Marine Power System Design
The design of your system will determine the safety, power and energy output, how long it will last and how reliable it will be....... Do It Right.
General Design Parameters - Once you have decided that you would like to add power with a RES, you have some basic choices from which to choose: wind, solar, and hydro. Chosing among these three is dependent upon your location and use of the boat. While a RES is good for many circumstances, others may point to (or dictate) an effecient, properly sized generator in place of or addition to the RES. Although the decision for most is a basic financial analysis, a RES is selected in many circumstances from the reliability, low maintenance and ethical perspectives as well. In case you skipped over the Electrical Requirements page which contained download for a spreadsheet for analysing system needs and cost, here it is again. In our region (NE FL) the all-up cost of installing wind or solar on a $/amp-hr basis is pretty much a toss up. One thing that can skew that number is the mounting costs of the solar panels (some folks spend more on the mount than the entire rest of the system as in the case of an elaborate arch). Obviously if you live in a windy overcast area, wind power will likely take honors, but if you live on the gulf coast of the FL peninsula, solar usually wins out. For most cruising boats on the Eastern US coast, the combination of both seems to make the most sense, as it provides a more continuous source of power, potentially reducing the size and/or discharge level of the battery bank. Commercially, when selecting your charging sources and their control systems, it is a good idea to go with suppliers that have a proven track record and will stand behind their products. From the technical side, it is imperative that all other components and the wiring are capable of handling the absolute maximum power the source is capable of generating.
Solar Panels - Once you have decided to mount panels, you want to mount them in an area least likely to be shaded a large percentage of the time. The best place on a sailboat is usually on the very back of the boat as high up as possible or hinged on a side rail. On a power boat, the options generally improve since ther are no spars and rigging to contend with. Bimini mounts are possible but will usually require some reinforcement. The construction of the aluminum frame of panel will dictate the size and number of support brackets that will be required. Make sure the panel has been certified to the salt mist standard IEC 61701. Most solar panels are pretty rugged, but they are not indestructable, so mount them in areas least likely to be walked on or hit by something solid. Boats used to be be limited to 12V panels to match the electrical system, but with certain new MPPT controlers, higher voltage panels will work fine and are generally less expensive as they are more common. The rated output of a solar panel, is NOT its maximum capability as they are rated at standard conditions for comparative purposes. There is also a variation of +/- some percent specified. Some are rated +5% and -0%, meaning the rated power is a minimum. It is likely the output of a panel with this rating will have a slightly higher output than one of +/- 5%. To calculate the maximum power for controller and wire sizing, take the rated value and increase it by the maximum +% variation and then use the temperature coefficient to calculate the additional power that may be generated by colder ambient temperatures.
Wind Turbines - Under many conditions, the rated power of a wind turbine is nowhere near as important as its power curve and cut in speed. A turbine rated at 400W may actually generate less energy than one rated much lower that is designed to operate more efficiently at lower wind speeds. To select the best turbine for your needs, it is better to look at the average wind speed in the areas you plan to spend the most time and select a turbine that will generate the most energy for those conditions. Like a solar panel, location is important. Higher is generally better and a relatively unobstructed air flow path will prevent major losses over time. Whatever support structure you use should be mounted rigidly to resist forces in all directions. The mounts should also be damped (insulated) for vibration to minimize noise on the boat
Hydrodynamic Generators -Towable hydro systems make sense primarily only on bluewater passage making sailboats. They are pretty much stand-alone systems that require some setup before each use. For the cruiser that makes frequent long passages offshore, they have advantages over traditional wind and solar, but for occasional passages, the expense is probably not justified.
Regulation - Most charging sources on a boat (e.g. the engine alternator) have some sort of regulation, which may or not be adjustable. A RES is no different, unless its size is very small (usually <5W). Wind turbines and most solar controllers operate by shutting off the source when the battery voltage is sensed to be at some certain threshold and reconnects whan the voltage has dropped by some preset amount. The solar controller will simply "open circuit" the connection, while the wind controller will have to "short" the output, as in the AIR series turbines. To help maximize battery life, most of the better solar controllers go one step further and have a multi-stage cycle when the batteries approach full charge. All of this means that you will have sources progressively cutting out as the batteries approach the top of the charge cycle. This generally poses to real problem electrically, but may not be the most efficient use of your RES.
Charge Controllers - Type: You have a choice between Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT) for the solar controller. The MPPT, although more expensive, almost always is the better investment. It can yield significantly more power from the same panel. The maximum power acheivable from a solar panel usually occurs at a voltage well above the nominal system voltage of 12V. A MPPT controller allows the panel to operate at its optimum voltage, whereas a PWM controller locks the panel output to the battery voltage effectively derating its output. Some MPPT controllers also have a wide voltage imput range, significantly widening your selection of panel types. If shading is a big factor, a separate controller for each panel can also lower total energy costs. Locating: Ideally, controllers should be mounted in well ventilated, dry locations near the breakers for the panels or turbine. They should be reasonably accessible for service and since most have some type of display for charge status, their face should be visable.
Wind turbines generally come with their own regulation systems designed specifically for their own electrial systems. However, it is sometimes desirable to over-ride that system with a diversion controller that allows the turbine to continue to operate after the batteries are charged, harnessing that energy for other purposes.
Batteries - The most popular types to choose from for marine use are the conventional flooded, Absorbed Glass Mat and Gel. Each of these are lead acid and there are significant differences among each of these types with regard to maintenance, charging voltage, charging rates and cost. Of these three, the AGM and flooded, dominate the marine market. If you don't want to maintain your batteries, go with the more expensive AGM as they offer other advantages as well. If you don't mind doing a monthly water check, flooded are probably a better value. This is an obvious over-simplification of battery selection. The specific pros and cons as well a good technical overview of all battery types is given in this link to Battery University.
The capacity (A-hrs) of your bank will be dictated by need, physical space and budget. The most basic rule of thumb is a total battery rating of 4X your daily use at anchor. This guarantees a minimum of 2 days of normal use without engine generated power and without drawing the batteries to damaging charge levels. The spreadsheet download above provides a slightly more detailed calculation based on your specific needs. Just because your budget allows it, more batteries than you need will increase your cost disproportionately to the advantages it may provide. Too few will require more engine time, deeper than desired discharges or reduced energy consumption.
The individual battery size and manufacturer is also important. Most prefer a battery that one person can safely handle in and out of the boat; however, a larger battery usually has more amp-hrs for the volume (size). Golf cart batteries are often selected for flooded type systems as they are the largest segment of deep-cycle battery manufactured and hence the least expensive per amp hour. They are also usually designed for deep discharge abuse.
Interface with other charging sources- Often