I think at this point early on, I should explain a few concepts to always be kept in mind with passive solar projects in general. I will tend to use as a reference point my area in the Northern hemisphere, and for those readers in the Southern Hemisphere, everything applies EXCEPT where I indicate ‘North” without modifiers, it should be considered to be “South” to apply accurately to your area in the Southern Hemisphere. Normally this would not need to be mentioned, but there are people who do not pay attention to the written word very closely and forget to transpose their ordinal positions.
The primary concept in solar design is solar aspect. Many do not take this account. When siting a solar collection system, be it a green house, be it a Trombe wall, etc, you need to maximize your olar aspect. in short- locate your “Solar South”. Sometimees due south on the compass is not your Solar South. In short- it is the midpoint of exposed sun shining on your target area. Terrain can alter this, as can trees. Your solar South is the midpoint of the exposed sun as it traverses the sky.
Another factor is sun angle. At what time of year are hoping to maximize your exposure for maximum heat gain? If you are on the 45th parallel, you want the southern exposure ideally angled to the sun in a manner that produces least reflection. December 21, give or take is typically the lowest point in the sky for the sun in the Northern hemisphere. The Earth’s axis of rotation also happens to be roughly 23.5 degrees, so the further north you are short of the Aectic circle, you want the south facing wall to actually be roughly 68.5 degrees as measured in relation to the level ground. This is for absolute maximum solar gain on the shortest day of the year. This also is the angle to mount your solar PhotoVoltaic panels for maximum output for that day. Essentially all this is taking your lattitude, adding the tilt of the earth’s axis to come up with your desired position for the shortest period of daylight. In this way the panel is as perpendicular to the sun as you will have for a fixed panel or single axis tracking system.
These are the “ideals” and often reality forces some adjustments because something is not ideal, or maybe you would prefer maximum output at the longest day of the year, or some other time. Sometimes those adjustments can lead to neglibible losses, such as having vertical orientation/structure for the solar panel at 45 degrees lattitude, be it a heat panel, Photo Voltaic Panel, or green house, and sometimes the compromises impact total output so other compromises need to be made, such as different mounting hardware for solar panels to achieve a more desired position, or the addition of heliostats or mirrors.
Some forget the detail of “Solar Aspect” when siting houses on a city lot or anywhere for that matter, or when siting a green house. If you bought a house already constructed, this is out of your hands. You can readilly spot siting mistakes for greenhouses because it is usually someone who sited a green house on the North side of a house in shade much of the day in the Northern hemisphere. In the Southern Hemisphere, it is opposite, but that is the only real difference between hemispheres of the planet. The only things you can grow in a North facing greenhouse in North America or Northern hemisphere in general is usually moss, woodland wildflowers, or shade loving plants like Hosta. Plants that produce the food you eat do not do well in anything less than 8 hours of sunlight. Anyone who suggests a green house on the north side of building where it will be shaded most of the time really does not grasp the purpose of a green house or fundamental principles of solar collection or plant propagation. Sometimes it may be the only area available, but as long as it is not shaded on the shortest day of the year, and able to be in the sunlight for 8 hours during the day it is still workable- just do not expect a greenhouse in shade attached to the North side of a building to be useful for anything but storage.
The advantages of a South facing greenhouse in the Northern lattitudes is the ability to use it as a green house in winter for growing food, and potentially as a source for warm air to blow into the house to reduce heating costs. Some plants that are day length sensitive will not do anything during this time of year with low sun angle even with ample water and fertilizer, such as tomatoes. Peppers that are content and getting adequate light will blossom 3 times a year and set smaller fruit in winter plus they are self fertile, which makes them ideal candidates as potted plants you can bring into the house to grow as a houseplant. However there are many greens that don’t care about daylight length and among them are most Lettuce varieties, Spinach, Bok Choi/Oriental cabbage, and even regular cabage, however for space considerations, the leafy types are better choices over the heading and semi-heading varieties which require a great deal of space. Most oriental cabage family members do not require a large amount of space to produce useful yields, so are most suitable for passive greenhouse cropping.
Ideally the greenhouse will have good drainage away from it, otherwise you need to elevate the base to get the floor of the greenhouse above any potential standing water line. If glazing it with plastic, you will be losing the UV component of sunlight, and if you glaze with glass, make sure it has no significant tint to it. In other words, if the glass has a grey tint to it- don’t use it anywhere except the North side of south facing green house. The reason is simple- that tint is a thin metallic coating between two panes of nitogen filled glass. That tint reflects heat in the inftared range of the spectrum and some visible light. I will discuss more on radiant barriers and reflective materials in a later post, which is part of what E- Glass is. You just do not want that E glass film on the glazing of a greenhouse/glass house because nothing will grow. Plants are green to your eyesight if not color blind because all other waveltengths have been absorbed by the leaf- except Green- which is reflected. Plants need those parts of the spectrum filtered out by E-glass. They might survive for a time, but they will not thrive under E glass.
if your greenhouse is free-standing, and you want to increase the performance, consider earth berming. Digging into the ground for the greenhouse is asking for trouble if you are in a snow zone, or the ground routinely freezes because water will infiltrate from below if the ground is easily saturated. The best choice is to adequately design the walls so they can at least prtially be buried, and they should be the North end of the green house for two reasons. 1) You do not want to block the sun, and 2) You want some insulation from the North winds. if you design it in such a way that you enter from the East side at the North end, and earth berm around the door on the east side and the west side for a short distance, say 10 feet on a greenhouse that is 30 geet long North to South, with the North 10 feet of each East and West Wall behind earth berm, this is ideal. The soil will have a certain heat capacity and make for a pretty good heat storage area. The actual details for what is acceptable construction design in your area will vary due to building codes, so I will not discuss this further due to those differences.
Inside of the greenhouse, you want heat storage capacity, which can be achieved in several ways, with one simply being a rock bed that is heated by the sun. another is with Calcium Chloride solution and/or motor vehicle antifreeze. With this last one, great care must be excercized to protect animals because Ethylene Glycol will kill cats, Dogs, and even humans. It is powertfully sweet. Propylene Glycol should also be used with the same cautions. Large amounts will kill you, and modest amounts will kill children and animals in spite of the labeling as “safe”. It is after all used in small amounts in dispensed ice cream product to allow it to flow, and most regulating agencies generally regard it as safe—-In those small quantities. You also do want the planting area to be elevated, so you can do a raised bed with foam insulation underneath the floor through the middle of the space and not then no need to worry too much about it freezing out completely.
In a later post I will show you a setup I used that works pretty good with rock as the storage medium, but rocks lack a certain amount of heat capacity when only heated by the sun. The greenhouse those are in is actually one I designed as a winter greenhouse, so it has a conventional roof with shingles and insulation. It was also my first experiment with radiant barrier- simply aluminum foil. When placed between the roof deck and the foam insulation, it made a signifcant difference in heat gain reduction in the summer. On a hot day that would normally produce a temp of 120 degrees F or more without the insulation and radiant barrier, the space was only slighly warmer than ambient air temp inside of the green house. On the day I tested with a calibrated thermometer, the space near the roof deck was only about 98 degrees with an ambient temperature in the green house of about 85 degrees. Not bad at all, considering a reasonable expectation to be in excess of 130 degrees F. Your measurements will likely vary and color of the shingles/reflectivity has a great deal to do with that too. My colors are lightly grey shaded white with stripes of green shingles. What can I say? I only had a certain amount of the two shingle colors, neither of which in an amount adequate to cover the roof completely, so I striped in with green to get the most coverage with what white shingles I had.
When considering passive heat storage methods in an existing structure, you need to consider the solar aspect of the structure. If there are any full length windows facing South, or nearly so, such as a sliding patio door- using 5 gallon stackable containers painted black, you can get considerable heat stored in plain water, on a small trombe wall. you can use regular water Since you know the space is going to stay above 32 degrees F for most circumstances, you do not need to add Calcium Chloride, Sodium Chloride, nor glycols, however water bed conditioner should be added to algae growth to a minimum, but it will not hurt anything to let it grow i fyou chose- it just tends get rather adversely aromatic.. You do also need to make accomodations for the expanding water as it warms, and the resultingair flow into and out of the containers, so leave some clear headspace inside of the jug. In many cases putting 4 gallons of water into a 5 gallon jug is sufficient. In the first project I used old floor wax containers with a snap cap lid. I did poke a small hole in the caps to deal with slow air pressure changes, and the caps will pop loose if there is a rapid change, but stay in place. The reason for this was to keep evaporation down.
To maximize their effectiveness, putting in a lined window shade that can pass between the window and the Trombe Wall of water filled jugs at night and open them to sunlight int he morning so you can insulate them once the sun goes down so they release collected heat into the house and collect solar energy when the sun is shining, you are maximizing their effectiveness. If the windows happen to be modern E rated windows, the heat gain is nominal at best, but the temperature extremes you may feel in their vicinity priot to installation will be reduced and evened out as the stored water will ahve a buffering effect on the extremes of hot and cold.
If you do not have a south facing window to take advantage or, or they happen to be E rated windows, you can still take advantage of the passive heat capabilities of a water filled jug Ttombe wall- simply place the containers near heat registers or baseboard heating so the heat coming from those conventional heat sources washes over the passive storage, once an equillibrium point has been achieved in summer or winter, the actual energy used in the house will go down some, but the extremes of heat and cold in cycling furnaces, heat pumps or, baseboard heaters will be less noticed as the “peaks” will be levelled off.
One other consideration is the plastic jug itself. More and more Politically Correct plastic is being used, which means the jugs will break down in sunlight, so coating with paint is not a bad idea as the paint will bear the brunt of the solor degradation. Polyetheylene is not readilyy accepting of spray paint however. Those types of jugs will accept the film layer of paint, but the paint film is not able to adhere strongly so can flake off. With this in mind keeping the jugs touched up or painted a color that matches the room is almost essential.
After all, the bottom line is simply gathering in as much sunlight as possible when siting an outdoor project, and noting the optimal direction to face the project may not always be facing due South, (or in the southern hemisphere- Due North.) because of terrain, trees, etc.
If you want to use solar calculators to try optimize your design(s), an excellent one stop website with many projects is is the Build it Solar Website. My blog is dealing primarily with what I build on the cheap (frugallity and limited budget), the fellow who runs Build it Solar, gathers in many projects from all areas. He is in Montana, so a harsher climate than mine many times in Winter, but his website and projects are different in scope and sometimes a little larger in plan and execution, and sometimes with a higher price tag.
Passive Solar Projects 