SOLAR POWER DESIGN ANALYSIS
The following is a step-by-step procedure for design analysis necessary for design of a solar power system.
1. Estimate Power Demand. The first step to any system design is determining the appliances and electrical power consumption expected. For this analysis we need to know the specific appliances to be used, their electrical power demand (in watts), the number of hours per day that the appliances are expected to be used during peak usage periods, and the degree of simultaneity of usage, or how many appliances can be reasonably be expected to be used at the same time. A table summarizing typical power demands of typical household appliances, equipment, and tools is given here. To calculate the power demands of your application, download the power demand estimation excel spreadsheet and enter in information for all of the appliances and equipment for your application according to the instructions at the bottom of the sheet.
2. Design Required Charging Source. The number of daily watt-hours calculated for power demand and reported in cell G68 (or column G in the row titled Total Daily Consumption if you added or subtracted rows from the spreadsheet. This represents the total energy needed on a daily basis. In order to arrive at a solar panel array capable of providing this, it is necessary to determine the average hours of day of insolation and to apply corrections as needed for latitude. In semi-equatorial regions (including Costa Rica) there is relatively little variation in the length of day throughout the year, since at the Equator there is twelve hours of daylight year round. In practice, the variation in daily insolation in Costa Rica varies mostly due to seasonal cloud cover. Hence in the dry season, ten hours may be expected, but in the rainy season only six hours of sun can be expected, considerably less at high elevations where there is greater cloud cover. I typically assume an average of eight hours. The total daily power demand in watt-hours divided by the number of hours of insolation determines the amount of power required from the solar array. For example, if we have 32 kilowatt-hours of calculated power demand and an assumed eight hours of average daily sunshine, this presumes a 4 kilowatt array. If we plan to use 224-watt panels, we will need 17.86 panels. Solar arrays should be deployed in parallel, so even numbers of panels should be arrayed. At this point the designer can introduce a safety factor or not. For this application, I would recommend deployment of 20 224-watt panels, which provides a 10% safety margin over the hypothetical estimated total power demand of 32 kilowatt-hours per day.
3. Design Inverter Capacity. The value in Column H of the spreadsheet under the line heading Total Daily Consumption provides an estimation of the amount of power required for all the appliances listed in Column H to be operated simultaneously. This will require a bit of an iterative process. While a conservative design would be to assume at peak capacity everything is on at the same time, this may amount to an instantaneous power demand of several kilowatts, and the larger the inverter the more expensive. For modestly anointed homes (no AC, gas stove, hybrid clothes dryer, gas hot water heater, and DC-refrigeration), an inverter greater than 3.5 kilowatts may be somewhat excessive. However, please note that a 3.5 kilowatt inverter may not even carry a fully electric clothes dryer alone, hence the need to plan for the use of energy efficient or hybrid appliances. Also, stoves, hot water heaters, and clothes dryers are typically 240 volt, which requires two inverters, essentially doubling the cost. Besides the wattage of the inverter there is one other design decision that will also bare upon the cost of the battery bank and will affect system performance. A decision must be made as to whether 12-volt, 24-volt, or 48-volt inverter is to be used. A design analysis of this decision is beyond the scope of this discussion. In general terms, smaller systems can be done in 12-volt, particularly if economy is vital. Cadillac-Bentley systems are 48-volt, and for most mid-range applications, 24-volt is the most reasonable input voltage.
4. Design Battery Capacity. The battery bank provides leeway for divergence from assumptions. For instance we assumed eight hours of average daily insolation. However, we may have a six-day stormy period in which we have only three hours of daily sunshine. Either we need to have a backup power source (such as a fossil fuel generator) or ample battery capacity, or both to make sure that the final system is capable of performing adequately under non-ideal circumstances. Typical design practice is to provide three days of independent capacity in the battery bank. So, based on the 32-kw-hour daily power demand, three days amounts to 96 kw-hour capacity. Since batteries can not be drawn down completely, divide by a factor of 0.8 to accommodate for the battery draw-down limit and another factor of 0.9 to account for inverter efficiency, and that brings the total reserve design capacity to 133 kw-hours. To determine the amp-hour capacity of the battery bank required, divide by the voltage of the inverter. If we assume a 24-volt system, this means that the amp-hour capacity is 133,000 watts / 24 volts = 5,542 amp-hours. The total number of batteries required will depend on what type of battery and what battery bank configuration is used, so the design process at this point is an iterative process in which the end point varies as a function of the owner's pocketbook since batteries have a variety of life-span ratings and corresponding costs. For a fifteen year battery lifespan I would deploy a parallel array of two sets of twelve 2170 amp-hour 2-volt cells for a total of 24 batteries. This would be equivalent in a five year life expectancy of 40 six-volt 500-amp-hour L-16s arrayed in ten series of four.
5. Charge Controller. Sizing of the charge controller is done according to the charging source and varies as a function of the voltage of the configuration, and the number and wattage of the panels, according to the formula P = VA. So for the system shown in the diagram above, each pair of panels provides 224 watts at 24 volts and produces 9.3 amps. Since there are four couplets of panels, this amounts to a total amperage of 37.3. Adding a 25% safety margin, this presumes a minimum charge controller capacity rating of 47 amps. In practice this corresponds to a charge controller with a 60-amp capacity.
6. Final Details. Final considerations involve the manner of solar panel deployment, which can be either fixed or on a frame that is designed to follow the sun with either a passive solar tracking mechanism based on freon gas, or a motorized tracking mechanism that uses a small fraction of the DC power being generated. Tracking arrays are vital components in high-latitude deployments where they can boost yield by as much as 30%. In the tropics they are unlikely to achieve more than 8% increase in power production, which may not be worth the annual maintenance of chlorofluorocarbon-gas based passive tracking arrays. For motorized arrays the increase in power production is further reduced by the power demands of the tracking motor. Location of the batteries, inverter, and controller with respect to the solar panels is an important consideration. DC-power does not travel very efficiently, so dramatic line losses can be expected for overly long expanses of undersized cables. Final location and distances should be used to gage the cable sizes that are required to keep losses to a minimum, but generally speaking, for battery saddles, cable sizes of triple-O may be considered optimal, and for cabling connecting other parts of the system, the sizes employed should be determined as a function of the distances and expected loads to be transmitted. Batteries produce explosive hydrogen gas and should always be kept in a well-ventilated location. Contemporary sine-wave inverters are all programmable to optimize functionality and the programming should be undertaken to accommodate for the best use of facility resources and needs.
Osa Water Works offers all components at competitive pricing based on whole system orders and does not maintain an inventory. All pricing is based on current market values at the United States wholesale distribution companies where we purchase our equipment. Shipments into the country are made through the port of Golfito, and in most cases orders can be filled within one month of receiving an order and deposit.
Osa Water Works provides installation and support throughout Costa Rica and Panama. For an assessment of your individual solar power needs, contact us by email or telephone to discuss the details of your application.
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