EMERGENCY POWER SOLAR CELLS

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1 EMERGENCY POWER SOLAR CELLS Solar panels require no fuel other than sunlight. They produce no emissions, no fumes and have no rotating parts. Other than an occasional wipe down to remove dust, they are virtually maintenance-free. Solar panels are used with batteries to store electrical energy produced during the day for use at night or when the sun is shaded by clouds. This combination of solar panels and batteries allows 24-hour operation with only occasional help from a generator when solar conditions are less than optimal. Types of Solar Cells SOLAR SYSTEM COMPONENTS Single or Monocrystalline Cell This cell was the most popular and has been produced for the longest period of time. It consists of a single grown silicon crystal that is sliced into thin wafers, processed chemically, and finally covered with electrodes to collect the electric charge. They are typically blue or black and have shiny appearance. Efficiencies of 18% - 25% are commercially available. Polycrystalline Cell This cell type is now the most popular. It is made by slicing wafers from cast square silicon ingots. This cheaper grade of silicon results in a cell with less efficiency but cheaper in cost. These cells appear frosted. Typical efficiencies are 12% - 13%. Ribbon Cell This cell is formed by drawing flat thin films from molten silicon and results in a polycrystalline structure that is cheaper to produce since no bulk wafer cutting is required. Efficiencies are typically 13% - 14%. Amorphous or Thin-Film Cell These cells are usually created by applying doped silicon or other photo-voltaic material to the back of a plate of glass or a flexible insulator. Efficiencies of 6% to 10% are commercially available. Solar Panels Solar cells are connected in parallel to increase current, and in series to increase the voltage output. When cells are connected together they form solar panels. Solar panels can then be connected together to form solar arrays. Arrays can be connected in series or parallel to increase current or voltage output. A single silicon solar cell, regardless of the type, produces about 0.5 VDC. A 12 volt solar panel consists of 36 cells and produces 18 VDC output. The amount of current produced by a specific type of solar cell is a function of its active surface area, efficiency and the intensity of the solar radiation or insolation. Standard Test Condition (STC) insolation is 1000 W/m 2 at 25 ºC. Solar Panels will have a maximum power output that is the product of its output voltage and current at STC. Drawing more current from a solar panel than its maximum rated current will produce a rapid drop in output voltage. This point is referred to as the voltage knee as seen in Figure 1, at about 0.55 volts. Also note the variance in the voltage knee with temperature. As the cell temperature 1

2 drops, its output voltage increases. You will get more output on a cold winter day than in the summer time! Figure 1: Solar Cell I-V Curve Typical solar panel specifications are discussed below: Cell Type - Typically mono or ploycrystalline silicon. Pmax - Maximum power output at STC. Vmp - Voltage output at maximum power. This voltage, some times referred to as Vpp, is usually between 17 and 18 VDC for a 12 volt panel. Voc - Voltage output at open circuit. This is the no-load output voltage and should be about 21 VDC for a 12 volt panel. Imp - Current output at maximum power. Isc - Short circuit current. STC - Standard Test Conditions Insolation at 1000 W/m 2 at 25 ºC Maximum Efficiency The efficiency should conform to the cell type previously discussed. Maximum System Voltage The capability of the panel to be series connected into larger arrays. Number of cells Number of cells connected in series to form an array. This number is usually 36 for a nominal 12 volt system or 72 for a nominal 24 volt system. 2

3 Table 1, by SRoeCo Solar, compares cost and efficiency for a variety of different solar panels. Arrays are compared by assembling each type of panel into an array to produce 1000 watts. Each assembled solar array is then compared by cost per watt and panel area (efficiency). The Cost per PTC (PVUSA Test Conditions) is the best cost per watt estimate at average operating conditions. Red means the bottom 5 of the category and Green means the top 5. Table 1: Cost Comparison of Solar Panels More detailed information on each solar panel can be found at: A closer look at this class of panel shows that they are designed to operate at voltages that are not compatible with 12 volt DC batteries, so a more complex charge controller like the Power Point Tracker (discussed below) is required. 3

4 Charge Controller The charge controller acts like a battery charger between the solar panel and the battery pack. It prevents overcharge of the battery pack and disconnects the solar array from the battery when inadequate power is being delivered by the solar array. All modern, high performance charge controllers will approximate the three-stage battery charge algorithm to both maximize battery charging efficiency and minimize overcharging. There are four basic designs that a charge controller can use to control the incoming current from the solar array. Series Controlling This type of charge controller is in series between the solar array and battery pack and periodically disconnects the solar array from the battery with a relay or solid state device. Various pulse width and pulse repetition rate techniques are used by the solid state versions to approximate the three-stage charge algorithm. Shunt Controlling This type of charge controller is connected across the solar array and periodically short circuits it to bypass the solar array current. Recall that as the current exceeds Imp and approaches Isc, the cell voltage drops to near zero, so very little power is actually dissipated in this type of controller. Diversion In this case, when the charge controller decides that the battery is charged, excess solar array power is diverted to another battery pack or load (water heater, etc.) Array Shedding This type of control is used for very large commercial systems. The charge controller function is broken down in sub-array controllers which are sequentially disconnected as the battery pack becomes charged. New Development Power Point Tracker or Maximum Power Point Tracking is a new feature offered in some charge controllers. This feature allows a higher voltage solar array to charge a lower voltage battery pack with minimum loss. This type of charge controller is effectively an adjustable step-down switching regulator coupled with a three stage battery charger. See: for further information. Load Sizing of Solar Panels DESIGNING A SOLAR SYSTEM The selection of solar panels to provide 100% of the required power to continuously operate a high-performance amateur radio station 24 hours a day requires careful design. The steps in load sizing include: 1. Evaluation of the average daily insolation. 2. Calculation of the total required solar panel power. 3. Selection of solar panels. 4. Selection of the storage batteries. 5. Selection of a Charge Controller. Note that the solar array must not only supply the required station power during the day light hours but sufficient additional power to replenish the storage batteries for night time operation. 4

5 Insolation The most productive hours of sunshine for solar electric generation are from 9 AM to 3 PM. The morning and later afternoon sun is less intense and the angle of the sun is not optimum for the solar panels. The average sun hours varies with geographic location as shown in Figure 2. The zone definitions are shown in Table 2. Figure 2: Solar Insolation Map Table 2: Zone Definitions for Figure 2 Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 6 6 hours 5.5 hours 5 hours 4.5 hours 4.2 hours 3.5 hours A web calculator for solar insolation by city can be found at: For central Texas, the average solar insolation is 5 hours per day or 5000 watts/m 3 per day. Required Solar Power 5

6 The required solar power will be the average power calculated for the storage battery case. The storage batteries can provide the intermittent peak transmit current. Since the solar array can only supply power during the maximum insolation period, it must supply the full 24 hour requirement in this shorter period of time. The additional power is used to charge the storage batteries. Example: Assume an average power requirement of 150 watts (11.9 amperes at 12.6 volts DC). In 24 hours, a total of AH or 3,598.6 Watt-Hours of energy will be consumed. The solar panel must supply this amount of total power ( watts) in the available 5 hours (zone 3) of insolation or watts per hour. Part of the energy will directly power the radio station and the remainder will be used to charge the battery pack for night-time operation. Selection of Solar Panel(s) In selecting the solar panel array, I used the PTC watt rating found in Table 1 for the least expensive solar array in dollars/watt. The surface area comparison (efficiency) is not important since this will be a ground mounted system. Example: For the required 750 watts, four Canadian Solar CS6P-230 panels are connected in parallel to supply a total of 844 watts. Detailed specifications for this panel can be found in Appendix 1. Note that this solar panel has a Vmp= 29.8 volts, so either a 24 volt storage battery system or a Power Point Tracker charge controller must be used to maintain solar panel efficiency. The cost for this panel is $624/each. Battery Storage Requirement The storage battery pack is required to deliver station power for 24 hours minus the insolation time. Example: The storage battery pack for the above example would be required to deliver 11.9 amperes at 12.6 volts for only 19 out of 24 hours, since the solar panel array would cover the remaining 5 hours. This allows a reduction of storage battery requirements from AH to AH. Assuming 50% discharge, the battery pack will need a total capacity of AH at 12.6 VDC. This can be realized with five, 100 AH, 12 volt lead acid storage batteries connected in parallel. The Charge Controller, selected next, must not exceed the maximum charge rate for the battery pack. Example: The Power Sonic PS battery has a capacity of 100 AH and can safely be charged at 20 amperes, so the initial charge rate should not exceed 100 amperes. Charge Controller When solar panels have a maximum output voltage that is significantly greater than the storage battery voltage, a simple charge controller would introduce too much loss. A Power Point Tracker charge controller must be use to convert the higher solar panel Vmp to a voltage and current compatible with the battery pack without sacrificing solar panel efficiency. 6

7 The FLEXmax 60, shown in Appendix 2, can provide both the required step-down capability and a three-stage battery charging algorithm with an efficiency of 94.5%. Further, the maximum charge rate for this controller is limited to 60 amperes, which is less than the maximum charge rate of 100 amperes for the selected battery pack. The efficiency of the FLEXmax charging a 12 volt battery system is shown in Figure 3. The applicable efficiency curve for the text example is the 34 volt solar array at 750 watts output (94.5%). The cost of this charge controller is $539. Figure 3: Efficiency of the FLEXmax Charge Controller Total Cost Solar Panels - $ (quantity 4 of Candian Solar CS6P-230) Controller - $ (quantity 1 of FLEXmax 60) Batteries - $ (quantity 5 of Power Sonic PS ) Total - $ Partial Implementation of Solar Power System The implementation of a 100% solar powered station is very costly and not practical for most of us. The more practical solution is to start with a single solar panel to maintain a charge on a battery pack. The solar panel would simply extend the operating time of the battery pack and a generator would then be used to supplement any deficiency from the solar array. If a properly sized controller was initially purchased, then additional solar panels and batteries could be added in the future to increase capacity and reduce fossil fuel consumption. 7

8 Solar Panel Location For maximum efficiency, the solar array must be located such that no shade will fall on it during its maximum insolation period. Further, the elevation and azimuth must be adjusted to maximize solar ray angle. Azimuth and Elevation (northern hemisphere) The solar array should be pointed due south (true) and tilted as follows: Winter Latitude minus 15º Summer Latitude plus 15º An active azimuth/elevation sun tracker would provide the best solution. There are claims that an active azimuth/elevation sun tracker could increase the power output per year by as much as 70% in south Texas. A more complete discussion of solar array tilt can be found at: Shading If solar panels are wired in parallel, shading a part of an array will just reduce the power output. When solar panels are connected in series to produce higher voltage output, all the panels must carry the same current. If a panel in this series configuration stops producing current, the array will become reverse biased and dissipates power as heat. This heat can quickly destroy part or all of an expensive solar panel. The reverse bias situation is prevented by using bypass diodes across each panel. The bypass diode (rated for the array s maximum output current) will carry the current around a module that is not contributing current. Wind Load Solar Panels can present a significant wind load so careful design of their mechanical support structure is required. Disclaimer: Good engineering practice was used in the collection of information and the design of the example solar array power system. The author has not built or tested any part of the example system. It is suggested that you consult with a solar power expert before under taking the construction of a large, expensive solar power system. Lewis Thompson, AAA6TX 8

9 APPENDIX 1 SOLAR ARRAY EXAMPLE 9

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11 APPENDIX CHARGE CONTROLLER EXAMPLE 11

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