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SOLAR POWER DESIGN ANALYSIS

Introduction

Long the energetic mainstay of remote homes and businesses located beyond the reach of the Costa Rican national power grid, solar power is increasingly popular in conventional households where grid power is available as well as remote systems where it may be the only practical alternative available for home power supply.

Solar Power Configuration

Conventional solar power systems consist of the following elements:

1)  Solar panels.  Through the application of -- technology, photo-voltaic solar panels convert the sun's energy into direct current.  The efficiency of this form of energy conversion is usually around 5%.  Even highly efficient photovoltaic cells using combinations of metals can achieve an electrical power conversion efficiency of only up to 40% of the solar energy actually striking the panels.  Unlike other sources of energy, like hydroelectric, in which power generation is continuous, solar power generation is possible only during daylight hours and is much less efficient during cloudy or rainy days.  Power generation efficiency also varies as a function of the solar angle, so it is more efficient in low latitudes than in high ones.  For this reason, the number of solar panels deployed for a given house must be based on a rational decision predicated on the overall daily power demand anticipated and the average number of hours per day of sunshine, or the duration of the regional solar day, as well as the latitude of the deployment area.

2)  Inverter.  Solar panels produce direct current that cannot be used by conventional electrical appliances.  While 12- and 24-volt appliances are available on the market so that solar power systems can be designed to provide lighting, fans, and a few other applications without an inverter, most systems depend upon a power inverter to convert direct current to the alternating current that is used in conventional households.  Inverters come in a variety of sizes.  Proper inverter selection will depend on the number and types of appliances that need to be able to operate simultaneously in a household.  In this manner, a 2500 watt inverter can be expected to simultaneously operate a 1200-watt washing machine and a 1200-watt dishwasher.  However, if two 100-watt bulbs are on while the washing machine is on, then the dishwasher will not come on because of the inverter's 2500-watt capacity.  Inverters are normally rated for 120-Volt capacity, and 240-Volt power is normally obtained by deploying two identical inverters, though it is also possible to buy a 240-Volt rated inverter.  Just as AC output voltage can vary between 120 and 240 volts, inverters are rated for different input DC voltages, so systems are either 12-Volt, 24-Volt, or 48-Volt. 

3)  Batteries.  The advantage of solar power is that the use of batteries allows power that is generated by the panels to be stored in the battery bank.  This allows the use of power at night when there is no power generation potential whatsoever.  It also allows for the instantaneous use of more power than the rated capacity of the solar panels, since excess energy demands are pulled from power stored in the batteries that is subsequently replaced during periods of lesser power consumption.  Batteries come in a variety of configurations, from 2-Volt cells to combinations of 2-Volt cells that give rise to 6-Volt and 12-Volt batteries.  The deep-cycle batteries used in solar power systems differ from car batteries in that they are designed to be recharged over and over again.  Beyond the voltage of batteries, the amp-hour capacity rating is a key design variable as discussed in greater detail in the design section.  Variations in overall battery bank voltage and amp-hour capacities can be achieved through varying combinations of battery configurations in series and parallel. 

4)  Charge Controller.  In order to make it possible so that solar panels are functional across as much of the day as possible, the actual power production rating of panels is considerably higher at full noon-day sun than what batteries are designed to receive.  To ensure that batteries receive a modulated and steady source of power and to protect the batteries from overheating during periods of more intense sunshine, charge controllers are always installed to modulate the power entering the batteries from the solar panels.

5)  Example.  The following system configuration shows a typical remote installation capable of supplying refrigeration, washer/dryer, lights, fans, satellite Internet, television, Direct TV, dishwasher, coffee maker and other small appliances, and fans.  In this case, the refrigeration is assumed to be 24-volt and the dryer is assumed to be a hybrid dryer with electrical power to spin and heating from natural gas.

Alternative solar power systems can be 12- and 24-volt systems in which the inverter is not needed but which restricts the versatility of the power system to power only DC appliances.  Grid-tie systems include a connection to the electrical grid.  This enables dependence upon the grid for peak power demands and will enable a smaller deployment of panels to reduce the initial capital cost of the system.  In some societies, energy produced in excess of what is consumed can be transmitted through grid-tie inverters back through the electrical meter to sell power back to the local electrical grid.  While Costa Rica does not provide a convenient mechanism for power sales to the grid, grid-tie system can be set up to trickle-charge a battery bank so that when grid power is disrupted, the household inverter comes on and prevents the household (or business) from losing power.  This is particularly attractive in remote communities where power failures and brown-outs are nearly a daily occurrence.  Because inverters can accept a variety of inputs, other configurations include the use of fossil fuel generators to augment power supply as needed as well as hybrid alternative energy systems that include micro-hydroelectric and/or wind power as supplemental charging sources.

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