Solar Devices
There are many kinds of solar collectors, including: water heating collectors, air heating collectors, and more sophisticated systems using mirrors, lenses and even photovoltaic solar cells. In the last 10 years, collectors of all types have begun to appear on homes and business buildings. One of the appeals of these solar collectors is that they frequently can be added onto an existing house without major structural renovations. However, in order for a solar collector to make an economical and significant contribution to the structure's energy load, it must be integrated with a complete energy improvement plan for the building. Here are some questions a homeowner must ask before deciding to add on solar collectors.
Has every attempt been made to conserve energy?
What form of energy is needed at the point of use? How hot? How much?
How does the demand for energy fluctuate during a 24-hour day? Is there a need to store energy during the day for use at night?
How much do various alternative energy options cost?
What side benefits/detriments can you anticipate from this application of solar energy?
How much solar energy is available at or near your site? (Use your Solar Site Surveyor and local weather data to answer this one.)
The following example traces the investigation process that leads a person to install solar collectors. It shows how a clear understanding of personal requirements and social concern leads to an appropriate application of solar technology.
Sam is an experienced carpenter and plumber who has an older home that he is renovating. Insulation and weather-stripping have already saved him money on his heating bill. He studies his gas bill, though, and finds that much of his monthly bill goes to water heating. He is concerned about the environmental effects of burning fossil fuels, recent newspaper articles convince him that natural gas prices are going to continue increasing, and he wants to try his hand at solar energy. He measures carefully the amount of hot water he uses in an average week. It works out to about 20 gallons of 1200 water a day. Since most of his hot water demand is for showering and dishwashing in the evening, he doesn't need a continuous supply all day long. Since a solar system would heat the water during the day, this option fits with his needs.
Next Sam gets up on his roof with a compass, tripod and Solar Site Surveyor. After a short period of sketching and calculation, he discovers that he has enough roof area with good solar exposure to supply his needs. He reads more about solar water heating systems and compares the costs of commercial collectors with those of building his own. After considering all of the above information, he decides that he could build a flat plate collector system that will allow him to eliminate much of his gas consumption.
In his reading Sam learned that many home-built collectors do not work very well because the builder didn't get enough good information about solar collectors, or didn't follow a reliable design, or didn't use high quality materials. Sam was able to avoid these pitfalls and, as a result, his solar water heater performed up to his expectations.
{mospagebreak title=Solar Collectors}
Solar Collectors
A solar collector consists of an insulated, glazed box with a dark heat-absorbing surface inside. In a flat plate collector, a transfer fluid such as air or water is pumped through the device, absorbs heat and carries it to its final use.
The key features of a collector are efficient absorption of solar radiation and the efficient transfer of heat to the air or water within the collector. A flat metal plate, painted black, can serve as the primary absorber surface. Light energy is absorbed when it hits the dark surface and is changed to heat energy. Ideally, the inside surface of the glazing (especially if it is glass) reflects heat radiating out from the absorber plate back into the box, keeping more heat inside the collector. Heat is conducted from the absorber plate to tubes containing liquid or directly to air that passes across the plate. Since heat conduction requires good thermal contact, the tubes containing liquid are soldered or welded to the collector plate.
Liquid Solar Collectors
In a liquid flat plate solar collector, a liquid or "transfer fluid" (usually water, but occasionally propylene glycol or mineral oil) is passed through a sealed system of tubes or pipes that are soldered to an absorber plate. Complete plans are given in the following section for a model flat plate collector. Here are some of the basic features of a liquid solar collector system:
1) Circulation Pattern
Liquid can pass through a collector in parallel or series arrangements. In the parallel style, liquid from a header has a choice of several routes through the absorber so that a given drop of liquid makes a single pass over the length of the absorber collector. The plate maintains an even temperature over its entire surface. When tubes are arranged in series, water must pass back and forward across the surface of the collector. The part of the absorber plate near the water intake is kept very cool from incoming water while the plate near the outlet is quite hot. Series collectors can heat water to a higher temperature but at the price of poor efficiency.
2) Means of Circulation
Most home solar water heaters use a pump to circulate the fluid. Pumps are "thermally activated" - that is, they begin to operate when the temperature of the collector reaches a set point. A "snap disc" thermostat is a simple switch mounted on the collector plate that turns on the pump motor when the plate reaches a certain temperature. This prevents hot water in the storage tank from being pumped through a cold collector at night or on freezing days. More sophisticated differential thermostats use several sensors and computer logic to check whether the collector is warmer than the storage tank.
Instead of using mechanical pumps, one can also design solar systems so that the transfer fluid is circulated between collector and storage tank via natural convection. Since hot water is less dense than cold water, it rises to the top. When a collector is located below the storage tank and the highest part of the collector is connected to the highest part of the storage tank, the water will naturally circulate through the collector up into the tank. Cold water from the bottom of the tank will sink down to the bottom inlet of the collector. When this "thermo-siphon" is established, no pumps are needed. Generally, thermo-siphon flow is slow and cannot circulate water very far.
3) Storage
Solar heated water can be stored for days or weeks in a large insulated tank. The larger the tank, the longer the supply of hot water will last. Generally, it is not economical to store solar heated water for more than a week because of the size of tank required. There has been some research into seasonal storage where heat is collected and stored in the summer and then retrieved in winter. One means of seasonal storage is the "solar pond" where heat is stored at the bottom of a pond.
4) Freeze Protection
In frigid climates some kind of freeze protection must be provided. Since water expands when it freezes, water freezing inside a collector can rupture pipes and burst fittings. One type of freeze protection is the ''drain back" method. Distilled water is circulated through the collector in a closed loop that drains itself when temperatures drop. A heat exchanger transfers heat from the distilled water to potable domestic water. Another method of freeze protection is to use anti- freeze (similar to the anti-freeze added to automobile radiators), preferably a non-toxic type like propylene glycol.
5) Collector Housing
Housings are shallow boxes made from wood, metal, plastic or fiberglass. They must be extremely weather resistant to protect the interior of the collector. Insulation within the box must be able to resist high temperature, since a collector may attain 350-400 0 F if the pump should fail. Such temperatures could melt styrofoam-type insulation and have been known to cause fires.
{mospagebreak title=Air Collectors}
Air Collectors
Solar hot air collectors are gaining popularity because they are simpler and less troublesome than liquid collectors. These collectors have an advantage over passive direct gain solar heating because, on sunny days, they allow you to turn the heat on and off with dampers as needed. They can also be added to existing homes without major structural remodeling. An air collector consists of an insulated housing, glazing and a flat absorber plate. A 1 to 3-in space between the absorber plate and the rear insulation allows air to pass behind and pick up heat from the absorber. Ducts supply unheated air to the bottom of the collector panel and withdraw solar heated air from the top. Small fans are used to circulate the solar heated air to where it is needed. Instead of using fans, wall-mounted collectors can also circulate air by the thermo-siphon effect. Reverse thermo-siphoning (where hot air from the house feeds into the collector at night, is cooled and returns back to the house) is prevented with "backdraft dampers" consisting of a thin plastic film placed over high vent. (See the July 1979 Solar Age Magazine for plans for Thermo-siphoning Air Panel)
A small solar hot air system should dump heat directly into a room. Larger systems trap so much heat that excess heat must be stored for later use. A rock storage bin is a common solution. A large insulated bin in the basement is filled with small stones. Plenums or air ducts are located underneath and above the bin to pass warm air from the collector through the rocks. Throughout the day the rocks increase in temperature, and heat is stored in their thermal mass. At night when heat is needed in the house, cold air is blown from the house into the rock bins to be heated and returned to the house.
Demonstration Collector
The demonstration flat plate collector described here will teach you about construction techniques used on larger scale collectors, and it will provide you with an experimental system with that you can explore the importance of each component of a flat plate collector. This working knowledge of materials and techniques will be useful if you are planning to build your own system, or it could help you evaluate a commercially available collector.
Since hot water and hot air collectors have features in common such as housing, insulation and glazing, the collector described below will be a combination panel that can function as either a hot air or hot water collector. By replacing the absorber plate water unit with a simple black metal plate and opening air vent holes in the top and bottom of the collector, you change it from a hot water to a hot air collector.
MATERIALS 41 "x27" of 18-gauge galvanized iron or 1/2 of 4'x8' sheet of 1/2" plywood and 200" of 1 "xl pine 10 It of 1/2" copper pipe 4) 1/2" copper T's 2) 1/2" copper elbows 2) 1/2"copper to 3/4" hose adapters 28-1/2" x 16" 3 mill copper sheet 30"x16" flat fiberglass glazing 1/2 sheet of 3/4" fiberglass duct-board 10 ft of 1 " aluminum corner reinforcing 15 ft of 1/2" angle iron for stand 20) 1/8" pop rivets 28"x 14" corrugated iron or aluminum for air collector barbecue black spray paint 50/50 solder and paste flux One gallon can 6' of hose with female hose fittings 4) female hose fittings 2) male hose fittings.
INSTRUCTIONS
1) Sheet Metal Housing
Lay out the entire pattern for the housing by enlarging the plans onto the sheet of 1 8-gauge galvanized iron. Cut it out with snips or shears. Cut the water pipe holes and the rear air vent holes as indicated using a chisel and hammer or light cutting torch.
Fold the outer edge completely over on itself to create a reinforced and protected edge 3/4-in wide. Fold it once more to 900 to form a ledge that can support the glazing. Make major folds along dotted lines using a folding brake. If your shop lacks a brake, you can use wood blocks and clamps to pinch the sheet metal along the fold lines. Use a board to fold the extending edge over the clamped blocks. A hammer can then be used to complete the fold. Pop rivet or spot-weld the corner flaps to make a solid box.
If you lack sheet metal facilities, you could build the housing from 1/2-in plywood assembled with glue and 1-in x 1-in wood corner reinforcing. Cut the wood so that the inside dimensions match the inside of the sheet metal plans. Paint the housing inside and out with enamel paint.
2) Insulation
Cut pieces of 3/4-in rigid duct-board insulation to fit the sides and slip them in underneath the reinforced glazing ledge. Cut two pieces to slide neatly into the back of the housing. Remember that for high temperature applications, styrofoam cannot be used. Polyisocyanurate foam may be used as a second (lower) layer of insulation if extremely high temperatures aren't expected. Cut the air vent holes through the rear insulation and save these plugs to fill the holes when the collector is used in the hot water mode. Seal the insulation with caulk or heat sensitive duct-board tape.
3) Collector Plate
Cut all of the sections of 1/2-in copper pipe with a pipe cutter to the dimensions shown in the "A Collector Plate" diagram. De-burr the cut pipes and clean oxidation off all joining surfaces with steel wool. Assemble the entire network of pipes and connectors to confirm a good fit. Take them apart again and coat the end of each pipe with soldering flux. Reassemble the grid and solder each joint by holding a propane torch to the joint, Heat the joint until it sizzles and just begins to turn colors. Touch the solder to the fitting to see if it melts and flows easily. Apply the solder all around until it begins to drip. Solder 3/4-in threaded hose connections to the header ends. Warning: Fumes from solder acid flux is toxic so be sure to work in a well-ventilated area.
Next, cut the copper foil absorber plate. It will be necessary to form hollow grooves in the foil to match the collector tube spacing. These dents will make a good solder connection between the plate and the pipes so that heat is efficiently conducted into the water. Make a jig by nailing two boards to a table with a 5/8-in gap between them. Position the foil with a scrap piece of 1/2-in copper tube over the jig. Pound the tube with a rubber mallet until a dent is formed in the copper foil. As you deform the foil, its width will decrease, so position each crease one at a time and check its spacing against the spacing on the pipe network. Clean the tube assembly and foil with steel wool, cover with flux, and solder them together by holding the torch to the tubing until solder flows underneath. Always apply heat to the more massive piece to be joined and allow the heat to be conducted into the thinner material. When the plate is finished, test it by hooking the hose adapters to a water tap to see if any joints leak. Re-solder any bad joints. Clean the completed absorber with acetic acid and spray it with high temperature flat black paint.
4) Glazing
Measure the final dimensions of your housing. Cut the fiberglass glazing with snips or a fine saw to fit these dimensions. Cut a glazing cap from thin aluminum corner reinforcing. The cap is cut from one piece the length of the perimeter of the housing. Miter the top surface of the glazing cap at each corner then bend it around the perimeter of the housing. Pop-rivet the last corner of the glazing cap so that it forms a continuous ring. Trial-fit the glazing on the front of the collector and slide the glazing cap over it. Adjust if needed. A neoprene gasket should be used under the glazing cap on full-scale collectors.
5) Stand
Make a folding stand from angle iron, metal shelf supports, or wood. The frame should have holes drilled at regular intervals so the stand can be adjusted. Attach the collector to the stand with sheet metal screws driven through the stand into the sides of the housing.
6) Storage Tank
Build a storage tank out of a one gallon metal can covered with rigid insulation. Solder 3/4-in hose connectors to the top and bottom of the can. Attach the storage tank to the top of the collector housing with a shelf bracket and sheet metal screws. A long stem thermometer should be pushed through the insulation into the top of the storage tank.
7) Assembly
The collector plate should fit into the housing, snugly pinched between the side insulation. Allow a 1-in air gap between the glazing and the collector plate. Spacers can be cut from the duct board to hold the plate in position. Attach glazing with glazing cap fastened to housing with two or more sheet metal screws. Press insulation plugs into the rear vent hole when using the collector in the water mode.
Cut a 2-ft and a 4-ft section of garden hose and press female hose fittings into the ends. Attach the short hose from the top of the collector to the top of the storage tank. Attach the longer hose from the bottom of the collector to the bottom of the storage tank. Fill the storage tank with water and gently shake the whole collector to dislodge air bubbles that might be trapped in the hoses or the tubing. After a half-hour in the sun, the top connection should feel warm while the bottom connection will be cold, indicating that a thermo-siphon flow is occurring. It may be necessary to tilt the collector so that the header is inclined towards the output hose. Experiment with a pump inserted into the system. Small water pumps with hose connections can be ordered from the Surplus Center Catalog listed in the Appendix.
8) Using the Collector in a Hot Air Mode
Remove the glazing, glazing cap and the water absorber plate. Pop out the rear air vent plugs. Make a metal absorber plate from corrugated aluminum or iron that has been sprayed black. Make sure there is a free passage of air from one vent to the other behind the plate. To assure free passage of air, cut small blocks of rigid duct-board and use them to support the plate above the rear insulation. Reassemble the collector. When placed in the sun, warm air can be felt pouring out of the top hole due to the thermo-siphon effect. See Rock Storage Design Problem later in this chapter. Collector Efficiency
So your collector makes some hot water. What does that tell you about its efficiency? The temperature alone doesn't tell you much about whether your collector is producing as much hot water as it theoretically could. Collector efficiency is the ratio of heat energy delivered by the collector divided by the solar energy failing on the collector. To measure the collector's efficiency, you must know exactly how much water was warmed, by how many degrees, in what amount of time. Then you have to compare this figure to solar radiation received by the collector during that period of time.
Begin your evaluation by measuring how much usable heat your collector delivered in one hour in the sun (Ed = Energy Delivered). British Thermal Unit (BTU) is the amount of energy it takes to heat one pound of water one degree F. Measure the temperature of the water in the storage tank before the one hour test begins (Ti = initial temperature) and immediately after (Tf = final temperature). Calculate the difference. This is the number of degrees that the collector has raised the water temperature. Multiply this figure by the pounds of water that were heated. Water weighs about 8.3 pounds per gallon at room temperature.
Now you must compare this figure with the total amount of energy that hit the collector area in that time (Ea = Energy Available). The simplest way to measure Ea is to measure the insolation with a hand-held insolation meter. (An insolation meter you can build is described later in this chapter.) Hold the meter parallel to the surface of the collector and read the number of BTUS/ft2/hr. Multiply this reading by the area of the collector (in square feet). Finally, divide the BTUs collected by the BTUs available to get percent efficiency .
% Efficiency = Ed /Ea * 100 = (Tf-Ti) * (lbs.water) / Ea (BTUs/ft') x Collector area
If you don't have a calibrated insolation meter or if you want to compare fiqures for the sake of interest, check the solar intensity chart for that hour and day of the year. This chart predicts BTU S/ft2 /hr assuming perfectly clear skies. It does not take into account daily local weather conditions such as degree of cloud cover.
The efficiency of your collector will drop markedly as the outside temperature decreases and as the heat transfer fluid temperature increases. This is due to increased heat loss of the collector. Your collector will be most efficient when it first begins, because the water is cold and heat transfer from collector to fluid is proportional to the difference in temperature between the plate and the water in the tubes. After the collector has been operating for a while, the collector will be receiving warm water from the storage tank. The absorber plate will be operating at a higher temperature and, consequently, will be losing heat to the outside environment more rapidly. In addition, heat will not be easily conducted from the plate into the warm water in the tubes because the temperature differential between water and absorber is smaller.
If you were to measure and graph the temperature of the storage tank every 10 minutes for several hours, you would get a curve that gradually levels off as the temperature ceases to increase. The steep slope at the beginning of the graph (lower left hand corner) indicates that much heat is being absorbed, and the collector is working very efficiently. Later the efficiency decreases to near zero, when the water becomes nearly as hot as the collector. The slope of the graph at any point on the curve approximates the collector efficiency at that point in time.
To maximize the efficiency of your collector, you should insulate it thoroughly. Experiment with additional layers of glazing or with reflectors that will focus more solar energy onto the absorber. If you need 1200 water for bathing, it is easier to maintain a large tank at 1200 than it is to heat a small tank to 1800 that must then be diluted with cold water to use. This is one example of how evaluation of real needs helps the designer fine tune an invention for peak performance.
{mospagebreak title=Insolation Meter}
Insolation Meter
When conducting collector efficiency experiments, one must know exactly how much energy was available to the collector given the time and atmospheric conditions for that period. An insolation meter consists of a solar cell connected to a small volt meter. A variable resistor in the circuit allows you to trim the output of the cell so that it stays within the range of the meter. The cell is mounted on a hinged bracket so that you can make measurements at different angles.
MATERIALS
1 ft2 of 1/4" plywood 1 1 "*l /2" silicon solar cell 1 plexiglass scrap 1/8"*2''* 2 1/2" 1 hinge 1 0-100 ohm variable resistor 1 0-50ma Milliammeter wire, silicone, glue, screws
INSTRUCTIONS
1) Cut plywood housing. Drill and jigsaw mounting holes sized to fit the ammeter and resistor. Assemble box with glue and small nails leaving the back panel off.
2) Mount solar cell to back side of plexiglass with silicone caulk. Cover back completely. Attach plexiglass to hinge with screws. Attach hinge to housing. 3) Insert meter and variable resistor Into their mounting holes. Connect wires as shown. Attach rear housing panel with screws for easy disassembly.
4) Attach paper mask to front of meter with rubber cement. Units of measurement should be BTU S/ft2 /hr. Calibrate the meter using another precalibrated meter or look up the predicted solar intensity for that hour and day from the Solar Site Surveyor intensity diagrams. Tilt the sensor until you get maximum reading. Adjust the variable resistor so that the highest reading is about 3/4 of the maximum on the scale. Repeat on several clear days to get an average. Mark these values on the mask.
Design Problems
1) TEST TRANSMITTANCE OF GLAZING A significant factor in the efficiency of a solar collector is the transparency of the glazing material. No glazing material will transmit 100% of incoming solar radiation. High quality (low-iron) glass can transmit up to about 92% of solar radiation while some plastics have transmittance percentages in the mid 80's.
Collect a sampling of various glazing materials, place these in front of the insolation meter's solar cell on a sunny day, and record the readings. Calculate the percent transmission of the materials by comparing these meter readings with readings made with no glazing in front of the solar cell.
% Transmission = (Gain with Glazing) * 100 (Gain without Glazing)
Try multiple layers or combinations of glazing. In real-world applications, you must choose the appropriate number of layers of glazing on the basis of heat loss. More layers of glazing decrease heat loss, but also decrease the amount of light reaching the collector surface. Your decision on the optimum glazing will depend on the temperature at which the collector will normally operate and on average outside temperatures. High collector temperatures and/or low outside temperatures favor multiple glazings; low collector temperatures and/or moderate outside temperatures favor single glazing.
2) DAYLIGHTING
Buildings that fully use daylight instead of artificial light save on their lighting electrical use. Use the Insolation Meter to measure the light levels in a local building. Make a remodeling plan that would add more windows to increase solar heating and lighting. Consider where in the building the most light is needed. What could be done to steer light into these dark corners? Remember that any glazed surface allows heat to escape, so don't forget double and triple glazing. And glazed surfaces on roofs tend to leak, so be sure to seal them well.
3) HOSE-ON-THE-ROOF COLLECTOR
Design and build this simple collector. It consists of a coil of black garden hose or 3/4-in PVC on the roof. The coil is held in a frame and protected by a glazed, insulated box. No absorber plate is used since most of the sunlight falls directly on the hose.
4) TRICKLE STYLE COLLECTOR
This idea occurred to an inventor who felt hot water from a sudden rainstorm pouring off a hot tin roof in summer. His idea was refined to produce the trickle style collector. Water is pumped into a header (a pipe that supplies a series of vertical parallel paths) that distributes tiny trickles of water down the collector plate. The corrugations in the absorber plate form channels for the water that absorbs heat as it flows down the plate. The heated water is collected in a trough at the bottom of the absorber. The entire collector is covered with glazing to retain heat and to prevent evaporation.
Design and build a trickle style collector. If you have a tin roof, you might use that as your collector plate. A copper or plastic pipe with a hole drilled for each corrugation will serve as a header. This could make a fine swimming pool or hot tub heater.
5) BREAD BOX WATER HEATER
The simplest solar water heater is the "batch" type. It consists of a tank filled with water and placed in the sun. Rather than maintaining a continuous flow of fluid as with flat plate collectors, an entire batch is heated at a time. A breadbox water heater consists of a metal tank filled with water that is placed in a glazed, insulated box with hinged, insulated lids for nighttime insulation. The lids and interior of the box may be lined with reflective foil so they can double as a solar concentrator during the day. The tank is filled with a hose once a day and drained as the hot water is needed.
Design and build a breadbox water heater using a recycled water heater tank stripped of insulation. Do not use old drums that may have contained poisonous materials.
6) ROCK STORAGE BIN
Build a model rock storage bin for your collector. Cover two 5-ft lengths of 4-in flexible dryer vent duct with thin fiberglass insulation. Use duct tape to attach a flexible 4-in metal connector to the end of the duct. This connector should slide tightly into the rear vent holes. The duct may be routed now into a box filled with rocks or water-filled plastic jugs to simulate a thermal storage area. You could also experiment with eutectic salts (see Glossary) as a heat storage medium. Arrange the ducts so that a one-way flow occurs through the box. Seal the box. If convection due to the thermo-siphon effect is not strong enough, you may want to install a small air blower in the duct to increase circulation.
7) SOLAR FOOD DEHYDRATOR
The techniques and principles of collectors and ovens are basic to solar design and as such can be applied to many other inventions.
Design and build a solar food dehydrator. The purpose of a dehydrator is to create a warm (1000 F maximum) bug free and well-ventilated space where food can dry. Food is cut thin and placed on nylon screens stretched over frames. This allows the natural convection to pass around the food on all sides. Hot air is provided by direct solar gain through the glazed lid of the cabinet or by a thermo-siphoning air collector that leads into the drying cabinet. Check the books Dry It You'll Like It and VITA plans for more on dehydrators.
{mospagebreak title=Solar Concentrators} Solar Concentrators
Flat plate collectors and solar hot air systems are well suited for tasks such as home heating, food drying and hot water. However, other tasks such as boiling water, cooking food or melting certain materials require much higher temperatures. These higher temperatures can be achieved using solar concentrators.
Several solar concentrating devices are described in this section. In order to reach high temperatures, one must concentrate the solar energy falling over a large surface onto a much smaller surface (focal area). Although light rays normally travel in perfectly straight paths, they do bend when they pass through a lens, and thus can be concentrated. One example of this principle is using a magnifying glass to burn holes in paper. A concentrating effect can also be achieved using mirrors that focus sunlight onto a small area. The ratio of intensification achieved by a lens or mirror can be measured by comparing its frontal surface area with the area of its focal point.
Warning: All of the solar concentrating devices described here are potentially dangerous. Do not look directly at the focal points without eye protection. Temperatures at the focal point are hot enough to burn your skin or clothes. Wear fireproof gloves when working near the focal point.
Sag Lens
A large lens can be made with a sheet of clear thin plastic and a rigid ring such as the rim of an old bicycle wheel. Stretch the plastic over the rim and tie it in place with tape or string. Hold the rim level and pour a few cups of water into it. The water will make a gentle curve in the plastic and form a lens shape. The center of the "sag lens" will be thicker than the edges while the surface of the water will be flat. When the sun is high in the sky, test underneath the lens to find the focal point. Try adding more water to see if the focus changes. The usefulness of this lens is limited because it has to be held horizontally. What would happen if you filled your sag lens with clear polyester casting resin?
Try stretching a thin sheet of aluminized mylar over the rim. Instead of a sag lens, you have a sag mirror. You might try stretching the mylar over an old garbage can with a hole in the bottom attached to a vacuum cleaner, The vacuum will pull the mylar mirror into a focus curve that can be tilted to face the sun.
Fresnel Lens Furnace
Large lenses are exceedingly heavy and expensive to make because they are ground from thick slabs of glass. Imagine slicing a large lens into thousands of concentric rings each with a prism shape corresponding to sections of a lens. The focusing of the lens is due to the angles between the top and bottom surface of the lens. If each concentric ring were to be pressed into a flat plastic sheet, you would have a Fresnel lens that is light, powerful and inexpensive. The Fresnel Lens Furnace described here is especially powerful and dangerous. Just looking at the focal point with unprotected eyes can damage your vision. Always use welding goggles and look at it only for short periods of time. This furnace has enough power to melt glass and small stones and can burn your hand in an instant if you accidentally touch the focal point.
MATERIALS 1) ES70717 fresnel lens from Edmund Scientific 6) 3/4" plastic pipe elbows 2) 3/4" plastic pipe "T's 14'of 3/4" plastic pipe 8' of plastic storm window molding and gasket Plastic pipe cement 3/4" sheet metal screws Tuna fish can High temperature insulation
INSTRUCTIONS
1) Cut the plastic window frame so it fits around the lens. Miter the corners. Set the lens into the rubber gasket and press into frame. Secure the corners of the frame with screws.
2) Cut the 3/4-in pipe into specified lengths. Assemble the base as shown in the exploded view. Glue pieces in place with pipe cement, making sure that the "T"s point straight up. Do not glue the 30-in uprights in place; this way the frame can be easily disassembled and stored.
3) Cut a notch in a 1-1 /2-in section of 3/4-in pipe to fit the lens frame. Mount this bracket halfway up the long side of the lens with glue and screws. Cut a slot in a 3/4-in elbow and slide the bracket into this elbow using some petroleum jelly for lubrication. Lock the bracket in place with a screw through the slot.
4) A crucible is a heat-proof container that can be held in the focal point of the furnace. Make a crucible from a tuna fish can packed with high temperature insulation from a ceramics supplier. Attach a section of broomstick to the side of the can for a handle. The broomstick can be supported in a ring stand or held in the hand. Unexposed, developed photographic film (it should be jet black) mounted in 35mm slide frames can make temporary goggles, but it is better to use dark arc welding goggles.
5) Beautiful jewelry can be made by melting colored glass on copper discs with the furnace. Obtain 1-in copper discs, flux and enamels from an art supply store. Wet the copper with water and sprinkle it with powdered glass flux. Place the coated copper in the crucible carefully and position it in the ring stand so that the focal point is as big as the blank. When the flux melts, remove it from the focus, add colored enamels and reheat. Don't forget your eye protection.
Solar Power Tower
Currently most thermal electric power plants burn coal and other fossil fuels to heat steam that runs turbines and spin generators. There is growing concern about the global effects of burning fossil fuels. In particular the carbon dioxide level in the atmosphere is steadily increasing, and this may produce climate changes. Burning coal also produces sulphurous emissions that cause acid rain hundreds and even thousands of miles away. Is there an alternative to coal fired boilers?
The federal government has invested in many projects researching focusing collectors and large-scale solar thermal power plants. One of the most ambitious of these projects is a Solar Power Tower in Barstow, California, that uses several hundred mirrors controlled by computers to focus light onto a boiler high on a tower. The steam produced is then used to generate electricity. If this proves economical, we may see large solar thermal plants that concentrate the sun's heat for other industrial processes as well.
You can experiment with this idea at home using mirrored tiles. Buy a box of 1-ft square mirror tiles from a home improvement store or collect free mirror scraps from a local glass store. Cut each tile into quarters. Prepare a mounting stand for each mirror using a 1/2-in dowel, wing nuts and a block of wood glued to the back of each mirror with construction adhesive. Drill 1/2-in holes in a series of boards on which the mirrors will be mounted. Now each mirror can be individually directed and will hold its position.
Make a boiler from an old kettle, painted black, and then suspended from a tripod made from metal tubing. Arrange the boiler so that it is between the sun and the rows of mirrors. Cover all of the mirrors temporarily with paper. Focus each mirror until its reflection is directly on the boiler, lock the position with the wing nut, cover it again and move quickly to the next mirror. When the last mirror is focused, remove all of the covers and see how long it takes to boil the water in the kettle. After a while, the sun will have moved enough so it will be necessary to realign the mirrors.
You can try other kinds of boilers. A pressure cooker is a safe steam generator as long as you don't interfere with its pressure release valve (the weight on the top of the cover). Pipe the steam to the ground with an insulated hose. Perhaps the steam could be used to power a model turbine or steam engine. Try encasing the boiler in a glazed cylinder to slow heat loss.
Curved Reflectors
The Solar Power Tower used flat mirrors, each held at a slightly different angle. A focusing mirror duplicates these various angles with a continuous smooth curve. There are many kinds of curves such as circles, ellipses and the parabola. By making a flexible reflector, you can study the focusing characteristics of each of these curves to find out which is best.
Make a flexible reflector by gluing aluminum foil on flexible posterboard, sheet metal or fiberglass sheet. Sketch out various curves, using techniques from a geometry text. Bend the reflector to match these curves and direct it towards the sun. A circle or ellipse can produce an odd shaped focal point that may be good enough for some purposes. However, only the parabola can produce a sharp focal point.
The focal point of a parabola depends on its size and the shallowness or depth of the curve. A shallow parabola will produce a distant focal point while the steeper curve focuses closer to the parabola. A parabolic reflector may be shaped like a dish or a trough. The dish produces a point focus and the trough produces a focal line. There are two methods for designing a parabola: the point plotting and the T-square method.
A parabola follows from the general formula:
D= 1/16 * (W*W/F)-(X*X/41)
w width of collector f focal length x distance from center line to any point D depth of parabola at any point
For example, to make ribs for a cylindrical parabolic collector utilizing 4-ft x 8-ft pieces of plywood, 4-ft would be a convenient collector width. An efficient focal length is approximately half the width, say 2-ft. Plug the appropriate values into the equation and solve for D at one-in intervals of x. Plot D and x on a graph to get your curve. This would make a simple microcomputer program.
A parabola can also be drawn using a T square. For a curve of width w, draw a line across a piece of paper w long. At the midpoint construct a perpendicular line of length f, the desired focal length. Place a nail at the focal point and place a carpenter's square on the paper such that the heel (the right angle corner) sits on the line w and the body (the long side) rests against the nail. Making sure to keep the body of the square against the focal point and the heel of the square on the horizontal line, move the square a few degrees at a time. Mark along the tongue edge (the short side) of the square after each movement. Continue marking the line until you have reached a distance of 1/4 w from the center line.
Parabolic Trough Collector
In a parabolic trough collector, a long pipe containing water is located along a focal line. Build this collector carefully and you will have a steam generator or high temperature water heater. You can also use it as an experimental apparatus to explore tracking drives, glazed absorber tubes, heat pipes and much more.
MATERIALS 3) pieces 1 "xl "x 23" pine (for frame) 3) pieces 3"xl 3"xl /2" plywood (for ribs) 20) pieces 3"xl 7"xl /2" plywood (for tube support) 1) piece 24"x13" fiberglass or aluminum 1) piece 24"x13" aluminum foil or aluminized mylar 1) piece 26-1/2 long 1 " OD copper pipe with cap 10) #6 1 " round head screws 1) 2"xl "xl /8" iron scrap
INSTRUCTIONS
1) Draft a parabola 13-in wide with a 4-1/2-in focal point using one of the methods described earlier. Cut out plywood ribs from this pattern with 1/2-in wide notches in middle and ends. Notch the three 1 "x 1" spars as shown in the plans.
2) Assemble ribs, spars and fiberglass or aluminum sheet with screws and glue and make sure the reflector surface is even and smooth. Attach the aluminum or fiberglass to frame with epoxy, hot glue or small nails. Carefully apply reflective foil to this surface with rubber cement or white glue.
3) Attach collector tube supports with screws and glue. Slide collector tube in place with end cap soldered in position.
4) Drill and thread a 1/4-20 hole in an 1/8-in thick scrap of iron. Attach this to the bottom of the middle spar. This hole will receive a 1/4-20 bolt from a photographic tripod.
Tracking Mechanisms
How can a device be mounted so that it points directly at the sun throughout the day? At each moment during the day, the sun has a given altitude and azimuth. A stationary object can track the sun if the vertical and horizontal angles of its face are changed periodically to follow the sun's course through the sky. Astronomers have developed mounts for telescopes that follow the movement of the stars so that long photo exposures can be made. This same technology can be applied to tracking our own star, the sun. Tracking mechanisms are used on all kinds of focusing collectors. Even photovoltaic devices (see next section) can be made to track the sun so that maximum energy is collected for each hour of the day.
Equatorial mounts pivot around two axes. One axis points toward the North Star (Polaris) and the other axis is positioned parallel to the equator. This results in a simplified tracking system since in a given day, angle adjustments have to be made on only one axis to achieve tracking. The equatorial mount is moved at exactly the speed of the earth's rotation, about 15 degrees per hour (i.e. 3600/24 hr). To complete your equatorial mount, install a clock drive that rotates the mount through the equatorial plane at a rate of 150/hr.
The equatorial mount can be used for solar applications where precise focusing of the sun is required. A mirror or other focusing device is mounted on an inclined axis corresponding to the solar angle for that day. This must be adjusted at least every week due to the seasonal change in sun angles as the earth passes around the su,n. Some solar tracking systems such as the solar power tower are completely computer controlled. To turn an equatorial tracker automatically, you may be able to adapt the clockworks from a windup clock. Otherwise you will need a small motor with appropriate gearing.
Other tracking schemes employ solar cells as sensors that activate drive motors through a relay when the sun moves out of alignment. These systems can get fouled up when clouds temporarily block the sun. Still another device uses gas canisters filled with freon or alcohol that are connected, by a tube. When the sun shines on one canister the fluid is vaporized and condenses in the shaded canister. The shifting weight causes the carefully balanced device to rotate with the movement of the sun.
Solar Oven
Solar cooking is an important innovation especially in developing countries where cooking fuel is scarce. By using the sun to cook, firewood is conserved, forests are protected, and money and time are saved. Solar cooking is an example in which a technology has a variety of beneficial effects on a culture.
A solar oven consists of a glazed, insulated box with reflectors that concentrate energy into the interior of the oven. After you understand the function of these various features, you can design your own oven or modify the design given here. For instance, the plans here call for a housing made from masonite though you could just as easily make it from plywood or sheet metal. After building your oven, you might try to improve its efficiency by adding more layers of glazing, by making bigger and more accurate reflectors, etc.
MATERIALS 1/2 of a 4'x8'sheet of 1/8" masonite 1/2 of a 4'x8'sheet 18-gauge galvanized iron Tempered glass, measured to fit finished oven 10' of aluminum corner reinforcing Tof 1 "x4" pine Tof 1 ''x6" pine ripped to 36 0 bevel Unbacked 3-1/2" fiberglass batt Barbecue black spray paint 75) aluminum pop rivets and shields Construction adhesive
INSTRUCTIONS
1) Cut out masonite pieces as drawn in plans. Cut aluminum corner reinforcing to fit each masonite corner. Check the fit before assembly. Lay a bead of construction adhesive along the inside of the corner reinforcing and set against the corresponding masonite piece. Temporarily C-clamp in place while you drill 1/8-in holes to receive pop-rivets. Use washer shields on inside and pop-rivet in place. Locate a rivet every six inches along joined edges. Before attaching the back piece, glue and nail oven door frame around rear opening.
2) The glazing frame serves to strengthen the housing and support the glazing and reflector. Saw or plane 1-in x 6-in pine to a 360 bevel 3-1/4-in wide for the top and bottom of the glazing frame. Saw 3-ft of 1-in x 4-in to 2-1/2-in for the sides of the frame. Cut the frame to dimensions indicated, then saw and chisel the ends of each frame piece to form a lap joint. Glue and clamp this. Glue and nail the completed glazing frame to the inside of oven housing. Route or chisel a 3/8-in deep recess along the top, inner edge of glazing frame. Alternatively, construct a double-pane glazing unit shown and omit the routed groove in glazing frame.
3) Construct the sheet metal oven liner by first cutting out the pieces shown in plans. Fold 1/2-in edges as shown and join with sheet metal screws, pop-rivets or spot welds. Clean the inside and spray it black with high temperature paint.
4) Press fiberglass into the oven's interior. Line all sides of the interior. Use gloves to avoid skin irritation. Leave room for the sheet metal liner but leave no gaps in the insulation. Press the oven liner into the oven housing and attach it along the inside of the glazing frame and along the rear opening frame with nails.
5) Construct the rear door; glue and nail masonite to the back. Check now to be sure that door fits easily into the rear opening of oven. Stuff it with, fiberglass and attach a rectangle of sheet metal to the inside. Finally glue felt weather-stripping to the oven door as shown.
6) Tempered glass will have to be ordered from a glass shop according to the dimensions of the routed recesses in the glazing frame. Allow an extra 1/8-in on all sides so the glass can expand as it heats. If you can find tempered glass from an old gas oven door, or window from an old automobile, build the oven to fit its shape and dimensions. Place glass on oven and hold it in place with silicone caulk or weatherproof tape.
7) Make reflectors from masonite or thin plywood paneling. Attach aluminum corner reinforcing to the base of each reflector for a mounting bracket. Cover the reflectors with aluminized mylar ("space blankets"), mirror tiles or aluminum foil. Attach the reflectors to the oven glazing frame with screws and adjust the angles so that a maximum amount of sun reaches the interior of the oven.
8) The oven should work best when the glazing is perpendicular to the sun's rays. Place wooden blocks under the rear of the oven to set angle. Sometimes, however, the contents of the oven can't be tilted and brick should be used inside to level the baking dish. You might also consider making a hanging oven rack that holds the food level for any tilt of the oven.
{mospagebreak title=Photovoltaics} Photovoltaics
It's time to re-roof your house, so you go down to the building supply store to check out the voltage roofs that are on sale. No, you are not seeing a typographical error, just a glimpse of the future. The time is approaching when you can be your own power company by installing photovoltaic solar cells on your roof.
From the user point of view, photovoltaic solar cells are beautifully simple. You place photovoltaic panels on your roof and when the sun shines, electricity is produced. You may need refinements such as batteries for night storage, a regulator to protect the batteries, and an inverter to produce alternating current (AC current) for certain applications. The major drawback is cost, but that is steadily decreasing.
Extremely specialized equipment is needed to produce silicon solar cells. High-grade silica sand is refined, melted, crystalized, impregnated, coated and tested. The small brittle wafers are then encased in a waterproof housing that protects them from the elements without blocking the sun. Encased cells are simply sandwiched between two rigid layers, usually plastic on the back and tempered glass on the front. A waterproof sealant such as silicone rubber is used on the edges to be sure no moisture or air can penetrate the solar cells. Although the cells themselves are glass-like, water and air can corrode the thin metallic conductors on their surface. An encased panel with many individual cells is called an array, and these are commercially available at certain rated voltages and amperages.
Most people who are installing photovoltaics today are living in remote areas where there are no conventional power sources. Installing photovoltaics can be cheaper than the cost of running new lines into remote sites. However, it is quite possible that, soon, reduced prices will mean that anyone can choose between producing his own power or buying it from the utility. Imagine the consequences of using this amazing technology on a national or global scale!
With some do-it-yourself ability, you can build your own experimental photovoltaic panel from individual cells. Check the supply catalogs listed in the Appendix to find special deals on factory "seconds" (cells that have small imperfections or cracks that make them unusable for high quality arrays but are still quite functional for our purposes).
Let us go through some of the problems that you will face in designing and building a photovoltaic battery charger.
Battery Charger
1) Sizing the System
For your demonstration avoid high cost by keeping the system small. If you should be interested in scaling up later, the same principles will apply. By staying with low voltages (12 or 24 volts DC), you can use highly efficient DC equipment. If you must have 110 V appliances, you will need an inverter that can change 12 V DC into 110 V AC. Inverters are expensive and lower the efficiency of your system. Some 110 V AC appliances can also work on DC depending on their motor wiring.
Voltage can be thought of as the pressure of the electricity; you can't put more air in a tire inflated to 30 pounds per square inch (psi) if your pump is only creating 25 psi. For charging a battery, you must have a voltage greater than the rating of the battery. Thus, to charge a 3 V battery, you might use 3.5 V supplied directly from a solar panel. Amperes, on the other hand, are a unit measuring how much electricity is available at that voltage. You might think of an ampere rating as the diameter of a water hose. A high amperage panel with a suitable voltage will charge a battery faster than a low amperage panel.
When you select cells from a catalog, try to get all the cells the same size. A single silicon solar cell is usually rated at .45 V, and the amperage varies according to its size. Catalogs will give both these specifications. Cells wired in series will give you the sum of all the individual cell voltages; wired in parallel, the amperage will be totaled. A combination of series and parallel circuits will give you exactly the volt and amp arrangement you need.
Example: You want to charge two 1.5 V batteries at 500 milliamperes (a milliamp, or mA, is 1/1000th of an amp). To charge both cells in series will require 3 V. Each cell produces .42 V so 3/.42 equals approximately 7 cells. In the catalog you can find cells rated at 250 mA. You will need 14 250 mA cells to produce 3 V at 500 mA. You wire 7 cells in series and each of these strings of 7 cells in parallel. The voltage of the 7 cells in series adds up to 3 V and the two chains in parallel equals 500 mA, thus giving you the voltage and current required.
2) Encasing
Choose a piece of plexiglass or glass for the front and back large enough to cover the cells, with 1-in to spare around the edge. Arrange the cells so that they are densely packed. Solder the cells together with fine wires going from the top of one cell to the bottom of the next for series arrangement. Allow insulated wires to extend several inches beyond the panel for outside connection. Clean the covering material and position the cells with a drop of silicone caulk underneath each cell to hold it in position on the backing material. Check your wiring with an electric meter to be sure that everything works; the panel will be very hard to take apart if the wiring is defective. Now lay a continuous 1/4-in bead of silicone around the perimeter of the backing sheet. Press the cover glazing into the caulk so that a seal is made all around. Do not press the top so hard that the cells are pinched and broken.
3) Attaching the System to a Battery
Buy a battery holder clip and attach it to a block of wood. Wire the batteries in series for the 3 volt charger. Place a small diode in the circuit to prevent back flow of electricity from the battery to the array when it is dark. Check the circuit with a volt meter to be sure that the current flows into the battery, but when the array is disconnected, there is no current coming from the battery through the diode. On a larger array you must install a voltage regulator that would prevent the batteries from being overcharged and damaged. A small solid-state regulator can be purchased from electronics stores.
DESIGN PROBLEMS
1) SOLAR ELECTRIC CAR
Electric cars may be common in the future especially around urban areas where distances are short and pollution is a concern. Electric cars are not likely to have the speed, range and power for intercity travel.
Design and build a model solar electric car. Begin with a model car that is powered by a single AA battery. Replace the disposable battery with a rechargeable nickel cadmium cell. Install a subminiature phono jack on the car housing and connect this to the battery terminals. Recharge the battery with your solar panel. Experiment to see how long the car will run on a given amount sunlight. Can you detect the difference between early morning light and noontime when recharging the battery?
You could also build a direct drive model using solar cells and a small DC motor. You can buy model conversion kits for $15 that include motor, cells, and gears that can be attached to most model cars.