AN ENGINEER’S CRITICAL ANALYSIS OF THE VPP SO FAR
In this article I want to explore what has happened since our AGL VPP Sunverge system was installed and made operational on 26/05/17, and what the changes to the VPP could mean. I have been closely monitoring the performance of the Sunverge system since inception, and would like to share my observations, and why I have chosen the Tesla Powerwall 2.
In March 2017 I published articles, amongst others, on “A Cost Benefit Analysis of the VPP Using 3 Years of Realtime Data”, “Statistical Analysis of Solar Energy Production in Adelaide” and “Why I, as a retired electrical engineer, joined the AGL VPP”.
I should state at the start that we currently have a Sunverge SIS-7048 DC coupled system, whereas most VPP customers will have an AC coupled system. There are many articles on the web that will describe the difference between these systems, but I will try and describe it in simple terms without complicating diagrams. I have a feeling that many participants in the VPP had no idea they weren’t getting prolonged backup facilities by having an AC coupled system.
I will start by clarifying the difference between power and energy because we very commonly talk about the “power system” and power poles. Power is instantaneous whereas energy is the use or storage of power over time. Another way to consider it is power can’t be stored but energy can. In simple electrical terms power is current multiplied by voltage. A 5 kW inverter is capable of power output of 5,000 watts which is 20 amps at 250 volts. If that inverter outputs 5 kW for 2 hours, that would be 10 kWh of energy.
Solar panels produce direct current (DC) and a DC coupled system has the solar panels connected directly to the solar battery via a charger. The voltage that solar panels produce is not constant, so the charger uses some clever electronics called MPPT (maximum power point tracking) to extract the absolute maximum energy from the panels under all conditions. The battery then powers an inverter to generate the 240 volt alternating current (AC) which powers the home and exports any excess to the grid at the feed-in tariff rate. In the event of a prolonged grid outage, the whole system can be isolated from the grid, and the solar panels will continue to charge the battery, and the inverter will provide as much energy as can be generated each day from the sun to power up the backup circuits, or critical load panel. In the event of a grid outage, should solar production exceed household and battery demand, solar production will be automatically limited or stopped to protect the battery from overcharging, but will restart when required.
In an AC coupled system, the solar panels connect directly to an MPPT solar (grid-tied) inverter, and the resultant 240 volt AC current is used to charge the battery, power the home or export to the grid. A second inverter, connected to the battery, is used to power the system when solar energy is not available, such as at night time.
The first important thing to know about inverters designed to connect to the grid is that they must be able to synchronise their output to both the grid voltage and the grid frequency of 50 cycles per second (50 Hz). The second vital point is that if the grid is down, they must stop working in order to protect any power workers who might be working on the grid. To do this, solar inverters look for a 240 volt 50 Hz AC grid voltage and frequency on their output side before switching on. A DC coupled system can just isolate itself from the grid and keep working because the panels are connected via DC to the battery charger and battery. However, an AC coupled system will only produce solar energy to charge the battery if the solar inverter can see the required 240 volts and 50 Hz frequency at its output. This is normally provided by the grid, but if the grid is out, this voltage must come from the battery inverter. The problem arises when the battery runs down and shuts off to protect itself. The solar inverter then can’t operate because it can’t see a 50 Hz voltage with which to synchronise from either the grid or the battery inverter, and so it can’t restart until the grid becomes live again. Therefore, in an AC coupled system with backup circuitry, all you get is most of what’s in the battery at the time of the outage, and from then on, your solar is useless. You have all those panels and no way to utilise them.
The obvious question then is why are AC coupled systems so popular? The reasons are all pretty well related to cost, as you might expect.
One of the most common scenarios is adding batteries to an already installed solar system. In this case an AC coupled system just requires connecting to the output of the existing solar inverter, and to the grid. From the installer’s point of view, this is safe and simple because they don’t have to disturb the existing solar inverter and the solar panel wiring behind that. Unfortunately many existing installations could be non-compliant because standards have changed over time or because of previous non-compliant workmanship. In a DC coupled installation to an existing solar system, as in our case, the installer is required to remove the existing solar inverter, and re-route the solar panel cables into the cabinet of the DC coupled system. In our case, during this process, it was discovered that our installation was non-compliant, and considerable work was needed to correct that situation. I give great recognition to CME who did this work at no cost to us.
An existing solar inverter will have its own MPPT controller built in, so removing the solar inverter means that the DC system has to have an MPPT charge controller built in, which adds to the cost. Also, a simple one way DC to AC inverter is cheaper to make than a hybrid inverter that is designed to generate AC from the battery, and also convert AC to DC to charge the battery from the grid.
The net result of this is that AC coupled systems are cheaper to make and cheaper to install. They are also slightly more efficient if most of your electricity is used during the day when solar energy is available.
In an AC coupled system, during the daytime, the solar panel DC current has to pass once through the solar inverter to be converted to AC (with the attendant losses associated with DC to AC conversion), and then is used in-house, or to charge the battery, or is exported to the grid, with no battery charge/discharge losses involved. At night, however, the energy used is retrieved from the battery via the battery inverter, so this solar generated electricity has passed through two inverters, an AC-DC battery charger, and includes battery charge/discharge losses.
DC coupled systems are more efficient at night because the electricity from the solar panels has passed through a highly efficient DC-DC MPPT charger, once through the hybrid inverter, and also includes battery charge/discharge losses. During the day the solar panel electricity has passed through the MPPT charger to the battery, and then through an inverter, and again includes battery charge/discharge losses.
This entire article is too large to post directly on the Community, so please follow the link below to access the complete document. In this document I look at the following in detail:
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