Hydroelectric power on a residential scale
It is well known that energy is generated by building dams over giant underwater turbines; however it is possible to use micro hydro generators (<100kW) or pico hydro generators (<5kW) on more modest water flows. In this section we explore where the technology can be used in a small scale area, for example the home or a community project. More about industrial size dams and solutions can be found in the green commercial section.
Obviously, there is a fundamental requirement on a steady stream of moving water, however they have an advantage over solar power (both solar PV and solar heating) and wind, in that they can run day and night and in any weather conditions provided the we don’t have a prolonged drought period where streams and brooks can dry up.
The amount of energy produced is reliant on two things:
THE FLOW OF WATER
The flow of water is simply the quantity of water flowing in the water source, which is measured in litres per second.
The other key factor is the head – this refers to the pressure at which the water hits the turbine blades, and is the vertical distance from the water source to the generator. The larger the distance that the water falls before it hits the blade, the higher the head. Ideally both the flow and the head will be high, however if one of these is particularly high, while the other is low there is still the potential for a rich source of electricity.
You can estimate the number of kilowatts of energy produced by multiplying the flow (litres/sec) by the head (m) and multiplying by 9.81 (gravitational constant). Remember a typical house uses 4500kWh per year.
How does micro hydroelectric work?
The type of turbine that is used varies depending on the type of flow available, however typically a residential generator uses a pipe to collect water from a river or a stream. Using gravity the water moves through the pipe ‘downhill’ and a generator situated within the pipe acts to change the kinetic energy from the water flow into electrical energy.
When you have high head (the vertical distance from the water source to the generator), you are best using an impulse turbine (such as a Pelton turbine). This turbine is not submerged in the water, instead it sits in the air, and consists of buckets around a central hub. The nozzle at the end of the pipe converts the water into a fast moving jet. This jet of water is directed at the buckets, and the force of the the water causes the turbine to spin generating the power. The smallest type of high head turbine requires a head of at least 10-14 metres, and a water flow of 3-4 litres/ second, and this is rated at producing 200 watts of power.
For medium head water flows, it is best to use a reaction turbine. With a 3-12 metre head and a water flow of 45 litres/ second, you can get a reaction turbine that will produce about 3000 watts of power. Obviously as with the high head turbines, if either the head or the flow increases, you will see dramatic increases in the potential electricity your system is capable of generating.
For low head water flows, you obviously require a high flow rate, and in this situation an old style water wheel is the best. So the water fills the buckets which fill up, then pulling the wheel down, so the next bucket is filled, and this process is continued so the wheel spins (albeit very slowly). However the advantage of this type of system is that any potential blockages just simply wash through the system. Gearing can be used in conjunction with water wheels to increase the speed that the generator spins to help electricity production. Water wheels are also aesthetically pleasing on the eye!
Summary of micro hydroelectric power
If you are lucky enough to have a water flow source on your property that either has high head or sizeable flow, a micro hydroelectric generating system may be the perfect solution for your energy needs. Despite potential seasonal fluctuations in flow and head, a micro hydroelectric system will provide you with electricity 24/7, with very little maintenance necessary.
Types of micro-hydro facilities
Small scale hydroelectric facilities are generally classified into three sizes:
- micro-hydro – up to 5 kW
- mini-hydro – between 5 kW and 20 kW
- small commercial hydro – between 20 kW and 10 MW.
Micro-hydro systems for houses and buildings are less than 5 kW, and in many cases less than 1 kW. Micro-hydro systems are best suited to rural areas on streams or waterways that flow all year round. The more vertical distance (head) you have between the point where you take the water and where the turbine is located, the more electricity you can generate.
How hydroelectricity works
Hydroelectricity systems use the force of running water to turn turbine blades, which spin a shaft connected to a generator. On rural sites they can be set up wherever water falls from a higher lever to a lower level, for example a waterfall, hillside, stream, or where a reservoir discharges into a river.
The type of turbine you need depends on the vertical distance the water falls and the rate the water flows. Pelton wheels are the type of turbine most commonly used for small scale domestic generation.
Small scale hydro systems don’t usually need water storage. A portion of the stream or river is temporarily diverted through a pipe system to the micro-hydro turbine and generator. It’s then returned to the same stream or river. This type of system has far less impact on the environment than large scale hydro schemes. If your small scale hydro scheme does need a dam or other form of water storage, you’ll need to get consent.
Does it make financial sense?
If you are looking at micro-hydro for purely economic reasons, then EECA strongly advises you do your sums carefully before you buy – especially if you are already connected to the electricity grid.
Costs vary depending on your location and requirements. Each micro-hydro system is designed to suit the specific features of a property. For a domestic system with a basic layout, expect to pay at least $10,000 to $15,000. Despite the high initial set up cost, running costs are low.
Factors influencing cost
- Size – larger systems are generally cheaper per kW.
- Geography and geology – depending on your site, it might take a few days or weeks to install your system.
- Damming – if you need to build a dam to store water it will cost more.
- Earth works and flood protection.
- Length of water pipes and electrical cables.
- Building and resource consents.
As the generating capacity of micro-hydro systems is quite small, you will most likely need to remain connected to the electricity grid and electricity continues to be purchased from a retailer during times when the micro-hydro system does not generate enough electricity.
Some households choose to go ‘off-grid’, disconnecting from the grid entirely and purchasing batteries to provide electricity when the micro-hydro system is not generating enough electricity to meet demand. Although battery prices are falling, this approach won’t make economic sense for most households that are already grid-connected. However, for new remote homes or farms facing a high cost to connect to the grid, going ‘off-grid’ with micro-hydro and batteries may be cost-effective.
Excess micro-hydro electricity can be sold to a retailer
At times, micro-hydro will generate more electricity than can be used, allowing grid-connected households to sell electricity to an electricity retailer. The retailer buys this electricity at a ‘buy-back rate’- these vary but are lower than the amount companies charge you for electricity. This means households with micro-hydro get greater value out of using the electricity they generate themselves, rather than selling it back to a retailer.
When considering micro-hydro, consider the buy-back rates on offer. You may need to switch from your current retailer to access a buy-back rate. The retailer offering the best buy-back rate may not necessarily charge the lowest for the electricity they sell.
Checklist for building a micro-hydro system
- Get expert advice – SEANZ (Sustainable Electricity Association of New Zealand) is the representative body for the onsite renewable electricity generation industry. EECA recommends consumers use SEANZ members to provide advice, quotes and undertake micro-hydro installation work.
- Make sure the waterway is suitable – ideally it will have a good flow of water year round, and enough vertical drop over a small horizontal distance.
- Ensure the water supply is reliable – compared with wind or solar generation, micro- hydro systems provide a constant flow of electricity (as long as the volume of water flowing remains constant).
- Check out your rights to the water – you may need a resource consent from your regional council before using water to generate electricity.
- You need prior approval to connect micro-hydro to the electricity grid, so contact your lines company before you start and talk to your electricity retailer also. A micro-hydro installation expert will be able to assist with this.
- Although you may undertake some of the work to install micro-hydro yourself, all electrical work needs to comply with and be certified to NZ electrical standards. Your micro-hydro installation must comply with any local council regulations, so check with your council before going ahead.
- Make allowances for maintenance – of mechanical, electrical and hydraulic equipment. This may only involve a few hours a month. Intake screens need to be kept clear of silt and debris, and collection lakes may require de-silting every few years.