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letters to HP

Not There Yet?

Hello, The following quote from President Bush prompted me to write this letter while listening to my stereo hooked to the "unit of the future" he describes. I am not a time traveler but a citizen who wishes our domestic RE industry could garner even a fraction of the support oil and nuclear industries receive.

I had an interesting opportunity to go see some research and development being done on solar energy. I'm convinced, someday in the relative near future we'll be able to have units on our houses that will be able to power electronics within our houses, and hopefully, with excess energy, be able to feed them back in the system. That's possible. We're not there yet, but it's coming.

—President George W. Bush, October 4, 2005, Rose Garden Press Conference

I really enjoy the magazine as it continues to improve issue to issue. Despite my battles here in Boulder, Colorado, with the building department over electrical systems, tower heights, and other nonsense, I'm very proud to run my house and editing studio "electronics" off of twelve domestically produced PV panels (my wind turbine should be installed next spring)!

I have to recommend a Home Power subscription to the White House so they can keep up on what's happening, because I really wonder where their RE information comes from (the 1960s?). Obviously, nobody around the President reads your fine publication.

Keep up the important work! Regards, Eric A. Thanks for the encouraging words, Eric. Needless to say, Home Power hasn't reached everyone quite yet. In fact, the White House grounds were outfitted with a 9 KW solar-electric array in September 2002. With that in mind, however, the President's misunderstanding reminds me of one of the more satisfying aspects of the renewable energy movement—it's growing from the grass roots on up, rather than the other way. On behalf of Home Power, a huge thanks to you and all of the countless, happy end users of solar electricity who've chosen to make one person's "relative near future" their present. We're sending a complimentary subscription to the White House. Scott Russell • [email protected]

Grounding Perspective

After reviewing the pole mount installation article in HP109, Code Corner columnist John Wiles had the following suggestions related to system grounding:

• Many ground terminals are not listed for the termination of multiple ground wires. If this is the case, multiple ground wires should be bonded together with connectors approved for that purpose, and a single conductor should be terminated at the ground lug.

• UL has in the past concluded that star washers are generally not suitable for breaking down and preventing reoxidation of the film on aluminum module frames. Tin-plated copper lay-in lugs installed flat against the module frame will likely provide the best ground connection over time, and better resist the possibility of galvanic corrosion compared to solid copper lugs.

Solar Woodshop?

I am planning to construct a 16- by 20-foot workshop for light woodworking projects. I am curious as to the limitations solar electricity has to service this type of structure. Electrical requirements would include lighting, tools (table saw, router, sander), and possibly occasional air conditioning. I have just completed a subscription form to your magazine, and would appreciate any support information you can provide me on my workshop endeavor. If this type of project was ever discussed in any past editions, could you please identify for me that edition? Thank you, Charlie Hensel • [email protected]

Hi Charlie, I can think of no limitations. Designing a PV system for a workshop is just like sizing any other system. You start by either estimating or measuring your daily usage, and then size the PV array, battery, and inverter to meet that.

When designing your shop electrical system, here are a few things to keep in mind. First, many power tools have high surge requirements. For example, your table saw may draw four of five times as much power during start-up compared to when it's running. Make sure that the inverter you choose for the system has high enough surge capacity to effectively start any power tools you'll have in the shop. Second, air conditioning is a substantial electrical load. The need for air conditioning is typically during the summer months when there is usually a lot of sunshine, so small, room-sized air conditioners can often be run without much trouble. Finally, in most personal workshops, more than one tool is rarely used at a time, which means it may be easier and cheaper to meet your needs with a solar-electric system than a home that has multiple occupants and cycling loads.

Another suggestion—before you build your workshop, site it so you'll have a south-facing roof with good solar access. That way your system designer will have an excellent starting place when time comes. This sounds like a great project—best of luck. Michael Welch • [email protected] com

Shading Kills Output

Mark Byington did some good work in his "Bypass Diodes" article in HP107, but unfortunately he also perpetuated a myth spread by RE foes that PVs "don't work" when shaded or under cloudy skies. In his tests, Mark didn't shade cells, he covered them so that they were in the dark. In shade, there is light (you can still see—right?), and the cells make electricity, just less of it. The shaded cells are not completely disabled as in Mark's tests. Raining Sky, Houston, Texas

Hello Raining Sky, A covered PV module produces about the same energy as a module in full shade—little to none. Here are some measurements from a Daystar insolation meter to illustrate the point, measured at noon on July 27, 2005, in Los Altos, California:

Full sun: 1,178 watts per square meter

Full shade: 33 watts per square meter

Daystar sensor covered: 0 watts per square meter

As shown in the measurements, the meter is measuring about 97 percent electricity reduction in full shade. Mottled or partial shade will produce results in between full sun and full shade. Regards, Mark Byington • [email protected]

Outhouse Solar Lights

I have an awesome stylish and comfortable (all my neighbors are jealous) single-seat outhouse in the Adirondack Mountains of New York. I was exploring lighting options— handheld flashlight, LED lantern, gas lantern. All had serious drawbacks: flashlight—hard to hold and read HP back issues; LED lantern—too dim to read HP back issues; gas lantern—too hot, noisy, and wasteful.

Then it hit me—solar lawn lights. Intermatic Inc. makes a three-floodlight kit with a separate PV module and attached NiMH AA battery charger that includes rechargeable batteries. The lights come with plenty of cord and each has a separate plug on the module. The lights have an option of a lawn spike or a two-screw "deck" mount. I mounted the three adjustable-angle mini-floodlights to the ceiling, neatly stapling the wire down to a small project box that has three automotive toggle switches (all from Radio Shack). Keeping the polarity in order, I then ran the remaining wire up and out a small hole under the eave of the tin roof to the module.

The circuit of the controller on the module only allows electricity to flow at night. So I suppose the switches could be eliminated and the lights left on all night. I wanted to have relatively full batteries when I turned on the lights, so I added the switches. Total costs were US$40 for the light kit, US$15 for the switches and box, and US$2 for the copper-coated adjustable pipe hanger to mount the module. The total was US$77 and the neighbors almost fainted. Soon no one will want flush toilets! Peace, Dean Russell • [email protected]


I read the article on the Good Life Center in HP108 where the caretakers said that they'd eventually like to generate all of their own electricity and disconnect from the grid. If they are meeting all of their needs, aren't they generating more than they need at times? If they stay attached to the grid, this extra would mean the utility would need to use less nuclear and coal power and they'd pollute less. Also the grid would then be available as an emergency backup generator, and it pollutes less than any small generator. David Darnell • [email protected]

Hi David, All that seems true, but we made an error in our terminology within that article that might have misled you, and we did not catch it before print. The Good Life center is not "grid-tied" in that they do not do sell back electricity, and the particular inverter they use is not capable of it. It only uses the grid as backup when the batteries get low. For true grid-tie, they would need a battery-based inverter that specifically does intertie, and those inverters are available at a somewhat higher price.

There are some costs associated with staying tied to the utility in the form of a monthly minimum bill. Going off-grid would eliminate those costs. Also, there is personal preference. It is important to some folks to be independent of the utility. You're correct that with their system they are wasting energy once the batteries are full, and sending the excess energy out to the grid would be a more efficient use of those solar-electric panels. Michael Welch • [email protected]

Rainwater Collection

Michael Durland, I enjoyed your article in Home Power. It is exciting to see others harvesting rainwater! Every drop of water we use in our home comes from harvested rainwater. I catch water from my metal roof in gutters protected by leaf guards. Half of my roof water is directed to one downspout and the other half flows to another. From there the water flows into a roof washer, which I made simply from a length of 4-inch PVC for each downspout, with a 1-inch ball valve on the bottom and a sweep "T" on the top. After the washer is full, the water flows directly into a series of polyethylene tanks.

Each tank is 1,800 gallons in capacity and there are three tanks. One roof washer feeds one tank, the other roof washer feeds the other, and I pump water from the third with a Dankoff booster pump, after it has passed through a 10-micron filter. After entering a pressure tank, that is the water that is used for the low-flush toilet, shower, clothes washer, and washing dishes. I have water filters installed at the kitchen sink and at the bathroom. They filter sediment, bacteria, and cysts, and the one in the kitchen additionally filters for chemicals.

Since February of this year, when I began metering the flow of water usage in our house, my family of four (two adults and two children) has averaged 48 gallons of water a day. That is only 12 gallons a person for flushing into our composting toilet, washing clothes and dishes, cooking, and bathing! Since June of this year, our household has used 3,562 gallons of water while harvesting 17,943 gallons from our roof!

Besides writing you to share my positive experience using rainwater, I am writing you to ask some questions. For some time I have desired to have my water tested, to ensure that it is safe to use and drink. You agree, as you state in your article, "To be safe, always test your harvested rainwater before using it in your home or garden." Where do you recommend getting such testing? So far, all I have been able to find is some very inexpensive and yet not very informative testing, or testing services that let you specify for each item you want tested. There is a plethora, and if you were to choose each one, it would total in the thousands of dollars.

What is your opinion on the filtration system I am using? Should I add UV? How much would that cost, and where could I get it? Rainwater is, as you say, distilled. But is it not possible for it to pick up pollutants as it falls? And does rain not form as a result of dust particles in the air, which could consist of pollutants? Thanks, Shawn Swartz, Earthaven Ecovillage • [email protected]

Shawn, It sounds like you are doing a good job of catching, filtering, and using rainwater. Rainwater does collect pollutants as it falls, and rain will wash pollutants and bacteria off the roof. The county regulations where I live are very strict about using untreated rainwater for household use. If you are just using filters (not recommended), you should filter down to 0.5 micron because this is what it takes to remove cysts. Filtering alone will not remove bacteria. Water can be treated with chlorine, ozone, or ultraviolet (UV) light . If treating with an ultraviolet light, a 5-micron filter can be used. I use two large sediment filters down to 1 micron and then a 5-micron carbon filter.

I would definitely recommend using an ultraviolet light to remove all the bacteria from the water. There are several UV units on the market. I use only NSF (National Sanitation Foundation) approved units with light transmittance sensors and automatic water shutoffs. This will ensure that only treated water flows to the house in case the power fails, the lights go out, or the light transmittance is too poor for the UV light to do its job. The NSF approved units usually come with a manual bypass valve, so in an emergency situation untreated water can still be allowed to flow. The units without a sensor and automatic water shut-off will continue to pass water through even though the water is not being treated properly. I have used several different UV units and am currently using the UV Pure unit from Hallet. It is a well-engineered product. One of the things I like about it is that the two lightbulbs are outside the water stream, so changing bulbs is a snap. The list price for this UV system is almost US$2,000.

Testing your water is a valid concern. I use a local laboratory that will do basic bacteria testing for US$18 and a full inorganic test for US$290. At the very least, you should get a bacteria test done on the water. This will test for the presence of coliform bacteria, including E. coli. The time your water is in transit to the lab is a concern, so find a lab close by. If you are using a galvanized roof, you could test for zinc and if using asphalt shingles you should test for petroleum products. I hope I have answered your questions. Feel free to e-mail me with any other concerns or questions. Michael Durland, PurRain Watertanks, Deer Harbor, Washington • [email protected]


Hello all, I read with interest the article titled "Be Cool" in HP108 and have a couple of comments. We built our house using some of the ideas for keeping a cool space in summer heat, and implementation of the ideas we missed will not come soon (new roofing materials for example, or new windows). In the meantime, our summers in northern lower Michigan have grown generally warmer over the past several years. We are using an idea from our experience in southern heat, mentioned but not emphasized in the article. It claims that closing the house up after a cool night works in dry climates with a 30-degree difference between day and night temperatures. Our place a mile away from Lake Michigan assures us of high humidity, and temperature swings are often far less than optimal. But we learned in the humid South that it still helps to close up the house in the day, especially if, as we usually experience here, the hot spells are only a few days long. In the morning, as soon as we notice that the outside has gotten warmer than the house, we close it up tight. It feels like coming into an air-conditioned space when we come back in at noon.

We also have added a DC vent fan high in the ceiling (venting to the outside), which is less effective than hoped (it is on a timer, so we can run it for several hours in the late evening) and a muffin-type equipment-cooling fan (also 12 VDC), which is surprisingly effective. We set it to blow right onto us at night, and it is quiet, "white" sound with a tiny energy draw. You can find these at Radio Shack, but lots cheaper at a distributor like Hosfelt Electronics ( As always, a great issue—thanks for consistently good articles! Jim Sluyter • [email protected]

Code Frustration

Hi HP, I have been a subscriber for the last seven years. I installed 1 KW of PV on the roof about five years ago and have been running guerilla since then. I also have two roof-mounted solar hot water panels, which have been in service for the last two years. I am planning to permit the existing system at the same time as the new equipment.

The new installation will be two trackers each with 3 KW of PV for 7 KW total. So following the manufacturers' instructions and numerous Home Power articles, I designed the electrical system taking into consideration ampacity, conduit fill, and required derating. I included the utility-provided second meter, as well as AC and DC disconnects. This is where the fun stopped due to unreasonable and expensive building department requirements for engineering stamps for the foundation design, soil study, and electrical system.

The building official explained that adding another outlet to an existing structure is "prescribed" by the 2002 NEC and doesn't require an electrical engineer stamp. So my question for you is, does the NEC "prescribe" adding a utility-intertie inverter, second solar utility meter, and all the other required safety stuff? If so, where? Or is it customary to jump through all these hoops?

As to foundation design for the trackers, Wattsun runs a spreadsheet when provided with the "lateral soil bearing pressure" from a boring. I have ordered a soil boring for the determination of the soil bearing pressure at US$600 cost. The city then wants an Arizona engineer's stamp for Wattsun's foundation design before it will approve a permit. For the existing equipment, a structural engineer must review and sign off on the garage roof truss calculations to certify that they will bear the weight of the eight 120 W PV panels and two hot water panels. Do you have any recommendation on how I could save the thousands of dollars in studies and engineering reports, and get this RE system producing? Name Withheld

Hi, I can tell you the engineering story on our Wattsun tracker installation. Wattsun recommends a 3-foot-diameter cylindrical hole bored 7 feet deep in their generic engineered tracker plans. In our location, there was simply no way to get the equipment needed onto our site's wet and heavy clay soils unless we wanted the truck to be a semipermanent installation in our field until the dry season. We needed to dig our foundation by shovel so we could get started on the foundation in the winter. And because we knew we would hit a hardpan about 4 feet below the ground, we needed an alternate set of plans. The first engineer we talked with wanted US$2,000. We took it to another engineer down the street who charged us US$600.

The county wanted us to have the engineering calculations cover 80 mph wind loads. All we needed to tell our engineer was that we have heavy clay soils (I'm guessing that they already have the soil bearing pressures calculated for our area), the height of the pole, the area of the array, and the weights involved, and he was able to do the calculations, complete with a drawing. We needed a 4- by 4- by 4-foot hole with rebar reinforcement on 1-foot grids and four, 1-inch J-bolts with a 30-inch embedment for bolting Wattsun's flanged pole.

The foundation for the tracker and the concrete pour were the biggest hurdles for our project. The rest took no more than two weeks (including 200+ feet of trenching) while working at a very casual pace. My advice is to shop for a quote from several engineers, and do as much of the legwork as possible by providing your engineer with data sheets and installation manuals so that he or she understands the project quickly without a lot of study time or research. Good luck. Linda Pinkham • [email protected]

Hello, You are not alone in your efforts to get a PV system installed where the permitting and inspection officials require all of the "I"s dotted and the "T"s crossed. I hear these stories on a regular basis from people in various parts of the country.

I am in the process of designing and building my retirement home and will be doing most of the construction, including plumbing, HVAC, and electrical, myself. As is my current home, it will more than likely be off grid, with a possible grid backup rather than a generator. Even here in the wild and wooly Southwest, I have to get engineers' stamps, apply for permits, and get inspections. Being an electrical engineer doesn't carry any weight with the local inspectors or the local regulations.

How do I work with the local inspectors? I treat them like people who know what they are doing and who are only trying to comply with various national and local codes and regulations established by others. I find that in many areas, they know far more than I do. If I do my homework and study the codes and handbooks that they suggest, they are willing to spend hours answering my beginner questions.

So, I have plumbing codes and handbooks, residential building codes and handbooks, fuel gas codes and handbooks, HVAC software, the NEC Handbook, all New Mexico codes, plus numerous books on these subjects written by the pros in each field.

The inspectors and I have had some very good discussions. One of their most frequent complaints is about plumbers and electricians who finish a project before taking out a permit and getting the necessary progress inspections. I would not be surprised if your guerilla PV system might be viewed the same way by your inspectors.

Over the years, many, many people have been killed by falling roofs and nearly all building codes for residential construction now require that roofing trusses be designed by a firm that will have a professional engineer stamp the plans. Of course, the local truss builder uses truss design software provided by the company who has the professional engineer who stamps the final plans. Since there are few "listed" residential roofing systems, this is the only way to ensure public safety.

A PV module and mounting rack may add up to 4 pounds per square foot to the dead load of a roof. This can represent a 40 percent increase in the design load for trusses in the Southwest— not an insignificant amount. A 40-gallon integrated-storage hot water collector can add nearly 500 pounds over a 32-square-foot area or less for a 15-pound-per-square-foot increase in the loading. Either of these could result in structural failure of an existing roof, especially during heavy seasonal rains and winds. I think it would be in your best interests to get an engineering assessment of the roofing trusses supporting your systems. The inspectors have no way of knowing that you may be using fly-weight PV modules and racks, and low-volume solar hot water collectors.

To get some inspector understanding on the electrical PV system, you might work with a local electrician. You could also download all of the "Perspectives on PV" articles that I have written for the International Association of Electrical Inspectors News from the Web site ( and give them to the local inspectors. These articles will bring the inspectors up to speed on PV systems and the code requirements for such systems. You can find the new Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices manual at

A neat, detailed diagram showing all of your system, plus the code calculations and how your system meets the NEC Article 690 PV requirements should be of help in showing the inspectors that yes, PV systems are included in the NEC, just like receptacle outlets.

Many PV installers around the country routinely supply detailed diagrams and calculations to the inspectors before they start any work. In more than a few cases, the inspectors welcome the information, do some studying themselves, and make positive recommendations. Good luck. John Wiles • [email protected]

Heat Pumps & Cooling

I live in Texas and plan to build a home in the next several years. Although your magazine has helped me with planning how to orient the house, and the types of windows and insulation to use, I still have not found an alternative for cooling in the hot Texas summers. Our summers are far too hot and humid to use evaporative cooling. I looked into solar heat absorption air conditioning units only to find them to be far too large for a single-family home. Geothermal heat pumps seem like the best option so far, but still need substantial power to run the compressor, pump, and fan motor. Am I missing anything? Christopher Deines • [email protected]

Hi Christopher, You aren't missing anything. Ground-coupled (aka geothermal) heat pumps are the most efficient way to cool (and heat) your home using electricity. If you have a grid-tied PV system, then most (if not all) of the electricity consumption can be offset by sunshine.

Having said this, please pay very special attention to the thermal details of the home. Adding extra insulation (use at least R-40 in the walls and R-60 in the roof) really pays off. Sheath the home (outside of the studs and their R-16 insulation) with 2 inches of rigid foam (4- by 8-foot sheets). We've done this and it works very well. Pay special attention to windows, doors, and any place where air infiltration is present. Good luck and stay cool. Richard Perez • [email protected]

Change the Status Quo

I've just discovered Home Power, and am really impressed by all the information available in it. It truly is a gold mine of RE information. Although I can understand the magazine, I don't really see myself putting a plan into action anytime soon. Unfortunately, my background is not at all electrical or technical (I'm a cellist in the opera orchestra) and I have big RE pipe dreams.

I live in a rather large condo complex in West Palm Beach, Florida. Our parking garage is about the size of two football fields and I'm thinking it is perfect for a grid-tied PV system on top. I've gotten some information from Shell Solar in Boca Raton, but really don't know how to approach such a project. Do you know anyone that would be willing to help me come up with a rough plan for my condo owners' board?

After looking through DSIRE (Database of State Incentives for Renewable Energy;, I get the feeling that the RE movement has a much better foothold in the Northwest and I would like to help change that. Sincerely, Benjamin Salsbury, West Palm Beach, Florida • [email protected]

Hi Benjamin, It seems like you have a great site, and your idea is a good one. I've seen parking structures used for PV systems before, like the solar carport installed by Sacred Power at the Indian Pueblo Cultural Center in Albuquerque, New Mexico. I hope someone in your area sees your letter and helps you convince the condo owners' board that this will be a worthwhile and cost-effective project. Linda Pinkham • [email protected]

Overcurrent Protection

Dear Editor, Batteries are still dangerous. With more and more utility-interactive PV systems being installed, I see a dangerous trend evolving where stand-alone or utility-interactive systems with batteries have been installed.

The energy storage batteries in PV systems can deliver far higher current than almost any wire can withstand. If there is no appropriately rated overcurrent protection in a conductor between the battery and some device (charge controller, load, etc.), that conductor may be destroyed (and possibly start a fire) if it develops a short circuit. Short circuits happen when terminals get loose and wires contact nearby wires, or when insulation fails due to mishandling or improper installation. Please note that the rating of the overcurrent device must be consistent with the conductor ampacity. A 250-amp circuit breaker will not protect a #6 conductor. Devices connected to batteries should also have a means of disconnect, not only for installation and routine maintenance, but also to allow them to be quickly disconnected should an internal failure occur.

Please install safe RE systems with appropriately rated and located overcurrent devices on those circuits connected to batteries. The life and home you save may be yours or someone else's. John Wiles • [email protected]

New to the Crew

Home Power would like to welcome Kim Bowker and Jacie Gray to the crew.

Kim is working with Connie Said in advertising sales and program development. She graduated from the Jordan Energy Institute in 1996 with a degree in Applied Environmental Technology. After graduation, she worked with Mick Sagrillo at Lake Michigan Wind and Sun doing everything from system design and sales to welding tower kits. More recently, Kim was operations manager with Conergy in Santa Fe, New Mexico, after working her way up through a variety of positions there.

Jacie Gray is the latest and greatest addition to our customer service department. Jacie lives in the mountains of southern Oregon in a straw bale home she and her son built from the ground up. She is concerned about saving our remaining natural resources, and it is important to her to work for what she believes in, so working on the HP crew is a good fit.

Welcome aboard Kim and Jacie!

Height Limitations on Top-of-Pole Mounts

Dear Home Power, Thank you for your article, "How to Install A Pole-Mounted Solar-Electric Array: Part 1" in HP108. I waited with bated breath for the issue to come in the mail because I am going to install a few such mounts in the near future and had many unanswered questions. This type of nuts-and-bolts information is often hard to come by if you are not a professional. Your article contained valuable tips that should allow me to install my own pole-mounts successfully. I still have a few questions before I install, so here goes.

In my area, a single storm can routinely dump 36 to 48 inches of snow. Snow often builds up on the ground here up to 48 inches in depth. (Our design snow load here is 100 pounds per square foot, and the peak wind speed is 80 mph, for 100 square feet of modules, on a 6-inch-diameter pole.) I feel that it would be good design, given these variables, to have the bottom of the panels at least 72 inches above the ground, so the snow that sloughs off or is removed will build up in a pile under the panels. Your pole and hole sizing for top-of-pole mounts chart states that 84 inches is the tallest a 6-inch-diameter pole can be above ground level. In fact, some major mount manufacturers recommend not exceeding 72 inches in height. Since the panels would hang about 40 to 48 inches below the top of the pole, that would leave their height at a mere 24 to 36 inches above the height of the ground in winter—they would be buried in snow. I have seen many photovoltaic panels pole-mounted 120 to 144 inches above ground level on 6-inch poles, and cannot figure out how it is done. What causes 72 to 84 inches to be the height limit? Is it the thickness of pipe or the weight of concrete? What strategies could I use to get that needed height? Thanks for a great publication. Lizah Eszterhas, Trinity Center, California • [email protected]

Hi Lizah, The height limit imposed on pole-mounted PV arrays is a function of the strength of the materials involved, the size and tilt angle of the array, and the force exerted by wind on the array. The basic idea is that as the pole gets taller, it will sway more in the wind. When this oscillation exceeds a certain amount, the pole will begin to fatigue and eventually fail, usually by breaking at the ground level. The size of the array figures in because the more square footage of modules you have, the more force will be exerted by the wind, causing the pole to begin swaying, and the more weight you have at the end of the pole, the larger the oscillations are going to be. Also, the closer to vertical the tilt angle is, the more the array will act like a sail to catch the wind.

Going with a larger diameter pipe or a pipe with a thicker wall increases the amount of swaying that can be tolerated by the pipe. Reducing the array size, thereby decreasing the wind loading and the weight at the end of the pole, will decrease how much the pipe is affected by the wind, as will flattening the tilt angle of the array.

Those are the general facts. With the information you provided and a few assumptions I made, I did some calculations using our in-house pole-top calculator. Based on the physical characteristics of a 6-inch-diameter schedule 40 steel pipe, I determined that you could have a pole height of 120 inches with 100 square feet of modules mounted on a UniRac PoleTop rack in an 80 mph wind zone, as long as the tilt angle doesn't exceed 45 degrees.

The 120-inch pole height should put the bottom edge of the array close to 72 inches off the ground, depending on what size PV module you're using. The pipe will need to be embedded 7 feet in the ground in a hole 32 inches in diameter filled to ground level with concrete.

So far, so good, but I see a big problem with that 100 pounds per square foot of snow loading. A 6-inch schedule 40 pipe at 120-inch height will be close to its maximum loading capability with just the wind pressure alone on the array. Tilting the array more towards vertical will dump the snow better, but if the wind picks up, it'll knock that big array right over. With that much snow loading, I don't see this scenario working very well. So my advice is to consider putting up two top-of-pole mounts with half the square footage on each. Reducing the array size to 60 square feet would allow you to tilt the rack to 60 degrees, which would dump the snow much better and help maximize your winter solar energy harvest too. Steve Fain, UniRac Inc. • [email protected]

Rainfall Energy

Has anyone researched or designed a system to capture rainfall energy in a residential setting? I mean the water from gutters, flow from roofs, and other capture devices. Thank you. Juanita Carlson • [email protected]

Hi Juanita, Collecting rainwater from your roof for domestic or irrigation use is a great idea, but the energy available from water flowing off a roof is very small indeed. My late colleague Don Kulha and I once joked that you might be able to make an LED flow sensor powered by a turbine in a downspout, but that would be all you could power.

Let's see if our joke was right...A 1,000-square-foot roof will collect about 600 gallons of rainwater per inch of rain. Let's say it rains 1/2 inch in a 10-hour period. That's 300 gallons total or 30 gallons per hour or 1/2 gallon per minute. Let's say the height from your gutters to your turbine is 10 feet. We can use a simple formula of head (height) times flow divided by 13 to guesstimate that you would get an output of 0.38 watts—3.8 watt-hours for the ten hours. This is a very small amount of energy. A tiny, 1-watt solar-electric module will do this every day in most locations.

The lesson is to let large areas of land collect rainwater, and then find places with lots of head to run a pipe down, so you can generate meaningful amounts of energy. Trying to make energy for your house using the rainfall and head on your roof is like trying to make energy for your house with the tiny PV module in your calculator. Best, Ian Woofenden • [email protected] ¿nt

A Clean and Quiet Revolution

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