Each of us has his/her own reason for implementing a solar photovoltaic (PV) system. A solar PV system is a practical solution for those not served by a utility company. Some of us implement alternative electric systems because we want to cut our electric bill, or because we like the idea of “green” living. Many of us implement small PV systems because we expect weather-related power outages, or because of an unreliable electric service provider.
Whatever your reason for using alternative energy, you’ve chosen to invest your time and money in a responsible way. As the 2008 election approaches, this would be a good time to consider which of the candidates would be the best choice for those of us who choose to act in such a responsible manner. Maybe the following stories will help you decide:
Ant & the Grasshopper
Two Different Versions! Two different Morals!
OLD VERSION: The ant works hard in the withering heat all summer long, building his house and laying up supplies for the winter.
The grasshopper thinks the ant is a fool and laughs and dances and plays the summer away. Come winter, the ant is warm and well fed.
The grasshopper has no food or shelter, so he dies out in the cold.
MORAL OF THE STORY: Be responsible for yourself!
-------------------------------------------
MODERN VERSION:
The ant works hard in the withering heat all summer long, building his
house and laying up supplies for the winter.
The grasshopper thinks the ant is a fool and laughs and dances and plays the summer away.
Come winter, the shivering grasshopper calls a press conference and demands
to know why the ant should be allowed to be warm and well fed while others are cold and starving.
CBS, NBC, PBS, CNN, and ABC show up to provide pictures of the shivering grasshopper next to a video of the ant in his comfortable home with a table
filled with food. America is stunned by the sharp contrast.
How can this be, that in a country of such wealth, this poor grasshopper is allowed to suffer so ?
Kermit the Frog appears on Oprah with the grasshopper, and everybody cries when they sing, 'It's Not Easy Being green.'
Jesse Jackson stages a demonstration in front of the ant's house where the news stations film the group singing, 'We shall overcome.' Jesse then has the group kneel down to pray to God for the grasshopper's sake.
Nancy Pelosi & John Kerry exclaim in an interview with Larry King that the ant has gotten rich off the back of the grasshopper, and both call for an immediate tax hike on the ant to make him pay his fair share.
Finally, the EEOC drafts the Economic Equity & Anti-Grasshopper Act retroactive to the beginning of the summer.
The ant is fined for failing to hire a proportionate number of green bugs and, having nothing left to pay his retroactive taxes, his home is confiscated by the government.
Hillary gets her old law firm to represent the grasshopper in a defamation suit against the ant, and the case is tried before a panel of federal judges that Bill Clinton appointed from a list of single-parent welfare recipients.
The ant loses the case.
The story ends as we see the grasshopper finishing up the last bits of the ant's food while the government house he is in, which just happens to be
the ant's old house, crumbles around him because he doesn't maintain it.
The ant has disappeared in the snow.
The grasshopper is found dead in a drug related incident and the house, now abandoned, is taken over by a gang of spiders who terrorize the once
peaceful neighborhood.
MORAL OF THE STORY: Be careful how you vote in 2008
- - Thanks JA for this story. sj
Monday, January 28, 2008
Thursday, January 24, 2008
Load Switcher Project Update
My goal is to get as much as I can from my solar photovoltaic system, and at the same time protect my batteries from over-discharging. I want to be able to use battery power as much as possible, and only switch to grid-supplied AC when battery state of charge (SOC) has declined to a preset value. The first step, the installation of a “Transfer Switch”, has already been done. Next, I’ll need a way to switch my inverter on and off, depending upon the battery state of charge (SOC) at any given time. When the inverter is switched on, the load is powered by the batteries. When the inverter is switched off, the transfer switch automatically connects the load to grid-supplied AC power instead.
I’ve considered three ways to accomplish the task, and thought about the pro’s and con’s of each:
1. A simple circuit that allows precise control of low and high voltage threshold settings to open and close a relay.
A considerable amount of time is needed for development and testing. Cost is also an issue. While the finished product may do the job, it may not be as efficient as a commercially available product that can perform the same function. This device will not be easily expandable.
2. A microprocessor-based controller that can easily be reprogrammed to open and close a relay based on battery voltage.
Hardware and software development time will be tremendous, unless I go with a commercially available product, but that will be expensive. However, the result will be a product that performs a simple task at first, but can be easily expanded to include many more functions. A real-time-clock can be included, greatly enhancing control, monitoring, and logging functionality.
3. A commercially available product that can be programmed or configured to connect and disconnect the load based on battery voltage.
The one-time cost will be significant, but the system will be up and running in a short time. Expansion capabilities are considerable, but do not include a real-time-clock (RTC). An RTC would be useful because I will want to take advantage of low night time electric rates to run a battery charger.
I’ve already built a simple device, but it’s going to take more time to get it working as well as I would like. Meanwhile, I’m not making the most of the available energy from the sun right now. Building a microprocessor-based device would take even longer. I want to get things going fairly quickly, and the time I have available for this project is limited. With these things in mind, I’ve decided to go with option number 3. Specifically, I’m considering the Morningstar Relay Driver and MSView software.
Calling the Morningstar product a “Relay Driver” is, in my opinion, a big mistake, and perhaps the reason I overlooked this option earlier. The name implies that its function is simply to turn on and off one or more relays, based on external signals. In reality, it does much more than that. Using the MSView software, the device can be programmed to perform a variety of functions. Most importantly to me, it can control a relay based on high and low battery voltage setpoints. I’ll use that relay to turn my inverter on when batteries are at a high SOC, and switch it off when battery voltage is low, with the transfer switch selecting the appropriate source of AC for the loads. I’ll use just one relay at first, but will add functionality later. The Morningstar Relay Driver can control up to four relays.
Once I get load control set up and tested, I probably want to control a battery charger. I’ll use the Morningstar Relay Driver to close a relay based on battery voltage, and I’ll use a timer to only allow the charger to be powered-up when electric rates are low.
Shown below is my simple circuit in the testing phase. I’ll soon be abandoning this project in favor of the Morningstar Relay Driver.
John
I’ve considered three ways to accomplish the task, and thought about the pro’s and con’s of each:
1. A simple circuit that allows precise control of low and high voltage threshold settings to open and close a relay.
A considerable amount of time is needed for development and testing. Cost is also an issue. While the finished product may do the job, it may not be as efficient as a commercially available product that can perform the same function. This device will not be easily expandable.
2. A microprocessor-based controller that can easily be reprogrammed to open and close a relay based on battery voltage.
Hardware and software development time will be tremendous, unless I go with a commercially available product, but that will be expensive. However, the result will be a product that performs a simple task at first, but can be easily expanded to include many more functions. A real-time-clock can be included, greatly enhancing control, monitoring, and logging functionality.
3. A commercially available product that can be programmed or configured to connect and disconnect the load based on battery voltage.
The one-time cost will be significant, but the system will be up and running in a short time. Expansion capabilities are considerable, but do not include a real-time-clock (RTC). An RTC would be useful because I will want to take advantage of low night time electric rates to run a battery charger.
I’ve already built a simple device, but it’s going to take more time to get it working as well as I would like. Meanwhile, I’m not making the most of the available energy from the sun right now. Building a microprocessor-based device would take even longer. I want to get things going fairly quickly, and the time I have available for this project is limited. With these things in mind, I’ve decided to go with option number 3. Specifically, I’m considering the Morningstar Relay Driver and MSView software.
Calling the Morningstar product a “Relay Driver” is, in my opinion, a big mistake, and perhaps the reason I overlooked this option earlier. The name implies that its function is simply to turn on and off one or more relays, based on external signals. In reality, it does much more than that. Using the MSView software, the device can be programmed to perform a variety of functions. Most importantly to me, it can control a relay based on high and low battery voltage setpoints. I’ll use that relay to turn my inverter on when batteries are at a high SOC, and switch it off when battery voltage is low, with the transfer switch selecting the appropriate source of AC for the loads. I’ll use just one relay at first, but will add functionality later. The Morningstar Relay Driver can control up to four relays.
Once I get load control set up and tested, I probably want to control a battery charger. I’ll use the Morningstar Relay Driver to close a relay based on battery voltage, and I’ll use a timer to only allow the charger to be powered-up when electric rates are low.
Shown below is my simple circuit in the testing phase. I’ll soon be abandoning this project in favor of the Morningstar Relay Driver.
John
Friday, January 18, 2008
The Need for Automatic Control in Off-Grid Solar PV Systems
To get as much as I can from my off-grid system I manually connect loads to it, watch as battery voltage declines, and remove the loads before the battery voltage falls to an unacceptably low level. Since I’m not always home, and because I’m not always monitoring voltage while I am at home, I often fail to use the available energy efficiently. I also run the risk of over-discharging the batteries, possibly causing irreversible damage to them. I’ve outlined a plan to automate my system previously, and I’ve made significant progress toward implementing that plan. It’s an ambitious project, but well worth the effort. Read on for additional justification, and a progress report.
I now have a Transfer Switch installed and functioning. When I turn on the inverter, loads attached to the circuit are powered by it. When I switch off the inverter, the loads are automatically switched to grid-supplied power instead. I’m using my refrigerator and a chest freezer as the test loads. Installing the Transfer Switch was “Step 1” of my plan. Automating the switching on and off of the inverter based on battery voltage is the next step.
Recently, I switched on the inverter with the loads attached in the afternoon as I often do. I monitored the battery voltage at regular intervals, intending to switch off the inverter before bedtime. Unfortunately, I fell asleep without switching the inverter off. I woke up at 4:00am to find that my refrigerator and freezer had no power. This wasn’t supposed to happen; the Transfer Switch was supposed to connect the loads to grid-supplied AC if the inverter stopped functioning. I found that battery voltage had dropped to 11.7 volts. The Transfer Switch was “chattering”. Analyzing the situation, this is what I determined: When battery voltage dropped to a point where the inverter could no longer function, the Transfer Switch disconnected the load, connecting it instead to the grid. With no load on the batteries, battery voltage quickly increased to the point where the inverter could once again function. This, in turn, caused the Transfer Switch to reconnect the inverter to the load. This cycle kept repeating at a rapid pace, which resulted in the Transfer Switch chatter. Eventually the breaker for the grid-supplied AC tripped. The tripped breaker didn’t stop the cycle, it just prevented grid-supplied AC from getting to the load. I had to manually intervene by turning off the inverter and resetting the breaker.
The incident described above demonstrates an interesting battery characteristic. When a load is applied, voltage drops. When the load is removed, battery voltage increases, even when the battery is deeply discharged. It is for this reason that I decided to use low and high set-points in my control circuit. And to facilitate differences in battery bank sizes and type of batteries, I’ve decided to make those set-points adjustable. I’ll make them adjustable to one-hundredth of a volt, which is much better control than some commercially available equipment provides. Once batteries are disconnected, they will not reconnect until they’re once again fully charged. After I’ve determined the appropriate settings, I should not experience a problem like the one I experienced recently.
I’ll get many of the parts needed for the project tomorrow (Saturday), and will hopefully have a portion of the circuit built and tested by the end of this weekend. Check in on me later for a progress report.
John
I now have a Transfer Switch installed and functioning. When I turn on the inverter, loads attached to the circuit are powered by it. When I switch off the inverter, the loads are automatically switched to grid-supplied power instead. I’m using my refrigerator and a chest freezer as the test loads. Installing the Transfer Switch was “Step 1” of my plan. Automating the switching on and off of the inverter based on battery voltage is the next step.
Recently, I switched on the inverter with the loads attached in the afternoon as I often do. I monitored the battery voltage at regular intervals, intending to switch off the inverter before bedtime. Unfortunately, I fell asleep without switching the inverter off. I woke up at 4:00am to find that my refrigerator and freezer had no power. This wasn’t supposed to happen; the Transfer Switch was supposed to connect the loads to grid-supplied AC if the inverter stopped functioning. I found that battery voltage had dropped to 11.7 volts. The Transfer Switch was “chattering”. Analyzing the situation, this is what I determined: When battery voltage dropped to a point where the inverter could no longer function, the Transfer Switch disconnected the load, connecting it instead to the grid. With no load on the batteries, battery voltage quickly increased to the point where the inverter could once again function. This, in turn, caused the Transfer Switch to reconnect the inverter to the load. This cycle kept repeating at a rapid pace, which resulted in the Transfer Switch chatter. Eventually the breaker for the grid-supplied AC tripped. The tripped breaker didn’t stop the cycle, it just prevented grid-supplied AC from getting to the load. I had to manually intervene by turning off the inverter and resetting the breaker.
The incident described above demonstrates an interesting battery characteristic. When a load is applied, voltage drops. When the load is removed, battery voltage increases, even when the battery is deeply discharged. It is for this reason that I decided to use low and high set-points in my control circuit. And to facilitate differences in battery bank sizes and type of batteries, I’ve decided to make those set-points adjustable. I’ll make them adjustable to one-hundredth of a volt, which is much better control than some commercially available equipment provides. Once batteries are disconnected, they will not reconnect until they’re once again fully charged. After I’ve determined the appropriate settings, I should not experience a problem like the one I experienced recently.
I’ll get many of the parts needed for the project tomorrow (Saturday), and will hopefully have a portion of the circuit built and tested by the end of this weekend. Check in on me later for a progress report.
John
Tuesday, January 01, 2008
My Solar Electric System and New Year's Resolution
Plans for my off-grid photovoltaic (PV) system have been pretty much the same for the past two years; enlarge the system and use more of the available power. I’ll continue to do both of these things in 2008, but I’ll also make a major change in the way I use energy from the system. In the past I’ve carefully monitored battery voltage and manually switched loads on and off. By doing this manually I not only miss opportunities to use energy from the sun, I risk damaging the batteries by over discharging them. To resolve these problems, and to relieve myself of the chore of manually switching between power sources, I plan to automate this task.
I’ll automate the switching on and off of loads using ideas from a recent post, “Getting the Most from an Off-grid System”. I’ve decided to use a transfer switch, and a circuit of my own design, to accomplish this task. Refer to the drawing below:
How it works:
The Transfer Switch is configured to use power from the inverter as the primary source of AC power for the load, only switching to grid-supplied AC power when the inverter is switched off. The inverter will be automatically switched on when the battery state of charge (SOC) is high, and switched off when the battery SOC has dropped to a predetermined value.
Thumbwheel switches, precision resistors, and a regulated reference voltage allow precise settings of the “Low Voltage Threshold” and the “High Voltage Threshold”. The thumbwheel switches and resistors create two voltage dividers. The output of the low voltage threshold voltage divider is equal to the low voltage threshold thumbwheel setting, and the output from the high voltage threshold divider is equal to the high voltage threshold thumbwheel setting. These two reference voltages are fed into two comparators.
When the battery voltage falls below the “Low Voltage Threshold” setting, comparator 1 changes state, triggering the Flip Flop, and the inverter is turned off. The Transfer Switch senses the loss of AC voltage from the inverter and switches the load to grid-supplied AC power.
When the battery voltage rises above the “High Voltage Threshold” setting, comparator 2 changes state, resetting the Flip Flop, and the inverter is switched on. The Transfer Switch senses the AC voltage from the inverter, and connects the load to the inverter.
Initially, I’ll set the low voltage threshold voltage to 12.25 volts. That voltage represents an approximate 75% state of charge (SOC). I’ll set the high voltage threshold at 14.75 volts, ensuring that the batteries are fully charged before allowing them to power the load.
It’s interesting to note that the battery voltage will not reach the high voltage threshold setting unless the sun is shining and the batteries have been fully charged. I’ll experiment with other settings in an attempt to improve system efficiency without endangering the batteries.
Benefits:
Since I’m not able to constantly monitor battery voltage, I’ve missed opportunities to use as much of the free power that my solar electric system is capable of providing. Instead, I disconnect the load when I think that the battery SOC may fall below 80%. Once I’ve implemented this plan, I’ll be able to use more of the available power from the system without the fear of damaging the batteries. Additionally, this automation will help to keep the batteries at a high enough SOC to ensure the availability of power in the event of a grid power failure.
With a Twist:
Because of a utility company plan that results in low rates at night, I’m thinking about storing energy in a larger battery bank at night when rates are low, and using that energy to power loads during the day when utility rates are high. And because my PV array is still small, I’m thinking about using a battery charger to supplement the charging that now comes from my PV array. The charger will be turned on via a timer in the early morning hours when rates are lowest, and turned off later in the morning before rates go up. When cloudy conditions limit the amount of charging my PV panels are able to provide, the battery charger will take up the slack and the batteries should be fully charged each morning. Confident that I’ll have plenty of stored energy in the morning, I’ll add more to the daytime battery load, and therefore save money on my electric bill.
With the charger switched on during the early morning hours, it’s likely that the batteries will quickly become fully charged. When that happens, the inverter will once again be used to power the load. Because the charger is still connected, and switched on via the timer, it too will provide power to the load. However, this will occur in the early morning hours when electric rates are low, and I’ll be taking advantage of the lowest rates, cutting my electric bill.
The graph below shows the expected results over a 24 hour period. As a result of charging from the PV panels during the day, and charging from the battery charger at night, the load will be powered by the batteries most of the time.
John
I’ll automate the switching on and off of loads using ideas from a recent post, “Getting the Most from an Off-grid System”. I’ve decided to use a transfer switch, and a circuit of my own design, to accomplish this task. Refer to the drawing below:
How it works:
The Transfer Switch is configured to use power from the inverter as the primary source of AC power for the load, only switching to grid-supplied AC power when the inverter is switched off. The inverter will be automatically switched on when the battery state of charge (SOC) is high, and switched off when the battery SOC has dropped to a predetermined value.
Thumbwheel switches, precision resistors, and a regulated reference voltage allow precise settings of the “Low Voltage Threshold” and the “High Voltage Threshold”. The thumbwheel switches and resistors create two voltage dividers. The output of the low voltage threshold voltage divider is equal to the low voltage threshold thumbwheel setting, and the output from the high voltage threshold divider is equal to the high voltage threshold thumbwheel setting. These two reference voltages are fed into two comparators.
When the battery voltage falls below the “Low Voltage Threshold” setting, comparator 1 changes state, triggering the Flip Flop, and the inverter is turned off. The Transfer Switch senses the loss of AC voltage from the inverter and switches the load to grid-supplied AC power.
When the battery voltage rises above the “High Voltage Threshold” setting, comparator 2 changes state, resetting the Flip Flop, and the inverter is switched on. The Transfer Switch senses the AC voltage from the inverter, and connects the load to the inverter.
Initially, I’ll set the low voltage threshold voltage to 12.25 volts. That voltage represents an approximate 75% state of charge (SOC). I’ll set the high voltage threshold at 14.75 volts, ensuring that the batteries are fully charged before allowing them to power the load.
It’s interesting to note that the battery voltage will not reach the high voltage threshold setting unless the sun is shining and the batteries have been fully charged. I’ll experiment with other settings in an attempt to improve system efficiency without endangering the batteries.
Benefits:
Since I’m not able to constantly monitor battery voltage, I’ve missed opportunities to use as much of the free power that my solar electric system is capable of providing. Instead, I disconnect the load when I think that the battery SOC may fall below 80%. Once I’ve implemented this plan, I’ll be able to use more of the available power from the system without the fear of damaging the batteries. Additionally, this automation will help to keep the batteries at a high enough SOC to ensure the availability of power in the event of a grid power failure.
With a Twist:
Because of a utility company plan that results in low rates at night, I’m thinking about storing energy in a larger battery bank at night when rates are low, and using that energy to power loads during the day when utility rates are high. And because my PV array is still small, I’m thinking about using a battery charger to supplement the charging that now comes from my PV array. The charger will be turned on via a timer in the early morning hours when rates are lowest, and turned off later in the morning before rates go up. When cloudy conditions limit the amount of charging my PV panels are able to provide, the battery charger will take up the slack and the batteries should be fully charged each morning. Confident that I’ll have plenty of stored energy in the morning, I’ll add more to the daytime battery load, and therefore save money on my electric bill.
With the charger switched on during the early morning hours, it’s likely that the batteries will quickly become fully charged. When that happens, the inverter will once again be used to power the load. Because the charger is still connected, and switched on via the timer, it too will provide power to the load. However, this will occur in the early morning hours when electric rates are low, and I’ll be taking advantage of the lowest rates, cutting my electric bill.
The graph below shows the expected results over a 24 hour period. As a result of charging from the PV panels during the day, and charging from the battery charger at night, the load will be powered by the batteries most of the time.
John
Friday, December 21, 2007
Getting the Most from a Small Off-Grid PV System
Although my off-grid system is small, I’m proud to say that by using it I’m reducing the amount of CO2 that would otherwise be released into the atmosphere and I’m reducing my electric bill. However, it’s a challenge to use as much solar-generated energy as possible, while still being gentle to the batteries. Allowing too much power to be sucked out of the batteries each night will shorten their life. On the other hand, I don’t want to under-use the system by not using enough of the available energy each day. Unused energy is wasted energy. The purpose of this article is to explore strategies for getting the most from a small off-grid PV system.
Strategy #1
A 100-watt solar panel might produce 500-watt/hours of electricity on a sunny day. But, due to system inefficiencies, it’s likely that only about 300-watt/hours of energy will make it to the load. Using solar-generated energy during peak sunlight hours is one way to improve efficiency. When power from the solar panels goes straight to the inverter, overall efficiency is much greater, since the losses associated with storing and retrieving electrical energy in batteries are eliminated.
The AC load can be plugged into the PV system’s inverter during the day and then to grid-supplied power at night, or switched automatically via a timer as illustrated below. A refrigerator or freezer is an ideal load for this scenario, since the amount of electricity required is about the same from day-to-day. The timer can be adjusted to match the power generated by the PV system to the power required by the load.
Strategy #2
A no-cost way to squeeze every available electron from an off-grid system is to constantly monitor battery voltage, disconnecting the load when the battery falls to a certain level, and reconnecting it when the battery is once again fully charged. Obviously, monitoring the system 24 hours a day is not practical. However, most DC to AC inverters include a low-battery “alarm” feature. The alarm sounds when the battery voltage drops below a certain level, eliminating the need for constant monitoring. Still, listening for an alarm is only a modest improvement over constant monitoring. No one wants to respond to an alarm in the middle of the night. Another drawback of this strategy is that the low-voltage alarm level is usually not adjustable. Typically, the alarm threshold voltage is about 10.6 volts (for a 12-volt inverter), which, depending upon battery type, translates to about a 75% depth of discharge. Most inverters will shut-down when battery voltage falls a little below the alarm threshold. While this strategy will work to a certain extent, you shouldn’t allow your battery state of charge (SOC) to fall that low. Allowing battery voltage to drop to those depths on a frequent basis will shorten the life of the battery. This strategy is not recommended on a day-to-day basis, but could be used in emergency situations, such as a grid power failure.
Strategy #3
To automate the process of disconnecting and reconnecting the load based on battery voltage, a charge controller with built-in low voltage disconnect (LVD) capabilities can be used. An LVD-capable charge controller is a great strategy for unattended systems, such as a weekend cottage. If a load is accidentally left on, the battery will be protected.
Although this is a good solution for applications where power to the load flows through the charge controller, it is not viable in systems where power to the load does not. The input current of an inverter is usually higher that the charge controller can handle, making it necessary to connect the inverter directly to the battery.
The diagram below shows typical wiring for a small, off-grid, PV system. Notice that the inverter is connected directly to the battery, and therefore will not be shut down when the Charge Controller’s low voltage disconnect kicks in. Only the DC output from the Charge Controller will be shut down in this scenario. Battery voltage will continue to decline (because of the AC load), even after the Low Voltage Disconnect kicks in. At some point the inverter will stop functioning. This is not a good strategy unless you’re only using the DC output of the Charge Controller to power a load, or unless you’re carefully monitoring the battery voltage while powering an AC load.
Strategy #4
Some Charge Controllers can be configured as “Load Controllers”, providing another strategy for getting the most from a small system. A Load Controller does not replace a Charge Controller, it is an add-on to the system. An advantage of using a Load Controller is that disconnect and reconnect thresholds can be precisely set. Settings are determined by the type of batteries, and by the size of the load. When properly set, the disconnect set-point prevents the battery from discharging too much, and the reconnect set-point value is high enough to ensure that the battery is not damaged by chronic undercharging. In other words, the disconnect and reconnect settings are optimized. The extra hardware does, of course, add considerable cost to the system.
Shown below is a typical application of a Load Controller. In the example, the Load Controller uses the batteries as the primary source of energy, but switches to a secondary source of energy when the batteries are depleted to a preset level. Once the batteries are recharged, they are again used to power the load. The beauty of this system is that it uses as much “free” energy from the sun as is available, only using a more costly source of energy when necessary.
Strategy #5
Those with electronics skills may consider the solution outlined below. This diagram represents a simple circuit that can replace the “Load Controller” and relay in the diagram above. Disconnect and reconnect voltage levels are set via two potentiometers. The potentiometers could be replaced by thumbwheel switches and precision resistors, providing better control of the threshold voltages. Notice that the relay wiring doesn’t allow AC from the inverter and AC from the power grid to be applied to the load simultaneously. Allowing that to happen would probably result in damage to the inverter.
Strategy #6
The battery-protecting switchover functions described above are also available in equipment designed for larger systems, but that equipment may not be appropriate for small systems. In addition to the high cost, this equipment typically uses more power for its operation than equipment designed for smaller systems. While this power drain may not be significant for a large system, it represents a significant percentage of the overall energy production of a small system.
Conclusion
In compiling this information it has been my intention to demonstrate optimization while holding down the cost. You may benefit from using one or more of the strategies outlined here. I welcome other suggestions for accomplishing the task.
John
Strategy #1
A 100-watt solar panel might produce 500-watt/hours of electricity on a sunny day. But, due to system inefficiencies, it’s likely that only about 300-watt/hours of energy will make it to the load. Using solar-generated energy during peak sunlight hours is one way to improve efficiency. When power from the solar panels goes straight to the inverter, overall efficiency is much greater, since the losses associated with storing and retrieving electrical energy in batteries are eliminated.
The AC load can be plugged into the PV system’s inverter during the day and then to grid-supplied power at night, or switched automatically via a timer as illustrated below. A refrigerator or freezer is an ideal load for this scenario, since the amount of electricity required is about the same from day-to-day. The timer can be adjusted to match the power generated by the PV system to the power required by the load.
Strategy #2
A no-cost way to squeeze every available electron from an off-grid system is to constantly monitor battery voltage, disconnecting the load when the battery falls to a certain level, and reconnecting it when the battery is once again fully charged. Obviously, monitoring the system 24 hours a day is not practical. However, most DC to AC inverters include a low-battery “alarm” feature. The alarm sounds when the battery voltage drops below a certain level, eliminating the need for constant monitoring. Still, listening for an alarm is only a modest improvement over constant monitoring. No one wants to respond to an alarm in the middle of the night. Another drawback of this strategy is that the low-voltage alarm level is usually not adjustable. Typically, the alarm threshold voltage is about 10.6 volts (for a 12-volt inverter), which, depending upon battery type, translates to about a 75% depth of discharge. Most inverters will shut-down when battery voltage falls a little below the alarm threshold. While this strategy will work to a certain extent, you shouldn’t allow your battery state of charge (SOC) to fall that low. Allowing battery voltage to drop to those depths on a frequent basis will shorten the life of the battery. This strategy is not recommended on a day-to-day basis, but could be used in emergency situations, such as a grid power failure.
Strategy #3
To automate the process of disconnecting and reconnecting the load based on battery voltage, a charge controller with built-in low voltage disconnect (LVD) capabilities can be used. An LVD-capable charge controller is a great strategy for unattended systems, such as a weekend cottage. If a load is accidentally left on, the battery will be protected.
Although this is a good solution for applications where power to the load flows through the charge controller, it is not viable in systems where power to the load does not. The input current of an inverter is usually higher that the charge controller can handle, making it necessary to connect the inverter directly to the battery.
The diagram below shows typical wiring for a small, off-grid, PV system. Notice that the inverter is connected directly to the battery, and therefore will not be shut down when the Charge Controller’s low voltage disconnect kicks in. Only the DC output from the Charge Controller will be shut down in this scenario. Battery voltage will continue to decline (because of the AC load), even after the Low Voltage Disconnect kicks in. At some point the inverter will stop functioning. This is not a good strategy unless you’re only using the DC output of the Charge Controller to power a load, or unless you’re carefully monitoring the battery voltage while powering an AC load.
Strategy #4
Some Charge Controllers can be configured as “Load Controllers”, providing another strategy for getting the most from a small system. A Load Controller does not replace a Charge Controller, it is an add-on to the system. An advantage of using a Load Controller is that disconnect and reconnect thresholds can be precisely set. Settings are determined by the type of batteries, and by the size of the load. When properly set, the disconnect set-point prevents the battery from discharging too much, and the reconnect set-point value is high enough to ensure that the battery is not damaged by chronic undercharging. In other words, the disconnect and reconnect settings are optimized. The extra hardware does, of course, add considerable cost to the system.
Shown below is a typical application of a Load Controller. In the example, the Load Controller uses the batteries as the primary source of energy, but switches to a secondary source of energy when the batteries are depleted to a preset level. Once the batteries are recharged, they are again used to power the load. The beauty of this system is that it uses as much “free” energy from the sun as is available, only using a more costly source of energy when necessary.
Strategy #5
Those with electronics skills may consider the solution outlined below. This diagram represents a simple circuit that can replace the “Load Controller” and relay in the diagram above. Disconnect and reconnect voltage levels are set via two potentiometers. The potentiometers could be replaced by thumbwheel switches and precision resistors, providing better control of the threshold voltages. Notice that the relay wiring doesn’t allow AC from the inverter and AC from the power grid to be applied to the load simultaneously. Allowing that to happen would probably result in damage to the inverter.
Strategy #6
The battery-protecting switchover functions described above are also available in equipment designed for larger systems, but that equipment may not be appropriate for small systems. In addition to the high cost, this equipment typically uses more power for its operation than equipment designed for smaller systems. While this power drain may not be significant for a large system, it represents a significant percentage of the overall energy production of a small system.
Conclusion
In compiling this information it has been my intention to demonstrate optimization while holding down the cost. You may benefit from using one or more of the strategies outlined here. I welcome other suggestions for accomplishing the task.
John
Friday, December 14, 2007
Driving a PHEV is Like Buying Gasoline at 20 Cents a Gallon
While you might think a Plug-in-Hybrid-Electric-Vehicle (PHEV) is simply an ordinary hybrid with a plug added for charging, there is another significant difference between the two. Typical hybrid cars on the road today have a gasoline motor and an electric motor, and either of them can be used to drive the wheels. On the other hand, power to the wheels on the new breed of PHEV’s will come ONLY from the electric motor. The gasoline motor is used to recharge batteries. This design is inherently more fuel-efficient, since the gasoline motor will run at a constant speed. It’s called a “series” hybrid, and the Chevrolet Volt is an example. The plug-in Volt is expected to be available in 2010. It will have a 16kwh lithium-ion battery that will take 6 to 6 ½ hours to charge, and a fully-charged battery can propel the car for the first 40 miles. The gasoline motor kicks-in after that, extending the range of the car. For those who drive less than 40 miles per day, which is most of us, the car will use no gasoline at all.
Based on that information, it’s easy to calculate operating expenses. Assuming that electricity costs ten cents per kwh, the cost to charge the battery should not exceed $1.60, which is ten cents per kwh times 16kwh. In reality, the battery will not be fully drained, and therefore the actual cost will be less than that. But for no more than $1.60 worth of electricity, the car will go 40 miles. It takes $3.00 to $6.00 worth of gasoline to go that far in a typical gasoline-powered car!
Nighttime electric rates are much less than daytime rates in many locations, and I’ve signed up for a plan that gives me rates as low as two cents per kwh in the early morning hours. My rate will change from day to day and hour to hour, but if it averages less than four cents per kwh at night, I’ll be able to charge my Volt for as little as twenty six cents. Needless to say, I can hardly wait to own one. When compared to a car that gets 30 mpg, this is equivalent to gasoline at 20 cents per gallon for the first 40 miles of driving each day!
In reality, state and federal legislators will soon realize that I’m not paying my fair share of road use taxes, and somehow I’ll be forced to make up the difference, but I’ll certainly have some unbelievably inexpensive transportation in the mean time.
Here’s the info from Chevy: http://www.chevrolet.com/pop/electriccar/2007/process_en.jsp
John
Based on that information, it’s easy to calculate operating expenses. Assuming that electricity costs ten cents per kwh, the cost to charge the battery should not exceed $1.60, which is ten cents per kwh times 16kwh. In reality, the battery will not be fully drained, and therefore the actual cost will be less than that. But for no more than $1.60 worth of electricity, the car will go 40 miles. It takes $3.00 to $6.00 worth of gasoline to go that far in a typical gasoline-powered car!
Nighttime electric rates are much less than daytime rates in many locations, and I’ve signed up for a plan that gives me rates as low as two cents per kwh in the early morning hours. My rate will change from day to day and hour to hour, but if it averages less than four cents per kwh at night, I’ll be able to charge my Volt for as little as twenty six cents. Needless to say, I can hardly wait to own one. When compared to a car that gets 30 mpg, this is equivalent to gasoline at 20 cents per gallon for the first 40 miles of driving each day!
In reality, state and federal legislators will soon realize that I’m not paying my fair share of road use taxes, and somehow I’ll be forced to make up the difference, but I’ll certainly have some unbelievably inexpensive transportation in the mean time.
Here’s the info from Chevy: http://www.chevrolet.com/pop/electriccar/2007/process_en.jsp
John
Subscribe to:
Posts (Atom)
