Solar Powered, Multi-seated, Internetted Computer System Final Report December 3rd, 2008
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- Navigate this page:
- Wire Sizing and Connections
- Maximum Solar Power Output
- Inverter to Battery Wiring
- DC-AC Inverter
- Charge Controller
- Conclusion
Sizing Battery ArrayNearly all large rechargeable batteries in common use are Lead-Acid type, although there are three variations, flooded, gelled electrolyte (“Gell Cells”) and absorbed glass matt (“AGM”). Flooded is the oldest and cheapest technology used but can be dangerous, in case of a malfunction acid can spill. Gell Cells contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass, therefore even if the battery where cracked open, no acid would spill. Gell batteries need to be charged at a slower rate (capacity / 20) but this is not a concern in the PV setup as the panels will not be outputting nearly this much current. AGM batteries are the newest technology and have all the advantages of Gell Cells without the charging limitations. All deep cycle batteries are rated in amp-hours. An amp-hour (Amps x Hours) is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. The accepted AH rating time period for batteries used in solar electric and backup power systems is the "20 hour rate". This means that it is discharged down to 10.5 volts over a 20 hour period while the total amp-hours it supplies is measured (Windsun). The compensated total power consumption per day value is used again to calculate minimum battery array size. 2540.16 Watt Hrs/12 Volts = 211.68 AmpHrs/day Number of days of autonomy to support: 1 (8hrs) 211.68 ∗ 1 = 211.68 AmpHrs “Battery life [deep cycle] is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD [depth of discharge]. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis (Windsun).” Depth of discharge for battery: 0.5 211.68 / 0.5 = 423.6 AmpHrs This means that after 8 hrs of use without sun the battery will be discharged to 50% 8 Hrs of autonomy and battery depth of discharge at 0.80 (Half the life-span of 0.50): 264.60 Amp Hrs 6 Hrs of autonomy and battery depth of discharge at 0.50: 264.6 Watts ∗ 6 Hrs = 1587.6 Watt Hrs / day ∗ 1.2 = 1905.12 Watt Hrs / day 1905.12 Watt Hrs/12 Volts = 158.76 AmpHrs 142.8 AmpHrs / 0.5 = 317.52 Amp Hrs 4 Hrs of autonomy and depth of discharge at 0.50: 238 Watts ∗ 4 Hrs = 1058.4 Watt Hrs / day ∗ 1.2 = 1270.08 Watt Hrs / day 1270.08 Watt Hrs / 12 Volts = 105.84 Amp Hrs 105.84 Amp Hrs / 0.5 = 211.68 Amp Hrs Wire Sizing and Connections:Another important consideration for the system is the electrical wiring. All wiring needs to safely accommodate the amount of current draw of the system with an acceptable amount of losses. In a DC system losses quickly become an issue. This is especially a concern PV systems as they can only handle a small voltage drop as there must be enough potential to charge the battery array, and of course it is good practice to keep energy loss sourced from the sun to a minimum. Generally a 3% drop between PV array and batteries is acceptable. Also, “any type of connection bigger than AWG 10 should have a proper compression connector, with appropriate joint compound and preparation. This does require special tools and dies. Otherwise you are running the risk of burning up your connections if you get any kind of heavy current flowing. (SolarForum)” Losses associated with transmission of DC power: CM = (22.2 ∗ A ∗ D)/VD CM = Circular Mills In Copper A = current in amps D = one-way cable distance in feet VD = Voltage Drop 22.2 = Constant for Copper For wiring from the PV panels to charge controller the maximum PV short circuit current specification (from PV data sheet) is used. Maximum Solar Power Output:24 Volt Systems:
Figure 6 12 Volt Systems:
Figure 7 Using the loss equation above the following result was obtained for the selected system: Distance: 50ft Voltage Drop: 0.72 Current: 15.42 Amps Circular Mills: 23772.5 AWG: 6 Inverter to Battery WiringFor current level estimates from the battery to inverter maximum power draw levels are used although this distance is generally short and maximum available wire gauge is recommended. This is also due to the fact that the system will encounter surge currents as various components are ‘turned on’. Since the system used as an example here is not continuously running and is to be turned off every night and back on in the morning this was a serious issue that needed to be tested. (Refer to Figure 9). Maximum Power Draw:
Figure 8 Assuming the inverter that will be sourced in deployment area is operating at 90% efficiency: 270 Watts = 300 Watts x 90% Maximum current draw in 12 Volt system = 300 Watts / 12 Volts = 25 Amps Maximum current draw in 24 Volt system = 300 Watts / 24 Volts = 12.5 Amps 
Figure 9 Figure 9 shows DC current draw as measured during power-up of Lenovo S10 Workstation (custom configuration) and L193p Monitor. Although the system is only drawing 5 amps while running the surge current spikes are clearly visible. This is indeed one of the reasons why proper electrical connections are crucial. DC-AC InverterSince the computer and monitors are designed to plug into AC power and accessory plugs for phone charging are a project specification an inverter is necessary. There are two types of inverters, pure sine wave and modified sine. Most devices will work from modified sine, this is what common uninterrupted power supplies provide and what was selected for this system. It is important to make sure that the inverter is rated to provide enough power for everything running off of it. Charge ControllerThe charge controller chosen for this system is the Outback Power FlexMax 60. This decision was based on versatility, efficiency, robustness, and availability in deployment area. The Outback can accept a wide range of voltage inputs as well as various battery arrays, this was important for this specific system as ultimately whatever solar panels are in stock at the time of deployment in the region will be used.  Figure 10 - Outback charge controller. Note, the efficiency curves (Figure 10 and Figure 11) are for the Flexmax80, they are identical to the Flexmax60 other than the fact that the FX60 does not accept 85 and 100V. The highlighted area on the graph represents the highest efficiency while charging a 12V battery array. The charge controller is operating at about 95.5% efficiency with an input Voltage between 17-34V. Typically a 12V PV panel's Voltage at Peak Power is around 17 Volts. 
Figure 11 - Outback charge controller. The highlighted area in this graph represents the optimum efficiency if the system where charging a 24V battery array. The charge controller is operating at about 98% efficiency with an input Voltage around 34V. Two 12V panels in series will typically have 34 Volt equivalent Voltage at peak power. In an ideal setup the FlexMax 60 would operate at 98.1% efficiency with an input of 68V while charging a battery array at 48V. This would be the case with the optimum PV panel chosen in section 1, the Kaneka G-EA060 as the VPM is 67Volts.
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