A Stand-Alone PV System Molly Hoffman I suspect that you have been overwhelmed by responses from renewable energy (RE) people. Suddenly, there is a publication that speaks directly to those of us who are using RE and to those who have been dreaming of the possibilities. It is exciting to see people responding with helpful information from their own experience. Your articles on system components have been very good down-to-earth stuff: information we can really use. Having been inspired by seeing other peoples' responses, I decided to contribute our experiences. In the way of a brief personal introduction: Ken, my husband, and I have lived in northeast Minnesota for the past eight years. Ken is a civil engineer, but has worked as a land surveyor most of his professional life. He is registered in MN, he is legally a professional surveyor. That is how we earn our living. We are a company of two people, what you might call a very small business. Our house, therefore, is our office. We needed electricity to conduct our business, to power calculating equipment, to recharge an electronic distance meter's battery pack and lighting adequate for drafting work (while I prepare final survey drawings). The system we have set-up has been the perfect answer for us. We have always been conservative in our use of any energy. Our system is compatible with our desire to be less consumptive of the world's energy. Nuff said, so at least you know a little of who we are. System Site Our homesite is located in northeastern Minnesota, 30 miles from Lake Superior, in what is known as lake country, the best known portion of which is the Boundary Waters Canoe Wilderness Area (BWCWA). The altitude, in a state without mountain ranges, is fairly high at 1,900 feet. The forest is boreal and typical of the rather cold climate. Snow arrives permanently in November (sometimes earlier), accumulates from 2 1/2 to 3 feet and melts in March and April (a late snow storm may occur in May). Winters tend to be cloudy and it always seems there is a flake of snow in the air. These climatic conditions have influenced greatly the type of system we have set up. Our system is not typical in many respects. It reflects our personal choices in the way we live. Photovoltaic systems are inherently flexible and seem easy to bend to the character and requirements of their owner. In the fall and winter of 1986-87 we built a small house (16' x 24' with a 6' x 8' entry). We decided from the first nail pounded that we did not want or need utility line power. Our need for electricity was small. We were in a break-even situation in comparison to the cost of bringing in commercial power versus the cost of our PV system. We decided that we would prefer to take responsibility for producing our own power and adapt, however it is necessary, to be comfortable with this option. We have a 12 VDC system, batteries charged with photovoltaic panels. We decided to run our system without a generator. It was strictly a decision based on our personal preference and not what is usually recommended by most conventional wisdom and experience. We want to create power without the maintenance, noise and fuel dependence needed for a generator. Our Present Energy Demands We meet our heating and cooking needs with wood and LP gas. We built our house with hand tools and therefore do not own power tools. We have not had a TV for the past eight years, our hardwood floors and shakeableÐsized rugs do not need a vacuum. All our curtains, chair pads, quilts, bedspreads and some clothing have been sewn on an old, but serviceable, treadle sewing machine. We have no electric well pump, but rather a water storage system inside our house. We have good well equipped with a freeze-proof hand pump and a comfortable outhouse. Since this has been our mode of life for six of the past eight years it has posed no adjustment problems. We use 12 VDC electricity for refrigeration, lighting, radio (modified to 12 VDC), and power our inverter for 120 vac production. The 300 Watt inverter supplies a programmable calculator & printer, recharges survey instrument battery packs and supplies appliances such as a shaver and toothbrush. By building a set of cabinets on an inside wall of our unheated entry, we are able to turn off our refrigerator during the coldest, darkest winter months and use the cabinets as a passive refrigerator. When the outside temperature occasionally dips below -35¡F., we will get partially frozen milk on the lower shelves but for the most part it has been an easy arrangement to manage. Following are graphs detailing maximum daily power use: INSERT GRAPH Power Sources- Photovoltaics We use two 66 Watt Solec and two 48 Watt Kyocera PV panels mounted on aluminum angle frames with 3 adjustment angles for spring/fall, summer and winter. The frames are grounded with 6 gauge copper wire to 8 ft. ground rods driven 7 feet into the ground. The panels are mounted at the roof peak and therefore, even in the flattened summer position, are never close to the hot surface of the roof and have good air flow for cooling. All four panels regularly produce more than their rated capacity. Regulation and Storage The power from the PV panels is brought through a wiring center (from Steve Willey of Backwoods Solar Electric Systems, 8530-HP Rapid Lightning Creek Rd., Sandpoint, ID 83864, 208 263-4290) which provides a blocking diode and a charge regulator. The power then flows into two 6 Volt LÐ16 Trojan batteries (rated 350 Ampere-hours) wired in series to produce a 12 VDC power source. The batteries rest on a hand built dolly with heavy duty wheels and are housed in a cabinet in the house. The cabinet is vented to the outside air. It has a top access lid for a regular servicing and a removable side panel so that the batteries can be rolled out on a dolly. The cabinet is large enough to accommodate four LÐ16 batteries to allow us some future flexibility. All current carrying wires leaving the cabinet are fused for fire protection. Distribution Cables and wires to and from the battery cabinet are run in an interior house wall which has a removable panel for complete access. Power from the batteries is supplied to fused 12 VDC house circuits on the wiring center board and to our Heart 300X inverter. INSERT BLOCK DIAGRAM 12 VDC House Circuits and Appliances We have wired 12 VDC house circuits so that we have outlets and overhead lights on switches, two swag lamps modified for 12 VDC use with compact fluorescent bulbs, a radio also modified for use with 12 VDC and two small 12 VDC fixtures for reading lights by our bed. We used standard ac grounded outlets and wired them in such that accidents with ac appliances are impossible. We used switches rated for higher current than most ac switches, they are the "loud" clicking type. We used ivory colored switches, outlets and cover plates for all these 12 VDC circuits. For refrigeration we have a SUNFROST, 10 cubic foot, 12 VDC refrigerator without a freezer. It is wired on its own circuit from the wiring center. We do not operate the refrigerator during the winter months as previously noted. 120 vac Circuit & Inverter Our only 120 vac circuit consists of four grounded outlets located where 120 vac is needed. These outlets are wired in the usual ac convention. To distinguish these outlets from the 12 VDC outlets brown colored receptacles and cover plates were used. We wired grounded plugs on both ends of heavy flexible wire and this is our connection from the plug receptacle on the inverter to an outlet in our 120 vac circuit. This 120 vac circuit is energized by the small Heart inverter only when ac power is needed. The inverter could be left on continuously, but we are in the habit of switching it off when ac is not being used. Some of our 120 vac loads are too small to cause the Heart to switch from its idle mode to the operating 120 vac mode. We found it necessary to use a small night light, which when switched on is just enough to activate the Heart into 120 vac mode. The inverter is grounded with a copper wire attached to an 8 ft. ground rod driven 7 feet into the ground. Cost The total cost of the basic system was $3,027. INSERT PIE GRAPH Additional costs which complete the system are: ¥ $169 for 4 overhead 12 VDC fluorescent fixtures and bulbs, modifications to two swag lamps, modifications to radio and 2 12 VDC Osram co-pilot lamps. ¥ $191 for Refrigerator cable, house wiring, outlets, switches, cover plates, conduit, miscellaneous nuts and bolts. ¥ $33 for System instrumentation - hydrometer & multimeter. ¥ $1,553 for a SUNFROST 10 cubic foot VDC refrigerator ($1,395 + $158 shipping). Without this refrigerator our system would be very difficult to manage. It is attractive, quiet and remarkably efficient. That's it, including all the nuts and bolts. It doesn't work out well to calculate our cost per kiloWattÐhour because we are not yet fully using all the power generated by our panels. At present we have no maintenance costs and do not anticipate any in the near future. System Operation Without the obvious benefits of a generator to "even out the low spots", we opted for a system where the PVs are our greatest expense. We need to generate maximum power at all times, especially when only limited solar insolation is available. Partly cloudy days are a frequent event because of our altitude and proximity to Lake Superior. It has worked out well so far (with only one year experience to speak from) and we seldom use more than 20% of our battery capacity. We have alot of excess power generated both summer and winter and intend to use some of this power in the future. An option on our wiring center makes it possible to take off and use this excess electricity as it is available. It is possible therefore power such things as a slow pump for water, a small water heating element, a fan, etc..... Our wiring center is equipped with expanded scale analog meters to monitor battery voltage, house power use and power produced by the PV panels. We use rechargeable batteries to keep flashlights and other battery operated devices functioning. These small batteries are recharged from an option on our wiring center board. Maintenance of the system consists of changing the panel angle seasonally, occasionally washing the panels and checking the electrolyte in the batteries. We hope that with shallow cycling, the life of our batteries will be long and that the inverter will also be trouble free as it is not given hard use. Ours is not a conventional set-up, I guess. But then the whole idea of setting up a system to supply electric power demands without utility assistance is not conventional either. Because of the excess power generated and not used, our system does not figure well in the present methods of cost analysis, but then we feel it doesn't have to. It is something we could afford and has worked wonderfully well for us and that is what counts. Molly and Ken Hoffman Gunflint Trail, Box 30 Grand Marais, MN 55604 218-388-4455 Editor's Note: from this article it is obvious that the control system and wiring center is nexus of this system. For a detailed discussion of this control system see our "Things that Work!" article in this issue. RP