Power, Fuel & Energy

     Rig Revealed     Part I    Part III

 Part II - Solar Power, You can go green but it's expensive!

Introduction - 12/23/2009

      In this part we'll examine the details of our solar system design, how it went together, its operation, cost and the electricity it produces. It's a technical presentation and chronicles one specific solar system's installation from start to finish and on into operation. If you are considering a similar project perhaps the information presented here will be of assistance. Solar systems are setup to be either grid tie or not. In grid tie systems solar electricity is blended with AC power that normally comes over a utility's power grid. (A special "grid tie" inverter is required to interface solar electric to the grid.) Solar electric power then offsets what is purchased from the utilities. If you collect more solar power than you use, the excess power is fed back into the grid for others to use. Power companies may pay you for this excess power. Non grid tie systems are isolated from any power grid and can operate independently. Solar power is collected and what isn't used is stored in a battery bank for consumption during times when the sun isn't available or obscured. Grid tie or not refers to how power is disposed of and doesn't influence basic PV (short for photovoltaic) array collection methods. The goal being always to collect as much solar power as is possible. Our solar system is a non grid tie system. In spite of the warm and fuzzy feeling we get from collecting "free" power there are limitations, drawbacks and cost implications. The power might be free but the equipment needed to collect, manage and store it costs money. We'll take a look at this later in the presentation. Yes you too can go green and save the planet but it's expensive.

      Harnessing the sun's power is anything but new and many schemes exist to convert solar radiation into something useful. Photosynthesis is perhaps the oldest. The most common man made by products of solar conversion schemes are heat and electricity. The sun is a star at the center of our solar system and it produces its energy by a process known as nuclear fusion where hydrogen is converted to helium. The conversion releases light and heat. Scientists estimate the sun converts 4 million tonnes of matter every second. At 93 million miles away the sun's light takes about 8 minutes to reach Earth. Light and heat are the two most obvious components of solar radiation and are also Earth's primary energy source. In its natural form, this energy is clean and renewable. If manufactured goods are required to process or convert solar radiation, environmental impacts exist. Regardless of what you are told and the current trends in propaganda harvesting any form of energy has environmental impacts. Without any scientific equipment we readily observe the sun's energy as heating on our skin and visible light through our eyes. The sun's energy is slightly attenuated by Earth's atmosphere. Measurements taken at the equator on a clear day at solar noon when a vertical stick casts no shadow reveals the sun depositing 1000 watts per square meter. All other locations on Earth will receive somewhat less than this. Time of day, geographic location, season, cloud cover and atmospheric conditions all influence how much solar radiation is available. Insolation tables exist to document the amount of solar energy falling on a given area. These data are tabulated over many years to form a long term average. Your specific area on a given day may deviate from insolation data because the sky is always changing. Converting the sun's energy into electricity is accomplished via the photovoltaic (PV for short) cell which was developed at Bell Labs in 1954. PV cells or panels produce electricity when exposed to light. Many such cells can be grafted onto a substrate to make panels of larger size that deliver more power. Panels themselves can be connected together into banks to increase solar collection. Panels are connected in parallel, series or a combination of both to establish a nominal working voltage close to that of the final load. Commercial grade panels last between 25 and 30 years and are maintenance free except for periodic cleaning.

     The BP 170SX panels we've used measure approximately 62" long by 31" wide for a total of 1,922 square inches per panel. This converts to 1.24 square meters (39.37" to the meter). If the panels were 100% efficient they would collect 1,240 watts each under ideal conditions. The manufacturer rates the output at 170 watts when placed in sun radiating 800 watts per square meter. This means only 21.25% of the available power is being collected or 78.75% is wasted. I think the panel efficiency is much less (like around 9-12%) than what the manufacturer claims. Independent laboratory analysis would be required to establish true power collection. There is also a derating factor that needs to be applied to solar panels operating in hot climates. This runs around a 1-2% loss on hot days and means hot panels produce less electricity than cold panels. That said, in 2008 this was the best PV technology could deliver and if you needed to install a solar system you ran with it. Perhaps more efficient panels are in the future but I'd say it isn't going to be the near future.

The Bus-Stead Solar System

     In anticipation of holding up in remote areas that lack electricity, we decided to max out the roof area with solar panels to collect as much solar power as possible. We were able to fit 8 170 watt panels on the roof and still maintain acceptable walking space to allow access to vents, pipes, A/C units, satellite dome and other electronics. I made custom brackets to allow the panels to be safely mounted to the fiberglass roof. Using a unique surface mount approach allowed complete flexibility in positioning the panels. The brackets also allow each panel to be tipped up on any of their 4 sides. Two axis motor drive tracking is the best way to position solar panels for maximum collection. On the bus this just isn't practical. Simply mounting the panels flat is the easiest way to install them and it is also required for traveling. However, fixed flat panel mounts are the least efficient position to collect solar energy. Fixed inclined panel mounts are better and should aim panels due south. Tip them up from the horizontal by a nominal angle equal to the area's lattitude. Seasonal adjustments can be made to improve collection by subtracting 15 degrees in summer and adding 15 degrees in winter to the nominal angle.

     The Bus-Stead system allows tipping the panels up by a "do everything" angle. This single angle approach was done in the interest of keeping the design, setup and breakdown simple. Even though the tipping angle isn't perfect it significantly increases collection by almost 100% over flat panels. Our panels have been fitted with a front edge air foil to help reduce wind resistance, noise, mounting stress and vibrations. In addition to 6 mounting points, each panel has a safety tether to make sure it can't fly off the roof of the bus should the primary mounts fail. During a day's worth of travel, panels can be subjected to 70+ MPH winds for extended periods of time. A common ground wire ties all the metal panel frames back to the chassis. This acts to ground out static electricity and helps prevent damaging voltage spikes during an electrical storm. The complete roof top solar array is organized into banks A and B each of which contain 4 panels connected in parallel. Special U/V resistant cabling is used with water tight connections to bring each of the 8 panel outputs back to a combiner box (not shown at right). The combiner box is a waterproof containment that houses two 8 pole terminal blocks that receive all 16 wires coming from the A and B banks. Wires enter the combiner box through water tight bulkhead connections. Bus bar combs strap 4 panels together to form two parallel connected banks. The system ground wire passes through to the chassis. Each bank has its own in line fuse to protect it from overload due to a possible short circuit in the system. Open circuit voltages run around 43VDC in the combiner box. As the array is loaded this voltage will drop to a nominal 24VDC. From the combiner box two main feed trunks connect the A and B banks to their own solar controllers. The solar controllers then manage the solar electric, panel sweeping and charging of the battery banks.

 

Roof Mounted Components
Solar Panels 8 BP 170SX 170 Watt solar panels. VOC = 43 VDC
Mounting Brackets 48 hinged mounting brackets providing 6 mounting points per panel.
Tether Brackets 8 safety tether brackets one per panel. Prevents panels from leaving roof.
MC Cables 8 sets of U/V resistant cables with water tight connectors.
Ground Wire System ground wire daisy chaining through all 8 panel frames.
Elevation Poles Up to 16 poles (2 per panel) to elevate selected panels for improved collection.
Combiner Box Watertight NEMA 4 enclosure where exposed connections exist.
Trunk Lines Two sets of trunk lines that direct power to solar contollers. These lines run in conduit where exposed to weather.
Air Foils 6 polycarbonate sheets mounted to leading edges of panels requiring smooth wind profiles. Two panels didn't need this.
Radial Supports 32 plastic clips to reduce shear stress on hinges/brackets and provide additional support.
Restraints Misc tie wraps and wire sheathing to hold wires in place and organize cable runs.

 

Roof Mounted Solar System Components
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Managing Solar Electricity

     A full treatment of this complex subject is beyond the scope of this presentation. A more practical delivery here is to present the inner works of the Bus-Stead solar system and how its components manage the solar power collected by the roof top array. The previous section described the physical solar panels themselves, mountings, wiring and adjustments. At this point we have a means of roof top collection and electricity coming in over the trunk lines. The challenge is to manage this power in the most efficient way. The first step is to position the solar panels to keep incident light angles at 90 degrees or as close as possible. Computerized 2 axis solar tracking is the only way to do this by constantly moving the panels to follow the sun's movement throughout the day. This just isn't practical on the roof of the bus. Fixed inclined panels are the only practical alternative and this approach is used with the understanding it wastes power. The second step is to load the panels properly for maximum power transfer to the load. This is handled quite well by the better solar controllers.

     In a DC circuit power measured in watts is calculated by the product of voltage and current. For example, a 40VDC source providing 10 amps is delivering 400 watts of power (10 x 40 = 400). Let's examine a solar panel with an open circuit voltage of 40VDC and a short circuit current of 10 amps. Keep in mind that power requires both voltage and current to be present. If either quantity is zero then no power is produced and no work gets done. Open circuit means no current is flowing so 40VDC at 0 amps means 0 watts. Short circuit means maximum current is flowing but no voltage is present so 10 amps at 0 volts means 0 watts. These are the extreme "powerless" ends of a panel's performance and provides helpful design parameters for the maximum voltage and current that can be expected. Working backwards from the open circuit voltage a panel's actual terminal voltage will begin to drop as current is drawn. The amount of current flowing is inversely proportional to the resistance of the load up to the maximum amperage the panel is capable of at a dead short. As this resistance moves towards 0 ohms (a unit of resistance) voltage will also approach 0. In between these two extremes the panel will be providing both voltage and current and thus be delivering power. An infinite combination of voltages and currents are available along the way. What's important is that only a few of them produce maximum power transfer and these "gravy" values are the ones we want. And these maximum power points (best combinations of voltage and current values) change with solar radiation and temperature. Keeping a panel near its maximum power transfer is important. As a rough guideline, PV panels usually transfer maximum power when loaded to 70% of their open circuit voltage. For our example 40VDC open circuit panel would be 28VDC. For a specific panel, on a given day, at a certain time and temperature this maximum power point needs to be empirically determined. Once that power point is determined it may only be good for a short time. This situation is handled by the solar controller.

     With the advent of the power MOSFET and imbedded processor applications many complex power handling problems can now be handled in unique ways. Good quality solar controllers use both power MOSFETs and imbedded microprocessors to manage maximum power transfer from solar arrays to load. Solar controllers also handle charging algorithms for battery banks, keep track of power and log useful data along the way. One of the biggest problems with solar panels, aside from determining the best power point, is their nominal operating voltage doesn't match the needs for direct charging of battery banks. Normal power fluxuations can be expected because the sun's intensity varies throughout the day. As previously mentioned, the Bus-Stead system includes a battery bank that must be charged. 12 volt AGM battery banks require upwards of 14.4 volts to begin the absorption charging phase. So the most basic function a solar controller performs is DC to DC conversion where power coming in at one voltage and is stepped down to another more useful voltage. The challenge is to accomplish this with minimal power loss. Good solar controllers do this converison at about 85% efficiency or better. It's a good idea to get the DC solar array voltage about twice what the load requires. So for 12 volt battery banks, 24-28 volts would be good. This helps the solar controller do its job with better efficiency. Buck and boost controllers use a transformer to assist with the DC voltage step down. Transformers are very efficient and can deliver better than 98% power transfer but work only with AC power. DC coming in from the solar array is converted to AC and processed through a transformer then rectified back into DC at a lower voltage. Power is lost in the conversion process from DC to AC on the way in to the transformer and back from AC to DC on the way out. Hence forth the overall efficiency is somewhat lower than 98%. The solar controller also can present itself as a varying load to the solar array. It measures the open circuit voltage of the solar array it is connected to and sweeps it looking for just the right combination of voltage and current that transfers maximum power. Once found it locks on this and begins passing the converted DC on to the load. Sweeping can occur every few minutes or every few hours based on a parameter you set inside the controller. In this way the controller tunes itself throughout the day to the changing conditions outside and endeavors to always be at or close to the maximum power transfer point. While doing that on the solar array side the controller also monitors the battery bank and applies the appropriate charging levels. This prevents damaging over charging currents from being applied. If batteries are floating, the charge controller becomes demand driven and adds power into the system as loads come online. Data logging is also going on and a history is built up of daily power collection for recall later. Some controllers will store up to 128 days worth of data to include daily power, daily peaks, sunrise, sunset, etc. Owing to the size of the Bus-Stead array two solar controllers are employed to handle peak collection periods. These occur in the summer months. A solar controller is limited to the amount of power coming into it. On heavily overcast days solar power is a mere fraction of that available on clear days. If power requirements exceed what's coming in on the solar array it needs to come from another place like your battery bank, generator or utility company. If alternate power sources aren't available you sit in the dark. Under ideal conditions you collect enough solar electricity to keep your batteries charged and provide the extra that's needed to run your house loads. At night your battery bank needs to be large enough (and fully charged by sunset) to cover house loads until the next morning. If direct sunlight isn't available you better have another source of power at hand. On the bus electricity is needed to run the control boards in the refrigerator, safety systems, lights, etc. Going without power isn't an option.

     Power MOSFETs and microprocessors have greatly improved inverters too. If you need to convert the DC coming in from your solar array to standard 120VAC then you'll need an inverter. On the bus we have a 2800 watt pure sine wave unit with crystal controlled frequency. That means we can run any AC device that requires an accurate 60 Hertz sine wave. The inverter also functions as a battery charger. When solar power is low the charger circuit in the inverter can float the house batteries and provide for all the DC house loads. A depleted battery bank can be completely recharged and there's enough power to spare to run all the DC loads while charging is in progress. A remote information and control panel exists to monitor all these functions and make adjustments from inside the living space. The inverter needs 120VAC to run its charger. That comes from shore power or the generator.

     One of the challenges with non grid tie solar systems is storing what you don't use. In grid tie systems you simply feed extra power back into the grid for someone else to use. With a stand alone system (non grid tie) like what we have in the bus, any power that isn't used is gone forever. It's pointless to go to the trouble of setting up a robust solar array that generates electricity that you can't use or store. The dilemma of solar power is it's there when you might not need all of it and not there at night when you do need it. The solution is to store what isn't consumed for use at night or on a rainy "low sun" day. The most common mechanism for storing solar electricity is a battery bank comprised of flooded lead acid or AGM batteries. These banks are expensive, heavy, bulky and have limited service life. Also you don't get out of a battery bank the same number of watts you put in to charge it. For example, if you charge with 100 watts you can expect to draw between 38-58 watts back out of the fully charged battery bank. Batteries also self discharge without any load being applied. This is a wasteful process and energy is lost as heat in both the battery and charging circuit. It's also why you'll never see electric cars make sense. But if you need electricity in remote areas away from the power grid it's most likely going to come from PV panels, batteries and/or a generator.

     Late fall, winter and early spring are the leanest solar periods and you may find it a challenge to stay ahead of your power needs. Long winter nights and cold temperatures reduce solar collection and battery storage capacity. Because batteries produce electricity by a chemical reaction, their storage capacity needs to be derated for cold climates. Chemical reactions occur at slower rates or with reduced intensities in cold weather so charging parameters need to be adjusted accordingly. Terminal voltages are lower in the cold which means less power output from batteries. Batteries can freeze which quickly sends them to the recycle pile. Don't leave them in freezing weather uncharged. As batteries go, lead acid batteries have the best overall charge efficiency compared to other chemistries. The Bus-Stead uses AGM chemistries which are very durable against overcharging and over depleting. AGMs charge faster and bounce back quicker from abuse than flooded lead acid batteries do. AGMs don't require constant monitoring of the electrolyte which is a huge advantage if the batteries aren't easily accessible as often times is the case when they are installed in a battery bay. As a rule of thumb battery banks should be sized at 3X or 4X your amp hour requirements. Space may not exist for a battery bank this large, however. The Bus-Stead battery bank consists of 4 8D batteries limited by the available space in the battery bay. In winter this bank will discharge in about 10-12 hours and it can't be recharged fully if the skies aren't clear.