An entirely passive 100% solar-heated bungalow


David Delaney
October 31, 2003
revised February 18, 2004

NB: This document records early design thoughts.
For later, better, designs, see
1) Thermosyphon solar air heater and attic heat store for 100% solar heating
2) Passive solar heating system with high-temperature heat-store wall and air-to-air heat exchanger


The notes and documents referenced below record the progress of the design of a solar house. I live in Ottawa, Ontario, Canada, 45.3N, 75.6W. I want a house that gets all its space heat and most of its hot water heat from the sun.  For various reasons, I want a house that will be comfortable during a prolonged power outage in the winter.

The house will be necessarily unusual.  Since I want it to be maintainable without specialized knowledge, its systems must be very simple. It must require no unusual mechanical or electrical components. The fewer mechanical and electrical components, the better.

Ottawa  has a cold cloudy winter.  December is the worst case month for solar design. On average there are 824 heating degree-days in December, giving an average ambient temperature of -10C (14F).  I will use a design average temperature of -18C (0F).  The December solar radiation on a vertical south facing wall is 2.15 kWh/m2/day (682 Btu/ft2/day) (RETScreen method. Data from NASA satellite observations calculated and presented at http://eosweb.larc.nasa.gov/sse/.) These data imply that the house will have to be very well insulated. It will also have to have a large heat collector (large air heater) and store heat obtained during a sequence of good sun days and deliver it to heat the house for fairly long periods of no sun, say 10 days, in a specialized heat store separate from the living space.

My work is based primarily on the work of Norman Saunders and William Shurcliff. William Shurcliff was an innovative solar thermal thinker and a great documenter of solar thermal ideas. Norman Saunders was one of the most creative of the solar heating pioneers. His goal was to provide all space heat and much domestic water heat from solar thermal energy--no backup heat, even in the cold and cloudy winter of Massachussets.  This goal requires storing heat for later use. Shurcliff describes three of Saunders's houses in "Super solar houses--Saunders's  100% solar, low-cost designs",  Brick House Publishing, 1983. These are active, not passive, solar houses--they require fans. Here is an excerpt describing one of these houses, the Cliff House.

Norman Saunders's Cliff House as described by William A. Shurcliff, (pdf)
(Reproduced by permission of William A. Shurcliff.)

Although admirable, the Cliff House is far from meeting my goals. It needs fans and complicated electronic controls,  which I might be willing to use for convenience, but which would have to be unnecessary for basic comfort when the power fails. I want a house that can be completely passive and comfortable during extended power failures. I include local solar electricity in the category of power that might fail, primarily because I want the heating system to be conceptually shallow. No one needing to restore heat should have to understand solar electricity.  Although the Cliff House will not work in a power failure, some of its principles seem likely to be useful in achieving my goals. The most promising features of the Cliff House are its use of a sunspace on the south wall as a solar air heater, its use of air as a heat transport medium, and its use of natural convection to move hot air from the air heater to an overhead heat store and to return cool air from the heat store to the air heater.

Air as a heat transport medium offers the possibility of a simple solar heat collection system that cannot freeze or leak, and requires no plumbing.  A thermosyphon (natural convection) air heater with an overhead heat store eliminates the need for dampers and control systems to keep cold night air out of the heat store, while still operating without human attention.

The absence of control systems, actuators, and fans in the in the heat collection system leads us to ask whether we need them in the heat distribution system.  The following drawing shows a (rather tall) bungalow with an entirely passive heat distribution system to go with its entirely passive heat collection system.

The ceiling over the north part of the floor of the  bungalow is higher than the ceiling over the south part of the floor. The heat store is located above the lower (south) ceiling.  The  north wall of the heat store has manually dampered slots at the top and bottom. Hot air convects out of the top of the north wall of the heat store and moves across the high northern ceiling, falls to the level of the lower ceiling, and reenters the heat  store at the bottom of its north wall.  The hot ceiling and upper walls of the taller space heat the lower part of the bungalow by radiation.  The radiative heat transfer might be suppplemented by forced convection produced by a push down ceiling fan, but radiation alone should be enough to keep the house livable even if the pushdown fan is not working.

One of the drawbacks of the Cliff House (from the perspective of my requirements) is that only part of its heat store is located overhead.  Its main heat store is located under the first floor slab. The basement heat store is composed of approximately 50 tonnes of small stones. It provides both a very large thermal mass and a very efficient transfer of heat from hot air to its thermal mass. Although the attic thermal mass of the Cliff House, which is composed of 11 tonnes of water in 50 drums of water (55 US gallons each),  has approximately the same thermal mass as the 50 tonnes of stones in the basement,  it aborbs heat from the air only slowly, because of the relatively small surface area of the drums of water. The 50 tonnes of small stones in the basement store, on the other hand, have a very large surface area. The Cliff House has a powerful blower to drive a large volume of hot air through a duct from the attic water store to the basement stone store.  Since blower runs at full power only when the air heater is very hot, and runs at much lower power most of the time (it runs 24 hours a day), the Cliff House needs a complicated electronic control system.  (There are other uses for the control system that I will not go into here.)

I intend to dispense with the basement heat store and the control system of the Cliff House. I will make up for their loss by increasing both the total thermal mass of the attic heat store, and its heat-transfer surface area.  The attic thermal mass will consist of two components, 11 tonnes of water in 50 drums, and 30 tonnes of concrete blocks stacked to provide a large heat-transfer surface area. The reduced mass of the concrete blocks (30 tonnes, as compared to the 50 or more tonnes of stones in the basement of the Cliff House) will be made up by larger temperature excursions in the concrete block stack. The decision to place such a large thermal mass overhead poses some architectural difficulties.

The thermal mass of the heat store will probably be a lot bigger than necessary.  The reasoning  and data required to decide how big a thermal mass should be in order to be just slightly bigger than necessary are beyond me.  The heat store design must ensure that the only drawbacks of its thermal mass being  too big are in the cost and the space it consumes.  Being too big must not degrade its performance. The keys to good performance of a heat store with an oversized thermal mass are

  • thermal stratification of the thermal mass, so that heat may be stored and retrieved over a wide range of states of the thermal mass at a temperature not far below that at which the heat was received from the solar air heater, and
  • priority to the house and the domestic hot water for claims on high temperature heat from the solar air heater.
  • When these conditions are met, the occupants of the house will 1) not have to wait for warmth on a sunny day while a cold thermal mass heats up,  and 2) will not have a cold night and morning after a moderately sunny day that started with a cold thermal mass.  Thermal stratification also maximizes the usability of the heat in the store.

    The ceiling and upper walls of the house (a high ceiling bungalow) will be heated by natural convection from the heat store controlled by manual dampers. Distribution of heat downward from the ceiling area will be by thermal radiation and a low power ceiling fan.  Thermal radiation alone should be sufficient to keep the house liveable when the ceiling fan is not working.

    The pages at the links below give more detail to the thermal principles I plan to incorporate in my house.

    1) General thermal scheme: Organizing the air flow between a thermosyphon solar air heater and a thermal mass located above it.  Norman Saunders reduced the energy loss from the solar air heater of his Cliff House by having the cool air enter the air heater from the top and descend next to the glazing. The pattern of the air flow in the Cliff House requires the north-south thickness of the air heater to be greater than the north-south extent of the elevated thermal mass. The linked page describes a way to remove this constraint while still having the cool air descend next to the glazing. The air heater can be much thinner, and/or the north-south extent of the elevated thermal mass can be much larger, or the thermal mass can be positioned more centrally in the building.  I believe this idea is new.

    2) Thermal mass concepts:

    4) Ventilation scheme: Ventilation for a solar heated building with a large thermosyphon air heater. No fans during  power failures means no heat recovery ventilator (HRV) during power failures. Here's a novel way to get a lot of fresh air into an entirely passive solar house: I believe I am the first to suggest it.

    Acknowledgement

    Nick Pine is a source of inspiration. Although he does not necessarily agree with all of my decisions, he has given extremely valuable feedback at crucial points in the process I am following. Find him in his lair at alt.solar.thermal.
     
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