Monday 15 December 2014

AC Coupling – Converting your Grid Tied PV System into a High Efficiency Battery Backup System

Posted at 3:32 PM

This time we’ll be talking about grid-tied systems that utilize batteries for energy storage. When the grid is absent we can rely on the batteries to power our “critical loads” these loads generally consist of refrigerators, freezers, well pump, some lighting circuits, etc. When we have a grid-tied system with battery back up the batteries are most often maintained in a ‘float’ charge where the battery is maintained at full capacity waiting for that critical moment. We’ll look at why strictly grid-tied systems are most often used, how batteries are traditionally incorporated into grid-tied systems and focus on a newer type of battery based system that couples the PV on the AC side of a battery based system to gain higher efficiencies.  

The vast majority of solar PV systems installed today are strictly grid-tied systems that do not utilize batteries for energy storage. These systems are very simple and efficient and require the least amount of components; the solar array, a utility-interactive inverter (to convert the solar modules DC current to AC current) and installation components for a code compliant system. Instead of storing the electrical energy in a battery bank excess energy produced goes onto the electrical grid virtually spinning the utility meter backwards. The energy that is delivered to the grid is ‘stored’ as a credit that is subtracted from the energy consumed every month. Often times the energy delivered to the utility exceeds the amount of energy consumed by the home or business and when the credit amount reaches a certain monetary value set by the utility, the utility will cut the system owner a check. Since there is no energy storage on site there is no way to utilize the energy from the solar array when the connection between the grid and the solar PV is interrupted due to a power outage or other event. This type of system is most often used because it is the least expensive and because of the higher efficiency that it delivers.  

Not being able to use your solar array during a power outage can be frustrating and some people opt for a system that utilizes batteries to store energy for those events when the grid is absent. In most areas around Wisconsin the utility is reliable and the amount of times that backup power is needed is generally few and far between. Investing in a PV system with battery backup from the get go has always been the most expensive system due to the number of components and complexity of the system, but you had the best of both worlds being able to have off-grid capability and be able to sell excess energy (after batteries were charged up) to the utility instead of diverting it to dump loads or wasting that energy.   Traditionally a PV array is connected or ‘coupled’ to the DC side of a battery-based system . The array is wired at a lower voltage to better match the battery bank and a charge controller is used to manage the energy from the PV array to effectively charge the battery. When the battery is full many battery inverters will sell excess PV power to the grid. In this configuration there are more efficiency losses in the system as a whole (when compared to a strictly grid-interactive inverter). We may see only 75% efficiency in this type of system as we have more components; charge controller, batteries, inverter(s) and we also have more potential for losses in the multiple wire runs at lower voltages. With only 75% efficiency you have an array that is selling significantly less power to the utility.  

AC coupling a PV system has many advantages. The main advantage is having the higher efficiency of the grid-tied system converter the PV power. We may see only 10% losses overall in the DC to AC conversion (selling potentially 15% more power to the utility). Another advantage is that it can easily be added to an existing PV system. AC coupling uses a strictly grid-tied inverter and a battery based inverter. During grid operation the grid-tied inverter is connected to the ‘critical load’ panel and the battery-based inverter is allowing the inverter will use power from PV or the grid to maintain the batteries in their ‘float’ charge. See the illustration below showing energy flow during operation when the grid is present.   During grid operation the batteries are maintained at the float charge, ready and waiting.  

During off-grid operation (when power is lost from the utility) the battery-based inverter acts as the grid signal for the grid-tied inverter and allows you to utilize the PV during the outage. If during the outage all the ‘critical loads’ are being met and the battery is at full capacity. The power is diverted from the battery using a charge controller and a resistive diversion load, such as; a DC water heating element or a DC air heater. The illustration below shows the off-grid operation.

Only ‘critical loads’ are powered during off-grid operation. The system added to an existing PV system can be very cost effective and give the piece of mind that backup power offers. The configuration of the PV array doesn’t need to be changed at all and only the battery-based inverter, battery bank, battery management components and a ‘critical load’ panel need to be added. Most importantly the high efficiency of the grid-tied inverter is maintained. It may be only once or twice a year when the battery system is asked to perform it’s crucial task and the other 360 days of the year you can utilize your PV at a very high efficiency, powering your loads and selling excess power to the grid. With the traditional DC coupled array during those other 360 days you may be wasting ¼ of your array potential only operating at 75%.  

Solar is hands down a very rewarding investment. AC coupling allows you to have an even more rewarding experience by giving you the battery back up capacity when the grid is absent while utilizing your existing, high efficiency PV array. We’d love to retrofit all of the central Wisconsin grid-tied systems out there, so give us a call!