SOLAR ENGINEERING & PROFESSOINAL SOLAR SYSTEMS COMPANY PROFILE.
Solar Engineering Bulletin 07 1
2
According to these calculations, it would take 34.3 days to recharge the battery to 100% state-of-charge if all of the assump- tions made above held true for those 34.3 days. Of course, that is never going to be the case, which makes our 34.3 day conclusion of little to no value. The fundamental job of the PV array is to replenish all the daily energy the load consumes back into the battery on a given day. Further, the array must do so during the worst-case time of the year when solar levels are lowest and it must do so while overcoming system losses. If the PV array is large enough to accom- plish the above, and if the battery bank is adequately sized to carry the system through periods of inclement weather, then the system will perform successfully all year long. Nowhere in the above information did we specify or even consider the time required to recharge the battery. Does that imply the battery is sufficiently recharged to keep the system operational? Absolutely! But, we do not size the PV array nor the battery bank on that basis. As a result, we simply don’t worry about how long it takes to re- charge the battery. Properly designed PV systems always have a sufficiently sized solar array to offset the daily load draw under worst- case solar conditions. Those same systems also have an adequate amount of battery autonomy to carry the system through periods of inclement weather, when solar levels are below the daily average. We verify this through advanced sizing software based on math and statistical analysis. And we prove this through thousands of industrial SunWize PV systems success- fully operating worldwide. Call us for more information about battery charging and system sizing. Customers often ask how long it takes for a photovoltaic (PV) array to recharge the battery bank in a solar electric system. The answer is it doesn’t matter!! The battery bank is in a constantly chang- ing state because it is always being either charged or discharged. Unlike a UPS backup system, PV’s power source is not steady state, but is dynamic in nature. So, the time needed to recharge a battery bank from a PV array constantly changes. Consider a PV array using a single 110 watt PV module (a Mitsubishi model MF- 110) and a single 120 amp-hour battery (Concorde model PVX-1040T) powering a 750mA, 12VDC continuous load. Assume the system is operating in the Houston, TX area. Houston gets 3.6 hours per day of sun on average in the winter and 4.8 hours per day in August (at a 50-degree PV array tilt angle, facing south). There are many factors to consider: • What does it take to recharge the battery? • Since the levels of solar radiation change monthly, what time of year are we considering? • What level of battery discharge are we considering at the start of our recharge cycle? • How are we interpreting the solar radiation – as an average daily amount for a given month, or something less or more than that? • What battery recharge efficiency are we going to assume since the efficiency changes as the battery state-of-charge increases? • Are we taking into account the out- door temperature? • Is the load constant during the recharge period? Calculating the recharge time relies on a series of assumptions. Let’s determine the recharge time based on the following assumptions: Solar Engineering Bulletin JANUARY 2007 How Long Does It Take to Recharge Solar System Batteries? SunWize Industrial Sales Offices Arizona Mike Savino, tel: 480-626-7769 Colorado Charlie Bachman, tel: 303-604-1125 Illinois Mark Wilkerson, tel: 815-256-2222 Maryland Bruce Wilson, tel: 410-480-7175 Nevada Robert Damrau, tel: 775-969-3131 Washington State David Love, tel: 360-705-4322 For Sales in Latin America Dean Middleton, email: [email protected] For Sales in Europe, Africa or the Middle East John DeBoever, email: [email protected] 1. Assume it is winter time at 3.6 hours/day of peak sun. 2. Assume the load is a constant 750mA at 12VDC. 3. Assume the battery is fully dis- charged to the 20% state-of- charge point when the recharge begins. 4. Assume there are 10% system losses. ⇒ The peak current rating of our 110 watt PV module is 6.43 amps. The output of the array for an average winter day is: 6.43 amps x 3.6 hours/day x 0.9 (system losses) = 20.8 amp hours/day ⇒ 80% of the battery = 0.8 x 120 amp-hours = 96Ah ⇒ The load is 0.75 amps continuous = 18Ah/day ⇒ The difference between the array output and the load is: 20.8 – 18 = 2.8 Ah/day ⇒ 96Ah/2.8 Ah/day = 34.3 days to recharge (assuming no tempera- ture impact on battery recharg- ing efficiency)
Transcript of Solar Engineering Bulletin 07 1
According to these calculations, it would take 34.3 days to recharge the battery to 100% state-of-charge if all of the assump-tions made above held true for those 34.3 days. Of course, that is never going to be the case, which makes our 34.3 day conclusion of little to no value. The fundamental job of the PV array is to replenish all the daily energy the load consumes back into the battery on a given day. Further, the array must do so during the worst-case time of the year when solar levels are lowest and it must do so while overcoming system losses. If the PV array is large enough to accom-plish the above, and if the battery bank is adequately sized to carry the system through periods of inclement weather, then the system will perform successfully all year long. Nowhere in the above information did we specify or even consider the time required to recharge the battery. Does that imply the battery is sufficiently recharged to keep the system operational? Absolutely! But, we do not size the PV array nor the battery bank on that basis. As a result, we simply don’t worry about how long it takes to re-charge the battery. Properly designed PV systems always have a sufficiently sized solar array to offset the daily load draw under worst-case solar conditions. Those same systems also have an adequate amount of battery autonomy to carry the system through periods of inclement weather, when solar levels are below the daily average. We verify this through advanced sizing software based on math and statistical analysis. And we prove this through thousands of industrial SunWize PV systems success-fully operating worldwide. Call us for more information about battery charging and system sizing.
Customers often ask how long it takes for a photovoltaic (PV) array to recharge the battery bank in a solar electric system. The answer is it doesn’t matter!! The battery bank is in a constantly chang-ing state because it is always being either charged or discharged. Unlike a UPS backup system, PV’s power source is not steady state, but is dynamic in nature. So, the time needed to recharge a battery bank from a PV array constantly changes. Consider a PV array using a single 110 watt PV module (a Mitsubishi model MF-110) and a single 120 amp-hour battery (Concorde model PVX-1040T) powering a 750mA, 12VDC continuous load. Assume the system is operating in the Houston, TX area. Houston gets 3.6 hours per day of sun on average in the winter and 4.8 hours per day in August (at a 50-degree PV array tilt angle, facing south). There are many factors to consider:
• What does it take to recharge the battery?
• Since the levels of solar radiation change monthly, what time of year are we considering?
• What level of battery discharge are we considering at the start of our recharge cycle?
• How are we interpreting the solar radiation – as an average daily amount for a given month, or something less or more than that?
• What battery recharge efficiency are we going to assume since the efficiency changes as the battery state-of-charge increases?
• Are we taking into account the out-door temperature?
• Is the load constant during the recharge period?
Calculating the recharge time relies on a series of assumptions. Let’s determine the recharge time based on the following assumptions:
Solar Eng ineer ing Bul let in JANUARY 2007
How Long Does It Take to Recharge Solar System Batteries?
SunWize Industrial Sales Offices Arizona Mike Savino, tel: 480-626-7769 Colorado Charlie Bachman, tel: 303-604-1125 Illinois Mark Wilkerson, tel: 815-256-2222
Maryland Bruce Wilson, tel: 410-480-7175 Nevada Robert Damrau, tel: 775-969-3131 Washington State David Love, tel: 360-705-4322
For Sales in Latin America Dean Middleton, email: [email protected] For Sales in Europe, Africa or the Middle East John DeBoever, email: [email protected]
1. Assume it is winter time at 3.6 hours/day of peak sun.
2. Assume the load is a constant 750mA at 12VDC.
3. Assume the battery is fully dis-charged to the 20% state-of-charge point when the recharge begins.
4. Assume there are 10% system losses.
⇒ The peak current rating of our
110 watt PV module is 6.43 amps. The output of the array for an average winter day is: 6.43 amps x 3.6 hours/day x 0.9 (system losses) = 20.8 amp hours/day
⇒ 80% of the battery = 0.8 x 120 amp-hours = 96Ah
⇒ The load is 0.75 amps continuous = 18Ah/day
⇒ The difference between the array output and the load is: 20.8 – 18 = 2.8 Ah/day
⇒ 96Ah/2.8 Ah/day = 34.3 days to recharge (assuming no tempera-ture impact on battery recharg-ing efficiency)
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