We’ve been living off-grid using a 2.5kw Proven to power our straw-bale house in West Cork, Ireland, writes Quentin Gargan.
Our farm had the grid (to keep freezers going) and we wanted to put up a second grid tied turbine to power our electric car. One thing led to another and I ended up developing a 2.5kw downwind turbine which is grid tied.
BUT we found customers wanting an off-grid version. When we looked into this, we realised that most off-grid systems have a 48V generator. Low voltages, high current, heavy cables and high losses.
More importantly, a modern grid tie inverter has a power point tracking system which ramps up the power taken from the coils as the turbine speeds up. Battery systems draw no power until the generator gets to 48V, and then the power drawn is quite random.
So we developed a way of using high voltage turbines (up to 560V) with their grid tied inverters, running into batteries with a charge controller that manages both bulk and float charge stages. This system is under test at the moment, and it looks good, and will work with most conventional turbines.
You can see our house and the original off-grid turbine at http://www.ardnashee.com, and we’ll put up a page on the new off-grid verion shortly.
The off-grid solution is generic and works with any wind turbine system. Using multiple inverters, it can manage turbines up to 20Kw, with loads up to 36kw. Packages are based on turbine sizes up to 3.6kw, or up to 6kw, and using multiple 6kw inverters above that level. Power output is in multiples of 3Kw. The system can work with either 24V DC or 48V DC.
Here is an outline of the system written for our own website – it gives an idea of the system and how it works. Hopefully this gives you the info you need.
It could be argued that the system suits continuous demand, such as telecoms, because stored power passes through the inverter twice, but the advantages of being able to use maximum power point tracking on a grid ties inverter way outweigh these losses.
Can you let me know where the post is so I can subscribe to it and keep an eye on it?
Kind regards, Quentin.
Most off-grid wind turbine systems use a lower voltage turbine to directly feed batteries, usually at 48V. A battery based inverter ramps this up to 230V for use throughout the home, often at low efficiency because of the voltages involved. We can supply the components for this system, but for wind turbines, this is inefficient in a number of ways;
Lower voltage, higher losses
Lower turbine voltages require higher currents to transmit the same amount of power. This is why power lines operate at such high voltages. Using low voltage results in larger cable sizes being needed, higher temperatures in the generator, less reliability, and reduced efficiency.
Optimising output for turbine speed.
On a grid tie system, wind turbine output is optimised by modern inverters which are capable of gradually loading the power taken from the turbine as its speed increases. The relationship between power available, and revs, is a curve which can be programmed into the inverter. The following graph shows a typical curve for voltage –vs- optimum power taken up from a turbine;
A battery based system will operate much more crudely. When the turbine voltage is below battery voltage, no power will be generated. Once it gets above battery voltage, the batteries control the voltage and loading of the turbine in a manner that cannot be controlled, resulting in reduced output from the blades which are not operating at their optimum speed.
We use a grid tie inverter to optimise the output from the wind turbine in the normal way. We then use a second charger and inverter to feed this power into the batteries and, when required, take it from the batteries to feed grid based appliances.
To manage this process, we need to also use a microprocessor based controller which is programmed to manage both inverters and to ensure prolonged battery life by monitoring battery conditions and temperature to control the charging process. The controller can also provide logging onto a standard SD card for monitoring, and can start up a backup generator if required.
This system has the advantage of optimising output from the turbine. Any wind power consumed at time of use does not need to go through the battery and can be supplied at efficiencies of up to 96% directly from the grid tied inverter. Battery inverter losses only apply to power which is stored for later use from surplus energy provided by the turbine. This has tremendous advantages in systems such as remote signal stations where power is used at all times. The controller can also be programmed to manage solar photovoltaic panels should they be incorporated into the system.
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