The Hydrological Cycle
Water in nature is a useful source of energy. It comes directly in mechanical form, without the losses associated with heat engines and fuel cells. No expenditure of fossil fuels is involved, hence its carbon footprint is zero.
Solar heat evaporates water, mostly from the oceans, where it is mixed into the lower atmosphere and moved around by the wind. Through meteorological processes, it falls back to earth as precipitation. For energy recovery purposes it is the fraction falling on high ground which is the most useful. This makes its way down hill to the sea under the influence of the earth`s gravity. When intercepted by a water wheel or water turbine this descending stream of water can be used to produce usable energy.
A cubic metre of water falling through a distance of one metre every second can produce 9.8 kW (13 Horse power) of mechanical power at close to 90% efficiency. This is readily convertible to electrical power at over 95% efficiency.
Recorded Flow Rates on the River Ure
Westwich lock Near Newby Hall Ripon: Mean Flow between 1958 & 2004 was 21.28 cubic metres per sec; Mean for 2004 was 24.5 m3/s.
Kilgram Bridge Near Thornton Steward: Mean Flow between 1967 & 2004 was 15.84 cubic metres per sec; Mean for 2004 was 17.9 m3/s.
Note 1 cubic metre per second is equivalent to 35.31 cubic feet per second or 220.72 imperial gallons per second or one metric Tonne per second (i.e. 1000 kg per second) or 9 810 Newtons per second.
Power From Water
Work has to be done to lift a quantity of water from the bottom of the waterfall to the top, usually a natural consequence of the water(hydrological) cycle. This energy comes from the sun (star drive). Whilst poised at the top of the waterfall it possesses energy (i.e. the capacity to do work) by virtue of its position, termed Potential Energy.
This energy can be released in a controlled manner when the water falls back down to the bottom of the waterfall. The water can be used to supply work via a turbine, the water being conveyed to the turbine via a pipeline termed the penstock.
The mechanical work (in Joules) required to lift the water is given by multiplying the weight of the water in Newtons by the height (gross head) it is lifted through in metres . This corresponds to the maximum work recoverable from the water if it is allowed to descend back through the same distance. To allow for losses in the pipeline etc, the term net head is sometimes used. The losses in the turbine due to factors such as leakage past the runner and unrecovered Kinetic Energy in the water leaving the turbine are taken into account by the term Hydraulic Efficiency. The efficiency and the way it varies with site operating conditions and turbine type, is considered in the next plate.
Power is defined as the rate of doing work and is measured in Joules per second or Watts.
The theoretical maximum power in watts is given by multiplying the available head of water in metres by the rate of flow in Newtons per second. Each category of turbine will be more, or less, efficient at extracting this power for a given range of head and flow rates.
Energy & Power
5.1 litres of water has a mass of 5.1 kg and weighs 50 Newtons. If this amount of
water is allowed to fall through a distance of 2 metres it will release 100 Joules of
energy. If this action takes place continuously, that is 5.1 litres falling through 2 metres every second, then if the system were 100% efficient, this could be used to generate 100 Watts of electrical power.
By comparison:
A person sat in a chair at rest will on average generate about 60 watts of heat.
It takes about 20 Joules to blow a 5 A fuse.
On average an electric shock of 1 200 Joules, hand to hand, is sufficient to kill an average person.
A digestive biscuit contains 250 000 Joules of energy.