"Hands-On" Solar Power the CMC Energy Efficient Building Students 1989/90 The Colorado Mountain College (CMC) Energy Efficient Building Technology Program teaches the design and installation of solar and energy efficient building systems. Participants receive two semesters of hands-on building construction experience. This article written by the class and details our recent state-of-the-art passive solar remodel. Client and class member, John D'Angelo volunteered his mobile home for the project. He purchased a trailer with solar retrofit potential. Four different solar systems were designed: 1) a larger window for direct gain heating and a better view; 2) a solar hot air collector; 3) a passive domestic solar hot water heater; 4) a solar heated natural gas generator. INSERT TRAILER ART AND PHOTO But First, INSULATE! Comprehensive weatherization is a pre-requisite for solar space heating. Air leakage reduction (stopping drafts) should always precede "supply side" thinking. Caulking, insulation, storm windows, skirting, etc. are cost effective pre-solar heating procedures. John took advantage of his student status and qualified for the local energy center weatherization program. Now the solar power will work efficiently and help heat the trailer into the evening hours. It doesn't make sense to collect free solar heat if you allow it to leak out as fast as it comes in. Also, efficient domestic hot water strategies should always proceed any solar hot water heater. Quality low-flow shower heads, low flow plumbing fixtures and pipe insulation should always come first. To complete John's solar retrofit he designed a solar powered natural gas generator. He dislikes cooking with electric (frustrating heat control and incompatible with his future PV system) and decided to produce "home grown" natural gas. Site Preparation We choose to build a permanent complete foundation system to support the heavy collectors. FIGURE 2 - Concrete Foundation Detail "It's called concrete - not cement," reminded CMC instructors to the CMC class of 1990. Cement is just one ingredient in concrete. The pour was small, 8' x 10', well planned and much work. We used a transit to establish proper elevation and excavated a rectangular slab. A plywood form with reinforced corners was built with drywall screws and cross-taped to establish square corners. The sill plate was attached directly to the inside of the form and anchor bolts were countersunk into the sill plate every few feet. Either pressure treated lumber or redwood can be used. By attaching the sill plate to the form before the concrete was poured, a nailing surface for the wall plate was permanently in place. This enabled us to easily screed the concrete level. The thickened edge slab perimeter was reinforced with # 4-1/2" rebar that was lapped and tied together to the dangling countersunk anchor bolts. Dirt was temporarily backfilled against the form and we were ready for the concrete. The pour was short. We moved lots of concrete quickly. The concrete is rated at 4000 p.s.i. and reinforced with plastic fibers. By adding these fibers to the mix, we could eliminate the standard reinforcing 6" x 6" remesh. There are many techniques for finishing concrete and I think we practiced nearly all of them. We took turns with the various floats, trowels, and brooms and experimented with different finishing techniques. Hours later, we covered up our work with rigid insulation to prevent it from freezing. Concrete needs three days without freezing to cure properly. Our slab was designed to support the weight of heavy collectors, but this foundation system works well for attached greenhouses and sun spaces. Daytime Solar Heating Daytime heating of our houses accounts for approximately one-third of our heating bills. Solar space heating systems that have no supplemental heat storage (like expensive rock boxes or water barrels) can offset a good portion of the daytime heating requirements. They are especially appropriate in living spaces where people are home during the day. Two types of these systems that we installed on this trailer are called Direct Gain (DG) and Fan-assisted Air Panel (FAPS). Direct Gain Systems Direct Gain Systems use a window to allow the sun's heat into the house and some form of movable insulation (MI) to keep the heat in at night. The window or glazing is best at a vertical position to allow the low winter sun to penetrate. Effective movable insulation prevents nighttime heat loss and should have an edge seal, high R (insulation) value, a radiant barrier, and a vapor barrier. Simple MI can be a removable piece of rigid insulation cut to the exact size of the window. Thick homemade drapes with velcro on the edges will help keep the "building envelope" tight and warm. It is important to design DG systems to avoid overheating during the sunny winter days. In Colorado, we recommend the window area be no more that 15% of the heated floor area. "Too much of a good thing" like south facing windows often create uncomfortably hot and glary living areas. The size window we installed on John's trailer can effectively heat only half the trailer. The window should be facing within 30 degrees of true south- the optimum range of orientation for all solar collectors. Facing east or west by 30 degrees only effects year round efficiency by 10%. East facing windows can provide early morning warm-up, but west facing windows often cause overheating in the spring, fall, and summer. A limiting factor of DG systems is that they are only appropriate for rooms with walls facing close to south. Fan-assisted Air Panels To solar heat north facing rooms, Fan-assisted Air Panels (FAPS) are a good strategy. FIGURE 3 - Distribution System The duct work, and a small solar electric blower move the air through a closed loop between the house and the collector. House air is pulled from a return grill in the north bedroom floor, through duct work, into the blower box and then pushed into the solar collector. A very short hot air supply duct (to minimize heat loss) supplies the warm air back into the house. The temperature of the air entering the house is 75¡ to 110¡ F. and varies with the amount of daily solar insolation. Higher temperatures may seem desirable, but actually very hot air lowers efficiency. It is more desirable to have lots of warm air than a smaller volume of hotter air. Since house air is sucked into the insulated return duct on the north side of the house, John receives the added benefit of improved warm air distribution. The blower on the FAP system distributes the heat from the DG window and the wood stove to the cooler end of the trailer. Collector Tilt In northern latitudes the rule of thumb for optimum wintertime tilt angle is latitude plus 15 degrees. For most of North America, this results in tilt angles of 45 to 65 degrees from horizontal. However, we prefer to mount solar air collectors directly to the vertical south facing wall. Ease of installation, avoidance of summertime overheating and increased ground reflection from snow have proven strong determinants. John's combination of solar systems and his trailer's south wall dictated a compromise tilt angle solution. For integrated aesthetics and simplicity we installed the air, water and natural gas generator systems all at 45 degrees. Air Collector Specifications The solar air collector was designed and built by the class in our campus workshop. Prior site inspection assured proper positioning and duct size. As with all solar air collectors, our system consisted of a frame, covered with a glazing and containing an absorber plate with an insulated air channel. The frame was built of 24 and 26 gauge paintlock sheet metal fastened with pop-rivets and screws. All joints, seams, and connections were sealed with pure, clear silicone caulk to prevent air leakage. Sheet metal was chosen because it is durable, fire-resistant and inexpensive. Wood should never be used in solar air collectors. Despite how wood is treated, it can cause problems by out gassing and warping. Wood frames do not remain air tight and have charred from long-term exposure to the high temperatures typically reached inside the collector. The collector gets extremely hot when the blower is off. A "stagnated" collector regularly reaches interior temperatures more than 250¡ F. The total collector frame size was determined by the glass unit dimensions. A 34" x 76" single pane of tempered, low iron, translucent, 5/32" thick glass was supported by the entire perimeter of the metal frame and protected with flashing. The glazing is isolated from the metal with E.P.D.M. tape and sealed in place with pure silicone caulk. The generous and continuous silicone bead gives structural support for the glass unit. The advantages of glass, versus plastic, as the glazing material are its high transmissivity and extremely long life span. For safety, always use tempered glass units. The standard sizes of low iron glass are 34" x 76", 34" x 96", 46" x 76", 46" x 96" and 46" x 120." Low iron "solar" glass offers maximum solar transmissivity. These units are available in either transparent (clear) or translucent (frosted) with solar transmittance gain the same for both types of glass. For aesthetics, we suggest translucent glass. A special manufactured selective surface was our choice of material for the absorber plate. It's high absorptivity and low emissivity soaks up the sun's rays and doesn't re-radiate them back out the glass. Our thin (.002") copper selective surface absorber was pop-riveted to the sides of the frame and supported in the middle by an air channel guide. Black, pure silicone caulk was used to seal the seams. The criteria for designing a successful solar system absorber are high conductivity, maximum surface area, and durability. Insulating the back and sides of the collector improves system performance. Only high temperature insulation is considered. We used 3/4" polyisocyanurate rigid board insulation. To prevent an insulation "meltdown," do not use any styrofoam insulation products. It is important to avoid any possibility, however remote, of the insulation out-gassing and causing air quality problems. We completely isolated the air flow channel with sheet metal thereby eliminating the possibility of long-term degradation that could result in an air quality concern. Proper selection of materials and attention to detail will insure a high performance solar air collector. Always use non-toxic materials that can withstand high temperatures. Caulk and re-caulk to prevent air leakage. Pure silicone is proven to be the best when sealing metal to metal and glass to metal. The total cost of the project was $451.01 including duct work and electrical parts. The solar panel was loaned to John from the CMC program. Solar Air System Distribution A 6", 26 GA round duct insulated with 2 layers of foil ray, (foil coated bubble pack duct insulation); two registers - Inlet 6" x 12", Outlet 4"x 12"; 1 -12 volts, 5 amp shaded pole DC blower; 1 ply wood blower box - shop built, insulated, caulked with removable access panel. Air System Controls The controls of a conventional 110 volt FAP system consists of 2 thermostats. A regular heating thermostat is mounted at a central location in the space to be heated. Another thermostat is placed in an accessible spot within the warm air duct. The heating thermostat is set at a desired room temperature and functions like a normal furnace thermostat. The warm air thermostat is set to go on at 110¡ F and off at 90¡ F. In this system two conditions have to be met for the blower to turn on: 1) the room temperature has to drop below a desired level, and; 2) the air inside the collector has to reach at least 110¡ F. When the room reaches the set temperature or the air inside the panel falls below 90¡ F. the system shuts off. John is a solar enthusiast and decided to have a Photovoltaic (solar electric) panel installed to power the blower. FIGURE 4 - Control Schematic We mounted a 12 volt - 50 watt nominal PV module on the roof of his trailer at a 45 degree tilt. It is directly wired to the 12 volt DC blower with an on-off switch inside the trailer. Now, the sun does the control function. As sunlight heats the air inside the air collector, the PV module provides electricity powering the blower. The advantage of this system is that it works proportionally without any complicated devices. As solar energy increases the temperature inside the air collector, it also also increases electricity from the PV module. Therefore as the blower speed increases more warm solar air is blown into the trailer. Elegantly, the solar powered electricity is proportional to the amount of heat produced by the collector, thus making this control strategy almost ideal. A Blower Box for the FAPS To provide convenient installation of the electric blower for the solar air system, we built a plywood plenum box. FIGURE 5 - Blower Box This box simplifies duct work and maintenance procedures. The blower blows house air into the collector, keeping the collector under positive pressure. This prevents active cold air leakage into the hot collector. Installing the blower in the cool air duct also allows increased blower life by preventing motor overheating. Passive Solar Domestic Hot Water System Design John choose a batch solar hot water heater because it is so simple and almost maintenance free. There are no pumps, controls, sensors or mechanical BS. It is a black water tank inside an insulated box. A reflective surface inside the box increases the amount of solar energy striking the water tank. One commercially available unit is the Cornell Model 480. It has a fiberglass box with polyisocyanurate insulation, a steel tank wrapped with selective surface and an enhanced multi-layered glazing. Because it is insulated and contains a 42 gallons of water it has withstood outside temperatures of -35¡ F. Pipes that go into the tank must be heavily insulated or have electric heat tape to prevent freezing. We recommend both for Colorado's cold winters. Do-it-yourselfers should avoid building solar water heaters with wood or insulation that is not heat tolerant. FIGURE 6 - Batch Heater Diagram The solar water heater should provide 100% of John's hot water for eight months of the year and work as a preheater for the other four months. One 42 gallon batch heater usually provides an adequate amount of hot water for 2 people. For larger households, two or more batch heaters can be hooked up in series. Typically 20% of the total household energy goes to heat hot water. We estimate this system will provide 60-80% of John's hot water when supplemented with other efficiency measures such as energy efficient shower heads. Installation Details We installed the collector at 45¡ for aesthetic reasons and to use water's natural stratification to always obtain the warmest water possible. With the collector oriented and tilted correctly, plumbing began. Copper pipe (3/4" and 1/2") was used throughout. High temperature, low lead, 95/5 solder was used to solder all joints. A tempering valve was installed to prevent "scalding" water temperatures from reaching faucets. It is an important safety equipment item. Water in passive SDHW systems can easily reach 160¡ F on a summer day. In John's system, four thermometers will be placed on lines going into and out of both the solar hot water heater and the small electric heater (17 gallon, 120 VAC electric). By knowing these temperature differences, John can evaluate solar system performance. Manually operated ball valves were placed at strategic points in the system. John's plumbing enables him to have three distinct modes of operation: solar only, preheat and auxiliary only. Please refer to the valving schematic. FIGURE 7 - Plumbing Diagram All ball valves are placed close together and labeled. The valves are easily visible and accessible under the kitchen cabinet. Providing for convenient operation and maintenance is part of good system design and installation. Natural Gas Generator The last section of the system is a natural gas generator (commonly known as a methane digester). This unit will provide John with gas cooking and supplementary heat during the winter time. The unit is experimental. John made natural gas from cow manure years ago and thought it would be exciting to do it on a home size scale. The system has two basic units. A 65 gallon plastic tank and a gas storage unit. The tank lies in a horizontal position inside a direct gain solar space. There will be an inlet to load the tank with raw materials and on outlet to remove the "dijested" material. There will be several tubes in the top of the tank to place temperature sensors and for thermostatic control of a small heat pad for auxiliary heat. A natural gas meter will be placed in the line so John can collect performance data. Gas will be generated 24 hours a day so a plastic storage tank is necessary. John likes plastic because it can be recycled, is not effected by methane, is easy to work with, lasts a lifetime and is inexpensive. To obtain the best performance, the ideal liquid temperature is 98¡F. John estimates 85% of the energy required to heat the liquid will come from direct solar gain and the balance will come from an auxiliary heat source. John does not know exactly how the unit will do during the cold winter nights. In the winter he plans to use some auxiliary heat from the 450 watt heat blanket to keep the liquid at the optimum temperature. He is counting on the thermal mass of the liquid and superinsulation to moderate the temperature swings. In the summer the solar glazing will be covered most of the time except when heat is required. He plans to have a temperature swing of 20¡F., from 100¡ to 80¡. The closer he can maintain a constant temperature the better his gas production. His goal is to have natural gas year around with a minimal amount of effort and energy. John plans to write a follow up article for HP about the generator's performance. Summary & Access The project was a great "hands-on" learning experience and fun for all. The class knows after they were done it was another small step toward a cleaner environment. Many thanks and appreciation goes to those who wrote different parts of this article and actively participated in the project: Students: Gary Beckwith, Marlene Brown, John D'Angelo, Evan Lawrence, Juan Livingstone, Zoe Shinno, Markus Stoffel, and Mark Wolf. Instructors: Johnny Weiss and Steve McCarney. For further information on the Energy Efficient Building Technology program write Colorado Mountain College (CMC) , P.O. Box 10001PB, Glenwood Springs, CO. 81602 or call 1-800-621-9602 in CO or 1- 800-621-8559 outside CO. For any information on the trailer project contact John D'Angelo, 0171 Hwy 133 C-2, Carbondale, CO or 303-963-9632.