Sustainable Living Center Oregon
Solar Heat Grabber©
Do It Yourself (DIY) solar air heating collectors are one of the better solar projects because they are easy to build, inexpensive, and offer a very quick payback on the cost of the materials.
How do solar air collectors work?
In solar collectors, the sun shines through the glazing, and hits an absorber, and heats it. The air flows through the input opening and flows over or inside or through the absorber picking up heat as it goes. This heated air then flows out the collector’s outlet opening.
In full sun during the summer, the incoming solar energy at 40-degree latitude is about 600 watts per square meter of collector area. One square meter is equal to 10.7 square ft.
Of this 600 watts about 10% is absorbed or reflected by the glazing and never gets to the absorber. Of the remaining solar energy, about 95% is absorbed by the absorber. So, for the 600 watts/sm that arrives at the collector face, about 510-watts/sm ends up actually heating up the absorber.
Most of this 510 w/sm that made it into the absorber ends up going down one of two paths: one part is picked up by the air flowing through the collector and ends up heating the Grabber, and the other part ends up being lost out of the glazing. The job of the collector designer is to maximize the first part and minimize the 2nd part.
The degree heat output the collector can be calculated as:
Degree Output = (Temperature Rise)(Airflow)(.075/lbs/cu ft)(.24 BTU/lbs/degree)
For example, with a 60-degree rise in the Solar Heat Grabber© with a 38 cfm fan attached to a 4-inch output pipe would produce 4.29 degrees of heat every minute or 258 degrees of heat per hour.
Heat Output = (60 degree)(3.97 /minute)(.075/lbs/)(.24 BTU/lbs/degree)= 4.29 degrees/minute
Temperature rise is the increase in air temperature from the inlet to the outlet of the collector — often around 50 to 60F for well-designed collectors. For example, air might enter at 60F and exit at 120F.
Airflow is the volume of air flowing through the openings expressed in cubic foot per minute (cfm)
Airflow = fan cpm /Area of opening in inches*12
For a 4” opening and a 38 cubic feet per minute fan
Area= 3.1417*radius squared = 12.56
Airflow = 38 cpm/12.56 X 12 = 3.97 cpm
Air density and Specific Heat are physical properties of air and cannot be controlled. Air density is 0.075 lbs per cubic foot under standard conditions. Specific Heat of air is about 0.24 BTU per lb per degree F.
While there are no hard and fast rules, a temperature rise through the collector of about 50 to 60F works well in that is warm enough to feel warmth coming out of a heater vent. If the room temperature is 60F, then the collector outlet temperature will be about 120F. Moving air that is much cooler than this will not feel warm. Going for a temperature rise greater than 60F usually means a hotter collector absorber and increased heat loss out the glazing.
Types of Solar Absorbers
Two of the more popular designs are the Pop Can Heat Collector and Window Screen Heat Absorber collector. The pop can absorber uses columns of ordinary aluminum soda pop cans with the ends cut out. The sun shines on the black painted pop cans and heats them. The air flows through the inside of the can columns, picks up the heat, and delivers it to the room. The screen absorber uses 2 layers of ordinary black window insect screen as the absorber. The sun shines on the screen, heats it, and the air flows through the screen picks up the heat.
The biggest question facing someone wanting to build one of these collectors is to decide which of the many collector designs out there to build. It’s fairly easy to look at the designs and evaluate construction difficulty and cost, but the big missing factor is the factor is the efficiency of each design.
We use the discipline of Terra Value Engineering© and determined that the Window Screen Heat Collector that is the best for heat collection at the lowest cost and least labor in building. We called our design the “Solar Heat Grabber©”
We ranked this the best because it has a great combination of high performance, low cost, and is a very easy build. This collector also has a low-pressure drop, which means a computer fan with just the small solar panel can be used to drive it.
Heat Collection Box
Always our first question when using Terra Value Engineering© is: “What functions does the device do?”
The second question is “What exists that we could reuse?”
We elected to use a non-working upright freezer. The major reason is that a freezer is already insulated, has an air-tight door and we can find them free on craigslist.
These were steps we followed to prepare the freezer.
1. Responsibly remove any leftover Freon.
2. Remove the shelves
3. Cut the 4” intake opening at the top of the Box
4. Cut the 4” exhaust opening on the Bottom in the opposite corner of intake opening
Install the Glazing
Solexx is installed as the glazing. Solexx™ is a unique triple-wall glazing material. Solexx provides superior insulation to hold in the warmth. Solexx has one of the highest R factors of 2.3 R on the market, so it holds in the heat better than the competition. Solexx offers a 10-year warranty with an even longer life expectancy. It withstands snow, wind, and hail and won’t fade or yellow over time. Best of all it requires very little maintenance. Two and half feet of 52” wide triple-wall Solexx costs about $20 and will be 2 times more efficient in holding heat in the box than glass.
A section 48” by 26” in the center of the door of the freezer door is cut completely out. Flexible, snug fitting PVC Solexx H-Channel is attached at the edge of this opening and the Solexx glazing is slipped into the H-Channel for a flush finish and tight seal.
Solar Computer Air Fans
Salvaging a CPU fan from an old computer is generally easy. If you do not have any, they are less the $5 on eBay. Their small size and relatively small power requirements make it practical to connect CPU fans ONLY to solar panels to them work. No need for a battery. To operate a single CPU fan, use one or two small solar panels. These types of solar panels contain a silica material that collects solar energy and converts it into direct current, or DC electricity. The voltage is sent directly through a series of output wires and can be connected directly to the CPU fan
Most CPU fans require between 5 and 12 volts of electricity to operate properly. That means you will have to generate at least 5 volts of continuous power through solar panels to operate a single fan at low speeds, or up to 12 volts to operate at a high speed. Most small solar panels create about 3 volts of electricity. That means you will likely need to connect at least two small solar panels in a series before the fan will operate properly.
Five Ways to Use the Solar Heat Grabber©
1. Heat a Room
Add a Window Screen Absorber on how to heat a room. A wooden frame is made to fit inside of Heat Collection Box. If you are going to use the Solar Heat Grabber© for any of the options listed below, the Window Screen Absorber must be hinged to be opened to service the shelf behind the Window Screen Absorber.
The top of the wooden frame in made wider than the bottom. Two pieces of screen are cut and placed on each side of the frame. The frame with the screen is placed inside the box above the air input opening and covering the output opening.
2. Solar Food Dehydrator
Food dehydration is not a new concept. From raisins to prunes to jerky, food has been dried for storage and later consumption for years. Solar dehydration simply refers to using the sun’s energy to complete that process. If built correctly, a solar dehydrator will not need any electricity and will be able to dry foods in even the most humid climates.
Whether solar drying could be right for your location, consider that that the Center is in Lincoln City, OR, at latitude equivalent to Bangor, Maine. We receive lots of rain, but fortunately, we also receive enough sunshine during the summer and fall harvest seasons to successfully dry all of our crops with solar energy. If you can get two days of sunshine in a row with some regularity, solar food drying will work for you.
But for those times when the sun is hiding, a backup heating system still can help. A 200-watt light bulb as heating elements can be added to finish drying produce any time the weather turned cloudy.
Removal of moisture prevents bacteria from ruining your valued fruits and vegetables. Drying is a form of preservation. Dehydration will occur between 100 and 140 degrees Fahrenheit. Any lower and bacteria can grow, any higher and it will be cooking. In order to achieve this balance, the rear door may need to be left ajar.
Different fruits and vegetables have different optimum drying temperature ranges. Research what you are drying to find this out. Remember to store your result in a dry place.
The original freezer shelves are cut apart and reinstalled, The Window Screen Absorber must to build to hinge above the cold air intake and lean toward the front of the freezer. The shelves would be cut to fit behind the Absorber with the narrowest shelve near the bottom of the freezer. The width of the shelves would increase with the top shelves being the widest.
3. Chicken Fodder System
Chicken feed is some of the most expensive feed on the market, making hydroponically grown fodder an easy choice for people wanting backyard chickens. Sprouted fodder is considered a complete ration for poultry and contains all of the essential nutrients, vitamins and minerals needed for optimal meat and egg production. The Chicken Fodder System allows any size chicken operation to grow their own feed year round in a compact growing area with minimal labor. Feeding fodder to poultry, from broilers to layers, will significantly reduce feed costs and improve the quality of poultry products. Chickens thrive on fresh vegetation. The nutritional composition of fodder makes it a full feed option for poultry and will improve their overall health and performance.
With a Chicken Fodder System, 1-¾ pounds of barley grain can be turn into 12 – 15 lbs. of fodder. This grows enough fodder to feed about 15 – 20 chickens every day.
The same shelves that were used for the Solar Food Dehydrator are used for the Chicken Fodder System, except a few changes. The fodder is grown in trays. The trays with the fodder are tilted and are placed on the shelves. Each tray drains into the one below it as shown in the figure.
4. Bokashi Mix Dryer
Most people doing bokashi composting get hung up on bokashi mix. Places selling bokashi supplies will always sell bokashi mix to go with the bin. It tends to be expensive, and you have to keep buying it! Fortunately, you can make your own mix very easily.
Nearly all bokashi mix recipes call for using “wheat bran” as the main ingredient. This can be hard to find and quite expensive. However, You don’t actually have to use wheat, or any other type of bran!
Bokashi mix in this context is simply a substrate that contains a live (or dormant at least) culture of microbes ready to ferment your kitchen scraps. To start with you will need the base substrate. This can be any carbon-rich medium.
Look at a list of possible substrates:
The problem of making Bokashi Mix is storing it after it is made. To store it, the mix has to be dried. The mix can be placed on trays on the shelves of the Solar Heat Grabber© to be dried and stored.
5. Stove Wood Dryer
A 5, 30, or 55-gallon Rocket Stove can use a wide range of dry organic materials to cook, heat or making high quality Biochar.
Biochar makes a great soil amendment or water filtration media. The primary air is regulated to achieved by constructing a structure that allows the Rocket Stove to function as a smokeless burner. A small pile of 6 inches of very dry wood scrap as the input material can provide up to an hour of smoke free cooking or heating time.
The wood can be just branches gathered from the yard, but they MUST be very dry. During the winter in Oregon, getting dry wood can be a problem. If these small branches are placed on the shelves in the Solar Heat Grabber© they will be dried and stored ready to be used.