflow loop cooler and denser than the warmer air in the sunspace. The heavier north air falls down through the northern ZED TBZ rooms and enters the crawlspace under the floor.
The subtle TBZ Winter nighttime airflow paths are very different from time-to-time and from house-to-house, depending on temperature differentials, wind velocity and direction. Airflow is MUCH slower at night with the sun not shining. The pressure differentials are much smaller. Slow, non-turbulent, “LAMINAR AIRFLOW” IS A VERY GOOD INSULATOR. Millions of tiny blankets of air have extremely small temperature differentials from one to the next, which greatly reduce undesirable heat transfer. The temperature differential between the interior living quarters and the outside temperature extremes are spread almost evenly over the area between the inner and outer walls. Even if the inner and outer walls are single pane glass, the HEAT TRANSFER RATE THROUGH LAMINAR AIR LAYERS IS EXTREMELY SMALL. This scientific principle is difficult for novice energy engineers to understand, since I have only seen it explained fully in PhD-level aerodynamic Fluid Mechanics textbooks. There is a brief introduction available at: http://ocw.mit.edu/NR/rdonlyres/Architecture/4-42JFundamentals-of-Energy-in-BuildingsFall2003/00573271-7E66-46CB-AC28-19CAEA27931E/0/442Glickman1529.pdf
The double shell TBZ is designed to minimize turbulence, which can happen at the top and bottom of the north wall and around any blockage in the airflow loop. Many factors are involved including cost, common building materials and practices, thermo¬dynamics, aerodynamics, and especially Fluid Mechanics. Dr. Daniel Vasicek, a PhD in Aeronautical Engineering, brought the following to my attention. If you consider the shape of the double-envelope house, what we are doing is conceptually very similar to the design of an airplane wing. Fluid Mechanics are a surprisingly important part of the ZED double-shell home design (as well as the basis for a major segment of aeronautical engineering). I thank Dan for his valuable help. The science is very sophisticated, but the construction is easy to implement.
Dr. L. D. Landau and Dr. E. M. Lifshitz (U.S.S.R. Academy of Sciences) describe Heat Transfer in a Boundary Layer in their comprehensive text entitled: "Fluid Mechanics". Their valuable book is sometimes used in PhD level Fluid Mechanics courses in the U. S. (Non-learning American scientists tend to overlook such important topics that are not intuitive, and are difficult to conceptualize.) The physics and mathematical description of heat flow across a boundary layer is complicated and probably explains why American energy engineers and most passive solar designers have overlooked this important point, and disregarded the valuable cost-effective double shell architectural design pattern for three decades. The following is a simplified summary of the applicable science. According to Landau and Lifshitz:
"It is easy to see that, in flow past a heated body the heating of the fluid (air) occurs almost exclusively in the (turbulent) wake, while outside the wake the fluid temperature does not change. The processes of thermal conduction in the main stream are unimportant. Hence the temperature varies only in the region reached by fluid that has been heated in the boundary layer. We know that the streamlines from the boundary layer enter the main stream only beyond the line of separation where they go into the region of the turbulent wake. From the wake, however, the streamlines do not emerge at all. Thus, the fluid which flows past the surface of the heated body in the boundary layer goes entirely into the wake and remains there. We see that the heat becomes distributed through the regions where the vortices are non-zero.”
“In the turbulent region itself, A VERY CONSIDERABLE EXCHANGE OF HEAT OCCURS, which is because of the intensive mixing of the fluid characteristic of any turbulent flow (including Winter wind chill or ceiling fan operation). This mechanism of heat transfer may be called ‘turbulent conduction.’ Thus, the processes of heat transfer in laminar and in turbulent flow are fundamentally different. In laminar flow, the processes of heat transfer ("wakes") are absent. In turbulent flow, however, heat transfer occurs and rapidly equalizes the temperatures in various parts of the stream."
What was "easy to see" for Landau and Lifshitz has been obscured from the vision of almost every previous solar designer throughout history. Except for the valuable input from my friend Dr. Vasicek, I would have overlooked this essential explanation myself. (Thank you again Dan.)
Never Use Poorly Insulated, Unprotected North Facing Glass(In the Northern Hemisphere)–
Use double shell design, salt box style, insulated interior window shutters, etc.
Never Ignore The Potential Of Passive Solar Heating –
Lots of vertical south facing glass in a non-air-conditioned double-shell thermal buffer zone (sunspace, sunroom, solarium, tropical greenhouse, conservatory, etc.). Do NOT use a lot of glass on any wall of any single shell building. Location-specific south glass overhang design is critical for climates with significant heating and cooling requirements.
Never Ignore The Use Of Self-Regulating Natural Convection To Equalize The Temperature In North And South Rooms-
Double shell design, Thermal Buffer Zones,,, etc.
Never Use Roof Angled Glass (Skylights-)
They create a Solar Furnace in the summer. In the winter, warm air rises and heat is rapidly lost through the cold conductive glass. Roof angled glass is always bad everywhere in every season. Even on sunny winter days, roof angled glass provides LESS solar gain than vertical south facing glass (in the northern hemisphere, if your from down under it would be "north facing glass"). (see Never Use Roof Angled Glass).
Never Use A Fireplace or A Clothes Dryer In Living Quarters-They suck out your expensive clean conditioned air, which must be replaced by unconditioned dirty air. ZED locates the laundry room in the un-air-conditioned thermal buffer zone. Folding and ironing can be done in air conditioned rooms. (See Conventional Laundry Room Energy Disaster )
Never Design A Complex Exterior with a high ratio of surface area to interior space –
Excessive surface area causes unnecessary undesirable heat transfer summer r and winter. A simple rectangle costs less, performs better, and has less surface area and less turbulent airflow. Simple architectural features can be added to a rectangle to improve street appeal. Look at our Florida ZED Home for some ideas.
Solar water heaters can reduce your water-heating bill by 80 percent or more, depending on your location. After basic weatherization, they are one of the most cost effective energy improvements you can make in many American climatic zones. Our government is strongly encouraging you to install one. They are currently eligible for a large 30% federal tax credit, up to $2,000. Ignoring the potential of solar hot water heating is a serious energy wasting mistake, especially in warm and moderate climates with minimal freezing.
Throughout most of the U.S., the AVERAGE ANNUAL solar gain potential is significant, ranging from over 1,000 BTU/sq.ft./day to over 1,800 BTU’s in the clear skies of the arid Southwestern U.S. Sunshine can heat water, even on cloudy days.
On cloudy Winter days, solar gain potential is below average (sometimes only 30%). In the Summer, solar gain potential is often over 3,000 BTU/sq.ft./day in many areas. Solar hot water is feasible and cost-effective most of the year in most locations.
There are multiple types of Zero Energy Home solar hot water systems. The simplest is a cheap “passive” solar hot water heater (called a “thermosiphon” - used in Southern U.S. tropical areas that seldom freeze). Since hot water can be 20% of a conventional home’s utility bills, even partial solar hot water heating can be cost effective much of the time. Thermal hot water storage is inexpensive, easy to construct, and efficient. It costs much less to store thermal energy than to convert sunlight into electricity and store it in batteries (which would have to be replaced every six years or so). An insulated 150-gallon hot water tank can hold the equivalent of 20 kWh of electricity with a temperature difference of only 54°F (30°C) between the minimum and maximum temperature. There is no difficulty in storing solar hot water energy from sundown until morning showers. (In Chapter Twelve we discuss solar-powered air conditioning and methods to store cold thermal energy - used to cool and dehumidify a building overnight, while the sun is not shining in a hot humid climate).
The first Low Cost Water Heater to discussed ia a passive solar thermosiphon hot water heater is basically a solar collector (made up of black water tubes with fins designed to absorb solar radiant energy), connected to an insulated hot water storage tank, which must be higher than the collector.
When the sun shines on the solar collector, the warmer water expands somewhat and becomes lighter than colder water in the storage tank. “Natural convection” causes the warmer water to rise into the tank (like warm air rises up a chimney), and the coldest water then descends into the collector, where it is warmed and re circulates back into storage in a continuous gravity-fed “flow loop” that gradually warms the stored water, depending on the available BTU’s/sq.ft. of solar gain potential, and solar collector square footage. The system is engineered to economically meet the large-or-small desired hot water needs of the owner.
An inexpensive, efficient, renewable-solar-energy thermosiphon needs no electric pump or electronic controller. It is completely self regulating and harmonious with nature. Cold water flows into the system from local water pressure, and heated water is made available for domestic use.
Thermosiphons in areas that occasionally freeze (like Northern Florida) need a temperature-activated automatic mechanical valve that protects the whole system from damage by freezing. Alternatively, the entire system can be isolated with valves, and manually drained, in locations with extended freezing conditions. Thermosiphons are not perfect systems for everyone, but in many areas they are the least expensive way to eliminate (most) hot water bills.
We know that a radiant heat source makes us feel warmer in the Winter, even if the air around us is cool. On a Winter day, sunshine on our skin feels good on a cool day, whereas the same temperature air at night would be much less comfortable. Ancient Romans understood radiant heating, and they developed ways to heat bathhouse floors with warm water or hot air – the first human form of central heating system.
Today, hydronic radiant floors (HRF) run water through concrete floors, or under wooden floors. Radiation from a HRF comfortably warms our bodies (instead of just warming the air around us). 50% of human thermal comfort is radiation, 30% convection and 20% evaporation.
When using utility-company power, an HRF is more efficient than baseboard heating or forced air heating, since the HRF water only needs to be heated to a lower temperature than other systems, and less energy is lost than through forced air heating systems. The HRF does not require the space needed by forced air ductwork, and the HRF does not blow around dust, dust mites, mold, pollen and other air pollution, yielding better, less-allergenic, interior air quality (IAQ).
HRF pipes can be zoned so they only heat rooms during times when they are cooler, and the room is going to be occupied. As the sun moves through its daily and season path changes, different zones can be heated at different times of different days. Zonal requirement differences may include morning, evening, and night solar-and-lifestyle patterns. Weekends may need different heating than work-and-school days.
HRF does not need high temperature water. In many cases, 85-to-100-degree water is comfortable for a reasonably well insulated home. That is easy to achieve while the sun is shining with a minimal solar hot water heating system, in almost all cold American, European, Asian, and similar climates.
If the HRF is in a well-insulated concrete floor, the combination of solar-heated water, and thermal lag may be sufficient to keep many homes comfortably warm 24-hours of all but the most extreme days.
We sincerely wish all of our readers an Abundant New Life In Harmony With Nature
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