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Following the off-grid project I began back in 2006, it was now time to start investigating the next step in renewable electricity production, the grid-tie system. You may recall that when I first discussed this there were no grid-tie systems allowed in Ireland. Since that time, however, ESB Networks, under pressure from various stakeholders and the common man, have finally begun issuing accreditations to companies and products which they feel meet the grid-tie specifications for the Irish market. However, the number of accredited installers are few on the ground so far, and that is also limiting, to an extent, the number of systems and options available. Grants: Again, nothing appears to have changed from when I last checked in 2006; there are still no grants available to for micro-electricity generation, and that does not look likely to change in the near future. So yet again, ir order to go green, we need to fund all of this ourselves. I sure hope that lottery win is just around the corner.... Grid-Tie Explained: Grid-Tie refers to the linking of the AC output generated from an inverter to the household mains supply. In fact, it should be more accurately described as "House-Tie", as it is tied directly to the house supply, not the external Utility grid. However, by extension the house supply itself is fed from the Utility Grid, hence the term which we are now stuck with. It sometimes goes by the name of Grid-Connect rather than Grid-Tie. The system comprises of an energy generating source, typically wind or solar, in the same way as we have for the off-grid systems. The output from the wind/solar is then fed directly into the Grid-Tie inverter, which in turn creates the AC power which is then plugged into any suitable ring-main. The system does not make use of batteries, so the system components are simplified : wind turbine ----> isolator switch ----> grid-tie inverter --/--> AC ring-main solar PV ----> isolator switch ----> grid-tie inverter --/--> AC ring-main One thing to bear in mind is that, unlike a batter charging system, most Grid-Tie inverters are specifically engineered to handle either wind or solar, but not both. The characteristics of power from the two systems are sufficiently different that a different inverter is required depending on the supply source. One of the main safety nets of the system is that it only allows AC power to be fed to the ring-main circuit if it detects AC available on the circuit. So, if the electricity goes off, say due to a power cut, then the Grid-Tie inverter senses that there is no current flowing on the circuit and cuts its own supply also. This ensures that 230AC is never fed through the house consumer unit and outwards into the local Utility Grid. Otherwise, if someone was working to restore power lines, there is a possibility of electrocution as the line would be live. "Islanding" is the term used for this, so grid-tie inverters have dedicated circuitry to prevent islanding and shut down automatically. How It Reduces Electricity Bills: While the system is operating normally, it supplies power to the house grid so that any connected appliances are then using that power. If the power draw is larger than can be supplied by the inverter, then the supplemental power is drawn from the Utility Grid. So, for example, lets assume the inverter is outputting 300watts of power at a given point in time. If the total load on the ring main is 310watts, then you are only billed for the 10watts that you draw through your consumer unit; you don't get charged for the 300watts which is being generated and consumed locally. That's where the savings are made. And naturally, the larger the system the more power is available to the ring-main and thus you benefit from larger reductions on your ESB bill. Excess Generation And Net Billing: Well, it's not all good news here I'm afraid. What if you are generating 300watts but only using 290watts? Well, this time the extra 10watts is termed 'spillage', and this is where it all gets a bit muddy. The term 'Net Billing' means that this 10watts is fed outwards into the Utility Grid and then you get a credit against this on your next bill. But, in order for that to happen we need: a) a metering system at the house which identifies the 10watts and records it as a "net amount". b) a Utility provider which notes your "net amount" and agrees to credit you for this, further reducing your bill. It is these 2 points which are still being debated by the powers that be. There is a draft legislation which has suggested a rollout of smart metering to the whole of the population should begin this year (2008). This smart meter can record both incoming and outgoing electricity flow from the premises, along with other functions. But then, who will pay for the new meters, and does the ESB have the facilities and infrastructure to actually employ these meters in the manner suggested? Finally, the big question: how much, if any, will the ESB pay me for the unit of electricity that I generate myself and export to the Utility Grid? I have already heard rumours that suggest the price paid for generated units will be pro-rated based on this time of the day it is exported to the GRID and will never be higher than 30% of the current price per unit. If that is the case, perhaps we should be ensuring no 'spillage' ever takes place whenever a surplus is present, and that all electricity generated on site is consumed on site. Also, from an eco-friendly and C02 point of view, it is very wasteful to allow excess electricity to be fed back to the Utility Grid only for it to be subsequently fed back into the home again when demand exceeds supply ! System Options:
As shown in the picture above the general layout of my current off-grid system has 2 wind inputs + a dedicated solar input, all feeding into the battery bank and then on to the standalone inverter. The inverter then powers specific circuits in the house and can be swapped over using a changeover switch. So, rather than create a totally separate Grid-Tie system, wouldn't it make sense to integrate the new Grid-Tie system such that power from it could also be used in my off-grid system? Bearing in mind the issues with 'net metering' and losing any hard earned electricity back to the GRID, surely it makes sense to link the systems together so that the EXCESS energy or 'spillage' from the Grid-Tie system is fed back into the batteries and is then stored for use in the off-grid system ...?
Although the diagram looks simple enough, the question is: can this be done. Almost every Grid-Tie system installed up to now does not interface with an off-grid systems, but I want to change that and link both together. So that 10watts of spillage we mentioned above now becomes 10watts of power feeding into the batteries. That's the plan. I can only see 2 ways to achieve this. The first is to obtain a Grid-Tie inverter with dedicated battery charging all built in to a single box. This would be ideal, but sadly such a product does not exist, and if it did the electronics would of course become that much more complicated. The second method is to divert the 230VAC power coming from the Grid-Tie inverter into a battery charger. This seems simple enough but of course it raises a number of issues such as:
As you can see there are a number of questions that need answering. At present I am posing these questions to various companies in the renewable sector and will feed back my results in due course. More information to follow...
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