Friends:
I wrote a draft white paper to discuss the issues relevant to PV Assist systems. If you are up for along read, please feel free. If you have suggestions, I welcome them. This idea is new to me so I am learning as I go. Also, I could not easily past the chart so I included a link to it. Also, the table did not quite line u, but I think you canfigure it out. Thanks, William Miller GENERATOR / BATTERY WITH PV ASSIST August 19, 2013 DRAFT, PRINCIPLES DISCUSSED ARE UNDER RESEARCH Introduction: Whenever we design an Off-grid Alternative Energy systems, we usually design the system so that non-fossil battery charging sources (solar, wind, hydro) provide a majority of the energy for battery charging. Sometimes we come across systems that have loads that are so large that it is not practical to rely solely on non-fossil sources. A large generator will be needed to run on a daily basis. We call these systems Generator / Battery Systems with PV Assist, or "PV Assist" for short. We use the term PV to describe all RE charging sources since PV is most common in our area. We have found that battery charging in PV Assist systems using traditional settings may cause premature failure in battery arrays for reasons described below. We want everyone to enjoy the longest battery life possible so we have developed some alternative settings for battery charging that will hopefully achieve longer battery life. It has always been the goal of solar technicians to minimize generator run time. In retrospect, this may not be the best idea in PV Assist systems. The consequence may be reduced battery life. Batteries wear out when they are "cycled". A cycle is defined as a discharge and charge. Normally, an off-grid system will charge batteries during the day and discharge them at night, resulting in one cycle per day. In the case of a PV Assist, the batteries may cycle multiple times per day. Your battery bank is rated for a finite number of cycles. If they cycle 4 times per day, they wear out 4 times faster than if they cycled once per day. In an attempt to preserve the longevity of a battery bank in a PV Assist system, we are experimenting with alternative generator auto start and inverter / charge battery charge settings. In general, we suggest that if batteries are going to be cycled multiple times per day, that they be cycled less "deeply." In other words, do not discharged the batteries to as low a voltage as one would normally. In order to achieve this goal, the generator will run more frequently than in a traditional hybrid off-grid system. In these systems it is already a given that generator run time will occur every day, even multiple times per day. We are essentially embracing generator run time to preserve battery life. We are trading off greater "wear-and-tear" on the generator for less wear-and-tear on the batteries. The decision to do this belongs with the system owner. Battery DOD versus life span: See figure 1 below that compares the depth of discharge (DOD) of a battery to battery life. Note that the maximum expected cycles is 4,000. If you cycle 4 times per day at this DOD, expect 1,000 days of life, or three years. If you cycle to 80% you can expect 1000 cycles, or 250 days. This is a very short life span and this scenario needs to be avoided. The sweet spot is to be determined for each unique situation, but for discussion we are assuming we want 3000 cycles, or about 2 years. This is not long for a battery bank, but keep in mind this design assumes we have undersized the battery bank significantly. Essentially you are buying two or three small battery banks in the span of time one properly sized battery bank would last. To achieve 3,000 cycles, the DOD must be limited to about 70%. The lowest we can allow the battery bank voltage to get, when measured at rest, is about 49.0 volts. Keep in mind that batteries being discharged will exhibit a voltage lower than the at-rest voltage. Also keep in mind that surge loads will cause the battery voltage to temporarily sag. The above described logic drives our suggested system settings below. http://www.trojanbatteryre.com/PDF/datasheets/L16REB_TrojanRE_Data_Sheets.pd f Generator Auto Start: We will require that generators in a PV Assist system be able to start automatically based on both battery voltage and load. Most common inverter/charger systems are capable of this. The two conditions for auto start are described below. Battery start: In the case of a battery start condition, the master inverter monitors the battery voltage. Generally, if the voltage becomes slightly low for 24 hours, or the voltage becomes moderately low for 2 hours or extremely low for 15 minutes, the generator will start. When the generator starts in these conditions, the system will complete a full three stage charge cycle. An explanation of a three stage charge cycle is beyond the scope of this paper. If we are assuming that the generator will start more than once per day, there is reason to desire a short run time. Charge settings will be modified to allow for a shorter battery charge cycle. One must be cautious to be sure that if loads decrease, settings are modified to reflect this reduction. If this is not done, there is a risk that the batteries will never recharge adequately. It is also required that system operators manually equalize the batteries every 30 days. This normally only applies to flooded batteries but we will present modified EQ settings for sealed (VRLA) batteries. In the case of VRLA batteries, this is not to actually equalize the batteries but to ensure a full absorption cycle. Load Start: For load start settings, the system senses how high the loads are (current draw on the system). If the loads exceed a programmed amount for a programmed time, the generator will run until the loads decrease below a lower programmed value for a programmed time delay. We will use this feature extensively for PV Assist systems. Load Start Levels: For this program, we are assuming that we do not want to subject a battery bank to any draw that would deplete it in less than 10 hours. If the draw exceeds this amount, the generator should start. Calculations show that if the draw exceeds 5% of the battery watt hours, the bank will deplete in 10 hours or less. Example: If a 48 volt battery bank has an amp/hour rating of 1,000, the watt/hour rating is 48,000. The generator should start if the load exceeds 2.4 KW. These calculations are based on an available amp/hour capacity 50% of name-plate. Load Start Delay: The excess load must be present for a programmable period of time before the generator will auto-start. If this load occurs for long enough the battery voltage will decline and the generator will auto-start for voltage reasons. This will cause a charge cycle, which is usually not necessary. Therefore we need to find a proper delay time that will start the generator if the excess load is expected from more than a short duration. It is up to the system designer to anticipate load values and load durations. Some examples are included below. Load Stop / Load Stop Delay: Load stop is the value below which the load must drop before the generator will auto-stop. The generator must fall below this value for a programmed period of time before the generator will stop. These values are difficult to predict unless the particulars of a system are known. The suggested starting point for Load Stop is 2.5% of bank watt hours (a 20 hour depletion). The stop delay is determined by factors discussed below. Quiet Time: Most modern inverter / chargers have a quiet time setting. When this is invoked the generator will not battery start for the 24 or 2 hours volt settings described above. We use this not only to allow for reduced generator run at night, but to disallow battery starting in the morning of a sunny day where the PV charge might prevent a generator start. We will embrace quiet time as valuable in PV Assist systems. PV Charging: We recommend that PV and other RE charging sources be set at traditional values. This will differ from the suggested settings below only in absorption duration. This value should be a minimum of 2 hours, or a taper setting that provides a similar absorption period. Settings: Below are suggested settings for three stage charging systems and for generator auto-start systems. These settings are for nominal 48 volt systems and can be interpolated for other nominal system voltages. These settings apply to flooded lead-acid and VRLA (sealed) batteries: Parameter FLA VRLA Absorb volts: 57.6 56.8 Absorb time: 0.5 hours 0.5 hours EQ Volts 61.0 56.8 EQ Time: 2.0 hours 2.0 hours 24 hour start: 48.8 48.8 2 hour start: 47.8 47.8 15 minute start: 47.0 47.0 Load Start: 5% of battery array watt/hours Load Start delay: Minimum setting Load Stop: 1% of battery array watt/hours Load Stop delay: Determined by load profile, see below Load profile: We need to determine the number of cycles of loads that exceed 5% of battery array watt hours. This is the number of times the generator will start on a daily basis. There should be a finite number of times a generator starts each day to avoid excess wear on the generator starting mechanism. If the load cycle number exceeds the desired generator starts per day, then the load cycle needs to be reduced or the delay time will need to be extended to exceed the off cycle of the 5% load. For example if the excess load comes on every two hours, the generator will start 12 times per day. If this is acceptable, then use a minimum Load Stop setting. The generator can shut down soon after the loading stops. If this number of generator starts is excessive, the load cycle count needs to be reduced or generator load start settings adjusted. For example, if the excess load is an appliance, the user needs to be instructed to use the appliance less often. Or, the load stop time needs to be increased so that the generator stays on for several cycles of the load. For example, if the uses of the appliance can be grouped so that most of the times it is used there is no more than 20 minutes between uses, set the stop delay for >20 minutes and the generator will remain on until the grouped use cycle is complete. An example might be a water heater that is used in the morning and evening, but not the rest of the day. The heater might come on three times in the morning and three times in the evening, cycling off for 30 minutes. In this case the load stop delay can be set at >30 minutes and the generator will run for a few hours morning and evening. The consumer should be encouraged to group other loading to occur during those hours. The cooking, laundry and workshop activities could occur during those hours, making use of the generator run. Summary: This program is still under development and we welcome your feedback on the results you achieve using these settings. End of paper.
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