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

Most of the Grid-Tie inverters available are specifically engineered to handle either wind or solar, but there are a few available at the lower end of the scale that can handle both. However these tend to be restrictive and usually the best option is to choose inverters specifically designed for the task.

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 230VAC 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 grid-tie 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 electricity bill.

Excess Generation And Net Billing:

What if you are generating 300watts but only using 290watts? Well, this time the extra 10watts is termed 'spillage', and, this is where the householder benefits from a payback for those extra 10watts if using correctly commissioned grid tie inverters. So 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.

So to fully implement this system requires a rollout of a smart meter or import/export meter, which is now viable following the introduction of the grid-tie framework. In 2009/2010 there were special incentives for the initial applications, which paid back the generator for the first 3 years at a cost equivalent to the cost of of buying electricity (19c), thereafter reducing to a  payback rate of 9c per unit, or appx 50%. That is a reasonable and fair ongoing rate if it proves correct, and is not subject to downward adjustment over time. For some of the early adopters there were also grants available of up to 40% of the system price for a limited number of technologies across each renewable sector (wind, hydro, solar..). I would imagine that anyone who got one of these grants and availed of the buy-in tariff for 3 years at the full payback price will stand a good chance of generating free electricity after appx 15-18 years of operation.  

For the remainder however, getting a feed in tariff of half the initial rate will mean even longer payback periods. And with the recession in full swing, its anyone's guess whether these tariffs will remain in place.

For me, preventing the 'spillage' from taking place, and consuming all electricity generated on site would be the best solution. 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! But in the cases of wind power, that would be the norm, especially in a typical scenario of late night wind coupled with low usage requirements.

System Options:

Originally I had hoped to install a hybrid grid-tie with battery backup system, but that has proved too difficult to plan. Therefore, this experiment will focus solely on using the existing solar PV and wind turbines to link into the house grid directly, instead of charging batteries for off-grid use. With 2 x 400W turbines available + 800W solar PV, this gives a potential of 1.4Kw.

As shown in the diagram above, the plan requires new grid-tie inverters for the PV panels, alongside new inverters for the wind turbines. The standalone pure sine inverter as used in off-grid use cannot be used for this purpose. There were a number of questions I faced when implementing this such as

• do I obtain a 100W inverter for each 100W solar panel ?
• how do I link a tri-phase 12V turbine to a grid-tie inverter, and what about excess winds ?
• what additional controls do I need in such a system ?
• how do I measure how well each of the the products are working  ?
• do I want to get paid for any excess I generate ?

Lots of questions - but one of the easiest to answer immediately was the last one -  do I want to get paid for any excess? No, I don't. There are 2 reasons for this - firstly with a maximum available of 1.4Kw the benefits would be very small. Secondly, the government introduced a new certificate called EN50438 for grid-tie inverters that can be wired into the consumer boards. These are almost identical to the G83 standard in the UK, except for small minor differences in disconnect/reconnect times. But it means the government will no longer accept G83 (UK) certified equipment, and the lack of cheap EN50438 inverters means the setup costs would be high (when you add in the cost of the electrician also).  Finally there would also be a cost to install the new smart meter. All these extras are required if you have a system big enough to want full grid-tie with net billing. 

So, lets see how we will configure the grid-tie system ....