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Microhydro Electricity Basics Hydropower is based on simple concepts. Moving water turns a turbine, the turbine spins a generator, and electricity is produced. Many other components may be in a system, but it all begins with the energy already within the moving water. Water power is the combination of head and flow. Both must be present to produce electricity. Consider a typical hydro system. Water is diverted from a stream into a pipeline, where it is directed downhill and through the turbine (flow). The vertical drop (head) creates pressure at the bottom end of the pipeline. The pressurized water emerging from the end of the pipe creates the force that drives the turbine. More flow or more head produces more electricity. Electrical power output will always be slightly less than water power input due to turbine and system inefficiencies. Head is water pressure, which is created by the difference in elevation between the water intake and the turbine. Head can be expressed as vertical distance (feet or meters), or as pressure, such as pounds per square inch (psi). Net head is the pressure available at the turbine when water is flowing, which will always be less than the pressure when the water is turned off (static head), due to the friction between the water and the pipe. Pipeline diameter has an effect on net head. Flow is water quantity, and is expressed as "volume per time," such as gallons per minute (gpm), cubic feet per second (cfs), or liters per minute (lpm). Design flow is the maximum flow for which your hydro system is designed. It will likely be less than the maximum flow of your stream (especially during the rainy season), more than your minimum flow, and a compromise between potential electrical output and system cost. Measuring Head & FlowBefore you can begin designing your hydro system or estimating how much electricity it will produce, you´ll need to make four essential measurements:
Head and flow are the two most important facts you need to know about your hydro site. You simply cannot move forward without these measurements. Your site´s head and flow will determine everything about your hydro system?pipeline size, turbine type, rotational speed, and generator size. Even rough cost estimates will be impossible until you´ve measured head and flow. When measuring head and flow, keep in mind that accuracy is important. Inaccurate measurements can result in a hydro system designed to the wrong specs, and one that produces less electricity at a greater expense.
Microhydro-Electric System Types Off-Grid Battery-Based Microhydro-Electric Systems Most small off-grid hydro systems are battery-based. Battery systems have great flexibility and can be combined with other energy sources, such as wind generators and solar-electric arrays, if your stream is seasonal. Because stream flow is usually consistent, battery charging is as well, and it´s often possible to use a relatively small battery bank. Instantaneous demand (watts) will be limited not by the water potential or turbine, but by the size of the inverter.
Off-Grid Batteryless Microhydro-Electric Systems If your stream has enough potential, you may decide to go with an AC-direct system. This consists of a turbine generator that produces AC output at 120 or 240 volts, which can be sent directly to standard household loads. The system is controlled by diverting energy in excess of load requirements to dump loads, such as water- or air-heating elements. This technique keeps the total load on the generator constant. A limitation of these systems is that the peak or surge loads cannot exceed the output of the generator, which is determined by the stream´s available head and flow. This type of system needs to be large to meet peak electrical loads, so it can often generate enough energy for all household needs, including water and space heating.
Grid-Tied Batteryless Microhydro-Electric Systems Systems of this type use a turbine and controls to produce electricity that can be fed directly into utility lines. These can use either AC or DC generators. AC systems will use AC generators to sync directly with the grid. An approved interface device is needed to prevent the system from energizing the grid when the grid is out of action and under repair. DC systems will use a specific inverter to convert the output of a DC hydro turbine to grid-synchronous AC. The biggest drawback of batteryless systems is that when the utility is down, your electricity will be out too. When the grid fails, these systems are designed to automatically shut down.
Calculation of Hydro Power Before embarking on any hydro power generation project it is essential to survey the proposed site to calculate the amount of available hydro power.The larger the flow - i.e. the more water there is, and the higher the head - i.e. the higher the distance the water falls - the more energy is available for conversion to electricity. Double the flow and double the power, double the head and double the power again. A low head site has a head of below 10 metres. In this case you need to have a good volume of water flow if you are to generate much electricity. A high head site has a head of above 20 metres. In this case you can get away with not having a large flow of water, because gravity will give what you have an energy boost. The key equation to remember is the following: Power = Head x Flow x Gravity where power is measured in Watts, head in metres, flow in litres per second, and acceleration due to gravity in metres per second per second. (The acceleration due to gravity is approximately 9.81 metres per second per second - i.e. each second an object is falling, its speed increases by 9.81 metres per second (until it hits its terminal velocity). Therefore it is very simple to calculate how much hydro power you can generate. Let's say for example that you have a flow of 20 litres per second with a head of 12 metres. Put those figures in the equation and you will see that: 12 x 20 x 9.81 = 2,354 Watts Efficiencies of around 70% can be expected which is to say that 70% of the hydraulic energy of the flowing water can be turned into mechanical energy spinning the turbine generator. The remaining 30% is lost. Energy is again lost in converting the mechanical energy into electrical energy (electricity) and so at the end of the day you can expect a complete system efficiency of around 50-60%.
In our previous example where 2.3kW of power was available - we can therefore expect to generate around 1.1-1.4kW of electricity. |
